Posts Tagged ‘ATP synthase’

Larry H. Benstein, MD, FCAP, Gurator and writer on mitochondrial function, respiration, and associated disorders

This is a condensed account of very recent published work on respiration and disturbed mitochondrail function.  We know that their is an equilibrium between respiration and autophagy in eukaryotic cells.  The Krebs Cycle produces 32 ATPs in oxidative phosphorylation, which is far more efficient than glycolysis.  There is also a different contribution of mitochondrial metabolism, in the balance, between tissues that are synthetic and those that are catabolic.  This is a subject long understood, essential for cellular energetics, and not adequately explored.


Gain-of-Function Mutant p53 Promotes Cell Growth and Cancer Cell Metabolism via Inhibition of AMPK Activation.

Zhou G1Wang J2Zhao M2Xie TX2Tanaka N2, et al.
Mol Cell. 
2014 Jun 19;54(6):960-974.   doi: 10.1016/j.molcel.2014.04.024. 

Many mutant p53 proteins (mutp53s) exert oncogenic gain-of-function (GOF) properties, but the mechanisms mediating these functions remain poorly defined.

We show here that GOF mutp53s inhibit AMP-activated protein kinase (AMPK) signaling in head and neck cancer cells.

Conversely, downregulation of GOF mutp53s enhances AMPK activation under energy stress, decreasing the activity of the anabolic factors acetyl-CoA carboxylase and ribosomal protein S6 and inhibiting aerobic glycolytic potential and invasive cell growth.

Under conditions of energy stress, GOF mutp53s, but not wild-type p53, preferentially bind to the AMPKα subunit and inhibit AMPK activation.

Given the importance of AMPK as an energy sensor and tumor suppressor that inhibits anabolic metabolism, our findings reveal that direct inhibition of AMPK activation is an important mechanism through which mutp53s can gain oncogenic function. PMID:24857548

Investigating and Targeting Chronic Lymphocytic Leukemia Metabolism with the HIV Protease Inhibitor Ritonavir and Metformin.

Adekola KUAydemir SDMa SZhou ZRosen STShanmugam M.
Leuk Lymphoma. 2014 May 14:1-23.

Chronic Lymphocytic Leukemia (CLL) remains fatal due to the development of resistance to existing therapies. Targeting abnormal glucose metabolism sensitizes various cancer cells to chemotherapy and/or elicits toxicity.

Examination of glucose dependency in CLL demonstrated variable sensitivity to glucose deprivation. Further evaluation of metabolic dependencies of CLL cells resistant to glucose deprivation revealed increased engagement of fatty acid oxidation upon glucose withdrawal.

Investigation of glucose transporter expression in CLL reveals up-regulation of glucose transporter GLUT4. Treatment of CLL cells with HIV protease inhibitor ritonavir, that inhibits GLUT4, elicits toxicity similar to that elicited upon glucose-deprivation.

CLL cells resistant to ritonavir are sensitized by co-treatment with metformin, potentially targeting compensatory mitochondrial complex 1 activity. Ritonavir and metformin have been administered in humans for treatment of diabetes in HIV patients, demonstrating the tolerance of this combination in humans. Our studies strongly substantiate further investigation of FDA approved ritonavir and metformin for CLL.

KEYWORDS:  Basic Biology; Chemotherapeutic approaches; Lymphoid Leukemia; Signal transduction             PMID: 24828872

Utilizing hydrogen sulfide as a novel anti-cancer agent by targeting cancer glycolysis and pH imbalance.

Lee ZW1Teo XYTay EYTan CHHagen TMoore PKDeng LW.
Br J Pharmacol. 2014 May 15.    doi: 10.1111/bph.12773

Many disparate studies have reported the ambiguous role of hydrogen sulfide (H2 S) in cell survival. The present study investigated the effect of H2 S on viability of cancer and non-cancer cells.

Cancer and non-cancer cells were exposed to H2 S (using sodium hydrosulfide, NaHS and GYY4137) and cell viability was examined by crystal violet assay. We then examined cancer cellular glycolysis process by in vitro enzymatic assays and pH regulator activity. Lastly, intracellular pH (pHi) was determined by ratiometric pHi measurement using BCECF staining.

Continuous, but not single, exposure to H2 S decreased cell survival more effectively in cancer cells, as compared to non-cancer cells. Slow H2 S-releasing donor, GYY4137, significantly increased glycolysis leading to overproduction of lactate. H2 S also decreased anion exchanger and sodium/proton exchanger activity. The combination of increased metabolic acid production and defective pH regulation resulted in an uncontrolled intracellular acidification leading to cancer cell death. In contrast, no significant intracellular acidification or cell death was observed in non-cancer cells.

Low and continuous exposure to H2 S targets metabolic processes and pH homeostasis in cancer cells, potentially serving as a novel and selective anti-cancer strategy.

KEYWORDS:  cancer cell death; cancer glucose metabolism; hydrogen sulfide; pH homeostasis          PMID: 24827113

Agonism of the 5-Hydroxytryptamine 1F Receptor Promotes Mitochondrial Biogenesis and Recovery from Acute Kidney Injury

Garrett SMWhitaker RMBeeson CC, and Schnellmann RG

Center for Cell Death, Injury, and Regeneration, Department of Drug Discovery and Biomedical Sciences, Medical University of South Carolina, Charleston, South Carolina (S.M.G., R.M.W., C.C.B., R.G.S.); and Ralph H. Johnson Veterans Affairs Medical Center, Charleston, South Carolina (R.G.S.)
Address correspondence to: Dr. Rick G. Schnellmann, Department of Drug Discovery and Biomedical Sciences, MUSC, Charleston, SC 29425.

Many acute and chronic conditions, such as acute kidney injury, chronic kidney disease, heart failure, and liver disease, involve mitochondrial dysfunction. Although we have provided evidence that drug-induced stimulation of mitochondrial biogenesis (MB) accelerates mitochondrial and cellular repair, leading to recovery of organ function, only a limited number of chemicals have been identified that induce MB.

The goal of this study was to assess the role of the 5-hydroxytryptamine 1F (5-HT1F) receptor in MB. Immunoblot and quantitative polymerase chain reaction analyses revealed 5-HT1F receptor expression in renal proximal tubule cells (RPTC). A MB screening assay demonstrated that two selective 5-HT1F receptor agonists,

  1. LY334370 (4-fluoro-N-[3-(1-methyl-4-piperidinyl)-1H-indol-5-yl]benzamide) and
  2. LY344864 (N-[(3R)-3-(dimethylamino)-2,3,4,9-tetrahydro-1H-carbazol-6-yl]-4-fluorobenzamide; 1–100 nM)

increased carbonylcyanide-p-trifluoromethoxyphenylhydrazone–uncoupled oxygen consumption in RPTC, and

  • validation studies confirmed both agonists increased mitochondrial proteins  in vitro.
    [e.g., ATP synthase β, cytochrome c oxidase 1 (Cox1), and NADH dehydrogenase (ubiquinone) 1β subcomplex subunit 8 (NDUFB8)]

Small interfering RNA knockdown of the 5-HT1F receptor

  • blocked agonist-induced MB.

Furthermore, LY344864 increased

  • peroxisome proliferator–activated receptor (PPAR) coactivator 1-α, Cox1, and
  • NDUFB8 transcript levels and
  • mitochondrial DNA (mtDNA) copy number

in murine renal cortex, heart, and liver.

Finally, LY344864 accelerated recovery of renal function, as indicated by

  • decreased blood urea nitrogen and kidney injury molecule 1 and
  • increased mtDNA copy number

following ischemia/reperfusion-induced acute kidney injury (AKI).

In summary, these studies reveal that

  • the 5-HT1F receptor is linked to MB, 5-HT1F receptor agonism promotes MB in vitro and in vivo, and

5-HT1F receptor agonism promotes recovery from AKI injury.

Induction of MB through 5-HT1F receptor agonism represents a new target and approach to treat mitochondrial organ dysfunction.


  • Portions of this work have been presented previously: Garrett SM, Wills LP, and Schnellmann RG (2012) Serotonin (5-HT) 1F receptor agonism as a potential treatment for acceleration of recovery from acute kidney injury.American Society of Nephrology Annual Meeting; 2012 Nov 1–4; San Diego, CA.

Ca2+ regulation of mitochondrial function in neurons.

Rueda CB1Llorente-Folch I1Amigo I1Contreras L1González-Sánchez P1Martínez-Valero P1Juaristi I1Pardo B1Del Arco A2Satrústegui J3

Biochim Biophys Acta. 2014 May 10. pii: S0005-2728(14)00126-1.
doi: 10.1016/j.bbabio.2014.04.010.

Calcium is thought to regulate respiration but it is unclear whether this is dependent on the increase in ATP demand caused by any Ca2+ signal or to Ca2+ itself.

[Na+]i, [Ca2+]i and [ATP]i dynamics in intact neurons exposed to different workloads in the absence and presence of Ca2+ clearly showed that

  • Ca2+-stimulation of coupled respiration is required to maintain [ATP]i levels.

Ca2+ may regulate respiration by

  1. activating metabolite transport in mitochondria from outer face of the inner mitochondrial membrane, or
  2. after Ca2+ entry in mitochondria through the calcium uniporter (MCU).

Two Ca2+-regulated mitochondrial metabolite transporters are expressed in neurons,

  1. the aspartate-glutamate exchanger ARALAR/AGC1/Slc25a12, a component of the malate-aspartate shuttle, with a Kd for Ca2+ activation of 300nM, and
  2. the ATP-Mg/Pi exchanger SCaMC-3/Slc25a23, with S0.5 for Ca2+ of 300nM and 3.4μM, respectively.

The lack of SCaMC-3 results in a smaller Ca2+-dependent stimulation of respiration only at high workloads, as caused by veratridine, whereas

  • the lack of ARALAR reduced by 46% basal OCR in intact neurons using glucose as energy source and the Ca2+-dependent responses to all workloads (veratridine, K+-depolarization, carbachol).

The lack of ARALAR caused a reduction of about 65-70% in the response to the high workload imposed by veratridine, and

  • completely suppressed the OCR responses to moderate (K+-depolarization) and small (carbachol) workloads,
  • effects reverted by pyruvate supply.

For K+-depolarization, this occurs in spite of the presence of large [Ca2+]mit signals and increased reduction of mitochondrial NAD(P)H.

These results show that ARALAR-MAS is a major contributor of Ca2+-stimulated respiration in neurons

  • by providing increased pyruvate supply to mitochondria.

In its absence and under moderate workloads, matrix Ca2+ is unable to stimulate pyruvate metabolism and entry in mitochondria suggesting a limited role of MCU in these conditions.

This article was invited for a Special Issue entitled: 18th European Bioenergetic Conference.    Copyright © 2014. Published by Elsevier B.V.

KEYWORDS:  ATP-Mg/Pi transporter; Aspartate–glutamate transporter; Calcium; Calcium-regulated transport; Mitochondrion; Neuronal respiration PMID: 24820519


Sestrin2 inhibits uncoupling protein 1 expression through suppressing reactive oxygen species.

Ro SH1Nam M2Jang I1Park HW1Park H1Semple IA1Kim M1et al.
Proc Natl Acad Sci U S A. 2014 May 27;111(21):7849-54.
doi: 10.1073/pnas.1401787111.

Uncoupling protein 1 (Ucp1), which is localized in the mitochondrial inner membrane of mammalian brown adipose tissue (BAT), generates heat by uncoupling oxidative phosphorylation. Upon cold exposure or nutritional abundance, sympathetic neurons stimulate BAT to express Ucp1 to induce energy dissipation and thermogenesis. Accordingly, increased Ucp1 expression reduces obesity in mice and is correlated with leanness in humans.

Despite this significance, there is currently a limited understanding of how Ucp1 expression is physiologically regulated at the molecular level. Here, we describe the involvement of Sestrin2 and reactive oxygen species (ROS) in regulation of Ucp1 expression. Transgenic overexpression of Sestrin2 in adipose tissues inhibited both basal and cold-induced Ucp1 expression in interscapular BAT, culminating in decreased thermogenesis and increased fat accumulation.

Endogenous Sestrin2 is also important for suppressing Ucp1 expression because BAT from Sestrin2(-/-) mice exhibited a highly elevated level of Ucp1 expression. The redox-inactive mutant of Sestrin2 was incapable of regulating Ucp1 expression, suggesting that Sestrin2 inhibits Ucp1 expression primarily through reducing ROS accumulation.

Consistently, ROS-suppressing antioxidant chemicals, such as butylated hydroxyanisole and N-acetylcysteine, inhibited cold- or cAMP-induced Ucp1 expression as well. p38 MAPK, a signaling mediator required for cAMP-induced Ucp1 expression, was inhibited by either Sestrin2 overexpression or antioxidant treatments.

Taken together, these results suggest that Sestrin2 and antioxidants inhibit Ucp1 expression through suppressing ROS-mediated p38 MAPK activation, implying a critical role of ROS in proper BAT metabolism.

KEYWORDS: aging; homeostasis; mouse; β-adrenergic signaling      PMID: 24825887     PMCID:  PMC4040599

Mitochondrial EF4 links respiratory dysfunction and cytoplasmic translation in Caenorhabditis elegans.

Yang F1Gao Y1Li Z2Chen L3Xia Z4Xu T5Qin Y6
Biochim Biophys Acta. 2014 May 15. pii: S0005-2728(14)00499-X.
doi: 10.1016/j.bbabio.2014.05.353.

How animals coordinate cellular bioenergetics in response to stress conditions is an essential question related to aging, obesity and cancer. Elongation factor 4 (EF4/LEPA) is a highly conserved protein that promotes protein synthesis under stress conditions, whereas its function in metazoans remains unknown.

Here, we show that, in Caenorhabditis elegans, the mitochondria-localized CeEF4 (referred to as mtEF4) affects mitochondrial functions, especially at low temperature (15°C).

At worms’ optimum growing temperature (20°C), mtef4 deletion leads to self-brood size reduction, growth delay and mitochondrial dysfunction.

Transcriptomic analyses show that mtef4 deletion induces retrograde pathways, including mitochondrial biogenesis and cytoplasmic translation reorganization.

At low temperature (15°C), mtef4 deletion reduces mitochondrial translation and disrupts the assembly of respiratory chain supercomplexes containing complex IV.

These observations are indicative of the important roles of mtEF4 in mitochondrial functions and adaptation to stressful conditions.

Copyright © 2014. Published by Elsevier B.V.

KEYWORDSC. elegans; EF4(LepA/GUF1); Mitochondrial dysfunction; Retrograde pathways; Translation    PMID:  24837196

The metabolite α-ketoglutarate extends lifespan by inhibiting ATP synthase and TOR.

Chin RM1Fu X2Pai MY3Vergnes L4Hwang H5Deng G6Diep S2, et al.
Nature  2014 Jun 19;509(7505):397-401. doi: 10.1038/nature13264. 

Metabolism and ageing are intimately linked. Compared with ad libitum feeding, dietary restriction consistently extends lifespan and delays age-related diseases in evolutionarily diverse organisms. Similar conditions of nutrient limitation and genetic or pharmacological perturbations of nutrient or energy metabolism also have longevity benefits.

Recently, several metabolites have been identified that modulate ageing; however, the molecular mechanisms underlying this are largely undefined. Here we show that α-ketoglutarate (α-KG), a tricarboxylic acid cycle intermediate, extends the lifespan of adult Caenorhabditis elegans.

ATP synthase subunit β is identified as a novel binding protein of α-KG using a small-molecule target identification strategy termed drug affinity responsive target stability (DARTS). The ATP synthase, also known as complex V of the mitochondrial electron transport chain, is the main cellular energy-generating machinery and is highly conserved throughout evolution.

Although complete loss of mitochondrial function is detrimental, partial suppression of the electron transport chain has been shown to extend C. elegans lifespan.

We show that α-KG inhibits ATP synthase and, similar to ATP synthase knockdown, inhibition by α-KG leads to reduced ATP content, decreased oxygen consumption, and increased autophagy in both C. elegans and mammalian cells.

We provide evidence that the lifespan increase by α-KG requires ATP synthase subunit β and is dependent on target of rapamycin (TOR) downstream.

Endogenous α-KG levels are increased on starvation and α-KG does not extend the lifespan of dietary-restricted animals, indicating that α-KG is a key metabolite that mediates longevity by dietary restriction.

Our analyses uncover new molecular links between a common metabolite, a universal cellular energy generator and dietary restriction in the regulation of organismal lifespan, thus suggesting new strategies for the prevention and treatment of ageing and age-related diseases.

PMID: 24828042




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Mitochondrial Metabolism and Cardiac Function

Curator: Larry H Bernstein, MD, FACP

This article is the SECOND in a four-article Series covering the topic of the Roles of the Mitochondria in Cardiovascular Diseases. They include the following;

The mitochondrion serves a critical role as a platform for
  • energy transduction,
  • signaling, and
  • cell death pathways
relevant to common diseases of the myocardium such as heart failure. This review focuses on the molecular regulatory events involved in mitochondrial energy metabolism.
This is followed by the derangements known to occur in the development of heart failure.

 Cardiac Energy Metabolism

All cellular processes are driven by ATP-dependent pathways. The heart has perpetually high energy demands related to
  • the maintenance of specialized cellular processes, including
    • ion transport,
    • sarcomeric function, and
    • intracellular Ca2+ homeostasis.
Myocardial workload (energy demand) and energy substrate availability (supply) are in continual flux. Thus, ATP-generating pathways must

  • respond proportionately to dynamic fluctuations in physiological demands and fuel delivery.
In order to support contractile activity, the human heart requires
  • a daily synthesis of approximately 30kg of ATP, via
    • oxidative phosphorylation at
    • the inner mitochondrial membrane.
These metabolic  processes are regulated, involving
  • allosteric control of enzyme activity,
  • signal transduction events, and
  • the activity of genes encoding
    • rate-limiting enzymes and proteins.
Catabolism of exogenous substrates ,such as
  • fatty acids,
  • glucose,
  • pyruvate,
  • lactate and
  • ketone bodies,
generates most of the reduced compounds,
  • NADH (nicotinamide adenine dinucleotide, reduced) and
  • FADH2 (flavin adenine dinucleotide, reduced),
which are necessary for mitochondrial electron transport (Fig. 1).
Fig 1  Fatty acid beta-oxidation and the Krebs cycle produce
  1. nicotinamide adenine dinucleotide, reduced (NADH) and
  2. flavin adenine dinucleotide, reduced(FADH2),
which are oxidized by complexes I and II, respectively, of
Electrons are transferred through the chain to the final acceptor, namely oxygen(O2).
The free energy from electron transfer
  1. is used to pump hydrogen out of the mitochondria and
  2. generate an electrochemical gradient across the inner mitochondrial membrane.
This gradient is the driving force for ATP synthesis via the ATP synthase. Alternatively,
H can enter the mitochondria by a mechanism not coupled to ATP synthesis, via
  • the uncoupling proteins(UCPs), which results in the dissipation of energy.

[ANT, adenine nucleotide translocase; CoA, coenzymeA; FAT, fatty acid transporter; GLUT, glucose transporter;

NAD, nicotinamide adenine dinucleotide; TCA, tricarboxylic acid].

Cardiac Energy Metabolic Pathways

 Oxidation of free fatty acids (FFAs) and glucose in mitochondria
  • accounts for the vast majority of ATP generation in the healthy adult heart.
FFAs are the preferred substrate in the adult myocardium,
  • supplying 70-90% of total ATP.
FAs derived from circulating triglyceride-rich lipoproteins and albumin bound nonesterified FAs
  • are oxidized in the mitochondrial matrix by the process of beta-oxidation (FAO), whereas
pyruvate derived from glucose and lactate
  • is oxidized by the pyruvate-dehydrogenase (PDH) complex,
    • localized within the inner mitochondrial membrane.
Acetyl-CoA, derived from both pathways,
  • enters the tricarboxylic acid (TCA) cycle.
Reduced flavin adenine dinucleotide (FADH2) and NADH are generated, respectively, via
  • substrate flux through the
The reducing equivalents enter the electron transport (ET) chain,
  • producing an electrochemical gradient across the mitochondrial membrane
  • that drives ATP synthesis in the presence of molecular oxygen (oxidative phosphorylation).
The relative contributions of each of these substrates are determined
  • by their availability
  • cardiac workload and
  • hormonal status
In the healthy, normal heart, the ATP requirement is largely met in the actively metabolic mitochondria by
  • the catabolism of free fatty acids (FFAs) via beta-oxidation,
  • the tricarboxylic acid cycle and
  • oxidative phosphorylation
giving rise to a greater ATP yield per C2 unit than with glucose.
The relative contribution of glucose to the mitochondrial acetyl-coenzyme A (CoA) pool increases
  • during the postprandial period,
    • when the heart is insulin stimulated, and
  • during exercise
  • hypoxia, or
  • ischemia
when glucose is favored as a more oxygen-efficient substrate than
  • FFAs (greater ATP yield per oxygen molecule consumed).
Substrate switching in the heart can also be achieved by
  • acute alterations in transcriptional regulation of key metabolic enzymes
  • in response to alterations in substrate levels and oxygen availability, or
  • indeed by the intracellular circadian clock.
This continual process of fine adjustment in fuel selection
  • allows cardiac mitochondria to function
  • under a range of metabolic conditions to meet the high energy demands of the heart.
Mitochondrial enzymes are encoded by both nuclear and mitochondrial genes.
All of the enzymes of
  1. beta-oxidation and the TCA cycle, and
  2. most of the subunits of Electron Transfer/Oxidative Phosphorylation,
    • are encoded by nuclear genes.
The mitochondrial genome is comprised of
  • 1 circular double-stranded chromosome that encodes
  • 13 ET chain subunits within complexes I, III, and IV.
Since mitochondrial number and function require both nuclear and mitochondrial-encoded genes,
  • coordinated mechanisms exist to regulate the 2 genomes and
  • determine overall cardiac oxidative capacity.
In addition, distinct pathways exist to coordinately regulate
  • nuclear genes encoding component mitochondrial pathways.

Early Postnatal Low-protein Nutrition, Metabolic Programming and
the Autonomic Nervous System in Adult Life.

JC de Oliveira, S Grassiolli, C Gravena, PCF de Mathias  Nutr Metab. 2012;9(80)

The developmental origins of health and disease (DOHaD) hypothesis stipulates that adult metabolic disease

  • may be programmed during the perinatal stage.

A large amount of evidence suggests that the etiology of obesity is not only related to food abundance

  • but also to food restriction during early life.

Protein restriction during lactation has been used as a rat model of metabolic programming

  • to study the impact of perinatal malnutrition on adult metabolism.

In contrast to protein restriction during fetal life, protein restriction during lactation did not appear to cause

  • either obesity or the hallmarks of metabolic syndrome, such as hyperinsulinemia, when individuals reached adulthood.

Protein restriction provokes body underweight and hypoinsulinemia.
Hypoinsulinemia programs adult rats to maintain normoglycemia,

  • pancreatic β-cells are less sensitive to secretion stimuli:
  1.  glucose and
  2. cholinergic agents.

These pancreatic dysfunctions are attributed to an imbalance of ANS activity

  • recorded in adult rats that experienced maternal protein restriction

Several studies have reported that the ANS activity is altered in under- or malnourished organisms. After weaning,

  • rats fed a chronically protein-deficient diet exhibited low activity of the vagus nerve,
  • whereas high sympathetic activity was recorded

These data were in agreement with a low insulin response to glucose.
Pancreatic islets isolated from protein-restricted rats showed

  • weak glucose and cholinergic insulin tropic responses
  • suggesting that pancreatic β-cell dysfunction may be attributed to altered ANS activity

Food abundance or restriction with regard to body weight control involves changes in

  • metabolic homeostasis and ANS balance activity.

Although the secretion of insulin by the pancreatic β-cells is increased in people who were overweight,

  • it is diminished in people who were underweight.

Changes in the ANS activity may constitute the mechanisms underlying the β-cell dysfunction:

  • the high PNS tonus observed in obese individuals constantly potentiates insulin secretion,
  • whereas the low activity reported in underweight individuals is associated with a weak cholinergic insulin tropic effect.

Under Nutrition Early in Life and Epigenetic Modifications, Association With Metabolic Diseases Risk

relevant to this issue is the role of epigenetic changes in the increased risk of developing metabolic diseases,

  • such as type 2 diabetes and obesity, later in life.

Epigenetic mechanisms, such as DNA methylation and/or nucleoprotein acetylation/methylation, are

  • crucial to the normal/physiological development of several tissues in mammals, and
  • they involve several mechanisms to guarantee fluctuations of enzymes and other proteins that regulate the metabolism.

The intrauterine phase of development is particularly important for the genomic processes related to genes associated with metabolic pathways.
This phase of life may be particularly important for nutritional disturbance. In humans who experienced the Dutch famine Winter in 1944–1945 and
in rats that were deprived of food in utero, epigenetic modifications were detected in

  • the insulin-like growth factor 2 (IGF2) and
  • pancreatic and duodenal home box 1 (Pdx1),

the major factors involved in pancreas development and pancreatic β-cell maturation.
The pancreas and the pancreatic β-cells develop during the embryonic phase, but the postnatal life is also crucial for

  • the maintenance processes that control the β-cell mass:
  1. proliferation,
  2. neogenesis
  3. apoptosis.

Nutritional Restriction to the Fetus: A Risk of Obesity Onset

If an abundant diet is offered to people who have been undernourished during the perinatal life,

  • this opportunity induces a metabolic shift toward the storage of energy and high fat tissue accumulation

The concept of Developmental Origins of Health and Disease extends to any type of stressful situations that may

  • predispose babies or pups to develop metabolic disorders when they reach adulthood.

Programmed Metabolism and Insulin Secretion-coupling Process

What are the mechanisms involved in the low glucose insulin tropic response observed in low protein-programmed lean rats?
The pancreatic β-cells secrete insulin when stimulated mostly by glucose. However, several nutrients, such as

  • amino acids,
  • fatty acids,
  • and their metabolites,

stimulate cellular metabolism and increase ATP production.

ATP-sensitive potassium channels (KATP) are inactivated by an increased ATP/ADP ratio. This provokes

  • membrane depolarization and
  • the activation of voltage-dependent calcium channels.

These ionic changes increase the intracellular calcium concentration, which is involved in

  • the export of insulin to the bloodstream.

Glucose may also stimulate insulin secretion by alternative pathways involving KATP channels.

Programmed Metabolism and Insulin Tropic Effects of Neurotransmitters

Insulin release is modulated by non-nutrient secretagogues, such as neurotransmitters, which

  • enhance or inhibit glucose-stimulated insulin secretion.

Pancreatic β-cells contain several receptors for neurotransmitters and Neuropeptide, such as

  • adrenoceptors and cholinergic muscarinic receptors (mAChRs).

These receptors are stimulated by efferent signals from the central nervous system, including the ANS,

  • throughout their neural ends for pancreatic β-cells.

During blood glucose level oscillations, the β-cells receive inputs from

  • the parasympathetic and sympathetic systems to participate in glycemic regulation.

Overall, acetylcholine promotes the potentiation of glucose-induced insulin secretion,

  • whereas noradrenaline and adrenaline inhibit this response.

Functional studies of mAChR subtypes have revealed that M1 and particularly M3 are the receptors that are involved in

  • the insulin tropic effect of acetylcholine.

Interestingly, it was reported that M3mAChR gene knockout mice are

  • underweight,
  • hypophagic and
  • hypoinsulinemic,

as are adult rats that were protein-restricted during lactation.
The pancreatic islets from M3mAChR mice (-/-) showed a reduced secretory response to cholinergic agonists.
In studies using transgenic mice in which the pancreatic β-cell M3mAChRs are chronically stimulated,

  • an improvement of glycemic control has been observed

Adult male rat offspring from whose mothers were protein-restricted during lactation

  • exhibit a low PNS activity.

Evidence suggests that ANS changes may contribute to the impairment of glycemic homeostasis in metabolically programmed rats.

Pathways involved in cardiac energy metabolism.

FA and glucose oxidation are the main ATP-generating pathways in the adult mammalian heart.
Acetyl-CoA derived from FA and glucose oxidation is
  • further oxidized in the TCA cycle to generate NADH and FADH2, which
  • enter the ET/oxidative phosphorylation pathway and drive ATP synthesis.
Genes encoding enzymes involved at multiple steps of these metabolic pathways
  1. uptake,
  2. esterification,
  3. mitochondrial transport,
  4. and oxidation
are transcriptionally regulated by PGC-1a
  • with its nuclear receptor partners, including PPARs and ERRs .
Glucose uptake/oxidation and electron transport/oxidative phosphorylation pathways are also regulated by PGC-1a via
  • other transcription factors, such as MEF-2 and NRF-1.

[Cyt c, cytochrome c]

 Fetal metabolism of carbohydrate utilization

This reviewer poses the question of whether the fetal cardiac metabolism, which is characterized by a (facultative) anaerobic glycolysis,
  • results in lactate production that is not redirected into the TCA cycle.
An unexamined, but related question is whether there is an associated change in the ratio of
  • mitochondrial to cytoplasmic malate dehydrogenase isoenzyme activity (m-MDH:c-MDH).
The fetal heart operates without oxygenation from a functioning lung, bathed in amniotic fluid.
An enzymatic feature might be expressed in a facultative anaerobic cytplasmic glycolytic pathway characterized by
  • a decrease in the h-type lactate dehydrogenase (LD) isoenzyme(s) (LD1, LD2) with a predominance of
  • the m-type LD isoenzymes (LD3, LD4, LD5).
The observation here is that the heart muscle is a syncytium, and it functions at a highly regulated rate,
  • not with the spurts of activity seen in skeletal muscle.
In another article in this series, there are morphological changes that occur in the heart mitochondria, and
  • there are three locations, as if the organelle itself were an organ.
The normal functioning myocardium can utilize lactic acid accumulated in the bloodstream during extreme exercise as fuel.
This is a virtue of mitochondrial function.  There is a significant functional difference between the roles of the h- and m-type LD isoenzymes.
The h-type is a regulatory enzyme that forms a complex as NADH is converted to NAD+ between the
  • LD (H4, H3M; LD1, LD2),
  • oxidized pyridine nucleotide coenzyme, and
  • pyruvate
The complex forms in 200 msec as observed in the Aminco-Morrow stop-flow analyzer.  This is not the case for the m-type isoenzyme.
I presume that it is not a factor in embryonic heart.  It would become a factor after birth with the expansion of the lungs.
This would also bring to the discussion the effect of severe restrictive lung disease on cardiac metabolism.

Related References:

LH Bernstein,  patents: Malate dehydrogenase method,  The lactate dehydrogenase method
LH Bernstein, J Everse. Determination of the isoenzyme levels of lactate dehydrogenase. Methods Enzymol 1975; 41 47-52    ICID: 825516
LH Bernstein, J Everse, N Shioura, PJ Russell. Detection of cardiac damage using a steady state assay for lactate dehydrogenase isoenzymes in serum.   J Mol Cell Cardiol 1974; 6(4):297-315  ICID: 825597
LH Bernstein, MB Grisham, KD Cole, J Everse . Substrate inhibition of the mitochondrial and cytoplasmic malate dehydrogenases. J Biol Chem 1978; 253(24):8697-8701. ICID: 825513
R Belding, L Bernstein, G Reynoso. An evaluation of the immunochemical LD1 method in routine clinical practice. Clin Chem 1981; 27(10):1027-1028.   ICID: 844981
J Adan, L H Bernstein, J Babb. Lactate dehydrogenase isoenzyme-1/total ratio: accurate for determining the existence of myocardial infarction. Clin Chem 1986; 32(4):624-628.  ICID: 825540
MB Grisham, LH Bernstein, J Everse. The cytoplasmic malate dehydrogenase in neoplastic tissues; presence of a novel isoenzyme? Br J Cancer 1983; 47(5):727-731. ICID: 825551
LH Bernstein, P Scinto. Two methods compared for measuring LD-1/total LD activity in serum. Clin Chem 1986; 32(5):792-796.   ICID: 825581

PGC-1a: an inducible integrator of transcriptional circuits

 The PPAR³ coactivator-1 (PGC-1) family of transcriptional coactivators is involved in regulating mitochondrial metabolism and biogenesis.
PGC-1a was the first member discovered through its functional interaction with the nuclear receptor PPAR³ in brown adipose tissue (BAT).
There are two PGC-1a related coactivators,
  1. PGC-1² (also called PERC) and
  2. PGC-1–related coactivator (PRC).
PRC coactivates transcription in mitochondrial biogenesis, with PGC-1a and PGC-1² . Both are expressed in tissues with high oxidative capacity, such as
  1. heart
  2. slow-twitch skeletal muscle, and
  3. BAT
They serve critical roles in the regulation of mitochondrial functional capacity. PGC-1a  also regulates
  • hepatic gluconeogenesis and
  • skeletal muscle glucose uptake.
PGC-1² appears to be important in regulating energy metabolism in the heart, but
  • PGC-1a is distinct from other PGC-1 family members, indeed from most coactivators, in its broad responsiveness to
  1. developmental alterations in energy metabolism and
  2. physiological and pathological cues at the level of expression and transactivation.
In the heart, PGC-1a expression increases at birth coincident with an increase in cardiac oxidative capacity and
  • a perinatal shift from reliance on glucose metabolism to the oxidation of fats for energy.
PGC-1a is induced by physiological stimuli that increase ATP demand and
  • stimulate mitochondrial oxidation, including
  1. cold exposure,
  2. fasting, and
  3. exercise.
Activation of this regulatory cascade increases cardiac mitochondrial oxidative capacity in the heart. In cardiac myocytes in culture, it
  1. increases mitochondrial number,
  2. upregulates expression of mitochondrial enzymes, and
  3. increases rates of FA oxidation and coupled respiration.
Thus, PGC-1a is an inducible coactivator that coordinately regulates
  • cardiac fuel selection and
  • mitochondrial ATP-producing capacity.
 PGC-1a activates expression of nuclear respiratory factor-1 (NRF-1) and NRF-2 and
  • directly coactivates NRF-1 on its target gene promoters.
NRF-1 and NRF-2 regulate expression of mitochondrial transcription factor A (Tfam),
  • a nuclear-encoded transcription factor that binds regulatory sites on mitochondrial DNA and is essential for
  1. replication,
  2. maintenance, and
  3. transcription of the mitochondrial genome.
Furthermore, NRF-1 and NRF-2 regulate the expression of nuclear genes encoding
  • respiratory chain subunits and other proteins required for mitochondrial function.
PGC-1a  also
  • coactivates the PPAR and ERR nuclear receptors, critical regulators of myocardial FFA utilization.
  • regulates genes involved in the cellular uptake and mitochondrial oxidation of FFAs.
  • is an integrator of the transcriptional network regulating mitochondrial biogenesis and function.
Numerous signaling pathways, by increasing either PGC-1a expression or activity, such as –
  • Ca2+-dependent,
  • NO,
  • MAPK, and
  • beta-adrenergic pathways (beta3/cAMP),
    • activate the PGC-1a directly
Additionally, the p38_MAPK pathway
  • selectively activates PPARa, which may bring about synergistic activation in the presence of PGC-1a,
  • whereas ERK-MAPK has the opposite effect.
These signaling pathways transduce physiological stimuli to the PGC-1a pathway:
  1. stress
  2. fasting
  3. exercise
PGC-1a, in turn, coactivates transcriptional partners,which regulate mitochondrial biogenesis and FA-oxidation pathways:
  • NRF-1 and -2,
  • ERRa, and
  • PPARa,
 Insights into the physiological responsiveness of the PGC-1a pathway come from
  • identification of signal transduction pathways that modulate the activity of PGC-1a and its downstream partners.
PGC-1a is upregulated in response to beta-adrenergic signaling, consistent with the involvement of this pathway in thermogenesis.
The stress-activated  p38_MAPK activates PGC-1a by increasing PGC-1a protein stability and promoting dissociation of a repressor.
p38 increases mitochondrial FAO through selective activation of the PGC-1a partner, PPARa. Conversely, the ERK-MAPK pathway
  • inactivates the PPARa/RXRa complex via direct phosphorylation.
Therefore, distinct limbs of the MAPK pathway exert
  • opposing regulatory influences on the PGC-1a cascade.
Recently, NO has emerged as a novel signaling molecule proposed to integrate pathways involved in
  • regulating mitochondrial biogenesis by inducing mitochondrial proliferation.

 A Paradox

Mitochondria are like little cells within our cells. They are the energy producing organelles of the body. The more energy a certain tissue requires
  • such as the brain and the heart
    • the more mitochondria those cells contain.
Conventional transmission electron microscopy of mammalian cardiac tissue reveals mitochondria to be
  1. elliptical individual organelles situated either in clusters beneath the sarcolemma (subsarcolemmal mitochondria, SSM) or
  2. in parallel, longitudinal rows ensconced within the contractile apparatus (interfibrillar mitochondria, IFM).
The two mitochondrial populations differ in their cristae morphology, with
  1. a lamelliform orientation in SSM, whereas
  2. the cristae orientation in IFM is tubular.
The morphology of mitochondria is responsive to changes in cardiomyocytes.
 Mitochondrial oxidative phosphorylation relies
  • not only on the activities of individual complexes, but also on
  • the coordinated action of supramolecular assemblies (respirasomes) of the electron transport chain (ETC) complexes
in both normal and failing heart.
Mitochondria have their own set of DNA and
  • the more energy they generate,
  • the more DNA-damaging free radicals they produce.
Mitochondrial DNA damage is incurred by generation of energy in ATP production, so that
  • the process that sustains life also is the source of toxic damage that causes the dysfunction and mitogeny in the cell.
In human mtDNA mutant cybrids with impaired mitochondrial respiration, the recovery of mitochondrial function
  • correlates with the formation of respirasomes suggesting that
  • respirasomes represent regulatory units of mitochondrial oxidative phosphorylation
    • by facilitating the electron transfer between the catalytic sites of the ETC.
We recently reported a decrease in mitochondrial respirasomes in CHF that fits in the category of a new mitochondrial cytopathy.
 ATP utilized by the heart is synthesized mainly by means of oxidative phosphorylation in the inner mitochondrial membrane,
  • a process that involves the coupling of electron transfer and oxygen consumption with phosphorylation of ADP to ATP.
The catabolism of exogenous substrates (FAs, glucose, pyruvate, lactate, and ketone bodies) provides the reduced intermediates,
  1. NADH (nicotinamide adenine dinucleotide, reduced) and
  2. FADH2 (flavin adenine dinucleotide, reduced),
as donors for mitochondrial electron transport.
The contribution of glucose to the acetyl CoA pool in the heart is
  • increased by insulin during the postprandial period and during exercise.
 All cells and tissues require
  • adenine,
  • pyridine, and
  • flavin nucleotides for energy
by way of Krebs cycle metabolism of fatty acids and carbohydrate substrates.
If DNA holds the blueprint for the proper function of a cell, then any change in the blueprint will change how the cell functions.
If the mitochondria do not function properly, then they cannot fulfill their role in producing energy:
  •  the cell will lose its ability to function adequately.

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Author and Curator: Ritu Saxena, Ph.D.

Consultants: Aviva Lev-Ari, PhD, RN and Pnina G. Abir-Am, PhD


Section I   : Mitochondrial diseases and molecular understanding

Section II  : Diagnosis and therapy of mitochondrial diseases

Section III: Mitochondria, metabolic syndrome and research


Mitochondrial cytopathy in adults – current understanding:

Mitochondrial cytopathies are a diverse group of inherited and acquired disorders that result in inadequate energy production leading to illnesses. Several syndromes have been linked to mutations in mitochondrial DNA. Some key features common to mitochondrial diseases are listed as follows:

  • Diverse manifestations of mitochondrial diseases: Although all mitochondrial diseases have the same characteristic of inadequate energy production as compared to the demand, they seem to show diverse manifestations in the form of organs being affected, age of onset and the rate of progression. Reason lies in the unique genetic makeup of mitochondria. The percentage of mtDNA carrying defects varies when the ovum divides and one daughter cells receiving more defective mtDNA and the other receiving less. Hence, successive divisions may lead to accumulation of defects in one of the developing organs or tissues. Since the process in which defective mtDNA becomes concentrated in an organ is random, this may account for the differing manifestations among patients with the same genetic defect. Also, somatic mutations and mutations occurring as a result of exposure to environmental toxins may cause mitochondrial diseases.

As stated by Robert K. Naviaux, founder and co-director of the Mitochondrial and Metabolic Disease Center (MMDC) at the University of California, San Diego;  

“It is a hallmark of mitochondrial diseases that identical mtDNA mutations may not produce identical diseases…the converse is also true, different mutations can lead to the same diseases.”

  • Postmitotic tissues are more vulnerable to mitochondrial diseases: Postmitotic tissues such as those in the brain, muscles, nerves, retinas, and kidneys, are vulnerable for several reasons. Apart from the fact that these tissues have high-energy demands, healthier neighboring cells unlike that observed in skin cannot replace the diseased cells. Thus, mutations in mtDNA accumulate over a period of time resulting in progressive dysfunction of individual cells and hence the organ itself.
  • High rate of mtDNA mutation: MtDNA mutates at rate that is six-seven times higher than the rate of mutation of nuclear DNA. First reason is the absence of histones on mtDNA and second is the exposure of mtDNA to free radicals due to their close proximity to electron transport chain. Additionally, lack of DNA repair enzymes results in mutant tRNA, rRNA and protein transcripts

Spectrum of mitochondrial diseases:

Following is the list of mitochondrial diseases occurring as a result of either mtDNA mutations, alteration in mitochondrial function or those diseases that sometimes might be associated with mitochondrial dysfunction.

  • Disorders associated with mtDNA mutations-

MELAS, MERRF, NARP, Myoneurogastrointestinal disorder and encephalopathy (MNGIE), Pearson Marrow syndrome Kearns-Sayre-CPEO, Leber hereditary optic neuropathy (LHON), Aminoglycoside-associated deafness, Diabetes with deafness

  • Mendelian disorders of mitochondrial function related to fuel homeostasis-

Luft disease, Leigh syndrome (Complex I, COX, PDH), Alpers Disease, MCAD, SCAD, SCHAD, VLCAD, LCHAD, Glutaric aciduria II, Lethal infantile cardiomyopathy, Friedreich ataxia, Maturity onset diabetes of young Malignant hyperthermia, Disorders of ketone utilization, mtDNA depletion syndrome, Reversible COX deficiency of infancy, Various defects of the Krebs Cycle, Pyruvate dehydrogenase deficiency, Pyruvate carboxylase deficiency, Fumarase deficiency, Carnitine palmitoyl transferase deficiency

  • Disorders sometimes associated with mitochondrial function-

Hemochromatosis, Wilson disease, Batten disease, Huntington disease, Menkes disease, Lesch-Nyhan syndrome, Aging, Type II diabetes mellitus, Atherosclerotic heart disease, Parkinson disease, Alzheimer dementia, Congestive heart failure, Niacin-responsive hypercholesterolemia, Postpartum cardiomyopathy, Alcoholic myopathy, Cancer metastasis, Irritable bowel syndrome Gastroparesis-GI dysmotility, Multiple sclerosis, Systemic lupus erythematosis, Rheumatoid arthritis.



Owing to the diversity of symptoms, there is no accepted criterion for diagnosis. Also, due to overlapping symptoms of several diseases with those of mitochondrial dysfunction illnesses, it is important to evaluate the patient for other conditions. A diagnosis could involve combination of molecular genetic, pathologic, or biochemical data in a patient who has clinical features consistent with the diagnosis including mutational analysis on blood lymphocytes and possibly muscle biopsy for visual and biochemical analysis.

The two main biochemical features in most mtDNA disorders are:

  1. Respiratory chain deficiency and
  2. Lactic acidosis.

Skeletal muscle is chosen to study the pathogenic consequence of mtDNA mutations because of the formation of ragged-red fibers (RRF) through mitochondrial proliferation and massive mitochondrial accumulation in many pathogenic situations. RRF can be detected in two ways. Mitochondrial fibers in a subset of these fibers are shown by red or purple stained area by Gomori trichrome stain; the normal or less-affected fibers stain blue or turquoise. Deep purple areas show accumulations of mitochondria as activity of succinate dehydrogenase (SDH) in the case of mitochondrial mutation.

The primary care physician should remember this relatively simple rule of thumb: “When a common disease has features that set it apart from the pack, or involves 3 or more organ systems, think mitochondria.”


There are no cures for mitochondrial diseases; therefore, the treatment is focused on alleviating symptoms and enabling normal functioning of the affected organs. Most patients have used cofactor and vitamins; however, there is no overwhelming evidence that they are helpful in most patients.

  • Coenzyme Q10 (CoQ10) is the best-known cofactor used in treating mitochondrial cytopathies with no known side effects. CoQ10, residing in the inner mitochondrial membrane, functions as the mobile electron carrier and is a powerful antioxidant with benefits such as reduction in lactic acid levels, improved muscle strength, decreased muscle fatigue and so on.
  • Levocarnitine (L-carnitine, carnitine), is a cofactor required for the metabolism of fatty acids. Levocarnitine therapy improves strength, reversal of cardiomyopathy, and improved gastrointestinal motility, which can be a major benefit to those with poor motility due to their disease. Intestinal cramping and pain are the major side effects.
  • Creatine phosphate, synthesized from creatine can accumulate in small amounts in the body, and can act as storage for a high-energy phosphate bond. Muscular creatine may be depleted in mitochondrial cytopathy, and supplemental creatine phosphate has been shown to be helpful in some patients with weakness due to their disease.
  • B Vitamin, are necessary for the function of several enzymes associated with energy production. The need for supplemental B vitamin therapy is not proven, aside from rare cases of thiamine (vitamin B1)-responsive pyruvate dehydrogenase deficiency.

Research – Restriction enzyme for gene therapy of Mitochondria diseases:

Mitochondrial DNA (mtDNA) is the only extrachromosomal DNA in humans and defects in this genome are now recognized as important causes of various diseases. Presently, there is no effective treatment for patients suffering from diseases that harbor mutations in mtDNA.

Tanaka et al discovered a gene therapy method to treat a mitochondrial disease associated with mtDNA heteroplasmy. Heteroplasmy is where mutant and wild-type mtDNA molecules co-exist within cells. This syndrome of neurogenic muscle weakness, ataxia and retinitis pigmentosa (NARP) is caused by mutations in mtDNA leading to amino acid replacement in the resulting protein that codes for a subunit of mitochondrial ATP synthase. Level of mutant mtDNA is crucial for the disease as above a certain threshold level of mtDNA, the disease becomes biochemically and clinically apparent. Authors hypothesized that a possible method to treat patients was by selectively destroying mutant mtDNA, thereby only allowing propagation of wild-type mtDNA. Since restriction endonucleases can recognize highly specific sequences, they were utilized for gene therapy. Tanaka et al utilized Sma1, a restriction endonuclease to destroy mutant mtDNA, leading to increase in wild-type mtDNA levels.

Thus, authors concluded, “ the present results indicate that the use of a mitochondrion-targeted restriction enzyme which specifically recognizes a mutant mtDNA provides a novel strategy for gene therapy of mitochondrial diseases.”



Mitochondria are double-membrane organelles located in the cytoplasm and often referred to as the “powerhouse” of the cell. In simple terms, they convert energy into forms that are usable by the cell. Mitochondria are semi-autonomous in that they are only partially dependent on the cell to replicate and grow. They have their own DNA, ribosomes, and can make their own proteins. They are the sites of cellular respiration that generates fuel for the cell’s activities. Mitochondria are also involved in other cell processes such as cell division, cellular growth and cell death. Multiple essential cellular functions are mediated by thousands of mitochondrial-specific proteins, encoded by both the nuclear and mitochondrial genomes.

Interestingly, mitochondria take on many different shapes and along with serving several different metabolic functions. In fact, each mitochondrion’s shape is characteristic of the specialized cell in which it resides. The number of mitochondria too varies in difference cell types, with as high as 500-2000 in some nucleated cells and as low as zero in RBCs and 2-6 in platelets.

The standard sequence to which all human mtNDNA is compared is referred to as the “Cambridge Sequence.” It was sequenced from several different human mtDNAs by a Medical Research Council (MRC) labora- tory based at Cambridge, UK, in 1981 and as a part of this work, Fred Sanger, the received his second Nobel Prize. Several variations in the form of polymorphisms are observed from the Cambridge sequence in the mtDNA of different individuals.

Metabolic syndrome:

Metabolic syndrome is a cluster of conditions — increased blood pressure, a high blood sugar level, excess body fat around the waist or abnormal cholesterol levels — that occur together, increasing your risk of heart disease, stroke and diabetes. Metabolic syndrome is becoming more and more common in the United States. In the future, it may overtake smoking as the leading risk factor for heart disease. In general, a person who has metabolic syndrome is twice as likely to develop heart disease and five times as likely to develop diabetes as someone who doesn’t have metabolic syndrome.

The five conditions described below are metabolic risk factors. You must have at least three metabolic risk factors to be diagnosed with metabolic syndrome.

  • A large waistline. This also is called abdominal obesity or “having an apple shape.” Excess fat in the stomach area is a greater risk factor for heart disease than excess fat in other parts of the body, such as on the hips.
  • A high triglyceride level (or you’re on medicine to treat high triglycerides). Triglycerides are a type of fat found in the blood.
  • A low HDL cholesterol level (or you’re on medicine to treat low HDL cholesterol). HDL sometimes is called “good” cholesterol. This is because it helps remove cholesterol from your arteries. A low HDL cholesterol level raises your risk for heart disease.
  • High blood pressure (or you’re on medicine to treat high blood pressure). Blood pressure is the force of blood pushing against the walls of your arteries as your heart pumps blood. If this pressure rises and stays high over time, it can damage your heart and lead to plaque buildup.
  • High fasting blood sugar (or you’re on medicine to treat high blood sugar). Mildly high blood sugar may be an early sign of diabetes.

Role of Mitochondria in Metabolic Syndrome & Diabetes:

Impaired mitochondrial function has recently emerged as a potential causes of insulin resistance and/or diabetes progression, risk factors of metabolic syndrome.

Mitochondria plays several key functions including generation of ATP, and generating metabolites via Tricarboxylic acid cycle that function in cytosolic pathways, oxidative catabolism of amino acids, ketogenesis, urea cycle; the generation of reactive oxygen species (ROS); the control of cytoplasmic calcium; and the synthesis of all cellular Fe/S clusters, protein cofactors essential for cellular functions such as protein translation and DNA repair. These roles define the mitochondria to be involved in metabolic homeostasis and hence, a major candidate for metabolic syndrome and its associated risk factor including diabetes, obesity and insulin resistance.

Research and Therapeutic relevance:

Understanding the underlying molecular mechanism of aberrant role of mitochondria is important in developing therapeutic agents for mitochondria-associated diseases. In the recent issue of Mitonews, several papers have been published using the products of MitoSciences, which describe research pertaining to the importance of mitochondria in obesity and diabetes. Some recent research articles based on mitochondrial research (also mentioned in MitoNews) have been briefly discussed here:

  • Metabolic inflexibility and Metabolic syndrome: Metabolic inflexibility is defined as the failure of insulin-resistant patients to appropriately adjust mitochondrial fuel selection in response to nutritional cues. Although the phenomenon has been emphasized an important aspect of metabolic syndrome, the molecular mechanisms have not yet been fully deciphered. In a recent article by Muoio et al, published in Cell Metabolism journal, essential role of the mitochondrial matrix enzyme, carnitine acetyltransferase (CrAT) has been identified in regulating substrate switching and glucose tolerance. CrAT regulates mitochondrial and intracellular Carbon trafficking by converting acetyl-CoA to its membrane permeant acetylcarnitine ester. Using muscle muscle-specific Crat knockout mice, primary human skeletal myocytes, and human subjects undergoing L-carnitine supplementation, authors have suggested a model wherein CrAT combats nutrient stress, promotes metabolic flexibility, and enhances insulin action by permitting mitochondrial efflux of excess acetyl moieties that otherwise inhibit key regulatory enzymes such as pyruvate dehydrogenase. These findings offer therapeutically relevant insights into the molecular basis of metabolic inflexibility.
  • Rosiglitazone and obesity: Eepicardial adipose tissue (EAT) has been described in humans as a functioning brown adipose tissue (BAT) and has been shown in animal models to have a lower glucose oxidation rate and higher fatty acid (FA) metabolism. In obese individuals, epicardial adipose tissue (EAT) is “hypertrophied”. EAT is a source of BAT may be a source of proinflamatory cytokines. Distel et al published their studies using a rat model of obesity and insulin resistance treated with rosiglitazone. They observed that rosiglitazone, promoted a BAT phenotype in the EAT depot characterized by an increase in the expression levels of genes encoding proteins involved in mitochondrial processing and density PPARγ coactivator 1 alpha (PGC-1α), NADH dehydrogenase 1 and cytochrome oxidase (COX4) resulting in significant up-regulation of PGC1-α and COX4 protein. The authors concluded that PPAR-γ agonist could induce a rapid browning of the EAT that probably contributes to the increase in lipid turnover. Thus, important insights into the mechanism of fat metabolism and involvement of mitochondrial proteins with a therapy were presented in the article.
  • Mitochondrial dysfunction and diabetic neuropathy: Animal models of diabetic neuropathy show that mitochondrial dysfunction occurs in sensory neurons that may contribute to distal axonopathy. The adenosine monophosphate-activated protein kinase (AMPK) and peroxisome proliferator-activated receptor γ coactivator-1α (PGC-1α) signalling axis senses the metabolic demands of cells and regulates mitochondrial function. Studies in muscle, liver and cardiac tissues have shown that the activity of AMPK and PGC-1α is decreased under hyperglycaemia. Chowdhury et al using type 1 and type 2 diabetic rat and mice models studied the hypothesis that deficits in AMPK/PGC-1 signalling in sensory neurons underlie impaired axonal plasticity, suboptimal mitochondrial function and development of neuropathy. The authors have shown there is a significant reduction in phospho-AMPK, phopho-ACC, total PGC-1α, NDUFS3and COXIV in sensory neurons of the dorsal root ganglia of 14 week old diabetic mice with marked signs of thermal hypoalgesia. These results were associated with an impaired neuronal bioenergetic profile and a decrease in the activity of mitochondrial complex I, complex IV and citrate synthase. The fact that resveratrol treatment reversed the changes observed in vitro and in vivo suggest that the development of distal axonopathy in diabetic neuropathy is linked to nutrient excess and mitochondrial dysfunction via defective signalling of the AMPK/PGC-1α pathway.
  • ROS and diabetes: Mitochondria generated reactive oxygen species (ROS) has been associated with kidney damage occurring in diabetes. Rosca et al, published an article investigating the source and site of ROS production by kidney cortical tubule mitochondria in streptozotocin-induced type 1 diabetes in rats. The authors observed that in diabetic mitochondria, the fatty acid oxidation enzymes were elevated with increased oxidative phosphorylation and increased ROS production. The authors observed ROS production with fatty acid oxidation remained unchanged by limiting electron flow in ETC complexes, changes in ETC substrate processing and that the ROS supported by pyruvate also remained unaltered. The authors hence concluded that mitochondrial fatty acid oxidation is the source of increased ROS production in kidney cortical tubules in early diabetes


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