Posts Tagged ‘control of proliferation’

Neural stem cells aging

Larry H. Bernstein, MD, FCAP, Curator



Aging Stem Cells Provide Clues into Tumor Development

With the enzyme Eyeless knocked out, cells over-proliferate (shown in green) in a fruit fly’s larval brain during reactivated Notch signaling. [University of Oregon]


Investigators at the University of Oregon (UO) have recently uncovered molecular events experienced by stem cells as they age, which could provide new avenues toward the discovery of novel therapies for cancer and neurological disorders. The researchers noticed that these changes arise in Drosophila during the development of the central nervous and at that time a specific protein is expressed, blocking tumor formation.

The UO researchers were focused on the larval stage of fruit fly development, as this is when stem cells generate most of the neurons that form the adult’s brain. During this process the stem cells are rapidly dividing in order to populate the central nervous system of the fly, and they rely heavily on the Notch signaling pathway—a developmentally important signal transduction pathway that has also been linked to cancer.

In previous studies, scientists have described scenarios where Notch signaling ran efficiently and stem cells produce neurons that populate the adult central nervous system. However, with too much Notch, stem cells lose control and over-proliferate—forming large tumors. In humans, adult T-cell leukemia is tied to overactive Notch signaling.

“Stem cells have a really tough job because they have to divide to make the millions of neurons in our brain,” explained Howard Hughes Medical Institute Investigator and senior author Chris Doe, Ph.D., professor of biology at the UO. “If they don’t divide enough, it results in microcephaly or other small brain diseases, but if they divide too much, they make tumors. They have to stay right on that boundary of dividing to make neurons but not dividing excessively and forming a tumor. It’s really walking a tightrope.”

The findings from this study were published recently in Current Biology through an article entitled “Aging Neural Progenitors Lose Competence to Respond to Mitogenic Notch Signaling.”

In the current study, the scientists discovered that if they waited for stem cells to divide a few times and age a bit, they quit responding to Notch. Moreover, the stem cells could not be pushed by high doses of Notch signaling to form tumors.

As the UO researchers looked closer, they uncovered a host of age-related molecular changes. As the stem cells get older and around the same time they begin to resist tumor formation, the stem cells begin expressing a transcription factor protein, known as Eyeless in Drosophila and Pax6 in humans. Its presence blocks Notch signaling.

Dr. Doe and his team described the genetic knockout of Eyeless in these stem cells, which led Notch signaling to overwhelm the precise growth balance and form tumors within the fruit flies.

“If we can identify the stem cells that are relied upon during development, maybe we could find a way to use them later to recreate conditions that might be therapeutic,” noted Dylan Farnsworth, a doctoral candidate at the UO. “If you do it incorrectly, you risk over-proliferation and the development of masses—and cancer.”

“This paper shows that Eyeless is important for winding down the lifespan of the stem cells that are giving rise to the adult brain,” Dr. Doe added. “It’s a stop signal that says it is time to cease responding to Notch signals.”

The UO researchers were excited by their findings and believe that with more extensive research, their system could provide a roadmap for fine-tuning the timing of stem cell-based therapies to restart healthy activity in adult stem cells.


Aging Neural Progenitors Lose Competence to Respond to Mitogenic Notch Signaling

Dylan R. Farnsworth, Omer Ali Bayraktar, and Chris Q. Doe
Cell 7 Dec 2015; 25(23):3058–3068  DOI: http://dx.doi.org/10.1016/j.cub.2015.10.027


  • Aging INPs lose competence to respond to constitutively active Notch signaling
  • The late temporal factor Eyeless blocks Notch-induced target gene expression
  • Eyeless blocks Notch-induced INP tumor formation


Drosophila neural stem cells (neuroblasts) are a powerful model system for investigating stem cell self-renewal, specification of temporal identity, and progressive restriction in competence. Notch signaling is a conserved cue that is an important determinant of cell fate in many contexts across animal development; for example, mammalian T cell differentiation in the thymus and neuroblast specification in Drosophila are both regulated by Notch signaling. However, Notch also functions as a mitogen, and constitutive Notch signaling potentiates T cell leukemia as well as Drosophila neuroblast tumors. While the role of Notch signaling has been studied in these and other cell types, it remains unclear how stem cells and progenitors change competence to respond to Notch over time. Notch is required in type II neuroblasts for normal development of their transit amplifying progeny, intermediate neural progenitors (INPs). Here, we find that aging INPs lose competence to respond to constitutively active Notch signaling. Moreover, we show that reducing the levels of the old INP temporal transcription factor Eyeless/Pax6 allows Notch signaling to promote the de-differentiation of INP progeny into ectopic INPs, thereby creating a proliferative mass of ectopic progenitors in the brain. These findings provide a new system for studying progenitor competence and identify a novel role for the conserved transcription factor Eyeless/Pax6 in blocking Notch signaling during development.



Supplemental Information

Figure S1, related to Figure 1. Notchintra is nuclear and at qualitatively similar levels when expressed in young or old INP lineages. (A-C’) R9D11-gal4, R16B06-gal and OK107-gal4 driving UAS-Notchintra result in efficient Notchintra protein expression, as visualized by antibody staining. Yellow dashed outlines show INP lineages in central brain labeled by each driver. Scale bar = 10 µm

Figure S2, related to Figure 3. R16B06-gal4 labels old INPs in third instar larval brains. (A) Expression pattern of R16B06-gal4 in both brain lobes of third instar larva. R9D11-gal4 marks young INPs and is shown for comparison. (B-B’’) High magnification images show Dpn+ INPs distal to their parental Type II NB (white dashed line) are labeled by R16B06-gal4. Arrow indicates direction of age progression in lineage. (C-C’’’) Distal INPs (yellow dashed line) labeled by R16B06-gal4 express the old INP specific transcription factor Eyeless (Ey) but not the young INP specific transcription factor Dichaete (D). Images are a single, one micron plane. All panels show third instar larvae; scale bar = 10 µm

Figure S3, related to Figure 7. Notchintra signaling can induce expression of the Notch response element reporter (NRE-PGR) in old INPs but not in GMCs. (A) In wild type, the NRE reporter is expressed at high levels in Type II neuroblasts (arrowhead) and shows progressively weaker levels in the progeny. There are low levels in old INPs (dashed yellow outline; identified by Ey expression), but is not detectable in Prospero (Pros)+ GMCs (arrow). (B) Expression of Notchintra in the old INPs and their progeny using R16B06-gal4 results in elevated expression of the NRE reporter (dashed yellow outline; compare to level in adjacent cells); only Dpn+ INPs show elevated levels of the reporter, Prospero (Pros)+ GMCs show no detectable expression. (C-C’’’) The Notch target E(spl)mγ is expressed in both young and old INPs. (C) Young INPs expressing Dichaete (small white circles) and (C’) old INPs expressing Eyeless (small dashed yellow circles) are positive for Deadpan and the Notch target E(spl)mγ- GFP fusion reporter. Asterisk marks Type II NB, arrow indicates direction of lineage from young to old. All panels show third instar larvae; scale bar = 10 µm.

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