Turning genetic information into working proteins
Larry H Bernstein, MD, FCAP, Curator
Leaders in Pharmaceutical Intelligence
Series 2; 3.3
James E. Darnell Jr. (1930— )
Vincent Astor Professor Emeritus
2002 Albert Lasker Award for Special Achievement in Medical Science
Responsible for the various tasks required in turning genetic information into working proteins, ribonucleic acids are one of the most essential players in the life of a cell. First discovered in 1868, RNA today remains the subject of intense scientific scrutiny. Over the course of a career dedicated to understanding the intricate workings of gene transcription, Rockefeller University scientist James E. Darnell Jr. has revealed some of RNA’s most secretive and surprising mechanisms. For his half-century of illuminating research, Dr. Darnell received the 2002 Albert Lasker Award for Special Achievement in Medical Science.
In 1963, Dr. Darnell described a phenomenon he termed “RNA processing,” a step in the process of gene transcription, which had only recently been elucidated in bacterial systems. Working with mammalian cells — which differ from bacterial cells in that they contain a nucleus, where RNA is created — Dr. Darnell observed that very long strings of RNA disappear from the cell nucleus and that subsequently, shorter RNAs resembling the absent longer ones appear in the cytoplasm. Mammalian cells, he concluded, must distill their massive, immature nuclear RNA into shorter, mature forms that are individually coded for specific purposes by specific segments of the genome.
Dr. Darnell carried the principles of his finding — which he made in ribosomal RNA, part of the construction crew that builds cellular proteins — to other long nuclear RNA, including the longest one, which he named heterogeneous nuclear RNA (hnRNA). His hypothesis, that hnRNA is the precursor of the better known messenger RNA — which carries the genetic blueprint for protein building — soon bore fruit when he found a structural correlation between the two. Certain hnRNAs and nearly all messenger RNAs have a “tail” of adenine nucleotides at one end. Dr. Darnell followed this discovery with the observation that when an hnRNA string with an adenine tail disappears from the nucleus, a messenger RNA with the same tail then appears in the cytoplasm, suggesting a causal link between the two. When he found a second similarity — a cap at the end of the string opposite the adenine tail — he faced a conundrum. Scientific dogma had it that the order of nucleotides in any RNA mirrors that of DNA, whether the RNA is modeled from somewhere in the middle of the DNA or from one of the ends. The matching of a nuclear RNA to its cytoplasmic product by two end pieces glued together was surprising, but the concept was soon proven by colleagues at other institutions and called RNA splicing.
After a brief sojourn in Paris to work in François Jacob’s lab, Darnell worked at MIT, the Albert Einstein College of Medicine, and Rockefeller University on the relationship between mRNA and hnRNA. hnRNA was believed to be the precursor to mRNA, and despite making some key discoveries, Darnell admits that he could not free his imagination from the idea of colinearity and envision an hnRNA spliced to produce a smaller mRNA.
At this time, Darnell turned his attention to the question he had pondered since Paris: how were genes regulated in animal cells? This led to the discovery of the STAT and the Jak-STAT pathway of transcription control.
With the knowledge of RNA processing and splicing, Dr. Darnell next examined how cells begin the process of transcription and how they activate particular segments of DNA. Having moved to Rockefeller University in 1974, he found in the early 1980s that cells retain their specificity only in the context of their natural environment. Away from other liver cells, for example, a single liver cell stops producing liver-specific RNA, though it continues to make RNA for more generic cellular tasks. To pinpoint the signals responsible, which he believed must be coming from outside the cell, Dr. Darnell took a closer look at interferons (IFN), proteins that warn a cell when it’s time to raise its genetic defenses against harmful microbes.
Dr. Darnell’s laboratory studies how signals from the cell surface affect transcription of genes in the nucleus. Originally using interferon as a model cytokine, the Darnell group discovered that cell transcription was quickly changed by binding of cytokines to the cell surface. Introducing IFNβ into cell cultures, he watched as a particular type of mRNA accumulated in the cytoplasm, unaccompanied by any new protein synthesis. Analyzing the mRNA led him to the segment of DNA that had been activated, and the lack of new proteins told him that the cell contained its own, usually dormant, IFN-responsive transcription factor. By isolating a particular stretch of DNA from IFN-treated cells, he was able to call out of hiding the proteins that make up that factor, which, partly because they respond to signals very quickly, he called “STATs.” Dr. Darnell then traced the chemical relay that activates the STATs after IFN contact, called the Jak-Stat pathway.
The bound interferon led to the tyrosine phosphorylation of latent cytoplasmic proteins now called STATs (signal transducers and activators of transcription) that dimerize by reciprocal phosphotyrosine-SH2 interchange. They accumulate in the nucleus, bind DNA and drive transcription. This pathway has proved to be of wide importance, with seven STATs now known in mammals that take part in a wide variety of developmental and homeostatic events in all multicellular animals. Crystallographic analysis defined functional domains in the STATs, and current attention is focused on two areas: how the STATs complete their cycle of activation and inactivation, which requires regulated tyrosine dephosphorylation; and how persistent activation of STAT3 that occurs in a high proportion of many human cancers contributes to blocking apoptosis in cancer cells. Current efforts are devoted to inhibiting STAT3 with modified peptides that can enter cells.
Dr. Darnell received his M.D. in 1955 from the Washington University School of Medicine. His career has included poliovirus research with Harry Eagle at the National Institute of Allergy and Infectious Diseases, research with François Jacob at the Pasteur Institute in Paris and academic appointments at the Massachusetts Institute of Technology, the Albert Einstein College of Medicine and Columbia University. In 1974 Dr. Darnell joined Rockefeller as Vincent Astor Professor, and from 1990 to 1991 he was vice president for academic affairs.
A member of the National Academy of Sciences since 1973, he has received numerous awards, including the 2012 Albany Medical Center Prize in Medicine and Biomedical Research, the 2003 National Medal of Science, the 2002 Albert Lasker Award for Special Achievement in Medical Science, the 1997 Passano Award, the 1994 Paul Janssen Prize in Advanced Biotechnology and Medicine and the 1986 Gairdner Foundation International Award.
He is the coauthor with S.E. Luria of General Virology and the founding author with Harvey Lodish and David Baltimore of Molecular Cell Biology, now in its seventh edition. His book RNA, Life’s Indispensable Molecule was published in July 2011 by Cold Spring Harbor Laboratory Press. He is a member of the American Academy of Arts and Sciences and a foreign member of The Royal Society and The Royal Swedish Academy of Sciences.
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