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An Epilogue to a Protein Saga – But Not the Last Word

A high magnification image of the cortical brain region from a dementia patient diagnosed with frontotemporal lobar degeneration with TDP-43 pathology. TDP-43 clumps were detected by immunohistochemistry using anti-TDP-43 antibodies (brown color).  The TDP-43 clumps were shown to contain disulfide cross-linking suggesting they result from oxidative stress in the brain. Todd Cohen, PhD, Perelman School of MedicineEvery saga needs occasional updates. TDP-43 -- a protein important in gene expression that can undergo pathologic misfolding -- is no different. Earlier reports on the protein were outlined in a Penn Med news blog, which describes its pathology and genetics related to neurodegenerative disease. But now the field is maturing and researchers are linking TDP-43 to a well-established clinical area - the role of oxidative stress in the demise of nerve cells.

A major hurdle in understanding neurodegenerative disease is gaining a clear picture of what leads up to neuron death. In recent years, much data has been published indicating that increased oxidative stress plays a role in neuron degeneration and death. Oxidative stress is an imbalance between the production of reactive oxygen molecules and the body's ability to get rid of them. Disturbances in the normal oxygen state of tissues can damage all components of the cell, including proteins and DNA.

Most recently, Todd Cohen, PhD, a postdoctoral fellow in the lab of Virginia M.Y. Lee, PhD at Penn’s Center for Neurodegenerative Disease Research, studied how TDP-43 reacts to oxidative stress, published in the EMBO Journal in December. Stress induces the protein to move from the nucleus to the cell cytoplasm. Its ability to fold properly is also altered.

A related review of TDP-43-mediated neurodegeneration in Nature Reviews Neuroscience by Lee, John Q. Trojanowski, MD, PhD, director of Penn’s Institute on Aging, and Edward B. Lee, MD, PhD, assistant professor of Pathology and Laboratory Medicine, suggests that neurodegeneration could come from loss of function of the protein as it misfolds and is no longer available to regulate gene expression or from a gain of toxic properties as it forms clumps in the cell, disrupting normal day-to-day functions.

In the EMBO Journal paper, Cohen explained how TDP-43 reacts to stress chemically and showed that it is reversible. The team found that oxidizing chemicals caused cross linking between sulfur molecules of the amino acid cysteine in TDP-43 proteins in culture. This linking caused the protein to misfold. What’s more, they also saw sulfur-sulfur bonding between two or more proteins, which was the start of the accumulation of debilitating TDP-43 clumps.

Even more surprising, the team was able to break up the TDP-43 clumps in cultured cells and in post- mortem human brain tissue from FTLD (frontotemporal lobar degeneration)-TDP patients. They also showed that these disaggregated proteins became functional again.

The “big take-home message,” says Cohen, is that antioxidant therapy -- particularly a well-known one called NAC – could prevent the sulfur cross-linking in the first place. That would prevent the protein misfolding and multi-protein clumps seen in many neurodegenerative diseases. Future studies - first in animal models and eventually in humans - he says, could evaluate whether taking NAC or related antioxidant supplements could be an effective treatment strategy to prevent Lou Gehrig’s disease or FTLD.

Read More

Cohen TJ, Hwang AW, Unger T, Trojanowski JQ, & Lee VM (2011). Redox signalling directly regulates TDP-43 via cysteine oxidation and disulphide cross-linking.The EMBO journal PMID: 22193716 

Lee EB, Lee VM, & Trojanowski JQ (2011). Gains or losses: molecular mechanisms of TDP43-mediated neurodegeneration. Nature reviews. Neuroscience, 13 (1), 38-50 PMID: 22127299

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