A new study uncovering the mechanism behind amyotrophic lateral sclerosis (ALS or also Lou Gehrig’s disease) has brought doctors a step closer to identifying drug targets for the malady. Tsukasa Uchida and collaborators at Kyoto University have identified proteins associated with cancer suppression and prevention of hypoxia as key players in the progression of ALS.
One of the main factors triggering the onset of ALS is the malfunctioning of nerve support cells called oligodendrocytes. Recent studies have implied that the misfolding and accumulation of the protein TDP-43 in oligodendrocytes was likely linked to the development of ALS, as with other neurological diseases like Alpha-synuclein in Parkinson’s. The current research takes this a step further, explaining how this protein ends up accelerating ALS’ characteristic decline in muscle strength.
Uchida’s team found that the von Hippel Lindau (VHL) protein — associated with a gene most notable for cancer suppression — strongly binds to malformed versions of TDP-43 in oligodendrocytes.
“TDP-43 appears to be a very fragile protein, and becomes fragmented in the cytoplasm. When this happens it binds to VHL, which is typically only found in blood vessels, but surprisingly enough, we also found them in oligodendrocytes,” explains Uchida.
VHL forms a complex with cullin 2 (CUL2), a protein that rescues the cell in hypoxic conditions, then facilitating the breakdown of malformed TDP-43 even under normal conditions.
“CUL2 was known to break down other proteins, but again, our study reports for the first time that it’s also involved in the breakdown of TDP-43,” says Uchida.
Additionally, the team found that when VHL becomes overly abundant, VHL/TDP-43 complex accumulates in the cytoplasm, forming a protein cluster thought to be detrimental to the functioning of oligodendrocytes.
“VHL overload in the cytoplasm and the imbalance between VHL and CUL2 seems to be the root cause of oligodendrocyte dysfunction,” says Makoto Urushitani, a senior author of the study. “Once we have a clearer idea of how the VHL and CUL2 balance is maintained, I believe we’ll be able to make a huge contribution to the treatment of ALS.”
Source: Kyoto University
Image Source: The image is credited to Eiri Ono/Kyoto University
Original Research: Full open access research for “CUL2-mediated clearance of misfolded TDP-43 is paradoxically affected by VHL in oligodendrocytes in ALS” by Tsukasa Uchida, Yoshitaka Tamaki, Takashi Ayaki, Akemi Shodai, Seiji Kaji, Toshifumi Morimura, Yoshinori Banno, Kazuchika Nishitsuji, Naomi Sakashita, Takakuni Maki, Hirofumi Yamashita, Hidefumi Ito, Ryosuke Takahashi and Makoto Urushitani in Scientific Reports. Published online January 11 2016 doi:10.1038/srep19118
CUL2-mediated clearance of misfolded TDP-43 is paradoxically affected by VHL in oligodendrocytes in ALS
The molecular machinery responsible for cytosolic accumulation of misfolded TDP-43 in amyotrophic lateral sclerosis (ALS) remains elusive. Here we identified a cullin-2 (CUL2) RING complex as a novel ubiquitin ligase for fragmented forms of TDP-43. The von Hippel Lindau protein (VHL), a substrate binding component of the complex, preferentially recognized misfolded TDP-43 at Glu246 in RNA-recognition motif 2. Recombinant full-length TDP-43 was structurally fragile and readily cleaved, suggesting that misfolded TDP-43 is cleared by VHL/CUL2 in a step-wise manner via fragmentation. Surprisingly, excess VHL stabilized and led to inclusion formation of TDP-43, as well as mutant SOD1, at the juxtanuclear protein quality control center. Moreover, TDP-43 knockdown elevated VHL expression in cultured cells, implying an aberrant interaction between VHL and mislocalized TDP-43 in ALS. Finally, cytoplasmic inclusions especially in oligodendrocytes in ALS spinal cords were immunoreactive to both phosphorylated TDP-43 and VHL. Thus, our results suggest that an imbalance in VHL and CUL2 may underlie oligodendrocyte dysfunction in ALS, and highlight CUL2 E3 ligase emerges as a novel therapeutic potential for ALS.
“CUL2-mediated clearance of misfolded TDP-43 is paradoxically affected by VHL in oligodendrocytes in ALS” by Tsukasa Uchida, Yoshitaka Tamaki, Takashi Ayaki, Akemi Shodai, Seiji Kaji, Toshifumi Morimura, Yoshinori Banno, Kazuchika Nishitsuji, Naomi Sakashita, Takakuni Maki, Hirofumi Yamashita, Hidefumi Ito, Ryosuke Takahashi and Makoto Urushitani in Scientific Reports. Published online January 11 2016 doi:10.1038/srep19118