The proteolytic degradation that accompanies neurodegeneration in animals may be mediated not by apoptotic proteases, but by a group of aspartyl and calcium-dependent proteases, at least in the roundworm, researchers report in today's Nature.
First author Popi Syntichaki and colleagues working in Nektarious Tavernarakis's lab at the Institute of Molecular Biology and Biotechnology, Heraklion, Greece, came to this conclusion after probing the proteolytic pathways that accompany neurodegeneration in the nematode C. elegans. In these animals, hyperactivity of the ion channel protein mec-4 leads to the death of touch receptor neurons in a manner that resembles the excitotoxic destruction of mammalian neurons by neurotransmitters such as glutamate.
Syntichaki et al. found that in animals with congenital loss of aspartyl protease activity, mec-4 induction of neurodegeneration failed. The authors also showed that the aspartyl protease inhibitor, pepstatin A, protected the six touch receptor neurons from mec-4, as did starvation, which also reduces aspartyl protease activity.
The authors extended these observations to neurons that degenerate in response to other toxic genes, such as mutants of degenerin. In all cases, reduction in aspartyl protease activity was protective, indicating that these enzymes are general mediators of neuronal cell death. But which protease is the culprit? To answer this question the authors systematically suppressed the expression of each C. elegans aspartyl proteases. Of the six in the genome, two, asp-3 and asp-4, were found to be necessary for neurodegeneration, while silencing of a third, asp-1, partly protected against cell death.
Syntichaki et al. then looked at what might trigger activation of these grim reapers. Knowing that rising intracellular calcium and calcium-dependent cysteine proteases, such as calpain, have been implicated in cell death in mammals and flatworms, the authors revisited their mec-4 necrosis model, but this time they administered the calpain inhibitor MDL-28170 to some of the animals. In these worms the number of dying neurons was reduced by as much as twofold. Again, capitalizing on the tractability of the flatworm model and the availability of the genome sequence, the authors were able to narrow the candidate pool for cell death mediators down to two out of 17 calpain homologs, clp-1 and clp-5 (also known as Tra-3).
All told, these experiments seem to link calcium and neurodegeneration through calpain and aspartyl proteases. However, as the authors mention, they never achieved complete blockage of neurodegeneration by RNAi-mediated silencing of these enzymes, which hints that other pathways may also be involved.—Tom Fagan
No Available References
- Syntichaki P, Xu K, Driscoll M, Tavernarakis N. Specific aspartyl and calpain proteases are required for neurodegeneration in C. elegans. Nature. 2002 Oct 31;419(6910):939-44. PubMed.