Editors' ChoiceNeuroscience

Calpains Cleave and Activate TRPC5

Sci. Signal.  22 May 2012:
Vol. 5, Issue 225, pp. ec142
DOI: 10.1126/scisignal.2003241

Calcium signaling and mitogen-activated protein kinase (MAPK) activity have both been implicated in regulation of neurite outgrowth and growth cone guidance. Kaczmarek et al. provide evidence that these two pathways may converge on proteases of the calpain family to cleave and potentiate the activity of the nonselective cation channel transient receptor potential C5 (TRPC5), which had been previously implicated in neurite growth. Semaphorin 3A (Sema3A) is a secreted axon guidance cue that can activate MAPK signaling and can trigger attraction, repulsion, or growth cone collapse, depending on the context. Growth cone collapse induced by Sem3A treatment was significantly greater in hippocampal neurons from wild-type mice compared with those from Trpc5-null mice. Application of calpain inhibitors reduced the enhancement in growth cone collapse induced by Sema3A only in cultures from wild-type mice. To examine the effect of calpain on TRPC5 activity, the authors switched to heterologous expression systems. Expression of a constitutively active form of calpain increased the basal current in human embryonic kidney (HEK) cells stably expressing mouse TRPC5. Electrophysiology performed on inside-out patches showed that calcium-activated calpain-1 or calpain-2 or MAPK-phosphorylated calpain-2 stimulated TRPC5 single-channel activity, peaking after several minutes. Exposure of homogenates from cells expressing a tagged form of mouse TRPC5 to calpain-1 or calpain-2 resulted in cleavage of TRPC5. An in silico prediction tool indicated that Thr857 may be a calpain cleavage site, and mutation of this residue reduced, but did not completely eliminate, calpain-mediated cleavage of TRPC5. Whole-cell, patch-clamp analysis showed that a mutant form of TRPC5 truncated at Thr857 was functional. Although the authors were unable to verify these results in cultured hippocampal neurons, which may relate to the low abundance of TRPC5 and its presumed localization in the neurites, the data suggest that calpain-mediated potentiation of TRPC5 activity may regulate neurite outgrowth.

J. S. Kaczmarek, A. Riccio, D. E. Clapham, Calpain cleaves and activates the TRPC5 channel to participate in semaphorin 3A-induced neuronal growth cone collapse. Proc. Natl. Acad. Sci. U.S.A. 109, 7888–7892 (2012). [Abstract] [Full Text]