In this week’s issue of Nature, two groups report the crystal structure of the calcium-dependent protease calpain in complex with its endogenous inhibitor protein, calpastatin. Tudor Moldoveanu and Douglas Green at St. Jude Children’s Research Hospital in Memphis, Tennessee, along with Kalle Gehring of McGill University in Montreal, Canada, as well as Rachael Hanna, Robert Campbell, and Peter Davies at the Queen’s University in Kingston, Ontario, have solved essentially identical structures of the calcium-bound form of the protease with its active site plugged by the calpain peptide.

The new structures help explain how the relatively floppy calpastatin protein takes the protein into a choke hold. The two well-ordered ends of the inhibitory domain grab hold of either side of calpain, sending the less structured intervening segment looping through the protease active site.

Calpains are potential targets in AD and stroke. Calpain 1 is located at synapses, and its activity increases in AD brain. Substrates for calpains include proteins that control APP production and tau phosphorylation. Calpain cleaves the Cdk5 regulator p35 to its constitutively active p25 form, which is also elevated in AD brain and appears to be involved in neurodegeneration. Calpains participate in calcium-initiated cell death, which has been implicated in neurotoxicity due to amyloid-β. A report last summer indicated that calpain inhibitors prevent the synaptic toxicity of Aβ and improve memory in a mouse model of AD (see ARF related news story).

However attractive the target, finding specific small-molecule inhibitors of these serine proteases has been difficult (reviewed in Carragher, 2006). Previous inhibitors have tended to hit other proteases as well, for example, cathepsins and even the proteasome. The detailed structure of the complex will come in handy to guide the design of new, hopefully more targeted, inhibitors.—Pat McCaffrey

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  1. The relationship between calpain and its endogenous inhibitor protein calpastatin in Alzheimer disease (AD) has been explored in a study in this week’s Journal of Neuroscience (Rao et al., 2008). The study shows that neuronal calpastatin becomes markedly depleted in AD brain due to abnormally activated caspases 1 and 3 and calpains. Calpastatin depletion is temporally and spatially related to calpain activation in neurons, which in turn is associated with a calpain-related cascade of events leading to neurofibrillary degeneration, including ERK activation, hyperphosphorylation of tau and neurofilaments, and caspase and calpain cleavage of these cytoskeletal proteins. In mice, a similar cascade of molecular events induced by kainate excitotoxicity is substantially ameliorated by maintaining calpastatin at high levels by transgenesis.

    The findings strongly suggest that calpastatin depletion represents a tipping point for catastrophic calpain overactivation and downstream events leading to neurodegeneration in AD. They strengthen the case for using mimetics of calpastatin (i.e., highly selective calpain inhibitors) in the therapy of AD. The timely reports by Hanna et al. and Moldoveanu et al. defining the crystal structure of the calcium-dependent protease calpain in complex with calpastatin provide a crucial new tool to overcome the vexing problem of designing inhibitors with high selectivity for calpains.

    References:

    . Marked calpastatin (CAST) depletion in Alzheimer's disease accelerates cytoskeleton disruption and neurodegeneration: neuroprotection by CAST overexpression. J Neurosci. 2008 Nov 19;28(47):12241-54. PubMed.

References

News Citations

  1. Target Practice: A Trio of Papers to Ponder for Potential Therapies

Paper Citations

  1. . Calpain inhibition: a therapeutic strategy targeting multiple disease states. Curr Pharm Des. 2006;12(5):615-38. PubMed.

Further Reading

Papers

  1. . Comprehensive behavioral phenotyping of calpastatin-knockout mice. Mol Brain. 2008;1:7. PubMed.
  2. . Marked calpastatin (CAST) depletion in Alzheimer's disease accelerates cytoskeleton disruption and neurodegeneration: neuroprotection by CAST overexpression. J Neurosci. 2008 Nov 19;28(47):12241-54. PubMed.
  3. . The novel calpain inhibitor A-705253 potently inhibits oligomeric beta-amyloid-induced dynamin 1 and tau cleavage in hippocampal neurons. Neurochem Int. 2008 Sep;53(3-4):79-88. PubMed.
  4. . Prevention of axonal injury using calpain inhibitor in chronic progressive experimental autoimmune encephalomyelitis. Brain Res. 2008 Oct 21;1236:206-15. PubMed.
  5. . Inhibition of calpains improves memory and synaptic transmission in a mouse model of Alzheimer disease. J Clin Invest. 2008 Aug;118(8):2796-807. PubMed.

Primary Papers

  1. . Calcium-bound structure of calpain and its mechanism of inhibition by calpastatin. Nature. 2008 Nov 20;456(7220):409-12. PubMed.
  2. . Concerted multi-pronged attack by calpastatin to occlude the catalytic cleft of heterodimeric calpains. Nature. 2008 Nov 20;456(7220):404-8. PubMed.
  3. . Structural biology: Enzyme knocked for a loop. Nature. 2008 Nov 20;456(7220):337-8. PubMed.