I recommend this paper as a persuasive examination of the normalizing effects of calpain inhibitors on synaptic and learning deficits in PS1/APP mice—but some clarification or further electrophysiological analysis is first needed to characterize the signaling deficits. In particular, I'm referring to increased frequency of mEPSCs (it would also be useful to see the traces) in the PS/APP cultured neurons (Fig 1D), and the normalized mEPSCs measured in glutamate in 1E. Since the baselines are vastly different between the wild-type and APP/PS1, it actually appears that the frequency of mEPSCs maxes out to a similar level across all conditions. There appears to be no increase in the APP/PS1 cells just because they are already releasing transmitter at a high level. Paired pulse facilitation experiments would have been helpful here to further evaluate presynaptic effects; one might expect a reduction in paired pulse facilitation (PPF) in the APP/PS1 cultures under these conditions, which is consistent with increased synaptic strength—which is not observed here. The input/output curves...  Read more

I recommend this paper as a persuasive examination of the normalizing effects of calpain inhibitors on synaptic and learning deficits in PS1/APP mice—but some clarification or further electrophysiological analysis is first needed to characterize the signaling deficits. In particular, I'm referring to increased frequency of mEPSCs (it would also be useful to see the traces) in the PS/APP cultured neurons (Fig 1D), and the normalized mEPSCs measured in glutamate in 1E. Since the baselines are vastly different between the wild-type and APP/PS1, it actually appears that the frequency of mEPSCs maxes out to a similar level across all conditions. There appears to be no increase in the APP/PS1 cells just because they are already releasing transmitter at a high level. Paired pulse facilitation experiments would have been helpful here to further evaluate presynaptic effects; one might expect a reduction in paired pulse facilitation (PPF) in the APP/PS1 cultures under these conditions, which is consistent with increased synaptic strength—which is not observed here. The input/output curves suggest reduced synaptic strength.

The role of calcium in the presynaptic transmitter release dynamics is fundamental, and may affect calpain activation. As an aside, but perhaps noteworthy, is the inclusion of 60 micromolar calcium in the patch pipette used to record postsynaptic events. This concentration is 100 times the normal cytosolic levels in neurons, and I am curious how calpain activity may differ in the APP/PS1 neurons if more physiological levels were used.

Obviously, these are complicated dynamics, but a few extra experiments to characterize the synaptic dynamics would have been very enlightening. Since the role of calpain in the neuropathology is evident, more detailed information is certainly welcomed.

View all comments by Grace (Beth) Stutzmann

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...  Read more

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.

View all comments by Ralph Nixon