Marked Calpastatin (CAST) Depletion in Alzheimer's Disease Accelerates Cytoskeleton Disruption and Neurodegeneration: Neuroprotection by CAST Overexpression

Abstract

Increased activity of calpains is implicated in synaptic dysfunction and neurodegeneration in Alzheimer's disease (AD). The molecular mechanisms responsible for increased calpain activity in AD are not known. Here, we demonstrate that disease progression is propelled by a marked depletion of the endogenous calpain inhibitor, calpastatin (CAST), from AD neurons, which is mediated by caspase-1, caspase-3, and calpains. Initial CAST depletion focally along dendrites coincides topographically with calpain II and ERK 1/2 activation, tau cleavage by caspase-3, and tau and neurofilament hyperphosphorylation. These same changes, together with cytoskeletal proteolysis and neuronal cell death, accompany CAST depletion after intrahippocampal kainic acid administration to mice, and are substantially reduced in mice overexpressing human CAST. Moreover, CAST reduction by shRNA in neuronal cells causes calpain-mediated death at levels of calcium-induced injury that are sublethal to cells normally expressing CAST. Our results strongly support a novel hypothesis that CAST depletion by multiple abnormally activated proteases accelerates calpain dysregulation in AD leading to cytoskeleton disruption and neurodegeneration. CAST mimetics may, therefore, be neuroprotective in AD.

Calpain-Mediated Tau Cleavage: A Mechanism Leading to Neurodegeneration Shared by Multiple Tauopathies

“…Recently, it has been shown that calpastatin levels decreased as the AD pathological process progressed (64). Calpastatin decrease has been attributed to the proteolytic activity of caspase 3 and calpain (64).…”

Calpain Activation Promotes BACE1 Expression, Amyloid Precursor Protein Processing, and Amyloid Plaque Formation in a Transgenic Mouse Model of Alzheimer Disease

“…Mice-There is a depletion of CAST, the endogenous inhibitor of calpain, in the prefrontal cortex of AD brains (22,43); this calpastatin decrease may allow calpain activation. We generated transgenic mice overexpressing human CAST under the CaMKII␣ promoter (Fig.…”

“…Inhibition of calpain by synthetic inhibitors exerts neuroprotection in various models of brain injuries, such as ischemia or excitotoxicity-induced neuronal death (22)(23)(24). Interestingly, A␤ aggregation is associated with neuronal and astrocytic calcium dysregulation (25)(26)(27).…”

Interaction of recombinant and native Cav2.3 E-/R-type voltage-gated Ca2+ channels with the molecular chaperone Hsp70

“…In the event of forward directed signaling from the kainate receptor to Hsp70 and Ca v 2.3, a slight shift in the threshold of activation for Ca v 2.3 may increase the Ca 2+ influx at lower membrane potentials, and could activate Ca 2+ -dependent proteases known to be involved in neurodegeneration, as has been shown for calpain after calpastatin depletion in Alzheimer's disease. 49 Whatever will be the correct explanation for the observed differences between Ca v 2.3 control and Ca v 2.3-deficient mice after kainate injection and subsequent neurodegeneration, increased Hsp70 binding to microsomal membranes represents an important (albeit only an intermediate) step leading finally via other processes to hippocampal neurodegeneration after kainate injection, similar to what has been reported for the rat hippocampus. 12 Therefore, using the kainate model for both control and Ca v 2.3-deficient mice will help in future to elucidate in detail the molecular steps involved in kainateinduced and Ca v 2.3-mediated neuronal cell death.…”