Calpain mediates calcium-induced activation of the erk1,2 MAPK pathway and cytoskeletal phosphorylation in neurons: relevance to Alzheimer's disease

Beta-amyloid-induced dynamin 1 depletion in hippocampal neurons. A potential mechanism for early cognitive decline in Alzheimer disease

Synaptic dysfunction is one of the earliest events in the pathogenesis of Alzheimer disease (AD). However, the molecular mechanisms underlying synaptic defects in AD are largely unknown. We report here that beta-amyloid (Abeta), the main component of senile plaques, induced a significant decrease in dynamin 1, a protein that is essential for synaptic vesicle recycling and, hence, for memory formation and information processing. The Abeta-induced dynamin 1 decrease occurred in the absence of overt synaptic loss and was also observed in the Tg2576 mouse model of AD. In addition, our results provided evidence that the Abeta-induced decrease in dynamin 1 was likely the result of a calpain-mediated cleavage of dynamin 1 protein and possibly the down-regulation of dynamin 1 gene expression. These data suggest a mechanism to explain the early cognitive loss without a major decline in synapse number observed in AD and propose a novel therapeutic target for AD intervention.

Mutational effects at the tetramerization site of nonerythroid alpha spectrin

Spectrin, a prominent cytoskeletal protein, exerts its fundamental role in cellular function by forming a sub-membrane filamentous network. An essential aspect of spectrin network formation is the tetramerization of spectrin alphabeta heterodimers. We used laboratory methods, the yeast two-hybrid system and random mutagenesis, to investigate, for the first time, effects of amino acid mutations on tetramerization of nonerythroid (brain) spectrin (fodrin). Based on high sequence homology with erythroid spectrin, we assume the putative tetramerization region of nonerythroid alpha-spectrin at the N-terminal region. We introduced mutations in the region consisting of residues 1-45 and studied mutational effects on spectrin alphabeta association to form tetramers. We detected single, double, and triple mutations involving 24 residues in this region. These amino acid mutations of nonerythroid alpha-spectrin exhibit full, partial, or no effect on the association with nonerythroid beta-spectrin. Single amino acid mutations in the region of residues 1-9 (D2Y, G5V, V6D, and V8M) did not affect the association. However, seven single mutations (I15F, I15N, R18G, V22D, R25P, Y26N, and R28P) affected the alphabeta association. These mutations were clustered in the region predicted by sequence alignment to be crucial in nonerythroid alpha-spectrin for tetramerization, a region that spanned residues 12-36, corresponding to the partial domain Helix C' (residues 21-45) in erythroid alpha-spectrin. In addition, two other mutations, one upstream and one downstream of this region at positions 10 (E10D) and 37 (R37P), also affected the alphabeta association. Our results implied nonerythroid alpha-spectrin partial domain helix may be longer than Helix C' (residues 21-45 and a total of 25 residues) in erythroid alpha-spectrin and spanned at least residues 10-37. It is interesting to note that seven out of these nine single mutations (I15F, I15N, R18G, V22D, R25P, Y26N, R37P) were at the a, d, e or g heptad positions based on sequence alignment with erythroid alpha-spectrin. Four of the mutated residues (I15, R18, V22, R25) are conserved in both erythroid and nonerythroid spectrin. These positions were previously identified as hot spots in erythroid alpha-spectrin that lead to severe hematological symptoms. This study clearly demonstrated that single mutation in a region predicted to be critical functionally in nonerythroid alpha-spectrin indeed leads to functional abnormalities and may lead to neurological disorders.

Proteolytic mechanisms in necrotic cell death and neurodegeneration

Programmed neuronal cell death is required during development to achieve the accurate wiring of the nervous system. However, genetic or accidental factors can lead to the premature, non-programmed death of neurons during adult life. Inappropriate death of cells in the nervous system is the cause of multiple neurodegenerative disorders. Pathological neuronal death can occur by apoptosis, by necrosis or by a combination of both. Necrotic cell death underlies the pathology of devastating neurological diseases such as neurodegenerative disorders, stroke or trauma. However, little is known about the molecular mechanisms that bring about necrotic cell death. Proteases play crucial roles in neuron degeneration by exerting both regulatory and catabolic functions. Elevated intracellular calcium is the most ubiquitous feature of neuronal death with the concomitant activation of cysteine calcium-dependent proteases, calpains. Calpains and lysosomal, catabolic aspartyl proteases, play key roles in the necrotic death of neurons. In this review, we survey the recent literature on the role of cysteine and aspartyl proteases in necrosis and neurodegeneration, aiming to delineate common proteolytic mechanisms mediating cellular destruction.

Ritonavir does not inhibit calpain in vitro

Ritonavir, an inhibitor of HIV-1 protease, has been reported to also inhibit the Ca2+-dependent cysteine protease, calpain. We have investigated these claims with an in vitro study of the effect of ritonavir on the m-calpain and mu-calpain isoforms. Ritonavir failed to block either autolytic or hydrolytic calpain activity, but remained fully capable of inhibiting the HIV-1 protease. Any calpain-related effects of ritonavir in cells must, therefore, arise by a mechanism other than direct inhibition of calpains.