β-Amyloid-induced Dynamin 1 Depletion in Hippocampal Neurons

Abstract

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 ␤-amyloid (A␤), 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 A␤-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 A␤-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.Senile plaques, neurofibrillary tangles, synapse loss, and gross neurodegeneration are common findings in the brain of AD 1 patients (1-4). Numerous genetic, biochemical, and animal model studies have implicated the gradual buildup of A␤, the main component of senile plaques, as the catalyst for AD. However, the mechanistic link between A␤ accumulation and the progressive cognitive impairment associated with this disease has not been elucidated. Synapse loss seems to be the best morphological correlate of the functional deficits observed in the mid-to-late stages of AD (3, 4). In contrast, patients in the earliest stages of the disease show no significant decline in synapse number (5). Based on these findings, it has been hypothesized that a stage of synaptic dysfunction might precede frank synapse loss, plaque accumulation, and tangle formation in AD (6, 7). The mechanisms underlying such synaptic dysfunction remain unknown. It is tempting to speculate that proteins involved in synaptic vesicle biogenesis and/or recycling might play a critical role in AD. Data obtained recently seem to support this view. Thus, changes in the levels of a number of presynaptic proteins, including SNAP-25, syntaxin, and synaptotagmin, have been reported in AD (8). More recently, dynamin 1, a protein highly enriched in presynaptic terminals, has been shown to be significantly reduced in AD brains (9). Dynamin 1, a well studied neuron-specific mechanochemical GTPase, pinches off synaptic vesicles, freeing them from the membrane and allowing them to re-enter the synaptic vesicle pool to be refilled for future release (10, 11). The essential role for dynamin 1 in vesicle scission and synaptic function has been best supported by studies of the Drosophila model shibire, a temperature-sensitive mutant of a dynamin ortholog (12, 13). At restrictive temperatures these flies displayed a paralysis phenotype. This functional deficit was accompanied by the depletion of synaptic vesicles and the accumulation of invaginated pits at pre-synaptic membranes adjacent to the synaptic clefts. Collective...

ERK1/2 Activation Mediates Aβ Oligomer-induced Neurotoxicity via Caspase-3 Activation and Tau Cleavage in Rat Organotypic Hippocampal Slice Cultures

“…6). By contrast, synaptophysin levels, which correlate closely with synapse number and hence are commonly used to assay for loss of synapses (35,43,44), were not significantly altered, and cleavage of synaptophysin was not clearly seen under the same experimental conditions. To compare A␤1-42 oligomer-mediated Tau cleavage pattern in hippocampal slices to that generated by caspase-3, we subjected the total lysate of hippocampal slices to in vitro caspase-3 digestion.…”

“…Tau in Vitro Cleavage Assay-For experiments using purified caspase-3 (Sigma), total lysates were prepared from hippocampal slices and then centrifuged for 10 min at 16,000 ϫ g. The supernatants (20 l) were then incubated with recombinant caspase-3 (Sigma) for 1 h at 37°C, as described previously (35). The reactions were terminated by adding 1 volume of 2ϫ SDS sample buffer and boiled for 5 min.…”

β-Amyloid Causes Depletion of Synaptic Vesicles Leading to Neurotransmission Failure

“…Previous experiments suggested that A␤ reduced several presynaptic proteins (12,25,26). Therefore, we decided to examine the ultrastructural features of the synapses using electron microscopy in double-blind experiments.…”

“…On the other hand, presynaptic proteins such as SNAP-25, synaptophysin, and synaptotagmin have also been reported to be reduced in brains of patients with AD and after treatment with A␤ (10,11). Additionally, A␤ oligomers can alter dynamin-1, a neuron-specific mechanochemical GTPase that pinches off synaptic vesicles, allowing them to reenter the synaptic vesicle pool (12,13). Therefore, with the aim of shedding light on mechanisms associated with A␤-induced synaptic failure in the brain, we examined the acute and chronic effects of A␤ aggregates, a mixture of protofibrils and oligomers (14,15), on cultured hippocampal neurons.…”

Calpain inhibition prevents amyloid-β-induced neurodegeneration and associated behavioral dysfunction in rats

“…Recent data showing the involvement of calpain in Abmediated dysfunction of the synaptic vesicle machinery (Kelly et al, 2005;Kelly and Ferreira, 2006) could support such view.…”