No abstract

Dendritic spines have two major structural elements: postsynaptic densities (PSDs) and actin cytoskeletons. PSD proteins are proposed to regulate spine morphogenesis. However, other molecular mechanisms should govern spine morphogenesis, because the initiation of spine morphogenesis precedes the synaptic clustering of these proteins. Here, we show that synaptic clustering of drebrin, an actin-binding protein highly enriched in dendritic spines, governs spine morphogenesis. We immunocytochemically analyzed developing hippocampal neurons of low-density cultures. Filopodia-like dendritic protrusions were classified into two types: diffuse-type filopodia, which have diffuse distribution of drebrin, and cluster-type filopodia, which have drebrin clusters with filamentous actin (F-actin). Most cluster-type filopodia were synaptic filopodia. Postsynaptic drebrin clusters were found in both most synaptic filopodia and spines. Postsynaptic PSD-95 clusters, however, were found in only one-half of synaptic filopodia but in most spines. These data indicate that cluster-type filopodia are not mature spines but their precursors. Suppression of the upregulation of drebrin adult isoform (drebrin A) by antisense oligonucleotides against it attenuated synaptic clustering of PSD-95, as well as clustering of drebrin and F-actin. Furthermore, the restoration of drebrin A expression by injection of the expression vectors of drebrin A tagged with green fluorescent protein into the neurons treated with the antisense oligonucleotides induced synaptic reclustering of PSD-95 on clusters of the labeled drebrin A. These data indicated that the synaptic clustering of drebrin is necessary for that of PSD-95 in developing neurons. Together, these data suggest that synaptic clustering of drebrin is an essential step for spine morphogenesis.

It is widely thought that Alzheimer's disease (AD) begins as a malfunction of synapses, eventually leading to cognitive impairment and dementia. Homeostatic synaptic scaling is a mechanism that could be crucial at the onset of AD but has not been examined experimentally. In this process, the synaptic strength of a neuron is modified so that the overall excitability of the cell is maintained. Here, we investigate whether synaptic scaling mediated by L-␣-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptors (AMPARs) contributes to pathology in double knockin (2؋KI) mice carrying human mutations in the genes for amyloid precursor protein and presenilin-1. By using whole-cell recordings, we show that 2؋KI mice exhibit age-related downscaling of AMPARmediated evoked currents and spontaneous, miniature currents. Electron microscopic analysis further corroborates the synaptic AMPAR decrease. Additionally, 2؋KI mice show age-related deficits in bidirectional plasticity (long-term potentiation and longterm depression) and memory flexibility. These results suggest that AMPARs are important synaptic targets for AD and provide evidence that cognitive impairment may involve downscaling of postsynaptic AMPAR function.amyloid precursor protein ͉ glutamate ͉ presenilin E xtensive work on Alzheimer's disease (AD) has led to the hypothesis that the memory failure exhibited by patients in the early stages of AD results from synaptic disruption (1-3), without frank neuronal loss, which is caused by toxicity of the 42-aa variant of the amyloid ␤ protein (A␤ 42 ). Work in AD models (4) shows that A␤ 42 impairs synaptic plasticity in brain regions, such as the hippocampus, that are recognized early targets for AD. Transgenic (Tg) mice with familial AD mutations display disruptions of long-term potentiation (LTP), an electrophysiological correlate of memory encoding (5). The LTP impairment occurs before deposition of A␤ plaques (4, 6), making it a sensitive marker for early AD dysfunction. Notably, the late phase of LTP (called expression) is highly susceptible to disruption (6-9). LTP expression relies on alterations of L-␣-amino-3-hydroxy-5-methyl-4-isoxazolepropionic receptors (AMPARs), including phosphorylation by kinases and recruitment of AMPARs to the synaptic membrane (10). Conversely, AMPAR removal is thought to mediate the activitydependent decrease of excitatory transmission (11), which is elicited by the paradigm of long-term depression (LTD) (12). Importantly, LTD has been scarcely studied in AD models (4, 13), particularly Tg-AD mice.Recently, AMPARs have been implicated in a slower form of synaptic plasticity, termed homeostatic synaptic scaling, in which the total synaptic strength of a neuron is modified to regulate its excitability (14). Synaptic scaling involves, among other factors, the postsynaptic insertion and removal of AMPARs and changes in the turnover rate of functional receptors (14-16). Adjustments by synaptic scaling operate in vivo (17) and seem critical for regulating synaptic strength during lear...

Morphological changes in the dendritic spines have been postulated to participate in the expression of synaptic plasticity. The cytoskeleton is likely to play a key role in regulating spine structure. Here we examine the molecular mechanisms responsible for the changes in spine morphology, focusing on drebrin, an actin-binding protein that is known to change the properties of actin filaments. We found that adult-type drebrin is localized in the dendritic spines of rat forebrain neurons, where it binds to the cytoskeleton. To identify the cytoskeletal proteins that associated with drebrin, we isolated drebrin-containing cytoskeletons using immunoprecipitation with a drebrin antibody. Drebrin, actin, myosin, and gelsolin were co-precipitated. We next examined the effect of drebrin on actomyosin interaction. In vitro, drebrin reduced the sliding velocity of actin filaments on immobilized myosin and inhibited the actin-activated ATPase activity of myosin. These results suggest that drebrin may modulate the actomyosin interaction within spines and may play a role in the structure-based plasticity of synapses.