Local translation of proteins in distal dendrites is thought to support synaptic structural plasticity. We have previously shown that metabotropic glutamate receptor (mGluR1) stimulation initiates a phosphorylation cascade, triggering rapid association of some mRNAs with translation machinery near synapses, and leading to protein synthesis. To determine the identity of these mRNAs, a cDNA library produced from distal nerve processes was used to screen synaptic polyribosome-associated mRNA. We identified mRNA for the fragile X mental retardation protein (FMRP) in these processes by use of synaptic subcellular fractions, termed synaptoneurosomes. We found that this mRNA associates with translational complexes in synaptoneurosomes within 1-2 min after mGluR1 stimulation of this preparation, and we observed increased expression of FMRP after mGluR1 stimulation. In addition, we found that FMRP is associated with polyribosomal complexes in these fractions. In vivo, we observed FMRP immunoreactivity in spines, dendrites, and somata of the developing rat brain, but not in nuclei or axons. We suggest that rapid production of FMRP near synapses in response to activation may be important for normal maturation of synaptic connections.Changes in synaptic connectivity are likely to be a key mechanism by which nervous system organization is permanently changed by experience. Local translation of some proteins in dendrites is increasingly considered to be important for changes in synaptic structure and receptor composition. Certain mRNAs are known to be targeted to dendrites (1, 2), and polyribosomal aggregates are observed in or near dendritic spines, more frequently at newly forming synapses (3-5). Dendrites have been shown to be equipped with components necessary for protein synthesis (6), and synthesis of proteins has been demonstrated directly in synaptoneurosomes (7,8) and in preparations of dendrites isolated from hippocampal neurons in culture (9). Local translation of transfected reporter-tagged mRNA has been demonstrated in transected dendrites (10).Protein translation induced by metabotropic receptor stimulation has previously been proposed to play a role in longterm potentiation (LTP), a model for synaptic plasticity (1,11,12). LTP induction also alters levels of specific mRNAs in tissue slices (13), and isotope-tagged leucine is taken up in dendritic regions of hippocampal slices in response to stimulation (14). We have demonstrated that phosphoinositidelinked metabotropic glutamate receptors (mGluR1), known to trigger a phosphorylation cascade, cause certain mRNAs to associate rapidly with protein translation complexes in synaptoneurosomes (15); this process is modulated by ionotropic receptors (16). Furthermore, we have shown that depolarization by 40 mM K ϩ or stimulation by phosphoinositide receptorspecific mGluR agonists increases [ 35 S]methionine incorporation into trichloroacetic acid-precipitable polypeptides (15, 17), indicating that de novo protein synthesis at the synapse occurs as a result of t...

Protein synthesis occurs in neuronal dendrites, often near synapses. Polyribosomal aggregates often appear in dendritic spines, particularly during development. Polyribosomal aggregates in spines increase during experience-dependent synaptogenesis, e.g., in rats in a complex environment. Some protein synthesis appears to be regulated directly by synaptic activity. We use ''synaptoneurosomes,'' a preparation highly enriched in pinched-off, resealed presynaptic processes attached to resealed postsynaptic processes that retain normal functions of neurotransmitter release, receptor activation, and various postsynaptic responses including signaling pathways and protein synthesis. We have found that, when synaptoneurosomes are stimulated with glutamate or group I metabotropic glutamate receptor agonists such as dihydroxyphenylglycine, mRNA is rapidly taken up into polyribosomal aggregates, and labeled methionine is incorporated into protein. One of the proteins synthesized is FMRP, the protein that is reduced or absent in fragile X mental retardation syndrome. FMRP has three RNA-binding domains and reportedly binds to a significant number of mRNAs. We have found that dihydroxyphenylglycine-activated protein synthesis in synaptoneurosomes is dramatically reduced in a knockout mouse model of fragile X syndrome, which cannot produce full-length FMRP, suggesting that FMRP is involved in or required for this process. Studies of autopsy samples from patients with fragile X syndrome have indicated that dendritic spines may fail to assume a normal mature size and shape and that there are more spines per unit dendrite length in the patient samples. Similar findings on spine size and shape have come from studies of the knockout mouse. Study of the development of the somatosensory cortical region containing the barrel-like cell arrangements that process whisker information suggests that normal dendritic regression is impaired in the knockout mouse. This finding suggests that FMRP may be required for the normal processes of maturation and elimination to occur in cerebral cortical development.

Fragile X mental retardation is caused by absence of the RNAbinding protein fragile X mental retardation protein (FMRP), encoded by the FMR1 gene. There is increasing evidence that FMRP regulates transport and modulates translation of some mRNAs. We studied neurotransmitter-activated synaptic protein synthesis in fmr1-knockout mice. Synaptoneurosomes from knockout mice did not manifest accelerated polyribosome assembly or protein synthesis as it occurs in wild-type mice upon stimulation of group I metabotropic glutamate receptors. Direct activation of protein kinase C did not compensate in the knockout mouse, indicating that the FMRP-dependent step is further along the signaling pathway. Visual cortices of young knockout mice exhibited a lower proportion of dendritic spine synapses containing polyribosomes than did the cortices of wild-type mice, corroborating this finding in vivo. This deficit in rapid neurotransmitter-controlled local translation of specific proteins may contribute to morphological and functional abnormalities observed in patients with fragile X syndrome.dendrites ͉ metabotropic glutamate receptor ͉ mRNA ͉ plasticity ͉ ultrastructure F ragile X mental retardation syndrome is an inherited, Xlinked disorder. In most patients, methylation of an extreme expansion (200-1,000 repeats) of a (CGG)n trinucleotide repeat in the 5Ј UTR of the FMR1 gene blocks transcription of fmr1 mRNA (1). The resulting absence of fragile X mental retardation protein (FMRP) causes the syndrome, which is characterized by mental retardation, macroorchidism, and behavioral abnormalities (2). The brains of these patients exhibit an unusual, spindly appearance of the dendritic spines as well as an overabundance of spines (3, 4), a morphology that resembles early postnatal tissue.The function of FMRP is unknown; in neurons much of the protein is found in dendrites (5). FMRP contains RNA-binding elements (6) and is associated with actively translating polyribosomes in the brain (7-9). Several laboratories have described sets of mRNAs bound by FMRP (10-12), and specific motifs involved in FMRP binding of some mRNAs have been identified (13,14). Recently, we demonstrated (10) that several members of a subset of mRNAs bound by FMRP in intact cells are differentially distributed and͞or translated in dendritic, as compared to somatic, subcellular domains. This finding suggests direct involvement of FMRP in transport and͞or translation of mRNA in dendrites. Antar et al. (15) have demonstrated rapid transport of FMRP into dendrites upon KCl depolarization. We report here that a dynamic aspect of translation, neurotransmitter-induced rapid initiation, is directly impacted by the absence of FMRP.Protein translation in dendrites was suggested by early descriptions of postsynaptic polyribosomal aggregates (PRAs) during synaptogenesis and in the visual cortex of rats reared in complex environments, indicating the importance of local translation for synaptic plasticity (16,17). Components necessary for translation are present postsynaptic...

These observations suggest that early postnatal binge alcohol exposure results in long-term deficits of adult hippocampal neurogenesis, providing a potential basis for the deficits of hippocampus-dependent behaviors reported for this model.

These results indicate that abnormal dendritic outgrowth or maintenance contributes to reduced neuropil and prefrontal connectivity in schizophrenia. Short postmortem intervals and resulting high tissue quality suggest that these dystrophic changes reflect schizophrenia pathology rather than postmortem artifact.

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Exposure to alcohol during the brain growth spurt results in impaired cognition and learning in adulthood. This impairment is accompanied by permanent structural changes in the hippocampal formation. Exercise improves performance on hippocampal-dependent learning and memory tasks and increases adult neurogenesis in the rat hippocampal dentate gyrus. The present study examined the effects of wheel running during adolescence on dentate gyrus cell proliferation and neurogenesis after postnatal binge-like alcohol exposure. On postnatal days (PD) 4-9, pups were either intubated with alcohol in a binge-like manner, sham-intubated, or reared normally. On PD30-42 all animals were randomly assigned to two adolescent conditions: wheel running or inactive control. Animals were injected with BrdU every day between PD32-42 and perfused on PD42 or PD72. In inactive control animals at both PD42 and 72, cell proliferation and neurogenesis did not differ between postnatal treatment groups. Wheel running significantly increased the number of BrdU labeled cells on PD42 in all three postnatal treatments. On PD72, only the normal controls showed significant increases in survival of newly generated cells resulting from the wheel running. These results indicate that adolescent wheel running can induce comparable increases in cell proliferation and neurogenesis in alcohol-exposed and control rats, but the long-term survival of those newly-generated cells is impaired relative normal controls. Exercise may provide a means to stimulate neurogenesis, with implications for amelioration of hippocampal-dependent learning impairments associated with alcohol exposure. However, benefits requiring long-lasting survival of the newly generated cells will depend on identifying ways to promote survival.