BackgroundDendritic spines are small membranous protrusions on the neuronal dendrites that receive synaptic input from axon terminals. Despite their importance for integrating the enormous information flow in the brain, the molecular mechanisms regulating spine morphogenesis are not well understood. NESH/Abi-3 is a member of the Abl interactor (Abi) protein family, and its overexpression is known to reduce cell motility and tumor metastasis. NESH is prominently expressed in the brain, but its function there remains unknown.Methodology/Principal FindingsNESH was strongly expressed in the hippocampus and moderately expressed in the cerebral cortex, cerebellum and striatum, where it co-localized with the postsynaptic proteins PSD95, SPIN90 and F-actin in dendritic spines. Overexpression of NESH reduced numbers of mushroom-type spines and synapse density but increased thin, filopodia-like spines and had no effect on spine density. siRNA knockdown of NESH also reduced mushroom spine numbers and inhibited synapse formation but it increased spine density. The N-terminal region of NESH co-sedimented with filamentous actin (F-actin), which is an essential component of dendritic spines, suggesting this interaction is important for the maturation of dendritic spines.Conclusions/SignificanceNESH is a novel F-actin binding protein that likely plays important roles in the regulation of dendritic spine morphogenesis and synapse formation.

Dendritic spines are highly specialized actin‐rich structures on which the majority of excitatory synapses are formed in the mammalian CNS. SPIN90 is an actin‐binding protein known to be highly enriched in postsynaptic densities (PSDs), though little is known about its function there. Here, we show that SPIN90 is a novel binding partner for Shank proteins in the PSD. SPIN90 and Shank co‐immunoprecipitate from brain lysates and co‐localize in postsynaptic dendrites and act synergistically to mediate spine maturation and spine head enlargement. At the same time, SPIN90 causes accumulation of Shank and PSD‐95 within dendritic spines. In addition, we found that the protein composition of PSDs in SPIN90 knockout mice is altered as is the actin cytoskeleton of cultured hippocampal SPIN90 knockout neurons. Taken together, these findings demonstrate that SPIN90 is a Shank1b binding partner and a key contributor to the regulation of dendritic spine morphogenesis and brain function.

SPIN90, a 90-kDa Nck-interacting protein with a SH3 domain, plays a role in sarcomere formation and myofibril assembly, and its phosphorylation is modulated by cell adhesion and Erk activation. Here we demonstrate that SPIN90 participates in receptor-mediated endocytic pathway in fibroblasts. We identified syndapin (synaptic dynamin-binding protein) as a SPIN90 interacting protein using yeast two-hybrid screening. SPIN90 directly binds the SH3 domain of syndapin via its proline rich domain in vitro and in vivo and also associates with clathrin. Over-expression of SPIN90-full length in COS-7 cells inhibited transferrin uptake, a marker of endocytosis. Interestingly, SPIN90-PRD, a syndapin-binding domain, significantly inhibited endocytosis, and the inhibition was reversed by co-expression of syndapin. Depleting SPIN90 through antibody microinjection or Knocking it down using siRNAs also significantly inhibited transferrin internalization. Moreover, early endosomal marker proteins (EEA1 and Rab5) appeared to closely associate or partially co-localize with SPIN90 in endosomes and an internalized FITC-dextran and Texas Red-EGF were found on the endosomes in association with SPIN90. Time-lapse video showed that GFP-SPIN90 travels with moving vesicles within living cells. Taken together, these findings suggest that SPIN90 is implicated in receptor-mediated endocytic pathway in fibroblasts.

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