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Synapses are specialized contact sites mediating communication between neurons. Synaptogenesis requires the specific assembly of protein clusters at both sides of the synaptic contact by mechanisms that are barely understood. We studied the synaptic targeting of multi-domain proteins of the ProSAP/Shank family thought to serve as master scaffolding molecules of the postsynaptic density. In contrast to Shank1, expression of green-fluorescent protein (GFP)-tagged Pro-SAP1/Shank2 and ProSAP2/Shank3 deletion constructs in hippocampal neurons revealed that their postsynaptic localization relies on the integrity of the C-termini. The shortest construct that was perfectly targeted to synaptic sites included the last 417 amino acids of ProSAP1/Shank2 and included the C-terminal sterile alpha motif (SAM) domain. Removal of 54 residues from the N-terminus of this construct resulted in a diffuse distribution in the cytoplasm. Altogether, our data delineate a hitherto unknown targeting signal in both Pro-SAP1/Shank2 and ProSAP2/Shank3 and provide evidence for an implication of these proteins and their close homologue, Shank1, in distinct molecular pathways.

Adult retinal ganglion cells (RGCs) can regenerate their axons in vitro. Using proteomics, we discovered that the supernatants of cultured retinas contain isoforms of crystallins with crystallin ␤ b2 (crybb2) being clearly up-regulated in the regenerating retina. Immunohistochemistry revealed the expression of crybb within the retina, including in filopodial protrusions and axons of RGCs. Cloning and overexpression of crybb2 in RGCs and hippocampal neurons increased axonogenesis, which in turn could be blocked with antibodies against ␤-crystallin. Conditioned medium from crybb2-transfected cell cultures also supported the growth of axons. Finally real time imaging of the uptake of green fluorescent protein-tagged crybb2 fusion protein showed that this protein becomes internalized. These data are the first to show that axonal regeneration is related to crybb2 movement. The results suggest that neuronal crystallins constitute a novel class of neurite-promoting factors that likely operate through an autocrine mechanism and that they could be used in neurodegenerative diseases. Molecular & Cellular Proteomics 6:895-907, 2007. Adult retinal ganglion cells (RGCs)1 exhibit only a short and transient sprouting reaction after injury, and they fail to extend axons throughout the interior of the optic nerve (1). The failure of regeneration is commonly attributed to inhibitory factors associated with myelin components and/or the glial scar that includes cells and extracellular matrix proteins (2-7). The inhibitory myelin proteins have been shown to include NogoA, myelin-associated glycoprotein, and oligodendrocyte-myelin glycoprotein, all of which act through the Nogo receptor, NgR (8 -11). Expanding on this pathway of inhibition, blockage of signaling through NgR, applying antibodies against NogoA, and inactivation of RhoA (a downstream effector of NgR) signaling result in only modest axonal regeneration (12-14).There are several experimental conditions that permit the regrowth of RGC axons, including (i) replacing the distal segment of the cut optic nerve with a sciatic nerve segment (15), (ii) injuring the optic nerve and delayed culturing of retinal explants in vitro (16) or dissociation of RGCs and culturing of primary cells mainly in the postnatal retina (17), and (iii) injuring the lens (18 -21) or implanting a peripheral nerve fragment directly into the vitreous (22). Expanding on the mutual inflammatory mechanism of lens injury, stimulation of ocular macrophages with intravitreal injections of zymosan (19, 21, 23) also supports axonal growth. Explorations of alternative noninflammatory mechanisms have revealed that coculturing dissociated RGCs with injured lens tissue devoid of macrophages also improves the growth of axons (24). In an attempt to localize the growth factors within the lens, a coculture of retinal stripes with intact lens epithelium cells was shown to also support axonal growth in vitro (25), suggesting that independent mechanisms can result in the successful growth of axons. Consequently axon regene...

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The purpose of this study was to identify the gene expression profile of the regenerating retina in vitro. To achieve this goal, three experimental groups were studied: (1) an injury control group (OC-LI group) that underwent open crush (OC) of the optic nerve and lens injury (LI) in vivo; (2) an experimental group (OC-LI-R group) that comprised animals treated like those in the OC-LI group except that retinal axons were allowed to regenerate (R) in vitro; and (3) an experimental group (OC-LI-NR group) that comprised animals treated as those in the OC-LI group, except that the retinas were cultured in vitro with the retinal ganglion cell (RGC) layer facing upwards to prevent axonal regeneration (NR). Gene expression in each treatment group was compared to that of untreated controls. Immunohistochemistry was used to examine whether expression of differentially regulated genes also occurred at the protein level and to localize these proteins to the respective retinal cells. Genes that were regulated belonged to different functional categories such as antioxidants, antiapoptotic molecules, transcription factors, secreted signaling molecules, inflammation-related genes, and others. Comparison of changes in gene expression among the various treatment groups revealed a relatively small cohort of genes that was expressed in different subsets of cells only in the OC-LI-R group; these genes can be considered to be regeneration-specific. Our findings demonstrate that axonal regeneration of RGC involves an orchestrated response of all retinal neurons and glia, and could provide a platform for the development of therapeutic strategies for the regeneration of injured ganglion cells.