Regulation of Dendritic Spine Morphology by SPAR, a PSD-95-Associated RapGAP

Abstract

The PSD-95/SAP90 family of scaffold proteins organizes the postsynaptic density (PSD) and regulates NMDA receptor signaling at excitatory synapses. We report that SPAR, a Rap-specific GTPase-activating protein (RapGAP), interacts with the guanylate kinase-like domain of PSD-95 and forms a complex with PSD-95 and NMDA receptors in brain. In heterologous cells, SPAR reorganizes the actin cytoskeleton and recruits PSD-95 to F-actin. In hippocampal neurons, SPAR localizes to dendritic spines and causes enlargement of spine heads, many of which adopt an irregular appearance with putative multiple synapses. Dominant negative SPAR constructs cause narrowing and elongation of spines. The effects of SPAR on spine morphology depend on the RapGAP and actin-interacting domains, implicating Rap signaling in the regulation of postsynaptic structure.

Ontogeny of postsynaptic density proteins at glutamatergic synapses

“…Most of the SPAR protein that we saw in the hippocampus was the 170 kD form, which is cytosolic (Pak et al, 2001). Since SPAR associates with actin and we found gold labeling associated with presumptive actin filaments, it is possible that the labeling that we saw associated with the PSD represents SPAR (perhaps associated with actin) that is only loosely attached to the PSD.…”

“…1), SPAR antibody produced a light band at P2 and this increased in density with age. This band was about 170 kD, corresponding to the cytoplasmic component of SPAR labeling (Pak et al, 2001). The 190 kD band, reported previously to be present in the PSD, was much lighter in our material at all ages.…”

“…A C-terminal GKAP antibody (affinity-purified; amino acids 744-964) was characterized in hippocampus (and in adult cerebral cortex for immunogold) by Kim et al (1997) and Naisbitt et al (1997). SPAR antibody (affinity-purified; amino acids 1744-1760) was characterized in adult brain and hippocampal neurons in vitro (Pak et al, 2001(Pak et al, , 2003. Nr-CAM (837; recognizes extracellular domain) antibody distribution in developing brain (light microscopy; including hippocampus) was characterized in Lustig et al (2001).…”

Serum inducible kinase is a positive regulator of cortical dendrite development and is required for BDNF-promoted dendritic arborization

“…SNK may thus affect dendrite growth and arborization in part by regulating the activity of SPAR, which is expressed in immature neurons and has the ability to regulate actin dynamics [40]. However, given that SNK is expressed in the nucleus of immature neurons (Figure 1), we cannot rule out the possibility that SNK affects morphogenesis from the nucleus.…”

“…SNK has been shown to phosphorylate SPAR [12], a known regulator of spine morphology [40]. SNK may thus affect dendrite growth and arborization in part by regulating the activity of SPAR, which is expressed in immature neurons and has the ability to regulate actin dynamics [40].…”