A Role for RhoB in Synaptic Plasticity and the Regulation of Neuronal Morphology

Abstract

Actin-rich dendritic spines are the locus of excitatory synaptic transmission and plastic events such as long-term potentiation (LTP).Morphological plasticity of spines accompanies activity-dependent changes in synaptic strength. Several Rho GTPase family members are implicated in regulating neuronal and, in particular, spine structure via actin and the actin-binding protein cofilin. However, despite expression in hippocampus and cortex, its ability to modulate actin-regulatory proteins, and its induction during aging, RhoB has been relatively neglected. We previously demonstrated that LTP is associated with specific RhoB activation. Here, we further examined its role in synaptic function using mice with genetic deletion of the RhoB GTPase (RhoB Ϫ/Ϫ mice). Normal basal synaptic transmission accompanied reduced paired-pulse facilitation and post-tetanic potentiation in the hippocampus of RhoB Ϫ/Ϫ mice. Early phase LTP was significantly reduced in RhoB Ϫ/Ϫ animals, whereas the later phase was unaffected. In wild-type mice (RhoB ϩ/ϩ ), Western blot analysis of potentiated hippocampus showed significant increases in phosphorylated cofilin relative to nonpotentiated slices, which were dramatically impaired in RhoB Ϫ/Ϫ slices. There was also a deficit in phosphorylated Lim kinase levels in the hippocampus from RhoB Ϫ/Ϫ mice. Morphological analysis suggested that lack of RhoB resulted in increased dendritic branching and decreased spine number. Furthermore, an increase in the proportion of stubby relative to thin spines was observed. Moreover, spines demonstrated increased length along with increased head and neck widths. These data implicate RhoB in cofilin regulation and dendritic and spine morphology, highlighting its importance in synaptic plasticity at a structural and functional level.

ADF/cofilin: a crucial regulator of synapse physiology and behavior

“…In double mutants lacking ADF and cofilin1, the number of synaptic vesicles attached to the active zone was increased and consequently synaptic vesicle exocytosis was enhanced [24,25]. In line with a role of ADF/cofilin in vesicle exocytosis, inactivation of upstream regulators such as RhoB, ROCK2, PAK, LIMK1 or slingshot impaired vesicle exocytosis [30,44,50,51,63]. Interestingly, very similar to ADF/cofilin mutants, the number of docked vesicles and synaptic vesicle exocytosis was enhanced upon inactivation of profilin2 [39,64], an actin-binding protein that promotes actin polymerization by accelerating the ATP-ADP exchange on G-actin and by funneling G-actin to F-actin's plus ends [65].…”

“…Additionally, RhoB-and LIMK1-mediated inactivation of ADF/cofilin has been shown recently, and this pathway is relevant specifically for the early, but not the late phase of LTP [50]. In RhoB-deficient mice, basal levels of ADF/cofilin phosphorylation were unchanged, but the LTP-induced phosphorylation (inactivation) of ADF/cofilin did not exist, resulting in larger spines [50]. Together, several Rho GTPase-dependent signaling pathways have been identified to control ADF/cofilin activity during LTP.…”

“…It is conceivable that deregulation of ADF/cofilin activity contributes to the synaptic defects found in Rac and Cdc42 mutant mice [48,49]. Two other Rho GTPases, namely RhoA and RhoB, are relevant for spine morphology and synaptic plasticity, and both have been shown to mediate ADF/cofilin inactivation in dendritic spines [34,50]. Stabilization of mature spines depends on signaling through the receptor tyrosine kinase EphB2 and focal adhesion kinase (FAK) which mediate ADF/cofilin inactivation via a pathway that includes RhoA, RhoA kinases (ROCK) and LIMK1 [34].…”

“…Interestingly, the RhoA-ROCK-LIMK-ADF/cofilin pathway is activated upon estrogen treatment, and it has been speculated that the acute effects of female sex hormones on synaptic function, LTP and learning are mediated via this pathway [53]. Additionally, RhoB-and LIMK1-mediated inactivation of ADF/cofilin has been shown recently, and this pathway is relevant specifically for the early, but not the late phase of LTP [50]. In RhoB-deficient mice, basal levels of ADF/cofilin phosphorylation were unchanged, but the LTP-induced phosphorylation (inactivation) of ADF/cofilin did not exist, resulting in larger spines [50].…”

Prenatal inhibition of the tryptophan–kynurenine pathway alters synaptic plasticity and protein expression in the rat hippocampus

“…The two small GTPase enzymes RhoA and RhoB were examined since they have been shown to modulate or contribute to plastic changes induced electrophysiologically in the hippocampus (O'Kane et al 2003a,b;McNair et al, 2010). Their activity is closely related to functions of NMDA receptors and they also play major roles in the regulation of cytoskeletal proteins.…”