Inhibition of Rho via Arg and p190RhoGAP in the postnatal mouse hippocampus regulates dendritic spine maturation, synapse and dendrite stability, and behavior.

The RhoA (Rho) GTPase is a master regulator of dendrite morphogenesis. Rho activation in developing neurons slows dendrite branch dynamics, yielding smaller, less branched dendrite arbors. Constitutive activation of Rho in mature neurons causes dendritic spine loss and dendritic regression, indicating that Rho can affect dendritic structure and function even after dendrites have developed. However, it is unclear whether and how endogenous Rho modulates dendrite and synapse morphology after dendrite arbor development has occurred. We demonstrate that a Rho inhibitory pathway involving the Arg tyrosine kinase and p190RhoGAP is essential for synapse and dendrite stability during late postnatal development. Hippocampal CA1 pyramidal dendrites develop normally in arg-/- mice, reaching their mature size by postnatal day 21 (P21). However, dendritic spines do not undergo the normal morphological maturation in these mice, leading to a loss of hippocampal synapses and dendritic branches by P42. Coincident with this synapse and dendrite loss, arg-/- mice exhibit progressive deficits in a hippocampus-dependent object recognition behavioral task. p190RhoGAP localizes to dendritic spines, and its activity is reduced in arg-/- hippocampus, leading to increased Rho activity. Although mutations in p190rhogap enhance dendritic regression resulting from decreased Arg levels, reducing gene dosage of the Rho effector ROCKII can suppress the dendritic regression observed in arg-/- mice. Together, these data indicate that signaling through Arg and p190RhoGAP acts late during synaptic refinement to promote dendritic spine maturation and synapse/dendrite stability by attenuating synaptic Rho activity.

Abl family nonreceptor tyrosine kinases modulate short-term synaptic plasticity.

Abl family nonreceptor tyrosine kinases regulate cell morphogenesis through functional interactions with the actin cytoskeleton. The vertebrate Abl family kinases, Abl and Arg, are expressed in the adult mouse brain, where they may regulate actin cytoskeletal dynamics in mature neurons. Using immunoelectron microscopy, we have localized Abl and Arg to the pre- and postsynaptic compartments of synapses in the mouse hippocampal area CA1. Paired-pulse facilitation (PPF) was significantly reduced at the Schaffer collateral-CA1 (SC-CA1) excitatory synapses in hippocampal slices from abl-/- and arg-/- mice as compared with wild-type mice. Furthermore, treatment of wild-type slices with the specific Abl family kinase inhibitor STI571 also reduced PPF. Basal synaptic transmission, posttetanic potentiation (PTP), long-term potentiation (LTP), and long-term depression (LTD) were similar to wild-type controls in abl-/- and arg-/- slices and in STI571-treated wild-type slices. These results indicate that an important function of Abl and Arg is to modulate synaptic efficacy via a presynaptic mechanism during repetitive activation.