No abstract

Oligophrenin-1 (OPHN1) encodes a Rho-GTPase-activating protein (Rho-GAP) whose loss of function has been associated with X-linked mental retardation (MR). The pathophysiological role of OPHN1, however, remains poorly understood. Here we show that OPHN1 through its Rho-GAP activity plays a critical role in the activitydependent maturation and plasticity of excitatory synapses by controlling their structural and functional stability. Synaptic activity through NMDA receptor activation drives OPHN1 into dendritic spines, where it forms a complex with AMPA receptors, and selectively enhances AMPA-receptor-mediated synaptic transmission and spine size by stabilizing synaptic AMPA receptors. Consequently, decreased or defective OPHN1 signaling prevents glutamatergic synapse maturation and causes loss of synaptic structure, function, and plasticity. These results imply that normal activity-driven glutamatergic synapse development is impaired by perturbation of OPHN1 function. Thus, our findings link genetic deficits in OPHN1 to glutamatergic dysfunction and suggest that defects in early circuitry development are an important contributory factor to this form of MR.[Keywords: Rho GTPase-activating protein; Oligophrenin-1; glutamatergic function; AMPA receptor; actin cytoskeleton; mental retardation] Supplemental material is available at http://www.genesdev.org.

Synaptic vesicles (SVs) are retrieved by clathrin-mediated endocytosis at the nerve terminals. Phosphatidylinositol 4,5-bisphosphate [PI(4,5)P 2 ] drives this event by recruiting the components of the endocytic machinery. However, the molecular mechanisms that result in local generation of PI(4,5)P 2 remain unclear. We demonstrate here that AP-2 complex directly interacts with phosphatidylinositol 4-phosphate 5-kinase c661 (PIP5Kc661), the major PI(4,5)P 2 -producing enzyme in the brain. The b2 subunit of AP-2 was found to bind to the C-terminal tail of PIP5Kc661 and cause PIP5Kc661 activation. The interaction is regulated by PIP5Kc661 dephosphorylation, which is triggered by depolarization in mouse hippocampal neurons. Finally, overexpression of the PIP5Kc661 C-terminal region in hippocampal neurons suppresses depolarizationdependent SV endocytosis. These findings provide evidence for the molecular mechanism through which PIP5Kc661 locally generates PI(4,5)P 2 in hippocampal neurons and suggest a model in which the interaction trigger SV endocytosis.

Summary Neurons transmit information at chemical synapses by releasing neurotransmitters that are stored in synaptic vesicles (SVs) at the presynaptic site. After release, these vesicles need to be efficiently retrieved in order to maintain synaptic transmission [1–3]. In concurrence, malfunctions in SV recycling have been associated with cognitive disorders [4, 5]. Oligophrenin-1 (OPHN1) encodes a Rho-GTPase activating protein (Rho-GAP) whose loss-of-function causes X-linked mental retardation [6, 7]. OPHN1 is highly expressed in the brain and present both pre- and post-synaptically in neurons [8]. Previous studies report that postsynaptic OPHN1 is important for dendritic spine morphogenesis [8, 9], but its function at the presynaptic site remains largely unexplored. Here, we present evidence that reduced/defective OPHN1 signaling impairs SV cycling at hippocampal synapses. In particular, we show that OPHN1 knockdown affects the kinetic efficiency of endocytosis. We further demonstrate that OPHN1 forms a complex with endophilin A1, a protein implicated in membrane curvature generation during SV endocytosis [10–16], and, importantly, that OPHN1’s interaction with endophilin A1 and its Rho-GAP activity are important for its function in SV endocytosis. Our findings suggest that defects in efficient SV retrieval may contribute to the pathogenesis of OPHN1-linked cognitive impairment.