Summary The activity-dependent transcription factor, Myocyte Enhancer Factor-2 (MEF2), induces excitatory synapse elimination in mouse neurons which requires Fragile X Mental Retardation Protein (FMRP), an RNA binding protein implicated in human cognitive dysfunction and autism. We report here that protocadherin-10 (Pcdh10), an autism-spectrum disorders gene, is necessary for this process. MEF2 and FMRP cooperatively regulate the expression of Pcdh10. Upon MEF2 activation, PSD-95 is ubiquitinated by the ubiquitin E3 ligase, murine double minute-2 (Mdm2) and then binds to Pcdh10, which links it to the proteasome for degradation. Blockade of the Pcdh10 -proteasome interaction inhibits MEF2-induced PSD-95 degradation and synapse elimination. In FMRP-lacking neurons, elevated protein levels of eukaryotic translation elongation factor 1-alpha (EF1α), an Mdm2 interacting protein and FMRP target mRNA, sequester Mdm2 and prevent MEF2-induced PSD-95 ubiquitination and synapse elimination. Together our findings reveal novel roles for multiple autism-linked genes in activity-dependent synapse elimination.

SUMMARY Fragile X Syndrome (FXS), the most common genetic form of mental retardation and autism, is caused by loss of function mutations in an RNA binding protein, Fragile X Mental Retardation Protein (FMRP). Patients’ neurons, as well as those of the mouse model, Fmr1 knockout (KO), are characterized by an excess of dendritic spines, suggesting a deficit in excitatory synapse elimination. In response to neuronal activity, myocyte enhancing factor 2 (MEF2) transcription factors induce robust synapse elimination. Here, we demonstrate that MEF2 activation fails to eliminate functional or structural excitatory synapses in hippocampal neurons from Fmr1 KO mice. Similarly, inhibition of endogenous MEF2 increases synapse number in wildtype, but not Fmr1 KO neurons. MEF2-dependent synapse elimination is rescued in Fmr1 KO neurons by acute postsynaptic expression of FMRP, but not RNA binding mutants of FMRP. Our results reveal that active MEF2 and FMRP function together in an acute, cell autonomous mechanism to eliminate excitatory synapses.

SUMMARY Experience refines synaptic connectivity through neural activity-dependent regulation of transcription factors. Although activity-dependent regulation of transcription factors has been well described, it is unknown if synaptic activity and local, dendritic regulation of the induced transcripts are necessary for mammalian synaptic plasticity in response to transcription factor activation. Neuronal depolarization activates the Myocyte Enhancer Factor 2 (MEF2) family of transcription factors which suppresses excitatory synapse number. We report that activation of metabotropic glutamate receptor 5 (mGluR5) on the dendrites, but not cell soma, of hippocampal CA1 neurons is required for MEF2-induced functional and structural synapse elimination. We present evidence that mGluR5 is necessary for synapse elimination to stimulate dendritic translation of the MEF2-target gene Arc/Arg3.1. Arc is required for MEF2-induced synapse elimination, where it plays an acute, cell autonomous and postsynaptic role. This work reveals a role for dendritic activity in local translation of specific transcripts in synapse refinement.

Chemosensory information processing in the mouse accessory olfactory system guides the expression of social behavior. After salient chemosensory encounters, the accessory olfactory bulb (AOB) experiences changes in the balance of excitation and inhibition at reciprocal synapses between mitral cells (MCs) and local interneurons. The mechanisms underlying these changes remain controversial. Moreover, it remains unclear whether MC-interneuron plasticity is unique to specific behaviors, such as mating, or whether it is a more general feature of the AOB circuit. Here, we describe targeted electrophysiological studies of AOB inhibitory internal granule cells (IGCs), many of which upregulate the immediate-early gene after male-male social experience. Following the resident-intruder paradigm,-expressing IGCs in acute AOB slices from resident males displayed stronger excitation than nonexpressing neighbors when sensory inputs were stimulated. The increased excitability of -expressing IGCs was not correlated with changes in the strength or number of excitatory synapses with MCs but was instead associated with increased intrinsic excitability and decreased HCN channel-mediated currents. Consistent with increased inhibition by IGCs, MCs responded to sensory input stimulation with decreased depolarization and spiking following resident-intruder encounters. These results reveal that nonmating behaviors drive AOB inhibitory plasticity and indicate that increased MC inhibition involves intrinsic excitability changes in -expressing interneurons. The accessory olfactory bulb (AOB) is a site of experience-dependent plasticity between excitatory mitral cells (MCs) and inhibitory internal granule cells (IGCs), but the physiological mechanisms and behavioral conditions driving this plasticity remain unclear. Here, we report studies of AOB neuronal plasticity following male-male social chemosensory encounters. We show that the plasticity-associated immediate-early gene is selectively expressed in IGCs from resident males following the resident-intruder assay. After behavior,-expressing IGCs are more strongly excited by sensory input stimulation and MC activation is suppressed. -expressing IGCs do not show increased excitatory synaptic drive but instead show increased intrinsic excitability. These data indicate that MC-IGC plasticity is induced after male-male social chemosensory encounters, resulting in enhanced MC suppression by-expressing IGCs.

Rates of synapse formation and elimination change over the course of postnatal development, but little is known of molecular mechanisms that mediate this developmental switch. Here we report that the dendritic RNA binding protein Fragile X Mental Retardation Protein (FMRP) bi-directionally and cell autonomously regulates excitatory synaptic function which depends on developmental age as well as function of the activity-dependent transcription factor Myocyte-Enhancer Factor 2 (MEF2). Acute postsynaptic expression of FMRP in CA1 neurons of hippocampal slice cultures (during the first postnatal week; P6–7) promotes synapse function and maturation. In contrast, acute expression of FMRP or endogenous FMRP in more mature neurons (during the second postnatal week; P13–16) suppresses synapse number. The ability of neuronal depolarization to stimulate MEF2 transcriptional activity increases over this same developmental time period. Knockout of endogenous MEF2 isoforms causes acute postsynaptic FMRP expression to promote, instead of eliminate, synapses onto two week old neurons. Conversely, expression of active MEF2 in neonatal neurons results in a precocious FMRP-dependent synapse elimination. Our findings suggest that FMRP and MEF2 function together to fine tune synapse formation and elimination rates in response to neuronal activity levels over the course of postnatal development.