Regulation of Synaptic Structure and Function by FMRP-Associated MicroRNAs miR-125b and miR-132

MicroRNAs (miRNAs) are noncoding RNAs that suppress translation of specific mRNAs. The miRNA machinery interacts with fragile X mental retardation protein (FMRP), which functions as translational repressor. We show that miR-125b and miR-132, as well as several other miRNAs, are associated with FMRP in mouse brain. miR-125b and miR-132 had largely opposing effects on dendritic spine morphology and synaptic physiology in hippocampal neurons. FMRP knockdown ameliorates the effect of miRNA overexpression on spine morphology. We identified NMDA receptor subunit NR2A as a target of miR-125b and show that NR2A mRNA is specifically associated with FMRP in brain. In hippocampal neurons, NR2A expression is negatively regulated through its 3′ UTR by FMRP, miR-125b, and Argonaute 1. Regulation of NR2A 3′UTR by FMRP depends in part on miR-125b. Because NMDA receptor subunit composition profoundly affects synaptic plasticity, these observations have implications for the pathophysiology of fragile X syndrome, in which plasticity is altered.Deutsche Forschungsgemeinschaft (ED157/1, postdoctoral fellowship)National Cancer Institute (U.S.) (NCI PO1-CA42063)National Cancer Institute (U.S.) (NCI P30-CA14051)National Cancer Institute (U.S.) (Cancer Center Support (Core) Grant)National Cancer Institute (U.S.) (NCI K99-CA131474)Howard Hughes Medical Institute (Investigator

Role of Plk2 in synaptic function and plasticity

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Biology, 2007.Includes bibliographical references.Homeostatic forms of plasticity keep the spiking output of neurons within an optimal range in the face of changing activity levels of the surrounding network, but little is known about the underlying molecular mechanisms, particularly during heightened activity. We report in Chapter 2 that in hippocampal neurons experiencing elevated activity, the activity-inducible protein kinase, Polo-like kinase 2 (Plk2), was required for synaptic scaling in dissociated culture and for reducing membrane excitability in slice culture-two principal compensatory mechanisms underlying homeostatic plasticity. Inhibition of Plk2 activity in slice culture during elevated activity resulted in increased dendritic spine size and density as well as a "run-up" in synaptic strength that prevented subsequent LTP. Thus, the homeostatic functions of Plk2 allow synapses to remain within a modifiable range during prolonged heightened network activity. In Chapter 3, we show that the homeostatic prevention of run-up during elevated activity also depended on CDK5, which acted as a "priming" kinase for the phospho-dependent binding of PIk2 to its substrate SPAR, a postsynaptic RapGAP.(cont.) Overexpression of SPAR strengthened synapses, whereas RNAi knockdown of SPAR weakened synapses and disrupted homeostasis. Thus CDK5-dependent recruitment of Plk2 to SPAR, followed by Plk2-mediated degradation of SPAR, constitutes a likely molecular mechanism for Plk2-dependent homeostatic plasticity in neurons.by Daniel P. Seeburg.Ph.D