Temporal requirements of the fragile X mental retardation protein in modulating circadian clock circuit synaptic architecture

Abstract

Loss of fragile X mental retardation 1 (FMR1) gene function is the most common cause of inherited mental retardation and autism spectrum disorders, characterized by attention disorder, hyperactivity and disruption of circadian activity cycles. Pursuit of effective intervention strategies requires determining when the FMR1 product (FMRP) is required in the regulation of neuronal circuitry controlling these behaviors. In the well-characterized Drosophila disease model, loss of the highly conserved dFMRP causes circadian arrhythmicity and conspicuous abnormalities in the circadian clock circuitry. Here, a novel Sholl Analysis was used to quantify over-elaborated synaptic architecture in dfmr1-null small ventrolateral neurons (sLNvs), a key subset of clock neurons. The transgenic Gene-Switch system was employed to drive conditional neuronal dFMRP expression in the dfmr1-null mutant background in order to dissect temporal requirements within the clock circuit. Introduction of dFMRP during early brain development, including the stages of neurogenesis, neuronal fate specification and early pathfinding, provided no rescue of dfmr1 mutant phenotypes. Similarly, restoring normal dFMRP expression in the adult failed to restore circadian circuit architecture. In sharp contrast, supplying dFMRP during a transient window of very late brain development, wherein synaptogenesis and substantial subsequent synaptic reorganization (e.g. use-dependent pruning) occur, provided strong morphological rescue to reestablish normal sLNvs synaptic arbors. We conclude that dFMRP plays a developmentally restricted role in sculpting synaptic architecture in these neurons that cannot be compensated for by later reintroduction of the protein at maturity.

Drosophila DFMR1 Interacts with Genes of the Lgl-Pathway in the Brain Synaptic Architecture

“…In accordance to previous data of other authors, we found that lack of dfmr1-expression caused an overgrown sLNV synaptic terminal area (dorsal projections), while its over-expression resulted in a retraction of the terminal synaptic branching and collapse of the synaptic area (22,36,43). These results are shown in Fig.…”

“…Both models confirm the main clinical aspects of FraX -neuronal features: dendrite and axon overgrowth and synaptic changes: increased spine numbers, abnormal synaptic growth, morphology and function, altered synaptic plasticity and circadian rhythms, as well as learning and memory deficits (3,22,33,40). Work on Drosophila has shown that dFMRP is also required for the normal neurite extension, guidance and branching of different neurons throughout the nervous system (16,36,34, reviewed in 59).…”

Therapeutic Strategies in Fragile X Syndrome: Dysregulated mGluR Signaling and Beyond

“…Gatto and Broadie (2009) reported abnormalities in circadian clock circuits in dfmr1 mutant Drosophila, which were due to aberrant synaptic architecture and could be rescued by temporally controlled overexpression of dFMRP. In Fmr1 KO mice, dysregulated neuronal connectivity in the barrel cortex (Bureau et al, 2008) leads to defective glutamatergic synapse maturation, delayed and aberrant formation of sensory maps, and altered synaptic plasticity during the critical period (Harlow et al, 2010).…”

Activity-dependent FMRP requirements in development of the neural circuitry of learning and memory

“…,UAS-9557-3/TM6 line (a wild-type dfmr1 transgene under UAS control; Gatto and Broadie, 2009), with progeny raised at the permissive temperature (18°C) until the last pupal day of development (pupal day 4, P4). At P4, animals were shifted to the restrictive temperature (29°C) until day 1 post eclosion, thereby releasing Gal80 inhibition and activating dfmr1 expression in targeted neurons.…”

“…For optogenetic manipulations, UAS-ChR2(H134R)-mCherry generously provided by Leslie Griffith (Pulver et al, 2009) and UAS-eNpHR3.0-EGFP (this study, see below) were introduced using standard genetic techniques. For conditional dfmr1 manipulation, UAS-dfmr1-RNAi (TRiP, Harvard) and a wild-type dfmr1 rescue construct (UAS-9557-3; Gatto et al, 2009) were crossed into control and mutant backgrounds, with all progeny raised at the permissive temperature (18°C) until pupal day 4 (P4), and then shifted to the restrictive temperature (29°C). These experiments involved the following sets of parental pairings: (1) tubP-Gal80ts/CyO;dfmr1…”