Synaptic refinement and glial phagocytosis in a mouse model of fragile X syndrome

Fragile X syndrome (FXS) is the leading monogenic cause of both autism and intellectual disability and results from the silencing of the FMR1 gene, which codes for the translational regulator, FMRP. FXS is characterized by structural and functional synaptic defects, in particular, aberrant dendritic spine morphology and long-term depression (LTD) enhancement. Glia are known regulators of both synapse growth and elimination. Despite FMRP expression in normal glia and reports of altered astrocytic glutamate signaling in FXS, little is known about the role of FMRP in glia. That astrocytes may drive neuronal pathology in FXS was recently shown in an astrocyte-specific Fmr1 knockout (KO) mouse, which, similar to the Fmr1 mutant, displays increased dendritic spine length and density in cortical pyramidal neurons1.

To understand the role of glia in synapse elimination, the developing mouse visual system has been the most well characterized. In the mouse retinogeniculate circuit, retinal ganglion cell (RGC) axons extend from each eye to the dorsal lateral geniculate nucleus (dLGN), initially forming an excess of overlapping synapses from opposite-side (contralateral) and same-side (ipsilateral) RGCs. Postnatally, dLGN synapses are refined, so that by postnatal day 12, each dLGN neuron receives RGC inputs from a single eye — either contralateral, or ipsilateral. Because this refinement process is known to require glial phagocytosis, the retinogeniculate system is ideal for studying glial-mediated synapse elimination.

Carol Mason's laboratory has identified a developmental enhancement in ipsilateral and contralateral RGC refinement in the Fmr1 KO dLGN at postnatal day seven, correlated with an enhancement in astrocyte engulfment of RGC inputs. The goal of the proposed study is to investigate the cellular and molecular mechanisms that underlie these effects. Specifically, Mason hypothesizes that the interplay between glial and neuronal function is disrupted by the loss of FMRP in astrocytes, ultimately contributing to the neurological defects seen in fragile X syndrome. To investigate this hypothesis, Mason's laboratory, in collaboration with the laboratory of Mimi Shirasu-Hiza, will use astrocyte-specific Fmr1 KO and rescue lines to determine whether loss of FMRP specifically in astrocytes is both necessary and sufficient to drive altered astrocyte engulfment of RGC synapses and RGC segregation. Furthermore, they will identify the cell-type specific transcriptional changes in Fmr1 KO and WT dLGN using single-cell RNA sequencing, in order to understand the molecular underpinnings of these effects.