Date of Award

2021

Embargo Period

6-23-2023

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Neuroscience

College

College of Graduate Studies

First Advisor

Christopher W. Cowan

Second Advisor

John Woodward

Third Advisor

Heather Boger

Fourth Advisor

Stephen Tomlinson

Fifth Advisor

Michael Scofield

Abstract

MEF2C Haploinsufficiency Syndrome (MCHS) is a neurodevelopmental disorder caused by mutations or deletions of one allele of the MEF2C gene. MCHS is characterized by hypotonia, epilepsy, autism, absent speech, intellectual disability, and developmental delay. We developed a germline heterozygous Mef2c loss-of-function mutant mouse (Mef2c-Het) to investigate the pathophysiology of MCHS. Mef2c-Het mice have differentially expressed cortical genes linked to gene expression in both excitatory neurons and microglia, suggesting that these cell types might play major roles in MCHS. Microglia are the resident immune cells of the brain, and the microglia in Mef2c-Hets have increased Iba1 levels, a marker of activation, without changes in other neuroinflammatory markers, suggesting that these mice do not have basal neuroinflammation. Analysis of RNA-seq data from Mef2c-Het microglia, suggests that Mef2c heterozygous microglia have delayed maturation and/or abnormal basal functions. We hypothesized that reduction of Mef2c in microglia is sufficient to cause autistic-like behaviors. We generated Cx3cr1Cre/+;Mef2cfl/+ mice (Mef2c cHetCx3cr1) and observed that these mice have decreased social preference and increased jumping, a repetitive-type motor behavior also seen in the global Mef2c-Hets, indicating that Mef2c reduction in microglia is sufficient to produce autistic-like behaviors. Via electrophysiological recordings from somatosensory cortex layer 2/3 pyramidal neurons of Mef2c cHetCx3cr1 mice, we observed a decrease in the amplitude of local, evoked excitatory postsynaptic currents (eEPSC) without affecting paired-pulse ratio or miniature EPSCs, suggesting a change in synapse number or strength of local cortical inputs. Interestingly, analysis of the corpus callosum (CC), a long-range cortico-cortical white matter tract, in Mef2c-Hets revealed a thickened CC. Findings from diffusion MRI and electron microscopy of CC axons suggest that there are more long-range cortical axons traversing the CC at the somatosensory cortex. Interestingly, Mef2c conditional heterozygosity in either forebrain excitatory neurons (Mef2c cHetEmx1) or in microglia (Mef2c cHetCx3cr1) produced a thickened CC, suggesting roles for MEF2C in cortical neurons and microglia in the development of long-range vs local connectivity. Together, these studies suggest that MEF2C in microglia and neurons promote neurotypical brain function, and that MEF2C hypofunction in microglia is sufficient to produce a subset of phenotypes in a mouse model of MCHS.

Rights

All rights reserved. Copyright is held by the author.

Download

Share

COinS