The Role of TSC in Oligodendrocyte Differentiation and Myelination

Abstract

Tuberous Sclerosis Complex (TSC) is an autosomal dominant syndrome characterized by epilepsy, intellectual disability, and autism. Recent studies have suggested that white matter abnormalities, including hypomyelination, contribute to the cognitive deficits in TSC patients, but the mechanism has remained elusive. I used the neuron-specific Tsc1 knockout mice that display a marked decrease in myelin and show that oligodendrocytes are arrested at immature stages of development in vivo resulting in a reduction in the number of myelinating cells. I established an oligodendrocyte culture system and examined the effect of neuron-conditioned media and found that the Tsc1 mutant phenotype was replicable in vitro using medium collected from Tsc1 knockdown (TSC-KD) neurons, confirming that a secreted signal is responsible for inhibiting differentiation of the oligodendrocytes. I took an unbiased genome-wide approach and identified Connective Tissue Growth Factor (CTGF) as a putative candidate for the secreted signal. I confirmed that CTGF was upregulated in Tsc1 mutant neurons and characterized its spatial and developmental expression pattern in our mouse model. In vitro, CTGF was sufficient to inhibit differentiation of oligodendrocytes. The addition of CTGF neutralizing antibody to the TSC-KD neuronal media was able to reverse the suppression of oligodendrocyte maturation, strongly suggesting that CTGF is a major component of the oligodendrocyte inhibitory signal derived from Tsc mutant neurons. Since TSC mutation affects all cells, I investigated the role of TSC in oligodendrocytes. In response to TSC knockdown, oligodendrocytes demonstrate an upregulation of cellular stress marker. I also found a decrease in myelin protein genes, a finding that offers interesting implications for the role of TSC in hypomyelination. Furthermore, I expanded my research into Zellweger disease, a syndrome that involves TSC in its neuropathological manifestations including white matter deficits, and found that localization of TSC to the peroxisome is a critical factor in neuron development. Together, this body of work developed new approaches in Tuberous Sclerosis research in the brain to investigate a previously under-appreciated aspect of TSC pathology - myelination. I have demonstrated that the TSC pathway has important roles in neuron-oligodendrocyte communication and emphasize the critical importance of neuron-derived signals in the establishment of myelination.