Roles of activity-dependent transcription factor Mef2c in the synaptic and morphological organization of early postnatal Purkinje neurons
Kamath, Sandhya Prakash
Date of Issue2018
School of Biological Sciences
Numerous brain disorders arise from disruptions in one or more genetically-regulated processes that occur during embryonic and postnatal neuronal development. Amongst these processes, several are regulated by transcription factors but very little is understood about how they exert their functions. In this study, we first characterize the expression of the activity-dependent transcription factor myocyte enhancer factor 2c (Mef2c) and found that it is selectively expressed by Purkinje cells of the adult cerebellum. A spatio-temporal analysis further revealed that the onset of Mef2c expression occurs early postnatally which coincides with a period of dynamic synaptic and morphological changes in Purkinje cells. In order to uncover its roles in Purkinje cell development, we knocked down Mef2c specifically in Purkinje cells at different stages of development. We show that a loss of Mef2c at P3 results in a significant increase in spine numbers of Purkinje cells at P14. A sustained increase in dendritic arborization was also observed in the Purkinje cells lacking Mef2c during the first three weeks of postnatal development, while other processes such as migration from P0 to P4, viability and elimination of somatic processes from P4 to P8 remain unaffected. Furthermore, the knockdown Mef2c from P1 to P14 resulted in an increase in the number of GABAergic proteins in the soma and dendrites while the glutamatergic synapses remain unchanged. However, by P21, we observe a decrease in both the inhibitory synapses in the soma as well as the glutamatergic synapses on the dendrites. Because Mef2c has been shown to be an activity-dependent gene, we assessed the consequences of behavioral manipulation on Mef2c expression, and found that increase in locomotor activity from food deprivation results in an increase in Mef2c expression in Purkinje cells in the simple, CrusI and CrusII lobules of the cerebellum. Collectively, our studies revealed several insights about an activity-dependent transcriptional regulator of Purkinje cell development: Firstly, we determined that Mef2c is expressed selectively by postnatal Purkinje cells, serving as a novel molecular marker for analysis of Purkinje cells. From the loss of function experiments, we next show that Mef2c is an essential regulator of dendritic arborization and spinogenesis of Purkinje cells from birth until maturity. Quantification of GABAergic and glutamatergic synaptic proteins revealed that Mef2c differentially controls synaptic organization at two key developmental stages of Purkinje cell development. Since both disruption in Purkinje cell function as well as the Mef2c gene have been implicated in neurodevelopmental disorders such as autism, our identification of the temporal and cell-specific roles of Mef2c during Purkinje cell development provide insight to a key molecular player implicated in pathogenesis of cerebellum-associated disorders.