Ever since mutations in the methyl-CpG binding protein 2 (MeCP2) gene were found to be the primary cause of Rett syndrome, there has been a tremendous level of interest in understanding the biology and normal function of MeCP2. The MeCP2 gene is alternatively spliced to generated two isoforms commonly referred to as MeCP2-e1 and MeCP2-e2. The significance of these two isoforms is not known and it is widely believed that there is no functional difference. Indeed, most studies in which MeCP2 is ectopically expressed do not specify which isoforms is being used, and expression studies of MeCP2 rarely distinguish between the two. We find that the two isoforms are differentially regulated in postmitotic neurons that are degenerating. Moreover, the enhanced expression of MeCP2-e2 promotes neuronal death while elevated MeCP2-e1 expression has no such effect. In addition to the differential expression pattern and effect on neuronal viability, the two isofors bind the FoxG1 with dramatically different efficiency. FoxG1 is a transcription factor which promotes neuronal survival and whose mutations have also been linked to Rett syndrome.
The specific aims of the proposal are (1) To understand the mechanisms underlying MeCP2-e2-induced neurotoxicity, (2) To define the relationship between MeCP2 and FoxG1 in the regulation of neuronal survival, and (3) If time permits, we will analyze neuronal death in MeCP2-expressing transgenic mice. It is noteworthy that while both MeCP2 and FoxG1 proteins are widely and abundantly expressed in neurons of the mature brain, studies on them have so far focused overwhelmingly on their roles in the context of neurodevelopment. Our proposed work will fill this void. As described above, MeCP2 gene duplication in humans causes neurological dysfunction along with cerebellar neurodegeneration. The neurotoxic effect of elevated MeCP2-e2 that we observe in tissue culture models recapitulates this neurodegeneration and will shed insight into the molecular mechanisms underlying it.
Our proposal focuses on MeCP2, mutations of which account for over 90% of the cases of Rett syndrome, a neurodevelopmental disorder. While the role of MeCP2 during nervous system development has been studied, its function in mature neurons is not clear. In addition, the functional differences of the two different MeCP2 isoforms that are expressed in vertebrate organisms is not known. We show that the two MeCP2 isoforms do have separate functions which include the regulation of neuronal death. We propose to study how MeCP2 regulates the death of mature neurons and understand its relationship with FoxG1, another protein that has been linked to Rett syndrome. It is noteworthy that humans and mice with elevated levels of MeCP2 suffer a neurological disorder distinct from Rett syndrome. Besides helping us understand the basic biology of how the survival and death of neurons is controlled, our studies could shed insight into how elevated MeCP2 causes neurological dysfunction in humans.