Structure and expression of the zebrafish mest gene, an ortholog of mammalian imprinted gene PEG1/MEST

Defective neuronal migration and inhibition of bipolar to multipolar transition of migrating neural cells by Mesoderm-Specific Transcript, Mest, in the developing mouse neocortex

Brain developmental disorders such as lissencephaly can result from faulty neuronal migration and differentiation during the formation of the mammalian neocortex. The cerebral cortex is a modular structure, where developmentally, newborn neurons are generated as a neuro-epithelial sheet and subsequently differentiate, migrate and organize into their final positions in the cerebral cortical plate via a process involving both tangential and radial migration. The specific role of Mest, an imprinted gene, in neuronal migration has not been previously studied. In this work, we reduced expression of Mest with in utero electroporation of neuronal progenitors in the developing embryonic mouse neocortex. Reduction of Mest levels by shRNA significantly reduced the number of neurons migrating to the cortical plate. Also, Mest-knockdown disrupted the transition of bipolar neurons into multipolar neurons migrating out of the sub-ventricular zone region. The migrating neurons also adopted a more tangential migration pattern upon knockdown of the Mest message, losing their potential to attach to radial glia cells, required for radial migration. The differentiation and migration properties of neurons via Wnt-Akt signaling were affected by Mest changes. In addition, miR-335, encoded in a Mest gene intron, was identified as being responsible for blocking the default tangential migration of the neurons. Our results suggest that Mest and its intron product, miR-335, play important roles in neuronal migration with Mest regulating the morphological transition of primary neurons required in the formation of the mammalian neocortex.

Tissue-specific alternative polyadenylation at the imprinted gene Mest regulates allelic usage at Copg2

The gene Mest (also known as Peg1) is regulated by genomic imprinting in the mouse and only the paternal allele is active for transcription. MEST is similarly imprinted in humans, where it is a candidate for the growth retardation Silver-Russell syndrome. The MEST protein belongs to an ancient family of hydrolases but its function is still unknown. It is highly conserved in vertebrates although imprinted expression is only observed in marsupials and eutherians, thus a recent evolutionary event. Here we describe the identification of new imprinted RNA products at the Mest locus, longer variants of the RNA, called MestXL, transcribed >10 kb into the downstream antisense gene Copg2. During development MestXL is produced exclusively in the developing central nervous system (CNS) by alternative polyadenylation. Copg2 is biallelically expressed in the embryo except in MestXL-expressing tissues, where we observed preferential expression from the maternal allele. To analyze the function of the MestXL transcripts in Copg2 regulation, we studied the effects of a targeted allele at Mest introducing a truncation in the mRNA. We show that both the formation of the MestXL isoforms and the allelic bias at Copg2 are lost in the CNS of mutants embryos. Our results propose a new mechanism to regulate allelic usage in the mammalian genome, via tissue-specific alternative polyadenylation and transcriptional interference in sense-antisense pairs at imprinted loci.