Functions of a newly identified meiotic protein, SCML1, and a meiosis-specific nuclear structure, the dense-body, in mice

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Meiosis produces haploid gametes from diploid germ cells, thereby providing the basis for sexual reproduction in mammals. Generation of haploid gametes depends on meiosis-specific features of chromosome behaviour and regulation of protein expression. To gain novel insights into the molecular basis of haploid gamete generation, we screened for meiotic proteins with a potential role in essential meiosis-specific processes. Among others, our screen discovered a novel meiotic protein, which we identified as the previously unrecognized mouse orthologue of human SCML1, a protein of unknown function. Mouse SCML1 is a component of a meiosis-specific RNA-rich nuclear structure, the dense-body (DB). DBs form during the first meiotic prophase in mammals, including humans. Although the composition and the functions of the DB remain enigmatic, recent discoveries indicate possible functions for DBs in crucial aspects of meiosis, such as small non-coding RNA (smRNA) biology and meiotic sex chromosome silencing in spermatocytes.Our results show that ectopically expressed SCML1 builds DB-like structures in somatic cells, indicating that SCML1 might be a structural component of DBs. Therefore, we propose to address the functions of the meiotic DB through the analysis of SCML1. We will investigate the functions of SCML1 by analysing meiosis and gametogenesis in Scml1 knockout mice, which will be generated from our recently produced mouse-strain that carries a conditional knockout Scml1 allele. In particular, we will address putative roles of SCML1 in meiotic smRNA metabolism and/or in transposon silencing and sex chromosome silencing. The latter two are essential meiotic processes that have been linked to meiotic smRNA biology. Furthermore, we plan to purify DBs/SCML1 containing RNA-protein complexes, to identify and characterize proteins and RNA species that accumulate in DBs. This will be relevant for understanding the role of DBs in meiotic RNA biology. As part of these experiments, we will compare the RNA composition of SCML1 containing RNA-protein complexes in wildtype and in meiotic mutants that display abnormalities in smRNA metabolism (e.g.: Mili and Miwi mutants). In addition, we will use yeast-two hybrid and biochemical approaches to identify and characterize SCML1 protein interactions and to examine the molecular basis of SCML1-mediated formation of DB-like structures.I expect that these experiments will reveal SCML1 and DB functions in meiosis, and will allow us to better understand the mechanistic role of DBs in meiotic smRNA biology and related processes. These experiments will also likely impact on our understanding of human reproductive health, since smRNAs are believed to safeguard the germline from transposons and play essential roles during gametogenesis.