Dr. Michael Griswold, Regents Professor of Molecular Biosciences
Biotechnology Life Sciences 240
Griswold Lab, Biotech/Lifesciences 202
Washington State University
Pullman, WA 99164-7520
Research & Interests
Normal fertility in the male requires the constant production of large numbers of gametes over a long time period. Spermatogenesis is a very complex, highly organized and regulated process that involves mitosis, meiosis and unique pathways of differentiation. In general, spermatogenesis involves three major biological fundamentals: (a) the renewal of stem cells and the production and expansion of progenitor cells (mitosis) (b) the reduction, by one-half, of the chromosome numbers in progenitor cells (meiosis) and (c) the unique differentiation of haploid cells (spermiogenesis). Early progenitor cells or spermatogonia are defined as “undifferentiated” or A spermatogonia in the mouse. Once the spermatogonia enter the “differentiation” pathway they begin the series of differentiation steps leading to meiosis and spermiogenesis. The endocrine regulation of spermatogenesis occurs by the interplay of gonadotropins and steroids with the somatic cells of the seminiferous tubules (Sertoli cell and Leydig cells) and of vitamin A directly with the germinal cells.
The research in my laboratory has been directed towards the understanding of mammalian spermatogenesis at the molecular level. Our current studies are focused on the role of vitamin A in this process. In particular we are interested in the mechanisms by which retinoic acid (vitamin A) influences the commitment of germ cells to enter meiosis. These mechanisms are central to the timing of sperm production and the organization of gametogenesis.
In our initial approach we developed extensive mRNA and microRNA expression data bases for both germ cells and somatic cells in the testis using array technology and we are currently enhancing that information using next generation sequencing. Our databases cover nearly all aspects of spermatogenesis including cell specific expression and hormone responsive transcription and are used by investigators worldwide. Our latest emphasis has been on discovering the genes expressed in germ cells that enable the entry of these cells into meiosis. We then examine the role of these genes using a variety of genetic approaches with transgenic mice. The projects span the disciplines from biochemistry to genetics to cell biology.
Over the last five years, the major focus in the lab has been directed toward deciphering the involvement of retinoic acid, Stra8 and other key factors in the maturation of spermatogonia and the function of the testis. Much of this work stems from information we extracted from our large microarray databases generated from the Affymetrix GeneChip microarray platform. Our array data clearly identifies a retinoid responsive gene, termed Stra8, that is expressed in germ cells of the postnatal male and embryonic female as a precursor to entry into meiosis. Previous work in our laboratory and other laboratories has shown that a vitamin A deficiency blocks the conversion of undifferentiated spermatogonia to differentiating A1 spermatogonia. This observation suggests retinoic acid is required for the undifferentiated spermatogonia to enter into a differentiation pathway and ultimately into meiosis. Induction of the gene, Stra8, is a requirement and a reliable marker for this process. A knockout of the Stra8 gene in mice prevents the maturation of both male and female gametes and their ultimate completion of the meiotic pathway.
We have shown that exogenous retinoic acid will stimulate Stra8 and spermatogonial maturation in undifferentiated spermatogonia both in vivo and in culture. These results suggest that, in vivo, the exposure of germ cells to retinoic acid is tightly controlled via the somatic cells of the testis. Under normal conditions, the circulating form of vitamin A is exclusively retinol that must be converted to bioactive retinoic acid by testicular somatic cells. The action of retinoic acid and the subsequent changes in gene expression that occur, dramatically impact the maturation of spermatogonia and the onset of meiosis. We will more clearly define the response of spermatogonia to retinoic acid, to examine the response of the Stra8 gene in detail and identify other genes involved in the maturation process. Additional preliminary studies have shown that vitamin A down-regulates key genes involved in androgen biosynthesis so we also propose to generate more information on the response of somatic cells to retinoic acid.
This research could potentially determine the molecular mechanisms that initiate spermatogenesis in the male germline. Knowledge about the actions of retinoic acid and the genes responsible for the entry of spermatogonia into the differentiation pathway could lead to new approaches to contraception.
Just a few of our current projects:
- Examine the action of retinoids in testicular somatic cells and germ cells using isolated cells and transgenic animal models.
- Examine the role of other potential germ cell factors involved in spermatogonial maturation.
- Examine changes in gene expression after over-expression of Stra8 and other factors involved in spermatogonial maturation.
- Determine the elements of the Stra8 promoter that regulate the level of induction in the P19 pluripotent embryonal carcinoma cell line.