A major focus of the Cohen lab is centered on the role of the extracellular signal regulated kinase (ERK) signaling pathway in mammalian gametogenesis. Strong preliminary data suggests that ERK signaling plays an essential role at many levels of gametogenesis, but particularly in the process of meiosis and recombination, oocyte activation, and fertility. Specifically, we have shown that phosphorylated, thus activated, ERKs co-localize with key synaptonemal complex (SC) proteins such as SYCP3, suggesting that ERK catalytic activity may impact crucial events during chromosomal pairing and recombination at prophase I. While the role of ERK pathway activation following germinal vesicle breakdown has been characterized, at least in part, our preliminary studies provide strong evidence that within the oocyte and somatic cell compartments within the ovary, ERKs are strongly activated prior to the preovulatory surge of LH. Most important are preliminary observations that when fertilized, ERK-deficient oocytes are not capable of progressing normally past the first cleavage during embryonic development in vitro. This leads us to conclude that a number of important and fundamental gaps exist in our understanding of the role of the ERK pathways during gametogenesis and folliculogenesis. Moreover, while many similarities exist between male and female gametogenesis in terms of the molecular determinants involved in these events, there are well-documented differences in the relative stringencies of the checkpoints and other regulatory mechanisms that ensure the production of viable gametes that can, under certain circumstances, lead to the production of chromosomally aneuploid gametes, particularly of the female germline. Our overall hypothesis is that the ERK pathway plays a critical role in monitoring events in late prophase I and for the subsequent activation of downstream meiotic processes that result in the accurate segregation of chromosomes at the first meiotic division. Further, ERK activity is essential for the transition from dictyate arrest to spindle formation and oocyte arrest at meiosis II. We are testing these hypotheses using genetic mouse models in which specific recombination and repair proteins are perturbed, or in which both Erk1 and Erk2 are conditionally deleted from the germ cell.
Figure 1. Localization of ERKs on meiotic chromosomes during prophase I in male (left) and female (right) mice. Click on panel for larger images.
In 1990, Melanie Cobb and her colleagues described the mammalian homolog for the mitogen activated protein kinase (MAPK) found in yeast by demonstrating that insulin action induced phosphorylation of a catalytic activity that could phosphorylate microtubule-associated protein 2 (MAP-2). This novel isoform of the MAPKs was referred to as extracellular signal regulated kinase1 (ERK1). Steve Pelech’s group went on to identify what was termed meiosis activated kinase which became activated at the time of germinal vesicle breakdown in Xenopus oocytes. Jonathon Cooper’s group then showed that catalyticlly inactive forms of Xp42 (the Xenopus homolog of p42 ERK2) could block progesterone-induced meiotic maturation. It was later determined that these proteins were members of a relatively large group of serine threonine kinases that all shared substrate specificity for MAP-2. These MAPKs themselves were activated by dual phosphorylation at both threonine and tyrosine residues and targeted their substrates via a proline-directed phosphorylation site with the consensus of PXXS/TP, where the serine or threonine would become phosphorylated.
From 1991 to 1996, the organization of the canonical MAPK modules were defined as the association of a three kinase cascade where the MAPK was the terminal kinase. The prototypical ERK module consists of three kinases, c-Raf kinase, MAPK/ERK kinase or MEK and ERK isoforms 1 and 2. Oncogenic p21 Ras was shown to be a potent activator of ERK activity in transformed cells, while G-protein coupled receptors can also activate ERK signaling. Moreover, in the context of the reproductive axis, ERK signaling in the gonad is regulated by a number of growth factors and trophic hormones like LH, each playing an important role in folliculogenesis and control of aspects of meiosis. Collectively, two important underlying notions have emerged related to ligand-induced ERK signaling. First, the role of ERK signaling has clearly been linked to control of cell cycle progression and the regulation of cell proliferation such as with oncogenic mutations and cancer cell progression. Secondly, a less dogmatic view is the role of ERK signaling in more fully differentiated cells and tissues and the ability of ERK pathway signaling to control the cell function via integration of gene transcription. Both of these ideas are critical to the analysis of the role of ERK cascade in gonadal function, particularly in the regulation of meiotic progression.