Department of Cell Biology
263 Farmington Avenue
Farmington, CT 06030
My lab is interested in the regulation of meiosis in mammalian oocytes, as well as the cytoplasmic events that occur during the time an oocyte transitions from an immature cell to a mature egg capable of fertilization.
Mammalian oocytes are stored within ovarian follicles at the prophase stage of meiosis. They remain in prophase I as they and their associated follicles grow, and until a preovulatory surge of luteinizing hormone (LH) from the pituitary stimulates meiotic resumption to the metaphase II stage. During the period of oocyte growth, meiotic arrest is maintained due to an insufficient amount of maturation-inducing proteins, in particular the cell cycle kinase CDK1. However, as CDK1 accumulates in the oocyte, the oocyte becomes competent to mature and meiotic arrest shifts to a dependency on cAMP. Work in my lab has established that the oocyte produces cAMP by the constitutive activity of the Gs-coupled receptor, GPR3 (Mehlmann et al., 2004). GPR3 localizes in the plasma membrane and throughout the oocyte in early endosomes, and although it is constitutively active, its activity appears to be regulated via endocytosis (Lowther et al., 2011, 2013). Oocytes lacking GPR3 are unable to maintain meiotic arrest and resume meiosis spontaneously in the follicle, in the absence of an LH surge. At the same time that the oocyte is producing cAMP, cAMP hydrolysis is prevented in the oocyte due to the inhibition of the phosphodiesterase, PDE3A, by follicle-derived cGMP (Norris et al., 2009). High cGMP enters the oocyte through gap junctions and ensures that cAMP levels remain high until LH stimulates the follicle cells to reduce cGMP both in the follicle as well as the oocyte. This in turn activates PDE3A in the oocyte to hydrolyze cAMP and cause meiotic resumption. Available evidence indicates that similar mechanisms also function in human oocytes (DiLuigi et al., 2008).
Stages of oocyte development. 1) As oocytes start to grow, they begin to express Gpr3, which produces cAMP; the role of cAMP at this stage is unknown, but it might aid in the acquisition of meiotic competence. 2) As oocytes continue to grow, they accumulate CDK1 and other cell cycle proteins that allow them to become meiotically competent. 3) In response to LH, oocytes are stimulated to mature and undergo cytoplasmic changes that allow them become fertilizable and developmentally competent eggs (see below).
Once LH stimulates the oocyte to mature, it undergoes numerous cytoplasmic changes during its transition to a fertilizable egg. Many of these changes are not well understood. However, it is well established that during maturation, the oocyte’s endoplasmic reticulum (ER) undergoes a dramatic reorganization, such that the ER accumulates in the egg cortex (Mehlmann et al., 2005; Mann et al., 2010). As the primary Ca2+ storage organelle, the ER thereby becomes situated in the immediate region of the fertilizing sperm, which triggers Ca2+ release through an IP3-mediated signalling cascade. This release of Ca2+ is critical for successful development of the newly-formed embryo. Other changes that occur during oocyte maturation are an influx of Ca2+ into the ER lumen, as well as a rearrangement of cortical granules to the cortex opposite the meiotic spindle. In addition, the protein RGS2 becomes expressed during oocyte maturation to inhibit premature Ca2+ release prior to fertilization (Bernhardt et al., 2015).
Cytoplasmic changes during meiotic maturation in mouse oocytes. Movement of endoplasmic reticulum (ER; red) to the egg cortex; increase in the number and change in localization of cortical granules (black dots), translation of RGS2, and an influx of Ca2+ (blue dots) into the ER lumen.
ER in a mature mouse egg, labelled with the lipophilic dye, DiI. Notice the accumulation of ER in the cortex opposite the meiotic spindle (MS).
Our lab is interested in elucidating cytoplasmic changes that occur in the oocyte that are important for the egg to become developmentally competent, as well as the mechanisms that cause these cytoplasmic changes to occur. A complete understanding of these events will contribute to improving methods for maturing oocytes in vitro.
Bernhardt ,M.L., Lowther, K.M., Padilla-Banks, E., McDonough, C.E., Lee, K.N., Evsikov, A.V., Uliasz, T.F., Chidiac, P., Williams, C.J., and Mehlmann, L.M. 2015 Regulator of G-protein signaling 2 (RGS2) suppresses premature calcium release in mouse eggs. Development 2015 142: 2633-2640. pdf
Lowther, K.M. and Mehlmann, L.M. 2015. Embryonic Poly(A) Binding Protein (EPAB) is required during early stages of mouse oocyte development for chromatin organization, transcriptional silencing, and meiotic competence. BOR Papers in Press. Published on July 1, 2015 as DOI:10.1095/biolreprod.115.131359. pdf
Mehlmann, L.M. 2013. Signalling for meiotic resumption in granulosa cells, cumulus cells, and oocyte. In: Mammalian Oogenesis, Coticchio, De Santis, and Albertini, eds. Springer Publishing Company, New York, NY, pp. 171-182.
Lowther, K.M., Uliasz, T.F., Gotz, K.R., Nikolaev, V.O., and Mehlmann, L.M. 2013. Regulation of constitutive GPR3 signaling and surface localization by GRK2 and b-arrestin-2 overexpression in HEK293 cells. PLoS One 8: e65365. pdf
Mehlmann, L.M. 2013. Losing mom's message: requirement for DCP1A and DCP2 in the degradation of maternal transcripts during oocyte maturation. Biol Reprod. 88: 1-2. pdf
Guzeloglu-Kayisli, O., Lalioti, M.D., Aydiner, F., Sasson, I., Ilbay, O., Sakkas, D., Lowther, K.M., Mehlmann, L.M, and Seli, E. 2012. Embryonic poly(A)-binding protein (EPAB) is required for oocyte maturation and female fertility in mice. Biochem J. 446:47-58. pdf
Lowther, K.M., Nikolaev, V.O., and Mehlmann, L.M. 2011. Endocytosis in the mouse oocyte and its contribution to cAMP signaling during meiotic arrest. Reproduction 141:737-747. pdf
Mann, J.S., Lowther, K.M., and Mehlmann, L.M. 2010. Reorganization of the endoplasmic reticulum and development of Ca2+ release mechanisms during meiotic maturation of human oocytes. Biol. Reprod. 83:578–583. pdf
Lowther, K.M., Weitzman, V.N., Maier, D., and Mehlmann, L.M. 2009. Maturation, fertilization, and the structure and function of the endoplasmic reticulum in cryopreserved mouse oocytes. Biol. Reprod. 81:147-154. pdf
Norris, R.P., Ratzan, W.J., Freudzon, M., Mehlmann, L.M., Krall, J., Movsesian, M.A., Wang, H., Ke, H., Nikolaev, V.O., and Jaffe, L.A. 2009. Cyclic GMP from the surrounding somatic cells regulates cyclic AMP and meiosis in the mouse oocyte. Development 136:1869-1878. pdf
DiLuigi, A., Weitzman, V.N., Pace, M.C., Siano, L.J., Maier, D., and Mehlmann, L.M. 2008. Meiotic arrest in human oocytes is maintained by a Gs signaling pathway. Biol. Reprod. 78:667-672. pdf
Vaccari, S., Horner, K., Mehlmann, L.M., and Conti, M. 2008. Generation of mouse oocytes defective in cAMP synthesis and degradation: endogenous cyclic AMP is essential for meiotic arrest. Dev. Biol. 316:124-134. pdf
Norris, R.P., Freudzon, M., Mehlmann, L.M., Cowan, A.E., Simon, A.M., Paul, D.L., Lampe, P.D., and Jaffe, L.A. 2008. Luteinizing hormone causes MAP kinase-dependent phosphorylation and closure of connexin 43 gap junctions in mouse ovarian follicles: one of two paths to meiotic resumption. Development 135:3229-3238.pdf
Norris, R.P., Freudzon, L., Freudzon, M., Hand, A.R., Mehlmann, L.M., and Jaffe, L.A. 2007. A Gs-linked receptor maintains meiotic arrest in mouse oocytes, but luteinizing hormone does not cause meiotic resumption by terminating receptor-Gs signaling. Dev. Biol. 310:240-249. pdf
Mehlmann, L.M., Kalinowski, R.R., Ross, L.F., Parlow, A.F., Hewlett, E.L. and Jaffe, L.A. 2006. Meiotic resumption in response to luteinizing hormone is independent of a Gi family G protein or calcium in the mouse oocyte. Dev. Biol. 299:345-355. pdf
Mehlmann, L.M. 2005. Stops and Starts in Mammalian Oocytes: Recent Advances in Understanding the Regulation of Meiotic Arrest and Oocyte Maturation. Reproduction 130:791-798. pdf
Freudzon, L., Norris, R.P., Hand, A.R., Tanaka, S., Saeki, Y., Jones, T.L.Z., Rasenick, M.M., Berlot, C.H., Mehlmann, L.M., and Jaffe, L.A. 2005. Regulation of meiotic prophase arrest in mouse oocytes by GPR3, a constitutive activator of the Gs G protein. J. Cell Biol. 171:255-265. pdf
Mehlmann, L.M. 2005. Oocyte-specific expression of GPR3 is required for the maintenance of meiotic arrest in mouse oocytes. Dev. Biol. 288:397-404. pdf
Mehlmann, L.M., and Jaffe, L.A. 2005. SH2 domain-mediated activation of a SRC family kinase is not required to initiate Ca2+ release at fertilization in mouse eggs. Reproduction: 129: 557-564. pdf
Mehlmann, L.M., Saeki, Y., Tanaka, S., Brennan, T.J., Evsikov, A.V. Pendola, F.L., Knowles, B.B., Eppig, J.J., and Jaffe, L.A. 2004. The G s -linked receptor GPR3 maintains meiotic arrest in mammalian oocytes. Science: 306: 1947 - 1950.