Project Summary/Abstract
In mammalian ovaries, the mid-cycle surge of luteinizing hormone (LH) acts on the granulosa cells of
preovulatory follicles to trigger oocyte maturation, ovulation, and luteinization. Signaling through LH receptors
occurs primarily occurs through sequential Gs-mediated adenylyl cyclase activation, production of the second
messenger cAMP, and protein kinase A-dependent phosphorylation. However, whether and how the cAMP
signal spreads from the subset of cells that express the LH receptor (LHR) to achieve the many outcomes of
the LH surge is a long-standing question that remains to be resolved. To test this and other questions, intact
preovulatory follicles will be isolated from mice that globally express a newly developed fluorescent sensor for
cAMP called R-FlincA that can increase brightness by up to 600% upon cAMP binding. Using R-FlincA and
state-of-the-art light sheet microscopy, individual cells can be imaged as they elevate cAMP either directly in
response to LH, or by diffusion between cells. If diffusion is detected, it is hypothesized to be via gap junctions,
which connect all granulosa cells to each other, and to the oocyte. To test this, follicles will be incubated in a
gap junction inhibitor, carbenoxolone, prior to LH treatment. It is hypothesized that only some cells will elevate
cAMP, corresponding to LHR-expressing cells, with no diffusion between cells. Diffusion of cAMP through gap
junctions would present a paradox because the LH surge also causes a decrease in cAMP in the oocyte, which
is necessary for meiotic resumption. It is hypothesized that LH signaling through epidermal growth factor
receptor (EGFR) kinase closes gap juctions between granulosa cells and the oocyte, allowing cAMP to decline
in the oocyte while remaining elevated in granulosa cells. This will be tested using AG1478, an inhibitor of
EGFR kinase activity. In the presence of AG1478, it is predicted that cAMP will continue to diffuse through gap
junctions from granulosa cells into the oocyte, which would prevent oocyte maturation. The LH surge also
causes the rapid decrease of another cyclic nucleotide, cGMP, in the granulosa cells and oocyte. This
decrease in oocyte cGMP allows for the decrease in oocyte cAMP, which then triggers oocyte maturation into a
fertilizable egg. However, it is not known how the granulosa cell cAMP increase and cGMP decrease are
related. This will be investigated by breeding R-FlincA mice with another mouse line that expresses a
fluorescent sensor for cGMP. It is hypothesized that cells that initially elevate cAMP in response to LH will
rapidly exhibit decreased cGMP levels, suggesting that the cAMP increase is required to lower cGMP levels.
This project will contribute to understanding of how LH signaling leads to a fertilizable egg, and could lead to
improvements in human in vitro maturation (IVM), which depends on optimal cyclic nucleotide levels in the
oocyte. Thus this project could lead to clinical advances in reproductive health.