Naturally Occurring Mutations of the Luteinizing-Hormone Receptor: Lessons Learned about Reproductive Physiology and G Protein–Coupled Receptors

Abstract

The luteinizing hormone (LH) receptor (LHR) is a heptahelical receptor present primarily in the ovaries of females and the testes of males. This same receptor can bind with high affinity either pituitary LH or the nearly identical placental hormone human chorionic gonadotropin (hCG). In both males and females, the levels of LH remain quite low during childhood years, until puberty, at which time the hypothalamic-pituitary-gonadal axis matures. After puberty, the functions of LH are critical to normal reproductive function. In postpubertal males, LH stimulates testosterone synthesis in the Leydig cells of the testes, which, in turn, is necessary for both formation of male secondary sexual characteristics and spermatogenesis. In nonpregnant postpubertal females, LH plays several roles. During the follicular phase of the ovarian cycle, LH stimulates theca cells to synthesize androgens, which are then aromatized into estradiol in granulosa cells under the influence of follicle-stimulating hormone (FSH). The midcycle surge of LH induces follicular maturation and ovulation. Subsequently, during the luteal phase, LH induces the formation of the corpus luteum and stimulates progesterone synthesis. In the pregnant female, placental hCG binds to the LHR on ovarian luteal cells and causes the corpus luteum, which otherwise undergoes atresia, to be maintained and to continue steroid synthesis, which is necessary for the continuation of pregnancy. During pregnancy, if the fetus is male, placental hCG also stimulates fetal testicular Leydig cells to produce testosterone, which, in turn, mediates the differentiation of the external genitalia and induces the descent of the testes (see Roberts et al. 1999 [in this issue]). Clearly, the LHR plays a critical role in reproductive physiology in both males and females. The importance of the LHR signal-transduction pathway in normal reproductive functioning has been further underscored by the discovery, in recent years, of naturally occurring mutations of the human LHR gene, hLHR.