Learning New Tricks from an Old Dog: The Processing of the Intracellular Precursor of the Luteinizing Hormone Receptor (LHR) into the Mature Cell-Surface LHR Is a Regulated Process

The cloning of the cDNAs for the rat (1) and porcine (2) LH receptor (LHR) in 1989 provided us with new experimental tools that resulted in a tremendous explosion in our knowledge of the structure and functions of the LHR as well as the closely related FSH and TSH receptors (reviewed in Refs. 3–7). Some of the most important recent advances in this area include 1) the discovery that naturally occurring mutations of these three receptors are responsible for some endocrine disorders (7–9); 2) the generation of FSH receptor(10) and LHR-null mice (11, 12); and 3) the recent report of the crystal structure of a portion of the extracellular domain of the FSH receptor bound to FSH (13). One aspect of the biology of the LHR that has remained rather mysterious is the possibility that this receptor may be widely expressed in extragonadal tissues, thus making LH and chorionic gonadotropin (CG) pleiotropic rather than gonadal-specific hormones. Although there are many reports documenting the presence of LHR transcripts and/or protein in an ever-increasing number of extragonadal tissues (reviewed in Refs. 14–17), the significance of these findings remains, for the most part, poorly understood. In this issue, Apaja et al. (18) add new fuel to the fire by documenting that the maturation of the intracellular precursor of the rat (r) LHR into the mature cell-surface protein (i.e. the LHR that is exposed to circulating gonadotropins) is a regulated process. They show that the developing rodent gonads, as well as several extragonadal tissues, express only the immature form of the LHR, whereas other tissues such as the mature gonads, adult female adrenal, and the kidneys of pregnant rats express both the immature and the mature LHR. The studies of Apaja et al. (18) are particularly convincing because they studied the expression of the LHR using several experimental approaches such as expression of a LacZ reporter driven by the murine LHR promoter and nested PCR to assess the presence of LHR transcripts. Finally, affinity purification or immunoprecipitation was also used to assess the expression of different forms of the LHR protein. Rodent and porcine ovaries and testes express at least two forms of the LHR, an 85to 95and a 68to 75-kDa protein (20–25). Studies on the nature of these two forms of the LHR have been conducted mostly in mammalian cells transfected with the cDNAs for the porcine, rat, or human (h) LHR (reviewed in Ref. 4). The 85to 95-kDa band present in transfected cells is the mature LHR located at the cell surface, as judged by surface biotinylation of intact cells and its susceptibility to degradation by surface proteolysis, neuraminidase and PGNase F (22, 26, 27). In contrast, the 85to 95-kDa LHR is not susceptible to EndoH, a glycosidase that removes the type of carbohydrate side chains associated with immature glycoproteins that reside in the endoplasmic reticulum (21, 22, 28, 29). Conversely, the 68to 75-kDa appears to be located intracellularly because it is readily susceptible to EndoH digestion and it cannot be detected by surface biotinylation of intact cells (21, 22, 26, 28). This form of the LHR is also insensitive to surface proteolysis and neuraminidase digestion (22, 26–29). Biosynthetic labeling of heterologous cells transfected with rLHR also revealed that the 68to 75-kDa rLHR is a precursor of the 85to 95-kDa rLHR (22, 29). In transfected cells, the conversion of the immature to the mature form of the rLHR is a slow and inefficient process, however, and a large proportion of the immature rLHR is never converted to the mature receptor (22, 30–33). Importantly, the immature form of the rLHR can bind hCG with the same affinity as the mature rLHR (27), but the binding affinity of ovine LH for the mature rLHR is higher than its binding affinity for the rLHR precursor (34). Clearly then, although the immature rLHR has attained a conformation that permits hormone binding, this conformation is not the same as that of the mature form of the rLHR. In addition, because of its intracellular location, the immature LHR cannot come in contact with circulating gonadotropins. In documenting the presence of the immature, but not the mature, form of the LHR in the developing rat gonads, the studies of Apaja et al. (18) are interesting because they imply that these tissues are not sensitive to gonadotropin stimulation. Their finding is in perfect agreement with the phenotype of the LHR-null mice that display no abnormalities in sexual differentiation (11, 12). In fact, an important conclusion established from the phenotype of these animals is that sexual differentiation in rodents is independent of gonadotropin actions (11, 12). This, of course, is in contrast with the need for gonadotropin action during sexual differentiation in humans. 46XY individuals who are homozygous (or compound heterozygous) for loss-of-function mutations of the LHR display various degrees of feminization of their external genitalia (8, 9). Herein lies an important difference between sexual differentiation in rodents and humans that may be related to the processing (or the regulation of the processing) of the immature to mature forms of the LHR in these two Abbreviations: CG, Chorionic gonadotropin; LHR, LH receptor.