Growth Hormone Receptors In The

Early studies with radiolabeled GH suggested the presence of specific binding sites in CNS, but these were of low abundance and principally identified in hypothalamic regions and choroid plexus although a much wider distribution has been claimed (1). These studies have not been universally confirmed and are fraught with technical difficulties. A major concern is the specificity of binding because homologous ligands have rarely been used. For example, human GH (hGH) often gives the strongest binding in rat tissues, but its specificity is unclear since hGH also binds to and activates several rodent prolactin receptors. Much the same problems faced the demonstration of GH receptors (GHRs) in peripheral tissues with the exception of the liver. High resolution autoradiography has recently been used to confirm such sites in choroid plexus (2,3) and iodinated homologous ligands have been used in some studies, suggesting that specific GHRs may indeed be present in the CNS.

The successful purification of GHRs made possible the development of specific antibodies to measure GHR proteins (4). These have been useful to demonstrate GHRs in peripheral target tissues where GHRs are of low abundance (e.g., within the epiphyseal growth plate). Immunocytochemistry has also been used to confirm that GHRs are indeed expressed in the brain, particularly in fetal and young animals (5), and show a widespread distribution including regions of the CNS not obviously expressing GHR in later life. Similar immunohistochemical studies demonstrated GHR expression in human fetal brain tissue (6). One potential complication is that GHR can also give rise to a GH binding protein (GHBP) either by proteolytic cleavage of the full-length receptor or as a translation product of an alternatively spliced mRNA in rodents (7,8). Because of this, antibody localization studies employing an epitope directed against the extracellular domain of the GHR will also recognize GHBP moieties able to bind GH, but lacking an intracellular domain able to transduce a signal. This problem can be overcome by development of antibodies directed against intracellular portions of the GHR, or to a peptide sequence present only in the hydrophilic tail of the alternate GHBP splice product in rodents (8,9). More recent methodological advances with enhanced immunocytochemical staining have been applied with specific GHR or GHBP antisera in other areas of low GHR density, such as the rat epiphyseal growth plate, and can also be used to visualize GHRs in the CNS (E. F. Gevers, personal communication). However, the presence of GHBP, rather than full-length GHRs, may not be without function. Similar observations of truncated forms of the leptin receptor in choroid plexus have implicated extracellular domains as possible vectors for carrier-mediated transport of leptin from the circulation to the CNS (10,11),

Fig. 1. In situ hybridization histochemistry for GH receptor mRNA in coronal brain sections through (A) periventricular nucleus, (B) paraventricular nucleus, and (C) arcuate nucleus. Note the intense specific hybridization signal in the hippocampus in addition to the hypothalamic nuclei indicated by arrows. Scale bar represents 1 mm.

Fig. 1. In situ hybridization histochemistry for GH receptor mRNA in coronal brain sections through (A) periventricular nucleus, (B) paraventricular nucleus, and (C) arcuate nucleus. Note the intense specific hybridization signal in the hippocampus in addition to the hypothalamic nuclei indicated by arrows. Scale bar represents 1 mm.

so GHBP-like binding sites in the choroid plexus could conceivably serve a similar function for GH.

The purification of the GHR led rapidly to cloning of the GHR (12) and nucleotide probes based on these sequences could then be used to confirm the presence of transcripts coding for GHRs and to determine the sites of expression of the GHR gene in the CNS (13-15). GHR and GHBP gene expression is detectable in the rat CNS throughout life, from as early as embryonic d 15 (16), and in the whole embryo as early as d 12 (17), and perhaps much earlier (see below). It can be concluded that the GHR gene is indeed expressed in the brain and translated into protein. Although it remains necessary to demonstrate the presence of intracellular signaling pathways functionally coupled to GHR activation in sites where GHR protein is located, these are found in most cell types. Therefore, it can reasonably be concluded that functional GHRs are indeed expressed in the CNS in both fetal and adult life.

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