Beyond Reproduction: Pituitary Hormone Actions on Bone

The long-held belief that pituitary hormones act solely on master targets was first questioned when we documented G protein-coupled receptors for thyroid-stimulating hormone, follicle-stimulating hormone, adrenocorticotrophic hormone, oxytocin, and vasopressin on bone cells. These evolutionarily conserved hormones and their receptors are known to have primitive roles, and exist in invertebrate species as far down as coelenterates. It is not surprising therefore that each such hormone has multiple hitherto unrecognized functions in mammalian integrative physiology, and hence, becomes a potential target for therapeutic intervention. Here we discuss the skeletal actions of pituitary hormones.

The oxytocin-bone axis

We recently demonstrated a direct action of oxytocin (OT) on skeletal homeostasis, mainly mediated through stimulation of osteoblasts (OBs) formation and through the reciprocal modulation of osteoclast (OCs) formation and function. Thus, mice lacking the hormone or its receptor develop a low turnover osteoporosis that worsens with age in both sexes. The skeletons of OT (Ot) and OT receptor (Oxtr) null mice display a pronounced decrease in vertebral and femoral trabecular volume. At the cellular level, OBs from Ot KO and Oxtr KO mice exhibit lower mineralization activity and, at the mRNA level, all master genes for osteoblast differentiation are down-regulated. Moreover, OT has dual effects on OCs: it increases osteoclast formation both directly, by activating nuclear factor kB (NFkB) and mitogen-activated protein kinase (MAPK) signalling and, indirectly, through the up-regulation of receptor activator nuclear factor-kappaB ligand synthesis by OBs. On the other hand, it inhibits bone resorption by triggering cytosolic Ca(2+) release and nitric oxide synthesis in mature OCs. OT is locally produced by osteoblasts acting as paracrine-autocrine regulators of bone formation modulated by oestrogens. The oestrogen signal involved in this feedforward circuit is nongenomic because it requires an intact MAPK kinase signal transduction pathway, instead of the classical nuclear translocation of oestrogen receptor. The ability of oestrogen to increase bone mass in vivo is to some extent OXTR-dependent. Thus, Oxtr KO mice injected 17β-oestradiol did not show any effects on bone formation parameters, whereas the same treatment increases trabecular and cortical bone in wild-type mice. An intact OT autocrine-paracrine circuit appears to be essential for optimal skeletal remodelling.

Gonadotropin-releasing hormone receptor-coupled gene network organization

An early gene cDNA microarray was developed to study genes that are regulated immediately following gonadotropin-releasing hormone (GnRH) receptor activation. 956 selected candidate genes were printed in triplicate, a t statistic-based regulation algorithm was used for data analysis, and the response to GnRH in a time course from 1 to 6 h was determined. Measurements were highly reproducible within arrays, between arrays, and between experiments. Accuracy and algorithm reliability were established by real-time polymerase chain reaction assays of 60 genes. Gene changes ranging from 1.3- to 31-fold on the microarray were confirmed by real-time polymerase chain reaction. Many of the genes were found to be highly regulated. The regulated genes identified were all elevated at 1 h of treatment and returned nearly or completely to baseline levels of expression by 3 h of treatment. This broad, robust, and transient transcriptional response to constant GnRH exposure includes modulators of signal transduction (e.g. Rgs2 and IkappaB), cytoskeletal proteins (e.g. gamma-actin), and transcription factors (e.g. c-Fos, Egr1, and LRG21). The interplay of the activators, repressors, and feedback inhibitors identified embodies a combinatorial code to direct the activity of specific downstream secondary genes.

Insertional mutagenesis of the arginine cage domain of the gonadotropin-releasing hormone receptor

The pattern of side-chain conservation at the cytoplasmic side of the third transmembrane domain of rhodopsin family G protein-coupled receptors, Asp/Glu-Arg-Tyr/X-X-X-Ile/Val, defines a structural "arginine cage" domain. Previous computational and mutagenesis studies of the GnRH receptor indicated an important contribution of local interactions to the function of this domain. We have investigated the functional importance of the intrahelical position and orientation of the arginine cage using insertional mutagenesis. Introduction of a single Ala proximal to the conserved Asp-Arg of this domain caused loss of detectable ligand binding. Inserting a second Ala, however, restored high-affinity agonist binding. Further insertion of three or four Ala residues at this site generated receptors that bound agonist with an affinity 3- to 10-fold higher than that of the wild-type receptor. Loss of detectable coupling to inositol phosphate turnover in all these mutant receptors confirms that the structure required in this region for efficient signaling is highly constrained. In contrast, the recovery of agonist binding with the progressive insertion of two to four Ala residues indicates that specific orientations of this segment can stabilize high-affinity receptor conformations that are uncoupled from signal transduction.

Sinus elevation. Caldwell Luc approach--has it gone passed its use-by date?

The introduction of the osteotome technique in sinus elevation has diminished the usage of the Caldwell Luc approach. With the successful result obtained by adding platelet rich plasma to bone and/or bone substitute and the possible future use of rhBMP-2, the Caldwell Luc approach cannot be discarded. A review of the literature is presented.

Multiple interactions of the Asp(2.61(98)) side chain of the gonadotropin-releasing hormone receptor contribute differentially to ligand interaction

Mutation of Asp(2.61(98)) at the extracellular boundary of transmembrane helix 2 of the gonadotropin-releasing hormone (GnRH) receptor decreased the affinity for GnRH. Using site-directed mutagenesis, ligand modification, and computational modeling, different side chain interactions of Asp(2.61(98)) that contribute to high-affinity binding were investigated. The conservative Asp(2. 61(98))Glu mutation markedly decreased the affinity for a series of GnRH analogues containing the native His(2) residue. This mutant showed smaller decreases in affinity for His(2)-substituted ligands. The loss of preference for His(2)-containing ligands in the mutant receptor shows that Asp(2.61(98)) determines the specificity for His(2). Analysis of the affinities of a series of position 2-substituted ligands suggests that a hydrogen bond forms between Asp(2.61(98)) and the delta NH group of His(2) and that Asp(2. 61(98)) forms a second hydrogen bond with the ligand. Substitution of Asp(2.61(98)) with an uncharged residue further decreased the affinity for all ligands and also decreased receptor expression. Computational modeling indicates an intramolecular ionic interaction of Asp(2.61(98)) with Lys(3.32(121)) in transmembrane helix 3. The uncharged, Lys(3.32(121))Gln mutation also markedly decreased agonist affinity. The modeling and the similar phenotypes of mutants with uncharged substitutions for Asp(2.61(98)) or Lys(3.32(121)) are consistent with the presence of this helix 2-helix 3 interaction. These studies support a dual role for Asp(2.61(98)): formation of an interhelical interaction with Lys(3.32(121)) that contributes to the structure of the agonist binding pocket and an interaction with His(2) of GnRH that helps stabilize agonist complexing.

The functional microdomain in transmembrane helices 2 and 7 regulates expression, activation, and coupling pathways of the gonadotropin-releasing hormone receptor

Structural microdomains of G protein-coupled receptors (GPCRs) consist of spatially related side chains that mediate discrete functions. The conserved helix 2/helix 7 microdomain was identified because the gonadotropin-releasing hormone (GnRH) receptor appears to have interchanged the Asp(2.50) and Asn(7.49) residues which are conserved in transmembrane helices 2 and 7 of rhodopsin-like GPCRs. We now demonstrate that different side chains of this microdomain contribute specifically to receptor expression, heterotrimeric G protein-, and small G protein-mediated signaling. An Asn residue is required in position 2.50(87) for expression of the GnRH receptor at the cell surface, most likely through an interaction with the conserved Asn(1.50(53)) residue, which we also find is required for receptor expression. Most GPCRs require an Asp side chain at either the helix 2 or helix 7 locus of the microdomain for coupling to heterotrimeric G proteins, but the GnRH receptor has transferred the requirement for an acidic residue from helix 2 to 7. However, the presence of Asp at the helix 7 locus precludes small G protein-dependent coupling to phospholipase D. These results implicate specific components of the helix 2/helix 7 microdomain in receptor expression and in determining the ability of the receptor to adopt distinct activated conformations that are optimal for interaction with heterotrimeric and small G proteins.

Nuclear translocation of GAPDH-GFP fusion protein during apoptosis

Increased expression and nuclear accumulation of glyceraldehyde-3-phosphate dehydrogenase (GAPDH) are early, critical events in several forms of apoptosis. In order to investigate the subcellular trafficking of GAPDH in vivo, the localization of a GAPDH-green fluorescent protein (GFP) fusion was studied in PC12, HEK 293 and COS-1 cells. In control cells, fusion protein autofluorescence was largely restricted to the cytoplasm, rather than the nuclear concentration favored by GFP alone. In contrast, as early as 30 min after an insult, nuclear fluorescence increased in all cell lines studied. The fusion protein redistribution paralleled the dynamics of endogenous GAPDH. These data suggest that some nuclear GAPDH observed during apoptosis represents protein previously resident in the cytosol. This construct provides an in vivo monitor for an early change in apoptosis.

Computer generated mandibular model: surgical role

A life-size nylon model of a traumatized mandible was produced from CT scan data by the process of laser sintering. The model was used for pre-operative planning and for production of surgical aids in order to facilitate the restoration of a large bony defect. Vascularized iliac crest bone was harvested, titanium implants placed and the bone then grafted to the mandible.

Ultrastructure of S-91 mouse melanomas subjected to phenylalanine and tyrosine restriction in vivo

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