| HOME | HELP | FEEDBACK | SUBSCRIPTIONS | ARCHIVE | SEARCH | TABLE OF CONTENTS |
PTH-CALCITONIN-VITAMIN D-BONE |
Division of Endocrinology/Metabolism (T.L.C., J.Q.) Department of Medicine, University of Cincinnati and Children’s Hospital Research Foundation (M.C.C.) Cincinnati, Ohio 45267-0547
Address all correspondence and requests for reprints to: Thomas L. Clemens, Ph.D., Division of Endocrinology and Metabolism, Vontz Center for Molecular Studies, 3125 Eden Avenue, Cincinnati, Ohio 45267-0547. E-mail: clementl{at}UC.edu
 |
Introduction |
|---|
 Top Introduction References |
|---|
The multiplicity of PTHrP’s actions appear to manifest through tissue-specific processing and differential receptor/effector utilization. Several different cell types produce mid-region PTHrP fragments capable of raising intracellular free calcium and augmenting transplacental transfer of calcium (8). Novel C-terminal fragments are also produced (9) and have been shown to inhibit osteoclastic bone resorption. These different cleavage products that lack the PTH-like N-terminal region likely activate receptors distinct from the PTH1R, although these putative receptors have not yet been conclusively identified. However, an additional member of the small PTH/PTHrP receptor gene family was identified by Usdin and colleagues (10) and shown to be expressed in brain, pancreas, and several other tissues (11). This receptor was named the PTH type 2 receptor because it could be activated by PTH but not by PTHrP. Its natural ligand was recently isolated from bovine hypothalamus and found to be a small unmodified peptide of 39 amino acids referred to as tuberoinfundibular peptide 39 (12). The peptide has limited primary structure homology with PTH and is expressed in the median eminence and paraventricular nucleus as well as the dorsal horn of the spinal tract. Its biology remains unknown.
Among the many functions ascribed for PTHrP (reviewed in Ref. 1), the best documented is its role as a developmental growth and differentiation factor. Studies using genetically altered mice and clinical observations in patients with rare genetic disorders have begun to define distinct roles for PTHrP and its receptor in organogenesis. Mice lacking PTHrP demonstrate accelerated chondrocyte differentiation, with consequent premature ossification of bones formed through an endochondral process. PTHrP and the PTH1R function in a signaling cascade involving Indian Hedgehog, a member of the conserved Hedgehog family of secreted proteins, to regulate the pace of chondrocyte differentiation (13, 14). The cause of death in the immediate perinatal period appears to be asphyxiation due to a premature calcification of the rib cage. Genetic ablation of the PTH1R results in a similar but more severe phenotype of chondrodysplasia and growth plate abnormalities (13). Moreover, the PTH1R null mice die before birth at various stages of gestation depending on the genetic background (see below).
The results from studies on these knockout models clearly define a role for PTHrP and its receptor in a signaling network that controls the pace of chondrocyte proliferation and hence endochondral bone formation. However, the lethality associated with either PTHrP ligand or the receptor null mice limited information on additional possible roles of PTHrP in other developing organs. In a series of elegant studies, genetic strategies were devised to rescue the PTHrP null mice. Using a segment of the collagen II promoter, PTHrP expression was directed to the prehypertrophic chondrocytes in the PTHrP null background (15). These transgenic mice had exactly the opposite phenotype of the PTHrP null mice (i.e. accelerated proliferation of growth plate chondrocytes such that the skeleton remained cartilaginous). Moreover, this skeletal picture was essentially similar to that reported for a rare human syndrome called Jansen’s metaphyseal chondrodysplasia (JMC) in which afflicted individuals carry an activating mutation of the PTH1R receptor and demonstrate hypercalcemia, hypophosphatemia, and severe foreshortening of long bones (16). Intercrosses of mice expressing transgenes encoding either the wild-type PTHrP (17) construct or a construct encoding the JMC constitutively active PTH1R receptor (18) under the control of the collagen II promoter with the PTHrP null mice rescued the endochondral bone phenotype enabling mice to survive into adulthood. The rescued mice displayed additional phenotypes with abnormalities in skin, in mammary gland development, and of tooth eruption (17).
As indicated above, mice lacking the PTH1R exhibit chondrodysplasia that is more severe than that observed in the PTHrP null mice and die before birth. This raises the question as to whether the disturbances in skeletal development account for the prenatal lethality in these mice. In this issue of Endocrinology, Soegiarto and colleagues (19) attempt to rescue the PTH1R null mice by selectively targeting the JCM mutant PTH1R to the growth plate of the null mice using the collagen II promoter to direct transgene expression. Consistent with the initial reports (13), PTH1R null mice demonstrate skeletal abnormalities including excessive mineralization in the bones forming the base of the skull and sternum, delayed blood vessel invasion, short and misshaped limbs, overpopulation of the sternebrae with collagen X expressing hyperthrophic chondrocytes, and accumulation of matrix-producing osteoblasts in membraneous bone surfaces. Each of these skeletal abnormalities, with the exception of the delay in blood vessel invasion, is largely corrected in the PTH1R null mice carrying the JMC transgene. In spite of this, the PTH1R nulls still died before birth, thereby eliminating the skeletal abnormalities as the cause of death.
If defects in the developing skeleton do not cause prenatal death in PTH1R null mice, then what does? Here again, clues are evident from previous studies in genetically altered mouse models and from lethal mutations in humans. A survey of phenotypic abnormalities resulting from gene mutations either arising spontaneously in mice or humans or as the result of gene knockouts or insertions reveals that disruption of a surprisingly few systems result in prenatal death (20, 21). For example, mice and humans can be born with defects in their gastrointestinal, genitourinary, central nervous systems, lungs, and skeleton. However, there are several critical milestones during mouse and human development that must be successfully completed for intrauterine life to proceed. These include implantation, formation of the chorioallantoic placenta and yolk sac, and establishment and continued function of the cardiovascular system. Thus, there are hints that defects occurring both early and later in embryonic life may contribute to the intrauterine death of the PTH1R mice described in the paper by Soegiarto et al. (19). First, the PTH1R null mice are small, a characteristic associated with poor nutrition suggesting an inadequate supply of oxygen or nutrients. This could stem from defects in the formation of extraembryonic endoderm, as suggested by Verheijen et al. (22). Second, when the PTH1R null mutation was bred onto two other genetic backgrounds (C57BL6 and MF-1), the null embryos did not survive past midgestation (13). Histologic examination at this time revealed a diminution of organ size but no other gross developmental defects. The number of (-/-) fetuses appeared to meet Mendelian expectations at day 9.5; however, only 10% of the (-/-) fetuses were alive at day 12.5, and all were reported to be dead by day 14.5. The reason why some PTH1R knockout mice survive until birth whereas others die at earlier times during embryogenesis is likely related to genetic modifiers associated with background strain. Our own recent studies (23) pinpoint the time of death of PTH1R deficient C57BL6 mice to embryonic day E11.5–E12.5, coincident with the stage at which the PTH1R is expressed in the heart and when enhanced cardiomyocyte function becomes essential for survival. Indeed, earlier hints that PTHrP and its receptor played a role in early heart formation and/or function were apparent from the heart defects found in double transgenic mice with cardiac overexpression of PTHrP and the PTH1R (24, 25). Another piece of evidence for a functional role of the PTH1R in the developing human heart is evident from the abnormalities seen in cases of the rare fatal Blomstrand’s chondrodysplasia caused by an inactivating mutation of the PTH1R (26, 27). These infants also die before birth with coarctation of the aorta and hydrops fetalis; the latter condition is typically caused by high output heart failure. Furthermore, the extreme rarity of Blomstrand’s fetuses is also suspicious that the lack of a functional PTH1R is usually lethal early in gestation. In fact, the incidence of heart defects or malformations in still born fetuses is nearly one in ten and significantly higher for early spontaneous abortion (28). Unequivocal proof that the lack of PTH1R signaling in the heart is the major cause for prenatal death in the PTH1R null mice could again be obtained using genetic strategies to rescue the heart phenotype.
While the studies described provide persuasive evidence for a role for PTHrP and its receptor in a number of developmental processes including the heart, the reader should now recognize a conundrum. If PTHrP and its receptor function together to generate signals required for these developmental processes, then why are PTHrP null mice born alive, whereas the PTH1R null mice die before birth? There are at least two possible explanations for this apparent paradox. First, PTHrP is known to be produced by decidual tissues (29). Thus, transfer of PTHrP from these or other maternal sources to the fetus could in essence rescue the ligand knockout mice. It should be feasible to test this idea by examining heart development and function in PTHrP null fetuses cultured in vitro in the absence or presence of PTHrP. An alternative explanation for the longer survival in the PTHrP null mice is that there is another ligand capable of activating the PTH1R. It is undoubtedly not tuberoinfundibular peptide 39, the PTHR2 ligand, as this peptide does not activate the PTH1R. However, there could very well be additional ligands capable of activating the PTH1R. Further studies to address these possibilities, and to dissect the downstream targets in the PTHrP/PTH1R signaling pathway(s), represent the next challenge for biologists in this exciting field.
|
Footnotes |
|---|
Abbreviations: JMC, Jansen’s metaphyseal chondrodysplasia; PTH1R, PTH/PTHrP type 1 receptor; PTHrP, PTH-related protein.
Received September 24, 2001.
Accepted for publication September 24, 2001.
|
References |
|---|
|
TopIntroductionReferences |
|---|
| |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| HOME | HELP | FEEDBACK | SUBSCRIPTIONS | ARCHIVE | SEARCH | TABLE OF CONTENTS |
| Endocrinology | Endocrine Reviews | J. Clin. End. & Metab. |
| Molecular Endocrinology | Recent Prog. Horm. Res. | All Endocrine Journals |
