Decreased growth hormone receptor expression in long bones from toothless (osteopetrotic) rats and restoration by treatment with colony-stimulating factor-1

CSF1R as a Therapeutic Target in Bone Diseases: Obvious but Not so Simple

PURPOSE OF REVIEW

The purpose of the review is to summarize the expression and function of CSF1R and its ligands in bone homeostasis and constraints on therapeutic targeting of this axis.

RECENT FINDINGS

Bone development and homeostasis depends upon interactions between mesenchymal cells and cells of the mononuclear phagocyte lineage (MPS), macrophages, and osteoclasts (OCL). The homeostatic interaction is mediated in part by the systemic and local production of growth factors, macrophage colony-stimulating factor (CSF1), and interleukin 34 (IL34) that interact with a receptor (CSF1R) expressed exclusively by MPS cells and their progenitors. Loss-of-function mutations in CSF1 or CSF1R lead to loss of OCL and macrophages and dysregulation of postnatal bone development. MPS cells continuously degrade CSF1R ligands via receptor-mediated endocytosis. As a consequence, any local or systemic increase or decrease in macrophage or OCL abundance is rapidly reversible. In principle, both CSF1R agonists and antagonists have potential in bone regenerative medicine but their evaluation in disease models and therapeutic application needs to carefully consider the intrinsic feedback control of MPS biology.

Pleiotropic Impacts of Macrophage and Microglial Deficiency on Development in Rats with Targeted Mutation of the Csf1r Locus

We have produced Csf1r-deficient rats by homologous recombination in embryonic stem cells. Consistent with the role of Csf1r in macrophage differentiation, there was a loss of peripheral blood monocytes, microglia in the brain, epidermal Langerhans cells, splenic marginal zone macrophages, bone-associated macrophages and osteoclasts, and peritoneal macrophages. Macrophages of splenic red pulp, liver, lung, and gut were less affected. The pleiotropic impacts of the loss of macrophages on development of multiple organ systems in rats were distinct from those reported in mice. Csf1r^-/- rats survived well into adulthood with postnatal growth retardation, distinct skeletal and bone marrow abnormalities, infertility, and loss of visceral adipose tissue. Gene expression analysis in spleen revealed selective loss of transcripts associated with the marginal zone and, in brain regions, the loss of known and candidate novel microglia-associated transcripts. Despite the complete absence of microglia, there was little overt phenotype in brain, aside from reduced myelination and increased expression of dopamine receptor-associated transcripts in striatum. The results highlight the redundant and nonredundant functions of CSF1R signaling and of macrophages in development, organogenesis, and homeostasis.

Reduced growth hormone receptor immunoreactivity in osteoclasts adjacent to the erupting molar in the incisor-absent (osteopetrotic) rat

First molars fail to erupt in the incisor-absent (ia/ia) rat because of a defect in osteoclast function. Growth factors that regulate local bone metabolism include growth hormone (GH), insulin-like growth factor-I (IGF-I), epidermal growth factor (EGF) and interleukin-1 alpha (IL-1alpha). Since osteoclast function may be affected by these factors, the aim of this study was to determine the distribution of GH receptor (GHr), IGF-I, EGF and IL-1alpha, in osteoclasts located occlusal to the erupting first molar, in the 'eruption pathway', in normal and ia/ia rats. Sagittal sections of the first molar and adjacent bone from 3- and 9-d-old animals were examined. Osteoclasts were identified using tartrate-resistant acid phosphatase (TRAP). The TRAP-positive osteoclast cell numbers were higher in ia/ia animals at 3 and 9 days-of-age. In the ia/ia group, fewer osteoclasts were GHr- and IGF-I-positive at 3 d of age, and at 9 d of age fewer osteoclasts were GHr-positive. In the ia/ia rat, defective osteoclast function failed to resorb bone to provide an eruption pathway for the lower first molar. The expression of GHr, and to some degree IGF-I, by these osteoclasts was reduced, which may be related to their ability to differentiate and function.

The osteopetrotic mutation toothless (tl) is a loss-of-function frameshift mutation in the rat Csf1 gene: Evidence of a crucial role for CSF-1 in osteoclastogenesis and endochondral ossification

The toothless (tl) mutation in the rat is a naturally occurring, autosomal recessive mutation resulting in a profound deficiency of bone-resorbing osteoclasts and peritoneal macrophages. The failure to resorb bone produces severe, unrelenting osteopetrosis, with a highly sclerotic skeleton, lack of marrow spaces, failure of tooth eruption, and other pathologies. Injections of CSF-1 improve some, but not all, of these. In this report we have used polymorphism mapping, sequencing, and expression studies to identify the genetic lesion in the tl rat. We found a 10-base insertion near the beginning of the open reading of the Csf1 gene that yields a truncated, nonfunctional protein and an early stop codon, thus rendering the tl rat CSF-1(null). All mutants were homozygous for the mutation and all carriers were heterozygous. No CSF-1 transcripts were identified in rat mRNA that would avoid the mutation via alternative splicing. The biology and actions of CSF-1 have been elucidated by many studies that use another naturally occurring mutation, the op mouse, in which a single base insertion also disrupts the reading frame. The op mouse has milder osteoclastopenia and osteopetrosis than the tl rat and recovers spontaneously over the first few months of life. Thus, the tl rat provides a second model in which the functions of CSF-1 can be studied. Understanding the similarities and differences in the phenotypes of these two models will be important to advancing our knowledge of the many actions of CSF-1.

Roles of growth hormone and insulin-like growth factor 1 in mouse postnatal growth

To examine the relationship between growth hormone (GH) and insulin-like growth factor 1 (IGF1) in controlling postnatal growth, we performed a comparative analysis of dwarfing phenotypes manifested in mouse mutants lacking GH receptor, IGF1, or both. This genetic study has provided conclusive evidence demonstrating that GH and IGF1 promote postnatal growth by both independent and common functions, as the growth retardation of double Ghr/Igf1 nullizygotes is more severe than that observed with either class of single mutant. In fact, the body weight of these double-mutant mice is only approximately 17% of normal and, in absolute magnitude ( approximately 5 g), only twice that of the smallest known mammal. Thus, the growth control pathway in which the components of the GH/IGF1 signaling systems participate constitutes the major determinant of body size. To complement this conclusion mainly based on extensive growth curve analyses, we also present details concerning the involvement of the GH/IGF1 axis in linear growth derived by a developmental study of long bone ossification in the mutants.

The effect of glucocorticosteroid treatment on dentine formation in the Lewis rat, a histological study

Glucocorticosteroids are widely used in the treatment of chronic illnesses and have been reported to cause premature obliteration of the pulp space. During the active stages of dentinogenesis, odontoblasts are growth hormone receptor (GHr) positive. The aims of this study were to determine if the glucocorticosteroid, prednisone, affected the rate of dentine deposition and odontoblast expression of GHr in the rat molar. Following subcutaneous injection of 0.05 mg/kg, 1.0 mg/kg or 5.0 mg/kg prednisone for 20 days, immature and mature molars from rats aged 3 and 6 weeks respectively, were examined histologically. Distribution of GHr expression was determined immunohistochemically. No morphological differences were observed in molars from prednisone treated animals. Prednisone did not appear to enhance dentine deposition in immature molars but in mature molars significantly increased dentine deposition on the roof of the pulp chamber at a dosage of 5.0 mg/kg (p < 0.001). In all immature molars, odontoblasts and pulp cells expressed GHr immunoreactivity. In mature molars, odontoblasts and pulpal cells from controls did not show GHr immunoreactivity. However, odontoblasts and pulp cells were GHr immunoreactive in mature molars from animals treated with prednisone.

Alterations in the neural growth hormone axis following hypoxic-ischemic brain injury

Recently, there has been considerable interest in determining the role of the growth hormone receptor (GHR) in the central nervous system (CNS). The aim of this study was to investigate the role of circulating growth hormone (GH) and the neural GHR after hypoxic-ischemic (HI) brain injury in the 21-day old rat. We observed growth hormone receptor/binding protein (GHR/BP) immunoreactivity to be rapidly upregulated following a severe unilateral HI injury. There was a biphasic increase with an initial rise occurring in blood vessels within a few hours after injury followed by a secondary rise evident by 3 days post-hypoxia in microglia/macrophages, some of which are destined to express insulin-like growth factor-I (IGF-I). There was also an increased immunoreactivity in reactive astrocytes, some of which were in the process of dividing. Subsequently, we attempted to activate the endothelial GHR/BP which was found to be increased after injury by treating with 15 microgram g-1 day-1 s.c. bGH for 7 days. Circulating concentrations of IGF-I fell after injury and were restored with GH treatment (P=0.001), whereas treatment of normal animals had no effect on serum IGF-I. Peripheral GH treatment increased the cerebrospinal fluid (CSF) concentration of immunoreactive IGF-I in the injured rats (P=0.017). GH treatment also reversed the systemic catabolism caused by the injury but had no significant neuroprotective effects. These results indicate that GH therapy can be used to reverse the systemic catabolism that occurs after CNS injury. In addition, these data suggest a role for the neural GHR during the recovery from brain injury, both in terms of the induction of IGF-I and in terms of glial proliferation.

Insulin-like growth factor-I (IGF-I) and IGF-I receptor (IGF-IR) immunoreactivity in normal and osteopetrotic (toothless, tl/tl) rat tibia

Insulin-like growth factor-I (IGF-I) plays a major role in regulating cell growth. This study examined the immunohistochemical distribution of IGF-I and IGF-I receptor (IGF-IR) in tibias from normal and osteopetrotic (toothless, tl/tl) rats, following treatment with colony stimulating factor-1 (CSF-1). In normal rats, immunoreactivity for IGF-I and IGF-IR was detected in cells of the articular and epiphyseal cartilage, secondary ossification centres, zones of resting and proliferating chondrocytes and bone marrow. Bone marrow cells immunoreactive for IGF-I and IGF-IR were significantly reduced in the tl/tl rat (p < 0.001) compared with normal animals. Treatment of tl/tl rats with CSF-1 increased immunoreactivity for IGF-I and IGF-IR in bone marrow cells as well as the number of TRAP positive osteoclasts. This increase was the result of recruitment of a range of hematopoietic cell types, including eosinophils, polymorphs and a substantial number of monocyte-like cells demonstrating strong immunoreactivity to IGF-I/IGF-IR. The differences in relative immunoreactivity for IGF-I/IGF-IR by bone marrow cells in untreated and CSF-1-treated tl/tl rats indicate a CSF-1-dependent recruitment of cells bearing surface IGF-IRs which may be mediated by an increase in local or systemic IGF-I.

The identification of myogenic cells in skeletal muscle, with emphasis on the use of tritiated thymidine autoradiography and desmin antibodies

The identification of myogenic precursor cells (mpc) is a key factor in determining the early events in the myogenesis and regeneration of skeletal muscle. Although satellite cells have long been established as the providers of myoblastic cells, very little is really known (apart from their anatomical location in relation to muscle fibres and their ability to migrate) about the precise role of satellite cells in myogenesis. Numerous techniques for labelling mpc have been devised, but none of these has proven to be completely reliable in firmly establishing the origin of myogenic cells. The use of tritiated thymidine to label DNA in proliferating mpc (which are not specifically distinguishable at the time) and the subsequent location of their labelled progeny in myotube nuclei has revealed a great deal of data on the timing of myogenesis, but not about the nature of mpc themselves. DNA synthesis can also be detected by antibodies to the thymidine analogue, bromodeoxyuridine, and also by antibody staining for proliferating nuclear cell antigen. Like tritiated thymidine, these other markers are not specific for muscle but are general markers for DNA synthesis. In situ hybridisation of various muscle-specific genetic markers and their products has been informative, as has immunolabelling of myogenin, MyoD1 and desmin. Desmin labelling has been particularly instructive in identifying mpc because it is one of the first muscle-specific proteins to be produced in mpc. This review covers some of the techniques mentioned above and their usefulness in determining the early events in myogenesis.

Tissue engineering, morphogenesis, and regeneration of the periodontal tissues by bone morphogenetic proteins

Tissue engineering is the emerging field of science developing techniques for fabrication of new tissues for replacement based on principles of cell and developmental biology and biomaterials. Morphogenesis is the cascade of pattern formation and the attainment of form of the various organs and the organism as a whole. The periodontium consist of the periodontal ligament, cementum, and alveolar bone. Bone has considerable potential for regeneration and therefore is a prototypic model for tissue engineering. The three main ingredients for tissue engineering are regulatory signals, responding stem cells, and extracellular matrix. Recent advances in molecular biology of the bone morphogenetic proteins (BMPs) have set the stage for tissue engineering of bone and related tissues, including the periodontium. Bone-derived BMPs, with a collagenous matrix as carrier, induced cementum and alveolar bone regeneration in surgically created furcation defects in the primate. It is noteworthy that there was morphogenesis of periodontal ligament and a faithful insertion of Sharpey's fibers into cementum. In the same furcation model, recombinant human osteogenic protein-1 (rhOP-1, also known as BMP-7), in conjunction with the collagenous carrier, induced extensive cementogenesis with insertion of Sharpey's fibers into the newly formed cementum. The observation that BMPs induce cementogenesis and periodontal ligament formation indicates that these proteins may have multiple functions in vivo not limited to cartilage and bone induction. The rapid advances in the molecular biology of BMPs and their receptors bode well for novel strategies to engineer the regeneration of the periodontal tissues.