Biomedical subjects

Arndt F Schilling

Publications and source records attributed to Arndt F Schilling.

10 recordsLinked to original sources

PubMed2006

Leptin inhibits bone formation not only in rodents, but also in sheep.

UNLABELLED: This study examines the effect of long-term ICV administration of leptin in ewes. We found that central application significantly decreased osteoblast activity as measured by serum analysis as well as by histomorphometry, resulting in decreased trabecular bone volume. These data provide additional evidence that bone formation and therefore bone remodeling is at least in part centrally controlled. INTRODUCTION: Genetic studies in mice have identified leptin as a potent inhibitor of bone formation acting through the central nervous system and unraveled the central nature of bone mass control and its disorders. Although these studies have radically enhanced our understanding of skeletal physiology because they have established a hypothalamic regulation of bone remodeling through the sympathetic nervous system, controversy remains about the physiological relevance of these observations because leptin's effect on bone after intracerebroventricular (ICV) application has only been shown in mice. To address whether leptin has a role in regulating bone mass beyond rodents, we treated ewes with long-term ICV application of leptin and analyzed the bone phenotype after a treatment period of 3 months. MATERIALS AND METHODS: Three groups of corriedale sheep were compared: (1) control entire (control), (2) ovariectomy (OVX) and ICV application of cerebrospinal fluid (CSF); and (iii) OVX and ICV application of leptin (leptin). Analysis included histomorphometric characterization of iliac crest, spine and femur by histology and biomechanical testing and measurement of bone turnover parameters in serum and urine. RESULTS: Central application of leptin decreased bone formation by 70% and mineralizing surface (MS/BS, 39.4 +/- 3.3% versus 16.1 +/- 2.1%) significantly (p < 0.01). Whereas OVX increased osteoclast indices and urinary cross-lap excretion by two and three times, respectively, serum parameters of osteoblast activity were significantly reduced by ICV application of leptin (p < 0.01). Consequently, ewes treated with leptin were osteopenic (iliac crest BV/TV entire, 22.7 +/- 1.3%; CSF, 18.9 +/- 2.4%; leptin, 12.4 +/- 2.6%), whereas bone torsional failure load reflecting the cortex of the tibia was not yet changed after 3 months of treatment (p < 0.01). CONCLUSIONS: Taken together, these data suggest that leptin controls bone formation after ICV application, leading to reduction of trabecular bone mass in sheep. Most importantly, however, they show that the central regulation of bone formation is not limited to rodents, but is also found in large animals, providing further evidence that bone remodeling in vertebrates is centrally controlled.

Animals↗

PubMed2006

Calcitonin deficiency in mice progressively results in high bone turnover.

UNLABELLED: Although the pharmacological action of calcitonin (CT) as an inhibitor of bone resorption is well established, there is still some controversy regarding its physiological function. Unexpectedly, Calca-deficient mice lacking CT and alpha-calcitonin gene-related peptide (alphaCGRP) were described to have a high bone mass phenotype caused by increased bone formation with normal bone resorption. Here we show that these mice develop a phenotype of high bone turnover with age, suggesting that CT is a physiological inhibitor of bone remodeling. INTRODUCTION: The absence of significant changes in bone mineral density caused by decline or overproduction of CT in humans has raised the question, whether the pharmacological action of CT as an inhibitor of bone resorption is also of physiological relevance. To study the physiological role of mammalian CT, we have analyzed the age-dependent bone phenotype of two mouse models, one lacking CT and alphaCGRP (Calca-/-), the other one lacking only alphaCGRP (alphaCGRP-/-). MATERIALS AND METHODS: Bones from wildtype, Calca-/- -mice and alphaCGRP-/- -mice were analyzed at the ages of 6, 12 and 18 months using undecalcified histology. Differences of bone remodeling were quantified by static and dynamic histomorphometry as well as by measuring the urinary collagen degradation products. To rule out secondary mechanisms underlying the observed phenotype, we determined serum concentrations of relevant hormones using commercially available antibody-based detection kits. RESULTS: Whereas alphaCGRP-/- -mice display an osteopenia at all ages analyzed, the Calca-/- -mice develop a phenotype of high bone turnover with age. Histomorphometric analysis performed at the age of 12 months revealed significant increases of bone formation and bone resorption specifically in the Calca-/- -mice. This severe phenotype that can result in hyperostotic lesions, can not be explained by obvious endocrine abnormalities other than the absence of CT. CONCLUSIONS: In addition to the previously described increase of bone formation in the Calca-deficient mice, we have observed that there is also an increase of bone resorption with age. This suggests that CT has a dual action as an inhibitor of bone remodeling, which may explain why alterations of CT serum levels in humans do not result in major changes of bone mineral density.

Aging↗

PubMed2005

The high mobility group transcription factor Sox8 is a negative regulator of osteoblast differentiation.

Bone remodeling is an important physiologic process that is required to maintain a constant bone mass. This is achieved through a balanced activity of bone-resorbing osteoclasts and bone-forming osteoblasts. In this study, we identify the high mobility group transcription factor Sox8 as a physiologic regulator of bone formation. Sox8-deficient mice display a low bone mass phenotype that is caused by a precocious osteoblast differentiation. Accordingly, primary osteoblasts derived from these mice show an accelerated mineralization ex vivo and a premature expression of osteoblast differentiation markers. To confirm the function of Sox8 as a negative regulator of osteoblast differentiation we generated transgenic mice that express Sox8 under the control of an osteoblast-specific Col1a1 promoter fragment. These mice display a severely impaired bone formation that can be explained by a strongly reduced expression of runt-related transcription factor 2, a gene encoding a transcription factor required for osteoblast differentiation. Together, these data demonstrate a novel function of Sox8, whose tightly controlled expression is critical for bone formation.

Animals↗

PubMed2005

Mouse models in skeletal physiology and osteoporosis: experiences and data on 14,839 cases from the Hamburg Mouse Archives.

Our understanding of the developmental biology of the skeleton, like that of virtually every other subject in biology, has been transformed by recent advances in human and mouse genetics, but we still know very little, in molecular and genetic terms, about skeletal physiology. Thus, among the many questions that are largely unexplained are the following: why is osteoporosis mainly a women's disease? How is bone mass maintained nearly constant between the end of puberty and the arrest of gonadal functions? Molecular genetics has emerged as a powerful tool to study previously unexplored aspects of the physiology of the skeleton. Among mammals, mice are the most promising animals for this experimental work. The input that transgenic animals can offer to our field depends on our means of phenotypic characterization of the mouse skeleton. In fact, full appreciation of the skeletal characteristics of a given mouse model requires the application of standardized protocols for noninvasive imaging, histology, histomorphometry, biomechanics, and individually adapted in vitro and in vivo analysis. Over the past years we have established a mouse archive that consists of 14,839 cases from more than 120 different mouse models that we have phenotypically characterized in Hamburg. Today, this is one of the biggest databases on the mouse skeleton. This review focuses on one aspect of skeletal physiology, namely skeletal aging, and demonstrates that mouse models can be a valuable tool to gain insights in certain facets of skeletal physiology that have been unexplored previously.

Animals↗

PubMed2005

Essential role of RSK2 in c-Fos-dependent osteosarcoma development.

Inactivation of the growth factor-regulated S6 kinase RSK2 causes Coffin-Lowry syndrome in humans, an X-linked mental retardation condition associated with progressive skeletal abnormalities. Here we show that mice lacking RSK2 develop a progressive skeletal disease, osteopenia due to impaired osteoblast function and normal osteoclast differentiation. The phenotype is associated with decreased expression of Phex, an endopeptidase regulating bone mineralization. This defect is probably not mediated by RSK2-dependent phosphorylation of c-Fos on serine 362 in the C-terminus. However, in the absence of RSK2, c-Fos-dependent osteosarcoma formation is impaired. The lack of c-Fos phosphorylation leads to reduced c-Fos protein levels, which are thought to be responsible for decreased proliferation and increased apoptosis of transformed osteoblasts. Therefore, RSK2-dependent stabilization of c-Fos is essential for osteosarcoma formation in mice and may also be important for human osteosarcomas.

Animals↗

PubMed2004

Increased bone formation in mice lacking apolipoprotein E.

UNLABELLED: ApoE is a plasma protein that plays a major role in lipoprotein metabolism. Here we describe that ApoE expression is strongly induced on mineralization of primary osteoblast cultures. ApoE-deficient mice display an increased bone formation rate compared with wildtype controls, thereby showing that ApoE has a physiologic function in bone remodeling. INTRODUCTION: Apolipoprotein E (ApoE) is a protein component of lipoproteins and facilitates their clearance from the circulation. This is confirmed by the phenotype of ApoE-deficient mice that have high plasma cholesterol levels and spontaneously develop atherosclerotic lesions. The bone phenotype of these mice has not been analyzed to date, although an association between certain ApoE alleles and BMD has been reported. MATERIALS AND METHODS: Primary osteoblasts were isolated from newborn mouse calvariae and mineralized ex vivo. A genome-wide expression analysis was performed during the course of differentiation using the Affymetrix gene chip system. Bones from ApoE-deficient mice and wildtype controls were analyzed using radiography, micro CT imaging, and undecalcified histology. Cellular activities were assessed using dynamic histomorphometry and by measuring urinary collagen degradation products. Lipoprotein uptake assays were performed with (125)I-labeled triglyceride-rich lipoprotein-remnants (TRL-R) using primary osteoblasts from wildtype and ApoE-deficient mice. Serum concentrations of osteocalcin were determined by radioimmunoassay after hydroxyapatite chromatography. RESULTS: ApoE expression is strongly induced on mineralization of primary osteoblast cultures ex vivo. Mice lacking ApoE display a high bone mass phenotype that is caused by an increased bone formation rate, whereas bone resorption is not affected. This phenotype may be explained by a decreased uptake of triglyceride-rich lipoproteins by osteoblasts, resulting in elevated levels of undercarboxylated osteocalcin in the serum of ApoE-deficient mice. CONCLUSION: The specific induction of ApoE gene expression during osteoblast differentiation along with the increased bone formation rate observed in ApoE-deficient mice shows that ApoE has a physiologic role as a regulator of osteoblast function.

Alleles↗

PubMed2004

Decreased bone formation and osteopenia in mice lacking alpha-calcitonin gene-related peptide.

UNLABELLED: We recently described an unexpected high bone mass phenotype in mice lacking the Calca gene that encodes CT and alphaCGRP. Here we show that mice specifically lacking alphaCGRP expression display an osteopenia caused by a decreased bone formation. These results show that alphaCGRP is a physiological activator of bone formation and that the high bone mass phenotype of the Calca-deficient mice is caused by the absence of CT. INTRODUCTION: Calcitonin (CT) and alpha-calcitonin gene-related peptide (alphaCGRP) are two polypeptides without completely defined physiologic functions that are both derived from the Calca gene by alternative splicing. We have recently described an unexpected high bone mass phenotype in mice carrying a targeted deletion of the Calca gene. To uncover whether this phenotype is caused by the absence of CT or by the absence of alphaCGRP, we analyzed a mouse model, where the production of alphaCGRP is selectively abolished. MATERIALS AND METHODS: Bones from Calca(-/-) mice, alphaCGRP(-/-) mice, and their corresponding wildtype controls were analyzed using radiography, muCT imaging, and undecalcified histology. Cellular activities were assessed using dynamic histomorphometry and by measuring the urinary collagen degradation products. CT expression was determined using radioimmunoassay and RT-PCR. Immunohistochemistry was performed using an anti-CGRP antibody on decalcified bone sections. RESULTS: Unlike the Calca-deficient mice, the alphaCGRP-deficient mice do not display a high bone mass phenotype. In contrast, they develop an osteopenia that is caused by a reduced bone formation rate. Serum levels and thyroid expression of CT are not elevated in alphaCGRP-deficient mice. While CGRP expression is detectable in neuronal cell close to trabecular bone structures, the components of the CGRP receptor are expressed in differentiated osteoblast cultures. CONCLUSION: The discrepancy between the bone phenotypes of Calca(-/-) mice and alphaCGRP(-/-) mice show that the high bone mass phenotype of the Calca(-/-) mice is caused by the absence of CT. The osteopenia observed in the alphaCGRP(-/-) mice that have normal levels of CT further show that alphaCGRP is a physiologic activator of bone formation.

Alternative Splicing↗

PubMed2004

The Fos-related antigen Fra-1 is an activator of bone matrix formation.

Ectopic expression of the transcription factor Fra-1 in transgenic mice leads to osteosclerosis, a bone disorder characterized by increased bone mass. The molecular basis for this phenotype is unknown and Fra-1 functions cannot be studied by a conventional loss-of-function approach, since fra-1-knockout mice die in utero likely due to placental defects. Here we show that the lethality of fra-1-knockout mice can be rescued by specific deletion of Fra-1 only in the mouse embryo and not in the placenta. Mice lacking Fra-1 (fra-1(delta/delta)) are viable and develop osteopenia, a low bone mass disease. Long bones of fra-1(delta/delta) mice appear to have normal osteoclasts but express reduced amounts of bone matrix components produced by osteoblasts and chondrocytes such as osteocalcin, collagen1a2 and matrix Gla protein. The gene for matrix Gla protein seems to be a specific target of Fra-1 since its expression was markedly increased in the long bones of fra-1-transgenic mice. These results uncover a novel function of Fra-1 in regulating bone mass through bone matrix production by osteoblasts and chondrocytes.

Animals↗

PubMed2004

Resorbability of bone substitute biomaterials by human osteoclasts.

Third generation biomaterials are being designed with the aim that once implanted they will help the body to heal itself. One desirable characteristic of these materials in bone is their ability to be remodeled, i.e. that osteoclasts resorb the material and it is subsequently replaced by newly formed bone through osteoblastic activity. So far the only way to test this biological property of bone substitutes are animal experiments with all their limitations like ethics, costs and limited transferability to man. The present study was designed, to develop a human in vitro assay, allowing to generate human osteoclasts directly on the biomaterial. The assay was validated using calcium phosphate cement and PMMA as biomaterials. Quantification was performed by raster electron microscopy and computer assisted image analysis. Dentin was used as internal standard. Our assay shows iso-bone resorbability of calcium phosphate cement in comparison to unresorbable PMMA cement. Both current clinical orthopedic practice and future skeletal engineering may profit from the availability and use of a test system for the assessment of resorption quality. The assay presented here allows to address this question of resorbability and to select the best materials for the use as bone substitutes in specific patients.

Absorbable Implants↗

PubMed2004

Absence of mouse pleiotrophin does not affect bone formation in vivo.

Pleiotrophin (Ptn) is an extracellular matrix protein that regulates hippocampal synaptic plasticity and learning behavior in vivo. Since the overexpression of Ptn in transgenic mice leads to increased bone formation, we analyzed whether a deficiency in Ptn expression would have a negative effect on bone remodeling. Bones from Ptn-deficient mice and wild-type littermates were analyzed using radiography, muCT imaging and undecalcified histology. Biomechanical stability was determined in a three-point-bending assay. Cellular activities were assessed using dynamic histomorphometry and the determination of urinary collagen degradation products. Skeletons of Ptn-deficient mice have no gross abnormalities, displayed a normal size, and showed no differences in growth plate organization compared to wild-type littermates. There were no obvious differences in bone mass as determined by radiographic and muCT imaging. The absence of a bone remodeling phenotype in Ptn-deficient mice was further confirmed using static histomorphometry and biomechanical testing. Finally, the number, morphology, and function of osteoclasts, osteoblasts, and osteocytes were not altered in Ptn-deficient mice compared to wild-type littermates. The complete skeletal analysis of Ptn-deficient mice presented here demonstrates that the lack of Ptn in mice does not affect bone formation in vivo. Therefore, Ptn does not play a significant role in normal bone physiology.

Animals↗