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Rac deletion in osteoclasts causes severe osteopetrosis.

ABSTRACT: Cdc42 mediates bone resorption principally by stimulating osteoclastogenesis. Whether its sister GTPase, Rac, meaningfully impacts upon the osteoclast and, if so, by what means, is unclear. We find that whereas deletion of Rac1 or Rac2 alone has no effect, variable reduction of Rac1 in osteoclastic cells of Rac2(-/-) mice causes severe osteopetrosis. Osteoclasts lacking Rac1 and Rac2 in combination (Rac double-knockout, RacDKO), fail to effectively resorb bone. By contrast, osteoclasts are abundant in RacDKO osteopetrotic mice and, unlike those deficient in Cdc42, express the maturation markers of the cells normally. Hence, the osteopetrotic lesion of RacDKO mice largely reflects impaired function, and not arrested differentiation, of the resorptive polykaryon. The dysfunction of RacDKO osteoclasts represents failed cytoskeleton organization as evidenced by reduced motility of the cells and their inability to spread or generate the key resorptive organelles (i.e. actin rings and ruffled borders), which is accompanied by abnormal Arp3 distribution. The cytoskeleton-organizing capacity of Rac1 is mediated through its 20-amino-acid effector domain. Thus, Rac1 and Rac2 are mutually compensatory. Unlike Cdc42 deficiency, their combined absence does not impact upon differentiation but promotes severe osteopetrosis by dysregulating the osteoclast cytoskeleton.

SUBMITTER: Croke M

PROVIDER: S-EPMC3225269 | biostudies-literature | 2011 Nov

REPOSITORIES: biostudies-literature

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Rac deletion in osteoclasts causes severe osteopetrosis.

Croke Monica M Ross F Patrick FP Korhonen Matti M Williams David A DA Zou Wei W Teitelbaum Steven L SL

Journal of cell science 20111123 Pt 22

Cdc42 mediates bone resorption principally by stimulating osteoclastogenesis. Whether its sister GTPase, Rac, meaningfully impacts upon the osteoclast and, if so, by what means, is unclear. We find that whereas deletion of Rac1 or Rac2 alone has no effect, variable reduction of Rac1 in osteoclastic cells of Rac2(-/-) mice causes severe osteopetrosis. Osteoclasts lacking Rac1 and Rac2 in combination (Rac double-knockout, RacDKO), fail to effectively resorb bone. By contrast, osteoclasts are abund ...[more]

PMID: 22114304

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Project description:Bone homeostasis is a complex, multi-step process, which is based primarily on a tightly orchestrated interplay between bone formation and bone resorption that is executed by osteoblasts and osteoclasts (OCLs), respectively. The essential physiological balance between these cells is maintained and controlled at multiple levels, ranging from regulated gene expression to endocrine signals, yet the underlying cellular and molecular mechanisms are still poorly understood. One approach for deciphering the mechanisms that regulate bone homeostasis is the characterization of relevant pathological states in which this balance is disturbed. In this article we describe one such "error of nature," namely the development of acute recessive osteopetrosis (ARO) in humans that is caused by mutations in sorting nexin 10 (SNX10) that affect OCL functioning. We hypothesize here that, by virtue of its specific roles in vesicular trafficking, SNX10 serves as a key selective regulator of the composition of diverse membrane compartments in OCLs, thereby affecting critical processes in the sequence of events that link the plasma membrane with formation of the ruffled border and with extracellular acidification. As a result, SNX10 determines multiple features of these cells either directly or, as in regulation of cell-cell fusion, indirectly. This hypothesis is further supported by the similarities between the cellular defects observed in OCLs form various models of ARO, induced by mutations in SNX10 and in other genes, which suggest that mutations in the known ARO-associated genes act by disrupting the same plasma membrane-to-ruffled border axis, albeit to different degrees. In this article, we describe the population genetics and spread of the original arginine-to-glutamine mutation at position 51 (R51Q) in SNX10 in the Palestinian community. We further review recent studies, conducted in animal and cellular model systems, that highlight the essential roles of SNX10 in critical membrane functions in OCLs, and discuss possible future research directions that are needed for challenging or substantiating our hypothesis.

Project description:BackgroundOur recent studies demonstrate that the focal adhesion protein Kindlin-2 exerts crucial functions in the mesenchymal stem cells, mature osteoblasts and osteocytes in control of early skeletal development and bone homeostasis in mice. However, whether Kindlin-2 plays a role in osteoprogenitors remains unclear.Materials and methodsMice lacking Kindlin-2 expression in osterix (Osx)-expressing cells (i.e., osteoprogenitors) were generated. Micro-computerized tomography (μCT) analyses, histology, bone histomorphometry and immunohistochemistry were performed to determine the effects of Kindlin-2 deletion on skeletal development and bone mass accrual and homeostasis. Bone marrow stromal cells (BMSCs) from mutant mice (Kindlin-2 fl/fl ; Osx Cre ) and control littermates were isolated and determined for their osteoblastic differentiation capacity.ResultsKindlin-2 was highly expressed in osteoprogenitors during endochondral ossification. Deleting Kindlin-2 expression in osteoprogenitors impaired both intramembranous and endochondral ossifications. Mutant mice displayed multiple severe skeletal abnormalities, including unmineralized fontanel, limb shortening and growth retardation. Deletion of Kindlin-2 in osteoprogenitors impaired the growth plate development and largely delayed formation of the secondary ossification center in the long bones. Furthermore, adult mutant mice displayed a severe low-turnover osteopenia with a dramatic decrease in bone formation which exceeded that in bone resorption. Primary BMSCs isolated from mutant mice exhibited decreased osteoblastic differentiation capacity.ConclusionsOur study demonstrates an essential role of Kinlind-2 expression in osteoprogenitors in regulating skeletogenesis and bone mass accrual and homeostasis in mice.The translational potential of this articleThis study reveals that Kindlin-2 through its expression in osteoprogenitor cells controls chondrogenesis and bone mass. We may define a novel therapeutic target for treatment of skeletal diseases, such as chondrodysplasia and osteoporosis.

Project description:BackgroundOsteopetrosis is a skeletal disorder of humans and animals characterized by the formation of overly dense bones, resulting from a deficiency in the number and/or function of bone-resorbing osteoclast cells. In cattle, osteopetrosis can either be induced during gestation by viral infection of the dam, or inherited as a recessive defect. Genetically affected calves are typically aborted late in gestation, display skull deformities and exhibit a marked reduction of osteoclasts. Although mutations in several genes are associated with osteopetrosis in humans and mice, the genetic basis of the cattle disorder was previously unknown.ResultsWe have conducted a whole-genome association analysis to identify the mutation responsible for inherited osteopetrosis in Red Angus cattle. Analysis of >54,000 SNP genotypes for each of seven affected calves and nine control animals localized the defective gene to the telomeric end of bovine chromosome 4 (BTA4). Homozygosity analysis refined the interval to a 3.4-Mb region containing the SLC4A2 gene, encoding an anion exchanger protein necessary for proper osteoclast function. Examination of SLC4A2 from normal and affected animals revealed a approximately 2.8-kb deletion mutation in affected calves that encompasses exon 2 and nearly half of exon 3, predicted to prevent normal protein function. Analysis of RNA from a proven heterozygous individual confirmed the presence of transcripts lacking exons 2 and 3, in addition to normal transcripts. Genotyping of additional animals demonstrated complete concordance of the homozygous deletion genotype with the osteopetrosis phenotype. Histological examination of affected tissues revealed scarce, morphologically abnormal osteoclasts displaying evidence of apoptosis.ConclusionsThese results indicate that a deletion mutation within bovine SLC4A2 is associated with osteopetrosis in Red Angus cattle. Loss of SLC4A2 function appears to induce premature cell death, and likely results in cytoplasmic alkalinization of osteoclasts which, in turn, may disrupt acidification of resorption lacunae.

Project description:Osteopetrosis ("marble bone disease") is a descriptive term that refers to a group of rare, heritable disorders of the skeleton characterized by increased bone density on radiographs. The overall incidence of these conditions is difficult to estimate but autosomal recessive osteopetrosis (ARO) has an incidence of 1 in 250,000 births, and autosomal dominant osteopetrosis (ADO) has an incidence of 1 in 20,000 births. Osteopetrotic conditions vary greatly in their presentation and severity, ranging from neonatal onset with life-threatening complications such as bone marrow failure (e.g. classic or "malignant" ARO), to the incidental finding of osteopetrosis on radiographs (e.g. osteopoikilosis). Classic ARO is characterised by fractures, short stature, compressive neuropathies, hypocalcaemia with attendant tetanic seizures, and life-threatening pancytopaenia. The presence of primary neurodegeneration, mental retardation, skin and immune system involvement, or renal tubular acidosis may point to rarer osteopetrosis variants, whereas onset of primarily skeletal manifestations such as fractures and osteomyelitis in late childhood or adolescence is typical of ADO. Osteopetrosis is caused by failure of osteoclast development or function and mutations in at least 10 genes have been identified as causative in humans, accounting for 70% of all cases. These conditions can be inherited as autosomal recessive, dominant or X-linked traits with the most severe forms being autosomal recessive. Diagnosis is largely based on clinical and radiographic evaluation, confirmed by gene testing where applicable, and paves the way to understanding natural history, specific treatment where available, counselling regarding recurrence risks, and prenatal diagnosis in severe forms. Treatment of osteopetrotic conditions is largely symptomatic, although haematopoietic stem cell transplantation is employed for the most severe forms associated with bone marrow failure and currently offers the best chance of longer-term survival in this group. The severe infantile forms of osteopetrosis are associated with diminished life expectancy, with most untreated children dying in the first decade as a complication of bone marrow suppression. Life expectancy in the adult onset forms is normal. It is anticipated that further understanding of the molecular pathogenesis of these conditions will reveal new targets for pharmacotherapy.

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Rac deletion in osteoclasts causes severe osteopetrosis.

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Rac deletion in osteoclasts causes severe osteopetrosis.

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