Sbds is required for Rac2-mediated monocyte migration and signaling downstream of RANK during osteoclastogenesis

Abstract

Shwachman-Diamond syndrome (SDS) results from mutations in the SBDS gene, characterized by exocrine pancreatic insufficiency and hematologic and skeletal abnormalities. Neutropenia and neutrophil dysfunction are hallmark features of SDS; however, causes for the bone defects are unknown. Dysfunction of boneresorbing osteoclasts, formed by the fusion of monocytic progenitors derived from the same granulocytic precursors as neutrophils, could be responsible. We report that Sbds is required for in vitro and in vivo osteoclastogenesis (OCG).Sbds-null murine monocytes formed osteoclasts of reduced number and size because of impaired migration and fusion required for OCG. Phenotypically, Sbdsnull mice exhibited low-turnover osteoporosis consistent with findings in SDS patients. Western blotting of Rho GTPases that control actin dynamics and migration showed a 5-fold decrease in Rac2, whereas Rac1, Cdc42, and RhoA were unchanged or only mildly reduced. Although migration was rescued on Rac2 supplementation, OCG was not. This was attributed to impaired signaling downstream of receptor activator of nuclear factor-B (RANK) and reduced expression of the RANK-ligand-dependent fusion receptor DC-STAMP. We conclude that Sbds is required for OCG by regulating monocyte migration via Rac2 and osteoclast differentiation signaling downstream of RANK. Impaired osteoclast formation could disrupt bone homeostasis, resulting in skeletal abnormalities seen in SDS patients. (Blood. 2011;117(6): 2044-2053) IntroductionShwachman-Diamond syndrome (SDS) is an autosomal recessive disorder with hallmark features of bone marrow failure, exocrine pancreatic insufficiency, and skeletal abnormalities. [1][2][3] Neutropenia is the most common clinical manifestation of bone marrow failure, but patients may also experience pancytopenia, anemia, and thrombocytopenia, and be at increased risk of developing aplastic anemia and acute myeloid leukemia. 4 In early crosssectional studies, metaphyseal dysostosis was observed in 40% to 80% of SDS patients, and rib and/or thoracic cage abnormalities in 30% to 50% of SDS patients. [5][6][7] More recently, in a longitudinal study, all of the SDS patients studied displayed various skeletal abnormalities, including delayed appearance of secondary ossification centers, variable widening, and irregularity of the metaphyses in early childhood followed by progressive thickening and irregularity of the growth plates, and generalized osteopenia. 8 In addition, a subset of patients showed early signs of osteoporotic vertebral deformities and disturbances in bone homeostasis manifesting in low-turnover osteoporosis. 8,9 Oral and dental diseases and conditions, including periodontitis, delayed eruption of the permanent dentition, increased caries risk in primary and permanent dentitions, and increased soft tissue pathoses, have also been reported. 10 SDS results from mutations in the ubiquitously expressed, conserved Shwachman-Bedian-Diamond syndrome gene (SBDS). 11 The exact function of the SBDS protein remains unclear; h...

Rac regulates PtdInsP3 signaling and the chemotactic compass through a redox-mediated feedback loop

“…Finally, proteins were concentrated in Pierce protein concentrators, and protein concentration and purity were assessed by SDS-PAGE followed by Coomassie staining. Control experiments and data from previous studies 20,21 have verified that addition of TAT-Rac-WT rescues ROS generation and chemotaxis in leukocytes (data not shown).…”

38 Uncoupling of GTP hydrolysis from eIF6 release on the ribosome causes Shwachman-Diamond syndrome

“…Thus, the primary defect underlying SDS is distinct from the other ribosomopathies associated with hematological disease. We suggest that the diverse alternate functions proposed for SBDS in mammalian cells (Wessels et al 2006;Austin et al 2008;Rujkijyanont et al 2008;Ball et al 2009;Leung et al 2010) are downstream secondary consequences of the primary defect in 60S ribosomal subunit maturation. How do we explain the SDS phenotype?…”

“…Although cosedimentation of human SBDS with free cytoplasmic 60S ribosomal subunits in sucrose gradients (Ganapathi et al 2007) would be consistent with a conserved role for SBDS in 60S subunit maturation, the current model in mammalian cells posits that eIF6 removal is triggered following phosphorylation of Ser 235 by protein kinase C (PKC) and RACK1 (receptor for activated protein C) (Ceci et al 2003). Furthermore, diverse alternate functions for SBDS in mammalian cells have been suggested, including mitotic spindle stabilization (Austin et al 2008), chemotaxis (Wessels et al 2006), Fas ligand-induced apoptosis (Rujkijyanont et al 2008), cellular stress responses (Ball et al 2009), and Rac2-mediated monocyte migration (Leung et al 2010). Thus, despite the prior genetic studies in yeast, the mechanism of eIF6 release in mammalian cells is controversial, biochemical evidence supporting direct catalysis of eIF6 release by SBDS and EFL1 in eukaryotic cells is currently lacking, and the specific function of the SBDS protein, its mode of action, and the molecular mechanism of the cooperative interaction with EFL1 remain obscure.…”

Modulation of osteoclast differentiation and bone resorption by Rho GTPases

“…Indeed, Rac2 activation downstream the signaling pathway of Sbds, a poorly characterized ribosome-related protein, and Rac1 activation upon interaction of the cytoplasmic tail of matrix metalloprotease MT1-MMP with p130Cas, are required for precursors migration. 37,38 Similarly, filamin A regulates actin dynamics that control M-CSF-dependent precursors migration via Rac1 and Cdc42 combined activation. 39 Besides, we showed that the atypical Rho GTPase RhoU, which expression is strongly induced during early osteoclastogenesis, is involved in osteoclast precursors fusion.…”