Linking Hematopoiesis to Endochondral Skeletogenesis through Analysis of Mice Transgenic for Collagen X

Abstract

The vertebrate skeleton forms predominantly by endochondral ossification (EO), where the cartilaginous model of the axial and appendicular skeleton, as well as of certain cranial bones, is replaced by bony trabeculae and marrow. The distinctive feature of this process is comprised of hypertrophic cartilage where EO initiates and collagen X is predominant.1 Emergence of hypertrophic cartilage defines each skeletal element where marrow forms. Since the marrow provides niches for blood cell differentiation, alterations in the cartilage-to-bone and marrow transition of EO may affect stromal and hematopoietic constituents. We demonstrate here that mice transgenic (Tg) for collagen X develop both skeletal and hematopoietic abnormalities, and that the latter likely arise as a consequence of disrupted collagen X function. These data reveal an unforeseen link between endochondral skeletogenesis and establishment of the marrow microenvironment prerequisite for hematopoiesis.During embryogenesis, EO initiates in cartilage with hypertrophy, and progresses by transforming a preexisting non-calcified avascular cartilage to a calcifiable one permissive to vascularization.

Skeletogenesis and the Hematopoietic Niche

“…Further, similar conclusions about the necessity of EO-derived components, e.g. cells and matrix molecules, were obtained through analyses of several mouse models with skeleto-hematopoietic defects ( Table 1), including mice with disrupted collagen X function in the HME (Jacenko et al 1993;Gress and Jacenko 2000;Jacenko et al 2002;Sweeney et al 2008;Sweeney et al 2010). Discussed below are the data describing which cell type(s), associated matrix and soluble factors are necessary for blood cell development in the marrow, including: multipotent stromal cells (fibroblasts, pericytes, reticular cells, and adipocytes), osteoblasts, chondrocytes, endothelial cells, and cells of hematopoietic origin (hematopoietic stem/progenitor cells (HSPC), osteoclasts and macrophages).…”

“…1B boxed) (Jacenko et al 1993;Nilsson et al 1997;Gress and Jacenko 2000;Nilsson et al 2001;Jacenko et al 2002;Yoshimoto et al 2003;Arai, F. et al 2004;Balduino et al 2005;Sweeney et al 2008;Kohler et al 2009;Lo Celso et al 2009;Xie et al 2009;Sweeney et al 2010). This skeleto-hematopoietic link is strongly supported by several animal models where alterations in process of EO leads to hematopoietic defects (Table 1), including mouse models with altered: collagen X (Jacenko et al 1993;Gress and Jacenko 2000;Jacenko et al 2002;Sweeney et al 2008;Sweeney et al 2010), parathyroid hormone related protein (PTHrP) receptor in osteoblasts (Calvi et al 2001;Calvi et al 2003;Kuznetsov et al 2004;Wu et al 2008), osteoblast numbers (Visnjic et al 2001;Visnjic et al 2004;Zhu et al 2007), bone morphogenic protein (BMP) receptor type 1A in marrow cells (Zhang, J. et al 2003), osteoclast function (Blin-Wakkach et al 2004;Mansour et al 2011), retinoic acid receptor gamma (Purton et al 2006;, G s  in ostoblasts (Wu et al 2008), Dicer in ostoblasts (Raaijmakers et al 2010), glypican-3 (Viviano et al 2005, and perlecan (Rodgers et al 2008). Table 1 p r e s e n t s a l i s t o f m o u s e m o d e l s w i t h d e f e c t s i n hematopoiesis due to alterations in a component within the niche environment.…”

“…Further, these assays confirmed a connection between osteoblasts and B cell development. To assess the contribution of osteoblasts to hematopoiesis in vivo, several different mouse models have been generated that either increase osteoblasts (Calvi et al 2001;Calvi et al 2003;Zhang, J. et al 2003), decrease osteoblasts (Visnjic et al 2001;Visnjic et al 2004), disrupt EO-based trabecular bone formation (Jacenko et al 1993;Gress and Jacenko 2000;Jacenko et al 2001;Jacenko et al 2002;Sweeney et al 2008;Sweeney et al 2010), alter osteoblast signaling (Wu et al 2008), or modify osteoblast RNA processing (Table 1) (Raaijmakers et al 2010). These in vivo models have confirmed that osteoblasts can support hematopoiesis, are involved in B lymphopoiesis, make pro-hematopoietic cytokines, make cell-cell contact with HSPCs, and moreover, have implicated osterix expressing osteoprogenitors as mediators of hematopoiesis.…”

Misfolding of Collagen X Chains Harboring Schmid Metaphyseal Chondrodysplasia Mutations Results in Aberrant Disulfide Bond Formation, Intracellular Retention, and Activation of the Unfolded Protein Response

“…Interestingly, accumulation of mutant collagen X within hypertrophic chondrocytes of transgenic mice expressing the del13 mutation (a 13-bp deletion that is equivalent to the human NC1del10 SMCD mutation) triggered the UPR, as illustrated by enhanced splicing of XBP1. 4 To determine whether the UPR is associated specifically with the del13 mutation or whether missense mutations also elicit the UPR, we analyzed the relative proportions of spliced and unspliced XBP1 mRNAs in 293-EBNA and SaOS-2 cells transfected with wild-type, Y598D, N617K, G618V, and NC1del10 cDNAs. Cells were grown to confluence and incubated in fresh medium for 2 h prior to RNA extraction to avoid exposure of the cells to ER stress induced by glucose starvation.…”

“…The absence of a functional collagen X network in mice is associated with displacement of proteoglycans, altered mineral deposition, compression of the growth plate, and hematopoietic changes (3)(4)(5). Based on these studies, it has been proposed that collagen X interactions within the cartilage extracellular matrix establish the correct microenvironment for matrix mineralization and subsequent bone development.…”