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Loss of function mutations in the gene encoding the heparan sulfate proteoglycan Glypican-3 (GPC3) causes an X-linked disorder in humans known as Simpson-Golabi-Behmel Syndrome (SGBS). This disorder includes both pre- and postnatal overgrowth, a predisposition to certain childhood cancers, and a complex assortment of congenital defects including skeletal abnormalities. In this study, we have identified a previously unrecognized delay in endochondral ossification associated with the loss of Gpc3 function. Gpc3 knockout animals show a marked reduction in calcified trabecular bone, and an abnormal persistence of hypertrophic chondrocytes at embryonic day 16.5 (E16.5). These hypertrophic chondrocytes down-regulate Type X collagen mRNA expression and undergo apoptosis, suggesting a normal progression of hypertrophic chondrocyte cell fate. However, replacement of these cells by mineralized bone is delayed in association with a marked delay in the appearance of osteoclasts in the bone in vivo. This delay in vivo correlates with a significant reduction in the capacity to form osteoclasts from bone marrow macrophage precursors in vitro in response to M-CSF and RANKL, and with a reduction in the numbers of bone-marrow-derived cells expressing the markers CD11b and Gr-1. Together, these results indicate selective impairment in the development of the common hematopoietic lineage from which monocyte/macrophages and PMNs are derived. This is the first report of a requirement for heparan sulfate, and specifically Gpc3, in the lineage-specific differentiation of these cell types in vivo. Keywords: Heparan sulfate; Proteoglycans; Glypican-3; Endochondral ossification; Osteoclasts; Hematopoiesis

Glypicans represent a family of six cell surface heparan sulfate proteoglycans in vertebrates. Although no specific in vivo functions have thus far been described for these proteoglycans, spontaneous mutations in the human and induced deletions in the mouse glypican-3 (Gpc3) gene result in severe malformations and both pre- and postnatal overgrowth, known clinically as the Simpson-Golabi-Behmel syndrome (SGBS). Mice carrying mutant alleles of Gpc3 created by either targeted gene disruption or gene trapping display a wide range of phenotypes associated with SGBS including renal cystic dysplasia, ventral wall defects, and skeletal abnormalities that are consistent with the pattern of Gpc3 expression in the mouse embryo. Previous studies in Drosophila have implicated glypicans in the signaling of decapentaplegic, a BMP homolog. Our experiments with mice show a significant relationship between vertebrate BMP signaling and glypican function; GPC3-deficient animals were mated with mice haploinsufficient for bone morphogeneticc protein-4 (Bmp4) and their offspring displayed a high penetrance of postaxial polydactyly and rib malformations not observed in either parent strain. This previously unknown link between glypican-3 and BMP4 function provides evidence of a role for glypicans in vertebrate limb patterning and skeletal development and suggests a mechanism for the skeletal defects seen in SGBS. Key Words: heparan sulfate; glypican; proteoglycans; bone morphogenetic protein; limb; skeleton; polydactyly; Simpson-Golabi-Behmel syndrome.