The skeletal phenotype of the cav-1 −/− mouse, which lacks caveolae, was examined. µCT and histology showed increased trabecular and cortical bone caused by the gene deletion. Structural changes were accompanied by increased mechanical properties. Cell studies showed that cav-1 deficiency leads to increased osteoblast differentiation. These results suggest that cav-1 helps to maintain osteoblast progenitors in a less differentiated state. Introduction:The absence of caveolin-1 in cellular membranes causes dysregulated signaling. To understand the role of the caveolar microdomain in bone homeostasis, we examined the skeletal phenotype of 5-and 8-wk-old cav-1 −/− mice. Materials and Methods: High-resolution CT imaging showed a region-specific effect of cav-1 deficiency on the skeleton. At 5 wk, cav-1 −/− mice had increased epiphyseal bone volume (+58.4%, p ‫ס‬ 0.05); at 8 wk, metaphyseal bone volume was increased by 77.4% (p ‫ס‬ 0.008). Cortical bone at the femoral mid-diaphysis showed that the periosteal area of cav-1 −/− mice significantly exceeded that of cav-1 +/+ mice by 23.9% and 16.3% at 5 and 8 wk, respectively, resulting in increased mechanical properties (I max : +38.2%, p ‫ס‬ 0.003 and I mi : +23.7%, p ‫ס‬ 0.03). Results: Histomorphometry complemented CT results showing increased bone formation rate (BFR) at trabecular and cortical sites at 5 wk, which supported findings of increased bone at 8 wk in cav-1 −/− mice. Formal mechanical testing of the femoral diaphysis confirmed increased bone structure: stiffness increased 33% and postyield deflection decreased 33%. Stromal cells from cav-1 −/− marrow showed a 23% increase in von Kossa-positive nodules; osteoclastogenesis was also modestly increased in cav-1-deficient marrow. Knockdown of cav-1 with siRNA in wildtype stromal cells increased alkaline phosphatase protein and expression of osterix and Runx2, consistent with osteoblast differentiation. Conclusions: These data suggest that cav-1 helps to maintain a less differentiated state of osteoblast progenitor cells, and the absence of cav-1 causes bone to mature more rapidly. Caveolin-1 may thus be a target for altering skeletal homeostasis.

Growth plate chondrocytes produce proteoglycan-rich type II collagen extracellular matrix (ECM). During cell maturation and hypertrophy, ECM is reorganized via a process regulated by 1α,25 (OH) 2 D 3 and involving matrix metalloproteinases (MMPs), including MMP-3and MMP-2. 1α,25 (OH) 2 D 3 regulates MMP incorporation into matrix vesicles (MVs), where they are stored until released. Like plasma membranes (PM), MVs contain the 1α,25(OH) 2 D 3 -binding protein ERp60, phospholipase A 2 (PLA 2 ), and caveolin-1, but lack nuclear vitamin D receptors (VDRs). Chondrocytes produce 1α,25(OH) 2 D 3 (10 −8 M), which binds ERp60, activating PLA 2 , and resulting lysophospholipids lead MV membrane disorganization, releasing active MMPs. MV MMP-3 activates TGF-β1 stored in the ECM as large latent TGF-β1 complexes, consisting of latent TGF-β1 binding protein, latency associated peptide, and latent TGF-β1. Others have shown that MMP-2 specifically activates TGF-β2. TGF-β1 regulates 1α,25(OH) 2 D 3 -production, providing a mechanism for local control of growth factor activation. 1α,25(OH) 2 D 3 activates PKCα in the PM via ERp60-signaling through PLA 2 , lysophospholipid production, and PLCβ. It also regulates distribution of phospholipids and PKC isoforms between MVs and PMs, enriching the MVs in PKCζ. Direct activation of MMP-3 in MVs requires ERp60. However, when MVs are treated with 1α,25 (OH) 2 D 3 , PKCζ activity is decreased and PKCα is unaffected, suggesting a more complex feedback mechanism, potentially involving MV lipid signaling.

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