Perlecan/HSPG2 deficiency enhances BMP2 effect on bone matrix mineralization
Date
2012
Authors
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Publisher
University of Delaware
Abstract
Perlecan/HSPG2 is a heparan-sulfate proteoglycan abundantly expressed in all basement membranes of the body and in the extracellular matrix of cartilage. It holds important regulatory functions during endochondral ossification, as is evidenced by the severe skeletal dysplasias that result from lethal null mutations of the Hspg2 gene. Specific perlecan gene mutations that result in reduced levels of functional protein expression are non-lethal in humans and mice, and result in Schwartz-Jampel syndrome (SJS) in humans. SJS patients exhibit a milder range of skeletal abnormalities including shortened long bones and disorganized growth plate. Perlecan expression begins in the growth plate at the onset of endochondral ossification, but drops completely upon mineralization of the developing bone matrix at the chondro-osseous junction. In perlecan-deficient mouse growth plates, the enlarged and disorganized growth plates have also been associated with abnormal matrix mineralization. Based on these observations, I hypothesize that perlecan’s presence in the developing bone serves as a modulator of osteogenesis to ensure endochondral ossification occurs correctly, and that it does this through the attenuation of growth factor signaling via heparin sulfate chains. When perlecan is absent or deficient in the matrix, enhanced heparan-binding growth factor signaling is allowed to occur, resulting in widespread differentiation of osteoprogenitors and abnormal shape and progression of the growth plate. To test this hypothesis, I use an in vitro cell culture system of primary mouse embryonic fibroblasts (MEFs) derived from both WT and perlecan-deficient mice to determine if differentiation and mineralization potential are affected by perlecan deficiency. Perlecan-deficient MEFs were found to differentiate and mineralize, as visualized by alkaline phosphatase and von Kossa staining, respectively, both earlier and to a greater extent than their WT counterparts. Several key markers of mineralization were highly upregulated in perlecan-deficient cells relative to WT cells. Levels of osteocalcin, an important marker of bone mineralization, secreted by osteoblasts was also elevated in the media from perlecan-deficient vs. WT cells. Though much remains to be uncovered about the mechanism of how perlecan contributes to the maintenance of proper bone matrix structure, these studies clearly demonstrate a role for perlecan in the modulation of growth factor activity during osteogenesis.