Original Article Perlecan/Hspg2 Deficiency Alters the Pericellular Space of the Lacuno-Canalicular System Surrounding Osteocytic Processes in Cortical Bone

words/characters: 226/1,782; Manuscript words/characters: 8,118/55,174; Figures (black and white): 3; Figures (color): 3; Tables: 1 Conflict of Interest: All authors have no conflicts of interest. Initial Date Submitted April 30, 2010; Date Revision Submitted July 9, 2010; Date Final Disposition Set August 24, 2010 Journal of Bone and Mineral Research © 2010 American Society for Bone and Mineral Research DOI 10.1002/jbmr.236 Abstract Osteocytes project long, slender processes throughout the mineralized matrix of bone where they connect and communicate with effector cells. The interconnected cellular projections form the functional lacuno-canalicular system allowing fluid to pass for cell to cell communication and nutrient and waste exchange. Prevention of mineralization in the pericellular space of the lacunocanalicular pericellular space is crucial for uninhibited interstitial fluid movement. Factors contributing to the ability of the pericellular space of the lacuno-canalicular system to remain open and unmineralized are unclear. Immunofluorescence and immunogold localization by transmission electron microscopy demonstrated perlecan/HSPG2 signal localized to the osteocyte lacuno-canalicular system of cortical bone and this proteoglycan was found in the pericellular space of the lacunocanalicular system. In this study we examined osteocyte lacuno-canalicular morphology in mice deficient in the large heparan sulfate proteoglycan perlecan/HSPG2 in this tissue. Ultrastructural measurements with electron microscopy of perlecan/HSPG2 deficient mice demonstrated diminished osteocyte canalicular pericellular area, resulting from a reduction in the total canalicular area. Additionally, perlecan/HSPG2 deficient mice showed decreased canalicular density and a reduced number of transverse tethering elements per canaliculus. These data indicated that perlecan/HSPG2 contributed to the integrity of the osteocyte lacunocanalicular system by maintaining the size of the pericellular space, an essential task to promote uninhibited interstitial fluid movement in this mechanosensitive environment. This work thus identified a new barrier function for perlecan/HSPG2 in murine cortical bone.Osteocytes project long, slender processes throughout the mineralized matrix of bone where they connect and communicate with effector cells. The interconnected cellular projections form the functional lacuno-canalicular system allowing fluid to pass for cell to cell communication and nutrient and waste exchange. Prevention of mineralization in the pericellular space of the lacunocanalicular pericellular space is crucial for uninhibited interstitial fluid movement. Factors contributing to the ability of the pericellular space of the lacuno-canalicular system to remain open and unmineralized are unclear. Immunofluorescence and immunogold localization by transmission electron microscopy demonstrated perlecan/HSPG2 signal localized to the osteocyte lacuno-canalicular system of cortical bone and this proteoglycan was found in the pericellular space of the lacunocanalicular system. In this study we examined osteocyte lacuno-canalicular morphology in mice deficient in the large heparan sulfate proteoglycan perlecan/HSPG2 in this tissue. Ultrastructural measurements with electron microscopy of perlecan/HSPG2 deficient mice demonstrated diminished osteocyte canalicular pericellular area, resulting from a reduction in the total canalicular area. Additionally, perlecan/HSPG2 deficient mice showed decreased canalicular density and a reduced number of transverse tethering elements per canaliculus. These data indicated that perlecan/HSPG2 contributed to the integrity of the osteocyte lacunocanalicular system by maintaining the size of the pericellular space, an essential task to promote uninhibited interstitial fluid movement in this mechanosensitive environment. This work thus identified a new barrier function for perlecan/HSPG2 in murine cortical bone.