Osteocytes are critical to the maintenance of tissue quality and mechanical integrity of bone. As the primary mechanosensing cells, osteocytes orchestrate bone's adaptation processes under mechanical cues such as load-induced fluid flow. However, the in vivo mechanisms by which osteocytes, deeply embedded in mineralized matrix, detect and transduce mechanical signals remain elusive. Filling this knowledge gap is essential to the development of new osteoporosis treatments that exploit bone's intrinsic sensitivity to mechanical loading (a potent anabolic factor). Recent studies have found a fibrous pericellular matrix (PCM) that spans the entire fluid annulus (~80nm) within the lacunar-canalicular system (LCS) and tethers the cell processes to the canalicular wall matrix. Evidence increasingly suggests that these PCM tethering fibers act as mechanical sensors, capturing fluid drag force and initiating mechanotransduction cascades in osteocytes. However, rigorous testing of this concept has been hindered by a lack of quantitative tools for measuring the PCM ultrastructure and by the scarcity of data regarding PCM composition. Breakthroughs from our previous award cycle have overcome these barriers, allowing us to precisely define the functional roles of the PCM in bone. First, we invented a tracer velocimetry approach based on fluorescence recovery after photobleaching (FRAP) to quantify osteocytic PCM in intact bone. Second, we identified perlecan/HSPG2, a large heparan sulfate (HS) proteoglycan, to be an essential structural component of the PCM. Using a perlecan-deficient mouse model that mimics human Schwartz-Jampel syndrome (SJS) we discovered that perlecan deficiency results in not only decreased PCM fiber density but also attenuated responses to in vivo loading and unloading. These preliminary studies formed the cornerstone of our hypothesis that the osteocytic PCM regulates bone's adaptation to mechanical cues through mechanosensing in the LCS, which will be tested at the tissue, cellular, and molecular levels in the following three specific aims: 1) Quantify the effects of PCM alterations on bone adaptation to mechanical cues in vivo; 2) Quantify the effects of PCM alterations on osteocyte mechanosensing ex vivo; 3) Determine the mechanisms by which PCM perlecan forms functional mechanosensing tethers in the LCS in vitro. The proposed studies are important because PCM is the critical interface between osteocytes and the extracellular environment. Identifying the functional roles of the osteocytic PCM and one of its major components, perlecan, in bone adaptation could lead to the development of new osteoporosis treatments that exploit bone's intrinsic sensitivity to mechanical stimuli, a potent non- pharmaceutical factor in promoting bone formation. These studies will also advance our knowledge of the fundamental functions of the PCM, a uniquely functioning but overlooked structure found in nearly all mammalian cells including osteocytes.

Public Health Relevance

The proposal aims to elucidate the functional roles of osteocyte pericellular matrix (PCM) and one of its major components (perlecan) in bone (re)modeling. The work is important because PCM, as the critical interface between bone osteocytes and the extracellular environment, controls how bone perceives and responds to mechanical loading, which is one of the most potent anabolic factors in promoting bone formation. These proposed studies will not only guide the development of novel strategies such as patient-specific exercise regimens and new interventions for the treatment of osteoporosis, but also advance the fundamental knowledge on the PCM, a uniquely functioning but overlooked structure found in nearly all mammalian cells, in skeletal mechanobiology and certain pathologies.

Agency
National Institute of Health (NIH)
Institute
National Institute of Arthritis and Musculoskeletal and Skin Diseases (NIAMS)
Type
Research Project (R01)
Project #
2R01AR054385-07A1
Application #
8887424
Study Section
Skeletal Biology Structure and Regeneration Study Section (SBSR)
Program Officer
Alekel, D Lee
Project Start
2006-12-01
Project End
2020-03-31
Budget Start
2015-04-01
Budget End
2016-03-31
Support Year
7
Fiscal Year
2015
Total Cost
$410,853
Indirect Cost
$115,216
Name
University of Delaware
Department
Engineering (All Types)
Type
Schools of Engineering
DUNS #
059007500
City
Newark
State
DE
Country
United States
Zip Code
19716
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Lv, Mengxi; Zhou, Yilu; Chen, Xingyu et al. (2018) Calcium signaling of in situ chondrocytes in articular cartilage under compressive loading: Roles of calcium sources and cell membrane ion channels. J Orthop Res 36:730-738
Shoga, Janty S; Graham, Brian T; Wang, Liyun et al. (2017) Direct Quantification of Solute Diffusivity in Agarose and Articular Cartilage Using Correlation Spectroscopy. Ann Biomed Eng 45:2461-2474
Chiu, Yu-Chieh; Fong, Eliza L; Grindel, Brian J et al. (2016) Sustained delivery of recombinant human bone morphogenetic protein-2 from perlecan domain I - functionalized electrospun poly (?-caprolactone) scaffolds for bone regeneration. J Exp Orthop 3:25
Fan, Lixia; Pei, Shaopeng; Lucas Lu, X et al. (2016) A multiscale 3D finite element analysis of fluid/solute transport in mechanically loaded bone. Bone Res 4:16032
Wijeratne, Sithara S; Martinez, Jerahme R; Grindel, Brian J et al. (2016) Single molecule force measurements of perlecan/HSPG2: A key component of the osteocyte pericellular matrix. Matrix Biol 50:27-38
Srinivasan, Padma P; Parajuli, Ashutosh; Price, Christopher et al. (2015) Inhibition of T-Type Voltage Sensitive Calcium Channel Reduces Load-Induced OA in Mice and Suppresses the Catabolic Effect of Bone Mechanical Stress on Chondrocytes. PLoS One 10:e0127290

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