Skeletal Adaptation to Intramedullary Pressure-Induced Interstitial Fluid Flow Is Enhanced in Mice Subjected to Targeted Osteocyte Ablation

Abstract

Interstitial fluid flow (IFF) is a potent regulatory signal in bone. During mechanical loading, IFF is generated through two distinct mechanisms that result in spatially distinct flow profiles: poroelastic interactions within the lacunar-canalicular system, and intramedullary pressurization. While the former generates IFF primarily within the lacunar-canalicular network, the latter generates significant flow at the endosteal surface as well as within the tissue. This gives rise to the intriguing possibility that loading-induced IFF may differentially activate osteocytes or surface-residing cells depending on the generating mechanism, and that sensation of IFF generated via intramedullary pressurization may be mediated by a non-osteocytic bone cell population. To begin to explore this possibility, we used the Dmp1-HBEGF inducible osteocyte ablation mouse model and a microfluidic system for modulating intramedullary pressure (ImP) to assess whether structural adaptation to ImP-driven IFF is altered by partial osteocyte depletion. Canalicular convective velocities during pressurization were estimated through the use of fluorescence recovery after photobleaching and computational modeling. Following osteocyte ablation, transgenic mice exhibited severe losses in bone structure and altered responses to hindlimb suspension in a compartment-specific manner. In pressure-loaded limbs, transgenic mice displayed similar or significantly enhanced structural adaptation to Imp-driven IFF, particularly in the trabecular compartment, despite up to ∼50% of trabecular lacunae being uninhabited following ablation. Interestingly, regression analysis revealed relative gains in bone structure in pressure-loaded limbs were correlated with reductions in bone structure in unpressurized control limbs, suggesting that adaptation to ImP-driven IFF was potentiated by increases in osteoclastic activity and/or reductions in osteoblastic activity incurred independently of pressure loading. Collectively, these studies indicate that structural adaptation to ImP-driven IFF can proceed unimpeded following a significant depletion in osteocytes, consistent with the potential existence of a non-osteocytic bone cell population that senses ImP-driven IFF independently and potentially parallel to osteocytic sensation of poroelasticity-derived IFF.

Osteocyte Mechanobiology

“…According to this theory, whereas the calcified matrix is a mechanically rigid material, mechanical loads induces poro-elastic interactions, micro deformation and microstrain of the matrix (maximally on the order of 0.2%). These micro deformations drive the flow of interstitial fluid within the lacuna-canalicular spaces (11, 19). It has been calculated that the magnitude of pressure on an osteocyte in vivo is in the order of 5 Pa. Loading of long bones also increases the intramedullary cavity pressure and generates interstitial fluid flow (IFF) at the endosteal surface as well as within the lacuno-canalicular network (19).…”

Biomechanical forces in the skeleton and their relevance to bone metastasis: Biology and engineering considerations

“…Intramedullary pressurization of bone marrow can induce fluid flow [59, 60], and pressurization resulting from externally applied loads can peak as high as 130 Pa [61] and can generate shear stresses up to 5 Pa at the interface between marrow and bone tissue [62]. Importantly, such changes in intramedullary pressure can modulate cellular functions in the bone marrow independent of matrix strains [59, 63, 64], suggesting that breast cancer cells may similarly change their behavior.…”

Mechanically stimulated bone cells secrete paracrine factors that regulate osteoprogenitor recruitment, proliferation, and differentiation

“…Interestingly, bone anabolism in response to loading is maintained in mouse models with depleted osteocytes suggesting a potential coordination by another mechanosensitive cell [22].…”