Differential response of osteocytes to tensile and compressive cues during loading, unloading, spaceflight and artificial gravity

Abstract

In bone, osteocytes are vital contributors to the processes of mechanosensation and mechanotransduction. Recently, there has been an increased interest in differential bone adaptation in response to different mechanical cues. To date, there are no reports about osteocytes' differential response to tension versus compression forces. We aimed to study the differential effects of tension and compression cues on osteocytes in vivo, in mice tibia at steady state and when subjected to different mechanical challenges. Areas in mice tibial midshaft subjected to tensile and compressive forces have been previously identified by utilizing finite element modeling. Mice were subjected to 1) mechanical loading, 2) unloading though hindlimb suspension, and 3) spaceflight and artificial gravity. Samples were analyzed using Backscatter Scanning Electron Microscopy (BSEM) imaging to visualize the morphological changes in osteocyte lacunae, as well as light microscopy to visualize the morphology of the Lacuno- Canalicular Network (LCN). Bulk RNA sequencing was performed to explore whole genome differential gene expression associated with the morphological changes. Depending upon whether the osteocytes were subjected to tension or compression, they behaved differently in terms of 1) peri-lacunar remodeling, 2) macro-remodeling and 3) their transcriptome; in which lacuna were lower in density and larger in size larger in compression cortices. The compression cortices were thicker and more porous than tension cortices, and the osteocytes’ transcriptome revealed differentially express neuronal genes that are involved in dendrite formation and regulation, ion transport genes, which regulate the ionic microenvironment, and Extra-Cellular Matrix (ECM) related genes, which regulate ECM resorption or formation.