Project Summary During pregnancy and lactation, the female skeleton undergoes significant bone loss and bone microstructure deteioration to provide calcium for fetal/infant growth. While weaning induces substantial bone recovery, reproduction-induced bone loss is only partially recovered after weaning. Our objective in the last funding cycle was to uncover protective mechanisms behind the lower fracture risk in women with histories of reproduction and lactation. Using a rat model, we demonstrated that structural adaptation of the maternal skeleton during pregnancy and lactation exerted a protective effect against estrogen deficiency-induced bone loss later in life. Moreover, we discovered enhanced responsiveness to external loading in the rat maternal bone both during lactation and later when subjected to estrogen deficiency by ovariectomy (OVX). During lactation, osteocytes (Ocys) are known to actively remodel their surrounding matrix via perilacunar-canalicular remodeling (PLR). Intriguingly, we also discovered enhanced Ocy PLR in the maternal bone post OVX, which appeared to be a result of “memory” or reactivation of Ocys’ PLR response during lactation. The activated PLR during lactation and post OVX leads to an enlarged lacunar-canalicular system (LCS) and an altered microenvironment of Ocys. Using a multiscale poroelastic model of the LCS, we further demonstrated that the PLR-induced alterations in the Ocy pericellular environment would amplify the mechanical and biochemical signal transduction to Ocys, which could in turn enhance mechanical adaptation of maternal bone to maintain its load-bearing function. These new and exciting findings provide a strong scientific premise for our central hypothesis that Ocy PLR-mediated skeletal adaptation increases Ocys’ mechano-sensing, which in turn enhances the mechano-response of maternal bone during lactation and post OVX. The objective of this renewal continues to be defining maternal bone adaptation mechanisms during challenging physiological events such as lactation and menopause. However, our focus advances from bone micro- and ultra-structural mechanisms (in the last funding cycle) to cellular mechanisms behind maternal bone adaptation and skeletal health (in this renewal). A cutting-edged imaging platform allows us to directly quantify mechano-sensitivity of the Ocy network by measuring in situ Ca2+ oscillations in mechanically loaded bones. In the Aim 1, we will establish the causal role of osteocyte PLR as an important mechanism to regulate bone mechano-sensitivity. In the Aim 2, by employing osteocyte fate mapping in a mouse model, we will for the first time interrogate the mechano-responses between osteocytes with and without exposure to prior lactation or lactation-associated hormonal changes within the same bone. The proposed research project will define a novel and critical function of ostecytes through PLR to regulate the balance between mineral resorption and mechanical integrity of th...