Hormonal Regulation of Osteocyte Perilacunar and Canalicular Remodeling in the Hyp Mouse Model of X-Linked Hypophosphatemia

Abstract

Osteocytes remodel their surrounding perilacunar matrix and canalicular network to maintain skeletal homeostasis. Perilacunar/canalicular remodeling is also thought to play a role in determining bone quality. X-linked hypophosphatemia (XLH) is characterized by elevated serum fibroblast growth factor 23 (FGF23) levels, resulting in hypophosphatemia and decreased production of 1,25 dihydroxyvitamin D (1,25D). In addition to rickets and osteomalacia, long bones from mice with XLH (Hyp) have impaired whole-bone biomechanical integrity accompanied by increased osteocyte apoptosis. To address whether perilacunar/canalicular remodeling is altered in Hyp mice, histomorphometric analyses of tibia and 3D intravital microscopic analyses of calvaria were performed. These studies demonstrate that Hyp mice have larger osteocyte lacunae in both the tibia and calvaria, accompanied by enhanced osteocyte mRNA and protein expression of matrix metalloproteinase 13 (MMP13) and genes classically used by osteoclasts to resorb bone, such as cathepsin K (CTSK). Hyp mice also exhibit impaired canalicular organization, with a decrease in number and branching of canaliculi extending from tibial and calvarial lacunae. To determine whether improving mineral ion and hormone homeostasis attenuates the lacunocanalicular phenotype, Hyp mice were treated with 1,25D or FGF23 blocking antibody (FGF23Ab). Both therapies were shown to decrease osteocyte lacunar size and to improve canalicular organization in tibia and calvaria. 1,25D treatment of Hyp mice normalizes osteocyte expression of MMP13 and classic osteoclast markers, while FGF23Ab decreases expression of MMP13 and selected osteoclast markers. Taken together, these studies point to regulation of perilacunar/canalicular remodeling by physiologic stimuli including hypophosphatemia and 1,25D.

Increased FGF23 protects against detrimental cardio-renal consequences during elevated blood phosphate in CKD

“…Even with normal renal function, elevated FGF23 is known to negatively affect bone homeostasis, as underscored by the rachitic phenotype of patients with autosomal dominant hypophosphatemia (ADHR) (21) and X-linked hypophosphatemia (XLH) (58). Recent studies demonstrated that FGF23 neutralization using anti-FGF23 antibodies in the Hyp mouse model of XLH (59) or in XLH patients (60) was able to correct serum phosphorus levels to within the normal ranges and improve bone parameters. However, with compromised renal function, use of a neutralizing FGF23 antibody in a rat model of CKD led to the deaths of the animals (61).…”

Disrupted Protein Expression and Altered Proteolytic Events in Hypophosphatemic Dentin Can Be Rescued by Dentin Matrix Protein 1

“…The mechanism, however, by which the loss of one endopeptidase, PHEX, affects other proteolytic cascades is yet to be determined. Enhanced expression of cathepsin K and MMP13 have been localized to the Hyp mouse osteocyte niche (Tokarz et al, 2018). Cathepsin K is known to cleave collagens type I and II at multiple sites along the triple helix (McKleroy et al, 2013).…”

Ex Vivo Organ Cultures as Models to Study Bone Biology

“…(31) Moreover, osteocyte-enriched cultures have been used to study the effects of aging on osteocytic gene expression, (32) the role of Piezo on osteocyte mechanotransduction, (33) and the dysregulation of gene expression in osteocytes in X-linked hypophosphatemia. (34) This protocol can be used with bones isolated from WT and genetically modified mice. When testing treatments in ex vivo bone organ cultures, we recommend performing dose-and time-course experiments to determine the ideal conditions for your genes/proteins of interest.…”

“…Similarly, Stegen and colleagues employed ex vivo osteocyte‐enriched bone cultures to study the effects of oxygen in osteocytes and identified the oxygen‐sensing Phd2 as an important regulator of sclerostin and bone mass . Moreover, osteocyte‐enriched cultures have been used to study the effects of aging on osteocytic gene expression, the role of Piezo on osteocyte mechanotransduction, and the dysregulation of gene expression in osteocytes in X‐linked hypophosphatemia …”