Autocrine and Paracrine Regulation of the Murine Skeleton by Osteocyte-Derived Parathyroid Hormone-Related Protein

Abstract

Parathyroid hormone-related protein (PTHrP) and parathyroid hormone (PTH) have N-terminal domains that bind a common receptor, PTHR1. N-terminal PTH (teriparatide) and now a modified N-terminal PTHrP (abaloparatide) are US Food and Drug Administration (FDA)-approved therapies for osteoporosis. In physiology, PTHrP does not normally circulate at significant levels, but acts locally, and osteocytes, cells residing within the bone matrix, express both PTHrP and the PTHR1. Because PTHR1 in osteocytes is required for normal bone resorption, we determined how osteocyte-derived PTHrP influences the skeleton. We observed that adult mice with low PTHrP in osteocytes (targeted with the Dmp1(10kb)-Cre) have low trabecular bone volume and osteoblast numbers, but osteoclast numbers were unaffected. In addition, bone size was normal, but cortical bone strength was impaired. Osteocytederived PTHrP therefore stimulates bone formation and bone matrix strength, but is not required for normal osteoclastogenesis. PTHrP knockdown and overexpression studies in cultured osteocytes indicate that osteocyte-secreted PTHrP regulates their expression of genes involved in matrix mineralization. We determined that osteocytes secrete full-length PTHrP with no evidence for secretion of lower molecular weight forms containing the N-terminus. We conclude that osteocyte-derived full-length PTHrP acts through both PTHR1 receptor-mediated and receptor-independent actions in a paracrine/autocrine manner to stimulate bone formation and to modify adult cortical bone strength.

Parathyroid Hormone Signaling in Osteocytes

“…Recent genetic evidence has confirmed a role for PTHrP expression in 9.6 kB DMP1‐Cre‐expressing cells in physiologic bone remodeling . Mice in which PTHrP (encoded by the PTHLH gene) is deleted using this Cre driver show low cancellous bone mass, reduced osteoblast activity, high local sclerostin levels, and reduced cortical bone strength.…”

Adolescent Inhalant Abuse Results in Adrenal Dysfunction and a Hypermetabolic Phenotype with Persistent Growth Impairments

“…Thresholds were set at a lower threshold of 0.632 mg/cm 3 CaHA for trabecular bone and 0.825 mg/cm 3 CaHA for cortical bone, based on multilevel Otsu thresholding of the entire dataset. Tissue mineral density was quantified as described [25]. To determine bone mechanical properties at the end of the abstinence period (air n = 10, pair-fed n = 10, CIT n = 12), 3-point bending was conducted as described [24] (see online Supplementary File 2).…”

TGFβ Inhibition Stimulates Collagen Maturation to Enhance Bone Repair and Fracture Resistance in a Murine Myeloma Model

“…Histomorphometric analysis of osteoclast and osteoblast numbers and surfaces on endocortical bone were measured using Osteomeasure (OsteoMetrics, Atlanta, GA, USA) using standard guidelines . Immunohistochemistry was performed on paraffin‐embedded tibial sections using a rabbit anti‐PTHrP (R87) antibody (1:5000) specific to the amino terminus of PTHrP (1‐34) or a rabbit anti‐VEGFA antibody (Abcam, Cambridge, UK; ab46154, 1:1000) as previously described . Contralateral tibiae were embedded in LR White medium resin (TAAB Laboratories, Aldermaston, UK) and 10 μm sections were used for dynamic histomorphometry analysis of alizarin/calcein labels .…”

“…BV/TV = bone volume/tissue volume; Tb.N = trabecular number; Tb.Th = trabecular thickness; N.Oc/Ec.S = osteoclast number/endocortical surface; Oc.S/Ec.S = osteoclast surface/endocortical surface; N.Ob/Ec.S = osteoblast number/endocortical surface; Ob.S/Ec.S = osteoblast surface/endocortical surface. performed on paraffin-embedded tibial sections using a rabbit anti-PTHrP (R87) antibody (49) (1:5000) specific to the amino terminus of PTHrP (1-34) or a rabbit anti-VEGFA antibody (Abcam, Cambridge, UK; ab46154, 1:1000) as previously described. (49,50) Contralateral tibiae were embedded in LR White medium resin (TAAB Laboratories, Aldermaston, UK) and 10 μm sections were used for dynamic histomorphometry analysis of alizarin/calcein labels.…”

“…performed on paraffin-embedded tibial sections using a rabbit anti-PTHrP (R87) antibody (49) (1:5000) specific to the amino terminus of PTHrP (1-34) or a rabbit anti-VEGFA antibody (Abcam, Cambridge, UK; ab46154, 1:1000) as previously described. (49,50) Contralateral tibiae were embedded in LR White medium resin (TAAB Laboratories, Aldermaston, UK) and 10 μm sections were used for dynamic histomorphometry analysis of alizarin/calcein labels. (47) Serum bone turnover markers TRAP5b (Oxford Biosystems, Oxford, UK) and P1NP (Immuno Diagnostic Systems, Tyne & Wear, UK) were measured by ELISA.…”