Constitutive protein kinase A activity in osteocytes and late osteoblasts produces an anabolic effect on bone

Abstract

Osteocytes have been implicated in the control of bone formation. However, the signal transduction pathways that regulate the biological function of osteocytes are poorly defined. Limited evidence suggests an important role for the G s /cAMP pathway in osteocyte function. In the present study, we explored the hypothesis that cAMP-dependent kinase A (PKA) activation in osteocytes plays a key role in controlling skeletal homeostasis. To test this hypothesis, we mated mice harboring a Cre-conditional, mutated PKA catalytic subunit allele that encodes a constitutively active form of PKA (CαR) with mice expressing Cre under the control of the osteocyte-specific promoter, DMP1. This allowed us to direct the expression of CαR to osteocytes in double transgenic progeny. Examination of Cre expression indicated that CαR was also expressed in late osteoblasts. Cortical and trabecular bone parameters from 12-week old mice were determined by μCT. Expression of CαR in osteocytes and late osteoblasts altered the shape of cortical bone proximal to the tibia-fibular junction (TFJ) and produced a significant increase in its size. In trabecular bone of the distal femur, fractional bone volume, trabecular number, and trabecular thickness were increased. These increases were partially the results of increased bone formation rates (BFRs) on the endosteal surface of the cortical bone proximal to the TFJ as well as increased BFR on the trabecular bone surface of the distal femur. Mice expressing CαR displayed a marked increase in the expression of osteoblast markers such as osterix, runx2, collagen 1α1, and alkaline phosphatase (ALP). Interestingly, expression of osteocyte marker gene, DMP1, was significantly up-regulated but the osteocyte number per bone area was not altered. Expression of SOST, a presumed target for PKA signaling in osteocytes, was significantly down-regulated in females. Importantly, no changes in bone resorption were detected. In summary, constitutive PKA signaling in osteocytes and late osteoblasts led to a small expansion of the size of the cortical bone proximal to the TFJ and an increase in trabecular bone in female mice. This was associated with down-regulation of SOST and up-regulation of several osteoblast marker genes. Activation of the PKA pathway in osteocytes and late osteoblasts is sufficient for the initiation of an anabolic skeletal response.

Connexin 43 Channels in Osteocytes Regulate Bone Responses to Mechanical Unloading

“…Nevertheless, our transgenic models with overexpression of dominant negative mutants offer a unique opportunity to dissect the specific involvement of the two types of channels by Cx43. Specifically, the 10 kb Dmp1 promoter has been extensively used to drive expression/deletion primarily in osteocytes (Kao et al, 2013;Kamiya et al, 2016). In previous immunohistochemical studies of Cx43 transgenic mice, we observed mutant Cx43 protein (GFP-labeled) only in osteocytes, not in other bone cell types.…”

The PTH-Gαs-Protein Kinase A Cascade Controls αNAC Localization To Regulate Bone Mass

“…More recently, increased trabecular bone mass and reduced Sost expression were reported in mice specifically expressing a constitutively active form of PKA (CA-PKA) in late osteoblasts and in osteocytes (50). A similar phenotype has been observed in transgenic mice expressing a constitutively active form of PTH1R (CA-PTH1R) in osteocytes (51,52).…”

Heterotrimeric G proteins in the control of parathyroid hormone actions

“…Accordingly, it has recently been shown that the increase in bone mass by constitutively active PTH1R depends on Gsα expression in the osteoblast lineage (Sinha, et al 2016). Moreover, in transgenic mice expressing a constitutively active PKA mutant in late osteoblasts and osteocytes, trabecular bone mass is increased and sclerostin expression is reduced (Kao, et al 2013). Expression of the same PKA mutant in mature osteoblasts also lead to increased bone mass with improved bone architecture and mechanical bone properties (Tascau, et al 2016).…”