Differential regulation of RGS-2 by constant and oscillating PTH concentrations

Engineering of a NIR-activable hydrogel-coated mesoporous bioactive glass scaffold with dual-mode parathyroid hormone derivative release property for angiogenesis and bone regeneration

Osteogenesis, osteoclastogenesis, and angiogenesis play crucial roles in bone regeneration. Parathyroid hormone (PTH), an FDA-approved drug with pro-osteogenic, pro-osteoclastogenic and proangiogenic capabilities, has been employed for clinical osteoporosis treatment through systemic intermittent administration. However, the successful application of PTH for local bone defect repair generally requires the incorporation and delivery by appropriate carriers. Though several scaffolds have been developed to deliver PTH, they suffer from the weaknesses such as uncontrollable PTH release, insufficient porous structure and low mechanical strength. Herein, a novel kind of NIR-activable scaffold (CBP/MBGS/PTHrP-2) with dual-mode PTHrP-2 (a PTH derivative) release capability is developed to synergistically promote osteogenesis and angiogenesis for high-efficacy bone regeneration, which is fabricated by integrating the PTHrP-2-loaded hierarchically mesoporous bioactive glass (MBG) into the N-hydroxymethylacrylamide-modified, photothermal agent-doped, poly(N-isopropylacrylamide)-based thermosensitive hydrogels through assembly process. Upon on/off NIR irradiation, the thermoresponsive hydrogel gating undergoes a reversible phase transition to allow the precise control of on-demand pulsatile and long-term slow release of PTHrP-2 from MBG mesopores. Such NIR-activated dual-mode delivery of PTHrP-2 by this scaffold enables a well-maintained PTHrP-2 concentration at the bone defect sites to continually stimulate vascularization and promote osteoblasts to facilitate and accelerate bone remodeling. In vivo experiments confirm the significant improvement of bone reparative effect on critical-size femoral defects of rats. This work paves an avenue for the development of novel dual-mode delivery systems for effective bone regeneration.

PTH and Vitamin D

PTH and Vitamin D are two major regulators of mineral metabolism. They play critical roles in the maintenance of calcium and phosphate homeostasis as well as the development and maintenance of bone health. PTH and Vitamin D form a tightly controlled feedback cycle, PTH being a major stimulator of vitamin D synthesis in the kidney while vitamin D exerts negative feedback on PTH secretion. The major function of PTH and major physiologic regulator is circulating ionized calcium. The effects of PTH on gut, kidney, and bone serve to maintain serum calcium within a tight range. PTH has a reciprocal effect on phosphate metabolism. In contrast, vitamin D has a stimulatory effect on both calcium and phosphate homeostasis, playing a key role in providing adequate mineral for normal bone formation. Both hormones act in concert with the more recently discovered FGF23 and klotho, hormones involved predominantly in phosphate metabolism, which also participate in this closely knit feedback circuit. Of great interest are recent studies demonstrating effects of both PTH and vitamin D on the cardiovascular system. Hyperparathyroidism and vitamin D deficiency have been implicated in a variety of cardiovascular disorders including hypertension, atherosclerosis, vascular calcification, and kidney failure. Both hormones have direct effects on the endothelium, heart, and other vascular structures. How these effects of PTH and vitamin D interface with the regulation of bone formation are the subject of intense investigation.

Role of Regulators of G Protein Signaling Proteins in Bone Physiology and Pathophysiology

Regulators of G protein signaling (RGS) proteins enhance the intrinsic GTPase activity of α subunits of the heterotrimeric G protein complex of G protein-coupled receptors (GPCRs) and thereby inactivate signal transduction initiated by GPCRs. The RGS family consists of nearly 37 members with a conserved RGS homology domain which is critical for their GTPase accelerating activity. RGS proteins are expressed in most tissues, including heart, lung, brain, kidney, and bone and play essential roles in many physiological and pathological processes. In skeletal development and bone homeostasis as well as in many bone disorders, RGS proteins control the functions of various GPCRs, including the parathyroid hormone receptor type 1 and calcium-sensing receptor and also regulate various critical signaling pathways, such as Wnt and calcium oscillations. This chapter will discuss the current findings on the roles of RGS proteins in regulating signaling of key GPCRs in skeletal development and bone homeostasis. We also will examine the current updates of RGS proteins' regulation of calcium oscillations in bone physiology and highlight the roles of RGS proteins in selected bone pathological disorders. Despite the recent advances in bone and mineral research, RGS proteins remain understudied in the skeletal system. Further understanding of the roles of RGS proteins in bone should not only provide great insights into the molecular basis of various bone diseases but also generate great therapeutic drug targets for many bone diseases.

Regulators of G-protein-signaling proteins: negative modulators of G-protein-coupled receptor signaling

Regulators of G-protein-signaling (RGS) proteins are a category of intracellular proteins that have an inhibitory effect on the intracellular signaling produced by G-protein-coupled receptors (GPCRs). RGS along with RGS-like proteins switch on through direct contact G-alpha subunits providing a variety of intracellular functions through intracellular signaling. RGS proteins have a common RGS domain that binds to G alpha. RGS proteins accelerate GTPase and thus enhance guanosine triphosphate hydrolysis through the alpha subunit of heterotrimeric G proteins. As a result, they inactivate the G protein and quickly turn off GPCR signaling thus terminating the resulting downstream signals. Activity and subcellular localization of RGS proteins can be changed through covalent molecular changes to the enzyme, differential gene splicing, and processing of the protein. Other roles of RGS proteins have shown them to not be solely committed to being inhibitors but behave more as modulators and integrators of signaling. RGS proteins modulate the duration and kinetics of slow calcium oscillations and rapid phototransduction and ion signaling events. In other cases, RGS proteins integrate G proteins with signaling pathways linked to such diverse cellular responses as cell growth and differentiation, cell motility, and intracellular trafficking. Human and animal studies have revealed that RGS proteins play a vital role in physiology and can be ideal targets for diseases such as those related to addiction where receptor signaling seems continuously switched on.

Parathyroid hormone (1-34) modulates odontoblast proliferation and apoptosis via PKA and PKC-dependent pathways

Parathyroid hormone (PTH) plays a key role in the development and homeostasis of mineralized tissues such as bone and dentine. We have reported that PTH (1-34) administration can increase dentine formation in mice and that this hormone modulates in vitro mineralization of odontoblast-like cells. The purpose of the present study was to investigate whether PTH (1-34) participates in the proliferative and apoptotic signaling of odontoblast-like cells (MDPC23). MDPC23 cells were exposed to 50 ng/ml hPTH (1-34) or vehicle for 1 (P1), 24 (P24), or 48 (P48) hours, and the cell proliferation, apoptosis, and cell number were evaluated. To examine whether changes in the proliferative and apoptotic signaling in response to PTH involve protein kinases A (PKA) and/or C (PKC), MDPC23 cells were exposed to PTH with or without PKC or PKA signaling pathway inhibitors. Overall, the results showed that the PKA pathway acts in response to PTH exposure maintaining levels of cell proliferation, while the PKC pathway is mainly involved for longer exposure to PTH (24 or 48 h), leading to the reduction of cell proliferation and increase of apoptosis. The exposure to PTH reduced the cell number in relation to the control group in a time-dependent manner. In conclusion, PTH modulates odontoblast-like cell proliferative and apoptotic response in a time-dependent manner. Both PKC and PKA pathways participate in PTH-induced modulation in an antagonist mode.

Role of regulator of G protein signaling proteins in bone

Regulators of G protein signaling (RGS) proteins are a family with more than 30 proteins that all contain an RGS domain. In the past decade, increasing evidence has indicated that RGS proteins play crucial roles in the regulation of G protein coupling receptors (GPCR), G proteins, and calcium signaling during cell proliferation, migration, and differentiation in a variety of tissues. In bone, those proteins modulate bone development and remodeling by influencing various signaling pathways such as GPCR-G protein signaling, Wnt, calcium oscillations and PTH. This review summarizes the recent advances in the understanding of the regulation of RGS gene expression, as well as the functions and mechanisms of RGS proteins, especially in regulating GPCR-G protein signaling, Wnt signaling, calcium oscillations signaling and PTH signaling during bone development and remodeling. This review also highlights the regulation of different RGS proteins in osteoblasts, chondrocytes and osteoclasts. The knowledge from the recent advances of RGS study summarized in the review would provide the insights into new therapies for bone diseases.

Interplay between local versus soluble transforming growth factor-beta and fibrin scaffolds: role of cells and impact on human mesenchymal stem cell chondrogenesis

Structural extracellular matrix molecules gain increasing attention as scaffolds for cartilage tissue engineering owing to their natural role as a growth factor repository. We recently observed that a collagen-type I/III (Col-I/III) matrix, human recombinant transforming growth factor-beta (TGF-β) protein, and fibrin hydrogel (FG) combined to a biphasic construct provided sufficient long-term TGF-β support to drive in vitro chondrogenesis of human mesenchymal stem cells (hMSC). Here we ask whether FG and Col-I/III can both retain TGF-β, describe the influence of cell seeding on TGF-β release, and compare the molecular path of hMSC chondrogenic differentiation under soluble versus local TGF-β supply. Release of growth factor from scaffolds augmented with increasing amounts of TGF-β was analyzed over 7 days and chondrogenesis was assessed over 42 days. Low TGF-β release rates from Col-I/III as opposed to higher release from FG indicated that both molecules retained TGF-β, with Col-I/III being the superior storage component. Cell seeding enhanced TGF-β retention in FG by about threefold and almost stopped release beyond 24 h. TGF-β remained bioactive and supported MSC chondrogenesis without impairing the amount of proteoglycan and collagen-type II deposition per cell and per construct compared to standard scaffold-free MSC pellets supplied with soluble TGF-β. Local TGF-β, however, mediated lower cell content, less collagen-type X relative to collagen-type II deposition and no matrix metalloproteinase-13 up-regulation. In conclusion, cells quickly halted release of local TGF-β from FG, turning FG and Col-I/III into attractive TGF-β repositories capable to drive full hMSC chondrogenesis, but via a modulated differentiation pathway. Since only part of the changes was reproduced by transient soluble TGF-β supply, release kinetics alone could not explain the molecular differences, suggesting that local TGF-β acts distinct from its soluble counterpart.