Pannexin 3 Regulates Intracellular ATP/cAMP Levels and Promotes Chondrocyte Differentiation

Abstract

Cartilage plays an important role in mechanical load resistance and in skeletal structure support. It also serves as the skeletal template for endochondral ossification by which most bones in the body, such as long bones, are formed. In endochondral ossification, cartilage development is initiated by mesenchymal cell condensation, followed by a series of proliferation and differentiation processes. Cells undergoing condensation differentiate into chondrocytes, which then proliferate, produce type II collagen and form the proliferative zone of the cartilage molds. As development proceeds, chondrocytes in the center of the cartilage molds (prehypertrophic zone) cease proliferating and differentiate into type X collagen-producing hypertrophic chondrocytes to form the hypertrophic zone. Terminally differentiated hypertrophic chondrocytes mineralize the surrounding matrix. Eventually these cells die by apoptosis and are replaced by osteoblasts that form trabecular bone.The regulation of chondrocyte proliferation and differentiation must be tightly coordinated to allow formation of properly sized cartilage and bone (1). Parathyroid hormone-related peptide (PTHrP) 2 and parathyroid hormone (PTH) sustain chondrocyte proliferation and delay differentiation of the growth plate (2). PTHrP is expressed by perichondrial cells and chondrocytes in the upper region of growing cartilage. Mutant mice that are deficient in PTHrP (3), PTH (4), or its receptor (5) have short proliferative zones and accelerated chondrocyte differentiation, which results in abnormal endochondral bone formation. In contrast, mice that overexpress PTHrP have enlarged proliferative zones and delayed chondrocyte terminal differentiation (6). Humans with an activating mutation in the PTH/ PTHrP receptor develop Jansen metaphyseal chondrodysplasia, characterized by disorganization of the growth plates and delayed chondrocyte terminal differentiation (7). These results suggest that PTH/PTHrP signaling regulates skeletal development by promoting cell proliferation and inhibiting hypertrophic differentiation of chondrocytes.The binding of PTH/PTHrP to its receptor activates both G s and G q family heterotrimeric G proteins (8, 9). The activation of G s is necessary for cAMP production and protein kinase A (PKA) activation, which leads to phosphorylation of the cAMPresponse element-binding (CREB) family of transcription factors. CREB then induces genes such as the cyclin D1 and cyclin A genes. The activated cyclin/cyclin-dependent kinases in turn phosphorylate the retinoblastoma protein and its relative factors, which then dissociates the E2F transcription factor and subsequently activates the target genes necessary for DNA replication and cell cycle progression. Thus, CREB is a direct target of PKA and a downstream target of PTH/PTHrP/cAMP signaling and is required for chondrocyte proliferation (10, 11). How proliferation signaling is down-regulated in the prehypertrophic zone to stop proliferation and allow the switch to the postmitotic state is not well unde...

Pannexin channels are not gap junction hemichannels

“…6 Pannexin membrane channels have been shown to be formed by six (Panx1), 7 or eight (Panx2) monomers. 6 The oligomeric number of a Panx3 membrane channel [8][9][10] has not yet been determined, but the expectation is that it Figure 1. Pannexins are predicted to be tetra-spanning membrane proteins with both the amino and carboxy termini exposed to the cytoplasm.…”

Purinergic signalling in the musculoskeletal system

“…ATP transport from chondrocyte primary cilium was, at least in part, via connexin 43 hemichannels [302,332]. Release of ATP via pannexin 3 hemichannels has also been proposed [333]. The authors suggested than pannexin 3 functioned to switch chondrocyte cell fate from proliferation to differentiation by regulating the intracellular ATP/cAMP levels.…”

The cellular life of pannexins

“…15,35 Although Panx3 was found within intracellular compartments in rat cochlear bone, 46 it was also found at the cell surface and within intracellular compartments in murine chondrocytes and chondrogenic cell lines (ATDC5). 21 Panx2, on the other hand, has been localized primarily to intracellular compartments 9,38,39 and displays only limited cell surface distribution that is increased when coexpressed with Panx1. 9 In the developing dentate gyrus, Panx2 was found to be expressed in intracellular punctatelike structures of neural progenitor cells.…”