IKK

Cells are colour rendered; nuclei are shown in purple, distended rER in blue and primary cilia in red

Cells are colour rendered; nuclei are shown in purple, distended rER in blue and primary cilia in red. unfolded protein response. Eventually, there is reduced proliferation and spatially dysregulated apoptosis of chondrocytes in the cartilage growth plate, which is likely to be the cause of disrupted linear bone growth and the resulting short-limbed dwarfism in the mutant mice. INTRODUCTION Chondrocyte proliferation and differentiation within the cartilage growth plate of endochondral bones is the driving force behind longitudinal bone growth. For example, chondrocyte proliferation and matrix deposition in the growth plate is the basis of long bone growth, while apoptosis of hypertrophic chondrocytes plays a pivotal role in the transition from chondrogenesis to osteogenesis (1). The regulation Rabbit Polyclonal to PAK5/6 (phospho-Ser602/Ser560) and control of chondrocyte proliferation, hypertrophy and apoptosis in the relevant zones of the growth plate is therefore critical for normal bone growth (2,3) and any disruption to the balance between proliferation and hypertrophy can lead to skeletal defects and in particular the chondrodysplasias (4). The chondrodysplasias are a clinically and genetically heterogeneous group Amygdalin of diseases that affect the development of the skeleton (5). There are over 200 different phenotypes, which range in severity from relatively mild to severe and lethal forms. Although individually rare, as a group of diseases the chondrodysplasias have an overall incidence of at least 1 per 4000 and result in a significant healthcare responsibility. Many of the individual phenotypes have been grouped into bone dysplasia families on the basis of a similar clinical and radiographic presentation and members of the same family are postulated to share common disease mechanisms (6). Pseudoachondroplasia (PSACH) and multiple epiphyseal dysplasia (MED) are a family of autosomal dominant skeletal dysplasias, which share common Amygdalin phenotypic characteristics but encompass a wide spectrum of severity, ranging from severe to mild phenotypes, respectively (7,8). PSACH results exclusively from mutations in the gene encoding cartilage oligomeric matrix protein (COMP) (9), the fifth member of the thrombospondin protein family. Some forms of MED are allelic with PSACH and also result from mutations, however, MED is genetically heterogeneous and can also result from mutations in the genes encoding matrilin-3 (and (18,19). All of the MED mutations identified in are missense mutations which primarily affect conserved residues that comprise the -sheet of the single A-domain of matrilin-3 (20-22); although a single missense mutation has also been identified in the -1 helix of the A-domain (23). Interestingly, a missense mutation (p.Thr303Met) in the first EGF-domain of matrilin-3 has been implicated in susceptibility to hand osteoarthritis (24,25) and spinal disc degeneration (26) suggesting a role for mutations in the pathophysiology of more common cartilage-related diseases (27). Previous studies of mutations using transfected cells as an assay have suggested that mutant matrilin-3 is retained within the rough endoplasmic reticulum (rER) of chondrocytes (22,28), where it exists as an unfolded intermediate and is associated with ERp72 (22), a chaperone protein known to be involved in mediating disulphide bond formation (29). Although data is very limited, due primarily to the scarcity of relevant samples, the microscopic analysis of an iliac crest biopsy from a 10-year old boy with MED, caused by a p.Arg121Trp mutation in matrilin-3, shows evidence of enlarged cisternae of rER due to the retention of matrilin-3 (22). In this context, the Amygdalin consequences of mutations on the trafficking of matrilin-3 appear similar to those caused by mutations in MED and PSACH (8). For example, mutations in the type III repeats of COMP have been shown to cause the retention of mutant COMP in enlarged cisternae of rER, along with the Amygdalin co-retention of other ECM molecules. This accumulation of protein has been proposed to elicit a cell stress response and results in an apparent increase in apoptosis (30-35). However, the over reliance on expression systems to study the effects of and mutations has meant that fundamental questions concerning the effect of mutant protein expression on chondrocyte proliferation and apoptosis in the growth plate have remained unresolved. Although a series of knock-out mice have been generated for COMP (36) and the matrilin family of ECM proteins, such as matrilin-3 (37), matrilin-1 (38,39) and matrilin-2 (40), all of these mice have apparently normal skeletal development. More recently, a second matrilin-3-deficient mouse strain was shown to have premature chondrocyte maturation, increased bone mineral density and osteoarthritis, but no chondrodysplasia (41). Overall, Amygdalin these data indicate a functional redundancy within the matrilin and thrombospondin families of proteins and strongly suggest that PSACH and MED are caused by dominant-negative (antimorphic) mechanisms..