We have identified a novel missense mutation in a pseudoachondroplasia (PSACH) patient in one of the type III repeats of cartilage oligomeric matrix protein (COMP). Enlarged lamellar rough endoplasmic reticulum vesicles were shown to contain accumulated COMP along with type IX collagen, a cartilage-specific component. COMP was secreted and assembled normally into the extracellular matrix of tendon, demonstrating that the accumulation of COMP in chondrocytes was a cellspecific phenomenon. We believe that the intracellular storage of COMP causes a nonspecific aggregation of cartilage-specific molecules and results in a cartilage matrix deficient in required structural components leading to impaired cartilage growth and maintenance. These data support a common pathogenetic mechanism behind two clinically related chondrodysplasias, PSACH and multiple epiphyseal dysplasia. Mutations in COMP,1 cartilage oligomeric matrix protein, have been associated with a bone dysplasia family that includes pseudoachondrodysplasia (PSACH) and multiple epiphyseal dysplasia (MED). PSACH and MED (EDM1) were localized to chromosome 19p13.1, the region that contains the gene for COMP (1-4), and specific base substitutions, deletions, and duplications have been subsequently identified (5, 6). Both PSACH and MED are inherited as dominant disorders.The clinical features of the two diseases are similar and can range from mild to severe forms (7). PSACH is characterized by a disproportionate short stature and joint laxity with a waddling gait that appears with the onset of walking. MED patients generally present with mild short stature and hip pain later in childhood. The radiological abnormalities of MED are restricted to the epiphyses whereas there is additional involvement of metaphyses and the spinal column in patients with PSACH. Patients with both disorders manifest symptoms of precocious osteoarthritis. The diseases appear to be allelic variants resulting from different mutations in the same gene.Another extracellular matrix component of cartilage, type IX collagen, has also been implicated in MED. This form of MED (EDM2) affects primarily the growth centers of the knees and has been mapped to chromosome 1p32 (8) in a family that had previously shown no linkage to COMP (chromosome 19p13). COL9A2 was also mapped to this locus (9). The major complaint of this MED family was early onset osteoarthritis, especially in the knees. Only a few individuals exhibited mild short stature, distinguishing this family from others with MED. Recently, a mutation in the ␣2 chain of type IX collagen was identified in this large family (10). It is not clear how mutations in these two genes result in a similar phenotype.COMP is the fifth member of the thrombospondin (TSP) family of extracellular matrix glycoproteins (3). It was originally discovered in cartilage extracts and has been immunolocalized to developing as well as mature cartilage (11,12). COMP has also been found within and around tendon bundles (13). Rotary shadow imaging shows the molecule to b...

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It is widely accepted that vitreous humor-derived FGFs are required for the differentiation of anterior lens epithelial cells into crystallin-rich fibers. We show that BMP 2, 4, and 7 can induce the expression of markers of fiber differentiation in primary lens cell cultures to an extent equivalent to FGF or medium conditioned by intact vitreous bodies (VBCM). Abolishing BMP 2/4/7 signaling with noggin inhibited VBCM from upregulating fiber marker expression. Remarkably, noggin and anti-BMP antibodies also prevented purified FGF (but not unrelated stimuli) from upregulating the same fiber-specific proteins. This effect is attributable to inhibition of BMPs produced by the lens cells themselves. Although BMP signaling is required for FGF to enhance fiber differentiation, the converse is not true. Expression of noggin in the lenses of transgenic mice resulted in a postnatal block of epithelial-to-secondary fiber differentiation, with extension of the epithelial monolayer to the posterior pole of the organ. These results reveal the central importance of BMP in secondary fiber formation and show that although FGF may be necessary for this process, it is not sufficient. Differentiation of fiber cells, and thus proper vision, is dependent on cross-talk between the FGF and BMP signaling pathways.

The chicken FK506-binding protein FKBP65, a peptidylprolyl cis-trans isomerase, is a rough endoplasmic reticulum protein that contains four domains homologous to FKBP13, another rough endoplasmic reticulum PPIase. Analytical ultracentrifugation suggests that in FKBP65 these four domains are arranged in a linear extended structure with a length of about 26 nm and a diameter of about 3 nm. All four domains are therefore expected to be accessible to substrates. The specificity of FKBP65 towards a number of peptide substrates was determined. The specific activity of FKBP65 is generally lower than that of FKBP12 when expressed as a per domain activity. The substrate specificity of FKBP65 also differs from that of FKBP12. Inhibition studies show that only one of the four domains can be inhibited by FK506, a powerful inhibitor of all other known FKBPs. Furthermore, the same domain seems to be susceptible to inhibition by cyclosporin A. No other FKBPs were shown to be inhibited by cyclosporin A. It is also shown that FKBP65 can catalyse the re-folding of type III collagen in vitro with a kcat/Km = 4.3 x 10(3) M-1.s-1.

TGFβ signaling is linked to posterior capsule opacification (PCO), the most common complication of cataract surgery. TGFβ can induce primary lens epithelial cells to differentiate into two disparate, PCO-causing abnormal cell phenotypes, a variation of the TGFβ paradox. Analysis of the signaling pathways downstream of TGFβ reveals novel therapeutic targets for PCO.