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1.  Absence of FKBP10 in Recessive Type XI Osteogenesis Imperfecta Leads to Diminished Collagen Cross-Linking and Reduced Collagen Deposition in Extracellular Matrix 
Human mutation  2012;33(11):1589-1598.
Recessive osteogenesis imperfecta (OI) is caused by defects in genes whose products interact with type I collagen for modification and/or folding. We identified a Palestinian pedigree with moderate and lethal forms of recessive OI caused by mutations in FKBP10 or PPIB, which encode endoplasmic reticulum resident chaperone/isomerases FKBP65 and CyPB, respectively. In one pedigree branch, both parents carry a deletion in PPIB (c.563_566delACAG), causing lethal type IX OI in their two children. In another branch, a child with moderate type XI OI has a homozygous FKBP10 mutation (c.1271_1272delCCinsA). Proband FKBP10 transcripts are 4% of control and FKBP65 protein is absent from proband cells. Proband collagen electrophoresis reveals slight band broadening, compatible with ≈10% overmodification. Normal chain incorporation, helix folding, and collagen Tm support a minimal general collagen chaperone role for FKBP65. However, there is a dramatic decrease in collagen deposited in culture despite normal collagen secretion. Mass spectrometry reveals absence of hydroxylation of the collagen telopeptide lysine involved in cross-linking, suggesting that FKBP65 is required for lysyl hydroxylase activity or access to type I collagen telopeptide lysines, perhaps through its function as a peptidylprolyl isomerase. Proband collagen to organics ratio in matrix is approximately 30% of normal in Raman spectra. Immunofluorescence shows sparse, disorganized collagen fibrils in proband matrix.
PMCID: PMC3470738  PMID: 22718341
osteogenesis imperfecta; Bruck syndrome; FKBP65; FKBP10; PPIB; peptidylprolyl isomerase
2.  Deficient degradation of homotrimeric type I collagen,α1(I)3 glomerulopathy in oim mice 
Molecular genetics and metabolism  2011;104(3):373-382.
Col1a2-deficient (oim) mice synthesize homotrimeric type I collagen due to nonfunctional proα2(I) collagen chains. Our previous studies revealed a postnatal, progressive type I collagen glomerulopathy in this mouse model, but the mechanism of the sclerotic collagen accumulation within the renal mesangium remains unclear. The recent demonstration of the resistance of homotrimeric type I collagen to cleavage by matrix metalloproteinases (MMPs), led us to investigate the role of MMP-resistance in the glomerulosclerosis of Col1a2-deficient mice. We measured the pre- and post-translational expression of type I collagen and MMPs in glomeruli from heterozygous and homozygous animals. Both the heterotrimeric and homotrimeric isotypes of type I collagen were equally present in whole kidneys of heterozygous mice by immunohistochemistry and biochemical analysis, but the sclerotic glomerular collagen was at least 95–98% homotrimeric, suggesting homotrimeric type I collagen is the pathogenic isotype of type I collagen in glomerular disease. Although steady-state MMP and Col1a1 mRNA levels increased with the disease progression, we found these changes to be a secondary response to the deficient clearance of MMP-resistant homotrimers. Increased renal MMP expression was not sufficient to prevent homotrimeric type I collagen accumulation.
PMCID: PMC3205245  PMID: 21855382
collagen; extracellular matrix; glomerular sclerosis; fibrosis; matrix metalloproteinase
3.  COL1 C-propeptide Cleavage Site Mutations Cause High Bone Mass Osteogenesis Imperfecta 
Human mutation  2011;32(6):598-609.
Osteogenesis imperfecta (OI) is most often caused by mutations in the type I procollagen genes (COL1A1/COL1A2). We identified two children with substitutions in the type I procollagen C-propeptide cleavage site, which disrupt a unique processing step in collagen maturation and define a novel phenotype within OI. The patients have mild OI caused by mutations in COL1A1 (Patient 1: p.Asp1219Asn) or COL1A2 (Patient 2: p.Ala1119Thr), respectively. Patient 1 L1-L4 DXA z-score was +3.9 and pQCT vBMD was +3.1; Patient 2 had L1-L4 DXA z-score of 0.0 and pQCT vBMD of −1.8. Patient BMD contrasts with radiographic osteopenia and histomorphometry without osteosclerosis. Mutant procollagen processing is impaired in pericellular and in vitro assays. Patient dermal collagen fibrils have irregular borders. Incorporation of pC-collagen into matrix leads to increased bone mineralization. FT-IR imaging confirms elevated mineral/matrix ratios in both patients, along with increased collagen maturation in trabecular bone, compared to normal or OI controls. Bone mineralization density distribution revealed a marked shift toward increased mineralization density for both patients. Patient 1 has areas of higher and lower bone mineralization than controls; Patient 2’s bone matrix has a mineral content exceeding even classical OI bone. These patients define a new phenotype of high BMD OI and demonstrate that procollagen C-propeptide cleavage is crucial to normal bone mineralization.
PMCID: PMC3103631  PMID: 21344539
Osteogenesis imperfecta; C-propeptide; collagen; C-proteinase; mineralization; high bone mass
4.  Chaperoning osteogenesis: new protein-folding-disease paradigms 
Trends in cell biology  2010;21(3):168-176.
Recent discoveries of severe bone disorders in patients with deficiencies in several endoplasmic reticulum chaperones are reshaping the discussion of type I collagen folding and related diseases. Type I collagen is the most abundant protein in all vertebrates and a crucial structural molecule for bone and other connective tissues. Its misfolding causes bone fragility, skeletal deformities and other tissue failures. Studies of newly discovered bone disorders indicate that collagen folding, chaperones involved in the folding process, cellular responses to misfolding, and related bone pathologies may not follow conventional protein folding paradigms. In this review, we examine the features that distinguish collagen folding from that of other proteins and describe findings that are beginning to reveal how cells manage collagen folding and misfolding. We discuss implications of these studies on general protein folding paradigms, unfolded protein response in cells and protein folding diseases.
PMCID: PMC3057343  PMID: 21183349
5.  Lack of Cyclophilin B in Osteogenesis Imperfecta with Normal Collagen Folding 
The New England journal of medicine  2010;362(6):521-528.
Osteogenesis imperfecta is a heritable disorder that causes bone fragility. Mutations in type I collagen result in autosomal dominant osteogenesis imperfecta, whereas mutations in either of two components of the collagen prolyl 3-hydroxylation complex (cartilage-associated protein [CRTAP] and prolyl 3-hydroxylase 1 [P3H1]) cause autosomal recessive osteogenesis imperfecta with rhizomelia (shortening of proximal segments of upper and lower limbs) and delayed collagen folding. We identified two siblings who had recessive osteogenesis imperfecta without rhizomelia. They had a homozygous start-codon mutation in the peptidyl-prolyl isomerase B gene (PPIB), which results in a lack of cyclophilin B (CyPB), the third component of the complex. The proband’s collagen had normal collagen folding and normal prolyl 3-hydroxylation, suggesting that CyPB is not the exclusive peptidyl-prolyl cis–trans isomerase that catalyzes the rate-limiting step in collagen folding, as is currently thought.
PMCID: PMC3156560  PMID: 20089953
6.  Carcinomas contain an MMP-resistant isoform of type I collagen exerting selective support to invasion 
Cancer research  2010;70(11):4366-4374.
Collagen fibers affect metastasis in two opposing ways, by supporting invasive cells but also generating a barrier to invasion. We hypothesized that these functions might be performed by different isoforms of type I collagen. Carcinomas are reported to contain α1(I)3 homotrimers, a type I collagen isoform normally not present in healthy tissues, but the role of the homotrimers in cancer pathophysiology is unclear. In this study, we found that these homotrimers were resistant to all collagenolytic matrix metalloproteinases (MMPs). MMPs are massively produced and utilized by cancer cells and cancer-associated fibroblasts for degrading stromal collagen at the leading edge of tumor invasion. The MMP-resistant homotrimers were produced by all invasive cancer cell lines tested, both in culture and in tumor xenografts, but they were not produced by cancer-associated fibroblasts, thereby comprising a specialized fraction of tumor collagen. We observed the homotrimer fibers to be resistant to pericellular degradation, even upon stimulation of the cells with pro-inflammatory cytokines. Further, we confirmed an enhanced proliferation and migration of invasive cancer cells on the surface of homotrimeric vs. normal (heterotrimeric) type I collagen fibers. In summary, our findings suggest that invasive cancer cells may utilize homotrimers for building MMP-resistant invasion paths, supporting local proliferation and directed migration of the cells while surrounding normal stromal collagen is cleaved. Because the homotrimers are universally secreted by cancer cells and deposited as insoluble, MMP-resistant fibers, they offer an appealing target for cancer diagnostics and therapy.
PMCID: PMC2880213  PMID: 20460529
collagen homotrimers; MMP; collagen degradation; cell-matrix interactions; collagenases
7.  Molecular Mechanism of Type I Collagen Homotrimer Resistance to Mammalian Collagenases* 
The Journal of Biological Chemistry  2010;285(29):22276-22281.
Type I collagen cleavage is crucial for tissue remodeling, but its homotrimeric isoform is resistant to all collagenases. The homotrimers occur in fetal tissues, fibrosis, and cancer, where their collagenase resistance may play an important physiological role. To understand the mechanism of this resistance, we studied interactions of α1(I)3 homotrimers and normal α1(I)2α2(I) heterotrimers with fibroblast collagenase (MMP-1). Similar MMP-1 binding to the two isoforms and similar cleavage efficiency of unwound α1(I) and α2(I) chains suggested increased stability and less efficient unwinding of the homotrimer triple helix at the collagenase cleavage site. The unwinding, necessary for placing individual chains inside the catalytic cleft of the enzyme, was the rate-limiting cleavage step for both collagen isoforms. Comparative analysis of the homo- and heterotrimer cleavage kinetics revealed that MMP-1 binding promotes stochastic helix unwinding, resolving the controversy between different models of collagenase action.
PMCID: PMC2903388  PMID: 20463013
Collagen; Enzyme Kinetics; Extracellular Matrix; Metalloprotease; Protein Degradation; Collagen Homotrimer; Matrix Metalloproteinase; Tissue Remodeling
8.  Procollagen Triple Helix Assembly: An Unconventional Chaperone-Assisted Folding Paradigm 
PLoS ONE  2007;2(10):e1029.
Fibers composed of type I collagen triple helices form the organic scaffold of bone and many other tissues, yet the energetically preferred conformation of type I collagen at body temperature is a random coil. In fibers, the triple helix is stabilized by neighbors, but how does it fold? The observations reported here reveal surprising features that may represent a new paradigm for folding of marginally stable proteins. We find that human procollagen triple helix spontaneously folds into its native conformation at 30–34°C but not at higher temperatures, even in an environment emulating Endoplasmic Reticulum (ER). ER-like molecular crowding by nonspecific proteins does not affect triple helix folding or aggregation of unfolded chains. Common ER chaperones may prevent aggregation and misfolding of procollagen C-propeptide in their traditional role of binding unfolded polypeptide chains. However, such binding only further destabilizes the triple helix. We argue that folding of the triple helix requires stabilization by preferential binding of chaperones to its folded, native conformation. Based on the triple helix folding temperature measured here and published binding constants, we deduce that HSP47 is likely to do just that. It takes over 20 HSP47 molecules to stabilize a single triple helix at body temperature. The required 50–200 µM concentration of free HSP47 is not unusual for heat-shock chaperones in ER, but it is 100 times higher than used in reported in vitro experiments, which did not reveal such stabilization.
PMCID: PMC2000351  PMID: 17925877

Results 1-8 (8)