A type I collagen reporter gene construct for protein engineering studies. Functional equivalence of transfected reporter COL1A1 and endogenous gene products during biosynthesis and in vitro extracellular matrix accumulation

Complex mechanical conditioning of cell-seeded agarose constructs can influence chondrocyte biosynthetic activity

Articular cartilage with its inherently poor capacity for self-regeneration represents a primary target for tissue engineering strategies, with approaches focusing on the in vitro generation of neo-cartilage using chondrocyte-seeded 3D scaffolds subjected to mechanical conditioning. Although uniaxial compression regimens have significantly up-regulated proteoglycan synthesis, their effects on the synthesis of collagen have been modest. Articular cartilage is subjected to shear forces during joint motion. Accordingly, this study utilized an apparatus to apply biaxial loading to chondrocytes seeded within agarose constructs with endplates. The chondrocytes yielded a monotonic increase in proteoglycan synthesis both in free swelling culture up to day 8 and when the constructs were subjected to dynamic compression alone (15% amplitude at a frequency of 1 Hz for 48 h). However, when dynamic shear (10% amplitude at 1 Hz) was superimposed on dynamic compression, total collagen synthesis was also up-regulated, within 3 days of culture, without compromising proteoglycan synthesis. Histological analysis revealed marked collagen deposition around individual chondrocytes. A significant proportion (50%) of collagen was released into the culture medium, suggesting that it had only been partially synthesized in its mature state. The overall biosynthetic activity was enhanced more when the biaxial stimulation was applied in a continuous mode as opposed to intermittent loading. Results of the present study strongly suggest that proteoglycan and collagen synthesis may be triggered by uncoupled mechanosensitive cellular responses. The proposed in vitro model and the prescribed conditioning protocols demonstrated that a short pre-culture period is preferable to long free swelling culture condition as it enables a significantly higher up-regulation of collagen. Biotechnol. Bioeng. 2017;114: 1614-1625. © 2017 Wiley Periodicals, Inc.

Biology of Hsp47 (Serpin H1), a collagen-specific molecular chaperone

Hsp47, a collagen-specific molecular chaperone that localizes in the endoplasmic reticulum (ER), is indispensable for molecular maturation of collagen. Hsp47, which is encoded by the SERPINH1 gene, belongs to the serpin family and has the serpin fold; however, it has no serine protease inhibitory activity. Hsp47 transiently binds to procollagen in the ER, dissociates in the cis-Golgi or ER-Golgi intermediate compartment (ERGIC) in a pH-dependent manner, and is then transported back to the ER via its RDEL retention sequence. Hsp47 recognizes collagenous (Gly-Xaa-Arg) repeats on triple-helical procollagen and can prevent local unfolding and/or aggregate formation of procollagen. Gene disruption of Hsp47 in mice causes embryonic lethality due to impairments in basement membrane and collagen fibril formation. In Hsp47-knockout cells, the type I collagen triple helix forms abnormally, resulting in thin and frequently branched fibrils. Secretion of type I collagens is slow and plausible in making aggregates of procollagens in the ER of hsp47-knocked out fibroblasts, which are ultimately degraded by autophagy. Mutations in Hsp47 are causally associated with osteogenesis imperfecta. Expression of Hsp47 is strongly correlated with expression of collagens in multiple types of cells and tissues. Therefore, Hsp47 represents a promising target for treatment of collagen-related disorders, including fibrosis of the liver, lung, and other organs.

Deletion of the collagen-specific molecular chaperone Hsp47 causes endoplasmic reticulum stress-mediated apoptosis of hepatic stellate cells

Chronic liver injury, often caused by alcoholism and viral hepatitis, causes liver fibrosis via the induction of collagen production. In liver fibrosis, hepatic stellate cells (HSCs) are activated and transform into myofibroblasts, which actively produce and secrete collagen into the extracellular matrix. Hsp47 (heat shock protein 47) is a collagen-specific molecular chaperone that is essential for the maturation and secretion of collagen. Here, we used the Cre-LoxP system to disrupt the Hsp47 gene in isolated HSCs from Hsp47 floxed mice. Immature type I procollagen accumulated and partially aggregated in Hsp47-KO HSCs. This accumulation was augmented when autophagy was inhibited, which induced expression of the endoplasmic reticulum (ER) stress-inducible proteins BiP (immunoglobulin heavy chain-binding protein) and Grp94 (94-kDa glucose-regulated protein). The inhibition of autophagy in Hsp47-KO HSCs also induced CHOP (CCAAT/enhancer-binding protein homologous protein), which is an ER stress-induced transcription factor responsible for apoptosis. These data suggest that apoptosis is induced through ER stress by procollagen accumulation in Hsp47-KO HSCs when autophagy is inhibited. Thus, Hsp47 could be a promising therapeutic target in liver fibrosis.

Autophagic elimination of misfolded procollagen aggregates in the endoplasmic reticulum as a means of cell protection

Type I collagen is a major component of the extracellular matrix, and mutations in the collagen gene cause several matrix-associated diseases. These mutant procollagens are misfolded and often aggregated in the endoplasmic reticulum (ER). Although the misfolded procollagens are potentially toxic to the cell, little is known about how they are eliminated from the ER. Here, we show that procollagen that can initially trimerize but then aggregates in the ER are eliminated by an autophagy-lysosome pathway, but not by the ER-associated degradation (ERAD) pathway. Inhibition of autophagy by specific inhibitors or RNAi-mediated knockdown of an autophagy-related gene significantly stimulated accumulation of aggregated procollagen trimers in the ER, and activation of autophagy with rapamycin resulted in reduced amount of aggregates. In contrast, a mutant procollagen which has a compromised ability to form trimers was degraded by ERAD. Moreover, we found that autophagy plays an essential role in protecting cells against the toxicity of the ERAD-inefficient procollagen aggregates. The autophagic elimination of aggregated procollagen occurs independently of the ERAD system. These results indicate that autophagy is a final cell protection strategy deployed against ER-accumulated cytotoxic aggregates that are not able to be removed by ERAD.

Proteasomal degradation of unassembled mutant type I collagen pro-alpha1(I) chains

We have previously shown that type I procollagen pro-alpha1(I) chains from an osteogenesis imperfecta patient (OI26) with a frameshift mutation resulting in a truncated C-propeptide, have impaired assembly, and are degraded by an endoplasmic reticulum-associated pathway (Lamandé, S. R., Chessler, S. D., Golub, S. B., Byers, P. H., Chan, D., Cole, W. G., Sillence, D. O. and Bateman, J. F. (1995) J. Biol. Chem. 270, 8642-8649). To further explore the degradation of procollagen chains with mutant C-propeptides, mouse Mov13 cells, which produce no endogenous pro-alpha1(I), were stably transfected with a pro-alpha1(I) expression construct containing a frameshift mutation that predicts the synthesis of a protein 85 residues longer than normal. Despite high levels of mutant mRNA in transfected Mov13 cells, only minute amounts of mutant pro-alpha1(I) could be detected indicating that the majority of the mutant pro-alpha1(I) chains synthesized are targeted for rapid intracellular degradation. Degradation was not prevented by brefeldin A, monensin, or NH(4)Cl, agents that interfere with intracellular transport or lysosomal function. However, mutant pro-alpha1(I) chains in both transfected Mov13 cells and OI26 cells were protected from proteolysis by specific proteasome inhibitors. Together these data demonstrate for the first time that procollagen chains containing C-propeptide mutations that impair assembly are degraded by the cytoplasmic proteasome complex, and that the previously identified endoplasmic reticulum-associated degradation of mutant pro-alpha1(I) in OI26 is mediated by proteasomes.

In vitro expression analysis of collagen biosynthesis and assembly

While the generalised pathway of collagen biosynthesis is well understood, the specific molecular interactions that drive chain recognition and assembly and the formation of tissue-specific extracellular supramolecular structures have not been elucidated. This review focuses on the use of in vitro collagen expression systems to explore some of these fundamental questions on the molecular basis of normal and mutant collagen assembly. Three in vitro expression/assembly systems are discussed. Firstly, a simple cell-free transcription/translation system to study the initial stages of collagen chain assembly. Secondly, a novel T7-driven high level expression system, using a recombinant vaccinia virus expressing T7 RNA polymerase, in transiently transfected cells which allows appropriate postranslational modification and collagen folding. Thirdly, the more complex questions of normal and mutant collagen extracellular matrix assembly are addressed by stable transfection and expression in cells which allow the formation of a 'tissue equivalent' matrix during long-term culture.

The type I collagen pro alpha 1(I) COOH-terminal propeptide N-linked oligosaccharide. Functional analysis by site-directed mutagenesis

The C-propeptides of the pro alpha 1(I) and pro alpha 2(I) chains of type I collagen are each substituted with a single high-mannose N-linked oligosaccharide. Conservation of this motif among the fibrillar collagens has led to the proposal that the oligosaccharide has structural or functional importance, but a role in collagen biosynthesis has not been unambiguously defined. To examine directly the function of the pro alpha 1(I) C-propeptide N-linked oligosaccharide, the acceptor Asn residue was changed to Gln by site-directed mutagenesis. In transfected mouse Mov13 and 3T6 cells, unglycosylated mutant pro alpha 1(I) folded and assembled normally into trimeric molecules with pro alpha 2(I). In biosynthetic pulse-chase experiments mutant pro alpha 1(I) were secreted at the same rate as wild-type chains; however, following secretion, the chains were partitioned differently between the cell layer and medium, with a greater proportion of the mutant pro alpha 1(I) being released into the medium. This distribution difference was not eliminated by the inclusion of yeast mannan indicating that the high-mannose oligosaccharide itself was not binding to the matrix or the fibroblast surface after secretion. Subtle alterations in the tertiary structure of unglycosylated C-propeptides may have decreased their affinity for a cell-surface component. Further support for a small conformational change in the mutant C-propeptides came from experiments suggesting that unglycosylated pro alpha 1(I) chains were cleaved in vitro by the purified C-proteinase slightly less efficiently than wild-type chains. Mutant and normal pro alpha 1(I) were deposited with equal efficiency into the 3T6 cell accumulated matrix, thus the reduced cleavage by C-proteinase and altered distribution in the short pulse-chase experiments were not functionally significant in this in vitro extracellular matrix model system.