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Role of Cys41 in the N-terminal domain of lumican in ex vivo collagen fibrillogenesis by cultured corneal stromal cells.

ABSTRACT: The keratan sulphate proteoglycan lumican regulates collagen fibrillogenesis to maintain the integrity and function of connective tissues such as cornea. We examined the role of a highly conserved cysteine-containing domain proximal to the N-terminus of lumican in collagen fibrillogenesis using site-specific mutagenesis to prepare plasmid DNA encoding wild-type murine lumican (Cys(37)-Xaa(3)-Cys(41)-Xaa-Cys-Xaa(9)-Cys) and a Cys-->Ser (C/S) mutant (Cys(37)-Xaa(3)-Ser(41)-Xaa-Cys-Xaa(9)-Cys). cDNAs were cloned into the pSecTag2A vector, and cultures of MK/T-1 cells (an immortalized cell line from mouse keratocytes) were transfected with the cDNAs. Stable transformants were selected and cloned in the presence of Zeocin. All stable transformants maintained a dendritic morphology and growth rate similar to those of parental MK/T-1 cells. Western blot analysis with anti-lumican antibody detected a 42 kDa lumican protein secreted into the culture medium of both wild-type and C/S mutant lumican cell lines. Ultrastructural analyses by transmission electron microscopy showed both cell lines to form a multi-layered stroma ex vivo, but the matrix assembled by the two cell lines differed. Compared with the mutant cell line, the wild-type cells assembled a more organized matrix with regions containing orthogonal collagen fibrils. In addition, the fibrils in the extracellular matrix formed by the mutant cell line exhibited alterations in fibril packing and structure. Immunostaining analysed by confocal microscopy showed a further difference in this matrix, with the marked occurrence of lumican and collagen I co-localization in the lumican wild-type cells, but a lack thereof in the lumican C/S mutant cells. The results indicate that the cysteine-rich domain of lumican is important in collagen fibrillogenesis and stromal matrix assembly.

SUBMITTER: Carlson EC

PROVIDER: S-EPMC1223108 | biostudies-other | 2003 Feb

REPOSITORIES: biostudies-other

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Project description:Previous work from our laboratories has demonstrated that: (a) the striated collagen fibrils of the corneal stroma are heterotypic structures composed of type V collagen molecules coassembled along with those of type I collagen, (b) the high content of type V collagen within the corneal collagen fibrils is one factor responsible for the small, uniform fibrillar diameter (25 nm) characteristic of this tissue, (c) the completely processed form of type V collagen found within tissues retains a large noncollagenous region, termed the NH2-terminal domain, at the amino end of its alpha 1 chain, and (d) the NH2-terminal domain may contain at least some of the information for the observed regulation of fibril diameters. In the present investigation we have employed polyclonal antibodies against the retained NH2-terminal domain of the alpha 1(V) chain for immunohistochemical studies of embryonic avian corneas and for immunoscreening a chicken cDNA library. When combined with cDNA sequencing and molecular rotary shadowing, these approaches provide information on the molecular structure of the retained NH2-terminal domain as well as how this domain might function in the regulation of fibrillar structure. In immunofluorescence and immunoelectron microscopy analyses, the antibodies against the NH2-terminal domain react with type V molecules present within mature heterotypic fibrils of the corneal stroma. Thus, epitopes within at least a portion of this domain are exposed on the fibril surface. This is in marked contrast to mAbs which we have previously characterized as being directed against epitopes located in the major triple helical domain of the type V molecule. The helical epitopes recognized by these antibodies are antigenically masked on type V molecules that have been assembled into fibrils. Sequencing of the isolated cDNA clones has provided the conceptual amino acid sequence of the entire amino end of the alpha 1(V) procollagen chain. The sequence shows the location of what appear to be potential propeptidase cleavage sites. One of these, if preferentially used during processing of the type V procollagen molecule, can provide an explanation for the retention of the NH2-terminal domain in the completely processed molecule. The sequencing data also suggest that the NH2-terminal domain consists of several regions, providing a structure which fits well with that of the completely processed type V molecule as visualized by rotary shadowing.

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Role of Cys41 in the N-terminal domain of lumican in ex vivo collagen fibrillogenesis by cultured corneal stromal cells.

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