Type XVII collagen (BP 180) in the developing avian cornea

PURPOSE

Previous sequence analyses of hemidesmosomal BP 180/collagen XVII cDNA from human skin and of a similar chicken corneal cDNA showed some similarities, but major differences as well. The authors examined whether, in one species, the same mRNA is present in cornea and skin. They also studied the developmental expression of the molecule and compared it to the transmembrane hemidesmosome component, alpha 6 beta 4 integrin, and to the formation of hemidesmosomes themselves.

METHODS

Cornea and skin BP 180/collagen XVII cDNAs were cloned by reverse transcription-polymerase chain reaction (RT-PCR) and sequenced. Developmental expression was evaluated by quantitative RT-PCR, immunoblotting, and immunofluorescence microscopy. alpha 6 beta 4 integrin was evaluated by immunofluorescence microscopy, and hemidesmosome formation was assessed by electron microscopy.

RESULTS

The same alpha 1 (XVII) collagen/BP 180 mRNA is present in cornea and skin. The appearance of alpha 1 (XVII) collagen mRNA and protein shows similar temporal patterns of expression, with changes in the mRNA preceding those of the protein by approximately 2 days. The appearance of mature hemidesmosomes lags still further. Immunofluorescence histochemistry of alpha 1 (XVII) collagen and alpha 6 beta 4 integrin shows that their developmental appearance is regulated closely.

CONCLUSIONS

The differences between human BP 180/collagen XVII and the chicken corneal molecule represent species divergence. The appearance of alpha 1 (XVII) collagen mRNA and protein is regulated closely, with the protein lagging. Mature hemidesmosomes, once present, have a low turnover rate. The developmental appearance of alpha 1 (XVII) collagen and alpha 6 beta 4 integrin are regulated closely. However, the component responsible for initiating hemidesmosome formation remains unknown.

Domains of type X collagen: alteration of cartilage matrix by fibril association and proteoglycan accumulation

During endochondral bone formation, hypertrophic cartilage is replaced by bone or by a marrow cavity. The matrix of hypertrophic cartilage contains at least one tissue-specific component, type X collagen. Structurally type X collagen contains both a collagenous domain and a COOH-terminal non-collagenous one. However, the function(s) of this molecule have remained largely speculative. To examine the behavior and functions of type X collagen within hypertrophic cartilage, we (Chen, Q., E. Gibney, J. M. Fitch, C. Linsenmayer, T. M. Schmid, and T. F. Linsenmayer. 1990. Proc. Natl. Acad. Sci. USA. 87:8046-8050) recently devised an in vitro system in which exogenous type X collagen rapidly (15 min to several hours) moves into non-hypertrophic cartilage. There the molecule becomes associated with preexisting cartilage collagen fibrils. In the present investigation, we find that the isolated collagenous domain of type X collagen is sufficient for its association with fibrils. Furthermore, when non-hypertrophic cartilage is incubated for a longer time (overnight) with "intact" type X collagen, the molecule is found both in the matrix and inside of the chondrocytes. The properties of the matrix of such type X collagen-infiltrated cartilage become altered. Such changes include: (a) antigenic masking of type X collagen by proteoglycans; (b) loss of the permissiveness for further infiltration by type X collagen; and (c) enhanced accumulation of proteoglycans. Some of these changes are dependent on the presence of the COOH-terminal non-collagenous domain of the molecule. In fact, the isolated collagenous domain of type X collagen appears to exert an opposite effect on proteoglycan accumulation, producing a net decrease in their accumulation, particularly of the light form(s) of proteoglycans. Certain of these matrix alterations are similar to ones that have been observed to occur in vivo. This suggests that within hypertrophic cartilage type X collagen has regulatory as well as structural functions, and that these functions are achieved specifically by its two different domains.

Stromal assemblies containing collagen types IV and VI and fibronectin in the developing embryonic avian cornea

The morphogenesis of type IV collagen-containing structures in the stromal matrix of the developing avian cornea was investigated using immunofluorescence and immunoelectron microscopic histochemistry. Two forms of type IV collagen-containing structures were seen; these differed in their probable origin, structure, molecular composition, and developmental fate. The major form of stromal type IV collagen-containing material, termed "strings," was observed only after swelling of the primary stroma and the onset of mesenchymal invasion. These strings are presumed to be products of the stromal cells. In immunofluorescence histochemistry they appeared as linear segments of type IV collagen-specific immunoreactivity. In immunoelectron microscopy, they appeared initially as electron-dense sausages of variable length and orientation. They frequently were associated with cell surfaces and, in fortuitous sections, appeared to connect adjacent cells. The strings also contained type VI collagen and fibronectin, but very little, if any, of the basement membrane components laminin and heparin sulfate proteoglycan (HSPG). As the stroma continued to expand in thickness, more of these structures were observed in a radial orientation, becoming quite long and less tortuous. Later in development, as stromal condensation proceeded, they disappeared. We suggest that the strings function to stabilize the stromal matrix, and perhaps to limit the rate and/or extent of stromal expansion, during a phase of rapid swelling and matrix deposition. The other form of type IV collagen-containing stromal material appeared as irregularly shaped plaques of basement membrane-like material identical to those previously described in mature corneas. These are likely derived from the corneal endothelial cells. They contained other basement membrane-associated components (laminin, HSPG) and fibronectin, but not type VI collagen. This material persists in mature corneas as sparse irregular stromal plaques and as matrix in the interface between Descemet's membrane and the corneal stroma.

Long-range movement and fibril association of type X collagen within embryonic cartilage matrix

A recent immunoelectron microscopic study of type X collagen in developing cartilage gave results that could be explained by movement of the molecule from one region of the cartilage matrix to another, there becoming associated with preexisting collagen fibrils. In the present study, to test the feasibility of this model we incubated pieces of nonhypertrophic, embryonic chicken sternal cartilage (which has no endogenous type X collagen) in medium with type X collagen and then used immunofluorescence and immunoelectron microscopy to evaluate movement of the molecule through the matrix. The results show that type X collagen molecules can indeed pass through embryonic sternal cartilage matrix and subsequently become fibril-associated.

The spatial organization of Descemet's membrane-associated type IV collagen in the avian cornea

The organization of type IV collagen in the unconventional basement membrane of the corneal endothelium (Descemet's membrane) was investigated in developing chicken embryos using anti-collagen mAbs. Both immunofluorescence histochemistry and immunoelectron microscopy were performed. In mature embryos (greater than 15 d of development), the type IV collagen of Descemet's membrane was present as an array of discrete aggregates of amorphous material at the interface between Descemet's membrane and the posterior corneal stroma. Immunoreactivity for type IV collagen was also observed in the posterior corneal stroma as irregular plaques of material with a morphology similar to that of the Descemet's membrane-associated aggregates. This arrangement of Descemet's membrane-associated type IV collagen developed from a subendothelial mat of type IV collagen-containing material. This mat, in which type IV collagen-specific immunoreactivity was always discontinuous, first appeared at the time a confluent endothelium was established, well before the onset of Descemet's membrane formation. Immunoelectron microscopy of mature corneas revealed that the characteristic nodal matrix of Descemet's membrane itself was unreactive for type IV collagen, but was penetrated at intervals by projections of type IV collagen-containing material. These projections frequently appeared to contact cell processes from the underlying corneal endothelium. This spatial arrangement of type IV collagen suggests that it serves to suture the corneal endothelium/Descemet's membrane to the dense interfacial matrix of the posterior stroma.