Development of a Functional Skin Matrix Requires Deposition of Collagen V Heterotrimers

Abstract

Collagen V is a minor component of the heterotypic I/III/V collagen fibrils and the defective product in most cases of classical Ehlers Danlos syndrome (EDS). The present study was undertaken to elucidate the impact of collagen V mutations on skin development, the most severely affected EDS tissues, using mice harboring a targeted deletion of the ␣2(V) collagen gene (Col5a2). Contrary to the original report, our studies indicate that the Col5a2 deletion (a.k.a. the pN allele) represents a functionally null mutation that affects matrix assembly through a complex sequence of events. First the mutation impairs assembly and/or secretion of the ␣1(V) 2 ␣2(V) heterotrimer with the result that the ␣1(V) homotrimer is the predominant species deposited into the matrix. Second, the ␣1(V) homotrimer is excluded from incorporation into the heterotypic collagen fibrils and this in turn severely impairs matrix organization. Third, the mutant matrix stimulates a compensatory loop by the ␣1(V) collagen gene that leads to additional deposition of ␣1(V) homotrimers. These data therefore underscore the importance of the collagen V heterotrimer in dermal fibrillogenesis. Furthermore, reduced thickness of the basement membranes underlying the epidermis and increased apoptosis of the stromal fibroblasts in pN/pN skin strongly indicate additional roles of collagen V in the development of a functional skin matrix.The physiological and biomechanical properties of different extracellular matrices depend in large part on the timely and tissue-specific deposition of collagen trimers (or types) that assemble into unique macromolecular aggregates. Among them, the banded fibrils represent the most abundant and widely distributed class of collagen assemblies. Fibril-forming collagens include five distinct types (I to III, V, and XI), which are found in most connective tissues (I, III, and V) or only in cartilage and vitreous (II and XI). Genetic studies have underscored the critical contribution of fibrillar collagens to organismal function by correlating mutations in the ␣ chain subunits with the genesis of several connective tissue disorders (17). Relevant to the present study, they have established the role of collagen V in regulating collagen I fibrillogenesis and in maintaining tissue integrity.Collagen V is a quantitatively minor component of tissues rich in collagen I, such as dermis, tendons/ligaments, bones, blood vessels, and cornea. Unlike other tissues, where collagen V represents only 1 to 3% of the total collagen fiber content, the relative concentration of this collagen type in cornea is significantly higher, 20 to 25% (3). Collagen V copolymerizes with collagens I and III to form heterotypic I/III/V fibrils in which the triple helical portion of the molecule is embedded and the amino-terminal globular domain projects onto the surface (4, 19). Very thin (5 to 10 nm in diameter) fibrils of collagen V have also been reported immediately near basement membranes and extending into the adjacent interstitial matrix (5,12,14,24). There...

Homozygosity and Heterozygosity for Null Col5a2 Alleles Produce Embryonic Lethality and a Novel Classic Ehlers-Danlos Syndrome–Related Phenotype

“…15 However, the small in-frame deletion in this allele should not have interfered with either assembly or secretion of a1(V) 2 a2(V) heterotrimers, and appropriately sized a2(V) chains were observable on immunoblots of mutant skin. 14 Heterozygotes for this allele were essentially phenotypically normal, but homozygotes died postnatally, with approximately 5% surviving after weaning, living a normal life span, 14,15 and having fragile, hyperextendible skin that contained unusually large-diameter collagen fibrils that included fibril aggregates, 14 a skin phenotype thus reminiscent of cEDS. However, the recessive nature of the phenotype of these mice, along with spinal abnormalities (kypholordosis) not characteristic of cEDS, argues against the possibility that such mice represent a model for cEDS.…”

“…Moreover, phenotypic differences between mice with the previously reported mutant Col5a2 allele and the mice described here, with truly null Col5a2 alleles, indicate that the previously described allele was not functionally null. 14,15 Instead, it seems probable that the non-cEDS phenotype of the previous mutant Col5a2 model results from expression of a stable aberrant a2(V) chain that is incorporated into aberrant a1(V) 2 a2(V) heterotrimers and into a consequently aberrant ECM. 14,15 We have described, for the first time to our knowledge, the phenotypic consequences of null Col5a2 alleles.…”

“…14,15 Instead, it seems probable that the non-cEDS phenotype of the previous mutant Col5a2 model results from expression of a stable aberrant a2(V) chain that is incorporated into aberrant a1(V) 2 a2(V) heterotrimers and into a consequently aberrant ECM. 14,15 We have described, for the first time to our knowledge, the phenotypic consequences of null Col5a2 alleles. Col5a2 À/À homozygotes die at approximately 12 dpc, with pallor, congestion of blood, and an absence of clearly visible vasculature because of a paucity of blood in the cardiovascular system.…”

“…14 A subsequent study on the same mice claimed the mutated allele to be functionally null. 15 We report the creation and characterization of mice with the first true null Col5a2 allele. Contrary to mice homozygous for the previously described Col5a2 mutant allele, 14 Col5a2 À/À homozygous null mice are early embryonic lethal, consistent with the early embryonic lethality of the previously described Col5a1 À/À mice.…”

Hemizygous deletion of COL3A1, COL5A2, and MSTN causes a complex phenotype with aortic dissection: a lesson for and from true haploinsufficiency

“…[33][34][35][36] This difference in the haploinsufficiencies of COL5A2 and COL5A1 may be because of the notion/evidence that homotrimeric formation of a2(V) chains is not possible, whereas a1(V) chains can assemble into stable homotrimers (ie, COL5A1 can replace COL5A2). 37,38 This may have hampered the identification of cases with COL5A2 haploinsufficiency because of lack of sufficient clinical signs. Although AD may occur in EDS I/II, 39 none of the few (B10) COL5A2 mutations reported so far have been associated with AD.…”