Low-threshold mechanoreceptor neurons (LTMs) of the dorsal root ganglia (DRG) are essential for touch sensation. They form highly specialized terminations in the skin and display stereotyped projections in the spinal cord. Functionally defined LTMs depend on neurotrophin signaling for their postnatal survival and functioning, but how these neurons arise during development is unknown. Here, we show that specific types of LTMs can be identified shortly after DRG genesis by unique expression of the MafA transcription factor, the Ret receptor and coreceptor GFRalpha2, and find that their specification is Ngn2 dependent. In mice lacking Ret, these LTMs display early differentiation defects, as revealed by reduced MafA expression, and at later stages their central and peripheral projections are compromised. Moreover, in MafA mutants, a discrete subset of LTMs display altered expression of neurotrophic factor receptors. Our results provide evidence that genetic interactions involving Ret and MafA progressively promote the differentiation and diversification of LTMs.

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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...

A heparin binding region is known to be present within the triple helical part of the ␣1(V) chain. Here we show that a recombinant ␣1(V) fragment (Ile 824 to Pro 950 ), referred to as HepV, is sufficient for heparin binding at physiological ionic strength. Both native individual ␣1(V) chains and HepV are eluted at identical NaCl concentrations (0.35 M) from a heparin-Sepharose column, and this binding can be inhibited specifically by the addition of free heparin or heparan sulfate. In contrast, a shorter 23-residue synthetic peptide, containing the putative heparin binding site in HepV, fails to bind heparin. Interestingly, HepV promotes cell attachment, and HepV-mediated adhesion is inhibited specifically by heparin or heparan sulfate, indicating that this region might behave as an adhesive binding site. The same site is equally functional on triple helical molecules as shown by heparin-gold labeling. However, the affinities for heparin of each of the collagen V molecular forms tested are different and increase with the number of ␣1(V) chains incorporated in the molecules. Molecular modeling of a sequence encompassing the putative HepV binding sequence region shows that all of the basic residues cluster on one side of the helical face. A highly positively charged ring around the molecule is thus particularly evident for the ␣1(V) homotrimer. This could strengthen its interaction with the anionic heparin molecules. We propose that a single heparin binding site is involved in heparin-related glycosaminoglycanscollagen V interactions, but the different affinities observed likely modulate cell and matrix interactions between collagen V and heparan sulfate proteoglycans in tissues.Collagen V is a fibrillar collagen that plays an important role in fibrillogenesis, and it also acts as an adhesive substrate for a large variety of cells and binds to a number of extracellular components through its major triple helical domain (1). Collagen V interacts with matrix proteoglycans such as the two small proteoglycans decorin and biglycan (2), the proteoglycan form of macrophage colony-stimulating factor (3), the cell surface proteoglycan syndecan-1 (4, 5), and as shown recently, the membrane spanning proteoglycan NG2 (6). Some of these interactions are mediated by the core proteins, but others depend on the glycosaminoglycan chains such as the heparan sulfate chains.Apart from in vitro binding of collagen V to membranespanning proteoglycans, the suggestion that heparan sulfate interacts with collagen V was supported by inhibition experiments showing a reduction of cell attachment to collagen V in the presence of heparin (7). It has been shown already that cell focal adhesion on fibronectin requires the cooperation of both cell transmembrane proteoglycans and integrin receptors (8).Because we have demonstrated already that cell-collagen V interactions involved integrins (9, 10), the binding of membrane-spanning proteoglycans could reinforce cell attachment to collagen V and, in that sense, would be of physiological importance....