Bromine is an essential trace element for assembly of collagen IV scaffolds in tissue development and architecture.

Bromine is ubiquitously present in animals as ionic bromide (Br(-)) yet has no known essential function. Herein, we demonstrate that Br(-) is a required cofactor for peroxidasin-catalyzed formation of sulfilimine crosslinks, a posttranslational modification essential for tissue development and architecture found within the collagen IV scaffold of basement membranes (BMs). Bromide, converted to hypobromous acid, forms a bromosulfonium-ion intermediate that energetically selects for sulfilimine formation. Dietary Br deficiency is lethal in Drosophila, whereas Br replenishment restores viability, demonstrating its physiologic requirement. Importantly, Br-deficient flies phenocopy the developmental and BM defects observed in peroxidasin mutants and indicate a functional connection between Br(-), collagen IV, and peroxidasin. We establish that Br(-) is required for sulfilimine formation within collagen IV, an event critical for BM assembly and tissue development. Thus, bromine is an essential trace element for all animals, and its deficiency may be relevant to BM alterations observed in nutritional and smoking-related disease. PAPERFLICK:

Collagen IV of basement membranes: III. Chloride pressure is a primordial innovation that drives and maintains the assembly of scaffolds.

Collagen IV scaffold is a primordial innovation enabling the assembly of a fundamental architectural unit of epithelial tissues-a basement membrane attached to polarized cells. A family of six α-chains (α1 to α6) coassemble into three distinct protomers that form supramolecular scaffolds, noted as collagen IVα121, collagen IVα345, and collagen IVα121-α556. Chloride ions play a pivotal role in scaffold assembly, based on studies of NC1 hexamers from mammalian tissues. First, Cl- activates a molecular switch within trimeric NC1 domains that initiates protomer oligomerization, forming an NC1 hexamer between adjoining protomers. Second, Cl- stabilizes the hexamer structure. Whether this Cl--dependent mechanism is of fundamental importance in animal evolution is unknown. Here, we developed a simple in vitro method of SDS-PAGE to determine the role of solution Cl- in hexamer stability. Hexamers were characterized from 34 animal species across 15 major phyla, including the basal Cnidarian and Ctenophora phyla. We found that solution Cl- stabilized the quaternary hexamer structure across all phyla except Ctenophora, Ecdysozoa, and Rotifera. Further analysis of hexamers from peroxidasin knockout mice, a model for decreasing hexamer crosslinks, showed that solution Cl- also stabilized the hexamer surface conformation. The presence of sufficient chloride concentration in solution or "chloride pressure" dynamically maintains the native form of the hexamer. Collectively, our findings revealed that chloride pressure on the outside of cells is a primordial innovation that drives and maintains the quaternary and conformational structure of NC1 hexamers of collagen IV scaffolds.

Identification of brominated proteins in renal extracellular matrix: Potential interactions with peroxidasin.

Peroxidasin (PXDN) is an extracellular peroxidase, which generates hypobromous acid to form sulfilimine cross-links within collagen IV networks. We have previously demonstrated that mouse and human renal basement membranes (BM) are enriched in bromine due to PXDN-dependent post-translational bromination of protein tyrosine residues. The goal of the present study was identification of specific brominated sites within renal BM. A comprehensive analysis of brominated proteome of mouse glomerular matrix had been performed using liquid chromatography-tandem mass spectrometry. We found that out of over 200 identified proteins, only three were detectably brominated, each containing a single distinct brominated tyrosine site i.e., Tyr-1485 in collagen IV α2 chain, Tyr-292 in TINAGL1 and Tyr-664 in nidogen-2. To explain this highly selective bromination, we proposed that these proteins interact with PXDN within the glomerular matrix. Experiments using purified proteins demonstrated that both TINAGL1 and nidogen-2 can compete with PXDN for binding to collagen IV and that TINAGL1 can directly interact with PXDN. We propose that a protein complex, including PXDN, TINAGL1, nidogen-2 and collagen IV, may exist in renal BM.

Assessment of Collagen in Translational Models of Lung Research.

The extracellular matrix (ECM) plays an important role in lung health and disease. Collagen is the main component of the lung ECM, widely used for the establishment of in vitro and organotypic models of lung disease, and as scaffold material of general interest for the field of lung bioengineering. Collagen also is the main readout for fibrotic lung disease, where collagen composition and molecular properties are drastically changed and ultimately result in dysfunctional "scarred" tissue. Because of the central role of collagen in lung disease, quantification, determination of molecular properties, and three-dimensional visualization of collagen is important for both development and characterization of translational models of lung research. In this chapter, we provide a comprehensive overview on the various methodologies currently available for quantification and characterization of collagen including their detection principles, advantages, and disadvantages.

The Extracellular Matrix Receptor Discoidin Domain Receptor 1 Regulates Collagen Transcription by Translocating to the Nucleus.

BACKGROUND: The discoidin domain receptor 1 (DDR1) is activated by collagens, upregulated in injured and fibrotic kidneys, and contributes to fibrosis by regulating extracellular matrix production, but how DDR1 controls fibrosis is poorly understood. DDR1 is a receptor tyrosine kinase (RTK). RTKs can translocate to the nucleus via a nuclear localization sequence (NLS) present on the receptor itself or a ligand it is bound to. In the nucleus, RTKs regulate gene expression by binding chromatin directly or by interacting with transcription factors. METHODS: To determine whether DDR1 translocates to the nucleus and whether this event is mediated by collagen-induced DDR1 activation, we generated renal cells expressing wild-type or mutant forms of DDR1 no longer able to bind collagen. Then, we determined the location of the DDR1 upon collagen stimulation. Using both biochemical assays and immunofluorescence, we analyzed the steps involved in DDR1 nuclear translocation. RESULTS: We show that although DDR1 and its natural ligand, collagen, lack an NLS, DDR1 is present in the nucleus of injured human and mouse kidney proximal tubules. We show that DDR1 nuclear translocation requires collagen-mediated receptor activation and interaction of DDR1 with SEC61B, a component of the Sec61 translocon, and nonmuscle myosin IIA and ß-actin. Once in the nucleus, DDR1 binds to chromatin to increase the transcription of collagen IV, a major collagen upregulated in fibrosis. CONCLUSIONS: These findings reveal a novel mechanism whereby activated DDR1 translates to the nucleus to regulate synthesis of profibrotic molecules.

Proteolytic processing of lysyl oxidase-like-2 in the extracellular matrix is required for crosslinking of basement membrane collagen IV.

Lysyl oxidase-like-2 (LOXL2) is an enzyme secreted into the extracellular matrix that crosslinks collagens by mediating oxidative deamination of lysine residues. Our previous work demonstrated that this enzyme crosslinks the 7S domain, a structural domain that stabilizes collagen IV scaffolds in the basement membrane. Despite its relevant role in extracellular matrix biosynthesis, little is known about the structural requirements of LOXL2 that enable collagen IV crosslinking. In this study, we demonstrate that LOXL2 is processed extracellularly by serine proteases, generating a 65-kDa form lacking the first two scavenger receptor cysteine-rich domains. Site-specific mutagenesis to prevent proteolytic processing generated a full-length enzyme that is active in vitro toward a soluble substrate, but fails to crosslink insoluble collagen IV within the extracellular matrix. In contrast, the processed form of LOXL2 binds to collagen IV and crosslinks the 7S domain. Together, our data demonstrate that proteolytic processing is an important event that allows LOXL2-mediated crosslinking of basement membrane collagen IV.

Lysyl Oxidase-like-2 Cross-links Collagen IV of Glomerular Basement Membrane.

The 7S dodecamer is recognized as an important structural cross-linking domain of collagen IV networks that provide mechanical stability to basement membranes, a specialized form of extracellular matrix essential for the development and maintenance of tissue architecture. Although the 7S dodecamer is stabilized by covalent cross-linking, the molecular mechanism by which such cross-links are formed has not been revealed. Here, we aimed to identify the enzyme(s) that cross-links the 7S dodecamer and characterize its expression in the kidney glomerulus. Pharmacological inhibition of candidate extracellular matrix enzymes revealed that lysyl oxidase activity is required for cross-linking of 7S polypeptides. Among all lysyl oxidase family members, lysyl oxidase-like-2 (LOXL2) was identified as the isoform cross-linking collagen IV in mouse embryonal PFHR-9 cells. Biochemical analyses revealed that LOXL2 readily promoted the formation of lysyl-derived cross-links in the 7S dodecamer but not in the NC1 domain. We also established that LOXL2 is the main lysyl oxidase family member present in the glomerular extracellular matrix. Altogether, we demonstrate that LOXL2 is a novel component of the molecular machinery that forms cross-linked collagen IV networks, which are essential for glomerular basement membrane stability and molecular ultrafiltration function.

A unique covalent bond in basement membrane is a primordial innovation for tissue evolution.

Basement membrane, a specialized ECM that underlies polarized epithelium of eumetazoans, provides signaling cues that regulate cell behavior and function in tissue genesis and homeostasis. A collagen IV scaffold, a major component, is essential for tissues and dysfunctional in several diseases. Studies of bovine and Drosophila tissues reveal that the scaffold is stabilized by sulfilimine chemical bonds (S = N) that covalently cross-link methionine and hydroxylysine residues at the interface of adjoining triple helical protomers. Peroxidasin, a heme peroxidase embedded in the basement membrane, produces hypohalous acid intermediates that oxidize methionine, forming the sulfilimine cross-link. We explored whether the sulfilimine cross-link is a fundamental requirement in the genesis and evolution of epithelial tissues by determining its occurrence and evolutionary origin in Eumetazoa and its essentiality in zebrafish development; 31 species, spanning 11 major phyla, were investigated for the occurrence of the sulfilimine cross-link by electrophoresis, MS, and multiple sequence alignment of de novo transcriptome and available genomic data for collagen IV and peroxidasin. The results show that the cross-link is conserved throughout Eumetazoa and arose at the divergence of Porifera and Cnidaria over 500 Mya. Also, peroxidasin, the enzyme that forms the bond, is evolutionarily conserved throughout Metazoa. Morpholino knockdown of peroxidasin in zebrafish revealed that the cross-link is essential for organogenesis. Collectively, our findings establish that the triad-a collagen IV scaffold with sulfilimine cross-links, peroxidasin, and hypohalous acids-is a primordial innovation of the ECM essential for organogenesis and tissue evolution.

Comprehensive Characterization of Glycosylation and Hydroxylation of Basement Membrane Collagen IV by High-Resolution Mass Spectrometry.

Collagen IV is the main structural protein that provides a scaffold for assembly of basement membrane proteins. Posttranslational modifications such as hydroxylation of proline and lysine and glycosylation of lysine are essential for the functioning of collagen IV triple-helical molecules. These modifications are highly abundant posing a difficult challenge for in-depth characterization of collagen IV using conventional proteomics approaches. Herein, we implemented an integrated pipeline combining high-resolution mass spectrometry with different fragmentation techniques and an optimized bioinformatics workflow to study posttranslational modifications in mouse collagen IV. We achieved 82% sequence coverage for the α1 chain, mapping 39 glycosylated hydroxylysine, 148 4-hydroxyproline, and seven 3-hydroxyproline residues. Further, we employed our pipeline to map the modifications on human collagen IV and achieved 85% sequence coverage for the α1 chain, mapping 35 glycosylated hydroxylysine, 163 4-hydroxyproline, and 14 3-hydroxyproline residues. Although lysine glycosylation heterogeneity was observed in both mouse and human, 21 conserved sites were identified. Likewise, five 3-hydroxyproline residues were conserved between mouse and human, suggesting that these modification sites are important for collagen IV function. Collectively, these are the first comprehensive maps of hydroxylation and glycosylation sites in collagen IV, which lay the foundation for dissecting the key role of these modifications in health and disease.

Aquaporin 11 variant associates with kidney disease in type 2 diabetic patients.

Kidney disease, a common complication of diabetes, associates with poor prognosis. Our previous animal model studies linked aquaporin (AQP)11 to acute kidney injury, hyperglycemia-induced renal impairment, and kidney disease in diabetes. Here, we report the AQP11 rs2276415 variant as a genetic factor placing type 2 diabetic patients at greater risk for the development of kidney disease. We performed two independent retrospective case-control studies in 1,075 diabetic and 1,619 nondiabetic individuals who were identified in the Synthetic Derivative Database with DNA samples in the BioVU DNA repository at Vanderbilt University (Nashville, TN). A χ(2)-test and multivariable logistic regression analysis with adjustments for age, sex, baseline serum creatinine, and underlying comorbid disease covariates showed a significant association between rs2276415 and the prevalence of any event of acute kidney injury and chronic kidney disease (CKD) in diabetic patients but not in patients without diabetes. This result was replicated in the second independent study. Diabetic CKD patients over 55 yrs old with the minor AQP11 allele had a significantly faster progression of estimated glomerular filtration rate decline than patients with the wild-type genotype. Three-dimensional structural analysis suggested a functional impairment of AQP11 with rs2276415, which could place diabetic patients at a higher risk for kidney disease. These studies identified rs2276415 as a candidate genetic factor predisposing patients with type 2 diabetes to CKD.

Supramolecular organization of the α121-α565 collagen IV network.

Collagen IV is a family of 6 chains (α1-α6), that form triple-helical protomers that assemble into supramolecular networks. Two distinct networks with chain compositions of α121 and α345 have been established. These oligomerize into separate α121 and α345 networks by a homotypic interaction through their trimeric noncollagenous (NC1) domains, forming α121 and α345 NC1 hexamers, respectively. These are stabilized by novel sulfilimine (-S=N-) cross-links, a covalent cross-link that forms between Met(93) and Hyl(211) at the trimer-trimer interface. A third network with a composition of α1256 has been proposed, but its supramolecular organization has not been established. In this study we investigated the supramolecular organization of this network by determining the chain identity of sulfilimine-cross-linked NC1 domains derived from the α1256 NC1 hexamer. High resolution mass spectrometry analyses of peptides revealed that sulfilimine bonds specifically cross-link α1 to α5 and α2 to α6 NC1 domains, thus providing the spatial orientation between interacting α121 and α565 trimers. Using this information, we constructed a three-dimensional homology model in which the α565 trimer shows a good chemical and structural complementarity to the α121 trimer. Our studies provide the first chemical evidence for an α565 protomer and its heterotypic interaction with the α121 protomer. Moreover, our findings, in conjunction with our previous studies, establish that the six collagen IV chains are organized into three canonical protomers α121, α345, and α565 forming three distinct networks: α121, α345, and α121-α565, each of which is stabilized by sulfilimine bonds between their C-terminal NC1 domains.

Peroxidasin forms sulfilimine chemical bonds using hypohalous acids in tissue genesis.

Collagen IV comprises the predominant protein network of basement membranes, a specialized extracellular matrix, which underlie epithelia and endothelia. These networks assemble through oligomerization and covalent crosslinking to endow mechanical strength and shape cell behavior through interactions with cell-surface receptors. A recently discovered sulfilimine (S=N) bond between a methionine sulfur and hydroxylysine nitrogen reinforces the collagen IV network. We demonstrate that peroxidasin, an enzyme found in basement membranes, catalyzes formation of the sulfilimine bond. Drosophila peroxidasin mutants have disorganized collagen IV networks and torn visceral muscle basement membranes, pointing to a critical role for the enzyme in tissue biogenesis. Peroxidasin generates hypohalous acids as reaction intermediates, suggesting a paradoxically anabolic role for these usually destructive oxidants. This work highlights sulfilimine bond formation as what is to our knowledge the first known physiologic function for peroxidasin, a role for hypohalous oxidants in tissue biogenesis, and a possible role for peroxidasin in inflammatory diseases.

Molecular architecture of the Goodpasture autoantigen in anti-GBM nephritis.

BACKGROUND: In Goodpasture's disease, circulating autoantibodies bind to the noncollagenous-1 (NC1) domain of type IV collagen in the glomerular basement membrane (GBM). The specificity and molecular architecture of epitopes of tissue-bound autoantibodies are unknown. Alport's post-transplantation nephritis, which is mediated by alloantibodies against the GBM, occurs after kidney transplantation in some patients with Alport's syndrome. We compared the conformations of the antibody epitopes in Goodpasture's disease and Alport's post-transplantation nephritis with the intention of finding clues to the pathogenesis of anti-GBM glomerulonephritis. METHODS: We used an enzyme-linked immunosorbent assay to determine the specificity of circulating autoantibodies and kidney-bound antibodies to NC1 domains. Circulating antibodies were analyzed in 57 patients with Goodpasture's disease, and kidney-bound antibodies were analyzed in 14 patients with Goodpasture's disease and 2 patients with Alport's post-transplantation nephritis. The molecular architecture of key epitope regions was deduced with the use of chimeric molecules and a three-dimensional model of the alpha345NC1 hexamer. RESULTS: In patients with Goodpasture's disease, both autoantibodies to the alpha3NC1 monomer and antibodies to the alpha5NC1 monomer (and fewer to the alpha4NC1 monomer) were bound in the kidneys and lungs, indicating roles for the alpha3NC1 and alpha5NC1 monomers as autoantigens. High antibody titers at diagnosis of anti-GBM disease were associated with ultimate loss of renal function. The antibodies bound to distinct epitopes encompassing region E(A) in the alpha5NC1 monomer and regions E(A) and E(B) in the alpha3NC1 monomer, but they did not bind to the native cross-linked alpha345NC1 hexamer. In contrast, in patients with Alport's post-transplantation nephritis, alloantibodies bound to the E(A) region of the alpha5NC1 subunit in the intact hexamer, and binding decreased on dissociation. CONCLUSIONS: The development of Goodpasture's disease may be considered an autoimmune "conformeropathy" that involves perturbation of the quaternary structure of the alpha345NC1 hexamer, inducing a pathogenic conformational change in the alpha3NC1 and alpha5NC1 subunits, which in turn elicits an autoimmune response. (Funded by the National Institute of Diabetes and Digestive and Kidney Diseases.)

Kidney collecting duct cells make vasopressin in response to NaCl-induced hypertonicity.

Vasopressin has traditionally been thought to be produced by the neurohypophyseal system and then released into the circulation where it regulates water homeostasis. The questions of whether vasopressin could be produced outside of the brain and if the kidney could be a source of vasopressin are raised by the syndrome of inappropriate antidiuretic hormone secretion (vasopressin). We found that mouse and human kidneys expressed vasopressin mRNA. Using an antibody that detects preprovasopressin, we found that immunoreactive preprovasopressin protein was found in mouse and human kidneys. Moreover, we found that murine collecting duct cells made biologically active vasopressin, which increased in response to NaCl-mediated hypertonicity, and that water restriction increased the abundance of kidney-derived vasopressin mRNA and protein expression in mouse kidneys. Thus, we provide evidence of biologically active production of kidney-derived vasopressin in kidney tubular epithelial cells.

Goodpasture's disease: molecular architecture of the autoantigen provides clues to etiology and pathogenesis.

PURPOSE OF REVIEW: Goodpasture's disease is an autoimmune disorder characterized by the deposition of pathogenic autoantibodies in basement membranes of kidney and lung, which induces rapidly progressive glomerulonephritis and pulmonary hemorrhage. The target antigen is the α3NC1 domain of collagen IV, which is expressed in target organs as an α345 network. Recent studies of specificity and epitopes of Goodpasture's autoantibodies and discovery of novel posttranslational modification of the antigen, a sulfilimine bond, provide further insight into mechanisms of initiation and progression of Goodpasture's disease. RECENT FINDINGS: Analysis of the specificity of Goodpasture's autoantibodies revealed a distinct subset of circulating and kidney-bound antiα5NC1 antibody, which is associated with loss of kidney function. Structural integrity of the α345NC1 hexamer is stabilized by the novel sulfilimine crosslinks conferring immune privilege to the Goodpasture's autoantigen. Native antibodies may contribute to establishment of immune tolerance to autoantigen. Structural analysis of epitopes for autoantibodies and alloantibodies indicates a critical role of conformational change in the α345NC1 hexamer in eliciting an autoimmune response in Goodpasture's disease. SUMMARY: Understanding of the quaternary structure of the Goodpasture's autoantigen continues to provide insights into autoimmune mechanisms that serve as a basis for development of novel diagnostic tools and therapies for Goodpasture's disease.

EGF receptor-mediated FUS phosphorylation promotes its nuclear translocation and fibrotic signaling.

Excessive accumulation of collagen leads to fibrosis. Integrin α1ß1 (Itgα1ß1) prevents kidney fibrosis by reducing collagen production through inhibition of the EGF receptor (EGFR) that phosphorylates cytoplasmic and nuclear proteins. To elucidate how the Itgα1ß1/EGFR axis controls collagen synthesis, we analyzed the levels of nuclear tyrosine phosphorylated proteins in WT and Itgα1-null kidney cells. We show that the phosphorylation of the RNA-DNA binding protein fused in sarcoma (FUS) is higher in Itgα1-null cells. FUS contains EGFR-targeted phosphorylation sites and, in Itgα1-null cells, activated EGFR promotes FUS phosphorylation and nuclear translocation. Nuclear FUS binds to the collagen IV promoter, commencing gene transcription that is reduced by inhibiting EGFR, down-regulating FUS, or expressing FUS mutated in the EGFR-targeted phosphorylation sites. Finally, a cell-penetrating peptide that inhibits FUS nuclear translocation reduces FUS nuclear content and collagen IV transcription. Thus, EGFR-mediated FUS phosphorylation regulates FUS nuclear translocation and transcription of a major profibrotic collagen gene. Targeting FUS nuclear translocation offers a new antifibrotic therapy.

Loss of function of Colgalt1 disrupts collagen post-translational modification and causes musculoskeletal defects.

In a screen for organogenesis defects in N-ethyl-N-nitrosourea (ENU)-induced mutant mice, we discovered a line carrying a mutation in Colgalt1 [collagen beta(1-O)galactosyltransferase type 1], which is required for proper galactosylation of hydroxylysine residues in a number of collagens. Colgalt1 mutant embryos have not been previously characterized; here, we show that they exhibit skeletal and muscular defects. Analysis of mutant-derived embryonic fibroblasts reveals that COLGALT1 acts on collagen IV and VI, and, while collagen VI appears stable and its secretion is not affected, collagen IV accumulates inside of cells and within the extracellular matrix, possibly due to instability and increased degradation. We also generated mutant zebrafish that do not express the duplicated orthologs of mammalian Colgalt1 The double-homozygote mutants have muscle defects; they are viable through the larvae stage but do not survive to 10 days post-fertilization. We hypothesize that the Colgalt1 mutant could serve as a model of a human connective tissue disorder and/or congenital muscular dystrophy or myopathy.

Quantitative proteomic profiling of extracellular matrix and site-specific collagen post-translational modifications in an in vitro model of lung fibrosis.

Lung fibrosis is characterized by excessive deposition of extracellular matrix (ECM), in particular collagens, by fibroblasts in the interstitium. Transforming growth factor-ß1 (TGF-ß1) alters the expression of many extracellular matrix (ECM) components produced by fibroblasts, but such changes in ECM composition as well as modulation of collagen post-translational modification (PTM) levels have not been comprehensively investigated. Here, we performed mass spectrometry (MS)-based proteomics analyses to assess changes in the ECM deposited by cultured lung fibroblasts from idiopathic pulmonary fibrosis (IPF) patients upon stimulation with transforming growth factor ß1 (TGF-ß1). In addition to the ECM changes commonly associated with lung fibrosis, MS-based label-free quantification revealed profound effects on enzymes involved in ECM crosslinking and turnover as well as multiple positive and negative feedback mechanisms of TGF-ß1 signaling. Notably, the ECM changes observed in this in vitro model correlated significantly with ECM changes observed in patient samples. Because collagens are subject to multiple PTMs with major implications in disease, we implemented a new bioinformatic platform to analyze MS data that allows for the comprehensive mapping and site-specific quantitation of collagen PTMs in crude ECM preparations. These analyses yielded a comprehensive map of prolyl and lysyl hydroxylations as well as lysyl glycosylations for 15 collagen chains. In addition, site-specific PTM analysis revealed novel sites of prolyl-3-hydroxylation and lysyl glycosylation in type I collagen. Interestingly, the results show, for the first time, that TGF-ß1 can modulate prolyl-3-hydroxylation and glycosylation in a site-specific manner. Taken together, this proof of concept study not only reveals unanticipated TGF-ß1 mediated regulation of collagen PTMs and other ECM components but also lays the foundation for dissecting their key roles in health and disease. The proteomic data has been deposited to the ProteomeXchange Consortium via the MassIVE partner repository with the data set identifier MSV000082958.

Building collagen IV smart scaffolds on the outside of cells.

Collagen IV scaffolds assemble through an intricate pathway that begins intracellularly and is completed extracellularly. Multiple intracellular enzymes act in concert to assemble collagen IV protomers, the building blocks of collagen IV scaffolds. After being secreted from cells, protomers are activated to initiate oligomerization, forming insoluble networks that are structurally reinforced with covalent crosslinks. Within these networks, embedded binding sites along the length of the protomer lead to the "decoration" of collagen IV triple helix with numerous functional molecules. We refer to these networks as "smart" scaffolds, which as a component of the basement membrane enable the development and function of multicellular tissues in all animal phyla. In this review, we present key molecular mechanisms that drive the assembly of collagen IV smart scaffolds.

Extracellular chloride signals collagen IV network assembly during basement membrane formation.

Basement membranes are defining features of the cellular microenvironment; however, little is known regarding their assembly outside cells. We report that extracellular Cl(-) ions signal the assembly of collagen IV networks outside cells by triggering a conformational switch within collagen IV noncollagenous 1 (NC1) domains. Depletion of Cl(-) in cell culture perturbed collagen IV networks, disrupted matrix architecture, and repositioned basement membrane proteins. Phylogenetic evidence indicates this conformational switch is a fundamental mechanism of collagen IV network assembly throughout Metazoa. Using recombinant triple helical protomers, we prove that NC1 domains direct both protomer and network assembly and show in Drosophila that NC1 architecture is critical for incorporation into basement membranes. These discoveries provide an atomic-level understanding of the dynamic interactions between extracellular Cl(-) and collagen IV assembly outside cells, a critical step in the assembly and organization of basement membranes that enable tissue architecture and function. Moreover, this provides a mechanistic framework for understanding the molecular pathobiology of NC1 domains.