Matrix Biology

Collagen is the most abundant structural protein in the human body, and its supramolecular organization is central to tissue mechanics and cell-matrix interactions. Integrins, key mediators of these interactions, are essential for key biological processes including adhesion, migration, differentiation, and platelet aggregation. While mutations in collagen are known to cause connective tissue disorders such as Osteogenesis Imperfecta (OI) with phenotypes ranging from mild to perinatal lethal, how these mutations alter fibril level architecture, dynamics and integrin-mediated interactions remains poorly understood. Here, we generated collagen-rich extra-cellular matrix (ECM) from primary dermal fibroblasts of a healthy donor (WT) and from two OI patients carrying distinct glycine mutations: G610C, associated with moderate disease, and G907D, linked to perinatal lethality. Comparative biophysical studies reveal that both mutants retain the canonical D-banding of collagen I fibrils but differ markedly at the nanoscale. G907D fibrils exhibit greater local structural perturbations and increased molecular mobility relative to the non-lethal G610C. Importantly, integrin binding also diverges between mutants: G610C displays reduced affinity, whereas G907D exhibits enhanced affinity compared to WT. Together, these findings establish a mechanistic link between single-residue mutations, nanoscale fibril architecture and collagen-receptor interactions, and highlight how genetic or acquired collagen defects can drive ECM dysregulation.

Collagen biosynthesis within the extracellular matrix (ECM) relies on finely regulated enzymatic steps to ensure proper collagen maturation and fibrillar assembly. Among these, bone morphogenetic protein-1 (BMP1) and the canonical lysyl oxidase (LOX) act on the collagen telopeptide to promote procollagen processing and oxidative cross-linking, respectively. However, the mechanisms that ensure precise coordination of their activities remain poorly understood. Using NanoBiT assays, we identified and characterized a stable LOX/BMP1 protein complex that assembles intracellularly during trafficking through the ER/Golgi pathway and persists after secretion. Analysis of BMP1 and LOX domains involved in the interaction showed that BMP1 binding requires its CUB2/3 domains, while LOX recognition depends on a conserved, positively charged segment of LOX (residues 259-285) located immediately upstream of its catalytic domain. Formation of the LOX/BMP1 complex did not substantially alter LOX enzymatic activity but markedly enhanced LOX association with collagen type I through the carboxy-telopeptide region, facilitating the assembly of a ternary LOX/BMP1/collagen complex. This pre-assembled complex promoted efficient targeting of LOX to nascent collagen fibrils. Our findings reveal a previously unrecognized layer of regulation in collagen biosynthesis, in which LOX and BMP1 act as a functional unit to ensure precise localization and proper processing of collagen. This mechanism offers new insights into ECM assembly and identifies the LOX/BMP1 interface as a potentially druggable node for anti-fibrotic strategies.

Formation of 4-hydroxyproline (4Hyp) in -X-Pro-Gly- collagen sequences is essential for the thermal stability of collagen molecules. 4Hyp formation is catalyzed by collagen prolyl 4-hydroxylases (C- P4H). Here we identify specific roles for the two main C-P4H isoenzymes by 4Hyp analysis of type I and IV collagens. Loss of C-P4H-I mainly affected prolines preceded by an X-position amino acid with a positively charged or a polar uncharged side chain. In contrast, loss of C-P4H-II affected triplets with a negatively charged glutamate or aspartate in the X-position, and their hydroxylation was found to be important as loss of C-P4H-II alone resulted in reduced collagen melting temperature and altered assembly of collagen fibrils and basement membrane. The C-P4H isoenzyme differences in substrate specificity were explained by selective substrate binding to the active site resulting in differences in Km and Vmax values. In conclusion, this study provides a molecular level explanation for the need of multiple C-P4H isoenzymes to generate collagen molecules capable to assemble into intact extracellular matrix structures.

The dermal papilla (DP), a specialized fibroblast aggregate in mammalian skin, plays a pivotal role in hair follicle development and regeneration through epithelial-mesenchymal interactions. While its aggregated configuration is critical for its function, the mechanism maintaining this organization has remained unclear. Here, we show that human DP cells are embedded in a basement membrane (BM)-like extracellular matrix (ECM) rather than in conventional interstitial fibrillar ECM, such as collagen I. This BM-like ECM occupies the intercellular space as a diffuse, mesh-like structure, with minimal cell-cell adhesion. These fibroblasts interact with specific laminin isoforms containing 1, 2 or 4 chains via integrin 7{beta}1, resulting in weak adhesiveness. In vitro, DP spheroids remained aggregated in a BM extract-based matrix, Matrigel, but dispersed in collagen I gel in an 1{beta}1 integrin-dependent manner. Our findings suggest that the BM components within DP aggregates mediate cell-ECM interactions and maintain the cohesive DP structure in the absence of strong cell-cell contacts. These results reveal an unconventional role for the BM as an adhesive microenvironment that sustains fibroblast aggregation and offer a new perspective on mesenchymal tissue organization and in vitro culture substrates for DP cells.

The myotendinous junction (MTJ) is a critical interface between muscle fibers and tendons, essential for force transmission between muscle and bone. Laminin-2, a key extracellular matrix (ECM) component, is strongly enriched at this interface. Mutations in the LAMA2 gene cause LAMA2-related muscular dystrophy (LAMA2 MD), an early-onset severe congenital muscular dystrophy. Here, we examined the MTJ in dyW/dyWmice, a mouse model for LAMA2 MD. We find a strong disruption of MTJ morphology, including altered muscle fiber tips, collagen XXII mislocalization, and reduced muscle tendon interface. As MTJ loading is altered in dyW/dyW mice and MTJ maintenance requires loading and unloading, we also examined MTJ structures upon denervation-induced unloading. While muscle fiber tip morphology resembled that of dyW/dyW mice, collagen XXII distribution was not affected and the muscle-tendon interface was preserved. Finally, proteomic profiling via laser capture microdissection and mass spectrometry revealed significant regional and global shifts in MTJ protein composition in dyW/dyW and denervated mice. Across both models, we identified integrin-associated remodeling as a shared response linked to the perturbed muscle fiber tip morphology. These findings demonstrate that laminin-2 is required for MTJ stability, and that mechanical unloading contributes to the observed phenotype. Importantly, our results suggest that disruptions in MTJ structure and protein composition may contribute to the pathology observed in LAMA2 MD.

Aging tendons undergo disruptions in homeostasis, increased susceptibility to injury, and reduced capacity for healing. Exploring the mechanisms behind this disruption in homeostasis is essential for developing therapeutics aimed at maintaining tendon health through the lifespan. We have previously identified that the extracellular matrix protein, Cochlin, which is highly expressed in healthy flexor tendon, is consistently lost during both natural aging and upon depletion of Scleraxis-lineage cells in young animals, which recapitulates many aging-associated homeostatic disruptions. Therefore, we hypothesized that loss of Cochlin would disrupt tendon homeostasis, including alterations in collagen fibril organization, and impaired tendon mechanics. By 3-months of age, Cochlin-/- flexor tendons exhibited altered collagen structure, with these changes persisting through at least 9-months. In addition, Cochlin-/- tendons demonstrated significant declines in structural and material properties at 6-months, and structural properties at 9-months. While Cochlin-/- did not drastically change the overall tendon proteome, consistent decreases in proteins associated with RNA metabolism, extracellular matrix production and the cytoskeleton were observed in Cochlin-/-. Interestingly, homeostatic disruption via Cochlin-/- did not impair the tendon healing process. Taken together, these data define a critical role for Cochlin in maintaining tendon homeostasis and suggest retention or restoration of Cochlin as a potential therapeutic approach to retain tendon structure and function through the lifespan.

Genetic mutations in fibrillin microfibrils cause a range of serious inherited diseases such as Marfan syndrome (MFS) and Weill-Marchesani syndrome (WMS). These diseases typically show major dysregulation of tissue development and growth, particularly in skeletal long bones, but links between the mutations and the diseases are unknown. In this study we reveal the detailed cryo-EM structure of native fibrillin microfibrils from mammalian tissue. The major bead region showed pseudo 8-fold symmetry and a buried protease resistant N-terminal core. Based on this structure, we show a WMS deletion mutant induces a rearrangement with long-range effects blocking interaction with latent TGF{beta}-binding protein (LTBP)-1 at a remote site. Separate deletion of this binding site resulted in the assembly of shorter fibrillin microfibrils with structural alterations. These results establish that in complex extracellular protein assemblies, such as in fibrillin, mutations may have long-range structural consequences to disrupt growth factor signalling and cause disease.

Collagen has been postulated to be the most abundant protein in our body, making up one-third of the total protein content in mammals. However, to the best of our knowledge, a direct assessment of the total collagen levels of an entire mammal to confirm this estimate is missing. Here we measured hydroxyproline levels as a proxy for collagen content together with total protein levels of entire mice or of individual tissues. Collagen content normalized to the total protein is approximately 0.1% in the brain and liver, 1% in the heart and kidney, 4% in the muscle and lung, 6% in the colon, 20-40% in the skin, 25-35% in bones, and 40-50% in tendons of wild-type (CD1 and CB57BL/6) mice, consistent with previous reports. Mice consist of 37 mg of collagen and 265 mg of protein per g of body weight. To our surprise, we find that collagen is approximately 12% in females and 17% in males of the total protein content of entire wild-type (CD1 and CB57BL/6) mice. High-Performance Liquid Chromatography approaches confirmed a 10-12% collagen over total protein estimates for female mice. Collagen staining methods and extracellular matrix-enriched proteomics estimated 5-6% of collagens over the total protein extracted. Although collagen type I is the most abundant collagen, the most abundant proteins are albumin, hemoglobulin, histones, actin, serpina, and then collagen type I. Analyzing amino acid compositions of mice revealed glycine as the most abundant amino acid. Thus, we provide reference points for collagen, matrisome, protein, and amino acid composition of healthy wild-type mice that are important for tissue and biomaterial engineering and for the comparison of these factors in various disease models.

Fibrotic lesions accompany several pathological conditions including tumors. We show that expression of a dominant-active form of the Ras oncogene in Drosophila salivary glands (SGs) leads to redistribution of components of the basement membrane (BM) and fibrotic lesions. Similar to several types of mammalian fibrosis, the disturbed BM attracts clot components including insect transglutaminase and phenoloxidase. SG epithelial cells show reduced apico-basal polarity accompanied by a loss of secretory activity. Both the fibrotic lesions and the reduced cell polarity are alleviated by ectopic expression of the antimicrobial peptide Drosomycin (Drs), which also restores secretory activity of the SGs. In addition to ECM components, both Drs and F-actin localize to fibrotic lesions.

Heavy carbon isotopes in the tendons of people who grew up in the age of nuclear bomb testing have shown that the extracellular matrix (ECM), assembled during development, stays with us for life. However, recent work suggests that type-I collagen in ECM-rich mouse tendon exists in two pools: a permanent matrix, and a more soluble, circadian-regulated matrix. Despite this, the underlying regulation of such distinct pools is not understood. Here, we demonstrate using stable isotope labelling coupled with mass spectrometry proteomics that circadian and permanent matrix pools have significantly different half-lives. Furthermore, the properties of the matrix pools are altered during development and ageing. Tail tendon tissue was harvested from mice fed on a heavy-lysine diet; protein was then extracted for analysis using a sequential two-step protocol. The first, soluble fraction ( F1) was found to contain intracellular proteins, and a range of core and associated extracellular matrix proteins, including a pool of type-I collagen shown to be circadian-regulated. The remaining fraction ( F2) contained primarily collagens, including type-I collagen which did not show rhythmicity. In adult mice, matrix proteins extracted in the F1 pool had significantly shorter half-lives than F2, including type-I collagen which had half-lives of 4 {+/-} 2 days in F1, compared to 700 {+/-} 100 days in F2. Circadian-regulated matrix proteins were found to have significantly faster turnover than non-circadian in adult mice, but this distinction was lost in older animals. This work identifies protein turnover as the underlying mechanism for the circadian/permanent model of tendon matrix, and suggests a loss of circadian regulation as a characteristic of ECM ageing.

The Hippo pathway effectors YAP and TAZ are key regulators of cell proliferation, apoptosis, and differentiation, thereby maintaining tissue homeostasis and controlling organ size. While their roles in epithelial tissues and cancer are well established, their role in dermal fibroblast extracellular matrix (ECM) regulation is less understood. Here, we investigated the role of Yap/Taz during postnatal skin dermis development. During postnatal growth, mouse skin steadily grows and undergoes significant surface expansion. Postnatal deletion of Yap/Taz in dermal fibroblasts, the primary cells responsible for dermal ECM homeostasis, significantly impaired dermal maturation, as evidenced by marked deficiencies in collagen synthesis and deposition. Isolated fibroblasts from Yap/Taz knockout mice showed reduced proliferation and diminished expression of Yap/Taz target genes (Ccn2, Col1a1), which were rescued by reintroduction of active Yap/Taz. RNA-seq, and spatial transcriptomics and proteomics of Yap/Taz knockout skin revealed substantial downregulation of matrisome genes, including type I (Col1a1, Col1a2) and type III (Col1a3) collagens, which together constitute more than 90% of the skins collagen content. These findings demonstrate that Yap/Taz are essential for dermal ECM homeostasis, highlighting their therapeutic potential in skin regeneration, fibrosis, and aging-related ECM decline.

Asprosin, the C-terminal furin cleavage product of profibrillin-1, was reported to act as a hormone that circulates at nanomolar levels and is recruited to the liver where it induces G protein-coupled activation of the cAMP-PKA pathway and stimulates rapid glucose release into the circulation. Although derived upon C-terminal cleavage of fibrillin-1, a multidomain extracellular matrix glycoprotein with a ubiquitous distribution in connective tissues, little is known about the mechanisms controlling the bioavailability of asprosin in tissues. In the current view, asprosin is mainly produced by white adipose tissue from where it is released into the blood in monomeric form. Here, by employing newly generated specific asprosin antibodies we monitored the distribution pattern of asprosin in human and murine connective tissues such as placenta, and muscle. Thereby we detected the presence of asprosin positive extracellular fibers. Further, by screening established cell lines for asprosin synthesis we found that most cells derived from musculoskeletal tissues render asprosin into an oligomerized form. Our analyses show that asprosin already multimerizes intracellularly, but that stable multimerization via covalent bonds is facilitated by transglutaminase activity. Further, asprosin fiber formation requires an intact fibrillin-1 fiber network for proper linear deposition. Our data suggest a new extracellular storage mechanism of asprosin in an oligomerized form which may regulate its cellular bioavailability in tissues.

BackgroundEffective skin wound healing is essential for restoring tissue integrity following injury. Repair proceeds through phases of hemostasis, inflammation, proliferation, and remodeling, but molecular mechanisms governing these stages remain poorly defined. Vascular niche cells (VNCs)-including endothelial cells, vascular smooth muscle cells (SMCs), and fibroblasts-are central regulators of healing, but the lack of longitudinal in vivo human data has limited identification of VNC-derived signals that distinguish effective repair from pathological healing such as ulcers. Thus, defining the regulation of VNCs in wound healing addresses a critical knowledge gap. MethodsWe developed a protocol for wound healing using dermal forearm punch biopsies to track longitudinal repair in healthy volunteers. Single-cell and spatial transcriptomics were performed to identify and validate signaling activities within VNCs. ResultsWe spatiotemporally defined the inflammation, proliferation, and remodeling phases of human skin wound healing with a focus on VNCs. Spatial analysis localized this activity for VNCs and immune cells within a heterogenous granulation zone that later led to re-epithelializion. Angiogenesis was dominated by Vegf, Egf and Hif1 signaling. Extracellular matrix (ECM) remodeling occurred through Collagen, Laminin, Thrombospondin, and Fibronectin. SMCs emerged as dominant drivers of injury-induced remodeling including basement membrane and interstitial ECM components compared to fibroblasts. This SMC-led program was further defined by robust induction of TIMP1, an inhibitor of matrix degradation, which localized to granulation tissue and correlated with re-epithelialization and wound resolution. Lastly, we compared remodeling factors between healing and non-healing human diabetic foot ulcers (DFUs). SMCs in non-healing DFUs had deficient expression for core remodeling factors, including TIMP1, indicating SMC activity is needed for effective healing. ConclusionWe identified an SMC-driven model of wound repair in which TIMP1-dependent activity underpins granulation zone formation. Failure of this program defined a mechanistic basis for impaired healing in ulcers, identifying SMCs and TIMP1 as therapeutic targets.

Matrix metalloproteinase-3 (MMP-3 or stromelysin 1) participates in normal extracellular matrix (ECM) turnover during embryonic development, organ morphogenesis and wound healing, and in tissue-destructive diseases, such as aneurysm, cancer, arthritis and heart failure. Despite its ability to hydrolyse numerous proteins in the ECM, MMP-3 fails to cleave the triple helix of interstitial fibrillar collagens. Nonetheless, it can still bind to these collagens although the mechanism, location and role of binding are not known. We used the Collagen Toolkits, libraries of triple-helical peptides that embrace the entire helical domains of collagens II and III, to map MMP-3 interaction sites. The enzyme recognises five sites on collagen II and three sites on collagen III. They share a glycine-phenylalanine-hydroxyproline/alanine (GFO/A) motif that is recognised by the enzyme in a context-dependent manner. Neither MMP-3 zymogen (proMMP-3) nor the individual catalytic (Cat) and hemopexin (Hpx) domains of MMP-3 interact with the peptides, revealing cooperative binding of both domains to the triple helix. The Toolkit binding data combined with molecular modelling enabled us to deduce the putative collagen-binding mode of MMP-3, where all three collagen chains make contacts with the enzyme in the valley running across both Cat and Hpx domains. The observed binding pattern casts light on how MMP-3 could regulate collagen turnover and compete with various collagen-binding proteins regulating cell adhesion and proliferation.

In mammals, significant injury is generally followed by the formation of a fibrotic scar which provides structural integrity but fails to functionally restore damaged tissue. Spiny mice of the genus Acomys represent the first example of full skin autotomy in mammals. Acomys cahirinus has evolved extremely weak skin as a strategy to avoid predation and is able to repeatedly regenerate healthy tissue without scar after severe skin injury or full-thickness ear punches. Extracellular matrix (ECM) composition is a critical regulator of wound repair and scar formation and previous studies have suggested that alterations in its expression may be responsible for the differences in regenerative capacity observed between Mus musculus and A. cahirinus, yet analysis of this critical tissue component has been limited in previous studies by its insolubility and resistance to extraction. Here, we utilize a 2-step ECM-optimized extraction to perform proteomic analysis of tissue composition during wound repair after full-thickness ear punches in A. cahirinus and M. musculus from weeks 1 to 4 post-injury. We observe changes in a wide range of ECM proteins which have been previously implicated in wound regeneration and scar formation, including collagens, coagulation and provisional matrix proteins, and matricryptic signaling peptides. We additionally report differences in crosslinking enzyme activity and ECM protein solubility between Mus and Acomys. Furthermore, we observed rapid and sustained increases in CD206, a marker of pro-regenerative M2 macrophages, in Acomys, whereas little or no increase in CD206 was detected in Mus. Together, these findings contribute to a comprehensive understanding of tissue cues which drive the regenerative capacity of Acomys and identify a number of potential targets for future pro-regenerative therapies.

Heligmosomoides polygyrus, a mouse parasite, modulates host immunity by secreting modular transforming growth factor-{beta} (TGF{beta}) mimics (TGMs). The agonist TGM1 interacts with TGFBR1, TGFBR2, and the co-receptor CD44 through domains D1/2, D3, and D4/5, respectively. In contrast, the antagonist TGM6, which lacks D1/2, but retains TGFBR2 binding through D3, targets different subsets of cells compared to TGM1. The TGM6 co-receptor is unknown. Using X-ray crystallography and binding studies, we show that TGM6 preferentially binds mouse TGFBR2 over human TGFBR2, and that this is essential for its antagonistic function. We identified low-density lipoprotein receptor-related protein 1 (LRP1) and betaglycan (TGFBR3) as co-receptors for TGM6. LRP1 enhances TGM6 efficacy and is vital for its specific antagonistic effects by promoting TGFBR2 degradation, while betaglycan counteracts TGM6 in a TGFBR2-dependent manner. The modular organization of TGMs enabled us to rationally design TGM1/6 chimeras or TGM-D3 fusion with an affibody that recognizes a specific cell-surface receptor, thereby altering cell-type specificity and functionality. Furthermore, we developed a TGFBR2 nanobody that, on its own, has no inhibitory effect but, when fused to a receptor antibody, antagonizes TGF{beta} signaling in a cell-selective manner. Thus, we designed programmable agents that modulate TGF{beta} signaling only in target co-receptor-expressing cells.

The tendon-bone enthesis is a specialized fibrocartilaginous tissue crucial for muscle-to-bone force transmission, yet its postnatal development is not fully understood. Emerging evidence suggests hypoxia plays a pivotal role in enthesis maturation akin to its function in skeletal growth plates, with Hypoxia Inducible Factor 1 alpha (HIF-1) acting as a key regulator of cellular adaptation (e.g., cell survival, extracellular matrix (ECM) deposition). Here, we investigated the spatial and temporal dynamics of hypoxia in the murine Achilles tendon enthesis and elucidated the role of Hif1a in enthesis cell survival and ECM formation using Scleraxis-lineage conditional knockout (ScxCre; Hif1a cKO) mice. We found that while neonatal tendons rapidly resolve hypoxia after birth, the enthesis maintains a hypoxic niche through postnatal day 5, mirroring a gradient observed in growth plates. Disruption of HIF-1 in enthesis-resident cells resulted in pronounced deficits in grip strength, abnormal tendon-bone attachment morphology, disrupted calcaneal architecture, impaired mineralization, and significant ECM disorganization. Histological analyses revealed persistent cell death and loss of the characteristic fibrocartilaginous gradient in cKO entheses, including dysregulated collagen alignment. In vitro, HIF-1-deficient tendon fibroblasts exhibited blunted transcriptional responses to hypoxia, altered metabolic gene expression, and changes in ECM deposition. Collectively, our findings illuminate hypoxia as a sustained niche in the postnatal enthesis, with HIF-1 critically required for cell survival, ECM organization, and enthesis structural integrity. This work advances our understanding of enthesis biology and provides insights relevant to tendon-bone attachment disorders and regenerative strategies.

Extracellular matrix (ECM) pathologic remodeling underlies many fibrotic disorders, including muscular dystrophy. Tissue decellularization removes cellular components while leaving behind ECM components. We generated "on-slide" decellularized tissue slices from genetically distinct dystrophic mouse models. The ECM of dystrophin-and sarcoglycan-deficient muscles had marked thrombospondin 4 deposition, while dysferlin-deficient muscle had excess decorin. Annexins A2 and A6 were present on all dystrophic decellularized ECMs, but annexin matrix deposition was excessive in dysferlin-deficient muscular dystrophy. Adeno-associated viral expression of annexin A6 specifically in muscle resulted in annexin A6 deposition throughout the ECM, indicating muscle as a source of this ECM protein. C2C12 myoblasts seeded onto decellularized matrices displayed differential myoblast mobility. Dystrophin-deficient decellularized matrices inhibited myoblast mobility while dysferlin-deficient decellularized matrices enhanced myoblast movement. Myoblasts treated with recombinant annexin A6 increased mobillity similar to that seen on dysferlin-deficient decellularized matrix. These findings demonstrate specific fibrotic signatures elicit effects on myoblast activity. TEASERFibrosis in muscular dystrophy has differential effects on myoblasts HIGHLIGHTSO_LISpatial architecture and composition of the ECM differ across genetically distinct forms of muscular dystrophy, especially with respect to Annexin A6 protein deposition C_LIO_LIMatrix from dystrophin-mediated muscular dystrophy inhibits myoblast movement C_LIO_LIMatrix from dysferlin-deficient muscular dystrophy promotes myoblast motility C_LIO_LIAnnexin A6 was sufficient to enhance myoblast motility C_LI

Collagen-IV (Col-IV) scaffolds, a primordial basement membrane component, enabled animal multicellularity, evolution and adaptation. These scaffolds provide tensile strength and tether macromolecules, forming supramolecular complexes that interact with cell-surface receptors and influence cell-behavior. Triple-helical Col-IV protomers, composed of three -chains, with a trimeric globular NC1-domain at the C-terminus, oligomerize forming a NC1-hexamer structure that connects adjoining protomers of Col-IV121, Col-IV556--121, and Col-IV345 scaffolds. Hexamer formation and stability are driven by the extracellular chloride concentration-"chloride pressure". Hexamer structure is reinforced by six sulfilimine bonds forming covalent crosslinks that weld together trimeric NC1-domains of adjoining protomers. We recently found evidence that sulfilimine bonds, independent of chloride, stabilize the quaternary structure of the Col-IV345 hexamer of the Col-IV345 scaffold. Here, we sought to determine whether this function also pertains to the Col-IV121 scaffold that occurs ubiquitously across the animal kingdom, and whether bromine, a cofactor of peroxidasin in bond formation, are evolutionary conserved. We found that sulfilimine bonds stabilized the quaternary structure of the Col-IV121 hexamer of bovine, mouse and a basal cnidarian, Nematostella vectensis, and that the mechanism of bond formation mediated by peroxidasin and bromide is evolutionary conserved. Analyses of the crystal structure of the NC1-hexamer revealed that sulfilimine bonds covalently fasten a clasp-motif across the trimer-trimer interface, interlocking the domain-swapping region of neighboring subunits, which reinforces the hexamer quaternary structure imposed by chloride conformational constraints. Collectively, our findings reveal that the sulfilimine-bond reinforcement is a critical event in Col-IV scaffold assembly enabling multicellularity, evolution and adaptation of metazoans, beginning with ancient cnidarians.

Anchoring fibrils formed by collagen VII play a critical role in stabilizing the dermal-epidermal junction. The N-terminal non-collagenous (NC1) domain of collagen VII binds firmly to basement membrane components including collagen IV and has also been reported to interact with mesenchymal fibrillar collagens via its von Willebrand factor A-like domain 2 (A2 domain). To elucidate how collagen VII recognizes fibrillar collagen, we performed yeast two-hybrid screening using a triple-helical random peptide library, which resulted in the identification of a Met-Gly-{Phi} ({Phi}; aromatic amino acid residue) motif. Biochemical analysis with synthetic triple-helical peptides revealed a binding preference of Trp > Phe as the {Phi} residue by the A2 domain despite Trp being absent in native collagens. The crystal structure of the A2 domain in complex with the Nle (Met surrogate)-Gly-Trp-containing peptide revealed a unique mechanism by which two distinct hydrophobic pockets of the A2 domain accommodate the Nle and Trp residues corresponding to the Met-Gly-{Phi} motif, engaging all three chains of the triple helix. Subsequent molecular dynamics simulations demonstrated that the A2 domain recognizes the corresponding native Met-Gly-Phe motif in a similar manner, but with lower affinity, implying a transient interaction with mesenchymal collagens. The findings obtained in this work suggest models in which transient A2-triple helix interaction promotes the recruitment of collagen I and III fibrils into the arc-shaped structure of anchoring fibrils. This also provides a foundation for linking structural understanding to skin fragility diseases caused by collagen VII dysfunction.