Stepwise proteolytic activation of type I procollagen to collagen within the secretory pathway of tendon fibroblasts in situ

Designing collagens to shed light on the multi-scale structure-function mapping of matrix disorders

Collagens are the most abundant structural proteins in the extracellular matrix of animals and play crucial roles in maintaining the structural integrity and mechanical properties of tissues and organs while mediating important biological processes. Fibrillar collagens have a unique triple helix structure with a characteristic repeating sequence of (Gly-X-Y)n. Variations within the repetitive sequence can cause misfolding of the triple helix, resulting in heritable connective tissue disorders. The most common variations are single-point missense mutations that lead to the substitution of a glycine residue with a bulkier amino acid (Gly → X). In this review, we will first discuss the importance of collagen's triple helix structure and how single Gly substitutions can impact its folding, structure, secretion, assembly into higher-order structures, and biological functions. We will review the role of "designer collagens," i.e., synthetic collagen-mimetic peptides and recombinant bacterial collagen as model systems to include Gly → X substitutions observed in collagen disorders and investigate their impact on structure and function utilizing in vitro studies. Lastly, we will explore how computational modeling of collagen peptides, especially molecular and steered molecular dynamics, has been instrumental in probing the effects of Gly substitutions on structure, receptor binding, and mechanical stability across multiple length scales.

The proprotein convertase BLI-4 promotes collagen secretion prior to assembly of the Caenorhabditis elegans cuticle

Some types of collagens, including transmembrane MACIT collagens and C. elegans cuticle collagens, are N-terminally cleaved at a dibasic site that resembles the consensus for furin or other proprotein convertases of the subtilisin/kexin (PCSK) family. Such cleavage may release transmembrane collagens from the plasma membrane and affect extracellular matrix assembly or structure. However, the functional consequences of such cleavage are unclear and evidence for the role of specific PCSKs is lacking. Here, we used endogenous collagen fusions to fluorescent proteins to visualize the secretion and assembly of the first collagen-based cuticle in C. elegans and then tested the role of the PCSK BLI-4 in these processes. Unexpectedly, we found that cuticle collagens SQT-3 and DPY-17 are secreted into the extraembryonic space several hours before cuticle matrix assembly. Furthermore, this early secretion depends on BLI-4/PCSK; in bli-4 and cleavage-site mutants, SQT-3 and DPY-17 are not efficiently secreted and instead form large intracellular puncta. Their later assembly into cuticle matrix is reduced but not entirely blocked. These data reveal a role for collagen N-terminal processing in intracellular trafficking and the control of matrix assembly in vivo. Our observations also prompt a revision of the classic model for C. elegans cuticle matrix assembly and the pre-cuticle-to-cuticle transition, suggesting that cuticle layer assembly proceeds via a series of regulated steps and not simply by sequential secretion and deposition.

The proprotein convertase BLI-4 promotes collagen secretion during assembly of the Caenorhabditis elegans cuticle

Some types of collagens, including transmembrane MACIT collagens and C. elegans cuticle collagens, are N-terminally cleaved at a dibasic site that resembles the consensus for furin or other proprotein convertases of the subtilisin/kexin (PCSK) family. Such cleavage may release transmembrane collagens from the plasma membrane and affect extracellular matrix assembly or structure. However, the functional consequences of such cleavage are unclear and evidence for the role of specific PCSKs is lacking. Here, we used endogenous collagen fusions to fluorescent proteins to visualize the secretion and assembly of the first collagen-based cuticle in C. elegans and then tested the role of the PCSK BLI-4 in these processes. Unexpectedly, we found that cuticle collagens SQT-3 and DPY-17 are secreted into the extraembryonic space several hours before cuticle matrix assembly. Furthermore, this early secretion depends on BLI-4/PCSK; in bli-4 and cleavage-site mutants, SQT-3 and DPY-17 are not efficiently secreted and instead form large intracellular aggregates. Their later assembly into cuticle matrix is reduced but not entirely blocked. These data reveal a role for collagen N-terminal processing in intracellular trafficking and in the spatial and temporal restriction of matrix assembly in vivo . Our observations also prompt a revision of the classic model for C. elegans cuticle matrix assembly and the pre-cuticle-to-cuticle transition, suggesting that cuticle layer assembly proceeds via a series of regulated steps and not simply by sequential secretion and deposition.

Vitreous Humor: Composition, Characteristics and Implication on Intravitreal Drug Delivery

Purpose: Intravitreal administration of drug molecules is one of the most common routes for treating posterior segment eye diseases. However, the properties of vitreous humour changes with the time. A number of ocular complications such as liquefaction of the vitreous humour, solidification of the vitreous humour in the central vitreous cavity and detachment of the limiting membrane due to the shrinking of vitreous humour are some of the factors that can drastically affect the efficacy of therapeutics delivered via intravitreal route. Although significant research has been conducted for studying the properties of vitreous humour and its changes during the ageing process, there have been limited work to understand the effect of these changes on therapeutic efficacy of intravitreal drug delivery systems. Therefore, in this review we discussed both the coomposition and characteristics of the vitreous humour, and their subsequent influence on intravitreal drug delivery.Methods: Articles were searched on Scopus, PubMed and Web of Science up to March 2022.Results: In this review, we discussed the biological composition and biomechanical properties of vitreous humour, methods to study the properties of vitreous humour and the changes in these properties and their relevance in ocular drug delivery field, with the aim to provide a useful insight into these aspects which can aid the process of development of novel intravitreal drug delivery systems.Conclusions: The composition and characteristics of the vitreous humour, and how these change during natural aging processes, directly influence intravitreal drug delivery. This review therefore highlights the importance of understanding the properties of the vitreous and identifies the need to achieve greater understanding of how changing properties of the vitreous affect the therapeutic efficacy of drugs administered for the treatment of posterior eye diseases.

Visualized procollagen Iα1 demonstrates the intracellular processing of propeptides

Procollagen Iα1 with two tags reveals the different fates of processed propeptides, the rate-limiting step in collagen secretion, and a link between defects in intracellular processing and diseases.

The processing of type I procollagen is essential for fibril formation; however, the steps involved remain controversial. We constructed a live cell imaging system by inserting fluorescent proteins into type I pre-procollagen α1. Based on live imaging and immunostaining, the C-propeptide is intracellularly cleaved at the perinuclear region, including the endoplasmic reticulum, and subsequently accumulates at the upside of the cell. The N-propeptide is also intracellularly cleaved, but is transported with the repeating structure domain of collagen into the extracellular region. This system makes it possible to detect relative increases and decreases in collagen secretion in a high-throughput manner by assaying fluorescence in the culture medium, and revealed that the rate-limiting step for collagen secretion occurs after the synthesis of procollagen. In the present study, we identified a defect in procollagen processing in activated hepatic stellate cells, which secrete aberrant collagen fibrils. The results obtained demonstrated the intracellular processing of type I procollagen, and revealed a link between dysfunctional processing and diseases such as hepatic fibrosis.

Supply chain logistics - the role of the Golgi complex in extracellular matrix production and maintenance

The biomechanical and biochemical properties of connective tissues are determined by the composition and quality of their extracellular matrix. This, in turn, is highly dependent on the function and organisation of the secretory pathway. The Golgi complex plays a vital role in directing matrix output by co-ordinating the post-translational modification and proteolytic processing of matrix components prior to their secretion. These modifications have broad impacts on the secretion and subsequent assembly of matrix components, as well as their function in the extracellular environment. In this Review, we highlight the role of the Golgi in the formation of an adaptable, healthy matrix, with a focus on proteoglycan and procollagen secretion as example cargoes. We then discuss the impact of Golgi dysfunction on connective tissue in the context of human disease and ageing.

Collagen fibril assembly: New approaches to unanswered questions

Collagen fibrils are essential for metazoan life. They are the largest, most abundant, and most versatile protein polymers in animals, where they occur in the extracellular matrix to form the structural basis of tissues and organs. Collagen fibrils were first observed at the turn of the 20th century. During the last 40 years, the genes that encode the family of collagens have been identified, the structure of the collagen triple helix has been solved, the many enzymes involved in the post-translational modifications of collagens have been identified, mutations in the genes encoding collagen and collagen-associated proteins have been linked to heritable disorders, and changes in collagen levels have been associated with a wide range of diseases, including cancer. Yet despite extensive research, a full understanding of how cells assemble collagen fibrils remains elusive. Here, we review current models of collagen fibril self-assembly, and how cells might exert control over the self-assembly process to define the number, length and organisation of fibrils in tissues.

Giantin is required for intracellular N-terminal processing of type I procollagen

Knockout of the golgin giantin leads to skeletal and craniofacial defects driven by poorly studied changes in glycosylation and extracellular matrix deposition. Here, we sought to determine how giantin impacts the production of healthy bone tissue by focusing on the main protein component of the osteoid, type I collagen. Giantin mutant zebrafish accumulate multiple spontaneous fractures in their caudal fin, suggesting their bones may be more brittle. Inducing new experimental fractures revealed defects in the mineralization of newly deposited collagen as well as diminished procollagen reporter expression in mutant fish. Analysis of a human giantin knockout cell line expressing a GFP-tagged procollagen showed that procollagen trafficking is independent of giantin. However, our data show that intracellular N-propeptide processing of pro-α1(I) is defective in the absence of giantin. These data demonstrate a conserved role for giantin in collagen biosynthesis and extracellular matrix assembly. Our work also provides evidence of a giantin-dependent pathway for intracellular procollagen processing.

Prolyl and lysyl hydroxylases in collagen synthesis

Collagens are the most abundant proteins in the extracellular matrix. They provide a framework to build organs and tissues and give structural support to make them resistant to mechanical load and forces. Several intra- and extracellular modifications are needed to make functional collagen molecules, intracellular post-translational modifications of proline and lysine residues having key roles in this. In this article, we provide a review on the enzymes responsible for the proline and lysine modifications, that is collagen prolyl 4-hydroxylases, 3-hydroxylases and lysyl hydroxylases, and discuss their biological functions and involvement in diseases.

Dysregulated mesenchymal PDGFR-β drives kidney fibrosis

Kidney fibrosis is characterized by expansion and activation of platelet-derived growth factor receptor-β (PDGFR-β)-positive mesenchymal cells. To study the consequences of PDGFR-β activation, we developed a model of primary renal fibrosis using transgenic mice with PDGFR-β activation specifically in renal mesenchymal cells, driving their pathological proliferation and phenotypic switch toward myofibroblasts. This resulted in progressive mesangioproliferative glomerulonephritis, mesangial sclerosis, and interstitial fibrosis with progressive anemia due to loss of erythropoietin production by fibroblasts. Fibrosis induced secondary tubular epithelial injury at later stages, coinciding with microinflammation, and aggravated the progression of hypertensive and obstructive nephropathy. Inhibition of PDGFR activation reversed fibrosis more effectively in the tubulointerstitium compared to glomeruli. Gene expression signatures in mice with PDGFR-β activation resembled those found in patients. In conclusion, PDGFR-β activation alone is sufficient to induce progressive renal fibrosis and failure, mimicking key aspects of chronic kidney disease in humans. Our data provide direct proof that fibrosis per se can drive chronic organ damage and establish a model of primary fibrosis allowing specific studies targeting fibrosis progression and regression.

Flexor Tendon: Development, Healing, Adhesion Formation, and Contributing Growth Factors

Management of flexor tendon injuries of the hand remains a major clinical problem. Even with intricate repair, adhesion formation remains a common complication. Significant progress has been made to better understand the mechanisms of healing and adhesion formation. However, there has been slow progress in the clinical prevention and reversal of flexor tendon adhesions. The goal of this article is to discuss recent literature relating to tendon development, tendon healing, and adhesion formation to identify areas in need of further research. Additional research is needed to understand and compare the molecular, cellular, and genetic mechanisms involved in flexor tendon morphogenesis, postoperative healing, and mechanical loading. Such knowledge is critical to determine how to improve repair outcomes and identify new therapeutic strategies to promote tissue regeneration and prevent adhesion formation.

Fibronectin matrix as a scaffold for procollagen proteinase binding and collagen processing

The extracellular matrix (ECM) proteins fibronectin (FN) and type I collagen (collagen I) are codistributed in many tissues, and collagens have been shown to depend on an FN matrix for fibrillogenesis. Microscopic analysis of a fibroblast ECM showed colocalization of procollagen I with FN fibrils, and proteolytic cleavage of procollagen to initiate fibril formation was significantly reduced with inhibition of FN matrix assembly. We examined the role of FN matrix in procollagen processing by the C-propeptide proteinase bone morphogenetic protein 1 (BMP-1). We found that BMP-1 binds to a cell-assembled ECM in a dose-dependent manner and that, like procollagen, BMP-1 colocalizes with FN fibrils in the matrix microenvironment. Binding studies with FN fragments identified a binding site in FN’s primary heparin-binding domain. In solution, BMP-1–FN interactions and BMP-1 cleavage of procollagen I were both enhanced by the presence of heparin, suggesting a role for heparin in complex formation during proteolysis. Indeed, addition of heparin enhanced the rate of procollagen cleavage by matrix-bound BMP-1. Our results show that matrix localization of this proteinase facilitates the initiation of collagen assembly and suggest a model in which FN matrix and associated heparan sulfate act as a scaffold to organize enzyme and substrate for procollagen processing.

Molecular mechanisms and clinical manifestations of rare genetic disorders associated with type I collagen

Type I collagen is an important structural protein of bone, skin, tendon, ligament and other connective tissues. It is initially synthesized as a precursor form, procollagen, consisting of two identical pro-α1(I) and one proα2(I) chains, encoded by COL1A1 and COL1A2, respectively. The N- and C- terminal propeptides of procollagen are cleavage by N-proteinase and C-proteinase correspondingly, to form the central triple helix structure with Gly-X-Y repeat units. Mutations of COL1A1 and COL1A2 genes are associated with osteogenesis imperfecta, some types of Ehlers-Danlos syndrome, Caffey diseases, and osteogenesis imperfect/Ehlers- Danlos syndrome overlapping diseases. Clinical symptoms caused by different variations can be variable or similar, mild to lethal, and vice versa. We reviewed the relationship between clinical manifestations and type I collagen - related rare genetic disorders and their possible molecular mechanisms for different mutations and disorders.

The endoplasmic reticulum-resident collagen chaperone Hsp47 interacts with and promotes the secretion of decorin, fibromodulin, and lumican

The build-up of diversified and tissue-specific assemblies of extracellular matrix (ECM) proteins depends on secreted and cell surface-located molecular arrays that coordinate ECM proteins into discrete designs. The family of small leucine-rich proteins (SLRPs) associates with and dictates the structure of fibrillar collagens, which form the backbone of most ECM types. However, whether SLRPs form complexes with proteins other than collagens is unclear. Here, we demonstrate that heat shock protein 47 (Hsp47), a well-established endoplasmic reticulum-resident collagen chaperone, also binds the SLRPs decorin, lumican, and fibromodulin with affinities comparable with that in the Hsp47-type I collagen interaction. Furthermore, we show that a lack of Hsp47 inhibits the cellular secretion of decorin and lumican. Our results expand the understanding of the concerted molecular interactions that control the secretion and organization of a functional collagenous ECM.

A High-Throughput Assay for Collagen Secretion Suggests an Unanticipated Role for Hsp90 in Collagen Production

Collagen overproduction is a feature of fibrosis and cancer, while insufficient deposition of functional collagen molecules and/or the secretion of malformed collagen is common in genetic disorders like osteogenesis imperfecta. Collagen secretion is an appealing therapeutic target in these and other diseases, as secretion directly connects intracellular biosynthesis to collagen deposition and biological function in the extracellular matrix. However, small molecule and biological methods to tune collagen secretion are severely lacking. Their discovery could prove useful not only in the treatment of disease, but also in providing tools for better elucidating mechanisms of collagen biosynthesis. We developed a cell-based, high-throughput luminescent assay of collagen type I secretion and used it to screen for small molecules that selectively enhance or inhibit that process. Among several validated hits, the Hsp90 inhibitor 17-allylaminogeldanamycin (17-AAG) robustly decreases the secretion of collagen-I by our model cell line and by human primary cells. In these systems, 17-AAG and other pan-isoform Hsp90 inhibitors reduce collagen-I secretion post-translationally and are not global inhibitors of protein secretion. Surprisingly, the consequences of Hsp90 inhibitors cannot be attributed to inhibition of the endoplasmic reticulum's Hsp90 isoform, Grp94. Instead, collagen-I secretion likely depends on the activity of cytosolic Hsp90 chaperones, even though such chaperones cannot directly engage nascent collagen molecules. Our results highlight the value of a cell-based high-throughput screen for selective modulators of collagen secretion and suggest an unanticipated role for cytosolic Hsp90 in collagen secretion.

Towards understanding the dose and timing effect of hydrocortisone treatment on the scleral ossicle system within the chicken eye

Previous work, almost four decades ago, showed that hydrocortisone (HC) treatment reduces the number of skeletogenic condensations that give rise to the scleral ossicles in the chicken eye. The scleral ossicles are a ring of overlapping intramembranous bones, the sclerotic ring, and are present in most reptiles, including birds. The scleral condensations that give rise to the scleral ossicles are induced by a series of overlying thickenings (or papillae) of the conjunctival epithelium. Here, we further explore the effects of altering the dosage and timing of HC treatment on the morphology and number of skeletogenic condensations and conjunctival papillae. We show that high doses can completely obliterate the entire sclerotic ring. Significantly, the reduction in papillae number we observed was less extreme than that of the scleral condensations, indicating that additional factors contribute to the observed skeletogenic condensation loss. Via immunohistochemical analyses, we show that HC treatment alters the spatial expression pattern of several extracellular matrix components (tenascin-C, decorin and procollagen I) and also alters the vasculature network within the sclera. This research provides important insights into understanding the role of the scleral tissue components in ossicle development within the vertebrate eye.

Collagen IV trafficking: The inside-out and beyond story

Collagen IV networks endow basement membranes (BMs) with remarkable tensile strength and function as morphoregulatory substrata for diverse tissue-specific developmental events. A complex repertoire of intracellular and extracellular molecular interactions are required for collagen IV secretion and supramolecular assembly into BMs. These include intracellular chaperones such as Heat shock protein 47 (Hsp47) and the chaperone-binding trafficking protein Transport and Golgi organization protein 1 (Tango1). Mutations in these proteins lead to compromised collagen IV protomer stability and secretion, leading to defective BM assembly and function. In addition to intracellular chaperones, a role for extracellular chaperones orchestrating the transport, supramolecular assembly, and architecture of collagen IV in BM is emerging. We present evidence derived from evolutionarily distant model organisms that supports an extracellular collagen IV chaperone-like activity for the matricellular protein SPARC (Secreted Protein, Acidic, Rich in Cysteine). Loss of SPARC disrupts BM homeostasis and compromises tissue biomechanics and physiological function. Thus, the combined contributions of intracellular and extracellular collagen IV-associated chaperones and chaperone-like proteins are critical to ensure proper secretion and stereotypic assembly of collagen IV networks in BMs.

Uniform spatial distribution of collagen fibril radii within tendon implies local activation of pC-collagen at individual fibrils

Collagen fibril cross-sectional radii show no systematic variation between the interior and the periphery of fibril bundles, indicating an effectively constant rate of collagen incorporation into fibrils throughout the bundle. Such spatially homogeneous incorporation constrains the extracellular diffusion of collagen precursors from sources at the bundle boundary to sinks at the growing fibrils. With a coarse-grained diffusion equation we determine stringent bounds, using parameters extracted from published experimental measurements of tendon development. From the lack of new fibril formation after birth, we further require that the concentration of diffusing precursors stays below the critical concentration for fibril nucleation. We find that the combination of the diffusive bound, which requires larger concentrations to ensure homogeneous fibril radii, and lack of nucleation, which requires lower concentrations, is only marginally consistent with fully processed collagen using conservative bounds. More realistic bounds may leave no consistent concentrations. Therefore, we propose that unprocessed pC-collagen diffuses from the bundle periphery followed by local C-proteinase activity and subsequent collagen incorporation at each fibril. We suggest that C-proteinase is localized within bundles, at fibril surfaces, during radial fibrillar growth. The much greater critical concentration of pC-collagen, as compared to fully processed collagen, then provides broad consistency between homogeneous fibril radii and the lack of fibril nucleation during fibril growth.

p-Coumaric Acid Attenuates UVB-Induced Release of Stratifin from Keratinocytes and Indirectly Regulates Matrix Metalloproteinase 1 Release from Fibroblasts

Ultraviolet (UV) radiation-induced loss of dermal extracellular matrix is associated with skin photoaging. Recent studies demonstrated that keratinocyte-releasable stratifin (SFN) plays a critical role in skin collagen metabolism by inducing matrix metalloproteinase 1 (MMP1) expression in target fibroblasts. In the present study, we examined whether SFN released from UVB-irradiated epidermal keratinocytes increases MMP1 release from dermal fibroblasts, and whether these events are affected by p-coumaric acid (p-CA), a natural phenolic compound with UVB-shielding and antioxidant properties. HaCaT cells were exposed to UVB in the absence and presence of p-CA, and the conditioned medium was used to stimulate fibroblasts in medium transfer experiments. The cells and media were analyzed to determine the expressions/releases of SFN and MMP1. UVB exposure increased SFN release from keratinocytes into the medium. The conditioned medium of UVB-irradiated keratinocytes increased MMP1 release from fibroblasts. The depletion of SFN using a siRNA rendered the conditioned medium of UVB-irradiated keratinocytes ineffective at stimulating fibroblasts to release MMP1. p-CA mitigated UVB-induced SFN expression in keratinocytes, and attenuated the MMP1 release by fibroblasts in medium transfer experiments. In conclusion, the present study demonstrated that the use of UV absorbers such as p-CA would reduce UV-induced SFN-centered signaling events involved in skin photoaging.

Mechanisms of aberrant organization of growth plates in conditional transgenic mouse model of spondyloepiphyseal dysplasia associated with the R992C substitution in collagen II

Mutations in collagen II, a main structural protein of cartilage, are associated with various forms of spondyloepiphyseal dysplasia (SED), whose main features include aberrations of linear growth. Here, we analyzed the pathomechanisms responsible for growth alterations in transgenic mice with conditional expression of the R992C collagen II mutation. Specifically, we studied the alterations of the growth plates of mutant mice in which chondrocytes lacked their typical columnar arrangement. Our studies demonstrated that chondrocytes expressing the thermolabile R992C mutant collagen II molecules endured endoplasmic reticulum stress, had atypical polarization, and had reduced proliferation. Moreover, we demonstrated aberrant organization and morphology of primary cilia. Analyses of the extracellular collagenous deposits in mice expressing the R992C mutant collagen II molecules indicated their poor formation and distribution. By contrast, transgenic mice expressing wild-type collagen II and mice in which the expression of the transgene encoding the R992C collagen II was switched off were characterized by normal growth, and the morphology of their growth plates was correct. Our study with the use of a conditional mouse SED model not only indicates a direct relation between the observed aberration of skeletal tissues and the presence of mutant collagen II, but also identifies cellular and matrix elements of the pathomechanism of SED.

Nonmuscle myosin II powered transport of newly formed collagen fibrils at the plasma membrane

Collagen fibrils can exceed thousands of microns in length and are therefore the longest, largest, and most size-pleomorphic protein polymers in vertebrates; thus, knowing how cells transport collagen fibrils is essential for a more complete understanding of protein transport and its role in tissue morphogenesis. Here, we identified newly formed collagen fibrils being transported at the surface of embryonic tendon cells in vivo by using serial block face-scanning electron microscopy of the cell-matrix interface. Newly formed fibrils ranged in length from ~1 to ~30 µm. The shortest (1-10 µm) occurred in intracellular fibricarriers; the longest (~30 µm) occurred in plasma membrane fibripositors. Fibrils and fibripositors were reduced in numbers when collagen secretion was blocked. ImmunoEM showed the absence of lysosomal-associated membrane protein 2 on fibricarriers and fibripositors and there was no effect of leupeptin on fibricarrier or fibripositor number and size, suggesting that fibricarriers and fibripositors are not part of a fibril degradation pathway. Blebbistatin decreased fibricarrier number and increased fibripositor length; thus, nonmuscle myosin II (NMII) powers the transport of these compartments. Inhibition of dynamin-dependent endocytosis with dynasore blocked fibricarrier formation and caused accumulation of fibrils in fibripositors. Data from fluid-phase HRP electron tomography showed that fibricarriers could originate at the plasma membrane. We propose that NMII-powered transport of newly formed collagen fibrils at the plasma membrane is fundamental to the development of collagen fibril-rich tissues. A NMII-dependent cell-force model is presented as the basis for the creation and dynamics of fibripositor structures.