Cell-layer-associated proteolytic cleavage of the telopeptides of type I collagen in fibroblast culture

Variation in Hydrogel Formation and Network Structure for Telo-, Atelo- and Methacrylated Collagens

As the most abundant protein in the extracellular matrix, collagen has become widely studied in the fields of tissue engineering and regenerative medicine. Of the various collagen types, collagen type I is the most commonly utilised in laboratory studies. In tissues, collagen type I forms into fibrils that provide an extended fibrillar network. In tissue engineering and regenerative medicine, little emphasis has been placed on the nature of the network that is formed. Various factors could affect the network structure, including the method used to extract collagen from native tissue, since this may remove the telopeptides, and the nature and extent of any chemical modifications and crosslinking moieties. The structure of any fibril network affects cellular proliferation and differentiation, as well as the overall modulus of hydrogels. In this study, the network-forming properties of two distinct forms of collagen (telo- and atelo-collagen) and their methacrylated derivatives were compared. The presence of the telopeptides facilitated fibril formation in the unmodified samples, but this benefit was substantially reduced by subsequent methacrylation, leading to a loss in the native self-assembly potential. Furthermore, the impact of the methacrylation of the collagen, which enables rapid crosslinking and makes it suitable for use in 3D printing, was investigated. The crosslinking of the methacrylated samples (both telo- and atelo-) was seen to improve the fibril-like network compared to the non-crosslinked samples. This contrasted with the samples of methacrylated gelatin, which showed little, if any, fibrillar or ordered network structure, regardless of whether they were crosslinked.

Hyaluronic acid as a macromolecular crowding agent for production of cell-derived matrices

Cell-derived matrices (CDMs) provide an exogenous source of human extracellular matrix (ECM), with applications as cell delivery vehicles, substrate coatings for cell attachment and differentiation, and as biomaterial scaffolds. However, commercial application of CDMs has been hindered due to the prolonged culture time required for sufficient ECM accumulation. One approach to increasing matrix deposition in vitro is macromolecular crowding (MMC), which is a biophysical phenomenon that limits the diffusion of ECM precursor proteins, resulting in increased ECM accumulation at the cell layer. Hyaluronic acid (HA), a natural MMC highly expressed in vivo during fetal development, has been shown to play a role in ECM production, but has not been investigated as a macromolecule for increasing cell-mediated ECM deposition in vitro. In the current study, we hypothesized that HA can act as a MMC, and increase cell-mediated ECM production. Human dermal fibroblasts were cultured for 3, 7, or 14 days with 0%, 0.05%, or 0.5% high molecular weight HA. Ficoll 70/400 was used as a positive control. SDS-PAGE, Sircol, and hydroxyproline assays indicated that 0.05% HA-treated cultures had significantly higher mean collagen deposition at 14 days, whereas Ficoll 70/400-treated cultures had significantly lower collagen production compared to the HA and untreated controls. However, fluorescent immunostaining of ECM proteins and quantification of mean gray values did not indicate statistically significant differences in ECM production in HA or Ficoll 70/400-treated cultures compared to untreated controls. Raman imaging (a marker-free spectral imaging method) indicated that HA increased ECM deposition in human dermal fibroblasts. These results are consistent with decreases in CDM stiffness observed in Ficoll 70/400-treated cultures by atomic force microscopy. Overall, these results indicate that there are macromolecule- and cell type- dependent effects on matrix assembly, turnover, and stiffness in cell-derived matrices. STATEMENT OF SIGNIFICANCE: Cell-derived matrices (CDMs) are versatile biomaterials with many regenerative medicine applications, including as cell and drug delivery vehicles and scaffolds for wound healing and tissue regeneration. While CDMs have several advantages, their commercialization has been limited due to the prolonged culture time required to achieve CDM synthesis in vitro. In this study, we explored the use of hyaluronic acid (HA) as a macromolecular crowder in human fibroblast cell cultures to support production of CDM biomaterials. Successful application of macromolecular crowding will allow development of human cell-derived, xeno-free biomaterials that re-capitulate the native human tissue microenvironment.

Live Imaging of Type I Collagen Assembly Dynamics in Osteoblasts Stably Expressing GFP and mCherry-Tagged Collagen Constructs

Type I collagen is the most abundant extracellular matrix protein in bone and other connective tissues and plays key roles in normal and pathological bone formation as well as in connective tissue disorders and fibrosis. Although much is known about the collagen biosynthetic pathway and its regulatory steps, the mechanisms by which it is assembled extracellularly are less clear. We have generated GFPtpz and mCherry-tagged collagen fusion constructs for live imaging of type I collagen assembly by replacing the α2(I)-procollagen N-terminal propeptide with GFPtpz or mCherry. These novel imaging probes were stably transfected into MLO-A5 osteoblast-like cells and fibronectin-null mouse embryonic fibroblasts (FN-null-MEFs) and used for imaging type I collagen assembly dynamics and its dependence on fibronectin. Both fusion proteins co-precipitated with α1(I)-collagen and remained intracellular without ascorbate but were assembled into α1(I) collagen-containing extracellular fibrils in the presence of ascorbate. Immunogold-EM confirmed their ultrastuctural localization in banded collagen fibrils. Live cell imaging in stably transfected MLO-A5 cells revealed the highly dynamic nature of collagen assembly and showed that during assembly the fibril networks are continually stretched and contracted due to the underlying cell motion. We also observed that cell-generated forces can physically reshape the collagen fibrils. Using co-cultures of mCherry- and GFPtpz-collagen expressing cells, we show that multiple cells contribute collagen to form collagen fiber bundles. Immuno-EM further showed that individual collagen fibrils can receive contributions of collagen from more than one cell. Live cell imaging in FN-null-MEFs expressing GFPtpz-collagen showed that collagen assembly was both dependent upon and dynamically integrated with fibronectin assembly. These GFP-collagen fusion constructs provide a powerful tool for imaging collagen in living cells and have revealed novel and fundamental insights into the dynamic mechanisms for the extracellular assembly of collagen. © 2018 American Society for Bone and Mineral Research.

Increased stromal extracellular matrix synthesis and assembly by insulin activated bovine keratocytes cultured under agarose

Previously, pharmacological levels of insulin have been shown to stimulate the synthesis of normal corneal stromal collagen and proteoglycans by bovine keratocytes in culture. Here we compared insulin to physiological levels of IGF-I and found that IGF-I also stimulated the synthesis of these extracellular matrix components, but less than that of insulin. Keratocytes in monolayer culture secreted most of the collagen synthesized into the media in the form of procollagen, a precursor of collagen. We found that an overlay of 3% agarose on the keratocytes in culture enhanced the conversion of procollagen to collagen and increased the deposition of collagen and proteoglycans into the cell layer. The extracellular matrix associated with the keratocytes cultured under agarose exhibited a corneal stromal-like architecture. These results suggest that enhancing the conversion of procollagen to collagen is a key step in the formation of extracellular matrix by keratocytes in vitro. Agarose overlay of insulin activated keratocytes in culture is a useful model for studying corneal stromal extracellular matrix assembly in vitro.

Gene interactions in Caenorhabditis elegans define DPY-31 as a candidate procollagen C-proteinase and SQT-3/ROL-4 as its predicted major target

Zinc metalloproteases of the BMP-1/TOLLOID family (also known as astacins) are extracellular enzymes involved in important developmental processes in metazoans. We report the characterization of the Caenorhabditis elegans gene dpy-31, which encodes the first essential astacin metalloprotease identified in this organism. Loss-of-function mutations in dpy-31 result in cuticle defects, abnormal morphology, and embryonic lethality, indicating that dpy-31 is required for formation of the collagenous exoskeleton. DPY-31 is widely expressed in the hypodermal cells, which are responsible for cuticle secretion. We have investigated the dpy-31 function through reversion analysis. While complete reversion can be obtained only by intragenic suppressors, reversion of the Dpy-31 lethal phenotype also can be caused by dominant extragenic suppressors. Nine extragenic suppressors carry mutations in the uniquely essential collagen gene sqt-3, which we show is the same gene as rol-4. Most mutations exhibit the unusual property of exclusively dominant suppression and all affect the sequence of the SQT-3 collagen C terminus. This suggests that DPY-31 is responsible for C-terminal proteolytic processing of collagen trimers and is therefore a structural and functional homolog of vertebrate BMP-1. The results also demonstrate the critical importance of the collagen C-terminal sequence, which is highly conserved among all 49 members of the SQT-3 subfamily.

Use of Bmp1/Tll1 doubly homozygous null mice and proteomics to identify and validate in vivo substrates of bone morphogenetic protein 1/tolloid-like metalloproteinases

Bone morphogenetic protein 1 (BMP-1) and mammalian Tolloid (mTLD), two proteinases encoded by Bmp1, provide procollagen C-proteinase (pCP) activity that converts procollagens I to III into the major fibrous components of mammalian extracellular matrix (ECM). Yet, although Bmp1(-/-) mice have aberrant collagen fibrils, they have residual pCP activity, indicative of genetic redundancy. Mammals possess two additional proteinases structurally similar to BMP-1 and mTLD: the genetically distinct mammalian Tolloid-like 1 (mTLL-1) and mTLL-2. Mice lacking the mTLL-1 gene Tll1 are embryonic lethal but have pCP activity levels similar to those of the wild type, suggesting that mTLL-1 might not be an in vivo pCP. In vitro studies have shown BMP-1 and mTLL-1 capable of cleaving Chordin, an extracellular antagonist of BMP signaling, suggesting that these proteases might also serve to modulate BMP signaling and to coordinate the latter with ECM formation. However, in vivo evidence of roles for BMP-1 and mTLL-1 in BMP signaling in mammals is lacking. To remove functional redundancy obscuring the in vivo functions of BMP-1-related proteases in mammals, we here characterize Bmp1 Tll1 doubly null mouse embryos. Although these appear morphologically indistinguishable from Tll1(-/-) embryos, biochemical analysis of cells derived from doubly null embryos shows functional redundancy removed to an extent enabling us to demonstrate that (i) products of Bmp1 and Tll1 are responsible for in vivo cleavage of Chordin in mammals and (ii) mTLL-1 is an in vivo pCP that provides residual activity observed in Bmp1(-/-) embryos. Removal of functional redundancy also enabled use of Bmp1(-/-) Tll1(-/-) cells in a proteomics approach for identifying novel substrates of Bmp1 and Tll1 products.

MT1-MMP-dependent and -independent regulation of gelatinase A activation in long-term, ascorbate-treated fibroblast cultures: regulation by fibrillar collagen

Human skin fibroblasts were cultured long-term in the presence of ascorbic acid to allow formation of a three-dimensional collagen matrix, and the effects of this on activation of secreted matrix metalloproteinase-2 (MMP-2) were examined. Accumulation of collagen over time correlated with increased levels of both mature MMP-2 and cell-associated membrane type 1-MMP (MT1-MMP), and subsequently increased mRNA levels for MT1-MMP, providing temporal resolution of the "nontranscriptional" and "transcriptional" effects of collagen on MT-1MMP functionality. MMP-2 activation by these cultures was blocked by inhibitors of prolyl-4-hydroxylase, or when fibroblasts derived from the collagen alpha1(I) gene-deficient Mov-13 mouse were used. MMP-2 activation by the Mov-13 fibroblasts was rescued by transfection of a full-length alpha1(I) collagen cDNA, and to our surprise, also by transfection with an alpha1(I) collagen cDNA carrying a mutation at the C-proteinase cleavage, which almost abrogated fibrillogenesis. Although studies with ascorbate-cultured MT1-MMP-/- fibroblasts showed that MT1-MMP played a significant role in the collagen-induced MMP-2 activation, a residual MT1-MMP-independent activation of MMP-2 was seen which resembled the level of MMP-2 activation persisting when wild-type fibroblasts were cultured in the presence of both ascorbic acid and MMP inhibitors. We were also unable to block this residual activation with inhibitors specific for serinyl, aspartyl, or cysteinyl enzymes.

The C-proteinase that processes procollagens to fibrillar collagens is identical to the protein previously identified as bone morphogenic protein-1

Bone morphogenic protein-1 (BMP-1) was originally identified as one of several BMPs that induced new bone formation when implanted into ectopic sites in rodents. BMP-1, however, differed from other BMPs in that it its structure was not similar to transforming growth factor beta. Instead, it had a large domain homologous to a metalloendopeptidase isolated from crayfish, an epidermal growth-factor-like domain, and three regions of internal sequence homology referred to as CUB domains. Therefore, BMP-1 was a member of the "astacin families" of zinc-requiring endopeptidases. Many astacins have been shown to play critical roles in embryonic hatching, dorsal/ventral patterning, and early developmental decisions. Here, we have obtained amino acid sequences and isolated cDNA clones for procollagen C-proteinase (EC 3.4.24.19), an enzyme that is essential for the processing of procollagens to fibrillar collagens. The results demonstrate that procollagen C-proteinase is identical to BMP-1.

A mouse 3T6 fibroblast cell culture model for the study of normal and protein-engineered collagen synthesis and deposition into the extracellular matrix

Mouse 3T6 fibroblasts deposited an organized collagenous extracellular matrix during long-term culture in the presence of ascorbic acid. The matrix produced by the cells had a similar distribution of collagen types as the mouse dermal matrix, comprising predominantly type I with smaller amounts of types III and V collagens. By day 8 of culture more than 70% of the collagen in the 3T6 matrix was involved in covalent crosslinkages and required pepsin digestion for extraction. Incorporation of NaB3H4 into reducible crosslinks and aldehydes directly demonstrated the involvement of the alpha 1 (I)CB6 and alpha 2(I)CB3.5 in crosslinks. The pattern of reducible crosslinks in the in vitro 3T6 matrix was similar to that in mouse skin suggesting a comparable fibril organization. Processing of procollagen to collagen occurred efficiently throughout the culture period and the rate of collagen production was unaltered during 15 days of culture, indicating that the development of a collagenous matrix does not directly play a role in procollagen processing or biosynthetic regulation. The existence of a preformed matrix did however, increase the efficiency with which newly synthesised collagen was incorporated into the pericellular matrix. At day 0, when there was no measurable matrix present, 29% of the collagen synthesised was deposited, while by day 15, 88% of the collagen was laid down in the matrix. The development of this 3T6 culture system, where collagen is efficiently incorporated into an organized extracellular matrix, will facilitate detailed studies on matrix organization and regulation and provide a system in which protein-engineered mutant collagens can be expressed to determine their effects on the production of a functional extracellular matrix.

Characterization of a type I collagen alpha 2(I) glycine-586 to valine substitution in osteogenesis imperfecta type IV. Detection of the mutation and prenatal diagnosis by a chemical cleavage method

A chemical cleavage method for detecting mismatched bases in heteroduplexes formed between patient mRNA and control cDNA probes was employed to identify a single base mutation in a heterozygous case of osteogenesis imperfecta type IV. The parents' fibroblast mRNA did not contain the mutation. The region of the mRNA mismatch was amplified by using the polymerase chain reaction, cloned and sequenced. A point mutation of G to U at base-pair 2162 of the collagen alpha 2(I) mRNA resulted in the substitution of glycine by valine at amino acid position 586 of the helix. This substitution disrupted the critical Gly-Xaa-Yaa repeating unit of the collagen triple helix and resulted in helix destabilization, as evidenced by a decreased thermal stability. This local disturbance to helix propagation from the C-terminus to the N-terminus led to the overmodification of the collagen helix downstream towards the N-terminus. However, collagen secretion in vitro was normal, and the clinical phenotype probably resulted from the secretion into the extracellular matrix of the mutant collagen combined with a decrease in collagen production to 65% of control values. The rapid detection of the osteogenesis imperfecta mutation by using the chemical cleavage method afforded the opportunity to apply the technique to prenatal diagnosis in the next pregnancy of the mother of the osteogenesis imperfecta patient. The absence of a mismatched base in chorionic villus mRNA and control cDNA heteroduplexes indicated that the foetus did not carry the mutation, which was confirmed by the subsequent delivery of a normal baby.

Assessment of procollagen processing defects by fibroblasts cultured in the presence of dextran sulphate

The culture of skin fibroblasts in the presence of 0.01% (w/v) dextran sulphate results in complete proteolytic processing of procollagen to collagen. Processing occurs predominantly via a pN-collagen intermediate, suggesting that C-propeptide cleavage occurs early during the processing pathway. The processed collagen is associated with the cell-layer fraction. This method of inducing procollagen processing was evaluated for use in detecting procollagen processing abnormalities in heritable connective-tissue diseases. Abnormal type I procollagen processing was clearly demonstrated in two cases with known defects of pN-propeptide cleavage. In one, the cleavage deficiency was due to diminished N-proteinase activity (dermatosparaxis) and in the other case (Ehler's-Danlos syndrome type VIIA) the cleavage site was deleted. In a case of osteogenesis imperfecta (type II) the slow electrophoretic migration of type I collagen alpha-chains due to over-modification of lysine was readily demonstrated. Inefficient procollagen processing was also evident in this patient, as had been previously reported [de Wet, Pihlanjaniemi, Myers, Kelly & Prockop (1983) J. Biol. Chem. 258, 7721-7728]. Thus this method of culture in the presence of dextran sulphate provides a simple and rapid procedure for the detection of procollagen processing defects and electrophoretic abnormalities.

Regulation and organization of connective tissues

It is difficult to study the regulation and interactions of the connective tissue macromolecules in vivo. However, studies of genetically determined diseases of the connective tissues have yielded a large amount of new information in these areas. Specific molecular defects can then be correlated with the functional and pathological changes in the tissues. We have concentrated on this approach which takes advantage of the large number of families with genetic diseases who come to our Hospital from all parts of Australasia and also takes advantage of developments in molecular biology in our Unit which were initiated with a RACS John Mitchell Crouch Fellowship. In addition to these studies on naturally occurring mutations we are also studying specific mutations that we are able to produce in specific regions of the collagen molecule. Another approach takes advantage of a unique model culture system developed in our Unit. These studies will be supplemented by various collaborative projects such as current ones involving smooth muscle cells in atherosclerosis, bone cells metabolism and myelofibrosis in leukaemia.