Production of recombinant human type I procollagen homotrimer in the mammary gland of transgenic mice

Green biomanufacturing in recombinant collagen biosynthesis: trends and selection in various expression systems

Collagen, classically derived from animal tissue, is an all-important protein material widely used in biomedical materials, cosmetics, fodder, food, etc. The production of recombinant collagen through different biological expression systems using bioengineering techniques has attracted significant interest in consideration of increasing market demand and the process complexity of extraction. Green biomanufacturing of recombinant collagen has become one of the focus topics. While the bioproduction of recombinant collagens (type I, II, III, etc.) has been commercialized in recent years, the biosynthesis of recombinant collagen is extremely challenging due to protein immunogenicity, yield, degradation, and other issues. The rapid development of synthetic biology allows us to perform a heterologous expression of proteins in diverse expression systems, thus optimizing the production and bioactivities of recombinant collagen. This review describes the research progress in the bioproduction of recombinant collagen over the past two decades, focusing on different expression systems (prokaryotic organisms, yeasts, plants, insects, mammalian and human cells, etc.). We also discuss the challenges and future trends in developing market-competitive recombinant collagens.

Pharmacological Functions, Synthesis, and Delivery Progress for Collagen as Biodrug and Biomaterial

Collagen has been widely applied as a functional biomaterial in regulating tissue regeneration and drug delivery by participating in cell proliferation, differentiation, migration, intercellular signal transmission, tissue formation, and blood coagulation. However, traditional extraction of collagen from animals potentially induces immunogenicity and requires complicated material treatment and purification steps. Although semi-synthesis strategies such as utilizing recombinant E. coli or yeast expression systems have been explored as alternative methods, the influence of unwanted by-products, foreign substances, and immature synthetic processes have limited its industrial production and clinical applications. Meanwhile, macromolecule collagen products encounter a bottleneck in delivery and absorption by conventional oral and injection vehicles, which promotes the studies of transdermal and topical delivery strategies and implant methods. This review illustrates the physiological and therapeutic effects, synthesis strategies, and delivery technologies of collagen to provide a reference and outlook for the research and development of collagen as a biodrug and biomaterial.

Synthetic biology-guided design and biosynthesis of protein polymers for delivery

Vehicles derived from genetically engineered protein polymers have gained momentum in the field of biomedical engineering due to their unique designability, remarkable biocompatibility and excellent biodegradability. However, the design and production of these protein polymers with on-demand sequences and supramolecular architectures remain underexplored, particularly from a synthetic biology perspective. In this review, we summarize the state-of-the art strategies for constructing the highly repetitive genes encoding the protein polymers, and highlight the advanced approaches for metabolically engineering expression hosts towards high-level biosynthesis of the target protein polymers. Finally, we showcase the typical protein polymers utilized to fabricate delivery vehicles.

Nature-Based Biomaterials and Their Application in Biomedicine

Natural polymers, based on proteins or polysaccharides, have attracted increasing interest in recent years due to their broad potential uses in biomedicine. The chemical stability, structural versatility, biocompatibility and high availability of these materials lend them to diverse applications in areas such as tissue engineering, drug delivery and wound healing. Biomaterials purified from animal or plant sources have also been engineered to improve their structural properties or promote interactions with surrounding cells and tissues for improved in vivo performance, leading to novel applications as implantable devices, in controlled drug release and as surface coatings. This review describes biomaterials derived from and inspired by natural proteins and polysaccharides and highlights their promise across diverse biomedical fields. We outline current therapeutic applications of these nature-based materials and consider expected future developments in identifying and utilising innovative biomaterials in new biomedical applications.

Enhanced Collagen-like Protein for Facile Biomaterial Fabrication

We present a collagen-mimetic protein of bacterial origin based upon a modified subdomain of the collagen-like Sc12 protein from Streptococcus pyogenes, as an alternative collagen-like biomaterial platform that is highly soluble, forms stable, homogeneous, fluid-like solutions at elevated concentrations, and that can be efficiently fabricated into hydrogel materials over a broad range of pH conditions. This extended bacterial collagen-like (eBCL) protein is expressed in a bacterial host and purified as a trimeric assembly exhibiting a triple helical secondary structure in its collagen-like subdomain that is stable near physiological solution conditions (neutral pH and 37 °C), as well as over a broad range of pH conditions. We also show how this sequence can be modified to include biofunctional attributes, in particular, the Arg-Gly-Asp (RGD) sequence to elicit integrin-specific cell binding, without loss of structural function. Furthermore, through the use of EDC-NHS chemistry, we demonstrate that members of this eBCL protein system can be covalently cross-linked to fabricate transparent hydrogels with high protein concentrations (at least to 20% w/w). These hydrogels are shown to possess material properties and resistance to enzymatic degradation that are comparable or superior to a type I collagen control. Moreover, such hydrogels containing the constructs with the RGD integrin-binding sequence are shown to promote the adhesion, spreading, and proliferation of C2C12 and 3T3 cells in vitro. Due to its enhanced solubility, structural stability, fluidity at elevated concentrations, ease of modification, and facility of cross-linking, this eBCL collagen-mimetic system has potential for numerous biomedical material applications, where the ease of processing and fabrication and the facility to tailor the sequence for specific biological functionality are desired.

Bioprinting of Collagen: Considerations, Potentials, and Applications

Collagen is the most abundant extracellular matrix protein that is widely used in tissue engineering (TE). There is little research done on printing pure collagen. To understand the bottlenecks in printing pure collagen, it is imperative to understand collagen from a bottom-up approach. Here it is aimed to provide a comprehensive overview of collagen printing, where collagen assembly in vivo and the various sources of collagen available for TE application are first understood. Next, the current printing technologies and strategy for printing collagen-based materials are highlighted. Considerations and key challenges faced in collagen printing are identified. Finally, the key research areas that would enhance the functionality of printed collagen are presented.

Sources of Collagen for Biomaterials in Skin Wound Healing

Collagen is the most frequently used protein in the fields of biomaterials and regenerative medicine. Within the skin, collagen type I and III are the most abundant, while collagen type VII is associated with pathologies of the dermal-epidermal junction. The focus of this review is mainly collagens I and III, with a brief overview of collagen VII. Currently, the majority of collagen is extracted from animal sources; however, animal-derived collagen has a number of shortcomings, including immunogenicity, batch-to-batch variation, and pathogenic contamination. Recombinant collagen is a potential solution to the aforementioned issues, although production of correctly post-translationally modified recombinant human collagen has not yet been performed at industrial scale. This review provides an overview of current collagen sources, associated shortcomings, and potential resolutions. Recombinant expression systems are discussed, as well as the issues associated with each method of expression.

The Collagen Suprafamily: From Biosynthesis to Advanced Biomaterial Development

Collagen is the oldest and most abundant extracellular matrix protein that has found many applications in food, cosmetic, pharmaceutical, and biomedical industries. First, an overview of the family of collagens and their respective structures, conformation, and biosynthesis is provided. The advances and shortfalls of various collagen preparations (e.g., mammalian/marine extracted collagen, cell-produced collagens, recombinant collagens, and collagen-like peptides) and crosslinking technologies (e.g., chemical, physical, and biological) are then critically discussed. Subsequently, an array of structural, thermal, mechanical, biochemical, and biological assays is examined, which are developed to analyze and characterize collagenous structures. Lastly, a comprehensive review is provided on how advances in engineering, chemistry, and biology have enabled the development of bioactive, 3D structures (e.g., tissue grafts, biomaterials, cell-assembled tissue equivalents) that closely imitate native supramolecular assemblies and have the capacity to deliver in a localized and sustained manner viable cell populations and/or bioactive/therapeutic molecules. Clearly, collagens have a long history in both evolution and biotechnology and continue to offer both challenges and exciting opportunities in regenerative medicine as nature's biomaterial of choice.

Alternative uses for co-products: Harnessing the potential of valuable compounds from meat processing chains

Opportunities for exploiting the inherent value of protein-rich meat processing co-products, in the context of increased global demand for protein and for sustainable processing systems, are discussed. While direct consumption maybe the most profitable route for some, this approach is influenced greatly by local and cultural traditions. A more profitable and sustainable approach may be found in recognizing this readily available and under-utilised resource can provide high value components, such as proteins, with targeted high value functionality of relevance to a variety of sectors. Applications in food & beverages, petfood biomedical and nutrition arenas are discussed. Utilization of the raw material in its entirety is a necessary underlying principle in this approach to help maintain minimum waste generation. Understanding consumer attitudes to these products, in particular when used in food or beverage systems, is critical in optimizing commercialization strategies.

Bioengineered Collagens

There is a great deal of interest in obtaining recombinant collagen as an alternative source of material for biomedical applications and as an approach for obtaining basic structural and biological information. However, application of recombinant technology to collagen presents challenges, most notably the need for post-translational hydroxylation of prolines for triple-helix stability. Full length recombinant human collagens have been successfully expressed in cell lines, yeast, and several plant systems, while collagen fragments have been expressed in E. coli. In addition, bacterial collagen-like proteins can be expressed in high yields in E. coli and easily manipulated to incorporate biologically active sequences from human collagens. These expression systems allow manipulation of biologically active sequences within collagen, which has furthered our understanding of the relationships between collagen sequences, structure and function. Here, recombinant studies on collagen interactions with cell receptors, extracellular matrix proteins, and matrix metalloproteinases are reviewed, and discussed in terms of their potential biomaterial and biomedical applications.

Analysis of Tissue-Specific Expression of Human Type II Collagen cDNA Driven by Different Sizes of the Upstream Region of the β-Casein Promoter

To investigate the ability of 1.8 kb or 3.1 kb bovine beta-casein promoter sequences for the expression regulation of transgene in vivo, transgenic mice were produced with human type II collagen gene fused to 1.8 kb and 3.1 kb of bovine beta-casein promoter by DNA microinjection. Five and three transgenic founder mice were produced using transgene constructs with 1.8 kb and 3.1 kb of bovine beta-casein promoters respectively. Founder mice were outbred with the wild type to produce F1 and F2 progenies. Total RNAs were extracted from four tissues (mammary gland, liver, kidney, and muscle) of female F1 transgenic mice of each transgenic line following parturition. RT-PCR and Northern blot analysis revealed that the expression level of transgene was variable among the transgenic lines, but transgenic mice containing 1.8 kb of promoter sequences exhibited more leaky expression of transgene in other tissues compared to those with 3.1 kb promoter. Moreover, Western blot analysis of transgenic mouse milk showed that human type II collagen proteins secreted into the milk of lactating transgenic mice contained 1.8 kb and 3.1 kb of bovine beta-casein promoter. These results suggest that promoter sequences of 3.1 kb bovine beta-casein gene can be used for induction of mammary gland-specific expression of transgenes in transgenic animals.

Human recombinant type I collagen produced in plants

As a central element of the extracellular matrix, collagen is intimately involved in tissue development, remodeling, and repair and confers high tensile strength to tissues. Numerous medical applications, particularly, wound healing, cell therapy, bone reconstruction, and cosmetic technologies, rely on its supportive and healing qualities. Its synthesis and assembly require a multitude of genes and post-translational modifications, where even minor deviations can be deleterious or even fatal. Historically, collagen was always extracted from animal and human cadaver sources, but bare risk of contamination and allergenicity and was subjected to harsh purification conditions resulting in irreversible modifications impeding its biofunctionality. In parallel, the highly complex and stringent post-translational processing of collagen, prerequisite of its viability and proper functioning, sets significant limitations on recombinant expression systems. A tobacco plant expression platform has been recruited to effectively express human collagen, along with three modifying enzymes, critical to collagen maturation. The plant extracted recombinant human collagen type I forms thermally stable helical structures, fibrillates, and demonstrates bioactivity resembling that of native collagen. Deployment of the highly versatile plant-based biofactory can be leveraged toward mass, rapid, and low-cost production of a wide variety of recombinant proteins. As in the case of collagen, proper planning can bypass plant-related limitations, to yield products structurally and functionally identical to their native counterparts.

APPLICATIONS OF COLLAGEN IN MEDICAL DEVICES

Collagen is the most abundant natural protein found in living systems. While there is a whole family of different collagen types, each differing in sequence, the properties that make this protein so attractive as the building blocks for medical devices, are reflected largely by the unique fibrillar structure of the molecule, as well as defined functional regions that interact with the surrounding cells and other matrix components. As a commercial medical product, collagen can be part of the natural tissue used in the device, or it can be fabricated as a reconstituted product from animal or recombinant sources. Both types of uses have distinct properties that convey advantages and disadvantages to the end product. This review examines the chemistry and biology of collagen and describes some well-documented examples of collagen-based medical devices produced in one or other of these formats.

Bioactive polymer scaffold for fabrication of vascularized engineering tissue

Tissue engineering seeks strategies to design polymeric scaffolds that allow high-cell-density cultures with signaling molecules and suitable vascular supply. One major obstacle in tissue engineering is the inability to create thick engineered-tissue constructs. A pre-vascularized tissue scaffold appears to be the most favorable approach to avoid nutrient and oxygen supply limitations as well as to allow waste removal, factors that are often hurdles in developing thick engineered tissues. Vascularization can be achieved using strategies in which cells are cultured in bioactive polymer scaffolds that can mimic extracellular matrix environments. This review addresses recent advances and future challenges in developing and using bioactive polymer scaffolds to promote tissue construct vascularization.

Natural and Genetically Engineered Proteins for Tissue Engineering

To overcome the limitations of traditionally used autografts, allografts and, to a lesser extent, synthetic materials, there is the need to develop a new generation of scaffolds with adequate mechanical and structural support, control of cell attachment, migration, proliferation and differentiation and with bio-resorbable features. This suite of properties would allow the body to heal itself at the same rate as implant degradation. Genetic engineering offers a route to this level of control of biomaterial systems. The possibility of expressing biological components in nature and to modify or bioengineer them further, offers a path towards multifunctional biomaterial systems. This includes opportunities to generate new protein sequences, new self-assembling peptides or fusions of different bioactive domains or protein motifs. New protein sequences with tunable properties can be generated that can be used as new biomaterials. In this review we address some of the most frequently used proteins for tissue engineering and biomedical applications and describe the techniques most commonly used to functionalize protein-based biomaterials by combining them with bioactive molecules to enhance biological performance. We also highlight the use of genetic engineering, for protein heterologous expression and the synthesis of new protein-based biopolymers, focusing the advantages of these functionalized biopolymers when compared with their counterparts extracted directly from nature and modified by techniques such as physical adsorption or chemical modification.

The creation of transgenic pigs expressing human proteins using BAC-derived, full-length genes and intracytoplasmic sperm injection-mediated gene transfer

In most transgenic (Tg) animals created to date, a transgene consisting of the minimum promoter region linked to a cDNA has been used. However, transgenes on small plasmids are susceptible to the position effect, increasing the difficulty of controlling transgene expression. In this study, we attempted to create Tg pigs by intracytoplasmic sperm injection-mediated gene transfer (ICSI-MGT) using two large genomic transgenes derived from a bacterial artificial chromosome (BAC) containing the full genomic region encoding two human proteins, type I collagen and albumin. The production efficiencies (Tg piglets/live offspring) of type I collagen and albumin Tg pigs were 11.8% (6/51) and 18.2% (2/11), respectively. In all of the Tg pigs examined by real-time PCR analysis, tissue-specific expression of the transgene was confirmed (type I collagen: skin, tendon, vessels, genitalia; albumin: liver). The production of human proteins derived from BAC transgenes was also confirmed. Fluorescence in situ hybridization analysis indicated that the BAC transgenes transferred into porcine oocytes by ICSI-MGT were integrated into single or multiple sites on the host chromosomes. These data demonstrate that Tg pigs expressing human proteins in a tissue-specific manner can be created using a BAC transgenic construct and the ICSI-MGT method.

Expression, Purification and Characterization of Recombinant Human Gelatin in Pichia pastoris

Based on the idiographic character of collagenous domain of human type III collagen, a recombinant human gelatin monomeric gene (gel) was designed and synthesized. All hydrophobic amino acids (proline excluded) were replaced by hydrophilic amino acids to improve the hydrophilic properties, and the codons encoding amino acids were optimized according to Pichia pastoris bias usage. Then a recombinant human gelatin expression vector pPIC9KG6 containing six monomeric genes ligated in the same orientation was constructed successfully. After verificated the validity of construction by DNA sequencing, the recombinant vector pPIC9KG6 was electroporated into the Pichia pastoris GS115, and Mut+ pPIC9KG6 transformants were selected on the basis of G418 resistance. Then a high-level expression strain was picked up from transformants by analyzing their recombinant protein expression levels. SDS-PAGE analysis of cell lysate and fermentation supernatant of the high-level expression strain showed that recombinant human gelatin can be expressed intracellularly and secreted expression, and its expression level reaches 16.06 g per liter. Secreted recombinant human gelatin was purified from fermentation supernatant by gel filtration chromatography. By UV spectroscopy and FTIR and SEM, it was confirmed that purified recombinant human gelatin is similar to animal-derived gelatin in protein structure.

Evaluation of the immunogenicity and cell compatibility of avian collagen for biomedical applications

There have been concerns regarding the suitability of bovine collagen as a biomaterial since the emergence of bovine spongiform encephalopathy. Consequently, collagens from other species may be used if they can meet appropriate standards, including negligible or lack of immunogenicity. In this study, the potential immunogenicity of both monomeric and pepsin-solubilized chicken collagens have been compared with a commercial, pepsin-solubilized bovine collagen that is approved for biomedical implantation. All collagens were poor immunogens compared with ovalbumin. No IgE responses were detected in sera of three strains of mice, and no hypersensitivity reactions were found in guinea pigs in maximization and Buehler tests. IgG(1) antibodies were found although the titre was substantially lower than against ovalbumin. All responses in mice and rabbits were found only when immunizations were performed with adjuvant, and after multiple injections over a long period of time. The response from the monomeric chicken collagen was less than for pepsin-solubilized collagens. Collagen sponges prepared from the two chicken collagen preparations both supported the attachment and growth of mouse fibroblasts. These data indicate that chicken collagen, particularly when monomeric, may be useful in certain biomedical applications.

A Streptococcus pyogenes derived collagen-like protein as a non-cytotoxic and non-immunogenic cross-linkable biomaterial

A range of bacteria have been shown to contain collagen-like sequences that form triple-helical structures. Some of these proteins have been shown to form triple-helical motifs that are stable around body temperature without the inclusion of hydroxyproline or other secondary modifications to the protein sequence. This makes these collagen-like proteins particularly suitable for recombinant production as only a single gene product and no additional enzyme needs to be expressed. In the present study, we have examined the cytotoxicity and immunogenicity of the collagen-like domain from Streptococcus pyogenes Scl2 protein. These data show that the purified, recombinant collagen-like protein is not cytotoxic to fibroblasts and does not elicit an immune response in SJL/J and Arc mice. The freeze dried protein can be stabilised by glutaraldehyde cross-linking giving a material that is stable at >37 degrees C and which supports cell attachment while not causing loss of viability. These data suggest that bacterial collagen-like proteins, which can be modified to include specific functional domains, could be a useful material for medical applications and as a scaffold for tissue engineering.

Collagens as biomaterials

This paper reviews the structure, function and applications of collagens as biomaterials. The various formats for collagens, either as tissue-based devices or as reconstituted soluble collagens are discussed. The major emphasis is on the new technologies that are emerging that will lead to new and improved collagen-based medical devices. In particular, the development of recombinant collagens, especially using microorganism systems, is allowing the development of safe and reproducible collagen products. These systems also allow for the development of novel, non-natural structures, for example collagen like structures containing repeats of key functional domains or as chimeric structures where a collagen domain is covalently linked to another biologically active component.

Purification and characterization of a 44-kDa recombinant collagen I alpha 1 fragment from corn grain

This paper demonstrates that a fibrous, repetitive amino acid sequence collagen-related protein, a 44-kDa fragment of human collagen I alpha 1 (CIalpha1), was expressed in corn grain molecularly equivalent to that produced in recombinant yeast. The recombinant CIalpha1 was extracted and purified from early generation plants having low levels of recombinant protein accumulation. It was selectively extracted at low pH and purified by ion exchange and gel filtration chromatography, resulting in a 44-kDa CIalpha1 with >70% purity and 60% recovery. The N-terminal sequence, amino acid composition, and immunoreactivity closely matched those of an analogous 44-kDa CIalpha1 fragment produced by the yeast Pichia . The corn-derived 44-kDa CIalpha1 had an intact protein mass of 44088 Da, which is within 0.2% of the mass calculated from the expected sequence. Tandem mass spectrometry confirmed the primary sequence with 78% coverage. The amino acid composition analysis indicated a low level of prolyl hydroxylation. Glycoprotein staining revealed no glycosylation.

Cloning and characterization of ovine alphaS1-casein gene promoter: a transfection study in rat mammary gland cell line

Promoter regions of milk protein genes are frequently used to produce pharmaceutically and medically important proteins in the mammary gland of transgenic animals and also can be used for the construction of an inducible eukaryotic expression vector. The aim of the present study was to clone, sequence and characterize the regulatory elements in ovine alphaS1-CSNGP. For the first time we have cloned and sequenced region extending from - 2136 to +49 bp containing 5'-flanking region and exon I. Computational analysis of the sequence showed presence of core promoter elements viz., TATA box, CAAT box and initiator sequence. Mammary gland specific sequences included MGF/STAT 5, MPBF, Yu Lee 2, 4 and 5, Oka box C and hormone responsive elements (HRE) viz., GRE, PRE, PRL, IRE and also Polyoma enhancer 3 sequences. Computational analysis data is validated by following the reporter gene expression studies in rat breast cell line. Six reporter gene constructs under the control of full length, proximal, distal, minimal and proximal-distal fused promoter segments were constructed to assess the effect of presence or absence of few selected regulatory elements on expression ability of the promoter. Based on qualitative evaluation of fluorescence, the pGFP-F/VspI showed highest fluorescence followed by pGFP-P, pGFP-F/SpeI, pGFPminimal and pGFP-D.

Testing the utility of rationally engineered recombinant collagen-like proteins for applications in tissue engineering

Collagens are attractive proteins as materials for tissue engineering. Over the last decade, significant progress has been made in developing technologies for large-scale production of native-like human recombinant collagens. Yet, the rational design of customized collagen-like proteins for smart biomaterials to enhance the quality of engineered tissues has not been explored. We mapped the D4 domain of human collagen II as most critical for supporting migration of chondrocytes and used this information to genetically engineer a collagen-like protein consisting of tandem repeats of the D4 domain (mD4 collagen). This novel collagen has been utilized to fabricate a scaffold for support of chondrocytes. We determined superior qualities of cartilaginous constructs created by chondrocytes cultured in scaffolds containing the mD4 collagen in comparison to those formed by chondrocytes cultured in bare scaffolds or those coated with wild-type collagen II. Our results are a first attempt to rationally engineer collagen-like proteins with characteristics tailored for specific needs of cartilage engineering and provide a basis for rational engineering of similar proteins for a variety of biomedical applications.

Assessment of prokaryotic collagen-like sequences derived from streptococcal Scl1 and Scl2 proteins as a source of recombinant GXY polymers

Collagen triple helix, composed of the repeating Gly-Xaa-Yaa (GXY) sequence, is a structural element found in all multicellular animals and also in some prokaryotes. Long GXY polymers are highly regarded components used in food, cosmetic, biomedical, and pharmaceutical industries. In this study, we explore a new concept for the production of recombinant GXY polymers which are based on the sequence of "prokaryotic collagens", the streptococcal collagen-like proteins Scl1 and Scl2. Analysis of 50 Scl variants identified the amino acid distribution and GXY-repeat usage that are involved in the stabilization of the triple helix in Scls. Using circular dichroism spectroscopy and electron microscopy, we show that significantly different recombinant rScl polypeptides form stable, unhydroxylated homotrimeric triple helices that can be produced both intra- and extracellularly in the Escherichia coli. These rScl constructs containing 20 to 129 GXY repeats had mid-point melting temperatures between 32 and 39 degrees C. Altogether, Scl-derived collagens, which are different from the mammalian collagens, can form stable triple helices under physiological conditions and can be used for the production of recombinant GXY polymers with a wide variety of potential applications.

Recombinant microbial systems for the production of human collagen and gelatin

The use of genetically engineered microorganisms is a cost-effective, scalable technology for the production of recombinant human collagen (rhC) and recombinant gelatin (rG). This review will discuss the use of yeast (Pichia pastoris, Saccharomyces cerevisiae, Hansenula polymorpha) and of bacteria (Escherichia coli, Bacillus brevis) genetically engineered for the production of rhC and rG. P. pastoris is the preferred production system for rhC and rG. Recombinant strains of P. pastoris accumulate properly hydroxylated triple helical rhC intracellularly at levels up to 1.5 g/l. Coexpression of recombinant collagen with recombinant prolyl hydroxylase results in the synthesis of hydroxylated collagen with thermal stability similar to native collagens. The purified hydroxylated rhC forms fibrils that are structurally similar to fibrils assembled from native collagen. These qualities make rhC attractive for use in many medical applications. P. pastoris can also be engineered to secrete high levels (3 to 14 g/l ) of collagen fragments with defined length, composition, and physiochemical properties that serve as substitutes for animal-derived gelatins. The replacement of animal-derived collagen and gelatin with rhC and rG will result in products with improved safety, traceability, reproducibility, and quality. In addition, the rhC and rG can be engineered to improve the performance of products containing these biomaterials.

The Application of Recombinant Human Collagen in Tissue Engineering

Collagen is the main structural protein in vertebrates. It plays an essential role in providing a scaffold for cellular support and thereby affecting cell attachment, migration, proliferation, differentiation, and survival. As such, it also plays an important role in numerous approaches to the engineering of human tissues for medical applications related to tissue, bone, and skin repair and reconstruction. Currently, the collagen used in tissue engineering applications is derived from animal tissues, creating concerns related to the quality, purity, and predictability of its performance. It also carries the risk of transmission of infectious agents and precipitating immunological reactions. The recent development of recombinant sources of human collagen provides a reliable, predictable and chemically defined source of purified human collagens that is free of animal components. The triple-helical collagens made by recombinant technology have the same amino acid sequence as human tissue-derived collagen. Furthermore, by achieving the equivalent extent of proline hydroxylation via coexpression of genes encoding prolyl hydroxylase with the collagen genes, one can produce collagens with a similar degree of stability as naturally occurring material. The recombinant production process of collagen involves the generation of single triple-helical molecules that are then used to construct more complex three-dimensional structures. If one loosely defines tissue engineering as the use of a biocompatible scaffold combined with a biologically active agent (be it a gene or gene construct, growth factor or other biologically active agent) to induce tissue regeneration, then the production of recombinant human collagen enables the engineering of human tissue based on a human matrix or scaffold. Recombinant human collagens are an efficient scaffold for bone repair when combined with a recombinant bone morphogenetic protein in a porous, sponge-like format, and when presented as a membrane, sponge or gel can serve as a basis for the engineering of skin, cartilage and periodontal ligament, depending on the specific requirements of the chosen application.

Prospects and limitations of the rational engineering of fibrillar collagens

Recombinant collagens are attractive proteins for a number of biomedical applications. To date, significant progress was made in the large-scale production of nonmodified recombinant collagens; however, engineering of novel collagen-like proteins according to customized specifications has not been addressed. Herein we investigated the possibility of rational engineering of collagen-like proteins with specifically assigned characteristics. We have genetically engineered two DNA constructs encoding multi-D4 collagens defined as collagen-like proteins, consisting primarily of a tandem of the collagen II D4 periods that correspond to the biologically active region. We have also attempted to decrease enzymatic degradation of novel collagen by mutating a matrix metalloproteinase 1 cleavage site present in the D4 period. We demonstrated that the recombinant collagen alpha-chains consisting predominantly of the D4 period but lacking most of the other D periods found in native collagen fold into a typical collagen triple helix, and the novel procollagens are correctly processed by procollagen N-proteinase and procollagen C-proteinase. The nonmutated multi-D4 collagen had a normal melting point of 41 degrees C and a similar carbohydrate content as that of control. In contrast, the mutant multi-D4 collagen had a markedly lower thermostability of 36 degrees C and a significantly higher carbohydrate content. Both collagens were cleaved at multiple sites by matrix metalloproteinase 1, but the rate of hydrolysis of the mutant multi-D4 collagen was lower. These results provide a basis for the rational engineering of collagenous proteins and identifying any undesirable consequences of altering the collagenous amino acid sequences.

Mammary gland-specific secretion of biologically active immunosuppressive agent cytotoxic-T-lymphocyte antigen 4 human immunoglobulin fusion protein (CTLA4Ig) in milk by transgenesis

A major challenge in the field of transplantation is to prevent graft rejection and prolong graft survival. Tolerance induction is a promising way to achieve long-term graft survival without the need for potent immunosuppression and its associated side effects. The recent success of co-stimulatory blockade by the chimeric protein CTLA4Ig in the modulation of the recipient's immune system and the prolongation of graft survival in animal models suggests a possible application of CTLA4Ig in clinical transplantation. To produce sufficient amounts of CTLA4Ig for future clinical application, we sought to use the mammary gland as a bioreactor and produce CTLA4Ig in the milk of transgenic farm animals. Prior to the generation of transgenic farm animals, we tested our strategy in mice. Using the promoter of the sheep beta-lactoglobulin gene, we expressed our CTLA4Ig chimeric gene in the mammary gland of transgenic mice. The yield of CTLA4Ig was fivefold higher in transgenic milk than that from transfected cells. Purified milk-derived CTLA4Ig is biologically active and suppresses T cell activation. We showed that the production of CTLA4Ig in the milk has no adverse immunosuppression effect on the transgenic animals and the offsprings that were fed with the transgenic milk. The findings suggest that the approach to produce CTLA4Ig in milk by transgenesis is feasible; further studies involving farm animals are warranted.

Evaluation of different promoters and host strains for the high-level expression of collagen-like polymer in Escherichia coli

The increased expression of collagen-like polymer, CLP3.1-his which consists of 52 repeating peptide (GAPGAPGSQGAPGLQ), in Escherichia coli was investigated. The effects of three promoters, thermally inducible promoter, T7 promoter and T7lac promoter, and three Escherichia coli host strains, BL21, BL21(DE3) and BL21(DE3)[pLysS] which differ in stringency of suppressing basal transcription, were compared. Based on the CLP3.1-his expression level, solubility of CLP3.1-his in cells and basal transcription that occurred in the absence of induction, two expression systems, BL21(DE3) containing plasmid pJY-2 with T7lac promoter and BL21(DE3)[pLysS] containing plasmid pJY-1 with T7 promoter, were selected. With these two expression systems, CLP3.1-his expression levels greater than 40% (g/g) of total cellular proteins and CLP3.1-his concentrations of 0.1-0.2 gl(-1) can be achieved by using Luria-Bertani medium in shake flask batch cultures. The CLP3.1-his accumulated in the cells is totally soluble and no basal transcription was found before induction. These two high-level expression systems are promising for use in scale-up production.

Genetic engineering of fibrous proteins: spider dragline silk and collagen

Various strategies have been employed to genetically engineer fibrous proteins. Two examples, the subject of this review, include spider dragline silk from Nephila clavipes and collagen. These proteins are highlighted because of their unique mechanical and biological properties related to controlled release, biomaterials and tissue engineering. Cloning and expression of native genes and synthetic artificial variants of the consensus sequence repeats from the native genes has been accomplished. Expression of recombinant silk and collagen proteins has been reported in a variety of host systems, including bacteria, yeast, insect cells, plants and mammalian cells. Future utility for these proteins for biomedical materials is expected to increase as needs expand for designer materials with tailored mechanical properties and biological interactions to elicit specific responses in vitro and in vivo.

Mapping critical sites in collagen II for rational design of gene-engineered proteins for cell-supporting materials

Collagen II is the most abundant protein of cartilage and forms a network of fibrils extended by proteoglycans that enables cartilage to resist pressure. The surface of the collagen fibril serves as a platform for the attachment of collagen IX, growth factors, and cells. In this study we examined the mechanism of the interaction of chondrocytes with recombinant versions of procollagen II, in which one of the four blocks of 234 amino acids that define repeating D periods of the collagen triple helix has been deleted. Analysis of the attachment of chondrocytes to collagen II variants with deleted D periods indicated that the collagen II monomer contains randomly distributed sites critical for cell binding. However, as was shown by spreading and migration assays, the D4 period, which is between residues 703 to 936, contains amino acids critical for cell motility. We also showed that binding, spreading, and migration of chondrocytes through three-dimensional nanofibrillar collagenous matrices are controlled by an interaction of the collagen triple helix with beta1 integrins. The results of this study provide a basis for the rational design of a scaffold containing genetically engineered collagen with a high density of specific sites of interaction.

Production of recombinant human type I procollagen trimers using a four-gene expression system in the yeast Saccharomyces cerevisiae

The expression of stable recombinant human collagen requires an expression system capable of post-translational modifications and assembly of the procollagen polypeptides. Two genes were expressed in the yeast Saccharomyces cerevisiae to produce both propeptide chains that constitute human type I procollagen. Two additional genes were expressed coding for the subunits of prolyl hydroxylase, an enzyme that post-translationally modifies procollagen and that confers heat (thermal) stability to the triple helical conformation of the collagen molecule. Type I procollagen was produced as a stable heterotrimeric helix similar to type I procollagen produced in tissue culture. A key requirement for glutamate was identified as a medium supplement to obtain high expression levels of type I procollagen as heat-stable heterotrimers in Saccharomyces. Expression of these four genes was sufficient for correct assembly and processing of type I procollagen in a eucaryotic system that does not produce collagen.