Skin wounds represent a major healthcare problem owing to an increasing number of trauma and pathophysiological conditions. Normal wound healing process includes a very well-orchestrated and regulated process consisting of series of events such as haemostasis, inflammation, proliferation and extracellular matrix (ECM) remodelling.
The four healing phases involve interactions between various types of cells, bioactive factors and a supporting platform, which is usually the natural ECM secreted by cells. This normal healing process gets severely dysregulated in case of pathophysiological conditions.
Large trauma wounds due to burns or accidents result in loss of majority of skin tissue and thus fail to heal. Majority of non-healing wounds are associated with accidents or disease conditions like diabetes. Such conditions hamper the normal healing pattern of skin tissue and has been a major social and financial burden since decades.
The wound healing cascade begins with hemostasis and inflammation. This stage involves recruitment of blood platelets and immune cells to control loss of blood and clearing of pathogens. The initially recruited immune cells play a major role in secreting chemokines and growth factors, which attract cells and thereby lead the healing process to its next phase of proliferation.
The proliferation phase comprises of numerous events like granulation tissue development (formation of provisional ECM), angiogenesis (formation of blood vessels) and reepithelialization (formation of epidermal skin layer), thereby resulting in wound contraction. This particular phase is regulated through crosstalk between various cells, mainly macrophages, fibroblasts, endothelial cells and keratinocytes.
Recent innovative strategies in wound healing and skin regeneration
Skin tissue engineering is a complex process involving appropriate choice of biomaterial, cell selection and designing of suitable platform in order to mimic structural and functional properties of skin. In last five years, fabrication of a pro-regenerative construct has been a major focus of research in wound repair and regeneration.
The principal aim is to develop scaffolds containing cell instructive cues to restore the damaged skin in a regenerative manner as depicted in Cells not only require a platform to reside and proliferate, but also need instructive cues in the form of a suitable niche to grow in a regenerative microenvironment. These cues are fine orchestra of signalling molecules along with physical properties of scaffolds.
Functionalization of scaffolds with bioactive molecules is a commonly explored approach wherein growth factors, antimicrobial molecules, bioactive nanoparticles or liposomes, cell binding peptides and other specific additives are used for fostering chemical signalling in the construct. There are two main types of matrix functionalization approaches – 1) functionalize the construct that acts as a cargo to slowly deliver the bioactive compounds at wound site and 2) permanent binding of bioactive molecule or its functional mimic that stays with the construct and stimulate cells. Both the approaches help in accelerating the wound repair process.
Immunomodulation and vascularization based approach
Principal goals in wound management include vascularization in the regenerated tissue to achieve rapid wound healing and skin regeneration. Many pioneering approaches have utilized cellular and molecular biology for the development of pre-vascularized skin grafts over the past decade.
During the course of development of advanced cellular therapies, a greater comprehension of basic biology has been considered by the researchers in recent years. Targeting immune system for enhanced angiogenesis has recently been explored leading to significant advances in wound healing therapy. This approach stands apart from the orthodox approach of using only endothelial cells in making vascularized grafts.
Clearly, immune system has a major role in the healing process because inflammation is the very initial phase of wound healing. The early immune response delivers signals to the subsequent phases through cytokines and chemokines. These strides have ultimately led to advancements in skin regeneration.
Therefore, tailoring the immune response or modulating the behaviour of immune cells may impart major breakthroughs in wound repair and skin tissue engineering in near future. At the early phase of wound healing, inflammation occurs and recruitment of neutrophils and macrophages takes place. The macrophages release various signals, which can be either pro inflammatory or anti-inflammatory and thus fate of the wound is decided accordingly.
SiRNA based skin therapeutics
Among the nucleic acid based wound healing strategies, utilization of siRNA and miRNA based wound healing has gained momentum over the past few years. Small interfering RNA is a small, synthetic RNA of 21–25 nucleotides length that can specifically carry out silencing of intended genes through knockdown of messenger RNA (mRNA) specific to the gene of interest .
Both the siRNA and miRNA are short stretches of nucleotide RNA duplex and holds potential to inhibit mRNA translation through various ways as illustrated in. Reduction in gene expression has major implications in multiple disease management strategies. Among these wound healing treatments, siRNA owes a special mention due to its complicated interplay of myriad of cellular pathways that prevail among the four stages of the process.
MicroRNA based skin therapeutics
The discussion on nucleotide-based treatments relevant in wound healing is incomplete without incorporating the context of miRNAs. Although both miRNA and siRNA share similar structures (~25 nucleotide RNA duplex with 2 nucleotides 3’overhang), their mode of action is different.
Unlike siRNAs that are designed prioritizing specificity for binding, miRNAs are comparatively imprecise in terms of complementarity to their target and hence can target multiple genes as depicted. siRNA regulate the gene expression by endonucleolytic cleavage of mRNA; whereas miRNA do translational repression of the target mRNA and also degrade the target mRNA as illustrated. Upon reaching the cytoplasm, miRNA acts on the target mRNA by forming RISC containing miRNA, Dicer and Argonaute (Ago) protein, where the miRNA pairs with the 3′ UTR of target mRNA and either inhibit the translation of mRNA or degrade/ cleave the same.
The two major approaches of miRNA-based therapy include – 1) restoration of miRNA expression by delivering synthetic miRNA via scaffold or viral vector-based carriers and 2) inhibiting or silencing a pathology associated miRNA by chemically modified anti-miRNA oligonucleotides or antagonistic sequence (antagomir). The association of miRNA to wound healing is well established now.
An update on the clinical trials
The clinical implementation of technology is necessary to validate proof of concept and pre-clinical studies performed in animal models. Clinical study not only reveals the treatment outcomes, but also unfolds the basic information like cost of treatment, patient safety and risks involved during the particular treatment.
Efficacy of wound dressings and skin grafts have been constantly under evaluation through the regulated clinical trials on patients in multiple clinical set-ups over a period of 5–10 year . The aim of clinical study is to transfer the technology from bench to bedside for harnessing the emerging wound management strategies and realistic advancements over the conventional methods.
Clinicians and scientists are nowadays applying mathematical calculations, statistical methods and resource expenditure assessment to demonstrate the results in a more realistic and accurate manner. Herein, we try to summarize the outcomes of some recent trial reports on the bioengineered skin grafts, cryopreserved decellularized or cadaveric based skin constructs and emerging usage of SVF on patients.
Author: Dimple Chouhan, Namit Dey, Nandana Bhardwaj, Biman B. Mandal,