NIH Campus, Building 31, Room 4C32
Carl C. Baker, MD, PhD, NIAMS
Stephen I. Katz, MD, PhD, NIAMS
Vincent Falanga, MD, FACP, Boston University
The objective of this roundtable discussion was to provide NIAMS leadership with information about scientific opportunities, needs, and roadblocks in chronic skin wound healing biology and treatment. Consultation with outside experts working within this field pointed to topics, concerns, challenges, and innovations viewed by the scientific community. Emerging themes that arose from the discussion will help NIAMS frame its long-term scientific planning and priority-setting.
Key questions addressed in the roundtable included:
- What are the most promising areas of science in the field?
- What are the most pressing scientific needs?
- What can NIAMS/NIH contribute uniquely to fill gaps or address needs?
Wound healing research has received significant attention in NIAMS's funding portfolio and the number of components of the Institute's extramural program (inflammation, tissue engineering and scaffolds; muscle, tendon, and ligament injury and repair; stem cells, gene therapy) that support these projects indicates the field's cross-cutting nature. The cross-cutting nature of the theme is also demonstrated by the interest of other NIH institutes in the topic (National Institute of General Medical Sciences, National Institute of Diabetes and Digestive and Kidney Diseases, National Institute on Aging, and National Institute of Nursing Research). The National Institute of General Medical Sciences (NIGMS) funded four new centers for wound healing research in 2006
Specific topics of NIAMS-supported research include:
- inflammation; angiogenesis.
- keratinocyte migration/motility.
- keratinocyte growth and proliferation in response to wounding.
- stem cells in wound healing.
- extracellular matrix (ECM) and remodeling.
- use of biologicals (e.g., growth factors) to promote wound healing.
- cell-based therapies (e.g., engineered human skin equivalents, stem cells).
- prevention of scarring; keloids.
- gene therapy for chronic wounds (PDGFB).
- grafting of human skin equivalents (StrataGraft).
The clinical, economic, and societal burden of wound healing is mostly attributed to impaired wound healing and chronic wounds, commonly found in the elderly, the bed-ridden, and the diabetic population. The requirements for skin wound healing and the specific factors involved (such as cell types, growth factors, and cytokines) are still not fully understood. The physiological complexity of the problem demands reliable and broadly-accepted animal models that relate to human therapeutic research. Although transgenic mice provide superb opportunities to dissect the effects of individual genes on wound healing in vivo, there are significant differences between human and mouse skin—the tissue most involved in wound healing. Mouse skin contracts more than human skin in wound healing. However, mouse tail skin wounds do not contract and therefore may be an acceptable model for human wound healing. Many defects in human wound healing are seen only in older populations, which cannot be modeled easily in mice (older mice are difficult to maintain and suffer from other diseases that confound wound healing studies). Animal models for chronic wounds are also needed. Porcine skin closely resembles human skin, but many factors limit the utility of this species in most research projects. The field's attempts to develop animal wound healing models identify an important problem: there has been a lack of standardization in the application of many approaches (such as in vivo animal studies, clinical tissue sampling, and clinical trials), making it difficult to integrate results from various research groups. Closer collaborations between animal researchers and clinical scientists would lead to more relevant utilization of mouse models towards clinical endpoints, a better understanding of human wound healing, and identification of new targets.
Due to the limitation of animal models, a focus on signaling pathways and a systems biology approach provide the greatest opportunities in many areas of biomedical research, including wound healing. A diagram of cells recruited during various stages of dermal wound repair would help the wound healing community to focus efforts and develop research questions. A preliminary diagram describes the steps of coagulation, inflammation, proliferation and migration, and remodeling, with the relative number of platelets, neutrophils, macrophages, fibroblasts, keratinocytes, and lymphocytes, and their surrogate markers, at each stage1. Some cell types undergo complicated phenotypic shifts (for example, fibroblasts will produce different collagen types at different stages of wound healing) and other cell types (such as macrophages) have heterogeneous characteristics. Therefore, an updated diagram should display the cell types and subtypes that contribute to wound healing, their origins, and their gene expression profiles, which change as wounds evolve. It would be useful to have genetically engineered mice in which relevant cell types have been tagged with reporter molecules to allow individual cell populations to be followed throughout the different stages of wound healing. Resident microbes and defensins and other components of the innate immune system need to be considered in the context of chronic wound healing because targeting some of the mediators of innate immunity may be effective in promoting wound healing. This approach would be based on eradicating negative factors (such as tumor necrosis factor alpha: TNF-α), for example, with interference RNA, rather than overcoming a pathological system with positive factors (such as promoters of keratinocyte migration). Information derived from this systems-based approach may be employed in creating molecular profiles of wound-healing lesions. It would be an excellent tool for classifying patients, refining the selection of participants in clinical trials, and evaluating results of clinical studies, based on the molecular profile of the wound area.
1 A diagram, shared at the meeting, accompanies the printed summary.
It is also important to understand epithelial-mesenchymal interactions in wounds, as signals from one component affect the other. There is positional diversity of cell types, such as fibroblasts or keratinocytes; for example, there can be significant differences in gene expression patterns between fibroblasts from different anatomic locations, in the same person. Hence, the behavior of cells involved with wound healing may depend on the type and site of injury.
High throughput screening can be used to identify surrogate markers on cells, and elucidate signaling pathways, matrix degradation pathways, and their inhibitors. It is difficult to apply these methods to skin, skin equivalents, and scaffolds, due to logistical challenges involved in obtaining and processing adequate tissue samples from patients, and complicated sample preparation. Mouse models, particularly those available through NIH's Knock-Out Mouse Project (KOMP), would be useful for screening; however, there is a fairly long window of observation in some mouse wound healing systems. NIAMS should be involved in establishing a genetic database of mouse mutants with wound healing phenotypes. A better understanding of the human genetics of impaired wound healing would contribute to better design of studies with mouse models. In addition, in vitro organotypic cell culture models for wound healing, particularly for chronic wounds, would facilitate screening of therapeutic agents.
A notable observation in impaired wound healing is diminished keratinocyte migration, which may be due to changes in the underlying matrix components (such as collagen production), lack of a cell stimulus, changes in receptors or cell signaling that inhibit release of cells from matrix attachments, or other events. Defects in these components could also affect the movement of keratinocytes in the wound area. "Repairing the road" for keratinocyte migration, including modulating factors, is an important problem to address for research and therapy. Recent publications cite the role of electrical signals in keratinocyte migration, via phosphatidyl inositol signaling and phosphatase activity, bringing attention to signaling pathways as critical areas of understanding. Migration is also balanced with proliferation in wound healing; a cell cannot participate in both activities simultaneously. In chronic wounds, significant proliferation is observed at the wound edges, but migration is impaired. Understanding the signals that switch a keratinocyte from proliferation to migration is a significant research opportunity that could lead to the development of therapeutics that enhance the healing of chronic wounds.
Understanding and controlling the regenerative process is essential; the natural wound healing response is "over-exuberant" and can create additional morbidity in the form of hypertrophic scarring and fibrosis. Frequently, healed skin lacks the tensile strength of normal skin, so the quality of healing must be considered. There is tremendous interest in attracting endogenous cells to the wound, to conduct most of the wound healing processes; scaffolds and biomaterials may aid the homing and residency of these cells. These materials could also be used to deliver cells, growth factors, cytokines, or other agents. Functional bonds can be used to regulate release of these factors, for example, by lowering pH in the wound. The clinical endpoint must be considered in recruiting the requisite cell types to the appropriate location (i.e., skin layer) of the wound.
The promise of growth factors has not yielded empirical results, at least not with topical delivery. There are still prospects for delivery of growth factor genes with viral vectors, tempered with concern of maintaining persistent expression of the growth factor in the wound.
The differences between wounds (venous vs. diabetic ulcers; new vs. chronic; responsive vs. non-responsive to treatment) must be addressed in therapeutic design, including scaffolds development. There are concerns of employing scaffolds or biomaterials in a diseased tissue: can the body support the repaired segment or will it fail again? Mechanical load on chronic wounds can affect collagen turnover, susceptibility to collagenases, tissue structure, blood flow, immune cell populations, and many unknown factors, which must also be addressed in all efforts towards wound healing, including use of scaffolds and biomaterials. The emphasis must be on regeneration, as well as repair.
Despite the acknowledgment of the complications that normal skin flora create in impaired wound healing, very little is understood about their specific role. In general, there is a correlation between bacterial load and rates of wound healing. However, low levels of bacteria usually stimulate an immune response, which adds many new factors to the wound area and enhances wound healing. Bacteria in many chronic wounds form biofilms (barriers that resist therapeutic agents) and therefore escape treatment with antibiotics. Hence, there is a need to characterize the microbial population of chronic wounds. Community-associated, region-specific microbial populations can also demand highly specialized chronic wound treatment strategies. Methods such as microarray analysis can be employed to detect microbes which cannot be cultured, but caution must be taken in the use and application of this information. Additionally, understanding the factors that foster biofilm formation (towards the design of biofilm inhibitors), and antimicrobial selection and delivery are important topics to pursue.
The interest in scaffolds and biomaterials raises the persistent issue of communication and interdisciplinary research. Development of these materials requires the collaboration of biologists, material scientists, and clinicians/clinical researchers for well-designed research projects.
Improved communication between research groups, as well as within research groups, is imperative. The multidisciplinary nature of wound healing presents challenges in both of these areas. The lack of a clinical standard-of-care creates a difficulty in conducting clinical trials in wound healing. There must be consensus on the appropriate controls for clinical research, within the clinical community and with the Food and Drug Administration (FDA). There is a tremendous need for agreement in the development and conduct of protocols for pre-clinical research, clinical trials, and tissue sampling for efficient progress in the field. The FDA can provide critical advice in clinical study design.
Concerted leadership, improved approaches between research teams, and agreeing to methodological and measurement provided tremendous gains in cancer and lupus research; an NIH solicitation helped to develop the lupus assessment measures. Similar consensus-building efforts should be applied to wound healing research; instruments to measure and assess disease should be a priority.
Clinicians should be involved in the early stages of project development, as well as throughout a project, to design studies appropriate to clinical feasibility and clinical endpoints, to mitigate problems in clinical sampling and/or testing, and to ensure adherence to protocols. The new NIH policy to allow multiple PIs is expected to enhance clinicians' participation; the laboratory and clinical team leaders of multidisciplinary research projects can receive credit for obtaining grant funding and attention for project achievements. Non-clinical researchers need to gain understanding in clinical research, as well, and institutions need to develop infrastructure for conducting clinical trials, such as Good Manufacturing Practices (GMP) facilities, and the associated regulatory processes, such as filing Investigational New Drug (IND) applications with the FDA. NIH's new Clinical and Translational Science Awards (CTSAs) provide institutions with funding for these essential services.
ALLEN-HOFFMANN, B. Lynn, Ph.D.
Professor, Department of Pathology
University of Wisconsin
CHANG, Howard, M.D., Ph.D.
Assistant Professor, Department of Dermatology
Stanford University School of Medicine
COHEN, I. Kelman, M.D.
Emeritus Professor of Surgery, Division of Plastic and Reconstructive Surgery
Virginia Commonwealth University Health Science Center
COTSARELIS, George, M.D.
Associate Professor, Department of Dermatology
University of Pennsylvania
COULOMBE, Pierre, Ph.D.
Professor, Departments of Biological Chemistry and Dermatology
Johns Hopkins University School of Medicine
DAVIDSON, Jeffrey M., Ph.D.
Professor of Pathology, Department of Pathology
Vanderbilt University School of Medicine
DIPIETRO, Luisa A., D.D.S., Ph.D.
Professor and Director, Center for Wound Healing and Tissue Regeneration
College of Dentistry
University of Illinois
FALANGA, Vincent, M.D., FACP
Professor, Departments of Dermatology and Biochemistry
Boston University School of Medicine
Chairman, Department of Dermatology and Skin Surgery
Roger Williams Medical Center
IKEDA, Richard, Ph.D.
Health Scientist Administrator, Pharmacology, Physiology and Biological Chemistry
National Institute of General Medical Sciences
National Institutes of Health
LASKIN, Jeffrey, Ph.D.
Professor, Division of Toxicology
Robert Wood Johnson Medical School
Environmental and Occupational Medicine
Environmental and Occupational Health Science Institutes
University of Medicine and Dentistry of New Jersey
OLERUD, John, E., M.D.
George F. Odland Professor
Division of Dermatology
University of Washington
STOTTS, Nancy, R.N., Ed.D., FAAN
Professor, Physiological Nursing
University of California, San Francisco
TOMIC-CANIC, Marjana, Ph.D.
Associate Professor, Departments of Cell and Molecular Biology and Dermatology
Weill Cornell Medical College
Hospital for Special Surgery
WANG, Xiao-Jing, M.D., Ph.D.
Director, Molecular Biology - Head and Neck Cancer Research
Professor, Department of Otolaryngology
Oregon Health and Science University