Novel approaches for understanding the mechanisms of wound repair

Skin proteomics - analysis of the extracellular matrix in health and disease

INTRODUCTION

The skin protects the human body from external insults and regulates water and temperature homeostasis. A highly developed extracellular matrix (ECM) supports the skin and instructs its cell functions. Reduced functionality of the ECM is often associated with skin diseases that cause physical impairment and also have implications on social interactions and quality of life of affected individuals.

AREAS COVERED

With a focus on the skin ECM we discuss how mass spectrometry (MS)-based proteomic approaches first contributed to establishing skin protein inventories and then facilitated elucidation of molecular functions and disease mechanisms.

EXPERT OPINION

MS-based proteomic approaches have significantly contributed to our understanding of skin pathophysiology, but also revealed the challenges in assessing the skin ECM. The numerous posttranslational modifications of ECM proteins, like glycosylation, crosslinking, oxidation, and proteolytic maturation in disease settings can be difficult to tackle and remain understudied. Increased ease of handling of LC-MS/MS systems and automated/streamlined data analysis pipelines together with the accompanying increased usage of LC-MS/MS approaches will ensure that in the coming years MS-based proteomic approaches will continue to play a vital part in skin disease research. They will facilitate the elucidation of molecular disease mechanisms and, ultimately, identification of new druggable targets.

In vitro skin models to study epithelial regeneration from the hair follicle

The development of dermal equivalents (DEs) for the treatment of burns has contributed toward efficient wound closure. A collagen-glycosaminoglycan DE (C-GAG) grafted with hair follicles converted a full-thickness wound to partial-thickness resulting in complete wound closure and restored hair. In this study we compared the ability of both intact pilosebaceous units (PSU) or truncated hair follicles (THF) to regenerate a multilayered epidermis in vitro when implanted into de-epidermalized dermis (DED) or C-GAG with the epidermis generated in vivo using C-CAG. Keratinocytes explanted from the outer root sheath of PSU and THF in both DED and C-GAG but only formed a multilayered epidermis with PSU in DED. PSU were more effective at forming multilayered epidermis in DED than THF. Both DED and C-GAG skin expressed proliferation (PCNA), differentiation (K1, K10), hyperproliferation (K6, K16), basal (K14), putative stem cell (p63), extracellular matrix protein (Collagen IV), mesenchymal (vimentin) and adherens junction (β-catenin) markers. These data suggest DEs supported initial maintenance of the implanted hair follicles, in particular PSU, and provide an excellent model with which to investigate the regulation of hair follicle progenitor epithelial cells during epidermal regeneration. Although neither PSU nor THF formed multilayered epidermis in C-CAG in vitro, hair follicles implanted into engrafted C-GAG on a burns patient resulted in epithelial regeneration and expression of proliferation and differentiation markers in a similar manner to that seen in vitro. However, the failure of C-GAG to support epidermal regeneration in vitro suggests in vivo factors are essential for full epidermal regeneration using C-GAG.

A cell-based screening assay to identify pharmaceutical compounds that enhance the regenerative quality of corneal repair

The goal of this study was to develop and validate a simple but quantitative cell-based assay to identify compounds that might be used pharmaceutically to give tissue repair a more regenerative character. The cornea was used as the model, and some specific aspects of repair in this organ were incorporated into assay design. A quantitative cell-based assay was developed based on transcriptional promoter activity of fibrotic marker genes ACT2A and TGFB2. Immortalized corneal stromal cells (HTK) or corneal epithelial cells (HCLE) were tested and compared to primary corneal stromal cells. Cells were transiently transfected with constructs containing the firefly luciferase reporter gene driven by transcriptional promoters for the selected fibrotic marker genes. A selected panel of seven chemical test compounds was used, containing three known fibrosis inhibitors: lovastatin (LOV), tyrphostin AG 1296 (6,7-dimethoxy-3-phenylquinoxaline) and SB203580 (4-(4-fluorophenyl)-2-(4-methylsulfinylphenyl)-5-(4-pyridyl)1H-imidazole), and four potential fibrosis inhibitors: 5-iodotubercidin (4-amino-5-iodo-7-(β-D-ribofuranosyl)-pyrrolo(2,3-d)pyrimidine), anisomycin, DRB (5,6-dichloro-1-β-D-ribofuranosyl-benzimidazole) and latrunculin B. Transfected cells were treated with TGFB2 in the presence or absence of one of the test compounds. To validate the assay, compounds were tested for their direct effects on gene expression in the immortalized cell lines and primary human corneal keratocytes using RT-PCR and immunohistochemistry. Three "hits" were validated LOV, SB203580 and anisomycin. This assay, which can be applied in a high throughput format to screen large libraries of uncharacterized compounds, or known compounds that might be repurposed, offers a valuable tool for identifying new treatments to address a major unmet medical need. Anisomycin has not previously been characterized as antifibrotic, thus, this is a novel finding of the study.

Wound degradomics - current status and future perspectives

Proteases are pivotal modulators of extracellular matrix components and bioactive proteins at all phases of cutaneous wound healing and thereby essentially contribute to the successful reestablishment of skin integrity upon injury. As a consequence, disturbance of proteolytic activity at the wound site is a major factor in the pathology of chronic wounds. A large body of data acquired in many years of research provide a good understanding of how individual proteases may influence the repair process. The next challenge will be to integrate these findings and to elucidate the complex interactions of proteolytic enzymes, their inhibitors and substrates on a system-wide level. Here, we present novel approaches that might help to achieve this ambitious goal in cutaneous wound healing research.

Spatial mapping by imaging mass spectrometry offers advancements for rapid definition of human skin proteomic signatures

Investigations into the human skin proteome by classical analytical procedures have not addressed spatial molecular distributions in whole-skin biopsies. The aim of this study was to develop methods for the detection of protein signatures and their spatial disposition in human skin using advanced molecular imaging technology based on mass spectrometry technologies. This technology allows for the generation of protein images at specific molecular weight values without the use of antibody while maintaining tissue architecture. Two experimental approaches were employed: MALDI-MS profiling, where mass spectra were taken from discrete locations based on histology, and MALDI-IMS imaging, where complete molecular images were obtained at various MW values. In addition, proteins were identified by in situ tryptic digestion, sequence analysis of the fragment peptides and protein database searching. We have detected patterns of protein differences that exist between epidermis and dermis as well as subtle regional differences between the papillary and reticular dermis. Furthermore, we were able to detect proteins that are constitutive features of human skin as well as those associated with unique markers of individual variability.

Prostaglandin E1 and misoprostol increase epidermal growth factor production in 3D-cultured human annulus cells

BACKGROUND CONTEXT

Epidermal growth factor (EGF) is a peptide known to modulate a number of cellular responses including embryogenesis, cell proliferation, and cell survival. Little is known about EGF and its regulation in human annulus cells. Previous work has identified EGF and its receptor in control outer annulus disc tissue, but not in herniated tissue.

PURPOSE

To determine if human annulus cells express EGF in vitro, to determine if the epidermal growth factor-receptor (EGF-r) was expressed in vivo and in vitro in disc cells, to test the effect of EGF on annulus cell proliferation and proteoglycan production in vitro, and to test the effect of prostaglandin E1 (PGE1) and misoprostol on disc cell production of EGF in vitro.

STUDY DESIGN/SETTING

Studies were approved by the authors' Human Subjects Institutional review Board. Human disc tissue was used for immunocytochemistry, and human annulus cells were tested in vitro.

PATIENT SAMPLE

Thirty-four disc specimens were used for studies of proteoglycan production, cell proliferation, and EGF production in vitro. An additional nine discs were used for EGF-r immunolocalization.

METHODS

Disc tissue was used for immunocytochemical studies for the localization of EGF-r and as a source for cultured annulus cells. Monolayer culture was used to test proliferation responses to 0, 25, 50, or 75 ng/mL EGF over a 2-day culture period. Three-dimensional (3D) culture in a collagen sponge was used to test 100,000 cells cultured in a paired experimental design over 14 days for production of EGF and proteoglycans. Cells were exposed to control conditions, or to either misoprostol at 8 ng/mL or PGE1 at 10(-7)M. Conditioned media was harvested and assayed using an enzyme-linked immunosorbent assay (ELISA) assay with the Human Protein Cytokine Antibody Array I kit. Replicate EGF relative intensity values were averaged and normalized to controls assayed on the same membrane. 3D-cultured cells were also used to confirm EGF gene expression using microarray analysis. Standard statistical methods were used to analyze results.

RESULTS

Microarray analysis of mRNA from annulus cells in 3D culture confirmed expression of EGF, and immunocytochemistry verified the presence of EGF-r in vitro and in vivo. PGE1, at a dose of 10(-7)M, and misoprostol (a synthetic PGE1 analog) at a dose of 8 ng/mL, both significantly increased EGF levels in annulus cells cultured in 3D compared with control levels (p=.031 and .034, respectively). No significant difference, however, was seen in cell proliferation or in total sulfated proteoglycan production in EGF-exposed annulus cells.

CONCLUSIONS

Data showed that EGF is expressed and produced by annulus cells in vivo and in 3D culture, with significantly greater in vitro EGF produced in the presence of PGE1 or the PGE1 analog misoprostol. Misoprostol, developed for prevention/treatment of nonsteroidal anti-inflammatory-induced gastropathy, has now been reported to have some interesting anabolic effects stimulating osteoblasts during fracture healing and during ovariectomy in animal models. Exogenous EGF did not increase cell proliferation in monolayer, or total production of proteoglycans in 3D culture. Additional work is needed to further delineate the role of EGF in the human disc.

Intracellular adenosine triphosphate delivery enhanced skin wound healing in rabbits

Small unilamellar lipid vesicles were used to encapsulate adenosine triphosphate (ATP-vesicles) for intracellular energy delivery. This technique was tested in full-thickness skin wounds in 16 adult rabbits. One ear was rendered ischemic by using a minimally invasive surgery. The other ear served as a normal control. Four circular full-thickness wounds were created on the ventral side of each ear. ATP-vesicles or saline was used and the wounds were covered with Tegaderm (3M, St. Paul, MN). Dressing was changed and digital photos were taken daily until all the wounds were healed. The mean healing times of ATP-vesicles-treated wounds were significantly shorter than that of saline-treated wounds on ischemic and nonischemic ears. Histologic study indicated better-developed granular tissue and reepithelialization in the ATP-vesicles-treated wounds. The wounds treated by ATP-vesicles exhibited extremely fast granular tissue growth. More CD31 positive cells were seen in the ATP-vesicles-treated wounds. This preliminary study shows that direct intracellular delivery of ATP can accelerate the healing process of skin wounds on ischemic and nonischemic rabbit ears. The extremely fast granular tissue growth was something never seen or reported in the past.

Sulfur mustard downregulates iNOS expression to inhibit wound healing in a human keratinocyte model

BACKGROUND

Increased nitric oxide (NO) synthesized by inducible NO synthase (iNOS) is involved in inflammatory and pathological conditions. iNOS also regulates several biomarkers that accelerate normal wound healing. Effects of exposure to sulfur mustard (SM) on the skin include formation of blisters and slow-healing injuries. Promoting re-epithelialization is a challenging issue in the treatment of the delayed healing of SM-induced skin injuries.

OBJECTIVES

To clarify the role(s) of iNOS in wound healing and the effect of SM on iNOS expression in an in vitro wound assay to eventually develop therapies for SM skin injuries.

METHODS

A wound was created by scratching normal human epidermal keratinocytes grown in vitro. iNOS expression was monitored by Western blotting, fluorescence microscopy, and real-time RT-PCR. Wound healing was analyzed using digitalized image analysis software.

RESULTS

The level of iNOS peaked 24-48h after wounding. SM exposure strongly reduced iNOS protein and mRNA levels. Fluorescence microscopy revealed that induction of iNOS expression by wounding and inhibition of iNOS expression by SM occurred not only in the cells at the wound edge but also in cells in the surrounding area, suggesting that wounding may induce and SM may inhibit release of cytokines that stimulate iNOS expression. iNOS-specific small interfering RNAs caused a marked decrease of iNOS expression irrespective of wounding. Gene silencing also completely inhibited wound healing.

CONCLUSION

These results suggest that preventing SM-induced inhibition of iNOS may be a prospective strategy to promote wound healing in SM-exposed skin.

Double blind, randomized, placebo controlled clinical trial for the treatment of diabetic foot ulcers, using a nitric oxide releasing patch: PATHON

Background

Diabetes Mellitus constitutes one of the most important public health problems due to its high prevalence and enormous social and economic consequences. Diabetic foot ulcers are one of the chronic complications of diabetes mellitus and constitute the most important cause of non-traumatic amputation of inferior limbs. It is estimated that 15% of the diabetic population will develop an ulcer sometime in their lives. Although novel therapies have been proposed, there is no effective treatment for this pathology. Naturally produced nitric oxide participates in the wound healing process by stimulating the synthesis of collagen, triggering the release of chemotactic cytokines, increasing blood vessels permeability, promoting angiogenic activity, stimulating the release of epidermical growth factors, and by interfering with the bacterial mitochondrial respiratory chain. Topically administered nitric oxide has demonstrated to be effective and safe for the treatment of chronic ulcers secondary to cutaneous leishmaniasis. However, due to their unstable nitric oxide release, the topical donors needed to be applied frequently, diminishing the adherence to the treatment. This difficulty has led to the development of a multilayer polymeric transdermal patch produced by electrospinning technique that guarantees a constant nitric oxide release. The main objective of this study is to evaluate the effectiveness and safety of this novel nitric oxide releasing wound dressing for the treatment of diabetic foot ulcers.

Methods and design

A double-blind, placebo-controlled clinical trial, including 100 diabetic patients was designed. At the time of enrollment, a complete medical evaluation and laboratory tests will be performed, and those patients who meet the inclusion criteria randomly assigned to one of two groups. Over the course of 90 days group 1 will receive active patches and group 2 placebo patches. The patients will be seen by the research group at least every two weeks until the healing of the ulcer or the end of the treatment. During each visit the healing process of the ulcer, the patient's health status and the presence of adverse events will be assessed. Should the effectiveness of the patches be demonstrated an alternative treatment would then be available to patients.

Trial registration

NCT00428727.