In vivo evaluation of adipose- and muscle-derived stem cells as a treatment for nonhealing diabetic wounds using multimodal microscopy

Animal Models of Type 2 Diabetes Complications: A Review

Diabetes mellitus is a multifactorial metabolic disease, of which type 2 diabetes (T2D) is one of the most common. The complications of diabetes are far more harmful than diabetes itself. Type 2 diabetes complications include diabetic nephropathy (DN), diabetic heart disease, diabetic foot ulcers (DFU), diabetic peripheral neuropathy (DPN), and diabetic retinopathy (DR) et al. Many animal models have been developed to study the pathogenesis of T2D and discover an effective strategy to treat its consequences. In this sense, it is crucial to choose the right animal model for the corresponding diabetic complication. This paper summarizes and classifies the animal modeling approaches to T2D complications and provides a comprehensive review of their advantages and disadvantages. It is hopeful that this paper will provide theoretical support for animal trials of diabetic complications.

Human adipose-derived stem cell-loaded small intestinal submucosa as a bioactive wound dressing for the treatment of diabetic wounds in rats

Chronic nonhealing wounds are one of the most common and serious complications of diabetes, which can lead to disability of patients. Adipose-derived stem cells (ADSCs) have emerged as a promising tool for skin wound healing, but the therapeutic potential depends considerably on the cell delivery system. Small intestinal submucosa (SIS) is an extracellular matrix-based membranous scaffold with outstanding repair potential for skin wounds. In this study, we first fabricated a bioactive wound dressing, termed the SIS+ADSCs composite, by using human ADSCs as the seed cell and porcine SIS as the cell delivery vehicle. Then, we systematically investigated, for the first time, the healing potential of this wound dressing in a rat model of type 2 diabetes. In vitro studies revealed that SIS provided a favorable microenvironment for ADSCs and significantly promoted the expression of growth factors critical for chronic wound healing. After implantation in the full-thickness skin wounds of diabetic rats, the SIS+ADSCs composite showed a higher wound healing rate and wound healing quality than those in the PBS, ADSCs, and SIS groups. Along with the ability to modulate the polarization of macrophages in vivo, the SIS+ADSCs composite was potent at promoting wound angiogenesis, reepithelialization, and skin appendage regeneration. Taken together, these results indicate that the SIS+ADSCs composite has good therapeutic potential and high translational value for diabetic wound treatment.

Limited Treatment Options for Diabetic Wounds: Barriers to Clinical Translation Despite Therapeutic Success in Murine Models

Significance: Millions of people worldwide suffer from diabetes mellitus and its complications, including chronic diabetic wounds. To date, there are few widely successful clinical therapies specific to diabetic wounds beyond general wound care, despite the vast number of scientific discoveries in the pathogenesis of defective healing in diabetes. Recent Advances: In recent years, murine animal models of diabetes have enabled the investigation of many possible therapeutics for diabetic wound care. These include specific cell types, growth factors, cytokines, peptides, small molecules, plant extracts, microRNAs, extracellular vesicles, novel wound dressings, mechanical interventions, bioengineered materials, and more. Critical Issues: Despite many research discoveries, few have been translated from their success in murine models to clinical use in humans. This massive gap between bench discovery and bedside application begs the simple and critical question: what is still missing? The complexity and multiplicity of the diabetic wound makes it an immensely challenging therapeutic target, and this lopsided progress highlights the need for new methods to overcome the bench-to-bedside barrier. How can laboratory discoveries in animal models be effectively translated to novel clinical therapies for human patients? Future Directions: As research continues to decipher deficient healing in diabetes, new approaches and considerations are required to ensure that these discoveries can become translational, clinically usable therapies. Clinical progress requires the development of new, more accurate models of the human disease state, multifaceted investigations that address multiple critical components in wound repair, and more innovative research strategies that harness both the existing knowledge and the potential of new advances across disciplines.

Longitudinal Label-Free Two-Photon Microscopy of Cellular Healing Processes in Non-Ablative Fractional Laser Wounds

BACKGROUND AND OBJECTIVES

Wound healing is an important biomedical problem with various associated complications. Although cutaneous wound healing has been studied in vivo extensively using various optical imaging methods, early-stage cellular healing processes were difficult to study due to scab formation. The objective of this study is to demonstrate that minimal laser wounds and optical microscopy can access the detailed cellular healing processes of cutaneous wounds from the early stage.

STUDY DESIGN/MATERIALS AND METHODS

A non-ablative fractional laser (NAFL) and label-free two-photon microscopy (TPM) were used to induce minimal cutaneous wounds and to image the wounds in three-dimension. Sixteen hairless mice and a single human volunteer were used. NAFL wounds were induced in the hindlimb skin of the mice and in the forearm skin of the human subject. The NAFL wounds were longitudinally imaged during the healing period, starting from an hour post wound induction in the earliest and until 21 days. Cells in the wound and surrounding normal skin were visualized based on two-photon excited auto-fluorescence (TPAF), and cellular changes were tracked by analyzing longitudinal TPM images both qualitatively and quantitatively. Damage and recovery in the skin dermis were tracked by using the second harmonic generation (SHG) signal of collagen. Immunofluorescence and hematoxylin and eosin histology analysis were conducted to validate the TPM results of the murine skin.

RESULTS

Cellular healing processes in NAFL wounds and surroundings could be observed by longitudinal TPM. In the case of murine skin, various healing phases including inflammation, re-epithelization, granulation tissue formation, and late remodeling phase including collagen regeneration were observed in the same wounds owing to minimal or no scab formation. The re-epithelization process was analyzed quantitatively by measuring cell density and thickness of the epithelium in the wound surroundings. In the case of the human skin, the access inside the wound was blocked for a few days post wound induction due to scabs but the cellular changes in the wound surroundings were observed from the early stage. Cellular healing processes in the NAFL wound of the human skin were similar to those in murine skin.

CONCLUSIONS

The minimal NAFL wound model and label-free TPM demonstrated the cell level assessment of wound healing processes with applicability to human subjects. © 2021 Wiley Periodicals LLC.

Simultaneous label-free autofluorescence and multi-harmonic imaging reveals in vivo structural and metabolic changes in murine skin

Simultaneous quantification of multifarious cellular metabolites and the extracellular matrix in vivo has been long sought. Simultaneous label-free autofluorescence and multi-harmonic (SLAM) microscopy has achieved simultaneous four-channel nonlinear imaging to study tissue structure and metabolism. In this study, we implemented two laser systems and directly compared SLAM microscopy with conventional two-photon microscopy for in vivo imaging. We found that three-photon imaging of adenine dinucleotide (phosphate) (NAD(P)H) in SLAM microscopy using our tailored laser source provided better resolution, contrast, and background suppression than conventional two-photon imaging of NAD(P)H. We also integrated fluorescence lifetime imaging with SLAM microscopy, and enabled differentiation of free and bound NAD(P)H. We imaged murine skin in vivo and showed that changes in tissue structure, cell dynamics, and metabolism can be monitored simultaneously in real-time. We also discovered an increase in metabolism and protein-bound NAD(P)H in skin cells during the early stages of wound healing.

In Vivo Assessment of Engineered Skin Cell Delivery with Multimodal Optical Microscopy

The healing process is often significantly impaired under conditions of chronic or large area wounds, which are often treated clinically using autologous split-thickness skin grafts. However, in many cases, harvesting of donor tissue presents a serious problem such as in the case of very large area burns. In response to this, engineered biomaterials have emerged that attempt to mimic the natural skin environment or deliver a suitable therapy to assist in the healing process. In this study, a custom-built multimodal optical microscope capable of noninvasive structural and functional imaging is used to investigate both the engineered tissue microenvironment and the in vivo wound healing process. Investigation of various engineered scaffolds show the strong relationship among the microenvironment of the scaffold, the organization of the cells within the scaffold, and the delivery pattern of these cells onto the healing wound. Through noninvasive tracking of these processes and parameters, multimodal optical microscopy provides an important tool in the assessment of engineered scaffolds both in vitro and in vivo.

Investigating the healing mechanisms of an angiogenesis-promoting topical treatment for diabetic wounds using multimodal microscopy

Impaired skin wound healing is a significant comorbid condition of diabetes that is caused by poor microcirculation, among other factors. Studies have shown that angiogenesis, a critical step in the wound healing process in diabetic wounds, can be promoted under hypoxia. In this study, an angiogenesis-promoting topical treatment for diabetic wounds, which promotes angiogenesis by mimicking a hypoxic environment via inhibition of prolyl hydroxylase resulting in elevation or maintenance of hypoxia-inducible factor, was investigated utilizing a custom-built multimodal microscopy system equipped with phase-variance optical coherence tomography (PV-OCT) and fluorescence lifetime imaging microscopy (FLIM). PV-OCT was used to track the regeneration of the microvasculature network, and FLIM was used to assess the in vivo metabolic response of mouse epidermal keratinocytes to the treatment during healing. Results show a significant decrease in the fluorescence lifetime of intracellular reduced nicotinamide adenine dinucleotide, suggesting a hypoxic-like environment in the wounded skin, followed by a quantitative increase in blood vessel density assessed by PV-OCT. Insights gained in these studies could lead to new endpoints for evaluation of the efficacy and healing mechanisms of wound-healing drugs in a setting where delayed healing does not permit available methods for evaluation to take place.