A review of exosomes and their application in cutaneous medical aesthetics

Exosomes have gained recent popularity in aesthetic medicine; however, there is still a dearth of understanding on the etiology of exosomes, their physiologic function, and regenerative capabilities.

conveyed.These modifications either inhibit or stimulate disease pathogenesis. 1These intrinsic properties have elevated the potential utilization of exosomes not only in the therapeutic control of various pathogenesis, but also in the realm of cutaneous medical aesthetics.
In the skin, it has been demonstrated that exosome-mediated cell-to-cell communication and transfer plays a role in tissue homeostasis and maintenance of cellular function with a demonstrated interplay in the pathophysiology of chronic inflammatory skin diseases. 3,4In addition, extracellular vesicles released by mesenchymal stem cells (MSCs-EV) are noted to play a role in wound healing by delivering proangiogenic, anti-inflammatory, and antifibrotic properties, hence the recent introduction of exosomes derived from stem cells in conditioned media into the skin rejuvenation space. 2 Other types of stem-cell-derived extracellular vesicles (SC-EVs) have also been found to have a role in anti-aging. 5For instance, exosomes derived from human umbilical cord blood mesenchymal stem cells (UCB-MSCs) have been demonstrated to promote cell migration and collagen synthesis of human dermal fibroblasts (HDFs). 6The same group found that these UCB-MSCs-derived exosomes permeated the skin layer and were found to increase the synthesis of collagen 1 and elastin, the major building blocks of skin rejuvenation.
The goal of this review is to provide a summary of the mechanistic role of exosomes in aesthetic medicine as it relates to skin rejuvenation with a brief spotlight on wound healing and scar remodeling, as well as describe some of the existing and emerging commercially available therapies.The objective is to provide the novice dermatologist with an understanding of the mechanism and therapeutic applications of exosomes in the cutaneous aesthetic medicine.We performed a systematic literature review in Medline utilizing the terms "exosomes in cutaneous aesthetics" and "exosome physiology."In addition, the MeSH term "exosomes" along with various subheadings to include chemistry, classification, drug effects, metabolism, physiology, and ultrastructure were used to build a PubMed search.Reference lists from included studies and review articles were also perused to identify additional studies.We limited to articles published in the last 10 years.

| Wound healing
Exosomes have been found to play a role in angiogenesis, cell proliferation and differentiation, apoptosis, and inflammation, which has led to a surging interest in their therapeutic potential in wound healing. 3,7Exosomes derived from multiple sources, by modulating effector cells, can regulate wound healing (Figure A1).Various notable sources include MSCs, keratinocytes, endothelial cells, immune cells, and several body fluids.
In the skin, the complex interplay between the various cells involved in wound healing to include keratinocytes, macrophage, endothelial cells, fibroblasts, adipocytes, and other cells of the immune system have been found to be partially facilitated by skin-cellderived exosomes. 3,8For instance, mice studies found that injecting M2 macrophage-produced exosomes into skin wounds increased M2 populations of macrophages which are anti-inflammatory, leading to increased fibroblast migration, deposition of collagen and endothelial cell stimulation, while decreasing M1 macrophage (proinflammatory) populations. 3,9Diabetic wound healing mice studies also found that MSC-derived exosomes, through secretion of proinflammatory cytokines and molecules, promoted angiogenesis and proliferative effects. 10Mesoglycan a fibrinolytic compound recently found to have promising effects in skin wound healing was investigated by Belvedere et al. and demonstrated its ability to activate keratinocytes to produce extracellular vesicles, particularly exosomes.
In vitro assays of the secreted exosomes from these mesoglycantreated keratinocytes demonstrated pro enhanced migration and invasion of fibroblasts and endothelial cells. 11osomes derived from mesenchymal stem cells (MSCs-Exos) have been found to have better functionality and are easier to handle than cell-based products. 12They inhibit pro-inflammatory cytokines to promote anti-inflammatory effects and enhance remodeling of the extracellular matrix (ECM) to stimulate tissue regeneration. 3,7rthermore, human bone-marrow MSC (BMSCs)-derived exosomes were found to have promising potential in the acceleration of wound healing via mice studies by inhibiting the transforming growth factor beta (TGF-B)/Smad signal pathway (Figure A1).Attenuation of this pathway has been demonstrated in non-healing wounds. 13,14

| APPLI C ATI ON OF E XOSOME S IN S K IN AE S THE TI C S
Dermatologists are often faced with a plethora of various cosmetic concerns.Scarring, skin aging, pigmentary alterations, and hair loss are some of the more common concerns that dermatologists face in their day-to-day practice.Exosomes serve as a robust therapeutic option to simultaneously address a multitude of concerns given their recent application in scar improvement therapies, pigmentation correction, skin rejuvenation, and hair loss recovery.

| Exosomes in scar remodeling
Scar formation is an essential, yet oftentimes undesired outcome of cutaneous injury, whether accidental, pathological, or iatrogenic, such as surgical scars.Mechanistically, various factors that potentiate scar formation include increased collagen deposition, decreased fibroblast apoptosis, protracted inflammation, excessive angiogenesis, augmented TGF-B1 expression, and delayed keratinocyte function. 3Exosomes have been found to the involved in the intercellular communication required for scar formation. 15Several studies have investigated the action of exogenous exosomes derived from various sources on scar formation.
One study investigated the systemic administration of exosomes derived from adipose mesenchymal stem cells (ASCs-Exos) in mice wound models.They found that while early wound healing stages were characterized by increased collagen I and III, later stages demonstrated reduced scar formation leading them to believe that the ASC-Exos downregulated collagen expression at this time point. 16Another study found that remodeling of the extracellular matrix (ECM) via intravenous administration of ASC-Exos was a key component of achieving a scarless wound repair. 17Furthermore, exosomes derived from human induced pluripotent stem-cellderived mesenchymal stem cells (hiPSC-MSCs-Exos) were found to enhance re-epithelization, angiogenesis, collagen maturity, and led to reduced scar widths. 18Exosomes from menstrual-blood derived MSCs were found to increase neoangiogenesis and accelerate reepithelization of wounds in diabetic mice models, leading to reduced scar formation (Figure A1). 19The exosomes from umbilical cordderived MSCs (uMSCs-exos) were found to suppress the differentiation and excessive aggregation of myofibroblasts, subsequently leading to reduced fibrosis and scar development (Figure A1). 3 Overall, these findings indicate that exosomes from various sources have a key role in regulating fibroblast function and collagen remodeling or deposition to promote reduced scar formation.Exosomes are therefore a promising therapeutic approach to not only enhancement of wound healing but also possible scar prevention.

| Exosomes in skin rejuvenation
The effects and appearance of the aged skin are one of the more common reasons for seeking dermatologic cosmetic consultation.
Both internal and external factors contribute to skin aging to include UV radiation, air pollution, and behavioral choices such as smoking and tanning.Photodamage, in particular, leads to changes in the ECM to include decreased collagen and elastic fibers, leading to the clinical appearances of wrinkles, coarseness, dryness, and sallow skin texture. 20,21Overall, skin aging is characterized by degradation of skin matrix by metalloproteinases (MMPs) and reduced fibroblast function, hence reduced collagen formation and restoration. 16osomes, in their ability to modulate cell-cell communication and some functions of fibroblasts, have gained a lot of attention and potential therapeutic utility in skin rejuvenation in recent years.
One study examining the utility of transdermal delivery of exosomes derived from three-dimensional spheroids of HDFs discovered an increase in procollagen type 1 and decreased MMP-1 expression (Figure A1). 22Another study investigated the effect that application of exosomes derived from human induced pluripotent stem cells (iPSCs-Exos) had on UVB irradiated (aged) HDFs. 23Authors found that pretreatment with the iPCS-Exos impeded damage to the HDFs, decreased overexpression of MMP, and restored collagen type 1 expression. 3,5Furthermore, exosomes derived from embryonic stem cells were found to have the ability to reverse fibroblast senescence by regulating the TGF-B receptor 2 pathway (Figure A1). 22These properties have led to the recent introduction of various commercially available stem cell-derived extracellular vesicles in the skin rejuvenation space.

| CURRENT THER APEUTI C AND COMMERCIAL L ANDSC APE OF E X TR AVE S I CUL AR VE S I CLE S (E V )
There is yet to be FDA clearance for any injectable exosome therapy, and current use in dermatologic practice is confined to topical application.This is sometimes done in conjunction with micro-needling or laser therapy to enhance penetration.It is possible that injected exosomes will be regulated as drugs and biological products under section 351 of public Health Service (PHS) Act and the Federal Food Drug & Cosmetics (FD&C) Act, although to our knowledge, no publications on specific regulatory guidelines exist. 24The above guidelines would require that studies demonstrate safety, efficacy, purity, and potency of their specific product. 25It is therefore important for practitioners to implement caution when utilizing injected exosomes as adjunctive therapies.
What is also clear is that the efficacy of pre-clinical studies, and some clinical data, has highlighted the significant commercial potential of EV/exosomes therapy.Currently, over 30 international companies have EV products in the research and development phase. 24ree main categories are included: EVs of cellular origin (typically stem cells), engineered EVs (cell source genetically modified to overexpress the therapeutic component of interest), and fusion systems, in which EVs are combined with synthetic lipid nanoparticles. 24veral EV products currently undergoing research and development for chronic wound healing and inflammatory skin conditions are summarized in the included Table .One example is ExoWound by Exogenus Therapeutics, a combination of EVs derived from human umbilical cord blood mononuclear cells with a slow-release hydrogel.
Topical application of this product to wound in diabetic mice models led to accelerated wound closure. 26,27Another allogenic EV product (Syngenus) being developed by Regenus also delivered via hydrogel is being studied in the management of acne. 24other notable exosome product developed by BENEV Inc., in partnership with ExoCobio (Table A1) demonstrated improvement/ reversal of the aging process through mice studies in which small EVs (sEVs) from adipose MSCs (ADSC-sEVs) from young mice were injected into older mice. 28An improvement in motor coordination, grip strength, fatigue-resistance, fur regeneration, and renal function was demonstrated.Furthermore, a notable decrease in oxidate stress, inflammation, and senescence markers was noted. 28This exosome regenerative complex is now marketed for post-procedure topical application for the management of hair loss, skin aging, and dyspigmentation.
Combination of EVs with over-the-counter moisturizers and serums is a growing trend in the commercial space with the exosomes in these formulations primarily derived from adult human stem cells. 24Doses of extracellular vesicles in these products are difficult to ascertain.One product Zen, developed by Exoskin Simple, was found to have 500 000 EVs per mL. 24In contrast, the products marketed for clinical applications, primarily containing adult or neonatal stem cell-derived exosomes, were developed to deliver between 1 and 25 billion EVs (Table A1). 24Information on how companies define EVs in their products is often ambiguous. 29erall, these available products are formulated for topical administration or in combination procedures to enhance delivery such as micro-needling or laser treatments.

| LI M ITATI O N S
Exosomes are an emerging powerhouse in cutaneous medical aesthetics.Their multifunctionality has proven exceedingly promising in expanding therapeutic options for various pathologies.However, there are various limitations to consider and address for widespread, homogenous utilization in clinical practice to occur.
Currently, no standardized protocol for the isolation, storage, or identification of exosomes exists.The sourcing and isolation, purification, and identifications of exosomes are widely heterogenous and their specific mechanisms of actions remains mostly unclear.For instance, several isolation techniques described include ultracentrifugation, size-based filtration, size-exclusion chromatography, polymer precipitation, and novel combination techniques. 3,7other limitation is that pre-clinical studies of mice and animal while showing promising effects may not necessarily translate to clinical application due to a difference in physiological and pathological processes.Human clinical trials are just beginning to determine true safety and efficacy.

| CON CLUS ION
The therapeutic potential of exosomes in dermatology is wide and varied.From inflammatory skin conditions to cosmetic concerns, exosome use opens up a whole new avenue of management options for clinicians. 30

CO N FLI C T O F I NTE R E S T S TATE M E NT
No conflicts of interest.

F I G U R E A 1
Summarized mechanism of exosome regulation in wound healing, scar remodeling, and skin rejuvenation.mRNAs from saliva-derived exosomes were demonstrated to mediate proliferation, migration, and angiogenesis in in vitro studies of human umbilical endothelial cells (HUVECs) leading to accelerated wound healing.Bone marrow MSCs (BMSCs)-derived exosomes promoted keratinocyte and fibroblasts proliferation in mice studies as well as decreased apoptosis and inflammation of fibroblasts demonstrating potential for accelerating wound healing.Human amnion MSCs increased migration of epidermal cells.ASC-Exos increased tube formation of HUVECs and capillary density in skin flap neovascularization studies.Umbilical cord-derived exosomes inhibited differentiation fibroblasts to myofibroblasts leading to decreased scar formation.Skin rejuvenation characterized by increased collagen and elastin synthesis, improved keratinocytes and fibroblast function, antisenescence of HDFs, and changes in genotype and phenotype of UVB-irradiated HDFs were modulated by 3D-cultured HDFs, keratinocyte-derived exosomes, embryonic stem cells, and human-induced pluripotent stem cells (iPSCs).