Topical SCD-153, a 4-methyl itaconate prodrug, for the treatment of alopecia areata.

Alopecia areata is a chronic hair loss disorder that involves autoimmune disruption of hair follicles by CD8+  T cells. Most patients present with patchy hair loss on the scalp that improves spontaneously or with topical and intralesional steroids, topical minoxidil, or topical immunotherapy. However, recurrence of hair loss is common, and patients with extensive disease may require treatment with oral corticosteroids or oral Janus kinase (JAK) inhibitors, both of which may cause systemic toxicities with long-term use. Itaconate is an endogenous molecule synthesized in macrophages that exerts anti-inflammatory effects. To investigate the use of itaconate derivatives for treating alopecia areata, we designed a prodrug of 4-methyl itaconate (4-MI), termed SCD-153, with increased lipophilicity compared to 4-MI (CLogP 1.159 vs. 0.1442) to enhance skin and cell penetration. Topical SCD-153 formed 4-MI upon penetrating the stratum corneum in C57BL/6 mice and showed low systemic absorption. When added to human epidermal keratinocytes stimulated with polyinosinic-polycytidylic acid (poly I:C) or interferon (IFN)γ, SCD-153 significantly attenuated poly I:C-induced interleukin (IL)-6, Toll-like receptor 3, IL-1ß, and IFNß expression, as well as IFNγ-induced IL-6 expression. Topical application of SCD-153 to C57BL/6 mice in the resting (telogen) phase of the hair cycle induced significant hair growth that was statistically superior to vehicle (dimethyl sulfoxide), the less cell-permeable itaconate analogues 4-MI and dimethyl itaconate, and the JAK inhibitor tofacitinib. Our results suggest that SCD-153 is a promising topical candidate for treating alopecia areata.

Bacteria induce skin regeneration via IL-1ß signaling.

Environmental factors that enhance regeneration are largely unknown. The immune system and microbiome are attributed roles in repairing and regenerating structure but their precise interplay is unclear. Here, we assessed the function of skin bacteria in wound healing and wound-induced hair follicle neogenesis (WIHN), a rare adult organogenesis model. WIHN levels and stem cell markers correlate with bacterial counts, being lowest in germ-free (GF), intermediate in conventional specific pathogen-free (SPF), and highest in wild-type mice, even those infected with pathogenic Staphylococcus aureus. Reducing skin microbiota via cage changes or topical antibiotics decreased WIHN. Inflammatory cytokine IL-1ß and keratinocyte-dependent IL-1R-MyD88 signaling are necessary and sufficient for bacteria to promote regeneration. Finally, in a small trial, a topical broad-spectrum antibiotic also slowed skin wound healing in adult volunteers. These results demonstrate a role for IL-1ß to control morphogenesis and support the need to reconsider routine applications of topical prophylactic antibiotics.

Noncoding dsRNA induces retinoic acid synthesis to stimulate hair follicle regeneration via TLR3.

How developmental programs reactivate in regeneration is a fundamental question in biology. We addressed this question through the study of Wound Induced Hair follicle Neogenesis (WIHN), an adult organogenesis model where stem cells regenerate de novo hair follicles following deep wounding. The exact mechanism is uncertain. Here we show that self-noncoding dsRNA activates the anti-viral receptor toll like receptor 3 (TLR3) to induce intrinsic retinoic acid (RA) synthesis in a pattern that predicts new hair follicle formation after wounding in mice. Additionally, in humans, rejuvenation lasers induce gene expression signatures for dsRNA and RA, with measurable increases in intrinsic RA synthesis. These results demonstrate a potent stimulus for RA synthesis by non-coding dsRNA, relevant to their broad functions in development and immunity.

dsRNA Sensing Induces Loss of Cell Identity.

How cell and tissue identity persist despite constant cell turnover is an important biologic question with cell therapy implications. Although many mechanisms exist, we investigated the controls for site-specific gene expression in skin, given its diverse structures and functions. For example, the transcriptome of in vivo palmoplantar (i.e., volar) epidermis is globally unique, including Keratin 9 (KRT9). Although volar fibroblasts have the capacity to induce KRT9 in nonvolar keratinocytes, we show here that volar keratinocytes continue to express KRT9 in in vitro solo cultures. Despite this, KRT9 expression is lost with volar keratinocyte passaging, despite stable hypomethylation of its promoter. Coincident with KRT9 loss is a gain of the primitive keratin 7 and a signature of dsRNA sensing, including the double-stranded RNA (dsRNA) receptor DExD/H-Box Helicase 58 (DDX58/RIG-I). Exogenous dsRNA inhibits KRT9 expression in early passage volar keratinocytes or in vivo footpads of wild-type mice. Loss of DDX58 in passaged volar keratinocytes rescues KRT9 and inhibits KRT7 expression. Additionally, DDX58-null mice are resistant to the ability of dsRNA to inhibit KRT9 expression. These results show that the sensing of dsRNA is critical for loss of cell-specific gene expression; our results have important implications for how dsRNA sensing is important outside of immune pathways.

Adipose and Hair Function: An aPPARent Connection.

Adipose tissue plays essential roles in various aspects of skin physiology, from regulating hair follicle morphogenesis to wound healing. Peroxisome proliferator-activated receptor gamma is important for the maintenance of adipose tissue and has been implicated in some types of hair loss; however, its function during the hair cycle is still unclear. Sardella et al. investigate the role of peroxisome proliferator-activated receptor gamma in hair follicle morphogenesis using a novel global peroxisome proliferator-activated receptor gamma-null mouse.

Cutting edge: STAT6 signaling in eosinophils is necessary for development of allergic airway inflammation.

Eosinophils are critical cellular mediators in allergic asthma and inflammation; however, the signals that regulate their functions are unclear. The transcription factor STAT6 regulates Th2 cytokine responses, acting downstream of IL-4 and IL-13. We showed previously that eosinophil-derived IL-13 plays an important role in the recruitment of T cells to the lung and the subsequent development of allergic asthma. However, whether eosinophils respond to Th2 signals to control allergic airway inflammation is unclear. In this report, we show that STAT6(-/-) eosinophils are unable to induce the development of allergic lung inflammation, including recruitment of CD4(+) T cells, mucus production, and development of airways hyperresponsiveness. This is likely due to the reduced migration of STAT6(-/-) eosinophils to the lung and in response to eotaxin. These data indicate that, like Th cells, eosinophils need to respond to Th2 cytokines via STAT6 during the development of allergic airway inflammation.