Browsing by Author "Zhang, Jennifer Y"

Skin is the largest organ and primary barrier to the external environment. Functionally distinct tissues maintain the skin barrier under homeostatic conditions to protect underlying organs from water loss, UV radiation, noxious chemicals, mechanical pressure, and infectious pathogens. Protection from these challenges is dependent on dialogue between tissue-resident stem cells and heterologous cell populations in a microenvironment or “niche” to coordinate tissue regeneration throughout life. Hair follicles (HFs) are a defining feature of mammals and represent one of few mammalian tissues with complete regenerative capabilities. HF niche environments are specialized centers that are populated by heterologous cell types spanning many lineages. HFs contain harbor a reservoir of hair follicle stem cells (HFSCs) that are central to regenerative activity of the niche. Unlike stem cells in bone marrow, intestinal epithelium, and interfollicular epidermis, HFSC expressing Lgr5 are largely quiescent; HFSC activity is temporally restricted to episodic bouts of proliferation and differentiation as a primary means to preserve stem cell integrity. Lgr5+ HFSC are multipotent stem cells and can regenerate all cellular lineages of the epidermis, hair follicle, and sebaceous gland; this potency is powerful during homeostatic HF regeneration and during wound repair. Tissue regeneration after injury prompts the local production and integration of signals by local cells including epithelial cells and HFSCs to communicate with each other and to coordinate early stages of tissue regeneration. Damage to the epidermal barrier stimulates local follicles to produce a new hair shaft, a process termed wound-induced hair growth (WIHG). Despite advances in our understanding of HF niche dynamics, it is unclear how specific wound-derived factors modulate stem cell and immune activity to couple wound healing with hair growth. We discovered the cytokine, thymic stromal lymphopoietin (TSLP) is produced in response to skin injury and during the anagen stage of the hair follicle cycle. We generated animal models of skin tissue regeneration and observed that exogenous TSLP is a potent inducer of hair growth during normal hair growth cycles and after skin injury. Interestingly, we found that TSLP treatment may have lasting effects to the tissue to favor hair follicle regeneration. TSLP-treated wound beds only showed accelerated WIHG onset and shortened quiescence phase prior to the next hair cycle. Using flow cytometry and fluorescent immunostaining, we examined TSLP functions in the skin to find that TSLP promotes the activation and proliferation of HFSC. HFSC activation results in generation of transit amplifying cells required for tissue regeneration, functionally linking TSLP to tissue regeneration during hair growth and during wound healing. We used in vitro and ex vivo models to determine how our findings translated to human tissue. We designed reagents to discriminate between unique expressed variants in human and confirmed that only the full length TSLP variant (long form, lfTSLP) that shares functional homology with murine TSLP was upregulated after injury in human skin. Consistent with findings from other groups, we report that lfTSLP promoted proliferation and reduced expression of differentiation genes in primary human epidermal keratinocytes. To determine the functional requirement for TSLPR in HFSC, we generated a mouse model enabling spatial and temporal control of Tslpr gene expression in LGR5+ HFSC. Tslpr ablation in adult skin inhibited both wound-induced and exogenous TSLP-induced hair growth. Additionally, Tslpr ablation during postnatal development resulted in severely delayed generation of the fur coat. Interestingly, Tslpr ablation did not reduce the LGR5+ HFSC population, indicating that TSLPR is not required for stem cell survival in the skin. Exogenous TSLP treatment increased expression of the epidermal progenitor factor DDX6 and the cell cycle regulator cyclin D1 in a TSLPR-dependent manner, highlighting a novel function for TSLP in regulation of hair follicle activity during homeostasis and during wound healing. Together, these findings delineate TSLP as a novel and locally produced cytokine that directly stimulates hair follicle cell proliferation in the skin during development and during tissue regeneration after injury. Immune cells have potent roles to regulate HFSC activity during hair follicle regeneration during hair growth and after skin injury. In epithelial tissues, TSLP has potent immunomodulatory functions and can unleash pathogenic Type-2 immune responses. Using flow cytometry, we observed mouse skin treated with TSLP appeared largely normal, with no significant changes in immune cell distribution. However, mice lacking Tslpr on LGR5+ HFSC exhibited a two-fold increase in cutaneous immune cells, with minimal changes in the number of dendritic epidermal T cells, the most abundant immune cell population in mouse skin. Instead, we observed a significant increase in CD8+ cell abundance, irregular CD8+ cell distribution, and irregular hair follicle morphology. These results suggest a tissue-specific and pleiotropic function for TSLP to facilitate niche communications during tissue regeneration.