Hair Follicle Epidermal Stem Cells Define a Niche for Tactile Sensation

Abstract

31The heterogeneity and compartmentalization of stem cells is a common principle in many 32 epithelia, and is known to function in epithelial maintenance, but its other physiological 33 roles remain elusive. Here we show transcriptional and anatomical contributions of 34 compartmentalized epidermal stem cells (EpSCs) in tactile sensory unit formation in the 35 hair follicle (HF). EpSCs in the follicle upper-bulge, where mechanosensory lanceolate 36 complexes (LCs) innervate, express a unique set of extracellular matrix (ECM) and 37 neurogenesis-related genes. These EpSCs deposit an ECM protein EGFL6 into the collar 38 matrix, a novel ECM that tightly ensheathes LCs. EGFL6 is required for the proper 39 patterning, touch responses, and av integrin-enrichment of LCs. By maintaining a quiescent 40 original EpSC niche, the old bulge, EpSCs provide anatomically stable HF-LC interfaces, 41 irrespective of the stage of follicle regeneration cycle. Thus, compartmentalized EpSCs 42 provide a niche linking the HF and the nervous system throughout the hair cycle. 43 44 Results 95 Upper-bulge EpSCs are molecularly specialized for HF-nerve interactions 96We first examined the global transcriptional features of distinct EpSC populations in the 97 HF. To this end, we established FACS-based cell purification methods using several eGFP 98 reporter mouse lines that label different SC compartments to isolate cellular subpopulations 99 resident in the lower-isthmus (Lgr6 + ), upper-bulge (Gli1 + ), mid-bulge (CD34 + ), and hair 100 germ (Cdh3 + ) as well as unfractionated basal EpSCs (a6 integrin + ) ( Figure 1A and B, Figure 101

Spatial association of SEMA3C with nerve endings/terminal Schwann cells in hair follicle isthmus region

“…It was more evident when SEMA3C was costained with the epithelial cell marker Keratin 5 (Figure 2a). Proteins such as EGFL6 can be secreted by epithelial cells but deposited in collar matrix which tightly ensheathed the lanceolate complex, obviously ouside of the epithelial compartment (Cheng et al., 2018). Several literatures have documented that SEMA3C mRNA is expressed in epithelial cells (Gunnarsson et al., 2016; Nguyen et al., 2006; Zhang et al., 2008).…”

“…This specialized structure enables hair to sense environmental stimuli and is able to impact on expression of many molecules in neighboring epithelial cells, including GLI1(Brownell et al., 2011), LGR6 (Liao & Nguyen, 2014), and EGFL6 (Wei et al., 2020). In adult mice, EGFL6 is specifically expressed at the central isthmus area by epithelial cells, tightly sandwiched with longitude‐aligned nerve fibers and TSCs processes (Cheng et al., 2018; Wei et al., 2020). EGFL6 knockout leads to misalignment of nerve endings and TSCs processes and defect of touch responses (Cheng et al., 2018).…”

“…In adult mice, EGFL6 is specifically expressed at the central isthmus area by epithelial cells, tightly sandwiched with longitude‐aligned nerve fibers and TSCs processes (Cheng et al., 2018; Wei et al., 2020). EGFL6 knockout leads to misalignment of nerve endings and TSCs processes and defect of touch responses (Cheng et al., 2018). However, EGFL6‐specific enrichment at the central isthmus area occurs after innervation during hair follicle development, therefore, it is dispensable for docking of nerves and TSCs at the central isthmus area (Cheng et al., 2018; Wei et al., 2020).…”

“…EGFL6 knockout leads to misalignment of nerve endings and TSCs processes and defect of touch responses (Cheng et al., 2018). However, EGFL6‐specific enrichment at the central isthmus area occurs after innervation during hair follicle development, therefore, it is dispensable for docking of nerves and TSCs at the central isthmus area (Cheng et al., 2018; Wei et al., 2020). Thus, it remains unknown how nerve endings and TSCs precisely dock at the isthmus area during development.…”

Stem cell‐intrinsic mechanisms regulating adult hair follicle homeostasis

“…Furthermore, HFSCs are responsible for deposition of neurotrophins such as BDNF and of ECM proteins such as EGFL6, thus ensuring proper nerve patterning around the HF, which is critical for mechanosensory response and piloerection 167–169 . Similarly, HFSCs deposit nephronectin on one side of the bulge, thus establishing a niche for the arrector pili muscle (APM), a muscle string that connects the epidermis with the hair follicle causing piloerection 169,170 .…”