- The integumentary system is the largest organ system of the body that comprises of skin and its appendages such as hair follicles, sebaceous and sweat glands. Skin is divided into three distinct structural layers: the epidermis, dermis and hypodermis. The epidermis originates from the ectoderm and is composed of four layers specified as basal, spinous, granular and corneal layer. The function of epidermis is to protect against all sources of environmental insults, prevent water loss and undergo re-epithelialization after wounding. For its normal functioning, the epidermis continually replenishes itself through a process of continuous proliferation and terminal differentiation of keratinocytes from the basal layer.
The hair follicle is a complex appendage of skin, which give rise to the keratinized hair shaft. Hair follicle is formed during embryonic development and it goes through cycles of growth (anagen), regression (catagen) and quiescence (telogen). The bulge region of the hair follicle in the outer root sheath area contains slow-cycling stem cells which are responsible for normal hair cycling as well as cutaneous wound repair after injury.
CTIP2 (COUP-TF interacting protein 2) is a C2H2 zinc finger transcription factor that is expressed in various organs and tissues. It has been shown to play an important role in the development of thymocyte, tooth and corticospinal motor neuron. Expression of CTIP2 is observed in the developing murine epidermis and dermis during skin organogenesis and predominantly in epidermal keratinocytes in adult mice skin. It is also expressed in the embryonic and mature adult hair follicles, especially in bulge region. CTIP2 regulates epidermal proliferation and terminal differentiation during embryogenesis and adulthood. Here we show that CTIP2 controls hair follicle development, hair cycling and cutaneous wound healing. To study the role of CTIP2 in hair morphogenesis and hair cycling, we have utilized two different genetically modified mouse strains. First, we studied the effect of CTIP2 during hair follicle formation using Ctip2-null mice containing a germline deletion of Ctip2. Ctip2-null mice exhibited reduced hair follicle density and downregulation of EGFR and NOTCH1 expression. To analyze the consequence of loss-of–funtion of CTIP2 on postnatal hair cycling, we selectively ablated Ctip2 in the epidermis and hair follicles using the Cre-LoxP strategy to generate Ctip2[superscript ep-/-] mice. Ctip2[superscript ep-/-] mice showed a defect in postnatal hair cycling marked by early exit from telogen and premature entry into anagen. The premature induction of anagen is a result of stem cell activation, increase in cell proliferation and decrease in apoptosis-driven cell death in the hair follicles. This early activation of follicular stem cells eventually leads to their depletion and therefore ultimate loss of hair follicles and hair coat. Reduced expression of LHX2 and NFATC1, which are two important regulators of hair cycling, was observed in the bulge area of Ctip2[superscript ep-/-] mice hair follicles. Furthermore, chromatin immunoprecipitation (ChIP) assay revealed that CTIP2 is recruited to the promoter region of both Lhx2 and Nfatc1 and possibly directly regulates their expression. Altogether, our results suggest that CTIP2 modulates expression of factors that regulate hair follicle morphogenesis and normal hair cycling as well as control stem cell fate and survival in the HFs.
Considering CTIP2’s role in skin formation and homeostasis we hypothesized that CTIP2 is involved in the epidermal regeneration process during wound closure after injury. To test our hypothesis, we used Ctip2[superscript ep-/-] mouse model where Ctip2 is conditionally inactivated in epidermis and hair follicle. Selective ablation of Ctip2 in the epidermis and HFs lead to the delay in wound healing. The slower healing property was an outcome of impaired keratinocyte activation, proliferation, differentiation and migration. Moreover, defects in cell-cell adhesion and ECM development were also observed in the healing wounds of Ctip2ep-/- mice due to the lack of E-cadherin and insufficient expression of alpha smooth muscle actin. Hair follicle stem cell factors such as K15, CD34, CD133, NFATC1 and LRIG1 that are crucial for wound re-epithelialization were aberrantly expressed in wound adjacent region of Ctip2[superscript ep-/-] mice. Our results identify CTIP2 as a key regulator of epidermal stem cells during skin wound healing and an important player in efficient wound closure after injury.