AP-2α/AP-2ß Transcription Factors Are Key Regulators of Epidermal Homeostasis.

AP-2 transcription factors regulate ectodermal development, but their roles in epidermal homeostasis in adult skin are unknown. We find that AP-2α is the predominant AP-2 family member in adult epidermis, followed by AP-2ß. Through inactivation of AP-2α, AP-2ß, or both in keratinocytes, we assessed the effects of a gradient of epidermal AP-2 activity on skin function. We find that (i) loss of AP-2ß in keratinocytes is compensated for by AP-2α, (ii) loss of AP-2α impairs terminal keratinocyte differentiation and hair morphogenesis, and (iii) the combined loss of AP-2α/AP-2ß results in more severe skin and hair abnormalities. Keratinocyte differentiation defects precede progressive neutrophilic skin inflammation. Inducible inactivation of AP-2α/AP-2ß in the adult phenocopies these manifestations. Transcriptomic analyses of epidermis lacking AP-2α or AP-2α/AP-2ß in keratinocytes demonstrate a terminal keratinocyte differentiation defect with upregulation of alarmin keratins and of several immune pathway regulators. Moreover, our analyses suggest a key role of reduced AP-2α-dependent gene expression of CXCL14 and the keratin 15 gene K15 as an early pathogenic event toward the manifestation of skin inflammation. Thus, AP-2α and AP-2ß are critical regulators of epidermal homeostasis in adult skin.

KCTD1/KCTD15 complexes control ectodermal and neural crest cell functions, and their impairment causes aplasia cutis.

Aplasia cutis congenita (ACC) is a congenital epidermal defect of the midline scalp and has been proposed to be due to a primary keratinocyte abnormality. Why it forms mainly at this anatomic site has remained a long-standing enigma. KCTD1 mutations cause ACC, ectodermal abnormalities, and kidney fibrosis, whereas KCTD15 mutations cause ACC and cardiac outflow tract abnormalities. Here, we found that KCTD1 and KCTD15 can form multimeric complexes and can compensate for each other's loss and that disease mutations are dominant negative, resulting in lack of KCTD1/KCTD15 function. We demonstrated that KCTD15 is critical for cardiac outflow tract development, whereas KCTD1 regulates distal nephron function. Combined inactivation of KCTD1/KCTD15 in keratinocytes resulted in abnormal skin appendages but not in ACC. Instead, KCTD1/KCTD15 inactivation in neural crest cells resulted in ACC linked to midline skull defects, demonstrating that ACC is not caused by a primary defect in keratinocytes but is a secondary consequence of impaired cranial neural crest cells, giving rise to midline cranial suture cells that express keratinocyte-promoting growth factors. Our findings explain the clinical observations in patients with KCTD1 versus KCTD15 mutations, establish KCTD1/KCTD15 complexes as critical regulators of ectodermal and neural crest cell functions, and define ACC as a neurocristopathy.

AP-2α/AP-2ß transcription factors are key regulators of epidermal homeostasis.

AP-2 transcription factors regulate ectodermal development but their roles for epidermal homeostasis in the adult skin are unknown. We find that AP-2α is the predominant AP-2 family member in adult epidermis, followed by AP-2ß. Through inactivation of AP-2α, AP-2ß, or both in keratinocytes we assessed the effects of a gradient of epidermal AP-2 activity on skin function. We find that (1) loss of AP-2ß in keratinocytes is compensated for by AP-2α, (2) loss of AP-2α impairs terminal keratinocyte differentiation and hair morphogenesis, and (3) the combined loss of AP-2α/AP-2ß results in more severe skin and hair abnormalities. Keratinocyte differentiation defects precede a progressive neutrophilic skin inflammation. Inducible inactivation of AP-2α/AP-2ß in the adult phenocopies these manifestations. Transcriptomic analyses of epidermis lacking AP-2α or AP-2α/AP-2ß in keratinocytes demonstrate a terminal keratinocyte differentiation defect with upregulation of alarmin keratins and of several immune pathway regulators. Moreover, our analyses suggest a key role of loss of AP-2α-dependent gene expression of CXCL14 and KRT15 as an early pathogenic event towards the manifestation of skin inflammation. Thus, AP-2α/AP-2ß are critical regulators of epidermal homeostasis in the adult skin.