Inducible deletion of epidermal Dicer and Drosha reveals multiple functions for miRNAs in postnatal skin

MicroRNAs (miRNAs) regulate the expression of many mammalian genes and play key roles in embryonic hair follicle development; however, little is known of their functions in postnatal hair growth. We compared the effects of deleting the essential miRNA biogenesis enzymes Drosha and Dicer in mouse skin epithelial cells at successive postnatal time points. Deletion of either Drosha or Dicer during an established growth phase (anagen) caused failure of hair follicles to enter a normal catagen regression phase, eventual follicular degradation and stem cell loss. Deletion of Drosha or Dicer in resting phase follicles did not affect follicular structure or epithelial stem cell maintenance, and stimulation of anagen by hair plucking caused follicular proliferation and formation of a primitive transient amplifying matrix population. However, mutant matrix cells exhibited apoptosis and DNA damage and hair follicles rapidly degraded. Hair follicle defects at early time points post-deletion occurred in the absence of inflammation, but a dermal inflammatory response and hyperproliferation of interfollicular epidermis accompanied subsequent hair follicle degradation. These data reveal multiple functions for Drosha and Dicer in suppressing DNA damage in rapidly proliferating follicular matrix cells, facilitating catagen and maintaining follicular structures and their associated stem cells. Although Drosha and Dicer each possess independent non-miRNA-related functions, the similarity in phenotypes of the inducible epidermal Drosha and Dicer mutants indicates that these defects result primarily from failure of miRNA processing. Consistent with this, Dicer deletion resulted in the upregulation of multiple direct targets of the highly expressed epithelial miRNA miR-205. © 2012. Published by The Company of Biologists Ltd

Dicer is required for maintenance of hair follicle stem cells in adult skin

Multiple miRNAs are expressed in developing and postnatal skin and hair follicles, and constitutive deletion of the miRNA processing enzyme Dicer in embryonic mouse skin causes failure of hair follicle morphogenesis and subsequent follicular degradation. To determine whether Dicer and miRNA functions are required in established hair follicles and epidermis in the adult mouse we generated Krt5-rtTA tetO-Cre Dicerfl/fl mice in which the Dicer gene can be inducibly deleted in the basal epidermis and hair follicle outer root sheath, including hair follicle stem cells, by dosage with oral doxycycline. Inducible Dicer deletion starting at postnatal day (P) 20, when hair follicles are just entering the first postnatal growth cycle, resulted in loss of external hair starting within 10days of doxycycline treatment. Induction of mutation during the resting stage of the hair follicle growth cycle did not immediately result in histological abnormalities or loss of hair. However, expression of the hair follicle bulge stem cell markers K15 and CD34 was lost soon after induction. Subsequent hair plucking stimulated a new cycle of hair follicle growth in control littermate, but not induced Dicer mutant skin. These results demonstrate a requirement for Dicer in maintaining the ability of adult hair follicles to grow and regenerate, and indicate that Dicer is required for maintenance of hair follicle epithelial stem cells