**9. aPKC and aging**

**8. The roles of other components of the aPKC-Par complex in hair loss**

In the epidermis, aPKCζ, another isoform of aPKC, localizes in the cytoplasm and nucleus of basal cells, although its expression level is much lower than that of aPKCλ in the skin of newborns and adults (40-fold and 10-fold, respectively) [21]. Although the activity of aPKCζ in epithelial polarity in vitro is distinguishable from that of aPKCλ, mice lacking aPKCζ at the whole-body level were viable and showed no obvious skin phenotypes [60]. This may be attributed to the low expression of aPKCζ in the epidermis, and aPKCλ may compensate for the loss of aPKCζ. However, these results do not exclude the possibility that aPKCλ and aPKCζ synergistically regulate epithelial cell polarity, oriented cell division, epidermal differentiation, and HFSC maintenance. Indeed, the combined deletion of the aPKCλ/ι and aPKCζ isoforms in podocytes leads to defective glomerular maturation with incomplete capillary formation and mesangiolysis, and causes severe proteinuria and perinatal death [61]. Thus, studies on mutant mice with simultaneous epidermal inactivation of aPKCλ and

aPKCζ would help provide further information on the synergism between the two.

Because aPKCs are localized to tight junctions, in aPKCλ cKO mice, the aPKCλ-Par6-Par3 complex at tight junctions was supposed to be absent or impaired in the granular layer. However, in the mutant mice, the overall multilayered architecture of the epidermis appeared to be normal, or rather hyperplasic [22, 23], suggesting that in contrast to simple epithelia, junctional aPKCλ is dispensable for establishing the polarity of the stratified epidermis, and that aPKCλ localized to the apical surface of basal cells during mitosis is more critical for maintaining epidermal homeostasis. Par6 and aPKCs form a stable heterodimer through their respective Phox/Bem1 (PB1) domains [7, 8], and aPKC-mediated phosphorylation is required for the dissociation of Par3 from the ternary complex. Thus, analysis of the dynamics of Par6 and Par3 or the Par6-Par3 complex in the absence of aPKCλ is helpful to understand the role

The difference between junctional and non-junctional aPKC has been demonstrated by twostep chemical skin carcinogenesis experiments using epidermis-specific Par3 knockouts [62]. The epidermal loss of Par3 reduced papilloma formation and promoted keratoacanthoma formation, indicating that Par3 acts as a tumor promoter for papilloma and as a tumor suppressor for keratoacanthoma. In the absence of Par3, the aPKC-Par6 complex localized to the cytoplasm [62]. These results imply that the junctional aPKC-Par6 complex with Par3 is involved in papilloma formation, whereas the non-junctional, cytoplasmic aPKC-Par6

The role of other components of the aPKC-Par complex (Par3 and Par6) in HFSC maintenance is unknown. Par3 is expressed throughout the interfollicular epidermis and the hair follicles,

complex without Par3 is involved in keratoacanthoma formation.

**8.2. Junctional aPKC vs non-junctional aPKC**

of junctional aPKCλ.

**8.3. Par proteins**

**8.1. aPKC**

38 Hair and Scalp Disorders

Is alopecia observed in aPKCλ cKO mice relevant to human diseases? As few inflammatory cells were present around the hair follicles of mutant mice, the mutant mice model is unlikely to be a disease model for alopecia areata, which involves perifollicular T cell infiltration and autoimmune responses to hair antigens. Progressive hair loss in aPKCλ cKO mice was similar to alopecia observed in collagen XVII (COL17A1/BP180/BPAG2, a structural component of the hemidesmosome) knockout mice and aged mice. In humans, COL17A1 deficiency causes a subtype of congenital junctional epidermolysis bullosa [63]. The patients also show premature hair loss (alopecia) with hair follicle atrophy [64, 65], suggesting that COL17A1 plays a role in hair follicle homeostasis. Consistent with this finding, *Col17a1*-deficient mice also show premature hair loss [66]. Mouse Col17a1 is preferentially localized along the dermal-epidermal junction of bulge keratinocytes, and loss of Col17a1 prevents the expression of HFSC markers, such as K15, CD34, and α6-integrin [67]. Similar to aPKCλ cKO mice, *Col17a1*-deficient mice showed progressive hair loss, hair follicles in sustained anagen, HFSC depletion, and deficient stemness of the HFSC population [67], indicating that Col17a1 is essential for HFSC maintenance. In addition, *Col17a1*-deficient HFSCs coexpress K15 and K1 in the bulge, and show increased K1 expression in the upper junctional zone and IFE, implying that the fate of HFSCs changes to epidermal differentiation.

A recent study clarified that accumulation of DNA damage in HFSCs leads to proteolysis of COL17A1 that triggers HFSC aging [68]. Importantly, aged HFSCs lose their stem cell signature and commit to epidermal differentiation, and they are finally eliminated from the epidermis [68]. The progressive depletion of HFSCs and the cell fate change observed in aPKC cKO mice are similar to aged mice. Thus, it would be interesting to examine whether the expression of Col17a1 is decreased in mutant mice and whether aPKCλ is involved in the induction and maintenance of Col17a1.
