Target Discovery and Validation Reviews and Protocols

220 Vijayaraj, Söhl, and Magin
absence of hair and nail defects and also for the, at least partial survival, of
K6a(–/–)/K6b(–/–) double-deficient mice (96). However, other modifier genes
deviating in both genetic backgrounds could influence either directly or indirectly
the abundance and expression pattern of yet unknown genes or the novel
K6hf gene itself.
The contribution to wound healing of K6a and K6b also was investigated
by keratin explant assays, indicating that the lack of these keratins renders
keratinocytes more motile, including changes in the actin filament organization
that might support either an increase in cell fragility or a more rapid
wound healing (98). In contrast, mice overexpressing K6a proteins with different
C-terminal truncations were largely found with severe blistering and
died shortly after birth. Those mice that survived showed lesions in the interfollicular
epidermis and hair follicles and developed severe alopecia followed
by the destruction of the outer root sheath (ORS) cells. The latter finding indicates
that the innermost ORS cells are uniquely sensitive to expression of
even slightly altered K6a proteins, suggesting that mutations affecting the
human K6a ortholog expressed in this cell layer could result in alopecia in
humans as well (99).
Although the precise mechanism is far from being understood, the relative
amounts between K6a, K6b, K16, and K17 seem to be important to establish
a less stable but functional keratin cytoskeleton. It is also likely that these keratins,
but especially K6a and K6b, provide attachment sites for associated
proteins (40,96). The transgenic expression of human K16 in the basal epidermis
of K14(–/–) mice led to a rearrangement of the keratin Ifs and normalized
the epidermis in neonatal animals, but these animals later developed extensive
alopecia and chronic epidermal ulcers in areas of frequent physical contact,
and alterations in other stratified epithelia (100). As control, mice expressing
a K14 cDNA under regulatory elements of the K16 gene also rescue the blistering
phenotype of the K14 null mice with only a small percentage exhibiting
minor alopecia. Despite their high sequence similarity, K16 and K14 are
not functionally equivalent in the epidermis and other stratified epithelia, and
it is primarily a carboxy-terminal segment of approx 105 amino acids of K16
that defines these differences (100). Moreover, comparative data from skin
explant assay and wound healing studies revealed a delay in wound healing
as a result from hair shaft fragility and apoptosis in hair matrix cells. This
pathology reverted to normalcy as a consequence of the K14 transgene
expression, implying that the composition of the keratin cytoskeleton can
influence cell migration (101).
K17 predominantly occurs in epidermal appendages (36,41), but it also
occurs in basal cells of stratified epithelia (4). It forms IFs with K5 and any of
the different K6 proteins currently identified (93). In humans, K17 mutations