Keratin proteins perform several functions in skin cells, including those of providing mechanical support and protection against injury. But it seems they also have a more active part to play in healing wounds.
Like the musculoskeletal framework of humans or the steel-girder scaffolds of buildings, the intermediate filaments of the cytoskeleton shield cells from mechanical forms of injury1,2,3. Intermediate filaments are made up of a large family of tissue-specific proteins, including desmin in muscle, neurofilaments in neurons and keratins in the epithelial cells that line organs such as the skin1,2,3. Although intermediate filaments are well known for their protective properties, it seems that they may also have a role in damage repair. On page 362 of this issue, Kim et al.4 provide a direct mechanistic link between an increase in the expression of the gene for keratin K17 and the characteristic increase in protein synthesis and cell growth seen in the cells around wounds.
Wound healing involves the orchestration of numerous events in the recovering cells and those adjacent to them5,6, such as cell growth, cell division and migration, and the upregulation of many genes — including those encoding several intermediate filament proteins (Fig. 1). In response to skin injury, for example, the cells surrounding the wound ramp up protein synthesis and enlarge to help seal the wound. In these cells, there is a rapid increase in the expression of several keratins (K6, K16 and K17)6.
Kim et al. began their study by pursuing their previous observation that injuries in mouse embryos lacking K17 show a striking delay in wound closure4,6. They noted that the wound-edge cells from such embryos did not swell up as usual, and were about 40% smaller than those of normal embryos. Moreover, these cells did not show the full protein-synthetic activity that typically accompanies cell growth. This initial link with cell growth led Kim et al. to explore the role of a regu-latory enzyme called mammalian target of rapamycin (mTOR). This enzyme can regulate cell growth and proliferation in other systems by controlling protein synthesis7. Kim et al. found that the activation of mTOR seen in normal cells is reduced in cells that lack K17.
So how does K17 affect mTOR signalling? The authors found that K17 binds to an adaptor protein called 14-3-3σ, which belongs to the 14-3-3 protein family4. The 14-3-3 proteins bind to more than 100 other proteins, prim-arily at particular phosphoserine/phosphothreonine residues — that is, serine (Ser) and threonine (Thr) residues that have a phosphate group attached. One of their many functions is to regulate where their binding partners can reside in the cell8. Mutation of two sites on the K17 protein that looked likely to act as 14-3-3 binding motifs (Thr 9 and Ser 44) not only blocked the binding of K17 to 14-3-3, but also prevented the normal activation of mTOR and translocation of 14-3-3 from the nucleus to the cytoplasm in cultured cells. The investigators then went full circle by showing that reintroducing K17 into keratinocytes lacking K17 restored the movement of 14-3-3 from the nucleus to the cytoplasm, leading to stimulation of mTOR signalling and increased protein synthesis and cell size. By contrast, introducing K17 that was mutated at its 14-3-3-binding sites had no signifi-cant effect.
These findings directly implicate the keratin cytoskeleton in the regulation of protein synthesis and cell size, highlighting an overlooked non-mechanical function for skin keratins that may extend to intermediate filament proteins in other tissues. Two other intermediate filament proteins, K18 (a major keratin in secretory epithelia)9 and vimentin (found in fibroblasts and endothelial cells, among others)10, are known to bind to 14-3-3. Notably, mutation of K18 at the Ser 33 site that regulates 14-3-3 binding stops the movement of 14-3-3 from the nucleus during liver regeneration9. Given that increased expression of intermediate filaments is a protective response to injury in many tissues3,6, such upregulation of these filaments, coupled to interaction with 14-3-3, may have similar ‘healing’ roles in other tissues. However, intermediate filament induction will probably have many roles that may or may not require 14-3-3, as well as being mechanical and non-mechanical in nature.
The 14-3-3 proteins are already implicated in wound healing, as they can facilitate migration of keratinocyte skin cells to close the wound by regulating the adhesion molecules that hold the cells together11. Presumably, this is a late event in wound healing, as K17 binding to 14-3-3 probably occurs earlier and regulates other 14-3-3 binding partners (for example, modulators of the mTOR pathway4) to allow increased protein synthesis and progression of wound repair (Fig. 1). The regulation, stoichiometry and precise timing of K17 binding to 14-3-3 after tissue injury remain to be determined. For instance, although Kim et al.4 examined Thr 9 and Ser 44 in K17, it is not clear whether both of these need to be phosphorylated for 14-3-3 to bind. Developing genetic models9, such as those in mice where K17–14-3-3 binding is blocked, and reagents (for example, antibodies specific for the phosphate tag) to track the precise timing of K17–14-3-3 binding, should further clarify this regulatory pathway.
The induction of K17 now becomes one of several established ‘command posts’ for the regulation of protein synthesis4,7. It may well be that K17 induction is part of a regulatory loop that can, depending on context, further enhance protein synthesis and promote cell growth.
Additional questions are raised by this work. For instance, what is the significance of nuclear 14-3-3 and its redistribution during wound healing? 14-3-3 is linked to the regulation of cell division, and can affect gene expression by binding to the histone proteins that are associated with chromosomes8,12. So are these processes involved in the injury response? Also, can some of the skin and nail disorders caused by mutations in the K17 gene3 be explained by signalling effects mediated by 14-3-3 proteins? K17 is induced in several conditions, including psoriasis and some cancers6, so what is its role in these disorders? Further areas for investigation include the possible interaction of K17 with 14-3-3 proteins other than the σ-isoform and involvement of K17–14-3-3 binding in other known functions of 14-3-3 (ref. 8). By linking the induction of keratin in response to skin damage with signalling through 14-3-3 proteins and the consequent effects on protein synthesis and cell growth, Kim et al. have made an exciting advance towards solving such problems.