Review
‘Hard’ and ‘soft’ principles defining the structure, function and regulation of keratin intermediate filaments

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Abstract

Keratins make up the largest subgroup of intermediate filament proteins and represent the most abundant proteins in epithelial cells. They exist as highly dynamic networks of cytoplasmic 10–12 nm filaments that are obligate heteropolymers involving type I and type II keratins. The primary function of keratins is to protect epithelial cells from mechanical and nonmechanical stresses that result in cell death. Other emerging functions include roles in cell signaling, the stress response and apoptosis, as well as unique roles that are keratin specific and tissue specific. The role of keratins in a number of human skin, hair, ocular, oral and liver diseases is now established and meshes well with the evidence gathered from transgenic mouse models. The phenotypes associated with defects in keratin proteins are subject to significant modulation by functional redundancy within the family and modifier genes as well. Keratin filaments undergo complex regulation involving post-translational modifications and interactions with self and with various classes of associated proteins.

Introduction

Keratins are the major structural proteins in epithelial cells. They occur as a cytoplasmic network of 10–12 nm wide intermediate filaments (IFs) (Fig. 1). These proteins are encoded by a large multigene family whose >50 individual members can be partitioned into two major sequence types. The pairwise regulation of type I (K9–K23; Ha1–Ha8) and type II (K1–K8; Hb1–Hb6) keratin genes reflects the composition of keratin polymers, which are built from heterodimers. Keratin gene pairs exhibit epithelial tissue type specific and differentiation-specific regulation, the molecular and functional basis of which are largely unknown. A major role fulfilled by keratins is to act as a resilient yet adaptable scaffold that endows epithelial cells with the ability to sustain mechanical and nonmechanical stresses. Additional functions manifested in a sequence-dependent and context-dependent fashion are emerging as well, although there is increasing experimental evidence for functional redundancy. In this text, we review the significant and more recent inroads that have been made towards our understanding of the assembly, structure, properties, function and regulation of keratin polymers.

Section snippets

Just when you think you know them all...

Novel keratin genes keep popping up. Schweizer and colleagues 1., 2•. completed a thorough investigation of the subfamily of keratin genes expressed in hard epithelia (hair, nail and oral filiform papillae). They had previously described the existence of nine human ‘hard’ type I keratin genes organized as a cluster along with other type I genes on chromosome 17q12–21. More recently, they reported the existence of six functional ‘hard’ type II keratin genes, clustered in a similar fashion within

Keratin heteropolymerization is a truly hard principle

Most vertebrate cytoplasmic IF proteins can easily be ascribed to one of the four main sequence groups (I–IV) based on sequence homology within the signature central α-helical domain. This is often not the case with invertebrate cytoplasmic IF sequences, which are being cloned and characterized in increasing numbers, as reported for ‘C’ and ‘D’ IF proteins from the tunicate (urochordate) Styela [9••]. Whereas proteins C and D do not show obvious homology to vertebrate keratins, they are found

Insights into IF architecture — going atomic!

It is now known that the display of bona fide heptad repeats of hydrophobic residues in the context of an α helix is not sufficient to promote efficient coiled-coil dimerization. A 13-residue motif, designated ‘trigger site’, affords special stability to the α-helical fold and coiled-coil conformation (through intrachain and interchain ion pairings), and is able to nucleate the formation of long-range coiled-coil dimers 11., 12.. Wu et al. [13•] applied a directed mutagenesis screen to test the

The hard and soft sides of keratin mechanics

Given the widespread function of scaffolding, studying the mechanical properties of keratins is timely [17•], although it has been largely ignored until recently. Rheological methods designed to study the flow properties of materials when placed under stress have been applied to examine the micromechanics of pure keratinfilament suspensions in vitro. As expected, on the basis of the structural features of keratin filaments along with previous studies of cytoskeletal polymers including vimentin

Keratin gene knockouts reveal the prevalence of functional redundancy

Keratin 8 is the first keratin gene that was inactivated by gene targeting and homologous recombination [24]. A number of additional keratin genes have been inactivated since then (Table 1) and a flurry of activity has occurred on this front in recent months. As could be anticipated from the phenotype displayed by mice null for the K14 gene [25], K5 null mice show severe blistering in the skin and oral mucosa, and die shortly after birth [26•]. K5 null basal cells do not exhibit any cytoplasmic

More soft and hard outcomes from transgenic systems

Gene knockout studies have been complemented with several transgenic overexpression models that have illuminated or suggested several keratin functions. As such, studies in mice that overexpress Arg89→Cys K18 as well as in K8-null mice demonstrated the importance of an intact IF network in imparting protection to hepatocytes from several stresses including the toxins griseofulvin and microcystin-LR, partial hepatectomy, collagenase liver perfusion, Fas and tissue necrosis factor (TNF)-mediated

Keratin human diseases continue to illuminate new principles

The role of keratin mutations in causing several epidermal, oral, ocular and hair-related diseases is well established [36]. These diseases are genetically well defined, typically autosomal dominant and less commonly recessive. In general, disease severity correlates with the location and nature of the mutation within the keratin backbone (Fig. 1a) although variations on this theme exist [61]. As discussed above, alterations in the organization of keratin IFs can translate into weaker

Functional regulation at the protein level

Keratin regulation can be envisaged to occur intrinsically via interactions with self (see section on mechanics), or extrinsically via interaction/association with nonkeratin proteins that result in keratin phosphorylation, glycosylation, transglutamination, ubiquitination, proteolytic cleavage, or association with other cytoplasmic or cytoskeletal elements 17•., 66.. The importance of keratin-associated proteins (KAPs; Fig. 3) is amply supported by the diseases that are associated with the

Conclusions

The major goal of this review was to assemble a set of generally accepted ‘hard’ and emerging ‘soft’ principles, listed in Table 2, that are based on our current knowledge of keratin IF protein structure, regulation and functions. Although the proposed ‘soft’ principles will ultimately dissolve or harden, the keratin field as a whole is advancing and expanding at a much more rapid pace than it was previously, given the emergence of transgenic mouse models, the association with a number of

Update

Recent work demonstrated that K8/K18, in the context of K8-null mouse hepatocytes in vitro and in vivo, are essential for protection from Fas antibody (but not TNF)-mediated apoptosis [96••]. This effect may be related to modulation of Fas density at the cell surface [96••]. A role for K8/K18 in significantly dampening TNF-mediated apoptosis in cultured cells is further supported by the recent finding of K18 association with the TNF receptor1 associated death domain (TRADD) protein [97••]. In

Acknowledgements

Because of space limitations, we relied on excellent reviews that have been published within the past four years to refer to numerous previous classic studies. We thank past and present members of the Coulombe and Omary laboratories for their essential contributions. We thank Robert Oshima for his comments, and Kris Morrow and Soichiro Yamada for help in preparing the figures. This work was supported by a Department of Veterans Affairs Career Development and Merit Awards, National Institutes of

References and recommended reading

Papers of particular interest, published within the annual period of review,have been highlighted as:

  • • of special interest

  • •• of outstanding interest

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