Keratins and protein synthesis: the plot thickens

Damaged Keratin Filament Network Caused by KRT5 Mutations in Localized Recessive Epidermolysis Bullosa Simplex

Epidermolysis bullosa simplex (EBS) is a blistering dermatosis that is mostly caused by dominant mutations in KRT5 and KRT14. In this study, we investigated one patient with localized recessive EBS caused by novel homozygous c.1474T > C mutations in KRT5. Biochemical experiments showed a mutation-induced alteration in the keratin 5 structure, intraepidermal blisters, and collapsed keratin intermediate filaments, but no quantitative change at the protein levels and interaction between keratin 5 and keratin 14. Moreover, we found that MAPK signaling was inhibited, while desmosomal protein desmoglein 1 (DSG1) was upregulated upon KRT5 mutation. Inhibition of EGFR phosphorylation upregulated DSG1 levels in an in vitro model. Collectively, our findings suggest that this mutation leads to localized recessive EBS and that keratin 5 is involved in maintaining DSG1 via activating MAPK signaling.

A comparative study of materials assembled from recombinant K31 and K81 and extracted human hair keratins

Natural biopolymers have found success in tissue engineering and regenerative medicine applications. Their intrinsic biocompatibility and biological activity make them well suited for biomaterials development. Specifically, keratin-based biomaterials have demonstrated utility in regenerative medicine applications including bone regeneration, wound healing, and nerve regeneration. However, studies of structure-function relationships in keratin biomaterials have been hindered by the lack of homogeneous preparations of materials extracted and isolated from natural sources such as wool and hair fibers. Here we present a side-by-side comparison of natural and recombinant human hair keratin proteins K31 and K81. When combined, the recombinant proteins (i.e. rhK31 and rhK81) assemble into characteristic intermediate filament-like fibers. Coatings made from natural and recombinant dimers were compared side-by-side and investigated for coating characteristics and cell adhesion. In comparison to control substrates, the recombinant keratin materials show a higher propensity for inducing involucrin and hence, maturation in terms of potential skin cell differentiation.

Epithelial Keratins Modulate cMet Expression and Signaling and Promote InlB-Mediated Listeria monocytogenes Infection of HeLa Cells

The host cytoskeleton is a major target for bacterial pathogens during infection. In particular, pathogens usurp the actin cytoskeleton function to strongly adhere to the host cell surface, to induce plasma membrane remodeling allowing invasion and to spread from cell to cell and disseminate to the whole organism. Keratins are cytoskeletal proteins that are the major components of intermediate filaments in epithelial cells however, their role in bacterial infection has been disregarded. Here we investigate the role of the major epithelial keratins, keratins 8 and 18 (K8 and K18), in the cellular infection by Listeria monocytogenes. We found that K8 and K18 are required for successful InlB/cMet-dependent L. monocytogenes infection, but are dispensable for InlA/E-cadherin-mediated invasion. Both K8 and K18 accumulate at InlB-mediated internalization sites following actin recruitment and modulate actin dynamics at those sites. We also reveal the key role of K8 and K18 in HGF-induced signaling which occurs downstream the activation of cMet. Strikingly, we show here that K18, and at a less extent K8, controls the expression of cMet and other surface receptors such TfR and integrin β1, by promoting the stability of their corresponding transcripts. Together, our results reveal novel functions for major epithelial keratins in the modulation of actin dynamics at the bacterial entry sites and in the control of surface receptors mRNA stability and expression.

Types I and II Keratin Intermediate Filaments

Keratins-types I and II-are the intermediate-filament-forming proteins expressed in epithelial cells. They are encoded by 54 evolutionarily conserved genes (28 type I, 26 type II) and regulated in a pairwise and tissue type-, differentiation-, and context-dependent manner. Here, we review how keratins serve multiple homeostatic and stress-triggered mechanical and nonmechanical functions, including maintenance of cellular integrity, regulation of cell growth and migration, and protection from apoptosis. These functions are tightly regulated by posttranslational modifications and keratin-associated proteins. Genetically determined alterations in keratin-coding sequences underlie highly penetrant and rare disorders whose pathophysiology reflects cell fragility or altered tissue homeostasis. Furthermore, keratin mutation or misregulation represents risk factors or genetic modifiers for several additional acute and chronic diseases.

Homo- and heteropolymer self-assembly of recombinant trichocytic keratins

In the past two decades, keratin biomaterials have shown impressive results as scaffolds for tissue engineering, wound healing, and nerve regeneration. In addition to its intrinsic biocompatibility, keratin interacts with specific cell receptors eliciting beneficial biochemical cues. However, during extraction from natural sources, such as hair and wool fibers, natural keratins are subject to extensive processing conditions that lead to formation of unwanted by-products. Additionally, natural keratins suffer from limited sequence tunability. Recombinant keratin proteins can overcome these drawbacks while maintaining the desired chemical and physical characteristics of natural keratins. Herein, we present the bacterial expression, purification, and solution characterization of human hair keratins K31 and K81. The obligate heterodimerization of the K31/K81 pair that results in formation of intermediate filaments is maintained in the recombinant proteins. Surprisingly, we have for the first time observed new zero- and one-dimensional nanostructures from homooligomerization of K81 and K31, respectively. Further analysis of the self-assembly mechanism highlights the importance of disulfide crosslinking in keratin self-assembly.

Kaempferol targets Krt-14 and induces cytoskeletal mineralization in osteoblasts: A mechanistic approach

Kaempferol (KEM) has been observed to stimulate Krt-14 protein which subsequently contributes to matrix maturation and mineralization in rat primary osteoblast cells. Incorporation of Krt-14 siRNA results in reduced mRNA and protein expression after 48h post transfection and remained low for 9days. At day 9 Krt-14 siRNA significantly reduced mineralization without concomitant change in the cell number. ColI and OCN gene expression was reduced in Krt-14 siRNA-treated osteoblast cells. Soluble osteocalcin and collagen levels were markedly decreased in conditioned medium as well as in acid-salt soluble cell-ECM layer treated with Krt-14 siRNA compared to control siRNA treated cells corroborated at the ultrastructral level by AFM. Further, knockdown of Krt-14 and inhibitors against AMPK and mTOR, repressed the activation of mTOR and mineralization attenuated by KEM confirmed the role of Krt-14 in mineralization. These findings strongly suggest that Krt-14 regulates osteoblast mineralization by organizing osteoblast derived ECM.

Gene expression profiling in pachyonychia congenita skin

BACKGROUND

Pachyonychia congenita (PC) is a skin disorder resulting from mutations in keratin (K) proteins including K6a, K6b, K16, and K17. One of the major symptoms is painful plantar keratoderma. The pathogenic sequelae resulting from the keratin mutations remain unclear.

OBJECTIVE

To better understand PC pathogenesis.

METHODS

RNA profiling was performed on biopsies taken from PC-involved and uninvolved plantar skin of seven genotyped PC patients (two K6a, one K6b, three K16, and one K17) as well as from control volunteers. Protein profiling was generated from tape-stripping samples.

RESULTS

A comparison of PC-involved skin biopsies to adjacent uninvolved plantar skin identified 112 differentially-expressed mRNAs common to patient groups harboring K6 (i.e., both K6a and K6b) and K16 mutations. Among these mRNAs, 25 encode structural proteins including keratins, small proline-rich and late cornified envelope proteins, 20 are related to metabolism and 16 encode proteases, peptidases, and their inhibitors including kallikrein-related peptidases (KLKs), and serine protease inhibitors (SERPINs). mRNAs were also identified to be differentially expressed only in K6 (81) or K16 (141) patient samples. Furthermore, 13 mRNAs were identified that may be involved in pain including nociception and neuropathy. Protein profiling, comparing three K6a plantar tape-stripping samples to non-PC controls, showed changes in the PC corneocytes similar, but not identical, to the mRNA analysis.

CONCLUSION

Many differentially-expressed genes identified in PC-involved skin encode components critical for skin barrier homeostasis including keratinocyte proliferation, differentiation, cornification, and desquamation. The profiling data provide a foundation for unraveling the pathogenesis of PC and identifying targets for developing effective PC therapeutics.

Deregulated HOX genes in ameloblastomas are located in physical contiguity to keratin genes

The expression of the HOX gene network in mid-stage human tooth development mostly concerns the epithelial tooth germ compartment and involves the C and D HOX loci. To further dissect the HOX gene implication with tooth epithelium differentiation we compared the expression of the whole HOX network in human ameloblastomas, as paradigm of epithelial odontogenic tumors, with tooth germs. We identified two ameloblastoma molecular types with respectively low and high number of active HOX C genes. The highly expressing HOX C gene ameloblastomas were characterized by a strong keratinized phenotype. Locus C HOX genes are located on chromosome 12q13-15 in physical contiguity with one of the two keratin gene clusters included in the human genome. The most posterior HOX C gene, HOX C13, is capable to interact with hair keratin genes located on the other keratin gene cluster in physical contiguity with the HOX B locus on chromosome 17q21-22. Inside the HOX C locus, a 2.2 kb ncRNA (HOTAIR) able to repress transcription, in cis, along the entire HOX C locus and, in trans, at the posterior region of the HOX D locus has recently been identified. Interestingly both loci are deregulated in ameloblastomas. Our finding support an important role of the HOX network in characterizing the epithelial tooth compartment. Furthermore, the physical contiguity between locus C HOX and keratin genes in normal tooth epithelium and their deregulation in the neoplastic counterparts suggest they may act on the same mechanism potentially involved with epithelial tumorigenesis.

Against the rules: human keratin K80: two functional alternative splice variants, K80 and K80.1, with special cellular localization in a wide range of epithelia

Of the 54 human keratins, five members have, at present, only been characterized at the gene level. In this study we have investigated the expression patterns of keratin K80, whose gene is located at the centromeric end of the type II keratin gene domain. K80 possesses a number of highly unusual properties. Structurally, it is distinctly closer to type II hair keratins than to type II epithelial keratins. Nonetheless, it is found in virtually all types of epithelia (stratified keratinizing/non-keratinizing, hard-keratinizing, as well as non-stratified tissues, and cell cultures thereof). This conspicuously broad expression range implies an unprecedented in vivo promiscuity of K80, which involves more than 20 different type I partners for intermediate filament (IF) formation. Throughout, K80 expression is related to advanced tissue or cell differentiation. However, instead of being part of the cytoplasmic IF network, K80 containing IFs are located at the cell margins close to the desmosomal plaques, where they are tightly interlaced with the cytoplasmic IF bundles abutting there. In contrast, in cells entering terminal differentiation, K80 adopts the "conventional" cytoplasmic distribution. In evolutionary terms, K80 is one of the oldest keratins, demonstrable down to fish. In addition, KRT80 mRNA is subject to alternative splicing. Besides K80, we describe a smaller but fully functional splice variant K80.1, which arose only during mammalian evolution. Remarkably, unlike the widely expressed K80, the expression of K80.1 is restricted to soft and hard keratinizing epithelial structures of the hair follicle and the filiform tongue papilla.