W. Michael O'Guin

Chapter 5 Patterns of Keratin Expression Define Distinct Pathways of Epithelial Development and Differentiation

Publisher Summary The precise localization of particular keratin polypeptides eventually enables to sub-classify both embryonic and adult epithelial cells and tissues with respect to their relative degree of differentiation. This capacity should be exceptionally useful in studying the various aspects of epithelial differentiation. These localization studies are largely dependent upon immunological procedures involving monoclonal antibodies. While a great deal has been learned about the biochemical and immunological properties of keratin polypeptides through the use of broadly cross-reacting monoclonal antibodies such as AE-1 and AE-3, they are of limited usefulness in the tissue localization of individual keratins within various epithelia. Any studies using conventional or monoclonal antibodies that recognize multiple keratin polypeptides must always be performed in conjunction with the immunochemical analysis of the keratins recognized in a given tissue before conclusions may be made about their localization. The current “state of the art” monoclonal antibody studies on keratin localization involve highly selective antibodies (usually recognizing a single keratin polypeptide) that demonstrate an extraordinary degree of keratin specificity. Despite the relative absence of comprehensive studies on the precise tissue localization of individual keratin polypeptides, the apparent ubiquity of the keratin pair concept and the theory of differentiation-specific pairs are so uniformly consistent that the total keratin polypeptide composition of any epithelial tissue, given only the species of origin and its histological characteristics, can be accurately predicted.

Bone Morphogenetic Protein Signaling Regulates Postnatal Hair Follicle Differentiation and Cycling

Hair follicle morphogenesis and cycling were examined in transgenic mice that overexpress the bone morphogenetic protein (BMP) inhibitor Noggin under the control of the neuron-specific enolase promoter. The Noggin transgene was misexpressed in the proximal portion of the hair follicle, primarily the matrix cells, apart from the usual expression in neurons. Transgene expression appeared only after induction of both the primary (tylotrich) and secondary (nontylotrich) pelage hair follicles had already occurred, thus allowing examination of the role of BMP signaling in follicles that had been induced normally in the presence of BMPs. The overexpression of Noggin in these animals resulted in a dramatic loss of hair postnatally. There was an apparently normal, but shortened period of postnatal hair follicle morphogenesis, followed by premature initiation of hair follicle cycling via entry into the first catagen transformation. This resulted in a complete loss of hair shafts from the nontylotrich hair follicles in these mice while the tylotrich hair follicles were normal. The onset of anagen of the first postnatal hair follicle cycle was also accelerated in the transgenic mice. Our results show that BMP signaling is specifically required for proper proliferation and differentiation during late morphogenesis of nontylotrich hair follicles and that inhibition of this signaling pathway may be one of the triggers for the onset of catagen when the follicles are in anagen and the onset of anagen when the follicles are in telogen. Ectopic sebocyte differentiation was another hallmark of the phenotype of these transgenic mice suggesting that BMP signaling may be an important determinant of lineage selection by common progenitor cells in the skin. BMPs likely promote a hair follicle-type differentiation pathway of keratinocytes while suppressing the sebaceous differentiation pathway of skin epithelium.

Immunofluorescence staining of keratin filaments in cultured epithelial cells

A procedure is described for the identification of cultured epithelial cells by indirect immunofluorescence staining of keratins. Additionally, some associated techniques such as double staining, keratin antigen unmasking, cytostatic drug treatment and photography are briefly outlined.

Keratohyalin, Trichohyalin and Keratohyalin‐Trichohyalin Hybrid Granules: An Overview

Filaggrin and trichohyalin are the two major intermediate filament associated proteins which interact with keratin filaments in the skin. These two proteins initially accumulate in cytoplasmic granules called keratohyalin or trichohyalin granules which provide prominent morphological hallmarks of differentiation in the epidermis and the inner root sheath of hair follicles, respectively. The contents of each of these granules are modified and subsequently released into the cytoplasm of the fully mature cells where they function in the role of aggregating keratin filament bundles. We are beginning to identify several important aspects relative to the unique biological functions of both filaggrin and trichohyalin during the late stages of keratinocyte differentiation. This overview summarizes recent work on these proteins and will also highlight the existence of novel cytoplasmic granules, keratohyalin‐trichohyalin hybrid granules, in dorsal tongue epithelia.

EXPRESSION OF KERATOHYALIN‐TRICHOHYALIN HYBRID GRANULES IN MOLLUSCUM CONTAGIOSUM

Background. Recently, in the filiform papillae epithelium of mouse dorsal tongue, we showed the presence of hybrid granules in which filaggrin and trichohyalin were both present, but physically segregated. Further, trichohyalin was also detected in scattered granular cells of a number of hyperplastic skin diseases.

Trichohyalin expression in skin tumors : Retrieval of trichohyalin antigenicity in tissues by microwave irradiation

Background. The antitrichohyalin antibody AE 15 is effective for identifying the cell lineage that undergoes the pathway of inner root sheath‐type differentiation. Unfortunately, the AE 15 does not react with trichohyalin in tissue that is formalin‐fixed and embedded in paraffin according to routine procedures.