Keratohyalin granules are heterogeneous in ridged and non-ridged human skin: evidence from anti-filaggrin immunogold labelling of normal skin and skin of autosomal dominant ichthyosis vulgaris patients

Characterization of the human ridged and non-ridged skin: a comprehensive histological, histochemical and immunohistochemical analysis

The structure of the human skin is directly dependent on its location and the mechanical forces to which it is subjected. In the present work, we have performed a comprehensive analysis of the human ridged and non-ridged skin to identify the differences and similarities between both skin types. For this purpose, human skin samples were obtained from dorsal hand skin (DHS), palmar hand skin (PHS), dorsal foot skin (DFS) and plantar foot skin (PFS) from the same cadaveric donors. Histological, histochemical and semiquantitative and quantitative immunohistochemical analyses were carried out to evaluate the epidermis, dermis and basement membrane. Results show that the epithelial layer of ridged skin had larger cell number and size than non-ridged skin for most strata. Melanocytes and Langerhans cells were more abundant in non-ridged skin, whereas Merkel cells were preferentially found in ridged skin. The expression pattern of CK5/6 was slightly differed between non-ridged and ridged skin. Involucrin expression was slightly more intense in non-ridged skin than in ridged skin. Collagen was more abundant in foot skin dermis than in hand skin, and in ridged skin as compared to non-ridged skin. Elastic fibers were more abundant in DHS. Biglycan was more abundant in foot skin than in hand skin. No differences were found for blood and lymphatic vessels. The basement membrane laminin was preferentially found in foot skin. These results revealed important differences at the epithelial, dermal and basement membrane levels that could contribute to a better knowledge of the human skin histology.

Orchestrated control of filaggrin-actin scaffolds underpins cornification

Epidermal stratification critically depends on keratinocyte differentiation and programmed death by cornification, leading to formation of a protective skin barrier. Cornification is dynamically controlled by the protein filaggrin, rapidly released from keratohyalin granules (KHGs). However, the mechanisms of cornification largely remain elusive, partly due to limitations of the observation techniques employed to study filaggrin organization in keratinocytes. Moreover, while the abundance of keratins within KHGs has been well described, it is not clear whether actin also contributes to their formation or fate. We employed advanced (super-resolution) microscopy to examine filaggrin organization and dynamics in skin and human keratinocytes during differentiation. We found that filaggrin organization depends on the cytoplasmic actin cytoskeleton, including the role for α- and β-actin scaffolds. Filaggrin-containing KHGs displayed high mobility and migrated toward the nucleus during differentiation. Pharmacological disruption targeting actin networks resulted in granule disintegration and accelerated cornification. We identified the role of AKT serine/threonine kinase 1 (AKT1), which controls binding preference and function of heat shock protein B1 (HspB1), facilitating the switch from actin stabilization to filaggrin processing. Our results suggest an extended model of cornification in which filaggrin utilizes actins to effectively control keratinocyte differentiation and death, promoting epidermal stratification and formation of a fully functional skin barrier.

Ichthyosis vulgaris: novel FLG mutations in the German population and high presence of CD1a+ cells in the epidermis of the atopic subgroup

BACKGROUND

Ichthyosis vulgaris (IV) is a genetic disorder with a prevalence of 1:250-1000 caused by filaggrin (FLG) mutations, which also predispose to atopic diseases.

OBJECTIVES

To study the genotype/phenotype relationship in IV and to analyse whether the suggested skin barrier defect is associated with differences of epidermal dendritic cells.

PATIENTS/METHODS

We evaluated a cohort of 26 German patients with IV, established an IV severity score and analysed epidermal ultrastructure, histology, filaggrin and CD1a antigens. Mutations were screened by restriction enzyme analysis. Particular sequencing techniques allowed the complete FLG analysis to reveal novel mutations.

RESULTS

The combined null allele frequency of R501X and 2282del4 was 67.3%. Patients also showed the mutations S3247X and R2447X as well as five novel FLG mutations: 424del17 and 621del4 (profilaggrin S100 domain), 2974delGA (repeat 2), R3766X (repeat 10(1)) and E4265X (repeat 10(2)). Their combined allele frequency in controls was <0.7%. No mutation was found in one IV patient, all in all approximately 27% were heterozygous, and the majority (approximately 69%) showed two null alleles. The IV severity score and ultrastructure showed a significant correlation with genotypes. Interestingly, CD1a cell counts showed a significant difference between nonatopic and atopic IV patients both with eczema and without eczema.

CONCLUSIONS

We confirm that the mutations R501X and 2282del4 represent the most frequent genetic cause in German IV patients. The novel mutations are probably population and family specific. The observed differences of CD1a cells support the hypothesis that there is a barrier defect that predisposes to atopic manifestations, possibly independent of atopic eczema.

Cathepsin D is involved in the regulation of transglutaminase 1 and epidermal differentiation

We previously demonstrated that the aspartate protease cathepsin D is activated by ceramide derived from acid sphingomyelinase. Increased expression of cathepsin D in the skin has been reported in wound healing, psoriasis and skin tumors. We explored specific functions of cathepsin D during epidermal differentiation. Protein expression and enzymatic activity of cathepsin D increased in differentiated keratinocytes in both stratified organotypic cultures and in mouse skin during epidermal barrier repair. Treatment of cultured keratinocytes with exogenous cathepsin D increased the activity of transglutaminase 1, known to cross-link the cornified envelope proteins involucrin and loricrin during epidermal differentiation. Inhibition of cathepsin D by pepstatin A suppressed the activity of transglutaminase 1. Cathepsin D-deficient mice revealed reduced transglutaminase 1 activity and reduced protein levels of the cornified envelope proteins involucrin and loricrin. Also, amount and distribution of cornified envelope proteins involucrin, loricrin, filaggrin, and of the keratins K1 and K5 were significantly altered in cathepsin D-deficient mice. Stratum corneum morphology in cathepsin D-deficient mice was impaired, with increased numbers of corneocyte layers and faint staining of the cornified envelope only, which is similar to the human skin disease lamellar ichthyosis. Our findings suggest a functional link between cathepsin D activation, transglutaminase 1 activity and protein expression of cornified envelope proteins during epidermal differentiation.

Absence of the granular layer and keratohyalin define a morphologically distinct subset of individuals with ichthyosis vulgaris

The clinical diagnosis of ichthyosis vulgaris (IV) can be difficult. Abnormalities in the granular layer and the ultrastructure of keratohyalin granules (KHG) suggest that morphology may be helpful. To clarify morphologic findings in IV, 41 clinically affected individuals and 21 unaffected family members or age- and sex-matched controls were studied by light microscopy. In these, the granular layer was totally absent in approximately 50% of affected individuals, while present in all controls. Forty-seven individuals in the light microscopy group were also studied by electron microscopy. Keratohyalin granules were absent in all affected individuals lacking the granular layer by light microscopy. Clinical severity usually correlated with the lack of a granular layer and KHG. Absence of the granular layer was consistent in different anatomic sites and in serial biopsies taken over a 1-3-year period. In a subset of clinically affected, unrelated subjects with moderate to severe involvement, four out of 11 (36%) had similar findings. Keratinocytes cultured from affected individuals with no KHG expressed virtually no detectable profilaggrin protein in vitro. The data suggest that a subset of individuals with moderate to severe IV have a consistently absent granular layer and KHG. Absence of the granular layer and lack of KHG correlated almost perfectly; thus light microscopy offers a convenient means of identifying this subtype of IV. However, both morphologic types of IV were observed within single families. Therefore, the relationship between granular layer abnormalities and IV is complex and requires the study of more affected families. One interpretation of the data is that IV is a multigenic disorder in which one of the genes alters profilaggrin expression. We propose this clinical and histologic phenotype as useful for identifying the gene(s) involved and also for determining whether it represents a modifier or a major locus of the disorder.

High-pressure freezing provides new information on human epidermis: simultaneous protein antigen and lamellar lipid structure preservation. Study on human epidermis by cryoimmobilization

Current transmission electron microscopy techniques do not permit simultaneous visualization of skin ultrastructure and stratum corneum extracellular lipids. We developed a new procedure, which entails application of high-pressure freezing followed by freeze-substitution with acetone containing uranyl acetate, followed by low temperature embedding in HM20. Electrospray ionization mass spectrometry showed that the amount of lipids lost during preparation was minimal. The ultrastructure of cryoprocessed skin was compared with that of conventionally prepared skin samples. Cryoprocessing, but not conventional processing, enabled visualization of lipid stacks within epidermal lamellar bodies, as well as the extracellular lipid domains of the stratum corneum and the ultrastructure within keratinocytes. Anti-filaggrin immunocytochemistry also showed, e.g., excellent preservation of filaggrin on cryoprocessed samples. Additionally, the cytosol of keratinocytes appeared to be organized in "microdomain"-like areas. Finally, the stratum corneum appeared more compact with smaller intercellular spaces and hence tighter cell-cell interactions, after cryoprocessing, than after conventional tissue preparation for transmission electron microscopy. We conclude here that only cryoprocessing preserves skin in a close to native state.

The major inherited disorders of cornification. New advances in pathogenesis

This article provides a synopsis of the major (most common) inherited disorders of cornification. It also reviews the recent advances that have been made for each disorder and their practical applications.

Morphology of the keratin filament network in palm and sole skin: evidence for site-dependent features based on stereological analysis

Ridged or glabrous skin of palms and soles has a specialized function and can be preferentially involved in various disorders of keratinization. To better define the morphological features of ridged skin, we carried out a qualitative and quantitative (stereological) analysis of normal epidermis from the palm and sole of four subjects. Skin from the upper arm was examined for control purposes. The study focused on the appearance and arrangement of the keratin filament network in relation to epidermal differentiation. Whereas palm and sole epidermis was essentially similar both qualitatively and quantitatively, it differed markedly from the epidermis from the arm. The volume density of keratin filaments was significantly higher (P < 0.03) in all subcorneal layers of the palm and sole compared with the arm. The volume density of the keratin filaments increased markedly from the basal to the upper spinous layer of ridged skin and they formed denser aggregates in the upper spinous and granular layers, providing an extensive matrix for the deposition of keratohyalin. The presence of dense keratin aggregates appeared to be a distinct ultrastructural feature of human ridged skin. Such keratin aggregates have not been described in normal skin from other sites, but showed some resemblance to the keratin clumps seen in non-ridged skin of patients with the Dowling-Meara form of epidermolysis bullosa simplex.

Monoclonal antibodies to human epidermal filaggrin, some not recognizing profilaggrin

To improve understanding of human profilaggrin processing to filaggrin, we produced seven monoclonal antibodies against epidermal filaggrin (AHF1-7). They were characterized on human epidermis by indirect immunofluorescence, immunogold labeling, and immunoblotting and found to be directed against seven different epitopes of (pro)filaggrin. AHF1-5 labeled the keratohyalin granules and the fibrous matrix of the lower corneocytes, and recognized filaggrin and profilaggrin. AHF6 also labeled the keratohyalin granules and the corneocyte matrix, but only recognized filaggrin. In addition to this reactivity within the upper epidermis, AHF4-6 stained the cytoplasm of the basal cells, and cross-reactivity of AHF5 and AHF6 with cytokeratin K14 was revealed on immunoblots. It is interesting that AHF7 recognized filaggrin, but not profilaggrin, and labeled only the corneocyte matrix and not the keratohyalin granules. This indicates that filaggrin and cytokeratins share several antigenic determinants and that filaggrin bears at least one epitope absent from its precursor. The original series of monoclonal antibodies described here appears to be a powerful tool for studying human profilaggrin processing in normal conditions and in the keratinization disorders in which processing is altered.

Corneocytes undergo systematic changes in element concentrations across the human inner stratum corneum

Using analytical electron microscopy of freeze-dried cryosections, physiologic elements were visualized within individual cells across the human inner stratum corneum. Human corneocytes undergo systematic changes in element composition as they advance through this region. Phosphorus is largely excluded from the stratum corneum, undergoing a precipitous drop in concentration at the granular/stratum corneum interface. The cellular potassium concentration has a profile similar to that of phosphorus but with a slower decline, thus migrating further into the stratum corneum. In contrast, the cellular chloride concentration increases in the innermost corneocyte layer, increases further in the subsequent layer or two (as potassium declines), and then decreases to values comparable to those in the innermost corneocyte. The cellular sodium concentration (per unit volume of tissue) is relatively unaltered in transit across the inner stratum corneum. The initial potassium and chloride movements are oppositely directed and have the appearance of creating an electrical charge imbalance. The position-dependent alterations in corneocyte elemental composition may reflect sequential stages of chemical maturation occurring intracellularly during stratum corneum transit, an example of which is the breakdown of filaggrin that occurs over this same region of the inner stratum corneum.

The rheumatoid arthritis-associated autoantibodies to filaggrin label the fibrous matrix of the cornified cells but not the profilaggrin-containing keratohyalin granules in human epidermis

Since they were first described, serum IgG antibodies to the stratum corneum of rat oesophagus epithelium, highly specific for rheumatoid arthritis (RA), have been consensually called antikeratin antibodies (AKA). However, we recently demonstrated that they actually recognize three new proteins of rat oesophagus epithelium distinct from cytokeratins, and also human epidermal filaggrin. In this work we provided further evidence that AKA and RA-associated anti-filaggrin autoantibodies are the same antibodies. Moreover, analysing by indirect immunofluorescence on human skin a large series of 212 well characterized RA sera and anti-filaggrin autoantibodies purified from RA sera by affinity chromatography, we demonstrated the specific binding of AKA to the stratum corneum of human epidermis and the absence of any staining of the granular keratinocytes. This binding was confirmed and the AKA antigen precisely localized in human epidermis by immunoelectron microscopy. The antigen was found to be restricted to the filaggrin-containing intracellular fibrous matrix of the corneocytes, up to the desquamating cells. In contrast, MoAbs directed to human filaggrin and to profilaggrin, its precursor, not only stained the intracellular matrix of the lower corneocytes but also the keratohyalin granules of the granular cells, where profilaggrin is stored. These results reinforced by the absence of immunoblotting reactivity of RA sera to profilaggrin suggest that the epitopes recognized by AKA are absent from profilaggrin. Their identification may provide more insight into the pathogenesis of RA.

Ultrastructural identification of basic abnormalities as clues to genetic disorders of the epidermis

The present article discusses specific, directly gene-dependent ultrastructural markers of dominantly inherited epidermal disorders that serve as clues to their underlying molecular genetic abnormalities. These are epidermolysis bullosa simplex Koebner and Weber-Cockayne with rupture or non-assembly of basal cell keratins and point mutations in keratins 5 and 14. Clumping of basal cell keratins is pathognomonic of EB Dowling-Meara and caused by mutations in hot spots of the rod domain of K5 and K 14. Clumps and aggregates of basal keratins occur side by side in the same cell and thus do not indicate specific different types of mutations. Similar clumping of suprabasal keratins in bullous CIE Brocq and in palmoplantar keratoderma Voerner have been assigned to identical types of mutations in the same critical position of the rod domain in K 1, K 10, and K 9, respectively. Highly unusual tubular keratins are pathognomonic of another dominant palmoplantar keratoderma type the genetic basis of which still awaits elucidation. Shell formation of (low molecular weight?) keratins in ichthyosis hystrix Curth-Macklin is not linked to the keratin gene clusters on chromosomes 12 and 17 and might be related to regulatory genes of keratin expression. Suprabasal shells in congenital reticular ichthyosiform erythroderma do not consist of keratins but resemble glycoprotein networks. Finally, the keratohyalin abnormality in ichthyosis vulgaris was the clue for the identification of a filaggrin deficiency, at the same time giving evidence to the heterogeneity of keratohyalin proteins.

Is filaggrin really a filament-aggregating protein in vivo?

Filaggrin, a basic protein of the stratum corneum, was named as such because of its capability to aggregate keratin intermediate filaments in vitro. To investigate its filament-aggregating capability in vivo, we performed immunoelectron microscopy in three autosomal dominant genodermatoses serving as in vivo models of abnormalities of keratin filament aggregation. In bullous congenital ichthyosiform erythroderma Brocq and epidermolytic palmoplantar keratoderma Voerner suprabasal clumping of keratin filaments prevents the normal spreading of keratohyalin between keratin filaments. Keratohyalin granules, either isolated or attached to clumped keratins, were specifically labelled by the anti-filaggrin antibody, whereas tonofilament clumps did not show any reaction. In epidermolysis bullosa herpetiformis Dowling-Meara the abnormal filament aggregation occurred in basal cell keratins where no reaction of the anti-filaggrin antibody was detected. In high level keratinocytes with normal distribution of tonofilaments, normal stellate keratohyalin reacted specifically. In all instances keratin filament clumping occurred independently of, and spatially separated from, the first signs of profilaggrin synthesis and keratohyalin granule formation. Thus, in these disorders, filaggrin is not involved in filament aggregation.

Mitten deformity in severe generalized recessive dystrophic epidermolysis bullosa: histological, immunofluorescence, and ultrastructural study

Light and electron microscopy and indirect immunofluorescence techniques were used to study the nature of the mitten deformity in five adult patients with severe generalized recessive dystrophic epidermolysis bullosa undergoing release of hand and finger contractures. Although the mitten appeared largely to be clinically separated from the underlying fixed digits, histology showed mostly normal keratinocytes beneath a thickened stratum corneum. The lower margin of the mitten was formed just below the lamina densa of the basement membrane, at a level similar to that of the usual blister formation in this condition. No anchoring fibrils and only a few distinct dermal structures were noted. A substantial portion of the mitten, however, consisted of necrotic keratinocytes without an intact basement membrane. This finding suggests that the mitten is not suitable for use as an epidermal autograft and confirms the rationale for taking split thickness skin grafts to close skin defects in patients with recessive dystrophic epidermolysis bullosa undergoing plastic surgery.

Congenital ichthyosis with hypogonadism and growth retardation--a new syndrome with peculiar ultrastructural features

A male patient presented with a congenital ichthyosis clinically characterized by generalized erythroderma, fine scaling on the trunk and palmoplantar hyperkeratoses with severely affected nails. The acanthotic epidermis was characterized by hyperproliferation with a large quantity of mitoses and extremely suppressed keratinization without a normal granular layer. The horny layer was parakeratotic and contained remnants of cell debris and lipid droplets. Ultrastructurally the prickle cell layer was characterized by binuclear cells, oedematization of the keratinocytes and isolated dyskeratotic cells. Some suprabasal cells showed unusual morphological features, containing nuclei with cytoplasmic pseudoinclusions, sometimes leading to a complete disintegration of the nuclear structure, and bowl- and lens-shaped accumulations of a filamentous material. Instead of normal tonofibrils, the aggregated material consisted of fine interlacing filaments. The latter are compared with the filamentous shells in ichthyosis hystrix Curth-Macklin and congenital reticular ichthyosiform erythroderma. The clinical symptomatology--congenital ichthyosis, growth retardation, secondary hypogonadism, hepatomegaly--and the ultrastructural characteristics of the keratinization disorder indicate that the present case cannot be considered as a subtype of the recessively inherited ichthyosis congenita group, but suggest a new syndrome as a separate nosologic entity.