Endogenous substrates for epidermal transglutaminase

Protein composition of cornified cell envelopes of epidermal keratinocytes

Terminally differentiated mammalian epidermal cells are lined with a 15 nm thick layer of proteins cross-linked by isodipeptide and disulfide bonds, called the cornified cell envelope (CE). A number of proteins, including involucrin, loricrin, cystatin A, filaggrin, a cysteine-rich protein (CRP) and the ‘small proline-rich’ proteins (SPRRs) have been reported to be components of this complex, but little information has been obtained as to their relative abundances because the acute insolubility of the CEs has precluded direct methods of analysis. To address this question, we have determined the amino acid compositions of isolated CEs, and then modelled them in terms of linear combinations of the candidate proteins. The results show that stratum corneum CEs have a loricrin content of 65–70% (w/w) in human, and 80–85% in mouse. In human epidermal CEs, the secondary contributors are filaggrin and CRP (each approximately 10%), with smaller amounts of involucrin, SPRR and cystatin A (2-5% each) also present. Mouse epidermal CEs have about the same amount of filaggrin and somewhat more SPRR, but only trace amounts of the other proteins. In marked contrast, the major constituents of the CEs of cultured keratinocytes induced to terminal differentiation in vitro are cystatin A, involucrin and CRP (each approximately 30%). No significant amount of loricrin was detected except in sloughed mouse cells, which represent a more advanced state of terminal differentiation than attached cells.(ABSTRACT TRUNCATED AT 250 WORDS)

Cohesion and desquamation of epidermal stratum corneum

This article attempts to provide a comprehensive review on the roles of various classes of molecules in the cohesion and desquamation of the stratum corneum. In the first part of this monograph we review the field of epidermal differentiation in vivo and vitro, describing the expression and functions of a number of key structural molecules that characterize the process. In the second part we emphasize terminal differentiation and the biogenesis of the stratum corneum. The stratum corneum is a cell layer unique to fully differentiated squamous epithelia such as skin. While it is a dead stratum, it nevertheless is in a homeostatic process of continual shedding and renewal in synchrony with basal cell replication. It is also a degradative layer containing many proteinases and glycosidases in which a variety of intracellular and intercellular macromolecules are degraded. We highlight the molecules localized within the intercorneal matrix that are most likely to play a role in cohesion and desquamation, including: glycoproteins, lipids and enzymes. Because it is difficult to study the stratum corneum and desquamation in the native tissue, we discuss a number of model systems that have been used. The stratum corneum can be dispersed into single squames in different ways; these include mechanical dispersion as well as agents such as detergents and enzymes. The solubilized molecules and the structures remaining can then be studied as to their specific roles in desquamation. Using this approach it is possible to reconstitute multilayered structures that resemble a real stratum corneum. We have shown that glycoproteins play a key role in squame reaggregation and that this process can be modulated with amino sugars in a lectin-like fashion. Cohesion and desquamation can also be studied in tissue culture. Depending on the culture system, the extent of terminal differentiation and squame accumulation varies. Yet desquamation does not normally occur. It can be induced however by the inclusion of exogenous agents such as IFN-gamma which are found in the native epidermis but are absent in vitro. Modulation of desquamation by other exogenous agents is likely to yield further knowledge of how shedding occurs in vivo. Insight has also come from studies of scaling skin disorders. The glycoprotein and lipid profiles are altered in the stratum corneum in many diseases of aberrant terminal differentiation. A number of abnormalities in the levels of cytokines and growth factors have also been reported in the lesional tissue of such diseases.(ABSTRACT TRUNCATED AT 400 WORDS)

Evidence for filaggrin as a component of the cell envelope of the newborn rat

A substrate of transglutaminase, specific to the epidermis, was identified, by fluorescent and radioactive labelling with the lysine analogues dansylcadaverine and [14C]putrescine respectively, in newborn-rat epidermal homogenates and whole-skin organ cultures. The labelled analogues were preferentially incorporated into the stratum-corneum protein filaggrin in a Ca2+-dependent manner in both 'in vitro' systems. When filaggrin was labelled in vivo with [3H]histidine and then incubated with rat epidermal preparations, the label was rendered SDS/thiol-insoluble. Incorporation of [3H]filaggrin into the insoluble envelope fraction was Ca2+-dependent and inhibited by EDTA and exogenous amines. Antisera to newborn-rat filaggrin cross-reacted with purified newborn-rat cell envelopes, and this reaction was blocked by adsorbing the antiserum with purified filaggrin. Quantification of the 'envelope-bound' filaggrin showed it to be a significant component, accounting for approx. 10% of the cell-envelope protein.

Effect of retinoic acid on cornified envelope formation: difference between spontaneous envelope formation in vivo or in vitro and expression of envelope competence

A large number of cross-linked envelopes form spontaneously when cell lines derived from chemically induced mouse skin papillomas are cultured in medium containing 1.2 mM calcium. This phenomenon is associated with high activity of the cross-linking enzyme, epidermal transglutaminase (TGase). The influence of retinoic acid (RA) on envelope formation was studied in detail in a papilloma cell line, PE. Retinoic acid (3 microM) completely blocked cornified envelope (CE) production but reduced TGase activity only 50%. A rabbit antiserum was produced against sonicated CEs isolated from newborn mouse skin. On Western blots of epidermal extracts, diffuse staining was observed for particulate proteins of suprabasal, but not basal, cells and similar immunoreactive material was absent from the cytosolic fraction of both cell layers. The antibody also recognized particulate proteins from PE cells induced to differentiate by calcium, but not from cells grown in the presence of high calcium and RA. The antiserum appears to recognize partially cross-linked CE precursor proteins judging by the diffuse staining, the molecular weight range of the proteins stained, and their origin in the particulate cellular fraction. Cross-linked envelopes could be induced in RA-treated PE cells by permeabilization with 0.75 M NaCl or 50 micrograms/ml A23187. However, this treatment failed to cause the appearance of proteins recognized by the antiserum. Preincubation of the antiserum with purified fragments of CEs from newborn mouse epidermis, but not with cross-linked envelopes from permeabilized, RA-treated PE cells, removed immunoreactivity. These results indicate that the cross-linked envelopes formed in RA-treated cells after permeabilization lack a set of proteins contained in CEs from stratum corneum and may even be composed of different proteins. Retinoic acid appears to prevent CE formation in part by inhibiting activation of epidermal TGase but in addition by influencing the synthesis of precursor proteins.

Identification of new components of the cornified envelope of human and bovine epidermis

Antibodies raised in rabbits to purified cornified envelopes (CEs) of cultured human keratinocytes reacted in a peripheral fashion with the granular and spinous layers of human, cow, rat, and mouse epidermis. This reaction could not be abolished by absorption of the antibody with purified human involucrin to which the antibody reacted by immunoblot, thus indicating the presence of an additional antigenic determinant(s). Antibodies raised to CEs of human epidermis stained the cytoplasm of epidermal cells and gave a strong reaction to cytokeratins and a weak one to involucrin, indicating that in tissue the keratins are also cross-linked. The antibody prepared to bovine CEs reacted with keratins, but when absorbed with prekeratin it gave a peripheral staining pattern with epidermis and reacted strongly with a 126 kD component of the neutral buffer extract of cow snout epidermis and weakly with 205 kD and 85 kD ones. This antibody reacted with human involucrin by immunoblot while an antibody to involucrin stained 143 kD, 119 kD, 113 kD, and 107 kD polypeptides in the bovine extract. These latter 3 bands were shown to be substrates of transglutaminase. Further, a monoclonal antibody to bovine CEs reacted with the 119 kD and 113 kD bands and gave a peripheral staining pattern in the epidermis. Proof that the 126 kD protein was a precursor of the envelope was obtained by preparing an antibody to it and demonstrating peripheral staining of epidermal cells. These results point out the value of preparing antibodies to CE as an additional approach to studying the composition of CEs and demonstrate previously undescribed components in human and bovine tissue.

Enzymatic cross-linking of involucrin and other proteins by keratinocyte particulates in vitro

A transglutaminase-catalyzed cross-linking process characteristic of keratinocytes leads to the formation of the insoluble corneocyte envelope. The essentials of this process take place in vitro in a reconstituted system derived from subcellular fractions. A particulate fraction containing membrane-bound envelope precursor proteins and the enzyme transglutaminase is combined with cytosolic proteins; when the enzyme is activated by Ca++, cytosolic proteins are removed from solution and cross-linked to particulate proteins. This interaction is cell-type-specific, since particulates derived from fibroblasts and also containing transglutaminase activity cannot substitute for those of keratinocytes. Involucrin, a cytosolic protein known to be a precursor of the envelope, is more efficiently cross-linked than other cytosolic proteins. The cross-linking of proteins of the particulate fraction (membrane proteins) is promoted by the presence of involucrin.

Epidermal and hair follicle transglutaminases and crosslinking in skin

Epidermal and hair follicle transglutaminases crosslink structural proteins in the skin by epsilon-(gamma-glutamyl)-lysine bonds. This crosslinking produces protein polymers that are extremely insoluble and, until recently, difficult to characterize. Epidermal transglutaminase is localized to the granular layer of the epidermis. It catalyzes the crosslinking of a soluble cytoplasmic precursor to form the cornified envelope that lines the inner membrane of the mature keratinocyte in the stratum corneum. Hair follicle transglutaminase is localized to the inner root sheath and medulla of the hair follicle. It crosslinks a poorly characterized citrulline-rich protein. The enzymes and their substrates have been shown to be important markers of normal differentiation. Regulation of these processes is currently under investigation.

Transglutaminase-mediated cross-linking in mammalian epidermis

The epsilon-(gamma-glutamyl)lysine isopeptide bond has been identified in certain structural proteins of the hair fibre and the epidermis. The major cross-linked proteins are not keratins and generally have little or no cysteine, but have a high glutamic acid/glutamine residue content. In the hair fibre the cross-link appears in the citrulline-containing proteins of the medulla and the inner root sheath of the follicle. In the epidermis the cross-linked proteins are involved in the formation of the cornified envelope of the stratum corneum cells. In both cases, the cross-linked proteins contribute the characteristic property of chemical resistance to their biological structures.

Biochemistry of transglutaminases and cross-linking in the skin

Transglutaminase is a calcium-dependent enzyme found widely in nature. It catalyzes the formation of epsilon-(gamma-glutamyl)lysine bonds that participate in processes varying from fibrin clot formation to epidermal cell envelope formation. Epidermal transglutaminase is localized to the granular layer of the epidermis. It catalyzes the covalent cross-linking of a soluble cytoplasmic substrate into large polymers to form the cornified envelope that lines the inner membrane of keratinocytes in the stratum corneum. The soluble precursor from epidermis has been named keratolinin, and from keratinocyte culture, it has been named involucrin. Hair follicle transglutaminase is biochemically and immunochemically distinct from its epidermal counterpart. It has been localized to the inner root sheath and medulla of the hair follicle. The substrate of hair follicle transglutaminase has been poorly defined but appears to be rich in the amino acid citrulline. Transglutaminase has been shown to be an important marker of normal differentiation. There is a rise in its activity at the time of keratinization, and transglutaminase activity has been shown to be greatly decreased in basal cell epithelioma and in psoriasis. Keratinocyte cell culture has proven most helpful in delineating the processes of normal differentiation and keratinization, since the formation of the cell envelope in culture appears to parallel the formation in vivo.

Ornithine decarboxylase may be a multifunctional protein

Ornithine decarboxylase may undergo posttranslational modifications which alter its function. Both transamidation of glutamine residues in the enzyme catalyzed by TGase and phosphorylation of serine and threonine residues catalyzed by a polyamine-stimulated protein kinase have been demonstrated. Data are presented which suggest that these modifications result in translocation of the modified protein to the nucleolus where it regulates the activity of RNA polymerase I to transcribe rDNA, the only active nucleolar genes. Transamidation of specific proteins with primary amines catalyzed by intracellular TGase may be an important posttranslational modification, capable of altering genetic transcription. The rapid half-life of ODC (10-15 min) may be related to rapid posttranslational modification with loss of enzymatic activity rather than to protein degradation.

Isolation of two immunologically related transglutaminase substrates from cultured human keratinocytes

Transglutaminase substrates A and B were identified in soluble extracts of cultured keratinocytes and human epidermis by their reactions with dansyl cadaverine in the presence of Ca++ ion. Substrate B was present in substantial amounts in both extracts whereas A, easily seen in cell extracts, was decreased and sometimes not detected in tissue extracts. Substrates A and B from cultured cells were separated by Sephadex G-75 chromatography and isolated by preparative sodium dodecyl sulfate (SDS) gel electrophoresis by which A had a mol wt of 125,000 and B had a mol wt of 12,000. Amino acid analysis of A, B, and cornified envelope were similar but not identical. The isopeptide bond is not a significant structural feature of A inasmuch as its content is less than 0.25 bonds/molecule. Antibodies raised to A cross-reacted with B and vice versa and A showed partial identity to B when reacted with anti-B. Anti-A reacted with epidermis being adsorbed by the edges of cornifying cells but only weakly by cells of the Malpighian layer. Anti-B also reacted with cornifying, but its reaction was more intense with the cytoplasm of Malpighian cells. Substrate A appears to be incorporated into cornified envelope immediately after its appearance in cells of the granular layer and seems similar to a protein isolated by a different method. Substrate B, convertible by transglutaminase to higher molecular weight species, may also participate in cornified envelope assembly and shares some structural similarities to A.

gamma-Glutamylamine cyclotransferase. An enzyme involved in the catabolism of epsilon-(gamma-glutamyl)lysine and other gamma-glutamylamines

gamma-Glutamylamine cyclotransferase, an enzyme found in a number of animal tissues and cells, catalyzes the conversion of epsilon-(L-gamma-glutamyl)-L-lysine to free lysine and 5-oxo-L-proline as well as the release of free amines and the formation of 5-oxo-L-proline from a variety of other L-gamma-glutamylamines. Among its substrates are both the mon- and di-gamma-glutamyl derivatives of putrescine, spermidine and spermine, and a derivative of epsilon-(L-gamma-glutamyl)-L-lysine in which both the alpha-amino group and the carboxyl group of the lysine moiety are blocked. The enzyme does not act on most gamma-glutamyl-alpha-amino acids, nor is it active toward the epsilon-lysyl derivatives of L-aspartic acid or D-glutamic acid. Derivatives of epsilon-(L-gamma-glutamyl)-L-lysine in which the alpha-amino or the alpha-carboxyl function of the glutamyl moiety is blocked also do not serve as substrates. The specificity of gamma-glutamylamine cyclotransferase is in accordance with the proposal that it functions biologically in the latter stages of the catabolism of products of the action of transglutaminases. Some suggestions as to the manner in which gamma-glutamylamine cyclotransferase serves this function are made based on present knowledge of protein degradation.

Epidermal protein synthesis in newborn rat skin

Protein synthesis in newborn rat epidermis was assessed by in vivo labeling of epidermal proteins with radioactive glycine and the isolation of neutral buffer, sodium-dodecylsulfates (SDS) dithiothreitol soluble proteins, and 0.1 M NaOH soluble protein fractions of the epidermis. There was an increase in the labeling of proteins in the buffer and SDS fractions for 5 h after injection and then no further increase at 19 h after injection. In the 0.1 M NaOH soluble proteins and the final pellet after these extractions there was a progressive increase in glycine incorporation up to 19 h after injection. SDS gel electrophoresis of the labeled proteins showed early synthesis and continued synthesis of proteins of 56,000 and 65,000 daltons corresponding to the main fibrous proteins (alpha-keratins) of the epidermis. At 19 h a new protein band of 45,000 daltons first appeared and was associated with a decrease in the labeling of very high molecular weight (or highly aggregated) proteins. This later protein is presumptively identified as keratohyalin. Free amino acid pools were associated with both the neutral buffer and SDS extractable fractions.