Differential Localization of Distinct Keratin mRNA-Species in Mouse Tongue Epithelium by In Situ Hybridization with Specific cDNA Probes

The tongue of the adult mouse is covered by a multilayered squamous epithelium which is continuous on the ventral surface, however interrupted on the dorsal surface by many filiform and few fungiform papillae. The filiform papillae themselves are subdivided into an anterior and posterior unit exhibiting different forms of keratinization. Thus, the entire epithelium shows a pronounced morphological diversity of well recognizable tissue units. We have used a highly sensitive in situ hybridization technique to investigate the differential expression of keratin mRNAs in the tongue epithelium. The hybridization probes used were cDNA restriction fragments complementary to the most specific 3'-regions of any given keratin mRNA. We could show that independent of the morphologically different tongue regions, all basal cells uniformly express the mRNA of a type I 52-kD keratin, typical also for basal cells of the epidermis. Immediately above the homogenous basal layer a vertically oriented specialization of the keratin expression occurs within the morphological tissue units. Thus the dorsal interpapillary and ventral epithelium express the mRNAs of a type II 57-kD and a type 1 47-kD keratin pair. In contrast, in the anterior unit of the filiform papillae, only the 47-kD mRNA is present, indicating that this keratin may be coexpressed in tongue epithelium with different type II partners. In suprabasal cells of both, the fungiform papillae and the posterior unit of the filiform papillae, a mRNA of a type I 59-kD keratin could be detected; however, its type II 67-kD epidermal counterpart seems not to be present in these cells. Most surprisingly, in distinct cells of both types of papillae, a type I 50-kD keratin mRNA could be localized which usually is associated with epidermal hyperproliferation. In conclusion, the in situ hybridization technique applied has been proved to be a powerful method for detailed studies of differentiation processes, especially in morphologically complex epithelia. ERATINS comprise a group of probably more than 20 related proteins that are responsible for the formation of intermediate filaments in epithelial cells. According to their charge properties the proteins can be subdivided into two distinct subclasses and one distinguishes between the basic to neutral type II keratins and the acidic type I keratins. As a rule, the complexity of a keratin pattern increases from simple to stratified epithelia, being most complex in terminally differentiating epithelia. In the epidermis, for example, the keratin expression is compartmentalized. Developmental studies (2, 19), investigations with antibodies to keratins (1, 22, 25), and analysis of sectioned epidermis (6) or epidermal cell fractions (22, 23) have shown that basal epidermal cells express a coordinate set of at least one basic and one acidic subunit which is different from the keratin pair of the living suprabasal compartment. The complexity is furthermore increased by the formation of posttranslationally derived stratum corneum equivalents mainly of the suprabasal keratin subunits (2, 3, 19, 22). Investigations on the ordered keratin expression in stratified epithelia are difficult in cases where a continuous epithelium displays either regular, or more or less focal, morphological variations. This is the case in the adult mouse tail epidermis which exhibits an alternating pattern of paraand orthokeratotic subregions (18). By far the most heterogenous epithelium is that of the adult mouse tongue, which is basically covered by a multilayered squamous epithelium. On the dorsal surface, however, it is interrupted by a multitude of papillary projections which themselves seem to be morphologically subdivided (4, 10). In the present study we have investigated the keratin expression in the tongue epithelium by means of in situ hybridization with various, highly specific keratin cDNAs. We have recently improved the experimental conditions of this technique (17; Knapp, B., M. Rentrop, J. Schweizer, and H. Winter, manuscript submitted for publication) and show here that especially in morphologically complex epithelia it represents an excellent tool for exact studies of differentiation processes. Parts of this investigation have been presented at the 5th © The Rockefeller University Press, 0021-9525/86/12/2583/9 $1.00 The Journal of Cell Biology, Volume 103 (No. 6, Pt. 2), Dec. 1986 2583-2591 2583 on S etem er 3, 2017 jcb.rress.org D ow nladed fom Joint Meeting of the European Society for Dermatological Research and the Society for Investigative Dermatology, Geneva, 22-25 June 1986. Materials and Methods Dissection of Tongue About 7-wk-old NMRl-mice were killed by cervical dislocation. The tongues were cut offat the apical region of the sulcus terminalis, so that the removed portion comprised the freely movable anterior two-thirds of the organ (4).