ENAMEL-LIKE ANTIGENS IN HAGFISH: POSSIBLE EVOLUTIONARY SIGNIFICANCE.

Abstract

The enamel proteins are a family of acidic glycoproteins which represent two groups of polypeptides: (i) amelogenins of approximately 10-30 kd (kilodaltons) which represent 80% of the total enamel protein in most mammalian species and (ii) enamelins of approximately 40-72 kd which represent -5-20% of total protein (Termine et al., 1980; Zeichner-David et al., in press). During mammalian tooth development, the inner enamel epithelial cells of the enamel organ differentiate into ameloblasts which form the extracellular matrix (see extensive reviews by Poole, 1967, 1971; Slavkin, 1974; Slavkin et al., 1981). Comparable features of morphogenesis and differentiation have been described for tooth development in Chondrichthyes (Moss, 1968, 1977) as well as in Teleostei (Shellis, 1975, 1978). It is generally agreed that during terminal epithelial differentiation of ameloblasts, enamel proteins are synthesized and secreted into the enamel matrix (Orvig, 1967; Poole, 1967; Peyer, 1968; Slavkin et al., 1981). To determine the function and structural roles of enamel proteins during vertebrate tooth development, various biochemical comparisons of the enamel polypeptides have been made. For the most part, these comparisons have emphasized the similarities amongst enamel acidic glycoproteins. Partial amino acid sequences of several different mammalian amelogenins indicate remarkable homology in primary structure when comparing 4-5 kd (kilodaltons) polypeptides with 25-30 kd amelogenins (Fukae et al., 1980; Fincham et al., in press). Comparative data on the amino acid compositions and sizes of the principal amelogenins of the developing enamel of cow, hamster, human, pig and sheep show homology (Fincham et al., 1982). In contrast to the similarities reported for amelogenins in various mammals, very little is known about enamelins in different mammalian species (Termine etal., 1980). We have begun to investigate the developmental process of enamel formation in a number of different vertebrates through the use of indirect immunofluorescent microscopy and biochemistry. Several recent studies have reported that rabbit antibodies raised against several different sources of mammalian amelogenins are antigenically cross-reactive with the enamel matrices of lower and higher vertebrates including Chondrichthyes, Teleostei, Amphibia, Reptilia and Mammalia (Graver et al., 1978; Graham et al., 1980; Herold et al., 1980; Slavkin et al., 1982). We now show that rabbit antimouse amelogenin antisera are antigenically cross-reactive with Pacific hagfish (Eptatretus stoutii). The cyclostomes comprise lampreys and hagfish (or myxinoids), which are eel-like aquatic vertebrates, considered as the remaining survivors of a nearly extinct group of agnathan or jawless Paleozoic fish known as ostracoderms (Stensio, 1968; Hardisty and Potter, 1971; Hardisty, 1978). Further, cyclostomes possess a cartilaginous skeleton which is not mineralized (Hardisty, 1978). Therefore, the presence of enamel-like antigens in the outer covering of hagfish teeth, and the presence of a relatively high molecular weight class of enamel-like Droteins, suggests remark-