Annexins are Ca2+-regulated membrane binding proteins implicated in a wide range of membrane-related and signal transduction events, including the endocytosis of membrane receptors and Ca2+-regulated as well as constitutive secretion. To date, 10 unique members of this multigene family have been identified in a variety of cell types and tissues of higher vertebrates, with different members showing distinct tissue distributions in the adult organisms. To establish whether annexins also function in embryonic development, we analyzed the expression pattern during vertebrate morphogenesis using the medaka fishOryzias latipesas a model system. From a larval medaka cDNA library, we isolated four types of clones, which were shown by sequence analysis to encode four different annexins (herein referred to as max 1-4). A comparison with known annexin sequences in the databases revealed that two medaka annexins (max 1 and 2) are highly similar in sequence to mammalian annexins V and IV, respectively, whereas the other two medaka annexins (max 3 and 4) are probably novel members of the family most closely related to mammalian annexins I and XI. Using whole-mount RNAin situhybridization, we showed that the expression of the different medaka annexins during embryogenesis was strictly regulated at both the spatial and the temporal level. High levels of max 1, 2, and 3 transcripts were present in the developing stomach, gut, liver, air-bladder, and rectum during somitogenesis, thus identifying the digestive tract as the prime region of annexin expression. Interestingly, two structures playing crucial roles in neuronal patterning showed a distinct expression of annexins. The mesendoderm of the anterior prechordal plate of neurula-stage embryos was a site of max 4 transcription, and the floor plate of somitogenesis-stage embryos showed expression of max 2 and 3 to differing rostrocaudal extends along the brain and spinal cord. These results suggest specific functions of different annexins during vertebrate morphogenesis.
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