Background
Annexins form a family of structurally related proteins, which bind to negatively charged phospholipids in a Ca2+-dependent manner [1]. They are characterized by a bipartite structure with a conserved C-terminal core domain and a unique N-terminal domain that varies in length and amino acid composition. The C-terminal domain is formed by either a four- or eightfold repeat of approximately 70 amino acids, each repeat carrying a Ca2+-binding site, and is responsible for phospholipid binding. The N-terminal regions are thought to confer functional diversity to the annexin proteins [2]. The biochemical features in vitro were analyzed extensively, but the in vivo functions of annexins remain unclear.
Annexin A7, the first annexin to be described, was isolated as the agent that mediated aggregation of chromaffin granules and fusion of membranes and phospholipids in the presence of Ca2+-ions [3]. Expression studies demonstrated the distribution of Annexin A7 in a wide variety of tissues and cells mainly enriched in the cytosol in close association with membranous structures, but it was also described in the nucleus of adrenal chromaffin cells [4]. The presence of an alternatively spliced cassette exon gives rise to two Annexin A7 isoforms corresponding in molecular mass to 47 kDa and 51 kDa. The isoforms differ in their N-terminal domain and exhibit a tissue-specific expression pattern. The 47 kDa isoform is present in all tissues except for skeletal muscle, where the 51 kDa isoform is exclusively present. Heart muscle, brain tissue and red blood cells contain both isoforms [5-8]. Previous studies indicated that the subcellular localization of Annexin A7 changes during myoblast differentiation. In undifferentiated cells the protein is equally distributed between cytosol and membrane fractions while in differentiated cells it is exclusively present in the membrane fraction [7]. Reports by Clemen et al. [9] and Herr et al. [8,10] demonstrated roles for Annexin A7 in shape and osmotic resistance of red blood cells, platelet aggregation velocity, and in the velocity of spreading astrocytic Ca2+-waves. Annexin A7 is also involved in the maintenance of regular cardiac electrophysiology and Ca2+-homeostasis [Schrickel et al., submitted]. Detailed reports on appearance and distribution of Annexin A7 during brain development are not available. In the present study we focus on the distribution of Annexin A7 in the developing brain of mice embryos between E5 and E16, and in the adult mouse brain.