Towards an understanding of the molecular basis of immune responses in sponges: The marine demosponge Geodia cydonium as a model

The phylogenetic position of the phylum Porifera (sponges) is near the base of the kingdom Metazoa. During the last few years, not only rRNA sequences but, more importantly, cDNA/genes that code for proteins have been isolate d and characterized from sponges, in particular from the marine demosponge Geodia cydonium. The analysis of the deduced amino acid sequences of these proteins allowed a molecular biological approach to the question of the mon ophyly of the Metazoa. Molecules of the extracellular matrix/basal lamina, with the integrin receptor, fibronectin, and galectin as prominent examples , and of cell-surface receptors (tyrosine kinase receptor), elements of sen sory systems (crystallin, metabotropic glutamate receptor) as well as homol ogs/modules of an immune system (immunoglobulin-like molecules, scavenger r eceptor cysteine-rich [SRCR]- and short consensus repeats [SCR]-repeats), c lassify the Porifera as true Metazoa. As living fossils, provided with simp le, primordial molecules allowing cell-cell and cell-matrix adhesion as wel l as processes of signal transduction as known in a more complex manner fro m higher Metazoa, sponges also show pecularities not known in later phyla. In this paper, the adhesion molecules presumably involved in the sponge imm une system are reviewed; these are the basic adhesion molecules (galectin, integrin, fibronectin, and collagen) and especially the highly polymorphic adhesion molecules, the receptor tyrosine kinase as well as the polypeptide s comprising scavenger receptor cysteine-rich (SRCR) and short consensus re peats (SCR) modules. In addition, it is reported that in the model sponge s ystem of G. cydonium, allogeneic rejection involves an upregulation of phen ylalanine hydroxylase, an enzyme initiating the pathway to melanin synthesi s. Microsc. Res. Tech. 44:218-236, 1999. (C) 1999 Wiley-Liss, Inc.