Cell death in Porifera: molecular players in the game of apoptotic cell death in living fossils

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Abstract
Apoptosis represents the morphological manifestation of programmed cell death and, paradoxically at first sight, it is a prerequisite for metazoan life. Thus, apoptosis is responsible for the demise of cells during many physiological processes. It is also accountable for the death of cells following exposure to countless stimuli. Therefore, it is obvious that apoptosis must be regulated by a complex network of various molecular signaling pathways. Research during the past 20 years has led to the identification of major functional groups of molecules involved in apoptotic pathways. These include members of the Bcl-2 superfamily, members of the TNF family, caspases, and their activators. Yet, the evolutionary conservation of those elements of the apoptotic machinery was only established from nematode to man. Sponges (phylum Porifera) are characterized by a remarkable regeneration capacity and longevity. Furthermore, they represent the phylogenetically oldest still extant metazoan taxon. Thus, research on these living fossils opens a window to the past, to the dawn of metazoan life. It allows us to trace the evolution of programmed cell death and its core components. This review summarizes the key findings and concepts which have emerged from studies of apoptosis in Porifera.