Autophagosome and phagosome.

Autophagy and phagocytosis are evolutionarily ancient processes functioning in capture and digestion of material found in the cellular interior and exterior, respectively. In their most primordial form, both processes are involved in cellular metabolism and feeding, supplying cells with externally obtained particulate nutrients or using portions of cell's own cytoplasm to generate essential nutrients and energy at times of starvation. Although autophagy and phagocytosis are commonly treated as completely separate biological phenomena, they are topologically similar and can be, at least morphologically, viewed as different manifestations of a spectrum of related processes. Autophagy is the process of sequestering portions of cellular interior (cytosol and intracellular organelles) into a membranous organelle (autophagosome), whereas phagocystosis is its topological equivalent engaged in sequestering cellular exterior. Both autophagosomes and phagosomes mature into acidified, degradative organelles, termed autolysosomes and phagolysosomes, respectively. The basic role of autophagy as a nutritional process, and that of phagocytosis where applicable, has survived in present-day organisms ranging from yeast to man. It has in addition evolved into a variety of specialized processes in metazoans, with a major role in cellular/cytoplasmic homeostasis. In humans, autophagy has been implicated in many health and disease states, including cancer, neurodegeneration, aging and immunity, while phagocytosis plays a role in immunity and tissue homeostasis. Autophagy and phagocytosis cooperate in the latter two processes. In this chapter, we briefly review the regulatory and execution stages of both autophagy and phagocytosis.