[Role of defective intracellular proteolysis in human degenerative diseases]

Although intracellular protein synthesis has been studied extensively, protein degradation and disposal, know as proteolysis, has been relatively neglected. Modern studies which led two Nobel prizes (de Duve in 1950 and Herschko, Rose and Ciechanover in 1980) established that proteolysis is ensured by two separate but complementary mechanisms: lysosomes responsible for auto and heterophagy and the Ubiquitin-Proteasome System (UPS). The UPS involves ubiquitin, a small molecule consisting of 76 amino acids found in all eukaryotic cells that ensures the identification of the protein to be degraded and its transport to the proteasome, an intracellular complex with enzymes which degrade unneeded or damaged proteins. The proteasome, acting as a composting agent, ensures the enzymatic dissociation of the protein. In this degradation process, as infinite screw, ubiquitin, peptides and amino acids are released and made available for a new cycle. Knowledge of the UPS and its related disorders is continually expanding. Concurrent with lysosomes which work in acidic environment, it is currently known that the UPS provides 80% to 90% of the proteolysis of the short-life proteins and ensures, as chaperon-molecules, the right conformation and hence the correct function of the proteins. The proteolytic activity generates abnormal residues (tau protein, amyloid and related proteins) and various soluble and insoluble wastes. Some are precipitated as inclusion-bodies or aggregosomes, identified years ago by pathologists. These aggregosomes affect almost exclusively long-lived cells (nervous and muscular, macophages). Pigment deposits, such as lipofuscines made by the peroxydation of cell membranes, are the most abundant. Due to their diverse chemical composition, they cannot be empoyed for a scientific classification. Failures of these systems are numerous. They vary not according to the chemical nature of the abnormal protein and wastes but the life span of the targeted cells and the nature of proteolysis. In this article, therefore, the following distinction should be made:--Lysosomal failures. They represent hereditary metabolic disorders involving all categories of cells. They are characterized by the accumulation of homogeneous material related to the underlying disease. Young people are predominantly affected--UPS failures. They represent sporadic conditions principally involving long-lived cells. The accumulated material is heterogeneous, composed of abnormal proteins and various "garbage-like" waste, including pigments. The elderly are predominatly affected, suggesting an epigenetic wear and tear process. Hypothetically, most the sporadic neurodegenerative diseases, from retinal macular degeneration and its associated drüsen to Alzheimer's disease, Parkinson's disease may represent fairly good examples of the UPS deficit.