Therapeutic Strategies for Lysosomal Storage Diseases by Targeting Glial Cells

The Lysosomal System: Functions And Uses

The lysosome is an organelle which can be spherical, ovoid, or sometimes tubular, and having a variable size between 0.1 and 2 μm, present in most plant and animal cells (Appelqvist et al., 2013). In humans, lysosomes are present in around a hundred copies in all cells of the body, except in red blood cells. Two distinct classes of proteins are present at the lysosome: a) acid hydrolases, approximately 50 in number, activated by the acidic pH between 4.5 and 5 at the lumen of the lysosome, b) membrane proteins, which are 25 in number (Mindell, 2012). These enzymes and proteins are all essential for the lysosome to perform its functions. These consist in particular of degrading macromolecules, polysaccharides, glycosaminoglycans and complex lipids into their elementary molecules respectively: amino acids, monosaccharides and free fatty acids. Lysosomal degradation products are transported outside the lysosome via specific transporters located in the lysosomal membrane (Saftig & Klumperman, 2009), or via vesicular membrane trafficking for reuse in biosynthetic pathways (Ruivo et al., 2009). (Figure 1).