Abstract
Lysosomes are acidic cytoplasmic organelles that are present in all nucleated mammalian cells and are involved in a variety of cellular processes including repair of the plasma membrane, defense against pathogens, cholesterol homeostasis, bone remodeling, metabolism, apoptosis and cell signaling. Defects in lysosomal enzyme activity have been associated with a variety of neurological diseases including Parkinson’s Disease, Lysosomal Storage Diseases, Alzheimer's disease and Huntington's disease. Fluorogenic lysosomal staining probes were synthesized for labeling lysosomes and other acidic organelles in a live-cell format and were shown to be capable of monitoring lysosomal metabolic activity. The new targeted substrates were prepared from fluorescent dyes having a low pKa value for optimum fluorescence at the lower physiological pH found in lysosomes. They were modified to contain targeting groups to direct their accumulation in lysosomes as well as enzyme-cleavable functions for monitoring specific enzyme activities using a live-cell staining format. Application to the staining of cells derived from blood and skin samples of patients with Metachromatic Leukodystrophy, Krabbe and Gaucher Diseases as well as healthy human fibroblast and leukocyte control cells exhibited localization to the lysosome when compared with known lysosomal stain LysoTracker® Red DND-99 as well as with anti-LAMP1 Antibody staining. When cell metabolism was inhibited with chloroquine, staining with an esterase substrate was reduced, demonstrating that the substrates can be used to measure cell metabolism. When applied to diseased cells, the intensity of staining was reflective of lysosomal enzyme levels found in diseased cells. Substrates specific to the enzyme deficiencies in Gaucher or Krabbe disease patient cell lines exhibited reduced staining compared to that in non-diseased cells. The new lysosome-targeted fluorogenic substrates should be useful for research, diagnostics and monitoring the effect of secondary therapeutic agents on lysosomal enzyme activity in drug development for the lysosomal storage disorders and allied diseases.
Highlights
Lysosomes are acidic cytoplasmic organelles that are present in all nucleated mammalian cells
Lysosomes are involved in metabolism and catabolism of foreign molecules that are brought into the cell by endocytosis, acting as a first line of defense against foreign bacterial or viral infection
These fluorogenic substrates contain a weakly basic targeting group, similar to that used with other lysosomal stains such as the LysoTracker1 dyes, that acts to sequester their localization to this organelle
Summary
Lysosomes are acidic cytoplasmic organelles that are present in all nucleated mammalian cells. More than 50 acidic hydrolytic enzymes have been identified that are involved in ordered lysosomal degradation of proteins, lipids, carbohydrates and nucleic acids. Functional deficiencies in these lysosomal enzymes are indicative of a number of disease states. Acidic organelles have been shown to be responsible for digestion of high molecular weight proteins, oligosaccharides, glycolipids or peptides by the cell. They are often involved in therapeutic drug metabolism
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