Lysosomal metabolism of lipids

Role of Phospholipids in Endocytosis, Phagocytosis, and Macropinocytosis

Endocytosis, phagocytosis, and macropinocytosis are fundamental processes that enable cells to sample their environment, eliminate pathogens and apoptotic bodies, and regulate the expression of surface components. While a great deal of effort has been devoted over many years to understanding the proteins involved in these processes, the important contribution of phospholipids has only recently been appreciated. This review is an attempt to collate and analyze the rapidly emerging evidence documenting the role of phospholipids in clathrin-mediated endocytosis, phagocytosis, and macropinocytosis. A primer on phospholipid biosynthesis, catabolism, subcellular distribution, and transport is presented initially, for reference, together with general considerations of the effects of phospholipids on membrane curvature and charge. This is followed by a detailed analysis of the critical functions of phospholipids in the internalization processes and in the maturation of the resulting vesicles and vacuoles as they progress along the endo-lysosomal pathway.

Phase coexistence in a triolein-phosphatidylcholine system. Implications for lysosomal membrane properties

The effects of tri- and monoglycerides on phospholipid (POPC) membranes were studied using spectroscopical methods. Triolein was found to form two types of POPC-rich membranes, both with POPC or as a three-component system with monopalmitin. These two membrane types were determined as co-existing phases based on their spontaneous and stable separation and named heavy and light phase according to their sedimentation behaviour. Marked differences were seen in the physical properties of these phases, even though only minor compositional variation was detected. The light, less polar phase was found to be less ordered and more fluid and seemed to allow significantly lower amount of water penetration into the membrane-water interface than pure POPC membrane. The heavy phase, apart from their slightly altered water penetration, resembled more a pure POPC membrane. As triglycerides are present in lysosomal membranes, the present results can be seen as an implication for polarity-based water permeability barrier possibly contributing to the integrity of lysosomes.

Screening for the drug-phospholipid interaction: correlation to phospholipidosis

Phospholipid bilayers represent a complex, anisotropic environment fundamentally different from bulk oil or octanol, for instance. Even "simple" drug association to phospholipid bilayers can only be fully understood if the slab-of-hydrocarbon approach is abandoned and the complex, anisotropic properties of lipid bilayers reflecting the chemical structures and organization of the constituent phospholipids are considered. The interactions of drugs with phospholipids are important in various processes, such as drug absorption, tissue distribution, and subcellular distribution. In addition, drug-lipid interactions may lead to changes in lipid-dependent protein activities, and further, to functional and morphological changes in cells, a prominent example being the phospholipidosis (PLD) induced by cationic amphiphilic drugs. Herein we briefly review drug-lipid interactions in general and the significance of these interactions in PLD in particular. We also focus on a potential causal connection between drug-induced PLD and steatohepatitis, which is induced by some cationic amphiphilic drugs.

Alzheimer's, atherosclerosis, and aggregates: a role for bacterial degradation

Several of the most prevalent and severe age-related diseases, notably Alzheimer's disease and atherosclerosis, feature the accumulation of non-degradable aggregates within the lysosomes of disease-affected cells. At an early point in disease progression, the breakdown of lysosomal contents by the resident catabolic enzymes stops working properly. A return of lysosomal enzymatic activity to pre-disease levels may restore aggregate elimination. In this review, a method of bioremediation-derived lysosomal enzyme enhancement is proposed, featuring the cellular introduction of microbial-isolated enzymes, or xenoenzymes. The benefits and challenges of using xenoenzymes to break down aggregates are discussed. As the size of our elderly population grows, the incidence of age-related diseases will increase, necessitating the exploration of radical, but potentially powerful, therapeutic strategies.

Engineering away lysosomal junk: medical bioremediation

Atherosclerosis, macular degeneration, and neurodegenerative diseases such as Alzheimer's disease, are associated with the intracellular accumulation of substances that impair cellular function and viability. Reversing this accumulation may be a valuable therapy, but the accumulating substances resist normal cellular catabolism. On the other hand, these substances are naturally degraded in the soil and water by microorganisms. Thus, we propose the concept of "medical bioremediation," which derives from the successful field of in situ environmental bioremediation of petroleum hydrocarbons. In environmental bioremediation, communities of microorganisms mineralize hydrophobic organics using a series of enzymes. In medical bioremediation, we hope to utilize one or several microbial enzymes to degrade the intracellular accumulators enough that they can be cleared from the affected cells. Here, we present preliminary, but promising results for the bacterial biodegradation of 7-ketocholesterol, the main accumulator of foam cells associated with atherosclerosis. In particular, we report on the isolation of several Nocardia strains able to biodegrade 7-ketocholesterol and as an ester of 7-ketocholoesterol. We also outline key intermediates in the biodegradation pathway, a key step towards identifying the key enzymes that may lead to a therapy.

Medical bioremediation: prospects for the application of microbial catabolic diversity to aging and several major age-related diseases

Several major diseases of old age, including atherosclerosis, macular degeneration and neurodegenerative diseases are associated with the intracellular accumulation of substances that impair cellular function and viability. Moreover, the accumulation of lipofuscin, a substance that may have similarly deleterious effects, is one of the most universal markers of aging in postmitotic cells. Reversing this accumulation may thus be valuable, but has proven challenging, doubtless because substances resistant to cellular catabolism are inherently hard to degrade. We suggest a radically new approach: augmenting humans' natural catabolic machinery with microbial enzymes. Many recalcitrant organic molecules are naturally degraded in the soil. Since the soil in certain environments - graveyards, for example - is enriched in human remains but does not accumulate these substances, it presumably harbours microbes that degrade them. The enzymes responsible could be identified and engineered to metabolise these substances in vivo. Here, we survey a range of such substances, their putative roles in age-related diseases and the possible benefits of their removal. We discuss how microbes capable of degrading them can be isolated, characterised and their relevant enzymes engineered for this purpose and ways to avoid potential side-effects.

Modulation of endosomal cholesteryl ester metabolism by membrane cholesterol

Cells acquire cholesterol in part by endocytosis of cholesteryl ester containing lipoproteins. In endosomes and lysosomes cholesteryl ester is hydrolyzed by acidic cholesteryl ester hydrolase producing cholesterol and fatty acids. Under certain pathological conditions, however, such as in atherosclerosis, excessive levels of cholesteryl ester accumulate in lysosomes for reasons that are poorly understood. Here, we have studied endosomal and lysosomal cholesteryl ester metabolism in cultured mouse macrophages and with cell-free extracts. We show that net hydrolysis of cholesteryl ester is coupled to the transfer of cholesterol to membranes. When membrane cholesterol levels are low, absorption of cholesterol effectively drives cholesteryl ester hydrolysis. When cholesterol levels in acceptor membranes approach saturation or when cholesterol export is blocked, cholesterol is re-esterified in endosomes. These results reveal a new facet of cellular cholesterol homeostasis and provide a potential explanation for cholesteryl ester accumulation in lysosomes of atherosclerotic cells.

Lysosome membrane permeability to anions

The permeability of rat liver lysosomes to some inorganic and aliphatic organic anions was investigated, using an osmotic-protection methodology. Lysosomes were incubated at 25 degreesC in 250 mOsm solutions of potassium salts of the anions, in the presence of valinomycin, and the latency of lysosomal hexosaminidase measured at intervals. Lysosomes suspended in 250 mM sucrose at 25 degreesC were stable for up to 4 h. When suspended in 250 mOsm solutions of potassium salts of inorganic acids, latency was lost at rates indicating anion permeance decreasing in the order thiocyanate, nitrate and iodide>bromide>chloride>sulfate. This rank order does not correspond with the anion selectivity of any known anion transporter, and is closer to that of the lyotropic series. Results with the potassium salts of aliphatic organic acids indicate little correlation between permeation and hydrocarbon chain length, although formate was more rapidly permeant than acetate and its higher homologs. By contrast, oxalate was less permeable than other dicarboxylic acids. The presence of one or more hydroxy groups decreased permeance. A correlation between permeance and the acid's lowest pKa suggested that penetration was due principally to the entry of the undissociated acid, but there is evidence that the (much more abundant) singly charged anionic form is also significantly permeant.