Lysosomes are cytoplasmic organelles delimited by a single membrane and filled with a variety of hydrolytic enzymes active at acidic pH and collectively capable to degrade the vast majority of macromolecules entering lysosomes via endocytosis, phagocytosis or autophagy. In this review, we describe the lipid composition and the dynamic properties of lysosomal membrane, the main delivery pathways of lipids to lysosomes and their catabolism inside lysosomes. Then, we present the consequences of a lipid accumulation as seen in various lysosomal storage diseases on lysosomal functions. Finally, we discuss about the possible involvement of lysosomes in lipotoxicity.

NCLs (neuronal ceroid lipofuscinoses) form a group of eight inherited autosomal recessive diseases characterized by the intralysosomal accumulation of autofluorescent pigments, called ceroids. Recent data suggest that the pathogenesis of NCL is associated with the appearance of fragmented mitochondria with altered functions. However, even if an impairement in the autophagic pathway has often been evoked, the molecular mechanisms leading to mitochondrial fragmentation in response to a lysosomal dysfunction are still poorly understood. In this study, we show that fibroblasts that are deficient for the TPP-1 (tripeptidyl peptidase-1), a lysosomal hydrolase encoded by the gene mutated in the LINCL (late infantile NCL, CLN2 form) also exhibit a fragmented mitochondrial network. This morphological alteration is accompanied by an increase in the expression of the protein BNIP3 (Bcl2/adenovirus E1B 19 kDa interacting protein 3) as well as a decrease in the abundance of mitofusins 1 and 2, two proteins involved in mitochondrial fusion. Using RNAi (RNA interference) and quantitative analysis of the mitochondrial morphology, we show that the inhibition of BNIP3 expression does not result in an increase in the reticulation of the mitochondrial population in LINCL cells. However, this protein seems to play a key role in cell response to mitochondrial oxidative stress as it sensitizes mitochondria to antimycin A-induced fragmentation. To our knowledge, our results bring the first evidence of a mechanism that links TPP-1 deficiency and oxidative stress-induced changes in mitochondrial morphology.

The accumulation of pinocytosed sucrose in lysosomes constitutes a convenient means to mimic a lysosomal storage and to study how such storage impacts the biology of the organelle and beyond. Several reports have shown that lysosomal storages can perturb the redistribution of lysosomal lipids, cholesterol in particular. Our goal was to analyse the effect of the intralysosomal accumulation of sucrose on the transcription factors Sterol Regulatory Element Binding Proteins (SREBPs): key actors of the regulation of lipid metabolism. We show that 143B cells grown in the presence of sucrose present a modified distribution of non-esterified cholesterol, which is associated with an increase in the activity of SREBPs. The activation of these transcription factors is associated with an increase in the expression of some of their target genes: HMG-CoA synthase and mevalonate kinase, and with an increase in total lipid biosynthesis. Finally, using siRNA interference we demonstrate that SREBP-2 but not SREBP-1a is responsible for the increase in the expression of the genes encoding these two enzymes.