Lipoprotéine | EPFL Graph Search

Chez les vertébrés, les lipoprotéines sont de grands complexes de protéines et de lipides, hydrosolubles par fonction, qui transportent massivement les lipides dans tout l’organisme. La coque externe est une monocouche de phospholipides contenant du cholestérol libre et une ou plusieurs molécules protéiques appelées apolipoprotéines (par exemple apo-A, Apo-B, etc.) ; la partie centrale contient des triglycérides, des esters de cholestérol et de petites quantités d’autres substances hydrophobes, comme des vitamines liposolubles. Les lipoprotéines sont hydrolysées par la lipoprotéine lipase. Les chylomicrons synthétisés par l'entérocyte et les VLDL, synthétisés par le foie, sont des lipoprotéines. Classification selon leur densité Les différents types de lipoprotéines peuvent être classés du moins dense au plus dense, c'est-à-dire du ratio lipide sur protéine le plus élevé au ratio lipide sur protéine le plus faible (autrement dit, plus le pourcentage de lipide est grand pa

Mitochondrial respiratory chain dysfunction disrupts intracellular cholesterol homeostasis

Christopher Tadhg James Wall

Mitochondrial diseases are rare and severe conditions with debilitating symptoms. Biochemical defects in mitochondria however are common. The difference between these two frequencies is suspected to lie in the capability of the cells to adapt to the homeostatic disbalance. Cells activate retrograde signalling pathways in response to mitochondrial dysfunction, activating cellular transcriptional programs to correct for the disbalance. The full extent of the retrograde signals that are induced by mitochondrial dysfunctions still remains unknown. Studying these retrograde signalling pathways can lead to insights how to make the difference between restoring the homeostasis and a detrimental disease. Investigating the transcriptional responses to artificially induced mitochondrial dysfunction has proven to be lucrative in unveiling novel responses to mitochondrial defects. In this work we studied the transcriptional responses to three chemical models of mitochondrial respiratory chain dysfunction. In line with recently published work, we found a commonly upregulated stress response in all models. Interestingly, shared between all the models was a downregulation of genes enriched in cholesterol biosynthesis pathways. Upon closer examination this encompassed, almost without exception, all transcripts of the mevalonate pathway. This coordinated downregulation of transcripts had functional consequences on the output of the pathway, lowering the abundance of cholesterol precursor metabolites. Other metabolites in more upstream regions of the pathway were changed differently depending on the complex affected, suggesting other regulatory influences involved. Upon closer mechanistic investigation to the cause of the transcriptional suppression we found the main transcription factor responsible for mevalonate pathway gene transcription, the Sterol Response Element Binding Protein 2 (SREBP2), to be inhibited during complex I impairment. This was most evident in sterol depleted conditions where SREBP2 did not activate to normal levels. A decreased activation of SREBP2 is signature for an elevated concentration of ER-cholesterol. Indeed, other ER-bound cholesterol sensitive proteins confirmed this, showing a similar decreased abundance following complex I impairment in both normal and low lipid conditions. To investigate the source of this cholesterol we measured various cholesterol levels. No changes in the levels of total cholesterol (free cholesterol plus esterified cholesterol) were detected. Also, the fraction of free cholesterol was not significantly affected by complex I inhibition. Using the fluorescent cholesterol stain filipin, we were able to measure a modest but reproducible increase in intracellular free cholesterol. This provided novel mechanistic insight into the inhibition of ER sterol sensors and mevalonate pathway activity by mitochondrial dysfunction. Several well-known retrograde signalling pathways AMPK, ISR and mTOR did not explain the changes observed. The exact mechanism through which mitochondria incite an increase in intracellular free cholesterol remains unknown. Taken together this work can provide a mechanistic link between mitochondrial function and intracellular cholesterol homeostasis and could possibly link mitochondrial function to circulating lipoprotein homeostasis. Further work should focus on elucidating the possible mechanism, where differences in complex functions could provide a hint in the right direction.

EPFL2022

Outer membrane lipoprotein NlpI scaffolds peptidoglycan hydrolases within multi-enzyme complexes in Escherichia coli

Ann-Kristin Hov

The peptidoglycan (PG) sacculus provides bacteria with the mechanical strength to maintain cell shape and resist osmotic stress. Enlargement of the mesh-like sacculus requires the combined activity of peptidoglycan synthases and hydrolases. In Escherichia coli, the activity of two PG synthases is driven by lipoproteins anchored in the outer membrane (OM). However, the regulation of PG hydrolases is less well understood, with only regulators for PG amidases having been described. Here, we identify the OM lipoprotein NlpI as a general adaptor protein for PG hydrolases. NlpI binds to different classes of hydrolases and can specifically form complexes with various PG endopeptidases. In addition, NlpI seems to contribute both to PG elongation and division biosynthetic complexes based on its localization and genetic interactions. Consistent with such a role, we reconstitute PG multi-enzyme complexes containing NlpI, the PG synthesis regulator LpoA, its cognate bifunctional synthase, PBP1A, and different endopeptidases. Our results indicate that peptidoglycan regulators and adaptors are part of PG biosynthetic multi-enzyme complexes, regulating and potentially coordinating the spatiotemporal action of PG synthases and hydrolases.

WILEY2020

Post-translational regulation of neuronal differentiation

Jaipreet Singh Loomba

Glycosphingolipids (GSLs) are amphipathic lipid moieties that make up only 3% of the total lipid content of the cell and are almost exclusively expressed at the Plasma Membrane(PM). They are key drivers of signalling hotspots known as âlipid-raftsâ as well as acting as primary signalling molecules themselves. Pathologically, perturbations in their synthesis (Knock-Out mice models) and disorders in their metabolism(Lysosomal storage disorders), both converge on neurological symptoms. In fact, during neuronal development, stem cells have been known to completely remodel their glycosphingolipid profiles, so much so, that the developmental stages of the nervous system have been associated with the expression of different glycosphingolipids. This coordinated change in the expression of glycosphingolipids has been previously attributed to the transcriptional shift in the the expression of genes encoding specific Glycosphingolipid Synthesizing Enzymes (GSEs) observed over neuronal development and, more recently, shown to be internally regulated by the GSLs themselves whereby globosides negatively regulate the expression of the ganglioside synthesizing enzyme GM3S via a neuronal transcription factor AUTS2. In this thesis I show that there exists an extra layer of post-translational regulation that is active over neuronal development that destabilizes the Gb3 synthesizing enzyme, Gb3S, on one hand while stabilizing the GM3S on the other. The regulatory mechanism achieves this effect by doubling the rate of degradation of Gb3S in neuronal cells, probably via membrane trafficking events. The exact details of the stabilization effect of GM3S remains to be determined as the results could not ascertain if it was the same mechanism or a completely different one. My study goes on to show that this regulatory mechanism is activated early during development, preceding the changes in canonical stem and neuronal markers. Morphologically, the study points towards a post-compaction but pre-cavitation (E2.5-E3.5) point of activation for the mechanism of Gb3S destabalization, relative to early mouse embryological development. Furthermore, I found that the Gb3S interactor and AUTS2 transcriptional target UCHL1 is a key regulatory unit of the machinery such that inhibiting its activity results in a specific rescue of the Gb3S enzyme.

EPFL2021