Lipid transfer proteins and instructive regulation of lipid kinase activities: Implications for inositol lipid signaling and disease

The sterol-regulating human ARV1 binds cholesterol and phospholipids through its conserved ARV1 homology domain

Evidence suggests that ARV1 regulates sterol movement within the cell. Saccharomyces cerevisiae cells lacking ScArv1 have defects in sterol trafficking, distribution, and biosynthesis. HepG2 cells treated with hARV1 antisense oligonucleotides accumulate cholesterol in the endoplasmic reticulum. Mice lacking Arv1 have a lean phenotype when fed a high fat diet and show no signs of liver triglyceride or cholesterol accumulation, suggesting a role for Arv1 in lipid transport. Here, we explored the direct lipid-binding activity of recombinant human ARV1 using in vitro lipid-binding assays. ARV1 lipid-binding activity was observed within the first N-terminal 98 amino acids containing the conserved ARV1 homology domain (AHD). The zinc-binding domain and conserved cysteine clusters within the AHD were necessary for lipid binding. Both full-length ARV1 and the AHD bound cholesterol, several phospholipids, and phosphoinositides with high affinity. The AHD showed the highest binding affinity for monophosphorylated phosphoinositides. Several conserved amino acids within the AHD were necessary for phospholipid binding. Biochemical studies suggested that ARV1 exists as a dimer in cells, with oligomerization being critical for ARV1 function, as amino acid mutations predicted to have a negative effect on dimerization caused weakened or complete loss of lipid binding. Our results show for the first time that human ARV1 can directly bind cholesterol and phospholipids. How this activity may function to regulate lipid binding and maintain proper lipid trafficking and/or transport in cells requires further studies.

Expanding functions of the phosphatidylinositol/phosphatidate lipid transporter, PITPNC1 in physiology and in pathology

PITPNC1 was the last of the PITPs to be identified and has been characterized as a binding protein for phosphatidylinositol and phosphatidate. In mammals, PITPNC1 is expressed as two splice variants whilst in zebrafish is expressed from two separate genes. The two splice variants have different expression profiles with the long splice variant having a prominent role in the brain. Several physiological functions have been identified including neuronal and metabolic functions. PITPNC1 also plays a significant role in cancer and has been identified as a risk factor in type 2 diabetes. Here, we review our current understanding of PITPNC1 in cell physiology and pathology.

Reduced expressions of TCA cycle genes during aging in humans and mice

Aging is associated with a decline in physiological functions and an increased risk of metabolic disorders. The liver, a key organ in metabolism, undergoes significant changes during aging that can contribute to systemic metabolic dysfunction. This study investigates the expression of genes involved in the tricarboxylic acid (TCA) cycle, a critical pathway for energy production, in the aging liver. We analyzed RNA sequencing data from the Genotype-Tissue Expression (GTEx) project to assess age-related changes in gene expression in the human liver. To validate our findings, we conducted complementary studies in young and old mice, examining the expression of key TCA cycle genes using quantitative real-time PCR. Our analysis of the GTEx dataset revealed a significant reduction in the expression of many genes that are critical for metabolism, including fat mass and obesity associated (FTO) and adiponectin receptor 1 (ADIPOR1). The most overrepresented pathway among the statistically enriched ones was the TCA cycle, with multiple genes exhibiting downregulation in older humans. This reduction was consistent with findings in aging mice, which also showed decreased expression of several TCA cycle genes. These results suggest a conserved pattern of age-related downregulation of TCA cycle, potentially leading to diminished mitochondrial function and energy production in the liver. The reduced expression of TCA cycle genes in the aging liver may contribute to metabolic dysfunction and increased susceptibility to age-related diseases. Understanding the molecular basis of these changes provides new insights into the aging process and highlights potential targets for interventions aimed at promoting healthy aging and preventing metabolic disorders.

Dopamine internalization via Uptake2 and stimulation of intracellular D5-receptor-dependent calcium mobilization and CDP-diacylglycerol signaling

Dopamine stimulates CDP-diacylglycerol biosynthesis through D1-like receptors, particularly the D5 subtype most of which is intracellularly localized. CDP-diacylglycerol regulates phosphatidylinositol-4,5-bisphosphate-dependent signaling cascades by serving as obligatory substrate for phosphatidylinositol biosynthesis. Here, we used acute and organotypic brain tissues and cultured cells to explore the mechanism by which extracellular dopamine acts to modulate intracellular CDP-diacylglycerol. Dopamine stimulated CDP-diacylglycerol in organotypic and neural cells lacking the presynaptic dopamine transporter, and this action was selectively mimicked by D1-like receptor agonists SKF38393 and SKF83959. Dopaminergic CDP-diacylglycerol stimulation was blocked by decynium-22 which blocks Uptake2-like transporters and by anti-microtubule disrupters of cytoskeletal transport, suggesting transmembrane uptake and guided transport of the ligands to intracellular sites of CDP-diacylglycerol regulation. Fluorescent or radiolabeled dopamine was saturably transported into primary neurons or B35 neuroblastoma cells expressing the plasmamembrane monoamine transporter, PMAT. Microinjection of 10-nM final concentration of dopamine into human D5-receptor-transfected U2-OS cells rapidly and transiently increased cytosolic calcium concentrations by 316%, whereas non-D5-receptor-expressing U2-OS cells showed no response. Given that U2-OS cells natively express PMAT, bath application of 10 μM dopamine slowly increased cytosolic calcium in D5-expressing cells. These observations indicate that dopamine is actively transported by a PMAT-implicated Uptake2-like mechanism into postsynaptic-type dopaminoceptive cells where the monoamine stimulates its intracellular D5-type receptors to mobilize cytosolic calcium and promote CDP-diacylglycerol biosynthesis. This is probably the first demonstration of functional intracellular dopamine receptor coupling in neural tissue, thus challenging the conventional paradigm that postsynaptic dopamine uptake serves merely as a mechanism for deactivating spent or excessive synaptic transmitter.

Restoration of PITPNA in Type 2 diabetic human islets reverses pancreatic beta-cell dysfunction

Defects in insulin processing and granule maturation are linked to pancreatic beta-cell failure during type 2 diabetes (T2D). Phosphatidylinositol transfer protein alpha (PITPNA) stimulates activity of phosphatidylinositol (PtdIns) 4-OH kinase to produce sufficient PtdIns-4-phosphate (PtdIns-4-P) in the trans-Golgi network to promote insulin granule maturation. PITPNA in beta-cells of T2D human subjects is markedly reduced suggesting its depletion accompanies beta-cell dysfunction. Conditional deletion of Pitpna in the beta-cells of Ins-Cre, Pitpnaflox/flox mice leads to hyperglycemia resulting from decreasing glucose-stimulated insulin secretion (GSIS) and reducing pancreatic beta-cell mass. Furthermore, PITPNA silencing in human islets confirms its role in PtdIns-4-P synthesis and leads to impaired insulin granule maturation and docking, GSIS, and proinsulin processing with evidence of ER stress. Restoration of PITPNA in islets of T2D human subjects reverses these beta-cell defects and identify PITPNA as a critical target linked to beta-cell failure in T2D.

MOSPD2 is an endoplasmic reticulum-lipid droplet tether functioning in LD homeostasis

Membrane contact sites between organelles are organized by protein bridges. Among the components of these contacts, the VAP family comprises ER-anchored proteins, such as MOSPD2, that function as major ER-organelle tethers. MOSPD2 distinguishes itself from the other members of the VAP family by the presence of a CRAL-TRIO domain. In this study, we show that MOSPD2 forms ER-lipid droplet (LD) contacts, thanks to its CRAL-TRIO domain. MOSPD2 ensures the attachment of the ER to LDs through a direct protein-membrane interaction. The attachment mechanism involves an amphipathic helix that has an affinity for lipid packing defects present at the surface of LDs. Remarkably, the absence of MOSPD2 markedly disturbs the assembly of lipid droplets. These data show that MOSPD2, in addition to being a general ER receptor for inter-organelle contacts, possesses an additional tethering activity and is specifically implicated in the biology of LDs via its CRAL-TRIO domain.

Roles of PI3K/AKT/mTOR Axis in Arteriovenous Fistula

Renal failure is a worldwide disease with a continuously increasing prevalence and involving a rising need for long-term treatment, mainly by haemodialysis. Arteriovenous fistula (AVF) is the favourite type of vascular access for haemodialysis; however, the lasting success of this therapy depends on its maturation, which is directly influenced by many concomitant processes such as vein wall thickening or inflammation. Understanding the molecular mechanisms that drive AVF maturation and failure can highlight new or combinatorial drugs for more personalized therapy. In this review we analysed the relevance of critical enzymes such as PI3K, AKT and mTOR in processes such as wall thickening remodelling, immune system activation and inflammation reduction. We focused on these enzymes due to their involvement in the modulation of numerous cellular activities such as proliferation, differentiation and motility, and their impairment is related to many diseases such as cancer, metabolic syndrome and neurodegenerative disorders. In addition, these enzymes are highly druggable targets, with several inhibitors already being used in patient treatment for cancer and with encouraging results for AVF. Finally, we delineate how these enzymes may be targeted to control specific aspects of AVF in an effort to propose a more specialized therapy with fewer side effects.

Alleviation of the Adverse Effect of Dietary Carbohydrate by Supplementation of Myo-Inositol to the Diet of Nile Tilapia (Oreochromis niloticus)

This study investigated the effect of dietary myo-inositol (MI) on alleviating the adverse effect of the high carbohydrate diet in Nile tilapia (Oreochromis niloticus). Six diets contained either low carbohydrate (LC 30%) or high carbohydrate (HC 45%) with three levels of MI supplementation (0, 400 and 1200 mg/kg diet) to each level of the carbohydrate diet. After an 8-week trial, the fish fed 400 mg/kg MI under HC levels had the highest weight gain and fatness, but the fish fed 1200 mg/kg MI had the lowest hepatosomatic index, visceral index and crude lipid in the HC group. The diet of 1200 mg/kg MI significantly decreased triglyceride content in the serum and liver compared with those fed the MI supplemented diets regardless of carbohydrate levels. Dietary MI decreased triglyceride accumulation in the liver irrespective of carbohydrate levels. The content of malondialdehyde decreased with increasing dietary MI at both carbohydrate levels. Fish fed 1200 mg/kg MI had the highest glutathione peroxidase, superoxide dismutase, aspartate aminotransferase and glutamic-pyruvic transaminase activities. The HC diet increased the mRNA expression of key genes involved in lipid synthesis (DGAT, SREBP, FAS) in the fish fed the diet without MI supplementation. Dietary MI significantly under expressed fatty acid synthetase in fish fed the HC diets. Moreover, the mRNA expression of genes related to lipid catabolism (CPT, ATGL, PPAR-α) was significantly up-regulated with the increase of dietary MI levels despite dietary carbohydrate levels. The gene expressions of gluconeogenesis, glycolysis and MI biosynthesis were significantly down-regulated, while the expression of the pentose phosphate pathway was up-regulated with the increase of MI levels. This study indicates that HC diets can interrupt normal lipid metabolism and tend to form a fatty liver in fish. Dietary MI supplement can alleviate lipid accumulation in the liver by diverging some glucose metabolism into the pentose phosphate pathway and enhance the antioxidant capacity in O. niloticus.