Non-vesicular phosphatidylinositol transfer plays critical roles in defining organelle lipid composition.

Phosphatidylinositol (PI) is the precursor lipid for the minor phosphoinositides (PPIns), which are critical for multiple functions in all eukaryotic cells. It is poorly understood how phosphatidylinositol, which is synthesized in the ER, reaches those membranes where PPIns are formed. Here, we used VT01454, a recently identified inhibitor of class I PI transfer proteins (PITPs), to unravel their roles in lipid metabolism, and solved the structure of inhibitor-bound PITPNA to gain insight into the mode of inhibition. We found that class I PITPs not only distribute PI for PPIns production in various organelles such as the plasma membrane (PM) and late endosomes/lysosomes, but that their inhibition also significantly reduced the levels of phosphatidylserine, di- and triacylglycerols, and other lipids, and caused prominent increases in phosphatidic acid. While VT01454 did not inhibit Golgi PI4P formation nor reduce resting PM PI(4,5)P2 levels, the recovery of the PM pool of PI(4,5)P2 after receptor-mediated hydrolysis required both class I and class II PITPs. Overall, these studies show that class I PITPs differentially regulate phosphoinositide pools and affect the overall cellular lipid landscape.

Messenger RNA transport on lysosomal vesicles maintains axonal mitochondrial homeostasis and prevents axonal degeneration.

In neurons, RNA granules are transported along the axon for local translation away from the soma. Recent studies indicate that some of this transport involves hitchhiking of RNA granules on lysosome-related vesicles. In the present study, we leveraged the ability to prevent transport of these vesicles into the axon by knockout of the lysosome-kinesin adaptor BLOC-one-related complex (BORC) to identify a subset of axonal mRNAs that depend on lysosome-related vesicles for transport. We found that BORC knockout causes depletion of a large group of axonal mRNAs mainly encoding ribosomal and mitochondrial/oxidative phosphorylation proteins. This depletion results in mitochondrial defects and eventually leads to axonal degeneration in human induced pluripotent stem cell (iPSC)-derived and mouse neurons. Pathway analyses of the depleted mRNAs revealed a mechanistic connection of BORC deficiency with common neurodegenerative disorders. These results demonstrate that mRNA transport on lysosome-related vesicles is critical for the maintenance of axonal homeostasis and that its failure causes axonal degeneration.

Loss of Ezh2 in the medial ganglionic eminence alters interneuron fate, cell morphology and gene expression profiles.

Introduction: Enhancer of zeste homolog 2 (Ezh2) is responsible for trimethylation of histone 3 at lysine 27 (H3K27me3), resulting in repression of gene expression. Here, we explore the role of Ezh2 in forebrain GABAergic interneuron development. Methods: We removed Ezh2 in the MGE by generating Nkx2-1Cre;Ezh2 conditional knockout mice. We then characterized changes in MGE-derived interneuron fate and electrophysiological properties in juvenile mice, as well as alterations in gene expression, chromatin accessibility and histone modifications in the MGE. Results: Loss of Ezh2 increases somatostatin-expressing (SST+) and decreases parvalbumin-expressing (PV+) interneurons in the forebrain. We observe fewer MGE-derived interneurons in the first postnatal week, indicating reduced interneuron production. Intrinsic electrophysiological properties in SST+ and PV+ interneurons are normal, but PV+ interneurons display increased axonal complexity in Ezh2 mutant mice. Single nuclei multiome analysis revealed differential gene expression patterns in the embryonic MGE that are predictive of these cell fate changes. Lastly, CUT&Tag analysis revealed that some genomic loci are particularly resistant or susceptible to shifts in H3K27me3 levels in the absence of Ezh2, indicating differential selectivity to epigenetic perturbation. Discussion: Thus, loss of Ezh2 in the MGE alters interneuron fate, morphology, and gene expression and regulation. These findings have important implications for both normal development and potentially in disease etiologies.

Metabolic routing maintains the unique fatty acid composition of phosphoinositides.

Phosphoinositide lipids (PPIn) are enriched in stearic- and arachidonic acids (38:4) but how this enrichment is established and maintained during phospholipase C (PLC) activation is unknown. Here we show that the metabolic fate of newly synthesized phosphatidic acid (PA), the lipid precursor of phosphatidylinositol (PI), is influenced by the fatty acyl-CoA used with preferential routing of the arachidonoyl-enriched species toward PI synthesis. Furthermore, during agonist stimulation the unsaturated forms of PI(4,5P)2 are replenished significantly faster than the more saturated ones, suggesting a favored recycling of the unsaturated forms of the PLC-generated hydrolytic products. Cytidine diphosphate diacylglycerol synthase 2 (CDS2) but not CDS1 was found to contribute to increased PI resynthesis during PLC activation. Lastly, while the lipid transfer protein, Nir2 is found to contribute to rapid PPIn resynthesis during PLC activation, the faster re-synthesis of the 38:4 species does not depend on Nir2. Therefore, the fatty acid side-chain composition of the lipid precursors used for PI synthesis is an important determinant of their metabolic fates, which also contributes to the maintenance of the unique fatty acid profile of PPIn lipids.

ORP3 phosphorylation regulates phosphatidylinositol 4-phosphate and Ca2+ dynamics at plasma membrane-ER contact sites.

Oxysterol-binding protein (OSBP)-related proteins (ORPs) mediate non-vesicular lipid transfer between intracellular membranes. Phosphoinositide (PI) gradients play important roles in the ability of OSBP and some ORPs to transfer cholesterol and phosphatidylserine between the endoplasmic reticulum (ER) and other organelle membranes. Here, we show that plasma membrane (PM) association of ORP3 (also known as OSBPL3), a poorly characterized ORP family member, is triggered by protein kinase C (PKC) activation, especially when combined with Ca2+ increases, and is determined by both PI(4,5)P2 and PI4P After activation, ORP3 efficiently extracts PI4P and to a lesser extent phosphatidic acid from the PM, and slightly increases PM cholesterol levels. Full activation of ORP3 resulted in decreased PM PI4P levels and inhibited Ca2+ entry via the store-operated Ca2+ entry pathway. The C-terminal region of ORP3 that follows the strictly defined lipid transfer domain was found to be critical for the proper localization and function of the protein.

Monitoring Non-vesicular Transport of Phosphatidylserine and Phosphatidylinositol 4-Phosphate in Intact Cells by BRET Analysis.

Non-vesicular lipid transport via lipid transfer proteins (LTPs) at membrane contact sites (MCSs) is critical for the maintenance of the lipid composition of biological membranes. The ability to measure lipid transfer activity of diverse LTPs in live cells without interrupting the fine structural organization is essential to better understand the contribution of non-vesicular lipid transport to membrane organization. Here, we describe a semiquantitative method to analyze phosphatidylinositol 4-phosphate (PI4P) and phosphatidylserine (PS) changes at the plasma membrane (PM) as they relate to LTP functions. This live cell method is based on bioluminescence resonance energy transfer (BRET) measurements between a luciferase-tagged lipid-recognizing module and a PM-targeted fluorescent acceptor. Oxysterol-binding protein-related protein (ORP) 5 is a PI4P/PS lipid transfer protein which is stably tethered to the ER and also dynamically interacts with PM PI4P/PI(4,5)P2 via its pleckstrin homology (PH) domain. We show that this method is able to detect PI4P and PS changes in the PM upon acute recruitment of an ORP5 construct to the PM. This method is convenient and robust, utilizing population of cells in 96-well plates analyzed in a plate reader. We will also highlight potential further applications extending the method for other LTPs and other lipid cargoes.

Ahnak promotes tumor metastasis through transforming growth factor-ß-mediated epithelial-mesenchymal transition.

Previously, we reported a molecular mechanism by which Ahnak potentiates transforming growth factor-ß (TGFß) signaling during cell growth. Here, we show that Ahnak induces epithelial-mesenchymal transition (EMT) in response to TGFß. EMT phenotypes, including altered in cell morphology, and expression patterns of various EMT marker genes were detected in HaCaT keratinocytes transfected with Ahnak-specific siRNA. Knockdown of Ahnak expression in HaCaT keratinocytes resulted in attenuated cell migration and invasion. We found that Ahnak activates TGFß signaling via Smad3 phosphorylation, leading to enhanced Smad3 transcriptional activity. To validate function of Ahnak in EMT of B16F10 cells having high metastatic and tumorigenic properties, we established B16F10 cells with stable knockdown of Ahnak. N-cadherin expression and Smad3 phosphorylation were significantly decreased in B16F10-shAhnak cells, compared to B16F10-shControl cells after treatment of TGFß. Moreover, TGFß failed to induce cell migration and cell invasion in B16F10-shAhnak cells. To determine whether Ahnak regulates the metastatic activity of B16F10 cells, we established a lung metastasis model in C57BL/6 mice via tail vein injection of B16F10-shAhnak cells. Lung metastasis was significantly suppressed in mice injected with B16F10-shAhnak cells, compared to those injected with B16F10-shControl cells. Taken together, we propose that TGFß-Ahnak signaling axis regulates EMT during tumor metastasis.

AHNAK Loss in Mice Promotes Type II Pneumocyte Hyperplasia and Lung Tumor Development.

AHNAK is known to be a tumor suppressor in breast cancer due to its ability to activate the TGFß signaling pathway. However, the role of AHNAK in lung tumor development and progression remains unknown. Here, the Ahnak gene was disrupted to determine its effect on lung tumorigenesis and the mechanism by which it triggers lung tumor development was investigated. First, AHNAK protein expression was determined to be decreased in human lung adenocarcinomas compared with matched nonneoplastic lung tissues. Then, Ahnak -/- mice were used to investigate the role of AHNAK in pulmonary tumorigenesis. Ahnak -/- mice showed increased lung volume and thicker alveolar walls with type II pneumocyte hyperplasia. Most importantly, approximately 20% of aged Ahnak -/- mice developed lung tumors, and Ahnak -/- mice were more susceptible to urethane-induced pulmonary carcinogenesis than wild-type mice. Mechanistically, Ahnak deficiency promotes the cell growth of lung epithelial cells by suppressing the TGFß signaling pathway. In addition, increased numbers of M2-like alveolar macrophages (AM) were observed in Ahnak -/- lungs, and the depletion of AMs in Ahnak -/- lungs alleviated lung hyperplastic lesions, suggesting that M2-like AMs promoted the progression of lung hyperplastic lesions in Ahnak-null mice. Collectively, AHNAK suppresses type II pneumocyte proliferation and inhibits tumor-promoting M2 alternative activation of macrophages in mouse lung tissue. These results suggest that AHNAK functions as a novel tumor suppressor in lung cancer.Implications: The tumor suppressor function of AHNAK, in murine lungs, occurs by suppressing alveolar epithelial cell proliferation and modulating lung microenvironment. Mol Cancer Res; 16(8); 1287-98. ©2018 AACR.

PI(4,5)P2 controls plasma membrane PI4P and PS levels via ORP5/8 recruitment to ER-PM contact sites.

Phosphatidylinositol 4,5-bisphosphate (PI(4,5)P2) is a critically important regulatory lipid of the plasma membrane (PM); however, little is known about how cells regulate PM PI(4,5)P2 levels. Here, we show that the phosphatidylinositol 4-phosphate (PI4P)/phosphatidylserine (PS) transfer activity of the endoplasmic reticulum (ER)-resident ORP5 and ORP8 is regulated by both PM PI4P and PI(4,5)P2 Dynamic control of ORP5/8 recruitment to the PM occurs through interactions with the N-terminal Pleckstrin homology domains and adjacent basic residues of ORP5/8 with both PI4P and PI(4,5)P2 Although ORP5 activity requires normal levels of these inositides, ORP8 is called on only when PI(4,5)P2 levels are increased. Regulation of the ORP5/8 attachment to the PM by both phosphoinositides provides a powerful means to determine the relative flux of PI4P toward the ER for PS transport and Sac1-mediated dephosphorylation and PIP 5-kinase-mediated conversion to PI(4,5)P2 Using this rheostat, cells can maintain PI(4,5)P2 levels by adjusting the availability of PI4P in the PM.

Quantifying lipid changes in various membrane compartments using lipid binding protein domains.

One of the largest challenges in cell biology is to map the lipid composition of the membranes of various organelles and define the exact location of processes that control the synthesis and distribution of lipids between cellular compartments. The critical role of phosphoinositides, low-abundant lipids with rapid metabolism and exceptional regulatory importance in the control of almost all aspects of cellular functions created the need for tools to visualize their localizations and dynamics at the single cell level. However, there is also an increasing need for methods to determine the cellular distribution of other lipids regulatory or structural, such as diacylglycerol, phosphatidic acid, or other phospholipids and cholesterol. This review will summarize recent advances in this research field focusing on the means by which changes can be described in more quantitative terms.

Lenz-Majewski mutations in PTDSS1 affect phosphatidylinositol 4-phosphate metabolism at ER-PM and ER-Golgi junctions.

Lenz-Majewski syndrome (LMS) is a rare disease characterized by complex craniofacial, dental, cutaneous, and limb abnormalities combined with intellectual disability. Mutations in thePTDSS1gene coding one of the phosphatidylserine (PS) synthase enzymes, PSS1, were described as causative in LMS patients. Such mutations render PSS1 insensitive to feedback inhibition by PS levels. Here we show that expression of mutant PSS1 enzymes decreased phosphatidylinositol 4-phosphate (PI4P) levels both in the Golgi and the plasma membrane (PM) by activating the Sac1 phosphatase and altered PI4P cycling at the PM. Conversely, inhibitors of PI4KA, the enzyme that makes PI4P in the PM, blocked PS synthesis and reduced PS levels by 50% in normal cells. However, mutant PSS1 enzymes alleviated the PI4P dependence of PS synthesis. Oxysterol-binding protein-related protein 8, which was recently identified as a PI4P-PS exchanger between the ER and PM, showed PI4P-dependent membrane association that was significantly decreased by expression of PSS1 mutant enzymes. Our studies reveal that PS synthesis is tightly coupled to PI4P-dependent PS transport from the ER. Consequently, PSS1 mutations not only affect cellular PS levels and distribution but also lead to a more complex imbalance in lipid homeostasis by disturbing PI4P metabolism.

Lenz-Majewski syndrome: How a single mutation leads to complex changes in lipid metabolism.

Lenz-Majewski syndrome (LMS) is a rare disease presenting with complex physical and mental abnormalities. Whole exome sequencing performed on five LMS-affected individuals has identified gain-of-function mutations in the PTDSS1 gene encoding phosphatidylserine synthase 1 (PSS1) enzyme. These mutations all rendered PSS1 insensitive to PS-mediated product inhibition. In a recent study we showed that uncontrolled PS production by these mutant PSS1 enzymes lead to the accumulation of PS in the ER where it is not detected in normal cells. This increased PS in the ER in turn, activated the Sac1 phosphatase, which is responsible for the dephosphorylation of the minor lipid, phosphatidylinositol 4-phosphate (PI4P) in the ER. Increased Sac1 activity decreased PI4P levels both in the Golgi and the plasma membrane thereby dissipating the PI4P gradients set up by PI 4-kinase enzymes (PI4Ks) between these membranes and the ER. Such PI4P gradients at membrane contact sites have been shown to support the transports of structural lipids such as cholesterol and PS out of the ER by non-vesicular lipid transfer. Therefore, uncontrolled production of PS not only affects the PS status of cells but also initiates an avalanche of changes in the metabolism of other membrane lipids via affecting PI4P gradients throughout the cell. Recognition of the close metabolic interaction between PS synthesis and PI4P metabolism provided a new clue to better understand the molecular underpinning of this rare and severe disease.

Obesity Resistance and Enhanced Insulin Sensitivity in Ahnak-/- Mice Fed a High Fat Diet Are Related to Impaired Adipogenesis and Increased Energy Expenditure.

OBJECTIVE: Recent evidence has suggested that AHNAK expression is altered in obesity, although its role in adipose tissue development remains unclear. The objective of this study was to determine the molecular mechanism by which Ahnak influences adipogenesis and glucose homeostasis. DESIGN: We investigated the in vitro role of AHNAK in adipogenesis using adipose-derived mesenchymal stem cells (ADSCs) and C3H10T1/2 cells. AHNAK-KO male mice were fed a high-fat diet (HFD; 60% calories from fat) and examined for glucose and insulin tolerances, for body fat compositions, and by hyperinsulinemic-euglycemic clamping. Energy expenditures were assessed using metabolic cages and by measuring the expression levels of genes involved in thermogenesis in white or brown adipose tissues. RESULTS: Adipogenesis in ADSCs was impaired in AHNAK-KO mice. The loss of AHNAK led to decreased BMP4/SMAD1 signaling, resulting in the downregulation of key regulators of adipocyte differentiation (P<0.05). AHNAK directly interacted with SMAD1 on the Pparγ2 promoter. Concomitantly, HFD-fed AHNAK-KO mice displayed reduced hepatosteatosis and improved metabolic profiles, including improved glucose tolerance (P<0.001), enhanced insulin sensitivity (P<0.001), and increased energy expenditure (P<0.05), without undergoing alterations in food intake and physical activity. CONCLUSION: AHNAK plays a crucial role in body fat accumulation by regulating adipose tissue development via interaction with the SMAD1 protein and can be involved in metabolic homeostasis.