Arabidopsis Ca2+-ATPases 1, 2, and 7 in the endoplasmic reticulum contribute to growth and pollen fitness.

Generating cellular Ca2+ signals requires coordinated transport activities from both Ca2+ influx and efflux pathways. In Arabidopsis (Arabidopsis thaliana), multiple efflux pathways exist, some of which involve Ca2+-pumps belonging to the Autoinhibited Ca2+-ATPase (ACA) family. Here, we show that ACA1, 2, and 7 localize to the endoplasmic reticulum (ER) and are important for plant growth and pollen fertility. While phenotypes for plants harboring single-gene knockouts (KOs) were weak or undetected, a triple KO of aca1/2/7 displayed a 2.6-fold decrease in pollen transmission efficiency, whereas inheritance through female gametes was normal. The triple KO also resulted in smaller rosettes showing a high frequency of lesions. Both vegetative and reproductive phenotypes were rescued by transgenes encoding either ACA1, 2, or 7, suggesting that all three isoforms are biochemically redundant. Lesions were suppressed by expression of a transgene encoding NahG, an enzyme that degrades salicylic acid (SA). Triple KO mutants showed elevated mRNA expression for two SA-inducible marker genes, Pathogenesis-related1 (PR1) and PR2. The aca1/2/7 lesion phenotype was similar but less severe than SA-dependent lesions associated with a double KO of vacuolar pumps aca4 and 11. Imaging of Ca2+ dynamics triggered by blue light or the pathogen elicitor flg22 revealed that aca1/2/7 mutants display Ca2+ transients with increased magnitudes and durations. Together, these results indicate that ER-localized ACAs play important roles in regulating Ca2+ signals, and that the loss of these pumps results in male fertility and vegetative growth deficiencies.

Zinc transporter ZIP7 is a novel determinant of ferroptosis.

Ferroptosis is a newly described form of regulated cell death triggered by oxidative stresses and characterized by extensive lipid peroxidation and membrane damages. The name of ferroptosis indicates that the ferroptotic death process depends on iron, but not other metals, as one of its canonical features. Here, we reported that zinc is also essential for ferroptosis in breast and renal cancer cells. Zinc chelator suppressed ferroptosis, and zinc addition promoted ferroptosis, even during iron chelation. By interrogating zinc-related genes in a genome-wide RNAi screen of ferroptosis, we identified SLC39A7, encoding ZIP7 that controls zinc transport from endoplasmic reticulum (ER) to cytosol, as a novel genetic determinant of ferroptosis. Genetic and chemical inhibition of the ZIP7 protected cells against ferroptosis, and the ferroptosis protection upon ZIP7 knockdown can be abolished by zinc supplementation. We found that the genetic and chemical inhibition of ZIP7 triggered ER stresses, including the induction of the expression of HERPUD1 and ATF3. Importantly, the knockdown of HERPUD1 abolished the ferroptosis protection phenotypes of ZIP7 inhibition. Together, we have uncovered an unexpected role of ZIP7 in ferroptosis by maintaining ER homeostasis. These findings may have therapeutic implications for human diseases involving ferroptosis and zinc dysregulations.

Defective endoplasmic reticulum-mitochondria contacts and bioenergetics in SEPN1-related myopathy.

SEPN1-related myopathy (SEPN1-RM) is a muscle disorder due to mutations of the SEPN1 gene, which is characterized by muscle weakness and fatigue leading to scoliosis and life-threatening respiratory failure. Core lesions, focal areas of mitochondria depletion in skeletal muscle fibers, are the most common histopathological lesion. SEPN1-RM underlying mechanisms and the precise role of SEPN1 in muscle remained incompletely understood, hindering the development of biomarkers and therapies for this untreatable disease. To investigate the pathophysiological pathways in SEPN1-RM, we performed metabolic studies, calcium and ATP measurements, super-resolution and electron microscopy on in vivo and in vitro models of SEPN1 deficiency as well as muscle biopsies from SEPN1-RM patients. Mouse models of SEPN1 deficiency showed marked alterations in mitochondrial physiology and energy metabolism, suggesting that SEPN1 controls mitochondrial bioenergetics. Moreover, we found that SEPN1 was enriched at the mitochondria-associated membranes (MAM), and was needed for calcium transients between ER and mitochondria, as well as for the integrity of ER-mitochondria contacts. Consistently, loss of SEPN1 in patients was associated with alterations in body composition which correlated with the severity of muscle weakness, and with impaired ER-mitochondria contacts and low ATP levels. Our results indicate a role of SEPN1 as a novel MAM protein involved in mitochondrial bioenergetics. They also identify a systemic bioenergetic component in SEPN1-RM and establish mitochondria as a novel therapeutic target. This role of SEPN1 contributes to explain the fatigue and core lesions in skeletal muscle as well as the body composition abnormalities identified as part of the SEPN1-RM phenotype. Finally, these results point out to an unrecognized interplay between mitochondrial bioenergetics and ER homeostasis in skeletal muscle. They could therefore pave the way to the identification of biomarkers and therapeutic drugs for SEPN1-RM and for other disorders in which muscle ER-mitochondria cross-talk are impaired.

Diosgenyl Saponin Inducing Endoplasmic Reticulum Stress and Mitochondria-Mediated Apoptotic Pathways in Liver Cancer Cells.

Diosgenin and diosgenyl saponins as the major bioactive compounds isolated from dietary fenugreek seeds, yam roots, etc. possessed strong antitumor effects. To understand their detailed antitumor mechanisms, a fluorophore-appended derivative of diosgenin [Glc/CNHphth-diosgenin (GND)] was synthesized, starting from diosgenin and glucosamine hydrochloride in overall yields of 7-12% over 7-10 steps. Co-localization of GND with organelle-specific stains, transmission electron microscopy, and relative protein analyses demonstrated that GND crossed the plasma membrane through organic anion-transporting polypeptide 1B1 and distributed in the endoplasmic reticulum (ER), lysosome, and mitochondria. In this process, GND induced ER swelling, mitochondrial damage, and autophagosome and upregulating IRE-1α to induce autophagy and apoptosis. Furthermore, autophagy inhibitor chloroquine delayed the appearance of cleaved poly(ADP-ribose) polymerase and inhibited cleaved caspase 8, which indicated that GND induced autophagy to activate caspase-8-dependent apoptosis. These observations suggested that diosgenyl saponin was a potent anticancer agent that elicited ER stress and mitochondria-mediated apoptotic pathways in liver cancer.

Perturbed mitochondria-ER contacts in live neurons that model the amyloid pathology of Alzheimer's disease.

The use of fixed fibroblasts from familial and sporadic Alzheimer's disease patients has previously indicated an upregulation of mitochondria-ER contacts (MERCs) as a hallmark of Alzheimer's disease. Despite its potential significance, the relevance of these results is limited because they were not extended to live neurons. Here we performed a dynamic in vivo analysis of MERCs in hippocampal neurons from McGill-R-Thy1-APP transgenic rats, a model of Alzheimer's disease-like amyloid pathology. Live FRET imaging of neurons from transgenic rats revealed perturbed 'lipid-MERCs' (gap width <10 nm), while 'Ca2+-MERCs' (10-20 nm gap width) were unchanged. In situ TEM showed no significant differences in the lipid-MERCs:total MERCs or lipid-MERCs:mitochondria ratios; however, the average length of lipid-MERCs was significantly decreased in neurons from transgenic rats as compared to controls. In accordance with FRET results, untargeted lipidomics showed significant decreases in levels of 12 lipids and bioenergetic analysis revealed respiratory dysfunction of mitochondria from transgenic rats. Thus, our results reveal changes in MERC structures coupled with impaired mitochondrial functions in Alzheimer's disease-related neurons.This article has an associated First Person interview with the first author of the paper.

Chemical Strike against a Dominant-Inherited MUC1-Frameshifted Protein Associated with Progressive Kidney Disease.

In a recent paper by Dvela-Levitt et al., chemical screening using an immunofluorescent assay identified a compound that caused removal of a dominant-inherited misfolded secretory protein, mucin1-frameshifted, from an intracellular location in immortalized renal epithelial cells of a patient affected with progressive medullary cystic kidney disease. This illustrates the power of chemical screening at the cellular level to address specific proteinopathies and the utility of such compounds to illuminate novel cellular pathways that can clear toxic proteins.

Dietary fatty acids promote lipid droplet diversity through seipin enrichment in an ER subdomain.

Exogenous metabolites from microbial and dietary origins have profound effects on host metabolism. Here, we report that a sub-population of lipid droplets (LDs), which are conserved organelles for fat storage, is defined by metabolite-modulated targeting of the C. elegans seipin ortholog, SEIP-1. Loss of SEIP-1 function reduces the size of a subset of LDs while over-expression of SEIP-1 has the opposite effect. Ultrastructural analysis reveals SEIP-1 enrichment in an endoplasmic reticulum (ER) subdomain, which co-purifies with LDs. Analyses of C. elegans and bacterial genetic mutants indicate a requirement of polyunsaturated fatty acids (PUFAs) and microbial cyclopropane fatty acids (CFAs) for SEIP-1 enrichment, as confirmed by dietary supplementation experiments. In mammalian cells, heterologously expressed SEIP-1 engages nascent lipid droplets and promotes their subsequent expansion in a conserved manner. Our results suggest that microbial and polyunsaturated fatty acids serve unexpected roles in regulating cellular fat storage by promoting LD diversity.

The multiple roles of the unfolded protein response regulator IRE1α in cancer.

Cancer is associated with a number of conditions such as hypoxia, nutrient deprivation, cellular redox, and pH changes that result in accumulation of unfolded or misfolded proteins in the endoplasmic reticulum (ER) and trigger a stress response known as the unfolded protein response (UPR). The UPR is a conserved cellular survival mechanism mediated by the ER transmembrane proteins activating transcription factor 6, protein kinase-like endoplasmic reticulum kinase, and inositol-requiring enzyme 1α (IRE1α) that act to resolve ER stress and promote cell survival. IRE1α is a kinase/endoribonuclease (RNase) with multiple activities including unconventional splicing of the messenger RNA (mRNA) for the transcription factor X-Box Binding Protein 1 (XBP1), degradation of other mRNAs in a process called regulated IRE1α-dependent decay (RIDD) and activation of a pathway leading to c-Jun N-terminal kinase phosphorylation. Each of these outputs plays a role in the adaptive and cell death responses to ER stress. Many studies indicate an important role for XBP1 and RIDD functions in cancer and new studies suggest that these two functions of the IRE1α RNase can have opposing functions in the early and later stages of cancer pathogenesis. Finally, as more is learned about the context-dependent role of IRE1α in cancer development, specific small molecule inhibitors and activators of IRE1α could play an important role in counteracting the protective shield provided by ER stress signaling in cancer cells.

ATP redirects cytokine trafficking and promotes novel membrane TNF signaling via microvesicles.

Cellular stress or injury induces release of endogenous danger signals such as ATP, which plays a central role in activating immune cells. ATP is essential for the release of nonclassically secreted cytokines such as IL-1ß but, paradoxically, has been reported to inhibit the release of classically secreted cytokines such as TNF. Here, we reveal that ATP does switch off soluble TNF (17 kDa) release from LPS-treated macrophages, but rather than inhibiting the entire TNF secretion, ATP packages membrane TNF (26 kDa) within microvesicles (MVs). Secretion of membrane TNF within MVs bypasses the conventional endoplasmic reticulum- and Golgi transport-dependent pathway and is mediated by acid sphingomyelinase. These membrane TNF-carrying MVs are biologically more potent than soluble TNF in vivo, producing significant lung inflammation in mice. Thus, ATP critically alters TNF trafficking and secretion from macrophages, inducing novel unconventional membrane TNF signaling via MVs without direct cell-to-cell contact. These data have crucial implications for this key cytokine, particularly when therapeutically targeting TNF in acute inflammatory diseases.-Soni, S., O'Dea, K. P., Tan, Y. Y., Cho, K., Abe, E., Romano, R., Cui, J., Ma, D., Sarathchandra, P., Wilson, M. R., Takata, M. ATP redirects cytokine trafficking and promotes novel membrane TNF signaling via microvesicles.

Downregulation of the auxin transporter gene SlPIN8 results in pollen abortion in tomato.

KEY MESSAGE: SlPIN8 is expressed specifically within tomato pollen, and that it is involved in tomato pollen development and intracellular auxin homeostasis. The auxin (IAA) transport protein PIN-FORMED (PIN) plays key roles in various aspects of plant development. The biological role of the auxin transporter SlPIN8 in tomato development remains unclear. Here, we examined the expression pattern of the SlPIN8 gene in vegetative and reproductive organs of tomato. RNA interference (RNAi) transgenic lines specifically silenced for the SlPIN8 gene were generated to identify the role of SlPIN8 in pollen development. We found that SlPIN8 mRNA is expressed specifically within tomato pollen. In the anthers, the highest mRNA expression and ß-glucuronidase (GUS) activity of promoter-SlPIN8-GUS was detected during late stages of anther development, when pollen maturation occurred. The downregulation of SlPIN8 did not drastically affect the vegetative growth of tomato. However, in SlPIN8-RNAi transgenic plants, approximately 80% of the pollen grains were identified to be abnormal and lack viability; they were shriveled and flattened. Furthermore, the downregulation of SlPIN8 affected the gene expression of some anther development-specific proteins. SlPIN8-RNAi transgenic plants induced seedless fruits because of defective pollen function rather than defective female gametophyte function. In addition, SlPIN8 was found to localize to the endoplasmic reticulum, consistent with the changes in the auxin levels of SlPIN8-RNAi lines, whereas the level of free IAA was increased in SlPIN8-overexpressing protoplasts, indicating that SlPIN8 is involved in intracellular auxin homeostasis.

Phosphatidylserine synthesis is essential for viability of the human fungal pathogen Cryptococcus neoformans.

Phospholipids are an integral part of the cellular membrane structure and can be produced by a de novo biosynthetic pathway and, alternatively, by the Kennedy pathway. Studies in several yeast species have shown that the phospholipid phosphatidylserine (PS) is synthesized from CDP-diacylglycerol and serine, a route that is different from its synthesis in mammalian cells, involving a base-exchange reaction from preexisting phospholipids. Fungal-specific PS synthesis has been shown to play an important role in fungal virulence and has been proposed as an attractive drug target. However, PS synthase, which catalyzes this reaction, has not been studied in the human fungal pathogen Cryptococcus neoformans Here, we identified and characterized the PS synthase homolog (Cn Cho1) in this fungus. Heterologous expression of Cn CHO1 in a Saccharomyces cerevisiae cho1Δ mutant rescued the mutant's growth defect in the absence of ethanolamine supplementation. Moreover, an Sc cho1Δ mutant expressing Cn CHO1 had PS synthase activity, confirming that the Cn CHO1 encodes PS synthase. We also found that PS synthase in C. neoformans is localized to the endoplasmic reticulum and that it is essential for mitochondrial function and cell viability. Of note, its deficiency could not be complemented by ethanolamine or choline supplementation for the synthesis of phosphatidylethanolamine (PE) or phosphatidylcholine (PC) via the Kennedy pathway. These findings improve our understanding of phospholipid synthesis in a pathogenic fungus and indicate that PS synthase may be a useful target for antifungal drugs.

ADAM22 and ADAM23 modulate the targeting of the Kv1 channel-associated protein LGI1 to the axon initial segment.

The distribution of the voltage-gated Kv1 K+ channels at the axon initial segment (AIS) influences neuronal intrinsic excitability. The Kv1.1 and Kv1.2 (also known as KCNA1 and KCNA2, respectively) subunits are associated with cell adhesion molecules (CAMs), including Caspr2 (also known as CNTNAP2) and LGI1, which are implicated in autoimmune and genetic neurological diseases with seizures. In particular, mutations in the LGI1 gene cause autosomal dominant lateral temporal lobe epilepsy (ADLTE). Here, by using rat hippocampal neurons in culture, we showed that LGI1 is recruited to the AIS where it colocalizes with ADAM22 and Kv1 channels. Strikingly, the missense mutations S473L and R474Q of LGI1 identified in ADLTE prevent its association with ADAM22 and enrichment at the AIS. Moreover, we observed that ADAM22 and ADAM23 modulate the trafficking of LGI1, and promote its ER export and expression at the overall neuronal cell surface. Live-cell imaging indicated that LGI1 is co-transported in axonal vesicles with ADAM22 and ADAM23. Finally, we showed that ADAM22 and ADAM23 also associate with Caspr2 and TAG-1 (also known as CNTN2) to be selectively targeted to different axonal sub-regions. Hence, the combinatorial expression of Kv1-associated CAMs may be critical to tune intrinsic excitability in physiological and epileptogenic contexts.

Systemic Upregulation of MTP2- and HMA2-Mediated Zn Partitioning to the Shoot Supplements Local Zn Deficiency Responses.

Low bioavailable concentrations of the micronutrient zinc (Zn) limit agricultural production on 40% of cultivated land. Here, we demonstrate that plant acclimation to Zn deficiency involves systemic regulation. Physiological Zn deficiency of Arabidopsis thaliana shoots results in increased root transcript levels of the membrane transport protein-encoding genes METAL TRANSPORT PROTEIN2 (MTP2) and HEAVY METAL ATPASE2 (HMA2), which are unresponsive to the local Zn status of roots. MTP2 and HMA2 act additively in the partitioning of Zn from roots to shoots. Chimeric GFP fusion proteins of MTP2 complement an mtp2 mutant and localize in the endoplasmic reticulum (ER) membrane of the outer cell layers from elongation to root hair zone of lateral roots. MTP2 restores Zn tolerance in a hypersensitive yeast mutant. These results are consistent with cell-to-cell movement of Zn toward the root vasculature inside the ER-luminal continuum through the desmotubules of plasmodesmata, under Zn deficiency. The previously described Zn deficiency response comprises transcriptional activation of target genes, including ZINC-REGULATED TRANSPORTER IRON-REGULATED TRANSPORTER PROTEIN genes ZIP4 and ZIP9, by the F-group bZIP transcription factors bZIP19 and bZIP23. We show that ZIP4 and ZIP9 respond to the local Zn status in both roots and shoots, in contrast to the systemic regulation identified here. Our findings are relevant for crop management and improvement toward combating human nutritional Zn deficiency that affects 30 to 50% of the world's population.

High-content screen for modifiers of Niemann-Pick type C disease in patient cells.

Niemann-Pick type C (NPC) disease is a rare lysosomal storage disease caused primarily by mutations in NPC1. NPC1 encodes the lysosomal cholesterol transport protein NPC1. The most common NPC1 mutation is a missense mutation (NPC1I1061T) that causes misfolding and rapid degradation of mutant protein in the endoplasmic reticulum. Cholesterol accumulates in enlarged lysosomes as a result of decreased levels of lysosomal NPC1I1061T protein in patient cells. There is currently no cure or FDA-approved treatment for patients. We sought to identify novel compounds that decrease lysosomal cholesterol storage in NPC1I1061T/I1061T patient fibroblasts using a high-content screen with the cholesterol dye, filipin and the lysosomal marker, LAMP1. A total of 3532 compounds were screened, including 2013 FDA-approved drugs, 327 kinase inhibitors and 760 serum metabolites. Twenty-three hits were identified that decreased both filipin and LAMP1 signals. The majority of hits (16/21) were histone deacetylase (HDAC) inhibitors, a previously described class of modifiers of NPC cholesterol storage. Of the remaining hits, the antimicrobial compound, alexidine dihydrochloride had the most potent lysosomal cholesterol-reducing activity. Subsequent analyses showed that alexidine specifically increased levels of NPC1 transcript and mature protein in both control and NPC patient cells. Although unsuitable for systemic therapy, alexidine represents a unique tool compound for further NPC studies and as a potent inducer of NPC1. Together, these findings confirm the utility of high-content image-based compound screens of NPC1 patient cells and support extending the approach into larger compound collections.

Visualization of Multicolored in vivo Organelle Markers for Co-Localization Studies in Oryza sativa.

Eukaryotic cells consist of a complex network of thousands of proteins present in different organelles where organelle-specific cellular processes occur. Identification of the subcellular localization of a protein is important for understanding its potential biochemical functions. In the post-genomic era, localization of unknown proteins is achieved using multiple tools including a fluorescent-tagged protein approach. Several fluorescent-tagged protein organelle markers have been introduced into dicot plants, but its use is still limited in monocot plants. Here, we generated a set of multicolored organelle markers (fluorescent-tagged proteins) based on well-established targeting sequences. We used a series of pGWBs binary vectors to ameliorate localization and co-localization experiments using monocot plants. We constructed different fluorescent-tagged markers to visualize rice cell organelles, i.e., nucleus, plastids, mitochondria, peroxisomes, golgi body, endoplasmic reticulum, plasma membrane, and tonoplast, with four different fluorescent proteins (FPs) (G3GFP, mRFP, YFP, and CFP). Visualization of FP-tagged markers in their respective compartments has been reported for dicot and monocot plants. The comparative localization of the nucleus marker with a nucleus localizing sequence, and the similar, characteristic morphology of mCherry-tagged Arabidopsis organelle markers and our generated organelle markers in onion cells, provide further evidence for the correct subcellular localization of the Oryza sativa (rice) organelle marker. The set of eight different rice organelle markers with four different FPs provides a valuable resource for determining the sub-cellular localization of newly identified proteins, conducting co-localization assays, and generating stable transgenic localization in monocot plants.

Drosophila TG-A transglutaminase is secreted via an unconventional Golgi-independent mechanism involving exosomes and two types of fatty acylations.

Transglutaminases (TGs) play essential intracellular and extracellular roles by covalently cross-linking many proteins. Drosophila TG is encoded by one gene and has two alternative splicing-derived isoforms, TG-A and TG-B, which contain distinct N-terminal 46- and 38-amino acid sequences, respectively. The TGs identified to date do not have a typical endoplasmic reticulum (ER)-signal peptide, and the molecular mechanisms of their secretion under physiologic conditions are unclear. Immunocytochemistry revealed that TG-A localizes to multivesicular-like structures, whereas TG-B localizes to the cytosol. We also found that TG-A, but not TG-B, was modified concomitantly by N-myristoylation and S-palmitoylation, and N-myristoylation was a pre-requisite for S-palmitoylation. Moreover, TG-A, but not TG-B, was secreted in response to calcium signaling induced by Ca2+ ionophores and uracil, a pathogenic bacteria-derived substance. Brefeldin A and monensin, inhibitors of the ER/Golgi-mediated conventional pathway, did not suppress TG-A secretion, whereas inhibition of S-palmitoylation by 2-bromopalmitate blocked TG-A secretion. Ultracentrifugation, electron microscopy analyses, and treatments with inhibitors of multivesicular body formation revealed that TG-A was secreted via exosomes together with co-transfected mammalian CD63, an exosomal marker, and the secreted TG-A was taken up by other cells. The 8-residue N-terminal fragment of TG-A containing the fatty acylation sites was both necessary and sufficient for the exosome-dependent secretion of TG-A. In conclusion, TG-A is secreted through an unconventional ER/Golgi-independent pathway involving two types of fatty acylations and exosomes.

Cyanophycinase CphE from P. alcaligenes produced in different compartments of N. benthamiana degrades high amounts of cyanophycin in plant extracts.

One of the major constraints in pig and poultry farming is the supply of protein-rich forage, containing sufficient amounts of key amino acids such as arginine (Ufaz and Galili 2008). Since these are underrepresented in plant proteins, the usage of plants as feed is limited. The heterologous production of the cyanobacterial storage polymer cyanophycin granule polypeptide (CGP) in plastids increases the amount of arginine substantially (Huhns et al. 2008; Huhns et al. 2009; Nausch et al. 2016a). CGP degradation releases arginine-aspartate dipeptides. CGP is stable in plants because its degradation is exclusively restricted to bacterial cyanophycinases (CGPases; Law et al. 2009). Since animals are also unable to digest CGP, CGPases need to be co-delivered with CGP-containing plant feed in order to release the dipeptides in the gastrointestinal tract of animals during digestion. Therefore, an extracellular CGPase, CphE from Pseudomonas alcaligenes DIP-1, was targeted to the cytosol, ER, and apoplasm of Nicotiana benthamiana. Translocation to the chloroplast was not successful. Although CphE accumulated in high amounts in the cytosol, only moderate levels were present in the ER, while the enzyme was nearly undetectable in the apoplasm. This correlates with the higher instability of post-translationally modified CphE in crude plant extracts. In addition, the production in the ER led to an increased number and size of necroses compared with cytosolic expression and might therefore interfere with the endogenous metabolism in the ER. Due to the high and robust enzyme activity, even moderate CphE concentrations were sufficient to degrade CGP in plant extracts.

Perilipin-2 Deletion Impairs Hepatic Lipid Accumulation by Interfering with Sterol Regulatory Element-binding Protein (SREBP) Activation and Altering the Hepatic Lipidome.

Perilipin-2 (PLIN2) is a constitutively associated cytoplasmic lipid droplet coat protein that has been implicated in fatty liver formation in non-alcoholic fatty liver disease. Mice with or without whole-body deletion of perilipin-2 (Plin2-null) were fed either Western or control diets for 30 weeks. Perilipin-2 deletion prevents obesity and insulin resistance in Western diet-fed mice and dramatically reduces hepatic triglyceride and cholesterol levels in mice fed Western or control diets. Gene and protein expression studies reveal that PLIN2 deletion suppressed SREBP-1 and SREBP-2 target genes involved in de novo lipogenesis and cholesterol biosynthetic pathways in livers of mice on either diet. GC-MS lipidomics demonstrate that this reduction correlated with profound alterations in the hepatic lipidome with significant reductions in both desaturation and elongation of hepatic neutral lipid species. To examine the possibility that lipidomic actions of PLIN2 deletion contribute to suppression of SREBP activation, we isolated endoplasmic reticulum membrane fractions from long-term Western diet-fed wild type (WT) and Plin2-null mice. Lipidomic analyses reveal that endoplasmic reticulum membranes from Plin2-null mice are markedly enriched in ω-3 and ω-6 long-chain polyunsaturated fatty acids, which others have shown inhibit SREBP activation and de novo lipogenesis. Our results identify PLIN2 as a determinant of global changes in the hepatic lipidome and suggest the hypothesis that these actions contribute to SREBP-regulated de novo lipogenesis involved in non-alcoholic fatty liver disease.

AtSRP1, SMALL RUBBER PARTICLE PROTEIN HOMOLOG, functions in pollen growth and development in Arabidopsis.

To identify novel roles of SMALL RUBBER PARTICLE PROTEIN Homolog in the non-rubber-producing plant Arabidopsis (AtSRP1), we isolated a T-DNA-insertion knock-out mutant (FLAG_543A05) and investigated its functional characteristics. AtSRP1 is predominantly expressed in reproductive organs and is localized to lipid droplets and ER. Compared to wild-type (WT) Arabidopsis, atsrp1 plants contain small siliques with a reduced number of heterogeneously shaped seeds. The size of anther and pollen grains in atsrp1 is highly irregular, with a lower grain number than WT. Therefore, AtSRP1 plays a novel role related to pollen growth and development in a non-rubber-producing plant.

Functional Roles of N-Linked Glycosylation of Human Matrix Metalloproteinase 9.

Matrix metalloproteinase-9 (MMP-9) is a secreted endoproteinase with a critical role in the regulation of the extracellular matrix and proteolytic activation of signaling molecules. Human (h)MMP-9 has two well-defined N-glycosylation sites at residues N38 and N120; however, their role has remained mostly unexplored partly because expression of the N-glycosylation-deficient N38S has been difficult due to a recently discovered single nucleotide polymorphism-dependent miRNA-mediated inhibitory mechanism. hMMP-9 cDNA encoding amino acid substitutions at residues 38 (modified-S38, mS38) or 120 (N120S) were created in the background of a miRNA-binding site disrupted template and expressed by transient transfection. hMMP-9 harboring a single mS38 replacement secreted well, whereas N120S, or a double mS38/N120S hMMP-9 demonstrated much reduced secretion. Imaging indicated endoplasmic reticulum (ER) retention of the non-secreted variants and co-immunoprecipitation confirmed an enhanced strong interaction between the non-secreted hMMP-9 and the ER-resident protein calreticulin (CALR). Removal of N-glycosylation at residue 38 revealed an amino acid-dependent strong interaction with CALR likely preventing unloading of the misfolded protein from the ER chaperone down the normal secretory pathway. As with other glycoproteins, N-glycosylation strongly regulates hMMP-9 secretion. This is mediated, however, through a novel mechanism of cloaking an N-glycosylation-independent strong interaction with the ER-resident CALR.