LysoPC acyltransferase/PC transacylase activities in plant plasma membrane and plasma membrane-associated endoplasmic reticulum

Guizhi Shaoyao Zhimu Decoction ameliorates gouty arthritis in rats via altering gut microbiota and improving metabolic profile

BACKGROUND

The incidence of gouty arthritis (GA) has gradually increased, and modern drug therapies have obvious side effects. Guizhi Shaoyao Zhimu Decoction (GSZD), a classic prescription in Traditional Chinese Medicine for treating various osteoarthritis, has shown significant advantages in curing GA.

PURPOSE

To verify the therapeutic effect of GSZD on GA and investigate its potential pharmacological mechanism via integrated analysis of the gut microbiota and serum metabolites for the first time.

METHODS

The chemical composition of GSZD was determined using UPLC-MS. The GA rat model was established by the induction of a high-purine diet combined with local injection. We examined the effects and mechanisms of GSZD after 21 d using enzyme-linked immunosorbent assays, 16S rRNA, and non-targeted metabolomics. Finally, correlation analysis and validation experiment were performed to explore the association among the gut microbiota, serum metabolites, and GA-related clinical indices.

RESULTS

In total, 19 compounds were identified as GSZD. High-purine feedstuff with local injection-induced arthroceles were significantly attenuated after GSZD treatment. GSZD improved bone erosion and reduced the serum levels of inflammatory factors (lipopolysaccharide, tumor cell necrosis factor-α, and interleukin) and key indicators of GA (uric acid). 16S rRNA analysis indicated that GSZD-treated GA rats exhibited differences in the composition of the gut microbiota. The abundance of flora involved in uric acid transport, including Lactobacillus, Ruminococcaceae, and Turicibacter, was elevated to various degrees, whereas the abundance of bacteria involved in inflammatory responses, such as Blautia, was markedly reduced after treatment. Moreover, serum metabolite profiles revealed 27 different metabolites associated with the amelioration of GA, which primarily included fatty acids, glycerophospholipids, purine metabolism, amino acids, and bile acids, as well as primary metabolic pathways, such as glycerophospholipid metabolism and alanine. Finally, correlation analysis of the heat maps and validation experiment demonstrated a close relationship among inflammatory cytokines, gut microbial phylotypes, and metabolic parameters.

CONCLUSION

This study demonstrated that GSZD could modulate the gut microbiota and serum metabolic homeostasis to treat GA. In addition, the application of gut microbiota and serum metabolomics correlation analyses sheds light on the mechanism of Traditional Chinese Medicine compounds in the treatment of bone diseases.

Metabolically Distinct Pools of Phosphatidylcholine Are Involved in Trafficking of Fatty Acids out of and into the Chloroplast for Membrane Production

The eukaryotic pathway of galactolipid synthesis involves fatty acid synthesis in the chloroplast, followed by assembly of phosphatidylcholine (PC) in the endoplasmic reticulum (ER), and then turnover of PC to provide a substrate for chloroplast galactolipid synthesis. However, the mechanisms and classes of lipids transported between the chloroplast and the ER are unclear. PC, PC-derived diacylglycerol, phosphatidic acid, and lyso-phosphatidylcholine (LPC) have all been implicated in ER-to-chloroplast lipid transfer. LPC transport requires lysophosphatidylcholine acyltransferase (LPCAT) activity at the chloroplast to form PC before conversion to galactolipids. However, LPCAT has also been implicated in the opposite chloroplast-to-ER trafficking of newly synthesized fatty acids through PC acyl editing. To understand the role of LPC and LPCAT in acyl trafficking we produced and analyzed the Arabidopsis (Arabidopsis thaliana) act1 lpcat1 lpcat2 triple mutant. LPCAT1 and LPCAT2 encode the major lysophospholipid acyltransferase activity of the chloroplast, and it is predominantly for incorporation of nascent fatty acids exported form the chloroplast into PC by acyl editing. In vivo acyl flux analysis revealed eukaryotic galactolipid synthesis is not impaired in act1 lpcat1 lpcat2 and uses a PC pool distinct from that of PC acyl editing. We present a model for the eukaryotic pathway with metabolically distinct pools of PC, suggesting an underlying spatial organization of PC metabolism as part of the ER-chloroplast metabolic interactions.

Lipid Trafficking at Membrane Contact Sites During Plant Development and Stress Response

The biogenesis of cellular membranes involves an important traffic of lipids from their site of synthesis to their final destination. Lipid transfer can be mediated by vesicular or non-vesicular pathways. The non-vesicular pathway requires the close apposition of two membranes to form a functional platform, called membrane contact sites (MCSs), where lipids are exchanged. These last decades, MCSs have been observed between virtually all organelles and a role in lipid transfer has been demonstrated for some of them. In plants, the lipid composition of membranes is highly dynamic and can be drastically modified in response to environmental changes. This highlights the importance of understanding the mechanisms involved in the regulation of membrane lipid homeostasis in plants. This review summarizes our current knowledge about the non-vesicular transport of lipids at MCSs in plants and its regulation during stress.

Multiscale Structural Analysis of Plant ER-PM Contact Sites

Membrane contact sites are recognized across eukaryotic systems as important nanostructures. Endoplasmic reticulum (ER)-plasma membrane (PM) contact sites (EPCS) are involved in excitation-contraction coupling, signaling, and plant responses to stress. In this report, we perform a multiscale structural analysis of Arabidopsis EPCS that combines live cell imaging, quantitative transmission electron microscopy (TEM) and electron tomography over a developmental gradient. To place EPCS in the context of the entire cortical ER, we examined green fluorescent protein (GFP)-HDEL in living cells over a developmental gradient, then Synaptotagmin1 (SYT1)-GFP was used as a specific marker of EPCS. In all tissues examined, young, rapidly elongating cells showed lamellar cortical ER and higher density of SYT1-GFP puncta, while in mature cells the cortical ER network was tubular, highly dynamic and had fewer SYT1-labeled puncta. The higher density of EPCS in young cells was verified by quantitative TEM of cryo-fixed tissues. For all cell types, the size of each EPCS had a consistent range in length along the PM from 50 to 300 nm, with microtubules and ribosomes excluded from the EPCS. The structural characterization of EPCS in different plant tissues, and the correlation of EPCS densities over developmental gradients illustrate how ER-PM communication evolves in response to cellular expansion.

A Shotgun Proteomic Approach Reveals That Fe Deficiency Causes Marked Changes in the Protein Profiles of Plasma Membrane and Detergent-Resistant Microdomain Preparations from Beta vulgaris Roots

In the present study we have used label-free shotgun proteomic analysis to examine the effects of Fe deficiency on the protein profiles of highly pure sugar beet root plasma membrane (PM) preparations and detergent-resistant membranes (DRMs), the latter as an approach to study microdomains. Altogether, 545 proteins were detected, with 52 and 68 of them changing significantly with Fe deficiency in PM and DRM, respectively. Functional categorization of these proteins showed that signaling and general and vesicle-related transport accounted for approximately 50% of the differences in both PM and DRM, indicating that from a qualitative point of view changes induced by Fe deficiency are similar in both preparations. Results indicate that Fe deficiency has an impact in phosphorylation processes at the PM level and highlight the involvement of signaling proteins, especially those from the 14-3-3 family. Lipid profiling revealed Fe-deficiency-induced decreases in phosphatidic acid derivatives, which may impair vesicle formation, in agreement with the decreases measured in proteins related to intracellular trafficking and secretion. The modifications induced by Fe deficiency in the relative enrichment of proteins in DRMs revealed the existence of a group of cytoplasmic proteins that appears to be more attached to the PM in conditions of Fe deficiency.

Identification of Arabidopsis GPAT9 (At5g60620) as an Essential Gene Involved in Triacylglycerol Biosynthesis

The first step in the biosynthesis of nearly all plant membrane phospholipids and storage triacylglycerols is catalyzed by a glycerol-3-phosphate acyltransferase (GPAT). The requirement for an endoplasmic reticulum (ER)-localized GPAT for both of these critical metabolic pathways was recognized more than 60 years ago. However, identification of the gene(s) encoding this GPAT activity has remained elusive. Here, we present the results of a series of in vivo, in vitro, and in silico experiments in Arabidopsis (Arabidopsis thaliana) designed to assign this essential function to AtGPAT9. This gene has been highly conserved throughout evolution and is largely present as a single copy in most plants, features consistent with essential housekeeping functions. A knockout mutant of AtGPAT9 demonstrates both male and female gametophytic lethality phenotypes, consistent with the role in essential membrane lipid synthesis. Significant expression of developing seed AtGPAT9 is required for wild-type levels of triacylglycerol accumulation, and the transcript level is directly correlated to the level of microsomal GPAT enzymatic activity in seeds. Finally, the AtGPAT9 protein interacts with other enzymes involved in ER glycerolipid biosynthesis, suggesting the possibility of ER-localized lipid biosynthetic complexes. Together, these results suggest that GPAT9 is the ER-localized GPAT enzyme responsible for plant membrane lipid and oil biosynthesis.

Tracking the metabolic pulse of plant lipid production with isotopic labeling and flux analyses: Past, present and future

Metabolism is comprised of networks of chemical transformations, organized into integrated biochemical pathways that are the basis of cellular operation, and function to sustain life. Metabolism, and thus life, is not static. The rate of metabolites transitioning through biochemical pathways (i.e., flux) determines cellular phenotypes, and is constantly changing in response to genetic or environmental perturbations. Each change evokes a response in metabolic pathway flow, and the quantification of fluxes under varied conditions helps to elucidate major and minor routes, and regulatory aspects of metabolism. To measure fluxes requires experimental methods that assess the movements and transformations of metabolites without creating artifacts. Isotopic labeling fills this role and is a long-standing experimental approach to identify pathways and quantify their metabolic relevance in different tissues or under different conditions. The application of labeling techniques to plant science is however far from reaching it potential. In light of advances in genetics and molecular biology that provide a means to alter metabolism, and given recent improvements in instrumentation, computational tools and available isotopes, the use of isotopic labeling to probe metabolism is becoming more and more powerful. We review the principal analytical methods for isotopic labeling with a focus on seminal studies of pathways and fluxes in lipid metabolism and carbon partitioning through central metabolism. Central carbon metabolic steps are directly linked to lipid production by serving to generate the precursors for fatty acid biosynthesis and lipid assembly. Additionally some of the ideas for labeling techniques that may be most applicable for lipid metabolism in the future were originally developed to investigate other aspects of central metabolism. We conclude by describing recent advances that will play an important future role in quantifying flux and metabolic operation in plant tissues.

Plasma membrane electron pathways and oxidative stress

SIGNIFICANCE

Several redox compounds, including respiratory burst oxidase homologs (Rboh) and iron chelate reductases have been identified in animal and plant plasma membrane (PM). Studies using molecular biological, biochemical, and proteomic approaches suggest that PM redox systems of plants are involved in signal transduction, nutrient uptake, transport, and cell wall-related processes. Function of PM-bound redox systems in oxidative stress will be discussed.

RECENT ADVANCES

Present knowledge about the properties, structures, and functions of these systems are summarized. Judging from the currently available data, it is likely that electrons are transferred from cytosolic NAD(P)H to the apoplast via quinone reductases, vitamin K, and a cytochrome b561. In tandem with these electrons, protons might be transported to the apoplastic space.

CRITICAL ISSUES

Recent studies suggest localization of PM-bound redox systems in microdomains (so-called lipid or membrane rafts), but also organization of these compounds in putative and high molecular mass protein complexes. Although the plant flavocytochrome b family is well characterized with respect to its function, the molecular mechanism of an electron transfer reaction by these compounds has to be verified. Localization of Rboh in other compartments needs elucidation.

FUTURE DIRECTIONS

Plant members of the flavodoxin and flavodoxin-like protein family and the cytochrome b561 protein family have been characterized on the biochemical level, postulated localization, and functions of these redox compounds need verification. Compositions of single microdomains and interaction partners of PM redox systems have to be elucidated. Finally, the hypothesis of an electron transfer chain in the PM needs further proof.

Plant cell wall secretion and lipid traffic at membrane contact sites of the cell cortex

Plant cell wall secretion is the result of dynamic vesicle fusion events at the plasma membrane. The importance of the lipid bilayer environment of the plasma membrane and its interactions with the endomembrane system through vesicle traffic are well recognized. Recent advances in yeast molecular biology and biochemistry lead us to re-examine the hypothesis that non-vesicular traffic of lipids through close contact sites of the plasma membrane and endoplasmic reticulum could also be important in plant cell wall biosynthesis. Non-vesicular traffic is the extraction and transfer of individual lipid molecules from a donor bilayer to a target bilayer, usually with the assistance of lipid transfer proteins.

Phospholipases and the Network of Auxin Signal Transduction with ABP1 and TIR1 as Two Receptors: A Comprehensive and Provocative Model

Three types of phospholipases, phospholipase D, secreted phospholipase A(2), and patatin-related phospholipase A (pPLA) have functions in auxin signal transduction. Potential linkage to auxin receptors ABP1 or TIR1, their rapid activation or post-translational activation mechanisms, and downstream functions regulated by these phospholipases is reviewed and discussed. Only for pPLA all aspects are known at least to some detail. Evidence is gathered that all these signal reactions are located in the cytosol and seem to merge on regulation of PIN-catalyzed auxin efflux transport proteins. As a consequence, auxin concentration in the nucleus is also affected and this regulates the E3 activity of this auxin receptor. We showed that ABP1, PIN2, and pPLA, all outside the nucleus, have an impact on regulation of auxin-induced genes within 30 min. We propose that regulation of PIN protein activities and of auxin efflux transport are the means to coordinate ABP1 and TIR1 activity and that no physical contact between components of the ABP1-triggered cytosolic pathways and TIR1-triggered nuclear pathways of signaling is necessary to perform this.

Alteration of plasma membrane-bound redox systems of iron deficient pea roots by chitosan

Iron is essential for all living organisms and plays a crucial role in pathogenicity. This study presents the first proteome analysis of plasma membranes isolated from pea roots. Protein profiles of four different samples (+Fe, +Fe/Chitosan, -Fe, and -Fe/Chitosan) were compared by native IEF-PAGE combined with in-gel activity stains and DIGE. Using DIGE, 89 proteins of interest were detected in plasma membrane fractions. Data revealed a differential abundance of several spots in all samples investigated. In comparison to the control and -FeCh the abundance of six protein spots increased whereas 56 spots decreased in +FeCh. Altered protein spots were analyzed by MALDI-TOF-TOF mass spectrometry. Besides stress-related proteins, transport proteins and redox enzymes were identified. Activity stains after native PAGE and spectrophotometric measurements demonstrated induction of a ferric-chelate reductase (-Fe) and a putative respiratory burst oxidase homolog (-FeCh). However, the activity of the ferric-chelate reductase decreased in -Fe plants after elicitor treatment. The activity of plasma membrane-bound class III peroxidases increased after elicitor treatment and decreased under iron-deficiency, whereas activity of quinone reductases decreased mostly after elicitor treatment. Possible functions of proteins identified and reasons for a weakened pathogen response of iron-deficient plants were discussed.

The signal molecule lysophosphatidylcholine in Eschscholzia californica is rapidly metabolized by reacylation

In cultured cells of California poppy (Eschscholzia californica), lysophosphatidylcholine (LPC) triggers a signal path that finally induces alkaloid biosynthesis. LPC is transiently generated by elicitor-activated phospholipase A(2) of the plasma membrane. Externally added LPC is rapidly acylated by a membrane-bound enzyme that shows the highest specific activity in the purified plasma membrane. The fatty acid incorporated into the sn-2 position of LPC is preferentially linoleic (18:2), which is the most abundant acyl component in the PC species of Eschscholzia cells, but a minor component of the pool of free fatty acids. The fatty acid at the sn-1 position of LPC is less important for substrate specificity. The capacity of LPC acylation by intact cells or isolated plasma membranes by far exceeds the rate of LPC generation by activated phospholipase A(2) and is not limited by the availability of acyl donors. Metabolites other than phosphatidylcholine (PC) were not significantly produced from labeled LPC within 20 min, indicating that lysophospholipases are not significantly contributing to the short-time metabolism of LPC. It is concluded that reacylation to PC is the dominating process in the detoxication of LPC and ensures the transient character of its steady state concentrations, even at maximum phospholipase A(2) activities.

Membrane phospholipids as a phosphate reserve: the dynamic nature of phospholipid-to-digalactosyl diacylglycerol exchange in higher plants

It is well established that phosphate deficiency induces the replacement of membrane phospholipid with non-phosphorous lipids in extra-plastidial membranes (e.g. plasma membrane, tonoplast, mitochondria). The predominant replacement lipid is digalactosyl diacylglycerol (DGDG). This paper reports that the phospholipid-to-DGDG replacement is reversible, and that when oat seedlings are re-supplied with radio-labelled phosphate, it is initially recovered primarily in phosphatidylcholine (PC). Within 2 d, the shoot contains more than half of the lipid-associated radiolabel, reflecting phosphate translocation. Oat was also cultivated in different concentrations of phosphate and the DGDG/PC ratio in roots and phospholipase activities in isolated plasma membranes was assayed after different times of cultivation. The DGDG/PC ratio in root tissue correlated more closely with plasma membrane-localized phospholipase D, yielding phosphatidic acid (PA), than with plasma membrane-localized PA phosphatase, the activity that results in a decreased proportion of phospolipids. The lipid degradation data did not reflect a significant involvement of phospholipase C, although a putative phospholipase C analogue, non-specific phospholipase C4 (NPC4), was present in oat roots. The correlation between increased phospholipase D activity and DGDG/PC ratio is consistent with a model where phospholipid-to-DGDG replacement involves formation of PA that readily is removed from the plasma membrane for further degradation elsewhere.