异养微生物固定CO2的合成生物学研究进展

人类活动造成大气二氧化碳（CO₂）浓度不断升高,使当今世界面临着气候变化的重大危机。微生物CO₂固定为实现地球“碳中和”提供了一条有前景的绿色发展路线。与自养微生物相比,异养微生物具有更快的生长速度和更先进的遗传工具,但是其固定CO₂的能力还很有限。近年来,基于合成生物学技术强化异养微生物CO₂固定受到诸多关注,主要包括优化能量供给、改造羧化途径以及基于异养微生物间接固定CO₂。本综述将围绕上述3个方面重点讨论异养微生物CO₂固定的研究进展,为将来更好地利用微生物CO₂固定技术实现“碳达峰、碳中和”提供参考。     

Metabolic Enzyme Alterations and Astrocyte Dysfunction in a Murine Model of Alexander Disease With Severe Reactive Gliosis

Alexander disease (AxD) is a rare and fatal neurodegenerative disorder caused by mutations in the gene encoding glial fibrillary acidic protein (GFAP). In this report, a mouse model of AxD (GFAP^Tg;Gfap^+/R236H) was analyzed that contains a heterozygous R236H point mutation in murine Gfap as well as a transgene with a GFAP promoter to overexpress human GFAP. Using label-free quantitative proteomic comparisons of brain tissue from GFAP^Tg;Gfap^+/R236H versus wild-type mice confirmed upregulation of the glutathione metabolism pathway and indicated proteins were elevated in the peroxisome proliferator-activated receptor (PPAR) signaling pathway, which had not been reported previously in AxD. Relative protein-level differences were confirmed by a targeted proteomics assay, including proteins related to astrocytes and oligodendrocytes. Of particular interest was the decreased level of the oligodendrocyte protein, 2-hydroxyacylsphingosine 1-beta-galactosyltransferase (Ugt8), since Ugt8-deficient mice exhibit a phenotype similar to GFAP^Tg;Gfap^+/R236H mice (e.g., tremors, ataxia, hind-limb paralysis). In addition, decreased levels of myelin-associated proteins were found in the GFAP^Tg;Gfap^+/R236H mice, consistent with the role of Ugt8 in myelin synthesis. Fabp7 upregulation in GFAP^Tg;Gfap^+/R236H mice was also selected for further investigation due to its uncharacterized association to AxD, critical function in astrocyte proliferation, and functional ability to inhibit the anti-inflammatory PPAR signaling pathway in models of amyotrophic lateral sclerosis (ALS). Within Gfap⁺ astrocytes, Fabp7 was markedly increased in the hippocampus, a brain region subjected to extensive pathology and chronic reactive gliosis in GFAP^Tg;Gfap^+/R236H mice. Last, to determine whether the findings in GFAP^Tg;Gfap^+/R236H mice are present in the human condition, AxD patient and control samples were analyzed by Western blot, which indicated that Type I AxD patients have a significant fourfold upregulation of FABP7. However, immunohistochemistry analysis showed that UGT8 accumulates in AxD patient subpial brain regions where abundant amounts of Rosenthal fibers are located, which was not observed in the GFAP^Tg;Gfap^+/R236H mice.     

The effects of weak selection on neutral diversity at linked sites

The effects of selection on variability at linked sites have an important influence on levels and patterns of within-population variation across the genome. Most theoretical models of these effects have assumed that selection is sufficiently strong that allele frequency changes at the loci concerned are largely deterministic. These models have led to the conclusion that directional selection for selectively favorable mutations, or against recurrent deleterious mutations, reduces nucleotide site diversity at linked neutral sites. Recent work has shown, however, that fixations of weakly selected mutations, accompanied by significant stochastic changes in allele frequencies, can sometimes cause higher diversity at linked sites when compared with the effects of fixations of neutral mutations. This study extends this work by deriving approximate expressions for the mean conditional times to fixation and loss of mutations subject to selection, and analyzing the conditions under which selection increases rather than reduces these times. Simulations are used to examine the relations between diversity at a neutral site and the fixation and loss times of mutations at a linked site that is subject to selection. It is shown that the long-term level of neutral diversity can be increased over the purely neutral value by recurrent fixations and losses of linked, weakly selected dominant or partially dominant favorable mutations, or linked recessive or partially recessive deleterious mutations. The results are used to examine the conditions under which associative overdominance, as opposed to background selection, is likely to operate.     

Screening Neutral and Acidic IgG N-Glycans by High Density Electrophoresis

IgG carries bi-antennary N-linked glycans which differ in degrees of galactosylation, core fucosylation and bisecting N-acetyl glucosamine. The majority of these are non-sialyated closely related neutral structures which can be resolved by HPLC analysis, but which are difficult to separate in techniques such as fluorophore-coupled carbohydrate electrophoresis. Derivatisation with the singly charged fluorophore, 2-amino benzoic acid and separation in gels with a 30% monomer content in tris/glycine buffer enabled separation of neutral glycans. In particular, agalactosyl glycans with either a core fucose substitution or bisecting N-acetyl galactosamine could be resolved. Good separation of mono- and di-galactosylated glycans was also achieved with this system. It was shown that IgG can be separated from serum by size-exclusion and anion exchange chromatography with minimal contamination, with complete glycan release accomplished by the enzyme peptide-N-glycosidase F (F. meningosepticum). This method of resolving IgG glycans could be used to monitor patients in which glycosylation changes may have a diagnostic value, as in rheumatoid arthritis. It could also be used to monitor recombinant IgG glycosylation where routine screening is required in the biotechnology industry.     

Direct selection on allozymes is not required to explain heterogeneity among marker loci across a Mytilus hybrid zone

Unequal differentiation between two types of loci (allozyme and DNA markers) across a Mytilus hybrid zone has recently been claimed as evidence for direct selection on some allozyme loci. We provide here a counter-example: a noncoding DNA locus that exhibits as much differentiation as the incriminated allozymes do. The levels of genetic differentiation varied widely among both allozymes and noncoding DNA markers and no clear difference emerged between the two types of markers. This suggests that the strong interlocus variance in genetic differentiation has been confounded with a discrepancy between marker types as a result of an insufficient and unbalanced locus sampling. Heterogeneity in differentiation among neutral loci can be created by stochastic variance during the allopatric divergence preceding a secondary contact. In hybrid zones, a further source of variance is differential introgression among chromosomal regions after the secondary contact owing to the local influence of selected genes on more or less distant markers. However, the degree of differentiation alone gives no way to distinguish indirect pseudo-selection (a regular and ubiquitous feature of hybrid zones) from direct selection. More generally, we suggest that comparative neutrality tests based on discrepancies among marker types have to be applied with caution when the presence of semi-permeable genetic barriers to gene exchange is suspected.     

Perspective: detecting adaptive molecular polymorphism: lessons from the MHC

Abstract. In the 1960s, when population geneticists first began to collect data on the amount of genetic variation in natural populations, balancing selection was invoked as a possible explanation for how such high levels of molecular variation are maintained. However, the predictions of the neutral theory of molecular evolution have since become the standard by which cases of balancing selection may be inferred. Here we review the evidence for balancing selection acting on the major histocompatibility complex (MHC) of vertebrates, a genetic system that defies many of the predictions of neutrality. We apply many widely used tests of neutrality to MHC data as a benchmark for assessing the power of these tests. These tests can be categorized as detecting selection in the current generation, over the history of populations, or over the histories of species. We find that selection is not detectable in MHC datasets in every generation, population, or every evolutionary lineage. This suggests either that selection on the MHC is heterogeneous or that many of the current neutrality tests lack sufficient power to detect the selection consistently. Additionally, we identify a potential inference problem associated with several tests of neutrality. We demonstrate that the signals of selection may be generated in a relatively short period of microevolutionary time, yet these signals may take exceptionally long periods of time to be erased in the absence of selection. This is especially true for the neutrality test based on the ratio of nonsynonymous to synonymous substitutions. Inference of the nature of the selection events that create such signals should be approached with caution. However, a combination of tests on different time scales may overcome such problems.     

Content and composition of phosphoglycerols and neutral lipids at different developmental stages of the eggs of the codling moth,Cydia pomonella (Lepidoptera: Tortricidae)

Phosphoglycerol, triacylglycerol, diacylglycerol, and free fatty acid content was studied in eggs of the codling moth Cydia pomonella at the white, red ring, and black head developmental stages. The composition of total phosphoglycerols and of the three classes of neutral lipids was also analyzed. The highest total lipid content was found in eggs at the white stage, the amount decreasing during development mainly as a result of a diminution in the quantity of phosphoglycerols, which account for approximately 50% of total content at all stages of egg development. The amount of triacylglycerols and free fatty acids changes significantly during development, whereas only minor changes were found in diacyglycerol levels. The total phosphoglycerol acyl composition of eggs at the white and red ring stages is similar, whereas differences are evident at the black head stage of development. Triacylglycerols and free fatty acids are enriched in saturated fatty acids in all analyzed stages. The acyl profile of diacylglycerols is different at each stage. The unsaturation index decreases in diacylglycerols and free fatty acids as a function of egg development. The results of the present paper suggest that triacylglycerols may constitute an important source of energy during the final period of egg development while phosphoglycerols may function as fuel during the beginning. Phosphoglycerols could be precursors for the triacylglycerol biosynthesis that takes place between white and red ring stages.     

Translocation of phosphatidylthreonine and -serine to mitochondria diminishes exponentially with increasing molecular hydrophobicity

Some cultured cells contain significant amounts of a rarely recognized phospholipid, phosphatidylthreonine. Since phosphatidylthreonine is a structural analog of phosphatidylserine, the question rises whether it is transported to mitochondria and decarboxylated to phosphatidylisopropanolamine therein. We studied this issue with hamster kidney cell-line using a novel approach, i.e. electrospray mass-spectrometry and stable isotope-labeled precursors. Scanning for a neutral loss of 155, which is characteristic for phosphatidylisopropanolamine, indicated that this lipid is indeed present. The identity of phosphatidylisopropanolamine was supported by the following: (i) it co-chromatographed with phosphatidylethanolamine; (ii) its molecular species profile was similar to that of phosphatidylethanolamine; (iii) its head group was labeled from ¹³C-threonine; and (iv) its concentration increased in parallel with phosphatidylthreonine. Tests with solubilized decarboxylase and subcellular fractionation studies indicated that the low cellular content of phosphatidylisopropanolamine is due to inefficient decarboxylation, rather than poor translocation of phosphatidylthreonine to mitochondria. Importantly, the average hydrophobicity of phosphatidylisopropanolamine molecular species was significantly less than that of phosphatidylthreonine species, indicating that hydrophilic phosphatidylthreonine species translocate to mitochondria far more rapidly than hydrophobic ones. Parallel results were obtained for phosphatidylserine. These findings imply that efflux from the ER membrane could be the rate-limiting step in the phosphatidylthreonine and -serine translocation to mitochondria.     

Phosphoglucomutase and trehalase isoenzymes of Venezuelan simulium vectors of Onchocerca volvulus

Phosphoglucomutase (PGM) and trehalase (Tre) isoenzymes of five species of Simulium blackflies (Diptera: Simuliidae), vectors of onchocerciasis in Venezuela, were investigated by means of a portable electrophoresis field kit. Tre differed between S. incrustatum and S. oyapockense s.l. Electrophoretic variation of Tre in other members of the S. amazonicum and S. incrustatum groups merit further investigation. PGM appears to be more useful for separating populations within species complexes. Multiple populations and/or seasonal changes in population structure of S. guianense s.l., S. exiguum s.l. and S. metallicum s.l. were inferred from elecrophoretic variation of PGM.