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Plasma metabolite concentrations and labeling enrichments are common measures of organismal metabolism. In mice, blood is often collected by tail snip sampling. Here, we systematically examined the effect of such sampling, relative to gold-standard sampling from an in-dwelling arterial catheter, on plasma metabolomics and stable isotope tracing. We find marked differences between the arterial and tail circulating metabolome, which arise from two major factors: handling stress and sampling site, whose effects were deconvoluted by taking a second arterial sample immediately after tail snip. Pyruvate and lactate were the most stress-sensitive plasma metabolites, rising ~14 and ~5-fold. Both acute handling stress and adrenergic agonists induce extensive, immediate production of lactate, and modest production of many other circulating metabolites, and we provide a reference set of mouse circulatory turnover fluxes with noninvasive arterial sampling to avoid such artifacts. Even in the absence of stress, lactate remains the highest flux circulating metabolite on a molar basis, and most glucose flux into the TCA cycle in fasted mice flows through circulating lactate. Thus, lactate is both a central player in unstressed mammalian metabolism and strongly produced in response to acute stress. [ABSTRACT FROM AUTHOR]

Cellular heterogeneity is an inherent property of cell populations with a wide spectrum of biological manifestations, ranging from barely observable variations that enhance organismal adaptation, to life‐threatening differences. Single‐cell metabolomics can reveal molecular information and variations in metabolite concentrations between cells that are masked in cell‐population studies. These differences are quantitatively captured by the abundance distributions for the population and their statistical analysis can reveal the presence of latent subpopulations. In recent years, mass spectrometry (MS), combined with novel sampling and ionization techniques, has become an important tool for single‐cell metabolomics. These new techniques must contend with significant challenges in the form of small cell sizes and volumes, ultratrace metabolite amounts per cell, and potentially interfering high turnover rates and rapid diffusion. Owing to the ultrasmall sample volume, low abundance of some metabolites, and poor ionization efficiencies, metabolite detection, identification, and accurate quantitation remain a major challenge. The ability of some techniques to analyze tissue‐embedded cells opens the door for spatial metabolomics, potentially revealing cellular synergism in organ level metabolism. In this Concept article, we present a bird's eye view of these major themes in single cell metabolomics and some of the relevant MS‐based methods. [ABSTRACT FROM AUTHOR]

Mass spectrometry imaging (MSI) enables simultaneous spatial mapping for diverse molecules in biological tissues. Matrix‐assisted laser desorption ionization (MALDI) mass spectrometry (MS) has been a mainstream MSI method for a wide range of biomolecules. However, MALDI‐MSI of biological homopolymers used for energy storage and molecular feedstock is limited by, e.g., preferential ionization for certain molecular classes. Matrix‐free nanophotonic ionization from silicon nanopost arrays (NAPAs) is an emerging laser desorption ionization (LDI) platform with ultra‐trace sensitivity and molecular imaging capabilities. Here, we show complementary analysis and MSI of polyhydroxybutyric acid (PHB), polyglutamic acid (PGA), and polysaccharide oligomers in soybean root nodule sections by NAPA‐LDI and MALDI. For PHB, number and weight average molar mass, polydispersity, and oligomer size distributions across the tissue section and in regions of interest were characterized by NAPA‐LDI‐MSI. [ABSTRACT FROM AUTHOR]

Mass spectrometry imaging (MSI) has become an important analytical tool for the label‐free chemical imaging of diverse molecules in biological specimens. This minireview surveys some emerging methods in the context of factors that can lead to inaccurate information in MSI, chemical and spatial aberrations, along with their common sources. Matrix‐assisted laser desorption ionization, based on organic matrices, has become the most widely used MSI technique for biomolecules. However, due to inherent limitations associated with the use of organic matrices, for example, heterogeneous matrix‐analyte cocrystallization, and spectral interferences due to the matrix, laser desorption ionization (LDI) from inorganic and nanophotonic platforms has emerged as an alternative MSI modality with complementary advantages. In this review, inorganic and nanophotonic platforms for LDI‐MSI, their applications in imaging, notable merits, and limitations are described. [ABSTRACT FROM AUTHOR]

SUMMARY: The establishment of the nitrogen‐fixing symbiosis between soybean and Bradyrhizobium japonicum is a complex process. To document the changes in plant metabolism as a result of symbiosis, we utilized laser ablation electrospray ionization‐mass spectrometry (LAESI‐MS) for in situ metabolic profiling of wild‐type nodules, nodules infected with a B. japonicum nifH mutant unable to fix nitrogen, nodules doubly infected by both strains, and nodules formed on plants mutated in the stearoyl‐acyl carrier protein desaturase (sacpd‐c) gene, which were previously shown to have an altered nodule ultrastructure. The results showed that the relative abundance of fatty acids, purines, and lipids was significantly changed in response to the symbiosis. The nifH mutant nodules had elevated levels of jasmonic acid, correlating with signs of nitrogen deprivation. Nodules resulting from the mixed inoculant displayed similar, overlapping metabolic distributions within the sectors of effective (fix+) and ineffective (nifH mutant, fix−) endosymbionts. These data are inconsistent with the notion that plant sanctioning is cell autonomous. Nodules lacking sacpd‐c displayed an elevation of soyasaponins and organic acids in the central necrotic regions. The present study demonstrates the utility of LAESI‐MS for high‐throughput screening of plant phenotypes. Overall, nodules disrupted in the symbiosis were elevated in metabolites related to plant defense. Significance Statement: Biological nitrogen fixation is important for sustainable agriculture. The present study examined metabolites in soybean nodules using laser ablation electrospray ionization‐mass spectrometry, a high‐throughput and powerful approach for in situ metabolic profiling. The results provide information regarding those metabolic pathways affected in nodules disrupted in the symbiosis as a result of mutation. [ABSTRACT FROM AUTHOR]