Cell Biology

The activation of the Takeda G-protein receptor 5 (TGR5, also known as the G protein-coupled bile acid receptor 1, GPBAR1) in enteroendocrine L-cells results in secretion of the anti-diabetic hormone Glucagon-Like Peptide 1 (GLP-1) into systemic circulation. Consequently, recent research has focused on identification and development of TGR5 agonists as type 2 diabetes therapeutics. However, the clinical application of TGR5 agonists has been hampered by side effects of these compounds that primarily result from their absorption into circulation. Here we describe an in vitro screening protocol to evaluate the TGR5 agonism, GLP-1 secretion, and gut-restricted properties of small molecules. The protocol involves differentiating gut epithelial and endocrine cells together in transwells to assess both the pharmacodynamics of TGR5 agonists and the toxicity of compounds to the intestinal monolayer. As a proof of concept, we demonstrate the use of the protocol in evaluating properties of naturally occurring bile acid metabolites that are potent TGR5 agonists. This protocol is adapted from Chaudhari et al. (2021).

The natural environment of microbial cells like bacteria and yeast is often a complex community in which growth and internal organization reflect morphogenetic processes and interactions that are dependent on spatial position and time. While most of research is performed in simple homogeneous environments (e.g., bulk liquid cultures), which cannot capture full spatiotemporal community dynamics, studying biofilms or colonies is complex and usually does not give access to the spatiotemporal dynamics at single cell level. Here, we detail a protocol for generation of a microfluidic device, the “yeast machine”, with arrays of long monolayers of yeast colonies to advance the global understanding of how intercellular metabolic interactions affect the internal structure of colonies within defined and customizable spatial dimensions. With Saccharomyces cerevisiae as a model yeast system we used the “yeast machine” to demonstrate the emergence of glucose gradients by following expression of fluorescently labelled hexose transporters. We further quantified the expression spatial patterns with intra-colony growth rates and expression of other genes regulated by glucose availability. In addition to this, we showed that gradients of amino acids also form within a colony, potentially opening similar approaches to study spatiotemporal formation of gradients of many other nutrients and metabolic waste products. This approach could be used in the future to decipher the interplay between long-range metabolic interactions, cellular development, and morphogenesis in other same species or more complex multi-species systems at single-cell resolution and timescales relevant to ecology and evolution.

Differential exposure of tumor cells to microenvironmental cues greatly impacts cell phenotypes, raising a need for position based sorting of tumor cells amenable to multiple OMICs and functional analyses. One such key determinant of tumor heterogeneity in solid tumors is its vasculature. Proximity to blood vessels (BVs) profoundly affects tumor cell phenotypes due to differential availability of oxygen, gradient exposure to blood-borne substances and inputs by angiocrine factors. To unravel the whole spectrum of genes, pathways and phenotypes impacted by BVs and to determine spatial domains of vascular influences, we developed a methodology for sorting tumor cells according to their relative distance from BVs. The procedure exemplified here using glioblastoma (GBM) model is based on differential uptake of intra-venously injected, freely-diffusing fluorescent dye that allows separation of stroma-free tumor cells residing in different, successive microenvironments amenable for subsequent OMICs and functional analyses. This reliable, easy to use, cost effective strategy can be extended to all solid tumors to study the impact of vasculature or the lack of it.

Trehalose (and glycogen) is a major storage carbohydrate in many cells, including S. cerevisiae. Typically, trehalose (a disaccharide of glucose) is synthesized and stored through gluconeogenesis. However, trehalose can also be made directly from glucose, if glucose-6-phosphate is channeled away from glycolysis or pentose phosphate pathway. Therefore, analyzing trehalose synthesis, utilization or its accumulation, can be used as a sentinel read-out for either gluconeogenesis or rewired glucose utilization. However, the steady-state measurements alone of trehalose cannot unambiguously distinguish the nature of carbon flux in a system. Here, we first summarize simple steady-state enzymatic assays to measure trehalose (and glycogen), that will have very wide uses. Subsequently, we describe methods of highly sensitive, quantitative LC-MS/MS based to measure trehalose. We include methods of ¹³C stable-isotope based pulse-labeling experiments (using different carbon sources) with which to measure rates of trehalose synthesis, from different carbon metabolism pathways. This approach can be used to unambiguously determine the extent of carbon flux into trehalose coming from gluconeogenesis, or directly from glucose/glycolysis. These protocols collectively enable comprehensive steady-state as well as carbon flux based measurements of trehalose. This permits a dissection of carbon flux to distinguish between cells in a gluconeogenic state (conventionally leading to trehalose synthesis), or cells with rewired glucose metabolism (also leading to trehalose synthesis). While the methods presented are optimized for yeast, these methods can be easily adapted to several types of cells, including many microbes.

Metabolic flux analyses are needed to provide insights into metabolic regulation that occurs in cells. The current protocol describes fast and reproducible methods for determining glycolysis and de novo lipogenesis of hepatocytes. Primary culture of hepatocytes is an ‘in vitro’ model useful to study liver glucose and lipid metabolism (Denechaud et al., 2016). The protocol is divided in 2 parts. Part I: Glycolysis experiment is assessed using the Seahorse extracellular flux (XF) analyser. Glycolysis is determined via the measurement of the extracellular acidification rate (ECAR) of the media, which come predominately from the cellular excretion of lactic acid after the conversion of glucose in pyruvate. Part II: De novo lipogenesis experiment determines the radioactive C¹⁴ incorporation in triglycerides (TG) from acetate C¹⁴ precursor. After 2 h acetate supplementation to the media lipids are extracted and separated by TLC (Thin Layer Chromatography) prior quantification of newly synthetized TG labelled.

An important component of this methodology is to assess the role of the tumor microenvironment on tumor growth and survival. To tackle this problem, we have adapted the original approach of Warburg (Warburg, 1923), by combining thin tissue slices with Stable Isotope Resolved Metabolomics (SIRM) to determine detailed metabolic activity of human tissues. SIRM enables the tracing of metabolic transformations of source molecules such as glucose or glutamine over defined time periods, and is a requirement for detailed pathway tracing and flux analysis. In our approach, we maintain freshly resected tissue slices (both cancerous and non- cancerous from the same organ of the same subject) in cell culture media, and treat with appropriate stable isotope-enriched nutrients, e.g., ¹³C₆-glucose or ¹³C₅, ¹⁵N₂-glutamine. These slices are viable for at least 24 h, and make it possible to eliminate systemic influence on the target tissue metabolism while maintaining the original 3D cellular architecture. It is therefore an excellent pre-clinical platform for assessing the effect of therapeutic agents on target tissue metabolism and their therapeutic efficacy on individual patients (Xie et al., 2014; Sellers et al., 2015).

Mice are widely used for human tumor xenograft studies of cancer development and drug efficacy and toxicity. Stable isotope tracing coupled with metabolomic analysis is an emerging approach for assaying metabolic network activity. In mouse models there are several routes of tracer introduction, which have particular advantages and disadvantages that depend on the model and the questions addressed. This protocol describes the bolus i.v. route via repeated tail vein injections of solutions of stable isotope enriched tracers including ¹³C₆-glucose and ¹³C₅,¹⁵N₂-glutamine. Repeated injections give higher enrichments and over longer labeling periods than a single bolus. Multiple injections of glutamine are necessary to achieve adequate enrichment in engrafted tumors.

Brown adipose tissue (BAT) has the unique ability to dramatically increase mitochondrial uncoupled fuel oxidation for thermogenesis in response to adrenergic stimulation. A key parameter in assessing brown adipocyte thermogenic capacity is mitochondrial uncoupling as determined by respiration. Measuring mitochondrial oxygen consumption rate (OCR) therefore provides valuable information to study the regulation and dysregulation of fuel metabolism and energy expenditure. Adding measurements of mitochondrial membrane potential allows for more in-depth interpretation of the respirometry data. Here we provide protocols for measuring respiration in adherent intact and plasma membrane permeabilized brown adipocytes using the Seahorse XF Analyzer. In the protocol Part I, a combination of norepinephrine and free fatty acids are used to induce uncoupled respiration. The ATP Synthase inhibitor oligomycin, the chemical uncoupler FCCP, and the complex III inhibitor Antimycin A are then used to measure coupled, maximal, and non-mitochondrial oxygen consumption, respectively. In the protocol Part II, the plasma membrane is permeabilized with recombinant perfringolysin O, a cholesterol-dependent cytolysin that oligomerizes into pores exclusively in the plasma membrane. This permits experimental control of metabolite availability without separating mitochondria from the native cell environment.

Cells use glucose to generate energy by two different metabolic processes: lactic fermentation and aerobic respiration. In the first common series of reactions, glucose is converted into pyruvate. In anaerobic conditions, pyruvate is transformed into lactate, this process yields to 2 ATP molecules per glucose molecule. In the presence of oxygen, pyruvate is imported into mitochondria where it is used in the Krebs (or TCA) cycle and oxydative phosphorylation. The global process of oxydative phosphorylation yields to 32 ATP per glucose molecule. For reasons not fully understood, in some pathological cases like cancer, cells use anaerobic glycolysis even in the presence of oxygen, in which case the process is called aerobic glycolysis (or Warburg effect). This results in an increased uptake of glucose and lactate production. Measure of intracellular ATP content and lactate concentrations can provide a readout of aerobic glycolyis.

This method allows to evaluate the degradation of lactate during cellular respiration. During this metabolic process, carbon atoms of lactate can be transformed in carbon dioxide. For this purpose, the radioactive lactate is added to the cells and the amount of radioactive carbon dioxide liberated is monitored. The radioactive carbon dioxide generated during cellular respiration is released into the culture medium and it is further converted into gas through the addition of sulfuric acid to culture media. A piece of Whatman paper wet with phenyl-ethylamine-methanol is placed inside the petri dish to trap radioactive carbon dioxide whose production is then evaluated by scintillator counting.