Using measures of metabolic flux to align screening and clinical development: Avoiding pitfalls to enable translational studies

Screening campaigns, especially those aimed at modulating enzyme activity, often rely on measuring substrate→product conversions. Unfortunately, the presence of endogenous substrates and/or products can limit one's ability to measure conversions. As well, coupled detection systems, often used to facilitate optical readouts, are subject to interference. Stable isotope labeled substrates can overcome background contamination and yield a direct readout of enzyme activity. Not only can isotope kinetic assays enable early screening, but they can also be used to follow hit progression in translational (pre)clinical studies. Herein, we consider a case study surrounding lipid biology to exemplify how metabolic flux analyses can connect stages of drug development, caveats are highlighted to ensure reliable data interpretations. For example, when measuring enzyme activity in early biochemical screening it may be enough to quantify the formation of a labeled product. In contrast, cell-based and in vivo studies must account for variable exposure to a labeled substrate (or precursor) which occurs via tracer dilution and/or isotopic exchange. Strategies are discussed to correct for these complications. We believe that measures of metabolic flux can help connect structure-activity relationships with pharmacodynamic mechanisms of action and determine whether mechanistically differentiated biophysical interactions lead to physiologically relevant outcomes. Adoption of this logic may allow research programs to (i) build a critical bridge between primary screening and (pre)clinical development, (ii) elucidate biology in parallel with screening and (iii) suggest a strategy aimed at in vivo biomarker development.

Mapping Lipogenic Flux: A Gold LDI-MS Approach for Imaging Neutral Lipid Kinetics

Spatial characterization of triglyceride metabolism is an area of significant interest which can be enabled by mass spectrometry imaging via recent advances in neutral lipid laser desorption analytical approaches. Here, we extend recent advancements in gold-assisted neutral lipid imaging and demonstrate the potential to map lipid flux in rodents. We address here critical issues surrounding the analytical configuration and interpretation of the data for a group of select triglycerides. Specifically, we examined how the signal intensity and spatial resolution would impact the apparent isotope ratio in a given analyte (which is an important consideration when performing MS based kinetics studies of this kind) with attention given to molecular ions and not fragments. We evaluated the analytics by contrasting lipid flux in well characterized mouse models, including fed vs fed states and different dietary perturbations. In total, the experimental paradigm described here should enable studies of hepatic lipogenesis; presumably, this logic can be enhanced via the inclusion of ion mobility and/or fragmentation. Although this study was carried out in robust models of liver lipogenesis, we expect that the model system could be expanded to a variety of tissues where zonated (or heterogeneous) lipid synthesis may occur, including solid tumor metabolism.

Isotope Fractionation during Gas Chromatography Can Enhance Mass Spectrometry-Based Measures of 2H-Labeling of Small Molecules

Stable isotope tracers can be used to quantify the activity of metabolic pathways. Specifically, 2H-water is quite versatile, and its incorporation into various products can enable measurements of carbohydrate, lipid, protein and nucleic acid kinetics. However, since there are limits on how much 2H-water can be administered and since some metabolic processes may be slow, it is possible that one may be challenged with measuring small changes in isotopic enrichment. We demonstrate an advantage of the isotope fractionation that occurs during gas chromatography, namely, setting tightly bounded integration regions yields a powerful approach for determining isotope ratios. We determined how the degree of isotope fractionation, chromatographic peak width and mass spectrometer dwell time can increase the apparent isotope labeling. Relatively simple changes in the logic surrounding data acquisition and processing can enhance gas chromatography-mass spectrometry measures of low levels of 2H-labeling, this is especially useful when asymmetrical peaks are recorded at low signal:background. Although we have largely focused attention on alanine (which is of interest in studies of protein synthesis), it should be possible to extend the concepts to other analytes and/or hardware configurations.

Engineering the Next Generation of Matched Models for Chemical Translational Biology

In order to achieve patient personalization and translate compounds through the discovery phase into the clinic, high throughput test models should be designed to be as closely matched to the patient as possible. Engineering high throughput and physiologically relevant biological models is the idealized scenario for testing next generation modulators. I present here a cautionary example of a misaligned model as well as my viewpoint on how overcoming this bottleneck is one of the next frontiers in chemical biology.

Impact of Extracellular Fatty Acids and Oxygen Tension on Lipid Synthesis and Assembly in Pancreatic Cancer Cells

Lipid oxidation and biosynthesis are crucial for cell survival, especially for rapidly proliferating cancer cells in a heterogeneous metabolic environment. The storage of high-energy lipid reservoirs competitively advantages the cancer cell over non-neoplastic tissue. Disrupting lipid biosynthetic processes, through modulation of fatty acid (FA) esterification or de novo lipogenesis (DNL), is of interest in drug discovery. Mimicking the in vivo environment in vitro is also vital for testing the efficacy of potential drug compounds. We present here a stable isotope tracer-based approach for examining the impact of exogenous FA and oxygen tension on the pathways that affect lipid biosynthesis, including the rates of metabolic flux. By applying tandem mass spectrometry (MS/MS) analyses to studies using parallel tracers, we characterized the impact of FA bioavailability on the positional enrichment within specific lipids. Our observations suggest that adding bioavailable FA as a carbon source preferentially biases the cellular metabolism away from DNL and toward esterification of free fatty acid pools. Additionally, we have found that this FA addition, under hypoxic conditions, led to a biased increase in the total triglyceride pool (nearly 5-fold, as compared to phospholipids), regardless of the isotope tracer utilized. We discuss the implications of this metabolic flexibility on studies that aim to characterize apparent drug efficacy.

Key Concepts Surrounding Studies of Stable Isotope-Resolved Metabolomics

"Omics"-based analyses are widely used in numerous areas of research, advances in instrumentation (both hardware and software) allow investigators to collect a wealth of data and therein characterize metabolic systems. Although analyses generally examine differences in absolute or relative (fold-) changes in concentrations, the ability to extract mechanistic insight would benefit from the use of isotopic tracers. Herein, we discuss important concepts that should be considered when stable isotope tracers are used to capture biochemical flux. Special attention is placed on in vivo systems, however, many of the general ideas have immediate impact on studies in cellular models or isolated-perfused tissues. While it is somewhat trivial to administer labeled precursor molecules and measure the enrichment of downstream products, the ability to make correct interpretations can be challenging. We will outline several critical factors that may influence choices when developing and/or applying a stable isotope tracer method. For example, is there a "best" tracer for a given study? How do I administer a tracer? When do I collect my sample(s)? While these questions may seem straightforward, we will present scenarios that can have dramatic effects on conclusions surrounding apparent rates of metabolic activity.

Spatial and temporal studies of metabolic activity: contrasting biochemical kinetics in tissues and pathways during fasted and fed states

The regulation of nutrient homeostasis, i.e., the ability to transition between fasted and fed states, is fundamental in maintaining health. Since food is typically consumed over limited (anabolic) periods, dietary components must be processed and stored to counterbalance the catabolic stress that occurs between meals. Herein, we contrast tissue- and pathway-specific metabolic activity in fasted and fed states. We demonstrate that knowledge of biochemical kinetics that is obtained from opposite ends of the energetic spectrum can allow mechanism-based differentiation of healthy and disease phenotypes. Rat models of type 1 and type 2 diabetes serve as case studies for probing spatial and temporal patterns of metabolic activity via [²H]water labeling. Experimental designs that capture integrative whole body metabolism, including meal-induced substrate partitioning, can support an array of research surrounding metabolic disease; the relative simplicity of the approach that is discussed here should enable routine applications in preclinical models.

Characterization of Brain Metabolism by Nuclear Magnetic Resonance

The noninvasive, quantitative ability of nuclear magnetic resonance (NMR) spectroscopy to characterize small molecule metabolites has long been recognized as a major strength of its application in biology. Numerous techniques exist for characterizing metabolism in living, excised, or extracted tissue, with a particular focus on 1 H-based methods due to the high sensitivity and natural abundance of protons. With the increasing use of high magnetic fields, the utility of in vivo 1 H magnetic resonance spectroscopy (MRS) has markedly improved for measuring specific metabolite concentrations in biological tissues. Higher fields, coupled with recent developments in hyperpolarization, also enable techniques for complimenting 1 H measurements with spectroscopy of other nuclei, such as 31 P and 13 C, and for combining measurements of metabolite pools with metabolic flux measurements. We compare ex vivo and in vivo methods for studying metabolism in the brain using NMR and highlight insights gained through using higher magnetic fields, the advent of dissolution dynamic nuclear polarization, and combining in vivo MRS and ex vivo NMR approaches.

Stromal cell-derived factor-1 promoted angiogenesis and inflammatory cell infiltration in aneurysm walls

OBJECT

A small percentage of cerebral aneurysms rupture, but when they do, the effects are devastating. Current management of unruptured aneurysms consists of surgery, endovascular treatment, or watchful waiting. If the biology of how aneurysms grow and rupture were better known, a novel drug could be developed to prevent unruptured aneurysms from rupturing. Ruptured cerebral aneurysms are characterized by inflammation-mediated wall remodeling. The authors studied the role of stromal cell-derived factor-1 (SDF-1) in inflammation-mediated wall remodeling in cerebral aneurysms.

METHODS

Human aneurysms, murine carotid artery aneurysms, and murine intracranial aneurysms were studied using immunohistochemistry. Flow cytometry analysis was performed on blood from mice developing carotid or intracranial aneurysms. The effect of SDF-1 on endothelial cells and macrophages was studied by chemotaxis cell migration assay and capillary tube formation assay. Anti-SDF-1 blocking antibody was given to mice and compared with control (vehicle)-administered mice for its effects on the walls of carotid aneurysms and the development of intracranial aneurysms.

RESULTS

Human aneurysms, murine carotid aneurysms, and murine intracranial aneurysms all expressed SDF-1, and mice with developing carotid or intracranial aneurysms had increased progenitor cells expressing CXCR4, the receptor for SDF-1 (p < 0.01 and p < 0.001, respectively). Human aneurysms and murine carotid aneurysms had endothelial cells, macrophages, and capillaries in the walls of the aneurysms, and the presence of capillaries in the walls of human aneurysms was associated with the presence of macrophages (p = 0.01). Stromal cell-derived factor-1 promoted endothelial cell and macrophage migration (p < 0.01 for each), and promoted capillary tube formation (p < 0.001). When mice were given anti-SDF-1 blocking antibody, there was a significant reduction in endothelial cells (p < 0.05), capillaries (p < 0.05), and cell proliferation (p < 0.05) in the aneurysm wall. Mice given anti-SDF-1 blocking antibody developed significantly fewer intracranial aneurysms (33% vs 89% in mice given control immunoglobulin G, respectively; p < 0.05).

CONCLUSIONS

These data suggest SDF-1 is associated with angiogenesis and inflammatory cell migration and proliferation in the walls of aneurysms, and may have a role in the development of intracranial aneurysms.

Modified murine intracranial aneurysm model: aneurysm formation and rupture by elastase and hypertension

Introduction

Cerebral aneurysms occur in up to 5% of the population. There are several murine models of aneurysms; however, all have limitations and none reproducibly model aneurysm rupture. To fulfill this need, we modified two current rodent aneurysm models to create a murine model which reproducibly produces intracranial aneurysms and rupture.

Methods

The left common carotid arteries and the right renal arteries were ligated in C57BL/6 female mice with a hypertensive diet. One week later, small burr holes were created with a stereotactic frame using the following stereotactic measurements: 1.2 mm rostral and 0.7 mm lateral to the right of the bregma. A 26 G needle was gradually advanced via the burr hole until contact with the skull base, upon which the needle was pulled back 0.3 mm. Five, 10 and 20 μL of 10 U/mL elastase solution and 10 μL of 1 U/mL elastase solution were stereotactically injected into the basal cisterns. Angiotensin II was then continually infused at a dose of 1000 ng/kg/min via an osmotic pump placed subcutaneously. In the control mice, 20 μL bromophenol blue solution was injected. Three weeks later, or earlier if mice expired prior to 3 weeks, the circle of Willis was inspected by microscopy for aneurysm formation and/or signs of rupture. Histological analyses were then performed to evaluate elastic lamina destruction, inflammatory cell and macrophage infiltration, absence of intimal endothelial cells and thickening of the smooth muscle layer within the aneurysm wall. To compare with human aneurysms, human aneurysm specimens (n=35; 34 unruptured and 1 ruptured) and normal control superficial temporal arteries (STAs) (n=9) were examined.

Results

All mice given 5, 10 and 20 μL of 10 U/mL elastase solution developed intracranial aneurysms within the circle of Willis; 40%, 60% and 50% of mice had ruptured aneurysms, respectively. In mice given 10 μL of 1.0 U/mL elastase solution, 90% developed intracranial aneurysms and 20% had ruptured aneurysms. Aneurysms were confirmed by examining the destruction of the elastic lamina. Aneurysms consistently demonstrated CD45 positive inflammatory cell and F4/80 positive macrophage infiltration within the aneurysm wall which was not present in the circle of Willis of normal sham-operated mice. These results were similar to those in human aneurysms and STA control arteries.

Conclusions

We modified two current rodent aneurysm models to create a murine model that produces consistent aneurysms and rupture and can be used for studying cerebral aneurysm formation, rupture and treatment.

Monocyte chemotactic protein-1 promotes inflammatory vascular repair of murine carotid aneurysms via a macrophage inflammatory protein-1α and macrophage inflammatory protein-2-dependent pathway

BACKGROUND

Up to 5% of the population may have a brain aneurysm. If the brain aneurysm ruptures, there is >50% mortality, and more than one third of survivors are dependent. Brain aneurysms detected before rupture can be treated to prevent rupture, or ruptured aneurysms can be treated to prevent rerupture. Endovascular coiling of brain aneurysms is the treatment of choice for some aneurysms; however, up to one quarter of aneurysms may recur. The coiled aneurysms that do not recur are characterized by inflammatory intra-aneurysmal tissue healing; therefore, we studied the biology of this process, specifically the role of monocyte chemotactic protein-1 (MCP-1), a cytokine known for tissue healing.

METHODS AND RESULTS

We created coils with a 50:50 poly-dl-lactic glycolic acid (PLGA) coating that released MCP-1 at 3 different doses (100 μg/mL, 1 mg/mL, and 10 mg/mL) and performed a dose-response study for effect on intra-aneurysmal tissue healing in a murine carotid aneurysm model. We then demonstrated that MCP-1 (100 μg/mL)-releasing coils promote significantly greater aneurysm tissue in-growth than bare platinum or PLGA-only coils. We show that MCP-1 recruits the migration of fibroblasts, macrophages, smooth muscle cells, and endothelial cells in vitro in cell migration assays and in vivo in murine carotid aneurysms. Using gfp(+) bone marrow-transplant chimeric mice, we demonstrate that the MCP-1-recruited fibroblasts and macrophages are derived from the bone marrow. We demonstrate that this MCP-1-mediated vascular inflammatory repair occurs via a macrophage inflammatory protein (MIP)-1α- and MIP-2-dependent pathway. MCP-1 released from coiled murine aneurysms causes significant upregulation of MIP-1α and MIP-2 expression by cytokine array assay. Blocking MIP-1α and MIP-2 with antagonist antibody causes a significant decrease in MCP-1-mediated intra-aneurysmal tissue healing.

CONCLUSION

Our findings suggest that MCP-1 has a critical role in promoting inflammatory intra-aneurysmal tissue healing in an MIP-1α- and MIP-2-dependent pathway.