ReviewReview: Microfluidic applications in metabolomics and metabolic profiling
Introduction
The goal of this review is to summarize the use of microfluidics as a novel tool set for metabolomics, metabolic profiling, and other metabolite related biological studies. To the knowledge of the authors this is the first such review generated, and thus we will include articles dating back as far as 2000 to give the reader a comprehensive view of the state of the art. While this is the first review of this topic, a number of important reviews on topics ranging from microfluidic and general analytical methods applicable to metabolomics [1], [2], [3], [4], [5], [6] to metabolomic and metabolic profiling [7], [8], [9], [10], [11], [12] have been published previously. For specific topics, it is suggested the reader consult these works. These reviews are by no means a comprehensive list of all reviews in this field given that several dozen review articles have been published over the last nine years ranging in scope from very specific to more general.
Before discussing specific applications of microfluidics to metabolomics we will first define metabolomics and the various subsets of this discipline. While many people are familiar with genomics and proteomics, the terminology associated with metabolomics is not as clearly defined. For the sake of this review, we define metabolomics as five separate categories: (1) metabolomics, (2) metabonomics, (3) metabolic fingerprinting, (4) metabolic profiling, and (5) targeted metabolic profiling [13]. Metabolomics is the overall study of metabolite expression at any given point in time. Commonly this is done using qualitative techniques like nuclear magnetic resonance (NMR) spectroscopy and mass spectrometry (MS) coupled to a high-resolution separation technique like liquid (LC) or gas chromatography (GC). These techniques provide the ability to detect many species in a single sample. In metabonomics, the goal is to follow changes in the concentrations of large numbers of metabolic markers over time [14]. Again, NMR and MS techniques are common in this form of metabolomics. Often times metabonomic studies make use of radiolabeled nutrients or other inputs to quantitatively compare different profiles. To date, metabolomics and metabonomics make up the largest research segment in metabolomics largely because they can be used to determine biomarkers of disease as well as help in elucidating specific pathways associated with biological changes. The remaining three areas of metabolomics have more specific applications. Metabolic fingerprinting seeks to measure a global profile of metabolites with identification of specific profiles based on pattern recognition. In these applications, non-specific detection methods are used reducing the analysis cost and often increasing method throughput. The major downfall of metabolic fingerprinting is the inability to identify specific biomarkers for a disease state or therapeutic endpoint. Metabolic profiling seeks to measure a specific subset of compounds such as amino acids or carbohydrates. A common area of research in the field of metabolic profiling is lipidomics which seeks to measure lipids present in a sample. Lipids represent an important class of metabolites that are of significant interest because of their multitude of roles in biology. Metabolic profiling can use a wide variety of techniques depending on the specific analytes in question. The final area of metabolomics is targeted metabolic profiling. In this area of research, one or two analytes are tracked with time. This area of research has the longest history and is often excluded from the discussion of metabolomics because of the limited number of analytes profiled. It is, however, a very useful tool for understanding biological systems.
As mentioned above, the most common tools in metabolomics are NMR, GC–MS, and LC–MS because they are capable of detecting and identifying many analytes in a single sample. Microfluidics has only recently been added to the metabolomics toolbox but has the potential to a play a significant role in this field. For example, extensive, time-consuming sample preparation techniques consisting of multiple extractions and derivatization reactions are commonly associated with metabolomic studies [15]. These steps could be carried out in a microfluidic format with reduced time and increased efficiency. To date the vast majority of microfluidic systems have focused on coupling selective separation and detection systems for determination of biological samples. In addition, a number of microfluidic systems have been developed that integrate biological systems (cells) with a microfluidic system to perform metabolomic studies. This review will first cover microseparation techniques since this is the most common use of microfluidics in metabolomics. The review then covers sample preparation techniques such as filtration and solid phase extraction that have been developed for microfluidics and are applicable to metabolomic studies. Finally, we will cover integrated microfluidic systems that combine aspects of biology with microanalytical systems. Table 1 summarizes pertinent information for research papers introduced in this review, including type of metabonomics, analyte investigated, detector type, and microfluidic material.
Section snippets
Microseparations
As discussed above, many approaches for metabolomic analysis utilize a high-resolution separation technique to aid in the identification or measurement of complex mixtures of metabolites. Separations on the microfluidic format are typically performed by electrophoretic means, such as microchip capillary electrophoresis (microchip-CE), or through pressure driven flow, such as miniaturized-LC. The following sections introduce examples of microchip-CE and miniaturized-LC for various applications
Sample preparation
One of the goals of microfluidics research is to incorporate sample pre-processing and analysis platforms onto the same device. Integrated microfluidic devices such as these are termed lab on a chip (LOC) or micro-total analysis systems (μTAS), and have many potential benefits for metabolic analyses. The rapid analysis times inherent to these systems make them attractive alternatives to currently used analytical methodologies. In addition, autonomous operation of these systems would further
Microfluidics for cellular analysis
Microfluidics technologies are capable of manipulating and mixing small volumes of solutions and reagents using networks of channels and reaction chambers. These features of microfluidic devices also make them well suited for the analysis and/or growth of cells and tissues. There have been extensive applications of microfluidics for miniaturized cell culture, isolation, trapping, concentration, and chemical cellular monitoring after exposure to a stimulus. Other recent review articles have
Conclusions
This review has presented recent publications that utilize microfluidic devices for metabolomics, metabonomics, metabolic fingerprinting, and targeted metabolic profiling applications. While the majority of bioanalyical microfluidics applications are focused towards proteomic and genomic systems, there is a growing trend towards developing microsystems for the determination of small molecules and metabolites. Furthermore, microsystems that can detect and measure metabolic changes in time and in
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