Electroanalytical eavesdropping on single cell communication

Streamlining the interface between electronics and neural systems for bidirectional electrochemical communication

Seamless neural interfaces conjoining neurons and electrochemical devices hold great potential for highly efficient signal transmission across neural systems and the external world. Signal transmission through chemical sensing and stimulation via electrochemistry is remarkable because communication occurs through the same chemical language of neurons. Emerging strategies based on synaptic interfaces, iontronics-based neuromodulation, and improvements in selective neurosensing techniques have been explored to achieve seamless integration and efficient neuro-electronics communication. Synaptic interfaces can directly exchange signals to and from neurons, in a similar manner to that of chemical synapses. Hydrogel-based iontronic chemical delivery devices are operationally compatible with neural systems for improved neuromodulation. In this perspective, we explore developments to improve the interface between neurons and electrodes by targeting neurons or sub-neuronal regions including synapses. Furthermore, recent progress in electrochemical neurosensing and iontronics-based chemical delivery is examined.

Echem methods and electrode types of the current in vivo electrochemical sensing

For a long time, people have been eager to realize continuous real-time online monitoring of biological compounds. Fortunately, in vivo electrochemical biosensor technology has greatly promoted the development of biological compound detection. This article summarizes the existing in vivo electrochemical detection technologies into two categories: microdialysis (MD) and microelectrode (ME). Then we summarized and discussed the electrode surface time, pollution resistance, linearity and the number of instances of simultaneous detection and analysis, the composition and characteristics of the sensor, and finally, we also predicted and prospected the development of electrochemical technology and sensors in vivo.

Plasmodium chabaudi Affects Mast Cell Degranulation as Measured by Carbon-Fiber Microelectrode Amperometry

Mast cells (MCs) are effector cells of the immune system commonly known for their role in asthma and allergy. They are present throughout biological systems in various tissues, serving as an interface between the biological system and environment. Previous work characterizing the impact of malaria on MCs revealed contradictory results, showing minimal to strong correlation between MC degranulation and disease progression. This work seeks to reveal how MC degranulation is impacted in the presence of malaria, induced by Plasmodium chabaudi, using a mouse model and a single cell measurement technique that reveals exquisite biophysical detail about any impacts to the degranulation process. It was hypothesized that the malaria parasites would impact MC degranulation response during live infection, and the differences would be revealed via carbon-fiber microelectrode amperometry. In fact, the data collected show that different stages of malaria infection affect MC degranulation differently, affirming the importance of considering different infection stages in future studies of malarial immune response. Furthermore, a comparison of MC degranulation response to that measured from platelets under similar circumstances shows similar trends in quantitative degranulation, suggesting that MC and platelet exocytosis machinery are affected similarly despite their distinct biological roles. However, based on the small number of mouse replicates, the studies herein suggest that there should be further study about cellular and disease processes. Overall, the work herein reveals important details about the role of MCs in malaria progression, relevant during treatment decisions, as well as a potentially generalizable impact on chemical messenger secretion from cells during malarial progression.

Carbon-Fiber Nanoelectrodes for Real-Time Discrimination of Vesicle Cargo in the Native Cellular Environment

Carbon-fiber microelectrodes have proven to be an indispensable tool for monitoring exocytosis events using amperometry. When positioned adjacent to a cell, a traditional microdisc electrode is well suited for quantification of discrete exocytotic release events. However, the size of the electrode does not allow for intracellular electrochemical measurements, and the amperometric approach cannot distinguish between the catecholamines that are released. In this work, carbon nanoelectrodes were developed to permit selective electrochemical sampling of nanoscale vesicles in the cell cytosol. Classical voltammetric techniques and electron microscopy were used to characterize the nanoelectrodes, which were ∼5 μm long and sharpened to a nanometer-scale tip that could be wholly inserted into individual neuroendocrine cells. The nanoelectrodes were coupled with fast-scan cyclic voltammetry to distinguish secretory granules containing epinephrine from other catecholamine-containing granules encountered in the native cellular environment. Both vesicle subtypes were encountered in most cells, despite prior demonstration of populations of chromaffin cells that preferentially release one of these catecholamines. There was substantial cell-to-cell variability in relative epinephrine content, and vesicles containing epinephrine generally stored more catecholamine than the other vesicles. The carbon nanoelectrode technology thus enabled analysis of picoliter-scale biological volumes, revealing key differences between chromaffin cells at the level of the dense-core granule.

Insight into the Effects of Plasmodium chabaudi on Platelets Using Carbon-Fiber Microelectrode Amperometry

Platelets are anuclear circulating cell bodies within the bloodstream commonly known for their roles in clot formation during vascular injury to prevent blood loss. They also have significant impact in a range of diseases, including malaria. However, the role of platelets in malaria is controversial, with contradicting evidence suggesting either that they assist in destruction of malarial parasites or facilitate a severe form of malaria. Precedent work suggests that the timing of infection is critical in determining whether platelets switch roles from being protective to deleterious. As such, the work herein makes use of the unique mechanistic perspective offered by carbon-fiber microelectrode amperometry (CFMA) to understand how platelet secretion is impacted in malarial infection stages (ascending parasite count versus descending parasite count). Malarial platelet behavior was compared to platelets from noninfected control mice by probing their exocytotic function. Results suggest that mouse malaria caused by the parasite Plasmodium chabaudi, during both ascending and descending infection stages, reduces platelet exocytotic events and delays platelet granule fusion; in addition, platelets are more impacted by the disease early in the infection stages. In all, understanding platelet behavior in the malarial context may present new therapeutic routes to treat or cure malaria.

Static and Dynamic Measurement of Dopamine Adsorption in Carbon Fiber Microelectrodes Using Electrochemical Impedance Spectroscopy

In this study, electrochemical impedance spectroscopy was used for the first time to study the adsorption of dopamine in carbon fiber microelectrodes. In order to show a proof-of-concept, static and dynamic measurements were taken at potentials ranging from -0.4 to 0.8 V versus Ag|AgCl to demonstrate the versatility of this technique to study dopamine without the need of its oxidation. We used electrochemical impedance spectroscopy and single frequency electrochemical impedance to measure different concentrations of dopamine as low as 1 nM. Moreover, the capacitance of the microelectrodes surface was found to decrease due to dopamine adsorption, which is dependent on its concentration. The effect of dissolved oxygen and electrochemical oxidation of the surface in the detection of dopamine was also studied. Nonoxidized and oxidized carbon fiber microelectrodes were prepared and characterized by optical microscopy, scanning electron microscopy, cyclic voltammetry, and electrochemical impedance spectroscopy. Optimum working parameters of the electrodes, such as frequency and voltage, were obtained for better measurement. Electrochemical impedance of dopamine was determined at different concentration, voltages, and frequencies. Finally, dynamic experiments were conducted using a flow cell and single frequency impedance in order to study continuous and real-time measurements of dopamine.

Tuning Selectivity of Fluorescent Carbon Nanotube-Based Neurotransmitter Sensors

Detection of neurotransmitters is an analytical challenge and essential to understand neuronal networks in the brain and associated diseases. However, most methods do not provide sufficient spatial, temporal, or chemical resolution. Near-infrared (NIR) fluorescent single-walled carbon nanotubes (SWCNTs) have been used as building blocks for sensors/probes that detect catecholamine neurotransmitters, including dopamine. This approach provides a high spatial and temporal resolution, but it is not understood if these sensors are able to distinguish dopamine from similar catecholamine neurotransmitters, such as epinephrine or norepinephrine. In this work, the organic phase (DNA sequence) around SWCNTs was varied to create sensors with different selectivity and sensitivity for catecholamine neurotransmitters. Most DNA-functionalized SWCNTs responded to catecholamine neurotransmitters, but both dissociation constants (K_d) and limits of detection were highly dependent on functionalization (sequence). K_d values span a range of 2.3 nM (SWCNT-(GC)₁₅ + norepinephrine) to 9.4 μM (SWCNT-(AT)₁₅ + dopamine) and limits of detection are mostly in the single-digit nM regime. Additionally, sensors of different SWCNT chirality show different fluorescence increases. Moreover, certain sensors (e.g., SWCNT-(GT)₁₀) distinguish between different catecholamines, such as dopamine and norepinephrine at low concentrations (50 nM). These results show that SWCNTs functionalized with certain DNA sequences are able to discriminate between catecholamine neurotransmitters or to detect them in the presence of interfering substances of similar structure. Such sensors will be useful to measure and study neurotransmitter signaling in complex biological settings.

Neuropeptide-Induced Mast Cell Degranulation and Characterization of Signaling Modulation in Response to IgE Conditioning

As tissue-resident immune cells, mast cells are frequently found in close proximity to afferent neurons and are subjected to immunoactive mediators secreted by these neurons, including substance P (SP) and calcitonin gene-related peptide (CGRP). Neurogenic inflammation is thought to play an important role in the pathophysiology of many diseases. Unraveling the cellular mechanisms at the interface between the immune response and the peripheral nervous system is important for understanding how these diseases arise and progress. In this work, mast cell degranulation following direct exposure to CGRP and SP was studied both at the bulk and single-cell levels to characterize the mouse peritoneal mast cell response to neuropeptides and compare this response to well-studied mast cell activation pathways. Results show that mast cells secrete fewer chemical messenger-filled granules with increased IgE preincubation concentrations. The biophysical characteristics of mast cell degranulation in response to SP and CGRP is in many ways similar to calcium ionophore-induced mast cell degranulation; however, neuropeptide-stimulated mast cells secrete reduced chemical messenger content per secretion event, resulting in an overall relative decrease in secreted chemical messengers.

Nanoparticle-Templated Molecular Recognition Platforms for Detection of Biological Analytes

Molecular recognition of biological analytes with optical nanosensors provides both spatial and temporal biochemical information. A recently developed sensing platform exploits near-infrared fluorescent single-wall carbon nanotubes combined with electrostatically pinned heteropolymers to yield a synthetic molecular recognition technique that is maximally transparent through biological matter. This molecular recognition technique is known as corona phase molecular recognition (CoPhMoRe). In CoPhMoRe, the specificity of a folded polymer toward an analyte does not arise from a pre-existing polymer-analyte chemical affinity. Rather, specificity is conferred through conformational changes undergone by a polymer that is pinned to the surface of a nanoparticle in the presence of an analyte and the subsequent modifications in fluorescence readout of the nanoparticles. The protocols in this article describe a novel single-molecule microscopy tool (near-infrared fluorescence and total internal reflection fluorescence [nIRF TIRF] hybrid microscope) to visualize the CoPhMoRe recognition process, enabling a better understanding of synthetic molecular recognition. We describe this requisite microscope for simultaneous single-molecule visualization of optical molecular recognition and signal transduction. We elaborate on the general procedures for synthesizing and identifying single-walled carbon nanotube-based sensors that employ CoPhMoRe via two biologically relevant examples of single-molecule recognition for the hormone estradiol and the neurotransmitter dopamine. © 2016 by John Wiley & Sons, Inc.

Platelet membrane variations and their effects on δ-granule secretion kinetics and aggregation spreading among different species

Platelet exocytosis is regulated partially by the granular/cellular membrane lipids and proteins. Some platelets contain a membrane-bound tube, called an open canalicular system (OCS), which assists in granular release events and increases the membrane surface area for greater spreading. The OCS is not found in all species, and variations in membrane composition can cause changes in platelet secretion. Since platelet studies use various animal models, it is important to understand how platelets differ in both their composition and granular release to draw conclusions among various models. The relative phospholipid composition of the platelets with (mouse, rabbit) and without (cow) an OCS was quantified using UPLC-MS/MS. Cholesterol and protein composition was measured using an Amplex Red Assay and BCA Assay. TEM and dark field platelet images were gathered and analyzed with Image J. Granular release was monitored with single cell carbon fiber microelectrode amperometry. Cow platelets contained greater amounts of cholesterol and sphingomyelin. In addition, they yield greater serotonin release and longer δ granule secretion times. Finally, they showed greater spreading area with a greater range of spread. Platelets containing an OCS had more similarities in their membrane composition and secretion kinetics compared to cow platelets. However, cow platelets showed greater fusion pore stability which could be due to extra sphingomyelin and cholesterol, the primary components of lipid rafts. In addition, their greater stability may lead to many granules assisting in spreading. This study highlights fundamental membrane differences and their effects on platelet secretion.

Single-cell analysis of mast cell degranulation induced by airway smooth muscle-secreted chemokines

BACKGROUND

Asthma is a chronic inflammatory disease characterized by narrowed airways, bronchial hyper-responsiveness, mucus hyper-secretion, and airway remodeling. Mast cell (MC) infiltration into airway smooth muscle (ASM) is a defining feature of asthma, and ASM regulates the inflammatory response by secreting chemokines, including CXCL10 and CCL5. Single cell analysis offers a unique approach to study specific cellular signaling interactions within large and complex signaling networks such as the inflammatory microenvironment in asthma.

METHODS

Carbon-fiber microelectrode amperometry was used to study the effects of ASM-secreted chemokines on mouse peritoneal MC degranulation.

RESULTS

MC degranulation in response to CXCL10 and CCL5 was monitored at the single cell level. Relative to IgE-mediated degranulation, CXCL10- and CCL5-stimulated MCs released a decreased amount of serotonin per granule with fewer release events per cell. Decreased serotonin release per granule was correlated with increased spike half-width and rise-time values.

CONCLUSIONS

MCs are directly activated by ASM-associated chemokines. CXCL10 and CCL5 induce less robust MC degranulation compared to IgE- and A23187-stimulation. The kinetics of MC degranulation are signaling pathway-dependent, suggesting a biophysical mechanism of regulated degranulation that incorporates control over granule trafficking, transport, and docking machinery.

GENERAL SIGNIFICANCE

The biophysical mechanisms, including variations in number of exocytotic release events, serotonin released per granule, and the membrane kinetics of exocytosis that underlie MC degranulation in response to CXCL10 and CCL5 were characterized at the single cell level. These findings clarify the function of ASM-derived chemokines as instigators of MC degranulation relative to classical mechanisms of MC stimulation.

Analytical characterization of the role of phospholipids in platelet adhesion and secretion

The cellular phospholipid membrane plays an important role in cell function and cell-cell communication, but its biocomplexity and dynamic nature presents a challenge for examining cellular uptake of phospholipids and the resultant effects on cell function. Platelets, small anuclear circulating cell bodies that influence a wide variety of physiological functions through their dynamic secretory and adhesion behavior, present an ideal platform for exploring the effects of exogenous phospholipids on membrane phospholipid content and cell function. In this work, a broad range of platelet functions are quantitatively assessed by leveraging a variety of analytical chemistry techniques, including ultraperformance liquid chromatography-tandem electrospray ionization mass spectrometry (UPLC-MS/MS), vasculature-mimicking microfluidic analysis, and single cell carbon-fiber microelectrode amperometry (CFMA). The relative enrichments of phosphatidylserine (PS) and phosphatidylethanolamine (PE) were characterized with UPLC-MS/MS, and the effects of the enrichment of these two phospholipids on both platelet secretory behavior and adhesion were examined. Results show that, in fact, both PS and PE influence platelet adhesion and secretion. PS was enriched dramatically and decreased platelet adhesion as well as secretion from δ-, α-, and lysosomal granules. PE enrichment was moderate and increased secretion from platelet lysosomes. These insights illuminate the critical connection between membrane phospholipid character and platelet behavior, and both the methods and results presented herein are likely translatable to other mammalian cell systems.

Review of methods to probe single cell metabolism and bioenergetics

Single cell investigations have enabled unexpected discoveries, such as the existence of biological noise and phenotypic switching in infection, metabolism and treatment. Herein, we review methods that enable such single cell investigations specific to metabolism and bioenergetics. Firstly, we discuss how to isolate and immobilize individuals from a cell suspension, including both permanent and reversible approaches. We also highlight specific advances in microbiology for its implications in metabolic engineering. Methods for probing single cell physiology and metabolism are subsequently reviewed. The primary focus therein is on dynamic and high-content profiling strategies based on label-free and fluorescence microspectroscopy and microscopy. Non-dynamic approaches, such as mass spectrometry and nuclear magnetic resonance, are also briefly discussed.

Microfluidic platforms for the investigation of intercellular signalling mechanisms

Intercellular signalling has been identified as a highly complex process, responsible for orchestrating many physiological functions. While conventional methods of investigation have been useful, their limitations are impeding further development. Microfluidics offers an opportunity to overcome some of these limitations. Most notably, microfluidic systems can emulate the in-vivo environments. Further, they enable exceptionally precise control of the microenvironment, allowing complex mechanisms to be selectively isolated and studied in detail. There has thus been a growing adoption of microfluidic platforms for investigation of cell signalling mechanisms. This review provides an overview of the different signalling mechanisms and discusses the methods used to study them, with a focus on the microfluidic devices developed for this purpose.

Achieving reproducible performance of electrochemical, folding aptamer-based sensors on microelectrodes: challenges and prospects

Combining specific recognition capabilities with the excellent spatiotemporal resolution of small electrodes represents a promising methodology in bioanalytical and chemical sensing. In this paper, we report the development of reproducible electrochemical, aptamer-based (E-AB) sensors on a gold microelectrode platform. Specifically, we develop microscale sensors (25 μm diameter) for two representative small molecule targets–adenosine triphosphate and tobramycin. Furthermore, we report on the challenges encountered at this size scale including small-magnitude signals and interference from the irreversible reduction of dissolved oxygen and present methods to circumvent these challenges. Through the electrochemical deposition of dendritic gold nanostructures, we demonstrate microscale sensors with improved performance by increasing signal-to-noise and consequently sensitivity. Finally, we report on the use of the nonspecific adsorption of serum proteins as an additional layer of surface passivation for stable sensor performance. The sensor development here represents general guidelines for fabricating electrochemical, folding aptamer-based sensors on small-scale electrodes.

Experimental tools to study molecular recognition within the nanoparticle corona

Advancements in optical nanosensor development have enabled the design of sensors using syntheticmolecular recognition elements through a recently developed method called Corona Phase MolecularRecognition (CoPhMoRe). The synthetic sensors resulting from these design principles are highly selective for specific analytes, and demonstrate remarkable stability for use under a variety of conditions. An essential element of nanosensor development hinges on the ability to understand the interface between nanoparticles and the associated corona phase surrounding the nanosensor, an environment outside of the range of traditional characterization tools, such as NMR. This review discusses the need for new strategies and instrumentation to study the nanoparticle corona, operating in both in vitro and in vivo environments. Approaches to instrumentation must have the capacity to concurrently monitor nanosensor operation and the molecular changes in the corona phase. A detailed overview of new tools for the understanding of CoPhMoRe mechanisms is provided for future applications.

On-chip evaluation of neutrophil activation and neutrophil-endothelial cell interaction during neutrophil chemotaxis

Neutrophils are always surrounded by/interacting with other components of the immune system; however, the current mechanistic understanding of neutrophil function is largely based on how neutrophils respond to a single chemical signal in a simplified environment. Such approaches are unable to recapitulate the in vivo microenvironment; thus, cell behavior may not fully represent the physiological behavior. Herein, we exploit a microfluidic model of the complex in vivo milieu to investigate how cell-cell interactions influence human neutrophil migration and surface marker expression. Neutrophil migration against a bacterially derived chemoattractant (formyl-met-leu-phe, fMLP), with and without preactivation by interleukins (interleukin-2 or interleukin-6), was evaluated in the presence and absence of endothelial support cells. Preactivation by interleukins or interaction with endothelial cells resulted in altered migration rates compared to naïve neutrophils, and migration trajectories deviated from the expected movement toward the fMLP signal. Interestingly, interaction with both interleukins and endothelial cells simultaneously resulted in a slight compensation in the deviation-on endothelial cells, 34.4% of untreated neutrophils moved away from the fMLP signal, while only 15.2 or 22.2% (interleukin-2-or interleukin-6-activated) of preactivated cells moved away from fMLP. Neutrophils interacting with interleukins and/or endothelial cells were still capable of prioritizing the fMLP signal over a competing chemoattractant, leukotriene B4 (LTB4). Fluorescence imaging of individual human neutrophils revealed that neutrophils treated with endothelial-cell-conditioned media showed up-regulation of the surface adhesion molecules cluster determinant 11b and 66b (CD11b and CD66b) upon stimulation. On the other hand, CD11b and CD66b down-regulation was observed in untreated neutrophils. These results leverage single cell analysis to reveal that the interaction between neutrophils and endothelial cells is involved in surface marker regulation and thus chemotaxis of neutrophils. This study brings new knowledge about neutrophil chemotaxis in the context of cell-to-cell communications, yielding both fundamental and therapeutically relevant insight.

Selective catecholamine recognition with NeuroSensor 521: a fluorescent sensor for the visualization of norepinephrine in fixed and live cells

A method for the selective labeling and imaging of catecholamines in live and fixed secretory cells is reported. The method integrates a tailored approach using a novel fluorescence-based turn-on molecular sensor (NeuroSensor 521) that can exploit the high concentration of neurotransmitters and acidic environment within secretory vesicles for the selective recognition of norepinephrine and dopamine. The utility of the method was demonstrated by selectively labeling and imaging norepinephrine in secretory vesicles such that discrimination between norepinephrine- and epinephrine-enriched populations of chromaffin cells was observed. This method was validated in fixed cells by co-staining with an anti-PNMT antibody.