Compartmental microfluidic system for studying muscle–neuron communication and neuromuscular junction maintenance

Recent advances in microfluidic platforms for single-cell analysis in cancer biology, diagnosis and therapy

Understanding molecular, cellular, genetic and functional heterogeneity of tumors at the single-cell level has become a major challenge for cancer research. The microfluidic technique has emerged as an important tool that offers advantages in analyzing single-cells with the capability to integrate time-consuming and labour-intensive experimental procedures such as single-cell capture into a single microdevice at ease and in a high-throughput fashion. Single-cell manipulation and analysis can be implemented within a multi-functional microfluidic device for various applications in cancer research. Here, we present recent advances of microfluidic devices for single-cell analysis pertaining to cancer biology, diagnostics, and therapeutics. We first concisely introduce various microfluidic platforms used for single-cell analysis, followed with different microfluidic techniques for single-cell manipulation. Then, we highlight their various applications in cancer research, with an emphasis on cancer biology, diagnosis, and therapy. Current limitations and prospective trends of microfluidic single-cell analysis are discussed at the end.

Toward Building the Neuromuscular Junction: In Vitro Models To Study Synaptogenesis and Neurodegeneration

The neuromuscular junction (NMJ) is a unique, specialized chemical synapse that plays a crucial role in transmitting and amplifying information from spinal motor neurons to skeletal muscles. NMJ complexity ensures closely intertwined interactions between numerous synaptic vesicles, signaling molecules, ion channels, motor neurons, glia, and muscle fibers, making it difficult to dissect the underlying mechanisms and factors affecting neurodegeneration and muscle loss. Muscle fiber or motor neuron cell death followed by rapid axonal degeneration due to injury or disease has a debilitating effect on movement and behavior, which adversely affects the quality of life. It thus becomes imperative to study the synapse and intercellular signaling processes that regulate plasticity at the NMJ and elucidate mechanisms and pathways at the cellular level. Studies using in vitro 2D cell cultures have allowed us to gain a fundamental understanding of how the NMJ functions. However, they do not provide information on the intricate signaling networks that exist between NMJs and the biological environment. The advent of 3D cell cultures and microfluidic lab-on-a-chip technologies has opened whole new avenues to explore the NMJ. In this perspective, we look at the challenges involved in building a functional NMJ and the progress made in generating models for studying the NMJ, highlighting the current and future applications of these models.

In Vitro Modelling of Nerve-Muscle Connectivity in a Compartmentalised Tissue Culture Device

Motor neurons project axons from the hindbrain and spinal cord to muscle, where they induce myofibre contractions through neurotransmitter release at neuromuscular junctions. Studies of neuromuscular junction formation and homeostasis have been largely confined to in vivo models. In this study we have merged three powerful tools - pluripotent stem cells, optogenetics and microfabrication - and designed an open microdevice in which motor axons grow from a neural compartment containing embryonic stem cell-derived motor neurons and astrocytes through microchannels to form functional neuromuscular junctions with contractile myofibers in a separate compartment. Optogenetic entrainment of motor neurons in this reductionist neuromuscular circuit enhanced neuromuscular junction formation more than two-fold, mirroring the activity-dependence of synapse development in vivo. We incorporated an established motor neuron disease model into our system and found that coculture of motor neurons with SOD1G93A astrocytes resulted in denervation of the central compartment and diminished myofiber contractions, a phenotype which was rescued by the Receptor Interacting Serine/Threonine Kinase 1 (RIPK1) inhibitor Necrostatin. This coculture system replicates key aspects of nerve-muscle connectivity in vivo and represents a rapid and scalable alternative to animal models of neuromuscular function and disease.

Engineering complex muscle-tissue interfaces through microfabrication

Skeletal muscle is a tissue with a complex and hierarchical architecture that influences its functional properties. In order to exert its contractile function, muscle tissue is connected to neural, vascular and connective compartments, comprising finely structured interfaces which are orchestrated by multiple signalling pathways. Pathological conditions such as dystrophies and trauma, or physiological situations such as exercise and aging, modify the architectural organization of these structures, hence affecting muscle functionality. To overcome current limitations of in vivo and standard in vitro models, microfluidics and biofabrication techniques have been applied to better reproduce the microarchitecture and physicochemical environment of human skeletal muscle tissue. In the present review, we aim to critically discuss the role of those techniques, taken individually or in combination, in the generation of models that mimic the complex interfaces between muscle tissue and neural/vascular/tendon compartments. The exploitation of either microfluidics or biofabrication to model different muscle interfaces has led to the development of constructs with an improved spatial organization, thus presenting a better functionality as compared to standard models. However, the achievement of models replicating muscle-tissue interfaces with adequate architecture, presence of fundamental proteins and recapitulation of signalling pathways is still far from being achieved. Increased integration between microfluidics and biofabrication, providing the possibility to pattern cells in predetermined structures with higher resolution, will help to reproduce the hierarchical and heterogeneous structure of skeletal muscle interfaces. Such strategies will further improve the functionality of these techniques, providing a key contribution towards the study of skeletal muscle functions in physiology and pathology.

Targeting the Sigma-1 Receptor via Pridopidine Ameliorates Central Features of ALS Pathology in a SOD1G93A Model

Amyotrophic Lateral Sclerosis (ALS) is a fatal neurodegenerative disease affecting both the upper and lower motor neurons (MNs), with no effective treatment currently available. Early pathological events in ALS include perturbations in axonal transport (AT), formation of toxic protein aggregates and Neuromuscular Junction (NMJ) disruption, which all lead to axonal degeneration and motor neuron death. Pridopidine is a small molecule that has been clinically developed for Huntington disease. Here we tested the efficacy of pridopidine for ALS using in vitro and in vivo models. Pridopidine beneficially modulates AT deficits and diminishes NMJ disruption, as well as motor neuron death in SOD1G93A MNs and in neuromuscular co-cultures. Furthermore, we demonstrate that pridopidine activates the ERK pathway and mediates its beneficial effects through the sigma-1 receptor (S1R). Strikingly, in vivo evaluation of pridopidine in SOD1G93A mice reveals a profound reduction in mutant SOD1 aggregation in the spinal cord, and attenuation of NMJ disruption, as well as subsequent muscle wasting. Taken together, we demonstrate for the first time that pridopidine improves several cellular and histological hallmark pathologies of ALS through the S1R.

The Effect of Microfluidic Geometry on Myoblast Migration

In vitro systems comprised of wells interconnected by microchannels have emerged as a platform for the study of cell migration or multicellular models. In the present study, we systematically evaluated the effect of microchannel width on spontaneous myoblast migration across these microchannels-from the proximal to the distal chamber. Myoblast migration was examined in microfluidic devices with varying microchannel widths of 1.5⁻20 µm, and in chips with uniform microchannel widths over time spans that are relevant for myoblast-to-myofiber differentiation in vitro. We found that the likelihood of spontaneous myoblast migration was microchannel width dependent and that a width of 3 µm was necessary to limit spontaneous migration below 5% of cells in the seeded well after 48 h. These results inform the future design of Polydimethylsiloxane (PDMS) microchannel-based co-culture platforms as well as future in vitro studies of myoblast migration.

CHRFAM7A alters binding to the neuronal alpha-7 nicotinic acetylcholine receptor

INTRODUCTION

CHRFAM7A is a uniquely-human gene that encodes a human-specific variant of the alpha-7 nicotinic acetylcholine receptor (α7nAchR). While the homopentameric α7nAChR consists of 5 equal subunits, previous studies demonstrated that CHRFAM7A expression disrupts the formation of α7nAChR homopentamers. Here we use a rat neuronal cell line expressing CHRFAM7A and a transgenic mouse expressing CHRFAM7A to define the alpha-bungarotoxin (α-BTX) binding in vitro and in vivo.

METHODS

Rat PC12 cells were stably transfected with human CHRFAM7A. α-BTX, a protein that irreversibly binds the α7nAchR, was utilized to assess the capacity for CHRFAM7A to interfere with α 7AchR subunits using immunohistochemistry and flow cytometry. To evaluate the effects of CHRFAM7A on α7nAchR at the neuromuscular junction in vivo, transgenic mice were engineered to express the uniquely human gene CHRFAM7A under the control of the EF1-α promoter. Using this model, muscle was harvested and CHRFAM7A and CHRNA7 gene expression evaluated by PCR. Binding of α-BTX to the α7nAchR in muscle was compared in sibling-matched wild-type C57 mice by immunostaining the neuromuscular junction using α-BTX and neurofilament antibodies.

RESULTS

Expression of CHRFAM7A in transfected, but not vector cells, was confirmed by PCR and by immunoblotting using an antibody we raised to a peptide sequence unique to CHRFAM7A. CHRFAM7A decreased α-BTX binding as detected by immunohistochemistry and flow cytometry. In vivo, α-BTX co-stained with neurofilament at the neuromuscular junction in wild-type mice, however, α-BTX staining was decreased at the neuromuscular junction of CHRFAM7A transgenic mice.

CONCLUSION

CHRFAM7A expression interferes with the binding of α7nAchR to α-BTX. Understanding the contribution of this uniquely human gene to human disease will be important in the identification of potential therapeutic targets.

Challenges and Future Prospects on 3D in-vitro Modeling of the Neuromuscular Circuit

Movement of skeletal-muscle fibers is generated by the coordinated action of several cells taking part within the locomotion circuit (motoneurons, sensory-neurons, Schwann cells, astrocytes, microglia, and muscle-cells). Failures in any part of this circuit could impede or hinder coordinated muscle movement and cause a neuromuscular disease (NMD) or determine its severity. Studying fragments of the circuit cannot provide a comprehensive and complete view of the pathological process. We trace the historic developments of studies focused on in-vitro modeling of the spinal-locomotion circuit and how bioengineered innovative technologies show advantages for an accurate mimicking of physiological conditions of spinal-locomotion circuit. New developments on compartmentalized microfluidic culture systems (cμFCS), the use of human induced pluripotent stem cells (hiPSCs) and 3D cell-cultures are analyzed. We finally address limitations of current study models and three main challenges on neuromuscular studies: (i) mimic the whole spinal-locomotion circuit including all cell-types involved and the evaluation of independent and interdependent roles of each one; (ii) mimic the neurodegenerative response of mature neurons in-vitro as it occurs in-vivo; and (iii) develop, tune, implement, and combine cμFCS, hiPSC, and 3D-culture technologies to ultimately create patient-specific complete, translational, and reliable NMD in-vitro model. Overcoming these challenges would significantly facilitate understanding the events taking place in NMDs and accelerate the process of finding new therapies.

A closer look at neuron interaction with track-etched microporous membranes

Microporous membranes support the growth of neurites into and through micro-channels, providing a different type of neural growth platform to conventional dish cultures. Microporous membranes are used to support various types of culture, however, the role of pore diameter in relation to neurite growth through the membrane has not been well characterised. In this study, the human cell line (SH-SY5Y) was differentiated into neuron-like cells and cultured on track-etched microporous membranes with pore and channel diameters selected to accommodate neurite width (0.8 µm to 5 µm). Whilst neurites extended through all pore diameters, the extent of neurite coverage on the non-seeded side of the membranes after 5 days in culture was found to be directly proportional to channel diameter. Neurite growth through membrane pores reduced significantly when neural cultures were non-confluent. Scanning electron microscopy revealed that neurites bridged pores and circumnavigated pore edges – such that the overall likelihood of a neurite entering a pore channel was decreased. These findings highlight the role of pore diameter, cell sheet confluence and contact guidance in directing neurite growth through pores and may be useful in applications that seek to use physical substrates to maintain separate neural populations whilst permitting neurite contact between cultures.

Neuronal network maturation differently affects secretory vesicles and mitochondria transport in axons

Studying intracellular dynamics in neurons is crucial to better understand how brain circuits communicate and adapt to environmental changes. In neurons, axonal secretory vesicles underlie various functions from growth during development to plasticity in the mature brain. Similarly, transport of mitochondria, the power plant of the cell, regulates both axonal development and synaptic homeostasis. However, because of their submicrometric size and rapid velocities, studying the kinetics of these organelles in projecting axons in vivo is technically challenging. In parallel, primary neuronal cultures are adapted to study axonal transport but they lack the physiological organization of neuronal networks, which in turn may bias observations. We previously developed a microfluidic platform to reconstruct a physiologically-relevant and functional corticostriatal network in vitro that is compatible with high-resolution videorecording of axonal trafficking. Here, using this system we report progressive changes in axonal transport kinetics of both dense core vesicles and mitochondria that correlate with network development and maturation. Interestingly, axonal flow of both types of organelles change in opposite directions, with rates increasing for vesicles and decreasing for mitochondria. Overall, our observations highlight the need for a better spatiotemporal control for the study of intracellular dynamics in order to avoid misinterpretations and improve reproducibility.

Microfluidic devices for disease modeling in muscle tissue

Microfluidic devices have advanced significantly in recent years and are a promising technology for the field of tissue engineering. Highly sophisticated microfabrication techniques have paved the way for the development of complex ex vivo models capable of incorporating and measuring the real-time response of multiple cell types interacting together in a single system. Muscle-on-a-chip technology has drastically improved and serves as a drug screening platform for many muscular diseases such as muscular dystrophy, tendinosis, fibromyalgia, mitochondrial myopathy, and myasthenia gravis. This review seeks to communicate the gaps in knowledge of current muscular disease models and highlight the power of microfluidic devices in enabling researchers to better understand disease pathology and provide high throughput screening of therapeutics for muscular myopathies.

Integrating Mass Spectrometry with Microphysiological Systems for Improved Neurochemical Studies

Microphysiological systems, often referred to as "organs-on-chips", are in vitro platforms designed to model the spatial, chemical, structural, and physiological elements of in vivo cellular environments. They enhance the evaluation of complex engineered biological systems and are a step between traditional cell culture and in vivo experimentation. As neurochemists and measurement scientists studying the molecules involved in intercellular communication in the nervous system, we focus here on recent advances in neuroscience using microneurological systems and their potential to interface with mass spectrometry. We discuss a number of examples - microfluidic devices, spheroid cultures, hydrogels, scaffolds, and fibers - highlighting those that would benefit from mass spectrometric technologies to obtain improved chemical information.

miR126-5p Downregulation Facilitates Axon Degeneration and NMJ Disruption via a Non-Cell-Autonomous Mechanism in ALS

Axon degeneration and disruption of neuromuscular junctions (NMJs) are key events in amyotrophic lateral sclerosis (ALS) pathology. Although the disease's etiology is not fully understood, it is thought to involve a non-cell-autonomous mechanism and alterations in RNA metabolism. Here, we identified reduced levels of miR126-5p in presymptomatic ALS male mice models, and an increase in its targets: axon destabilizing Type 3 Semaphorins and their coreceptor Neuropilins. Using compartmentalized in vitro cocultures, we demonstrated that myocytes expressing diverse ALS-causing mutations promote axon degeneration and NMJ dysfunction, which were inhibited by applying Neuropilin1 blocking antibody. Finally, overexpressing miR126-5p is sufficient to transiently rescue axon degeneration and NMJ disruption both in vitro and in vivo Thus, we demonstrate a novel mechanism underlying ALS pathology, in which alterations in miR126-5p facilitate a non-cell-autonomous mechanism of motor neuron degeneration in ALS.SIGNIFICANCE STATEMENT Despite some progress, currently no effective treatment is available for amyotrophic lateral sclerosis (ALS). We suggest a novel regulatory role for miR126-5p in ALS and demonstrate, for the first time, a mechanism by which alterations in miR126-5p contribute to axon degeneration and NMJ disruption observed in ALS. We show that miR126-5p is altered in ALS models and that it can modulate Sema3 and NRP protein expression. Furthermore, NRP1 elevations in motor neurons and muscle secretion of Sema3A contribute to axon degeneration and NMJ disruption in ALS. Finally, overexpressing miR126-5p is sufficient to transiently rescue NMJ disruption and axon degeneration both in vitro and in vivo.

Tackling muscle fibrosis: From molecular mechanisms to next generation engineered models to predict drug delivery

Muscle fibrosis represents the end stage consequence of different diseases, among which muscular dystrophies, leading to severe impairment of muscle functions. Muscle fibrosis involves the production of several growth factors, cytokines and proteolytic enzymes and is strictly associated to inflammatory processes. Moreover, fibrosis causes profound changes in tissue properties, including increased stiffness and density, lower pH and oxygenation. Up to now, there is no therapeutic approach able to counteract the fibrotic process and treatments directed against muscle pathologies are severely impaired by the harsh conditions of the fibrotic environment. The design of new therapeutics thus need innovative tools mimicking the obstacles posed by the fibrotic environment to their delivery. This review will critically discuss the role of in vivo and 3D in vitro models in this context and the characteristics that an ideal model should possess to help the translation from bench to bedside of new candidate anti-fibrotic agents.

In Vitro Microfluidic Models for Neurodegenerative Disorders

Microfluidic devices enable novel means of emulating neurodegenerative disease pathophysiology in vitro. These organ-on-a-chip systems can potentially reduce animal testing and substitute (or augment) simple 2D culture systems. Reconstituting critical features of neurodegenerative diseases in a biomimetic system using microfluidics can thereby accelerate drug discovery and improve our understanding of the mechanisms of several currently incurable diseases. This review describes latest advances in modeling neurodegenerative diseases in the central nervous system and the peripheral nervous system. First, this study summarizes fundamental advantages of microfluidic devices in the creation of compartmentalized cell culture microenvironments for the co-culture of neurons, glial cells, endothelial cells, and skeletal muscle cells and in their recapitulation of spatiotemporal chemical gradients and mechanical microenvironments. Then, this reviews neurodegenerative-disease-on-a-chip models focusing on Alzheimer's disease, Parkinson's disease, and amyotrophic lateral sclerosis. Finally, this study discusses about current drawbacks of these models and strategies that may overcome them. These organ-on-chip technologies can be useful to be the first line of testing line in drug development and toxicology studies, which can contribute significantly to minimize the phase of animal testing steps.

Microphysiological Human Brain and Neural Systems-on-a-Chip: Potential Alternatives to Small Animal Models and Emerging Platforms for Drug Discovery and Personalized Medicine

Translational challenges associated with reductionist modeling approaches, as well as ethical concerns and economic implications of small animal testing, drive the need for developing microphysiological neural systems for modeling human neurological diseases, disorders, and injuries. Here, we provide a comprehensive review of microphysiological brain and neural systems-on-a-chip (NSCs) for modeling higher order trajectories in the human nervous system. Societal, economic, and national security impacts of neurological diseases, disorders, and injuries are highlighted to identify critical NSC application spaces. Hierarchical design and manufacturing of NSCs are discussed with distinction for surface- and bulk-based systems. Three broad NSC classes are identified and reviewed: microfluidic NSCs, compartmentalized NSCs, and hydrogel NSCs. Emerging areas and future directions are highlighted, including the application of 3D printing to design and manufacturing of next-generation NSCs, the use of stem cells for constructing patient-specific NSCs, and the application of human NSCs to 'personalized neurology'. Technical hurdles and remaining challenges are discussed. This review identifies the state-of-the-art design methodologies, manufacturing approaches, and performance capabilities of NSCs. This work suggests NSCs appear poised to revolutionize the modeling of human neurological diseases, disorders, and injuries.

Cardiac Meets Skeletal: What's New in Microfluidic Models for Muscle Tissue Engineering

In the last few years microfluidics and microfabrication technique principles have been extensively exploited for biomedical applications. In this framework, organs-on-a-chip represent promising tools to reproduce key features of functional tissue units within microscale culture chambers. These systems offer the possibility to investigate the effects of biochemical, mechanical, and electrical stimulations, which are usually applied to enhance the functionality of the engineered tissues. Since the functionality of muscle tissues relies on the 3D organization and on the perfect coupling between electrochemical stimulation and mechanical contraction, great efforts have been devoted to generate biomimetic skeletal and cardiac systems to allow high-throughput pathophysiological studies and drug screening. This review critically analyzes microfluidic platforms that were designed for skeletal and cardiac muscle tissue engineering. Our aim is to highlight which specific features of the engineered systems promoted a typical reorganization of the engineered construct and to discuss how promising design solutions exploited for skeletal muscle models could be applied to improve cardiac tissue models and vice versa.

Microfluidic device for the formation of optically excitable, three-dimensional, compartmentalized motor units

Motor units are the fundamental elements responsible for muscle movement. They are formed by lower motor neurons and their muscle targets, synapsed via neuromuscular junctions (NMJs). The loss of NMJs in neurodegenerative disorders (such as amyotrophic lateral sclerosis or spinal muscle atrophy) or as a result of traumatic injuries affects millions of lives each year. Developing in vitro assays that closely recapitulate the physiology of neuromuscular tissues is crucial to understand the formation and maturation of NMJs, as well as to help unravel the mechanisms leading to their degeneration and repair. We present a microfluidic platform designed to coculture myoblast-derived muscle strips and motor neurons differentiated from mouse embryonic stem cells (ESCs) within a three-dimensional (3D) hydrogel. The device geometry mimics the spinal cord-limb physical separation by compartmentalizing the two cell types, which also facilitates the observation of 3D neurite outgrowth and remote muscle innervation. Moreover, the use of compliant pillars as anchors for muscle strips provides a quantitative functional readout of force generation. Finally, photosensitizing the ESC provides a pool of source cells that can be differentiated into optically excitable motor neurons, allowing for spatiodynamic, versatile, and noninvasive in vitro control of the motor units.