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Parkinson's disease (PD) is a progressive neurological disorder associated with the loss of dopaminergic neurons (DNs) in the substantia nigra and the widespread accumulation of α-synuclein (α-syn) protein, leading to motor impairments and eventual cognitive dysfunction. In-vitro cell cultures and in-vivo animal models have provided the opportunity to investigate the PD pathological hallmarks and identify different therapeutic compounds. However, PD pathogenesis and causes are still not well understood, and effective inhibitory drugs for PD are yet to be discovered. Biologically simple but pathologically relevant disease models and advanced screening technologies are needed to reveal the mechanisms underpinning protein aggregation and PD progression. For instance, Caenorhabditis elegans (C. elegans) offers many advantages for fundamental PD neurobehavioral studies including a simple, well-mapped, and accessible neuronal system, genetic homology to humans, body transparency and amenability to genetic manipulation. Several transgenic worm strains that exhibit multiple PD-related phenotypes have been developed to perform neuronal and behavioral assays and drug screening. However, in conventional worm-based assays, the commonly used techniques are equipment-intensive, slow and low in throughput. Over the past two decades, microfluidics technology has contributed significantly to automation and control of C. elegans assays. In this review, we focus on C. elegans PD models and the recent advancements in microfluidic platforms used for manipulation, handling and neurobehavioral screening of these models. Moreover, we highlight the potential of C. elegans to elucidate the in-vivo mechanisms of neuron-to-neuron protein transfer that may underlie spreading Lewy pathology in PD, and its suitability for in-vitro studies. Given the advantages of C. elegans and microfluidics technology, their integration has the potential to facilitate the investigation of disease pathology and discovery of potential chemical leads for PD., (© The Author(s) 2019. Published by Oxford University Press. All rights reserved. For permissions, please e-mail: journals.permissions@oup.com.)

Behavioral responses of zebrafish larvae to environmental cues are important functional readouts that should be evoked on-demand and studied phenotypically in behavioral, genetical and developmental investigations. Very recently, it was shown that zebrafish larvae execute a voluntary and oriented movement toward the positive electrode of an electric field along a microchannel. Phenotypic characterization of this response was not feasible due to larva's rapid movement along the channel. To overcome this challenge, a microfluidic device was introduced to partially immobilize the larva's head while leaving its mid-body and tail unrestrained in a chamber to image motor behaviors in response to electric stimulation, hence achieving quantitative phenotyping of the electrically evoked movement in zebrafish larvae. The effect of electric current on the tail-beat frequency and response duration of 5–7 days postfertilization zebrafish larvae was studied. Investigations were also performed on zebrafish exposed to neurotoxin 6-hydroxydopamine and larvae carrying a pannexin1a (panx1a) gene knockout, as a proof of principle applications to demonstrate on-demand movement behavior screening in chemical and mutant assays. We demonstrated for the first time that 6-hydroxydopamine leads to electric response impairment, levodopa treatment rescues the response and panx1a is involved in the electrically evoked movement of zebrafish larvae. We envision that our technique is broadly applicable as a screening tool to quantitatively examine zebrafish larvae's movements in response to physical and chemical stimulations in investigations of Parkinson's and other neurodegenerative diseases, and as a tool to combine recent advances in genome engineering of model organisms to uncover the biology of electric response. [ABSTRACT FROM AUTHOR]

Significance: Selective plane illumination microscopy (SPIM) is an emerging fluorescent imaging technique suitable for noninvasive volumetric imaging of C. elegans. These promising microscopy systems, however, are scarce in academic and research institutions due to their high cost and technical complexities. Simple and low-cost solutions that enable conversion of commonplace wide-field microscopes to rapid SPIM platforms promote widespread adoption of SPIM by biologist for studying neuronal expressions of C. elegans. Aim: We sought to develop a simple and low-cost optofluidic add-on device that enables rapid and immobilization-free volumetric SPIM imaging of C. elegans with conventional fluorescent microscopes. Approach: A polydimethylsiloxane (PDMS)-based device with integrated optical and fluidic elements was developed as a low-cost and miniaturized SPIM add-on for the conventional wide-field microscope. The developed optofluidic chip contained an integrated PDMS cylindrical lens for on-chip generation of the light-sheet across a microchannel. Cross-sectional SPIM images of C. elegans were continuously acquired by the native objective of microscope as worms flowed in an L-shape microchannel and through the light sheet. Results: On-chip SPIM imaging of C. elegans strains demonstrated possibility of visualizing the entire neuronal system in few seconds at single-neuron resolution, with high contrast and without worm immobilization. Volumetric visualization of neuronal system from the acquired cross-sectional two-dimensional images is also demonstrated, enabling the standard microscope to acquire three-dimensional fluorescent images of C. elegans. The full-width at half-maximum width of the point spread function was measured as 1.1 and 2.4 μm in the lateral and axial directions, respectively. Conclusion: The developed low-cost optofluidic device is capable of continuous SPIM imaging of C. elegans model organism with a conventional fluorescent microscope, at high speed, and with single neuron resolution. [ABSTRACT FROM AUTHOR]

• Microchip shows semi-mobile C. elegans respond to electric field (electrotaxis). • Electrotaxis of tail-trapped worms is towards the cathode with head. • Electrotaxis of head-trapped worms is also towards the cathode with tail. • 6-OHDA affects the electrotaxis of semi-mobile Parkinson disease (PD) model worms. • L-dopa treatment rescues both neurodegeneration and electrotaxis in PD worms. Microfluidic-based electrotaxis assay is a quantitative movement phenotyping technique for behavioral studies on Caenorhabditis (C.) elegans which is currently space-consuming and limited for cell imaging. To address these limitations and show applications in Parkinson's Disease (PD) studies, we introduce a novel microfluidic device to investigate the electrotaxis of semi-mobile C. elegans and image them immediately after the behavioral assay. The device consisted of an electrotaxis screening channel integrated with a perpendicular tapered microchannel that was used for worm tail or head trapping for electrotaxis screening and full-body immobilization for neuron imaging. Semi-mobile C. elegans with trapped tails demonstrated electrotaxis orientation towards the cathode. Interestingly, the response of head-trapped worms was also with the tail towards the cathode, implying the involvement of the mid-body to tail sensory neurons in electrotaxis. Moreover, semi-mobile C. elegans electrotaxis assay time is 3-folds shorter and 20-folds less space-consuming, compared to our freely moving assay. This makes the technique amenable to parallelization for the design of multi-worm electrotaxis and neuronal screening devices. For biological application to PD studies, we showed that human α-synuclein protein accumulation or exposure to neurotoxin 6-OHDA affected the electrotaxis phenotypes of semi-mobile worms. Moreover, L-DOPA rescued the electrotaxis of 6-OHDA exposed worms. The above behavioral effects of 6-OHDA and L-DOPA corresponded well with the degeneration of dopaminergic neurons and rescue of dopamine transmission, confirmed by on-chip fluorescent imaging. Our technique can be used for PD pathology studies and potentially for other neurobehavioral disorders modeled in C. elegans. [ABSTRACT FROM AUTHOR]

In this paper, we report a novel microfluidic method to conduct a Caenorhabditis elegans electrotaxis movement assay and neuronal imaging on up to 16 worms in parallel. C. elegans is a model organism for neurodegenerative disease and movement disorders such as Parkinson's disease (PD), and for screening chemicals that alleviate protein aggregation, neuronal death, and movement impairment in PD. Electrotaxis of C. elegans in microfluidic channels has led to the development of neurobehavioral screening platforms, but enhancing the throughput of the electrotactic behavioral assay has remained a challenge. Our device consisted of a hierarchy of tree-like channels for worm loading into 16 parallel electrotaxis screening channels with equivalent electric fields. Tapered channels at the ends of electrotaxis channels were used for worm immobilization and fluorescent imaging of neurons. Parallel electrotaxis of worms was first validated against established single-worm electrotaxis phenotypes. Then, mutant screening was demonstrated using the NL5901 strain, carrying human α -synuclein in the muscle cells, by showing the associated electrotaxis defects in the average speed, body bend frequency (BBF), and electrotaxis time index (ETI). Moreover, chemical screening of a PD worm model was shown by exposing the BZ555 strain, expressing green fluorescence protein (GFP) in the dopaminergic neurons (DNs), to 6-hydroxydopamine neurotoxin. The neurotoxin-treated worms exhibited a reduction in electrotaxis swimming speed, BBF, ETI, and DNs fluorescence intensity. We envision our technique to be used widely in C. elegans-based movement disorder assays to accelerate behavioral and cellular phenotypic investigations. [ABSTRACT FROM AUTHOR]