Single cell electroporation method for axon tracing in cultured slices

Single-Cell Visualization Deep in Brain Structures by Gene Transfer

A projection neuron targets multiple regions beyond the functional brain area. In order to map neuronal connectivity in a massive neural network, a means for visualizing the entire morphology of a single neuron is needed. Progress has facilitated single-neuron analysis in the cerebral cortex, but individual neurons in deep brain structures remain difficult to visualize. To this end, we developed an in vivo single-cell electroporation method for juvenile and adult brains that can be performed under a standard stereomicroscope. This technique involves rapid gene transfection and allows the visualization of dendritic and axonal morphologies of individual neurons located deep in brain structures. The transfection efficiency was enhanced by directly injecting the expression vector encoding green fluorescent protein instead of monitoring cell attachment to the electrode tip. We obtained similar transfection efficiencies in both young adult (≥P40) and juvenile mice (P21-30). By tracing the axons of thalamocortical neurons, we identified a specific subtype of neuron distinguished by its projection pattern. Additionally, transfected mOrange-tagged vesicle-associated membrane protein 2-a presynaptic protein-was strongly localized in terminal boutons of thalamocortical neurons. Thus, our in vivo single-cell gene transfer system offers rapid single-neuron analysis deep in brain. Our approach combines observation of neuronal morphology with functional analysis of genes of interest, which can be useful for monitoring changes in neuronal activity corresponding to specific behaviors in living animals.

A Single-Neuron: Current Trends and Future Prospects

The brain is an intricate network with complex organizational principles facilitating a concerted communication between single-neurons, distinct neuron populations, and remote brain areas. The communication, technically referred to as connectivity, between single-neurons, is the center of many investigations aimed at elucidating pathophysiology, anatomical differences, and structural and functional features. In comparison with bulk analysis, single-neuron analysis can provide precise information about neurons or even sub-neuron level electrophysiology, anatomical differences, pathophysiology, structural and functional features, in addition to their communications with other neurons, and can promote essential information to understand the brain and its activity. This review highlights various single-neuron models and their behaviors, followed by different analysis methods. Again, to elucidate cellular dynamics in terms of electrophysiology at the single-neuron level, we emphasize in detail the role of single-neuron mapping and electrophysiological recording. We also elaborate on the recent development of single-neuron isolation, manipulation, and therapeutic progress using advanced micro/nanofluidic devices, as well as microinjection, electroporation, microelectrode array, optical transfection, optogenetic techniques. Further, the development in the field of artificial intelligence in relation to single-neurons is highlighted. The review concludes with between limitations and future prospects of single-neuron analyses.

Rho Guanine Nucleotide Exchange Factors Regulate Horizontal Axon Branching of Cortical Upper Layer Neurons

Axon branching is a crucial process for cortical circuit formation. However, how the cytoskeletal changes in axon branching are regulated is not fully understood. In the present study, we investigated the role of RhoA guanine nucleotide exchange factors (RhoA-GEFs) in branch formation of horizontally elongating axons (horizontal axons) in the mammalian cortex. In situ hybridization showed that more than half of all known RhoA-GEFs were expressed in the developing rat cortex. These RhoA-GEFs were mostly expressed in the macaque cortex as well. An overexpression study using organotypic cortical slice cultures demonstrated that several RhoA-GEFs strongly promoted horizontal axon branching. Moreover, branching patterns were different between overexpressed RhoA-GEFs. In particular, ARHGEF18 markedly increased terminal arbors, whereas active breakpoint cluster region-related protein (ABR) increased short branches in both distal and proximal regions of horizontal axons. Rho kinase inhibitor treatment completely suppressed the branch-promoting effect of ARHGEF18 overexpression, but only partially affected that of ABR, suggesting that these RhoA-GEFs employ distinct downstream pathways. Furthermore, knockdown of either ARHGEF18 or ABR considerably suppressed axon branching. Taken together, the present study revealed that subsets of RhoA-GEFs differentially promote axon branching of mammalian cortical neurons.

Visualization of Thalamocortical Axon Branching and Synapse Formation in Organotypic Cocultures

Axon branching and synapse formation are crucial processes for establishing precise neuronal circuits. During development, sensory thalamocortical (TC) axons form branches and synapses in specific layers of the cerebral cortex. Despite the obvious spatial correlation between axon branching and synapse formation, the causal relationship between them is poorly understood. To address this issue, we recently developed a method for simultaneous imaging of branching and synapse formation of individual TC axons in organotypic cocultures. This protocol describes a method which consists of a combination of an organotypic coculture and electroporation. Organotypic cocultures of the thalamus and cerebral cortex facilitate gene manipulation and observation of axonal processes, preserving characteristic structures such as laminar configuration. Two distinct plasmids encoding DsRed and EGFP-tagged synaptophysin (SYP-EGFP) were co-transfected into a small number of thalamic neurons by an electroporation technique. This method allowed us to visualize individual axonal morphologies of TC neurons and their presynaptic sites simultaneously. The method also enabled long-term observation which revealed the causal relationship between axon branching and synapse formation.

Synapse-dependent and independent mechanisms of thalamocortical axon branching are regulated by neuronal activity

Axon branching and synapse formation are critical processes for establishing precise circuit connectivity. These processes are tightly regulated by neural activity, but the relationship between them remains largely unclear. We use organotypic coculture preparations to examine the role of synapse formation in the activity-dependent axon branching of thalamocortical (TC) projections. To visualize TC axons and their presynaptic sites, two plasmids encoding DsRed and EGFP-tagged synaptophysin (SYP-EGFP) were cotransfected into a small number of thalamic neurons. Time-lapse imaging of individual TC axons showed that most branches emerged from SYP-EGFP puncta, indicating that synapse formation precedes emergences of axonal branches. We also investigated the effects of neuronal activity on axon branching and synapse formation by manipulating spontaneous firing activity of thalamic cells. An inward rectifying potassium channel, Kir2.1, and a bacterial voltage-gated sodium channel, NaChBac, were used to suppress and promote firing activity, respectively. We found suppressing neural activity reduced both axon branching and synapse formation. In contrast, increasing neural activity promoted only axonal branch formation. Time-lapse imaging of NaChBac-expressing cells further revealed that new branches frequently appeared from the locations other than SYP-EGFP puncta, indicating that enhancing activity promotes axonal branch formation due to an increase of branch emergence at nonsynaptic sites. These results suggest that presynaptic locations are hotspots for branch emergence, and that frequent firing activity can shift branch emergence to a synapse-independent process.

Regulation of thalamocortical axon branching by BDNF and synaptic vesicle cycling

During development, axons form branches in response to extracellular molecules. Little is known about the underlying molecular mechanisms. Here, we investigate how neurotrophin-induced axon branching is related to synaptic vesicle cycling for thalamocortical axons. The exogenous application of brain-derived neurotrophic factor (BDNF) markedly increased axon branching in thalamocortical co-cultures, while removal of endogenous BDNF reduced branching. Over-expression of a C-terminal fragment of AP180 that inhibits clathrin-mediated endocytosis affected the laminar distribution and the number of branch points. A dominant-negative synaptotagmin mutant that selectively targets synaptic vesicle cycling, strongly suppressed axon branching. Moreover, axons expressing the mutant synaptotagmin were resistant to the branch-promoting effect of BDNF. These results suggest that synaptic vesicle cycling might regulate BDNF induced branching during the development of the axonal arbor.

Optogenetic manipulation of neural and non-neural functions

Optogenetic manipulation of the neuronal activity enables one to analyze the neuronal network both in vivo and in vitro with precise spatio-temporal resolution. Channelrhodopsins (ChRs) are light-sensitive cation channels that depolarize the cell membrane, whereas halorhodopsins and archaerhodopsins are light-sensitive Cl(-) and H(+) transporters, respectively, that hyperpolarize it when exogenously expressed. The cause-effect relationship between a neuron and its function in the brain is thus bi-directionally investigated with evidence of necessity and sufficiency. In this review we discuss the potential of optogenetics with a focus on three major requirements for its application: (i) selection of the light-sensitive proteins optimal for optogenetic investigation, (ii) targeted expression of these selected proteins in a specific group of neurons, and (iii) targeted irradiation with high spatiotemporal resolution. We also discuss recent progress in the application of optogenetics to studies of non-neural cells such as glial cells, cardiac and skeletal myocytes. In combination with stem cell technology, optogenetics may be key to successful research using embryonic stem cells (ESCs) and induced pluripotent stem cells (iPSCs) derived from human patients through optical regulation of differentiation-maturation, through optical manipulation of tissue transplants and, furthermore, through facilitating survival and integration of transplants.

Organotypic coculture preparation for the study of developmental synapse elimination in mammalian brain

We developed an organotypic coculture preparation allowing fast and efficient identification of molecules that regulate developmental synapse elimination in the mammalian brain. This coculture consists of a cerebellar slice obtained from rat or mouse at postnatal day 9 (P9) or P10 and a medullary explant containing the inferior olive dissected from rat at embryonic day 15. We verified that climbing fibers (CFs), the axons of inferior olivary neurons, formed functional synapses onto Purkinje cells (PCs) in the cerebellum of cocultures. PCs were initially reinnervated by multiple CFs with similar strengths. Surplus CFs were eliminated subsequently, and the remaining CFs became stronger. These changes are similar to those occurring in developing cerebellum in vivo. Importantly, the changes in CF innervations in cocultures involved the same molecules required for CF synapse elimination in vivo, including NMDA receptor, type 1 metabotropic glutamate receptor and glutamate receptor δ2 (GluRδ2). We demonstrate that gain- and loss-of-function analyses can be efficiently performed by lentiviral-mediated overexpression and RNAi-induced knockdown of GluRδ2. Using this approach, we identified neuroligin-2 as a novel molecule that promotes CF synapse elimination in postsynaptic PCs. Thus, our coculture preparation will greatly facilitate the elucidation of molecular mechanisms of synapse elimination.

High-throughput single-cell manipulation in brain tissue

The complexity of neurons and neuronal circuits in brain tissue requires the genetic manipulation, labeling, and tracking of single cells. However, current methods for manipulating cells in brain tissue are limited to either bulk techniques, lacking single-cell accuracy, or manual methods that provide single-cell accuracy but at significantly lower throughputs and repeatability. Here, we demonstrate high-throughput, efficient, reliable, and combinatorial delivery of multiple genetic vectors and reagents into targeted cells within the same tissue sample with single-cell accuracy. Our system automatically loads nanoliter-scale volumes of reagents into a micropipette from multiwell plates, targets and transfects single cells in brain tissues using a robust electroporation technique, and finally preps the micropipette by automated cleaning for repeating the transfection cycle. We demonstrate multi-colored labeling of adjacent cells, both in organotypic and acute slices, and transfection of plasmids encoding different protein isoforms into neurons within the same brain tissue for analysis of their effects on linear dendritic spine density. Our platform could also be used to rapidly deliver, both ex vivo and in vivo, a variety of genetic vectors, including optogenetic and cell-type specific agents, as well as fast-acting reagents such as labeling dyes, calcium sensors, and voltage sensors to manipulate and track neuronal circuit activity at single-cell resolution.

Electroporation-induced inward current in voltage-clamped guinea pig ventricular myocytes

Electroporation induced by high-strength electrical fields has long been used to investigate membrane properties and facilitate transmembrane delivery of molecules and genes for research and clinical purposes. In the heart, electric field-induced passage of ions through electropores is a factor in defibrillation and postshock dysfunction. Voltage-clamp pulses can also induce electroporation, as exemplified by findings in earlier studies on rabbit ventricular myocytes: Long hyperpolarizations to ≤-110 mV induced influx of marker ethidium and irregular inward currents that were as large with external NMDG(+) as Na(+). In the present study, guinea pig ventricular myocytes were bathed with NMDG(+), Na(+) or NMDG(+) + La(3+) solution (36°C) and treated with five channel blockers. Hyperpolarization of myocytes in NMDG(+) solution elicited an irregular inward current (I (ep)) that reversed at -21.5 ± 1.5 mV. In myocytes hyperpolarized with 200-ms steps every 30 s, I (ep) occurred in "episodes" that lasted for one to four steps. Boltzmann fits to data on the incidence of I (ep) per experiment indicate 50% incidence at -129.7 ± 1.4 mV (Na(+)) and -146.3 ± 1.6 mV (NMDG(+)) (slopes ≈-7.5 mV). I (ep) amplitude increased with negative voltage and was larger with Na(+) than NMDG(+) (e.g., -2.83 ± 0.34 vs. -1.40 ± 0.22 nA at -190 mV). La(3+) (0.2 mM) shortened episodes, shifted 50% incidence by -35 mV and decreased amplitude, suggesting that it inhibits opening/promotes closing of electropores. We compare our findings with earlier ones, especially in regard to electropore selectivity. In the Appendix, relative permeabilities and modified excluded-area theory are used to derive estimates of electropore diameters consistent with reversal potential -21.5 mV.

Single neuron electroporation in manipulating and measuring the central nervous system

The development and application of single neuron electroporation largely advanced the use of traditional genetics in investigations of the central nervous system. This quick and accurate manipulation of the brain at individual neuron level allowed the gain and loss of functional analyses of different genes and/or proteins. This manuscript reviewed the development of the technique and discussed some technical aspects in practical manipulations. Then the manuscript summarized the potential applications with this technique. Last but not least, the technique showed prospective future when combined with other modern methods in neuroscience research.

Single-cell electroporation

Single-cell electroporation (SCEP) is a relatively new technique that has emerged in the last decade or so for single-cell studies. When a large enough electric field is applied to a single cell, transient nano-pores form in the cell membrane allowing molecules to be transported into and out of the cell. Unlike bulk electroporation, in which a homogenous electric field is applied to a suspension of cells, in SCEP an electric field is created locally near a single cell. Today, single-cell-level studies are at the frontier of biochemical research, and SCEP is a promising tool in such studies. In this review, we discuss pore formation based on theoretical and experimental approaches. Current SCEP techniques using microelectrodes, micropipettes, electrolyte-filled capillaries, and microfabricated devices are all thoroughly discussed for adherent and suspended cells. SCEP has been applied in in-vivo and in-vitro studies for delivery of cell-impermeant molecules such as drugs, DNA, and siRNA, and for morphological observations.

Role of pre- and postsynaptic activity in thalamocortical axon branching

Axonal branching is thought to be regulated not only by genetically defined programs but also by neural activity in the developing nervous system. Here we investigated the role of pre- and postsynaptic activity in axon branching in the thalamocortical (TC) projection using organotypic coculture preparations of the thalamus and cortex. Individual TC axons were labeled with enhanced yellow fluorescent protein by transfection into thalamic neurons. To manipulate firing activity, a vector encoding an inward rectifying potassium channel (Kir2.1) was introduced into either thalamic or cortical cells. Firing activity was monitored with multielectrode dishes during culturing. We found that axon branching was markedly suppressed in Kir2.1-overexpressing thalamic cells, in which neural activity was silenced. Similar suppression of TC axon branching was also found when cortical cell activity was reduced by expressing Kir2.1. These results indicate that both pre- and postsynaptic activity is required for TC axon branching during development.