Single-cell electroporation in Xenopus

Cell-autonomous and differential endocannabinoid signaling impacts the development of presynaptic retinal ganglion cell axon connectivity in vivo

Cannabis exposure during gestation evokes significant molecular modifications to neurodevelopmental programs leading to neurophysiological and behavioral abnormalities in humans. The main neuronal receptor for Δ⁹-tetrahydrocannabinol (THC) is the type-1 cannabinoid receptor CB₁R, one of the most abundant G-protein-coupled receptors in the nervous system. While THC is the major psychoactive phytocannabinoid, endocannabinoids (eCBs) are the endogenous ligands of CB₁R and are known to act as retrograde messengers to modulate synaptic plasticity at different time scales in the adult brain. Accumulating evidence indicates that eCB signaling through activation of CB₁R plays a central role in neural development. During development, most CB₁R localized to axons of projection neurons, and in mice eCB signaling impacts axon fasciculation. Understanding of eCB-mediated structural plasticity during development, however, requires the identification of the precise spatial and temporal dynamics of CB₁R-mediated modifications at the level of individual neurons in the intact brain. Here, the cell-autonomous role of CB₁R and the effects of CB₁R-mediated eCB signaling were investigated using targeted single-cell knockdown and pharmacologic treatments in Xenopus. We imaged axonal arbors of retinal ganglion cells (RGCs) in real time following downregulation of CB₁R via morpholino (MO) knockdown. We also analyzed RGC axons with altered eCB signaling following treatment with URB597, a selective inhibitor of the enzyme that degrades Anandamide (AEA), or JZL184, an inhibitor of the enzyme that blocks 2-Arachidonoylglycerol (2-AG) hydrolysis, at two distinct stages of retinotectal development. Our results demonstrate that CB₁R knockdown impacts RGC axon branching at their target and that differential 2-AG and AEA-mediated eCB signaling contributes to presynaptic structural connectivity at the time that axons terminate and when retinotectal synaptic connections are made. Altering CB₁R levels through CB₁R MO knockdown similarly impacted dendritic morphology of tectal neurons, thus supporting both pre- and postsynaptic cell-autonomous roles for CB₁R-mediated eCB signaling.

Assembly of imaging chambers and high-resolution imaging of early chick embryos

The chick embryo is a classical model system for developmental biology. Its flat disk morphology exemplifies the amniote embryos from reptiles, birds, and most mammals (including humans). It is also particularly amenable to manipulation and culture both in vitro and in ovo. As a consequence, studies with this system have been important for promoting our understanding of cell and tissue movements, lineages, inductive processes, and cell fate decisions. In recent years, imaging techniques have been devised that allow these questions to be studied at the cellular level at late neurulation and segmentation stages. Modern improvements to imaging techniques now allow direct visualization of molecular interactions and cellular events and are likely to be instrumental in linking these processes to whole tissue and embryo morphogenesis. Here, we describe a method for high-resolution (confocal or multiphoton) time-lapse imaging of chick embryos at stages spanning pregastrulation to early neurulation. We give detailed instructions for assembling the specialized imaging chambers required for this procedure.

Single-cell electroporation of Xenopus tadpole tectal neurons

Single-cell electroporation (SCE) is a versatile technique for delivering electrically charged macromolecules, including DNA, RNA, synthetic oligonucleotides, peptides, dyes, and drugs, to individual cells within intact tissues. Here, we describe methods for SCE of single tectal neurons within the albino Xenopus laevis tadpole for delivery of plasmid DNA-expressing protein fluorophores or fluorescent dye. Individual neurons labeled by this technique can then be imaged in three dimensions (3D) within the intact and living brain using in vivo two-photon microscopy for studies of morphology and growth. The SCE protocol is relatively simple and requires minimal and common laboratory equipment, including a fluorescent stereomicroscope, micropipette puller, and electrical stimulator. Once equipment is set up, learning to label cells with fluorescent dyes is straightforward and usually quickly achieved, because direct visualization with fluorescent microscopy offers immediate feedback of success. The main challenges are positioning the pipette tip in a cell body layer and optimizing pipette tip shape and stimulation parameters. Once fluorescent dye loading has been achieved, transfecting neurons with DNA should follow by using the same pipette tip parameters, but extending the stimulation parameters, because plasmid DNA is larger than dye and requires formation of larger pores. Detectable expression of protein fluorophores from transfected DNA typically takes 6-12 h. SCE for dye loading or DNA transfection of tadpole tectal neurons is highly efficient and can be learned in 1 or 2 d by novice laboratory personnel.