Two-photon axotomy and time-lapse confocal imaging in live zebrafish embryos

Summary

Here we describe a method for mounting zebrafish embryos for long-term imaging, two-photon imaging and tissue-damage techniques, and time-lapse confocal imaging.

Abstract

Zebrafish have long been utilized to study the cellular and molecular mechanisms of development by time-lapse imaging of the living transparent embryo. Here we describe a method to mount zebrafish embryos for long-term imaging and demonstrate how to automate the capture of time-lapse images using a confocal microscope. We also describe a method to create controlled, precise damage to individual branches of peripheral sensory axons in zebrafish using the focused power of a femtosecond laser mounted on a two-photon microscope. The parameters for successful two-photon axotomy must be optimized for each microscope. We will demonstrate two-photon axotomy on both a custom built two-photon microscope and a Zeiss 510 confocal/two-photon to provide two examples.

Zebrafish trigeminal sensory neurons can be visualized in a transgenic line expressing GFP driven by a sensory neuron specific promoter ¹. We have adapted this zebrafish trigeminal model to directly observe sensory axon regeneration in living zebrafish embryos. Embryos are anesthetized with tricaine and positioned within a drop of agarose as it solidifies. Immobilized embryos are sealed within an imaging chamber filled with phenylthiourea (PTU) Ringers. We have found that embryos can be continuously imaged in these chambers for 12-48 hours. A single confocal image is then captured to determine the desired site of axotomy. The region of interest is located on the two-photon microscope by imaging the sensory axons under low, non-damaging power. After zooming in on the desired site of axotomy, the power is increased and a single scan of that defined region is sufficient to sever the axon. Multiple location time-lapse imaging is then set up on a confocal microscope to directly observe axonal recovery from injury.

Protocol

Part 1: Mounting zebrafish embryos for long-term imaging

1. Prepare 1% low melt agarose solution for embedding. Dissolve agarose in DI water by heating in a microwave, aliquot into small tubes and store in a heating block at 42 degrees Celsius.
2. Select embryos for imaging and remove their chorions by gently pulling the chorion apart with forceps. Embryos can be placed into a 5% PTU Ringers solution at 22-24 hours post fertilization (hpf) to inhibit the formation of pigment. This improves the clarity of imaging and is essential for performing two-photon axotomies of sensory axons. If the natural pigment is allowed to form, autofluorescence obscures ....

Discussion

We have used the methods described to precisely axotomize peripheral sensory axons and to directly observe regeneration in the living zebrafish embryo. Long-term time-lapse confocal imaging in zebrafish can be used to observe many developmental processes in vivo. The two-photon axotomy procedure described can be modified for many different experimental goals. We have used the same general procedure to ablate entire trigeminal sensory neuron cell bodies, by zooming in on the cell body rather than on a branch o.......