Ex Vivo Oculomotor Slice Culture from Embryonic GFP-Expressing Mice for Time-Lapse Imaging of Oculomotor Nerve Outgrowth
Summary
An ex vivo slice assay allows oculomotor nerve outgrowth to be imaged in real time. Slices are generated by embedding E10.5 IslMN:GFP embryos in agarose, slicing on a vibratome, and growing in a stage-top incubator. The role of axon guidance pathways is assessed by adding inhibitors to the culture media.
Abstract
Accurate eye movements are crucial for vision, but the development of the ocular motor system, especially the molecular pathways controlling axon guidance, has not been fully elucidated. This is partly due to technical limitations of traditional axon guidance assays. To identify additional axon guidance cues influencing the oculomotor nerve, an ex vivo slice assay to image the oculomotor nerve in real-time as it grows towards the eye was developed. E10.5 IslMN-GFP embryos are used to generate ex vivo slices by embedding them in agarose, slicing on a vibratome, then growing them in a microscope stage-top incubator with time-lapse photomicroscopy for 24-72 h. Control slices recapitulate the in vivo timing of outgrowth of axons from the nucleus to the orbit. Small molecule inhibitors or recombinant proteins can be added to the culture media to assess the role of different axon guidance pathways. This method has the advantages of maintaining more of the local microenvironment through which axons traverse, not axotomizing the growing axons, and assessing the axons at multiple points along their trajectory. It can also identify effects on specific subsets of axons. For example, inhibition of CXCR4 causes axons still within the midbrain to grow dorsally rather than ventrally, but axons that have already exited ventrally are not affected.
Introduction
The ocular motor system provides an elegant system for investigating axon guidance mechanisms. It is relatively uncomplicated, consisting of three cranial nerves innervating six extraocular muscles (EOMs) which move the eye, and the levator palpebrae superioris (LPS) which lifts the eyelid. The oculomotor nerve innervates the LPS and four EOMs – the inferior oblique and the medial, inferior, and superior rectus muscles. The other two nerves, the trochlear and abducens, each only innervate one muscle, the superior oblique and lateral rectus muscle, respectively. Eye movements provide an easy readout, showing if innervation was appropriate, missing, or aberrant. Additionally, there are human eye movement disorders that result from deficits in neuronal development or axon guidance, collectively termed the congenital cranial disinnervation disorders (CCDDs)1.
Despite these advantages, the ocular motor system is rarely used in axon guidance studies2,3,4,5,6,7,8,9,10, due to technical drawbacks. In vitro axon guidance assays have many disadvantages11. Co-culture assays, in which neuronal explants are cultured together with explants of target tissue12 or transfected cells13, depend on both symmetry of the explant and precise positioning between the explant and target tissue. Stripe assays14,15, in which two cues are laid down in alternating stripes and axons are assessed for preferential growth on one stripe, only indicate that one substrate is preferable to the other, not that either is attractive or repulsive, or physiologically relevant. Microfluidics chambers can form precise chemical gradients, but subject growing axons to shear stress16,17,18, which can affect their growth. Moreover, in each of these approaches, collecting explants or dissociated cells requires that outgrowing axons be axotomized and thus these assays actually examine axon regeneration, rather than initial axon outgrowth. Finally, these in vitro approaches remove the microenvironment that influences axons and their responses to cues along different points of their course, and traditionally only test one cue in isolation. Compounding these disadvantages, the small size of each nucleus in the ocular motor system makes dissection technically challenging for either explants or dissociated cultures. Additionally, primary cultures of ocular motor neurons are usually heterogeneous, have significant cell death, and are density dependent, requiring pooling of cells from multiple embryos (Ryosuki Fujiki, personal communication). In vivo methods, however, including knockout mouse models, are inappropriate to use for screening, given the time and expense required.
Methods developed to culture whole embryos19 allow labeling of migrating cells20 or blockade of specific molecules21, but whole embryo cultures require incubation in roller bottles which precludes real-time imaging of labeled structures. Surgical techniques that allow manipulation of the embryo and then subsequent further development either in the uterus or in the abdomen of the mother (maintaining the placental connection)22 are also possible, but these also do not allow time-lapse imaging.
To overcome the obstacles of in vitro assays and allow rapid screening of signaling pathways, an ex vivo embryonic slice culture technique was developed23, adapted from a previously published protocol for peripheral nerve outgrowth24. Using this protocol, the developing oculomotor nerve can be imaged over time in the presence of many of the surrounding structures along its trajectory, including EOM targets. By adding small molecule inhibitors, growth factors, or guidance cues to the culture media, we can assess guidance perturbations at multiple points along the axon trajectory, allowing more rapid assessment of potential growth and guidance factors.
Protocol
Representative Results
Discussion
This ex vivo slice culture protocol provides significant advantages over traditional axon guidance assays23. The size of each cranial motor nucleus is not a limiting factor, and no difficult dissection is necessary. The endogenous microenvironment through which the axons travel is maintained, allowing modification of one signaling pathway while maintaining other signaling pathways. Additionally, effects can be assessed at different points along the axon trajectory. Since axon guidance requires mul…
Disclosures
The authors have nothing to disclose.
Acknowledgements
Funding provided by the National Eye Institute [5K08EY027850], National Institute of Child Health and Development [U54HD090255], Harvard-Vision Clinical Scientist Development Program [5K12EY016335], the Knights Templar Eye Foundation [Career Starter Grant], and the Children’s Hospital Ophthalmology Foundation [Faculty Discovery Award]. ECE is a Howard Hughes Medical Institute investigator.
Materials
| 24-Well Tissue Culture Plate | Genesee Scientific | 25-107 | |
| 6-Well Tissue Culture Plate | Genesee Scientific | 25-105 | |
| Disposable Pasteur Pipet (Flint Glass) | VWR | 14672-200 | |
| Fine Forceps | Fine Science Tools | 11412-11 | |
| Fluorobrite DMEM | Thermo Fisher Scientific | A1896701 | |
| Glucose (200 g/L) | Thermo Fisher Scientific | A2494001 | |
| Hank's Balanced Salt Solution (1X) | Thermo Fisher Scientific | 14175-095 | |
| Heat Inactivated Fetal Bovine Serum | Atlanta Biologicals | S11550H | |
| HEPES Buffer Solution (1M) | Thermo Fisher Scientific | 15630106 | |
| L-Glutamine (250 nM) | Thermo Fisher Scientific | 25030081 | |
| Loctite Superglue | Loctite | ||
| Low Melting Point Agarose | Thermo Fisher Scientific | 16520050 | |
| Millicell Cell Culture Insert (30mm, hydrophilic PTFE, 0.4 um) | Millipore Sigma | PICM03050 | |
| Moria Mini Perforated Spoon | Fine Science Tools | 10370-19 | |
| Penicillin/Streptomycin (10,000 U/mL) | Thermo Fisher Scientific | 15140122 | |
| Petri Dish (100 x 15mm) | Genesee Scientific | 32-107G | |
| Phosphate Buffered Saline (1X, pH 7.4) | Thermo Fisher Scientific | 10010049 | |
| Razor Blades | VWR | 55411-050 | |
| Surgical Scissors – Blunt | Fine Science Tools | 14000-12 | |
| Ti Eclipse Perfect Focus with TIRF | Nikon | ||
| Vibratome (VT 1200S) | Leica | 1491200S001 | |
| Vibratome Blades (Double Edge, Stainless Steel) | Ted Pella, Inc. | 121-6 |
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