Embryonic neurons are born in the ventricular zone of the neural tube, but migrate to reach appropriate targets. Facial branchiomotor (FBM) neurons are a useful model to study neuronal migration. This protocol describes the wholemount ex vivo culture of mouse embryo hindbrains to investigate mechanisms that regulate FBM migration.
Embryonic neurons are born in the ventricular zone of the brain, but subsequently migrate to new destinations to reach appropriate targets. Deciphering the molecular signals that cooperatively guide neuronal migration in the embryonic brain is therefore important to understand how the complex neural networks form which later support postnatal life. Facial branchiomotor (FBM) neurons in the mouse embryo hindbrain migrate from rhombomere (r) 4 caudally to form the paired facial nuclei in the r6-derived region of the hindbrain. Here we provide a detailed protocol for wholemount ex vivo culture of mouse embryo hindbrains suitable to investigate the signaling pathways that regulate FBM migration. In this method, hindbrains of E11.5 mouse embryos are dissected and cultured in an open book preparation on cell culture inserts for 24 hr. During this time, FBM neurons migrate caudally towards r6 and can be exposed to function-blocking antibodies and small molecules in the culture media or heparin beads loaded with recombinant proteins to examine roles for signaling pathways implicated in guiding neuronal migration.
Embryonic neurons are born in the ventricular zone of the brain, but subsequently migrate to new destinations to reach appropriate target regions that are located at a large distance. The correct positioning of neuronal cell bodies in appropriate places along the dorso-ventral and anterior-posterior axes of the developing brain is essential for the correct wiring, survival, and function of these neurons after the migratory stage1-4. Similar to the molecular mechanisms that control axon guidance5-7, combinatorial sets of attractive and repulsive cues are thought to guide migrating neurons1,8. However, due to the interactions of multiple cell types, the signals controlling neuronal migration have been less extensively studied than those involved in axon guidance, which can be studied cell autonomously. The developing hindbrain of vertebrates has been used in several recent studies to understand the molecular and cellular mechanisms of neuronal migration, for example in chick, mouse, and zebrafish1-4,9. This organ contains several different types of neurons, including several subtypes of precerebellar and motor neurons5,7,10,11.
Hindbrain motorneuron are born in the ventricular zone close to the floorplate, and they differentiate into specific subsets according to their rhombomere of origin1,12. The facial branchiomotor (FBM) neurons are generated in rhombomere (r) 4 in the hindbrain and extend their axons dorsally through an r4 exit point into the second branchial arch to innervate the facial muscles2,9,13. FBM neurons of zebrafish and mice provide excellent models to study the molecular and cellular mechanisms of neuronal migration in a process that is readily visualized, because these neurons reproducibly translocate their somata in a spatiotemporally well-defined process. In mice, FBM neurons first migrate caudally through r5 and then both caudally and ventrally to reach their final position on the pial side of the hindbrain in the territory of r6, where they form the paired nuclei of the VIIth cranial nerve (VIIn)10,11,14. In zebrafish, FBM neurons initially migrate ventrally and then change direction at the r4-r5 boundary to continue migrating towards the pial surface in a laminin-dependent manner4,12,15,16. This migration proceeds over a period of several days in development and can be divided into phases of tangential and radial migration, allowing the identification of molecules that mediate these two distinct processes. In contrast, the FBM neurons of the embryonic chick hindbrain remain in r43,13,17-19.
During their migration, FBM neurons can be identified, like other types or motor neurons, through their expression of the homoeodomain transcription factor islet 1 (ISL1)14. Thus, wholemount immunofluorescence staining or in situ hybridization for this marker at different developmental stages reveals the distinct migratory stream of FBM somata extending from r4 to r6 in the zebrafish or mouse4,15,16. Moreover, fluorescent transgenic reporters such as ISL1-GFP have been used as suitable tools to visualize migrating FBM neurons in zebrafish3,17-19. In addition to their suitability for imaging, many investigators have studied the migration of FBM neurons in developing zebrafish, because their free-living embryos can be manipulated easily with cell transplantation techniques and pharmacological compounds applied directly to the aquarium water. In contrast, the mouse embryo develops enclosed in the uterus, precluding the implantation of beads carrying guidance cues or the administration of function-blocking antibodies that do not cross the placental barrier. Moreover, pharmacological compounds administered to the pregnant mother may have undesired side effects that can indirectly impair embryogenesis. Circumventing this limitation, we have developed an ex vivo culture method for whole mouse hindbrain that is compatible with FBM neuron migration and survival for 24 hr after explanting7,16. This method allows easy pharmacological manipulation, implantation of beads carrying guidance cues or administration of function-blocking antibodies and could also be adapted to study the migration of other neuronal subtypes in the hindbrain at different developmental stages.
This protocol describes the wholemount culture of E11.5 mouse hindbrains in a transwell system to study the migration of FBM neurons. This protocol allows mouse hindbrain motorneurons to be kept alive and migrating for a period of 24 hr, enabling ex vivo manipulation. This method has numerous experimental advantages for investigators seeking to identify the molecular and cellular mechanisms of neuronal migration. Whereas traditional migration assays explant small neural tissue pieces into matrix on culture dishes and enable observation of individual neurons as they respond to exogenous stimuli, a major advantage of the transwell assay is its suitability to manipulate migrating neurons within the host organ environment and therefore a more physiological context. Importantly, substances can be readily applied to the ex vivo hindbrain explants to test their effect on neuronal migration, circumventing possible side effects associated with administering these substances to a pregnant mouse. Finally, the ex vivo model also allows the testing of substances that do not cross the placental barrier, such as function-blocking antibodies. Due to these advantages, the ex vivo hindbrain culture provides an alternative and complementary method to using zebrafish embryos, which can be treated with water soluble small molecules in the aquarium water, or to in utero electroporation of embryonic brains, which requires the use of specialized equipment and is more difficult to master than the culture technique described here. Another advantage of the protocol described here is its amenability to implanting beads soaked in recombinant protein or other reagents, therefore allowing the application of a standard embryological method developed to manipulate chick embryos to a mouse model of neuronal migration. In particular, the ex vivo culture model may be applied to hindbrains of genetically engineered mice defective in specific molecules implicated in neuronal migration, such as growth or guidance factor receptors, and combined with bead implants to test if responsiveness to ligands is lost. In addition to pharmacological manipulation, the ex vivo culture protocol could also be adapted to electroporate expression vectors that could manipulate expression of genes of interest; appropriate methods for electroporation have previously been described22,23. This protocol may also be adapted to visualize neuron migration by time-lapse microscopy in hindbrain explants from transgenic mice containing fluorescently labeled FBM neurons, e.g. Isl1-Cre; Rosa26Yfp 21. Finally, this protocol may also be used to study other types of migrating neurons in the hindbrain, such as those that form the inferior olive, although this would require the use of hindbrains at older embryonic stages and may require culture for up to 48 hr, depending on neuronal viability ex vivo.
Critical steps and troubleshooting
For the success of this protocol, it is crucial that embryos are collected early on day E11.5, closer to E11.25, when FBM neuron migration has just begun. However, it is not always possible to catch embryos at this developmental stage due to natural mating variability of mice, and accordingly, there may be some variability in the extend of FBM migration between different experiments. Variability in FBM migration may also be observed if the experiment is not completed within the time frame assigned, about 3 hr, as can be seen in Figure 3E. Hindbrain tissue from E11.25 mouse embryos is delicate. When dissecting and throughout the explant procedure, it is important to not tear the hindbrain tissue in the areas from r4-r6 where the FBM neurons are located. Due to the delicate nature of the dissection process, and because the speed at which hindbrains are placed into culture influences outcome, the procedure might take a couple of practice runs to master, in particular before precious samples or reagents are used. Finally, it is important that the hindbrain tissue is placed into an open book configuration on the culture insert, because folding of the hindbrain tissue during culture will prevent normal FBM migration (see Figure 3F).
The authors have nothing to disclose.
MT is supported by a PhD studentship [ref. 092839/Z/10/Z] and CR by a New Investigator Award [ref. 095623/Z/11/Z] from the Wellcome Trust.
Eppendorf round-bottomed reagent tube, 2.0 ml (Safe-Lock) | VWR | 211-2120 | |
Cell culture plates, 12 well | Thermo Scientific | 150628 | |
Plastic cell culture dish, 100 mm | Thermo Scientific | 150288 | |
15 mm Netwell insert with Mesh Polyester Membrane | Corning | 3477 | |
Watchmaker forceps, no. 5 | Dumont | 91150-20 | |
Phosphate buffered saline | Sigma | P4417 | |
Laminin mouse protein | Life Technologies | 23017-015 | |
Primary antibody, Isl1 | Developmental Hybridoma Bank | 39.4D5 | Dilution 1/100 |
AlexaFluor 488-conjugated goat anti mouse secondary antibody | Life Technologies | A11029 | Dilution 1/200 |
Neurobasal medium | Life Technologies | 21103 | |
B27 supplement (50x) | Life Technologies | 17504-044 | |
Leibovitz’s L-15 | Life Technologies | 21083-027 | |
Penicillin/ streptomycin | Life Technologies | 15070 | |
Glucose | VWR | 101174Y | |
Heat-inactivated goat serum | Sigma | G9023 | |
Triton X-100 | Sigma | T8787 | |
Paraformaldehyde | Sigma | P6148 | |
Slowfade Antifade Kit | Life Technologies | S-2828 | Alternative mounting solutions may be used |
Microscope slides | VWR | 631-0912 | |
Cover glass | VWR | 631-0137 | |
Affi-Gel Heparin Beads | Biorad | 153-6173 | Glass, latex, or coloured beads may be used alternatively |
Recombinant human VEGF165 | R&D systems | 293-VE | Resuspend in PBS, store aliquots at -80oC |
Stereo Microscope, Leica MZ16 | Leica | ||
Confocal laser scanning microscope LSM710 | Zeiss |