Developmental Biology

Neuronal migration is a critical step for the development of neuronal circuits in the brain. Immature new neurons (neuroblasts) generated in the postnatal ventricular-subventricular zone (V-SVZ) show a remarkable potential to migrate for a long distance at a high speed in the postnatal mammalian brain, and are thus a powerful model to analyze the molecular and cellular mechanisms of neuronal migration. Here we describe a methodology for in vitro time-lapse imaging of the primary cilium and its related structures in migrating V-SVZ-derived neuroblasts using confocal or superresolution laser-scanning microscopy. The V-SVZ tissues are dissected from postnatal day 0-1 (P0-1) mouse brains and dissociated into single cells by trypsinization and gentle pipetting. These cells are then transduced with a plasmid(s) encoding a gene(s) of interest, aggregated by centrifugation, and cultured for 2 days in Matrigel. Time-lapse images of migratory behaviors of cultured neuroblasts and their ciliary structures, including the ciliary membrane and basal body, are acquired by confocal or superresolution laser-scanning microscopy. This method provides information about the spatiotemporal dynamics of neuroblasts’ morphology and ciliary structures, and is widely applicable to various types of migrating neuronal and nonneuronal cells in various species.

Cell-type specific transcriptional programs underlie the development and maintenance of organs. Not only distinct cell types within a tissue, even cells with supposedly identical cell fates show a high degree of transcriptional heterogeneity. Inevitable, low cell numbers are a major hurdle to study transcriptomes of pure cell populations. Here we describe DigiTAG, a high-throughput method that combines transposase fragmentation and molecular barcoding to retrieve high quality transcriptome data of rare cell types in Drosophila melanogaster. The protocol showcases how DigiTAG can be used to analyse the transcriptome of rare neural stem cells (type II neuroblasts) of Drosophila larval brains, but can also be utilized for other cell types or model systems.

Hirschsprung disease (HSCR), also named aganglionic megacolon, is a severe congenital malformation characterized by a lack of enteric nervous system (ENS) in the terminal regions of the bowel (Bergeron et al., 2013). As the ENS notably regulates motility in the whole gastrointestinal track, the segment without neurons remains tonically contracted, resulting in functional intestinal obstruction and accumulation of fecal material (megacolon). HSCR occurs when enteric neural progenitors of vagal neural crest origin fail to fully colonize the developing intestines. These “enteric” neural crest cells (ENCCs) have to migrate in a rostro-caudal direction during a fixed temporal window, which is between embryonic day (e) 9.5 and e14.5 in the mouse (Obermayr et al., 2013). Recently, our group generated a new HSCR mouse model called Holstein in which migration of ENCCs is impaired because of increased collagen VI levels in their microenvironment (Soret et al., 2015). Here, we describe the method that allowed us to demonstrate the cell-autonomous nature of this migration defect. In this system adapted from a previously described heterotopic grafting approach (Breau et al., 2006), the donor tissue is a fully colonized segment of e12.5 midgut while the host tissue is an aneural segment of e12.5 hindgut. Extent of ENCC migration in host tissue is assessed after 24 h of culture and is greatly facilitated when donor tissue has a transgenic background such as the Gata4-RFP (Pilon et al., 2008) that allows endogenous labeling of ENCCs with fluorescence. Depending of the genetic background of donor and host tissues, this approach can allow evaluating both cell-autonomous and non-cell-autonomous defects of ENCC migration.

Transplantation in mouse brain slices is a powerful tool in order to study axon targeting and migrational events during development. Taking advantage of donors and recipients belonging to different genotypes, this technique allows researchers to assess the contribution of donor and/or recipient tissue by performing various combinations and to study cell-autonomous functions or effects that are influenced by the recipient’s environment (Bastakis, et al., 2015). Here we describe the transplantation procedure on sagittal brain slices containing olfactory bulb (OB). Specifically, we have transplanted the proximal-to-the-cortex part of dorsal OB to the same region on a recipient slice. Transplanted slices can be cultured for up to 3 days before their morphology is disfigured due to growth in 3D. Re-sectioning of these slices allows for a more detailed immunohistochemical analysis.

Photoreceptors are specialized retinal neurons able to respond to light in order to generate visual information. Among photoreceptors, cones are involved in colors discrimination and high-resolution central vision and are selectively depleted in macular degenerations and cone dystrophies. A possible therapeutic solution for these disorders is to replace degenerating cells with functional cones. Here, we describe a simple protocol for the rapid production of large amount of cone photoreceptors from human pluripotent stem cells. The differentiation protocol is based on the “default pathway” of neural induction using the BMP, TGFβ and WNT antagonist COCO.

In the vertebrate central nervous system (CNS), different neural precursor populations such as neural stem cells (NSCs), intermediate neurogenic progenitors (INPs) and immature neurons have to migrate from their places of birth to their location of function. Coordinated migration is mediated by direct cell-cell interactions and by extracellular matrix components, chemoattractants as well as repellents. The migration potential of such populations as well as the responsiveness to chemoattractive compounds can be addressed in isolated cells using in vitro migration assays. Here we describe two migration assays, a matrigel migration assay and a Boyden chamber migration assay, which allow the in vitro assessment of neural migration under defined conditions (Ladewig, Koch and Brüstle, 2014). A matrigel matrix is a soluble basement membrane extract. The major components of matrigel matrix are collagens, laminin and proteoglycans, which provide the substrate for migrating cells. In the matrigel assay migration can be analyzed using a phase contrast microscope. The Boyden chamber assay (Richards and McCullough, 1984) is based on microchemotaxis chambers, which consist of two compartments separated by a membrane with a defined pore size. Cells can be plated in the upper compartment and allowed to migrate through the pores towards the lower compartment, in which a potential chemotactic agent is loaded. Cell migration can be analyzed following fixing and immunohistochemical staining. In principle, the described protocols should be applicable to other cell populations such as endothelial cells or cancer cells using conditions adapted to the individual needs of the specific cell type.

Successful neural circuit formation relies on the accurate navigation of axons towards their targets during development. Axons are guided by a combination of short-range and long-range, attractive and repulsive cues. The commissural axons of the developing spinal cord have provided an informative in vivo model for the identification of multiple axon guidance molecules and mechanisms. These axons extend ventrally from the dorsal spinal cord and cross the midline at the floor plate, before making a sharp rostral turn towards the head. This simple trajectory has facilitated the identification of many axon guidance molecules, because perturbation of the stereotypical guidance decisions as a result of genetic manipulations can be easily identified. The open-book assay is a method to assess the trajectory of spinal commissural axons. The spinal cord is dissected out, opened at the roof plate and pinned flat. Punctate injections of the lipophilic fluorescent dye, DiI, are used to trace commissural axon trajectories prior to microscopy and analysis.

This protocol will be useful to introduce the genes of interest into the cerebellar granule cells at early stages of development. Since the granule cell precursors are localized in the external granule layer before migration, DNA plasmids can be specifically incorporated into the granule cells by injecting DNA solution into the cerebellar fissures followed by application of electric pulses. This technique can be performed prior to the preparation of either dissociated or organotypic culture, which can be used to study the molecular mechanisms of cell migration, axon elongation and synapstogenesis during development.

This technique will allow using brain slices to study several aspects of cortical development (i.e. neurogenesis), as well as neuronal differentiation (i.e. neuronal migration, axon and dendrite formation) in situ. This protocol is suitable for various embryonic stages (Calderon de Anda et al., 2010; Ge et al., 2010).