A primer on using in ovo electroporation to analyze gene function

Chicken embryo as a model in second heart field development

Previously, a single source of progenitor cells was thought to be responsible for the formation of the cardiac muscle. However, the second heart field has recently been identified as an additional source of myocardial progenitor cells. The chicken embryo, which develops in the egg, outside the mother can easily be manipulated in vivo and in vitro. Hence, it was an excellent model for establishing the concept of the second heart field. Here, our review will focus on the chicken model, specifically its role in understanding the second heart field. In addition to discussing historical aspects, we provide an overview of recent findings that have helped to define the chicken second heart field progenitor cells. A better understanding of the second heart field development will provide important insights into the congenital malformations affecting cardiac muscle formation and function.

Species-specific sensitivity to TGFβ signaling and changes to the Mmp13 promoter underlie avian jaw development and evolution

Precise developmental control of jaw length is critical for survival, but underlying molecular mechanisms remain poorly understood. The jaw skeleton arises from neural crest mesenchyme (NCM), and we previously demonstrated that these progenitor cells express more bone-resorbing enzymes including Matrix metalloproteinase 13 (Mmp13) when they generate shorter jaws in quail embryos versus longer jaws in duck. Moreover, if we inhibit bone resorption or Mmp13, we can increase jaw length. In the current study, we uncover mechanisms establishing species-specific levels of Mmp13 and bone resorption. Quail show greater activation of and sensitivity to transforming growth factor beta (TGFβ) signaling than duck; where intracellular mediators like SMADs and targets like Runt-related transcription factor 2 (Runx2), which bind Mmp13, become elevated. Inhibiting TGFβ signaling decreases bone resorption, and overexpressing Mmp13 in NCM shortens the duck lower jaw. To elucidate the basis for this differential regulation, we examine the Mmp13 promoter. We discover a SMAD-binding element and single nucleotide polymorphisms (SNPs) near a RUNX2-binding element that distinguish quail from duck. Altering the SMAD site and switching the SNPs abolish TGFβ sensitivity in the quail Mmp13 promoter but make the duck promoter responsive. Thus, differential regulation of TGFβ signaling and Mmp13 promoter structure underlie avian jaw development and evolution.

In Ovo Gain- and Loss-of-Function Approaches to Study Gut Morphogenesis

The polarity of cellular components is essential for cellular shape changes, oriented cell migration, and modulating intra- and intercellular mechanical forces. However, many aspects of polarized cell behavior-especially dynamic cell shape changes during the process of morphogenesis-are almost impossible to study in cells cultured in plastic dishes. Avian embryos have always been a treasured model system to study vertebrate morphogenesis for developmental biologists. Avian embryos recapitulate human biology particularly well in the early stages due to their flat disc gastruloids. Since avian embryos can be manipulated in ovo they present paramount opportunities for highly localized targeting of genetic mechanisms during cellular and developmental processes. Here, we review the application of these methods for both gain of function and loss of function of a gene of interest at a specific developmental stage during left-right (LR) asymmetric gut morphogenesis. These tools present a powerful premise to investigate various polarized cellular activities and molecular processes in vivo in a reproducible manner.

Transport and Secretion of the Wnt3 Ligand by Motor Neuron-like Cells and Developing Motor Neurons

The vertebrate neuromuscular junction (NMJ) is formed by a presynaptic motor nerve terminal and a postsynaptic muscle specialization. Cumulative evidence reveals that Wnt ligands secreted by the nerve terminal control crucial steps of NMJ synaptogenesis. For instance, the Wnt3 ligand is expressed by motor neurons at the time of NMJ formation and induces postsynaptic differentiation in recently formed muscle fibers. However, the behavior of presynaptic-derived Wnt ligands at the vertebrate NMJ has not been deeply analyzed. Here, we conducted overexpression experiments to study the expression, distribution, secretion, and function of Wnt3 by transfection of the motor neuron-like NSC-34 cell line and by in ovo electroporation of chick motor neurons. Our findings reveal that Wnt3 is transported along motor axons in vivo following a vesicular-like pattern and reaches the NMJ area. In vitro, we found that endogenous Wnt3 expression increases as the differentiation of NSC-34 cells proceeds. Although NSC-34 cells overexpressing Wnt3 do not modify their morphological differentiation towards a neuronal phenotype, they effectively induce acetylcholine receptor clustering on co-cultured myotubes. These findings support the notion that presynaptic Wnt3 is transported and secreted by motor neurons to induce postsynaptic differentiation in nascent NMJs.

From the Farm to the Lab: How Chicken Embryos Contribute to the Field of Teratology

Congenital anomalies and its causes, particularly, by external factors are the aim of the field called teratology. The external factors studied by teratology are known as teratogens and can be biological or environmental factors for example, chemicals, medications, recreational drugs, environmental pollutants, physical agents (e.g., X-rays and maternal hyperthermia) and maternal metabolic conditions. Proving the teratogenicity of a factor is a difficult task requiring epidemiology studies as well as experimental teratology evidence from the use of animal models, one of which is the chicken embryo. This model in particular has the advantage of being able to follow development live and in vivo, with rapid development hatching around 21 days, is cheap and easy to manipulate and to observe development. All this allows the chicken embryo to be used in drug screening studies, teratogenic evaluation and studies of mechanisms of teratogenicity. The chicken embryo shares morphological, biochemical and genetic similarities with humans as well as mammalian species, making them ideal to ascertain the actions of teratogens, as well as screen drugs to test for their safety. Pre-clinical trials for new drugs are carried out in rodents and rabbits, however, chicken embryos have been used to screen new compounds or analogs of thalidomide as well as to investigate how some drugs can lead to congenital malformations. Indeed, the chicken embryo has proved valuable in understanding how many congenital anomalies, seen in humans, arise following teratogen exposure. The aim of this review is to highlight the role of the chicken embryo as an experimental model for studies in teratology, exploring its use in drug screening studies, phenotypic evaluation and studies of teratogenic mechanisms of action. Here, we discuss many known teratogens, that have been evaluated using the chicken embryo model including some medicines, such as, thalidomide, valproic acid; recreational drugs including alcohol; environmental influences, such as viruses, specifically ZIKV, which is a newly discovered human teratogen. In addition, we discuss how the chicken embryo has provided insight on the mechanisms of teratogenesis of many compounds and also how this impact on drug safety.

Stable integration of an optimized inducible promoter system enables spatiotemporal control of gene expression throughout avian development

Precisely altering gene expression is critical for understanding molecular processes of embryogenesis. Although some tools exist for transgene misexpression in developing chick embryos, we have refined and advanced them by simplifying and optimizing constructs for spatiotemporal control. To maintain expression over the entire course of embryonic development we use an enhanced piggyBac transposon system that efficiently integrates sequences into the host genome. We also incorporate a DNA targeting sequence to direct plasmid translocation into the nucleus and a D4Z4 insulator sequence to prevent epigenetic silencing. We designed these constructs to minimize their size and maximize cellular uptake, and to simplify usage by placing all of the integrating sequences on a single plasmid. Following electroporation of stage HH8.5 embryos, our tetracycline-inducible promoter construct produces robust transgene expression in the presence of doxycycline at any point during embryonic development in ovo or in culture. Moreover, expression levels can be modulated by titrating doxycycline concentrations and spatial control can be achieved using beads or gels. Thus, we have generated a novel, sensitive, tunable, and stable inducible-promoter system for high-resolution gene manipulation in vivo.

Evolutionary loss of foot muscle during development with characteristics of atrophy and no evidence of cell death

Many species that run or leap across sparsely vegetated habitats, including horses and deer, evolved the severe reduction or complete loss of foot muscles as skeletal elements elongated and digits were lost, and yet the developmental mechanisms remain unknown. Here, we report the natural loss of foot muscles in the bipedal jerboa, Jaculus jaculus. Although adults have no muscles in their feet, newborn animals have muscles that rapidly disappear soon after birth. We were surprised to find no evidence of apoptotic or necrotic cell death during stages of peak myofiber loss, countering well-supported assumptions of developmental tissue remodeling. We instead see hallmarks of muscle atrophy, including an ordered disassembly of the sarcomere associated with upregulation of the E3 ubiquitin ligases, MuRF1 and Atrogin-1. We propose that the natural loss of muscle, which remodeled foot anatomy during evolution and development, involves cellular mechanisms that are typically associated with disease or injury.

Intrinsic muscles are a group of muscles deep inside the hands and feet. They help to control the precise movements required, for example, for a pianist to play their instrument or for certain animals to climb with remarkable agility.

Some animals, such as horses and deer, have evolved in such a way that they no longer grasp objects with hands and feet. Where intrinsic muscles were once present in the hands and feet of their ancestors, these animals now have strong ligaments that prevent over-extension of the wrist and ankle joints during hard landings.

Given their size, it is difficult to study horses and deer in the laboratory and understand how they lost their intrinsic muscles during evolution. Tran et al. therefore focused on a small rodent called the lesser Egyptian jerboa, which also displays long legs with strong ligaments and no intrinsic muscles.

Newborn jerboas have foot muscles that look very much like the intrinsic muscles found in mice, but these muscles disappear within 4 days of birth. A mechanism called programmed cell death is often responsible for specific tissues disappearing during development, but the experiments of Tran et al. revealed that this was not the case in jerboas. Instead, their intrinsic muscles were degraded by processes triggered by genes that disassemble underused muscles. In mice and humans, fasting, nerve injuries, or immobility trigger this type of muscle degradation, but in jerboas these processes appear to be a normal part of development.

This unexpected discovery shows that development and disease-like processes are linked, and that more studies of nontraditional research animals may help scientists better understand these connections.

Nuclear FGFR2 regulates musculoskeletal integration within the developing limb

BACKGROUND

Bent bone dysplasia syndrome (BBDS), a congenital skeletal disorder caused by dominant mutations in fibroblast growth factor receptor 2 (FGFR2), is characterized by bowed long bones within the limbs. We previously showed that the FGFR2 mutations in BBDS enhance nuclear and nucleolar localization of the receptor; however, exactly how shifts in subcellular distribution of FGFR2 affect limb development remained unknown.

RESULTS

Targeted expression of the BBDS mutations in the lateral plate mesoderm of the developing chick induced angulated hindlimbs, a hallmark feature of the disease. Whole-mount analysis of the underlying skeleton revealed bent long bones with shortened bone collars and, in severe cases, dysmorphic epiphyses. Epiphyseal changes were also correlated with joint dislocations and contractures. Histological analysis revealed that bent long bones and joint defects were closely associated with irregularities in skeletal muscle patterning and tendon-to-bone attachment. The spectrum of limb phenotypes induced by the BBDS mutations were recapitulated by targeted expression of wild-type FGFR2 appended with nuclear and nucleolar localization signals.

CONCLUSIONS

Our results indicate that the bent long bones in BBDS arise from disruptions in musculoskeletal integration and that increased nuclear and nucleolar localization of FGFR2 plays a mechanistic role in the disease phenotype. 248:233-246, 2019. © 2018 Wiley Periodicals, Inc.

Embryological and Genetic Manipulation of Chick Development

The ability to combine embryological manipulations with gene function analysis in an amniote embryo makes the chick a valuable system for the vertebrate developmental biologist. This chapter describes methods for those unfamiliar with the chick system wishing to initiate experiments in their lab. After outlining methods to prepare chick embryos, protocols are provided for introducing beads or cells expressing secreted factors, and for culturing tissue explants as a means of assessing development in vitro. Approaches to achieve gain of function and loss of function (morpholino oligonucleotides) in chick are outlined, and methods for introducing these reagents by electroporation are detailed.

Neural crest and the origin of species-specific pattern

For well over half of the 150 years since the discovery of the neural crest, the special ability of these cells to function as a source of species-specific pattern has been clearly recognized. Initially, this observation arose in association with chimeric transplant experiments among differentially pigmented amphibians, where the neural crest origin for melanocytes had been duly noted. Shortly thereafter, the role of cranial neural crest cells in transmitting species-specific information on size and shape to the pharyngeal arch skeleton as well as in regulating the timing of its differentiation became readily apparent. Since then, what has emerged is a deeper understanding of how the neural crest accomplishes such a presumably difficult mission, and this includes a more complete picture of the molecular and cellular programs whereby neural crest shapes the face of each species. This review covers studies on a broad range of vertebrates and describes neural-crest-mediated mechanisms that endow the craniofacial complex with species-specific pattern. A major focus is on experiments in quail and duck embryos that reveal a hierarchy of cell-autonomous and non-autonomous signaling interactions through which neural crest generates species-specific pattern in the craniofacial integument, skeleton, and musculature. By controlling size and shape throughout the development of these systems, the neural crest underlies the structural and functional integration of the craniofacial complex during evolution.

The Use of Chick Embryos to Study Wnt Activity Gradients

The chick spinal cord provides a valuable model for assessing Wnt signaling activity. Loss or gain of function constructs that are transfected by electroporation can be directed to a single side of the spinal cord, thus leaving the contralateral side as an internal control. Here, we describe a method for measuring Wnt signaling via the use of BAT-Gal, a β-catenin dependent Wnt reporter.

Neuregulin1 fine-tunes pre-, post-, and perisynaptic neuromuscular junction development

BACKGROUND

Neuromuscular junction (NMJ) development is a multistep process mediated by coordinated interactions between the nerve terminal, target muscle, and perisynaptic Schwann cell that require constant back-and-forth communication. Retrograde and anterograde growth and differentiation factors have been postulated to participate in this communication. While neuregulin1 (NRG1) has been shown to be potent anterograde signal that activates acetylcholine receptor (AChR) transcription and clustering in vitro, its roles in NMJ development in vivo remain elusive.

RESULTS

Using the model of chicken embryo, we measured the effects of NRG1 signaling during NMJ development in ovo using quantitative, sequential measures of AChR cluster size and density, pre- and postsynaptic apposition, and the alignment of perisynaptic Schwann cells. Using in ovo electroporation at early stages and a targeted soluble neuregulin antagonist through all developmental stages, we found soluble NRG1 regulates AChR cluster density and size at the earliest stage prior to nerve-AChR cluster contact. Once the nerve contacts with muscle AChRs, NRG1 has pronounced effects on presynaptic specialization and on the alignment of perisynaptic Schwann cells at endplates.

CONCLUSION

These findings suggest that, while NRG1 may not be critical for overall development, it appears to be important in fine-tuning pre-, post-, and perisynaptic development of the NMJ. Developmental Dynamics 246:368-380, 2017. © 2017 Wiley Periodicals, Inc.

A reliable and flexible gene manipulation strategy in posthatch zebra finch brain

Songbird models meaningfully contribute to many fields including learned vocal communication, the neurobiology of social interactions, brain development, and ecology. The value of investigating gene-brain-behavior relationships in songbirds is therefore high. Viral infections typically used in other lab animals to deliver gene editing constructs have been less effective in songbirds, likely due to immune system properties. We therefore leveraged the in vivo electroporation strategy used in utero in rodents and in ovo in poultry, and apply it to posthatch zebra finch songbird chicks. We present a series of experiments with a combination of promoters, fluorescent protein genes, and piggyBac transposase vectors to demonstrate that this can be a reliable, efficient, and flexible strategy for genome manipulation. We discuss options for gene delivery experiments to test circuit and behavioral hypotheses using a variety of manipulations, including gene overexpression, CRISPR/Cas9 gene editing, inducible technologies, optogenetic or DREADD cellular control, and cell type-specific expression.

Quantitative single cell gene expression profiling in the avian embryo

BACKGROUND

Single cell gene profiling has been successfully applied to cultured cells. However, isolation and preservation of a cell's native gene expression state from an intact embryo remain problematic.

RESULTS

Here, we present a strategy for in vivo single cell profiling that optimizes cell identification, isolation and amplification of nucleic acids with nominal bias and sufficient material detection. We first tested several photoconvertible fluorescent proteins to selectively mark a cell(s) of interest in living chick embryos then accurately identify and isolate the same cell(s) in fixed tissue slices. We determined that the dual color mDendra2 provided the optimal signal/noise ratio for this purpose. We developed proper procedures to minimize cell death and preserve gene expression, and suggest nucleic acid amplification strategies for downstream analysis by microfluidic reverse transcriptase quantitative polymerase chain reaction or RNAseq. Lastly, we compared methods for single cell isolation and found that our fluorescence-activated cell sorting (FACS) protocol was able to preserve native transcripts and generate expression profiles with much higher efficiency than laser capture microdissection (LCM).

CONCLUSIONS

Quantitative single cell gene expression profiling may be accurately applied to interrogate complex cell dynamics events during embryonic development by combining photoconversion cell labeling, FACS, proper handling of isolated cells, and amplification strategies.

Tol2-Mediated Delivery of miRNAs to the Chicken Otocyst Using Plasmid Electroporation

The avian embryo has a well-documented history as a model system for the study of neurogenesis, morphogenesis, and cell fate specification. This includes studies of the chicken inner ear that employ in ovo electroporation, in conjunction with the Tol2 system, to yield robust long-term transgene expression. Capitalizing on the success of this delivery method, we describe a modified version of the Tol2 expression vector that readily accepts the insertion of a microRNA-encoding artificial intron. This offers a strategy to investigate the possible roles of different candidate microRNAs in ear development by overexpression. Here, we describe the general design of this modified vector and the electroporation procedure. This approach is expected to facilitate phenotypic screening of candidate miRNAs to explore their bioactivity in vivo.

Stable and bicistronic expression of two genes in somite- and lateral plate-derived tissues to study chick limb development

Background

Components of the limb musculoskeletal system have distinct mesoderm origins. Limb skeletal muscles originate from somites, while the skeleton and attachments (tendons and connective tissues) derive from limb lateral plate. Despite distinct mesoderm origins, the development of muscle, skeleton and attachments is highly coordinated both spatially and temporally to ensure complete function of the musculoskeletal system. A system to study molecular interactions between somitic-derived tissues (muscles) and lateral-plate-derived tissues (skeletal components and attachments) during limb development is missing.

Results

We designed a gene delivery system in chick embryos with the ultimate aim to study the interactions between the components of the musculoskeletal system during limb development. We combined the Tol2 genomic integration system with the viral T2A system and developed new vectors that lead to stable and bicistronic expression of two proteins at comparable levels in chick cells. Combined with limb somite and lateral plate electroporation techniques, two fluorescent reporter proteins were co-expressed in stoichiometric proportion in the muscle lineage (somitic-derived) or in skeleton and their attachments (lateral-plate-derived). In addition, we designed three vectors with different promoters to target muscle cells at different steps of the differentiation process.

Conclusion

Limb somite electroporation technique using vectors containing these different promoters allowed us to target all myogenic cells, myoblasts or differentiated muscle cells. These stable and promoter-specific vectors lead to bicistronic expression either in somitic-derived myogenic cells or lateral plate-derived cells, depending on the electroporation sites and open new avenues to study the interactions between myogenic cells and tendon or connective tissue cells during limb development.

A transgenic quail model that enables dynamic imaging of amniote embryogenesis

Embryogenesis is the coordinated assembly of tissues during morphogenesis through changes in individual cell behaviors and collective cell movements. Dynamic imaging, combined with quantitative analysis, is ideal for investigating fundamental questions in developmental biology involving cellular differentiation, growth control and morphogenesis. However, a reliable amniote model system that is amenable to the rigors of extended, high-resolution imaging and cell tracking has been lacking. To address this shortcoming, we produced a novel transgenic quail that ubiquitously expresses nuclear localized monomer cherry fluorescent protein (chFP). We characterize the expression pattern of chFP and provide concrete examples of how Tg(PGK1:H2B-chFP) quail can be used to dynamically image and analyze key morphogenetic events during embryonic stages X to 11.

Avians as a model system of vascular development

For more than 2,000 years, philosophers and scientists have turned to the avian embryo with questions of how life begins (Aristotle and Peck Generations of Animals. Loeb Classics, vol. XIII. Harvard University Press, Cambridge, 1943; Needham, A history of embryology. Abelard-Schuman, New York, 1959). Then, as now, the unique accessibility of the embryo both in terms of acquisition of eggs from domesticated fowl and ease at which the embryo can be visualized by simply opening the shell has made avians an appealing and powerful model system for the study of development. Thus, as the field of embryology has evolved through observational, comparative, and experimental embryology into its current iteration as the cellular and molecular biology of development, avians have remained a useful and practical system of study.

RNA-Seq analysis of differential gene expression in electroporated chick embryonic spinal cord

In ovo electroporation of the chick neural tube is a fast and inexpensive method for identification of gene function during neural development. Genome wide analysis of differentially expressed transcripts after such an experimental manipulation has the potential to uncover an almost complete picture of the downstream effects caused by the transfected construct. This work describes a simple method for comparing transcriptomes from samples of transfected embryonic spinal cords comprising all steps between electroporation and identification of differentially expressed transcripts. The first stage consists of guidelines for electroporation and instructions for dissection of transfected spinal cord halves from HH23 embryos in ribonuclease-free environment and extraction of high-quality RNA samples suitable for transcriptome sequencing. The next stage is that of bioinformatic analysis with general guidelines for filtering and comparison of RNA-Seq datasets in the Galaxy public server, which eliminates the need of a local computational structure for small to medium scale experiments. The representative results show that the dissection methods generate high quality RNA samples and that the transcriptomes obtained from two control samples are essentially the same, an important requirement for detection of differential expression genes in experimental samples. Furthermore, one example is provided where experimental overexpression of a DNA construct can be visually verified after comparison with control samples. The application of this method may be a powerful tool to facilitate new discoveries on the function of neural factors involved in spinal cord early development.

Parameters affecting efficiency of in ovo electroporation of the avian neural tube and crest

BACKGROUND

Many variations in avian in ovo transfection of the neural tube/crest have been reported, but never compared quantitatively.

RESULTS

Genome integrating pT2K-CAGGS-GFP and pCAGGS-T2TP transposase plasmids were co-electroporated into quail E2 embryo trunk neural tube and the proportion of GFP-expressing neural cells was counted 1 and 7 days later. Electroporation efficiency increased with plasmid concentration and pulse number but plateaued at, respectively, above 1.25 µg/µL and 3 pulses. Bilateral electroporation transfected more cells than unilateral but less than that anticipated by doubling the unilateral treatment. Holding the concentration of GFP plasmid constant and varying the transposase plasmid concentration revealed an optimum ratio of, in this case, 4:1 (1.2 µg/µL:0.3 µg/µL). Leaving transfected embryos to E9 confirmed that expression was maintained in vivo with the transposase system, but declined with non-integrated plasmid. Transfection of neural crest cells was low if electroporated less than 6-8 hr before emigration. We propose this indicates loss of epithelial integrity well prior to exit. We suggest this event be termed epithelio-mesenchymal transition sensu stricto, whereas the term delamination be reserved for the later emigration from the neural epithelium.

CONCLUSIONS

Co-electroporation in ovo must take into account plasmid(s) concentration and ratio, pulse number, pulse directionality, and timing.

Retrograde migration of pectoral girdle muscle precursors depends on CXCR4/SDF-1 signaling

In vertebrates, muscles of the pectoral girdle connect the forelimbs with the thorax. During development, the myogenic precursor cells migrate from the somites into the limb buds. Whereas most of the myogenic precursors remain in the limb bud to form the forelimb muscles, several cells migrate back toward the trunk to give rise to the superficial pectoral girdle muscles, such as the large pectoral muscle, the latissimus dorsi and the deltoid. Recently, this developing mode has been referred to as the "In-Out" mechanism. The present study focuses on the mechanisms of the "In-Out" migration during formation of the pectoral girdle muscles. Combining in ovo electroporation, tissue slice-cultures and confocal laser scanning microscopy, we visualize live in detail the retrograde migration of myogenic precursors from the forelimb bud into the trunk region by live imaging. Furthermore, we present for the first time evidence for the involvement of the chemokine receptor CXCR4 and its ligand SDF-1 during these processes. After microsurgical implantations of CXCR4 inhibitor beads in the proximal forelimb region of chicken embryos, we demonstrate with the aid of in situ hybridization and live-cell imaging that CXCR4/SDF-1 signaling is crucial for the retrograde migration of pectoral girdle muscle precursors. Moreover, we analyzed the MyoD expression in CXCR4-mutant mouse embryos and observed a considerable decrease in pectoral girdle musculature. We thus demonstrate the importance of the CXCR4/SDF-1 axis for the pectoral girdle muscle formation in avians and mammals.

In ovo electroporation of miRNA-based-plasmids to investigate gene function in the developing neural tube

When studying gene function in vivo during development, gene expression has to be controlled in a precise temporal and spatial manner. Technologies based on RNA interference (RNAi) are well suited for such studies, as they allow for the efficient silencing of a gene of interest. In contrast to challenging and laborious approaches in mammalian systems, the use of RNAi in combination with oviparous animal models allows temporal control of gene silencing in a fast and precise manner. We have developed approaches using RNAi in the chicken embryo to analyze gene function during neural tube development. Here we describe the construction of plasmids that direct the expression of one or two artificial microRNAs (miRNAs) to knock down expression of endogenous protein/s of interest upon electroporation into the spinal cord. The miRNA cassette is directly linked to a fluorescent protein reporter, for the direct visualization of transfected cells. The transcripts are under the control of different promoters/enhancers which drive expression in genetically defined cell subpopulations in the neural tube. Mixing multiple RNAi vectors allows combinatorial knockdowns of two or more genes in different cell types of the spinal cord, thus permitting the analysis of complex cellular and molecular interactions in a fast and precise manner. The technique that we describe can easily be applied to other cell types in the neural tube, or even adapted to other organisms in developmental studies.

Hybrid voltage sensor imaging of eGFP-F expressing neurons in chicken midbrain slices

BACKGROUND

Dendritic computation is essential for understanding information processing in single neurons and brain circuits. Optical methods are suited best to investigate function and biophysical properties of cellular compartments at high spatial and temporal resolution. Promising approaches include the use of voltage sensitive dyes, genetically encoded voltage sensors, or hybrid voltage sensors (hVoS) consisting of fluorescent proteins and voltage-dependent quenchers that, so far, are not available in avian neuroscience.

NEW METHOD

We have adapted a hVoS system for a chicken midbrain slice preparation by combining genetically expressed farnesylated eGFP with dipicrylamine (DPA). Depending on the cellular potential, DPA is shifted in the membrane, resulting in quenching of eGFP fluorescence linearly to the membrane potential by Förster resonance electron transfer.

RESULTS

In ovo electroporation resulted in labelled neurons throughout the midbrain with a high level of fine structural detail. After application of DPA, we were able to optically record electrically evoked action potentials with high signal-to-noise ratio and high spatio-temporal resolution.

COMPARISON WITH EXISTING METHODS

Standard methods available for avian neuroscience such as whole-cell patch clamp yield insufficient data for the analysis of dendritic computation in single neurons. The high spatial and temporal resolution of hVoS data overcomes this limitation. The results obtained by our method are comparable to hVoS data published for mammals.

CONCLUSIONS

With the protocol presented here, it is possible to optically record information processing in single avian neurons at such high spatial and temporal resolution, that cellular and subcellular events can be analysed.

A subset of chicken statoacoustic ganglion neurites are repelled by Slit1 and Slit2

Mechanosensory hair cells in the chicken inner ear are innervated by bipolar afferent neurons of the statoacoustic ganglion (SAG). During development, individual SAG neurons project their peripheral process to only one of eight distinct sensory organs. These neuronal subtypes may respond differently to guidance cues as they explore the periphery in search of their target. Previous gene expression data suggested that Slit repellants might channel SAG neurites into the sensory primordia, based on the presence of robo transcripts in the neurons and the confinement of slit transcripts to the flanks of the prosensory domains. This led to the prediction that excess Slit proteins would impede the outgrowth of SAG neurites. As predicted, axonal projections to the primordium of the anterior crista were reduced 2-3 days after electroporation of either slit1 or slit2 expression plasmids into the anterior pole of the otocyst on embryonic day 3 (E3). The posterior crista afferents, which normally grow through and adjacent to slit expression domains as they are navigating towards the posterior pole of the otocyst, did not show Slit responsiveness when similarly challenged by ectopic delivery of slit to their targets. The sensitivity to ectopic Slits shown by the anterior crista afferents was more the exception than the rule: responsiveness to Slits was not observed when the entire E4 SAG was challenged with Slits for 40 h in vitro. The corona of neurites emanating from SAG explants was unaffected by the presence of purified human Slit1 and Slit2 in the culture medium. Reduced axon outgrowth from E8 olfactory bulbs cultured under similar conditions for 24 h confirmed bioactivity of purified human Slits on chicken neurons. In summary, differential sensitivity to Slit repellents may influence the directional outgrowth of otic axons toward either the anterior or posterior otocyst.

Combination of in ovo electroporation and time-lapse imaging to study migrational events in chicken embryos

BACKGROUND

During embryonic development cell migration plays a principal role in several processes. In past decades, many studies were performed to investigate migrational events, occurring during embryonic organogenesis, neurogenesis, gliogenesis or myogenesis, just to name a few. Although different common techniques are already used for this purpose, one of their major limitations is the static character. However, cell migration is a sophisticated and highly dynamic process, wherefore new appropriate technologies are required to investigate this event in all its complexity.

RESULTS AND CONCLUSIONS

Here we report a novel approach for dynamic analysis of cell migration within embryonic tissue. We combine the modern transfection method of in ovo electroporation with the use of tissue slice culture and state-of-the-art imaging techniques, such as confocal laser scanning microscopy or spinning disc confocal microscopy, and thus, develop a method to study live the migration of myogenic precursors in chicken embryos. The conditions and parameters used in this study allow long-term imaging for up to 24 hr. Our protocol can be easily adapted for investigations of a variety of other migrational events and provides a novel conception for dynamic analysis of migration during embryonic development.

Developmental imaging: the avian embryo hatches to the challenge

The avian embryo provides a multifaceted model to study developmental mechanisms because of its accessibility to microsurgery, fluorescence cell labeling, in vivo imaging, and molecular manipulation. Early two-dimensional planar growth of the avian embryo mimics human development and provides unique access to complex cell migration patterns using light microscopy. Later developmental events continue to permit access to both light and other imaging modalities, making the avian embryo an excellent model for developmental imaging. For example, significant insights into cell and tissue behaviors within the primitive streak, craniofacial region, and cardiovascular and peripheral nervous systems have come from avian embryo studies. In this review, we provide an update to recent advances in embryo and tissue slice culture and imaging, fluorescence cell labeling, and gene profiling. We focus on how technical advances in the chick and quail provide a clearer understanding of how embryonic cell dynamics are beautifully choreographed in space and time to sculpt cells into functioning structures. We summarize how these technical advances help us to better understand basic developmental mechanisms that may lead to clinical research into human birth defects and tissue repair.

A detailed description of an economical setup for electroporation of chick embryos in ovo

One of the challenges of the postgenomic era is characterizing the function and regulation of specific genes. For various reasons, the early chick embryo can easily be adopted as an in vivo assay of gene function and regulation. The embryos are robust, accessible, easily manipulated, and maintained in the laboratory. Genomic resources centered on vertebrate organisms increase daily. As a consequence of optimization of gene transfer protocols by electroporation, the chick embryo will probably become increasingly popular for reverse genetic analysis. The challenge of establishing chick embryonic electroporation might seem insurmountable to those who are unfamiliar with experimental embryological methods. To minimize the cost, time, and effort required to establish a chick electroporation assay method, we describe and illustrate in great detail the procedures involved in building a low-cost electroporation setup and the basic steps of electroporation.

In ovo expression of microRNA in ventral chick midbrain

Non-coding RNAs are additional players in regulating gene expression. Targeted in ovo electroporation of specific areas provides a unique tool for spatial and temporal control of ectopic microRNA expression. However, ventral brain structures like ventral midbrain are rather difficult to reach for any manipulations. Here, we demonstrate an efficient way to electroporate miRNA into ventral midbrain using thin platinum electrodes. This method offers a reliable way to transfect specific areas of the midbrain and a useful tool for in vivo studies.

SCO-spondin from embryonic cerebrospinal fluid is required for neurogenesis during early brain development

The central nervous system (CNS) develops from the neural tube, a hollow structure filled with embryonic cerebrospinal fluid (eCSF) and surrounded by neuroepithelial cells. Several lines of evidence suggest that the eCSF contains diffusible factors regulating the survival, proliferation, and differentiation of the neuroepithelium, although these factors are only beginning to be uncovered. One possible candidate as eCSF morphogenetic molecule is SCO-spondin, a large glycoprotein whose secretion by the diencephalic roof plate starts at early developmental stages. In vitro, SCO-spondin promotes neuronal survival and differentiation, but its in vivo function still remains to be elucidated. Here we performed in vivo loss of function experiments for SCO-spondin during early brain development by injecting and electroporating a specific shRNA expression vector into the neural tube of chick embryos. We show that SCO-spondin knock down induces an increase in neuroepithelial cells proliferation concomitantly with a decrease in cellular differentiation toward neuronal lineages, leading to hyperplasia in both the diencephalon and the mesencephalon. In addition, SCO-spondin is required for the correct morphogenesis of the posterior commissure and pineal gland. Because SCO-spondin is secreted by the diencephalon, we sought to corroborate the long-range function of this protein in vitro by performing gain and loss of function experiments on mesencephalic explants. We find that culture medium enriched in SCO-spondin causes an increased neurodifferentiation of explanted mesencephalic region. Conversely, inhibitory antibodies against SCO-spondin cause a reduction in neurodifferentiation and an increase of mitosis when such explants are cultured in eCSF. Our results suggest that SCO-spondin is a crucial eCSF diffusible factor regulating the balance between proliferation and differentiation of the brain neuroepithelial cells.

Specialized filopodia direct long-range transport of SHH during vertebrate tissue patterning

The ability of signaling proteins to traverse tissues containing tightly packed cells is of fundamental importance for cell specification and tissue development, however, how this is achieved at a cellular level remains poorly understood¹. For over a century, the vertebrate limb bud has served as a paradigm to study cell signaling during embryonic development². Here we optimize single cell real-time imaging to delineate the cellular mechanisms for how signaling proteins, such as Sonic Hedgehog (Shh), that possess membrane-bound covalent lipid modifications transverse long distances within the limb bud in vivo. By directly imaging Shh ligand production under native regulatory control, our findings show that Shh is unexpectedly produced in the form of a particle that remains associated with the cell via long cytoplasmic extensions that span several cell diameters. We show that these cellular extensions are a specialized class of actin-based filopodia with novel cytoskeletal features that have not been previously described. Strikingly, particles containing Shh traffic along these extensions with a net anterograde movement within the field of Shh cell signaling. We further show that in Shh responding cells specific subsets of Shh co-receptors, including Cdo and Boc, actively distribute and co-localize in specific micro-domains within filopodial extensions, far from the cell body. Stabilized interactions are formed between filopodia containing Shh ligand and those containing co-receptors over a long-range. These results suggest that contact-mediated release propagated by specialized filopodia contributes to the delivery of Shh at a distance. Together, these studies identify an important mode of communication between cells that significantly extends our understanding of ligand movement and reception during vertebrate tissue patterning.

Different gene transfer methods at the very early, early, late and whole embryonic stages in chicken

New technologies in gene transfer combined with experimental embryology make the chicken embryo an excellent model system for gene function studies. The techniques of in ovo electroporation, in vitro culture for ex ovo electroporation and retrovirus-mediated gene transfer have already been fully developed in chicken. Yet to our knowledge, there are no definite descriptions on the features and application scopes of these techniques. The survival rates of different in vitro culture methods were compared and the EGFP expression areas of different gene transfer techniques were explored. It was that the optimal timings of removing embryo for EC culture and Petri dish system was at E1.5 and E2.5, respectively; and optimal timing of injecting retrovirus is at E0. Results indicated that the EC culture, in ovo electroporation, the Petri dish system and retrovirus-mediated method are, respectively, suitable for the very early, early, late and whole embryonic stages in chicken. Comparison of different gene transfer methods and establishment of optimal timings are expected to provide a better choice of the efficient method for a particular experiment.

Gene profiling in the avian embryo using laser capture microdissection and RT-qPCR

The dynamic nature of the developing embryo makes it challenging to understand complex morphogenetic events using information from large-scale gene expression patterns. What would be more insightful is molecular profiling of small numbers of cells selectively surveyed at specific developmental stages. However, detecting gene expression profile information from small numbers of cells (<10) in homogenous tissue has remained a major challenge. Here, we describe the use of laser capture microdissection (LCM), immunohistochemistry (IHC), and RT-qPCR to extract gene profile information in distinct embryo tissue more precisely than is possible with any other method. We use the chick embryo model system and combine electroporation and dual-label IHC to specifically identify cells for harvest by LCM without significant degradation of total RNA. We describe the development of a pre-amplification protocol for small subpopulations of cells to produce sensitive RT-qPCR results. The gene-specific pre-amplification efficiently and linearly amplifies only gene transcripts of interest from the harvested material without the need for RNA isolation. By combining the above techniques with microfluidic RT-qPCR, we robustly analyze the expression of ∼300 genes from as few as 10 cells harvested by LCM. Together, this protocol presents a confident isolation and means of sensitive expression analysis of small cell numbers from tissues and overcomes a technical hurdle that limits gene profiling.

Electroporation: past, present and future

Gene transfer by electroporation has become an indispensable method for the study of developmental biology. The technique is applied not only in chick embryos but also in mice and other organisms. Here, a short history and perspectives of electroporation for gene transfer in vertebrates are described.

Axon sorting within the spinal cord marginal zone via Robo-mediated inhibition of N-cadherin controls spinocerebellar tract formation

The axons of spinal projection neurons transmit sensory information to the brain by ascending within highly organized longitudinal tracts. However, the molecular mechanisms that control the sorting of these axons within the spinal cord and their directed growth to poorly defined targets are not understood. Here, we show that an interplay between Robo and the cell adhesion molecule, N-cadherin, sorts spinal commissural axons into appropriate longitudinal tracts within the spinal cord, and thereby facilitates their brain targeting. Specifically, we show that d1 and d2 spinal commissural axons join the lateral funiculus within the spinal cord and target the cerebellum in chick embryos, and that these axons contribute to the spinocerebellar projection in transgenic reporter mice. Disabling Robo signaling or overexpressing N-cadherin on these axons prevents the formation of the lateral funiculus and the spinocerebellar tract, and simultaneously perturbing Robo and N-cadherin function rescues both phenotypes in chick embryos. Consistent with these observations, disabling Robo function in conditional N-cadherin knock-out mice results in a wild-type-like lateral funiculus. Together, these findings suggest that spinal projection axons must be sorted into distinct longitudinal tracts within the spinal cord proper to project to their brain targets.

In ovo electroporation of miRNA-based plasmids in the developing neural tube and assessment of phenotypes by DiI injection in open-book preparations

Commissural dI1 neurons have been extensively studied to elucidate the mechanisms underlying axon guidance during development(1,2). These neurons are located in the dorsal spinal cord and send their axons along stereotyped trajectories. Commissural axons initially project ventrally towards and then across the floorplate. After crossing the midline, these axons make a sharp rostral turn and project longitudinally towards the brain. Each of these steps is regulated by the coordinated activities of attractive and repulsive guidance cues. The correct interpretation of these cues is crucial to the guidance of axons along their demarcated pathway. Thus, the physiological contribution of a particular molecule to commissural axon guidance is ideally investigated in the context of the living embryo. Accordingly, gene knockdown in vivo must be precisely controlled in order to carefully distinguish axon guidance activities of genes that may play multiple roles during development. Here, we describe a method to knockdown gene expression in the chicken neural tube in a cell type-specific, traceable manner. We use novel plasmid vectors(3) harboring cell type-specific promoters/enhancers that drive the expression of a fluorescent protein marker, followed directly by a miR30-RNAi transcript(4) (located within the 3'-UTR of the cDNA encoding the fluorescent protein) (Figure 1). When electroporated into the developing neural tube, these vectors elicit efficient downregulation of gene expression and express bright fluorescent marker proteins to enable direct tracing of the cells experiencing knockdown(3). Mixing different RNAi vectors prior to electroporation allows the simultaneous knockdown of two or more genes in independent regions of the spinal cord. This permits complex cellular and molecular interactions to be examined during development, in a manner that is fast, simple, precise and inexpensive. In combination with DiI tracing of commissural axon trajectories in open-book preparations(5), this method is a useful tool for in vivo studies of the cellular and molecular mechanisms of commissural axon growth and guidance. In principle, any promoter/enhancer could be used, potentially making the technique more widely applicable for in vivo studies of gene function during development(6). This video first demonstrates how to handle and window eggs, the injection of DNA plasmids into the neural tube and the electroporation procedure. To investigate commissural axon guidance, the spinal cord is removed from the embryo as an open-book preparation, fixed, and injected with DiI to enable axon pathways to be traced. The spinal cord is mounted between coverslips and visualized using confocal microscopy.

Rediscovering the chick embryo as a model to study retinal development

The embryonic chick occupies a privileged place among animal models used in developmental studies. Its rapid development and accessibility for visualization and experimental manipulation are just some of the characteristics that have made it a vertebrate model of choice for more than two millennia. Until a few years ago, the inability to perform genetic manipulations constituted a major drawback of this system. However, the completion of the chicken genome project and the development of techniques to manipulate gene expression have allowed this classic animal model to enter the molecular age. Such techniques, combined with the embryological manipulations that this system is well known for, provide a unique toolkit to study the genetic basis of neural development. A major advantage of these approaches is that they permit targeted gene misexpression with extremely high spatiotemporal resolution and over a large range of developmental stages, allowing functional analysis at a level, speed and ease that is difficult to achieve in other systems. This article provides a general overview of the chick as a developmental model focusing more specifically on its application to the study of eye development. Special emphasis is given to the state of the art of the techniques that have made gene gain- and loss-of-function studies in this model a reality. In addition, we discuss some methodological considerations derived from our own experience that we believe will be beneficial to researchers working with this system.

Guidance of longitudinally projecting axons in the developing central nervous system

The directed and stereotypical growth of axons to their synaptic targets is a crucial phase of neural circuit formation. Many axons in the developing vertebrate and invertebrate central nervous systems (CNSs), including those that remain on their own (ipsilateral), and those that cross over to the opposite (commissural), side of the midline project over long distances along the anterior-posterior (A-P) body axis within precisely positioned longitudinally oriented tracts to facilitate the transmission of information between CNS regions. Despite the widespread distribution and functional importance of these longitudinal tracts, the mechanisms that regulate their formation and projection to poorly characterized synaptic targets remain largely unknown. Nevertheless, recent studies carried out in a variety of invertebrate and vertebrate model systems have begun to elucidate the molecular logic that controls longitudinal axon guidance.

In ovo eye electroporation

Electroporation has been used successfully to introduce macromolecules such as DNA into the chick embryo for at least 15 years. Purified plasmid DNA is microinjected into embryo and then a series of low voltage electrical pulses are applied to the embryo which allows naked DNA to enter cells. Following entrance into the cytoplasm, the DNA is transported to the nucleus where it is transiently expressed. This powerful technique is useful for studies involving overexpression, misexpression, and knockdown of genes of interest at a variety of developmental timepoints.

Knockdown of ephrin-A5 expression by 40% does not affect motor axon growth or migration into the chick hindlimb

Bidirectional signaling between Eph receptor tyrosine kinases and their cell-surface protein signals, the ephrins, comprises one mechanism for guiding motor axons to their proper targets. During projection of motor axons from the lateral motor column (LMC) motor neurons of the spinal cord to the hindlimb muscles in chick embryos, ephrin-A5 has been shown to be expressed in the LMC motor axons until they reach the base of the limb bud and initiate sorting into their presumptive dorsal and ventral nerve trunks, at which point expression is extinguished. We tested the hypothesis that this dynamic pattern of ephrin-A5 expression in LMC motor axons is important for the growth and guidance of the axons to, and into, the hindlimb by knocking down endogenous ephrin-A5 expression in the motor neurons and their axons. No perturbation of LMC motor axon projections was observed in response to this treatment, suggesting that ephrin-A5 is not needed for LMC motor axon growth or guidance.

Identification of spontaneous mutations within the long-range limb-specific Sonic hedgehog enhancer (ZRS) that alter Sonic hedgehog expression in the chicken limb mutants oligozeugodactyly and silkie breed

The evolutionarily conserved, non-coding ~800-base-pair (bp) zone of polarizing activity (ZPA) regulatory sequence (ZRS) controls Shh expression in the posterior limb. We report that the chicken mutant oligozeugodactyly (ozd), which lacks limb Shh expression, has a large deletion within the ZRS. Furthermore, the preaxial polydactylous, Silkie Breed chicken, which develops ectopic anterior limb Shh expression, has a single bp change within the ZRS. Using an in vivo reporter assay to examine enhancer function in the chick limb, we demonstrate that the wild-type ZRS drives β-galactosidase reporter expression in the ZPA of both wild-type and ozd limbs. The Silkie ZRS drives β-galactosidase in both posterior and anterior Shh domains in wild-type limb buds. These results support the hypothesis that the ZRS integrates positive and negative prepatterned regulatory inputs in the chicken model system and demonstrate the utility of the chicken limb as an efficient genetic system for gene regulatory studies.

Critical period of axoglial signaling between neuregulin-1 and brain-derived neurotrophic factor required for early Schwann cell survival and differentiation

During peripheral nervous system development, successful communication between axons and Schwann cells is required for proper function of both myelinated and nonmyelinated nerve fibers. Alternatively spliced proteins belonging to the neuregulin1 (NRG1) gene family of growth and differentiation factors are essential for Schwann cell survival and peripheral nerve development. Although recent studies have strongly implicated membrane-bound NRG1 forms (type III) in the myelination at late stages, little is known about the role of soluble, heparin-binding forms of NRG1 (type I/II) in regulating early Schwann cell development in vivo. These forms are rapidly released from axons in vitro by Schwann-cell-secreted neurotrophic factors and, unlike membrane-bound forms, have a unique ability to diffuse and adhere to heparan sulfate-rich cell surfaces. Here, we show that axon-derived soluble NRG1 translocates from axonal to Schwann cell surfaces in the embryonic chick between days 5 and 7, corresponding to the critical period of Schwann cell survival. Downregulating endogenous type I/II NRG1 signaling either with a targeted NRG1 antagonist or by shRNA blocks their differentiation from precursors into immature Schwann cells and increases programmed cell death, whereas upregulating NRG1 rescues Schwann cells. Exogenous BDNF also promotes Schwann cell survival through promoting the local release of axonal NRG1. Consistently, increased Schwann cell death occurs both in trkB knock-out mice and after knocking down axonal trkB in chick embryos, which can then be rescued with soluble NRG1. These findings suggest a localized, axoglial feedback loop through soluble NRG1 and BDNF critical for early Schwann cell survival and differentiation.

Reelin inhibits migration of sympathetic preganglionic neurons in the spinal cord of the chick

The present study examined the effects of Reelin in the migration of sympathetic preganglionic neurons (SPN) in the spinal cord of the chick. SPN in the chick first migrate from the neuroepithelium to the ventrolateral spinal cord. They then undergo a secondary migration to cluster adjacent to the central canal, forming the column of Terni (CT). During secondary migration, abundant Reelin is found in large areas of the ventral spinal cord; the only areas devoid of Reelin are areas occupied by SPN or somatic motor neurons and the pathway along which SPN migrate. Ectopic expression of Reelin in the pathway of SPN through electroporation of full-length Reelin DNA stopped SPN migration toward their destination. The spatiotemporal pattern of Reelin expression, along with the inhibition of SPN migration by exogenous Reelin, suggests that Reelin functions as a barrier to SPN migration during normal development of the spinal cord.

Effects of in ovo electroporation on endogenous gene expression: genome-wide analysis

Background

In ovo electroporation is a widely used technique to study gene function in developmental biology. Despite the widespread acceptance of this technique, no genome-wide analysis of the effects of in ovo electroporation, principally the current applied across the tissue and exogenous vector DNA introduced, on endogenous gene expression has been undertaken. Here, the effects of electric current and expression of a GFP-containing construct, via electroporation into the midbrain of Hamburger-Hamilton stage 10 chicken embryos, are analysed by microarray.

Results

Both current alone and in combination with exogenous DNA expression have a small but reproducible effect on endogenous gene expression, changing the expression of the genes represented on the array by less than 0.1% (current) and less than 0.5% (current + DNA), respectively. The subset of genes regulated by electric current and exogenous DNA span a disparate set of cellular functions. However, no genes involved in the regional identity were affected. In sharp contrast to this, electroporation of a known transcription factor, Dmrt5, caused a much greater change in gene expression.

Conclusions

These findings represent the first systematic genome-wide analysis of the effects of in ovo electroporation on gene expression during embryonic development. The analysis reveals that this process has minimal impact on the genetic basis of cell fate specification. Thus, the study demonstrates the validity of the in ovo electroporation technique to study gene function and expression during development. Furthermore, the data presented here can be used as a resource to refine the set of transcriptional responders in future in ovo electroporation studies of specific gene function.