A new paper in Development shows that bioelectric signals initiate the development of the eye in the African clawed frog, Xenopus laevis.
Embryonic induction, the process by which one tissue organises the patterning of neighbouring tissues, is a key concept in understanding animal development. The prototypic example of induction is Spemann’s organiser: a tissue that when grafted to the opposite side of the amphibian embryo can induce the development of ectopic body axes. Induction is generally mediated by secreted inducing molecules: either small molecules like Retinoic acid, or polypeptides such as members of the Wnt or BMP families. However, has the application of the technical innovations of molecular biology biased our perceptions of development to the detriment of other, more biophysical, mechanisms?
Using a voltage-sensitive dye, Pai et al. detected bilateral patches of hyperpolarisation in the anterior neural field of Xenopus embryos. Electrophysiological measurement confirmed that these cells maintained a ~10 mV transmembrane potential difference (Vmem) with their neighbours. By marking these hyperpolarised cell clusters in a fixation resistant manner and then sectioning the embryos, it was found that the cluster cells contribute to both the lens and the retinal layers of the eyes.
To disrupt the potential activity of hyperpolarisation in eye development, the researchers expressed constitutively active ion channels in cells giving rise to the anterior neural plate. They found that nearly 50% of the resulting embryos showed eye defects, ranging from absence to incomplete development. The loss of hyperpolarised cluster Vmem also disrupted the expression of two transcription factors important for eye patterning, Pax6 and Rx1. Interestingly, interfering with Pax6 activity (by injection of a dominant negative construct) caused the loss of the hyperpolarisation signal. Therefore, the hyperpolarised cluster Vmem was necessary for proper development of the eye and acts in a positive feedback loop with Pax6.
Pai et al. went on to perform gain of function experiments, in which they asked whether modulation of Vmem could induce ectopic eyes. Injection of mRNAs encoding dominant negative ion channels (the main one used was an ATP-sensitive potassium channel) into the early embryo had widespread effects. In a quarter of cases the endogenous eyes showed defects, while in 20% of cases ectopic eye tissue was found in other parts of the embryo. In 7.5% of cases well formed ectopic eyes were induced. These ectopic inductions occurred in many different locations, including the gut, mesoderm, and the tail. Manipulation of Vmem was also shown to be able to cause focal ectopic expression of the eye marker genes pax6 and rx1.
Presumably, the gain of function experiments work by the local production of Vmems due to mosaicism in distribution of the dominant negative mRNAs. I was slightly surprised that such differences weren’t caused by the constitutively active ion channel experiments as well. The fact that similar results are found with other ion channels, show that it is the biophysical character Vmem itself that is responsible for this inductive capability, and not a specific species of ion or gene product.
Although the idea that voltage gradients and electrical fields have roles in development is not new, this paper brings to the fore their potential importance. It demonstrates that restricted hyperpolarisation can act as an instructive signal necessary for patterning in vertebrate embryos. Further work is needed to dissect this pathway: Do the hyperpolarised cell clusters induce eye specification in neighbouring cells? or does the Vmem signal act in an autocrine manner? How does the Vmem signal fit into other signalling pathways involved in eye induction and patterning? Bioelectric effects are no doubt important for developmental patterning and morphogenesis. The use of voltage sensitive dyes and other neuroscience-derived techniques will help to test the extent of their roles in development in coming years.
Pai, V., Aw, S., Shomrat, T., Lemire, J., & Levin, M. (2011). Transmembrane voltage potential controls embryonic eye patterning in Xenopus laevis Development, 139 (2), 313-323 DOI: 10.1242/dev.073759