Retinoic acid, a derivative of Vitamin A is known to play a crucial role in cell growth and development of several tissues and organs in the embryonic stages, including the forebrain. A recent study has uncovered a function of retinoic acid (RA) that was not known before — RA signaling regulates the invagination, or infolding, of the developing forebrain in chicks.

The vertebrate brain doesn’t have two hemispheres to begin with. The invagination (or inward folding) of the forebrain roof plate is the first step in the process of formation of the hemispheres. The line along which the infolding occurs is called the dorsal forebrain midline. ​“Our study was initiated because we discovered, for the first time, that an enzyme that synthesizes RA is present just above the dorsal forebrain midline,” said Jonaki Sen, Assistant Professor at the Indian Institute of Technology, Kanpur, and the corresponding author of the paper. This discovery kick-started their investigations on the precise spatial domain of RA signaling and its role in the development of the chick forebrain.

This detection of the RA synthesizing enzyme, referred to as the source, was soon followed by the discovery of two domains of RA degrading enzymes (called a sink). Such an arrangement, with the sink juxtaposed right next to the source is common for RA signaling — it allows for precise control of the amount of RA present. The researchers then used electroporation techniques to show that the signaling was restricted to just the middle of the dorsal forebrain. This also fit very well with their observations of the source and the sink. The source was placed such that the entire dorsal forebrain can be bathed in RA, but the sink domains degrade the RA everywhere but the middle.

Next, they set out to examine the role of RA in this domain. One test was a loss of function experiment in which they blocked the RA signaling in the dorsal region, and found that there was no invagination. The region became a flat structure. This clearly showed that RA signaling was necessary for invagination to occur. 

On the other hand, they also did experiments in which they activated RA signaling in other locations lateral to the dorsal region. They found that this led to the expression of two genes in this region that were found in the midline. They also observed another feature of the midline here — a reduction in the proliferation, i.e., there were fewer cells dividing. However, there was no invagination observed. ​“From this we concluded that RA signaling imparts some characteristic of the midline in an ectopic location, but not all the characteristics. This means it is necessary, but not sufficient for an invagination to happen,” explained Sen. 

Sen emphasised that the regulatory role of RA signaling in forebrain development is seen only in chicks. She quoted studies by other researchers, which suggest that it has no role to play in the invagination of the dorsal forebrain midline in mice. However, similar mechanisms could be at play, with a different signaling pathway playing the role of RA. ​“Our observations are in keeping with the idea that there is an evolutionarily conserved mechanism which regulates the process of invagination. In the chick, we found that RA is a key player, but some other signal, which is yet to be identified, could play a similar role in the mouse,” she added. Their findings not only add further versatility to RA but also potentially reveal novel mechanisms of morphogenetic regulation. The group is now working on understanding how this kind of a signaling pathway is involved in bringing about morphological changes.