A summary of the paper “ Sox1a mediates the ability of the parapineal to impart habenular left-right asymmetry” eLife 2019;8:e47376 DOI: 10.7554/eLife.47376

The transparent larval zebrafish is excellent for studying brain development, allowing whole-embryo imaging and relatively simple genetic manipulation. Using these advantages, we study how the left and right side of the brain become different from each other (i.e. asymmetric) during embryogenesis.

The pineal is midline brain structure that contributes to the regulation of circadian rhythms in vertebrates. In zebrafish, the pineal is accompanied by an exclusively left-sided parapineal nucleus, function of which is not quite clear. It is known, however, that during embryonic development the parapineal emerges from the pineal and migrates to the left side (Figure 1). On this journey the parapineal cells instruct adjacent habenular neurons on the left side of the epithalamus (dorsal part of the thalamus) to become different from equivalent neurons on the right with regards to molecular properties and neuronal connectivity. In other words, the parapineal determines the development of left-right asymmetries in the habenulae. Eliminating or changing the side of the parapineal therefore leads to loss or reversal of habenular asymmetries, respectively (Figure 1).

Figure 1: The parapineal emerges from the pineal during zebrafish embryogenesis, as shown by confocal images of a transgenic line with GFP in the pineal complex (top pictures). The pineal complex lies at the midline of the epithalamus, flanked by left (green on the scheme) and right (magenta on the scheme) habenulae. The habenulae are innervated by the parapineal and themselves send neuronal projections to the midbrain interpeduncular nucleus (IPN). The side of the parapineal, on the left in wild-type (wt), determines which habenula acquires a left-type identity projecting to the dorsal part of the IPN (dIPN).

Figure 1: The parapineal emerges from the pineal during zebrafish embryogenesis, as shown by confocal images of a transgenic line with GFP in the pineal complex (top pictures). The pineal complex lies at the midline of the epithalamus, flanked by left (green on the scheme) and right (magenta on the scheme) habenulae. The habenulae are innervated by the parapineal and themselves send neuronal projections to the midbrain interpeduncular nucleus (IPN). The side of the parapineal, on the left in wild-type (wt), determines which habenula acquires a left-type identity projecting to the dorsal part of the IPN (dIPN).

We show that during embryogenesis the parapineal instructs the cells in the left habenula to initiate neurogenesis earlier compared to the right habenula. The parapineal is also required at later stages for the establishment neuronal connectivity specific to the left habenula.

Upon precise laser-ablation of the embryonic parapineal cells prior to their migration to the left side, both habenulae exhibit right habenula characteristics at larval stages. A similar phenotype can be observed upon mutating the sox1a gene, normally active in the parapineal. In sox1a mutants the parapineal forms properly, but the habenulae develop as if there was no parapineal. Finally, transplanting only a few parapineal cells to the right side of the embryonic brain induces left-sided character in neurons within the right habenula. The results of these experiments are schematically summarised in Figure 2.

Figure 2: One of the mechanisms leading to left-right asymmetries in the zebrafish habenulae is the different timing of neurogenesis on each side. In  sox1a-/-  mutants and upon parapineal ablation the early onset of neurogenesis in the left habenula is delayed and the habenulae acquire a double-right character. Hence, in the absence of the left-sided parapineal or upon loss of  sox1a-/-  in the parapineal the left habenula becomes identical to the right habenula. Conversely, transplantation of external parapineal cells to the right side induces left-habenula characteristics in the right habenula.

Figure 2: One of the mechanisms leading to left-right asymmetries in the zebrafish habenulae is the different timing of neurogenesis on each side. In sox1a-/- mutants and upon parapineal ablation the early onset of neurogenesis in the left habenula is delayed and the habenulae acquire a double-right character. Hence, in the absence of the left-sided parapineal or upon loss of sox1a-/- in the parapineal the left habenula becomes identical to the right habenula. Conversely, transplantation of external parapineal cells to the right side induces left-habenula characteristics in the right habenula.

The development of the zebrafish epithalamus serves as a great example of a genetically determined mechanism for brain asymmetry formation. The emergence of a parapineal nucleus in the pineal complex during evolution has in turn led to asymmetries in the habenulae, ensuring that appropriate lateralised character is propagated within left and right-sided circuitry. As we and others have previously shown, the asymmetric neuroanatomy translates into asymmetric function of the habenulae and lateralised behaviour of the fish. Hence, from studies in zebrafish a clearer picture is forming on how and why brain asymmetries emerge in largely­ bilaterally symmetric animals.

By Ingrid Lekk