Pitx2c ensures habenular asymmetry by restricting parapineal cell number

Abstract

Left-right (L/R) asymmetries in the brain are thought to underlie lateralised cognitive functions. Understanding how neuroanatomical asymmetries are established has been achieved through the study of the zebrafish epithalamus. Morphological symmetry in the epithalamus is broken by leftward migration of the parapineal, which is required for the subsequent elaboration of left habenular identity; the habenular nuclei flank the midline and show L/R asymmetries in marker expression and connectivity. The Nodal target pitx2c is expressed in the left epithalamus, but nothing is known about its role during the establishment of asymmetry in the brain. We show that abrogating Pitx2c function leads to the right habenula adopting aspects of left character, and to an increase in parapineal cell numbers. Parapineal ablation in Pitx2c loss of function results in right habenular isomerism, indicating that the parapineal is required for the left character detected in the right habenula in this context. Partial parapineal ablation in the absence of Pitx2c, however, reduces the number of parapineal cells to wild-type levels and restores habenular asymmetry. We provide evidence suggesting that antagonism between Nodal and Pitx2c activities sets an upper limit on parapineal cell numbers. We conclude that restricting parapineal cell number is crucial for the correct elaboration of epithalamic asymmetry.

Mutations in zebrafish pitx2 model congenital malformations in Axenfeld-Rieger syndrome but do not disrupt left-right placement of visceral organs

“…In contrast to humans, eye and tooth phenotypes were only observed in homozygous pitx2HD −/− zebrafish. In previous work, transient knockdown of Pitx2 expression using morpholino oligonucleotides caused defects in eye, craniofacial and asymmetric brain development (Bohnsack et al, 2012; Garric et al, 2014; Liu and Semina, 2012). pitx2HD mutants developed eye phenotypes that were consistent with Pitx2 morpholino injected embryos, but did not develop other defects (e.g.…”

The ancestral role of nodal signalling in breaking L/R symmetry in the vertebrate forebrain

“…The presence or absence of a parapineal organ appears as a highly variable trait across vertebrates and mechanisms responsible for this variability remain totally unknown. Interestingly, Pitx2 has recently been shown to be involved in restricting parapineal cell number in the zebrafish 41 . Thus, it is tempting to speculate that modulations of Pitx2 gene expression duration could account for parapineal variability.…”

Asymmetry of the Brain: Development and Implications

“…However, this function seems to be dependent on a tight control of pp size. Indeed, in a manner somewhat similar to the loss of Tcf7l2 function, the abrogation of Pitx2c activity leads to the right habenula adopting aspects of left character (60). In the latter context, however, there is a concomitant increase in pp cell numbers.…”

“…Indeed, Pitx2 is required, downstream of Nodal in the lateral plate mesoderm (LPM) for the LR-asymmetric morphogenesis of different internal body organs (25,129,145,169). Although Pitx2 controls pp size, it is intriguing that pitx2c loss-offunction does not randomize pp migration orientation (60). This suggests that Nodal targets other than Pitx2c must be involved in biasing pp laterality.…”