The Joubert Syndrome Gene arl13b is Critical for Early Cerebellar Development in Zebrafish

Abstract

Joubert syndrome is characterized by unique malformation of the cerebellar vermis. More than thirty Joubert syndrome genes have been identified, including ARL13B. However, its role in cerebellar development remains unexplored. We found that knockdown or knockout of arl13b impaired balance and locomotion in zebrafish larvae. Granule cells were selectively reduced in the corpus cerebelli, a structure homologous to the mammalian vermis. Purkinje cell progenitors were also selectively disturbed dorsomedially. The expression of atoh1 and ptf1, proneural genes of granule and Purkinje cells, respectively, were selectively down-regulated along the dorsal midline of the cerebellum. Moreover, wnt1, which is transiently expressed early in cerebellar development, was selectively reduced. Intriguingly, activating Wnt signaling partially rescued the granule cell defects in arl13b mutants. These findings suggested that Arl13b is necessary for the early development of cerebellar granule and Purkinje cells. The arl13b-deficient zebrafish can serve as a model organism for studying Joubert syndrome. Keywords Joubert syndrome Á arl13b Á Cerebellum Á Development Á Granule cell Á Purkinje cell Á Wnt Jian Zhu and Han-Tsing Wang have contributed equally to this work.

Aquatic models of human ciliary diseases

“…Either mutations in or loss of Ahi1 in zebrafish lead to problems with either crossing the midline or axonal elongation. Similar experiments have been done with other genes associated with Joubert syndrome, such as ARL13B (Zhu et al, 2020) and INPP5E (Luo, Lu, & Yang, 2012). A novel causative gene Pibf1 was identified by exome sequencing of human Joubert syndrome patients, and the cilia phenotypes were verified in Xenopus (Ott et al, 2019).…”

“…problems with either crossing the midline or axonal elongation. Similar experiments have been done with other genes associated with Joubert syndrome, such as ARL13B(Zhu et al, 2020) and INPP5E(Luo, Lu, & Yang, 2012). A novel causative gene Pibf1 was identified by exome sequencing of human Joubert syndrome patients, and the cilia phenotypes were verified in Xenopus(Ott et al, 2019).A novel ciliopathy recently identified in Xenopus involves the protein Dyrk1a (dual specificity tyrosine-(Y)-phosphorylation-regulated kinase 1 A).…”

Insights Gained From Zebrafish Models for the Ciliopathy Joubert Syndrome

“…An in-depth analysis of the CNS and specifically of the cerebellum in the available zebrafish mutants will be required to further tease apart the role of Hh signaling in cerebellar development in zebrafish and the role of primary cilia in regulating Hh signaling in the developing zebrafish cerebellum. To our knowledge, a single study has described some cerebellar anomalies in a zebrafish arl13b morphant (and mutant) model with a decrease in granule cells in the corpus cerebelli and alteration of expression levels of various cerebellar markers ( Zhu et al, 2020 ). Interestingly, this phenotype was found to be linked to dysregulation of another key developmental signaling pathway, namely Wnt signaling, while Hh signaling was not investigated in this model.…”

Patient-derived cellular models of primary ciliopathies

“…C. elegans , often combined with gene editing techniques, are also largely used to interpret the impact of genetic variants identified in patients and have been instrumental in studying the ciliary module organisation at the TZ 39–41. The zebrafish Danio rerio has also been pivotal to model ciliopathies such as PKD, NPHP and JS, enabling large-scale genetic screens and the characterisation of many ciliary proteins 42–44. Finally, a large number of mouse lines have been exploited for basic and preclinical studies of different ciliopathies 45.…”