γ-Tubulin small complex formation is essential for early zebrafish embryogenesis

A Zebrafish/Drosophila Dual System Model for Investigating Human Microcephaly

Microcephaly presents in neurodevelopmental disorders with multiple aetiologies, including bi-allelic mutation in TUBGCP2, a component of the biologically fundamental and conserved microtubule-nucleation complex, γ-TuRC. Elucidating underlying principles driving microcephaly requires clear phenotype recapitulation and assay reproducibility, areas where go-to experimental models fall short. We present an alternative simple vertebrate/invertebrate dual system to investigate fundamental TUBGCP2-related processes driving human microcephaly and associated developmental traits. We show that antisense morpholino knockdown (KD) of the Danio rerio homolog, tubgcp2, recapitulates human TUBGCP2-associated microcephaly. Co-injection of wild type mRNA pre-empts microcephaly in 55% of KD zebrafish larvae, confirming causality. Body shortening observed in morphants is also rescued. Mitotic marker (pH3) staining further reveals aberrantly accumulated dividing brain cells in microcephalic tubgcp2 KD morphants, indicating that tubgcp2 depletion disrupts normal mitosis and/or proliferation in zebrafish neural progenitor brain cells. Drosophila melanogaster double knockouts (KO) for TUBGCP2 homologs Grip84/cg7716 also develop microcephalic brains with general microsomia. Exacerbated Grip84/cg7716-linked developmental aberration versus single mutations strongly suggests interactive or coinciding gene functions. We infer that tubgcp2 and Grip84/cg7716 affect brain size similarly to TUBGCP2 and recapitulate both microcephaly and microcephaly-associated developmental impact, validating the zebrafish/fly research model for human microcephaly. Given the conserved cross-phyla homolog function, the data also strongly support mitotic and/or proliferative disruption linked to aberrant microtubule nucleation in progenitor brain cells as key mechanistic defects for human microcephaly.

Tubgcp3 is a mitotic regulator of planarian epidermal differentiation

Tubgcp3/GCP3 (The centrosomal protein γ-tubulin complex protein 3) is a component of the γ-tubulin small complexes (γ-TuSCs) and γ-tubulin ring complexes (γ-TuRCs), which play critical roles in mitotic spindle formation during mitosis. However, its function in stem cell development has not been thoroughly elucidated. The planarian flatworm, which contains a large number of adult somatic stem cells (neoblasts), is a unique model to study stem cell lineage development in vivo. Here, we identified a homolog of Tubgcp3 in planarian Dugesia japonica, and found that Tubgcp3 is required for the maintenance of epidermal lineage. RNAi targeting Tubgcp3 resulted in tissue homeostasis and regeneration defect. Knockdown of Tubgcp3 reduced cell divisions and led to a loss of the mature epidermal cells. Our findings indicate that Tubgcp3 is a mitotic regulator and plays a crucial role in planarian epidermal differentiation.

Tubgcp3 Is Required for Retinal Progenitor Cell Proliferation During Zebrafish Development

The centrosomal protein γ-tubulin complex protein 3 (Tubgcp3/GCP3) is required for the assembly of γ-tubulin small complexes (γ-TuSCs) and γ-tubulin ring complexes (γ-TuRCs), which play critical roles in mitotic spindle formation during mitosis. However, its function in vertebrate embryonic development is unknown. Here, we generated the zebrafish tubgcp3 mutants using the CRISPR/Cas9 system and found that the tubgcp3 mutants exhibited the small eye phenotype. Tubgcp3 is required for the cell cycle progression of retinal progenitor cells (RPCs), and its depletion caused cell cycle arrest in the mitotic (M) phase. The M-phase arrested RPCs exhibited aberrant monopolar spindles and abnormal distributed centrioles and γ-tubulin. Moreover, these RPCs underwent apoptosis finally. Our study provides the in vivo model for the functional study of Tubgcp3 and sheds light on the roles of centrosomal γ-tubulin complexes in vertebrate development.