O'Neill, R. S. | Search bioRxiv

bioRxiv2021

Spd-2 Gene Duplication Suggests Cell-Type Specific Assembly Mechanisms of Pericentriolar Material

Centrosomes are multi-protein complexes that function as the major microtubule organizing center (MTOC) for the cell. While centrosomes play tissue-specific MTOC functions, little is known about how particular centrosome proteins are regulated across cell types to achieve these different functions. To investigate this cell type-specific diversity, we searched for gene duplications of centrosome genes in the Drosophila lineage with the aim of identifying centrosome gene duplications where each copy evolved for specialized functions. Through in depth functional analysis of a Spd-2 gene duplication in the Willistoni group, we discovered differences in the regulation of PCM in somatic and male germline cells. The parental gene, Spd-2A, is expressed in somatic cells, where it can function to organize pericentriolar material (PCM) and the mitotic spindle in larval brain neuroblasts. Spd-2A is absent during male meiosis, and even when ectopically expressed in spermatocytes it fails to rescue PCM and spindle organization. In contrast, the new gene duplicate, Spd-2B, is expressed specifically in spermatocytes. During male meiosis, Spd-2B localizes to centrosomes, organizes PCM and spindles, and is sufficient for proper male fertility. Experiments using chimeric transgenes reveal that differences in the C-terminal tails of Spd-2A and Spd-2B are responsible for these functional changes. Thus, Spd-2A and Spd-2B have evolved complementary functions by specializing for distinct subsets of cells. Together, our results demonstrate that somatic cells and male germline cells have fundamentally different requirements for PCM, suggesting that PCM proteins such as Spd-2 is differentially regulated across cell types to satisfy distinct requirements. HighlightsO_LISpd-2 gene duplication in the Willistoni group is rapidly evolving C_LIO_LIParent gene Spd-2A is expressed in somatic cells, whereas the gene duplicate Spd-2B is expressed in spermatocytes C_LIO_LISpd-2A organizes pericentriolar material during somatic cell mitosis C_LIO_LISpd-2B is specialized for pericentriolar material organization during male meiosis C_LI

cell biology↗

bioRxiv2021

Traip Mitotic Function Controls Brain Size

Microcephaly is a developmental failure to achieve proper brain size and neuron number. Mutations in diverse genes are linked to microcephaly, including several with DNA damage repair (DDR) functions; however, it is not well understood how these DDR gene mutations limit brain size. One such gene is TRAIP, which has multiple known functions in DDR. We characterized the Drosophila ortholog Traip, finding that loss of Traip causes a brain-specific defect in the Mushroom Body (MB). Traip mutant (traip-) MBs had reduced size and fewer neurons, but no neurodegeneration, consistent with human primary microcephaly disorders. Reduced neuron numbers in traip- were explained by premature caspase-dependent cell death of MB neuroblasts (MB-NBs). Many traip- MB-NBs had prominent chromosome bridges in anaphase, along with polyploidy, aneuploidy, or micronuclei. We found no evidence for an interphase DNA repair role for Traip in MB-NBs; instead, proper MB development requires Traip function during mitosis, where Traip localizes to centrosomes and mitotic spindles. Our results suggest that proper brain size is ensured by the recently described role for TRAIP in unloading stalled replication forks in mitosis, which suppresses DNA bridges and neural stem cell death to promote proper neuron number. Further, the mitotic nature of traip- MB-NB defects and Traip localization suggest a closer etiological link between DDR microcephaly genes like Traip and the centrosome/spindle-related genes more commonly associated with microcephaly.

developmental biology↗