Loss of function studies have identified a large number of proteins critical for the function of centrosomes. The loss of many of these proteins results in the loss of centrosomes from the cell, precluding analysis of other roles they might play beyond centrosome duplication or maintenance. Here we investigate roles for Asl beyond its well-established function in centriole duplication. We take advantage of Drosophila zygotic mutant animals that build centrioles in early development using maternally provided Asl. We then identify these centrioles in larval and adult tissues where they now reside in an environment genetically null for Asl. These remnant centrioles have proven to be a critical tool to investigate Asl, but similar analysis can be performed on remnant centrioles generated using zygotic mutants of other centriole duplication factors. The most striking finding was that the absence of Asl resulted in the massive (up to 30-fold) elongation of centrioles in both germline and somatic cells. Although long, these centrioles maintain normal radial distribution for all the tested proteins, and a characteristic distal centriole tip. In several cell types we found that these long asl mutant centrioles are formed in a manner that results in an extended proximal centriole end such that it spans the entire centriole. However, remnant centrioles in asl mutant hub cells do not grow long. We find this result quite revealing, as it supports the model that centriole elongation requires cells to be actively proceeding through the cell cycle. Our data suggest that Asls role in controlling centriole length is most important in highly proliferative cells where a given centriole has the highest chance of being maintained in the same cell over multiple cell cycles. This is a circumstance common to stem cells. Thus, our identification of significant length control defects in two distinct stem cells suggests that length control by this mechanism might be especially critical in stem cells. Direct protein binding and genetic analysis revealed that Cep97 functions downstream of Asl to exert a significant portion of its centriole length control. The ability of centriole targeted Cep97 to partially compensate for the loss of Asl is consistent with this model. Our data also suggests that Asl does not function exclusively through Cep97, since the long centrioles in cep97 mutants are shorter than those in similarly aged asl mutants. An interesting question raised by our findings is why cells have evolved a mechanism to control centriole length. One possibility is that evolution of the centriole has selected for a minimum centriole functional-unit that can efficiently accomplish its task, in this case serving as an MTOC. Any unessential centriole length might require additional cellular resources, which could provide selective pressure to evolve a centriole length control mechanism. We also identified a role for Asl in sperm basal body function. In the absence of Asl in developing spermatids, we found significant defects in the attachment of the basal body to the nucleus as well as in the assembly and / or maintenance of the flagellar axoneme. Our initial hypothesis was that these defects were a result of the longer, proximalized asl mutant centrioles, but our data proved otherwise. We believe these defects uncover a unique and independent role for Asl during the final stages of sperm development. Asl is associated with the outer surface of basal bodies throughout spermatogenesis and we show it undergoes a dramatic change in localization during the final stages of spermatid development. The scarcity of basal bodies in asl testis makes a detailed understanding of the role of Asl in flagella assembly especially challenging. Future identification of interactions of Asl with proteins involved in flagella assembly and the generation of separation of function mutations in Asl that retain centriole duplication function, but lose basal body functions, will be required to further our understanding of this novel role for Asl. Taken together, we have shown that Asl has roles additional in addition to recruiting Plk4 for centriole duplication. Asl functions to control absolute centriole length, the length of proximal centriole characteristics, and ensures proper basal body functions during spermatogenesis. Insight into additional roles beyond duplication could help shed light on how specific lesions within centriole proteins manifest human diseases. Our recent studies have also focused on the role of Asp in brain development. We discovered that Asp is regulated by the ubiquitous calcium-sensing protein calmodulin (CaM). CaM was localized near the spindle poles over 35 years ago (Welsh et al., 1978); our data now assigns a role for this CaM localization in directly regulating Asp to crosslink spindle MTs. Our work extends our functional understanding of the Asp-CaM complex in spindle pole focusing and centrosome-pole cohesion, in addition to the cell biology of microcephaly. Our Asp transgenes that localize to the spindle in a manner identical to that of the full-length protein, yet are defective in CaM binding, fail to maintain pole focusing and centrosome-pole cohesion. Further, our transgene analysis also highlighted a second mode of MT binding by Asp, mediated through its C-terminus and is independent of its known N-terminal MT binding domain.. We believe the stronger spindle pole and punctate localization of wild-type (WT) normally masks this AspC localization and possibly contributes to Asps ability to crosslink MTs. Furthermore, we also uncovered a novel mode of Asp-CaM complex behavior on spindles, highlighted by dynamic streaming of foci through the spindle lattice towards the pole. We suggest that Asp-CaM complexes, seen as discrete puncta that move poleward, reside at MT minus ends distributed throughout the spindle that are collectively transported and organized at poles. Biochemical analysis will be critical for establishing the relationship between the distribution of minus ends within the spindle, the ability of the Asp-CaM complex to bind MT minus ends, and how the dynamic nature of their movement contribute to pole focusing and centrosome-pole cohesion. Our results also shed light on the role of Asp in microcephaly. Interestingly, this phenotype is not dependent on the Asp-CaM complex. Both AspN and AspFLIQ rescued the brain size defects of the aspt25/Df despite showing no or reduced binding to CaM. These results are in agreement with previous work from the Basto lab that demonstrated normal head size in animals expressing an N-terminal Asp fragment in the hypomorphic asp allele background (Rujano et al., 2013). Importantly, our data using the null allele show that microcephaly is a result of the loss of Asp function and not a dominant negative effect of the hypomorphic asp alleles. Furthermore, we show that the microcephaly phenotype is not a consequence of unfocused spindle poles or detached centrosomes, as the AspN and AspFLIQ rescue fragments displayed both of these defects. Taken collectively, our analysis of the null asp allele uncovered a separation of function that requires both termini of Asp to maintain MT crosslinking and an unknown region of the N terminus to specify proper brain size.
| O'Neill, Ryan S; Schoborg, Todd A; Rusan, Nasser M (2018) Same but different: pleiotropy in centrosome-related microcephaly. Mol Biol Cell 29:241-246 |
| Lerit, Dorothy A; Shebelut, Conrad W; Lawlor, Kristen J et al. (2017) Germ Cell-less Promotes Centrosome Segregation to Induce Germ Cell Formation. Cell Rep 18:831-839 |
| Varadarajan, Ramya; Hammer, John A; Rusan, Nasser M (2017) A centrosomal scaffold shows some self-control. J Biol Chem 292:20410-20411 |
| Galletta, Brian J; Jacobs, Katherine C; Fagerstrom, Carey J et al. (2016) Asterless is required for centriole length control and sperm development. J Cell Biol 213:435-50 |
| Schoborg, Todd A; Rusan, Nasser M (2016) Taking Centrioles to the Elimination Round. Dev Cell 38:10-2 |
| Schoborg, Todd; Zajac, Allison L; Fagerstrom, Carey J et al. (2015) An Asp-CaM complex is required for centrosome-pole cohesion and centrosome inheritance in neural stem cells. J Cell Biol 211:987-98 |
| Pronobis, Mira I; Rusan, Nasser M; Peifer, Mark (2015) A novel GSK3-regulated APC:Axin interaction regulates Wnt signaling by driving a catalytic cycle of efficient ?catenin destruction. Elife 4:e08022 |
| Smyth, Jeremy T; Schoborg, Todd A; Bergman, Zane J et al. (2015) Proper symmetric and asymmetric endoplasmic reticulum partitioning requires astral microtubules. Open Biol 5: |
| Lerit, Dorothy A; Jordan, Holly A; Poulton, John S et al. (2015) Interphase centrosome organization by the PLP-Cnn scaffold is required for centrosome function. J Cell Biol 210:79-97 |
| Lerit, Dorothy A; Plevock, Karen M; Rusan, Nasser M (2014) Live imaging of Drosophila larval neuroblasts. J Vis Exp : |
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