The formation of a metaphase spindle, a bipolar microtubule array with centrally aligned chromosomes, is a prerequisite for the faithful segregation of a cell's genetic material. Using a full-genome RNA interference screen of Drosophila S2 cells, we identified about 200 genes that contribute to spindle assembly, more than half of which were unexpected. The screen, in combination with a variety of secondary assays, led to new insights into how spindle microtubules are generated; how centrosomes are positioned; and how centrioles, centrosomes, and kinetochores are assembled.

SUMMARY Signaling pathways that respond to DNA damage are essential for the maintenance of genome stability and are linked to many diseases, including cancer. Here, a genome-wide siRNA screen was employed to identify novel genes involved in genome stabilization by monitoring phosphorylation of the histone variant H2AX, an early mark of DNA damage. We identified hundreds of genes whose down-regulation led to elevated levels of H2AX phosphorylation (γH2AX) and revealed new links to cellular complexes and to genes with unclassified functions. We demonstrate a widespread role for mRNA processing factors in preventing DNA damage, which in some cases is caused by aberrant RNA-DNA structures. Furthermore, we connect increased γH2AX levels to the neurological disorder, Charcot-Marie-Tooth (CMT) syndrome, and we find a role for several CMT proteins in the DNA damage response. These data indicate that preservation of genome stability is mediated by a larger network of biological processes than previously appreciated.

Stochasticity inherent to biochemical reactions (intrinsic noise) and variability in cellular states (extrinsic noise) degrade information transmitted through signaling networks. We analyze the ability of temporal signal modulation, that is dynamics, to reduce noise-induced information loss. In the extracellular signal-regulated kinase (ERK), calcium (Ca2+), and nuclear factor kappa-B (NFκB) pathways, response dynamics resulted in significantly greater information transmission capacities compared to non-dynamic responses. Theoretical analysis demonstrated that signaling dynamics has a key role in overcoming extrinsic noise. Experimental measurements of information transmission in the ERK network under varying signal-to-noise confirmed our predictions and showed that signaling dynamics mitigate, and can potentially eliminate, extrinsic noise induced information loss. By curbing the information-degrading effects of cell-to-cell variability, dynamic responses substantially increase the accuracy of biochemical signaling networks.

The mitotic spindle assembles into a bipolar, microtubule-based protein machine during prometaphase. One proposed mechanism for this process is "search-and-capture," in which dynamically unstable microtubules (MTs) search space to capture chromosomes. Although existing theoretical estimates suggest that dynamic instability is efficient enough to allow capture within characteristic mitotic timescales, they are limited in scope and do not address the capture times for realistic numbers of chromosomes. Here we used mathematical modeling to explore this issue. We show that without any bias toward the chromosomes, search-and-capture is not efficient enough to explain the typical observed duration of prometaphase. We further analyze search-and-capture in the presence of a spatial gradient of a stabilizing factor that biases MT dynamics toward the chromosomes. We show theoretically that such biased search-and-capture is efficient enough to account for chromosome capture. We also show that additional factors must contribute to accelerate the spindle assembly for cells with large nuclear volumes. We discuss the possibility that a RanGTP gradient introduces a spatial bias into microtubule dynamics and thus improves the efficiency of search-and-capture as a mechanism for spindle assembly.

By integrating extensive RNAi with high-throughput image-processing methodology and mathematical modeling, we reach to a conclusion that metaphase spindle length is sensitive to alterations in microtubule dynamics and sister-chromatid cohesion, but robust against alterations of microtubule sliding force.

Recently, we have shown that a cancer causing truncation in adenomatous polyposis coli (APC) (APC1–1450) dominantly interferes with mitotic spindle function, suggesting APC regulates microtubule dynamics during mitosis. Here, we examine the possibility that APC mutants interfere with the function of EB1, a plus-end microtubule-binding protein that interacts with APC and is required for normal microtubule dynamics. We show that siRNA-mediated inhibition of APC, EB1, or APC and EB1 together give rise to similar defects in mitotic spindles and chromosome alignment without arresting cells in mitosis; in contrast inhibition of CLIP170 or LIS1 cause distinct spindle defects and mitotic arrest. We show that APC1–1450 acts as a dominant negative by forming a hetero-oligomer with the full-length APC and preventing it from interacting with EB1, which is consistent with a functional relationship between APC and EB1. Live-imaging of mitotic cells expressing EB1-GFP demonstrates that APC1–1450 compromises the dynamics of EB1-comets, increasing the frequency of EB1-GFP pausing. Together these data provide novel insight into how APC may regulate mitotic spindle function and how errors in chromosome segregation are tolerated in tumor cells.

The results, combined with quantitative analysis, suggest that these motors could act as mutual brakes to modulate the rate of pole-pole separation and could maintain a prometaphase spindle displaying small fluctuations in its steady-state length.

Sensory neurodegeneration occurs in mice defective in BPAG1, a gene encoding cytoskeletal linker proteins capable of anchoring neuronal intermediate filaments to actin cytoskeleton. While BPAG1 null mice fail to anchor neurofilaments (NFs), BPAG1/NF null mice still degenerate in the absence of NFs. We report a novel neural splice form that lacks the actin-binding domain and instead binds and stabilizes microtubules. This interaction is functionally important; in mice and in vitro, neurons lacking BPAG1 display short, disorganized, and unstable microtubules defective in axonal transport. Ironically, BPAG1 neural isoforms represent microtubule-associated proteins that when absent lead to devastating consequences. Moreover, BPAG1 can functionally account for the extraordinary stability of axonal microtubules necessary for transport over long distances. Its isoforms interconnect all three cytoskeletal networks, a feature apparently central to neuronal survival.