CKAP5 stabilizes CENP-E at kinetochores by regulating microtubule-chromosome attachments.

Stabilization of microtubule plus end-directed kinesin CENP-E at the metaphase kinetochores is important for chromosome alignment, but its mechanism remains unclear. Here, we show that CKAP5, a conserved microtubule plus tip protein, regulates CENP-E at kinetochores in human cells. Depletion of CKAP5 impairs CENP-E localization at kinetochores at the metaphase plate and results in increased kinetochore-microtubule stability and attachment errors. Erroneous attachments are also supported by computational modeling. Analysis of CKAP5 knockout cancer cells of multiple tissue origins shows that CKAP5 is preferentially essential in aneuploid, chromosomally unstable cells, and the sensitivity to CKAP5 depletion is correlated to that of CENP-E depletion. CKAP5 depletion leads to reduction in CENP-E-BubR1 interaction and the interaction is rescued by TOG4-TOG5 domain of CKAP5. The same domain can rescue CKAP5 depletion-induced CENP-E removal from the kinetochores. Interestingly, CKAP5 depletion facilitates recruitment of PP1 to the kinetochores and furthermore, a PP1 target site-specific CENP-E phospho-mimicking mutant gets stabilized at kinetochores in the CKAP5-depleted cells. Together, the results support a model in which CKAP5 controls mitotic chromosome attachment errors by stabilizing CENP-E at kinetochores and by regulating stability of the kinetochore-attached microtubules.

Capillary Hemangioma of the External Auditory Canal Extending to Middle Ear and Mastoid Cavity: A Rare Case Report with Review of Literature.

Capillary hemangioma  involving external auditory canal, middle ear and mastoid cavity is a very rare entity. Due to infrequent incidence but multiple overlapping clinical and radiological features amongst some common middle ear pathologies this benign vascular tumour often can be misdiagnosed. Histopathological report helps us to get definitive diagnosis. 40 years old female presented with diminished hearing and mass in right ear cavity for last 3 years. The otoscopic examination of right ear showed a pinkish polypoidal mass at the external auditory canal and tympanic membrane could not be visualized. Patient had a history of cortical mastoidectomy 2 years back for the similar problem. HRCT temporal bone revealed a single cavity with soft tissue density content in right mastoid cavity which was extending into right middle ear and external ear. Contrast Enhanced MRI showed a long polypoidal heterogeneously but strong enhancing T2 and STIR hyperintense lesion within mastoid antrum extending into middle ear cavity and external auditory canal forming an aural polyp. Excision of the mass was done by canal wall down mastoidectomy and also type IIIc tympanoplasty performed in same sitting. There is no recurrence noted after 6 months of follow up and hearing improvement noticed. Capillary haemangioma of mastoid antrum extending to middle ear and external auditory canal requires surgical excision as preferred treatment modality, pre-operative imaging guide us to choose the preferred surgical approaches.

Mechanics of Multicentrosomal Clustering in Bipolar Mitotic Spindles.

To segregate chromosomes in mitosis, cells assemble a mitotic spindle, a molecular machine with centrosomes at two opposing cell poles and chromosomes at the equator. Microtubules and molecular motors connect the poles to kinetochores, specialized protein assemblies on the centromere regions of the chromosomes. Bipolarity of the spindle is crucial for the proper cell division, and two centrosomes in animal cells naturally become two spindle poles. Cancer cells are often multicentrosomal, yet they are able to assemble bipolar spindles by clustering centrosomes into two spindle poles. Mechanisms of this clustering are debated. In this study, we computationally screen effective forces between 1) centrosomes, 2) centrosomes and kinetochores, 3) centrosomes and chromosome arms, and 4) centrosomes and cell cortex to understand mechanics that determines three-dimensional spindle architecture. To do this, we use the stochastic Monte Carlo search for stable mechanical equilibria in the effective energy landscape of the spindle. We find that the following conditions have to be met to robustly assemble the bipolar spindle in a multicentrosomal cell: 1) the strengths of centrosomes' attraction to each other and to the cell cortex have to be proportional to each other and 2) the strengths of centrosomes' attraction to kinetochores and repulsion from the chromosome arms have to be proportional to each other. We also find that three other spindle configurations emerge if these conditions are not met: 1) collapsed, 2) monopolar, and 3) multipolar spindles, and the computational screen reveals mechanical conditions for these abnormal spindles.