Efficiency of Organelle Capture by Microtubules as a Function of Centrosome Nucleation Capacity: General Theory and the Special Case of Polyspermia

Transport of organelles along microtubules is essential for the cell metabolism and morphogenesis. The presented analysis derives the probability that an organelle of a given size comes in contact with the microtubule aster. The question is asked how this measure of functionality of the microtubule aster is controlled by the centrosome. A quantitative model is developed to address this question. It is shown that for the given set of cellular parameters, such as size and total tubulin content, a centrosome nucleation capacity exists that maximizes the probability of the organelle capture. The developed general model is then applied to the capture of the female pronucleus by microtubules assembled on the sperm centrosome, following physiologically polyspermic fertilization. This application highlights an unintuitive reflection of nonlinearity of the nucleated polymerization of the cellular pool of tubulin. The prediction that the sperm centrosome should lower its nucleation capacity in the face of the competition from the other sperm is a stark illustration of the new optimality principle. Overall, the model calls attention to the capabilities of the centrosomal pathway of regulation of the transport-related functionality of the microtubule cytoskeleton. It establishes a quantitative and conceptual framework that can guide experiment design and interpretation

Drug targeting the actin cytoskeleton potentiates the cytotoxicity of low dose vincristine by abrogating actin-mediated repair of spindle defects

Antimicrotubule vinca alkaloids are widely used in the clinic but their toxicity is often dose limiting. Strategies that enhance their effectiveness at lower doses are needed. We show that combining vinca alkaloids with compounds that target a specific population of actin filaments containing the cancer-associated tropomyosin Tpm3.1 result in synergy against a broad range of tumor cell types. We discovered that low concentrations of vincristine alone induce supernumerary microtubule asters that form transient multi-polar spindles in early mitosis. Over time these asters can be reconstructed into functional bipolar spindles resulting in cell division and survival. These microtubule asters are organized by the nuclear mitotic apparatus protein (NuMA)-dynein-dynactin complex without involvement of centrosomes. However, anti-Tpm3.1 compounds at nontoxic concentrations inhibit this rescue mechanism resulting in delayed onset of anaphase, formation of multi-polar spindles, and apoptosis during mitosis. These findings indicate that drug targeting actin filaments containing Tpm3.1 potentiates the anticancer activity of low-dose vincristine treatment

Biophysics and Biofluid Dynamics of Primary Cilia: Evidence for and Against the Flow-Sensing Function

© 2017 the American Physiological Society. Primary cilia have been called “the forgotten organelle” for over 20 yr. As cilia now have their own journal and several books devoted to their study, perhaps it is time to reconsider the moniker “forgotten organelle.” In fact, during the drafting of this review, 12 relevant publications have been issued; we therefore apologize in advance for any relevant work we inadvertently omitted. What purpose is yet another ciliary review? The primary goal of this review is to specifically examine the evidence for and against the hypothesized flow-sensing function of primary cilia expressed by differentiated epithelia within a kidney tubule, bringing together differing disciplines and their respective conceptual and experimental approaches. We will show that understanding the biophysics/biomechanics of primary cilia provides essential information for understanding any potential role of ciliary function in disease. We will summarize experimental and mathematical models used to characterize renal fluid flow and incident force on primary cilia and to characterize the mechanical response of cilia to an externally applied force and discuss possible ciliary-mediated cell signaling pathways triggered by flow. Throughout, we stress the importance of separating the effects of fluid shear and stretch from the action of hydrodynamic drag

Sporadic clear cell renal cell carcinoma but not the papillary type is characterized by severely reduced frequency of primary cilia

Renal cysts and clear cell renal cell carcinoma are common clinical manifestations of people with germ-line mutations of the von Hippel-Lindau tumor suppressor gene, VHL. Recent cell biological evidence suggests that the VHL gene product, pVHL, functions to maintain the primary cilium, a microtubule-based antenna-like structure whose functional integrity is believed to have an important role in cell-cycle control. As VHL mutations are common in sporadic clear cell renal cell carcinoma, but not papillary renal cell carcinoma, we asked whether there is an association between VHL status and primary cilia in vivo. VHL status was assessed in 20 cases of clear cell renal cell carcinoma and 9 cases of papillary renal cell carcinoma by DNA sequencing and by immunohistochemical staining for the hypoxia-inducible factor-alpha target gene products CA9 and GLUT-1. Of 20, 18 clear cell renal cell carcinomas, but only 1 of 9 papillary renal cell carcinomas, displayed evidence of VHL inactivation. In clear cell renal cell carcinoma the frequency of ciliated tumor cells ranged from 0 to 22% (median value 7.8+/-6.0%), whereas cilia frequency was significantly higher (P<0.0001) in papillary renal cell carcinoma (range 12-83%, median value 43.3+/-21.3%). There was no correlation between Ki-67 staining and cilia frequency, suggesting that the observed differences between the tumor types in cilia frequency are not accounted for by differences in cellular proliferation rates and that primary cilia degeneration in sporadic clear cell renal cell carcinoma depends on VHL inactivation. We propose that the different ciliation status of clear cell and papillary renal cell carcinoma may contribute, at least in part, to the different biological behaviors of these tumor types

pVHL and GSK3beta are components of a primary cilium-maintenance signalling network

Defects in the structure or function of the primary cilium, an antennae-like structure whose functional integrity has been linked to the suppression of uncontrolled kidney epithelial cell proliferation, are a common feature of genetic disorders characterized by kidney cysts. However, the mechanisms by which primary cilia are maintained remain poorly defined. von Hippel-Lindau (VHL) disease is characterized by the development of premalignant renal cysts and arises because of functional inactivation of the VHL tumour suppressor gene product, pVHL. Here, we show that pVHL and glycogen synthase kinase (GSK)3beta are key components of an interlinked signalling pathway that maintains the primary cilium. Although inactivation of either pVHL or GSK3beta alone did not affect cilia maintenance, their combined inactivation leads to loss of cilia. In VHL patients, GSK3beta is subjected to inhibitory phosphorylation in renal cysts, but not in early VHL mutant lesions, and these cysts exhibit reduced frequencies of primary cilia. We propose that pVHL and GSK3beta function together in a ciliary-maintenance signalling network, disruption of which enhances the vulnerability of cells to lose their cilia, thereby promoting cyst formation