The ability to induce microtubule acetylation is a general feature of formin proteins

Kif4 interacts with EB1 and stabilizes microtubules downstream of Rho-mDia in migrating fibroblasts

Selectively stabilized microtubules (MTs) form in the lamella of fibroblasts and contribute to cell migration. A Rho-mDia-EB1 pathway regulates the formation of stable MTs, yet how selective stabilization of MTs is achieved is unknown. Kinesin activity has been implicated in selective MT stabilization and a number of kinesins regulate MT dynamics both in vitro and in cells. Here, we show that the mammalian homolog of Xenopus XKLP1, Kif4, is both necessary and sufficient for the induction of selective MT stabilization in fibroblasts. Kif4 localized to the ends of stable MTs and participated in the Rho-mDia-EB1 MT stabilization pathway since Kif4 depletion blocked mDia- and EB1-induced selective MT stabilization and EB1 was necessary for Kif4 induction of stable MTs. Kif4 and EB1 interacted in cell extracts, and binding studies revealed that the tail domain of Kif4 interacted directly with the N-terminal domain of EB1. Consistent with its role in regulating formation of stable MTs in interphase cells, Kif4 knockdown inhibited migration of cells into wounded monolayers. These data identify Kif4 as a novel factor in the Rho-mDia-EB1 MT stabilization pathway and cell migration.

Enhanced shedding of extracellular vesicles from amoeboid prostate cancer cells: potential effects on the tumor microenvironment

The gene encoding the cytoskeletal regulator DIAPH3 is lost at high frequency in metastatic prostate cancer, and DIAPH3 silencing evokes a transition to an amoeboid tumor phenotype in multiple cell backgrounds. This amoeboid transformation is accompanied by increased tumor cell migration, invasion, and metastasis. DIAPH3 silencing also promotes the formation of atypically large (> 1 μm) membrane blebs that can be shed as extracellular vesicles (EV) containing bioactive cargo. Whether loss of DIAPH3 also stimulates the release of nano-sized EV (e.g., exosomes) is not established. Here we examined the mechanism of release and potential biological functions of EV shed from DIAPH3-silenced and other prostate cancer cells. We observed that stimulation of LNCaP cells with the prostate stroma-derived growth factor heparin-binding EGF-like growth factor (HB-EGF), combined with p38MAPK inhibition caused EV shedding, a process mediated by ERK1/2 hyperactivation. DIAPH3 silencing in DU145 cells also increased rates of EV production. EV isolated from DIAPH3-silenced cells activated AKT1 and androgen signaling, increased proliferation of recipient tumor cells, and suppressed proliferation of human macrophages and peripheral blood mononuclear cells. DU145 EV contained miR-125a, which suppressed AKT1 expression and proliferation in recipient human peripheral blood mononuclear cells and macrophages. Our findings suggest that EV produced as a result of DIAPH3 loss or growth factor stimulation may condition the tumor microenvironment through multiple mechanisms, including the proliferation of cancer cells and suppression of tumor-infiltrating immune cells.

Essential and nonredundant roles for Diaphanous formins in cortical microtubule capture and directed cell migration

The Diaphanous formins mDia1, mDia2, and mDia3 are involved in the capture of cortical microtubules and ErbB2-dependent directed migration. These functions are independent of actin. They are mediated by mDia FH2 domains, which associate with distinct sets of proteins. Rab6IP2 is a novel interactor of mDia1 that contributes to microtubule tethering.

Formins constitute a large family of proteins that regulate the dynamics and organization of both the actin and microtubule cytoskeletons. Previously we showed that the formin mDia1 helps tether microtubules at the cell cortex, acting downstream of the ErbB2 receptor tyrosine kinase. Here we further study the contributions of mDia1 and its two most closely related formins, mDia2 and mDia3, to cortical microtubule capture and ErbB2-dependent breast carcinoma cell migration. We find that depletion of each of these three formins strongly disrupts chemotaxis without significantly affecting actin-based structures. Further, all three formins are required for formation of cortical microtubules in a nonredundant manner, and formin proteins defective in actin polymerization remain active for microtubule capture. Using affinity purification and mass spectrometry analysis, we identify differential binding partners of the formin-homology domain 2 (FH2) of mDia1, mDia2, and mDia3, which may explain their nonredundant roles in microtubule capture. The FH2 domain of mDia1 specifically interacts with Rab6-interacting protein 2 (Rab6IP2). Further, mDia1 is required for cortical localization of Rab6IP2, and concomitant depletion of Rab6IP2 and IQGAP1 severely disrupts cortical capture of microtubules, demonstrating the coinvolvement of mDia1, IQGAP1, and Rab6IP2 in microtubule tethering at the leading edge.

AtFH16, [corrected] an Arabidopsis type II formin, binds and bundles both microfilaments and microtubules, and preferentially binds to microtubules

Formins are well-known regulators that participate in the organization of the actin cytoskeleton in organisms. The Arabidopsis thaliana L. genome encodes 21 formins, which can be divided into two distinct subfamilies. However, type II formins have to date been less well characterized. Here, we cloned a type II formin, AtFH16, and characterized its biochemical activities on actin and microtubule dynamics. The results show that the FH1FH2 structure of AtFH16 cannot nucleate actin polymerization efficiently, but can bind and bundle microfilaments. AtFH16 FH1FH2 is also able to bind and bundle microtubules, and preferentially binds microtubules over microfilaments in vitro. In addition, AtFH16 FH1FH2 co-localizes with microtubules in onion epidermal cells, indicating a higher binding affinity of AtFH16 FH1FH2 for microtubules rather than microfilaments in vivo. In conclusion, AtFH16 is able to interact with both microfilaments and microtubules, suggesting that AtFH16 probably functions as a bifunctional protein, and may thus participate in plant cellular processes.

The microtubule end-binding protein EB2 is a central regulator of microtubule reorganisation in apico-basal epithelial differentiation

Microtubule end-binding (EB) proteins influence microtubule dynamic instability, a process essential for microtubule reorganisation during apico-basal epithelial differentiation. Here we establish for the first time that EB2, but not EB1, expression is critical for initial microtubule reorganisation during apico-basal epithelial differentiation, and that EB2 downregulation promotes bundle formation. EB2 siRNA knockdown during early stages of apico-basal differentiation prevented microtubule reorganisation, while its downregulation at later stages promoted microtubule stability and bundle formation. Interestingly, while EB1 is not essential for microtubule reorganisation its knockdown prevented apico-basal bundle formation and epithelial elongation. EB2 siRNA depletion in undifferentiated epithelial cells induced formation of straight, less dynamic microtubules with EB1 and ACF7 lattice association and co-alignment with actin filaments, a phenotype that could be rescued by formin inhibition. Importantly, in situ inner ear and intestinal crypt epithelial tissue revealed direct correlations between low level of EB2 expression and presence of apico-basal microtubule bundles, which were absent where EB2 was elevated. EB2 is evidently important for initial microtubule reorganisation during epithelial polarisation, while its downregulation facilitates EB1/ACF7 microtubule lattice association, microtubule-actin filament co-alignment and bundle formation. The spatiotemporal expression of EB2 thus dramatically influences microtubule organisation, EB1/ACF7 deployment and epithelial differentiation.