Stress-induced phosphorylation of CLIP-170 by JNK promotes microtubule rescue

The contribution of de novo coding mutations to meningomyelocele

Meningomyelocele (also known as spina bifida) is considered to be a genetically complex disease resulting from a failure of the neural tube to close. Individuals with meningomyelocele display neuromotor disability and frequent hydrocephalus, requiring ventricular shunting. A few genes have been proposed to contribute to disease susceptibility, but beyond that it remains unexplained1. We postulated that de novo mutations under purifying selection contribute to the risk of developing meningomyelocele2. Here we recruited a cohort of 851 meningomyelocele trios who required shunting at birth and 732 control trios, and found that de novo likely gene disruption or damaging missense mutations occurred in approximately 22.3% of subjects, with 28% of such variants estimated to contribute to disease risk. The 187 genes with damaging de novo mutations collectively define networks including actin cytoskeleton and microtubule-based processes, Netrin-1 signalling and chromatin-modifying enzymes. Gene validation demonstrated partial or complete loss of function, impaired signalling and defective closure of the neural tube in Xenopus embryos. Our results indicate that de novo mutations make key contributions to meningomyelocele risk, and highlight critical pathways required for neural tube closure in human embryogenesis.

β-Cell miRNA-503-5p Induced by Hypomethylation and Inflammation Promotes Insulin Resistance and β-Cell Decompensation

Chronic inflammation promotes pancreatic β-cell decompensation to insulin resistance because of local accumulation of supraphysiologic interleukin 1β (IL-1β) levels. However, the underlying molecular mechanisms remain elusive. We show that miR-503-5p is exclusively upregulated in islets from humans with type 2 diabetes and diabetic rodents because of its promoter hypomethylation and increased local IL-1β levels. β-Cell-specific miR-503 transgenic mice display mild or severe diabetes in a time- and expression-dependent manner. By contrast, deletion of the miR-503 cluster protects mice from high-fat diet-induced insulin resistance and glucose intolerance. Mechanistically, miR-503-5p represses c-Jun N-terminal kinase-interacting protein 2 (JIP2) translation to activate mitogen-activated protein kinase signaling cascades, thus inhibiting glucose-stimulated insulin secretion (GSIS) and compensatory β-cell proliferation. In addition, β-cell miR-503-5p is packaged in nanovesicles to dampen insulin signaling transduction in liver and adipose tissues by targeting insulin receptors. Notably, specifically blocking the miR-503 cluster in β-cells effectively remits aging-associated diabetes through recovery of GSIS capacity and insulin sensitivity. Our findings demonstrate that β-cell miR-503-5p is required for the development of insulin resistance and β-cell decompensation, providing a potential therapeutic target against diabetes.

ARTICLE HIGHLIGHTS

Promoter hypomethylation during natural aging permits miR-503-5p overexpression in islets under inflammation conditions, conserving from rodents to humans. Impaired β-cells release nanovesicular miR-503-5p to accumulate in liver and adipose tissue, leading to their insulin resistance via the miR-503-5p/insulin receptor/phosphorylated AKT axis. Accumulated miR-503-5p in β-cells impairs glucose-stimulated insulin secretion via the JIP2-coordinated mitogen-activated protein kinase signaling cascades. Specific blockage of β-cell miR-503-5p improves β-cell function and glucose tolerance in aging mice.

Phase separation of +TIP networks regulates microtubule dynamics

Regulation of microtubule dynamics is essential for diverse cellular functions, and proteins that bind to dynamic microtubule ends can regulate network dynamics. Here, we show that two conserved microtubule end-binding proteins, CLIP-170 and EB3, undergo phase separation and form dense liquid networks. When CLIP-170 and EB3 act together, the multivalency of the network increases, which synergistically increases the amount of protein in the dense phase. In vitro and in cells, these liquid networks can concentrate tubulin. In vitro, in the presence of microtubules, phase separation of EB3/CLIP-170 can enrich tubulin all along the microtubule. In this condition, microtubule growth speed increases up to twofold and the frequency of depolymerization events are strongly reduced compared to conditions in which there is no phase separation. Our data show that phase separation of EB3/CLIP-170 adds an additional layer of regulation to the control of microtubule growth dynamics.

Cytosolic JNK-dependent microtubule reassembly protects Jurkat leukemia cells from selenite-induced apoptosis

BACKGROUND

Selenite at high dosage exhibits great potential in curing tumors. It has been shown that selenite inhibits tumor growth through regulation of microtubule dynamics, however, the exact underlying mechanisms remained to be fully elucidated.

METHODS & RESULTS

Western blots were carried out to evaluate expression level of different molecules. Our current study discovered that selenite induced microtubule disassembly, cell cycle arrest and finally resulted in apoptosis in Jurkat leukemia cells, while during this process disassembled tubulins were re-organized after long-term exposure to selenite. Furthermore, JNK was activated in the cytoplasm of selenite-treated Jurkat cells, and inhibition of JNK activity successfully prevented the process of microtubule re-assembly. Moreover, inactivation of JNK further enhanced selenite-induced cell cycle arrest and apoptosis. According to the results from cell counting-8 assay, blockage of microtubule re-assembly by colchicine further inhibited Jurkat cell viability after exposure to selenite. Experiments in a xenograft model also proved that selenite could alter JNK activity, destroy microtubule structure and inhibit cell division in vivo. Moreover, TP53, MAPT and YWHAZ were identified to be three most confident interactors that link JNK to microtubule assembly using PPIs analysis.

CONCLUSION

Our study indicated that cytosolic JNK-dependent microtubule re-organization took a protective function during selenite-induced apoptosis, while inhibition of this process would finally enhance the anti-tumor effect of selenite.

The autophagy inducer SMER28 attenuates microtubule dynamics mediating neuroprotection

SMER28 originated from a screen for small molecules that act as modulators of autophagy. SMER28 enhanced the clearance of autophagic substrates such as mutant huntingtin, which was additive to rapamycin-induced autophagy. Thus, SMER28 was established as a positive regulator of autophagy acting independently of the mTOR pathway, increasing autophagosome biosynthesis and attenuating mutant huntingtin-fragment toxicity in cellular- and fruit fly disease models, suggesting therapeutic potential. Despite many previous studies, molecular mechanisms mediating SMER28 activities and its direct targets have remained elusive. Here we analyzed the effects of SMER28 on cells and found that aside from autophagy induction, it significantly stabilizes microtubules and decelerates microtubule dynamics. Moreover, we report that SMER28 displays neurotrophic and neuroprotective effects at the cellular level by inducing neurite outgrowth and protecting from excitotoxin-induced axon degeneration. Finally, we compare the effects of SMER28 with other autophagy-inducing or microtubule-stabilizing drugs: whereas SMER28 and rapamycin both induce autophagy, the latter does not stabilize microtubules, and whereas both SMER28 and epothilone B stabilize microtubules, epothilone B does not stimulate autophagy. Thus, the effect of SMER28 on cells in general and neurons in particular is based on its unique spectrum of bioactivities distinct from other known microtubule-stabilizing or autophagy-inducing drugs.

Tension of plus-end tracking protein Clip170 confers directionality and aggressiveness during breast cancer migration

The microtubule (MT) plus-end binding protein Clip170 is associated closely with breast cancer invasion and migration. In this study, Clip170 tension observed by a newly designed cpstFRET tension probe was suggested to be positive related to breast cancer aggressiveness, which could be regulated by α-tubulin detyrosination-induced MT disassembly. Clip170 phosphorylation induced by Ribosomal protein S6 kinase (RSK) could also increase its tension and promote the conversion of a discrete comet-like Clip-170 distribution into a spotty pattern during cancer metastasis. Heightened Clip170 tension was correlated with the formation of cortactin-associated filopodia and lamellipodia, and then promoted invasion and metastasis both in vitro and in vivo. Meanwhile, Clip170 tension enhanced at the leading edge in directional migration, accompanying with IQGAP1 subcellular distribution variation. Our work indicates that the malignancy and directionality during breast cancer migration depend on the magnitude and polarization of Clip170 tension, and we suggest Clip170 tension as a new potential drug target for breast cancer therapy.

A kinesin-1 variant reveals motor-induced microtubule damage in cells

Kinesins drive the transport of cellular cargoes as they walk along microtubule tracks; however, recent work has suggested that the physical act of kinesins walking along microtubules can stress the microtubule lattice. Here, we describe a kinesin-1 KIF5C mutant with an increased ability to generate damage sites in the microtubule lattice as compared with the wild-type motor. The expression of the mutant motor in cultured cells resulted in microtubule breakage and fragmentation, suggesting that kinesin-1 variants with increased damage activity would have been selected against during evolution. The increased ability to damage microtubules is not due to the enhanced motility properties of the mutant motor, as the expression of the kinesin-3 motor KIF1A, which has similar single-motor motility properties, also caused increased microtubule pausing, bending, and buckling but not breakage. In cells, motor-induced microtubule breakage could not be prevented by increased α-tubulin K40 acetylation, a post-translational modification known to increase microtubule flexibility. In vitro, lattice damage induced by wild-type KIF5C was repaired by soluble tubulin and resulted in increased rescues and overall microtubule growth, whereas lattice damage induced by the KIF5C mutant resulted in larger repair sites that made the microtubule vulnerable to breakage and fragmentation when under mechanical stress. These results demonstrate that kinesin-1 motility causes defects in and damage to the microtubule lattice in cells. While cells have the capacity to repair lattice damage, conditions that exceed this capacity result in microtubule breakage and fragmentation and may contribute to human disease.

Cytoskeleton and Associated Proteins: Pleiotropic JNK Substrates and Regulators

This review extensively reports data from the literature concerning the complex relationships between the stress-induced c-Jun N-terminal kinases (JNKs) and the four main cytoskeleton elements, which are actin filaments, microtubules, intermediate filaments, and septins. To a lesser extent, we also focused on the two membrane-associated cytoskeletons spectrin and ESCRT-III. We gather the mechanisms controlling cytoskeleton-associated JNK activation and the known cytoskeleton-related substrates directly phosphorylated by JNK. We also point out specific locations of the JNK upstream regulators at cytoskeletal components. We finally compile available techniques and tools that could allow a better characterization of the interplay between the different types of cytoskeleton filaments upon JNK-mediated stress and during development. This overview may bring new important information for applied medical research.

α-tubulin tail modifications regulate microtubule stability through selective effector recruitment, not changes in intrinsic polymer dynamics

Microtubules are non-covalent polymers of αβ-tubulin dimers. Posttranslational processing of the intrinsically disordered C-terminal α-tubulin tail produces detyrosinated and Δ2-tubulin. Although these are widely employed as proxies for stable cellular microtubules, their effect (and of the α-tail) on microtubule dynamics remains uncharacterized. Using recombinant, engineered human tubulins, we now find that neither detyrosinated nor Δ2-tubulin affect microtubule dynamics, while the α-tubulin tail is an inhibitor of microtubule growth. Consistent with the latter, molecular dynamics simulations show the α-tubulin tail transiently occluding the longitudinal microtubule polymerization interface. The marked differential in vivo stabilities of the modified microtubule subpopulations, therefore, must result exclusively from selective effector recruitment. We find that tyrosination quantitatively tunes CLIP-170 density at the growing plus end and that CLIP170 and EB1 synergize to selectively upregulate the dynamicity of tyrosinated microtubules. Modification-dependent recruitment of regulators thereby results in microtubule subpopulations with distinct dynamics, a tenet of the tubulin code hypothesis.

Predicting and Overcoming Taxane Chemoresistance

Taxanes are microtubule-targeting drugs used as cytotoxic chemotherapy to treat most solid tumors. The development of resistance to taxanes is a major cause of therapeutic failure and overcoming chemoresistance remains an important challenge to improve patient's outcome. Extensive efforts have been made recently to identify predictive biomarkers to select populations of patients who will benefit from taxane-based chemotherapy and avoid inefficient treatment of patients with innate resistance. This, together with the discovery of new mechanisms of resistance that include metabolic reprogramming and dialogue between tumor and its microenvironment, pave the way to a new era of personalized medicine. In this review, we recapitulate recent insights into taxane resistance and present promising emerging strategies to overcome chemoresistance in the future.