Arp2/3-branched actin regulates microtubule acetylation levels and affects mitochondrial distribution

Kinesin KIF3A regulates meiotic progression and spindle assembly in oocyte meiosis

Kinesin family member 3A (KIF3A) is a microtubule-oriented motor protein that belongs to the kinesin-2 family for regulating intracellular transport and microtubule movement. In this study, we characterized the critical roles of KIF3A during mouse oocyte meiosis. We found that KIF3A associated with microtubules during meiosis and depletion of KIF3A resulted in oocyte maturation defects. LC-MS data indicated that KIF3A associated with cell cycle regulation, cytoskeleton, mitochondrial function and intracellular transport-related molecules. Depletion of KIF3A activated the spindle assembly checkpoint, leading to metaphase I arrest of the first meiosis. In addition, KIF3A depletion caused aberrant spindle pole organization based on its association with KIFC1 to regulate expression and polar localization of NuMA and γ-tubulin; and KIF3A knockdown also reduced microtubule stability due to the altered microtubule deacetylation by histone deacetylase 6 (HDAC6). Exogenous Kif3a mRNA supplementation rescued the maturation defects caused by KIF3A depletion. Moreover, KIF3A was also essential for the distribution and function of mitochondria, Golgi apparatus and endoplasmic reticulum in oocytes. Conditional knockout of epithelial splicing regulatory protein 1 (ESRP1) disrupted the expression and localization of KIF3A in oocytes. Overall, our results suggest that KIF3A regulates cell cycle progression, spindle assembly and organelle distribution during mouse oocyte meiosis.

Branched Actin Maintains Acetylated Microtubule Network in the Early Secretory Pathway

In the early secretory pathway, the delivery of anterograde cargoes from the endoplasmic reticulum (ER) exit sites (ERES) to the Golgi apparatus is a multi-step transport process occurring via the ER-Golgi intermediate compartment (IC, also called ERGIC). While the role microtubules in ER-to-Golgi transport has been well established, how the actin cytoskeleton contributes to this process remains poorly understood. Here, we report that Arp2/3 inhibition affects the network of acetylated microtubules around the Golgi and induces the accumulation of unusually long RAB1/GM130-positive carriers around the centrosome. These long carriers are less prone to reach the Golgi apparatus, and arrival of anterograde cargoes to the Golgi is decreased upon Arp2/3 inhibition. Our data suggest that Arp2/3-dependent actin polymerization maintains a stable network of acetylated microtubules, which ensures efficient cargo trafficking at the late stage of ER to Golgi transport.

The Golgi microtubules regulate single cell durotaxis

Current understandings on cell motility and directionality rely heavily on accumulated investigations of the adhesion-actin cytoskeleton-actomyosin contractility cycles, while microtubules have been understudied in this context. Durotaxis, the ability of cells to migrate up gradients of substrate stiffness, plays a critical part in development and disease. Here, we identify the pivotal role of Golgi microtubules in durotactic migration of single cells. Using high-throughput analysis of microtubule plus ends/focal adhesion interactions, we uncover that these non-centrosomal microtubules actively impart leading edge focal adhesion (FA) dynamics. Furthermore, we designed a new system where islands of higher stiffness were patterned within RGD peptide coated polyacrylamide gels. We revealed that the positioning of the Golgi apparatus is responsive to external mechanical cues and that the Golgi-nucleus axis aligns with the stiffness gradient in durotaxis. Together, our work unveils the cytoskeletal underpinning for single cell durotaxis. We propose a model in which the Golgi-nucleus axis serves both as a compass and as a steering wheel for durotactic migration, dictating cell directionality through the interaction between non-centrosomal microtubules and the FA dynamics.

The cell pushes back: The Arp2/3 complex is a key orchestrator of cellular responses to environmental forces

The Arp2/3 complex orchestrates the formation of branched actin networks at the interface between the cytoplasm and membranes. Although it is widely appreciated that these networks are useful for scaffolding, creating pushing forces and delineating zones at the membrane interface, it has only recently come to light that branched actin networks are mechanosensitive, giving them special properties. Here, we discuss recent advances in our understanding of how Arp2/3-generated actin networks respond to load forces and thus allow cells to create pushing forces in responsive and tuneable ways to effect cellular processes such as migration, invasion, phagocytosis, adhesion and even nuclear and DNA damage repair.

Functional Regeneration of the Sensory Root via Axonal Invasion

Regeneration following spinal root avulsion is broadly unsuccessful despite the regenerative capacity of other PNS-located nerves. By combining focal laser lesioning to model root avulsion in zebrafish, time-lapse imaging, and transgenesis, we identify that regenerating DRG neurons fail to recapitulate developmental paradigms of actin-based invasion after injury. We demonstrate that inducing actin reorganization into invasive components via pharmacological and genetic approaches in the regenerating axon can rescue sensory axon spinal cord entry. Cell-autonomous induction of invasion components using constitutively active Src induces DRG axon regeneration, suggesting an intrinsic mechanism can be activated to drive regeneration. Furthermore, analyses of neuronal activity and animal behavior show restoration of sensory circuit activity and behavior upon stimulating axons to re-enter the spinal cord via invasion. Altogether, our data identify induction of invasive components as sufficient for functional sensory root regeneration after injury.

Dorsal root ganglion (DRG) sensory axons are unable to regenerate into the spinal cord after injury. Nichols and Smith demonstrate in zebrafish that injured DRG axons do not initiate actin-based invasion components during re-entry into the spinal cord. Pharmacological and cell-autonomous genetic manipulations that promote actin-mediated cell invasion to restore sensory behavior.

RAB7 GTPase regulates actin dynamics for DRP1-mediated mitochondria function and spindle migration in mouse oocyte meiosis

RAB7 is a small GTPase that belongs to the Rab family, and as a vesicle trafficking factor it is shown to regulate the transport to late endocytic compartments, autophagosome maturation and organelle function. In present study, we showed the critical roles of RAB7 GTPase on actin dynamics and mitochondria function in oocyte meiosis. RAB7 mainly accumulated at cortex and spindle periphery during oocyte maturation. RAB7 depletion caused the failure of polar body extrusion and asymmetric division, and Rab7 exogenous mRNA supplement could rescue the defects caused by RAB7 RNAi. Based on mass spectrometry analysis, we found that RAB7 associated with several actin nucleation factors and mitochondria-related proteins in oocytes. The depletion of RAB7 caused the decrease of actin dynamics, which further affected meiotic spindle migration to the oocyte cortex. In addition, we found that RAB7 could maintain mitochondrial membrane potential and the mitochondrial distribution in mouse oocytes, and this might be due to its effects on the phosphorylation of DRP1 at Ser616 domain. Taken together, our data indicated that RAB7 transported actin nucleation factor for actin polarization, which further affected the phosphorylation of DRP1 for mitochondria dynamics and the meiotic spindle migration in mouse oocytes.

Control of actin dynamics during cell motility

Actin polymerization is essential for cells to migrate, as well as for various cell biological processes such as cytokinesis and vesicle traffic. This brief review describes the mechanisms underlying its different roles and recent advances in our understanding. Actin usually requires "nuclei"-preformed actin filaments-to start polymerizing, but, once initiated, polymerization continues constitutively. The field therefore has a strong focus on nucleators, in particular the Arp2/3 complex and formins. These have different functions, are controlled by contrasting mechanisms, and generate alternate geometries of actin networks. The Arp2/3 complex functions only when activated by nucleation-promoting factors such as WASP, Scar/WAVE, WASH, and WHAMM and when binding to a pre-existing filament. Formins can be individually active but are usually autoinhibited. Each is controlled by different mechanisms and is involved in different biological roles. We also describe the processes leading to actin disassembly and their regulation and conclude with four questions whose answers are important for understanding actin dynamics but are currently unanswered.

New insights into cytoskeletal remodeling during platelet production

The past decade has brought unprecedented advances in our understanding of megakaryocyte (MK) biology and platelet production, processes that are strongly dependent on the cytoskeleton. Facilitated by technological innovations, such as new high-resolution imaging techniques (in vitro and in vivo) and lineage-specific gene knockout and reporter mouse strains, we are now able to visualize and characterize the molecular machinery required for MK development and proplatelet formation in live mice. Whole genome and RNA sequencing analysis of patients with rare platelet disorders, combined with targeted genetic interventions in mice, has led to the identification and characterization of numerous new genes important for MK development. Many of the genes important for proplatelet formation code for proteins that control cytoskeletal dynamics in cells, such as Rho GTPases and their downstream targets. In this review, we discuss how the final stages of MK development are controlled by the cellular cytoskeletons, and we compare changes in MK biology observed in patients and mice with mutations in cytoskeleton regulatory genes.

Local actin nucleation tunes centrosomal microtubule nucleation during passage through mitosis

Cells going through mitosis undergo precisely timed changes in cell shape and organisation, which serve to ensure the fair partitioning of cellular components into the two daughter cells. These structural changes are driven by changes in actin filament and microtubule dynamics and organisation. While most evidence suggests that the two cytoskeletal systems are remodelled in parallel during mitosis, recent work in interphase cells has implicated the centrosome in both microtubule and actin nucleation, suggesting the potential for regulatory crosstalk between the two systems. Here, by using both in vitro and in vivo assays to study centrosomal actin nucleation as cells pass through mitosis, we show that mitotic exit is accompanied by a burst in cytoplasmic actin filament formation that depends on WASH and the Arp2/3 complex. This leads to the accumulation of actin around centrosomes as cells enter anaphase and to a corresponding reduction in the density of centrosomal microtubules. Taken together, these data suggest that the mitotic regulation of centrosomal WASH and the Arp2/3 complex controls local actin nucleation, which may function to tune the levels of centrosomal microtubules during passage through mitosis.