Microtubule-depolymerizing kinesins in the regulation of assembly, disassembly, and length of cilia and flagella

A model of microtubule depolymerization by kinesin-8 motor proteins

The dimeric kinesin-8 motors have the biological function of depolymerizing microtubules (MTs) from the plus end. However, the molecular mechanism of the depolymerization promoted by the kinesin-8 motors is still undetermined. Here, a model is proposed for the MT depolymerization by the kinesin-8 motors. Based on the model, the dynamics of depolymerization in the presence of the single motor at the MT plus end under no load and under load on the motor is studied theoretically. The dynamics of depolymerization in the presence of multiple motors at the MT plus end is also analyzed. The theoretical results explain well the available experimental data. The studies can also be applicable to other families of kinesin motors such as kinesin-13 mitotic centromere-associated kinesin motors that have the ability to depolymerize MTs.

Molecular mechanism of interaction between kinesin motors affecting their residence times on microtubule lattice and end

Kinesin superfamily can be classified into 14 subfamilies. Some families of kinesin motors such as kinesin-1 are responsible for long-distance intracellular transports and thus the motors are required to reside on the microtubule (MT) lattice for a longer time than at the end. Some families such as kinesin-8 Kip3 and kinesin-5 Eg5 are responsible for the regulation of MT length by depolymerizing or polymerizing the MT from the plus end and thus the motors are required to reside at the MT end for a long time. Under the crowded condition of the motors, it was found experimentally that the residence times of the kinesin-8 Kip3 and kinesin-5 Eg5 at the MT end are reduced greatly compared to the single-motor case. However, the underlying mechanism of different families of kinesin motors having different MT-end residence times is unknown. The molecular mechanism by which the interaction between the two motors greatly reduces the residence time of the motor at the MT end is elusive. In addition, during the processive stepping on the MT lattice, when two kinesin motors meet it is unknown how the interaction between them affects their dissociation rates. To address the above unclear issues, here we make a consistent and theoretical study of the residence times of the kinesin-1, kinesin-8 Kip3 and kinesin-5 Eg5 motors on the MT lattice and at the end under both the single-motor condition and multiple-motors or crowded condition.

Atomic force microscopy reveals distinct protofilament-scale structural dynamics in depolymerizing microtubule arrays

One cannot help but marvel at the precise organization of microtubule polymers in cellular structures such as the axoneme and the spindle. However, our understanding of the biochemical mechanisms that sculpt these arrays comes largely from in vitro experiments with a small number (one or two) of microtubules. This is somewhat akin to studying the architecture of multilane highways by studying one-lane streets. Here, we directly visualize depolymerizing microtubule arrays at individual microtubule and protofilament resolution using atomic force microscopy. Our results reveal differences in microtubule depolymerase activity and provide insights into how these differences in enzymatic activity on the nanometer scale can result in the differential remodeling of multimicrotubule arrays on the micron-length scale.

The dynamic reorganization of microtubule-based cellular structures, such as the spindle and the axoneme, fundamentally depends on the dynamics of individual polymers within multimicrotubule arrays. A major class of enzymes implicated in both the complete demolition and fine size control of microtubule-based arrays are depolymerizing kinesins. How different depolymerases differently remodel microtubule arrays is poorly understood. A major technical challenge in addressing this question is that existing optical or electron-microscopy methods lack the spatial-temporal resolution to observe the dynamics of individual microtubules within larger arrays. Here, we use atomic force microscopy (AFM) to image depolymerizing arrays at single-microtubule and protofilament resolution. We discover previously unseen modes of microtubule array destabilization by conserved depolymerases. We find that the kinesin-13 MCAK mediates asynchronous protofilament depolymerization and lattice-defect propagation, whereas the kinesin-8 Kip3p promotes synchronous protofilament depolymerization. Unexpectedly, MCAK can depolymerize the highly stable axonemal doublets, but Kip3p cannot. We propose that distinct protofilament-level activities underlie the functional dichotomy of depolymerases, resulting in either large-scale destabilization or length regulation of microtubule arrays. Our work establishes AFM as a powerful strategy to visualize microtubule dynamics within arrays and reveals how nanometer-scale substrate specificity leads to differential remodeling of micron-scale cytoskeletal structures.

KIF22 promotes bladder cancer progression by activating the expression of CDCA3

Bladder cancer is a common malignant tumor of the urinary system and is associated with a high morbidity and mortality, due to the difficulty in the accurate diagnosis of patients with early‑stage bladder cancer and the lack of effective treatments for patients with advanced bladder cancer. Thus, novel therapeutic targets are urgently required for this disease. Kinesin family member 22 (KIF22) is a kinesin‑like DNA binding protein belonging to kinesin family, and is involved in the regulation of mitosis. KIF22 has also been reported to promote the progression of several types of cancer, such as breast cancer and melanoma. The present study demonstrates the high expression of KIF22 in human bladder cancer tissues. KIF22 was found to be associated with clinical features, including clinical stage (P=0.003) and recurrence (P=0.016), and to be associated with the prognosis of patients with bladder cancer. Furthermore, it was found that KIF22 silencing inhibited the proliferation of bladder cancer cells in vitro and tumor progression in mice. Additionally, it was noted that KIF22 transcriptionally activated cell division cycle‑associated protein 3 expression, which was also confirmed in tumors in mice. Taken together, the present study investigated the molecular mechanisms underlying the promotion of bladder cancer by KIF22 and provide a novel therapeutic target for the treatment of bladder cancer. Introduction.

De novo Transcriptome of the Non-saxitoxin Producing Alexandrium tamutum Reveals New Insights on Harmful Dinoflagellates

Many dinoflagellates species, especially of the Alexandrium genus, produce a series of toxins with tremendous impacts on human and environmental health, and tourism economies. Alexandrium tamutum was discovered for the first time in the Gulf of Naples, and it is not known to produce saxitoxins. However, a clone of A. tamutum from the same Gulf showed copepod reproduction impairment and antiproliferative activity. In this study, the full transcriptome of the dinoflagellate A. tamutum is presented in both control and phosphate starvation conditions. RNA-seq approach was used for in silico identification of transcripts that can be involved in the synthesis of toxic compounds. Phosphate starvation was selected because it is known to induce toxin production for other Alexandrium spp. Results showed the presence of three transcripts related to saxitoxin synthesis (sxtA, sxtG and sxtU), and others potentially related to the synthesis of additional toxic compounds (e.g., 44 transcripts annotated as "polyketide synthase"). These data suggest that even if this A. tamutum clone does not produce saxitoxins, it has the potential to produce toxic metabolites, in line with the previously observed activity. These data give new insights into toxic microalgae, toxin production and their potential applications for the treatment of human pathologies.

LF4/MOK and a CDK-related kinase regulate the number and length of cilia in Tetrahymena

The length of cilia is controlled by a poorly understood mechanism that involves members of the conserved RCK kinase group, and among them, the LF4/MOK kinases. The multiciliated protist model, Tetrahymena, carries two types of cilia (oral and locomotory) and the length of the locomotory cilia is dependent on their position with the cell. In Tetrahymena, loss of an LF4/MOK ortholog, LF4A, lengthened the locomotory cilia, but also reduced their number. Without LF4A, cilia assembled faster and showed signs of increased intraflagellar transport (IFT). Consistently, overproduced LF4A shortened cilia and downregulated IFT. GFP-tagged LF4A, expressed in the native locus and imaged by total internal reflection microscopy, was enriched at the basal bodies and distributed along the shafts of cilia. Within cilia, most LF4A-GFP particles were immobile and a few either diffused or moved by IFT. We suggest that the distribution of LF4/MOK along the cilium delivers a uniform dose of inhibition to IFT trains that travel from the base to the tip. In a longer cilium, the IFT machinery may experience a higher cumulative dose of inhibition by LF4/MOK. Thus, LF4/MOK activity could be a readout of cilium length that helps to balance the rate of IFT-driven assembly with the rate of disassembly at steady state. We used a forward genetic screen to identify a CDK-related kinase, CDKR1, whose loss-of-function suppressed the shortening of cilia caused by overexpression of LF4A, by reducing its kinase activity. Loss of CDKR1 alone lengthened both the locomotory and oral cilia. CDKR1 resembles other known ciliary CDK-related kinases: LF2 of Chlamydomonas, mammalian CCRK and DYF-18 of C. elegans, in lacking the cyclin-binding motif and acting upstream of RCKs. The new genetic tools we developed here for Tetrahymena have potential for further dissection of the principles of cilia length regulation in multiciliated cells.

Ciliary kinesins beyond IFT: Cilium length, disassembly, cargo transport and signalling

Cilia and flagella are microtubule-based antenna which are highly conserved among eukaryotes. In vertebrates, primary and motile cilia have evolved to exert several key functions during development and tissue homoeostasis. Ciliary dysfunction in humans causes a highly heterogeneous group of diseases called ciliopathies, a class of genetic multisystemic disorders primarily affecting kidney, skeleton, retina, lung and the central nervous system. Among key ciliary proteins, kinesin family members (KIF) are microtubule-interacting proteins involved in many diverse cellular functions, including transport of cargo (organelles, proteins and lipids) along microtubules and regulating the dynamics of cytoplasmic and spindle microtubules through their depolymerising activity. Many KIFs are also involved in diverse ciliary functions including assembly/disassembly, motility and signalling. We here review these ciliary kinesins in vertebrates and focus on their involvement in ciliopathy-related disorders.

Regulation of ciliary membrane protein trafficking and signalling by kinesin motor proteins

Primary cilia are antenna-like sensory organelles that regulate a substantial number of cellular signalling pathways in vertebrates, both during embryonic development as well as in adulthood, and mutations in genes coding for ciliary proteins are causative of an expanding group of pleiotropic diseases known as ciliopathies. Cilia consist of a microtubule-based axoneme core, which is subtended by a basal body and covered by a bilayer lipid membrane of unique protein and lipid composition. Cilia are dynamic organelles, and the ability of cells to regulate ciliary protein and lipid content in response to specific cellular and environmental cues is crucial for balancing ciliary signalling output. Here we discuss mechanisms involved in regulation of ciliary membrane protein trafficking and signalling, with main focus on kinesin-2 and kinesin-3 family members.

Intraflagellar transport: mechanisms of motor action, cooperation, and cargo delivery

Intraflagellar transport (IFT) is a form of motor-dependent cargo transport that is essential for the assembly, maintenance, and length control of cilia, which play critical roles in motility, sensory reception, and signal transduction in virtually all eukaryotic cells. During IFT, anterograde kinesin-2 and retrograde IFT dynein motors drive the bidirectional transport of IFT trains that deliver cargo, for example, axoneme precursors such as tubulins as well as molecules of the signal transduction machinery, to their site of assembly within the cilium. Following its discovery in Chlamydomonas, IFT has emerged as a powerful model system for studying general principles of motor-dependent cargo transport and we now appreciate the diversity that exists in the mechanism of IFT within cilia of different cell types. The absence of heterotrimeric kinesin-2 function, for example, causes a complete loss of both IFT and cilia in Chlamydomonas, but following its loss in Caenorhabditis elegans, where its primary function is loading the IFT machinery into cilia, homodimeric kinesin-2-driven IFT persists and assembles a full-length cilium. Generally, heterotrimeric kinesin-2 and IFT dynein motors are thought to play widespread roles as core IFT motors, whereas homodimeric kinesin-2 motors are accessory motors that mediate different functions in a broad range of cilia, in some cases contributing to axoneme assembly or the delivery of signaling molecules but in many other cases their ciliary functions, if any, remain unknown. In this review, we focus on mechanisms of motor action, motor cooperation, and motor-dependent cargo delivery during IFT.

Role of primary cilia in non-dividing and post-mitotic cells

The essential role of primary (non-motile) cilia during the development of multi-cellular tissues and organs is well established and is underlined by severe disease manifestations caused by mutations in cilia-associated molecules that are collectively termed ciliopathies. However, the role of primary cilia in non-dividing and terminally differentiated, post-mitotic cells is less well understood. Although the prevention of cells from re-entering the cell cycle may represent a major chore, primary cilia have recently been linked to DNA damage responses, autophagy and mitochondria. Given this connectivity, primary cilia in non-dividing cells are well positioned to form a signaling hub outside of the nucleus. Such a center could integrate information to initiate responses and to maintain cellular homeostasis if cell survival is jeopardized. These more discrete functions may remain undetected until differentiated cells are confronted with emergencies.

Kinesin-4 KIF21B is a potent microtubule pausing factor

Microtubules are dynamic polymers that in cells can grow, shrink or pause, but the factors that promote pausing are poorly understood. Here, we show that the mammalian kinesin-4 KIF21B is a processive motor that can accumulate at microtubule plus ends and induce pausing. A few KIF21B molecules are sufficient to induce strong growth inhibition of a microtubule plus end in vitro. This property depends on non-motor microtubule-binding domains located in the stalk region and the C-terminal WD40 domain. The WD40-containing KIF21B tail displays preference for a GTP-type over a GDP-type microtubule lattice and contributes to the interaction of KIF21B with microtubule plus ends. KIF21B also contains a motor-inhibiting domain that does not fully block the interaction of the protein with microtubules, but rather enhances its pause-inducing activity by preventing KIF21B detachment from microtubule tips. Thus, KIF21B combines microtubule-binding and regulatory activities that together constitute an autonomous microtubule pausing factor.

DOI:http://dx.doi.org/10.7554/eLife.24746.001

Microtubules are tiny tubes that cells use as rails to move various cell compartments and structures to different locations within the cell. They are made of building blocks called tubulin and form extensive networks across the cell. Depending on the cell’s needs, microtubule networks can be rapidly assembled and disassembled by adding or removing tubulin subunits at the ends of individual microtubules. While a lot is known about how cells regulate the growth and shrinkage of microtubules, much less is known about the factors that can pause these processes and thus stabilize a microtubule.

Proteins belonging to the kinesin family are molecular motors that can walk along microtubules and control how microtubules grow and shrink. A kinesin known as KIF21B is found in several types of cells including neurons and immune cells and genetic alterations in this protein have been linked with several neurodegenerative diseases. KIF21B is made up of three regions: a motor domain, a stalk and a tail domain that binds to microtubules. Recent studies have suggested that this kinesin affects the ability of one end of microtubules (known as the plus end) to grow.

Here, van Riel, Rai, Bianchi et al. used a biochemical approach to investigate the activity of KIF21B. The experiments show that KIF21B can walk to the plus end of microtubules and efficiently pause growth. Small numbers of KIF21B molecules are enough to inhibit microtubule growth and this activity depends on the motor domain and the tail domain of KIF21B working together. These experiments were performed a cell-free system and so the next challenge is to investigate how KIF21B works in living cells, including neurons and immune cells.

DOI:http://dx.doi.org/10.7554/eLife.24746.002

Five factors can reconstitute all three phases of microtubule polymerization dynamics

Cytoplasmic microtubules (MTs) undergo growth, shrinkage, and pausing. However, how MT polymerization cycles are produced and spatiotemporally regulated at a molecular level is unclear, as the entire cycle has not been recapitulated in vitro with defined components. In this study, we reconstituted dynamic MT plus end behavior involving all three phases by mixing tubulin with five Drosophila melanogaster proteins (EB1, XMAP215Msps, Sentin, kinesin-13Klp10A, and CLASPMast/Orbit). When singly mixed with tubulin, CLASPMast/Orbit strongly inhibited MT catastrophe and reduced the growth rate. However, in the presence of the other four factors, CLASPMast/Orbit acted as an inducer of pausing. The mitotic kinase Plk1Polo modulated the activity of CLASPMast/Orbit and kinesin-13Klp10A and increased the dynamic instability of MTs, reminiscent of mitotic cells. These results suggest that five conserved proteins constitute the core factors for creating dynamic MTs in cells and that Plk1-dependent phosphorylation is a crucial event for switching from the interphase to mitotic mode.

Transition zone assembly and its contribution to axoneme formation in Drosophila male germ cells

Ciliary transition zone (TZ) assembly is complex and incompletely understood. Vieillard et al. show that Drosophila Cby and Dila cooperate to assemble the TZ and membrane cap, which, together with microtubule remodeling by kinesin-13, is required for axoneme formation in male germ cells.

The ciliary transition zone (TZ) is a complex structure found at the cilia base. Defects in TZ assembly are associated with human ciliopathies. In most eukaryotes, three protein complexes (CEP290, NPHP, and MKS) cooperate to build the TZ. We show that in Drosophila melanogaster, mild TZ defects are observed in the absence of MKS components. In contrast, Cby and Azi1 cooperate to build the TZ by acting upstream of Cep290 and MKS components. Without Cby and Azi1, centrioles fail to form the TZ, precluding sensory cilia assembly, and no ciliary membrane cap associated with sperm ciliogenesis is made. This ciliary cap is critical to recruit the tubulin-depolymerizing kinesin Klp59D, required for regulation of axonemal growth. Our results show that Drosophila TZ assembly in sensory neurons and male germ cells involves cooperative actions of Cby and Dila. They further reveal that temporal control of membrane cap assembly by TZ components and microtubule elongation by kinesin-13 is required for axoneme formation in male germ cells.

Mechanism of ciliary disassembly

As motile organelles and sensors, cilia play pivotal roles in cell physiology, development and organ homeostasis. Ciliary defects are associated with a class of cilia-related diseases or developmental disorders, termed ciliopathies. Even though the presence of cilia is required for diverse functions, cilia can be removed through ciliary shortening or resorption that necessitates disassembly of the cilium, which occurs normally during cell cycle progression, cell differentiation and in response to cellular stress. The functional significance of ciliary resorption is highlighted in controlling the G1-S transition during cell cycle progression. Internal or external cues that trigger ciliary resorption initiate signaling cascades that regulate several downstream events including depolymerization of axonemal microtubules, dynamic changes in actin and the ciliary membrane, regulation of intraflagellar transport and posttranslational modifications of ciliary proteins. To ensure ciliary resorption, both the active disassembly of the cilium and the simultaneous inhibition of ciliary assembly must be coordinately regulated.