Microtubule +TIPs at a glance

Endogenously produced LG3/4/5-peptide protects testes against toxicant-induced injury

Laminin-α2 chain is one of the major constituent proteins of the basement membrane in the mammalian testis. The laminin-type globular (LG) domains of LG3, 4 and 5 (LG3/4/5, an 80 kDa fragment) can be cleaved from laminin-α2 chain at the C-terminus via the action of matrix metalloproteinase 9 (MMP-9). This LG3/4/5 is a biologically active fragment, capable of modulating the Sertoli cell blood–testis barrier (BTB) function by tightening the barrier both in vitro and in vivo. Overexpression of LG3/4/5 cloned into a mammalian expression vector pCI-neo in Sertoli cells in a Sertoli cell in vitro model with a functional BTB also protected Sertoli cells from cadmium chloride (CdCl₂, an environmental toxicant) mediated cell injury. Importantly, overexpression of LG3/4/5 in the testis in vivo was found to block or rescue cadmium-induced BTB disruption and testis injury. LG3/4/5 was found to exert its BTB and spermatogenesis promoting effects through corrective spatiotemporal expression of actin- and MT-based regulatory proteins by maintaining the cytoskeletons in the testis, illustrating the therapeutic implication of this novel bioactive fragment.

Glucose limitation and pka1 deletion rescue aberrant mitotic spindle formation induced by Mal3 overexpression in Schizosaccharomyces pombe

The cAMP-dependent protein kinase Pka1 is known as a regulator of glycogenesis, transition into meiosis, proper chromosome segregation, and stress responses in Schizosaccharomyces pombe. We demonstrated that both the cAMP/PKA pathway and glucose limitation play roles in appropriate spindle formation. Overexpression of Mal3 (1-308), an EB1 family protein, caused growth defects, increased 4C DNA content, and induced monopolar spindle formation. Overproduction of a high-affinity microtubule binding mutant (Q89R) and a recombinant protein possessing the CH and EB1 domains (1-241) both resulted in more severe phenotypes than Mal3 (1-308). Loss of functional Pka1 and glucose limitation rescued the phenotypes of Mal3-overexpressing cells, whereas deletion of Tor1 or Ssp2 did not. Growth defects and monopolar spindle formation in a kinesin-5 mutant, cut7-446, was partially rescued by pka1 deletion or glucose limitation. These findings suggest that Pka1 and glucose limitation regulate proper spindle formation in Mal3-overexpressing cells and the cut7-446 mutant.

Regulation of blood-testis barrier dynamics by the mTORC1/rpS6 signaling complex: An in vitro study

During spermatogenesis, developing germ cells that lack the cellular ultrastructures of filopodia and lamellipodia generally found in migrating cells, such as macrophages and fibroblasts, rely on Sertoli cells to support their transport across the seminiferous epithelium. These include the transport of preleptotene spermatocytes across the blood-testis barrier (BTB), but also the transport of germ cells, in particular developing haploid spermatids, across the seminiferous epithelium, that is to and away from the tubule lumen, depending on the stages of the epithelial cycle. On the other hand, cell junctions at the Sertoli cell–cell and Sertoli–germ cell interface also undergo rapid remodeling, involving disassembly and reassembly of cell junctions, which, in turn, are supported by actin- and microtubule-based cytoskeletal remodeling. Interestingly, the underlying mechanism(s) and the involving biomolecule(s) that regulate or support cytoskeletal remodeling remain largely unknown. Herein, we used an in vitro model of primary Sertoli cell cultures that mimicked the Sertoli BTB in vivo overexpressed with the ribosomal protein S6 (rpS6, the downstream signaling protein of mammalian target of rapamycin complex 1 [mTORC1]) cloned into the mammalian expression vector pCI-neo, namely, quadruple phosphomimetic and constitutively active mutant of rpS6 (pCI-neo/p-rpS6-MT) versus pCI-neo/rpS6-WT (wild-type) and empty vector (pCI-neo/Ctrl) for studies. These findings provide compelling evidence that the mTORC1/rpS6 signal pathway exerted its effects to promote Sertoli cell BTB remodeling. This was mediated through changes in the organization of actin- and microtubule-based cytoskeletons, involving changes in the distribution and/or spatial expression of actin- and microtubule-regulatory proteins.

Microtubules in cell migration

Directed cell migration is critical for embryogenesis and organ development, wound healing and the immune response. Microtubules are dynamic polymers that control directional migration through a number of coordinated processes: microtubules are the tracks for long-distance intracellular transport, crucial for delivery of new membrane components and signalling molecules to the leading edge of a migrating cell and the recycling of adhesion receptors. Microtubules act as force generators and compressive elements to support sustained cell protrusions. The assembly and disassembly of microtubules is coupled to Rho GTPase signalling, thereby controlling actin polymerisation, myosin-driven contractility and the turnover of cellular adhesions locally. Cross-talk of actin and microtubule dynamics is mediated through a number of common binding proteins and regulators. Furthermore, cortical microtubule capture sites are physically linked to focal adhesions, facilitating the delivery of secretory vesicles and efficient cross-talk. Here we summarise the diverse functions of microtubules during cell migration, aiming to show how they contribute to the spatially and temporally coordinated sequence of events that permit efficient, directional and persistent migration.

Proximity mapping of the microtubule plus-end tracking protein SLAIN2 using the BioID approach

The centrosome is the main microtubule-organizing center of animal cells, which plays key roles in critical cellular processes ranging from cell division to cellular signaling. Accordingly, defects in the structure and function of centrosomes cause various human diseases such as cancer and primary microcephaly. To elucidate the molecular defects underlying these diseases, the biogenesis and functions of the centrosomes have to be fully understood. An essential step towards addressing these questions is the identification and functional dissection of the full repertoire of centrosome proteins. Here, we used high-resolution imaging and showed that the microtubule plus-end tracking protein SLAIN2 localizes to the pericentriolar material at the proximal end of centrioles. To gain insight into its cellular functions and mechanisms, we applied in vivo proximity-dependent biotin identification to SLAIN2 and generated its proximity interaction map. Gene ontology analysis of the SLAIN2 interactome revealed extensive interactions with centriole duplication, ciliogenesis, and microtubule-associated proteins, including previously characterized and uncharacterized interactions. Collectively, our results define SLAIN2 as a component of pericentriolar material and provide an important resource for future studies aimed at elucidating SLAIN2 functions at the centrosome.

Disease-Associated Mutant Tau Prevents Circadian Changes in the Cytoskeleton of Central Pacemaker Neurons

A hallmark feature of Alzheimer's disease (AD) and other Tauopathies, like Frontotemporal Dementia with Parkinsonism linked to chromosome 17 (FTDP-17), is the accumulation of neurofibrillary tangles composed of the microtubule-associated protein Tau. As in AD, symptoms of FTDP-17 include cognitive decline, neuronal degeneration, and disruptions of sleep patterns. However, mechanisms by which Tau may lead to these disturbances in sleep and activity patterns are unknown. To identify such mechanisms, we have generated novel Drosophila Tauopathy models by replacing endogenous fly dTau with normal human Tau (hTau) or the FTDP-17 causing hTau^V337M mutation. This mutation is localized in one of the microtubule-binding domains of hTau and has a dominant effect. Analyzing heterozygous flies, we found that aged hTau^V337M flies show neuronal degeneration and locomotion deficits when compared to wild type or hTau^WT flies. Furthermore, hTau^V337M flies are hyperactive and they show a fragmented sleep pattern. These changes in the sleep/activity pattern are accompanied by morphological changes in the projection pattern of the central pacemaker neurons. These neurons show daily fluctuations in their connectivity, whereby synapses are increased during the day and reduced during sleep. Synapse formation requires cytoskeletal changes that can be detected by the accumulation of the end-binding protein 1 (EB1) at the site of synapse formation. Whereas, hTau^WT flies show the normal day/night changes in EB1 accumulation, hTau^V337M flies do not show this fluctuation. This suggests that hTau^V337M disrupts sleep patterns by interfering with the cytoskeletal changes that are required for the synaptic homeostasis of central pacemaker neurons.

Single-Molecule Turnarounds of Intraflagellar Transport at the C. elegans Ciliary Tip

Cilia are microtubule-based sensing hubs that rely on intraflagellar transport (IFT) for their development, maintenance, and function. Kinesin-2 motors transport IFT trains, consisting of IFT proteins and cargo, from ciliary base to tip. There, trains turn around and are transported back by IFT dynein. The mechanism of tip turnaround has remained elusive. Here, we employ single-molecule fluorescence microscopy of IFT components in the tips of phasmid cilia of living C. elegans. Analysis of the trajectories reveals that while motor proteins and IFT-A particle component CHE-11 mostly turn around immediately, the IFT-B particle component OSM-6 pauses for several seconds. Our data indicate that IFT trains disassemble into at least IFT-A, IFT-B, IFT-dynein, and OSM-3 complexes at the tip, where OSM-6 is temporarily retained or undergoes modification, prior to train reassembly and retrograde transport. The single-molecule approach used here is a valuable tool to study how directional switches occur in microtubule-based transport processes.

Cdc42 is involved in NC1 peptide-regulated BTB dynamics through actin and microtubule cytoskeletal reorganization

Noncollagenous domain 1 (NC1)-peptide is a biologically active peptide derived from the C-terminal region of collagen α3(IV) chain, a structural constituent protein at the basement membrane in the rat testis, likely via proteolytic cleavage of matrix metalloproteinase 9. Studies have shown that this NC1 peptide regulates testis function by inducing Sertoli cell blood-testis barrier (BTB) remodeling and is also capable of inducing elongate spermatid exfoliation through its disruptive effects on the organization of actin- and microtubule (MT)-based cytoskeletons at these cell adhesion sites. However, the underlying molecular mechanism remains unknown. NC1 peptide was found to exert its biologic effects through an activation of small GTPase cell division control protein 42 homolog (Cdc42) because cooverexpression of the dominant negative mutant of Cdc42 [namely, Cdc42-T17N (via a single mutation of amino acid residue 17 from the N terminus from Thr to Asn by site-directed mutagenesis, making it constitutively inactive)] and NC1 peptide was able to block the NC1 peptide-induced Sertoli cell tight junction-permeability barrier disruption. Their cooverexpression also blocked the NC1 peptide-induced misdistribution of BTB-associated proteins at the cell-cell interface and also disruptive cytoskeletal organization of F-actin and MTs through changes in spatial expression of the corresponding actin and MT regulatory proteins. Interestingly, NC1 peptide was also found to induce an up-regulation of phosphorylated (p)-ribosomal protein S6 (rpS6) (namely, p-rpS6-S235/S236) and a concomitant down-regulation of p-Akt1/2 (namely, p-Akt1-S473 and p-Akt2-S474), but these changes could not be blocked by overexpression of Cdc42-T17N. More importantly, NC1 peptide-induced Cdc42 activation was effectively blocked by treatment of Sertoli cell epithelium with a p-Akt1/2 activator SC79, which is also capable of blocking NC1 peptide-induced down-regulation of p-Akt1-S473 and p-Akt2/S474, but not p-rpS6-S235/S236 up-regulation. In summary, these findings illustrate that Cdc42 is working downstream of the mammalian target of rapamycin complex 1/rpS6/Akt1/2 signaling pathway to support NC1 peptide-mediated effects on Sertoli cell function in the testis using the rat as an animal model.-Su, W., Cheng, C. Y. Cdc42 is involved in NC1 peptide-regulated BTB dynamics through actin and microtubule cytoskeletal reorganization.

Mechanism of microtubule plus-end tracking by the plant-specific SPR1 protein and its development as a versatile plus-end marker

Microtubules are cytoskeletal polymers that perform diverse cellular functions. The plus ends of microtubules promote polymer assembly and disassembly and connect the microtubule tips to other cellular structures. The dynamics and functions of microtubule plus ends are governed by microtubule plus end-tracking proteins (+TIPs). Here we report that the Arabidopsis thaliana SPIRAL1 (SPR1) protein, which regulates directional cell expansion, is an autonomous +TIP. Using in vitro reconstitution experiments and total internal reflection fluorescence microscopy, we demonstrate that the conserved N-terminal region of SPR1 and its GGG motif are necessary for +TIP activity whereas the conserved C-terminal region and its PGGG motif are not. We further show that the N- and C-terminal regions, either separated or when fused in tandem (NC), are sufficient for +TIP activity and do not significantly perturb microtubule plus-end dynamics compared with full-length SPR1. We also found that exogenously expressed SPR1-GFP and NC-GFP label microtubule plus ends in plant and animal cells. These results establish SPR1 as a new type of intrinsic +TIP and reveal the utility of NC-GFP as a versatile microtubule plus-end marker.

Mapping the kinetochore MAP functions required for stabilizing microtubule attachments to chromosomes during metaphase

In mitosis, faithful chromosome segregation is orchestrated by the dynamic interactions between the spindle microtubules (MTs) emanating from the opposite poles and the kinetochores of the chromosomes. However, the precise mechanism that coordinates the coupling of the kinetochore components to dynamic MTs has been a long-standing question. Microtubule-associated proteins (MAPs) regulate MT nucleation and dynamics, MT-mediated transport and MT cross-linking in cells. During mitosis, MAPs play an essential role not only in determining spindle length, position, and orientation but also in facilitating robust kinetochore-microtubule (kMT) attachments by linking the kinetochores to spindle MTs efficiently. The stability of MTs imparted by the MAPs is critical to ensure accurate chromosome segregation. This review primarily focuses on the specific function of nonmotor kinetochore MAPs, their recruitment to kinetochores and their MT-binding properties. We also attempt to synthesize and strengthen our understanding of how these MAPs work in coordination with the kinetochore-bound Ndc80 complex (the key component at the MT-binding interface in metaphase and anaphase) to establish stable kMT attachments and control accurate chromosome segregation during mitosis.

Regulation of BTB dynamics in spermatogenesis - insights from the adjudin toxicant model

During spermatogenesis, cell organelles and germ cells, most notably haploid spermatids, are transported across the seminiferous epithelium so that fully developed spermatids line-up at the edge of the tubule lumen to undergo spermiation at stage VIII of the cycle. Studies have suggested that the microtubule (MT)-based cytoskeleton is necessary to support these cellular events. However, the regulatory molecule(s) and underlying mechanism(s) remain poorly understood. Herein, we sought to better understand this event by using an adjudin-based animal model. Adult rats were treated with adjudin at low-dose (10 mg/kg b.w.) which by itself had no notable effects on spermatogenesis. Rats were also treated with low-dose adjudin combined with overexpression of two endogenously produced blood-testis barrier (BTB) modifiers, namely rpS6 [ribosomal protein S6, the downstream signaling protein of mammalian target of rapamycin complex 1 (mTORC1)] and F5-peptide (a biological active peptide released from laminin-γ3 chain at the Sertoli-spermatid interface) versus the two BTB modifiers alone. Overexpression of these two BTB modifiers in the testis was shown to enhance delivery of adjudin to the testis, effectively inducing disruptive changes in MT cytoskeletons, causing truncation of MT conferred tracks that led to their collapse across the epithelium. The net result was massive germ cell exfoliation in the tubules, disrupting germ cell transport and cell adhesion across the seminiferous epithelium that led to aspermatogenesis. These changes were the result of disruptive spatial expression of several MT-based regulatory proteins. In summary, MT cytoskeleton supported by the network of MT regulatory proteins is crucial to maintain spermatogenesis.

MAP7 Prevents Axonal Branch Retraction by Creating a Stable Microtubule Boundary to Rescue Polymerization

Complex neural circuits are built from axonal branches that allow each neuron to connect with multiple targets. During development, maturation of nascent branches depends on stabilization of newly assembled or transported microtubules, which are thought to be regulated by microtubule-associated proteins (MAPs). However, because many known MAPs inhibit branch formation, it is not clear which MAP is responsible for regulating microtubule stability during branch development. Here, we show that MAP7, a less-well understood MAP that is localized to branch junctions, provides a key molecular mechanism to regulate microtubule stability during branch formation. In developing rodent sensory neurons of mixed sex, MAP7 is required for branch maturation mainly by preventing branch retraction. This function is mediated by the ability of MAP7 to control microtubule stability, as microtubules are more stable at branch junctions where MAP7 is localized. Consistently, nascent branches depleted of MAP7 have decreased stable microtubules but increased dynamic microtubules. Moreover, MAP7 binds to the acetylated and stable region of individual microtubules and avoids the dynamic plus end, thereby creating a boundary that prevents microtubule depolymerization and rescues microtubule polymerization. This unique binding property, which is not observed for other MAPs, can prevent branch retraction caused by laser-induced severing or nocodazole-induced microtubule depolymerization. Together, our study identifies a novel molecular mechanism mediated by MAP7 to regulate microtubule stability and strengthen branches at different stages of axonal branch morphogenesis.SIGNIFICANCE STATEMENT Development and maintenance of axonal branches rely on microtubule stability, but the underlying molecular mechanisms are not fully understood. Here, we show that MAP7, a unique protein that interacts with both microtubules and the motor protein kinesin-1, plays a key role at branch junctions. MAP7 stabilizes microtubules in nascent branches and prevents branch retraction during branch maturation or after laser-induced injury. MAP7 also binds to the acetylated region of microtubules to prevent depolymerization and rescue polymerization. This unique binding property supports a novel mechanism mediated by MAP7 to cooperate with other MAPs and control microtubule stability during axonal branch development. This mechanism could also impact microtubule regulation in branch regeneration after nerve injury.

CAMSAP2 Is a Microtubule Minus-End Targeting Protein That Regulates BTB Dynamics Through Cytoskeletal Organization

During spermatogenesis, microtubule (MT) cytoskeleton in Sertoli cells confers blood-testis barrier (BTB) function, but the regulators and mechanisms that modulate MT dynamics remain unexplored. In this study, we examined the role of calmodulin-regulated spectrin-associated protein (CAMSAP)2 (a member of the CAMSAP/Patronin protein family), and a minus-end targeting protein (-TIP) that binds to the minus-end (i.e., slow-growing end) of polarized MTs involved in determining MT length, in Sertoli cell function. CAMSAP2 was found to localize at discrete sites across the Sertoli cell cytosol, different from end-binding protein 1 (a microtubule plus-end tracking protein that binds to the plus-end of MTs), and colocalized with MTs. CAMSAP2 displayed a stage-specific expression pattern, appearing as tracklike structures across the seminiferous epithelium in adult rat testes that lay perpendicular to the basement membrane. CAMSAP2 knockdown by RNA interference was found to promote Sertoli cell tight junction (TJ) barrier function, illustrating its role in inducing TJ remodeling under physiological conditions. To further examine the regulatory role of CAMSAP2 in BTB dynamics, we used a perfluorooctanesulfonate (PFOS)-induced Sertoli cell injury model for investigations. CAMSAP2 knockdown blocked PFOS-induced Sertoli cell injury by promoting proper distribution of BTB-associated proteins at the cell-cell interface. This effect was mediated by the ability of CAMSAP2 knockdown to block PFOS-induced disruptive organization of MTs, but also F-actin, across cell cytosol through changes in cellular distribution/localization of MT- and actin-regulatory proteins. In summary, CAMSAP2 is a regulator of MT and actin dynamics in Sertoli cells to support BTB dynamics and spermatogenesis.

Tubulin Polymerization Promoting Protein, Ringmaker, and MAP1B Homolog Futsch Coordinate Microtubule Organization and Synaptic Growth

Drosophila Ringmaker (Ringer) is homologous to the human Tubulin Polymerization Promoting Proteins (TPPPs) that are implicated in the stabilization and bundling of microtubules (MTs) that are particularly important for neurons and are also implicated in synaptic organization and plasticity. No in vivo functional data exist that have addressed the role of TPPP in synapse organization in any system. Here, we present the phenotypic and functional characterization of ringer mutants during Drosophila larval neuromuscular junction (NMJ) synaptic development. ringer mutants show reduced synaptic growth and transmission and display phenotypic similarities and genetic interactions with the Drosophila homolog of vertebrate Microtubule Associated Protein (MAP)1B, futsch. Immunohistochemical and biochemical analyses show that individual and combined loss of Ringer and Futsch cause a significant reduction in MT loops at the NMJs and reduced acetylated-tubulin levels. Presynaptic over-expression of Ringer and Futsch causes elevated levels of acetylated-tubulin and significant increase in NMJ MT loops. These results indicate that Ringer and Futsch regulate synaptic MT organization in addition to synaptic growth. Together our findings may inform studies on the close mammalian homolog, TPPP, and provide insights into the role of MTs and associated proteins in synapse growth and organization.

Chemotherapy-Induced Peripheral Neuropathy and Changes in Cytoskeleton

Despite the different antineoplastic mechanisms of action, peripheral neurotoxicity induced by all chemotherapy drugs (anti-tubulin agents, platinum compounds, proteasome inhibitors, thalidomide) is associated with neuron morphological changes ascribable to cytoskeleton modifications. The "dying back" degeneration of distal terminals (sensory nerves) of dorsal root ganglia sensory neurons, observed in animal models, in in vitro cultures and biopsies of patients is the most evident hallmark of the perturbation of the cytoskeleton. On the other hand, in highly polarized cells like neurons, the cytoskeleton carries out its role not only in axons but also has a fundamental role in dendrite plasticity and in the organization of soma. In the literature, there are many studies focused on the antineoplastic-induced alteration of microtubule organization (and consequently, fast axonal transport defects) while very few studies have investigated the effect of the different classes of drugs on microfilaments, intermediate filaments and associated proteins. Therefore, in this review, we will focus on: (1) Highlighting the fundamental role of the crosstalk among the three filamentous subsystems and (2) investigating pivotal cytoskeleton-associated proteins.

Host proteins involved in microtubule-dependent HIV-1 intracellular transport and uncoating

Microtubules and microtubule-associated proteins are critical for cargo transport throughout the cell. Many viruses are able to usurp these transport systems for their own replication and spread. HIV-1 utilizes these proteins for many of its early events postentry, including transport, uncoating and reverse transcription. The molecular motor proteins dynein and kinesin-1 are the primary drivers of cargo transport, and HIV-1 utilizes these proteins for infection. In this Review, we highlight recent developments in the understanding of how HIV-1 hijacks motor transport, the key cellular and viral proteins involved, and the ways that transport influences other steps in the HIV-1 lifecycle.

Rudhira/BCAS3 couples microtubules and intermediate filaments to promote cell migration for angiogenic remodeling

Blood vessel formation requires endothelial cell (EC) migration that depends on dynamic remodeling of the cytoskeleton. Rudhira/Breast Carcinoma Amplified Sequence 3 (BCAS3) is a cytoskeletal protein essential for EC migration and sprouting angiogenesis during mouse development and is implicated in metastatic disease. Here, we report that Rudhira mediates cytoskeleton organization and dynamics during EC migration. Rudhira binds to both microtubules (MTs) and vimentin intermediate filaments (IFs) and stabilizes MTs. Rudhira depletion impairs cytoskeletal cross-talk, MT stability, and hence focal adhesion disassembly. The BCAS3 domain of Rudhira is necessary and sufficient for MT-IF cross-linking and cell migration. Pharmacologically restoring MT stability rescues gross cytoskeleton organization and angiogenic sprouting in Rudhira-depleted cells. Our study identifies the novel and essential role of Rudhira in cytoskeletal cross-talk and assigns function to the conserved BCAS3 domain. Targeting Rudhira could allow tissue-restricted cytoskeleton modulation to control cell migration and angiogenesis in development and disease.

Mal3 is a multi-copy suppressor of the sensitivity to microtubule-depolymerizing drugs and chromosome mis-segregation in a fission yeast pka1 mutant

The cAMP-dependent protein kinase Pka1 is known as a regulator of glycogenesis, transition into meiosis, chronological aging, and stress responses in the fission yeast, Schizosaccharomyces pombe. We demonstrated here that Pka1 is responsible for normal growth in the presence of the microtubule-destabilization drug TBZ and proper chromosome segregation. The deletion of the pka1 gene resulted in the TBZ-sensitive phenotype and chromosome mis-segregation. We isolated the mal3 gene as a multi-copy suppressor of the TBZ-sensitive phenotype in the pka1Δ strains. Overexpression of the CH domain (1–143) or the high-affinity microtubule binding mutant (1–143 Q89R) of Mal3 rescued the TBZ-sensitive phenotype in the pka1Δ and mal3Δ strains, while the EB1 domain (135–308) and the mutants defective in microtubule binding (1–143 Q89E) failed to do so in the same strains. Chromosome mis-segregation caused by TBZ in the pka1Δ or mal3Δ strains was suppressed by the overexpression of the Mal3 CH domain (1–143), Mal3 CH domain with the coiled-coil domain (1–197), or full-length Mal3. Overexpression of EB1 orthologs from Saccharomyces cerevisiae, Arabidopsis thaliana, Mus musculus, or Homo sapiens suppressed the TBZ-sensitive phenotype in the pka1Δ strains, indicating their conserved functions. These findings suggest that Pka1 and the microtubule binding of the Mal3 CH domain play a role in the maintenance of proper chromosome segregation.

Myosin VIIa Supports Spermatid/Organelle Transport and Cell Adhesion During Spermatogenesis in the Rat Testis

The biology of transport of spermatids and spermatid adhesion across the seminiferous epithelium during the epithelial cycle remains largely unexplored. Nonetheless, studies have implicated the role of motor proteins in these cellular events. In this article, we report findings to unravel the role of myosin VIIa, an F-actin-based barbed (+)-end-directed motor protein, to support cellular transport and adhesion in the testis. Using RNA interference to knock down myosin VIIa in Sertoli cells cultured in vitro as a study model was shown to perturb the Sertoli cell tight junction permeability barrier, mediated through disorganization of actin- or microtubule (MT)-based cytoskeletons owing to disruptive changes on the spatiotemporal expression of F-actin or MT-regulatory proteins. Consistent with these in vitro findings, knockdown of myosin VIIa in the testis in vivo also induced disorganization of the actin- and MT-based cytoskeletons across the seminiferous epithelium, mediated by disruptive changes in the spatiotemporal expression of actin- and MT-based regulatory proteins. More important, the transport of spermatids and organelles across the epithelium, as well as cell adhesion, was grossly disrupted. For instance, step 19 spermatids failed to be transported to the adluminal compartment near the tubule lumen to undergo spermiation; in this manner, step 19 spermatids were persistently detected in stage IX and XII tubules, intermingling with step 9 and 12 spermatids, respectively. Also, phagosomes were detected near the tubule lumen in stage I to III tubules when they should have been degraded near the base of the seminiferous epithelium via the lysosomal pathway. In summary, myosin VIIa motor protein was crucial to support cellular transport and adhesion during spermatogenesis.

CLIP-170 spatially modulates receptor tyrosine kinase recycling to coordinate cell migration

Endocytic sorting of activated receptor tyrosine kinases (RTKs), alternating between recycling and degradative processes, controls signal duration, location and surface complement of RTKs. The microtubule (MT) plus-end tracking proteins (+TIPs) play essential roles in various cellular activities including translocation of intracellular cargo. However, mechanisms through which RTKs recycle back to the plasma membrane following internalization in response to ligand remain poorly understood. We report that net outward-directed movement of endocytic vesicles containing the hepatocyte growth factor (HGF) Met RTK, requires recruitment of the +TIP, CLIP-170, as well as the association of CLIP-170 to MT plus-ends. In response to HGF, entry of Met into Rab4-positive endosomes results in Golgi-localized γ-ear-containing Arf-binding protein 3 (GGA3) and CLIP-170 recruitment to an activated Met RTK complex. We conclude that CLIP-170 co-ordinates the recycling and the transport of Met-positive endocytic vesicles to plus-ends of MTs towards the cell cortex, including the plasma membrane and the lamellipodia, thereby promoting cell migration.

Planar cell polarity protein Dishevelled 3 (Dvl3) regulates ectoplasmic specialization (ES) dynamics in the testis through changes in cytoskeletal organization

In the mammalian testes, such as in rats, the directional alignment of polarized elongating/elongated spermatids, in particular step 17-19 spermatids, across the plane of seminiferous epithelium resembles planar cell polarity (PCP) found in hair cells of the cochlea. It is obvious that spermatid PCP is necessary to support the simultaneous development of maximal number of elongating/elongated spermatids to sustain the daily production of > 50 million sperm per adult rat. Studies have shown that the testis indeed expresses multiple PCP proteins necessary to support spermatid PCP. Herein, using physiological and biochemical assays, and morphological analysis, and with the technique of RNA interference (RNAi) to knockdown PCP protein Dishevelled (Dvl) 1 (Dvl1), Dvl2, Dvl3, or Dvl1/2/3, Dvl proteins, in particular Dvl3, it was shown that Dvl3 played a crucial role of support Sertoli cell tight junction (TJ)-permeability barrier function through changes in the organization of actin- and microtubule (MT)-based cytoskeletons. More important, an in vivo knockdown of Dvl1/2/3 in the testis, defects of spermatid polarity were remarkably noted across the seminiferous epithelium, concomitant with defects of spermatid adhesion and spermatid transport, leading to considerably defects in spermatogenesis. More important, Dvl1/2/3 triple knockdown in the testis also impeded the organization of actin- and MT-based cytoskeletons owing to disruptive spatial expression of actin- and MT-regulatory proteins. In summary, PCP Dishevelled proteins, in particular, Dvl3 is a regulator of Sertoli cell blood-testis barrier (BTB)  and also spermatid PCP function through its effects on the actin- and MT-based cytoskeletons in Sertoli cells.

PKA-dependent phosphorylation of IP3K-A at Ser119 regulates a binding affinity with EB3

Microtubule-associated end-binding protein 3 (EB3) accumulates asymmetrically at the tip-end of growing microtubules, providing a central platform for linking various cellular components. EB3 orchestrates microtubule dynamics and targeting, enabling diverse processes within neurons. Inositol 1, 4, 5-trisphosphate 3-kinase A (IP3K-A; also known as ITPKA) is a neuron-enriched protein that binds to microtubules by PKA-dependent manners. In this study, we found that IP3K-A binds to EB3 and their binding affinity is precisely regulated by protein kinase A (PKA)-dependent phosphorylation of IP3K-A at Ser119 (pSer119). We also revealed that the complex of IP3K-A and EB3 dissociates and reassociates rapidly during chemically induced LTP (cLTP) condition. This dynamic rearrangement of IP3K-A and EB3 complex will contribute remodeling of microtubule cytoskeleton allowing effective structural plasticity in response to synaptic stimulations.

Molecular basis of the functions of the mammalian neuronal growth cone revealed using new methods

The neuronal growth cone is a highly motile, specialized structure for extending neuronal processes. This structure is essential for nerve growth, axon pathfinding, and accurate synaptogenesis. Growth cones are important not only during development but also for plasticity-dependent synaptogenesis and neuronal circuit rearrangement following neural injury in the mature brain. However, the molecular details of mammalian growth cone function are poorly understood. This review examines molecular findings on the function of the growth cone as a result of the introduction of novel methods such superresolution microscopy and (phospho)proteomics. These results increase the scope of our understating of the molecular mechanisms of growth cone behavior in the mammalian brain.

Dynamic Microtubules in Alzheimer's Disease: Association with Dendritic Spine Pathology

Alzheimer's disease (AD) is the most common incurable neurodegenerative disorder that affects the processes of memory formation and storage. The loss of dendritic spines and alteration in their morphology in AD correlate with the extent of patient's cognitive decline. Tubulin had been believed to be restricted to dendritic shafts, until recent studies demonstrated that dynamically growing tubulin microtubules enter dendritic spines and promote their maturation. Abnormalities of tubulin cytoskeleton may contribute to the process of dendritic spine shape alteration and their subsequent loss in AD. In this review, association between tubulin cytoskeleton dynamics and dendritic spine morphology is discussed in the context of dendritic spine alterations in AD. Potential implications of these findings for the development of AD therapy are proposed.

Dynein 1 supports spermatid transport and spermiation during spermatogenesis in the rat testis

In the mammalian testis, spermatogenesis is dependent on the microtubule (MT)-specific motor proteins, such as dynein 1, that serve as the engine to support germ cell and organelle transport across the seminiferous epithelium at different stages of the epithelial cycle. Yet the underlying molecular mechanism(s) that support this series of cellular events remain unknown. Herein, we used RNAi to knockdown cytoplasmic dynein 1 heavy chain (Dync1h1) and an inhibitor ciliobrevin D to inactivate dynein in Sertoli cells in vitro and the testis in vivo, thereby probing the role of dynein 1 in spermatogenesis. Both treatments were shown to extensively induce disruption of MT organization across Sertoli cells in vitro and the testis in vivo. These changes also perturbed the transport of spermatids and other organelles (such as phagosomes) across the epithelium. These changes thus led to disruption of spermatogenesis. Interestingly, the knockdown of dynein 1 or its inactivation by ciliobrevin D also perturbed gross disruption of F-actin across the Sertoli cells in vitro and the seminiferous epithelium in vivo, illustrating there are cross talks between the two cytoskeletons in the testis. In summary, these findings confirm the role of cytoplasmic dynein 1 to support the transport of spermatids and organelles across the seminiferous epithelium during the epithelial cycle of spermatogenesis.

Microtubule-severing enzymes: From cellular functions to molecular mechanism

McNally and Roll-Mecak review the molecular mechanism of microtubule-severing enzymes and their diverse roles in processes ranging from cell division to ciliogensis and morphogenesis.

Microtubule-severing enzymes generate internal breaks in microtubules. They are conserved in eukaryotes from ciliates to mammals, and their function is important in diverse cellular processes ranging from cilia biogenesis to cell division, phototropism, and neurogenesis. Their mutation leads to neurodegenerative and neurodevelopmental disorders in humans. All three known microtubule-severing enzymes, katanin, spastin, and fidgetin, are members of the meiotic subfamily of AAA ATPases that also includes VPS4, which disassembles ESCRTIII polymers. Despite their conservation and importance to cell physiology, the cellular and molecular mechanisms of action of microtubule-severing enzymes are not well understood. Here we review a subset of cellular processes that require microtubule-severing enzymes as well as recent advances in understanding their structure, biophysical mechanism, and regulation.

The small molecule AMBMP disrupts microtubule growth, ciliogenesis, cell polarity, and cell migration

2-Amino-4-(3,4-[methylenedioxy]benzylamino)-6-(3-methoxyphenyl)pyrimidine (AMBMP) is a small molecule that has been previously reported to be both a Wnt agonist and a microtubule (MT) regulator. Here we report a detailed analysis of AMBMPs effects on MTs and on MT associated cellular processes including cell polarity, ciliogenesis, and cell migration. Specifically, treatment of Xenopus embryos with AMBMP leads to defects similar to the MT depolymerizing drug nocodazole, including a failure to generate or polarize cilia (depending on the timing of treatment) and a loss of the cell movements associated with radial intercalation. The dramatic effect AMBMP has on basic MT based cellular functions suggests that its usefulness as a Wnt regulator is questionable. Moreover, it may be an important new tool for experimental or pharmacological manipulation of MTs.

Viscoelasticity of single cells-from subcellular to cellular level

This review deals with insights into complex cellular structures and processes obtained by measuring viscoelastic impedances of the cell envelope and the cytoplasm by colloidal bead microrheometry. I first introduce a mechanical cell model that allows us to understand their unique ability of mechanical self-stabilization by actin microtubule crosstalk. In the second part, I show how cell movements can be driven by pulsatile or propagating solitary actin gelatin waves (SAGW) that are generated on nascent adhesion domains by logistically controlled membrane recruitment of functional proteins by electrostatic-hydrophobic forces. The global polarization of cell migration is guided by actin-microtubule crosstalk that is mediated by the Ca⁺⁺ and strain-sensitive supramolecular scaffolding protein IQGAP. In the third part, I introduce the traction force microscopy as a tool to measure the forces between somatic cells and the tissue ´Here I show, how absolute values of viscoelastic impedances of the composite cell envelope can be obtained by deformation field mapping techniques. In the fourth part, it is shown how the dynamic mechanical properties of the active viscoplastic cytoplasmic space can be evaluated using colloidal beads as phantom endosomes. Separate measurements of velocity distributions of directed and random motions of phantom endosomes, yield local values of transport forces, viscosities and life times of directed motion along microtubules. The last part deals with biomimetic experiments allowing us to quantitatively evaluate the mechanical properties of passive and active actin networks on the basis of the percolation theory of gelation.

Opposing kinesin complexes queue at plus tips to ensure microtubule catastrophe at cell ends

In fission yeast, the lengths of interphase microtubule (iMT) arrays are adapted to cell length to maintain cell polarity and to help centre the nucleus and cell division ring. Here, we show that length regulation of iMTs is dictated by spatially regulated competition between MT‐stabilising Tea2/Tip1/Mal3 (Kinesin‐7) and MT‐destabilising Klp5/Klp6/Mcp1 (Kinesin‐8) complexes at iMT plus ends. During MT growth, the Tea2/Tip1/Mal3 complex remains bound to the plus ends of iMT bundles, thereby restricting access to the plus ends by Klp5/Klp6/Mcp1, which accumulate behind it. At cell ends, Klp5/Klp6/Mcp1 invades the space occupied by the Tea2/Tip1/Tea1 kinesin complex triggering its displacement from iMT plus ends and MT catastrophe. These data show that in vivo, whilst an iMT length‐dependent model for catastrophe factor accumulation has validity, length control of iMTs is an emergent property reflecting spatially regulated competition between distinct kinesin complexes at the MT plus tip.

EB1 contributes to microtubule bundling and organization, along with root growth, in Arabidopsis thaliana

Microtubules are involved in plant development and adaptation to their environment, but the sustaining molecular mechanisms remain elusive. Microtubule-end-binding 1 (EB1) proteins participate in directional root growth in Arabidopsis thaliana. However, a connection to the underlying microtubule array has not been established yet. We show here that EB1 proteins contribute to the organization of cortical microtubules in growing epidermal plant cells, without significant modulation of microtubule dynamics. Using super-resolution stimulated emission depletion (STED) microscopy and an original quantification approach, we also demonstrate a significant reduction of apparent microtubule bundling in cytoplasmic-EB1-deficient plants, suggesting a function for EB1 in the interaction between adjacent microtubules. Furthermore, we observed root growth defects in EB1-deficient plants, which are not related to cell division impairment. Altogether, our results support a role for EB1 proteins in root development, in part by maintaining the organization of cortical microtubules.

This article has an associated First Person interview with the first author of the paper.

Summary: EB1 proteins affect cortical-microtubule bundling and organization in Arabidopsis thaliana, without significant modulation of microtubule dynamics. They also participate in root growth, further linking microtubules to plant development.

Disruption of the psychiatric risk gene Ankyrin 3 enhances microtubule dynamics through GSK3/CRMP2 signaling

The ankyrin 3 gene (ANK3) is a well-established risk gene for psychiatric illness, but the mechanisms underlying its pathophysiology remain elusive. We examined the molecular effects of disrupting brain-specific Ank3 isoforms in mouse and neuronal model systems. RNA sequencing of hippocampus from Ank3+/- and Ank3+/+ mice identified altered expression of 282 genes that were enriched for microtubule-related functions. Results were supported by increased expression of microtubule end-binding protein 3 (EB3), an indicator of microtubule dynamics, in Ank3+/- mouse hippocampus. Live-cell imaging of EB3 movement in primary neurons from Ank3+/- mice revealed impaired elongation of microtubules. Using a CRISPR-dCas9-KRAB transcriptional repressor in mouse neuro-2a cells, we determined that repression of brain-specific Ank3 increased EB3 expression, decreased tubulin acetylation, and increased the soluble:polymerized tubulin ratio, indicating enhanced microtubule dynamics. These changes were rescued by inhibition of glycogen synthase kinase 3 (GSK3) with lithium or CHIR99021, a highly selective GSK3 inhibitor. Brain-specific Ank3 repression in neuro-2a cells increased GSK3 activity (reduced inhibitory phosphorylation) and elevated collapsin response mediator protein 2 (CRMP2) phosphorylation, a known GSK3 substrate and microtubule-binding protein. Pharmacological inhibition of CRMP2 activity attenuated the rescue of EB3 expression and tubulin polymerization in Ank3-repressed cells by lithium or CHIR99021, suggesting microtubule instability induced by Ank3 repression is dependent on CRMP2 activity. Taken together, our data indicate that ANK3 functions in neuronal microtubule dynamics through GSK3 and its downstream substrate CRMP2. These findings reveal cellular and molecular mechanisms underlying brain-specific ANK3 disruption that may be related to its role in psychiatric illness.

Genetically encoded tags for real time dissection of protein assembly in living cells

Simple methods with straightforward readouts that enable real-time interrogation of protein quaternary structure are much needed to facilitate the physicochemical characterization of proteins at the single-cell level. After screening over a series of microtubule (MT) binders, we report herein the development of two genetically encoded tags (designated as "MoTags" for the monomer/oligomer detection tag) that can be conveniently fused to a given protein to probe its oligomeric state in cellulo when combined with routine fluorescence microscopy. In their monomeric form, MoTags are evenly distributed in the cytosol; whereas oligomerization enables MoTags to label MT or track MT tips in an oligomeric state-dependent manner. We demonstrate here the broad utility of engineered MoTags to aid the determination of protein oligomeric states, dissection of protein structure and function, and monitoring of protein-target interactions under physiological conditions in living cells.

Protein⁻Protein Interactions with Connexin 43: Regulation and Function

Connexins are integral membrane building blocks that form gap junctions, enabling direct cytoplasmic exchange of ions and low-molecular-mass metabolites between adjacent cells. In the heart, gap junctions mediate the propagation of cardiac action potentials and the maintenance of a regular beating rhythm. A number of connexin interacting proteins have been described and are known gap junction regulators either through direct effects (e.g., kinases) or the formation of larger multifunctional complexes (e.g., cytoskeleton scaffold proteins). Most connexin partners can be categorized as either proteins promoting coupling by stimulating forward trafficking and channel opening or inhibiting coupling by inducing channel closure, internalization, and degradation. While some interactions have only been implied through co-localization using immunohistochemistry, others have been confirmed by biophysical methods that allow detection of a direct interaction. Our understanding of these interactions is, by far, most well developed for connexin 43 (Cx43) and the scope of this review is to summarize our current knowledge of their functional and regulatory roles. The significance of these interactions is further exemplified by demonstrating their importance at the intercalated disc, a major hub for Cx43 regulation and Cx43 mediated effects.

Visualizing the Actin and Microtubule Cytoskeletons at the B-cell Immune Synapse Using Stimulated Emission Depletion (STED) Microscopy

B cells that bind to membrane-bound antigens (e.g., on the surface of an antigen-presenting cell) form an immune synapse, a specialized cellular structure that optimizes B-cell receptor (BCR) signaling and BCR-mediated antigen acquisition. Both the remodeling of the actin cytoskeleton and the reorientation of the microtubule network towards the antigen contact site are essential for immune synapse formation. Remodeling of the actin cytoskeleton into a dense peripheral ring of F-actin is accompanied by polarization of the microtubule-organizing center towards the immune synapse. Microtubule plus-end binding proteins, as well as cortical plus-end capture proteins mediate physical interactions between the actin and microtubule cytoskeletons, which allow them to be reorganized in a coordinated manner. Elucidating the mechanisms that control this cytoskeletal reorganization, as well as understanding how these cytoskeletal structures shape immune synapse formation and BCR signaling, can provide new insights into B cell activation. This has been aided by the development of super-resolution microscopy approaches that reveal new details of cytoskeletal network organization. We describe here a method for using stimulated emission depletion (STED) microscopy to simultaneously image actin structures, microtubules, and transfected GFP-tagged microtubule plus-end binding proteins in B cells. To model the early events in immune synapse formation, we allow B cells to spread on coverslips coated with anti-immunoglobulin (anti-Ig) antibodies, which initiate BCR signaling and cytoskeleton remodeling. We provide step-by-step protocols for expressing GFP fusion proteins in A20 B-lymphoma cells, for anti-Ig-induced cell spreading, and for subsequent cell fixation, Immunostaining, image acquisition, and image deconvolution steps. The high-resolution images obtained using these procedures allow one to simultaneously visualize actin structures, microtubules, and the microtubule plus-end binding proteins that may link these two cytoskeletal networks.

Characterization of a novel EB1 acetylation site important for the regulation of microtubule dynamics and cargo recruitment

Microtubule plus ends undergo highly dynamic modifications to regulate different aspects of cellular activities. Most microtubule plus-end tracking proteins (+TIPs) are recruited to the microtubule ends by the master loading factor, end-binding protein 1 (EB1). These proteins coordinately regulate microtubule dynamics and cellular plasticity. Acetylation is known to modulate EB1 function; however, the molecular details of EB1 acetylation remain largely unclear. We mapped the acetylation pattern of EB1 and identified several previously uncharacterized sites of EB1 acetylation. We examined the effects of lysine-212 (K212) acetylation and found that acetylation of this site accelerates autophagy-mediated EB1 degradation. By time-lapse microscopy, we found that the acetylation-deficient K212R mutant increased the percentage of fast-growing and long-lived microtubules. Although K212 acetylation did not affect microtubule stability in vitro and the association of EB1 with microtubules, the K212R mutant significantly promoted microtubule regrowth in cells. Coimmunoprecipitation assays further revealed that the K212 site was critical for the recruitment of different +TIP cargoes. These data thus uncover a critical role for a novel EB1 acetylation site in regulating the dynamic structure of microtubules.

Actin nucleator Spire 1 is a regulator of ectoplasmic specialization in the testis

Germ cell differentiation during the epithelial cycle of spermatogenesis is accompanied by extensive remodeling at the Sertoli cell-cell and Sertoli cell-spermatid interface to accommodate the transport of preleptotene spermatocytes and developing spermatids across the blood-testis barrier (BTB) and the adluminal compartment of the seminiferous epithelium, respectively. The unique cell junction in the testis is the actin-rich ectoplasmic specialization (ES) designated basal ES at the Sertoli cell-cell interface, and the apical ES at the Sertoli-spermatid interface. Since ES dynamics (i.e., disassembly, reassembly and stabilization) are supported by actin microfilaments, which rapidly converts between their bundled and unbundled/branched configuration to confer plasticity to the ES, it is logical to speculate that actin nucleation proteins play a crucial role to ES dynamics. Herein, we reported findings that Spire 1, an actin nucleator known to polymerize actins into long stretches of linear microfilaments in cells, is an important regulator of ES dynamics. Its knockdown by RNAi in Sertoli cells cultured in vitro was found to impede the Sertoli cell tight junction (TJ)-permeability barrier through changes in the organization of F-actin across Sertoli cell cytosol. Unexpectedly, Spire 1 knockdown also perturbed microtubule (MT) organization in Sertoli cells cultured in vitro. Biochemical studies using cultured Sertoli cells and specific F-actin vs. MT polymerization assays supported the notion that a transient loss of Spire 1 by RNAi disrupted Sertoli cell actin and MT polymerization and bundling activities. These findings in vitro were reproduced in studies in vivo by RNAi using Spire 1-specific siRNA duplexes to transfect testes with Polyplus in vivo-jetPEI as a transfection medium with high transfection efficiency. Spire 1 knockdown in the testis led to gross disruption of F-actin and MT organization across the seminiferous epithelium, thereby impeding the transport of spermatids and phagosomes across the epithelium and perturbing spermatogenesis. In summary, Spire 1 is an ES regulator to support germ cell development during spermatogenesis.

mTORC1/rpS6 regulates blood-testis barrier dynamics and spermatogenetic function in the testis in vivo

The blood-testis barrier (BTB), conferred by Sertoli cells in the mammalian testis, is an important ultrastructure that supports spermatogenesis. Studies using animal models have shown that a disruption of the BTB leads to meiotic arrest, causing defects in spermatogenesis and male infertility. To better understand the regulation of BTB dynamics, we report findings herein to understand the role of ribosomal protein S6 (rpS6), a downstream signaling protein of mammalian target of rapamycin complex 1 (mTORC1), in promoting BTB disruption in the testis in vivo, making the barrier "leaky." Overexpression of wild-type rpS6 (rpS6-WT, the full-length cDNA cloned into the mammalian expression vector pCI-neo) and a constitutively active quadruple phosphomimetic mutant cloned into pCI-neo (p-rpS6-MT) vs. control (empty pCI-neo vector) was achieved by transfecting adult rat testes with the corresponding plasmid DNA using a Polyplus in vivo-jetPEI transfection reagent. On the basis of an in vivo functional BTB integrity assay, p-rpS6-MT was found to induce BTB disruption better than rpS6-WT did (and no effects in empty vector control), leading to defects in spermatogenesis, including loss of spermatid polarity and failure in the transport of cells (e.g., spermatids) and organelles (e.g., phagosomes), to be followed by germ exfoliation. More important, rpS6-WT and p-rpS6-MT exert their disruptive effects through changes in the organization of actin- and microtubule (MT)-based cytoskeletons, which are mediated by changes in the spatiotemporal expression of actin- and MT-based binding and regulatory proteins. In short, mTORC1/rpS6 signaling complex is a regulator of spermatogenesis and BTB by modulating the organization of the actin- and MT-based cytoskeletons.

CK2β regulates thrombopoiesis and Ca2+-triggered platelet activation in arterial thrombosis

CK2β is critically required for thrombopoiesis by regulating tubulin polymerization, MK fragmentation, and proplatelet formation. CK2β facilitates inositol triphosphate–mediated increase of cytosolic Ca2+ and is essential for platelet activation in arterial thrombosis in vivo.

Stim2-Eb3 Association and Morphology of Dendritic Spines in Hippocampal Neurons

Mushroom spines form strong synaptic contacts and are essential for memory storage. We have previously demonstrated that neuronal store-operated calcium entry (nSOC) in hippocampal neurons is regulated by STIM2 protein. This pathway plays a key role in stability of mushroom spines and is compromised in different mice models of Alzheimer's disease (AD). Actin was thought to be the sole cytoskeleton compartment presented in dendritic spines, however, recent studies demonstrated that dynamic microtubules with EB3 capped plus-ends transiently enter spines. We showed that STIM2 forms an endoplasmic reticulum (ER) Ca²⁺ -dependent complex with EB3 via Ser-x-Ile-Pro aminoacid motif and that disruption of STIM2-EB3 interaction resulted in loss of mushroom spines in hippocampal neurons. Overexpression of EB3 causes increase of mushroom spines fraction and is able to restore their deficiency in hippocampal neurons obtained from PS1-M146V-KI AD mouse model. STIM2 overexpression failed to restore mushroom dendritic spines after EB3 knockdown, while in contrast EB3 overexpression rescued loss of mushroom spines resulting from STIM2 depletion. We propose that EB3 is involved in regulation of dendritic spines morphology, in part due to its association with STIM2, and that modulation of EB3 expression is a potential way to overcome synaptic loss during AD.

Sperm Release at Spermiation Is Regulated by Changes in the Organization of Actin- and Microtubule-Based Cytoskeletons at the Apical Ectoplasmic Specialization-A Study Using the Adjudin Model

The mechanism that regulates sperm release at spermiation is unknown. Herein, we used an animal model wherein rats were treated with adjudin, 1-(2,4-dichlorobenzyl)-1H-indazole-3-carbohydrazide, via oral gavage to induce premature release of elongating/elongated spermatids, followed by round spermatids and spermatocytes. Spermatid release mimicking spermiation occurred within 6 to 12 hours following adjudin treatment and, by 96 hours, virtually all tubules were devoid of elongating/elongated spermatids. Using this model, we tracked the organization of F-actin and microtubules (MTs) by immunofluorescence microscopy, and the association of actin or MT regulatory proteins that either promote or demolish cytoskeletal integrity through changes in the organization of actin microfilaments or MTs by coimmunoprecipitation. Adjudin treatment induced an increase in the association of (1) epidermal growth factor receptor pathway substrate 8 (an actin barbed-end capping and bundling protein) or formin 1 (an actin nucleator) with actin and (2) end-binding protein 1 (an MT stabilizing protein) with MT shortly after adjudin exposure (at 6 hours), in an attempt to maintain spermatid adhesion to the Sertoli cell at the apical ectoplasmic specialization (ES). However, this was followed by a considerable decline of their steady-state protein levels, replacing with an increase in association of (1) actin-related protein 3 (a branched actin nucleator that converts actin filaments into a branched/unbundled network) with actin and (2) MT affinity-regulating kinase 4 (an MT destabilizing protein kinase) with MTs by 12 hours after adjudin treatment. These latter changes thus promoted actin and MT disorganization, leading to apical ES disruption and the release of elongating/elongated spermatids, mimicking spermiation. In summary, spermiation is a cytoskeletal-dependent event, involving regulatory proteins that modify cytoskeletal organization.

A novel immunofluorescence method to visualize microtubules in the antiparallel overlaps of microtubule-plus ends in the anaphase and telophase midzone

Cell division, in which duplicated chromosomes are separated into two daughter cells, is the most dynamic event during cell proliferation. Chromosome movement is powered mainly by microtubules, which vary in morphology and are organized into characteristic structures according to mitotic progression. During the later stages of mitosis, antiparallel microtubules form the spindle midzone, and the irregular formation of the midzone often leads to failure of cytokinesis, giving rise to the unequal segregation of chromosomes. However, it is difficult to analyze the morphology of these microtubules because microtubules in the antiparallel overlaps of microtubule-plus ends in the midzone are embedded in highly electron-dense matrices, impeding the access of anti-tubulin antibodies to their epitopes during immunofluorescence staining. Here, we developed a novel method to visualize selectively antiparallel microtubule overlaps in the midzone. When cells are air-dried before fixation, aligned α-tubulin staining is observed and colocalized with PRC1 in the center of the midzone of anaphase and telophase cells, suggesting that antiparallel microtubule overlaps can be visualized by this method. In air-dried cells, mCherry-α-tubulin fluorescence and β-tubulin staining show almost the same pattern as α-tubulin staining in the midzone, suggesting that the selective visualization of antiparallel microtubule overlaps in air-dried cells is not attributed to an alteration of the antigenicity of α-tubulin. Taxol treatment extends the microtubule filaments of the midzone in air-dried cells, and nocodazole treatment conversely decreases the number of microtubules, suggesting that unstable microtubules are depolymerized during the air-drying method. It is of note that the air-drying method enables the detection of the disruption of the midzone and premature midzone formation upon Aurora B and Plk1 inhibition, respectively. These results suggest that the air-drying method is suitable for visualizing microtubules in the antiparallel overlaps of microtubule-plus ends of the midzone and for detecting their effects on midzone formation.

Prognostic impact of RITA expression in patients with anal squamous cell carcinoma treated with chemoradiotherapy

BACKGROUND

RBP-J interacting and tubulin-associated protein (RITA) has been identified as a negative regulator of the Notch signalling pathway and its deregulation is involved in the pathogenesis of several tumour entities. RITA's impact on the response of anal squamous cell carcinoma (SCC) to anticancer treatment, however, remains elusive.

MATERIALS AND METHODS

In our retrospective study immunohistochemical evaluation of RITA was performed on 140 pre-treatment specimens and was correlated with clinical and histopathologic characteristics and clinical endpoints cumulative incidence of local control (LC), distant recurrence (DC), disease-free survival (DFS) and overall survival (OS).

RESULTS

We observed significant inverse correlations between RITA expression and tumour grading, the levels of HPV-16 virus DNA load, CD8 (+) tumour infiltrating lymphocytes and programmed death protein (PD-1) immunostaining. In univariate analyses, elevated levels of RITA expression were predictive for decreased local control (p = 0.001), decreased distant control (p = 0.040), decreased disease free survival (p = 0.001) and overall survival (p < 0.0001), whereas in multivariate analyses RITA expression remained significant for decreased local control (p = 0.009), disease free survival (p = 0.032) and overall survival (p = 0.012).

CONCLUSION

These data indicate that elevated levels of pretreatment RITA expression are correlated with unfavourable clinical outcome in anal carcinoma treated with concomitant chemoradiotherapy.

ReMAPping the microtubule landscape: How phosphorylation dictates the activities of microtubule-associated proteins

Classical microtubule-associated proteins (MAPs) were originally identified based on their co-purification with microtubules assembled from mammalian brain lysate. They have since been found to perform a range of functions involved in regulating the dynamics of the microtubule cytoskeleton. Most of these MAPs play integral roles in microtubule organization during neuronal development, microtubule remodeling during neuronal activity, and microtubule stabilization during neuronal maintenance. As a result, mutations in MAPs contribute to neurodevelopmental disorders, psychiatric conditions, and neurodegenerative diseases. MAPs are post-translationally regulated by phosphorylation depending on developmental time point and cellular context. Phosphorylation can affect the microtubule affinity, cellular localization, or overall function of a particular MAP and can thus have profound implications for neuronal health. Here we review MAP1, MAP2, MAP4, MAP6, MAP7, MAP9, tau, and DCX, and how each is regulated by phosphorylation in neuronal physiology and disease. Developmental Dynamics 247:138-155, 2018. © 2017 Wiley Periodicals, Inc.

Kinetochore-microtubule interactions in chromosome segregation: lessons from yeast and mammalian cells

Chromosome congression and segregation require robust yet dynamic attachment of the kinetochore with the spindle microtubules. Force generated at the kinetochore-microtubule interface plays a vital role to drive the attachment, as it is required to move chromosomes and to provide signal to sense correct attachments. To understand the mechanisms underlying these processes, it is critical to describe how the force is generated and how the molecules at the kinetochore-microtubule interface are organized and assembled to withstand the force and respond to it. Research in the past few years or so has revealed interesting insights into the structural organization and architecture of kinetochore proteins that couple kinetochore attachment to the spindle microtubules. Interestingly, despite diversities in the molecular players and their modes of action, there appears to be architectural similarity of the kinetochore-coupling machines in lower to higher eukaryotes. The present review focuses on the most recent advances in understanding of the molecular and structural aspects of kinetochore-microtubule interaction based on the studies in yeast and vertebrate cells.

CLASP1 regulates endothelial cell branching morphology and directed migration

Endothelial cell (EC) branching is critically dependent upon the dynamic nature of the microtubule (MT) cytoskeleton. Extracellular matrix (ECM) mechanosensing is a prominent mechanism by which cytoskeletal reorganization is achieved; yet how ECM-induced signaling is able to target cytoskeletal reorganization intracellularly to facilitate productive EC branching morphogenesis is not known. Here, we tested the hypothesis that the composition and density of the ECM drive the regulation of MT growth dynamics in ECs by targeting the MT stabilizing protein, cytoplasmic linker associated protein 1 (CLASP1). High-resolution fluorescent microscopy coupled with computational image analysis reveal that CLASP1 promotes slow MT growth on glass ECMs and promotes short-lived MT growth on high-density collagen-I and fibronectin ECMs. Within EC branches, engagement of either high-density collagen-I or high-density fibronectin ECMs results in reduced MT growth speeds, while CLASP1-dependent effects on MT dynamics promotes elevated numbers of short, branched protrusions that guide persistent and directed EC migration.

Summary: CLASP1 modulates microtubule dynamics with sub-cellular specificity in response to extracellular matrix density and composition. CLASP1 effects on microtubules promote short, branched protrusions that guide persistent and directional EC migration. This article has an associated First Person interview with the first author of the paper as part of the supplementary information.

H+- and Na+- elicited rapid changes of the microtubule cytoskeleton in the biflagellated green alga Chlamydomonas

Although microtubules are known for dynamic instability, the dynamicity is considered to be tightly controlled to support a variety of cellular processes. Yet diverse evidence suggests that this is not applicable to Chlamydomonas, a biflagellate fresh water green alga, but intense autofluorescence from photosynthesis pigments has hindered the investigation. By expressing a bright fluorescent reporter protein at the endogenous level, we demonstrate in real time discreet sweeping changes in algal microtubules elicited by rises of intracellular H⁺ and Na⁺. These results from this model organism with characteristics of animal and plant cells provide novel explanations regarding how pH may drive cellular processes; how plants may respond to, and perhaps sense stresses; and how organisms with a similar sensitive cytoskeleton may be susceptible to environmental changes.

The Microtubule-Stabilizing Protein CLASP1 Associates with the Theileria annulata Schizont Surface via Its Kinetochore-Binding Domain

T. annulata, the only eukaryote known to be capable of transforming another eukaryote, is a widespread parasite of veterinary importance that puts 250 million cattle at risk worldwide and limits livestock development for some of the poorest people in the world. Crucial to the pathology of Theileria is its ability to interact with host microtubules and the mitotic spindle of the infected cell. This study builds on our previous work in investigating the host and parasite molecules involved in mediating this interaction. Because it is not possible to genetically manipulate Theileria schizonts, identifying protein interaction partners is critical to understanding the function of parasite proteins. By identifying two Theileria surface proteins that are involved in the interaction between CLASP1 and the parasite, we provide important insights into the molecular basis of Theileria persistence within a dividing cell.

Theileria is an apicomplexan parasite whose presence within the cytoplasm of a leukocyte induces cellular transformation and causes uncontrolled proliferation and clonal expansion of the infected cell. The intracellular schizont utilizes the host cell’s own mitotic machinery to ensure its distribution to both daughter cells by associating closely with microtubules (MTs) and incorporating itself within the central spindle. We show that CLASP1, an MT-stabilizing protein that plays important roles in regulating kinetochore-MT attachment and central spindle positioning, is sequestered at the Theileria annulata schizont surface. We used live-cell imaging and immunofluorescence in combination with MT depolymerization assays to demonstrate that CLASP1 binds to the schizont surface in an MT-independent manner throughout the cell cycle and that the recruitment of the related CLASP2 protein to the schizont is MT dependent. By transfecting Theileria-infected cells with a panel of truncation mutants, we found that the kinetochore-binding domain of CLASP1 is necessary and sufficient for parasite localization, revealing that CLASP1 interaction with the parasite occurs independently of EB1. We overexpressed the MT-binding domain of CLASP1 in parasitized cells. This exhibited a dominant negative effect on host MT stability and led to altered parasite size and morphology, emphasizing the importance of proper MT dynamics for Theileria partitioning during host cell division. Using coimmunoprecipitation, we demonstrate that CLASP1 interacts, directly or indirectly, with the schizont membrane protein p104, and we describe for the first time TA03615, a Theileria protein which localizes to the parasite surface, where it has the potential to participate in parasite-host interactions.

IMPORTANCET. annulata, the only eukaryote known to be capable of transforming another eukaryote, is a widespread parasite of veterinary importance that puts 250 million cattle at risk worldwide and limits livestock development for some of the poorest people in the world. Crucial to the pathology of Theileria is its ability to interact with host microtubules and the mitotic spindle of the infected cell. This study builds on our previous work in investigating the host and parasite molecules involved in mediating this interaction. Because it is not possible to genetically manipulate Theileria schizonts, identifying protein interaction partners is critical to understanding the function of parasite proteins. By identifying two Theileria surface proteins that are involved in the interaction between CLASP1 and the parasite, we provide important insights into the molecular basis of Theileria persistence within a dividing cell.

Noncentrosomal microtubules regulate autophagosome transport through CAMSAP2-EB1 cross-talk

Microtubules (MTs) play essential roles in many steps of autophagy, an important degradation pathway in the maintenance of cellular homoeostasis. In many cells, MT networks are comprised of centrosomal MTs and noncentrosomal MTs. However, it is unknown whether noncentrosomal MTs and its binding proteins are involved in autophagy. Here, we show in HeLa cells that calmodulin-regulated spectrin-associated protein 2 (CAMSAP2), a noncentrosomal MT minus-end stabilizing protein, regulates retrograde transport of autophagosomes through MT dynamics. CAMSAP2 cooperates with EB1 to regulate end-binding protein 1 (EB1) behaviour at MT plus ends, MT growth directions and autophagosome transport. An association between CAMSAP2 and EB1 in the cytosol may modulate EB1 binding to MT plus ends. Collectively, our data indicate that noncentrosomal MTs regulate autophagy through a cross-talk between CAMSAP2 and EB1.