Microtubule Bundles Research Articles

Association of mitochondria with microtubules inhibits mitochondrial fission by precluding assembly of the fission protein Dnm1

Mitochondria are organized as tubular networks in the cell and undergo fission and fusion. Although several of the molecular players involved in mediating mitochondrial dynamics have been identified, the precise cellular cues that initiate mitochondrial fission or fusion remain largely unknown. In fission yeast (Schizosaccharomyces pombe), mitochondria are organized along microtubule bundles. Here, we employed deletions of kinesin-like proteins to perturb microtubule dynamics and used high-resolution and time-lapse fluorescence microscopy, revealing that mitochondrial lengths mimic microtubule lengths. Furthermore, we determined that compared with WT cells, mutant cells with long microtubules exhibit fewer mitochondria, and mutant cells with short microtubules have an increased number of mitochondria because of reduced mitochondrial fission in the former and elevated fission in the latter. Correspondingly, upon onset of closed mitosis in fission yeast, wherein interphase microtubules assemble to form the spindle within the nucleus, we observed increased mitochondrial fission. We found that the consequent rise in the mitochondrial copy number is necessary to reduce partitioning errors during independent segregation of mitochondria between daughter cells. We also discovered that the association of mitochondria with microtubules physically impedes the assembly of the fission protein Dnm1 around mitochondria, resulting in inhibition of mitochondrial fission. Taken together, we demonstrate a mechanism for the regulation of mitochondrial fission that is dictated by the interaction between mitochondria and the microtubule cytoskeleton.

Contribution of airway eosinophils in airway wall remodeling in asthma: Role of MMP-10 and MET.

Eosinophils play an important role in the pathophysiology of asthma being implicated in airway epithelial damage and airway wall remodeling. We determined the genes associated with airway remodeling and eosinophilic inflammation in patients with asthma. We analyzed the transcriptomic data from bronchial biopsies of 81 patients with moderate-to-severe asthma of the U-BIOPRED cohort. Expression profiling was performed using Affymetrix arrays on total RNA. Transcription binding site analysis used the PRIMA algorithm. Localization of proteins was by immunohistochemistry. Using stringent false discovery rate analysis, MMP-10 and MET were significantly overexpressed in biopsies with high mucosal eosinophils (HE) compared to low mucosal eosinophil (LE) numbers. Immunohistochemical analysis confirmed increased expression of MMP-10 and MET in bronchial epithelial cells and in subepithelial inflammatory and resident cells in asthmatic biopsies. Using less-stringent conditions (raw P-value<0.05, log2 fold change>0.5), we defined a 73-gene set characteristic of the HE compared to the LE group. Thirty-three of 73 genes drove the pathway annotation that included extracellular matrix (ECM) organization, mast cell activation, CC-chemokine receptor binding, circulating immunoglobulin complex, serine protease inhibitors, and microtubule bundle formation pathways. Genes including MET and MMP10 involved in ECM organization correlated positively with submucosal thickness. Transcription factor binding site analysis identified two transcription factors, ETS-1 and SOX family proteins, that showed positive correlation with MMP10 and MET expression. Pathways of airway remodeling and cellular inflammation are associated with submucosal eosinophilia. MET and MMP-10 likely play an important role in these processes.

The binding of Borealin to microtubules underlies a tension independent kinetochore-microtubule error correction pathway

Proper chromosome segregation depends upon kinetochore phosphorylation by the Chromosome Passenger Complex (CPC). Current models suggest the activity of the CPC decreases in response to the inter-kinetochore stretch that accompanies the formation of bi-oriented microtubule attachments, however little is known about tension-independent CPC phosphoregulation. Microtubule bundles initially lie in close proximity to inner centromeres and become depleted by metaphase. Here we find these microtubules control kinetochore phosphorylation by the CPC in a tension independent manner via a microtubule-binding site on the Borealin subunit. Disruption of Borealin-microtubule interactions generates reduced phosphorylation of prometaphase kinetochores, improper kinetochore-microtubule attachments and weakened spindle checkpoint signals. Experimental and modeling evidence suggests that kinetochore phosphorylation is greatly stimulated when the CPC binds microtubules that lie near the inner centromere, even if kinetochores have high inter-kinetochore stretch. We propose the CPC senses its local environment through microtubule structures to control phosphorylation of kinetochores.

Microtubule End-Clustering Maintains a Steady-State Spindle Shape

Each time a cell divides, the microtubule cytoskeleton self-organizes into the metaphase spindle: anellipsoidal steady-state structure that holds its stereotyped geometry despite microtubule turnover and internal stresses [1-6]. Regulation of microtubule dynamics, motor proteins, microtubule crosslinking, and chromatid cohesion can modulate spindle size and shape, and yet modulated spindles reach and hold a new steady state [7-11]. Here, we ask what maintains any spindle steady-state geometry. We report that clustering of microtubule ends by dynein and NuMA is essential for mammalian spindles to hold a steady-state shape. After dynein or NuMA deletion, the mitotic microtubule network is "turbulent"; microtubule bundles extend and bend against the cell cortex, constantly remodeling network shape. We find that spindle turbulence is driven by the homotetrameric kinesin-5 Eg5, and that acute Eg5 inhibition in turbulent spindles recovers spindle geometry and stability. Inspired by invitro work on activeturbulent gels of microtubules and kinesin [12, 13], we explore the kinematics of this invivo turbulent network. We find that turbulent spindles display decreased nematic order and that motile asters distort the nematic director field. Finally, we see that turbulent spindles can drive both flow of cytoplasmic organelles and whole-cell movement-analogous to the autonomous motility displayed by droplet-encapsulated turbulent gels [12]. Thus, end-clustering by dynein and NuMA is required for mammalian spindles to reach a steady-state geometry, and in their absence Eg5 powers a turbulent microtubule network inside mitotic cells.

Tau regulates the microtubule-dependent migration of glioblastoma cells via the Rho-ROCK signaling pathway.

The pathological significance of Tau (encoded by MAPT) in mechanisms driving cell migration in glioblastoma is unclear. By using an shRNA approach to deplete microtubule-stabilizing Tau in U87 cells, we determined its impact on cytoskeletal coordination during migration. We demonstrated here that the motility of these Tau-knockdown cells (shTau cells) was significantly (36%) lower than that of control cells. The shTau cells displayed a slightly changed motility in the presence of nocodazole, which inhibits microtubule formation. Such reduced motility of shTau cells was characterized by a 28% lower number of microtubule bundles at the non-adhesive edges of the tails. In accordance with Tau-stabilized microtubules being required for cell movement, measurements of the front, body and rear section displacements of cells showed inefficient tail retraction in shTau cells. The tail retraction was restored by treatment with Y27632, an inhibitor of Rho-ROCK signaling. Moreover, we clearly identified that shTau cells displayed relocation of the active phosphorylated form of p190-RhoGAP (also known as ARHGAP35), which inhibits Rho-ROCK signaling, and focal adhesion kinase (FAK, also known as PTK2) in cell bodies. In conclusion, our findings indicate that Tau governs the remodeling of microtubule and actin networks for the retraction of the tail of cells, which is necessary for effective migration.

Tubulin Polymerization-Promoting Protein Family Member 3 (Tppp3) Facilitates Microtubule Bundling and Network Formation via its Weak Interaction with Microtubules

Tubulin polymerization-promoting protein family member 3 (TPPP3) is a conserved protein factor found in cytoplasm associated with microtubules in many ciliated cells. The expression level of TPPP3 in mouse tracheal epithelial cells (mTEC) was reported to vary in concomitant with the formation of microtubule networks in the cytoplasm during epithelia development. These observations suggest that TPPP3 takes part in formation of the microtubule network. Here, we expressed several TPPP3 constructs and, by using conventional biochemical measurements and single molecule observations, examined its effects on cell morphism of HEK293 cells, the binding, the bundling, and the elongation of microtubules in vitro. TPPP3 formed microtubule networks in vitro in a concentration dependent manner and showed specific but weak binding (Kd = ca. 5 μM) to the microtubules. The molecular dissection combined with single molecule observations suggests that the ability to dimerize the TPPP3 molecules resides in the N-terminal domain and the ability to promote microtubule elongation resides in the C-terminal domain of TPPP3. Micro-rheology measurements of TPPP3-microtubule network were carried out, in which the Brownian motion of microspheres of 0.2 μm-diameter in the microtubule network was measured. The measurement showed that the network was crosslinked but remained flexible. This crosslinked flexibility enables the network to adopt the optimal mechanical configuration in response to the external force. These results suggest that during the development of the basal body array in ciliated cells, the loose but somehow restricted network provides the adaptability of the array to the external load. This work was supported by funding from the Dynamic Mechanisms and Fundamental Technology for Biological Systems and the Creation of Fundamental Technologies for Understanding and Control of Biosystem Dynamics, CREST, from Japan Science and Technology.

KIF18A's neck linker permits navigation of microtubule-bound obstacles within the mitotic spindle.

KIF18A (kinesin-8) is required for mammalian mitotic chromosome alignment. KIF18A confines chromosome movement to the mitotic spindle equator by accumulating at the plus-ends of kinetochore microtubule bundles (K-fibers), where it functions to suppress K-fiber dynamics. It is not understood how the motor accumulates at K-fiber plus-ends, a difficult feat requiring the motor to navigate protein dense microtubule tracks. Our data indicate that KIF18A's relatively long neck linker is required for the motor's accumulation at K-fiber plus-ends. Shorter neck linker (sNL) variants of KIF18A display a deficiency in accumulation at the ends of K-fibers at the center of the spindle. Depletion of K-fiber-binding proteins reduces the KIF18A sNL localization defect, whereas their overexpression reduces wild-type KIF18A's ability to accumulate on this same K-fiber subset. Furthermore, single-molecule assays indicate that KIF18A sNL motors are less proficient in navigating microtubules coated with microtubule-associated proteins. Taken together, these results support a model in which KIF18A's neck linker length permits efficient navigation of obstacles to reach K-fiber ends during mitosis.

Spermatological characteristics of Siphoderina aloysiae (Digenea, Cryptogonimidae), an intestinal parasite of Sciaena umbra (Teleostei, Sciaenidae)

The ultrastructural organisation of the spermatozoon of Siphoderina aloysiae (Digenea, Cryptogonimidae) is described based upon examination by transmission electron microscopy. Live digeneans were collected from the digestive tract of the Brown meagre Sciaena umbra (Teleostei: Sciaenidae), caught off the Gulf of Gabes in La Chebba (Tunisia). The male gamete of S. aloysiae is a filiform cell, tapered at both extremities, and exhibits typical characteristics of digenean sperm such as two axonemes showing the 9 + ‘1’ trepaxonematan pattern with different lengths. The first axoneme is disorganised and disappears near the anterior nuclear extremity. However, the second axoneme reaches the posterior extremity of the spermatozoon. The latter also contains a nucleus, two bundles of parallel cortical microtubules, external ornamentation of the plasma membrane, spine-like bodies and granules of glycogen. Cortical microtubules are absent in the anterior extremity of the spermatozoon and appear after the disappearance of the electron-dense material. In addition, the male gamete of S. aloysiae has two mitochondria. The first mitochondrion is associated with the external ornamentation of the plasma membrane and spine-like bodies. The second is more posteriorly located at the nuclear region after the disappearance of the first axoneme. The present study provides new data on the mature male gamete of S. aloysiae that may be useful for the understanding of digenean relationships and phylogenetic studies.

CLASP promotes microtubule bundling in metaphase spindle independently of Ase1/PRC1 in fission yeast.

ABSTRACTMicrotubules in the mitotic spindle are organised by microtubule-associated proteins. In the late stage of mitosis, spindle microtubules are robustly organised through bundling by the antiparallel microtubule bundler Ase1/PRC1. In early mitosis, however, it is not well characterised as to whether spindle microtubules are actively bundled, as Ase1 does not particularly localise to the spindle at that stage. Here we show that the conserved microtubule-associated protein CLASP (fission yeast Peg1/Cls1) facilitates bundling of spindle microtubules in early mitosis. The peg1 mutant displayed a fragile spindle with unbundled microtubules, which eventually resulted in collapse of the metaphase spindle and abnormal segregation of chromosomes. Peg1 is known to be recruited to the spindle by Ase1 to stabilise antiparallel microtubules in late mitosis. However, we demonstrate that the function of Peg1 in early mitosis does not rely on Ase1. The unbundled spindle phenotype of the peg1 mutant was not seen in the ase1 mutant, and Peg1 preferentially localised to the spindle even in early mitosis unlike Ase1. Moreover, artificial overexpression of Ase1 in the peg1 mutant partially suppressed unbundled microtubules. We thus conclude that Peg1 bundles microtubules in early mitosis, in a distinct manner from its conventional Ase1-dependent functions in other cell cycle stages.

The symbiotic role of the actin filament cytoskeleton.

This article is a Commentary on Zhang et al., 221: 1049–1059.

The microtubule-associated protein WDL3 mediates ABA-induced stomatal closure in &amp;lt;italic&amp;gt;Arabidopsis&amp;lt;/italic&amp;gt;

Stomatal movements control CO2 uptake for plant photosynthesis and water loss by transpiration, then determine plant productivity and water utilization efficiency. The microtubule dynamics is widely recognized to be essential for guard cell function. However, the molecular mechanisms underlying this process remain largely unknown. WDL3 belongs to the microtubule-associated protein WAVE-DAMPENED 2(WVD2)/WVD2-LIKE (WDL) family, which binds to and stabilizes microtubules against low-temperature and dilution-induced depolymerization. In the presence of WDL3, tubulins assemble into large microtubule bundles in vitro , otherwise only single filament patterns form. It has been intensively investigated that WDL3 participates in COP1(CONSTITUTIVE PHOTOMORPHOGENIC 1)-mediated hypocotyl cell elongation in darkness, but whether and how it modulates the microtubule behaviors during guard cell signaling transduction is still an open question. In this study, we dissected the interplay among WDL3, microtubule dynamics and Ca2+ in ABA-induced stomatal closure. We found that transpirational water loss from detached leaves occurred slowly in the WDL3 RNA interference transgenic line. Stomatal bioassay revealed that guard cell sensitivity to ABA was promoted in the WDL3 RNAi seedlings, and this phenotype could be partially blocked by paclitaxel, a microtubule stabilizing agent. On the other hand, the microtubule-disrupting drug oryzalin, enhanced ABA-triggered stomatal closure even further, and the WDL3 RNAi guard cells were more sensitive to oryzalin treatment. Based on the pharmacological results above, we next tested the effect of WDL3 on cortical microtubules in guard cells directly by Confocal microscopy. The differences in the configurations of microtubule filaments between WDL3 RNAi and wild type (WT) were analyzed after ABA treatment. Microscopic images showed that ABA-induced microtubule disassembly took faster in WDL3 RNAi guard cells than in WT, and the extent of filament bundling decreased significantly in WDL3 RNAi seedlings, as evaluated by the Image J software. This was consistent with the rapid ABA-induced stomatal closing in WDL3 RNAi material . Moreover, we examined the potential role of Ca2+ in this signal pathway. The cytosolic Ca2+ chelator—BAPTA generally alleviated ABA effects in both WT and WDL3 RNAi materials. ABA-induced stomatal closure and microtubule remodeling were much more delayed in WDL3 RNAi seedlings, indicating that Ca2+ acted upstream in the WDL3 mediated-ABA signaling pathway for stomatal movement. In addition, when exogenous ABA was applied, the Ca2+ influx monitored by the non-invasive micro-test technique (NMT) in WDL3 RNAi guard cells was more pronounced than in WT. Taken together, our results suggest that WDL3, probably coordinated with Ca2+, is involved in the precise regulation of microtubule architecture and dynamics, to accurately execute ABA-stimulated signaling transduction during stomatal movement. Thus it contributes to the proper control of leaf transpiration.

Geometry of antiparallel microtubule bundles regulates relative sliding and stalling by PRC1 and Kif4A

Motor and non-motor crosslinking proteins play critical roles in determining the size and stability of microtubule-based architectures. Currently, we have a limited understanding of how geometrical properties of microtubule arrays, in turn, regulate the output of crosslinking proteins. Here we investigate this problem in the context of microtubule sliding by two interacting proteins: the non-motor crosslinker PRC1 and the kinesin Kif4A. The collective activity of PRC1 and Kif4A also results in their accumulation at microtubule plus-ends ('end-tag'). Sliding stalls when the end-tags on antiparallel microtubules collide, forming a stable overlap. Interestingly, we find that structural properties of the initial array regulate microtubule organization by PRC1-Kif4A. First, sliding velocity scales with initial microtubule-overlap length. Second, the width of the final overlap scales with microtubule lengths. Our analyses reveal how micron-scale geometrical features of antiparallel microtubules can regulate the activity of nanometer-sized proteins to define the structure and mechanics of microtubule-based architectures.

The microtubule-associated protein HURP recruits the centrosomal protein TACC3 to regulate K-fiber formation and support chromosome congression

Kinetochore fibers (K-fibers) are microtubule bundles attached to chromosomes. Efficient K-fiber formation is required for chromosome congression, crucial for faithful chromosome segregation in cells. However, the mechanisms underlying K-fiber formation before chromosome biorientation remain unclear. Depletion of hepatoma up-regulated protein (HURP), a RanGTP-dependent microtubule-associated protein localized on K-fibers, has been shown to result in low-efficiency K-fiber formation. Therefore, here we sought to identify critical interaction partners of HURP that may modulate this function. Using co-immunoprecipitation and bimolecular fluorescence complementation assays, we determined that HURP interacts directly with the centrosomal protein transforming acidic coiled coil-containing protein 3 (TACC3), a centrosomal protein, both in vivo and in vitro through the HURP1-625 region. We found that HURP is important for TACC3 function during kinetochore microtubule assembly at the chromosome region in prometaphase. Moreover, HURP regulates stable lateral kinetochore attachment and chromosome congression in early mitosis by modulation of TACC3. These findings provide new insight into the coordinated regulation of K-fiber formation and chromosome congression in prometaphase by microtubule-associated proteins.

Fragmentation of the Golgi Apparatus in Neuroblastoma Cells Is Associated with Tau-Induced Ring-Shaped Microtubule Bundles.

Abnormal fibrillary aggregation of tau protein is a pathological condition observed in Alzheimer's disease and other tauopathies; however, the presence and pathological significance of early non-fibrillary aggregates of tau remain under investigation. In cell and animal models expressing normal or modified tau, toxic effects altering the structure and function of several membranous organelles have also been reported in the absence of fibrillary structures; however, how these abnormalities are produced is an issue yet to be addressed. In order to obtain more insights into the mechanisms by which tau may disturb intracellular membranous elements, we transiently overexpressed human full-length tau and several truncated tau variants in cultured neuroblastoma cells. After 48 h of transfection, either full-length or truncated tau forms produced significant fragmentation of the Golgi apparatus (GA) with no changes in cell viability. Noteworthy is that in the majority of cells exhibiting dispersion of the GA, a ring-shaped array of cortical or perinuclear microtubule (Mt) bundles was also generated under the expression of either variant of tau. In contrast, Taxol treatment of non-transfected cells increased the amount of Mt bundles but not sufficiently to produce fragmentation of the GA. Tau-induced ring-shaped Mt bundles appeared to be well-organized and stable structures because they were resistant to Nocodazole post-treatment and displayed a high level of tubulin acetylation. These results further indicate that a mechanical force generated by tau-induced Mt-bundling may be responsible for Golgi fragmentation and that the repeated domain region of tau may be the main promoter of this effect.

The mitotic spindle is chiral due to torques within microtubule bundles

Mitosis relies on forces generated in the spindle, a micro-machine composed of microtubules and associated proteins. Forces are required for the congression of chromosomes to the metaphase plate and their separation in anaphase. However, besides forces, torques may exist in the spindle, yet they have not been investigated. Here we show that the spindle is chiral. Chirality is evident from the finding that microtubule bundles in human spindles follow a left-handed helical path, which cannot be explained by forces but rather by torques. Kinesin-5 (Kif11/Eg5) inactivation abolishes spindle chirality. Our theoretical model predicts that bending and twisting moments may generate curved shapes of bundles. We found that bundles turn by about −2 deg µm−1 around the spindle axis, which we explain by a twisting moment of roughly −10 pNµm. We conclude that torques, in addition to forces, exist in the spindle and determine its chiral architecture.

A Switch in Microtubule Orientation during C. elegans Meiosis

In oocytes of many organisms, meiotic spindles form in the absence of centrosomes [1-5]. Such female meiotic spindles have a pointed appearance in metaphase with microtubules focused at acentrosomal spindle poles. At anaphase, the microtubules of acentrosomal spindles then transition to an inter-chromosomal array, while the spindle poles disappear. This transition is currently not understood. Previous studies have focused on this inter-chromosomal microtubule array and proposed a pushing model to drive chromosome segregation [6, 7]. This model includes an end-on orientation of microtubules with chromosomes. Alternatively, chromosomes were thought to associate along bundles of microtubules [8, 9]. Starting with metaphase, this second model proposed a pure lateral chromosome-to-microtubule association up to the final meiotic stages of anaphase. Here, we applied large-scale electron tomography [10] of staged C.elegans oocytes in meiosis to analyze the orientation of microtubules in respect to chromosomes. We show that microtubules at metaphase I are primarily oriented laterally to the chromosomes and that microtubules switch to an end-on orientation during progression through anaphase. We further show that this switch in microtubule orientation involves a kinesin-13 microtubule depolymerase, KLP-7, which removes laterally associated microtubules around chromosomes. From this, we conclude that both lateral and end-on modes of microtubule-to-chromosome orientations are successively used in C.elegans oocytes to segregate meiotic chromosomes.

Reinvestigation of the sperm ultrastructure of Hypoderaeum conoideum (Digenea: Echinostomatidae).

Spermatological characteristics of the digenean Hypoderaeum conoideum (Echinostomatidae) collected from Anas platyrhynchos in the Lac d'Annecy (France) were reinvestigated using transmission electron microscopy. The previous study on this species only describes the presence of two axonemes of unequal lengths, a mitochondrion, a posterior nucleus, and the disposition of cortical microtubules. The present ultrastructural study reveals that the mature spermatozoon of H. conoideum is a filiform cell tapered at both extremities. The sperm cell exhibits the characteristics of a digenean spermatozoa type V, namely two axonemes of the 9 + '1' pattern of trepaxonematan Platyhelminthes, external ornamentation of the plasma membrane associated with cortical microtubules, and located in the anterior part of the proximal region of the sperm cell, lateral expansions, two bundles of parallel cortical microtubules, maximum number of cortical microtubules in the anterior part of the spermatozoon, and presence of two mitochondria. In addition, the sperm cell of H. conoideum shows spine-like bodies and a posterior extremity with only the nucleus. The ultrastructural characters of the spermatozoon of H. conoideum are compared with those of other digeneans belonging to the superfamily Echinostomatoidea.

Ultrastructure of the spermatozoon of Aephnidiogenes senegalensis (Digenea, Aephnidiogenidae), an intestinal parasite of Pomadasys jubelini (Teleostei, Haemulidae) off the coast of Dakar (Senegal)

This study provides the first ultrastructural data on the mature spermatozoon of Aephnidiogenes senegalensis (Digenea, Lepocreadioidea, Aephnidiogenidae) parasite of the teleost fish Pomadasys jubelini (Haemulidae) captured near the coast of Dakar (Senegal). This is also the first ultrastructural data on the spermatozoon in the genus Aephnidiogenes. The mature spermatozoon of A. senegalensis is filiform, tapered at both ends and exhibits the general pattern described in the Lepocreadioidea, namely two axonemes of the 9 + ‘1’ pattern, an antero-lateral electron dense material, a nucleus, two mitochondria, external ornamentation of the plasma membrane in the anterior region of the spermatozoon, granules of glycogen and two bundles of parallel cortical microtubules. A peculiarity of the spermatozoon of A. senegalensis is the structure of the apical electron-dense material.

Bundles of Brain Microtubules Generate Electrical Oscillations

Microtubules (MTs) are long cylindrical structures of the cytoskeleton that control cell division, intracellular transport, and the shape of cells. MTs also form bundles, which are particularly prominent in neurons, where they help define axons and dendrites. MTs are bio-electrochemical transistors that form nonlinear electrical transmission lines. However, the electrical properties of most MT structures remain largely unknown. Here we show that bundles of brain MTs spontaneously generate electrical oscillations and bursts of electrical activity similar to action potentials. Under intracellular-like conditions, voltage-clamped MT bundles displayed electrical oscillations with a prominent fundamental frequency at 39 Hz that progressed through various periodic regimes. The electrical oscillations represented, in average, a 258% change in the ionic conductance of the MT structures. Interestingly, voltage-clamped membrane-permeabilized neurites of cultured mouse hippocampal neurons were also capable of both, generating electrical oscillations, and conducting the electrical signals along the length of the structure. Our findings indicate that electrical oscillations are an intrinsic property of brain MT bundles, which may have important implications in the control of various neuronal functions, including the gating and regulation of cytoskeleton-regulated excitable ion channels and electrical activity that may aid and extend to higher brain functions such as memory and consciousness.

Utilizing a Structural Mechanics Approach to Assess the Primary Effects of Injury Loads Onto the Axon and Its Components.

Diffuse axonal injury (DAI) occurs as a result of the transmission of rapid dynamic loads from the head to the brain and specifically to its neurons. Despite being one of the most common and most deleterious types of traumatic brain injury (TBI), the inherent cell injury mechanism is yet to be understood. Experimental observations have led to the formulation of different hypotheses, such as mechanoporation of the axolemma and microtubules (MTs) breakage. With the goal of singling out the mechanical aspect of DAI and to resolve the ambiguity behind its injury mechanism, a composite finite element (FE) model of a representative volume of an axon was developed. Once calibrated and validated against published experimental data, the axonal model was used to simulate injury scenarios. The resulting strain distributions along its components were then studied in dependence of strain rate and of typical modeling choices such as the applied MT constraints and tau proteins failure. Results show that oversimplifying the MT bundle kinematic entails a systematic attenuation (cf = 2.33) of the computed maximum MT strain. Nevertheless, altogether, results support the hypothesis of axolemma mechanoporation as a cell-injury trigger. Moreover, for the first time the interconnection between the axolemma and the MT bundle is shown to play a role in damage localization. The proposed FE axonal model is a valuable tool to understand the axonal injury mechanism from a mechanical perspective and could be used in turn for the definition of well-informed injury criteria at the tissue and organ level.