Tau co-organizes dynamic microtubule and actin networks

Inflammasome links traumatic brain injury, chronic traumatic encephalopathy, and Alzheimer's disease

Traumatic brain injury, chronic traumatic encephalopathy, and Alzheimer's disease are three distinct neurological disorders that share common pathophysiological mechanisms involving neuroinflammation. One sequela of neuroinflammation includes the pathologic hyperphosphorylation of tau protein, an endogenous microtubule-associated protein that protects the integrity of neuronal cytoskeletons. Tau hyperphosphorylation results in protein misfolding and subsequent accumulation of tau tangles forming neurotoxic aggregates. These misfolded proteins are characteristic of traumatic brain injury, chronic traumatic encephalopathy, and Alzheimer's disease and can lead to downstream neuroinflammatory processes, including assembly and activation of the inflammasome complex. Inflammasomes refer to a family of multimeric protein units that, upon activation, release a cascade of signaling molecules resulting in caspase-induced cell death and inflammation mediated by the release of interleukin-1β cytokine. One specific inflammasome, the NOD-like receptor protein 3, has been proposed to be a key regulator of tau phosphorylation where it has been shown that prolonged NOD-like receptor protein 3 activation acts as a causal factor in pathological tau accumulation and spreading. This review begins by describing the epidemiology and pathophysiology of traumatic brain injury, chronic traumatic encephalopathy, and Alzheimer's disease. Next, we highlight neuroinflammation as an overriding theme and discuss the role of the NOD-like receptor protein 3 inflammasome in the formation of tau deposits and how such tauopathic entities spread throughout the brain. We then propose a novel framework linking traumatic brain injury, chronic traumatic encephalopathy, and Alzheimer's disease as inflammasome-dependent pathologies that exist along a temporal continuum. Finally, we discuss potential therapeutic targets that may intercept this pathway and ultimately minimize long-term neurological decline.

Active and passive crosslinking of cytoskeleton scaffolds tune the effects of cell inclusions on composite structure

Incorporating cells within active biomaterial scaffolds is a promising strategy to develop forefront materials that can autonomously sense, respond, and alter the scaffold in response to environmental cues or internal cell circuitry. Using dynamic biocompatible scaffolds that can self-alter their properties via crosslinking and motor-driven force-generation opens even greater avenues for actuation and control. However, the design principles associated with engineering active scaffolds embedded with cells are not well established. To address this challenge, we design a dynamic scaffold material of bacteria cells embedded within a composite cytoskeletal network of actin and microtubules that can be passively or actively crosslinked by either biotin-streptavidin or multimeric kinesin motors. Using quantitative microscopy, we demonstrate the ability to embed cells of volume fractions 0.4-2% throughout the network without compromising the structural integrity of the network or inhibiting crosslinking or motor-driven dynamics. Our findings suggest that both passive and active crosslinking promote entrainment of cells within the network, while depletion interactions play a more important role in uncrosslinked networks. Moreover, we show that large-scale structures emerge with the addition of cell fractions as low as 0.4%, but these structures do not influence the microscale structural length scale of the materials. Our work highlights the potential of our composite biomaterial in designing autonomous materials controlled by cells, and provides a roadmap for effectively coupling cells to complex composite materials with an eye towards using cells as in situ factories to program material modifications.

Organelle perturbation in Alzheimer's disease: do intracellular amyloid-ß and the fragmented Golgi mediate early intracellular neurotoxicity?

Alzheimer's disease is a devastating and incurable neurological disease. Most of the current research has focused on developing drugs to clear the extracellular amyloid plaques in the brain of Alzheimer's disease patients. However, this approach is limited as it does not treat the underlying cause of the disease. In this review, we highlight the evidence in the field showing that the accumulation of intracellular toxic amyloid-ß could underpin very early events in neuronal death in both familial early-onset and sporadic late-onset alzheimer's disease. Indeed, intracellular amyloid-ß, which is produced within intracellular compartments, has been shown to perturb endosomal and secretory organelles, in different neuronal models, and the brain of Alzheimer's patients, leading to membrane trafficking defects and perturbation of neuronal function associated with cognition defects. The Golgi apparatus is a central transport and signaling hub at the crossroads of the secretory and endocytic pathways and perturbation of the Golgi ribbon structure is a hallmark of Alzheimer's disease. Here, we discuss the role of the Golgi as a major player in the regulation of amyloid-β production and propose that the Golgi apparatus plays a key role in a cellular network which can seed the onset of Alzheimer's disease. Moreover, we propose that the Golgi is central in an intracellular feedback loop leading to an enhanced level of amyloid-β production resulting in early neuronal defects before the appearance of clinical symptoms. Further advances in defining the molecular pathways of this intracellular feedback loop could support the design of new therapeutic strategies to target a primary source of neuronal toxicity in this disease.

Brain-derived tau oligomer polymorphs: distinct aggregations, stability profiles, and biological activities

Aggregation of microtubule-associated tau protein is a distinct hallmark of several neurodegenerative disorders such as Alzheimer's disease (AD), dementia with Lewy bodies (DLB), and progressive supranuclear palsy (PSP). Tau oligomers are suggested to be the primary neurotoxic species that initiate aggregation and propagate prion-like structures. Furthermore, different diseases are shown to have distinct structural characteristics of aggregated tau, denoted as polymorphs. Here, we investigate the structural and functional differences of amplified brain-derived tau oligomers (aBDTOs) from AD, DLB, and PSP. Our results indicate that the aBDTOs possess different structural and morphological features that impact neuronal function, gene regulation, and ultimately disease progression. The distinct tau oligomeric polymorphs may thus contribute to the development of clinical phenotypes and shape the progression of diseases. Our results can provide insight into developing personalized therapy to target a specific neurotoxic tau polymorph.

Differential DNA methylation profiles of Alzheimer's disease-related genomic pathways in the blood of cognitively-intact individuals with and without high impact chronic pain

Background

Chronic pain and Alzheimer's disease (AD) are prevalent in older age and their etiologies remain to be understoodand evidence supports potential associations between the two. Both high impact pain and AD have been previously associated with differences in the epigenome. Interactions with the epigenome may serve as a possible underlying mechanism linking high impact pain and AD.

Objective

To complete epigenetic canonical pathways analyses related to AD in individuals with and without high-impact knee pain.

Methods

This manuscript aimed to explore differences in DNA methylation patterns in genes and pathways associated with AD. Blood samples of cognitively intact, community-dwelling adults with high impact knee painmversus pain-free controls were compared on their DNA methylation levels of AD-related genes. Pathway enrichment analysis was performed on significantly different DNA Methylation probes by pain group.

Results

There were significant DNA methylation differences between the high impact versus the pain-free control groups in genes and pathways associated with AD (p < 0.05). We found a total of 17,563 differentially methylated CpG probes, including 13,411 hypermethylated CpG probes and 4152 hypomethylated CpG probes. Further, pathway analysis revealed these differences were significantly associated with AD-related pathways associated with AD progression.

Conclusions

This study sample showed AD-related DNA methylation differences and associated potential canonical pathways in those with and without high impact knee pain. These results highlight the need to study overlapping epigenetic modifications underlying high impact pain and AD pathologies. Further studies, including gene expression, are needed to further explore the relationship between epigenetics, chronic pain, and AD.

Untangling the Complexity and Impact of Tau Protein Ubiquitination

The microtubule-associated protein tau is an intrinsically disordered protein highly expressed in neuronal axons. In healthy neurons, tau regulates microtubule dynamics and neurite outgrowth. However, pathological conditions can trigger aberrant tau aggregation into insoluble filaments, a hallmark of neurodegenerative disorders known as tauopathies. Tau undergoes diverse posttranslational modifications (PTMs), suggesting complex regulation and potentially varied functions. Among PTMs, the role and mechanisms of ubiquitination in physiology and disease have remained enigmatic. The past three decades have witnessed the emergence of key studies on tau protein ubiquitination. In this concept, we discuss how these investigations have begun to shed light on the ubiquitination patterns of physiological and pathological tau, the responsible enzymatic machinery, and the influence of ubiquitination on tau aggregation. We also provide an overview of the semi-synthetic methods that have enabled in vitro investigations of conformational transitions of tau induced by ubiquitin modification. Finally, we discuss future perspectives in the field necessary to elucidate the molecular mechanisms of tau ubiquitination and clearance.

Defining Alzheimer's Disease through Proteomic CSF Profiling

Alzheimer disease (AD) is the main cause of dementia, and its complexity is not yet completely understood. Proteomic profiles can provide useful information to explore the pathways involved and the heterogeneity among AD patients. A proteomic analysis was performed in cerebrospinal fluid (CSF) samples from mild cognitive impairment due to AD (MCI-AD) and control individuals; both groups were classified by amyloid β42/amyloid β40 levels in CSF (data available in BioStudies database (S-BSST1456)). The analysis based on PLS regression and volcano plot identified 7 proteins (FOLR2, PPP3CA, SMOC2, STMN1, TAGLN3, TMEM132B, and UCHL1) mainly related to protein phosphorylation, structure maintenance, inflammation, and protein degradation. Enrichment analysis revealed the involvement of different biological processes related to neuronal mechanisms and synapses, lipid and carbohydrate metabolism, immune system and inflammation, vascular, hormones, and response to stimuli, and cell signaling and adhesion. In addition, the proteomic profile showed some association with the levels of AD biomarkers in CSF. Regarding the subtypes, two MCI-AD subgroups were identified: one could be related to synapsis and neuronal functions and the other to innate immunity. The study of the proteomic profile in the CSF of AD patients reflects the heterogeneity of biochemical pathways involved in AD.

MAD-microbial (origin of) Alzheimer's disease hypothesis: from infection and the antimicrobial response to disruption of key copper-based systems

Microbes have been suspected to cause Alzheimer's disease since at least 1908, but this has generally remained unpopular in comparison to the amyloid hypothesis and the dominance of Aβ and Tau. However, evidence has been accumulating to suggest that these earlier theories are but a manifestation of a common cause that can trigger and interact with all the major molecular players recognized in AD. Aβ, Tau and ApoE, in particular appear to be molecules with normal homeostatic functions but also with alternative antimicrobial functions. Their alternative functions confer the non-immune specialized neuron with some innate intracellular defenses that appear to be re-appropriated from their normal functions in times of need. Indeed, signs of infection of the neurons by biofilm-forming microbial colonies, in synergy with herpes viruses, are evident from the clinical and preclinical studies we discuss. Furthermore, we attempt to provide a mechanistic understanding of the AD landscape by discussing the antimicrobial effect of Aβ, Tau and ApoE and Lactoferrin in AD, and a possible mechanistic link with deficiency of vital copper-based systems. In particular, we focus on mitochondrial oxidative respiration via complex 4 and ceruloplasmin for iron homeostasis, and how this is similar and possibly central to neurodegenerative diseases in general. In the case of AD, we provide evidence for the microbial Alzheimer's disease (MAD) theory, namely that AD could in fact be caused by a long-term microbial exposure or even long-term infection of the neurons themselves that results in a costly prolonged antimicrobial response that disrupts copper-based systems that govern neurotransmission, iron homeostasis and respiration. Finally, we discuss potential treatment modalities based on this holistic understanding of AD that incorporates the many separate and seemingly conflicting theories. If the MAD theory is correct, then the reduction of microbial exposure through use of broad antimicrobial and anti-inflammatory treatments could potentially alleviate AD although this requires further clinical investigation.

Microtubule-associated protein, MAP1B, encodes functionally distinct polypeptides

Microtubule-associated protein, MAP1B, is crucial for neuronal morphogenesis and disruptions in MAP1B function are correlated with neurodevelopmental disorders. MAP1B encodes a single polypeptide that is processed into discrete proteins, a heavy chain (HC) and a light chain (LC); however, it is unclear if these two chains operate individually or as a complex within the cell. In vivo studies have characterized the contribution of MAP1B HC and LC to microtubule and actin-based processes, but their molecular mechanisms of action are unknown. Using in vitro reconstitution with purified proteins, we dissect the biophysical properties of the HC and LC and uncover distinct binding behaviors and functional roles for these MAPs. Our biochemical assays indicate that MAP1B HC and LC do not form a constitutive complex, supporting the hypothesis that these proteins operate independently within cells. Both HC and LC inhibit the microtubule motors, kinesin-3, kinesin-4, and dynein, and differentially affect the severing activity of spastin. Notably, MAP1B LC binds to actin filaments in vitro and can simultaneously bind and cross-link actin filaments and microtubules, a function not observed for MAP1B HC. Phosphorylation of MAP1B HC by dual-specificity, tyrosine phosphorylation-regulated kinase 1a negatively regulates its actin-binding activity without significantly affecting its microtubule-binding capacity, suggesting a dynamic contribution of MAP1B HC in cytoskeletal organization. Overall, our study provides new insights into the distinct functional properties of MAP1B HC and LC, underscoring their roles in coordinating cytoskeletal networks during neuronal development.

cUMP elicits interendothelial gap formation during Pseudomonas aeruginosa infection

Pseudomonas aeruginosa utilizes a type 3 secretion system to intoxicate host cells with the nucleotidyl cyclase ExoY. After activation by its host cell cofactor, filamentous actin, ExoY produces purine and pyrimidine cyclic nucleotides, including cAMP, cGMP, and cUMP. ExoY-generated cyclic nucleotides promote interendothelial gap formation, impair motility, and arrest cell growth. The disruptive activities of cAMP and cGMP during the P. aeruginosa infection are established; however, little is known about the function of cUMP. Here, we tested the hypothesis that cUMP contributes to endothelial cell barrier disruption during P. aeruginosa infection. Using a membrane permeable cUMP analog, cUMP-AM, we revealed that during infection with catalytically inactive ExoY, cUMP promotes interendothelial gap formation in cultured pulmonary microvascular endothelial cells (PMVECs) and contributes to increased filtration coefficient in the isolated perfused lung. These findings indicate that cUMP contributes to endothelial permeability during P. aeruginosa lung infection.NEW & NOTEWORTHY During pneumonia, bacteria utilize a virulence arsenal to communicate with host cells. The Pseudomonas aeruginosa T3SS directly introduces virulence molecules into the host cell cytoplasm. These molecules are enzymes that trigger interkingdom communication. One of the exoenzymes is a nucleotidyl cyclase that produces noncanonical cyclic nucleotides like cUMP. Little is known about how cUMP acts in the cell. Here we found that cUMP instigates pulmonary edema during Pseudomonas aeruginosa infection of the lung.

Prolonged depletion of profilin 1 or F-actin causes an adaptive response in microtubules

In addition to its well-established role in actin assembly, profilin 1 (PFN1) has been shown to bind to tubulin and alter microtubule growth. However, whether PFN1's predominant control over microtubules in cells occurs through direct regulation of tubulin or indirectly through the polymerization of actin has yet to be determined. Here, we manipulated PFN1 expression, actin filament assembly, and actomyosin contractility and showed that reducing any of these parameters for extended periods of time caused an adaptive response in the microtubule cytoskeleton, with the effect being significantly more pronounced in neuronal processes. All the observed changes to microtubules were reversible if actomyosin was restored, arguing that PFN1's regulation of microtubules occurs principally through actin. Moreover, the cytoskeletal modifications resulting from PFN1 depletion in neuronal processes affected microtubule-based transport and mimicked phenotypes that are linked to neurodegenerative disease. This demonstrates how defects in actin can cause compensatory responses in other cytoskeleton components, which in turn significantly alter cellular function.

Brain Slice Derived Nerve Fibers Grow along Microcontact Prints and are Stimulated by Beta-Amyloid(42)

BACKGROUND

Alzheimer's disease is characterized by extracellular beta-amyloid plaques, intraneuronal tau neurofibrillary tangles and excessive neurodegeneration. The mechanisms of neuron degeneration and the potential of these neurons to form new nerve fibers for compensation remain elusive. The present study aimed to evaluate the impact of beta-amyloid and tau on new formations of nerve fibers from mouse organotypic brain slices connected to collagen-based microcontact prints.

METHODS

Organotypic brain slices of postnatal day 8-10 wild-type mice were connected to established collagen-based microcontact prints loaded with polyornithine to enhance nerve fiber outgrowth. Human beta-amyloid(42) or P301S mutated aggregated tau was co-loaded to the prints. Nerve fibers were immunohistochemically stained with neurofilament antibodies. The physiological activity of outgrown neurites was tested with neurotracer MiniRuby, voltage-sensitive dye FluoVolt, and calcium-sensitive dye Rhod-4.

RESULTS

Immunohistochemical staining revealed newly formed nerve fibers extending along the prints derived from the brain slices. While collagen-only microcontact prints stimulated nerve fiber growth, those loaded with polyornithine significantly enhanced nerve fiber outgrowth. Beta-amyloid(42) significantly increased the neurofilament-positive nerve fibers, while tau had only a weak effect. MiniRuby crystals, retrogradely transported along these newly formed nerve fibers, reached the hippocampus, while FluoVolt and Rhod-4 monitored electrical activity in newly formed nerve fibers.

CONCLUSIONS

Our data provide evidence that intact nerve fibers can form along collagen-based microcontact prints from mouse brain slices. The Alzheimer's peptide beta-amyloid(42) stimulates this growth, hinting at a neuroprotective function when physiologically active. This "brain-on-chip" model may offer a platform for screening bioactive factors or testing drug effects on nerve fiber growth.

How cytoskeletal crosstalk makes cells move: Bridging cell-free and cell studies

Cell migration is a fundamental process for life and is highly dependent on the dynamical and mechanical properties of the cytoskeleton. Intensive physical and biochemical crosstalk among actin, microtubules, and intermediate filaments ensures their coordination to facilitate and enable migration. In this review, we discuss the different mechanical aspects that govern cell migration and provide, for each mechanical aspect, a novel perspective by juxtaposing two complementary approaches to the biophysical study of cytoskeletal crosstalk: live-cell studies (often referred to as top-down studies) and cell-free studies (often referred to as bottom-up studies). We summarize the main findings from both experimental approaches, and we provide our perspective on bridging the two perspectives to address the open questions of how cytoskeletal crosstalk governs cell migration and makes cells move.

Cell autonomous microglia defects in a stem cell model of frontotemporal dementia

Neuronal dysfunction has been extensively studied as a central feature of neurodegenerative tauopathies. However, across neurodegenerative diseases, there is strong evidence for active involvement of immune cells like microglia in driving disease pathophysiology. Here, we demonstrate that tau mRNA and protein are expressed in microglia in human brains and in human induced pluripotent stem cell (iPSC)-derived microglia like cells (iMGLs). Using iMGLs harboring the MAPT IVS10+16 mutation and isogenic controls, we demonstrate that a tau mutation is sufficient to alter microglial transcriptional states. We discovered that MAPT IVS10+16 microglia exhibit cytoskeletal abnormalities, stalled phagocytosis, disrupted TREM2/TYROBP networks, and altered metabolism. Additionally, we found that secretory factors from MAPT IVS10+16 iMGLs impact neuronal health, reducing synaptic density in neurons. Key features observed in vitro were recapitulated in human brain tissue and cerebrospinal fluid from MAPT mutations carriers. Together, our findings that MAPT IVS10+16 drives cell-intrinsic dysfunction in microglia that impacts neuronal health has major implications for development of therapeutic strategies.

Cytoskeletal crosstalk: A focus on intermediate filaments

The cytoskeleton, comprising actin microfilaments, microtubules, and intermediate filaments, is crucial for cell motility and tissue integrity. While prior studies largely focused on individual cytoskeletal networks, recent research underscores the interconnected nature of these systems in fundamental cellular functions like adhesion, migration, and division. Understanding the coordination of these distinct networks in both time and space is essential. This review synthesizes current findings on the intricate interplay between these networks, emphasizing the pivotal role of intermediate filaments. Notably, these filaments engage in extensive crosstalk with microfilaments and microtubules through direct molecular interactions, cytoskeletal linkers, and molecular motors that form molecular bridges, as well as via more complex regulation of intracellular signaling.

Collective dynamics of actin and microtubule and its crosstalk mediated by FHDC1

The coordination between actin and microtubule network is crucial, yet this remains a challenging problem to dissect and our understanding of the underlying mechanisms remains limited. In this study, we used travelling waves in the cell cortex to characterize the collective dynamics of cytoskeletal networks. Our findings show that Cdc42 and F-BAR-dependent actin waves in mast cells are mainly driven by formin-mediated actin polymerization, with the microtubule-binding formin FH2 domain-containing protein 1 (FHDC1) as an early regulator. Knocking down FHDC1 inhibits actin wave formation, and this inhibition require FHDC1's interaction with both microtubule and actin. The phase of microtubule depolymerization coincides with the nucleation of actin waves and microtubule stabilization inhibit actin waves, leading us to propose that microtubule shrinking and the concurrent release of FHDC1 locally regulate actin nucleation. Lastly, we show that FHDC1 is crucial for multiple cellular processes such as cell division and migration. Our data provided molecular insights into the nucleation mechanisms of actin waves and uncover an antagonistic interplay between microtubule and actin polymerization in their collective dynamics.

The role of microtubule-associated protein tau in netrin-1 attractive signaling

Direct binding of netrin receptors with dynamic microtubules (MTs) in the neuronal growth cone plays an important role in netrin-mediated axon guidance. However, how netrin-1 (NTN1) regulates MT dynamics in axon turning remains a major unanswered question. Here, we show that the coupling of netrin-1 receptor DCC with tau (MAPT)-regulated MTs is involved in netrin-1-promoted axon attraction. Tau directly interacts with DCC and partially overlaps with DCC in the growth cone of primary neurons. Netrin-1 induces this interaction and the colocalization of DCC and tau in the growth cone. The netrin-1-induced interaction of tau with DCC relies on MT dynamics and TUBB3, a highly dynamic β-tubulin isotype in developing neurons. Netrin-1 increased cosedimentation of DCC with tau and TUBB3 in MTs, and knockdown of either tau or TUBB3 mutually blocked this effect. Downregulation of endogenous tau levels by tau shRNAs inhibited netrin-1-induced axon outgrowth, branching and commissural axon attraction in vitro, and led to defects in spinal commissural axon projection in vivo. These findings suggest that tau is a key MT-associated protein coupling DCC with MT dynamics in netrin-1-promoted axon attraction.

Morphological and cytoskeleton changes in cells after EMT

Epithelial cells undergoing EMT experience significant alterations at transcriptional and morphological levels. However, changes in the cytoskeleton, especially cytoskeleton dynamics are poorly described. Addressing the question we induced EMT in three cell lines (MCF-7, HaCaT and A-549) and analyzed morphological and cytoskeletal changes there using immunostaining and life cell imaging of cells transfected with microtubule and focal adhesion markers. In all studied cell lines, cell area after EMT increased, MCF-7 and A-549 cells became elongated, while HaCaT cells kept the aspect ratio the same. We next analyzed three components of the cytoskeleton: microtubules, stress fibers and focal adhesions. The following changes were observed after EMT in cultured cells: (i) Organization of microtubules becomes more radial; and the growth rate of microtubule plus ends was accelerated; (ii) Actin stress fibers become co-aligned forming the longitudinal cell axis; and (iii) Focal adhesions had decreased area in all cancer cell lines studied and became more numerous in HaCaT cells. We conclude that among dynamic components of the cytoskeleton, the most significant changes during EMT happen in the regulation of microtubules.

Microtubules under mechanical pressure can breach dense actin networks

The crosstalk between the actin network and microtubules is essential for cell polarity. It orchestrates microtubule organization within the cell, driven by the asymmetry of actin architecture along the cell periphery. The physical intertwining of these networks regulates spatial organization and force distribution in the microtubule network. Although their biochemical interactions are becoming clearer, the mechanical aspects remain less understood. To explore this mechanical interplay, we developed an in vitro reconstitution assay to investigate how dynamic microtubules interact with various actin filament structures. Our findings revealed that microtubules can align and move along linear actin filament bundles through polymerization force. However, they are unable to pass through when encountering dense branched actin meshworks, similar to those present in the lamellipodium along the periphery of the cell. Interestingly, immobilizing microtubules through crosslinking with actin or other means allow the buildup of pressure, enabling them to breach these dense actin barriers. This mechanism offers insights into microtubule progression towards the cell periphery, with them overcoming obstacles within the denser parts of the actin network and ultimately contributing to cell polarity establishment.

Differential regulation of MAP2 by phosphorylation events in proline-rich versus C-terminal domains

MAP2 is a critical cytoskeletal regulator in neurons. The phosphorylation of MAP2 (MAP2-P) is well known to regulate core functions of MAP2, including microtubule (MT)/actin binding and facilitation of tubulin polymerization. However, site-specific studies of MAP2-P function in regions outside of the MT-binding domain (MTBD) are lacking. We previously identified a set of MAP2 phosphopeptides which are differentially expressed and predominantly increased in the cortex of individuals with schizophrenia relative to nonpsychiatric comparison subjects. The phosphopeptides originated not from the MTBD, but from the flanking proline-rich and C-terminal domains of MAP2. We sought to understand the contribution of MAP2-P at these sites on MAP2 function. To this end, we isolated a series of phosphomimetic MAP2C constructs and subjected them to cell-free tubulin polymerization, MT-binding, actin-binding, and actin polymerization assays. A subset of MAP2-P events significantly impaired these functions, with the two domains displaying different patterns of MAP2 regulation: proline-rich domain mutants T293E and T300E impaired MT assembly and actin-binding affinity but did not affect MT-binding, while C-terminal domain mutants S426E and S439D impaired all three functions. S443D also impaired MT assembly with minimal effects on MT- or actin-binding. Using heterologous cells, we also found that S426E but not T293E had a lower capability for process formation than the wild-type protein. These findings demonstrate the functional utility of MAP2-P in the proline-rich and C-terminal domains and point to distinct, domain-dependent regulations of MAP2 function, which can go on to affect cellular morphology.

Cytoskeletal adaptation following long-term dysregulation of actomyosin in neuronal processes

Microtubules, intermediate filaments, and actin are cytoskeletal polymer networks found within the cell. While each has unique functions, all the cytoskeletal elements must work together for cellular mechanics to be fully operative. This is achieved through crosstalk mechanisms whereby the different networks influence each other through signaling pathways and direct interactions. Because crosstalk can be complex, it is possible for perturbations in one cytoskeletal element to affect the others in ways that are difficult to predict. Here we investigated how long-term changes to the actin cytoskeleton affect microtubules and intermediate filaments. Reducing F-actin or actomyosin contractility increased acetylated microtubules and intermediate filament expression, with the effect being significantly more pronounced in neuronal processes. Changes to microtubules were completely reversible if F-actin and myosin activity is restored. Moreover, the altered microtubules in neuronal processes resulting from F-actin depletion caused significant changes to microtubule-based transport, mimicking phenotypes that are linked to neurodegenerative disease. Thus, defects in actin dynamics cause a compensatory response in other cytoskeleton components which profoundly alters cellular function.

Tau, microtubule dynamics, and axonal transport: New paradigms for neurodegenerative disease

The etiology of Tauopathies, a diverse class of neurodegenerative diseases associated with the Microtubule Associated Protein (MAP) Tau, is usually described by a common mechanism in which Tau dysfunction results in the loss of axonal microtubule stability. Here, we reexamine and build upon the canonical disease model to encompass other Tau functions. In addition to regulating microtubule dynamics, Tau acts as a modulator of motor proteins, a signaling hub, and a scaffolding protein. This diverse array of functions is related to the dynamic nature of Tau isoform expression, post-translational modification (PTM), and conformational flexibility. Thus, there is no single mechanism that can describe Tau dysfunction. The effects of specific pathogenic mutations or aberrant PTMs need to be examined on all of the various functions of Tau in order to understand the unique etiology of each disease state.

Transfusion with Blood Plasma from Young Mice Affects rTg4510 Transgenic Tau Mice Modeling of Alzheimer's Disease

Alzheimer's disease (AD) is characterized by the buildup of plaques and tangles in the brain. Tangles are formed when the stabilizing protein, tau, becomes hyperphosphorylated and clumps together. There are limited treatments for AD; therefore, the exploration of new treatments is warranted. Previous research showed that plasma transfusion from young donor mice improved spatial memory and increased synaptic proteins in old transgenic APP/PS1 mice, suggesting a remediation of memory and synaptic function. In the current study, plasma was transfused from 2-3-month-old young wildtype mice (WT) to 8-month-old rTg4510 mice expressing human tau (Tau). One week after the transfusions, behavior and tau pathology were examined. We found that Tau mice injected with plasma had lower expression of phosphorylated tau (ptau) in the brain, accompanied by fewer tau tangles in the cortex and CA1 region of the hippocampus and smaller tau tangles in the cortex, when compared to Tau mice injected with saline. Despite no improvement in behavior, the decreased level of ptau and tangles open the door to future studies involving plasma transfusions.

Microtubule remodelling as a driving force of axon guidance and pruning

The establishment of neuronal connectivity relies on the microtubule (MT) cytoskeleton, which provides mechanical support, roads for axonal transport and mediates signalling events. Fine-tuned spatiotemporal regulation of MT functions by tubulin post-translational modifications and MT-associated proteins is critical for the coarse wiring and subsequent refinement of neuronal connectivity. The defective regulation of these processes causes a wide range of neurodevelopmental disorders associated with connectivity defects. This review focuses on recent studies unravelling how MT composition, post-translational modifications and associated proteins influence MT functions in axon guidance and/or pruning to build functional neuronal circuits. We here summarise experimental evidence supporting the key role of this network as a driving force for growth cone steering and branch-specific axon elimination. We further provide a global overview of the MT-interactors that tune developing axon behaviours, with a special emphasis on their emerging versatility in the regulation of MT dynamics/structure. Recent studies establishing the key and highly selective role of the tubulin code in the regulation of MT functions in axon pathfinding are also reported. Finally, our review highlights the emerging molecular links between these MT regulation processes and guidance signals that wire the nervous system.

Evaluating the Association Between Genetically Proxied Neurodevelopmental Language Phenotypes and the Risk of Primary Progressive Aphasia

BACKGROUND AND OBJECTIVES

Primary progressive aphasia (PPA) is a neurodegenerative syndrome of progressive language decline. PPA has 3 main subtypes: logopenic, semantic, and agrammatic. Observational studies suggested an association between language-related neurodevelopmental phenotypes and an increased risk of PPA. We sought to assess such relationships through Mendelian randomization (MR) approach, which can suggest potentially causal associations.

METHODS

Genome-wide significant single-nucleotide polymorphisms (SNPs) associated with dyslexia (42 SNPs), developmental speech disorders (29 SNPs), and left-handedness (41 SNPs) were used as genetic proxies for the exposures. Eighteen of 41 SNPs of left-handedness were associated with structural asymmetry of the cerebral cortex. Genome-wide association study summary statistics were obtained from publicly available databases for semantic (308 cases/616 controls) and agrammatic PPA (269 cases/538 controls). The logopenic PPA (324 cases/3,444 controls) was approximated by proxy through the rubric of clinically diagnosed Alzheimer disease with salient language impairment. Inverse-weighted variance MR was performed as the main analysis for testing the relationship between the exposures and outcomes. Sensitivity analyses were completed to test the robustness of the results.

RESULTS

Dyslexia, developmental speech disorders, and left-handedness were not associated with any PPA subtype (p > 0.05). The genetic proxy of cortical asymmetry in left-handedness was significantly associated with agrammatic PPA (β = 4.3, p = 0.007), but not with other PPA subtypes. This association was driven by microtubule-related genes, primarily by a variant that is in complete linkage disequilibrium with MAPT gene. Sensitivity analyses were overall consistent with the primary analyses.

DISCUSSION

Our results do not support a causal association between dyslexia, developmental speech disorders, and handedness with any of the PPA subtypes. Our data suggest a complex association between cortical asymmetry genes and agrammatic PPA. Whether the additional association with left-handedness is necessary remains to be determined but is unlikely, given the absence of association between left-handedness and PPA. Genetic proxy of brain asymmetry (regardless of handedness) was not tested as an exposure due to lack of suitable genetic proxy. Furthermore, the genes related to cortical asymmetry associated with agrammatic PPA are implicated in microtubule-related proteins (TUBA1B, TUBB, and MAPT), which is keeping with the association of tau-related neurodegeneration in this PPA variant.

CLASP2 facilitates dynamic actin filament organization along the microtubule lattice

Coordination between the microtubule and actin networks is essential for cell motility, neuronal growth cone guidance, and wound healing. Members of the CLASP (cytoplasmic linker-associated protein) family of proteins have been implicated in the cytoskeletal cross-talk between microtubules and actin networks; however, the molecular mechanisms underlying the role of CLASP in cytoskeletal coordination are unclear. Here, we investigate CLASP2α's cross-linking function with microtubules and F-actin. Our results demonstrate that CLASP2α cross-links F-actin to the microtubule lattice in vitro. We find that the cross-linking ability is retained by L-TOG2-S, a minimal construct containing the TOG2 domain and serine-arginine-rich region of CLASP2α. Furthermore, CLASP2α promotes the accumulation of multiple actin filaments along the microtubule, supporting up to 11 F-actin landing events on a single microtubule lattice region. CLASP2α also facilitates the dynamic organization of polymerizing actin filaments templated by the microtubule network, with F-actin forming bridges between individual microtubules. Finally, we find that depletion of CLASPs in vascular smooth muscle cells results in disorganized actin fibers and reduced coalignment of actin fibers with microtubules, suggesting that CLASP and microtubules contribute to higher-order actin structures. Taken together, our results indicate that CLASP2α can directly cross-link F-actin to microtubules and that this microtubule-CLASP-actin interaction may influence overall cytoskeletal organization in cells.

Neuroprotective Effects of a Multi-Herbal Extract on Axonal and Synaptic Disruption in Vitro and Cognitive Impairment in Vivo

Background

Alzheimer's disease (AD) is a multifactorial disorder characterized by cognitive decline. Current available therapeutics for AD have limited clinical benefit. Therefore, preventive therapies for interrupting the development of AD are critically needed. Molecules targeting multifunction to interact with various pathlogical components have been considered to improve the therapeutic efficiency of AD. In particular, herbal medicines with multiplicity of actions produce cognitive benefits on AD. Bugu-M is a multi-herbal extract composed of Ganoderma lucidum (Antler form), Nelumbo nucifera Gaertn., Ziziphus jujuba Mill., and Dimocarpus longan, with the ability of its various components to confer resilience to cognitive deficits.

Objective

To evaluate the potential of Bugu-M on amyloid-β (Aβ) toxicity and its in vitro mechanisms and on in vivo cognitive function.

Methods

We illustrated the effect of Bugu-M on Aβ_25-35-evoked toxicity as well as its possible mechanisms to diminish the pathogenesis of AD in rat cortical neurons. For cognitive function studies, 2-month-old female 3×Tg-AD mice were administered 400 mg/kg Bugu-M for 30 days. Behavioral tests were performed to assess the efficacy of Bugu-M on cognitive impairment.

Results

In primary cortical neuronal cultures, Bugu-M mitigated Aβ-evoked toxicity by reducing cytoskeletal aberrations and axonal disruption, restoring presynaptic and postsynaptic protein expression, suppressing mitochondrial damage and apoptotic signaling, and reserving neurogenic and neurotrophic factors. Importantly, 30-day administration of Bugu-M effectively prevented development of cognitive impairment in 3-month-old female 3×Tg-AD mice.

Conclusion

Bugu-M might be beneficial in delaying the progression of AD, and thus warrants consideration for its preventive potential for AD.

Tau; One Protein, So Many Diseases

Tau, a member of the microtubule-associated proteins, is a known component of the neuronal cytoskeleton; however, in the brain tissue, it is involved in other vital functions beyond maintaining the cellular architecture. The pathologic tau forms aggregates inside the neurons and ultimately forms the neurofibrillary tangles. Intracellular and extracellular accumulation of different tau isoforms, including dimers, oligomers, paired helical filaments and tangles, lead to a highly heterogenous group of diseases named "Tauopathies". About twenty-six different types of tauopathy diseases have been identified that have different clinical phenotypes or pathophysiological characteristics. Although all these diseases are identified by tau aggregation, they are distinguishable based on the specific tau isoforms, the affected cell types and the brain regions. The neuropathological and phenotypical heterogeneity of these diseases impose significant challenges for discovering new diagnostic and therapeutic strategies. Here, we review the recent literature on tau protein and the pathophysiological mechanisms of tauopathies. This article mainly focuses on physiologic and pathologic tau and aims to summarize the upstream and downstream events and discuss the current diagnostic approaches and therapeutic strategies.

Actin limits egg aneuploidies associated with female reproductive aging

Aging-related centromeric cohesion loss underlies premature separation of sister chromatids and egg aneuploidy in reproductively older females. Here, we show that F-actin maintains chromatid association after cohesion deterioration in aged eggs. F-actin disruption in aged mouse eggs exacerbated untimely dissociation of sister chromatids, while its removal in young eggs induced extensive chromatid separation events generally only seen in advanced reproductive ages. In young eggs containing experimentally reduced cohesion, F-actin removal accelerated premature splitting and scattering of sister chromatids in a microtubule dynamics-dependent manner, suggesting that actin counteracts chromatid-pulling spindle forces. Consistently, F-actin stabilization restricted scattering of unpaired chromatids generated by complete degradation of centromeric cohesion proteins. We conclude that actin mitigates egg aneuploidies arising from age-related cohesion depletion by limiting microtubule-driven separation and dispersion of sister chromatids. This is supported by our finding that spindle-associated F-actin structures are disrupted in eggs of reproductively older females.

Lumenal components of cytoplasmic microtubules

The lumen of cytoplasmic microtubules is a poorly explored expanse of intracellular space. Although typically represented in textbooks as a hollow tube, studies over several decades have shown that the microtubule lumen is occupied by a range of morphologically diverse components. These are predominantly globular particles of varying sizes which appear to exist either in isolation, bind to the microtubule wall, or form discontinuous columns that extend through the lumenal space. Actin filaments with morphologies distinct from the canonical cytoplasmic forms have also now been found within the microtubule lumen. In this review, we examine the historic literature that observed these lumenal components in tissues from diverse species and integrate it with recent cryo-electron tomography studies that have begun to identify lumenal proteins. We consider their cell and tissue distribution, possible mechanisms of incorporation, and potential functions. It is likely that continuing work in this area will open a new frontier in cytoskeletal biology.

Septins mediate a microtubule-actin crosstalk that enables actin growth on microtubules

Cellular morphogenesis and processes such as cell division and migration require the coordination of the microtubule and actin cytoskeletons. Microtubule-actin crosstalk is poorly understood and largely regarded as the capture and regulation of microtubules by actin. Septins are filamentous guanosine-5'-triphosphate (GTP) binding proteins, which comprise the fourth component of the cytoskeleton along microtubules, actin, and intermediate filaments. Here, we report that septins mediate microtubule-actin crosstalk by coupling actin polymerization to microtubule lattices. Superresolution and platinum replica electron microscopy (PREM) show that septins localize to overlapping microtubules and actin filaments in the growth cones of neurons and non-neuronal cells. We demonstrate that recombinant septin complexes directly crosslink microtubules and actin filaments into hybrid bundles. In vitro reconstitution assays reveal that microtubule-bound septins capture and align stable actin filaments with microtubules. Strikingly, septins enable the capture and polymerization of growing actin filaments on microtubule lattices. In neuronal growth cones, septins are required for the maintenance of the peripheral actin network that fans out from microtubules. These findings show that septins directly mediate microtubule interactions with actin filaments, and reveal a mechanism of microtubule-templated actin growth with broader significance for the self-organization of the cytoskeleton and cellular morphogenesis.

Huntingtin and Other Neurodegeneration-Associated Proteins in the Development of Intracellular Pathologies: Potential Target Search for Therapeutic Intervention

Neurodegenerative diseases are currently incurable. Numerous experimental data accumulated over the past fifty years have brought us closer to understanding the molecular and cell mechanisms responsible for their development. However, these data are not enough for a complete understanding of the genesis of these diseases, nor to suggest treatment methods. It turns out that many cellular pathologies developing during neurodegeneration coincide from disease to disease. These observations give hope to finding a common intracellular target(s) and to offering a universal method of treatment. In this review, we attempt to analyze data on similar cellular disorders among neurodegenerative diseases in general, and polyglutamine neurodegenerative diseases in particular, focusing on the interaction of various proteins involved in the development of neurodegenerative diseases with various cellular organelles. The main purposes of this review are: (1) to outline the spectrum of common intracellular pathologies and to answer the question of whether it is possible to find potential universal target(s) for therapeutic intervention; (2) to identify specific intracellular pathologies and to speculate about a possible general approach for their treatment.

Amyloids on Membrane Interfaces: Implications for Neurodegeneration

Membrane interfaces are vital for various cellular processes, and their involvement in neurodegenerative disorders such as Alzheimer's and Parkinson's disease has taken precedence in recent years. The amyloidogenic proteins associated with neurodegenerative diseases interact with the neuronal membrane through various means, which has implications for both the onset and progression of the disease. The parameters that regulate the interaction between the membrane and the amyloids remain poorly understood. The review focuses on the various aspects of membrane interactions of amyloids, particularly amyloid-β (Aβ) peptides and Tau involved in Alzheimer's and α-synuclein involved in Parkinson's disease. The genetic, cell biological, biochemical, and biophysical studies that form the basis for our current understanding of the membrane interactions of Aβ peptides, Tau, and α-synuclein are discussed.

Controlled Tau Cleavage in Cells Reveals Abnormal Localizations of Tau Fragments

In several forms of dementia, such as Alzheimer's disease, the cytoskeleton-associated protein tau undergoes proteolysis, giving rise to fragments that have a toxic impact on neuronal homeostasis. How these fragments interact with cellular structures, in particular with the cytoskeleton, is currently incompletely understood. Here, we developed a method, derived from a Tobacco Etch Virus (TEV) protease system, to induce controlled cleavage of tau at specific sites. Five tau proteins containing specific TEV recognition sites corresponding to pathological proteolytic sites were engineered, and tagged with GFP at one end and mCherry at the other. Following controlled cleavage to produce GFP-N-terminal and C-terminal-mCherry fragments, we followed the fate of tau fragments in cells. Our results showed that whole engineered tau proteins associate with the cytoskeleton similarly to the non-modified tau, whereas tau fragments adopted different localizations with respect to the actin and microtubule cytoskeletons. These distinct localizations were confirmed by expressing each separate fragment in cells. Some cleavages - in particular cleavages at amino-acid positions 124 or 256 - displayed a certain level of cellular toxicity, with an unusual relocalization of the N-terminal fragments to the nucleus. Based on the data presented here, inducible cleavage of tau by the TEV protease appears to be a valuable tool to reproduce tau fragmentation in cells and study the resulting consequences on cell physiology.

CPNE7 regenerates periodontal ligament via TAU-mediated alignment and cementum attachment protein-mediated attachment

AIM

Once the periodontal ligament (PDL) is damaged, it is difficult to regenerate its characteristic structure. Copine7 (CPNE7) reportedly plays a functional role in supporting periodontal attachment and PDL alignment. Here we demonstrate the regulatory mechanism of CPNE7 coordination with cytoskeleton reorganization and cementum attachment protein (CAP)-mediated attachment in PDL regeneration.

MATERIALS AND METHODS

The expression and localization of CPNE7, α-TUBULIN, ACTIN, and microtubule associated protein tau (TAU) were investigated in vitro. The effects of recombinant CPNE7 (rCPNE7) and CPNE7-derived peptides (CPNE7-DP) on the regulation of CAP were analysed in vitro, and PDL repair capacity was analysed in vivo.

RESULTS

CPNE7 co-localized with F-ACTIN and induced α-TUBULIN expansion to the edge of human PDL cells (hPDLCs). ACTIN and α-TUBULIN protein expressions were not elevated in rCPNE7-treated hPDLCs. rCPNE7 elevated the protein expression of TAU, which co-localized with F-ACTIN and α-TUBULIN. Replantation studies on mice revealed that well-attached and well-aligned PDLs were repaired in the rCPNE7 group. CPNE7-DP directly up-regulate the expression of CAP in vitro and promote PDL regeneration in three-wall defect canine models in vivo.

CONCLUSIONS

Our findings suggest that CPNE7 helps in PDL repair by supporting PDL alignment through TAU-mediated cytoskeleton reorganization and direct regulation of CAP-mediated PDL attachments of PDLCs.

Reducing PDK1/Akt Activity: An Effective Therapeutic Target in the Treatment of Alzheimer's Disease

Alzheimer’s disease (AD) is a common age-related neurodegenerative disease that leads to memory loss and cognitive function damage due to intracerebral neurofibrillary tangles (NFTs) and amyloid-β (Aβ) protein deposition. The phosphoinositide-dependent protein kinase (PDK1)/protein kinase B (Akt) signaling pathway plays a significant role in neuronal differentiation, synaptic plasticity, neuronal survival, and neurotransmission via the axon–dendrite axis. The phosphorylation of PDK1 and Akt rises in the brain, resulting in phosphorylation of the TNF-α-converting enzyme (TACE) at its cytoplasmic tail (the C-terminal end), changing its internalization as well as its trafficking. The current review aimed to explain the mechanisms of the PDK1/Akt/TACE signaling axis that exerts its modulatory effect on AD physiopathology. We provide an overview of the neuropathological features, genetics, Aβ aggregation, Tau protein hyperphosphorylation, neuroinflammation, and aging in the AD brain. Additionally, we summarized the phosphoinositide 3-kinase (PI3K)/PDK1/Akt pathway-related features and its molecular mechanism that is dependent on TACE in the pathogenesis of AD. This study reviewed the relationship between the PDK1/Akt signaling pathway and AD, and discussed the role of PDK1/Akt in resisting neuronal toxicity by suppressing TACE expression in the cell membrane. This work also provides a perspective for developing new therapeutics targeting PDK1/Akt and TACE for the treatment of AD.

Macromolecular structures and proteins interacting with the microtubule associated tau protein

It is well established that neurodegenerative diseases known as tauopathies are characterized by the presence of filamentous forms of phosphorylated tau protein inside neurons. However, the causal relationship between the initial symptoms of a particular disease and the molecular events affecting tau and leading to the appearance of tangles of filamentous forms of this protein remains unknown. Even the main function (or functions) of tau inside neurons is debatable and controversial. Tau seems to be a multifunctional protein. I review here some of the most studied interactions of tau with different macromolecules and proteins, which can be classified according to the structural o functional unit within which the interaction works: Microtubule, Nuclear localization and DNA, Synaptic activity, RNA metabolism, Fats transport, Proteostasis, Amyloid Cascade Hypothesis, Mitochondria and Phosphorylation. Although this seems to be a broad spectrum of tau functions, interactome studies of tau reveal hundreds of plausible partners of tau, suggesting that it engages in an extensive network of interconnected regulatory interactions by means of its high capability to interact with all kinds of proteins and complex structures, combined with its vast number of post-translational modifications. I include also some thermodynamic data concerning the interaction of tau with some partners.

Tauopathies: the role of tau in cellular crosstalk and synaptic dysfunctions

Tauopathies are a group of neurodegenerative diseases among which are many of the most prevalent and with higher incidence worldwide, such as Alzheimer's disease (AD). According to the World Health Organization, this set of diseases will continue to increase their incidence, affecting millions of people by 2050. All of them are characterized by aberrant aggregation of tau protein in neurons and glia that are distributed in different brain regions according to their susceptibility. Numerous studies reveal that synaptic regulation not only has a neuronal component, but glia plays a fundamental role in it beyond its neuroinflammatory role. Despite this, it has not been emphasized how the glial inclusions of tau in this cell type directly affect this and many other essential functions, whose alterations have been related to the development of tauopathies. In this way, this review shows how tau inclusions in glia influence the synaptic dysfunctions that result in the cognitive symptoms characteristic of tauopathies. Thus, the mechanisms affected by inclusions in neurons, astrocytes, and oligodendrocytes are unraveled.

Microtubule-associated protein tau in murine kidney: role in podocyte architecture

Tau is a cytoskeletal protein that is expressed mainly in neurons and is involved in several cellular processes, such as microtubule stabilization, axonal maintenance, and transport. Altered tau metabolism is related to different tauopathies being the most important Alzheimer's disease where aberrant hyperphosphorylated and aggregated tau is found in the central nervous system. Here, we have analyzed that function in kidney by using tau knockout mice generated by integrating GFP-encoding cDNA into exon 1 of MAPT (here referred to as TauGFP/GFP). IVIS Lumina from PerkinElmer demonstrated GFP expression in the kidney. We then demonstrated by qPCR that the main tau isoform in the kidney is Tau4R. The GFP reporter allowed us to demonstrate that tau is found in the glomeruli of the renal cortex, and specifically in podocytes. This was further confirmed by immunohistochemistry. TauGFP/GFP mice present a podocyte cytoskeleton more dynamic as they contain higher levels of detyrosinated tubulin than wild-type mice. In addition, transmission electron microscopy studies demonstrated glomerular damage with a decrease in urinary creatinine. Our results prove that tau has an important role in kidney metabolism under normal physiological conditions.

The balance of mitochondrial fission and fusion in cortical axons depends on the kinases SadA and SadB

Neurons are highly polarized cells that display characteristic differences in the organization of their organelles in axons and dendrites. The kinases SadA and SadB (SadA/B) promote the formation of distinct axonal and dendritic extensions during the development of cortical and hippocampal neurons. Here, we show that SadA/B are required for the specific dynamics of axonal mitochondria. Ankyrin B (AnkB) stimulates the activity of SadA/B that function as regulators of mitochondrial dynamics through the phosphorylation of tau. Suppression of SadA/B or AnkB in cortical neurons induces the elongation of mitochondria by disrupting the balance of fission and fusion. SadA/B-deficient neurons show an accumulation of hyper-fused mitochondria and activation of the integrated stress response (ISR). The normal dynamics of axonal mitochondria could be restored by mild actin destabilization. Thus, the elongation after loss of SadA/B results from an excessive stabilization of actin filaments and reduction of Drp1 recruitment to mitochondria.

With the Permission of Microtubules: An Updated Overview on Microtubule Function During Axon Pathfinding

During the establishment of neural circuitry axons often need to cover long distances to reach remote targets. The stereotyped navigation of these axons defines the connectivity between brain regions and cellular subtypes. This chemotrophic guidance process mostly relies on the spatio-temporal expression patterns of extracellular proteins and the selective expression of their receptors in projection neurons. Axon guidance is stimulated by guidance proteins and implemented by neuronal traction forces at the growth cones, which engage local cytoskeleton regulators and cell adhesion proteins. Different layers of guidance signaling regulation, such as the cleavage and processing of receptors, the expression of co-receptors and a wide variety of intracellular cascades downstream of receptors activation, have been progressively unveiled. Also, in the last decades, the regulation of microtubule (MT) assembly, stability and interactions with the submembranous actin network in the growth cone have emerged as crucial effector mechanisms in axon pathfinding. In this review, we will delve into the intracellular signaling cascades downstream of guidance receptors that converge on the MT cytoskeleton of the growing axon. In particular, we will focus on the microtubule-associated proteins (MAPs) network responsible of MT dynamics in the axon and growth cone. Complementarily, we will discuss new evidences that connect defects in MT scaffold proteins, MAPs or MT-based motors and axon misrouting during brain development.

Presenilin 1 phosphorylation regulates amyloid-β degradation by microglia

Amyloid-β peptide (Aβ) accumulation in the brain is a hallmark of Alzheimer's Disease. An important mechanism of Aβ clearance in the brain is uptake and degradation by microglia. Presenilin 1 (PS1) is the catalytic subunit of γ-secretase, an enzyme complex responsible for the maturation of multiple substrates, such as Aβ. Although PS1 has been extensively studied in neurons, the role of PS1 in microglia is incompletely understood. Here we report that microglia containing phospho-deficient mutant PS1 display a slower kinetic response to micro injury in the brain in vivo and the inability to degrade Aβ oligomers due to a phagolysosome dysfunction. An Alzheimer's mouse model containing phospho-deficient PS1 show severe Aβ accumulation in microglia as well as the postsynaptic protein PSD95. Our results demonstrate a novel mechanism by which PS1 modulates microglial function and contributes to Alzheimer's -associated phenotypes.

Synaptic tau: A pathological or physiological phenomenon?

In this review, we discuss the synaptic aspects of Tau pathology occurring during Alzheimer's disease (AD) and how this may relate to memory impairment, a major hallmark of AD. Whilst the clinical diagnosis of AD patients is a loss of working memory and long-term declarative memory, the histological diagnosis is the presence of neurofibrillary tangles of hyperphosphorylated Tau and Amyloid-beta plaques. Tau pathology spreads through synaptically connected neurons to impair synaptic function preceding the formation of neurofibrillary tangles, synaptic loss, axonal retraction and cell death. Alongside synaptic pathology, recent data suggest that Tau has physiological roles in the pre- or post- synaptic compartments. Thus, we have seen a shift in the research focus from Tau as a microtubule-stabilising protein in axons, to Tau as a synaptic protein with roles in accelerating spine formation, dendritic elongation, and in synaptic plasticity coordinating memory pathways. We collate here the myriad of emerging interactions and physiological roles of synaptic Tau, and discuss the current evidence that synaptic Tau contributes to pathology in AD.

Alzheimer's Disease: A Molecular View of β-Amyloid Induced Morbific Events

Amyloid-β (Aβ) is a dynamic peptide of Alzheimer’s disease (AD) which accelerates the disease progression. At the cell membrane and cell compartments, the amyloid precursor protein (APP) undergoes amyloidogenic cleavage by β- and γ-secretases and engenders the Aβ. In addition, externally produced Aβ gets inside the cells by receptors mediated internalization. An elevated amount of Aβ yields spontaneous aggregation which causes organelles impairment. Aβ stimulates the hyperphosphorylation of tau protein via acceleration by several kinases. Aβ travels to the mitochondria and interacts with its functional complexes, which impairs the mitochondrial function leading to the activation of apoptotic signaling cascade. Aβ disrupts the Ca²⁺ and protein homeostasis of the endoplasmic reticulum (ER) and Golgi complex (GC) that promotes the organelle stress and inhibits its stress recovery machinery such as unfolded protein response (UPR) and ER-associated degradation (ERAD). At lysosome, Aβ precedes autophagy dysfunction upon interacting with autophagy molecules. Interestingly, Aβ act as a transcription regulator as well as inhibits telomerase activity. Both Aβ and p-tau interaction with neuronal and glial receptors elevate the inflammatory molecules and persuade inflammation. Here, we have expounded the Aβ mediated events in the cells and its cosmopolitan role on neurodegeneration, and the current clinical status of anti-amyloid therapy.

Tau Protein Interaction Partners and Their Roles in Alzheimer's Disease and Other Tauopathies

Tau protein plays a critical role in the assembly, stabilization, and modulation of microtubules, which are important for the normal function of neurons and the brain. In diseased conditions, several pathological modifications of tau protein manifest. These changes lead to tau protein aggregation and the formation of paired helical filaments (PHF) and neurofibrillary tangles (NFT), which are common hallmarks of Alzheimer's disease and other tauopathies. The accumulation of PHFs and NFTs results in impairment of physiological functions, apoptosis, and neuronal loss, which is reflected as cognitive impairment, and in the late stages of the disease, leads to death. The causes of this pathological transformation of tau protein haven't been fully understood yet. In both physiological and pathological conditions, tau interacts with several proteins which maintain their proper function or can participate in their pathological modifications. Interaction partners of tau protein and associated molecular pathways can either initiate and drive the tau pathology or can act neuroprotective, by reducing pathological tau proteins or inflammation. In this review, we focus on the tau as a multifunctional protein and its known interacting partners active in regulations of different processes and the roles of these proteins in Alzheimer's disease and tauopathies.

Cytoskeletal Arrest: An Anoxia Tolerance Mechanism

Polymerization of actin filaments and microtubules constitutes a ubiquitous demand for cellular adenosine-5′-triphosphate (ATP) and guanosine-5′-triphosphate (GTP). In anoxia-tolerant animals, ATP consumption is minimized during overwintering conditions, but little is known about the role of cell structure in anoxia tolerance. Studies of overwintering mammals have revealed that microtubule stability in neurites is reduced at low temperature, resulting in withdrawal of neurites and reduced abundance of excitatory synapses. Literature for turtles is consistent with a similar downregulation of peripheral cytoskeletal activity in brain and liver during anoxic overwintering. Downregulation of actin dynamics, as well as modification to microtubule organization, may play vital roles in facilitating anoxia tolerance. Mitochondrial calcium release occurs during anoxia in turtle neurons, and subsequent activation of calcium-binding proteins likely regulates cytoskeletal stability. Production of reactive oxygen species (ROS) formation can lead to catastrophic cytoskeletal damage during overwintering and ROS production can be regulated by the dynamics of mitochondrial interconnectivity. Therefore, suppression of ROS formation is likely an important aspect of cytoskeletal arrest. Furthermore, gasotransmitters can regulate ROS levels, as well as cytoskeletal contractility and rearrangement. In this review we will explore the energetic costs of cytoskeletal activity, the cellular mechanisms regulating it, and the potential for cytoskeletal arrest being an important mechanism permitting long-term anoxia survival in anoxia-tolerant species, such as the western painted turtle and goldfish.

New twists in actin-microtubule interactions

Actin filaments and microtubules are cytoskeletal polymers that participate in many vital cell functions including division, morphogenesis, phagocytosis, and motility. Despite the persistent dogma that actin filament and microtubule networks are distinct in localization, structure, and function, a growing body of evidence shows that these elements are choreographed through intricate mechanisms sensitive to either polymer. Many proteins and cellular signals that mediate actin–microtubule interactions have already been identified. However, the impact of these regulators is typically assessed with actin filament or microtubule polymers alone, independent of the other system. Further, unconventional modes and regulators coordinating actin–microtubule interactions are still being discovered. Here we examine several methods of actin–microtubule crosstalk with an emphasis on the molecular links between both polymer systems and their higher-order interactions.

The axonal radial contractility: Structural basis underlying a new form of neural plasticity

Axons are the longest cellular structure reaching over a meter in the case of human motor axons. They have a relatively small diameter and contain several cytoskeletal elements that mediate both material and information exchange within neurons. Recently, a novel type of axonal plasticity, termed axonal radial contractility, has been unveiled. It is represented by dynamic and transient diameter changes of the axon shaft to accommodate the passages of large organelles. Mechanisms underpinning this plasticity are not fully understood. Here, we first summarised recent evidence of the functional relevance for axon radial contractility, then discussed the underlying structural basis, reviewing nanoscopic evidence of the subtle changes. Two models are proposed to explain how actomyosin rings are organised. Possible roles of non-muscle myosin II (NM-II) in axon degeneration are discussed. Finally, we discuss the concept of periodic functional nanodomains, which could sense extracellular cues and coordinate the axonal responses. Also see the video abstract here: https://youtu.be/ojCnrJ8RCRc.

Cytoskeletal players in single-cell branching morphogenesis

Branching networks are a very common feature of multicellular animals and underlie the formation and function of numerous organs including the nervous system, the respiratory system, the vasculature and many internal glands. These networks range from subcellular structures such as dendritic trees to large multicellular tissues such as the lungs. The production of branched structures by single cells, so called subcellular branching, which has been better described in neurons and in cells of the respiratory and vascular systems, involves complex cytoskeletal remodelling events. In Drosophila, tracheal system terminal cells (TCs) and nervous system dendritic arborisation (da) neurons are good model systems for these subcellular branching processes. During development, the generation of subcellular branches by single-cells is characterized by extensive remodelling of the microtubule (MT) network and actin cytoskeleton, followed by vesicular transport and membrane dynamics. In this review, we describe the current knowledge on cytoskeletal regulation of subcellular branching, based on the terminal cells of the Drosophila tracheal system, but drawing parallels with dendritic branching and vertebrate vascular subcellular branching.