Rapid changes in phospho-MAP/tau epitopes during neuronal stress: cofilin-actin rods primarily recruit microtubule binding domain epitopes

Protein complexes from mouse and chick brain that interact with phospho-KXGS motif tau/microtubule associated protein antibody

Mouse monoclonal 12E8 antibody, which recognises conserved serine phosphorylated KXGS motifs in the microtubule binding domains of tau/tau-like microtubule associated proteins (MAPs), shows elevated binding in brain during normal embryonic development (mammals and birds) and at the early stages of human Alzheimer's disease (AD). It also labels ADF/cofilin-actin rods that form in neurites during exposure to stressors. We aimed to identify direct and indirect 12E8 binding proteins in postnatal mouse brain and embryonic chick brain by immunoprecipitation (IP), mass spectrometry and immunofluorescence. Tau and/or MAP2 were major direct 12E8-binding proteins detected in all IPs, and actin and/or tubulin were co-immunoprecipitated in most samples. Additional proteins were different in mouse versus chick brain IP. In mouse brain IPs, FSD1l and intermediate filament proteins - vimentin, α-internexin, neurofilament polypeptides - were prominent. Immunofluorescence and immunoblot using recombinant intermediate filament subunits, suggests an indirect interaction of these proteins with the 12E8 antibody. In chick brain IPs, subunits of eukaryotic translation initiation factor 3 (EIF3) were found, but no direct interaction between 12E8 and recombinant Eif3e protein was detected. Fluorescence microscopy in primary cultured chick neurons showed evidence of co-localisation of Eif3e and tubulin labelling, consistent with previous data demonstrating cytoskeletal organisation of the translation apparatus. Neither total tau or MAP2 immunolabelling accumulated at ADF/cofilin-actin rods generated in primary cultured chick neurons, and we were unable to narrow down the major antigen recognised by 12E8 antibody on ADF/cofilin-actin rods.

Tau: More than a microtubule-binding protein in neurons

Tau protein was discovered as a microtubule-associated protein nearly 50 years ago, and our understanding of tau has revolved around that role. Even with tau's rise to stardom as a central player in neurodegenerative disease, therapeutic efforts have largely been targeted toward cytoskeletal changes. While some studies hinted toward non-cytoskeletal roles for tau, it is only fairly recently that these ideas have begun to receive considerable attention. Many new binding partners for tau have been identified, including DNA, RNA, RNA-binding proteins, some receptors, and other tau molecules. The diversity of tau binding partners coupled with the discovery of tau other than axonal compartments such as nucleus, dendrites, and synapses have led to the proposal of novel functions for tau in roles such as nuclear stability, cell signaling, transcriptional processing, and protein synthesis. Tau self-assembly in particular has made an impact, leading to the hypothesis that a prion-like function of hyperphosphorylated tau is central to tauopathies. With tau emerging as a multifaceted protein that operates in many parts of the cell and with many molecular partners, the field of tau biology is primed for discoveries that can provide new perspectives on both the unique biochemistry of tau and the nature of devastating neurological diseases.

Cofilin promotes tau pathology in Alzheimer's disease

The molecular mechanisms mediating the aggregation and transmission of tau in AD remain unclear. Here, we show that the actin-binding protein cofilin is cleaved by a cysteine protease asparagine endopeptidase (AEP) at N138 in the brains of patients with AD. The AEP-generated cofilin 1-138 fragment interacts with tau and promotes its aggregation. The mixed fibrils consisting of cofilin 1-138 and tau are more pathogenic to cells than pure tau fibrils. Furthermore, overexpression of cofilin 1-138 in the brain facilitates the propagation of pathological tau aggregates and promotes AD-like cognitive impairments in tau P301S mice. However, mice infected with adeno-associated viruses (AAVs) encoding an AEP-uncleavable cofilin mutant show attenuated tau pathology and cognitive impairments compared with mice injected with AAVs encoding wild-type cofilin. Together, these observations support the role of the cofilin 1-138 fragment in the aggregation and transmission of tau pathology during the onset and progression of AD.

Cytoskeletal dysregulation and neurodegenerative disease: Formation, monitoring, and inhibition of cofilin-actin rods

The presence of atypical cytoskeletal dynamics, structures, and associated morphologies is a common theme uniting numerous diseases and developmental disorders. In particular, cytoskeletal dysregulation is a common cellular feature of Alzheimer’s disease, Parkinson’s disease, and Huntington’s disease. While the numerous activators and inhibitors of dysregulation present complexities for characterizing these elements as byproducts or initiators of the disease state, it is increasingly clear that a better understanding of these anomalies is critical for advancing the state of knowledge and plan of therapeutic attack. In this review, we focus on the hallmarks of cytoskeletal dysregulation that are associated with cofilin-linked actin regulation, with a particular emphasis on the formation, monitoring, and inhibition of cofilin-actin rods. We also review actin-associated proteins other than cofilin with links to cytoskeleton-associated neurodegenerative processes, recognizing that cofilin-actin rods comprise one strand of a vast web of interactions that occur as a result of cytoskeletal dysregulation. Our aim is to present a current perspective on cytoskeletal dysregulation, connecting recent developments in our understanding with emerging strategies for biosensing and biomimicry that will help shape future directions of the field.

Cofilin and Actin Dynamics: Multiple Modes of Regulation and Their Impacts in Neuronal Development and Degeneration

Proteins of the actin depolymerizing factor (ADF)/cofilin family are ubiquitous among eukaryotes and are essential regulators of actin dynamics and function. Mammalian neurons express cofilin-1 as the major isoform, but ADF and cofilin-2 are also expressed. All isoforms bind preferentially and cooperatively along ADP-subunits in F-actin, affecting the filament helical rotation, and when either alone or when enhanced by other proteins, promotes filament severing and subunit turnover. Although self-regulating cofilin-mediated actin dynamics can drive motility without post-translational regulation, cells utilize many mechanisms to locally control cofilin, including cooperation/competition with other proteins. Newly identified post-translational modifications function with or are independent from the well-established phosphorylation of serine 3 and provide unexplored avenues for isoform specific regulation. Cofilin modulates actin transport and function in the nucleus as well as actin organization associated with mitochondrial fission and mitophagy. Under neuronal stress conditions, cofilin-saturated F-actin fragments can undergo oxidative cross-linking and bundle together to form cofilin-actin rods. Rods form in abundance within neurons around brain ischemic lesions and can be rapidly induced in neurites of most hippocampal and cortical neurons through energy depletion or glutamate-induced excitotoxicity. In ~20% of rodent hippocampal neurons, rods form more slowly in a receptor-mediated process triggered by factors intimately connected to disease-related dementias, e.g., amyloid-β in Alzheimer’s disease. This rod-inducing pathway requires a cellular prion protein, NADPH oxidase, and G-protein coupled receptors, e.g., CXCR4 and CCR5. Here, we will review many aspects of cofilin regulation and its contribution to synaptic loss and pathology of neurodegenerative diseases.

Granulovacuolar degeneration bodies are neuron-selective lysosomal structures induced by intracellular tau pathology

Granulovacuolar degeneration bodies (GVBs) are membrane-bound vacuolar structures harboring a dense core that accumulate in the brains of patients with neurodegenerative disorders, including Alzheimer's disease and other tauopathies. Insight into the origin of GVBs and their connection to tau pathology has been limited by the lack of suitable experimental models for GVB formation. Here, we used confocal, automated, super-resolution and electron microscopy to demonstrate that the seeding of tau pathology triggers the formation of GVBs in different mouse models in vivo and in primary mouse neurons in vitro. Seeding-induced intracellular tau aggregation, but not seed exposure alone, causes GVB formation in cultured neurons, but not in astrocytes. The extent of tau pathology strongly correlates with the GVB load. Tau-induced GVBs are immunoreactive for the established GVB markers CK1δ, CK1ɛ, CHMP2B, pPERK, peIF2α and pIRE1α and contain a LAMP1- and LIMP2-positive single membrane that surrounds the dense core and vacuole. The proteolysis reporter DQ-BSA is detected in the majority of GVBs, demonstrating that GVBs contain degraded endocytic cargo. GFP-tagged CK1δ accumulates in the GVB core, whereas GFP-tagged tau or GFP alone does not, indicating selective targeting of cytosolic proteins to GVBs. Taken together, we established the first in vitro model for GVB formation by seeding tau pathology in primary neurons. The tau-induced GVBs have the marker signature and morphological characteristics of GVBs in the human brain. We show that GVBs are lysosomal structures distinguished by the accumulation of a characteristic subset of proteins in a dense core.

Oxidative stress differentially induces tau dissociation from neuronal microtubules in neurites of neurons cultured from different regions of the embryonic Gallus domesticus brain

Abnormal phosphorylation of microtubule-associated proteins such as tau has been shown to play a role in neurodegenerative disorders. It is hypothesized that oxidative stress-induced aggregates of hyperphosphorylated tau could lead to the microtubule network degradation commonly associated with neurodegeneration. We investigated whether oxidative stress induced tau hyperphosphorylation and focused on neurite degradation using cultured neurons isolated from the embryonic chick brain as a model system. Cells were isolated from the cerebrum, cerebellum, and tectum of 14-day-old chicks, grown separately in culture, and treated with tert-Butyl hydroperoxide (to simulate oxidative stress) for 48 hr. Relative expression and localization of tau or phospho-tau and β-tubulin III in neurites were determined using quantitative immunocytochemistry and confocal microscopy. In untreated cells, tau was tightly colocalized with β-tubulin III. Increasing levels of oxidative stress induced an increase in overall tau expression in neurites of cerebral and tectal but not the cerebellar neurons, coupled with a decrease in phospho-tau expression in tectal but not the cerebral or cerebellar neurons. In addition, oxidative stress induced the degeneration of the distal ends of the neurites and redistribution of phospho-tau toward the neuronal soma in the cerebral but not the tectal and cerebellar neurons. These results suggest that oxidative stress induces changes in tau protein that precede cytoskeletal degradation and neurite retraction. Additionally, there is a differential susceptibility of neuronal subpopulations to oxidative stress, which may offer potential avenues for investigation of the cellular mechanisms underlying the differential manifestations of neurodegenerative disorders in different regions of the brain.

Tau is required for the function of extrasynaptic NMDA receptors

Tau is a microtubule-associated neuronal protein found mainly in axons. However, increasing evidence indicates that it is also present in dendrites, where it serves as an essential mediator of synaptic NMDA (N-methyl-D-aspartate) receptor-dependent excitotoxicity. Of note, NMDA receptors can also be found outside synapses in the plasma membrane, and activation of extrasynaptic NMDA receptors has been shown to be more linked to excitotoxicity than the activation of synaptic ones. Little is known about the role of Tau in the activity of extrasynaptic NMDA receptors. Thus, we have used a Tau knockout mouse model (Tau^−/− mice) to analyze the consequences of Tau absence in extrasynaptic NMDA receptor activity. We demonstrate that absence of Tau leads to a decrease in functional extrasynaptic NMDA receptors in the hippocampus. We propose that this impairment in extrasynaptic NMDA receptor activity may contribute to the well-known neuroprotective effect associated with Tau deficiency under pathological conditions.

Activated cofilin exacerbates tau pathology by impairing tau-mediated microtubule dynamics

Alzheimer's disease (AD) is a progressive neurodegenerative disorder and the most common form of dementia. While the accumulation of Aβ is pivotal to the etiology of AD, both the microtubule-associated protein tau (MAPT) and the F-actin severing protein cofilin are necessary for the deleterious effects of Aβ. However, the molecular link between tau and cofilin remains unclear. In this study, we found that cofilin competes with tau for direct microtubule binding in vitro, in cells, and in vivo, which inhibits tau-induced microtubule assembly. Genetic reduction of cofilin mitigates tauopathy and synaptic defects in Tau-P301S mice and movement deficits in tau transgenic C. elegans. The pathogenic effects of cofilin are selectively mediated by activated cofilin, as active but not inactive cofilin selectively interacts with tubulin, destabilizes microtubules, and promotes tauopathy. These results therefore indicate that activated cofilin plays an essential intermediary role in neurotoxic signaling that promotes tauopathy.

Cofilin-actin rod formation in neuronal processes after brain ischemia

Functional impairment after brain ischemia results in part from loss of neuronal spines and dendrites, independent of neuronal death. Cofilin-actin rods are covalently linked aggregates of cofilin-1 and actin that form in neuronal processes (neurites) under conditions of ATP depletion and oxidative stress, and which cause neurite degeneration if not disassembled. ATP depletion and oxidative stress occur with differing severity, duration, and time course in different ischemic conditions. Here we evaluated four mouse models of brain ischemia to define the conditions that drive formation of cofilin-actin rods. Three of the models provide early reperfusion: transient middle cerebral artery occlusion (MCAo), transient bilateral common carotid artery occlusion (CCAo), and cardiac arrest / cardiopulmonary resuscitation (CA/CPR). Early reperfusion restores ATP generating capacity, but also induces oxidative stress. The fourth model, photothrombotic cortical infarction, does not provide reperfusion. Cofilin-actin rods were formed in each of these models, but with differing patterns. Where acute reperfusion occurred, rod formation was maximal within 4 hours after reperfusion. Where infarction occurred, rods continued to form for at least 24 hours after ischemic onset, and extended into the adjacent non-ischemic tissue. Interventions that limit cofilin-actin rod formation may help to preserve integrity of neuronal processes in permanent ischemia.

Neuronal cell culture from transgenic zebrafish models of neurodegenerative disease

We describe a protocol for culturing neurons from transgenic zebrafish embryos to investigate the subcellular distribution and protein aggregation status of neurodegenerative disease-causing proteins. The utility of the protocol was demonstrated on cell cultures from zebrafish that transgenically express disease-causing variants of human fused in sarcoma (FUS) and ataxin-3 proteins, in order to study amyotrophic lateral sclerosis (ALS) and spinocerebellar ataxia type-3 (SCA3), respectively. A mixture of neuronal subtypes, including motor neurons, exhibited differentiation and neurite outgrowth in the cultures. As reported previously, mutant human FUS was found to be mislocalized from nuclei to the cytosol, mimicking the pathology seen in human ALS and the zebrafish FUS model. In contrast, neurons cultured from zebrafish expressing human ataxin-3 with disease-associated expanded polyQ repeats did not accumulate within nuclei in a manner often reported to occur in SCA3. Despite this, the subcellular localization of the human ataxin-3 protein seen in cell cultures was similar to that found in the SCA3 zebrafish themselves. The finding of similar protein localization and aggregation status in the neuronal cultures and corresponding transgenic zebrafish models confirms that this cell culture model is a useful tool for investigating the cell biology and proteinopathy signatures of mutant proteins for the study of neurodegenerative disease.

Summary: This article describes the optimization and validation of a protocol for culturing of neurons from transgenic zebrafish for the study of neurodegenerative diseases.

Deficiency of Cks1 Leads to Learning and Long-Term Memory Defects and p27 Dependent Formation of Neuronal Cofilin Aggregates

In mitotic cells, the cyclin-dependent kinase (CDK) subunit protein CKS1 regulates S phase entry by mediating degradation of the CDK inhibitor p27. Although mature neurons lack mitotic CDKs, we found that CKS1 was actively expressed in post-mitotic neurons of the adult hippocampus. Interestingly, Cks1 knockout (Cks1-/-) mice exhibited poor long-term memory, and diminished maintenance of long-term potentiation in the hippocampal circuits. Furthermore, there was neuronal accumulation of cofilin-actin rods or cofilin aggregates, which are associated with defective dendritic spine maturation and synaptic loss. We further demonstrated that it was the increased p27 level that activated cofilin by suppressing the RhoA kinase-mediated inhibitory phosphorylation of cofilin, resulting in the formation of cofilin aggregates in the Cks1-/- neuronal cells. Consistent with reports that the peptidyl-prolyl-isomerase PIN1 competes with CKS1 for p27 binding, we found that inhibition of PIN1 diminished the formation of cofilin aggregates through decreasing p27 levels, thereby activating RhoA and increasing cofilin phosphorylation. Our results revealed that CKS1 is involved in normal glutamatergic synapse development and dendritic spine maturation in adult hippocampus through modulating p27 stability.

Revisiting rodent models: Octodon degus as Alzheimer's disease model?

Alzheimer's disease primarily occurs as sporadic disease and is accompanied with vast socio-economic problems. The mandatory basic research relies on robust and reliable disease models to overcome increasing incidence and emerging social challenges. Rodent models are most efficient, versatile, and predominantly used in research. However, only highly artificial and mostly genetically modified models are available. As these 'engineered' models reproduce only isolated features, researchers demand more suitable models of sporadic neurodegenerative diseases. One very promising animal model was the South American rodent Octodon degus, which was repeatedly described as natural 'sporadic Alzheimer's disease model' with 'Alzheimer's disease-like neuropathology'. To unveil advantages over the 'artificial' mouse models, we re-evaluated the age-dependent, neurohistological changes in young and aged Octodon degus (1 to 5-years-old) bred in a wild-type colony in Germany. In our hands, extensive neuropathological analyses of young and aged animals revealed normal age-related cortical changes without obvious signs for extensive degeneration as seen in patients with dementia. Neither significant neuronal loss nor enhanced microglial activation were observed in aged animals. Silver impregnation methods, conventional, and immunohistological stains as well as biochemical fractionations revealed neither amyloid accumulation nor tangle formation. Phosphoepitope-specific antibodies against tau species displayed similar intraneuronal reactivity in both, young and aged Octodon degus.In contrast to previous results, our study suggests that Octodon degus born and bred in captivity do not inevitably develop cortical amyloidosis, tangle formation or neuronal loss as seen in Alzheimer's disease patients or transgenic disease models.

Actin dynamics and cofilin-actin rods in alzheimer disease

Cytoskeletal abnormalities and synaptic loss, typical of both familial and sporadic Alzheimer disease (AD), are induced by diverse stresses such as neuroinflammation, oxidative stress, and energetic stress, each of which may be initiated or enhanced by proinflammatory cytokines or amyloid-β (Aβ) peptides. Extracellular Aβ-containing plaques and intracellular phospho-tau-containing neurofibrillary tangles are postmortem pathologies required to confirm AD and have been the focus of most studies. However, AD brain, but not normal brain, also have increased levels of cytoplasmic rod-shaped bundles of filaments composed of ADF/cofilin-actin in a 1:1 complex (rods). Cofilin, the major ADF/cofilin isoform in mammalian neurons, severs actin filaments at low cofilin/actin ratios and stabilizes filaments at high cofilin/actin ratios. It binds cooperatively to ADP-actin subunits in F-actin. Cofilin is activated by dephosphorylation and may be oxidized in stressed neurons to form disulfide-linked dimers, required for bundling cofilin-actin filaments into stable rods. Rods form within neurites causing synaptic dysfunction by sequestering cofilin, disrupting normal actin dynamics, blocking transport, and exacerbating mitochondrial membrane potential loss. Aβ and proinflammatory cytokines induce rods through a cellular prion protein-dependent activation of NADPH oxidase and production of reactive oxygen species. Here we review recent advances in our understanding of cofilin biochemistry, rod formation, and the development of cognitive deficits. We will then discuss rod formation as a molecular pathway for synapse loss that may be common between all three prominent current AD hypotheses, thus making rods an attractive therapeutic target. © 2016 Wiley Periodicals, Inc.

Lost after translation: missorting of Tau protein and consequences for Alzheimer disease

Tau is a microtubule-associated-protein that is sorted into neuronal axons in physiological conditions. In Alzheimer disease (AD) and other tauopathies, Tau sorting mechanisms fail and Tau becomes missorted into the somatodendritic compartment. In AD, aberrant amyloid-β (Aβ) production might trigger Tau missorting. The physiological axonal sorting of Tau depends on the developmental stage of the neuron, the phosphorylation state of Tau and the microtubule cytoskeleton. Disease-associated missorting of Tau is connected to increased phosphorylation and aggregation of Tau, and impaired microtubule interactions. Disease-causing mechanisms involve impaired transport, aberrant kinase activation, non-physiological interactions of Tau, and prion-like spreading. In this review we focus on the physiological and pathological (mis)sorting of Tau, the underlying mechanisms, and effects in disease.

Hirano bodies differentially modulate cell death induced by tau and the amyloid precursor protein intracellular domain

BACKGROUND

Hirano bodies are actin-rich paracrystalline inclusions found in brains of patients with Alzheimer's disease (AD), frontotemporal dementia (FTD), and in normal aged individuals. Although studies of post-mortem brain tissue provide clues of etiology, the physiological function of Hirano bodies remains unknown. A cell culture model was utilized to study the interactions of mutant tau proteins, model Hirano bodies, and GSK3β in human astrocytoma cells.

RESULTS

Most tau variants showed co-localization with model Hirano bodies. Cosedimentation assays revealed this interaction may be direct, as recombinant purified forms of tau are all capable of binding F-actin. Model Hirano bodies had no effect or enhanced cell death induced by tau in the absence of amyloid precursor protein intracellular domain (AICD). In the presence of AICD and tau, synergistic cell death was observed in most cases, and model Hirano bodies decreased this synergistic cell death, except for forms of tau that caused significant cell death in the presence of Hirano bodies only. A role for the kinase GSK3β is suggested by the finding that a dominant negative form of GSK3β reduces this synergistic cell death. A subset of Hirano bodies in brain tissue of both Alzheimer's disease and normal aged individuals was found to contain tau, with some Hirano bodies in Alzheimer's disease brains containing hyperphosphorylated tau.

CONCLUSION

The results demonstrate a complex interaction between tau and AICD involving activation of GSK3β in promoting cell death, and the ability of Hirano bodies to modulate this process.

Tau-induced neurodegeneration: mechanisms and targets

The accumulation of hyperphosphorylated tau is a common feature of several dementias. Tau is one of the brain microtubule-associated proteins. Here we discuss tau's functions in microtubule assembly and stabilization and with regard to its interactions with other proteins. We describe and analyze important post-translational modifications: hyperphosphorylation, ubiquitination, glycation, glycosylation, nitration, polyamination, proteolysis, acetylation, and methylation. We discuss how these post-translational modifications can alter tau's biological function. We analyze the role of mitochondrial health in neurodegeneration. We propose that microtubules could be a therapeutic target and review different approaches. Finally, we consider whether tau accumulation or its conformational change is related to tau-induced neurodegeneration, and propose a mechanism of neurodegeneration.

The cellular distribution and Ser262 phosphorylation of tau protein are regulated by BDNF in vitro

The brain-enriched microtubule-associated protein tau, a critical regulator of cytoskeletal dynamics, forms insoluble aggregates in a number of neurodegenerative diseases termed tauopathies, including Alzheimer's disease (AD). Hyperphosphorylation of tau protein is an important mechanism for aggregation, so many studies on the pathogenesis of AD and other tauopathies have focused on regulation of tau phosphorylation by kinases and phosphatases. Less studied are mechanisms of tau transcriptional and post-transcriptional regulation by extracellular signals such as BDNF and how such changes alter neuronal function. Previously, we reported that tau is required for morphological plasticity induced by BDNF. Here, we further explore tau modification during BDNF-induced changes in neuronal cell morphology. In undifferentiated SH-SY5Y cells lacking neurites, tau formed a sphere within the soma as revealed by immunocytochemistry. In contrast, tau was enriched in the neurites and sparse in the soma of SH-SY5Y cells induced to differentiate by retinoic acid (RA). Treatment with RA also increased total tau protein levels but decreased expression of tau phosphorylated at Ser262 as determined by Western blot. Both effects were further enhanced by subsequent BDNF treatment. Upregulation of tau protein and downregulation of p-Ser262 tau were correlated with total neurite length (R = .94 and R = −.98, respectively). When primary E18 hippocampal neurons were treated with nocodazole, a blocker of microtubule polymerization, nascent neurites were lost and tau shifted to the soma. This process of retrograde tau movement away from neurites was reversed by BDNF. These results indicate that tau is redistributed to neurites and dephosphorylated during RA- and BDNF-mediated differentiation.

The potential role of rho GTPases in Alzheimer's disease pathogenesis

Alzheimer's disease (AD) is characterized by a wide loss of synapses and dendritic spines. Despite extensive efforts, the molecular mechanisms driving this detrimental alteration have not yet been determined. Among the factors potentially mediating this loss of neuronal connectivity, the contribution of Rho GTPases is of particular interest. This family of proteins is classically considered a key regulator of actin cytoskeleton remodeling and dendritic spine maintenance, but new insights into the complex dynamics of its regulation have recently determined how its signaling cascade is still largely unknown, both in physiological and pathological conditions. Here, we review the growing evidence supporting the potential involvement of Rho GTPases in spine loss, which is a unanimously recognized hallmark of early AD pathogenesis. We also discuss some new insights into Rho GTPase signaling framework that might explain several controversial results that have been published. The study of the connection between AD and Rho GTPases represents a quite unchartered avenue that holds therapeutic potential.

Defective tubulin organization and proplatelet formation in murine megakaryocytes lacking Rac1 and Cdc42

Rac1 and Cdc42 have redundant functions in platelet biogenesis. Deficiency of Rac1 and Cdc42 results in highly abnormal megakaryocyte morphology associated with severely defective tubulin organization.

Quantitative imaging of microtubule alteration as an early marker of axonal degeneration after ischemia in neurons

Neuronal death can be preceded by progressive dysfunction of axons. Several pathological conditions such as ischemia can disrupt the neuronal cytoskeleton. Microtubules are basic structural components of the neuronal cytoskeleton that regulate axonal transport and neuronal function. Up-to-date, high-resolution observation of microtubules in living neuronal cells is usually accomplished using fluorescent-based microscopy techniques. However, this needs exogenous fluorescence markers to produce the required contrast. This is an invasive procedure that may interfere with the microtubule dynamics. In this work, we show, for the first time to our knowledge, that by using the endogenous (label-free) contrast provided by second harmonic generation (SHG) microscopy, it is possible to identify early molecular changes occurring in the microtubules of living neurons under ischemic conditions. This is done by measuring the intensity modulation of the SHG signal as a function of the angular rotation of the incident linearly polarized excitation light (technique referred to as PSHG). Our experiments were performed in microtubules from healthy control cultured cortical neurons and were compared to those upon application of several periods of oxygen and glucose deprivation (up to 120 min) causing ischemia. After 120-min oxygen and glucose deprivation, a change in the SHG response to the polarization was measured. Then, by using a three-dimensional PSHG biophysical model, we correlated this finding with the structural changes occurring in the microtubules under oxygen and glucose deprivation. To our knowledge, this is the first study performed in living neuronal cells that is based on direct imaging of axons and that provides the means of identifying the early symptoms of ischemia. Live observation of this process might bring new insights into understanding the dynamics and the mechanisms underlying neuronal degeneration or mechanisms of protection or regeneration.

Decreases in valosin-containing protein result in increased levels of tau phosphorylated at Ser262/356

VCP/p97 is a multifunctional AAA+-ATPase involved in vesicle fusion, proteasomal degradation, and autophagy. Reported dysfunctions of these processes in Alzheimer disease (AD), along with the linkage of VCP/p97 to inclusion body myopathy with Paget's disease and frontotemporal dementia (IBMPFD) led us to examine the possible linkage of VCP to the AD-relevant protein, tau. VCP levels were reduced in AD brains, but not in the cerebral cortex of an AD mouse model, suggesting that VCP reduction occurs upstream of tau pathology. Genetic reduction of VCP in a primary neuronal model led to increases in the levels of tau phosphorylated at Ser(262/356), indicating that VCP may be involved in regulating post-translational processing of tau in AD, demonstrating a possible functional linkage between tau and VCP.