Microtubule formation and neurite growth in cerebellar macroneurons which develop in vitro: evidence for the involvement of the microtubule-associated proteins, MAP-1a, HMW-MAP2 and Tau

Role of Tau Protein Hyperphosphorylation in Diabetic Retinal Neurodegeneration

Diabetic retinal neurodegeneration (DRN) is an early manifestation of diabetic retinopathy (DR) characterized by neurodegeneration that precedes microvascular abnormalities in the retina. DRN is characterized by apoptosis of retinal ganglion cells (involves alterations in retinal ganglion cells [RGCs], photoreceptors, amacrine cells and bipolar cells and so on), reactive gliosis, and reduced retinal neuronal function. Tau, a microtubule-associated protein, is a key mediator of neurotoxicity in neurodegenerative diseases, with functions in phosphorylation-dependent microtubule assembly and stabilization, axonal transport, and neurite outgrowth. The hyperphosphorylated tau (p-tau) loses its ability to bind to microtubules and aggregates to form paired helical filaments (PHFs), which further form neurofibrillary tangles (NFTs), leading to abnormal cell scaffolding and cell death. Studies have shown that p-tau can cause degeneration of RGCs in DR, making tau pathology a new pathophysiological model for DR. Here, we review the mechanisms by which p-tau contribute to DRN, including insulin resistance or lack of insulin, mitochondrial damage such as mitophagy impairment, mitochondrial axonal transport defects, mitochondrial bioenergetics dysfunction, and impaired mitochondrial dynamics, Abeta toxicity, and inflammation. Therefore, this article proposes that tau protein hyperphosphorylation plays a crucial role in the pathogenesis of DRN and may serve as a novel therapeutic target for combating DRN.

The Advancements of Marine Natural Products in the Treatment of Alzheimer's Disease: A Study Based on Cell and Animal Experiments

As life expectancy rises and the aging population grows, Alzheimer's disease (AD) has become a significant global health concern. AD is a complex neurodegenerative disorder with an unclear etiology. Current hypotheses primarily focus on β-amyloid (Aβ) aggregation, tau protein hyperphosphorylation, and neuroinflammation as key pathological processes. Given the limited efficacy of existing therapeutic strategies, there is an urgent need to explore novel treatment options. Marine natural products have garnered significant attention due to their unique chemical structures and diverse bioactivities, demonstrating potential for multi-target interventions in AD. This review systematically summarizes the roles of marine-derived compounds, including polysaccharides, carotenoids, and polyphenols, in modulating Aβ aggregation, mitigating tau protein pathology, and regulating gut-brain axis dysfunction. Furthermore, the challenges of current research are discussed, with an emphasis on improving blood-brain barrier permeability and optimizing drug delivery systems to facilitate clinical translation.

Alzheimer's Disease Pathology and Assistive Nanotheranostic Approaches for Its Therapeutic Interventions

Alzheimer's disease (AD) still prevails and continues to increase indiscriminately throughout the 21st century, and is thus responsible for the depreciating quality of health and associated sectors. AD is a progressive neurodegenerative disorder marked by a significant amassment of beta-amyloid plaques and neurofibrillary tangles near the hippocampus, leading to the consequent loss of cognitive abilities. Conventionally, amyloid and tau hypotheses have been established as the most prominent in providing detailed insight into the disease pathogenesis and revealing the associative biomarkers intricately involved in AD progression. Nanotheranostic deliberates rational thought toward designing efficacious nanosystems and strategic endeavors for AD diagnosis and therapeutic implications. The exceeding advancements in this field enable the scientific community to envisage and conceptualize pharmacokinetic monitoring of the drug, sustained and targeted drug delivery responses, fabrication of anti-amyloid therapeutics, and enhanced accumulation of the targeted drug across the blood-brain barrier (BBB), thus giving an optimistic approach towards personalized and precision medicine. Current methods idealized on the design and bioengineering of an array of nanoparticulate systems offer higher affinity towards neurocapillary endothelial cells and the BBB. They have recently attracted intriguing attention to the early diagnostic and therapeutic measures taken to manage the progression of the disease. In this article, we tend to furnish a comprehensive outlook, the detailed mechanism of conventional AD pathogenesis, and new findings. We also summarize the shortcomings in diagnostic, prognostic, and therapeutic approaches undertaken to alleviate AD, thus providing a unique window towards nanotheranostic advancements without disregarding potential drawbacks, side effects, and safety concerns.

Why kiss-and-hop explains that tau does not stabilize microtubules and does not interfere with axonal transport (at physiological conditions)

Tau is a microtubule-associated protein that is enriched in the axonal process of neurons. Post-translational modifications of tau have been implicated in the development of tauopathies characterized by defects in axonal transport, neuronal atrophy, and microtubule disassembly. Although tau is almost quantitatively bound to microtubules under physiological conditions, it does not significantly affect axonal transport. Furthermore, acute or chronic tau deficiency does not result in significant destabilization of neuronal microtubules, challenging the classical view that disease-related tau modifications directly cause axonal microtubule collapse. Here, we discuss how the rapid interaction kinetics of the tau-microtubule interaction, which we previously termed the kiss-and-hop interaction, explains why tau does not affect microtubule-dependent axonal transport but still allows tau to modulate microtubule polymerization. In contrast, tau modifications that slow down the kinetics of the tau-microtubule interaction and increase the residence time of tau at a microtubule interaction site can disrupt axonal transport and cause dendritic atrophy. We discuss the consequences of such a gain-of-toxicity mechanism in terms of the development of disease-modulating drugs that target the tau protein.

Synchrotron X-ray study of intrinsically disordered and polyampholytic Tau 4RS and 4RL under controlled ionic strength

Aggregated and hyperphosphorylated Tau is one of the pathological hallmarks of Alzheimer's disease. Tau is a polyampholytic and intrinsically disordered protein (IDP). In this paper, we present for the first time experimental results on the ionic strength dependence of the radius of gyration (Rg) of human Tau 4RS and 4RL isoforms. Synchrotron X-ray scattering revealed that 4RS Rg is regulated from 65.4 to 58.5 Å and 4RL Rg is regulated from 70.9 to 57.9 Å by varying ionic strength from 0.01 to 0.592 M. The Rg of 4RL Tau is larger than 4RS at lower ionic strength. This result provides an insight into the ion-responsive nature of intrinsically disordered and polyampholytic Tau, and can be implicated to the further study of Tau-Tau and Tau-tubulin intermolecular structure in ionic environments.

Liquid-liquid phase separation of protein tau: An emerging process in Alzheimer's disease pathogenesis

Metabolic reactions within cells occur in various isolated compartments with or without borders, the latter being known as membrane-less organelles (MLOs). The MLOs show liquid-like properties and are formed by a process known as liquid-liquid phase separation (LLPS). MLOs contribute to different molecules interactions such as protein-protein, protein-RNA, and RNA-RNA driven by various factors, such as multivalency of intrinsic disorders. MLOs are involved in several cell signaling pathways such as transcription, immune response, and cellular organization. However, disruption of these processes has been found in different pathologies. Recently, it has been demonstrated that protein aggregates, a characteristic of some neurodegenerative diseases, undergo similar phase separation. Tau protein is known as a major neurofibrillary tangles component in Alzheimer's disease (AD). This protein can undergo phase separation to form a MLO known as tau droplet in vitro and in vivo, and this process can be facilitated by several factors, including crowding agents, RNA, and phosphorylation. Tau droplet has been shown to mature into insoluble aggregates suggesting that this process may precede and induce neurodegeneration in AD. Here we review major factors involved in liquid droplet formation within a cell. Additionally, we highlight recent findings concerning tau aggregation following phase separation in AD, along with the potential therapeutic strategies that could be explored in this process against the progression of this pathology.

Caspase-cleaved tau is senescence-associated and induces a toxic gain of function by putting a brake on axonal transport

The microtubule-associated protein tau plays a central role in tauopathies such as Alzheimer's disease (AD). The exact molecular mechanisms underlying tau toxicity are unclear, but aging is irrefutably the biggest risk factor. This raises the question of how cellular senescence affects the function of tau as a microtubule regulator. Here we report that the proportion of tau that is proteolytically cleaved at the caspase-3 site (TauC3) doubles in the hippocampus of senescent mice. TauC3 is also elevated in AD patients. Through quantitative live-cell imaging, we show that TauC3 has a drastically reduced dynamics of its microtubule interaction. Single-molecule tracking of tau confirmed that TauC3 has a longer residence time on axonal microtubules. The reduced dynamics of the TauC3-microtubule interaction correlated with a decreased transport of mitochondria, a reduced processivity of APP-vesicle transport and an induction of region-specific dendritic atrophy in CA1 neurons of the hippocampus. The microtubule-targeting drug Epothilone D normalized the interaction of TauC3 with microtubules and modulated the transport of APP-vesicles dependent on the presence of overexpressed human tau. The results indicate a novel toxic gain of function, in which a post-translational modification of tau changes the dynamics of the tau-microtubule interaction and thus leads to axonal transport defects and neuronal degeneration. The data also introduce microtubule-targeting drugs as pharmacological modifiers of the tau-microtubule interaction with the potential to restore the physiological interaction of pathologically altered tau with microtubules.

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.

Amyloid-β with isomerized Asp7 cytotoxicity is coupled to protein phosphorylation

Neuronal dysfunction and loss associated with the accumulation of amyloid-β (Aβ) in the form of extracellular amyloid plaques and hyperphosphorylated tau in the form of intraneuronal neurofibrillary tangles represent key features of Alzheimer's disease (AD). Amyloid plaques found in the brains of AD patients are predominantly composed of Aβ42 and its multiple chemically or structurally modified isoforms. Recently, we demonstrated that Aβ42 with isomerised Asp7 (isoAβ42) which is one of the most abundant Aβ isoform in plaques, exhibited high neurotoxicity in human neuronal cells. Here, we show that, in SH-SY5Y neuroblastoma cells, the administration of synthetic isoAβ42 rather than intact Aβ42 resulted in a significantly higher level of protein phosphorylation, especially the phosphorylation of tau, tubulins, and matrin 3. IsoAβ42 induced a drastic reduction of tau protein levels. Our data demonstrate, for the first time, that isoAβ42, being to date the only known synthetic Aβ species to cause AD-like amyloidogenesis in an animal AD model, induced cell death by disabling structural proteins in a manner characteristic of that observed in the neurons of AD patients. The data emphasize an important role of isoAβ42 in AD progression and provide possible neurotoxicity paths for this particular isoform.

Tau45-230 association with the cytoskeleton and membrane-bound organelles: Functional implications in neurodegeneration

The dysregulation of posttranslational modifications of the microtubule-associated protein (MAP) tau plays a key role in Alzheimer's disease (AD) and related disorders. Thus, we have previously shown that beta amyloid (Aβ)-induced neurotoxicity was mediated, at least in part, by tau cleavage into the tau_45-230 fragment. However, the mechanisms underlying the toxicity of tau_45-230 remain unknown. To get insights into such mechanisms, we first determined the subcellular localization of this tau fragment in hippocampal neurons. Tau_45-230 was easily detectable in cell bodies and processes extended by these neurons. In addition, cell extraction experiments performed using Triton X-100 and saponin showed that a pool of tau_45-230 was associated with the cytoskeleton and the cytoskeleton plus membrane-bound organelles, respectively, in cultured hippocampal neurons. Furthermore, they suggested that these associations were independent of the presence of full-length tau. We also assessed whether this tau fragment could alter axonal transport. Our results indicated that tau_45-230 significantly reduced the number of organelles transported along hippocampal axons. This altered axonal transport did not correlate with changes in the total number of organelles present in these cells or in motor protein levels. Together these results suggested that tau_45-230 could exert its toxic effects by partially blocking axonal transport along microtubules thus contributing to the early pathology of AD.

The Microtubule-Associated Protein Tau and Its Relevance for Pancreatic Beta Cells

Structural and biochemical alterations of the microtubule-associated protein tau (MAPT) are associated with degenerative disorders referred to as tauopathies. We have previously shown that MAPT is present in human islets of Langerhans, human insulinomas, and pancreatic beta-cell line models, with biophysical similarities to the pathological MAPT in the brain. Here, we further studied MAPT in pancreatic endocrine tissue to better understand the mechanisms that lead to functional dysregulation of pancreatic beta cells. We found upregulation of MAPT protein expression in human insulinomas when compared to human pancreatic islets of Langerhans and an imbalance between MAPT isoforms in insulinomas tissue. We cloned one 3-repeat domain MAPT and transduced this into a beta-cell derived rodent cell line Rin-5F. Proliferation experiments showed higher growth rates and metabolic activities of cells overexpressing MAPT protein. We observed that a MAPT overexpressing cell line demonstrates altered insulin transcription, translation, and insulin secretion rates. We found the relative insulin secretion rates were significantly decreased in a MAPT overexpressing cell line and these findings could be confirmed using partial MAPT knock-down cell lines. Our findings support that MAPT may play an important role in insulin granule trafficking and indicate the importance of balanced MAPT phosphorylation and dephosphorylation for adequate insulin release.

Extracellular and Intracellular Signaling for Neuronal Polarity

Neurons are one of the highly polarized cells in the body. One of the fundamental issues in neuroscience is how neurons establish their polarity; therefore, this issue fascinates many scientists. Cultured neurons are useful tools for analyzing the mechanisms of neuronal polarization, and indeed, most of the molecules important in their polarization were identified using culture systems. However, we now know that the process of neuronal polarization in vivo differs in some respects from that in cultured neurons. One of the major differences is their surrounding microenvironment; neurons in vivo can be influenced by extrinsic factors from the microenvironment. Therefore, a major question remains: How are neurons polarized in vivo? Here, we begin by reviewing the process of neuronal polarization in culture conditions and in vivo. We also survey the molecular mechanisms underlying neuronal polarization. Finally, we introduce the theoretical basis of neuronal polarization and the possible involvement of neuronal polarity in disease and traumatic brain injury.

Neuronal degeneration, synaptic defects, and behavioral abnormalities in tau₄₅₋₂₃₀ transgenic mice

The complement of mechanisms underlying tau pathology in neurodegenerative disorders has yet to be elucidated. Among these mechanisms, abnormal tau phosphorylation has received the most attention because neurofibrillary tangles present in Alzheimer's disease (AD) and related disorders known as tauopathies are composed of hyperphosphorylated forms of this microtubule-associated protein. More recently, we showed that calpain-mediated cleavage leading to the generation of the 17kDa tau₄₅₋₂₃₀ fragment is a conserved mechanism in these diseases. To obtain insights into the role of this fragment in neurodegeneration, we generated transgenic mice that express tau₄₅₋₂₃₀ and characterized their phenotype. Our results showed a significant increase in cell death in the hippocampal pyramidal cell layer of transgenic tau₄₅₋₂₃₀ mice when compared to wild-type controls. In addition, significant synapse loss was detected as early as six months after birth in transgenic hippocampal neurons. These synaptic changes were accompanied by alterations in the expression of the N-methyl-d-aspartate glutamate (NMDA) receptor subunits. Furthermore, functional abnormalities were detected in the transgenic mice using Morris Water Maze and fear conditioning tests. These results suggest that the accumulation of tau₄₅₋₂₃₀ is responsible, at least in part, for neuronal degeneration and some behavioral changes in AD and other tauopathies. Collectively, these data provide the first direct evidence of the toxic effects of a tau fragment biologically produced in the context of these diseases in vertebrate neurons that develop in situ.

Wnt signaling through the Ror receptor in the nervous system

The receptor tyrosine kinase-like orphan receptor (Ror) proteins are conserved tyrosine kinase receptors that play roles in a variety of cellular processes that pattern tissues and organs during vertebrate and invertebrate development. Ror signaling is required for skeleton and neuronal development and modulates cell migration, cell polarity, and convergent extension. Ror has also been implicated in two human skeletal disorders, brachydactyly type B and Robinow syndrome. Rors are widely expressed during metazoan development including domains in the nervous system. Here, we review recent progress in understanding the roles of the Ror receptors in neuronal migration, axonal pruning, axon guidance, and synaptic plasticity. The processes by which Ror signaling execute these diverse roles are still largely unknown, but they likely converge on cytoskeletal remodeling. In multiple species, Rors have been shown to act as Wnt receptors signaling via novel non-canonical Wnt pathways mediated in some tissues by the adapter protein disheveled and the non-receptor tyrosine kinase Src. Rors can either activate or repress Wnt target expression depending on the cellular context and can also modulate signal transduction by sequestering Wnt ligands away from their signaling receptors. Future challenges include the identification of signaling components of the Ror pathways and bettering our understanding of the roles of these pleiotropic receptors in patterning the nervous system.

Opticin, a small leucine-rich proteoglycan, is uniquely expressed and translocated to the nucleus of chronic lymphocytic leukemia cells

Background

Opticin (OPTC) is a member of the small leucine-rich proteoglycan (SLRP) family and is localized particularly in certain extracellular matrices. We have previously reported the unique expression of another SLRP, fibromodulin (FMOD) in the leukemic cells of patients with chronic lymphocytic leukemia (CLL). OPTC is located in the same region as FMOD on chromosome 1 (1q32.1). Cluster up-regulation of genes may be observed in malignancies and the aim of the present study was to analyze the expression of OPTC in CLL cells.

Methods

The expression of OPTC was tested by RT-PCR and realtime qPCR in PBMC from CLL patients, other hematological malignancies and healthy controls. The presence of OPTC protein, and its subcellular localization, was investigated using fractionation methods where the obtained lysate fractions were analyzed by Western blotting. Deglycosylation experiments were performed to investigate the glycosylation status of the CLL OPTC.

Results

OPTC was expressed at the gene level in all patients with CLL (n = 90) and in 2/8 patients with mantle cell lymphoma (MCL) but not in blood mononuclear cells of healthy control donors (n = 20) or in tumor samples from nine other types of hematological malignancies. OPTC was detected by Western blot in all CLL samples analyzed (n = 30) but not in normal leukocytes (n = 10). Further analysis revealed a CLL-unique unglycosylated 37 kDa core protein that was found to be located preferentially in the cell nucleus and endoplasmic reticulum (ER) of the CLL cells.

Conclusions

A 37 kDa unglycosylated OPTC protein was detected in ER and in the nucleus of CLL cells and not in healthy control donors. The function of this OPTC core protein remains unclear but its CLL-specific expression and subcellular localization warrants further investigations in the pathobiology of CLL.

A proline/arginine-rich end leucine-rich repeat protein (PRELP) variant is uniquely expressed in chronic lymphocytic leukemia cells

Proline/arginine-rich end leucine-rich repeat protein (PRELP) belongs to the small leucine-rich proteoglycan (SLRP) family, normally expressed in extracellular matrix of collagen-rich tissues. We have previously reported on another SLRP, fibromodulin (FMOD) in patients with chronic lymphocytic leukemia (CLL). PRELP is structurally similar to FMOD with adjacent localization on chromosome 1 (1q32.1). As cluster-upregulation of genes may occur in malignancies, the aim of our study was to analyze PRELP expression in CLL. PRELP was expressed (RT-PCR) in all CLL patients (30/30), as well as in some patients with mantle cell lymphoma (3/5), but not in healthy donor leukocytes (0/20) or tumor samples from other hematological malignancies (0/35). PRELP was also detected in CLL cell-lines (4/4) but not in cell-lines from other hematological tumors (0/9). PRELP protein was detected in all CLL samples but not in normal leukocytes. Deglycosylation experiments revealed a CLL-unique 38 kDa core protein, with an intact signal peptide. This 38 kDa protein was, in contrast to the normal 55 kDa size, not detected in serum which, in combination with the uncleaved signal peptide, suggests cellular retention. The unique expression of a 38 kDa PRELP in CLL cells may suggest involvement in the pathobiology of CLL and merits further studies.

Tau-targeted treatment strategies in Alzheimer's disease

With populations ageing worldwide, the need for treating and preventing diseases associated with high age is pertinent. Alzheimer's disease (AD) is reaching epidemic proportions, yet the currently available therapies are limited to a symptomatic relief, without halting the degenerative process that characterizes the AD brain. As in AD cholinergic neurons are lost at high numbers, the initial strategies were limited to the development of acetylcholinesterase inhibitors, and more recently the NMDA receptor antagonist memantine, in counteracting excitotoxicity. With the identification of the protein tau in intracellular neurofibrillary tangles and of the peptide amyloid-β (Aβ) in extracellular amyloid plaques in the AD brain, and a better understanding of their role in disease, newer strategies are emerging, which aim at either preventing their formation and deposition or at accelerating their clearance. Interestingly, what is well established to combat viral diseases in peripheral organs - vaccination - seems to work for the brain as well. Accordingly, immunization strategies targeting Aβ show efficacy in mice and to some degree also in humans. Even more surprising is the finding in mice that immunization strategies targeting tau, a protein that forms aggregates in nerve cells, ameliorates the tau-associated pathology. We are reviewing the literature and discuss what can be expected regarding the translation into clinical practice and how the findings can be extended to other neurodegenerative diseases with protein aggregation in brain.

β-Amyloid carrying the Dutch mutation has diverse effects on calpain-mediated toxicity in hippocampal neurons

Hereditary cerebral hemorrhage with amyloidosis-Dutch type is a disorder associated with a missense mutation (E693Q) in the β-amyloid (Aβ)-coding region of the amyloid precursor protein (APP). This familial disease is characterized by cognitive deficits secondary to intracerebral hemorrhage and, in some cases, progressive Alzheimer's disease (AD)-like dementia. Although this mutation was the first ever reported in the human APP gene, little is known about the molecular mechanisms underlying the direct toxic effects of this mutated Aβ on central neurons. In the present study, we assessed the role of calpain-mediated toxicity in such effects using an AD primary culture model system. Our results showed that Dutch mutant Aβ (E22Q) induced calpain-mediated cleavage of dynamin 1 and a significant decrease in synaptic contacts in mature hippocampal cultures. These synaptic deficits were similar to those induced by wild-type (WT) Aβ. In contrast, calpain-mediated tau cleavage leading to the generation of a 17-kDa neurotoxic fragment, as well as neuronal death, were significantly reduced in E22Q Aβ-treated neurons when compared with WT Aβ-treated ones. This complex regulation of the calpain-mediated toxicity pathway by E22Q Aβ could have some bearing in the pathobiology of this familial AD form.

Cell-length-dependent microtubule accumulation during polarization

BACKGROUND

Breaking cell symmetry, known as polarization, requires dynamic reorganization of microtubules (MTs) and is essential to many cellular processes, including axon formation in neurons. A critical step in polarization is believed to be the "selective stabilization" of MTs, which hypothesizes a spatial and/or temporal shift toward net MT assembly in a preferred direction of growth.

RESULTS

We now find that a simpler "length-dependent" model, in which MT assembly parameters are spatially and temporally constant, predicts MT accumulation in the direction of growth because of longer mean first passage times in the longer direction. We experimentally tested both models by tracking MT assembly dynamics in polarizing embryonic chick forebrain neurons, and we confirmed that assembly is spatially and temporally constant during axon formation.

CONCLUSION

Cell polarization occurs most simply through cell-length-dependent accumulation of MTs without MT stabilization or capture. In this way, F-actin-mediated cell shape and size changes can be read out by dynamic MTs undergoing simple dynamic instability to ultimately break cell symmetry.

Ror1-Ror2 complexes modulate synapse formation in hippocampal neurons

Ror1 and Ror2, a small family of tyrosine kinase receptors, have been implicated in multiple aspects of brain development in C. elegans and X. laevis. More recently, we have shown that these receptors modulate the rate of neurite elongation in cultured rat hippocampal neurons. However, no information is available regarding a potential role of these receptors in other developmental milestones in mammalian central neurons. Neither is the identity known of the Ror ligand(s) and/or the signal transduction pathway(s) in which they participate. Here we report that the down regulation of either Ror1 or Ror2 led to a significant decrease in synapse formation in cultured hippocampal neurons. Simultaneous targeting of Ror proteins, however, did not result in an additive phenotype. Our results also indicated that Ror1 and Ror2 physically interact in the mouse brain, suggesting that they might function as heterodimers in central neurons. In addition, these Ror complexes interacted with Wnt-5a mediating its effects on synaptogenesis. Together, these data suggest that Ror proteins play a key role in Wnt-5a-activated signaling pathways leading to synapse formation in the mammalian CNS.

Microtubule binding and trapping at the tip of neurites regulate tau motion in living neurons

During the development of neurons, the microtubule-associated tau proteins show a graded proximo-distal distribution in axons. In tauopathies such as Alzheimer's disease, tau accumulates in the somatodendritic compartment. To scrutinize the determinants of tau's distribution and motion, we constructed photoactivatable green fluorescent protein (GFP)-tagged tau fusion proteins and recorded their distribution after focal activation in living cells. Simulation showed that the motion of tau was compatible with diffusion/reaction as opposed to active transport/reaction. Effective diffusion constants of 0.7-0.8 microm(2)/second were calculated in neurites of PC12 cells and primary cortical neurons. Furthermore, tau's amino terminal projection domain mediated binding and enrichment of tau at distal neurites indicating that the tip of a neurite acts as an adsorber trapping tau protein. Treatment with taxol, incorporation of disease-related tau modifications, experimentally induced hyperphosphorylation and addition of preaggregated amyloid beta peptides (Abeta) increased the effective diffusion constant compatible with a decreased binding to microtubules. Distal enrichment was present after taxol treatment but was suppressed at disease-relevant conditions. The data suggest that (i) dynamic binding of tau to microtubules and diffusion along microtubules and (ii) trapping at the tip of a neurite both contribute to its distribution during development and disease.

Microtubule assembly, organization and dynamics in axons and dendrites

During the past decade enormous advances have been made in our understanding of the basic molecular machinery that is involved in the development of neuronal polarity. Far from being mere structural elements, microtubules are emerging as key determinants of neuronal polarity. Here we review the current understanding of the regulation of microtubule assembly, organization and dynamics in axons and dendrites. These studies provide new insight into microtubules' function in neuronal development and their potential contribution to plasticity.

Antisense suppression of tau in cultured rat oligodendrocytes inhibits process formation

The microtubule-associated protein tau is integral to neuronal process development and has a role in the pathogenesis of several neurodegenerative conditions. We examined possible roles for tau in cultured oligodendrocyte process formation by using antisense oligonucleotide treatment. Inhibition of tau synthesis with single oligonucleotides resulted in decreased tau protein levels and significantly shorter cellular processes. Simultaneous use of two nonoverlapping oligonucleotides caused a major reduction in tau levels and severely inhibited process outgrowth. The timing of oligonucleotide addition to oligodendrocyte cultures was important, with addition of antisense at the time of plating into culture having the most significant effect on morphology through reduction of tau expression.

The novel calpain inhibitor A-705253 potently inhibits oligomeric beta-amyloid-induced dynamin 1 and tau cleavage in hippocampal neurons

We have previously shown that beta-amyloid (Abeta) oligomers induced dynamin 1 and tau cleavage in cultured hippocampal neurons. As a result of this cleavage, dynamin 1 levels decreased and a toxic tau fragment was generated. Abeta-induced cleavage of these proteins was calpain-mediated and impacted both synaptic vesicle recycling and the integrity of neuronal processes [Kelly, B.L., Vassar, R., Ferreira, A., 2005. Beta-amyloid-induced dynamin 1 depletion in hippocampal neurons. A potential mechanism for early cognitive decline in Alzheimer disease. J. Biol. Chem. 280, 31746-31753; Park, S.Y., Ferreira, A., 2005. The generation of a 17kDa neurotoxic fragment: an alternative mechanism by which tau mediates beta-amyloid-induced neurodegeneration. J. Neurosci. 25, 5365-5375; Kelly, B.L., Ferreira, A., 2006. Beta-amyloid-induced dynamin 1 degradation is mediated by N-methyl-d-aspartate receptors in hippocampal neurons. J. Biol. Chem. 281, 28079-28089, Kelly, B.L., Ferreira, A., 2007. Beta-amyloid disrupted synaptic vesicle endocytosis in cultured hippocampal neurons. Neuroscience 147, 60-70]. Building on previous reports, these results identified calpain as a potential target for therapeutic intervention in Alzheimer's disease. In the present study, we tested the ability of A-705253, a novel water-soluble calpain inhibitor with oral availability and enhanced metabolic stability, to prevent Abeta-induced dynamin 1 and tau cleavage in cultured hippocampal neurons. Quantitative Western blot analysis indicated that the incubation of these cells with A-705253 prior to the addition of oligomeric Abeta reduced both dynamin 1 and tau cleavage in a dose-dependent manner. In addition, our results showed that this calpain inhibitor significantly ameliorated the cleavage of these proteins when added simultaneously with oligomeric Abeta. Furthermore, our data indicated that the use of this calpain inhibitor could have some beneficial effects even when added after the cleavage of these proteins have been triggered by Abeta. Collectively, these results suggest that, indeed, specific calpain inhibitors could play an important role in the treatment of Alzheimer's disease.

Tau binding to microtubules does not directly affect microtubule-based vesicle motility

Tau protein is a major microtubule (MT)-associated brain protein enriched in axons. Multiple functional roles are proposed for tau protein, including MT stabilization, generation of cell processes, and targeting of phosphotransferases to MTs. Recently, experiments involving exogenous tau expression in cultured cells suggested a role for tau as a regulator of kinesin-1-based motility. Tau was proposed to inhibit attachment of kinesin-1 to MTs by competing for the kinesin-1 binding site. In this work, we evaluated effects of tau on fast axonal transport (FAT) by using vesicle motility assays in isolated squid axoplasm. Effects of recombinant tau constructs on both kinesin-1 and cytoplasmic dynein-dependent FAT rates were evaluated by video microscopy. Exogenous tau binding to endogenous squid MTs was evidenced by a dramatic change in individual MT morphologies. However, perfusion of tau at concentrations approximately 20-fold higher than physiological levels showed no effect on FAT. In contrast, perfusion of a cytoplasmic dynein-derived peptide that competes with kinesin-1 and cytoplasmic dynein binding to MTs in vitro rapidly inhibited FAT in both directions. Taken together, our results indicate that binding of tau to MTs does not directly affect kinesin-1- or cytoplasmic dynein-based motilities. In contrast, our results provide further evidence indicating that the functional binding sites for kinesin-1 and cytoplasmic dynein on MTs overlap.

Agrin induced morphological and structural changes in growth cones of cultured hippocampal neurons

The role of agrin in synaptogenesis has been extensively studied. On the other hand, little is known about the function of this extracellular matrix protein during developmental processes that precede the formation of synapses. Recently, agrin was shown to regulate the rate of neurite elongation and the behavior of growth cones in hippocampal and spinal neurons, respectively. However, the molecular mechanisms underlying these effects have not been completely elucidated. In the present study, we analyzed the morphological and molecular changes induced by agrin in growth cones of hippocampal neurons that developed in culture. Morphometric analysis showed a significant enlargement of growth cones of hippocampal neurons cultured in the presence of agrin. These agrin-induced growth cone changes were accompanied by the formation of loops of microtubules highly enriched in acetylated tubulin and an increase in the content of the microtubule-associated protein (MAP)1B. Together, these data provide further insights into the potential molecular mechanisms underlying the effects of agrin on neurite outgrowth in rat central neurons.

Beta-amyloid disrupted synaptic vesicle endocytosis in cultured hippocampal neurons

Neuronal death leading to gross brain atrophy is commonly seen in Alzheimer's disease (AD) patients. Yet, it is becoming increasingly apparent that the pathogenesis of AD involves early and more discrete synaptic changes in affected brain areas. However, the molecular mechanisms that underlie such synaptic dysfunction remain largely unknown. Recently, we have identified dynamin 1, a protein that plays a critical role in synaptic vesicle endocytosis, and hence, in the signaling properties of the synapse, as a potential molecular determinant of such dysfunction in AD. In the present study, we analyzed beta-amyloid (Abeta)-induced changes in synaptic vesicle recycling in rat cultured hippocampal neurons. Our results showed that Abeta, the main component of senile plaques, caused ultrastructural changes indicative of impaired synaptic vesicle endocytosis in cultured hippocampal neurons that have been stimulated by depolarization with high potassium. In addition, Abeta led to the accumulation of amphiphysin in membrane fractions from stimulated hippocampal neurons. Moreover, experiments using FM1-43 showed reduced dye uptake in stimulated hippocampal neurons treated with Abeta when compared with untreated stimulated controls. Similar results were obtained using a dynamin 1 inhibitory peptide suggesting that dynamin 1 depletion caused deficiency in synaptic vesicle recycling not only in Drosophila but also in mammalian neurons. Collectively, these results showed that Abeta caused a disruption of synaptic vesicle endocytosis in cultured hippocampal neurons. Furthermore, we provided evidence suggesting that Abeta-induced dynamin 1 depletion might play an important role in this process.

Nervous Rac: DOCK7 regulation of axon formation

Microtubules play an important role in neuronal polarity. In this issue of Neuron, Watabe-Uchida et al. link a novel Rac-mediated pathway that regulates microtubule dynamics to axon formation.

Caspase-3- and calpain-mediated tau cleavage are differentially prevented by estrogen and testosterone in beta-amyloid-treated hippocampal neurons

A growing body of evidence suggests that the proteolytic cleavage of the microtubule-associated protein tau, the main component of neurofibrillary tangles, might play a role in the molecular mechanisms underlying beta-amyloid (Abeta) -induced neurotoxicity in central neurons. In the present study, we analyzed whether sex hormones could prevent such tau cleavage, and hence, protect rat hippocampal neurons against Abeta toxicity. Our results indicated that estrogen and testosterone prevented caspase-3- and calpain-mediated tau cleavage, respectively. Thus, estrogen decreased the levels of caspase-3-cleaved 50-kDa truncated tau, while testosterone prevented the generation of a calpain-cleaved 17-kDa tau fragment. In addition, our results showed that the decrease in the levels of these tau proteolytic forms was accompanied by an increased cell survival in Abeta-treated neurons. Furthermore, our findings indicated that testosterone was more effective than estrogen in protecting hippocampal neurons against Abeta-induced cell death. Collectively, our data suggest that preventing the decline of estrogen and testosterone associated with normal aging might reduce the susceptibility of central neurons to Abeta-induced toxicity.

Tau associates with actin in differentiating PC12 cells

The microtubule-associated protein tau may be involved in cell morphogenesis and axonal maintenance. In addition to microtubules, tau has been shown to interact with actin in vitro. In the present study interaction of tau and actin was investigated in PC12 cells. No interaction between tau and actin was observed without NGF treatment. Under NGF stimulation, tau distributed at ends of cellular extensions, where it associated with actin in a microtubule-independent manner. F-actin disruption revealed that relocalization and assembly of F-actin at the ends of cellular extensions were necessary for NGF-induced tau reorganization and association with actin. A truncated tau-GFP (tau(1-186)-GFP, N-terminal of tau) did not associate with actin. However, tau23(174-352)-GFP (carboxyl-terminal of Tau23) did associate with actin and the requirement for NGF was lost. Nevertheless, NGF boosted tau23(174-352)-GFP interaction with actin and promoted colocalization at the ends of cellular extensions. This suggests that the C-terminal of tau is required for associating with actin and the tau N-terminal may play a regulatory role in this process. A possible role for tau-actin interaction in neurite outgrowth is postulated.

Progesterone-induced agrin expression in astrocytes modulates glia-neuron interactions leading to synapse formation

Experimental evidence recently obtained suggests that synaptogenesis is a tripartite event in which not only pre- and post-synaptic neurons but also glial cells play a key role. However, the molecular mechanisms by which glia modulate the formation of synapses in the CNS remain poorly understood. In the present study, we analyzed the role of astrocytes in synapse formation in cultured hippocampal rat neurons. For these experiments, hippocampal neurons were cultured in the presence or absence of a monolayer of astrocytes. Our results indicated that hippocampal neurons cultured in the presence of astrocytes formed more synapses than the ones cultured in their absence only when kept in N2 serum-free medium. To get insights into the potential molecular mechanisms underlying this effect, we analyzed the expression of proteins known to induce synapse formation in hippocampal neurons. A significant increase in agrin expression was detected in astrocytes cultured in N2 serum-free medium when compared with the ones cultured in serum containing medium. Experiments performed using different components of the N2 mixture indicated that progesterone induced the expression of agrin in astrocytes. Taken collectively, these results provide evidence supporting a role for astrocytes in synapse formation in central neurons. Furthermore, they identified agrin as a potential mediator of this effect, and astrocytes as a bridge between the endocrine and nervous systems during synaptogenesis.

Neuritogenesis and neuronal differentiation promoted by 2,4-dinitrophenol, a novel anti-amyloidogenic compound

Neurite outgrowth is a critical event in neuronal development, formation, and remodeling of synapses, response to injury, and regeneration. We examined the effects of 2,4-dinitrophenol (DNP), a recently described blocker of the aggregation and neurotoxicity of the beta-amyloid peptide, on neurite elongation of central neurons. Morphometric analysis of rat embryo hippocampal and cortical neuronal cultures showed that neurite outgrowth was stimulated by DNP. This effect was accompanied by increases in the neuronal levels of the microtubule-associated protein tau and of cyclic adenosine 3',5' monophosphate (cAMP). DNP also promoted cAMP accumulation, increased tau level, neurite outgrowth, and neuronal differentiation in the mouse neuroblastoma cell line N2A. We show that DNP-induced differentiation requires activation of the extracellular signal-regulated kinase (ERK). The finding that DNP promotes neuritogenesis and neuronal differentiation suggests that, in addition to its anti-amyloidogenic actions, it may be a useful lead compound in the development of novel therapeutic approaches targeting neurite dystrophy and synaptic dysfunction in neurodegenerative pathologies such as Alzheimer's disease.

The generation of a 17 kDa neurotoxic fragment: an alternative mechanism by which tau mediates beta-amyloid-induced neurodegeneration

Recently, we have shown that the microtubule-associated protein tau is essential for beta-amyloid (Abeta)-induced neurotoxicity in hippocampal neurons. However, the mechanisms by which tau mediates Abeta-induced neurite degeneration remain poorly understood. In the present study, we analyzed whether tau cleavage played a role in these events. Our results showed that pre-aggregated Abeta induced the generation of a 17 kDa tau fragment in cultured hippocampal neurons. The generation of this fragment was preceded by the activation of calpain-1. Conversely, inhibitors of this protease, but not of caspases, completely prevented tau proteolysis leading to the generation of the 17 kDa fragment and significantly reduced Abeta-induced neuronal death. Furthermore, the expression of this fragment in cultured hippocampal neurons induced the formation of numerous varicosity-bearing tortuous processes, as well as the complete degeneration of some of those neurite processes. These results suggest that Abeta-induced neurotoxicity may be mediated, at least in part, through the calpain-mediated generation of a toxic 17 kDa tau fragment. Collectively, these results provide insight into a novel mechanism by which tau could mediate Abeta-induced neurotoxicity.

Microtubule-associated protein MAP1A, MAP1B, and MAP2 proteolysis during soluble amyloid beta-peptide-induced neuronal apoptosis. Synergistic involvement of calpain and caspase-3

A growing body of evidence supports the notion that soluble oligomeric forms of the amyloid beta-peptide (Abeta) may be the proximate effectors of neuronal injuries and death in the early stages of Alzheimer disease. However, the molecular mechanisms associated with neuronal apoptosis induced by soluble Abeta remain to be elucidated. We recently demonstrated the involvement of an early reactive oxygen species-dependent perturbation of the microtubule network (Sponne, I., Fifre, A., Drouet, B., Klein, C., Koziel, V., Pincon-Raymond, M., Olivier, J.-L., Chambaz, J., and Pillot, T. (2003) J. Biol. Chem. 278, 3437-3445). Because microtubule-associated proteins (MAPs) are responsible for the polymerization, stabilization, and dynamics of the microtubule network, we investigated whether MAPs might represent the intracellular targets that would enable us to explain the microtubule perturbation involved in soluble Abeta-mediated neuronal apoptosis. The data presented here show that soluble Abeta oligomers induce a time-dependent degradation of MAP1A, MAP1B, and MAP2 involving a perturbation of Ca2+ homeostasis with subsequent calpain activation that, on its own, is sufficient to induce the proteolysis of isoforms MAP2a, MAP2b, and MAP2c. In contrast, MAP1A and MAP1B sequential proteolysis results from the Abeta-mediated activation of caspase-3 and calpain. The prevention of MAP1A, MAP1B, and MAP2 proteolysis by antioxidants highlights the early reactive oxygen species generation in the perturbation of the microtubule network induced by soluble Abeta. These data clearly demonstrate the impact of cytoskeletal perturbations on soluble Abeta-mediated cell death and support the notion of microtubule-stabilizing agents as effective Alzheimer disease drugs.

The dynein light chain Tctex-1 has a dynein-independent role in actin remodeling during neurite outgrowth

Coordinated microtubule and microfilament changes are essential for the morphological development of neurons; however, little is know about the underlying molecular machinery linking these two cytoskeletal systems. Similarly, the indispensable role of RhoGTPase family proteins has been demonstrated, but it is unknown how their activities are specifically regulated in different neurites. In this paper, we show that the cytoplasmic dynein light chain Tctex-1 plays a key role in multiple steps of hippocampal neuron development, including initial neurite sprouting, axon specification, and later dendritic elaboration. The neuritogenic effects elicited by Tctex-1 are independent from its cargo adaptor role for dynein motor transport. Finally, our data suggest that the selective high level of Tctex-1 at the growth cone of growing axons drives fast neurite extension by modulating actin dynamics and also Rac1 activity.

Neurite extension in central neurons: a novel role for the receptor tyrosine kinases Ror1 and Ror2

Neurite elongation and branching are key cellular events during brain development as they underlie the formation of a properly wired neuronal network. Here we report that the receptor tyrosine kinases Ror1 and Ror2 modulate the growth of neurites as well as their branching pattern in hippocampal neurons. Upon Ror1 or Ror2 suppression using antisense oligonucleotides or RNA interference (RNAi), neurons extended shorter and less branched minor processes when compared to those in control cells. In addition, Ror-depleted cells elongated longer, albeit less branched, axons than seen in control cells. Conversely, Ror overexpression both in non-neuronal cells and in hippocampal neurons resulted in the enhanced extension of short and highly branched processes. These phenotypes were accompanied by changes in the microtubule-associated proteins MAP1B and MAP2. Taken together, these results support a novel role for Ror receptors as modulators of neurite extension in central neurons.

CSF biomarkers for mild cognitive impairment and early Alzheimer's disease

A correct clinical diagnosis, early in the course of Alzheimer's disease (AD), is of importance given the currently available symptomatic treatment with acetylcholine esterase inhibitors. The development of disease-modifying drugs like beta-sheet breakers or gamma- and beta-secretase inhibitors, emphasizes the need of improved diagnostic accuracy, especially in patients with mild cognitive impairment (MCI) that have incipient AD. Therefore, diagnostic markers in the cerebrospinal fluid (CSF) have become a rapidly growing research field. Three cerebrospinal fluid biomarkers (the 42 amino acid form of beta-amyloid (A beta), total tau, and phospho tau) have been evaluated in numerous scientific papers. These CSF markers have high sensitivity to differentiate early and incipient AD from normal aging, depression, alcohol dementia and Parkinson's disease, but lower specificity against other dementias, such as frontotemporal and Lewy body dementia. If these biomarkers are used in combination with a careful medical history, clinical examination, standard laboratory tests and imaging techniques of the brain, the diagnostic accuracy may be appropriate for the clinical evaluation of MCI cases.

Differential subcellular localization of Ror tyrosine kinase receptors in cultured astrocytes

Ror1 and Ror2 belong to a family of tyrosine kinase receptors that are highly conserved among species. They are expressed throughout the organism, including the central nervous system. In the present study, we analyzed the expression and subcellular localization of Ror1 and Ror2 in astrocytes by means of reverse transcription-polymerase chain reaction, Western blot analysis, and immunocytochemistry. Our results indicated that both Ror1 and Ror2 are readily detectable in cultured astrocytes. They also showed that Ror1 and Ror2 are associated with different components of the cytoskeleton. While Ror1 co-localized with F-actin along stress fibers, Ror2 partially co-localized with microtubules. In addition, our results suggest that Ror1 and Ror2 undergo different posttranslational modifications in cultured astrocytes. Ror1 is highly glycosylated in these cells. In contrast, no glycosylation was detected in Ror2. Taken together, these results suggest distinct roles for these tyrosine kinase receptors in astrocytes.

alpha1 Integrin activation: a link between beta-amyloid deposition and neuronal death in aging hippocampal neurons

A growing body of evidence obtained using in vitro model systems indicates that the deposition of fibrillar beta-amyloid (Abeta) results in neurite degeneration and cell death in central neurons. Little is known, however, about the molecular mechanisms underlying these neurotoxic effects. We have shown previously that fibrillar Abeta induced sustained activation of the mitogen-activated protein kinase (MAPK) followed by hyperphosphorylation of tau proteins in aging hippocampal neurons. Furthermore, the blockage of MAPK activation using specific inhibitors prevented neurite degeneration in these cells. These results suggested that the MAPK signal transduction pathway could play a key role in Abeta-induced neurite degeneration. We sought to identify upstream elements of the MAPK signaling cascade activated by Abeta deposition. We evaluated the participation of the integrins in this pathway by monitoring the activation of MAPK in the presence of specific integrin inhibitors. Our results indicate that pretreatment of mature hippocampal neurons with either echistatin or alpha(1) integrin-blocking antibodies prevented Abeta-induced MAPK activation. In addition, the blockage of alpha(1) activation prevented cell death induced by Abeta. Similar results were obtained when alpha(1) and beta(1) integrin blocking antibodies were used combined. Taken collectively, these results identify alpha(1) integrin and the alpha(1) plus beta(1) integrin complexes as potential targets for therapeutic intervention in the Abeta signaling pathway in aging neurons.

CSF markers for Alzheimer's disease: total tau, phospho-tau and Abeta42

Today we have the first therapeutic compounds for treatment of Alzheimer's disease (AD) e.g. acetylcholine esterase inhibitors and in the near future we may expect new compounds such as gamma- and beta-secretase inhibitors. This has demanded increased accuracy in the diagnosis of AD and thus, among other possible approaches, diagnostic markers in the cerebrospinal fluid (CSF) have become a rapidly growing research field. Especially early in the course of the disease, when correct diagnosis is most difficult, such biomarkers would be especially valuable as one might expect the compounds to have the greatest potential of being effective. Two of the defining lesions in AD brains are senile plaques and neurofibrillary tangles with beta-amyloid (Abeta) and tau proteins as the main components respectively. Abeta and tau proteins are secreted to body fluids including plasma and cerebrospinal fluid (CSF). In this paper we review CSF markers for AD, with focus on their role in the clinical diagnosis. Reduced CSF levels of the 42 amino acid form of Abeta (Abeta42) and increased CSF levels of total tau (T-tau) in AD have been found in numerous studies, with high sensitivity figures. However, the specificity against other dementias is lower. The addition of phospho-tau (P-tau) seems to increase the specificity, since normal levels are found in other dementias and in cerebrovascular disease. An increasing number of studies suggests that these CSF markers perform well enough to have a role in the clinical work-up of patients with dementia if used together. We stress that the CSF markers should be combined with the clinical information and brain-imaging techniques.

Expression and subcellular localization of Ror tyrosine kinase receptors are developmentally regulated in cultured hippocampal neurons

Ror1 and Ror2 are two novel receptor tyrosine kinases that have been implicated in neuronal differentiation in Caenorhabditis elegans. As a first step toward elucidating their role in the mammalian brain, we analyzed their expression and localization patterns in hippocampal neurons. Our results showed that both receptors are expressed from early stages of development and that their protein levels peak during periods of active synapse formation. Immunocytochemical analysis indicated that Ror1 and Ror2 are highly concentrated in the growth cones of immature neurons and are present throughout the somatodendritic compartment of mature hippocampal cells. Further analysis indicated that they are present not only in the cell membrane but also in Triton- and saponin-insoluble fractions, suggesting that they may be associated with both the cytoskeleton and membrane-bound organelles. Taken collectively, our results suggest that Ror1 and Ror2 might play a role during early stages of development in mammalian central neurons.