Tau pathology generated by overexpression of tau

Serotonergic dysfunction may mediate the relationship between alcohol consumption and Alzheimer's disease

The impact of Alzheimer's disease (AD) and its related dementias is rapidly expanding, and its mitigation remains an urgent social and technical challenge. To date there are no effective treatments or interventions for AD, but recent studies suggest that alcohol consumption is correlated with the risk of developing dementia. In this review, we synthesize data from preclinical, clinical, and epidemiological models to evaluate the combined role of alcohol consumption and serotonergic dysfunction in AD, underscoring the need for further research on this topic. We first discuss the limitations inherent to current data-collection methods, and how neuropsychiatric symptoms common among AD, alcohol use disorder, and serotonergic dysfunction may mask their co-occurrence. We additionally describe how excess alcohol consumption may accelerate the development of AD via direct effects on serotonergic function, and we explore the roles of neuroinflammation and proteostasis in mediating the relationship between serotonin, alcohol consumption, and AD. Lastly, we argue for a shift in current research to disentangle the pathogenic effects of alcohol on early-affected brainstem structures in AD.

Astrocytic 4R tau expression drives astrocyte reactivity and dysfunction

The protein tau and its isoforms are associated with several neurodegenerative diseases, many of which are characterized by greater deposition of the 4-repeat (4R) tau isoform; however, the role of 4R tau in disease pathogenesis remains unclear. We created antisense oligonucleotides (ASOs) that alter the ratio of 3R to 4R tau to investigate the role of specific tau isoforms in disease. Preferential expression of 4R tau in human tau-expressing (hTau-expressing) mice was previously shown to increase seizure severity and phosphorylated tau deposition without neuronal or synaptic loss. In this study, we observed strong colocalization of 4R tau within reactive astrocytes and increased expression of pan-reactive and neurotoxic genes following 3R to 4R tau splicing ASO treatment in hTau mice. Increasing 4R tau levels in primary astrocytes provoked a similar response, including a neurotoxic genetic profile and diminished homeostatic function, which was replicated in human induced pluripotent stem cell-derived (iPSC-derived) astrocytes harboring a mutation that exhibits greater 4R tau. Healthy neurons cultured with 4R tau-expressing human iPSC-derived astrocytes exhibited a higher firing frequency and hypersynchrony, which could be prevented by lowering tau expression. These findings support a potentially novel pathway by which astrocytic 4R tau mediates reactivity and dysfunction and suggest that astrocyte-targeted therapeutics against 4R tau may mitigate neurodegenerative disease progression.

The secret life of extracellular vesicles in metal homeostasis and neurodegeneration

Biologically active metals such as copper, zinc and iron are fundamental for sustaining life in different organisms with the regulation of cellular metal homeostasis tightly controlled through proteins that coordinate metal uptake, efflux and detoxification. Many of the proteins involved in either uptake or efflux of metals are localised and function on the plasma membrane, traffic between intracellular compartments depending upon the cellular metal environment and can undergo recycling via the endosomal pathway. The biogenesis of exosomes also occurs within the endosomal system, with several major neurodegenerative disease proteins shown to be released in association with these vesicles, including the amyloid-β (Aβ) peptide in Alzheimer's disease and the infectious prion protein involved in Prion diseases. Aβ peptide and the prion protein also bind biologically active metals and are postulated to play important roles in metal homeostasis. In this review, we will discuss the role of extracellular vesicles in Alzheimer's and Prion diseases and explore their potential contribution to metal homeostasis.

The emerging role of peptidyl-prolyl isomerase chaperones in tau oligomerization, amyloid processing, and Alzheimer's disease

Peptidyl-prolyl cis/trans isomerases (PPIases), a unique family of molecular chaperones, regulate protein folding at proline residues. These residues are abundant within intrinsically disordered proteins, like the microtubule-associated protein tau. Tau has been shown to become hyperphosphorylated and accumulate as one of the two main pathological hallmarks in Alzheimer's disease, the other being amyloid beta (Ab). PPIases, including Pin1, FK506-binding protein (FKBP) 52, FKBP51, and FKBP12, have been shown to interact with and regulate tau biology. This interaction is particularly important given the numerous proline-directed phosphorylation sites found on tau and the role phosphorylation has been found to play in pathogenesis. This regulation then affects downstream aggregation and oligomerization of tau. However, many PPIases have yet to be explored for their effects on tau biology, despite the high likelihood of interaction based on proline content. Moreover, Pin1, FKBP12, FKBP52, cyclophilin (Cyp) A, CypB, and CypD have been shown to also regulate Ab production or the toxicity associated with Ab pathology. Therefore, PPIases directly and indirectly regulate pathogenic protein multimerization in Alzheimer's disease and represent a family rich in targets for modulating the accumulation and toxicity.

Biochemical studies in Normal Pressure Hydrocephalus (NPH) patients: change in CSF levels of amyloid precursor protein (APP), amyloid-beta (Aβ) peptide and phospho-tau

Normal Pressure Hydrocephalus (NPH) is one of the causes of dementia of the elderly characterized by impaired mental function, gait difficulties and urinary incontinence. Previously, it was proposed that some of the NPH patients may develop Alzheimer's disease (AD) like pathology. Aim of this study was to compare levels of different CSF biomarkers, including total secreted β-amyloid precursor protein (sAPP), sAPP-alpha form (sAPPα), amyloid-beta (Aβ) peptide, total-tau protein and hyperphosphorylated-tau protein in subjects from NPH and Non-NPH Control (NNC). CSF was collected from 23 NPH patients and 13 Non-NPH controls by lumber puncture. Western blot analysis was performed to measure levels of sAPP-total. ELISA was used separately to determine levels of sAPPα, Aβ peptide, total-tau and phospho-tau proteins. We found a significant decrease in levels of total secreted APP, sAPPα and Aβ (1-42) in the CSF sample of NPH patients vs. NNC. We did not observe any change in levels of total-tau or phospho-tau in NPH vs. NNC subjects. Notably, phospho-tau level was significantly increased in the NPH patients, who were suffering from the disease for more than one year, vs. NNC. Among five biomarkers studied, decreased sAPP, sAPPα and Aβ (1-42) levels in CSF can be molecular markers to distinguish NPH cases from NNC. Disease severity can also be assessed by increased levels of CSF phospho-tau protein and the ratio of phospho-tau to Aβ (1-42), which might be a useful tool for predicting conversion of NPH individuals to other neurodegenerative disorders including Alzheimer's disease (AD).

Trends in the molecular pathogenesis and clinical therapeutics of common neurodegenerative disorders

The term neurodegenerative disorders, encompasses a variety of underlying conditions, sporadic and/or familial and are characterized by the persistent loss of neuronal subtypes. These disorders can disrupt molecular pathways, synapses, neuronal subpopulations and local circuits in specific brain regions, as well as higher-order neural networks. Abnormal network activities may result in a vicious cycle, further impairing the integrity and functions of neurons and synapses, for example, through aberrant excitation or inhibition. The most common neurodegenerative disorders are Alzheimer’s disease, Parkinson’s disease, Amyotrophic Lateral Sclerosis and Huntington’s disease. The molecular features of these disorders have been extensively researched and various unique neurotherapeutic interventions have been developed. However, there is an enormous coercion to integrate the existing knowledge in order to intensify the reliability with which neurodegenerative disorders can be diagnosed and treated. The objective of this review article is therefore to assimilate these disorders’ in terms of their neuropathology, neurogenetics, etiology, trends in pharmacological treatment, clinical management, and the use of innovative neurotherapeutic interventions.

High homocysteine serum levels in young male schizophrenia and bipolar patients and in an animal model

Elevated plasma homocysteine has been found to be a risk factor for Alzheimer's disease as well as cerebral vascular disease, suggesting that some risk factors can accelerate or increase the severity of several CNS disease processes. We screened plasma total homocysteine levels of 193 schizophrenic patients vs. 762 controls for plasma homocysteine levels. The effect of schizophrenia was marked (p<0.0001) and mean homocysteine level was 16.3+/-12 (S.D.) microM in schizophrenic patients vs. 10.6+/-3.6 (S.D.) microM in healthy controls. The increase was almost entirely in young male schizophrenic patients. It seemed important to determine if this finding is already present in newly admitted schizophrenic patients. Serum homocysteine levels were studied in 184 consecutively admitted schizophrenic patients and 305 control subjects. Homocysteine levels were markedly increased in this population of newly admitted schizophrenic patients, especially in young males. However, no difference was found for CSF homocysteine levels between schizophrenia patients and controls. We also examined homocysteine levels in 41 euthymic outpatients with bipolar disorder. Functional deterioration in patients was rated as 'present' or 'absent' by consensus of two treating clinicians. Young male bipolar patients were found to have higher homocysteine levels than controls. Among the male subjects, bipolar patients showing deterioration had homocysteine levels which were significantly higher than other patients. We attempted to develop a model of homocysteine neurotoxicity in mice. Mice were fed homocysteine in water at a dose of 200 mg/kg per mouse per day. Independent samples of animals were studied at 2 to 6 months with behavioral tests including apomorphine-induced stereotypy and spatial learning and memory in the Morris Water Maze. Homocysteine levels were elevated up to 800% at months 5 and 6 by this procedure. No homocysteine-induced defects were found in any behavioral test until month 5 when mild but statistically significant abnormalities in the Morris Water Maze were detected.

Genetic disruptions of O/E2 and O/E3 genes reveal involvement in olfactory receptor neuron projection

The mammalian Olf1/EBF (O/E) family of repeated helix-loop-helix (rHLH)transcription factors has been implicated in olfactory system gene regulation,nervous system development and B-cell differentiation. Ebf(O/E1) mutant animals showed defects in B-cell lineage and brain regions where it is the only O/E family member expressed, but the olfactory epithelium appeared unaffected and olfactory marker expression was grossly normal in these animals. In order to further study the mammalian O/E proteins,we disrupted O/E2 and O/E3 genes in mouse and placed tau-lacZ and tau-GFP reporter genes under the control of the respective endogenous O/E promoters. Mice mutant for each of these genes display reduced viability and other gene-specific phenotypes. Interestingly, both O/E2 and O/E3 knockout mice as well as O/E2/O/E3 double heterozygous animals share a common phenotype:olfactory neurons (ORN) fail to project to dorsal olfactory bulb. We suggest that a decreased dose of O/E protein may alter expression of O/E target genes and underlie the ORN projection defect.

Rb binding protein Che-1 interacts with Tau in cerebellar granule neurons. Modulation during neuronal apoptosis

Che-1 is a recently identified human Rb binding protein that inhibits the Rb growth-suppressing function and regulates cell proliferation. Che-1 contacts the Rb and competes with HDAC1 for Rb-binding site, removing HDAC1 from the Rb/E2F cell cycle-regulated promoters. We have investigated the expression of Che-1 in neuronal cells and we showed that Che-1 directly interacts with Tau. Tau is a microtubule-associated protein involved in the assembly and stabilization of neuronal microtubules network that plays a crucial role modulating neuronal morphogenesis, axonal shape, and transport. In rat cerebellar granule neurons (CGNs) Che-1 partially colocalizes with Tau in the cytoplasm. Che-1 binds the amino-terminal region of Tau protein, which is not involved in microtubule interactions. Tau and Che-1 endogenous proteins coimmunoprecipitate from CGNs cellular lysates. In addition, Che-1/Tau interaction was demonstrated both in overexpressing COS-7 cells and CGNs by FRET analysis. Finally, we observed that Tau/Che-1 interaction is modulated during neuronal apoptosis.

Intracellular A-beta amyloid, a sign for worse things to come?

In this review the authors discuss the possible neuropathological role of intracellular amyloid-beta accumulation in Alzheimer's disease (AD) pathology. There is abundant evidence that at early stages of the disease, prior to A-beta amyloid plaque formation, A-beta peptides accumulate intraneuronally in the cerebral cortex and the hippocampus. The experimental evidence would indicate that intracellular amyloid-beta could originate both by intracellular biosynthesis and also from the uptake of amyloidogenic peptides from the extracellular milieu. Herein the aspects of the possible impact of intracellular amyloid-beta in human AD pathology are discussed, as well as recent observations from a rat transgenic model with a phenotype of intracellular accumulation of A-beta fragments in neurons of the hippocampus and cortex, without plaque formation. In this model, the intracellular amyloid-beta phenotype is accompanied by increased MAPK/ERK activity and tau hyperphosphorylation. Finally, the authors discuss the hypothesis that, prior to plaque formation, intracellular A-beta accumulation induces biochemical and pathological changes in the brain at the cellular level priming neurons to further cytotoxic attack of extracellular amyloidogenic peptides.