Phosphorylation of the tau protein sequence 199–205 in the hippocampal CA3 region of Syrian hamsters in adulthood and during aging

Effects of early tooth loss on chronic stress and progression of neuropathogenesis of Alzheimer's disease in adult Alzheimer's model AppNL-G-F mice

Introduction

Alzheimer's disease (AD), the most common neurodegenerative disease, is characterized by accumulated amyloid-β (Aβ) plaques, aggregated phosphorylated tau protein, gliosis-associated neuroinflammation, synaptic dysfunction, and cognitive impairment. Many cohort studies indicate that tooth loss is a risk factor for AD. The detailed mechanisms underlying the association between AD and tooth loss, however, are not yet fully understood.

Methods

We explored the involvement of early tooth loss in the neuropathogenesis of the adult AppNL-G-F mouse AD model. The maxillary molars were extracted bilaterally in 1-month-old male mice soon after tooth eruption.

Results

Plasma corticosterone levels were increased and spatial learning memory was impaired in these mice at 6 months of age. The cerebral cortex and hippocampus of AD mice with extracted teeth showed an increased accumulation of Aβ plaques and phosphorylated tau proteins, and increased secretion of the proinflammatory cytokines, including interleukin 1β (IL-1β) and tumor necrosis factor α (TNF-α), accompanied by an increased number of microglia and astrocytes, and decreased synaptophysin expression. AD mice with extracted teeth also had a shorter lifespan than the control mice.

Discussion

These findings revealed that long-term tooth loss is a chronic stressor, activating the recruitment of microglia and astrocytes; exacerbating neuroinflammation, Aβ deposition, phosphorylated tau accumulation, and synaptic dysfunction; and leading to spatial learning and memory impairments in AD model mice.

Ethanol sustains phosphorylated tau protein in the cultured neonatal rat hippocampus: Implications for fetal alcohol spectrum disorders

Fetal Alcohol Spectrum Disorders (FASDs) are comprised of developmental, behavioral, and cognitive abnormalities caused by prenatal alcohol exposure, affecting an estimated 2%-5% of children and costing $4 billion annually in the United States. While some behavioral therapies help, the neurobiological mechanisms that underpin FASDs need further elucidation for development of efficacious pharmacotherapeutics. The role of the tau protein in the hippocampus is likely to be involved. Tau catalyzes microtubule polymerization in developing neurons. However, this function can become disrupted by hyperphosphorylation. Many of the cognitive deficits observed in neurodegenerative tauopathies overlap to some degree with what is observed in juvenile developmental disabilities, such as FASDs (e.g., selective memory, executive dysfunction). Thus, tau protein phosphorylation may be one important mechanism of dysfunction in FASDs. The purpose of this study is to provide an empirical basis for a tauopathic characterization of FASDs. To do so, hippocampal slices were extracted from rats at postnatal day 10 (PND10); hippocampal slices were then exposed to 5 days of 50mM ethanol between 6 days in vitro (DIV) and 11DIV. Immunoblots were taken for Total and p-Tau (Threonine231) at 12DIV and 24DIV. Immunohistochemical fluorescent images were taken for p-Tau (Threonine231) at 12DIV and 24DIV. Separate p-Tau measures were taken for the cornu ammonis 1 (CA1), CA3, and dentate gyrus (DG). Total Tau protein expression remained unchanged between 12DIV and 24DIV regardless of EtOH condition. In the control group, longer DIV was associated with decreased p-Tau. However, in the EtOH-exposed group, p-Tau was sustained across DIV. This is the first study to show that EtOH exposure sustains tau Threonine231 phosphorylation in the perinatal hippocampus regardless of total tau expression. These findings could lead to innovative pharmacotherapeutic targets for the treatment of cognitive deficits seen in FASDs.

Tau Post-Translational Modifications: Potentiators of Selective Vulnerability in Sporadic Alzheimer's Disease

Sporadic Alzheimer's Disease (AD) is the most common form of dementia, and its severity is characterized by the progressive formation of tau neurofibrillary tangles along a well-described path through the brain. This spatial progression provides the basis for Braak staging of the pathological progression for AD. Tau protein is a necessary component of AD pathology, and recent studies have found that soluble tau species with selectively, but not extensively, modified epitopes accumulate along the path of disease progression before AD-associated insoluble aggregates form. As such, modified tau may represent a key cellular stressing agent that potentiates selective vulnerability in susceptible neurons during AD progression. Specifically, studies have found that tau phosphorylated at sites such as T181, T231, and S396 may initiate early pathological changes in tau by disrupting proper tau localization, initiating tau oligomerization, and facilitating tau accumulation and extracellular export. Thus, this review elucidates potential mechanisms through which tau post-translational modifications (PTMs) may simultaneously serve as key modulators of the spatial progression observed in AD development and as key instigators of early pathology related to neurodegeneration-relevant cellular dysfunctions.

Maturation of neuronal AD-tau pathology involves site-specific phosphorylation of cytoplasmic and synaptic tau preceding conformational change and fibril formation

In Alzheimer's disease (AD), tau-protein undergoes a multi-step process involving the transition from a natively unfolded monomer to large, aggregated structures such as neurofibrillary tangles (NFTs). However, it is not yet clear which events initiate the early preclinical phase of AD tauopathy and whether they have impact on the propagation of tau pathology in later disease stages. To address this question, we analyzed the distribution of tau species phosphorylated at T231, S396/S404 and S202/T205, conformationally modified at the MC1 epitope and fibrillary tau detected by the Gallyas method (Gallyas-tau), in the brains of 15 symptomatic and 20 asymptomatic cases with AD pathology as well as of 19 nonAD cases. As initial tau lesions, we identified phosphorylated-T231-tau diffusely distributed within the somatodendritic compartment (IC-tau) and phosphorylated-S396/pS404-tau in axonal lesions of the white matter and in the neuropil (IN-tau). The subcellular localization of pT231-tau in the cell body and pS396/pS404-tau in the presynapse was confirmed in hP301L mutant Drosophila larvae. Phosphorylated-S202/T205-tau, MC1-tau and Gallyas-tau were negative for these lesions. IC- and IN-tau were observed in all analyzed regions of the human brain, including early affected regions in nonAD cases (entorhinal cortex) and late affected regions in symptomatic AD cases (cerebellum), indicating that tau pathology initiation follows similar processes when propagating into previously unaffected regions. Furthermore, a sequence of AD-related maturation of tau-aggregates was observed, initiated by the appearance of IC- and IN-tau, followed by the formation of pretangles exhibiting pT231-tau, pS396/pS404-tau and pS202/pT205-tau, then by MC1-conformational tau, and, finally, by the formation of Gallyas-positive NFTs. Since cases classified as nonAD [Braak NFT stages < I (including a-1b)] already showed IC- and IN-tau, our findings suggest that these lesions are a prerequisite for the development of AD.

Hibernation Impairs Odor Discrimination - Implications for Alzheimer's Disease

Reversible formation of PHF-like phosphorylated tau, an early feature of Alzheimer's disease (AD) was previously shown to occur in torpor during hibernation in the Golden hamster (Syrian hamster, Mesocricetus auratus). Here, we tackled the question to what extent hibernating Golden hamsters can serve as a model for the early stage of AD. During early AD, anosmia, the loss of olfactory function, is a common and typical feature. We, thus, investigated tau phosphorylation, synaptic plasticity and behavioral physiology of the olfactory system during hibernation. Tau was phosphorylated on several AD-relevant epitopes, and distribution of PHF-like phosphorylated tau in the olfactory bulb was quite similar to what is seen in AD. Tau phosphorylation was not associated with a destabilization of microtubules and did not lead to fibril formation. Previously, we observed a transient spine reduction in pyramidal cells in the hippocampus, which is correlated with the distribution of phosphorylated tau. Here we show that granule cells in the olfactory bulb are devoid of phosphorylated tau and maintain their spines number during torpor. No reduction of synaptic proteins was observed. However, hibernation did impair the recall performance in a two-odor discrimination task. We conclude that hibernation is associated with a specific olfactory memory deficit, which might not be attributed to the formation of PHF-like phosphorylated tau within the olfactory bulb. We discuss a possible involvement of modulatory input provided by cholinergic neurons in the basal forebrain, which are affected by hibernation.

Modifications of the axon initial segment during the hibernation of the Syrian hamster

Mammalian hibernation is a natural process in which the brain undergoes profound adaptive changes that appear to protect the brain from extreme hypoxia and hypothermia. In addition to a virtual cessation of neural and metabolic activity, these changes include a decrease in adult neurogenesis; the retraction of neuronal dendritic trees; changes in dendritic spines and synaptic connections; fragmentation of the Golgi apparatus; and the phosphorylation of the microtubule-associated protein tau. Furthermore, alterations of microglial cells also occur in torpor. Importantly, all of these changes are rapidly and fully reversed when the animals arouse from torpor state, with no apparent brain damage occurring. Thus, hibernating animals are excellent natural models to study different aspects of brain plasticity. The axon initial segment (AIS) is critical for the initiation of action potentials in neurons and is an efficient site for the regulation of neural activity. This specialized structure-characterized by the expression of different types of ion channels and adhesion, scaffolding and cytoskeleton proteins-is subjected to morpho-functional plastic changes upon variations in neural activity or in pathological conditions. Here, we used immunocytochemistry and 3D confocal microscopy reconstruction techniques to measure the possible morphological differences in the AIS of neocortical (layers II-III and V) and hippocampal (CA1) neurons during the hibernation of the Syrian hamster. Our results indicate that the general integrity of the AIS is resistant to the ischemia/hypoxia conditions that are characteristic of the torpor phase of hibernation. In addition, the length of the AIS significantly increased in all the regions studied-by about 16-20% in torpor animals compared to controls, suggesting the existence of compensatory mechanisms in response to a decrease in neuronal activity during the torpor phase of hibernation. Furthermore, in double-labeling experiment, we found that the AIS in layer V of torpid animals was longer in neurons expressing phospho-tau than in those not labeled for phospho-tau. This suggests that AIS plastic changes were more marked in phospho-tau accumulating neurons. Overall, the results further emphasize that mammalian hibernation is a good physiological model to study AIS plasticity mechanisms in non-pathological conditions.

Changes in the Golgi Apparatus of Neocortical and Hippocampal Neurons in the Hibernating Hamster

Hibernating animals have been used as models to study several aspects of the plastic changes that occur in the metabolism and physiology of neurons. These models are also of interest in the study of Alzheimer's disease because the microtubule-associated protein tau is hyperphosphorylated during the hibernation state known as torpor, similar to the pretangle stage of Alzheimer's disease. Hibernating animals undergo torpor periods with drops in body temperature and metabolic rate, and a virtual cessation of neural activity. These processes are accompanied by morphological and neurochemical changes in neurons, which reverse a few hours after coming out of the torpor state. Since tau has been implicated in the structural regulation of the neuronal Golgi apparatus (GA) we have used Western Blot and immunocytochemistry to analyze whether the GA is modified in cortical neurons of the Syrian hamster at different hibernation stages. The results show that, during the hibernation cycle, the GA undergo important structural changes along with differential modifications in expression levels and distribution patterns of Golgi structural proteins. These changes were accompanied by significant transitory reductions in the volume and surface area of the GA elements during torpor and arousal stages as compared with euthermic animals.

The endocrine dyscrasia that accompanies menopause and andropause induces aberrant cell cycle signaling that triggers re-entry of post-mitotic neurons into the cell cycle, neurodysfunction, neurodegeneration and cognitive disease

This article is part of a Special Issue "SBN 2014". Sex hormones are physiological factors that promote neurogenesis during embryonic and fetal development. During childhood and adulthood these hormones support the maintenance of brain structure and function via neurogenesis and the formation of dendritic spines, axons and synapses required for the capture, processing and retrieval of information (memories). Not surprisingly, changes in these reproductive hormones that occur with menopause and during andropause are strongly correlated with neurodegeneration and cognitive decline. In this connection, much evidence now indicates that Alzheimer's disease (AD) involves aberrant re-entry of post-mitotic neurons into the cell cycle. Cell cycle abnormalities appear very early in the disease, prior to the appearance of plaques and tangles, and explain the biochemical, neuropathological and cognitive changes observed with disease progression. Intriguingly, a recent animal study has demonstrated that induction of adult neurogenesis results in the loss of previously encoded memories while decreasing neurogenesis after memory formation during infancy mitigated forgetting. Here we review the biochemical, epidemiological and clinical evidence that alterations in sex hormone signaling associated with menopause and andropause drive the aberrant re-entry of post-mitotic neurons into an abortive cell cycle that leads to neurite retraction, neuron dysfunction and neuron death. When the reproductive axis is in balance, gonadotropins such as luteinizing hormone (LH), and its fetal homolog, human chorionic gonadotropin (hCG), promote pluripotent human and totipotent murine embryonic stem cell and neuron proliferation. However, strong evidence supports menopausal/andropausal elevations in the LH:sex steroid ratio as driving aberrant mitotic events. These include the upregulation of tumor necrosis factor; amyloid-β precursor protein processing towards the production of mitogenic Aβ; and the activation of Cdk5, a key regulator of cell cycle progression and tau phosphorylation (a cardinal feature of both neurogenesis and neurodegeneration). Cognitive and biochemical studies confirm the negative consequences of a high LH:sex steroid ratio on dendritic spine density and human cognitive performance. Prospective epidemiological and clinical evidence in humans supports the premise that rebalancing the ratio of circulating gonadotropins:sex steroids reduces the incidence of AD. Together, these data support endocrine dyscrasia and the subsequent loss of cell cycle control as an important etiological event in the development of neurodegenerative diseases including AD, stroke and Parkinson's disease.

Changes in tau phosphorylation in hibernating rodents

Tau is a cytoskeletal protein present mainly in the neurons of vertebrates. By comparing the sequence of tau molecule among different vertebrates, it was found that the variability of the N-terminal sequence in tau protein is higher than that of the C-terminal region. The N-terminal region is involved mainly in the binding of tau to cellular membranes, whereas the C-terminal region of the tau molecule contains the microtubule-binding sites. We have compared the sequence of Syrian hamster tau with the sequences of other hibernating and nonhibernating rodents and investigated how differences in the N-terminal region of tau could affect the phosphorylation level and tau binding to cell membranes. We also describe a change, in tau phosphorylation, on a casein kinase 1 (ck1)-dependent site that is found only in hibernating rodents. This ck1 site seems to play an important role in the regulation of tau binding to membranes.

Tau Phosphorylation by GSK3 in Different Conditions

Almost a 20% of the residues of tau protein are phosphorylatable amino acids: serine, threonine, and tyrosine. In this paper we comment on the consequences for tau of being a phosphoprotein. We will focus on serine/threonine phosphorylation. It will be discussed that, depending on the modified residue in tau molecule, phosphorylation could be protective, in processes like hibernation, or toxic like in development of those diseases known as tauopathies, which are characterized by an hyperphosphorylation and aggregation of tau.

Early improved and late defective cognition is reflected by dendritic spines in Tau.P301L mice

Cognitive demise correlates with progressive brain tauopathy in dementing patients. Improved cognition of young Tau.P301L mice contrasts with dysfunction later in life and remains unexplained (Boekhoorn et al., 2006). To unravel early mechanisms, we composed a correlative time line of clinical symptoms, cognitive defects, and biochemical and pathological traits, including comprehensive analysis of dendritic spines in specified regions of the cortex and hippocampus of young and adult Tau.P301L mice. Remarkably, young Tau.P301L mice have not more, but more mature spines than wild-type mice, revealing the anatomical substrate for their improved cognition and LTP. Spine maturation remained high in the hippocampus of adult Tau.P301L mice. However, spines regressed in length paralleling impaired cognition and increased Tau phosphorylation (Terwel et al., 2005). Conversely, spine maturation was unaffected in adult Tau.4R mice, while spine density was increased and length reduced similar to Tau.P301L mice. To explain how protein Tau promoted spinogenesis, we analyzed hippocampal synaptosomes and dendritic spines for mouse and human Tau. While synaptosomes were positive for both mouse and human Tau, weak variable reaction in spines was observed only after fixation according to Bouin. Mouse Tau was absent from spines in wild-type mice, dissociating the pathological actions of Tau in transgenic mice by relocalization into dendrites and spines from the physiological actions of protein Tau in axons only. We conclude that mutant protein Tau modulates cognition and morphology of spines similarly and in both directions, with pathology later in life coinciding with increased phosphorylation and relocalization of Tau from axons to soma and processes.

Vacuolation and mineralisation as dominant age-related findings in hamster brains

Syrian golden hamsters (Mesocricetus auratus) are laboratory animals increasingly used for research and toxicological studies. Despite the need for an adequate knowledge of spontaneously occurring lesions, studies investigating the background pathology of different organ systems in hamsters are lacking. The aim of this study was to investigate the occurrence of spontaneous, age-dependent lesions in the central nervous system of this species. Multiple brain and spinal cord transverse sections of 520 hamsters of 1, 3, 6, 12, and 24 months of age were investigated using histology and immunohistochemistry. Vacuolation of grey matter neuropil and mineralisation especially in the brain stem were the most prominent findings. They gradually increased in severity and frequency with age. Vacuolation and mineralisation affected approximately 100% and 50% of 24-month-old hamsters, respectively. In addition, pigment deposition and mast cell infiltration were commonly detected. Whether vacuolation and mineralisation represent an incidental finding or are related to a cognitive dysfunction syndrome remains to be determined.

Chronic restraint stress alters the expression and distribution of phosphorylated tau and MAP2 in cortex and hippocampus of rat brain

Microtubule-associated proteins (MAPs) play a critical role in maintaining normal cytoskeletal architecture and functions. In the present study, we aim to explore the effects of the emotional stressor, chronic restraint stress, on the expression levels and localization of tau and MAP2. We found that after chronic restraint stress, soluble hyperphosphorylated tau was greatly increased, whereas MAP2 was decreased. Moreover, immunohistochemistry analysis demonstrated that phosphorylated tau and MAP2 displayed the similar subcellular distribution pattern after chronic restraint stress. Robust hyperphosphorylated tau immunolabeling was observed both in cortex and hippocampus of stressed animals and mainly located to perikaryal/dendritic elements. After stress, the MAP2 was mainly distributed in the perikaryal compartments, discontinuous dendrites and neuropil. Moreover, the distribution pattern of MAP2 in hippocampus significantly changed. Immunofluorescence double labeling indicated that hyperphosphorylated tau increased in the regions where there displayed an decrease of MAP2. These results suggest that the involvement of repeated restraint stress may not only induce phosphorylation state of tau and distribution of phosphorylated tau, but also alter the content and neuronal distribution of MAP2. Tau and MAP2 are most important MAPs for neuronal cells, the subcellular distribution change of them might be link to functional change of neurons after emotional stress.

Tau hyperphosphorylation and axonal damage induced by N,N-diethyldithiocarbamate (DEDTC) treatment along late postnatal development is followed by a rescue during adulthood

Axonal degeneration has been described as the pathological hallmark of peripheral neuropathies induced by DEDTC. In addition, axonal damage has also been observed in the brain of mice treated daily with DEDTC along postnatal development, though with this experimental model there was observed to be axonal recovery after treatment, during the adulthood. To focus on this axonal dynamic activity, damage-recovery, a key axonal protein, the microtubule associated protein tau, was analyzed in this DEDTC model. Tau is a phosphoprotein and its dynamic site-specific phosphorylation is essential for its proper function; in fact, high levels are correlated with cell dysfunction. Furthermore, the levels of tau phosphorylation are associated with dynamic microtubules during periods of high plasticity. Thus, phosphorylated tau at two sites of phosphorylation, Ser(199) and Ser(396), were evaluated during the second week of postnatal development and throughout adulthood. The results obtained by Western blot made it evident that the levels of p-tau Ser(199) and p-tau Ser(396) were higher in treated mice than in controls. Interestingly, by immunohistochemistry there was shown to be an increase in p-tau-immunolabeling in neuronal soma together with axonal tract alterations in treated animals with respect to controls, and the analyses of GSK3 beta and cdk5 revealed an increase in its activity in DEDTC-treated animals. Nevertheless, in the adult a general decline in p-tau was observed together with a rescue of axonal tract. All these data support the idea that the axonal damage induced by DEDTC treatment along postnatal development is followed by an axonal rescue during adulthood.

Reversible reduction in dendritic spines in CA1 of rat and ground squirrel subjected to hypothermia-normothermia in vivo: A three-dimensional electron microscope study

A study was made at electron microscope level of changes in the three-dimensional (3-D) morphology of dendritic spines and postsynaptic densities (PSDs) in CA1 of the hippocampus in ground squirrels, taken either at low temperature during hibernation (brain temperature 2-4 degrees C), or after warming and recovery to the normothermic state (34 degrees C). In addition, the morphology of PSDs and spines was measured in a non-hibernating mammal, rat, subjected to cooling at 2 degrees C at which time core rectal temperature was 15 degrees C, and then after warming to normothermic conditions. Significant differences were found in the proportion of thin and stubby spines, and shaft synapses in CA1 for rats and ground squirrels for normothermia compared with cooling or hibernation. Hypothermia induced a decrease in the proportion of thin spines, and an increase in stubby and shaft spines, but no change in the proportion of mushroom spines. The changes in redistribution of these three categories of spines in ground squirrel are more prominent than in rat. There were no significant differences in synapse density determined for ground squirrels or rats at normal compared with low temperature. Measurement of spine and PSD volume (for mushroom and thin spines) also showed no significant differences between the two functional states in either rats or ground squirrels, nor were there any differences in distances between neighboring synapses. Spinules on dendritic shafts were notable qualitatively during hibernation, but absent in normothermia. These data show that hypothermia results in morphological changes which are essentially similar in both a hibernating and a non-hibernating animal.

Hibernation model of tau phosphorylation in hamsters: selective vulnerability of cholinergic basal forebrain neurons - implications for Alzheimer's disease

Neurofibrillar tangles made up of 'paired helical filaments' (PHFs) consisting of hyperphosphorylated microtubule-associated protein tau are major hallmarks of Alzheimer's disease (AD). Tangle formation selectively affects certain neuronal types and systematically progresses throughout numerous brain areas, which reflects a hierarchy of neuronal vulnerability and provides the basis for the neuropathological staging of disease severity. Mechanisms underlying this selective neuronal vulnerability are unknown. We showed previously that reversible PHF-like phosphorylation of tau occurs during obligate hibernation. Here we extend these findings to facultative hibernators such as Syrian hamsters (Mesocricetus auratus) forced into hibernation. In this model, we showed in the basal forebrain projection system that cholinergic neurons are selectively affected by PHF-like phosphorylated tau, while gamma-aminobutyric acid (GABA)ergic neurons are largely spared, which shows strong parallels to the situation in AD. Formation of PHF-tau in these neurons apparently does not affect their function as pacemaker for terminating hibernation. We conclude that although formation of PHF-like phosphorylated tau in the mammalian brain follows a certain hierarchy, affecting some neurons more frequently than others, it is not necessarily associated with impaired neuronal function and viability. This indicates a more general link between PHF-like phosphorylation of tau and the adaptation of neurons under conditions of a 'vita minima'.