Expression of P301L-hTau in mouse MEC induces hippocampus-dependent memory deficit

Tau pathology in the dorsal raphe may be a prodromal indicator of Alzheimer's disease

Protein aggregation in brainstem nuclei is thought to occur in the early stages of Alzheimer's disease (AD), but its specific role in driving prodromal symptoms and disease progression is largely unknown. The dorsal raphe nucleus (DRN) contains a large population of serotonin (5-hydroxytryptamine; 5-HT) neurons that regulate mood, reward-related behavior, and sleep, which are all disrupted in AD. We report here that tau pathology is present in the DRN of individuals 25-80 years old without a known history of dementia, and its prevalence was comparable to the locus coeruleus (LC). By comparison, fewer cases were positive for other pathological proteins including α-synuclein, β-amyloid, and TDP-43. To evaluate how early tau pathology impacts behavior, we overexpressed human P301L-tau in the DRN of mice and observed depressive-like behaviors and hyperactivity without deficits in spatial memory. Tau pathology was predominantly found in neurons relative to glia and colocalized with a significant proportion of Tph2-expressing neurons in the DRN. 5-HT neurons were also hyperexcitable in P301L-tauDRN mice, and there was an increase in the amplitude of excitatory post-synaptic currents (EPSCs). Moreover, astrocytic density was elevated in the DRN and accompanied by an increase in IL-1α and Frk expression, which suggests increased inflammatory signaling. Additionally, tau pathology was detected in axonal processes in the thalamus, hypothalamus, amygdala, and caudate putamen. A significant proportion of this tau pathology colocalized with the serotonin reuptake transporter (SERT), suggesting that tau may spread in an anterograde manner to regions outside the DRN. Together these results indicate that tau pathology accumulates in the DRN in a subset of individuals over 50 years and may lead to behavioral dysregulation, 5-HT neuronal dysfunction, and activation of local astrocytes which may be prodromal indicators of AD.

Microglial-Targeted nSMase2 Inhibitor Fails to Reduce Tau Propagation in PS19 Mice

The progression of Alzheimer's disease (AD) correlates with the propagation of hyperphosphorylated tau (pTau) from the entorhinal cortex to the hippocampus and neocortex. Neutral sphingomyelinase2 (nSMase2) is critical in the biosynthesis of extracellular vesicles (EVs), which play a role in pTau propagation. We recently conjugated DPTIP, a potent nSMase2 inhibitor, to hydroxyl-PAMAM-dendrimer nanoparticles that can improve brain delivery. We showed that dendrimer-conjugated DPTIP (D-DPTIP) robustly inhibited the spread of pTau in an AAV-pTau propagation model. To further evaluate its efficacy, we tested D-DPTIP in the PS19 transgenic mouse model. Unexpectantly, D-DPTIP showed no beneficial effect. To understand this discrepancy, we assessed D-DPTIP's brain localization. Using immunofluorescence and fluorescence-activated cell-sorting, D-DPTIP was found to be primarily internalized by microglia, where it selectively inhibited microglial nSMase2 activity with no effect on other cell types. Furthermore, D-DPTIP inhibited microglia-derived EV release into plasma without affecting other brain-derived EVs. We hypothesize that microglial targeting allowed D-DPTIP to inhibit tau propagation in the AAV-hTau model, where microglial EVs play a central role in propagation. However, in PS19 mice, where tau propagation is independent of microglial EVs, it had a limited effect. Our findings confirm microglial targeting with hydroxyl-PAMAM dendrimers and highlight the importance of understanding cell-specific mechanisms when designing targeted AD therapies.

Clinical significance of the cognition-related pathogenic proteins in plasma neuronal-derived exosomes among normal cognitive adults over 45 years old with olfactory dysfunction

OBJECTIVES

Exosomal Phospho-Tau-181(P-T181-tau), Total tau (T-tau), and amyloid-β peptide 42 (Aβ42) have been proved the capacity for the amnestic mild cognitive impairment (MCI) and the diagnosis of Alzheimer's disease (AD). This study aimed to explore the cognitive function and the levels of P-T181-tau, T-tau, and Aβ42 in neuronal-derived exosomes (NDEs) extracted from plasma in normal cognitive adults over 45 years old with olfactory dysfunction.

METHODS

A cross-sectional survey of 29 participants aged over 45 was conducted. Plasma exosomes were isolated, precipitated, and enriched by immuno-absorption with anti- L1 cell adhesion molecule (L1CAM) antibody. NDEs were characterized by CD81, and extracted NDE protein (P-T181-tau, T-tau, and Aβ42) biomarkers were quantified by enzyme-linked immunosorbent assay (ELISAs). Olfactory performance was assessed by Sniffin' Sticks and cognitive performance was assessed by Montreal Cognitive Assessment (MoCA).

RESULTS

There was no significant difference between adults with olfactory dysfunction and without olfactory dysfunction regarding the cognitive function as measured by MoCA and all the participants showed normal cognition. Adults with olfactory dysfunction showed a higher concentration of P-T181-tau in plasma NDEs than did adults without olfactory dysfunction (P = 0.034). Both the levels of P-T181-tau (r = - 0.553, P = 0.003) and T-tau (r = - 0.417, P = 0.034) negatively correlated with the odor identification scores. In addition, the level of T-tau negatively correlated with MoCA scores (r = - 0.597, P = 0.002). The levels of P-T181-tau (r = - 0.464, P = 0.022) and T-tau (r = - 0.438, P = 0.032) negatively correlated with the delayed recall scores.

CONCLUSIONS

This study demonstrated that cognition-related pathogenic proteins including P-T181-tau in plasma NDEs were significantly increased in adults over 45 years old with olfactory dysfunction before the occurrence of cognitive impairment. The impaired odor identification and the delayed recall function were highly associated with the increased levels of P-T181-tau and T-tau in plasma NDEs.

T217-Phosphorylation Exacerbates Tau Pathologies and Tau-Induced Cognitive Impairment

BACKGROUND

Recent studies show that an increased T217-phosphorylation of tau in plasma could diagnose AD at an early stage with high accuracy and high specificity, while the potential toxic role of tau T217-phosphorylation is not known.

OBJECTIVE

To study the potential toxic role of tau T217-phosphorylation.

METHODS

We performed stereotactic brain injection, behavioral testing, immunohistochemistry and immunofluorescence, western blotting, Golgi staining, in vitro recombinant tau polymerization, and other measurements.

RESULTS

We first constructed tau T217-wild-type (T217), T217-phospho-mimic (T217E), and T217-non-phospho-mimic (T217A) plasmids or their virus vectors on the basis of wild-type tau. We found that expressing tau-T217E induced a significantly increased tau phosphorylation at multiple AD-associated sites with inhibited proteolysis and increased cleavage/fibrillization of tau, while expressing tau-T217A abolished the above changes of tau both in vitro and in vivo. By mutating T217E on tau-P301L, a dominant mutation identified in patients with frontotemporal dementia, we did not observe significant exacerbation of tau-P301L phosphorylation and cognitive impairment although the increased tau cleavage and propagation were shown.

CONCLUSION

T217-phosphorylation exacerbates wild-type tau hyperphosphorylation with aggravated tau cleavage/fibrillization and cognitive impairments, while overexpressing T217E on the basis P301L does not exacerbate tau phosphorylation or the P301L-induced cognitive deficits, although it aggravates tau cleavage and propagation.

Peripheral inflammation promotes brain tau transmission via disrupting blood-brain barrier

Abnormal aggregation of pathological tau protein is a neuropathological feature of Alzheimer's disease (AD). In the AD patients, the abnormal tau accumulation first appeared in entorhinal cortex (EC) and then propagated to the hippocampus with microglia activation and inflammation, but the mechanism is elusive. Here, we studied the role and mechanisms underlying periphery inflammation on brain tau transmission. By intraperitoneal injection of lipopolysaccharide (LPS) with brain medial entorhinal cortex (MEC)-specific overexpressing P301L human tau (P301L-hTau), we found that both acute and chronic administration of LPS remarkably promoted P301L-hTau transmission from MEC to the hippocampal subsets. Interestingly, the chronic LPS-induced P301L-hTau transmission was still apparent after blocking microglia activation. Further studies demonstrated that LPS disrupted the integrity of blood-brain barrier (BBB) and simultaneous intraperitoneal administration of glucocorticoid (GC) attenuated LPS-promoted P301L-hTau transmission. These data together suggest that a non-microglia-dependent BBB disruption contributes to peripheral LPS-promoted brain P301L-hTau transmission, therefore, maintaining the integrity of BBB can be a novel strategy for preventing pathological tau propagation in AD and other tauopathies.

Sowing the Seeds of Discovery: Tau-Propagation Models of Alzheimer's Disease

The propagation of pathological proteins throughout the brain is the primary physiological hallmark of the progression of Alzheimer's Disease (AD). A growing body of evidence indicates that hyperphosphorylated Tau proteins are spread transcellularly between neurons in a prionlike fashion, inducing misfolding and aggregation into neurofibrillary tangles which accumulate along specific connectivity pathways. Earlier transgenic rodent AD models did not capture this disease-relevant spread, and therefore, seeded Tau-propagation models have been developed. Here, mutant human Tau (as isolated protein or packaged into an adeno-associated virus (AAV) viral vector) is stereotaxically injected into select brain regions and its histopathological propagation to downstream neurons quantified. These models offer a faster and more direct mechanism to evaluate genetic components and therapeutic approaches which attenuate Tau spreading in vivo. Recently, these Tau-seeding models have revealed several new targets for AD drug discovery, including nSMase2, SIRT1, p300/CBP, LRP1, and TYROBP, as well as the potential therapeutics based on melatonin and chondroitinase ABC. Importantly, these Tau-propagation rodent models more closely phenocopy the progression of AD in humans and are therefore likely to improve preclinical studies and derisk future moves into clinical trials.

Intranasal Insulin Prevents Anesthesia-induced Cognitive Impairments in Aged Mice

BACKGROUND

Preclinical and clinical evidence suggests that elderly individuals are at increased risk of cognitive decline after general anesthesia. General anesthesia is also believed to be a risk factor for Postoperative Cognitive Dysfunction (POCD) and Alzheimer's Disease (AD). Intranasal administration of insulin, which delivers the drug directly into the brain, improves memory and cognition in both animal studies and small clinical trials. However, how insulin treatment improves cognitive function is poorly understood.

METHODS

Aged mice were pretreated with intranasal insulin or saline before anesthesia. Propofol was added intraperitoneally to the mice from 7th day of insulin/saline treatment, and general anesthesia was induced and maintained for 2 hours/day for 5 consecutive days. Mice were evaluated at 26th day when the mice were continued on insulin or saline administration for another 15 days.

RESULTS

We found that intranasal insulin treatment prevented anesthesia-induced cognitive impairments, as measured by novel object recognition test and contextual-dependent fear conditioning test. Insulin treatment also increased the expression level of Post-synaptic Density Protein 95 (PSD95), as well as upregulated Microtubule-associated Protein-2 (MAP-2) in the dentate gyrus of the hippocampus. Furthermore, we found that insulin treatment restored insulin signaling disturbed by anesthesia via activating PI3K/PDK1/AKT pathway, and attenuated anesthesia-induced hyperphosphorylation of tau at multiple AD-associated sites. We found the attenuation of tau hyperphosphorylation occurred by increasing the level of GSK3β phosphorylated at Ser9, which leads to inactivation of GSK-3β.

CONCLUSION

Intranasal insulin administration might be a promising therapy to prevent anesthesiainduced cognitive deficit in elderly individuals.

AD-Related N-Terminal Truncated Tau Is Sufficient to Recapitulate In Vivo the Early Perturbations of Human Neuropathology: Implications for Immunotherapy

The NH₂tau 26-44 aa (i.e., NH₂htau) is the minimal biologically active moiety of longer 20-22-kDa NH₂-truncated form of human tau-a neurotoxic fragment mapping between 26 and 230 amino acids of full-length protein (htau40)-which is detectable in presynaptic terminals and peripheral CSF from patients suffering from AD and other non-AD neurodegenerative diseases. Nevertheless, whether its exogenous administration in healthy nontransgenic mice is able to elicit a neuropathological phenotype resembling human tauopathies has not been yet investigated. We explored the in vivo effects evoked by subchronic intracerebroventricular (i.c.v.) infusion of NH₂htau or its reverse counterpart into two lines of young (2-month-old) wild-type mice (C57BL/6 and B6SJL). Six days after its accumulation into hippocampal parenchyma, significant impairment in memory/learning performance was detected in NH₂htau-treated group in association with reduced synaptic connectivity and neuroinflammatory response. Compromised short-term plasticity in paired-pulse facilitation paradigm (PPF) was detected in the CA3/CA1 synapses from NH₂htau-impaired animals along with downregulation in calcineurin (CaN)-stimulated pCREB/c-Fos pathway(s). Importantly, these behavioral, synaptotoxic, and neuropathological effects were independent from the genetic background, occurred prior to frank neuronal loss, and were specific because no alterations were detected in the control group infused with its reverse counterpart. Finally, a 2.0-kDa peptide which biochemically and immunologically resembles the injected NH₂htau was endogenously detected in vivo, being present in hippocampal synaptosomal preparations from AD subjects. Given that the identification of the neurotoxic tau species is mandatory to develop a more effective tau-based immunological approach, our evidence can have important translational implications for cure of human tauopathies.