Fimbria-fornix lesion does not affect APP levels and amyloid deposition in the hippocampus of APP+PS1 double transgenic mice

Neural stem cell-derived extracellular vesicles: The light of central nervous system diseases

Central nervous system (CNS) diseases are the leading cause of death worldwide. By performing compensatory functions and improving the inflammatory microenvironment, the transplantation of neural stem cells (NSCs) can promote functional recovery from brain injury, aging, brain tumours, and other diseases. However, the ability of NSCs to differentiate into neurons is limited, and they are associated with a risk of tumourigenicity. NSC-derived extracellular vesicles (NSC-EVs) can modulate the local microenvironment of the nervous system as well as distant neuronal functions. Thus, cell-free therapy may be a novel remedy for CNS disorders. This article reviews the characteristics, contents, and mechanisms of action of NSC-EVs as well as their roles and application prospects in various CNS diseases.

circRNA Acbd6 promotes neural stem cell differentiation into cholinergic neurons via the miR-320-5p-Osbpl2 axis

Neural stem cells (NSCs) persist in the dentate gyrus of the hippocampus into adulthood and are essential for both neurogenesis and neural circuit integration. Exosomes have also been shown to play vital roles in regulating biological processes of receptor cells as a medium for cell-to-cell communication signaling molecules. The precise molecular mechanisms of exosome-mediated signaling, however, remain largely unknown. Here, we found that exosomes produced by denervated hippocampi following fimbria–fornix transection could promote the differentiation of hippocampal neural precursor cells into cholinergic neurons in coculture with NSCs. Furthermore, we found that 14 circular RNAs (circRNAs) were upregulated in hippocampal exosomes after fimbria–fornix transection using high-throughput RNA-Seq technology. We further characterized the function and mechanism by which the upregulated circRNA Acbd6 (acyl-CoA-binding domain–containing 6) promoted the differentiation of NSCs into cholinergic neurons using RT–quantitative PCR, Western blot, ELISA, flow cytometry, immunohistochemistry, and immunofluorescence assay. By luciferase reporter assay, we demonstrated that circAcbd6 functioned as an endogenous miR-320-5p sponge to inhibit miR-320-5p activity, resulting in increased oxysterol-binding protein–related protein 2 expression with subsequent facilitation of NSC differentiation. Taken together, our results suggest that circAcbd6 promotes differentiation of NSCs into cholinergic neurons via miR-320-5p/oxysterol-binding protein–related protein 2 axis, which contribute important insights to our understanding of how circRNAs regulate neurogenesis.

Treatment with D3 removes amyloid deposits, reduces inflammation, and improves cognition in aged AβPP/PS1 double transgenic mice

One of the characteristic pathological hallmarks of Alzheimer's disease (AD) is neuritic plaques. The sequence of events leading to deposition of amyloid-β (Aβ) peptides in plaques is not clear. Here we investigate the effects of D3, an Aβ oligomer directed D-enantiomeric peptide that was obtained from a mirror image phage display selection against monomeric or small oligomeric forms of Aβ42, on Aβ deposition in aged AβPP/PS1 double transgenic AD-model mice. Using Alzet minipumps, we infused the brains of these AD model mice for 8 weeks with FITC-labeled D3, and examined the subsequent changes in pathology and cognitive deficits. Initial cognitive deficits are similar comparing control and D3-FITC-treated mice, but the treated mice show a significant improvement on the last day of testing. Further, we show that there is a substantial reduction in the amount of amyloid deposits in the animals treated with D3-FITC, compared to the control mice. Finally, the amount of activated microglia and astrocytes surrounding Aβ deposits is dramatically reduced in the D3-FITC-treated mice. Our findings demonstrate that treatments with the high affinity Aβ42 oligomer binding D-enantiomeric peptide D3 significantly decrease Aβ deposits and the associated inflammatory response, and improve cognition even when applied only at late stages and high age. Together, this suggests that the treatment reduces the level of Aβ peptide in the brains of AβPP/PS1 mice, possibly by increasing Aβ outflow from the brain. In conclusion, treatments with this D-peptide have great potential to be successful in AD patients.

Rapid β-amyloid deposition and cognitive impairment after cholinergic denervation in APP/PS1 mice

Although extensive evidence supports the role of β-amyloid (Aβ) in Alzheimer disease (AD), the neurotoxic mechanisms underlying AD pathogenesis are not understood. On the other hand, neuronal loss is the pathologic feature that best correlates with cognitive impairment. We hypothesized that cholinergic neurodegeneration may lead to Aβ deposition and tested this by inducing selective cholinergic lesions in APPswe/PS1dE9 mice with murine p75 saporin (mu p75-SAP). Intracerebroventricular lesions that removed approximately 50% of cholinergic innervation to the cortex and hippocampus were induced in animals with incipient (∼3 months) and marked (∼7 months of age) Aβ deposition. Cranial windows were implanted, and Aβ deposition was monitored in vivo using multiphoton microscopy. Deposition of Aβ was increased as soon as 7 days after the lesion, and this effect was maintained up to 3 months later. Postmortem studies using immunohistochemistry with an anti-Aβ antibody corroborated these findings in both cerebral cortex and hippocampus. Tau phosphorylation was also significantly increased after the lesions. Cholinergic denervation resulted in early memory impairment at 3 months of age that worsened with age (∼7 months); there was a synergistic effect between cholinergic denervation and the presence of APP/PS1 transgenes. Altogether, our data suggest that cholinergic denervation may trigger Aβ deposition and synergistically contribute to cognitive impairment in AD patients.

Artificial theta stimulation impairs encoding of contextual fear memory

Several experiments have demonstrated an intimate relationship between hippocampal theta rhythm (4-12 Hz) and memory. Lesioning the medial septum or fimbria-fornix, a fiber track connecting the hippocampus and the medial septum, abolishes the theta rhythm and results in a severe impairment in declarative memory. To assess whether there is a causal relationship between hippocampal theta and memory formation we investigated whether restoration of hippocampal theta by electrical stimulation during the encoding phase also restores fimbria-fornix lesion induced memory deficit in rats in the fear conditioning paradigm. Male Wistar rats underwent sham or fimbria-fornix lesion operation. Stimulation electrodes were implanted in the ventral hippocampal commissure and recording electrodes in the septal hippocampus. Artificial theta stimulation of 8 Hz was delivered during 3-min free exploration of the test cage in half of the rats before aversive conditioning with three foot shocks during 2 min. Memory was assessed by total freezing time in the same environment 24 h and 28 h after fear conditioning, and in an intervening test session in a different context. As expected, fimbria-fornix lesion impaired fear memory and dramatically attenuated hippocampal theta power. Artificial theta stimulation produced continuous theta oscillations that were almost similar to endogenous theta rhythm in amplitude and frequency. However, contrary to our predictions, artificial theta stimulation impaired conditioned fear response in both sham and fimbria-fornix lesioned animals. These data suggest that restoration of theta oscillation per se is not sufficient to support memory encoding after fimbria-fornix lesion and that universal theta oscillation in the hippocampus with a fixed frequency may actually impair memory.

Ectopic neurogenesis in the forebrain cholinergic system-related areas of a rat dementia model

Lesions to the fimbria fornix (FiFx) plus cingulate bundle (CB), the principal routes of communication of forebrain cholinergic regions, produce lasting impairment of spatial learning and memory in mice. We report that extensive neurogenesis takes place in the FiFx, CB, and basalis magnocellularis following FiFx plus CB transection. Immunofluorescence revealed that nestin-expressing cells were present in all 3 areas following lesion; the majority of nestin-positive cells were also positive for 5-bromo-2-deoxy-uridine, a marker of DNA synthesis. Nestin-positive proliferative cells were almost entirely absent from unlesioned tissue. Neurospheres cultured in vitro from lesioned FiFx displayed the characteristics of neural stem cells--proliferation, expression of embryonic markers, and multipotential differentiation into neurons, astrocytes, and oligodendrocytes. At early stages after transection, a small number of immature and migrating doublecortin-immunopositive neurons were detected in lesioned FiFx, where neuronal cell bodies are normally absent. At later stages, postlesion immature neurons developed into β-tubulin III-positive mature neurons. Lentivirus labeling assay implied that the injury-induced neurogenesis in FiFx may be from local neurogenic astrocytes but not from dentate gyrus. These results demonstrate that insult to cholinergic tracts can stimulate neural stem cell proliferation and neuronal regeneration not only in innervated regions but also in the projection pathways themselves. Ectopic neurogenesis in cholinergic system-related areas provides an additional mechanism for repair of cholinergic innervation following damage.

Cholinotrophic basal forebrain system alterations in 3xTg-AD transgenic mice

The cholinotrophic system, which is dependent upon nerve growth factor and its receptors for survival, is selectively vulnerable in Alzheimer's disease (AD). But, virtually nothing is known about how this deficit develops in relation to the hallmark lesions of this disease, amyloid plaques and tau containing neurofibrillary tangles. The vast majority of transgenic models of AD used to evaluate the effect of beta amyloid (Aβ) deposition upon the cholinotrophic system over-express the amyloid precursor protein (APP). However, nothing is known about how this system is affected in triple transgenic (3xTg)-AD mice, an AD animal model displaying Aβ plaque- and tangle-like pathology in the cortex and hippocampus, which receive extensive cholinergic innervation. We performed a detailed morphological and biochemical characterization of the cholinotrophic system in young (2-4 months), middle-aged (13-15 months) and old (18-20 months) 3xTg-AD mice. Cholinergic neuritic swellings increased in number and size with age, and were more conspicuous in the hippocampal-subicular complex in aged female than in 3xTg-AD male mice. Stereological analysis revealed a reduction in choline acetyltransferase (ChAT) positive cells in the medial septum/vertical limb of the diagonal band of Broca in aged 3xTg-AD mice. ChAT enzyme activity levels decreased significantly in the hippocampus of middle-aged 3xTg-AD mice compared to age-matched non-transgenic (or wild type) mice. ProNGF protein levels increased in the cortex of aged 3xTg-AD mice, whereas TrkA protein levels were reduced in a gender-dependent manner in aged mutant mice. In contrast, p75(NTR) protein cortical levels were stable but increased in the hippocampus of aged 3xTg-AD mice. These data demonstrate that cholinotrophic alterations in 3xTg-AD mice are age- and gender-dependent and more pronounced in the hippocampus, a structure more severely affected by Aβ plaque pathology.

Brn-4 is upregulated in the deafferented hippocampus and promotes neuronal differentiation of neural progenitors in vitro

Fimbria-fornix (FF), the septo-hippocampal pathway, was transected to model Alzheimer's disease (AD), which is characterized by loss of cholinergic afferent fibers in hippocampus. Various alternations may happen in the deafferented hippocampus. In this study, we determined the expression of Brn-4 in hippocampus after FF lesion. RT-PCR and Western blot showed that mRNA transcription and protein of Brn-4 increased significantly and reached to the peak at day 14 after FF lesion. Hybridization and immunohistochemistry indicated that Brn-4 signals in hippocampus and dentate gyrus (DG) of the deafferented side were significantly stronger than the normal side. More Brn-4 positive cells were identified in the DG of deafferented hippocampus. In the pyramidal and granular cells, Brn-4 positive cells were all NeuN positive neurons, whereas in the neurogenic area, subgranular zone (SGZ), only a part of Brn-4 positive cells were NeuN positive, and these Brn-4/NeuN double positive neurons in SGZ and hilus of DG increased significantly after the trauma induced by FF lesion. In vitro Brn-4 antibody attenuated the role of extract from deafferented hippocampus in promoting differentiation of hippocampal progenitors into MAP-2 positive neurons. This study demonstrated that after FF lesion, Brn-4 in the deafferented hippocampus was upregulated and might play an important role in inducing local progenitors to differentiate into neurons, which may compensate for the loss of cholinergic afferent fibers or other dysfunctions.

Immunotherapy as treatment for Alzheimer's disease

Alzheimer's disease (AD) is a progressive neurodegenerative disorder that is characterized pathologically by the deposition of beta-amyloid (A beta)-containing extracellular neuritic plaques, intracellular neurofibrillary tangles and neuronal loss. Much evidence supports the hypothesis that A beta peptide aggregation contributes to AD pathogenesis, however, currently approved therapeutic treatments do nothing to stop or reverse A beta deposition. The success of active and passive anti-A beta immunotherapies in both preventing and clearing parenchymal amyloid in transgenic mouse models led to the initiation of an active anti-A beta vaccination (AN1792) trial in human patients with mild-to-moderate AD, but was prematurely halted when 6% of inoculated patients developed aseptic meningoencephalitis. Autopsy results from the brains of four individuals treated with AN1792 revealed decreased plaque burden in select brain areas, as well as T-cell lymphocytes in three of the patients. Furthermore, antibody responders showed some improvement in memory task measures. These findings indicated that anti-A beta therapy might still be a viable option for the treatment of AD, if potentially harmful proinflammatory processes can be avoided. Over the past 6 years, this target has led to the development of novel experimental immunization strategies, including selective A beta epitope targeting, antibody and adjuvant modifications, as well as alternative routes and mechanisms of vaccine delivery, to generate anti-A beta antibodies that selectively target and remove specific A beta species without evoking autoimmunity. Results from the passive vaccination AD clinical trials that are currently underway will provide invaluable information about both the effectiveness of newly improved anti-A beta vaccines in clinical treatment, as well as the role of the A beta peptide in the pathogenesis of the disease.

The alpha7 nicotinic receptor agonist 4OH-GTS-21 protects axotomized septohippocampal cholinergic neurons in wild type but not amyloid-overexpressing transgenic mice

While activation of alpha7 nicotinic receptors protects neurons from a variety of apoptotic insults in vitro, little is known about this neuroprotective action in vivo, especially under amyloidogenic conditions that mimic Alzheimer's disease. We therefore investigated the effects of 4OH-GTS-21, a selective partial agonist for these receptors, on septohippocampal cholinergic and GABAergic neuron survival following fimbria fornix (FFX) lesions in three strains of mice: C57BL/6J wild type mice; human presenilin-1 mutant M146L (PS1) transgenic mice; and mice expressing both mutant PS1 and Swedish mutant K670N/M671L amyloid precursor protein (APP). Initial studies to demonstrated that 4OH-GTS-21 is likely brain permeant based on its ability to improve passive avoidance and Morris water task behaviors in nucleus basalis-lesioned rats. In FFX-lesioned mice, twice per day i.p. injections of 1 mg/kg of 4OH-GTS-21 for 2 weeks promoted the survival and prevented the atrophy of septal cholinergic neurons. Septal parvalbumin-staining GABAergic neurons were not protected by this treatment, although they also express alpha7 nicotinic receptors, suggesting an indirect, nerve growth factor (NGF)-mediated mechanism. No protection of cholinergic neurons was observed in similarly treated PS1 or APP/PS1 transgenic mice. 4OH-GTS-21 treatment actually reduced cholinergic neuronal size in APP/PS1 mice. Hippocampal amyloid deposition was not affected by FFX lesions or treatment with this alpha7 nicotinic receptor agonist in APP/PS1 mice under these conditions. These results indicate that brain alpha7 nicotinic receptors are potential targets for protecting at-risk brain neurons in Alzheimer's disease, perhaps via their effects on NGF receptors; however, this protection may be sensitive under some conditions to environmental factors such as inhibitory amyloid-peptides.

The denervated hippocampus provides proper microenvironment for the survival and differentiation of neural progenitors

The fate of neural stem/progenitor cells (NSCs/NPCs) in vivo lies on the local microenvironment. Whether the denervated hippocampus provides a stimulative role on the survival and differentiation of the anterior subventricular zone (SVZa) progenitors was investigated in the present study. In vivo the SVZa progenitors were transplanted into the denervated hippocampus and the contralateral side, and were found migrating along the subgranular layer. More implanted cells were found survived and differentiated into the Neurofilament 200 (NF-200) or beta-Tubulin-III positive neurons in the denervated than in the normal hippocampus at all points studied. In vitro the extracts from the denervated and normal hippocampus were used to induce differentiation of the SVZa progenitors. More progenitors incubated with the denervated hippocampal extract differentiated significantly into the MAP-2 or AChE positive neurons than those incubated with the normal hippocampal extract (P<0.05). We concluded that the deafferented hippocampus provided proper microenvironment for the survival and neuronal differentiation of neural progenitors.

Acetylcholinesterase inhibitors ameliorate behavioral deficits in the Tg2576 mouse model of Alzheimer's disease

RATIONALE

Acetylcholinesterase inhibitors are widely used for the treatment of patients with Alzheimer's disease (AD). However, the relationship between the capacity of such drugs to ameliorate the symptoms of AD and their ability to alter the underlying disease process is not well understood. Transgenic mice that overexpress the human form of amyloid precursor protein and develop deposits of beta-amyloid (Abeta) and behavioral deficits during adulthood are useful for investigating this question.

OBJECTIVES

The effects of administration of two acetylcholinesterase inhibitors, physostigmine and donepezil, on Abeta plaque formation and memory-related behaviors were investigated in the Tg2576-transgenic mouse model of AD. At 9-10 months of age, Tg2576-transgenic [Tg(+)] mice develop Abeta plaques and impairments on paradigms related to learning and memory as compared to transgene-negative [Tg(-)] mice.

METHODS

Beginning at 9 months of age, increasing doses of physostigmine (0.03, 0.1, and 0.3 mg/kg), donepezil (0.1, 0.3, and 1.0 mg/kg), or saline were administered over 6 weeks to cohorts of Tg(+) and Tg(-) mice. Performance on tests of spatial reversal learning and fear conditioning was evaluated at each drug dose throughout the period of drug administration. After drug administration was completed, the animals were sacrificed and Abeta plaque number was quantified.

RESULTS

Administration of physostigmine and donepezil improved deficits in contextual and cued memory in Tg(+) mice so that their behaviors became more similar to Tg(-) mice. However, administration of physostigmine and donepezil tended to improve cued memory and deficits in spatial learning in both Tg(+) and Tg(-) mice. Physostigmine administration demonstrated more prominent effects in improving contextual memory than donepezil, while donepezil was more effective than physostigmine in improving deficits in the acquisition of the spatial memory paradigm. Administration of neither drug altered the deposition of Abeta plaques.

CONCLUSIONS

These studies suggest that acetylcholinesterase inhibitors can ameliorate memory deficits in Tg(+) mice without necessarily altering the deposition of Abeta plaques. Tg2576 mice may be useful as an animal model to further investigate the mechanisms by which aceytlcholinesterase inhibitors improve cognitive deficits in patients with AD.

Transgenic AD model mice, effects of potential anti-AD treatments on inflammation and pathology

The extracellular deposition of amyloid (A) peptides in plaques, and neurofibrillary tangles are the two characteristic pathological features of Alzheimer's disease (AD). Plaques are surrounded by activated astrocytes and microglia, to study the relation between amyloid neuropathology and inflammation, we examined the changes in amyloid pathology in the hippocampus following three different treatments aimed at reducing the amyloid burden. (1) To investigate the effects of long-term cholinergic deafferentation, we lesioned the fimbria-fornix pathway in our AD-model mice at 7 months of age, and 11 months post-lesion the mice were sacrificed for histopathological analysis. The fimbria-fornix transection resulted in a substantial depletion of cholinergic markers in the hippocampus, but the lesion did not result in an alteration in hippocampal A deposition and inflammation (i.e., numbers or staining density of astrocytes and microglia). (2) To investigate the effects of estrogen, we ovariectomized mice and treated them with estrogen (sham-lesion, zero dose, low dose, and high dose) and studied the pathology at different postsurgery intervals. Estrogen depletion (i.e., ovariectomy) or estrogen replacement did not affect A deposition or inflammation at any time point. (3) In the final studies, we treated mice with flurbiprofen and an NO-donating derivative of flurbiprofen (HCT 1026) for several months (from 6 till 14 months of age), and studied the A pathology and inflammation in the brain. Sham treatment, flurbiprofen, and the low-dose HCT 1026 did not affect pathology; however, a higher dose of HCT 1026 reduced both A load and amount of microglial activation surrounding plaques.

Gender differences in the amount and deposition of amyloidbeta in APPswe and PS1 double transgenic mice

Transgenic mice carrying both the human amyloid precursor protein (APP) with the Swedish mutation and the presenilin-1 A246E mutation (APP/PS1 mice) develop Alzheimer's disease-like amyloidbeta protein (Abeta) deposits around 9 months of age. These mice show an age-dependent increase in the level of Abeta40 and Abeta42 and in the number of amyloid plaques in the brain. Abeta40 and Abeta42 levels were measured, and amyloid burden and plaque number were quantified, in the hippocampus at the age of 4, 12, and 17 months in both male and female APP/PS1 mice. In all mice, amyloid burden and plaque number increased markedly with age, with female mice bearing a heavier amyloid burden and higher plaque number compared to male mice of the same age, both at 12 and at 17 months of age. The level of both Abeta40 and Abeta42 significantly increased in female mice with age and was always significantly higher in female than in male mice of the same age. Further, there were significant correlations between amyloid burden and Abeta42 level in female mice and between amyloid burden and plaques in both female and male mice. Together these data show that female APP/PS1 mice accumulate amyloid at an earlier age and that they build up more amyloid deposits in the hippocampus than age-matched male mice. Together, these results provide new insights in the potential mechanisms of the observed gender differences in the pathogenesis of AD.