Aging reduces glial uptake and promotes extracellular accumulation of Aβ from a lentiviral vector

Microglia in Alzheimer's disease: pathogenesis, mechanisms, and therapeutic potentials

Alzheimer's disease (AD) is a neurodegenerative disorder characterized by protein aggregation in the brain. Recent studies have revealed the critical role of microglia in AD pathogenesis. This review provides a comprehensive summary of the current understanding of microglial involvement in AD, focusing on genetic determinants, phenotypic state, phagocytic capacity, neuroinflammatory response, and impact on synaptic plasticity and neuronal regulation. Furthermore, recent developments in drug discovery targeting microglia in AD are reviewed, highlighting potential avenues for therapeutic intervention. This review emphasizes the essential role of microglia in AD and provides insights into potential treatments.

SENP1 modulates chronic intermittent hypoxia-induced inflammation of microglia and neuronal injury by inhibiting TOM1 pathway

Chronic intermittent hypoxia (CIH) is a characteristic pathophysiological change of obstructive sleep apnea syndrome (OSAS). Inflammation of microglia induced by CIH, plays a vital role in OSAS-associated cognitive dysfunction. SUMO-specific proteases 1 (SENP1) has been implicated in tumor inflammatory microenvironment and cells migration. However, the role of SENP1 in CIH-induced neuroinflammation remains unknown. We aimed to investigate the effect of SENP1 on neuroinflammation and neuronal injury. After the preparation of SENP1 overexpression microglia and SENP1 knockout mouse, CIH microglia and mice were established using an intermittent hypoxia device. Results showed that CIH reduced the level of SENP1 and TOM1, induced the SUMOylation of TOM1, and promoted microglial migration, neuroinflammation, neuronal amyloid-beta 42 (Aβ₄₂) deposition and apoptosis in vitro and in vivo. After SENP1 overexpression in vitro, the enhanced SUMOylation of TOM1 was inhibited; the level of TOM1 and microglial migration were enhanced; neuroinflammation, neuronal Aβ₄₂ deposition and apoptosis were significantly reduced. However, the administration of siRNA-TOM1 suppressed microglial migration, neuroinflammation, neuronal Aβ₄₂ deposition and apoptosis. After SENP1 knockout in vivo, the SUMOylation enhancement of TOM1 was accelerated, microglial migration was inhibited. Neuroinflammation, neuronal Aβ₄₂ deposition and apoptosis, cognitive impairment was significantly exacerbated. Overall, the results demonstrated that SENP1 promoted microglial migration by alleviating the de-SUMOylation of TOM1, thus contributing to attenuate neuroinflammation, neuronal Aβ₄₂ deposition and neuronal apoptosis induced by CIH.

Apoptotic neurons and amyloid-beta clearance by phagocytosis in Alzheimer's disease: Pathological mechanisms and therapeutic outlooks

Neuronal survival and axonal renewal following central nervous system damage and in neurodegenerative illnesses, such as Alzheimer's disease (AD), can be enhanced by fast clearance of neuronal apoptotic debris, as well as the removal of amyloid beta (Aβ) by phagocytic cells through the process of efferocytosis. This process quickly inhibits the release of proinflammatory and antigenic autoimmune constituents, enhancing the formation of a microenvironment vital for neuronal survival and axonal regeneration. Therefore, the detrimental features associated with microglial phagocytosis uncoupling, such as the accumulation of apoptotic cells, inflammation and phagoptosis, could exacerbate the pathology in brain disease. Some mechanisms of efferocytosis could be targeted by several promising agents, such as curcumin, URMC-099 and Y-P30, which have emerged as potential treatments for AD. This review aims to investigate and update the current research regarding the signaling molecules and pathways involved in efferocytosis and how these could be targeted as a potential therapy in AD.

Development of a Human APOE Knock-in Mouse Model for Study of Cognitive Function After Cancer Chemotherapy

Cancer-related cognitive impairment in breast cancer patients exposed to multi-agent chemotherapy regimens is associated with the apolipoprotein E4 (APOE4) allele. However, it is difficult to determine the effects of specific agents on cognitive impairment in human studies. We describe the development of a human APOE knock-in congenic C57BL/6J mouse model to study cancer-related cognitive impairment. Female APOE3 and APOE4 homozygous mice were either left untreated or treated with the most commonly used breast cancer therapeutic agent, doxorubicin. APOE3 and APOE4 mice had similar behaviors in exploratory and anxiety assays, which were affected transiently by doxorubicin treatment. Spatial learning and memory were measured in a Barnes maze: after 4 days of training, control APOE3 and APOE4 mice were able to escape with similar latencies. In contrast, doxorubicin-treated APOE4 mice had markedly impaired learning compared to doxorubicin-treated APOE3 mice at all time points. Voxel-based morphometry of magnetic resonance images revealed that doxorubicin treatment caused significant changes in the cortex and hippocampus of in both APOE3 and APOE4 mouse brains, but the differences were significantly greater in the APOE4 brains. The results indicate that doxorubicin-exposed APOE4 mice recapitulate key aspects of human cancer-related cognitive impairment. These data support the usefulness of this novel preclinical model for future elucidation of the genetic and molecular interactions of APOE genotype with chemotherapy; this model can also allow extension to prospective studies of older mice to study these interactions in the context of aging.

Decreased immunoglobulin G in brain regions of elder female APOE4-TR mice accompany with Aβ accumulation

Background

Apolipoprotein E4 (APOE4) and ageing are the most important known risk factors for late-onset Alzheimer’s disease (AD). In the present study, we determined the alterations of IgG, CD19, and Aβ in various brain regions of uninfected male and female APOE3- and APOE4-TR mice at the age of 3 and 10 months to elucidate impacts of AD risk factors on alterations of brain IgG.

Results

Positive staining for IgG was distributed across the brain, including neocortex, entorhinal cortex, hippocampus, thalamus and cerebellum. IgG positive staining was mainly located on microglia, but not astrocytes. Some IgG positive neurons were also observed, but only in mediodorsal thalamic nucleus. Compared with APOE3-TR mice, 10-month-old female APOE4-TR mice had lower IgG level in AD susceptible brain regions such as neocortex, entorhinal cortex and hippocampus, but no significant changes in thalamus and cerebellum, two regions nearly intact in AD. In addition, the expression of CD19, a specific marker for mature B cells, was significantly reduced in the hippocampus of 10-month-old female APOE4-TR mice. Although there were no obvious differences in plasma IgG levels between APOE4- and age matched female APOE3-TR mice, significant decreased B cell amount in blood of 10-month-old female APOE4-TR mice have also been found. Moreover, more obvious positive staining for Aβ was observed in the cortex of 10-month-old female APOE4-TR mice than other groups.

Conclusions

Our study demonstrated that AD risk factors were associated with IgG alterations in various brain regions, which might result from the defects of humoral immunity and lead to the impairment of IgG-mediated clearance of Aβ by microglia, therefore facilitated AD progression.

Bilateral Implantation of Shear Stress Modifier in ApoE Knockout Mouse Induces Cognitive Impairment and Tau Abnormalities

Vascular cognitive impairment (VCI) encompasses all causes of cerebrovascular disease that lead to cognitive decline, or overt dementia, atherosclerotic disease being the most common contributor. However, few rodent models that mimic the pathology of VCI replicated the clinical cerebrovascular atherosclerosis. Here we aimed to investigate the mechanism underlying VCI in an Apolipoprotein E knockout (ApoE-KO) mouse model fed with western style food with implantation of bilateral shear stress modifiers. We established a cognitive decline in spatial learning and memory developed in the bilateral modifier treated mice. Brain imaging and pathological examinations demonstrated reduced glucose intake and neuronal loss in hippocampus. Although no amyloid plaques or neurofibrillary tangles (NFTs) were observed, tau pathology including hyperphosphorylation, paired helical filament formation and pathologic truncation were found at considerable higher extent in the bilateral modifier group 8 weeks post the procedure. In addition, gliosis and microglia activation were confirmed in corpus callosum (CC) and ventral striatum. Thus, this ApoE-KO mouse model faithfully replicates the stenosis of common carotid artery (CCA) and cognitive impairment following atherosclerotic deposition and global cerebral hypoperfusion. The close correlation of cognitive decline and tau pathology indicates the toxic tau species could be at least partially responsible for the neurodegenerative changes induced by the chronic hypoxia/ischemia.

Natural Peptides in Drug Discovery Targeting Acetylcholinesterase

Acetylcholinesterase-inhibitory peptide has gained much importance since it can inhibit acetylcholinesterase (AChE) and increase the availability of acetylcholine in cholinergic synapses, enhancing cholinergic transmission in pharmacological treatment of Alzheimer’s disease (AD). Natural peptides have received considerable attention as biologically important substances as a source of AChE inhibitors. These natural peptides have high potential pharmaceutical and medicinal values due to their bioactivities as neuroprotective and neurodegenerative treatment activities. These peptides have attracted great interest in the pharmaceutical industries, in order to design potential peptides for use in the prophylactic and therapy purposes. Some natural peptides and their derivatives have high commercial values and have succeeded in reaching the pharmaceutical market. A large number of peptides are already in preclinical and clinical pipelines for treatment of various diseases. This review highlights the recent researches on the various natural peptides and future prospects for AD management.

Studying the Progression of Amyloid Pathology and Its Therapy Using Translational Longitudinal Model of Accumulation and Distribution of Amyloid Beta

Long‐term effects of amyloid targeted therapy can be studied using a mechanistic translational model of amyloid beta (Aβ) distribution and aggregation calibrated on published data in mouse and human species. Alzheimer disease (AD) pathology is modeled utilizing age‐dependent pathological evolution for rate constants and several variants of explicit functions for Aβ toxicity influencing cognitive outcomes (Adas‐cog). Preventive Aβ targeted therapies were simulated to minimize the Aβ difference from healthy physiological levels. Therapeutic targeted simulations provided similar predictions for mouse and human studies. Our model predicts that: (1) at least 1 year (2 years for preclinical AD) of treatment is needed to observe cognitive effects; (2) under the hypothesis with functional importance of Aβ, a 15% decrease in Aβ (using an imaging biomarker) is related to 15–20% cognition improvement by immunotherapy. Despite negative outcomes in clinical trials, Aβ continues to remain a prospective target demanding careful assessment of mechanistic effect and duration of trial design.

Translocator Protein-18 kDa (TSPO) Positron Emission Tomography (PET) Imaging and Its Clinical Impact in Neurodegenerative Diseases

In vivo exploration of activated microglia in neurodegenerative diseases is achievable by Positron Emission Tomography (PET) imaging, using dedicated radiopharmaceuticals targeting the translocator protein-18 kDa (TSPO). In this review, we emphasized the major advances made over the last 20 years, thanks to TSPO PET imaging, to define the pathophysiological implication of microglia activation and neuroinflammation in neurodegenerative diseases, including Parkinson’s disease, Huntington’s disease, dementia, amyotrophic lateral sclerosis, multiple sclerosis, and also in psychiatric disorders. The extent and upregulation of TSPO as a molecular biomarker of activated microglia in the human brain is now widely documented in these pathologies, but its significance, and especially its protective or deleterious action regarding the disease’s stage, remains under debate. Thus, we exposed new and plausible suggestions to enhance the contribution of TSPO PET imaging for biomedical research by exploring microglia’s role and interactions with other cells in brain parenchyma. Multiplex approaches, associating TSPO PET radiopharmaceuticals with other biomarkers (PET imaging of cellular metabolism, neurotransmission or abnormal protein aggregates, but also other imaging modalities, and peripheral cytokine levels measurement and/or metabolomics analysis) was considered. Finally, the actual clinical impact of TSPO PET imaging as a routine biomarker of neuroinflammation was put into perspective regarding the current development of diagnostic and therapeutic strategies for neurodegenerative diseases.

Old Maids: Aging and Its Impact on Microglia Function

Microglia are highly active and vigilant housekeepers of the central nervous system that function to promote neuronal growth and activity. With advanced age, however, dysregulated inflammatory signaling and defects in phagocytosis impede their ability to perform the most essential of homeostatic functions, including immune surveillance and debris clearance. Microglial activation is one of the hallmarks of the aging brain and coincides with age-related neurodegeneration and cognitive decline. Age-associated microglial dysfunction leads to cellular senescence and can profoundly alter the response to sterile injuries and immune diseases, often resulting in maladaptive responses, chronic inflammation, and worsened outcomes after injury. Our knowledge of microglia aging and the factors that regulate age-related microglial dysfunction remain limited, as the majority of pre-clinical studies are performed in young animals, and human brain samples are difficult to obtain quickly post-mortem or in large numbers. This review outlines the impact of normal aging on microglial function, highlights the potential mechanisms underlying age-related changes in microglia, and discusses how aging can shape the recovery process following injury.

The Oral Iron Chelator, Deferasirox, Reverses the Age-Dependent Alterations in Iron and Amyloid-β Homeostasis in Rat Brain: Implications in the Therapy of Alzheimer's Disease

The altered metabolism of iron impacts the brain function in multiple deleterious ways during normal aging as well as in Alzheimer's disease. We have shown in this study that chelatable iron accumulates in the aged rat brain along with overexpression of transferrin receptor 1 (TfR1) and ferritin, accompanied by significant alterations in amyloid-β (Aβ) peptide homeostasis in the aging brain, such as an increased production of the amyloid-β protein precursor, a decreased level of neprilysin, and increased accumulation of Aβ42. When aged rats are given daily the iron chelator, deferasirox, over a period of more than 4 months starting from the 18th month, the age-related accumulation of iron and overexpression of TfR1 and ferritin in the brain are significantly prevented. More interestingly, the chelator treatment also considerably reverses the altered Aβ peptide metabolism in the aging brain implying a significant role of iron in the latter phenomenon. Further, other results indicate that iron accumulation results in oxidative stress and the activation of NF-κB in the aged rat brain, which are also reversed by the deferasirox treatment. The analysis of the results together suggests that iron accumulation and oxidative stress interact at multiple levels that include transcriptional and post-transcriptional mechanisms to bring about changes in the expression levels of TfR1 and ferritin and also alterations in Aβ peptide metabolism in the aging rat brain. The efficacy of deferasirox in preventing age-related changes in iron and Aβ peptide metabolism in the aging brain, as shown here, has obvious therapeutic implications for Alzheimer's disease.

APOE Genotype Alters Immunoglobulin Subtypes in Knock-In Mice

Apolipoprotein E (APOE) alleles are strongly related to the risk of Alzheimer's disease (AD). APOE genotype also affects inflammatory processes in response to damage. We tested whether APOE genotype affected the levels of specific immunoglobulins in healthy, uninfected APOE knock-in mice. We measured specific immunoglobulins in brain, spleen, and plasma. Levels of total IgG in brain and spleen were highest in APOE-ɛ3 mice, significantly higher than in APOE-ɛ2 and APOE-ɛ4 mice; no differences were observed for levels of total IgG in plasma. We also measured specific subtypes of IgG. IgG1 was only detectable in plasma and did not differ by APOE genotype. IgG3 was detectable in plasma and spleen, and also did not differ by APOE genotype. IgG2b showed the same pattern as levels of total IgG by APOE genotype, with the highest levels of IgG2b in brain, spleen, and plasma of APOE-ɛ3 mice. IgG2a showed an entirely different pattern, with significantly higher levels in spleen and plasma of APOE-ɛ4 mice compared to APOE-ɛ2 and APOE-ɛ3 mice. We also measured IgM and IgA in spleens and plasma of these mice. In spleen, APOE-ɛ4 mice had the lowest IgA levels and the highest levels of IgM; both being significantly different from APOE-ɛ2 mice. In total, murine IgG2a and IgM were highest in APOE-ɛ4 mice, while total IgG and Ig2b were highest in APOE-ɛ3 mice. These dramatically different distributions of immunoglobulins could allow for human AD risk biomarkers based on specific immunoglobulin subtypes.

Microglial cell dysregulation in brain aging and neurodegeneration

Aging is the main risk factor for neurodegenerative diseases. In aging, microglia undergoes phenotypic changes compatible with their activation. Glial activation can lead to neuroinflammation, which is increasingly accepted as part of the pathogenesis of neurodegenerative diseases, including Alzheimer’s disease (AD). We hypothesize that in aging, aberrant microglia activation leads to a deleterious environment and neurodegeneration. In aged mice, microglia exhibit an increased expression of cytokines and an exacerbated inflammatory response to pathological changes. Whereas LPS increases nitric oxide (NO) secretion in microglia from young mice, induction of reactive oxygen species (ROS) predominates in older mice. Furthermore, there is accumulation of DNA oxidative damage in mitochondria of microglia during aging, and also an increased intracellular ROS production. Increased ROS activates the redox-sensitive nuclear factor kappa B, which promotes more neuroinflammation, and can be translated in functional deficits, such as cognitive impairment. Mitochondria-derived ROS and cathepsin B, are also necessary for the microglial cell production of interleukin-1β, a key inflammatory cytokine. Interestingly, whereas the regulatory cytokine TGFβ1 is also increased in the aged brain, neuroinflammation persists. Assessing this apparent contradiction, we have reported that TGFβ1 induction and activation of Smad3 signaling after inflammatory stimulation are reduced in adult mice. Other protective functions, such as phagocytosis, although observed in aged animals, become not inducible by inflammatory stimuli and TGFβ1. Here, we discuss data suggesting that mitochondrial and endolysosomal dysfunction could at least partially mediate age-associated microglial cell changes, and, together with the impairment of the TGFβ1-Smad3 pathway, could result in the reduction of protective activation and the facilitation of cytotoxic activation of microglia, resulting in the promotion of neurodegenerative diseases.

Dissecting the Contribution of Vascular Alterations and Aging to Alzheimer's Disease

Alzheimer's disease (AD) is a neurodegenerative disease characterized by cognitive decline that afflicts as many as 45 % of individuals who survive past the age of 85. AD has been associated with neurovascular dysfunction and brain accumulation of amyloid-β peptide, as well as tau phosphorylation and neurodegeneration, but the pathogenesis of the disease is still somewhat unclear. According to the amyloid cascade hypothesis of AD, accumulation of amyloid-β peptide (Aβ) aggregates initiates a sequence of events leading to neuronal injury and loss, and dementia. Alternatively, the vascular hypothesis of AD incorporates the vascular contribution to the disease, stating that a primary insult to brain microcirculation (e.g., stroke) not only contributes to amyloidopathy but initiates a non-amyloidogenic pathway of vascular-mediated neuronal dysfunction and injury, which involves blood-brain barrier compromise, with increased permeability of blood vessels, leakage of blood-borne components into the brain, and, consequently, neurotoxicity. Vascular dysfunction also includes a diminished brain capillary flow, causing multiple focal ischemic or hypoxic microinjuries, diminished amyloid-β clearance, and formation of neurotoxic oligomers, which lead to neuronal dysfunction. Here we present and discuss relevant findings on the contribution of vascular alterations during aging to AD, with the hope that a better understanding of the players in the "orchestra" of neurodegeneration will be useful in developing therapies to modulate the "symphony".

Three dimensions of the amyloid hypothesis: time, space and 'wingmen'

The amyloid hypothesis, which has been the predominant framework for research in Alzheimer's disease (AD), has been the source of considerable controversy. The amyloid hypothesis postulates that amyloid-β peptide (Aβ) is the causative agent in AD. It is strongly supported by data from rare autosomal dominant forms of AD. However, the evidence that Aβ causes or contributes to age-associated sporadic AD is more complex and less clear, prompting criticism of the hypothesis. We provide an overview of the major arguments for and against the amyloid hypothesis. We conclude that Aβ likely is the key initiator of a complex pathogenic cascade that causes AD. However, we argue that Aβ acts primarily as a trigger of other downstream processes, particularly tau aggregation, which mediate neurodegeneration. Aβ appears to be necessary, but not sufficient, to cause AD. Its major pathogenic effects may occur very early in the disease process.

The effect of focal brain injury on beta-amyloid plaque deposition, inflammation and synapses in the APP/PS1 mouse model of Alzheimer's disease

Traumatic brain injury is a risk factor for Alzheimer's disease (AD), however the effect of such neural damage on the onset and progression of beta-amyloid (Aβ) plaque pathology is not well understood. This study utilized an in vivo model of focal brain injury to examine how localized damage may acutely affect the onset and progression of Aβ plaque deposition as well as inflammatory and synaptic changes, in the APP/PS1 (APPSWE, PSEN1dE9) transgenic model of AD relative to wild-type (Wt) mice. Acute focal brain injury in 3- and 9-month-old APP/PS1 and Wt mice was induced by insertion of a needle into the somatosensory neocortex, as compared to sham surgery, and examined at 24h and 7d post-injury (PI). Focal brain injury did not induce thioflavine-S stained or (pan-Aβ antibody) MOAB-2-labeled plaques at either 24h or 7d PI in 3-month-old APP/PS1 mice or Wt mice. Nine-month-old APP/PS1 mice demonstrate cortical Aβ plaques but focal injury had no statistically significant (p>0.05) effect on thioflavine-S or MOAB-2 plaque load surrounding the injury site at 24h PI or 7d PI. There was a significant (p<0.001) increase in cross-sectional cortical area occupied by Iba-1 positive microglia in injured mice compared to sham animals, however this response did not differ between APP/PS1 and Wt mice (p>0.05). For both Wt and APP/PS1 mice alike, synaptophysin puncta near the injury site were significantly reduced 24h PI (compared to sites distant to the injury and the corresponding area in sham mice; p<0.01), but not after 7d PI (p>0.05). There was no significant effect of genotype on this response (p>0.05). These results indicate that focal brain injury and the associated microglial response do not acutely alter Aβ plaque deposition in the APP/PS1 mouse model. Furthermore the current study demonstrated that the brains of both Wt and APP/PS1 mice are capable of recovering lost synaptophysin immunoreactivity post-injury, the latter in the presence of Aβ plaque pathology that causes synaptic degeneration.