Connection between periodontitis and Alzheimer's disease: possible roles of microglia and leptomeningeal cells

The Neuroprotective Effects of Brazilian Green Propolis on Neurodegenerative Damage in Human Neuronal SH-SY5Y Cells

Oxidative stress and synapse dysfunction are the major neurodegenerative damage correlated to cognitive impairment in Alzheimer's disease (AD). We have found that Brazilian green propolis (propolis) improves the cognitive functions of mild cognitive impairment patients living at high altitude; however, mechanism underlying the effects of propolis is unknown. In the present study, we investigated the effects of propolis on oxidative stress, expression of brain-derived neurotrophic factor (BDNF), and activity-regulated cytoskeleton-associated protein (Arc), the critical factors of synapse efficacy, using human neuroblastoma SH-SY5Y cells. Pretreatment with propolis significantly ameliorated the hydrogen peroxide- (H₂O₂-) induced cytotoxicity in SH-SY5Y cells. Furthermore, propolis significantly reduced the H₂O₂-generated reactive oxygen species (ROS) derived from mitochondria and 8-oxo-2'-deoxyguanosine (8-oxo-dG, the DNA oxidative damage marker) but significantly reversed the fibrillar β-amyloid and IL-1β-impaired BDNF-induced Arc expression in SH-SY5Y cells. Furthermore, propolis significantly upregulated BDNF mRNA expression in time- and dose-dependent manners. In addition, propolis induced Arc mRNA and protein expression via phosphoinositide-3 kinase (PI3K). These observations strongly suggest that propolis protects from the neurodegenerative damage in neurons through the properties of various antioxidants. The present study provides a potential molecular mechanism of Brazilian green propolis in prevention of cognitive impairment in AD as well as aging.

The Infectious Etiology of Alzheimer's Disease

BACKGROUND

Inflammation is a part of the first line of defense of the body against invasive pathogens, and plays a crucial role in tissue regeneration and repair. A proper inflammatory response ensures the suitable resolution of inflammation and elimination of harmful stimuli, but when the inflammatory reactions are inappropriate it can lead to damage of the surrounding normal cells. The relationship between infections and Alzheimer's Disease (AD) etiology, especially lateonset AD (LOAD) has been continuously debated over the past three decades.

METHODS

This review discusses whether infections could be a causative factor that promotes the progression of AD and summarizes recent investigations associating infectious agents and chronic inflammation with AD. Preventive and therapeutic approaches to AD in the context of an infectious etiology of the disease are also discussed.

RESULTS

Emerging evidence supports the hypothesis of the role of neurotropic viruses from the Herpesviridae family, especially Human herpesvirus 1 (HHV-1), Cytomegalovirus (CMV), and Human herpesvirus 2 (HHV-2), in AD neuropathology. Recent investigations also indicate the association between Hepatitis C virus (HCV) infection and dementia. Among bacteria special attention is focused on spirochetes family and on periodontal pathogens such as Porphyromonas gingivalis or Treponema denticola that could cause chronic periodontitis and possibly contribute to the clinical onset of AD.

CONCLUSION

Chronic viral, bacterial and fungal infections might be causative factors for the inflammatory pathway in AD.

Association between periodontal disease and dementia: A literature review

INTRODUCTION

Periodontal disease and dementia are very prevalent, especially in elderly populations. Multiple studies have shown a link between these diseases; however, the conditions are highly heterogeneous and so is the diagnostic methodology, which may hinder interpretation and comparison of the results. The aim of this article is to provide a critical review of the literature linking these 2 processes.

DEVELOPMENT

We retrieved 22 studies, most of which were retrospective, and analysed various methodological variables including study population, diagnosis of periodontitis, definition of dementia, adjusted variables, and results. The different aetiopathogenic mechanisms that may affect the progression and interaction of these 2 conditions were also analysed.

CONCLUSIONS

Although available evidence indicates a positive association between periodontitis and dementia, both the strength of that association and the presence of a causal relationship have yet to be determined.

Elucidating the link between the modifiable risk factors of Alzheimer's disease and neuroinflammation

Increased worldwide longevity through medical interventions, although beneficial, has allowed the age-related Alzheimer's disease (AD) to become an epidemic of the 21st century. AD pathology involves adverse activation of microglia, the immune cells of the brain and resulting chronic neuroinflammation. Certain diets, physical inactivity and Type 2 diabetes mellitus have been identified as the risk factors for developing AD, which may increase the risk of AD by neuroimmune mechanisms primarily through the overactivation of microglia. Thus, modifying these risk factors may represent an alternative therapeutic strategy for lowering the incidence of AD. We highlight the link between select modifiable risk factors and neuroimmune mechanisms, and demonstrate that by controlling microglial activation and neuroinflammation the prevalence of AD may be decreased.

Diurnal dynamic behavior of microglia in response to infected bacteria through the UDP-P2Y6 receptor system

It has long been believed that microglia morphologically transform into the activated state by retracting their long processes and consuming pathogens when bacteria infect into the brain parenchyma. In the present study, however, we showed for the first time that murine cortical microglia extend their processes towards focally injected Porphyromonas gingivalis. This P. gingivalis-induced microglial process extension was significantly increased during the light (sleeping) phase than the dark (waking) phase. In contrast, focally injected ATP-induced microglial process extension was significantly increased during the dark phase than the light phase. Furthermore, in contrast to the P2Y₁₂ receptor-mediated mechanism of ATP-induced microglial process extension, the P. gingivalis-mediated microglial process extension was mediated by P2Y₆ receptors. The infection of bacteria such as P. gingivalis to the brain parenchyma may induce the secretion of UDP from microglia at the site of infection, which in turn induces the process extension of the neighboring microglia.

Unified theory of Alzheimer's disease (UTAD): implications for prevention and curative therapy

The aim of this review is to propose a Unified Theory of Alzheimer's disease (UTAD) that integrates all key behavioural, genetic and environmental risk factors in a causal chain of etiological and pathogenetic events. It is based on three concepts that emanate from human's evolutionary history: (1) The grandmother-hypothesis (GMH), which explains human longevity due to an evolutionary advantage in reproduction by trans-generational transfer of acquired knowledge. Consequently it is argued that mental health at old-age must be the default pathway of humans' genetic program and not development of AD. (2) Therefore, mechanism like neuronal rejuvenation (NRJ) and adult hippocampal neurogenesis (AHN) that still function efficiently even at old age provide the required lifelong ability to memorize personal experiences important for survival. Cumulative evidence from a multitude of experimental and epidemiological studies indicate that behavioural and environmental risk factors, which impair productive AHN, result in reduced episodic memory performance and in reduced psychological resilience. This leads to avoidance of novelty, dysregulation of the hypothalamic-pituitary-adrenal (HPA)-axis and cortisol hypersecretion, which drives key pathogenic mechanisms of AD like the accumulation and oligomerization of synaptotoxic amyloid beta, chronic neuroinflammation and neuronal insulin resistance. (3) By applying to AHN the law of the minimum (LOM), which defines the basic requirements of biological growth processes, the UTAD explains why and how different lifestyle deficiencies initiate the AD process by impairing AHN and causing dysregulation of the HPA-axis, and how environmental and genetic risk factors such as toxins or ApoE4, respectively, turn into disease accelerators under these unnatural conditions. Consequently, the UTAD provides a rational strategy for the prevention of mental decline and a system-biological approach for the causal treatment of AD, which might even be curative if the systemic intervention is initiated early enough in the disease process. Hence an individualized system-biological treatment of patients with early AD is proposed as a test for the validity of UTAD and outlined in this review.

Nutrients, Microglia Aging, and Brain Aging

As the life expectancy continues to increase, the cognitive decline associated with Alzheimer's disease (AD) becomes a big major issue in the world. After cellular activation upon systemic inflammation, microglia, the resident immune cells in the brain, start to release proinflammatory mediators to trigger neuroinflammation. We have found that chronic systemic inflammatory challenges induce differential age-dependent microglial responses, which are in line with the impairment of learning and memory, even in middle-aged animals. We thus raise the concept of “microglia aging.” This concept is based on the fact that microglia are the key contributor to the acceleration of cognitive decline, which is the major sign of brain aging. On the other hand, inflammation induces oxidative stress and DNA damage, which leads to the overproduction of reactive oxygen species by the numerous types of cells, including macrophages and microglia. Oxidative stress-damaged cells successively produce larger amounts of inflammatory mediators to promote microglia aging. Nutrients are necessary for maintaining general health, including the health of brain. The intake of antioxidant nutrients reduces both systemic inflammation and neuroinflammation and thus reduces cognitive decline during aging. We herein review our microglia aging concept and discuss systemic inflammation and microglia aging. We propose that a nutritional approach to controlling microglia aging will open a new window for healthy brain aging.

Lessons from Microglia Aging for the Link between Inflammatory Bone Disorders and Alzheimer's Disease

Bone is sensitive to overactive immune responses, which initiate the onset of inflammatory bone disorders, such as rheumatoid arthritis and periodontitis, resulting in a significant systemic inflammatory response. On the other hand, neuroinflammation is strongly implicated in Alzheimer's disease (AD), which can be enhanced by systemic inflammation, such as that due to cardiovascular disease and diabetes. There is growing clinical evidence supporting the concept that rheumatoid arthritis and periodontitis are positively linked to AD, suggesting that inflammatory bone disorders are risk factors for this condition. Recent studies have suggested that leptomeningeal cells play an important role in transducing systemic inflammatory signals to brain-resident microglia. More importantly, senescent-type, but not juvenile-type, microglia provoke neuroinflammation in response to systemic inflammation. Because the prevalence of rheumatoid arthritis and periodontitis increases with age, inflammatory bone disorders may be significant sources of covert systemic inflammation among elderly people. The present review article highlights our current understanding of the link between inflammatory bone disorders and AD with a special focus on microglia aging.