Etiology and pathogenesis of Alzheimer's disease

Blood-brain barrier leakage in Alzheimer's disease: From discovery to clinical relevance

Alzheimer's disease (AD) is the most common form of dementia. AD brain pathology starts decades before the onset of clinical symptoms. One early pathological hallmark is blood-brain barrier dysfunction characterized by barrier leakage and associated with cognitive decline. In this review, we summarize the existing literature on the extent and clinical relevance of barrier leakage in AD. First, we focus on AD animal models and their susceptibility to barrier leakage based on age and genetic background. Second, we re-examine barrier dysfunction in clinical and postmortem studies, summarize changes that lead to barrier leakage in patients and highlight the clinical relevance of barrier leakage in AD. Third, we summarize signaling mechanisms that link barrier leakage to neurodegeneration and cognitive decline in AD. Finally, we discuss clinical relevance and potential therapeutic strategies and provide future perspectives on investigating barrier leakage in AD. Identifying mechanistic steps underlying barrier leakage has the potential to unravel new targets that can be used to develop novel therapeutic strategies to repair barrier leakage and slow cognitive decline in AD and AD-related dementias.

Inconsistencies and controversies surrounding the amyloid hypothesis of Alzheimer's disease

The amyloid hypothesis has driven drug development strategies for Alzheimer's disease for over 20 years. We review why accumulation of amyloid-beta (Aβ) oligomers is generally considered causal for synaptic loss and neurodegeneration in AD. We elaborate on and update arguments for and against the amyloid hypothesis with new data and interpretations, and consider why the amyloid hypothesis may be failing therapeutically. We note several unresolved issues in the field including the presence of Aβ deposition in cognitively normal individuals, the weak correlation between plaque load and cognition, questions regarding the biochemical nature, presence and role of Aβ oligomeric assemblies in vivo, the bias of pre-clinical AD models toward the amyloid hypothesis and the poorly explained pathological heterogeneity and comorbidities associated with AD. We also illustrate how extensive data cited in support of the amyloid hypothesis, including genetic links to disease, can be interpreted independently of a role for Aβ in AD. We conclude it is essential to expand our view of pathogenesis beyond Aβ and tau pathology and suggest several future directions for AD research, which we argue will be critical to understanding AD pathogenesis.

The genetics of Alzheimer's disease: a review and a discussion of the implications

This paper examines the evidence for a genetic etiology of Alzheimer's Disease. Three groups of cases are identified; the first being those with a consistently early age of onset, in which autosomal dominant inheritance can be established. These families may require genetic counseling, and represent an extraordinary resource for research. There are early onset cases without a strong genetic component, and later onset cases generally show only weak evidence of hereditary disease. Some of the apparent distinctions between these groups may result from methodologic problems, especially if the age of onset of symptoms is highly variable in the majority of families. Studies of associations between Alzheimer's Disease and Down's Syndrome suggest a role for genes on chromosome 21 in the genesis of the pathologic features, and testable hypotheses can be identified. Attempts to link the development of Alzheimer's Disease to the presence of specific inherited markers have generally been unsuccessful to date, but there are some promising findings. Recent progress in understanding the molecular basis of other inherited diseases is reviewed in relation to attempts to understand the possible molecular basis of Alzheimer's Disease. A limiting factor in future studies will probably be the availability of cells and tissues from pedigrees demonstrating autosomal dominant inheritance of this disorder.