Researchers at the Hong Kong University of Science and Technology (HKUST) have identified new therapeutic targets for Alzheimer’s disease (AD) by studying the patients’ brain with a newly-developed methodology.
This novel approach also enables researchers to measure the effects of potential drugs on AD patients, opening new directions for AD research and drug development.
Although the pathological mechanisms of AD have been studied for decades, the disease remains incurable. One reason is that conventional research approaches have limited capability to identify molecular targets for drug development.
Molecular and pathological pathway analysis generally examines AD patients‘ brains as a single unit, which usually underestimates the contributions of different brain cell types to AD and any abnormalities in them.
This is especially the case with less-common cell types such as microglia (the brain’s resident immune cells) and neurovascular cells (specifically endothelial cells), which only account for less than 5% and 1% of the total brain cell population, respectively.
However, a team led by Prof. Nancy Ip, Vice-President for Research and Development, Director of the State Key Laboratory of Molecular Neuroscience, and Morningside Professor of Life Science at HKUST, has more than circumvented this problem—they have also identified several new potential molecular targets in endothelial cells and microglia for AD drug development.
The team examined the functions of specific cell types in the postmortem brains of AD patients, which is typically impossible with conventional approaches, by using cutting-edge, single-cell transcriptome analysis, which can be used to characterize of the molecular changes in single cells.
This yielded a comprehensive profile of the cell-type-specific changes in the transcriptome in the brains of AD patients. Subsequent analysis identified cell subtypes and pathological pathways associated with AD, highlighting a specific subpopulation of endothelial cells found in the brains’ blood vessels.
Accordingly, the team discovered that increased angiogenesis (the formation of new blood vessels from current ones) and immune system activation in a subpopulation of endothelial cells are associated with the pathogenesis of AD, suggesting a link between the dysregulation of blood vessels and AD.
The researchers also identified novel targets for restoring neural homeostasis (the ability to maintain a relatively stable internal state despite external changes) in AD patients.
The team also leveraged their single-cell transcriptome analysis to study the mechanism by which the cytokine interleukin-33 (IL-33), an important protein for immune signaling, exerts beneficial actions, making it a possible AD therapeutic intervention.
The researchers found that IL-33 reduces AD-like pathology by stimulating the development of a specific subtype of microglia that helps clear amyloid-beta, a neurotoxic protein found in AD brains.
The team is also the first to capture data on the mechanisms by which microglia transform into an amyloid-beta-consuming phagocytic state, which is a major cellular mechanism for the removal of pathogens.
“The complex and heterogeneous cell composition within the brain makes it difficult to study disease mechanisms,” Prof. Ip explained.
“The advancement of single-cell technology has enabled us to identify specific cell subtypes and molecular targets, which is critical for developing new interventions for Alzheimer’s disease.”
The team has recently published their work in the prestigious scientific journals Proceedings of the National Academy of Sciences U S A (PNAS) and Cell Reports.
AD, the predominant form of dementia, currently affects over 50 million individuals worldwide and is projected to afflict 150 million people by 2050.
Its pathological hallmarks include the accumulation of extracellular amyloid-beta depositions and neurofibrillary tangles. Over time, ineffective clearance of these pathological hallmarks leads to cellular dysfunction in AD, resulting in memory loss, communication problems, reduced physical abilities, and eventually death.
Interleukin-33 (IL-33) is a dual function cytokine produced by endothelial and epithelial cells as well as fibroblast, macrophages, adipocytes, smooth muscle, and brain cells .
This cytokine is released as a full-length active protein that can be inactivated by caspase-1, caspase-3, and caspase-7-mediated cleavage [2, 3] or can processed by different proteases into shorter forms characterized by diverse biological activities [4–6].
The regulation of IL-33 biological activity is complex as it is the result of the interplay between the extra- and intracellular forms of this cytokine and its ability to bind two different isoforms of ST2, its cognate receptor. Binding of extracellular IL-33 to ST2 on leukocytes, astrocytes, and oligodendrocytes  mediates the biological effects of this cytokine .
On the other hand, when expressed intracellularly, IL-33 binds the p65 subunit of NF-kB ; the resulting IL-33/NF-kB p65 complex interferes with NF-kB-dependent transcription by impeding p65-mediated transactivation.
This causes a negative modulation of NF-kB activity, with a dampening effect on inflammation. An additional actor that modulates the biological activity of IL-33 is the soluble form of ST2 (sST2), a decoy receptor. As is usually the case with decoy receptors, IL-33 binding to sST2 limits its biological activity .
Th2 helper cells and mast cells express ST2 and respond to IL-33, and type 2 innate lymphoid cells (ILC2) are considered to be the signature IL-33-responsive cells . Thus, IL-33 targets ILC2 to produce IL-5 and IL-13, resulting in the recruitment of eosinophils, the activation of DC, and Th2 differentiation [12–16].
A growing body of evidence suggests that IL-33 also controls the accumulation and effector function of regulatory T cell (Treg), either directly or indirectly via ILC2 activation and macrophage polarization [17–19].
Thus, IL-33 causes microglial and macrophage polarization to an anti-inflammatory type-2 (M2) phenotype through the IL-33/ST2 signaling pathway; this results in IL-10 generation and reduces IL-1β and IL-6 production [20–22].
IL-33 plays a yet poorly understood role in the pathogenesis of Alzheimer’s disease (AD), a condition where deposition of extracellular amyloid beta (Aβ) plaques in the brain and neuronal cell death accompany neuroinflammation.
In AD patients, the concentration of proinflammatory cytokines, including interleukin IL-1β is increased, possibly as a result of the activation of the NLRP3 inflammasome, and immune regulatory mechanisms, including those mediated by Tregs and PDL-1, are impaired [23–27].
A more subtle form of neuroinflammation is also present in mild cognitive impairment (MCI), a subjective and objective decline in cognitive performance that is greater than expected for an individual’s age and education level, but does not meet criteria for the diagnosis of AD . Elderly MCI patients are at high risk for developing AD; this situation thus represents a borderline condition between normal aging and AD .
Correlates of MCI conversion to AD are still poorly defined, even if neuroinflammation is strongly suspected to play a role in this process [29–33]. Recent observations suggest a beneficial role of IL-33 in AD-associated neuroinflammation. To summarize, (1) ex vivo results in cellular models of AD indicated that over expression of IL-33 decreases Aβ secretion and (2) autoptic data indicate that IL-33 is significantly reduced in brains of AD patients.
Notably, in the APP/PS1 animal model of AD, IL-33 administration was shown to restore the phagolysosomal activity of the microglia, thus enhancing amyloid β (Aβ) clearance, and to polarize monocytes to an anti-inflammatory phenotype. This resulted in a significant amelioration of AD symptoms , suggesting the possibility that IL-33 is a neuroprotective cytokine in AD .
This hypothesis nevertheless is not supported by other results suggesting that IL-33 is present in high concentrations in the neuropathological lesions of AD brain and can exacerbate AD-associated neuroinflammation . In the attempt to clarify the role of the ST2/IL-33 axis in AD, we analyzed these proteins in AD, MCI, and healthy controls (HC) individuals and explored the effect of IL-33 supplementation in an in vitro system of Aβ-stimulated monocytes.
reference link : https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7276088/
More information: Shun-Fat Lau et al, Single-nucleus transcriptome analysis reveals dysregulation of angiogenic endothelial cells and neuroprotective glia in Alzheimer’s disease, Proceedings of the National Academy of Sciences (2020). DOI: 10.1073/pnas.2008762117
Shun-Fat Lau et al. IL-33-PU.1 Transcriptome Reprogramming Drives Functional State Transition and Clearance Activity of Microglia in Alzheimer’s Disease, Cell Reports (2020). DOI: 10.1016/j.celrep.2020.107530