Nicotinic Acetylcholine Receptors and Microglia as Therapeutic and Imaging Targets in Alzheimer's Disease.

Amyloid-ß (Aß) accumulation and tauopathy are considered the pathological hallmarks of Alzheimer's disease (AD), but attenuation in choline signaling, including decreased nicotinic acetylcholine receptors (nAChRs), is evident in the early phase of AD. Currently, there are no drugs that can suppress the progression of AD due to a limited understanding of AD pathophysiology. For this, diagnostic methods that can assess disease progression non-invasively before the onset of AD symptoms are essential, and it would be valuable to incorporate the concept of neurotheranostics, which simultaneously enables diagnosis and treatment. The neuroprotective pathways activated by nAChRs are attractive targets as these receptors may regulate microglial-mediated neuroinflammation. Microglia exhibit both pro- and anti-inflammatory functions that could be modulated to mitigate AD pathogenesis. Currently, single-cell analysis is identifying microglial subpopulations that may have specific functions in different stages of AD pathologies. Thus, the ability to image nAChRs and microglia in AD according to the stage of the disease in the living brain may lead to the development of new diagnostic and therapeutic methods. In this review, we summarize and discuss the recent findings on the nAChRs and microglia, as well as their methods for live imaging in the context of diagnosis, prophylaxis, and therapy for AD.

Analysis of Aß-induced neurotoxicity and microglial responses in simple two- and three-dimensional human iPSC-derived cortical culture systems.

The extracellular accumulation of amyloid-ß (Aß) in plaques and associated neurodegeneration are the pathological hallmarks of Alzheimer's disease (AD). These plaques are surrounded by microglia-the resident tissue macrophages of the brain parenchyma that originate from primitive macrophages from the embryonic yolk sac. Microglia, including a unique subpopulation called "disease-associated microglia" (DAM), are strongly implicated in AD pathology; however, their exact function and physiology remain largely unknown. Notably, simple cell and tissue culture systems that adequately recreate the brain microenvironment and can simulate critical aspects of AD pathology could fundamentally contribute to elucidating microglial function in disease development and progression. Thus, we added human-induced pluripotent stem cell (hiPSC)-induced primitive macrophages (hiMacs) to hiPSC-induced cortical neurons (cell model) and cortical organoids (tissue model). The treatment of these culture systems with the O-acyl isopeptide of Aß1-42, which reverts to natural extracellular Aß1-42 at neutral pH and starts self-aggregation, caused the degeneration of hiPSC-induced cortical neurons in 2D culture and within cortical organoid cultures. Notably, the hiMacs phagocytosed extracellular Aß and exhibited a DAM-like phenotype. In both cell and tissue organoid culture systems, neurodegeneration was attenuated by the addition of hiMacs. Moreover, in cortical organoids, Aß plaques formed more circular and fewer hotspot-like morphological structures in the vicinity of hiMacs. These findings demonstrate the utility of simple hiPSC-induced cortical cell and tissue culture systems supplemented with hiMacs for elucidating critical aspects of AD pathology, such as microglial function and physiology. Adopting such systems in routine research practice may lead to the development of novel therapeutic strategies for AD.

Mouse Bone Marrow-derived Microglia-like Cells Secrete Transforming Growth Factor-ß1 and Promote Microglial Aß Phagocytosis and Reduction of Brain Aß.

Accumulation of amyloid-ß (Aß) in brain tissue contributes to the pathophysiology of Alzheimer's disease (AD). We recently reported that intrahippocampal transplantation of mouse bone marrow-derived microglia-like (BMDML) cells suppresses brain amyloid pathology and cognitive impairment in a mouse model of AD. How these transplanted cells interact with resident microglia remains unknown. In the present study, we evaluated the effects of cytokines secreted from mouse BMDML cells on cultured mouse microglia. Conditioned medium from BMDML cells increased microglial Aß phagocytosis. High levels of transforming growth factor-ß1 (TGF-ß1) were present in the conditioned medium, and BMDML cells and microglia expressed Tgf-ß1 mRNA and TGF-ß receptor type 1 (TGF-ßR1) protein, respectively. BMDML conditioned medium also induced microglial Smad2/3 phosphorylation. A TGF-ßR1 inhibitor suppressed Smad2/3 phosphorylation and promotion of microglial Aß phagocytosis induced by conditioned medium. Recombinant mouse TGF-ß1 similarly increased microglial Aß phagocytosis and induced Smad2/3 phosphorylation, which were suppressed by the TGF-ßR1 inhibitor. Brain TGF-ß1 levels and resident microglial TGF-ß1R expression were increased by intrahippocampal injection of BMDML cells in a mouse model of AD. Cotreatment with the TGF-ßR1 inhibitor suppressed the ability of transplanted BMDML cells to increase microglial TGF-ß1R expression and decrease hippocampal Aß levels. Taken together, these findings suggested that transplanted BMDML cells secreted TGF-ß1 to stimulate Aß phagocytosis by resident microglia and decrease brain Aß pathology.