Hirotaka Watanabe

BACKGROUND: Microglia play a crucial role in brain homeostasis through phagocytosis of amyloid-β (Aβ) fibrils, a hallmark of Alzheimer disease (AD) pathology. The balance between Aβ production and clearance is critical for AD pathogenesis, with impaired clearance mechanisms potentially contributing to disease progression. G-protein coupled receptor 34 (GPR34), a microglia-enriched Gi/o-coupled receptor, is highly expressed in homeostatic microglia and may regulate phagocytic functions, yet its role in Aβ clearance remains poorly understood. METHODS: Using flow cytometry-based assays, we investigated the effect of a selective GPR34 agonist (M1) on Aβ uptake in mouse primary microglia and human induced pluripotent stem cell-derived microglia. We evaluated uptake specificity across different Aβ species and phagocytic substrates, and measured intracellular cyclic adenosine monophosphate (cAMP) levels to determine the signaling mechanism. We performed in vivo studies using human amyloid precursor protein knock-in mice with intrahippocampal M1 injections. Additionally, we analyzed GPR34 expression in Japanese AD patient brain samples using single-nucleus RNA sequencing and examined age-dependent expression changes across multiple datasets. RESULTS: M1 specifically enhanced uptake of Aβ fibrils through reduction of intracellular cAMP levels, without affecting monomeric or oligomeric Aβ internalization. Gpr34 knockdown experiments confirmed GPR34 as the molecular target of M1. An intrahippocampal injection of M1 significantly increased microglial Aβ uptake in vivo, an effect that required functional TREM2 signaling. GPR34 expression was significantly reduced in microglia from AD patients and showed age-dependent decline in both humans and mice. CONCLUSIONS: Our findings identify GPR34 as a promising therapeutic target for enhancing microglial Aβ clearance and highlight the potential of GPR34 agonists for AD treatment. The age-dependent decline in GPR34 expression may contribute to reduced Aβ clearance efficiency in aging brains, exacerbating amyloid accumulation. Pharmacological activation of GPR34 represents a novel strategy to counteract impaired Aβ clearance in both aging and AD brains, potentially modifying disease progression through enhancement of microglial phagocytic function.

Amyotrophic Lateral Sclerosis/Parkinsonism-Dementia Complex (ALS/PDC), a rare and complex neurological disorder, is predominantly observed in the Western Pacific islands, including regions of Japan, Guam, and Papua. This enigmatic condition continues to capture medical attention due to affected patients displaying symptoms that parallel those seen in either classical amyotrophic lateral sclerosis (ALS) or Parkinson's disease (PD). Distinctly, postmortem examinations of the brains of affected individuals have shown the presence of α-synuclein aggregates and TDP-43, which are hallmarks of PD and classical ALS, respectively. These observations are further complicated by the detection of phosphorylated tau, accentuating the multifaceted proteinopathic nature of ALS/PDC. The etiological foundations of this disease remain undetermined, and genetic investigations have yet to provide conclusive answers. However, emerging evidence has implicated the contribution of astrocytes, pivotal cells for maintaining brain health, to neurodegenerative onset, and likely to play a significant role in the pathogenesis of ALS/PDC. Leveraging advanced induced pluripotent stem cell technology, our team cultivated multiple astrocyte lines to further investigate the Japanese variant of ALS/PDC (Kii ALS/PDC). CHCHD2 emerged as a significantly dysregulated gene when disease astrocytes were compared to healthy controls. Our analyses also revealed imbalances in the activation of specific pathways: those associated with astrocytic cilium dysfunction, known to be involved in neurodegeneration, and those related to major neurological disorders, including classical ALS and PD. Further in-depth examinations revealed abnormalities in the mitochondrial morphology and metabolic processes of the affected astrocytes. A particularly striking observation was the reduced expression of CHCHD2 in the spinal cord, motor cortex, and oculomotor nuclei of patients with Kii ALS/PDC. In summary, our findings suggest a potential reduction in the support Kii ALS/PDC astrocytes provide to neurons, emphasizing the need to explore the role of CHCHD2 in maintaining mitochondrial health and its implications for the disease.

Genetic variants in the apolipoprotein E (APOE) gene affect the onset and progression of Alzheimer's disease (AD). The APOE Christchurch (APOE Ch) variant has been identified as the most prominent candidate for preventing the onset and progression of AD. In this study, we generated isogenic APOE3Ch/3Ch human induced pluripotent stem cells (iPSCs) from APOE3/3 healthy control female iPSCs and induced them into astrocytes. RNA expression analysis revealed the inherent resilience of APOE3Ch/3Ch astrocytes to induce a reactive state in response to inflammatory cytokines. Moreover, cytokine treatment changed astrocytic morphology with more complexity in APOE3/3 astrocytes, but not in APOE3Ch/3Ch astrocytes, indicating resilience of the rare variant to a reactive state. Interestingly, we observed robust morphological alterations containing more intricate processes when cocultured with iPSC-derived cortical neurons, in which APOE3Ch/3Ch astrocytes reduced complexity compared with APOE3/3 astrocytes. To assess the impacts of tau propagation effects, we next developed a sophisticated and sensitive assay utilizing cortical neurons derived from human iPSCs, previously generated from donors of both sexes. We showed that APOE3Ch/3Ch astrocytes effectively mitigated tau propagation within iPSC-derived neurons. This study provides important experimental evidence of the characteristic functions exhibited by APOE3Ch/3Ch astrocytes, thereby offering valuable insights for the advancement of novel clinical interventions in AD research.Significance Statement Alzheimer's disease (AD) is a degenerative disease that causes cognitive decline. Familial AD is a severe form caused by mutations in the PSEN1, PSEN2, or APP genes. One carrier of the PSEN1 mutation did not develop dementia. This carrier also had a rare variant of the APOE gene, the Christchurch variant. The APOE Christchurch variant may protect against familial AD. The mechanism of this protection is not fully understood. In the present study, we have successfully demonstrated that the APOE Christchurch variant suppresses the propagation of tau and exhibits a diminished capacity to convert native astrocytes into reactive astrocytes. These significant findings contribute novel insights to the field of the APOE gene and AD research.