An unrecognized mechanism of self-protection in degenerating neurons mediated by astrocytic YAP through Wnts/β-catenin/EAAT2 signaling in C9orf72-poly-GA mice

Rationale: Amyotrophic lateral sclerosis (ALS) is a fatal neurodegenerative disorder characterized by the progressive loss of motor neurons in the central nervous system (CNS). Non-neuronal cells, particularly astrocytes, have been recognized as pivotal contributors to ALS onset and progression. However, the underlying mechanisms of interactions between astrocytes and motor neurons during ALS remain unclear. Recent studies have identified the neuronal Hippo kinase mammalian sterile 20-like kinase 1 (MST1) as a key regulator of neurodegeneration in ALS. Yes-associated protein (YAP), a major downstream effector of the Hippo pathway, is predominantly expressed in astrocytes. However, the role of astrocytic YAP in ALS and its underlying mechanisms remain unexplored. Methods: To evaluate the function of YAP in ALS, we established a C9orf72-poly-GA mouse model (ALS mice) via intracerebroventricular injection of AAV viruses. Furthermore, mice with conditional knockout (CKO) of YAP in astrocytes (YAPGFAP-CKO mice) were generated and then YAPGFAP-CKO ALS mice and their littermate controls (YAPf/f ALS mice) were used as experimental subjects. Behavioral tests, immunostaining, Nissl staining, quantitative real-time PCR (qPCR), and Western blotting were used to assess the effects of astrocytic YAP deletion in ALS progression. In addition, we investigated the role and mechanism of astrocytic YAP in the pathogenesis of ALS by integrating RNA sequencing (RNA-seq) from primary cultured astrocytes with single-nucleus transcriptomic (snRNA-seq) from C9orf72-ALS/FTD patients. Then, in vitro experiments including primary cultured astrocytes and neurons were used to further elucidate the potential molecular mechanism of astrocytic YAP in ALS. Finally, we evaluated the therapeutic effects of the excitatory amino acid transporter-2 (EAAT2) activator LDN-212320 and the Hippo kinase MST1/2 inhibitor XMU-MP-1 as candidate treatments for ALS. Results: We found that YAP was upregulated and activated specifically in astrocytes, but not in neurons or microglia, within the motor cortex of ALS mice. Conditional knockout of YAP in astrocytes exacerbated motor deficits, neuronal loss, pathological translocation of TDP-43, inflammatory infiltration, and reduced astrocytic proliferation in ALS mice. Mechanistically, Wnts secreted by degenerating neurons and astrocytes activated YAP/β-catenin signaling and further promoted the expression of EAAT2 in astrocytes, which prevented neuronal glutamate excitotoxicity, neuronal loss, and motor dysfunction in ALS mice. Interestingly, treatment with LDN-212320 promoted EAAT2 expression and partially restored motor deficits and neuronal loss in YAPGFAP-CKO ALS mice. Finally, activation of YAP by XMU-MP-1 upregulated β-catenin and EAAT2 expression, and partially alleviated motor deficits and neurodegeneration in ALS mice. Conclusions: These results identify an unrecognized mechanism of self-protection in degenerating neurons mediated by astrocytic YAP through Wnts/β-catenin/EAAT2 signaling to prevent glutamate excitotoxicity of neurons in ALS mice, and provide a novel drug target for ALS.