LRP1 knockdown aggravates Aβ1–42-stimulated microglial and astrocytic neuroinflammatory responses by modulating TLR4/NF-κB/MAPKs signaling pathways

Neuroinflammation is an important pathological feature and an early event in the pathogenesis of Alzheimer's disease (AD), which is characterized by activation of microglia and astrocytes. Low-density lipoprotein receptor-related protein 1 (LRP1) is an endocytic receptor that is abundantly expressed in neurons, microglia, and astrocytes, and plays a critical role in AD pathogenesis. There is increasing evidence to show that LRP1 regulates inflammatory responses by modulating the release of pro-inflammatory cytokines and phagocytosis. However, the effects of LRP1 on β-amyloid protein (Aβ)-induced microglial and astrocytic neuroinflammatory responses and its underlying mechanisms have not been studied in detail. In the present study, knockdown of LRP1 significantly enhanced Aβ1-42-stimulated neuroinflammation by increasing the production of pro-inflammatory cytokines in both BV2 microglial cells and mouse primary astrocytes. Furthermore, it is revealed that LRP1 knockdown further led to the activation of nuclear factor-kappa B (NF-κB) and mitogen-activated protein kinases (MAPKs) signaling pathways. The phosphorylation of IκBα, p38, and JNK was significantly up-regulated in LRP1 knockdown BV2 microglial cells and primary astrocytes. Meanwhile, LRP1 knockdown increased expression of the NF-κB p65 subunit in the nucleus while decreased its expression in the cytoplasm. Besides, the upstream signaling adaptor molecules such as toll-like receptor 4 (TLR4), myeloid differentiation primary response protein 88 (MyD88), and tumor necrosis factor receptor-associated factor 6 (TRAF6) were also further increased. Moreover, blockade of NF-κB, p38, and JNK inhibited the production of tumor necrosis factor-α (TNF-α), interleukin-1β (IL-1β), and interleukin-6 (IL-6) induced by the knockdown of LRP1. Taken together, these findings indicated that LRP1 as an effective therapeutic target against AD and other neuroinflammation related diseases.