Microglial responses after phagocytosis: Escherichia coli bioparticles, but not cell debris or amyloid beta, induce matrix metalloproteinase‐9 secretion in cultured rat primary microglial cells

Abstract

Upon infection or brain damage, microglia are activated to play roles in immune responses, including phagocytosis and soluble factor release. However, little is known whether the event of phagocytosis could be a trigger for releasing soluble factors from microglia. In this study, we tested if microglia secrete a neurovascular mediator matrix metalloproteinase-9 (MMP-9) after phagocytosis in vitro. Primary microglial cultures were prepared from neonatal rat brains. Cultured microglia phagocytosed Escherichia coli bioparticles within 2 hr after incubation and started to secrete MMP-9 at around 12 hr after the phagocytosis. A TLR4 inhibitor TAK242 suppressed the E. coli-bioparticle-induced MMP-9 secretion. However, TAK242 did not change the engulfment of E. coli bioparticles in microglial cultures. Because lipopolysaccharides (LPS), the major component of the outer membrane of E. coli, also induced MMP-9 secretion in a dose-response manner and because the response was inhibited by TAK242 treatment, we assumed that the LPS-TLR4 pathway, which was activated by adhering to the substance, but not through the engulfing process of phagocytosis, would play a role in releasing MMP-9 from microglia after E. coli bioparticle treatment. To support the finding that the engulfing step would not be a critical trigger for MMP-9 secretion after the event of phagocytosis in microglia, we confirmed that cell debris and amyloid beta were both captured into microglia via phagocytosis, but neither of them induced MMP-9 secretion from microglia. Taken together, these data demonstrate that microglial response in MMP-9 secretion after phagocytosis differs depending on the types of particles/substances that microglia encountered. K E Y W O R D Samyloid beta, cell debris, Escherichia coli, matrix metalloproteinase-9, microglia, phagocytosis

Intestinal Barrier Dysfunction Exacerbates Neuroinflammation via the TLR4 Pathway in Mice With Heart Failure

“…The activation of TLR4 and its downstream pathway could promote the activation of microglia and facilitate the release of inflammatory factors in the brain (Leitner et al, 2019). Furthermore, previous studies have shown that the intestinal flora metabolite LPS is a vital activator of TLR4 that can specifically binds to TLR4 and activates its downstream proinflammatory signaling pathway (Kinsner et al, 2006;Hamanaka et al, 2020). Accordingly, we hypothesize that HF could facilitate intestinal barrier dysfunction and increase the release of LPS, which could further activate TLR4 and promote neuroinflammation.…”

Renal Denervation Attenuates Neuroinflammation in the Brain by Regulating Gut-Brain Axis in Rats With Myocardial Infarction

“…For example, LPS released by gut bacteria can breakdown the BBB and increase its permeability ( 29 ). Moreover, LPS can bind to the TLR4 to induce monocyte differentiation into M1-type microglia and activate microglia to express nitric oxide and pro-inflammatory cytokines by activating the TLR4 signaling ( 8 , 30 ). Evidently, intraperitoneal injections with LPS induced microglia activation and neuroinflammation in CNS in rats ( 39 ).…”

“…However, in pathological state, the imbalance of gut microbial compositions and the damages of gut barrier can produce and allow more harmful products into the systemic circulation ( 7 ). This process can further impair the BBB function and activate microglia, leading to neuroinflammation ( 8 ). Interestingly, recent studies have also reported intestinal dysfunction and microbiota imbalance in cardiovascular diseases ( 9 ).…”

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“…For example, LPS released by gut bacteria can breakdown the BBB and increase its permeability (29). Moreover, LPS can bind to the TLR4 to induce monocyte differentiation into M1-type microglia and activate microglia to express nitric oxide and pro-inflammatory cytokines by activating the TLR4 signaling (8,30). Evidently, intraperitoneal injections with LPS induced microglia activation and neuroinflammation in CNS in rats (39).…”

“…However, in pathological state, the imbalance of gut microbial compositions and the damages of gut barrier can produce and allow more harmful products into the systemic circulation (7). This process can further impair the BBB function and activate microglia, leading to neuroinflammation (8). Interestingly, recent studies have also reported intestinal dysfunction and microbiota imbalance in cardiovascular diseases (9).…”