CD14 and Toll-Like Receptor 4 Promote Fibrillar Aβ42 Uptake by Microglia Through A Clathrin-Mediated Pathway

We previously demonstrated that microglia play an essential role in clearance of amyloid-β (Aβ) in Alzheimer's disease (AD)-like pathology. Our prior work also showed that several receptors expressed on microglia participated in Aβ phagocytosis. However, clathrin-mediated endocytosis (CME), which is associated with production and release of Aβ in neurons, has received much less attention in the context of microglial Aβ uptake. To elucidate the detailed mechanisms of microglial Aβ uptake pathways, we focused on CD14 and Toll-like receptor 4 (TLR4), which have been shown to mediate fibrillar Aβ1 - 42 (fAβ42) phagocytosis in microglia. CD14 has also been known to control lipopolysaccharide-induced internalization of TLR4 in a clathrin-dependent manner. However, it remains unclear whether CD14 and TLR4 engage in CME in microglial fAβ42 uptake, including whether CD14 interacts with TLR4 in the process. In the present study, we found that CD14-positive microglia increased in an age-dependent manner in the cortex of AD model mice. Immunostaining showed that CD14 interacted with TLR4 to internalize fAβ42 in the mouse microglial cell line MG6. Knock-down of CD14 and TLR4 in MG6 cells significantly reduced intracellular fAβ42, showing their involvement in fAβ42 uptake. We also found that clathrin participated in fAβ42 uptake by MG6 cells. Furthermore, CD14 and TLR4 mediated fAβ42 uptake via clathrin-dependent mechanisms. These results indicate that CD14 and TLR4 participate not only in phagocytosis but also in clathrin-dependent fAβ42 internalization in microglia. These findings provide novel molecular understanding of microglial fAβ42 uptake, which could be of therapeutic relevance for AD.