Inhibitory effect of minocycline on amyloid β fibril formation and human microglial activation

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Abstract
Minocycline, a derivative of the antibiotic tetracycline, displays neuroprotective properties in various models of neurodegenerative diseases and is now used in clinical trials, because of its relative safety and tolerability. Minocycline passes the blood‐brain barrier and is presumed to inhibit microglial activation. In Alzheimer's disease brain, a number of proteins, including serum amyloid P component (SAP) and complement factors such as C1q, accumulate in amyloid β (Aβ) plaques. In a previous study, SAP and C1q were found to be required for clustering of activated microglia in Aβ plaques. Furthermore, SAP and C1q enhanced Aβ fibril formation and Aβ mediated cytokine release by human microglia in vitro. In the present study, we report that tetracycline and minocycline dose‐dependently reduce TNF‐α and IL‐6 release by adult human microglia upon stimulation with a combination of Aβ, SAP, and C1q. In addition, minocycline and to a lesser extent tetracycline inhibit fibril formation of Aβ as determined in a thioflavin‐S‐based fluorescence test. This inhibitory effect was observed with Aβ alone as well as with Aβ in combination with SAP and C1q. Our data suggest that minocycline and tetracycline at tolerable doses can inhibit human microglial activation. This activity in part is exerted by inhibition of (SAP and C1q enhanced) Aβ fibril formation.