Suppression of MT5-MMP Reveals Early Modulation of Alzheimer's Pathogenic Events in Primary Neuronal Cultures of 5xFAD Mice

We previously reported that membrane-type 5-matrix metalloproteinase (MT5-MMP) deficiency not only reduces pathological hallmarks of Alzheimer's disease (AD) in 5xFAD (Tg) mice in vivo but also impairs interleukin-1 beta (IL-1β)-mediated neuroinflammation and Aβ production in primary Tg immature neural cell cultures after 11 days in vitro. We now investigate the effect of MT5-MMP on incipient pathogenic pathways that are activated in cortical primary cultures at 21-24 days in vitro (DIV), during which time neurons are organized into a functional mature network. Using wild-type (WT), MT5-MMP^-/- (MT5^-/-), 5xFAD (Tg), and 5xFADxMT5-MMP^-/- (TgMT5^-/-) mice, we generated primary neuronal cultures that were exposed to IL-1β and/or different proteolytic system inhibitors. We assessed neuroinflammation, APP metabolism, synaptic integrity, and electrophysiological properties using biochemical, imaging and whole-cell patch-clamp approaches. The absence of MT5-MMP impaired the IL-1β-mediated induction of inflammatory genes in TgMT5^-/- cells compared to Tg cells. Furthermore, the reduced density of dendritic spines in Tg neurons was also prevented in TgMT5^-/- neurons. IL-1β caused a strong decrease in the dendritic spine density of WT neurons, which was prevented in MT5^-/- neurons. However, the latter exhibited fewer spines than the WT under untreated conditions. The spontaneous rhythmic firing frequency of the network was increased in MT5^-/- neurons, but not in TgMT5^-/- neurons, and IL-1β increased this parameter only in Tg neurons. In terms of induced somatic excitability, Tg and TgMT5^-/- neurons exhibited lower excitability than WT and MT5^-/-, while IL-1β impaired excitability only in non-AD backgrounds. The synaptic strength of miniature global synaptic currents was equivalent in all genotypes but increased dramatically in WT and MT5^-/- neurons after IL-1β. MT5-MMP deficiency decreased endogenous and overexpressed C83 and C99 levels but did not affect Aβ levels. C99 appears to be cleared by several pathways, including γ-secretase, the autophagolysosomal system, and also α-secretase, via its conversion to C83. In summary, this study confirms that MT5-MMP is a pivotal factor affecting not only neuroinflammation and APP metabolism but also synaptogenesis and synaptic activity at early stages of the pathology, and reinforces the relevance of targeting MT5-MMP to fight AD.