A degradomics strategy for the analysis of inflammation-associated neuronal vulnerability

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Inflammation is thought to play a pivotal role in the progression of neurodegenerative diseases by establishing conditions that increase the vulnerability of neurons to toxic insults. The activation of microglia and macrophages and perhaps their long-term sensitization are associated with the increased vulnerability. The inflammatory events that sensitize neurons very likely precede the development of severe pathology and cognitive decline. Although it is widely acknowledged that substances secreted from macrophages and microglia induce neuronal damage the precise mechanisms of pathogenesis are not well understood. In animal models of Alzheimer Disease (AD) the first signs of pathology in neurons include the dysregulation of calcium, development of focal swellings (beading) within axons and dendrites, actin aggregation, loss of transport systems and loss of trophic support. No current treatments are available that effectively reduce the inflammatory response and neural damage. We have shown that these early pathogenic changes can be can duplicated by exposing primary neurons to microglial conditioned medium (MCM) collected after treatment with amyloid beta oligomers (Aβo). Accumulation of excess intracellular calcium though NMDA receptors appears to be a triggering event although the medium does not contain toxic levels of glutamate or other excitatory compounds. Proteomic profiling of the medium revealed that MMP-9 was robustly secreted and correlated with the neurotoxic effects. We were able to show that exogenous MMP-9 added to neural cultures enhanced the sensitivity of the NMDA receptor. This sensitization may represent an initial trigger that increases neruonal vulnerability. A growing body of data supporting the essential role of MMP-9 in normal synaptic plasticity emphasizes the need to better understand the actions of MMP-9 under inflammatory conditions. However, many protein targets have been identified that could impact plasticity and neural function. In addition, the presence of endogenous inhibitors and the need for activation of secreted proMMP-9 introduces complexities that have made it difficult to evaluate the exact role of MMP- 9. Recent developments in the ability to assess degradomic profiles using Terminal Amine Isotopic Labeling of Substrates (TAILS) provides the opportunity to precisely define the activity of MMP-9 under different conditions. By quantifying proteins cleaved by MMP-9 in wild type and MMP-9 KO neurons challenged with MCM or the purified enzyme, we can generate a clear picture of the targets specific to each experimental condition as well as the contributions of intracellular versus exogenous MMP-9. The specific role of each cleavage event can then be tested in the culture model and subsequently validated in a mouse model of AD. Results from these studies will provide a new tool to characterize the functional activity of proteolytic enzymes in the context of normal and disease states. We further expect that the results will identify cleavage fragments that might be used as markers of early pathogenesis and for the screening of precisely targeted, protective small interfering molecules.