The role of medin, the most common human amyloid, in the pathology of Alzheimer disease

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Over the last 1.5 years, we have been investigating whether the most common human amyloid, medin (a fragment of the protein MFG-E8), may contribute to the aggregation of amyloid-beta. In two mouse models of Alzheimer disease, we now found that i) MFG-E8 levels increase with cerebral amyloid-beta deposition and MFG-E8 staining is localized to amyloid fibrils, and that ii) genetic deficiency in MFG-E8 decreases cerebral beta-amyloidosis and may alter amyloid conformation. As we could not purify recombinant medin for in vivo administration, we looked for an endogenous source and found that iii) extracellular aggregation of medin occurs in the aorta of aged wildtype (but not Mfge8 knockout) mice, strongly reminiscent of the human situation, and iv) intracerebral injection of aged wildtype aorta homogenate induces amyloid-beta aggregation in pre-depositing APP transgenic animals. Further data indicate that v) full-length MFG-E8 and a ~6 kDa fragment (which may correspond to medin) are increased in human Alzheimer patients compared to age-matched controls. As it has been reported that MFG-E8 may enhance phagocytosis of amyloid-beta, we also analysed microglial amyloid uptake but found it unchanged by MFG-E8 deficiency. Thus, our data strongly suggest a direct interaction between MFG-E8/medin and amyloid-beta that enhances amyloid-beta aggregation. Experiments in the current grant period will aim to confirm our preliminary data, e.g. the reduction of cerebral and vascular amyloid deposition and a change in amyloid conformation in APP x Mfge8 knockout mice. Further, they will include the quantification of medin levels in murine/human brain and cerebrospinal fluid (this analysis has so far been prevented by cross-reaction of available anti-medin antibodies with amyloid-beta on Western Blots and the similar molecular weight of the peptides, i.e. 4/5 kDa; however, we have just obtained a new monoclonal medin-specific antibody, which will allow us to perform these experiments). In addition, I would like to apply for a one-year project extension to firmly establish the in vivo interaction of amyloid-beta and medin. To this end, I propose to analyse the induction of amyloid-beta aggregation through intracerebral injection of i) young/old wildtype aorta and human aorta showing low/high levels of medin or of ii) enriched medin or medin-depleted aorta homogenate, and to iii) perform co-immunoprecipitation experiments to determine if medin/MFG-E8 interact directly with amyloid-beta in the brain. As medin aggregates are found in blood vessels of virtually anyone above 60 years of age, it is easily conceivable that medin aggregates also form in the ageing brain, particularly since brain MFG-E8 levels increase with ageing. The results of our studies will demonstrate if medin can promote amyloid-beta aggregation. Thus, the insights generated from this project could provide the basis for new therapeutic approaches for Alzheimer disease.