The mechanisms that control the clearance of Amyloid Precursor Protein (APP)-derived fragments in the brain remain poorly understood. APP expression is dramatically upregulated following neuronal injury and in a variety of neurodegenerative diseases, resulting in elevated levels of both amyloid (A?) peptides and soluble sAPP ectodomains that have potent biological activities. Growing evidence suggests that the misregulation of sAPP levels can be harmful to the nervous system, while abnormal sAPP levels may provide useful biomarkers for a variety of neurodegenerative conditions and injury responses. In contrast to extensive work on microglial responses to A?, the mechanisms by which these phagocytic cells regulate sAPP levels in the brain has remained largely unexplored. To address this issue, we have adapted the Lepidopteran species Manduca (a well-characterized model of innate immunity) to investigate the mechanisms of sAPP scavenging. As in humans, insect macrophages actively phagocytose cleaved membrane proteins and cellular debris to protect the nervous system, a process that can be readily analyzed both in vitro and in vivo. Recently, we discovered that Manduca macrophages play a prominent role in removing the shed ectodomains of APPL (the insect ortholog of APP). Both during development and following nerve injury, macrophages home to regions of neuronal migration, growth, and repair, where they phagocytose neuronally derived APPL ectodomains (sAPPLs). Using a plasmid expression library to screen for APPL-binding partners, we identified Manduca Contactin (msContactin) as a candidate sAPPL receptor that is expressed by insect macrophages. Whereas Contactins have traditionally been considered neuronal-specific receptors, we have now shown that both mouse microglia and macrophages also express specific subsets of Contactins, depending on their activation state. Initial trials using blocking antibodies indicate that msContactin is required for macrophage responses to sAPPLs (but not to A?), suggesting that distinct mechanisms are used to clear different APP fragments. These discoveries suggest a previously unrecognized role for Contactins in mediating non-inflammatory responses to sAPP.
Aim 1 will test the hypothesis that msContactin specifically regulates the homing and phagocytic responses of insect macrophages to sAPPL fragments, using our gene knockdown, re-expression, and imaging protocols in primary cell cultures and developing animals.
Aim 2 will use primary cultures of mouse microglia and macrophages to test whether Contactin-dependent responses to sAPP represents a novel signaling pathway used by mammalian phagocytic cells. Successful completion of these studies will provide new data for an R01 application, with the goal of comprehensively defining the mechanisms by which Contactin-dependent signaling regulates the microglial control of sAPPs in the nervous system. Public Heath Relevance: Understanding these mechanisms will support new therapeutic strategies for preventing the accumulation of toxic sAPP fragments associated with neurodegenerative disease and traumatic brain injury.
The proposed research will test the previously unrecognized role that Contactins may play in regulating macrophage-dependent scavenging of bioactive fragments produced by the cleavage of APP family proteins. Whereas amyloid peptides derived from APP feature prominently in Alzheimer's disease, APP is also strongly upregulated in neurodegenerative disease and following neuronal injury, where it undergoes rapid cleavage to release ectodomain fragments (sAPPs) that can be both beneficial and harmful to the nervous system. Using both a simple insect 'discovery' model and primary cultures of mouse macrophages and microglia, we will explore how Contactin receptors regulate macrophage homing and phagocytosis of sAPPs, which in turn will provide the framework for investigating how harnessing this previously unrecognized aspect of the innate immune response might be harnessed to mitigate the deleterious effects of a variety of diseases and injuries that affect the nervous system.
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