Physiologic Mechanisms of Action of APP and APLP2 in Axon Targeting

Massachusetts General Hospital, Boston MA

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Abstract

Abstract The amyloid precursor protein (APP) is thought to play a central role in the pathogenesis of Alzheimer's disease. In mammals, APP has two amyloid precursor like protein (APLP) homologs, and the physiological functions of this family of proteins remain poorly understood. Orthologs of APP are not present in yeast, but are conserved from worms to humans, suggesting they provide a fundamental role in intercellular communication. In preliminary data, we demonstrate that mouse lines with genetic deletion of either APP or APLP2 exhibit a loss of fidelity of axon projections of olfactory sensory neurons (OSNs), indicating an essential role for these proteins in the establishment and/or maintenance of the precise wiring diagram of the olfactory neural circuit. Next generation sequencing data derived from isolated OSNs indicates that APLP2 and APP are highly expressed in OSNs (ranked #26 and #29, respectively, of 27,390 annotated transcripts). We recently published that loss of function of the BACE1 protease, a key protease that processes proteins of the APP family as well as many other axon guidance molecules, also leads to connectivity errors in the mouse olfactory neural circuit. Here, we propose that APP and APLP2 function physiologically in the formation and maintenance of the precise axon projection map of the mouse olfactory neural circuit. This hypothesis is supported by several observations from others. APP associates with numerous adhesion molecules as well as proteins with established roles in axon guidance, such as netrin and contactin 4. Indeed, many of these associated proteins are expressed in OSNs, too, as evidenced by our next generation sequencing data. Alternatively spliced extracellular domains of APP and APLP2 can be modified posttranslationally by chondroitin sulfate. This alternatively spliced APLP2 isoform is expressed at high levels in OSNs. In addition, the extracellular domains of APP and APLP2 bind heparan sulfate in vitro. Both of these proteoglycans contribute to the environmental cues that repel or attract growth cones and are present in tissue embedding the olfactory neural circuit. To test the role of APP and APLP2 to precisely map axon projections, we deploy a combination of genetic tools developed to manipulate the mouse olfactory circuit to examine the consequences of selective deletion of APP and/or APLP2 exclusively in OSNs (Aim 1). Furthermore, we take advantage of the high levels of expression of APP and APLP2 to purify and identify proteins associated with APP and/or APLP2 derived from mouse OSNs (Aim 2). Together, the knowledge gained from these studies will advance our understanding of the physiologic function of this important protein family and may provide insight into the pathogenesis of Alzheimer's disease as well as into adverse consequences of the therapies under development for Alzheimer's disease to alter the levels of APP and its cleavage products.