Dosage of amyloid precursor protein affects axonal contact guidance in Down syndrome

Abstract

Amyloid precursor protein (APP), encoded on Hsa21, functions as a cell adhesion molecule (CAM) in axonal growth cones (GCs) of the developing brain. We show here that axonal GCs of human fetal Down syndrome (DS) neurons (and of a DS mouse model) overexpress APP protein relative to euploid controls. We investigated whether DS neurons generate an abnormal, APP-dependent GC phenotype in vitro. On laminin, which binds APP and β1 integrins (Itgb1), DS neurons formed enlarged and faster-advancing GCs compared to controls. On peptide matrices that bind APP only, but not on those binding exclusively Itgb1 or L1CAM, DS GCs were significantly enlarged (2.0-fold), formed increased close adhesions (1.8-fold), and advanced faster (1.4-fold). In assays involving alternating stripes of monospecific matrices, human control GCs exhibited no preference for any of the substrates, whereas DS GCs preferred the APP-binding matrix (cross-over decreased significantly from 48.2 to 27.2%). Reducing APP expression in DS GCs with siRNA normalized most measures of the phenotype, including substrate choice. These experiments show that human DS neurons exhibit an APP-dependent, abnormal GC phenotype characterized by increased adhesion and altered contact guidance. The results suggest that APP overexpression may perturb axonal pathfinding and circuit formation in developing DS brain.

The physiological role of the amyloid precursor protein as an adhesion molecule in the developing nervous system

“…; Sosa et al . ). In this context, hippocampal pyramidal neurons derived from Ts65Dn mice and cultured on laminin substrate exhibit aberrant growth cones with large adhesive areas and abundant filopodia (Fig.…”

“…GC of these neurons are large and grow faster than those derived of euploid human neurons (Sosa et al . ). Since several genes are over‐expressed in DS, it becomes necessary to identify and differentiate the effects of APP from other CAMs on adhesion properties and the overall cell phenotype.…”

“…In contrast, the same neurons present a phenotype similar to euploid neurons when plated on substrates that are recognized by integrin or L1 (Sosa et al . ). The dosage effect of APP in adhesions was confirmed when partial knockdown of APP in DS neurons restored growth cone adhesion and phenotype to a range similar to euploid neurons.…”

“…; Sosa et al . ). Intriguingly, neocortical pyramidal neurons of adult DS human and Ts65Dn mice exhibit reduced dendritic complexity including reduced length, branching, and spine density, while young Ts65Dn mice and DS humans showed abnormally enlarged dendrites with increased branching.…”

APP Receptor? To Be or Not To Be

“…This very important concept may indeed account for the described cell-cell-adhesion properties of APP and subsequent signaling through homophilic interactions (reviewed in ref [37] ). This effect can be exacerbated in a dose-dependent manner during initiation of axonal growth pathfinding [74, 75] . More recent studies showed clear indication that APP dimerization would facilitate synapse formation [58, 76, 77] .…”