Genetic Analysis Reveals that Amyloid Precursor Protein and Death Receptor 6 Function in the Same Pathway to Control Axonal Pruning Independent of β-Secretase

Abstract

In the developing brain, initial neuronal projections are formed through extensive growth and branching of developing axons, but many branches are later pruned to sculpt the mature pattern of connections. Despite its widespread occurrence, the mechanisms controlling pruning remain incompletely characterized. Based on pharmacological and biochemical analysis in vitro and initial genetic analysis in vivo, prior studies implicated a pathway involving binding of the Amyloid Precursor Protein (APP) to Death Receptor 6 (DR6) and activation of a downstream caspase cascade in axonal pruning. Here, we further test their involvement in pruning in vivo and their mechanism of action through extensive genetic and biochemical analysis. Genetic deletion of DR6 was previously shown to impair pruning of retinal axons in vivo. We show that genetic deletion of APP similarly impairs pruning of retinal axons in vivo and provide evidence that APP and DR6 act cell autonomously and in the same pathway to control pruning. Prior analysis had suggested that ␤-secretase cleavage of APP and binding of an N-terminal fragment of APP to DR6 is required for their actions, but further genetic and biochemical analysis reveals that ␤-secretase activity is not required and that high-affinity binding to DR6 requires a more C-terminal portion of the APP ectodomain. These results provide direct support for the model that APP and DR6 function cell autonomously and in the same pathway to control pruning in vivo and raise the possibility of alternate mechanisms for how APP and DR6 control pruning.

The crystal structure of DR6 in complex with the amyloid precursor protein provides insight into death receptor activation

“…Previous studies have elucidated the structures of the unbound APP extracellular domains (Wang and Ha 2004;Dahms et al 2010Dahms et al , 2012Hoopes et al 2010;Lee et al 2011; Xue et al 2011a,b;Coburger et al 2013Coburger et al , 2014. High-affinity binding of APP to death receptor 6 (DR6, also known as TNFRSF21), a death domain-containing member of the extended TNF receptor superfamily (Pan et al 1998), was recently documented (Nikolaev et al 2009) and shown to be mediated by the E2 domain of APP (Olsen et al 2014). DR6 has four cysteine-rich domain (CRD) modules in its extracellular region, followed by a transmembrane domain and death domain in its cytoplasmic region.…”

“…The structure of the unbound DR6 ectodomain (ECD) (Kuester et al 2011;Ru et al 2012) revealed an elongated architecture with a kink between CRD modules 2 and 3. Genetic analysis of DR6 and APP mutant mice showed that the animals share several coincident phenotypes, including behavioral deficits, altered synapse formation, and delayed axon pruning during development or following sensory deprivation Olsen et al 2014). These and other data Olsen et al 2014) indicate that APP and DR6 function cell-autonomously and in the same pathway to stimulate axon pruning and synapse elimination.…”

“…Genetic analysis of DR6 and APP mutant mice showed that the animals share several coincident phenotypes, including behavioral deficits, altered synapse formation, and delayed axon pruning during development or following sensory deprivation Olsen et al 2014). These and other data Olsen et al 2014) indicate that APP and DR6 function cell-autonomously and in the same pathway to stimulate axon pruning and synapse elimination. However, how APP and DR6 interact to mediate their actions is poorly understood.…”

Soluble APP functions as a vascular niche signal that controls adult neural stem cell number

“…Among different fragments of sAPP (Caille et al, 2004;Ohsawa et al, 1999;Olsen et al, 2014;Willem et al, 2015), the soluble Nterminal fragment (aa 18-286) and the 770 aa form sAPP(770) negatively regulate NSC growth but do not affect the number of neurospheres in the culture from acutely dissociated SVZ cells (Fig. S3).…”

The DR is out

“…In 2009, Genentech discovered an interaction between DR6 and amyloid precursor protein (APP), 3 a neuronal surface protein that undergoes a series of proteolytic cleavages to yield the b-amyloid (Ab) fragment. Ab aggregates form the amyloid plaques that are the hallmarks of AD and are thought to be a main upstream trigger for neurodegeneration.…”