Glutamate Receptor Trafficking and Protein Synthesis Mediate the Facilitation of LTP by Secreted Amyloid Precursor Protein-Alpha

Abstract

Secreted amyloid precursor protein-alpha (sAPP␣) has growth factor-like properties and can modulate long-term potentiation (LTP) and memory. Here, we demonstrate that exposure to sAPP␣ converts short-lasting LTP into protein-synthesis-dependent late LTP in hippocampal slices from male rats. sAPP␤ had no discernable effect. We hypothesized that sAPP␣ facilitated LTP via regulated glutamate receptor trafficking and de novo protein synthesis. We found using a linear mixed model that sAPP␣ stimulated trafficking of GluA2lacking AMPARs, as well as NMDARs to the extrasynaptic cell surface, in a calcium/calmodulin-dependent kinase II and protein kinase G-dependent manner. Both cell surface receptor accumulation and LTP facilitation were present even after sAPP␣ washout and inhibition of receptor trafficking or protein synthesis prevented all these effects. Direct visualization of newly synthesized proteins (FUNCAT-PLA) confirmed the ability of sAPP␣ to stimulate de novo protein synthesis and revealed GluA1 as one of the upregulated proteins. Therefore, sAPP␣ generates a coordinated synthesis and trafficking of glutamate receptors to the cell surface that facilitate LTP. Significance StatementSecreted amyloid precursor protein-alpha (sAPP␣) is a neurotrophic and neuroprotective protein that can promote synaptic plasticity and memory, yet the molecular mechanisms underlying these effects are still not well understood. Here, we show that sAPP␣ facilitates long-term potentiation (LTP) in a concentration-dependent fashion through cellular processes involving de novo protein synthesis and trafficking of both GluA2-lacking AMPARs and NMDARs to the extrasynaptic cell surface. sAPP␣ also enhances GluA1, but not GluA2, synthesis. The trafficking effects, along with the LTP facilitation, persist after sAPP␣ washout, revealing a metaplastic capability of exogenous sAPP␣ administration. sAPP␣ thus facilitates LTP through coordinated activation of protein synthesis and trafficking of glutamate receptors to the cell surface, where they are positioned for priming LTP.

Measuring mRNA translation in neuronal processes and somata by tRNA-FRET

“…Notably, this up-regulation of mRNA translation was blocked 6 hours post activation by prior application of GluR antagonists, consistent with reports of GluRdependent LTP(65)(66)(67). These results were further corroborated by immunocytochemical SUnSET.Examining sub-cellular compartments through the unique benefits of tRNA-FRET, we determined that somatic and dendritic protein synthesis equally contributed to the pulsatile pattern of mRNA translation up-regulation, consistent with previous observations(110), and further strengthened by our presentation of the constant ratio of ~1 between somatic and dendritic mRNA translation levels regardless of cLTP induction and GluR inhibition.Research regarding tRNA itself, which has been limited for a long time, has recently attracted renewed attention. Aside from its canonical role as the adaptor that facilitates mRNA translation to proteins, tRNA participates in many additional cellular pathways, including N-terminal conjugation of amino acids to proteins, signal transduction pathways that respond to nutrient deprivation, and regulation of apoptosis (16, 19, 27).…”

Secreted Amyloid Precursor Protein-Alpha Promotes Arc Protein Synthesis in Hippocampal Neurons

“…Despite many studies, the cell surface receptor(s) which transduce the sAPPα signal are yet to be conclusively identified, though specific candidates have emerged (Rice et al, 2017, 2019; Richter et al, 2018; Mockett et al, 2019). Here, we pharmacologically inhibited likely candidates mediating sAPPα’s plasticity-promoting effects and observed the effect on sAPPα-induced Arc protein levels (Figure 8).…”

“…Further, sAPPα is able to facilitate long-term potentiation (LTP; Taylor et al, 2008; Moreno et al, 2015), stimulate neurite outgrowth (Clarris et al, 1994), and regulate spine morphology (Hick et al, 2015). Recently, it has been shown that the molecular mechanisms underpinning these actions include enhancement of glutamate receptor trafficking, synaptodendritic protein synthesis and new gene transcription (Claasen et al, 2009; Chasseigneaux et al, 2011; Ryan et al, 2013; Mockett et al, 2019), yet these and other mechanisms have not been fully explored.…”

“…Notably, while the receptor/s mediating the actions of sAPPα have not been conclusively identified, candidate receptors and downstream pathways overlap with those identified in relation to the regulation of Arc. Somatodendritic translation of Arc mRNA is dependent on Ca 2+ signaling via ionotropic receptors, including the N -methyl-d-aspartate receptor (NMDA; Chen et al, 2017) and α7 nicotinic acetylcholine receptor (α7nAchR; Kristensen et al, 2007); these receptors are both candidates for mediating sAPPα’s actions (Taylor et al, 2008; Richter et al, 2018; Mockett et al, 2019). Furthermore, downstream signaling molecules such as protein kinase G (PKG), mitogen activated protein kinase (MAPK) and CaMKII have not only been shown to enhance Arc mRNA or regulate Arc protein expression (Huang et al, 2007; Gakhar-Koppole et al, 2008; Ota et al, 2010; Chasseigneaux et al, 2011), but also mediates the neuroprotective, neurotrophic and plasticity-enhancing effects of sAPPα (Furukawa et al, 1996a; Claasen et al, 2009; Mockett et al, 2019).…”

Amyloid Precursor Protein (APP) and GABAergic Neurotransmission

“…Aβ is well known for causing pathological dysregulation of postsynaptic trafficking of both the AMPA [39] and N -Methyl- d -aspartate (NMDA) [40]-type glutamate receptors. Notably, APP also has physiological roles in the function and trafficking of these glutamate receptors [41,42,43] and may thus be important for synaptic plasticity and learning/memory [44,45,46,47,48]. The actions of APP at the synapse are also known to be mediated by the secreted (s)APPs, mainly sAPPα generated by α-secretase cleavage [49,50,51,52,53].…”