Date Published: December 12, 2014
Publisher: Public Library of Science
Author(s): Ming Gan, Peizhou Jiang, Pamela McLean, Takahisa Kanekiyo, Guojun Bu, Ya-Ping Tang.
The low-density lipoprotein receptor-related protein 1 (LRP1) is a multifunctional endocytic receptor abundantly expressed in neurons. Increasing evidence demonstrates that LRP1 regulates synaptic integrity and function at the post synapses, at least partially by regulating glutamate receptors. The α-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid receptors (AMPARs) are critical ionotropic glutamate receptors consisting of homotetramer or heterotetramer of GluA1-4 subunits and play an essential role in synaptic transmission and synaptic plasticity. Our previous work has shown that neuronal deletion of the Lrp1 gene in mice leads to decreased level of GluA1 and reduced long-term potentiation. To understand the underlying mechanism, we investigated the cellular and functional consequences of LRP1 deletion in primary neurons. Here, we show that LRP1 interacts with and regulates the cellular distribution and turnover of GluA1. LRP1 knockdown in mouse primary neurons led to accelerated turnover and decreased cell surface distribution of GluA1, which correspond to decreased phosphorylation of GluA1 at S845 and S831 sites. Decreased LRP1 expression also attenuated AMPA-evoked calcium influx and reduced GluA1-regulated neurite outgrowth and filopodia density. Our results reveal a novel mechanism by which LRP1 controls synaptic integrity and function, specifically by regulating GluA1 trafficking, phosphorylation and turnover. They further demonstrate that LRP1-GluA1 pathway may hold promises as a therapeutic target for restoring synaptic functions in neurodegenerative diseases.
The low-density lipoprotein receptor-related protein 1 (LRP1) is a large endocytic receptor abundantly expressed in various brain cell types, including neurons and glial cells in brain parenchyma, and smooth muscle cells and pericytes in cerebrovasculature, where it mediates cellular uptake of diverse ligands including apolipoprotein E (apoE), α2-macroglobulin, and tissue plasminogen activator (tPA) , , . LRP1 is a highly efficient transport receptor with a rapid endocytosis rate and signal-mediated recycling by interacting with multiple adaptor proteins through several tyrosine-based motifs in its cytoplasmic tail region , . Furthermore, LRP1 also regulates signal transduction by coupling with other cell-surface signalling receptors including the platelet-derived growth factor receptor (PDGFR)  and the leptin receptor . In neurons, LRP1 is predominantly expressed in the postsynaptic region  and the cell body , where it regulates lipid transport  and the metabolism of amyloid-β (Aβ) peptides ,  whose accumulation is considered central to the pathogenesis of Alzheimer’s disease (AD). LRP1 is known to form a complex with N-methyl-d-aspartate receptors (NMDARs) through the multivalent scaffold protein, postsynaptic density protein 95 (PSD95) , which modulates synaptic transmission and synaptic plasticity , , .
Increasing evidence has demonstrated a critical role of LRP1 in maintaining synaptic functions and neuronal homeostasis , , , . When the Lrp1 gene was specifically deleted in differentiated neurons by synapsin promoter-driven Cre recombinase expression, severe behavioral and motor abnormalities were evident at 3–6 months of age . Furthermore, Lrp1 gene deletion in forebrain neurons in adult mice by αCaMKII-Cre led to compromised brain lipid metabolism and progressive, age-dependent synaptic dysfunction, cognitive impairments, and eventual neurodegeneration . One important pathway by which LRP1 regulates synaptic plasticity depends on functions of glutamate NMDARs receptors at post-synapses , , . LRP1 interacts with NMDARs through PSD95 and controls its trafficking and degradation , . In addition, LRP1 modulates NMDARs function by regulating the binding and function of tissue-type plasminogen activator (tPA) , . The LRP1 intracellular domain also regulates NMDAR-mediated signaling pathway . Although effects of LRP1-deletion on NMDARs are well studied, the interaction between LRP1 and AMPARs has received relatively less attention. Thus, in this study, we focused on the dissecting the interaction between LRP1 and the AMPAR subunit GluA1 in mouse primary neurons, and showed that PSD95, LRP1, and GluA1 form immunoprecipitable complexes. PSD95 is a specialized scaffold protein with multiple protein interacting domains that form the backbone of postsynaptic protein complexes at synaptic contact zone . PSD95 recruits and anchors GluA1 to the post-synaptic density , , ,  through transmembrane AMPAR regulatory proteins (TARPs) . Our results show that the knockdown of LRP1 decreases both GluA1 and PSD95 by accelerating their degradation without affecting their mRNA levels. We further demonstrate that depletion of LRP1 decreases the distribution of GluA1 on the cell surface, which results in suppressed phosphorylation of GluA1 at Ser-845 and Ser-831. While phosphorylation of S845 promotes GluA1 surface expression and affects its stability , phosphorylation of Ser831 mediates the delivery of GluA1 to the synapses  and increases their single channel conductance . These observations suggest that LRP1 suppresses GluA1 sorting to the degradation pathway and/or facilitates its recycling by influencing its phosphorylation and cell surface distribution. During LTP, AMPARs are recruited to the post-synaptic membrane from non-synaptic pools and interact with scaffold proteins for local concentration. AMPARs can also be targeted to lysosomes for degradation to reduce synaptic density during LTD . Thus, the deletion of neuronal LRP1 likely induces the disturbed LTP and cognitive dysfunction at least partially by suppressing GluA1 trafficking and accelerates its turnover.