The Molecular Mechanism Underlying RPS23RG1-mediated Synaptic Plasticity And Its Involvement In Alzheimer’s Disease

During learning and memory formation,synapse numbers and connections change dynamically,accompanied with signal transmitting from the presynaptic membrane to the postsynaptic membrane in neurons.There are thousands of molecules locating in synaptic structures,and their orders and precise combination at the synaptic site are critical for normal synaptic plasticity.In a variety of neurological diseases,including Alzheimer’s disease(AD),Parkinson’ disease(PD),schizophrenia,autism,etc.,synapses have been found to be dysfunctional.Therefore,identifying proteins involved in synaptic functions and revealing the underlying mechanisms will help us elucidate the molecular pathogenesis of neurological diseases.PSD-93 and PSD-95 are postsynaptic density scaffolding proteins crucial for regulation of postsynaptic membrane receptor transport and excitatory synaptic plasticity maintenance.It has been found that PSD-95 is ubiquitinated by the E3 ubiquitin ligase MDM2 and this process impacted synaptic functions.However,the detailed molecular pathways remain largely unclear.In addition,less is known on the homeostatic regulation of PSD-93.In previous studies,we have identified Rps23rglgne that originats through retroposition of ribosomal protein S23 mRNA,and found that RPS23RG1 overexpression reduces the levels of Aβ and tau phosphorylation,two critical proteins in the pathogenesis of AD.RPS23RG1 overexpression also offsets Aβ-induced synaptic dysfunction and learning/memory impairments.The RPS23RG1 level is decreased in the brain of AD patients and an AD model(Tg2576)mice.Together,RPS23RG1 clearly plays a key role in the pathogenesis of AD,however,its physiological function remains unknown.Here,we investigated the role of RPS23RG1 in synaptic functions and learning and memory.Firstly,we found that Rps23rg1 was ubiquitously expressed in all tissues examined and in all three neural cell types as neurons,microglia and astrocytes.RPS23RG1 protein was also detected in different synaptosomal fractions,in a pattern similar to that of PSD-95.Moreover,we confirmed that RPS23RG1/Rps23rg1 mRNA levels were significantly decreased in the brain of AD patients and APP/PS1 AD model mice.We also noted that RPS23RG1 protein levels had close correlation with PSD-93 and PSD-95 levels in AD patient samples,and PSD-93 and PSD-95 protein levels were decreased similarly to those of RPS23RGI in APP/PS1 mice.Next,we used a transcription activator-like effector nuclease(TALEN)-mediated targeted deletion strategy to generate Rps23rgl knockout(KO)mice.We found that homozygous Rps23rgl KO mice were born at an expected mendelian frequency and with a nearly 1:1 sex ratio,and had normal morphology and comparable body weights to those of wild type(WT)littermate controls.However,Rps23rgl KO mice had severe deficits in learning and memory,accompanied with reduced PSD-93 and PSD-95 levels,decreased spine maturation and impaired synaptic plasticity.Our mechanistic study reveals that RPS23RG1 binds to PSD-93 and PSD-95 through a "TTLAH" motif that is conserved in human and mouse RPS23RG1 carboxyl-terminus.Moreover,RPS23RG1 competed with MDM2 for the binding to PSD-93 and PSD-95.Loss of RPS23RG1 enhanced MDM2-PSD-93 and MDM2-PSD-95 interactions and thus PSD-93 and PSD-95 ubiquitination and degradation,whereas overexpression of RPS23RG1 had an opposite effect.In addition,we injected viruses expressing PSD-93 and PSD-95 bilaterally into lateral ventricles of PO mice to normalize PSD-93 and PSD-95 expression.Mouse behaviors were assayed at 2 month of age.The results show that restoring PSD-93 and PSD-95 levels can rescue synaptic and cognitive impairments in Rps23rg1 KO mice.It is demonstrated that the reduction of PSD-93 and PSD-95 levels is an important factor leading to synaptic and memory impairment in Rps23rgl KO mice.Finally,intraperitoneal injection of the human RPS23RG1 carboxyl-terminus peptide fused with a TAT transduction peptide to Rps23rg1 KO mice or APP/PS1 mice rescued synaptic and cognitive deficits by attenuating MDM2-mediated ubiquitination of PSD-93 and PSD-95.In summary,our results not only identify RPS23RG1 as a novel modulator for PSD-93 and PSD-95 stability and synaptic function,but also indicate that targeting RPS23RG1-mediated pathway has therapeutic potential for AD.