Neuronal surface P antigen (NSPA) modulates postsynaptic NMDAR stability through ubiquitination of tyrosine phosphatase PTPMEG

Abstract

Background Cognitive dysfunction (CD) is common among patients with the autoimmune disease systemic lupus erythematosus (SLE). Anti-ribosomal P autoantibodies associate with this dysfunction and have neuropathogenic effects that are mediated by cross-reacting with neuronal surface P antigen (NSPA) protein. Elucidating the function of NSPA can then reveal CD pathogenic mechanisms and treatment opportunities. In the brain, NSPA somehow contributes to glutamatergic NMDA receptor (NMDAR) activity in synaptic plasticity and memory. Here we analyze the consequences of NSPA absence in KO mice considering its structural features shared with E3 ubiquitin ligases and the crucial role of ubiquitination in synaptic plasticity. Results Electrophysiological studies revealed a decreased long-term potentiation in CA3-CA1 and medial perforant pathway-dentate gyrus (MPP-DG) hippocampal circuits, reflecting glutamatergic synaptic plasticity impairment in NSPA-KO mice. The hippocampal dentate gyrus of these mice showed a lower number of Arc-positive cells indicative of decreased synaptic activity and also showed proliferation defects of neural progenitors underlying less adult neurogenesis. All this translates into poor spatial and recognition memory when NSPA is absent. A cell-based assay demonstrated ubiquitination of NSPA as a property of RBR-type E3 ligases, while biochemical analysis of synaptic regions disclosed the tyrosine phosphatase PTPMEG as a potential substrate. Mice lacking NSPA have increased levels of PTPMEG due to its reduced ubiquitination and proteasomal degradation, which correlated with lower levels of GluN2A and GluN2B NMDAR subunits only at postsynaptic densities (PSDs), indicating selective trafficking of these proteins out of PSDs. As both GluN2A and GluN2B interact with PTPMEG, tyrosine (Tyr) dephosphorylation likely drives their endocytic removal from the PSD. Actually, immunoblot analysis showed reduced phosphorylation of the GluN2B endocytic signal Tyr1472 in NSPA-KO mice. Conclusions NSPA contributes to hippocampal plasticity and memory processes ensuring appropriate levels of adult neurogenesis and PSD-located NMDAR. PTPMEG qualifies as NSPA ubiquitination substrate that regulates Tyr phosphorylation-dependent NMDAR stability at PSDs. The NSPA/PTPMEG pathway emerges as a new regulator of glutamatergic transmission and plasticity and may provide mechanistic clues and therapeutic opportunities for anti-P-mediated pathogenicity in SLE, a still unmet need.

Progress in the mechanism of neuronal surface P antigen modulating hippocampal function and implications for autoimmune brain disease

“…Regarding the role of NSPA, it is interesting that NSPA-KO mice show less ubiquitinated and higher levels of PTPMEG/PTPN4, qualifying as substrate of NSPA in synaptic fractions [27 ▪▪ ]. This correlates with decreased levels of GluN2 NMDAR subunits at the PSD, very likely due to dephosphorylation of Tyr residues involved in the endocytic removal of these receptors, as indicated by the immunoblot analysis of GluN2B Tyr1472 [27 ▪▪ ].…”

“…These domains, even without RING1, have intrinsic catalytic activity for autoubiquitination and polyubiquitin chain formation, as suggested by the truncated version of the E3 ubiquitin ligase Parkin [38]. Therefore, NSPA seems to be an atypical E3 ubiquitin ligase most likely related to the RBR family [27 ▪▪ ].…”

“…Mice expressing a truncated version of NSPA (NSPA-TR) or lacking NSPA expression (NSPA-KO) reveal that NSPA somehow contributes to maintain appropriate levels of NMDAR transmission, neuronal activity, synaptic plasticity and memory processes in the hippocampus [23,27 ▪▪ ]. NSPA-TR mice have decreased NMDAR-transmission, impaired long-term potentiation (LTP) in the Schaffer collateral-CA1 pathway (CA3-CA1 synapses) and poor performance in episodic-like memory flexibility tests [23].…”

“…Further evidence of NSPA as an E3 ubiquitin ligase is provided by a substrate that links such role with glutamatergic synaptic transmission [27 ▪▪ ]. In a wide-screen yeast two-hybrid analysis of brain interacting proteins [39], NSPA appears interacting with the tyrosine-phosphatase PTPMEG (megakaryocyte protein tyrosine phosphatase), also known as PTPN4 (Protein Tyrosine Phosphatase Non-Receptor Type 4).…”

Roles of N-Methyl-D-Aspartate Receptors (NMDARs) in Epilepsy

“…It is also found that S-PrP interacts with LRP1/NMDAR system to activate ERK1/2, thereby promoting cell migration in Schwann cells (Mantuano et al, 2020). Neuronal surface P antigen (NSPA) regulates the postsynaptic stability of NMDAR by ubiquitination of tyrosine phosphatase PTPMEG (Espinoza et al, 2020). In addition, Cyclin B/CDK1 mediates NMDAR phosphorylation and regulates calcium kinetics and mitosis (Rosendo-Pineda et al, 2020).…”