Schnurri-2 Promotes the Expression of Excitatory Glutamate Receptors and Contributes to Neuropathic Pain

Highlights

• Increasing Shn2 upregulates GluN2D and GluR1 levels and leads to neuropathic pain.

• Knockdown of Shn2 reduces GluN2D and GluR1 levels, then alleviating neuropathic pain.

• Schnurri-2 is a potential therapeutic target for neuropathic pain.

Abstract

Neuropathic pain is a type of chronic pain with complex mechanisms, and current treatments have shown limited success in treating patients suffering from chronic pain. Accumulating evidence has shown that the pathogenesis of neuropathic pain is mediated by the plasticity of excitatory neurons in the dorsal horn of the spinal cord, which provides insights into the treatment of hyperalgesia. In this study, we found that Schnurri-2 (Shn2) was significantly upregulated in the L4–L6 segments of the spinal cord of C57 mice with spared nerve injury, which was accompanied by an increase in GluN2D subunit and glutamate receptor subunit 1 (GluR1) levels. Knocking down the expression of Shn2 using a lentivirus in the spinal cord decreased the GluN2D subunit and GluR1 levels in spared nerve injury mice and eventually alleviated mechanical allodynia. In summary, Shn2 regulates neuropathic pain, promotes the upregulation of GluN2D in glutamatergic neurons and increases the accumulation of GluR1 in excitatory neurons. Taken together, our study provides a new underlying mechanism for the development of neuropathic pain.

Introduction

Neuropathic pain, a complex chronic pain resulting from primary injury or nervous system dysfunction, is usually caused by cancerous, metabolic, infectious, or traumatic injury. Chronic neuropathic pain is a global disease burden, with a prevalence rate from 6.9% to 10% of the general population (Wang et al., 2020). It is a devastating consequence of injury or disease of the peripheral or central nervous system and characterized by spontaneous pain, hyperalgesia, and allodynia (Jensen et al., 2011, van Hecke, 2014). Neuropathic pain is chronic, severe, debilitating, and extremely difficult to treat (Finnerup et al., 2015); therefore, it is necessary to explore its detailed molecular mechanisms.

The α-amino-3-hydroxy-5-methyl-4-isoxazolpropionic acid receptor (AMPAR) is an ionic glutamate receptor that mediates the majority of rapid excitatory synaptic transmission in the mammalian central nervous system. AMPARs are crucially important for dynamic changes in neuronal synaptic efficiency and plasticity and in synapses at many postsynaptic neurons (Wu and Donohoe, 2019). The AMPAR is a polymer composed of four glutamate receptor subunits (GluR1–4), of which GluR1 and GluR2 are the most abundant on the postsynaptic neuron membrane in the superficial dorsal horn of the spinal cord (Kerr and Maxwell, 1998, Lu et al., 2002). GluR1 is a significant AMPAR subtype, and AMPARs are involved in pain hypersensitivity (Guo et al., 2020). Our previous study indicated that upregulation of Neuroligin1 contributed to neuropathic pain and may be involved in phosphorylation of cofilin and an increase in GluR1 (Ouyang et al., 2021).

Activation of N-methyl-D-aspartate receptors (NMDARs) in nociceptive neurons of the spinal dorsal horn plays a key role in the generation and maintenance of central sensitization (Petrenko et al., 2003). GluN2 subunits comprised four isoforms (GluN2A–D), each with distinct physiological properties (Paoletti et al., 2013). Based on available studies in male rodents, three of the four GluN2 isoforms are expressed in the dorsal horn-GluN2A, GluN2B, and GluN2D (Bourinet et al., 2014). In both sexes, GluN2A immunoreactivity is almost exclusively restricted to neuropil, while GluN2B and GluN2D are found in both the neuropil and the soma of dorsal horn neurons (Temi et al., 2021). It has been suggested that different NMDA receptor subtypes may participate differentially in nociception. For example, NMDA receptor subunits that participate in fast synaptic transmission, namely NR2A-2B, may take part in the transmission of acute noxious input (Momiyama, 2000). NMDA receptor subunits, like the GluN2D subunit, that are characterized as low-conductance channels (Momiyama and Feldmeyer, 1996, Misra et al., 2000), may contribute to the maintenance of more prolonged pain states such as neuropathic pain (Lawand et al., 1997). Little is known about their functional role in spinal cord to drive neuropathic pain, so it is necessary to make a further study for GluN2D.

Schnurri-2 (Shn2), also known as major histocompatibility complex binding protein 2 (MHC-2), closely binds to the MHC I gene enhancer in the MHC region of chromosome 6. It is a member of the large zinc finger transcription protein family, is expressed in the central nervous system, and participates in the development and maturation of the nervous system (Park et al., 2019). It is also closely associated with schizophrenia, neuroinflammation, immune deficiency, and other diseases (Hallie et al., 2016, Heidi et al., 2019). Downregulation of Shn2 can lead to neuropsychiatric disorders such as chronic brain inflammation and schizophrenia (Caitlin et al., 2020, Caitlin et al., 2020). The dendritic spines of hippocampal dentate granule cells in Shn2 knockout mice are longer and thinner than those of normal mice. This abnormal morphology can hinder the rapid lateral diffusion of AMPARs. In addition, studies have shown that the expression of the AMPAR subunit GluA1 is reduced by approximately 50% in the dentate molecular layer of Shn2 knockout mice (Nakao et al., 2017). Furthermore, Shn2 knockout mice showed weak activation of NMDARs (Kobayashi et al., 2018). However, the role of Shn2 in pain formation through regulation of GluR1 and NMDARs expression remains unclear.

Many studies have reported that Shn2 has an important impact on the occurrence of schizophrenia and other mental diseases through regulation of synaptic plasticity (Kobayashi et al., 2018). However, whether Shn2 participates in the formation of neuropathic pain and the upregulation of excitatory glutamate receptors and its mechanism have not been reported. In the present study, we evaluated the effect of Shn2 in mouse models of neuropathic pain. We found that Shn2 promoted the expression of excitatory glutamate receptors and contributed to neuropathic pain.