Papers of the Week

Calcineurin inhibitors, such as cyclosporine and tacrolimus (FK506), are commonly used immunosuppressants for preserving transplanted organs and tissues. However, these drugs can cause severe and persistent pain. GluA2-lacking, calcium-permeable AMPA receptors (CP-AMPARs) are implicated in various neurological disorders, including neuropathic pain. It is unclear whether and how constitutive calcineurin, a Ca/calmodulin protein phosphatase, controls synaptic CP-AMPARs. In this study, we found that blocking CP-AMPARs with IEM-1460 markedly reduced the amplitude of AMPAR-EPSCs in excitatory neurons expressing vesicular glutamate transporter-2 (VGluT2), but not in inhibitory neurons expressing vesicular GABA transporter, in the spinal cord of FK506-treated male and female mice. FK506 treatment also caused an inward rectification in the current-voltage relationship of AMPAR-EPSCs specifically in VGluT2 neurons. Intrathecal injection of IEM-1460 rapidly alleviated pain hypersensitivity in FK506-treated mice. Furthermore, FK506 treatment substantially increased physical interaction of α2δ-1 with GluA1 and GluA2 in the spinal cord and reduced GluA1/GluA2 heteromers in endoplasmic reticulum-enriched fractions of spinal cords. Correspondingly, inhibiting α2δ-1 with pregabalin, genetic knockout, or disrupting α2δ-1-AMPAR interactions with an α2δ-1 C-terminus peptide reversed inward rectification of AMPAR-EPSCs in spinal VGluT2 neurons caused by FK506 treatment. In addition, CK2 inhibition reversed FK506 treatment-induced pain hypersensitivity, α2δ-1 interactions with GluA1 and GluA2, and inward rectification of AMPAR-EPSCs in spinal VGluT2 neurons. Thus, the increased prevalence of synaptic CP-AMPARs in spinal excitatory neurons plays a major role in calcineurin inhibitor-induced pain hypersensitivity. Calcineurin and CK2 antagonistically regulate postsynaptic CP-AMPARs through α2δ-1-mediated GluA1/GluA2 heteromeric assembly in the spinal dorsal horn. Clinically used calcineurin inhibitors can cause severe pain, known as calcineurin inhibitor-induced pain syndrome (CIPS). However, its underlying mechanisms remain elusive. This study shows for the first time that calcineurin inhibition caused cell type-specific expression of synaptic Ca-permeable AMPARs in spinal cord excitatory neurons. Blocking spinal Ca-permeable AMPARs reduced CIPS. Calcineurin inhibition potentiated the α2δ-1 (previously known as a calcium channel subunit) interaction with GluA1 and GluA2 subunits, disrupting their intracellular assembly in the spinal cord. Additionally, inhibiting spinal CK2 diminished α2δ-1-AMPAR interactions and synaptic Ca-permeable AMPARs augmented by calcineurin inhibitors. Thus, calcineurin and CK2 dynamically control AMPAR phenotypes in spinal excitatory neurons through α2δ-1-mediated GluA1/GluA2 assembly. Targeting α2δ-1 and CK2 are effective strategies for treating CIPS.