Long March Toward Safe and Effective Analgesia by Enhancing Gene Expression of Kcc2: First Steps Taken

Abstract

Low intraneuronal chloride in spinal cord dorsal horn pain relay neurons is critical for physiologic transmission of primary pain afferents because low intraneuronal chloride dictates whether GABA-ergic and glycin-ergic neurotransmission is inhibitory. If the neuronal chloride elevates to pathologic levels, then spinal cord primary pain relay becomes leaky and exhibits the behavioral hallmarks of pathologic pain, namely hypersensitivity and allodynia. Low chloride in spinal cord dorsal horn neurons is maintained by proper gene expression of Kcc2 and sustained physiologic function of the KCC2 chloride extruding electroneutral transporter. Peripheral nerve injury and other forms of neural injury evoke greatly diminished Kcc2 gene expression and subsequent corruption of inhibitory neurotransmission in the spinal cord dorsal horn, thus causing derailment of the gate function for pain. Here I review key discoveries that have helped us understand these fundamentals, and focus on recent insights relating to the discovery of Kcc2 gene expression enhancing compounds via compound screens in neurons. One such study characterized the kinase inhibitor, kenpaullone, more in-depth, revealing its function as a robust and long-lasting analgesic in preclinical models of nerve injury and cancer bone pain, also elucidating its mechanism of action via GSK3β inhibition, diminishing delta-catenin phosphorylation, and facilitating its nuclear transfer and subsequent enhancement of Kcc2 gene expression by de-repressing Kaiso epigenetic transcriptional regulator. Future directions re Kcc2 gene expression enhancement are discussed, namely combination with other analgesics and analgesic methods, such as spinal cord stimulation and electroacupuncture, gene therapy, and leveraging Kcc2 gene expression-enhancing nanomaterials.

Spinal cord dorsal horn sensory gate in preclinical models of chemotherapy-induced painful neuropathy and contact dermatitis chronic itch becomes less leaky with Kcc2 gene expression-enhancing treatments

“…Spinal cord dorsal horn (SCDH) neuronal chloride is established as a critical determinant of sensory circuit integrity (Basbaum, 1999 ; Kahle et al, 2014 ; Liang et al, 2015 ; Liedtke, 2022 ). Low neuronal chloride in the CNS neurons of vertebrates is maintained by the chloride-extruding KCC2 electroneutral chloride transporter (Doyon et al, 2013 ; Mapplebeck et al, 2019 ; Yeo and Liedtke, 2020 ).…”

“…For stable KCC2 expression and function, which maintains steady chloride ion extrusion from neurons, the following features are key: appreciable gene expression of Kcc2 , proper post-translational modification of the KCC2 protein and its location to the outer plasma membrane of the neuron. Kcc2 gene expression is fundamental to all functions of the KCC2 protein, and attenuation of Kcc2 gene expression in pain relay neurons has been demonstrated as causal and key for chronic pain (Doyon et al, 2013 ; Gagnon et al, 2013 ; Kahle et al, 2014 ; Yeo and Liedtke, 2020 ; Yeo et al, 2021 ; Liedtke, 2022 ), with a clean demonstration of causality in several preclinical models of pathologic pain (Coull et al, 2003 ; Gagnon et al, 2013 ; Kahle et al, 2014 ). Suggestive evidence in human systems stems from observations in spinal circuits using organotypic spinal cultures, derived from early post-mortem material (Dedek et al, 2019 ).…”

“…With regards to Kcc2 gene expression, the DNA regulatory regions of the Kcc2 gene were found to be critical, namely REST-RE-1, Egr-EGR4, E-box-USF1/2, and most recently Kaiso sites (Yeo et al, 2013b ; Yeo et al, 2021 ; Liedtke, 2022 ). Kaiso sites depend on DNA methylation to function as transcriptional regulators, indicative of their key relevance in epigenetic gene regulation.…”