Diego Miranda

Background: Spasticity, a product of spinal cord injury (SCI), is characterized by hyperreflexia, muscle spasms, co-contraction of antagonist muscles and clonus.¹ After an SCI, there is a decrease in the expression of the potassium-chloride co-transporter, also known as KCC2, leading to an excitatory/inhibitory imbalance and self-sustained activity in motor neurons.¹ The current pharmaceutical treatment for spasticity symptoms is Baclofen, but it can cause severe side effects such as muscle weakness, fatigue, dizziness, and drowsiness.¹ However, downregulation of Calpain1, modulation of the BDNF-TrkB pathway, CLP257 therapy, and phosphorylation of Serine 937 have shown promise as alternative therapeutic targets to upregulate KCC2 expression thus alleviating spasticity symptoms while avoiding the side effects of Baclofen.^1-5

Objective: The purpose of this study was to explore the mechanisms of KCC2.

Search Methods: A search within the PubMed Database for research from 2018 to 2023 was done with the MeSH terms “spinal cord injury”, “spasticity”, and “KCC2 co-transporter”.

Results: The enzyme Calpain1 has been proven to have increased expression at the site of a SCI.¹ Animal models which were given a shRNA to knock down the Calpain1 gene were shown to have an effective reduction of spasticity and hyperreflexia as well as an upregulation of KCC2 levels when compared to animals who were given a scramble that did not knock down the gene.¹ The BDNF-TrkB pathway can cause phosphorylation of KCC2 and down regulate mKCC2 expression after a spared nerve injury (SNI).² However when administered K252a, a tyrosine kinase inhibitor, it was observed that mKCC2 levels remained similar to levels prior to the SNI.² It was also shown that in for exercise based physical therapy to be successful in modulating spasticity symptoms the BDNF-TrkB pathway was essential.³ When animals undergoing therapy, post SCI, had the BDNF pathway competitively inhibited they demonstrated levels of spasticity similar to the unexercised animal groups.³ CLP257 was also shown to increase membrane KCC2 expression similarly to exercise therapy, but when used in conjunction did not significantly enhance KCC2 levels.⁴ This demonstrated that it could be used alongside physical therapy to decrease the amount of therapy needed to achieve the same levels of KCC2 expression.⁴ In the auditory brainstem, phosphorylation of the Serine 937 KCC2 gene was shown to increase innate levels of KCC2.⁵ The animals with this mutation showed no abnormalities in their nutritional state, posture, motor skills, fur, fertility meaning that the mutation did not affect other genes. The subjects also demonstrated no change in the volume of auditory nuclei indicating that the mutation also had no effect on cell migration or cell viability.⁵

Conclusion: CLP257 therapy and modulation of the BDNF-TrkB signaling pathway in conjunction with exercise present as potential therapeutic agent targets for the upregulation of KCC2 expression. Phosphorylation of Serine 937 and modulation of Calpain1 activity also present as potential mechanisms that can be targeted, however further research is needed to further specify the broad effects of the therapies on spinal cord motor neurons.

Works Cited:

1. Kerzonkuf M, Verneuil J, Brocard C, et al. Knockdown of calpain1 in lumbar motoneurons reduces spasticity after spinal cord injury in adult rats. Mol Ther. Published online January 29, 2024. doi:10.1016/j.ymthe.2024.01.029
2. Beverungen H, Klaszky SC, Klaszky M, Côté MP. Rehabilitation Decreases Spasticity by Restoring Chloride Homeostasis through the Brain-Derived Neurotrophic Factor-KCC2 Pathway after Spinal Cord Injury. J Neurotrauma. 2020;37(6):846-859. doi:10.1089/neu.2019.6526
3. Hu Z, Yu X, Chen P, et al. BDNF-TrkB signaling pathway-mediated microglial activation induces neuronal KCC2 downregulation contributing to dynamic allodynia following spared nerve injury [published correction appears in Mol Pain. 2024 Jan-Dec;20:17448069231222686]. Mol Pain. 2023;19:17448069231185439. doi:10.1177/17448069231185439
4. Bilchak JN, Yeakle K, Caron G, Malloy D, Côté MP. Enhancing KCC2 activity decreases hyperreflexia and spasticity after chronic spinal cord injury. Exp Neurol. 2021;338:113605. doi:10.1016/j.expneurol.2021.113605
5. Radulovic T, Rajaram E, Ebbers L, et al. Serine 937 phosphorylation enhances KCC2 activity and strengthens synaptic inhibition. Sci Rep. 2023;13(1):21660. Published 2023 Dec 8. doi:10.1038/s41598-023-48884-x