Just in time! Identification of a novel mechanism for treating PIPN

The chemotherapeutic drug paclitaxel is commonly used in the effective clinical management of many cancers, including breast, ovarian, prostate and lung cancers. One of the most frequent dose-limiting complications arising from treatment is the emergence of paclitaxel induced peripheral neuropathy (PIPN). This side effect occurs in 60–70% of patients treated with paclitaxel and, for some, the emergence of this painful neuropathy is irreversible (Nyrop et al. 2019; Staff et al. 2020). In addition, symptoms associated with PIPN are notoriously difficult to control and, similar to other forms of neuropathic pain, are often refractory to treatment (Alles & Smith, 2018). This lack of treatment efficacy has made understanding the mechanisms underlying PIPN an urgent scientific question, which could provide new targets for the development of novel therapies. Neuropathic pain arises from a maladaptive response of the nervous system to damage. The mechanisms underlying pathophysiological changes during the initiation of neuropathic pain are unique and independent from those that maintain neuropathic pain over time. The cellular mechanisms accompanying each phase are important to differentiate because most clinical encounters occur during the maintenance phase, yet animal studies investigating neuropathy are generally performed over a shorter time course (Alles & Smith, 2018). At the cellular level, one of the processes shown to be involved in neuropathic pain is known as central sensitization (Woolf, 1983). This process results from disruption of synaptic transmission, where excitatory synaptic connections are enhanced and inhibitory connections are reduced, ultimately leading to aberrant sensory perception. Central sensitization has been observed in neurons of the dorsal horn of the spinal cord where primary sensory neurons make their initial central connections. Preclinical models have demonstrated that neuropathic pain produces increased numbers of mature dendritic spines on spinal neurons of the dorsal horn (Tan & Waxman, 2015). These spines can thus be used as a structural proxy for a neuron’s level of synaptic transmission efficacy. Membrane evagination, an initial step in the formation dendritic spines, is known to be regulated by the protein slit-robo GTPase activating protein 3 (srGAP3), among other cytoskeletal proteins (Spence et al. 2019). In mice, srGAP3 deficiency produced profound deficiency in learning and memory related functions. Additionally, srGAP3 has been shown to inhibit the activity of the Rho GTPase Rac1. This protein is a major regulator of actin filaments that determine the structure and morphology of dendritic spines (Endris et al. 2011). Previously, inhibition of RAC1 using the small molecule NSC23766 successfully ameliorated neuropathic pain and reduced aberrant dendritic spine formation in an animal model of spinal cord injury (Tan & Waxman, 2015). Together, these findings suggested that dual targeting of srGAP3 and RAC1 could prove advantageous in the treatment of PIPN. In this issue of The Journal of Physiology, an elegant series of experiments performed by Chen et al. (2020) found that srGAP3 levels in the dorsal horn of the spinal cord increased during the initiation phase of PIPN, which correlated with dendritic spine density and maturity. This result is consistent with previous studies showing that PIPN results in enhanced numbers of spines, as well as increased spine maturity, which together act to facilitate synaptic transmission. In addition, it was demonstrated that small interfering RNA (siRNA) induced knockdown of srGAP3, specifically during the initiation phase of PIPN, reduced mechanical allodynia in paclitaxel-treated rats. Furthermore, adeno-associated virus-mediated overexpression of srGAP3 induced mechanical allodynia and a neuropathic pain phenotype with commensurate alterations in dendritic spine density and structure. However, these effects were only observed when srGAP3 levels were altered during the initiation phase of PIPN and not during the maintenance phase. This suggests that srGAP3 is a mediator of neuropathic pain through the regulation of dendritic spine formation and maturation in the early stages of PIPN. To investigate the maintenance phase of PIPN, which is most clinically relevant, Chen et al. (2020) turned their attention to a different target: RAC1. SrGAP3 inhibits the activity of RAC1, which has been shown to be involved in neuropathic pain following nerve injury. They found that pharmacological antagonism of RAC1 during the maintenance phase, but not the initiation phase, was sufficient to reduce mechanical allodynia in paclitaxel-treated rats. These findings portray functionally coupled, but temporally distinct, roles for srGAP3 and RAC1 in the modulation of PIPN. This interplay was clearly demonstrated in the final experiment by Chen et al. (2020) in which srGAP3 was knocked down using siRNA during the initiation phase and RAC1 activity was suppressed with NSC23766 during the maintenance phase. The results reveal a robust reduction in mechanical allodynia, approaching baseline levels, on day 30 following paclitaxel administration. In addition, dendrites of superficial dorsal horn neurons in these animals showed markedly reduced spine density and maturation, which suggests this behavioural change was probably mediated through modulation of structural synaptic plasticity (Fig. 1). Although the roles of srGAP3 and RAC1 in regulating dendritic spine formation have been documented previously, there has been little work specifically examining these targets in the setting of PIPN. The findings identify unique roles for these two proteins at distinct time points in