PROJECT SUMMARY Chronic pain is a pathological state where sensory neurons become hyperexcitable leading to nociceptive neurotransmission in the absence of a painful stimulus. Genetic and functional studies have established the voltage-gated sodium channel NaV1.7 as a major contributor to human pain signaling. Although the regulation of NaV1.7 is poorly understood, it is thought to involve mechanisms related to surface trafficking and regulation via protein-protein interactions. For instance, upregulation of SUMOylation of cytosolic collapsin response mediator protein 2 (CRMP2) and VGSC β-subunits, and downregulation of Nedd4-2 (a cytosolic E3 ubiquitin ligase) in a model of spared nerve injury induced chronic pain result in functional upregulation of NaV1.7 channels. My studies have identified lymphocyte antigen 6 (Ly6) proteins as a novel class of NaV1.7 channel modulators. Ly6 proteins are extracellular glycoproteins that are a hallmark of different types of cancer and have a role in cell proliferation, cell migration, cell–cell interactions, immune cell maturation, macrophage activation, and cytokine production. Ly6 proteins show structural resemblance to the three-fingered snake venom toxins that are known to modulate nicotinic acetylcholine receptors and voltage-gated sodium channels. Of these group of proteins, Ly6e and Lynx1 are common between humans and rodents. RNA-sequencing databases showed that both Ly6e and Lyn1 expression increases in different populations of dorsal root ganglia (DRG) neurons after nerve injury. Moreover, my preliminary findings show that: (i) there are higher Ly6e signal levels in DRGs after spared nerve injury (SNI); (ii) overexpression of Ly6e is associated with increased sodium currents in DRGs, and NaV1.7 currents in HEK cells; and (iii) intrathecal injection of Ly6e plasmid induces pain- like behaviors in naïve rats. These data led me to hypothesize that: (i) modulation of NaV1.7 channels by Ly6e and Lynx1 may lead to altered expression and activity of these channels during chronic pain, and that (ii) interfering with NaV1.7-Ly6e/Lynx1 interactions may relieve pain. Thus, the goals of this proposal are to investigate the role of Ly6 proteins (i) in sensory neurons and (ii) as molecular determinants of the altered functional activity of NaV1.7 channels in pain states. My Specific Aims are: (1) Investigate the physiological function(s) of Ly6e and Lynx1 in primary sensory neurons from rodents with and without nerve injury; (2) Identify and characterize Ly6e and Lynx1 as modulators of NaV1.7 channels in rodent and human dorsal root ganglia neurons; and (3) Identify specific interaction domain(s) in NaV1.7, Ly6e and Lynx1 and validation of in vivo target engagement. These studies are anticipated to advance our understanding of the role of Ly6e and Lynx1 in the sensory system, and their role as modulators of a key pain-associated voltage-gated sodium channel, NaV1.7.