As described in my new book on nerve regeneration in the first chapter, peripheral nerve injury is a common clinical problem. It significantly affects the quality of life of the injured patients and represents a considerable economic burden for the community.
Although peripheral axons are principally capable of regenerating after a lesion, only about 10% of patients recover. Hence, the majority suffer permanent disabilities with irreversible symptoms such as impaired motor function, loss of sensation, and often pain. New therapeutic approaches must therefore be developed to enable sufficient and specific axon regeneration.
The research group led by Arthur English at the Institute of Cell Biology at Emory University School of Medicine in Atlanta (GA, USA) had already shown in earlier work that increasing the activity of injured nerve cells promises therapeutic success. Thus, brief low-frequency (20 Hz) electrical stimulation and moderate exercise significantly accelerates functional recovery after peripheral nerve injury.
However, not all patients with nerve injuries can be electrically stimulated or enrolled in an exercise program. Therefore, Anna Ecanow and colleagues developed bioluminescence optogenetics (BL-OG) as an alternative to activity-dependent therapies and published their results last year in the International Journal of Molecular Sciences.
BL-OG uses luminopsins, which are fusion proteins of light-sensitive ion channels (opsins) and light-emitting luciferase.
Schematic of the excitatory luminopsin. Gaussia luciferase (GLuc) is fused to a light-sensitive rhodopsin channel (VChR1). A fluorescent marker (EYFP) is located at its cytosolic end to visualize the channel. Upon addition of a luciferase substrate, such as coelenterazine (CTZ), photons (bioluminescence) are released and the cation channel of VChR1 is opened, electrically exciting the neuron (Fig. 1 from Ecanow et al., 2022, Int. J. Mol. Sci. 23:16084).
As a tool to deliver the light-sensitive ion channel into motor and sensory nerve fibers, they used adeno-associated viral vectors encoding either an excitatory luminopsin (eLMO3) or its mutant form (R115A). The latter produces bioluminescence but does not excite neurons. After injection into the sciatic nerve of mice and subsequent viral transduction of the associated neurons, the nerve was transected and repaired by a simple end-to-end anastomosis. Immediately after surgery, a single systemic (intraperitoneal) administration of CTZ, a luciferase substrate, was performed. As early as four weeks after the lesion, the compound muscle action potentials (the so-called M waves) in response to sciatic nerve stimulation were four times greater than in control animals expressing the mutant (inactive) luminopsin.
In addition, the number of motor and sensory neurons retrogradely labeled by the re-innervated muscles was significantly greater in experimental animals than in mice with mutant luminopsin and was not significantly different from those in intact mice. When the virus containing the light-sensitive ion channel was injected in a delayed manner so that luminopsin expression occurred only after nerve injury, evoked M waves recorded from reinnervated muscles were also significantly greater after axotomy. This result, which is of particular clinical interest, demonstrates that BL-OG has significant potential to improve axon regeneration and promote functional recovery after peripheral nerve lesion.
Reference:
Ecanow A, Berglund K, Carrasco D, Isaacson R, English AW (2022) Enhancing motor and sensory axon regeneration after peripheral nerve injury using bioluminescent optogenetics. International Journal of Molecular Sciences 23:16084.
Image credit: iStock/Ianm35