Towards the clinical translation of optogenetic skeletal muscle stimulation

Abstract

Paralysis is a frequent phenomenon in many diseases, and to date, only functional electrical stimulation (FES) mediated via the innervating nerve can be employed to restore skeletal muscle function in patients. Despite recent progress, FES has several technical limitations and significant side effects. Optogenetic stimulation has been proposed as an alternative, as it may circumvent some of the disadvantages of FES enabling cell type-specific, spatially and temporally precise stimulation of cells expressing light-gated ion channels, commonly Channelrhodopsin2. Two distinct approaches for the restoration of skeletal muscle function with optogenetics have been demonstrated: indirect optogenetic stimulation through the innervating nerve similar to FES and direct optogenetic stimulation of the skeletal muscle. Although both approaches show great promise, both have their limitations and there are several general hurdles that need to be overcome for their translation into clinics. These include successful gene transfer, sustained optogenetic protein expression, and the creation of optically active implantable devices. Herein, a comprehensive summary of the underlying mechanisms of electrical and optogenetic approaches is provided. With this knowledge in mind, we substantiate a detailed discussion of the advantages and limitations of each method. Furthermore, the obstacles in the way of clinical translation of optogenetic stimulation are discussed, and suggestions on how they could be overcome are provided. Finally, four specific examples of pathologies demanding novel therapeutic measures are discussed with a focus on the likelihood of direct versus indirect optogenetic stimulation.

Keywords: Electrical stimulation; Gene transfer; Immune response; Optical implants; Optogenetic stimulation; Skeletal muscle.

Fig. 1

Illumination of motor neurons. Schematic of a an optical cuff implant and b a TIOP device used to interact with a peripheral nerve. The reflective coating of the optical cuff is not illustrated in the schematic. Color-coded fascicles either express two different ChR (blue and red) or are not AAV transduced (rose)

Fig. 2

Illumination of skeletal muscles. Conceptual drawing of a a 2D LED array on a polymeric substrate encapsulated in silicone rubber or b a polymeric waveguide array with integrated mirrors and Y-splitters used for direct optical stimulation of a skeletal muscle. c Light reflected at 45° mirrors integrated into the waveguide structure of B