Stem cells and neural circuit engineering

Modelling Neuromuscular disease in vitro

Medical conditions that affect muscle function and/or its neuronal control represent some of the most common debilitating disorders, which are typically studied in rodent animal models in vivo or in cultured cells in vitro. Current animal models of neuromuscular disease suffer from a number of limitations, such as high costs and long generation times of transgenic animals, as well as genetic differences between rodents and humans. Likewise, in vitro models often do not reflect the complexity of cell-cell-interactions seen in vivo. Previously, we have used in vitro derivation of motor neurons from pluripotent stem cells (PSCs) to deciphers subtype-specific developmental programs (Machado et al., 2014), and also developed tools which allow us to sort and stimulate PSC-derived motor neurons. Building on these advances, my group is developing a human PSC-based tissue culture model of neuromuscular circuitry to study nerve-muscle connectivity in neuromuscular disorders. The key cellular components of these circuits (motor neurons, astrocytes and muscle) will be derived separately from PSCs in vitro, magnetically purified and assembled in compartmentalized tissue culture devices that segregate the cell bodies of neural cells from muscle fibres, yet allow motor axons to connect to their synaptic targets. PSC-motor neurons will be equipped with optogenetic actuators and fluorescent reporters, enabling us to selectively stimulate and record without mechanical interference. We will initially focus on amyotrophic lateral sclerosis (ALS), a neurodegenerative condition which leads to the loss of motor neurons, muscle denervation and muscle paralysis. The aim of this project is to map different disease phenotypes, such as initial degeneration of the cell body versus initial axonal degeneration, to specific mutations associated with ALS, and categorize different forms of ALS based on the underlying cell biology. Long-term, this may lead to more effective ALS therapies, because specific disease pathways could be targeted with customized drugs selected on the basis of patients’ genotypes and phenotypes

Optogenetic control of muscle function

The loss of motor neurons due to Spinal Cord Injury (SCI) or ALS disconnects the CNS from skeletal muscle and leads to the impairment of vital motor function, such as breathing and locomotion. As there is no natural mechanism for motor neuron regeneration in humans, such defects are usually irreversible, and currently, no established therapy exists that could reconstitute muscle function once motor neurons are lost. Motor neurons directly derived from PSCs could, in principle, be used to reconnect the CNS with muscle targets, but it is unclear how newly generated, embryonic-like neurons would integrate into lesioned adult spinal circuits. To circumvent this problem, we have developed an alternative approach that relies on stem cell-derived peripheral neural grafts which express an optogenetic actuator to establish neuromuscular junctions with recipient muscle. Due to the photosensitivity of the graft, muscle can then be paced with light flashes generated by an optical pacemaker device and transmitted to the graft via LED microimplants. In a recent proof-of-principle study, my group and collaborators have shown that such optogenetic motor neuron grafts can relay rhythmic contraction patterns from an artificial control system to skeletal muscle in vivo (Bryson et al., 2014). We are currently developing both the biological and the optoelectronic elements of the system further and we will test a prototype device on respiratory muscle in rodents and eventually in minipigs, a pre-clinical animal model that approximates human scale, anatomy and physiology. While such a neural prosthesis would not offer a cure for SCI or ALS, the quality of life of patients could be dramatically improved by artificially driving respiration and avoiding the need for mechanical ventilation. If successful, our approach could also be applied to other key motor functions, for example swallowing.