Illuminating ALS Motor Neurons With Optogenetics in Zebrafish.

Kazuhide Asakawa,Hiroshi Handa,Koichi Kawakami

doi:10.3389/fcell.2021.640414

Abstract

Amyotrophic lateral sclerosis (ALS) is a fatal neurological disorder characterized by progressive degeneration of motor neurons in the brain and spinal cord. Spinal motor neurons align along the spinal cord length within the vertebral column, and extend long axons to connect with skeletal muscles covering the body surface. Due to this anatomy, spinal motor neurons are among the most difficult cells to observe in vivo. Larval zebrafish have transparent bodies that allow non-invasive visualization of whole cells of single spinal motor neurons, from somas to the neuromuscular synapses. This unique feature, combined with its amenability to genome editing, pharmacology, and optogenetics, enables functional analyses of ALS-associated proteins in the spinal motor neurons in vivo with subcellular resolution. Here, we review the zebrafish skeletal neuromuscular system and the optical methods used to study it. We then introduce a recently developed optogenetic zebrafish ALS model that uses light illumination to control oligomerization, phase transition and aggregation of the ALS-associated DNA/RNA-binding protein called TDP-43. Finally, we will discuss how this disease-in-a-fish ALS model can help solve key questions about ALS pathogenesis and lead to new ALS therapeutics.

Highlights

Amyotrophic lateral sclerosis (ALS) is a fatal neurological disorder in which motor neurons in the brain and spinal cord are selectively degenerated, leading to progressive muscle weakness
Recent studies have achieved optogenetic induction of ALS pathology by controlling the biophysics of disease-associated proteins with an unprecedented spatiotemporal precision by light illumination (Shin et al, 2017; Mann et al, 2019; Zhang et al, 2019; Asakawa et al, 2020). In this mini-review, we describe the skeletal neuromuscular system of larval zebrafish and optogenetic approaches for controlling in vivo phase transition of TDP-43 in motor neurons pertinent to the study of ALS pathogenesis
While the pan-motor neuronal cytoplasmic opTDP43 aggregation induced by the 3 day light illumination did not lead to overt motor deficit, the same set of conditions with opTDP-43 carrying ALS-associated intrinsically disordered regions (IDRs) mutation (A315T) led to the failure to inflate the swim bladder and an impaired motor activity in a small population (13%) of larvae (Asakawa et al, 2020)

Summary

Illuminating ALS Motor Neurons With Optogenetics in Zebrafish

Spinal motor neurons align along the spinal cord length within the vertebral column, and extend long axons to connect with skeletal muscles covering the body surface. Due to this anatomy, spinal motor neurons are among the most difficult cells to observe in vivo. Larval zebrafish have transparent bodies that allow non-invasive visualization of whole cells of single spinal motor neurons, from somas to the neuromuscular synapses. This unique feature, combined with its amenability to genome editing, pharmacology, and optogenetics, enables functional analyses of ALS-associated proteins in the spinal motor neurons in vivo with subcellular resolution.

INTRODUCTION

Findings

Optogenetic Zebrafish ALS Model