Motor neuron differentiation in the developing and the regenerating adult spinal cord of zebrafish Danio rerio (Hamilton, 1822).

Zebrafish, in contrast to mammals, are capable of functional spinal cord regeneration. Spinal motor neurons are major targets for axons regenerating from the brainstem. Using immunohistochemical markers and transgenic reporter fish for motor neuron markers (H89, islet -1), this study demonstrates that large differentiated motor neurons are transiently lost after a spinal lesion, suggesting that these cells undergo cell death after a lesion and may be replaced by proliferation. Indeed, a massive and transient increase in the number of small, undifferentiated motor neurons, which were labelled by the proliferation marker bromodeoxyuridine, was observed. Proliferation and lineage tracing studies indicated significant proliferation only at the spinal ventricle and that a subset of olig2 expressing ependymo -radial glial cells are the likely motor neuron progenitor /stem cells in the lesioned spinal cord. A spinal lesion increased expression of sonic hedgehog (shh), an embryonic differentiation signal for motor neurons. Blocking this signal with an antagonist reduced progenitor cell proliferation and motor neuron differentiation. This suggests that shh is an important signal for motor neuron differentiation during adult motor neuron regeneration.To learn more about axonal differentiation of motor neurons, the role of the cell recognition molecule plexinA3 was investigated during the outgrowth of embryonic primary motor axons. The molecule is selectively expressed in primary motor neurons. Knockdown of expression led to ectopic exiting from the spinal cord and excessive branching of motor axons. Over -expression of full length plexinA3 rescued this effect, indicating specificity of experimental manipulations. Thus, plexinA3 expression is crucial for motor axon pathfinding during development. Overall, this study demonstrates that adult zebrafish are capable of motor neuron regeneration from endogenous progenitor /stem cells and that shh is an important regulator of motor neuron regeneration. PlexinA3 is crucial for motor axon differentiation in embryonic zebrafish. This study establishes adult spinal cord lesion as a model system for motor neuron regeneration, which may ultimately help to find ways to promote motor neuron regeneration also in human conditions, such as spinal cord injury or motor neuron disease