Motor neurons elongate axons over great distances during development, with some extending
from the spinal cord to the distal limb, but little is known about extracellular signals that
control this growth. The TGF-β superfamily includes two major classes of ligands: Nodallike
and BMP. BMP signalling is essential for neuromuscular synaptic growth and plasticity
in Drosophila; however, a similar role for the pathway in mammalian motor neurons has not
been described.
It has previously been shown that the E3-RING ubiquitin ligase Arkadia enhances signalling
through the Nodal branch of TGF-β signalling. A second Arkadia locus was previously found
to contain two isoforms; the C-terminal isoform (Arkadia2C) contains domains critical for
Arkadia’s activity. This project addresses the in vivo function of Arkadia2C in mice.
Arkadia2C was found to be expressed specifically in the nervous system throughout
embryonic development and adulthood. In a neuronal context Arkadia2C enhances signalling
through the Smad1/5/8 branch of the TGF-β pathway in a RING domain-dependent manner.
Mice null for Arkadia2C display motor neuron disease-like symptoms including weak motor
control and difficulties breathing and eating, leading to postnatal mortality in the majority of
mice. A subset of Arkadia2C null embryonic innervation defects was analysed; the majority
of the motor axons of the dorsal forelimb fail to form synapses with their target muscles while
the phrenic nerve exhibits shorter terminal branches. Postnatally, a failure to maintain
neuromuscular junctions was observed leading to atrophy of several muscles.
Active BMP signalling was observed in the neurons that innervate the limb and diaphragm
and genetic reduction of BMP signalling in asymptomatic Arkadia2C heterozygous mice
caused the appearance of the same muscle innervation defects observed in the null
individuals. Together, these findings suggest that Arkadia2C’s enhancement of the BMP
signalling pathway is critical for the growth and connectivity of certain motor neurons.