Evolution of the neuromuscular junction: Functional studies in a sea anemone

A nervous system and muscles are two unique animal features allowing them to perform coordinated movements. That way an animal is able to escape predators, hunt for food, find a mating partner or do other types of behavior ensuring its survival. These abilities have significantly contributed to the successful distribution and evolution of animals on this planet. The communication between the motor neuron and the muscle cell that leads to muscle contraction, is accomplished by a specialized type of synapse, the neuromuscular junction (NMJ). Thus, the acquisition of this structure marks an important step in animal evolution. A nervous system and muscles are present in almost all animals including vertebrates, arthropods (insects, arachnids, and crustaceans), mollusks and annelids, because they have been inherited from the last common ancestor of these bilaterian groups. The NMJ is best studied and understood in vertebrates. Here, the motor neuron releases the neurotransmitter acetylcholine at the NMJ that binds to cholinergic receptors at the muscle cells membrane. Unexpectedly, muscle innervation works completely different in arthropods such as insects and crayfish. At the NMJ in the fly Drosophila the excitatory neurotransmitter is glutamate and this molecule accordingly binds to glutamatergic receptors at the muscle cell. Due to the large differences of both systems, it is currently unclear what type of NMJ was present in the last common ancestor (the Urbilateria) and how this important mechanism evolved. Which of the two neurotransmitters did the Urbilateria use for muscle innervation? Or did he even use both of them? In order to answer these questions and to unravel the evolution of the NMJ in Bilateria, one needs to know the mechanics of the NMJ in a closely related outgroup. To this end, cnidarians (sea anemones and jellyfish), constitute an ideal outgroup, because they are closely related to Bilateria and likewise possess a nervous system and muscles. In particular the sea anemone Nematostella vectensis seems to be a well-suited candidate, because it has retained a number of evolutionary old features. Unfortunately, the mechanisms of muscle innervation in the sea anemone are still completely unknown. In order to fill this gap of knowledge, I will use modern molecular genetic methods to investigate the NMJ in Nematostella. From the data obtained herein, the origin of the neuromuscular system in cnidarians and bilaterians can be reconstructed, resulting in a better understanding of the evolution these groups as a whole.

The evolution of a third germ layer, the mesoderm, was a key event for the triploblastic Bilateria that led to the acquisition of a number of new tissues, including three types of muscles. Although contractile cells are also present in non-bilaterians without a mesoderm, evolutionary relationships to bilaterian muscle cells are ambiguous. Here we show that the molecular signatures of bilaterian muscle cells are already present in the diploblastic anthozoan Nematostella vectensis, constituting slow- and fast-contracting muscle cells. Like fast somatic muscles in bilaterians, fast-contracting muscles in N. vectensis show nervous stimulation. Unlike arthropods, but similar to vertebrates, these fast anthozoan muscles express acetylcholine and GABA receptors. e-protein mutants suggest that bHLH transcription factors are essential for the development of anthozoan muscles. These findings strongly indicate that visceral and somatic muscle cell types predate the evolution of Bilateria, and are independent of the acquisition of a mesoderm as a whole.