The cellular and molecular basis of cnidarian neurogenesis
Abstract
Neurogenesis initiates during early development and it continues through later
developmental stages and in adult animals to enable expansion, remodeling, and
homeostasis of the nervous system. The generation of nerve cells has been analyzed
in detail in few bilaterian model organisms, leaving open many questions
about the evolution of this process. As the sister group to bilaterians, cnidarians
occupy an informative phylogenetic position to address the early evolution of
cellular and molecular aspects of neurogenesis and to understand common principles
of neural development. Here we review studies in several cnidarian model
systems that have revealed significant similarities and interesting differences
compared to neurogenesis in bilaterian species, and between different cnidarian
taxa. Cnidarian neurogenesis is currently best understood in the sea anemone
Nematostella vectensis, where it includes epithelial neural progenitor cells that
express transcription factors of the soxB and atonal families. Notch signaling regulates
the number of these neural progenitor cells, achaete-scute and dmrt genes
are required for their further development and Wnt and BMP signaling appear
to be involved in the patterning of the nervous system. In contrast to many vertebrates
and Drosophila, cnidarians have a high capacity to generate neurons
throughout their lifetime and during regeneration. Utilizing this feature of cnidarian
biology will likely allow gaining new insights into the similarities and differences
of embryonic and regenerative neurogenesis. The use of different
cnidarian model systems and their expanding experimental toolkits will thus
continue to provide a better understanding of evolutionary and developmental
aspects of nervous system formation.