Characterization of expression of Sema1a variants in high-plasticity areas of the Gryllus bimaculatus nervous system

The well-conserved semaphorin family of guidance molecules is known to play multiple complex roles in directing the growth and orientation of dendrites and axons within the developing invertebrate central and peripheral nervous system. Additionally, the expression of select semaphorins is maintained within some highly plastic areas of the adult central nervous system, such as the mushroom bodies, where they are associated with guidance of newly-born neurons as well as with synapse formation and modification. Within the cricket species Gryllus bimaculatus, deafferentation of the prothoracic ganglia and subsequent dendritic rearrangement of the auditory interneurons is associated with fluctuations in the expression of transmembrane Sema1a and diffusible Sema2a. Here, we characterize the expression of two different variants of Gryllus Sema1a, termed Horch Sema1a and Extavour Sema1a, in tissues associated with both developmental neuronal guidance and adult structural plasticity: the embryonic limb buds, the mushroom bodies of the brain, and the non-deafferented adult prothoracic ganglion. Although we were unable to visualize the expression of Extavour Sema1a in any tissue, we demonstrate via phylogenetic analysis that both Sema1a variants have homologs in species across the Insecta class, suggesting that Extavour SEMA1a is a conserved protein sequence. We observe no expression of Horch Sema1a in the embryonic limb bud, and suspect that Extavour Sema1a, which has a high pairwise identity with Schistocerca Sema1a, could be facilitating guidance of the tibial pioneer neuron growth in the limb bud, along with Sema2a. In the adult brain, we observe a colocalization of Horch Sema1a and Sema2a in the mushroom bodies and in a vertical stripe across the calyx, which may be indicative of interactions between Horch SEMA1a and SEMA2a in maintaining synaptic plasticity and guiding newly-born Kenyon cells. We also report a colocalization of Horch Sema1a and Sema2a in the anterior and posterior of the prothoracic ganglia on the ventral side, in the region of auditory interneuron cell bodies, suggesting the possibility that auditory interneurons may express both Horch Sema1a and Sema2a, which could interact with each other or with Plexin receptors to regulate dendrite morphology at the midline.