A Subset of Circulating Hemocytes Expresses Genes Indicating Neural Precursor Identity

Adult neurogenesis in crayfish has been shown to require progenitors from an external source linked with the hematopoietic system, and some hemocyte (blood cell) types are attracted to a neurogenic niche in the brain. By using multiplex RNA-FISH techniques we have for the first time detected specific neural lineage marker transcripts expressed together in a small proportion of the circulating hemocytes (around 1%). This finding agrees with and confirms that there is only a small proportion of hemocytes which can develop further into neurons. Interestingly, these transcripts were co-expressed in the same cell as well as sometimes together with the transcript of the serotonin receptor 1 (5htr1+). Moreover, we could also show that several treatments, including serotonin, astakine, and lipopolysaccharide, as well as an acute brain injury, could induce a greater proportion of such neural marker positive hemocytes in the circulation as a response to these stimuli. Cells expressing neural lineage marker transcripts were also detected in the HPT, the hemocyte producing organ. Such neural lineage marker positive cells were shown to be present in the hemocyte-associated vascular plexus and the perineurial glia-like cells of the brain. These findings, therefore, for the first time demonstrate that the neural lineage markers are expressed in a distinct population of crustacean hemocytes, and provide additional molecular evidence that reinforces the hypothesis that these hemocytes may function as neural precursors in adult neurogenesis.