Longitudinal tracking of hemocyte populations in vivo indicates lineage relationships and supports neural progenitor identity in adult neurogenesis.

Adult neurogenesis, which takes place in both vertebrate and invertebrate species, is the process by which new neurons are born and integrated into existing functional neural circuits, long after embryonic development. Most studies in mammals suggest that self-renewing stem cells are the source of the new neurons, although the extent of self-renewal is a matter of debate. In contrast, research in the crayfish Procambarus clarkii has demonstrated that the neural progenitors producing adult-born neurons are capable of both self-renewing and consuming (non-self-renewing) divisions. However, self-renewing divisions are relatively rare, and therefore the production of adult-born neurons depends heavily on progenitors that are not replenishing themselves. Because the small pool of neural progenitors in the neurogenic niche is never exhausted throughout the long lives of these animals, we hypothesized that there must also be an extrinsic source of these cells. It was subsequently demonstrated that the neural progenitors originate in hemocytes (blood cells) produced by the immune system that travel in the circulation before ultimately integrating into niches where the neural lineage begins. The current study examines the developmental lineage of the three hemocyte types - hyaline (HC), semigranular (SGC) and granular (GC) cells - with the goal of understanding the origins of the progenitor cells that produce adult-born neurons. Longstanding qualitative metrics for hemocyte classification were validated quantitatively. Then, in a longitudinal study, proliferation markers were used to label the hemocytes in vivo, followed by sampling the circulating hemocyte population over the course of two months. Hemolymph samples were taken at intervals to track the frequencies of the different hemocyte types. These data reveal sequential peaks in the relative frequencies of HCs, SGCs and GCs, which were identified using qualitative and quantitative measures. These findings suggest that the three hemocyte types comprise a single cellular lineage that occurs in the circulation, with each type as a sequential progressive stage in hemocyte maturation beginning with HCs and ending with GCs. When combined with previously published data, this timeline provides additional evidence that HCs serve as the primary neural progenitor during adult neurogenesis in P. clarkii.

Edwards AJ ，Beltz BS 《Neural Development》

Adult neurogenesis in crayfish: Origin, expansion, and migration of neural progenitor lineages in a pseudostratified neuroepithelium.

Two decades after the discovery of adult-born neurons in the brains of decapod crustaceans, the deutocerebral proliferative system (DPS) producing these neural lineages has become a model of adult neurogenesis in invertebrates. Studies on crayfish have provided substantial insights into the anatomy, cellular dynamics, and regulation of the DPS. Contrary to traditional thinking, recent evidence suggests that the neurogenic niche in the crayfish DPS lacks self-renewing stem cells, its cell pool being instead sustained via integration of hemocytes generated by the innate immune system. Here, we investigated the origin, division and migration patterns of the adult-born neural progenitor (NP) lineages in detail. We show that the niche cell pool is not only replenished by hemocyte integration but also by limited numbers of symmetric cell divisions with some characteristics reminiscent of interkinetic nuclear migration. Once specified in the niche, first generation NPs act as transit-amplifying intermediate NPs that eventually exit and produce multicellular clones as they move along migratory streams toward target brain areas. Different clones may migrate simultaneously in the streams but occupy separate tracks and show spatio-temporally flexible division patterns. Based on this, we propose an extended DPS model that emphasizes structural similarities to pseudostratified neuroepithelia in other arthropods and vertebrates. This model includes hemocyte integration and intrinsic cell proliferation to synergistically counteract niche cell pool depletion during the animal's lifespan. Further, we discuss parallels to recent findings on mammalian adult neurogenesis, as both systems seem to exhibit a similar decoupling of proliferative replenishment divisions and consuming neurogenic divisions.

Brenneis G ，Beltz BS 《-》

Primary neuronal precursors in adult crayfish brain: replenishment from a non-neuronal source.

Benton JL ，Zhang Y ，Kirkhart CR ，Sandeman DC ，Beltz BS ... -  《BMC NEUROSCIENCE》

Adult neurogenesis in crayfish: Identity and regulation of neural progenitors produced by the immune system.

Adult-born neurons are incorporated into brain circuits in the crayfish Procambarus clarkii, as in many vertebrate and invertebrate species. Adult neurogenesis depends on several conserved features, including the presence of neurogenic niches housing progenitor cells and the expansion, migration, and differentiation of their daughters, the neural precursor cells. However, in contrast to mammalian species, the progenitors initiating the neurogenic lineage in P. clarkii do not undergo long-term self-renewal. A central question is the mode of replenishment of these cells. Experiments have shown that hemocytes generated by the immune system, and not other cell types, are attracted to and incorporated into the niche. The present studies highlight the interdependency of the immune and nervous systems in the generation of adult-born neurons, by demonstrating that hyaline hemocytes are the probable neural progenitor cells, and that serotonin and the cytokine astakine 1 regulate both immune function and adult neurogenesis.

Benton JL ，Li E ，Weisbach E ，Fukumura Y ，Quinan VC ，Chaves da Silva PG ，Edwards AJ ，Beltz BS ... -  《iScience》

Cells from the immune system generate adult-born neurons in crayfish.

Neurogenesis is an ongoing process in the brains of adult decapod crustaceans. However, the first-generation precursors that produce adult-born neurons, which reside in a neurogenic niche, are not self-renewing in crayfish and must be replenished. The source of these neuronal precursors is unknown. Here, we report that adult-born neurons in crayfish can be derived from hemocytes. Following adoptive transfer of 5-ethynyl-2'-deoxyuridine (EdU)-labeled hemocytes, labeled cells populate the neurogenic niche containing the first-generation neuronal precursors. Seven weeks after adoptive transfer, EdU-labeled cells are located in brain clusters 9 and 10 (where adult-born neurons differentiate) and express appropriate neurotransmitters. Moreover, the number of cells composing the neurogenic niche in crayfish is tightly correlated with total hemocyte counts (THCs) and can be manipulated by raising or lowering THC. These studies identify hemocytes as a source of adult-born neurons in crayfish and demonstrate that the immune system is a key contributor to adult neurogenesis.

Benton JL ，Kery R ，Li J ，Noonin C ，Söderhäll I ，Beltz BS ... -  《-》