Primitive and committed human hematopoietic progenitor cells interact with primary murine neural cells and are induced to undergo self-renewing cell divisions

Epac Activation Regulates Human Mesenchymal Stem Cells Migration and Adhesion

How to enhance the homing of human mesenchymal stem cells (hMSCs) to the target tissues remains a clinical challenge nowadays. To overcome this barrier, the mechanism responsible for the hMSCs migration and engraftment has to be defined. Currently, the exact mechanism involved in migration and adhesion of hMSCs remains unknown. Exchange protein directly activated by cAMP (Epac), a novel protein discovered in cAMP signaling pathway, may have a potential role in regulating cells adhesion and migration by triggering the downstream Rap family signaling cascades. However, the exact role of Epac in cells homing is elusive. Our study evaluated the role of Epac in the homing of hMSCs. We confirmed that hMSCs expressed functional Epac and its activation enhanced the migration and adhesion of hMSCs significantly. The Epac activation was further found to be contributed directly to the chemotactic responses induced by stromal cell derived factor-1 (SDF-1) which is a known chemokine in regulating hMSCs homing. These findings suggested Epac is connected to the SDF-1 signaling cascades. In conclusion, our study revealed that Epac plays a role in hMSCs homing by promoting adhesion and migration. Appropriate manipulation of Epac may enhance the homing of hMSCs and facilitate their future clinical applications.

Circulating phagocytes: the ancient and conserved interface between immune and neuroendocrine function

Immune and neuroendocrine functions display significant overlap in highly divergent and evolutionarily distant models such as molluscs, crustaceans, insects and mammals. Fundamental players in this crosstalk are professional phagocytes: macrophages in vertebrates and immunocytes in invertebrates. Although they have different developmental origins, macrophages and immunocytes possess comparable functions and differentiate under the control of evolutionarily conserved transcription factors. Macrophages and immunocytes share their pools of receptors, signalling molecules and pathways with neural cells and the neuro-endocrine system. In crustaceans, adult transdifferentiation of circulating haemocytes into neural cells has been documented recently. In light of developmental, molecular and functional evidence, we propose that the immune-neuroendocrine role of circulating phagocytes pre-dates the split of protostomian and deuterostomian superphyla and has been conserved during the evolution of the main groups of metazoans.

Adult neurogenesis in the decapod crustacean brain: a hematopoietic connection?

New neurons are produced and integrated into circuits in the adult brains of many organisms, including crustaceans. In some crustacean species, the first-generation neuronal precursors reside in a niche exhibiting characteristics analogous to mammalian neurogenic niches. However, unlike mammalian niches where several generations of neuronal precursors co-exist, the lineage of precursor cells in crayfish is spatially separated allowing the influence of environmental and endogenous regulators on specific generations in the neuronal precursor lineage to be defined. Experiments also demonstrate that the first-generation neuronal precursors in the crayfish Procambarus clarkii are not self-renewing. A source external to the neurogenic niche must therefore provide cells that replenish the first-generation precursor pool, because although these cells divide and produce a continuous efflux of second-generation cells from the niche, the population of first-generation niche precursors is not diminished with growth and aging. In vitro studies show that cells extracted from the hemolymph, but not other tissues, are attracted to and incorporated into the neurogenic niche, a phenomenon that appears to involve serotonergic mechanisms. We propose that, in crayfish, the hematopoietic system may be a source of cells that replenish the niche cell pool. These and other studies reviewed here establish decapod crustaceans as model systems in which the processes underlying adult neurogenesis, such as stem cell origins and transformation, can be readily explored. Studies in diverse species where adult neurogenesis occurs will result in a broader understanding of fundamental mechanisms and how evolutionary processes may have shaped the vertebrate/mammalian condition.

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

Background

Adult neurogenesis, the production and integration of new neurons into circuits in the brains of adult animals, is a common feature of a variety of organisms, ranging from insects and crustaceans to birds and mammals. In the mammalian brain the 1^st-generation neuronal precursors, the astrocytic stem cells, reside in neurogenic niches and are reported to undergo self-renewing divisions, thereby providing a source of new neurons throughout an animal's life. In contrast, our work shows that the 1^st-generation neuronal precursors in the crayfish (Procambarus clarkii) brain, which also have glial properties and lie in a neurogenic niche resembling that of vertebrates, undergo geometrically symmetrical divisions and both daughters appear to migrate away from the niche. However, in spite of this continuous efflux of cells, the number of neuronal precursors in the crayfish niche continues to expand as the animals grow and age. Based on these observations we have hypothesized that (1) the neuronal stem cells in the crayfish brain are not self-renewing, and (2) a source external to the neurogenic niche must provide cells that replenish the stem cell pool.

Results

In the present study, we tested the first hypothesis using sequential double nucleoside labeling to track the fate of 1^st- and 2^nd-generation neuronal precursors, as well as testing the size of the labeled stem cell pool following increasing incubation times in 5-bromo-2'-deoxyuridine (BrdU). Our results indicate that the 1^st-generation precursor cells in the crayfish brain, which are functionally analogous to neural stem cells in vertebrates, are not a self-renewing population. In addition, these studies establish the cycle time of these cells. In vitro studies examining the second hypothesis show that Cell Tracker™ Green-labeled cells extracted from the hemolymph, but not other tissues, are attracted to and incorporated into the neurogenic niche, a phenomenon that appears to involve serotonergic mechanisms.

Conclusions

These results challenge our current understanding of self-renewal capacity as a defining characteristic of all adult neuronal stem cells. In addition, we suggest that in crayfish, the hematopoietic system may be a source of cells that replenish the niche stem cell pool.

The natural and engineered 3D microenvironment as a regulatory cue during stem cell fate determination

The concept of using stem cells as self-renewing sources of healthy cells in regenerative medicine has existed for decades, but most applications have yet to achieve clinical success. A main reason for the lack of successful stem cell therapies is the difficulty in fully recreating the maintenance and control of the native stem cell niche. Improving the performance of transplanted stem cells therefore requires a better understanding of the cellular mechanisms guiding stem cell behavior in both native and engineered three-dimensional (3D) microenvironments. Most techniques, however, for uncovering mechanisms controlling cell behavior in vitro have been developed using 2D cell cultures and are of limited use in 3D environments such as engineered tissue constructs. Deciphering the mechanisms controlling stem cell fate in native and engineered 3D environments, therefore, requires rigorous quantitative techniques that permit mechanistic, hypothesis-driven studies of cell-microenvironment interactions. Here, we review the current understanding of 2D and 3D stem cell control mechanisms and propose an approach to uncovering the mechanisms that govern stem cell behavior in 3D.

Adult neurogenesis in the crayfish brain: proliferation, migration, and possible origin of precursor cells

The birth of new neurons and their incorporation into functional circuits in the adult brain is a characteristic of many vertebrate and invertebrate organisms, including decapod crustaceans. Precursor cells maintaining life-long proliferation in the brains of crayfish (Procambarus clarkii, Cherax destructor) and clawed lobsters (Homarus americanus) reside within a specialized niche on the ventral surface of the brain; their daughters migrate to two proliferation zones along a stream formed by processes of the niche precursors. Here they divide again, finally producing interneurons in the olfactory pathway. The present studies in P. clarkii explore (1) differential proliferative activity among the niche precursor cells with growth and aging, (2) morphological characteristics of cells in the niche and migratory streams, and (3) aspects of the cell cycle in this lineage. Morphologically symmetrical divisions of neuronal precursor cells were observed in the niche near where the migratory streams emerge, as well as in the streams and proliferation zones. The nuclei of migrating cells elongate and undergo shape changes consistent with nucleokinetic movement. LIS1, a highly conserved dynein-binding protein, is expressed in cells in the migratory stream and neurogenic niche, implicating this protein in the translocation of crustacean brain neuronal precursor cells. Symmetrical divisions of the niche precursors and migration of both daughters raised the question of how the niche precursor pool is replenished. We present here preliminary evidence for an association between vascular cells and the niche precursors, which may relate to the life-long growth and maintenance of the crustacean neurogenic niche.