Studies of the olfactory epithelium model system have demonstrated that production of neurons is regulated by negative feedback. Previously, we showed that a locally produced signal, the TGFβ ...superfamily ligand GDF11, regulates the genesis of olfactory receptor neurons by inhibiting proliferation of the immediate neuronal precursors (INPs) that give rise to them. GDF11 is antagonized by follistatin (FST), which is also produced locally. Here, we show that Fst(-/-) mice exhibit dramatically decreased neurogenesis, a phenotype that can only be partially explained by increased GDF11 activity. Instead, a second FST-binding factor, activin βB (ACTβB), inhibits neurogenesis by a distinct mechanism: whereas GDF11 inhibits expansion of INPs, ACTβB inhibits expansion of stem and early progenitor cells. We present data supporting the concept that these latter cells, previously considered two distinct types, constitute a dynamic stem/progenitor population in which individual cells alternate expression of Sox2 and/or Ascl1. In addition, we demonstrate that interplay between ACTβB and GDF11 determines whether stem/progenitor cells adopt a glial versus neuronal fate. Altogether, the data indicate that the transition between stem cells and committed progenitors is neither sharp nor irreversible and that GDF11, ACTβB and FST are crucial components of a circuit that controls both total cell number and the ratio of neuronal versus glial cells in this system. Thus, our findings demonstrate a close connection between the signals involved in the control of tissue size and those that regulate the proportions of different cell types.
The sensory neurons that subserve olfaction, olfactory receptor neurons (ORNs), are regenerated throughout life, making the neuroepithelium in which they reside the olfactory epithelium (OE) an ...excellent model for studying how intrinsic and extrinsic factors regulate stem cell dynamics and neurogenesis during development and regeneration. Numerous studies indicate that transcription factors and signaling molecules together regulate generation of ORNs from stem and progenitor cells during development, and work on regenerative neurogenesis indicates that these same factors may operate at postnatal ages as well. This review describes our current knowledge of the identity of the OE neural stem cell; the different cell types that are thought to be the progeny (directly or indirectly) of this stem cell; and the factors that influence cell differentiation in the OE neuronal lineage. We review data suggesting that (1) the ORN lineage contains three distinct proliferating cell types—a stem cell and two populations of transit amplifying cells; (2) in established OE, these three cell types are present within the basal cell compartment of the epithelium; and (3) the stem cell that gives rise ultimately to ORNs may also generate two glial cell types of the primary olfactory pathway: sustentacular cells (SUS), which lie within OE proper; and olfactory ensheathing cells (OEC), which envelope the olfactory nerve. In addition, we describe factors that are both made by and found within the microenvironment of OE stem and progenitor cells, and which exert crucial growth regulatory effects on these cells. Thus, as with other regenerating tissues, the basis of regeneration in the OE appears be a population of stem cells, which resides within a microenvironment (niche) consisting of factors crucial for maintenance of its capacity for proliferation and differentiation.
SNARE (soluble N-ethylmaleimide-sensitive fusion protein attachment protein receptor) proteins are supposed to mediate the docking and/or fusion of the vesicle with the plasma membrane. However, it ...is not clearly understood how this process is regulated. In a search for potential SNARE regulators, we recently identified septin 5 (Sept5) as a novel SNARE interacting protein. Septins were first identified as filamentous proteins required for cytokinesis in yeast. Several septins have now been identified in mammals but little is known about their functions. We have previously shown that Sept5 is predominantly expressed in the brain, where it associates with vesicles and membranes through its interaction with the SNARE domain of syntaxin 1A. Furthermore, Sept5 appears to inhibit exocytosis, possibly by regulating vesicle targeting and/or fusion events. To gain insight into the role of Sept5, we have mapped the Sept5 domains important for syntaxin binding. We also investigated the ability of Sept5 to bind to syntaxin when in various protein complexes. Although Sept5 cannot bind an nSec1-syntaxin complex, it can bind syntaxin in a SNARE complex. This interaction is occluded by the binding of alpha-SNAP, suggesting that Sept5 may regulate the availability of SNARE proteins through its interaction with syntaxin and the 7 S complex.
The regenerative capacity of many placode-derived epithelial structures makes them of interest for understanding the molecular control of epithelial stem cells and their niches. Here, we investigate ...the interaction between the developing epithelium and its surrounding mesenchyme in one such system, the taste papillae and sensory taste buds of the mouse tongue. We identify follistatin (FST) as a mesenchymal factor that controls size, patterning and gustatory cell differentiation in developing taste papillae. FST limits expansion and differentiation of Sox2 -expressing taste progenitor cells and negatively regulates the development of taste papillae in the lingual epithelium: in Fst -/- tongue, there is both ectopic development of Sox2 -expressing taste progenitors and accelerated differentiation of gustatory cells. Loss of Fst leads to elevated activity and increased expression of epithelial Bmp7 ; the latter effect is consistent with BMP7 positive autoregulation, a phenomenon we demonstrate directly. We show that FST and BMP7 influence the activity and expression of other signaling systems that play important roles in the development of taste papillae and taste buds. In addition, using computational modeling, we show how aberrations in taste papillae patterning in Fst -/- mice could result from disruption of an FST-BMP7 regulatory circuit that normally suppresses noise in a process based on diffusion-driven instability. Because inactivation of Bmp7 rescues many of the defects observed in Fst -/- tongue, we conclude that interactions between mesenchyme-derived FST and epithelial BMP7 play a central role in the morphogenesis, innervation and maintenance of taste buds and their stem/progenitor cells.
Septins are GTPases required for the completion of cytokinesis in diverse organisms, yet their roles in cytokinesis or other cellular processes remain unknown. Here we describe studies of a newly ...identified septin, CDCrel-1, which is predominantly expressed in the nervous system. This protein was associated with membrane fractions, and a significant fraction of the protein copurified and coprecipitated with synaptic vesicles. In detergent extracts, CDCrel-1 and another septin, Nedd5, immunoprecipitated with the SNARE protein syntaxin by directly binding to syntaxin via the SNARE interaction domain. Transfection of HIT-T15 cells with wild-type CDCrel-1 inhibited secretion, whereas GTPase dominant-negative mutants enhanced secretion. These data suggest that septins may regulate vesicle dynamics through interactions with syntaxin.
Celotno besedilo
Dostopno za:
DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, UILJ, UKNU, UL, UM, UPUK
To understand how signaling molecules regulate the generation of neurons from proliferating stem cells and neuronal progenitors in the developing and regenerating nervous system, we have studied ...neurogenesis in a model neurogenic epithelium, the olfactory epithelium (OE) of the mouse. Our studies have employed a candidate approach to test signaling molecules of potential importance in regulating neurogenesis and have utilized methods that include tissue culture, in situ hybridization and mouse genetics. Using these approaches, we have identified three distinct stages of stem and transit amplifying progenitor cells in the differentiation pathway of olfactory receptor neurons (ORNs) and have identified mechanisms by which the development of each of these progenitor cell types is regulated by signals produced both within the OE itself and by its underlying stroma. Our results indicate that regulation of olfactory neurogenesis is critically dependent on multiple signaling molecules from two different polypeptide growth factor superfamilies, the fibroblast growth factors and the transforming growth factor beta (TGF-beta) group. In addition, they indicate that these signaling molecules interact in at least two important ways: first, opposing signals converge on cells at specific developmental stages in the ORN pathway to regulate proliferation and differentiation; and second, these signaling molecules--particularly the TGF-betas and their antagonists--play key roles in feedback loops that regulate the size of progenitor cell pools and thereby neuron number, during development and regeneration.
SNARE proteins mediate the docking and/or fusion of the vesicle with the plasma membrane. However, it is not clearly understood how this process is regulated. In a search for potential SNARE ...regulators, we have identified a novel snare interacting protein, the septin CDCrel-1. Septins were first identified as filamentous proteins required for cytokinesis in yeast. However, in mammals little is known about their functions. I show here that cdcrel-1 is predominantly expressed in the brain where it associates with membranes via binding to syntaxin 1A. Wildtype CDCrel-1 transfected into HIT-T15 cells inhibits secretion while mutated forms of CDCrel-1 potentiate secretion, suggesting that cdcrel-1 may be regulating vesicle targeting and/or fusion events. I further map the CDCrel-1 domains important for syntaxin binding and investigate the ability of CDCrel-1 to bind to syntaxin when in various SNARE complexes. CDCrel-1 can bind syntaxin in a SNARE complex, but its binding is occluded by α-SNAP. This suggests that CDCrel-1 may act as a novel filamentous element, regulating the delivery and/or fusion of vesicles to the presynaptic membrane through its interaction with syntaxin and the 7S complex. The regulation of filaments may be via post-translational modifications. Indeed we have discovered a novel interaction between SUMO E3 PIAS proteins and CDCrel-1. The conjugation of SUMO to substrates is dependent upon an E1 and E2, whereas specificity is mediated by an E3. Although several SUMO-1 substrates have been characterized, conjugation solely by SUMO-2/3 has not been described. Here I describe the colocalization of CDCrel-1 with SUMO-2 and 3 but not SUMO-1. Transfection of SUMO-2/3 but not SUMO-1 causes a reorganization of CDCrel-1 distribution in CHO cells. Furthermore, CDCrel-1 sequesters the nuclear pool of SUMO-2/3 and of the E2 Ubc9 but not SUMO1 into the cytoplasm. Sumoylation of CDCrel-l is shown in vivo and putative SUMO modification sites on CDCrel-1 are investigated by deletion of lysine residues. These experiments strongly suggest that CDCrel-1 is sumoylated specifically by SUMO-2/3. Sumoylation of CDCrel-1 may therefore play a regulatory role in secretion and septin filament formation. Future work will be aimed at determining the functional significance of SUMO modified CDCrel-1.
Stem cell therapies to treat central nervous system (CNS) injuries and diseases face many obstacles, one of which is the fact that the adult CNS often presents an environment hostile to the ...development and differentiation of neural stem and progenitor cells. Close examination of two regions of the nervous system – the olfactory epithelium (OE), which regenerates, and the neural retina, which does not – have helped identify endogenous signals, made by differentiated neurons, which act to inhibit neurogenesis by stem/progenitor cells within these tissues. In this chapter, we provide background information on these systems and their neurogenic signaling systems, with the goal of providing insight into how manipulation of endogenous signaling molecules may enhance the efficacy of stem cell neurotherapeutics.
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