Integrating daughter cell self-renewal and fate-choice during retinogenesis
Dr Lucia Poggi, University of Heidelberg, Germany
One focus of regenerative medicine is to efficiently and safely replace retinal ganglion cells (RGCs), the output neurons of the retina, which are lost upon glaucoma and optic neuropathies leading to irreversible blindness. To this end, a tightly controlled expansion of the source cell (stem, progenitor or even differentiated neuronal cells) as well as its efficient and safe reprogramming into functional RGCs is needed. Achieving this requires that we understand the complex crosstalk of cell fate determinants, chromatin regulators and self-renewal factors that are at work during the normal genesis of RGCs in vivo. The bHLH transcription factor Ath5 (Atoh7) plays a crucial role in instructing RGC fate acquisition of retinal progenitor cells as well as induced pluripotent stem cells and Müller glia. Expression of ath5, however, is not decisive for RGC commitment and cells escape the RGC fate while progressing through self-renewal states. Poggi is interested in disentangling the integrated molecular interactions, from daughter cell inheritance to transcriptional regulation, which allow ath5-expressing progenitors to elude the RGC differentiation pathway upon asymmetric cell division. To understand this, Poggi began with interrogating gene regulatory networks controlled by Ath5, and to examine them in the physiological cellular context of the three-dimensionally patterned retinal tissue of the developing zebrafish embryo. Here Poggi provides insights suggesting how reciprocal feedbacks between Ath5 and factors influencing multipotency and self-renewal might intersect during asymmetric cell division, to restrict the RGC fate choice of retinal progenitor cells.
Asymmetric cell division in Ciona notochord tapering
Dr Michael Veeman, Kansas State University, USA
Tapering body parts are common in nature, but little is known about the developmental mechanisms by which taper arises. The simple 40 cell notochord of the invertebrate chordate Ciona forms a tapered rod, and the Veeman group are using it as a model for dissecting the mechanisms of tapering. Much of the taper reflects the cells at the front and back of the notochord being progressively smaller in volume than cells in the middle. Veeman used a genetic fate mapping strategy based on mosaic expression of an electroporated transgene to show that asymmetric division is the major driver of these anterior to posterior cell volume differences. Cells in the anterior of the notochord primordium divide to give smaller anterior daughters, and cells in the posterior of the primordium divide to give smaller posterior daughters. The volume asymmetries seen are modest compared with many cases of asymmetric division, but two consecutive rounds of division with complex patterns of asymmetry lead to important changes in cell size and ultimately help control the shape of an entire chordate organ. Veeman is currently investigating the cellular and molecular mechanisms driving these novel asymmetric divisions. The mitotic spindle becomes robustly oriented along the AP axis in these cells, but it is not clear if it is being displaced. The group are working to test an alternate hypothesis that some of these asymmetric divisions might involve a centred spindle in the context of an asymmetrically shaped cell.
Live imaging reveals new aspects of zebrafish neurogenesis
Dr Paula Alexandre, University College London, UK
During development of central nervous system (CNS) neural progenitors must be able to self-renew while producing neurons by undergoing series of asymmetric divisions. During early stages of zebrafish embryonic development, differentiated neurons appear regularly spaced along the anterior posterior axis of the spinal cord. This suggests that a mechanism that spaces neural progenitors’ asymmetric divisions or differentiating neurons must exist in this region. To determine the cellular and molecular mechanisms that can regulate neuronal patterning in the neural tube, Alexandre used live-imaging in zebrafish embryo to monitor neural progenitors’ divisions and neurons differentiating in vivo. Alexandre discovered that neuronal committed progenitors transiently elongate two long basal processes along the antero-posterior axis of the neural tube. The Alexandre group has evidence that signals delivered by these long processes may prevent neighbours from differentiating. This is the first cellular behaviour found in a vertebrate system that can regulate neuronal patterning in the neural tube.
Proliferation control in Drosophila neural stem cells
Dr Catarina Homem, CEDOC, Universidade Nova de Lisboa, Portugal
Stem cells are highly abundant during early development but become rare in most adult organs. Stem cell numbers must then be tightly regulated during development, but the molecular mechanisms causing stem cells to exit proliferation at a specific time are unclear. To address the mechanisms triggering stem cell exit during development Homem used Drosophila neural stem cells, the neuroblasts. Neuroblasts undergo size and fate asymmetric divisions to self-renew and generate more differentiated cells. Neuroblasts proliferate during development but all exit cell cycle and disappear before adulthood. Homem’s data show that changes in energy metabolism induced by the steroid hormone Ecdysone together with transcription regulator Mediator initiate an irreversible cascade of events leading to neuroblast differentiation. An increase in the levels of oxidative phosphorylation in neuroblasts leads to uncoupling between cell cycle from cell growth. This results in progressive reduction in neuroblast cell size and ultimately in terminal differentiation. Homem’s findings show that neuroblast size control can be modified by systemic hormonal signalling and reveal a unique connection between metabolism and proliferation in stem cells.
Centrosome asymmetry and Notch signalling in spinal cord neural progenitors
Dr Xavier Morin, Institut de Biologie de l'Ecole Normale Superieure, France
Unequal maturation of centrosomes has been associated with differential fate choices in several models of asymmetric cell division, including vertebrate neural stem cells. Nevertheless, signalling molecules taking advantage of the intrinsic centrosome asymmetry to instruct binary fate choices in sister cells have yet to be identified. Morin’s group found that the mono-ubiquitin ligase Mindbomb1 (Mib1), a regulator of the Notch pathway, localizes to centriolar satellites associated with the daughter centrosome of chick spinal cord progenitors. Using live imaging and fate tracking in the neural tube, we show that during asymmetric divisions, the centrosome carrying this pool of Mib1 is inherited by the prospective neuron. However, in symmetric divisions, a second pool of Mib1 associated with the Golgi apparatus in interphase is released during mitosis, aggregates on the free centrosome, and compensates for the initial Mib1 centrosomal asymmetry. Remarkably, as development of the neural tube proceeds, Mib1 is progressively lost from the Golgi apparatus, correlating with the reduction in symmetric proliferative divisions. Thus, the Golgi apparatus may represent a storage compartment compensating for the centrosome asymmetry of Mib1 in proliferative divisions. As neurogenesis progresses, this compensatory mechanism fades out and neurogenic divisions become predominant. Finally, Morin shows that preventing Mib1 centrosomal association in dividing cells hinders Notch signalling and delays neuronal differentiation in daughter cells. Thus, Morin establishes for the first time a link between centrosome asymmetry and Notch signalling, and propose that changes in the subcellular localization of Mib1 in neural progenitors are instrumental for the progression of neurogenesis.
How are mechanical factors controlled during brain formation?
Dr Yoichi Kosodo, Korea Brain Research Institute, South Korea
During brain development, neural stem cells define specific niches within different cortical layers to control their cellular environment. Generally, communication to other cells or surrounding matrix by biochemical signalling pathways such as protein-protein interactions or soluble factors have been well studied. Much less studied, however, is how physical properties of the niche can influence behaviour, growth, and differentiation of cells. Among physical properties, stiffness of the matrix has been shown to have direct effects on fate determination in certain stem cell types in vitro. While some studies indicate that stiffness may influence the fate of cultured neural stem cells by unknown mechanisms, little evidence exists on whether this principle holds true during the physiological development of the mammalian brain in vivo. Here, Kosodo has characterized tissue stiffness of the developing mouse brain cortex by using atomic force microscopy to seek a link to the cell fate determination of neural cells via mechanosignalling pathways, and found specific spatiotemporal shifts in stiffness during brain formation. The research aims to establish a novel concept for mammalian neurogenesis and brain development, and the outcomes will likely influence the fields of stem cell and developmental biology by clarifying unknown mechanosignalling mechanisms of somatic stem cells.