Asymmetric cell division and cell fate in plants

A variety of approaches has recently been employed to investigate how sister cells adopt distinct fates following asymmetric divisions during plant development. Surgical and drug studies have been used to analyze asymmetric divisions during both early embryogenesis in brown algae and pollen development in tobacco. Genetic screens have been used to identify genes in Arabidopsis thaliana that are required for specific asymmetric cell divisions during pollen and root development. These studies indicate that cell polarity and division orientation are closely tied to the process of cell fate specification, and suggest that differential inheritance of determinants and positional information may both be involved in the specification of cell fates following asymmetric cell division.     

Regulation of asymmetric cell division in the epidermis

For proper tissue morphogenesis, cell divisions and cell fate decisions must be tightly and coordinately regulated. One elegant way to accomplish this is to couple them with asymmetric cell divisions. Progenitor cells in the developing epidermis undergo both symmetric and asymmetric cell divisions to balance surface area growth with the generation of differentiated cell layers. Here we review the molecular machinery implicated in controlling asymmetric cell division. In addition, we discuss the ability of epidermal progenitors to choose between symmetric and asymmetric divisions and the key regulatory points that control this decision.     

Decreased p53 is associated with a decline in asymmetric stem cell self‐renewal in aged human epidermis

With age, the epidermis becomes hypoplastic and hypoproliferative. Hypoproliferation due to aging has been associated with decreased stem cell (SC) self‐renewal in multiple murine tissues. The fate of SC self‐renewal divisions can be asymmetric (one SC, one committed progenitor) or symmetric (two SCs). Increased asymmetric SC self‐renewal has been observed in inflammatory‐mediated hyperproliferation, while increased symmetric SC self‐renewal has been observed in cancers. We analyzed SC self‐renewal divisions in aging human epidermis to better understand the role of SCs in the hypoproliferation of aging. In human subjects, neonatal to 78 years, there was an age‐dependent decrease in epidermal basal layer divisions. The balance of SC self‐renewal shifted toward symmetric SC self‐renewal, with a decline in asymmetric SC self‐renewal. Asymmetric SC divisions maintain epidermal stratification, and this decrease may contribute to the hypoplasia of aging skin. P53 decreases in multiple tissues with age, and p53 has been shown to promote asymmetric SC self‐renewal. Fewer aged than adult ALDH+CD44+ keratinocyte SCs exhibited p53 expression and activity and Nutlin‐3 (a p53 activator) returned p53 activity as well as asymmetric SC self‐renewal divisions to adult levels. Nutlin‐3 increased Notch signaling (NICD, Hes1) and DAPT inhibition of Notch activation prevented Nutlin‐3 (p53)‐induced asymmetric SC self‐renewal divisions in aged keratinocytes. These studies indicate a role for p53 in the decreased asymmetric SC divisions with age and suggest that in aged keratinocytes, Notch is required for p53‐induced asymmetric SC divisions.     

Cortical domains and the mechanisms of asymmetric cell division

Asymmetric cell divisions are central to the generation of cell-fate diversity because factors that are present in a mother cell and distributed unequally at cell division can generate distinct daughters. The process o f asymmetric cell division can be described as consisting of three steps: setting up an asymmetric cue in the mother cell, localizing factors with respect to this cue, and positioning the plane o f cell division so that localized factors are partitioned asymmetrically between daughters. This review describes how specialized cortical domains play a key role in each of these steps and discusses our current understanding of the molecular nature o f cortical domains and the mechanisms by which they may orchestrate asymmetric cell divisions.     

Asymmetric Numb distribution is critical for asymmetric cell division of mouse cerebral cortical stem cells and neuroblasts

Stem cells and neuroblasts derived from mouse embryos undergo repeated asymmetric cell divisions, generating neural lineage trees similar to those of invertebrates. In Drosophila, unequal distribution of Numb protein during mitosis produces asymmetric cell divisions and consequently diverse neural cell fates. We investigated whether a mouse homologue m-numb had a similar role during mouse cortical development. Progenitor cells isolated from the embryonic mouse cortex were followed as they underwent their next cell division in vitro. Numb distribution was predominantly asymmetric during asymmetric cell divisions yielding a beta-tubulin III(-) progenitor and a beta-tubulin III(+) neuronal cell (P/N divisions) and predominantly symmetric during divisions producing two neurons (N/N divisions). Cells from the numb knockout mouse underwent significantly fewer asymmetric P/N divisions compared to wild type, indicating a causal role for Numb. When progenitor cells derived from early (E10) cortex undergo P/N divisions, both daughters express the progenitor marker Nestin, indicating their immature state, and Numb segregates into the P or N daughter with similar frequency. In contrast, when progenitor cells derived from later E13 cortex (during active neurogenesis in vivo) undergo P/N divisions they produce a Nestin(+) progenitor and a Nestin(-) neuronal daughter, and Numb segregates preferentially into the neuronal daughter. Thus during mouse cortical neurogenesis, as in Drosophila neurogenesis, asymmetric segregation of Numb could inhibit Notch activity in one daughter to induce neuronal differentiation. At terminal divisions generating two neurons, Numb was symmetrically distributed in approximately 80% of pairs and asymmetrically in 20%. We found a significant association between Numb distribution and morphology: most sisters of neuron pairs with symmetric Numb were similar and most with asymmetric Numb were different. Developing cortical neurons with Numb had longer processes than those without. Numb is expressed by neuroblasts and stem cells and can be asymmetrically segregated by both. These data indicate Numb has an important role in generating asymmetric cell divisions and diverse cell fates during mouse cortical development.     

Oriented asymmetric divisions that generate the stomatal spacing pattern in arabidopsis are disrupted by the too many mouths mutation

Wild-type stomata are spaced by intervening cells, a pattern disrupted in the Arabidopsis mutant too many mouths (tmm). To determine the mechanism of wild-type spacing and how tmm results in pattern violations, we analyzed the behavior of cells through time by using sequential dental resin impressions. Meristemoids are stomatal precursors produced by asymmetric division. We show that wild-type patterning largely results when divisions next to a preexisting stoma or precursor are oriented so that the new meristemoid is placed away. Because this placement is independent of cell lineage, these divisions may be oriented by cell-cell signaling. tmm randomizes this orientation and releases a prohibition on asymmetric division in cells at specific locations, resulting in stomatal clusters. TMM is thus necessary for two position-dependent events in leaves: the orientation of asymmetric divisions that pattern stomata, and the control of which cells will enter the stomatal pathway. In addition, our findings argue against most previous hypotheses of wild-type stomatal patterning.     

Upregulation of Mitimere and Nubbin acts through cyclin E to confer self-renewing asymmetric division potential to neural precursor cells

In the Drosophila CNS, neuroblasts undergo self-renewing asymmetric divisions, whereas their progeny, ganglion mother cells (GMCs), divide asymmetrically to generate terminal postmitotic neurons. It is not known whether GMCs have the potential to undergo self-renewing asymmetric divisions. It is also not known how precursor cells undergo self-renewing asymmetric divisions. Here, we report that maintaining high levels of Mitimere or Nubbin, two POU proteins, in a GMC causes it to undergo self-renewing asymmetric divisions. These asymmetric divisions are due to upregulation of Cyclin E in late GMC and its unequal distribution between two daughter cells. GMCs in an embryo overexpressing Cyclin E, or in an embryo mutant for archipelago, also undergo self-renewing asymmetric divisions. Although the GMC self-renewal is independent of inscuteable and numb, the fate of the differentiating daughter is inscuteable and numb-dependent. Our results reveal that regulation of Cyclin E levels, and asymmetric distribution of Cyclin E and other determinants, confer self-renewing asymmetric division potential to precursor cells, and thus define a pathway that regulates such divisions. These results add to our understanding of maintenance and loss of pluripotential stem cell identity.     

Who begets whom? Plant cell fate determination by asymmetric cell division

Asymmetric cell division generates cell types with different fates. Recent studies have improved our understanding of the molecular mechanisms involved in asymmetric cell division in Arabidopsis thaliana. Genetic approaches have identified candidate intrinsic factors and signaling components that mediate extrinsic cues. WOX genes appear to be putative intrinsic determinants acting in early embryonic asymmetric divisions. A non-canonical mechanism involving specific SHORT ROOT (SHR)-SCARECROW (SCR) nuclear complexes is implicated in ground tissue asymmetric divisions. Asymmetric stem cell division requires extrinsic organizer signaling, whereas the involvement of intrinsic stem cell segregants is unknown. Finally, new studies on stomatal development have identified several intrinsic acting factors that specify cell fate and an extrinsic signaling cascade that controls the number and plane of asymmetric divisions.     

BASL and EPF2 act independently to regulate asymmetric divisions during stomatal development

The initiation of stomatal development in the developing Arabidopsis epidermis is characterized by an asymmetric ‘entry’ division in which a small cell, known as a meristemoid, and a larger daughter cell is formed. The meristemoid may undergo further asymmetric divisions, regenerating a meristemoid each time, before differentiating into a guard mother cell which divides symmetrically to form a pair of guard cells surrounding a stomatal pore. Recently EPF2 and BASL have emerged as regulators of these asymmetric divisions and here we present results indicating that these two factors operate independently to control stomatal developmentKey words: stomata, development, meristemoids, asymmetric cell division, leaf epidermis, cell polarity, peptide signal     

Mammalian inscuteable regulates spindle orientation and cell fate in the developing retina

During mammalian neurogenesis, progenitor cells can divide with the mitotic spindle oriented parallel or perpendicular to the surface of the neuroepithelium. Perpendicular divisions are more likely to be asymmetric and generate one progenitor and one neuronal precursor. Whether the orientation of the mitotic spindle actually determines their asymmetric outcome is unclear. Here, we characterize a mammalian homolog of Inscuteable (mInsc), a key regulator of spindle orientation in Drosophila. mInsc is expressed temporally and spatially in a manner that suggests a role in orienting the mitotic spindle in the developing nervous system. Using retroviral RNAi in rat retinal explants, we show that downregulation of mInsc inhibits vertical divisions. This results in enhanced proliferation, consistent with a higher frequency of symmetric divisions generating two proliferating cells. Our results suggest that the orientation of neural progenitor divisions is important for cell fate specification in the retina and determines their symmetric or asymmetric outcome.     

Imaging hematopoietic precursor division in real time

SUMMARYStem cells are thought to balance self-renewal and differentiation through asymmetric and symmetric divisions, but whether such divisions occur during hematopoietic development remains unknown. Using a Notch reporter mouse, in which GFP acts as a sensor for differentiation, we image hematopoietic precursors and show that they undergo both symmetric and asymmetric divisions. In addition we show that the balance between these divisions is not hardwired but responsive to extrinsic and intrinsic cues. Precursors in a prodifferentiation environment preferentially divide asymmetrically, whereas those in a prorenewal environment primarily divide symmetrically. Oncoproteins can also influence division pattern: although BCR-ABL predominantly alters the rate of division and death, NUP98-HOXA9 promotes symmetric division, suggesting that distinct oncogenes subvert different aspects of cellular function. These studies establish a system for tracking division of hematopoietic precursors and show that the balance of symmetric and asymmetric division can be influenced by the microenvironment and subverted by oncogenes.     

Cytoskeleton and Early Development in Fucoid Algae

Cell polarization and asymmetric cell divisions play important roles during development in many multicellular eukaryotes. Fucoid algae have a long history as models for studying early developmental processes, probably because of the ease with which zygotes can be observed and manipulated in the laboratory. This review discusses cell polarization and asymmetric cell divisions in fucoid algal zygotes with an emphasis on the roles played by the cytoskeleton.     

Inscuteable-independent apicobasally oriented asymmetric divisions in the Drosophila embryonic CNS

Inscuteable is the founding member of a protein complex localised to the apical cortex of Drosophila neural progenitors that controls their asymmetric division. Aspects of asymmetric divisions of all identified apicobasally oriented neural progenitors characterised to date, in both the central and peripheral nervous systems, require inscuteable. Here we examine the generality of this requirement. We show that many identified neuroblast lineages, in fact, do not require inscuteable for normal morphological development. To elucidate the requirements for apicobasal asymmetric divisions in a context where inscuteable is not essential, we focused on the MP2 > dMP2 + vMP2 division. We show that for MP2 divisions, asymmetric localisation and segregation of Numb and the specification of distinct dMP2 and vMP2 identities require bazooka but not inscuteable. We conclude that inscuteable is not required for all apicobasally oriented asymmetric divisions and that, in some cellular contexts, bazooka can mediate apicobasal asymmetric divisions without inscuteable.     

Asymmetric distribution of the apical plasma membrane during neurogenic divisions of mammalian neuroepithelial cells

At the onset of neurogenesis in the mammalian central nervous system, neuroepithelial cells switch from symmetric, proliferative to asymmetric, neurogenic divisions. In analogy to the asymmetric division of Drosophila neuroblasts, this switch of mammalian neuroepithelial cells is thought to involve a change in cleavage plane orientation from perpendicular (vertical cleavage) to parallel (horizontal cleavage) relative to the apical surface of the neuroepithelium. Here, we report, using TIS21-GFP knock-in mouse embryos to identify neurogenic neuroepithelial cells, that at the onset as well as advanced stages of neurogenesis the vast majority of neurogenic divisions, like proliferative divisions, show vertical cleavage planes. Remarkably, however, neurogenic divisions of neuroepithelial cells, but not proliferative ones, involve an asymmetric distribution to the daughter cells of the apical plasma membrane, which constitutes only a minute fraction (1-2%) of the entire neuroepithelial cell plasma membrane. Our results support a novel concept for the cell biological basis of asymmetric, neurogenic divisions of neuroepithelial cells in the mammalian central nervous system.     

Spindle orientation and epidermal morphogenesis

Asymmetric cell divisions (ACDs) result in two unequal daughter cells and are a hallmark of stem cells. ACDs can be achieved either by asymmetric partitioning of proteins and organelles or by asymmetric cell fate acquisition due to the microenvironment in which the daughters are placed. Increasing evidence suggests that in the mammalian epidermis, both of these processes occur. During embryonic epidermal development, changes occur in the orientation of the mitotic spindle in relation to the underlying basement membrane. These changes are guided by conserved molecular machinery that is operative in lower eukaryotes and dictates asymmetric partitioning of proteins during cell divisions. That said, the shift in spindle alignment also determines whether a division will be parallel or perpendicular to the basement membrane, and this in turn provides a differential microenvironment for the resulting daughter cells. Here, we review how oriented divisions of progenitors contribute to the development and stratification of the epidermis.     

Symmetric versus asymmetric cell division during neurogenesis in the developing vertebrate central nervous system

The type and number of cell divisions of neuronal progenitors determine the number of neurons generated during the development of the vertebrate central nervous system. Over the past several years, there has been substantial progress in characterizing the various kinds of neuronal progenitors and the types of symmetric and asymmetric divisions they undergo. The understanding of the cell-biological basis of symmetric versus asymmetric progenitor cell division has been consolidated, and the molecular machinery controlling these divisions is beginning to be unravelled. Other recent advances include comparative studies of brain development in rodents and primates, as well as the identification of gene mutations in humans that affect the balance between the various types of cell division of neuronal progenitors.     

Timing cell-fate determination during asymmetric cell divisions

From invertebrates to mammals, cell-cycle progression during an asymmetric cell division is accompanied by precisely timed redistribution of cell-fate determinants so that they segregate asymmetrically to enable the two daughter cells to choose different fates. Interestingly, studies on how cell fates are specified in such divisions reveal that the same fate determinants can be reiteratively used to specify a variety of cell types through multiple rounds of cell divisions or to exert seemingly contradictory effects on cell proliferation and differentiation. Here I summarize the molecular mechanisms governing asymmetric cell division and review recent findings pointing to a novel mechanism for coupling intracellular signaling and cell-cycle progression. This mechanism uses changes in the morphology, subcellular distribution, and molecular composition of cellular organelles like the Golgi apparatus and centrosomes, which not only accompany the progression of cell cycle to activate but also temporally constrain the activity of fate determinants during asymmetric cell divisions.     

Distribution of CD133 reveals glioma stem cells self-renew through symmetric and asymmetric cell divisions

Malignant gliomas contain a population of self-renewing tumorigenic stem-like cells; however, it remains unclear how these glioma stem cells (GSCs) self-renew or generate cellular diversity at the single-cell level. Asymmetric cell division is a proposed mechanism to maintain cancer stem cells, yet the modes of cell division that GSCs utilize remain undetermined. Here, we used single-cell analyses to evaluate the cell division behavior of GSCs. Lineage-tracing analysis revealed that the majority of GSCs were generated through expansive symmetric cell division and not through asymmetric cell division. The majority of differentiated progeny was generated through symmetric pro-commitment divisions under expansion conditions and in the absence of growth factors, occurred mainly through asymmetric cell divisions. Mitotic pair analysis detected asymmetric CD133 segregation and not any other GSC marker in a fraction of mitoses, some of which were associated with Numb asymmetry. Under growth factor withdrawal conditions, the proportion of asymmetric CD133 divisions increased, congruent with the increase in asymmetric cell divisions observed in the lineage-tracing studies. Using single-cell-based observation, we provide definitive evidence that GSCs are capable of different modes of cell division and that the generation of cellular diversity occurs mainly through symmetric cell division, not through asymmetric cell division.     

Analysis of partner of inscuteable, a novel player of Drosophila asymmetric divisions, reveals two distinct steps in inscuteable apical localization

Asymmetric localization is a prerequisite for inscuteable (insc) to function in coordinating and mediating asymmetric cell divisions in Drosophila. We show here that Partner of Inscuteable (Pins), a new component of asymmetric divisions, is required for Inscuteable to asymmetrically localize. In the absence of pins, Inscuteable becomes cytoplasmic and asymmetric divisions of neuroblasts and mitotic domain 9 cells show defects reminiscent of insc mutants. Pins colocalizes with Insc and interacts with the region necessary and sufficient for directing its asymmetric localization. Analyses of pins function in neuroblasts reveal two distinct steps for Insc apical cortical localization: A pins-independent, bazooka-dependent initiation step during delamination (interphase) and a later maintenance step during which Baz, Pins, and Insc localization are interdependent.     

Binary cell death decision regulated by unequal partitioning of Numb at mitosis

An important issue in Metazoan development is to understand the mechanisms that lead to stereotyped patterns of programmed cell death. In particular, cells programmed to die may arise from asymmetric cell divisions. The mechanisms underlying such binary cell death decisions are unknown. We describe here a Drosophila sensory organ lineage that generates a single multidentritic neuron in the embryo. This lineage involves two asymmetric divisions. Following each division, one of the two daughter cells expresses the pro-apoptotic genes reaper and grim and subsequently dies. The protein Numb appears to be specifically inherited by the daughter cell that does not die. Numb is necessary and sufficient to prevent apoptosis in this lineage. Conversely, activated Notch is sufficient to trigger death in this lineage. These results show that binary cell death decision can be regulated by the unequal segregation of Numb at mitosis. Our study also indicates that regulation of programmed cell death modulates the final pattern of sensory organs in a segment-specific manner.