Biased segregation of DNA and centrosomes: moving together or drifting apart?

Nonrandom sister chromatid segregation mediates rDNA copy number maintenance in Drosophila

Although considered to be exact copies of each other, sister chromatids can segregate nonrandomly in some cases. For example, sister chromatids of the X and Y chromosomes segregate nonrandomly during asymmetric division of male germline stem cells (GSCs) in Drosophila melanogaster. Here, we demonstrate that the ribosomal DNA (rDNA) loci, which are located on the X and Y chromosomes, and an rDNA binding protein Indra are required for nonrandom sister chromatid segregation (NRSS). We provide the evidence that NRSS, following unequal sister chromatid exchange, is a mechanism by which GSCs recover rDNA copy number, counteracting the spontaneous copy number loss that occurs during aging. Our study reveals an unexpected role for NRSS in maintaining germline immortality through maintenance of a vulnerable genomic element, rDNA.

Dynamic properties of mitochondria during human corticogenesis

Mitochondria are signaling hubs responsible for the generation of energy through oxidative phosphorylation, the production of key metabolites that serve the bioenergetic and biosynthetic needs of the cell, calcium (Ca2+) buffering and the initiation/execution of apoptosis. The ability of mitochondria to coordinate this myriad of functions is achieved through the exquisite regulation of fundamental dynamic properties, including remodeling of the mitochondrial network via fission and fusion, motility and mitophagy. In this Review, we summarize the current understanding of the mechanisms by which these dynamic properties of the mitochondria support mitochondrial function, review their impact on human cortical development and highlight areas in need of further research.

Phase Separation in Asymmetric Cell Division

Asymmetric cell division (ACD) is a conserved strategy for achieving cell diversity. A cell can undergo an intrinsic ACD through asymmetric segregation of cell fate determinants or cellular organelles. Recently, a new biophysical concept known as biomolecular phase separation, through which proteins and/or RNAs autonomously form a highly concentrated non-membrane-enclosed compartment via multivalent interactions, has provided new insights into the assembly and regulation of many membrane-less or membrane-attached organelles. Intriguingly, biomolecular phase separation is suggested to drive asymmetric condensation of cell fate determinants during ACD as well as organization of cellular organelles involved in ACD. In this Perspective, I first summarize recent findings on the molecular basis governing intrinsic ACD. Then I will discuss how ACD might be regulated by formation of dense molecular assemblies via phase separation.

Controlled ploidy reduction of pluripotent 4n cells generates 2n cells during mouse embryo development

Cells with high ploidy content are common in mammalian extraembryonic and adult tissues. Cell-to-cell fusion generates polyploid cells during mammalian development and tissue regeneration. However, whether increased ploidy can be occasionally tolerated in embryonic lineages still remains largely unknown. Here, we show that pluripotent, fusion-derived tetraploid cells, when injected in a recipient mouse blastocyst, can generate diploid cells upon ploidy reduction. The generated diploid cells form part of the adult tissues in mouse chimeras. Parental chromosomes in pluripotent tetraploid cells are segregated through tripolar mitosis both randomly and nonrandomly and without aneuploidy. Tetraploid-derived diploid cells show a differentiated phenotype. Overall, we discovered an unexpected process of controlled genome reduction in pluripotent tetraploid cells. This mechanism can ultimately generate diploid cells during mouse embryo development and should also be considered for cell fusion-mediated tissue regeneration approaches.

Epigenetically distinct sister chromatids and asymmetric generation of tumor initiating cells

Cancer stem cells (CSC) are thought to be an important source of cancer cells in tumors of different origins. Mounting evidence suggests they are generated reversibly from existing cancer cells, and supply new cancer cells during tumor progression and following therapy. Elegant lineage mapping stud(ies are identifying progenitors, and in some cases differentiated cells, as targets of transformation in a variety of tumors. Recent evidence suggests resulting tumor initiating cells (TIC) might be distinct from CSC. Molecular pathways leading from cells of tumor origin to precancerous lesions and cancer cells are only beginning to be unraveled. We review a pathway where asymmetric division of precancerous cells generates TIC in a K-Ras-initiated model of lung cancer. And, we compare unexpected steps in this asymmetric division to those evident in well-studied stem cell models.

Subcellular Specialization and Organelle Behavior in Germ Cells

Gametes, eggs and sperm, are the highly specialized cell types on which the development of new life solely depends. Although all cells share essential organelles, such as the ER (endoplasmic reticulum), Golgi, mitochondria, and centrosomes, germ cells display unique regulation and behavior of organelles during gametogenesis. These germ cell-specific functions of organelles serve critical roles in successful gamete production. In this chapter, I will review the behaviors and roles of organelles during germ cell differentiation.

Symmetry from Asymmetry or Asymmetry from Symmetry?

The processes of DNA replication and mitosis allow the genetic information of a cell to be copied and transferred reliably to its daughter cells. However, if DNA replication and cell division were always performed in a symmetric manner, the result would be a cluster of tumor cells instead of a multicellular organism. Therefore, gaining a complete understanding of any complex living organism depends on learning how cells become different while faithfully maintaining the same genetic material. It is well recognized that the distinct epigenetic information contained in each cell type defines its unique gene expression program. Nevertheless, how epigenetic information contained in the parental cell is either maintained or changed in the daughter cells remains largely unknown. During the asymmetric cell division (ACD) of Drosophila male germline stem cells, our previous work revealed that preexisting histones are selectively retained in the renewed stem cell daughter, whereas newly synthesized histones are enriched in the differentiating daughter cell. We also found that randomized inheritance of preexisting histones versus newly synthesized histones results in both stem cell loss and progenitor germ cell tumor phenotypes, suggesting that programmed histone inheritance is a key epigenetic player for cells to either remember or reset cell fates. Here, we will discuss these findings in the context of current knowledge on DNA replication, polarized mitotic machinery, and ACD for both animal development and tissue homeostasis. We will also speculate on some potential mechanisms underlying asymmetric histone inheritance, which may be used in other biological events to achieve the asymmetric cell fates.

The Inherent Asymmetry of DNA Replication

Semiconservative DNA replication has provided an elegant solution to the fundamental problem of how life is able to proliferate in a way that allows cells, organisms, and populations to survive and replicate many times over. Somewhat lost, however, in our admiration for this mechanism is an appreciation for the asymmetries that occur in the process of DNA replication. As we discuss in this review, these asymmetries arise as a consequence of the structure of the DNA molecule and the enzymatic mechanism of DNA synthesis. Increasing evidence suggests that asymmetries in DNA replication are able to play a central role in the processes of adaptation and evolution by shaping the mutagenic landscape of cells. Additionally, in eukaryotes, recent work has demonstrated that the inherent asymmetries in DNA replication may play an important role in the process of chromatin replication. As chromatin plays an essential role in defining cell identity, asymmetries generated during the process of DNA replication may play critical roles in cell fate decisions related to patterning and development.

Klp10A, a stem cell centrosome-enriched kinesin, balances asymmetries in Drosophila male germline stem cell division

Asymmetric stem cell division is often accompanied by stereotypical inheritance of the mother and daughter centrosomes. However, it remains unknown whether and how stem cell centrosomes are uniquely regulated and how this regulation may contribute to stem cell fate. Here we identify Klp10A, a microtubule-depolymerizing kinesin of the kinesin-13 family, as the first protein enriched in the stem cell centrosome in Drosophila male germline stem cells (GSCs). Depletion of klp10A results in abnormal elongation of the mother centrosomes in GSCs, suggesting the existence of a stem cell-specific centrosome regulation program. Concomitant with mother centrosome elongation, GSCs form asymmetric spindle, wherein the elongated mother centrosome organizes considerably larger half spindle than the other. This leads to asymmetric cell size, yielding a smaller differentiating daughter cell. We propose that klp10A functions to counteract undesirable asymmetries that may result as a by-product of achieving asymmetries essential for successful stem cell divisions.

DOI:http://dx.doi.org/10.7554/eLife.20977.001

Centrosomes in spindle organization and chromosome segregation: a mechanistic view

Centrosomes are complex structures, which are embedded into the opposite poles of the mitotic spindle of most animals, acting as microtubule organizing centres. Surprisingly, in several biological systems, such as flies, chicken, or human cells, centrosomes are not essential for cell division. Nonetheless, they ensure faithful chromosome segregation. Moreover, mis-functioning centrosomes can act in a dominant-negative manner, resulting in erroneous mitotic progression. Here, I review the mechanisms by which centrosomes contribute to proper spindle organization and faithful chromosome segregation under physiological conditions and discuss how errors in centrosome function impair transmission of the genomic material in a pathological setting.

Histone H3 Threonine Phosphorylation Regulates Asymmetric Histone Inheritance in the Drosophila Male Germline

A long-standing question concerns how stem cells maintain their identity through multiple divisions. Previously, we reported that pre-existing and newly synthesized histone H3 are asymmetrically distributed during Drosophila male germline stem cell (GSC) asymmetric division. Here, we show that phosphorylation at threonine 3 of H3 (H3T3P) distinguishes pre-existing versus newly synthesized H3. Converting T3 to the unphosphorylatable residue alanine (H3T3A) or to the phosphomimetic aspartate (H3T3D) disrupts asymmetric H3 inheritance. Expression of H3T3A or H3T3D specifically in early-stage germline also leads to cellular defects, including GSC loss and germline tumors. Finally, compromising the activity of the H3T3 kinase Haspin enhances the H3T3A but suppresses the H3T3D phenotypes. These studies demonstrate that H3T3P distinguishes sister chromatids enriched with distinct pools of H3 in order to coordinate asymmetric segregation of "old" H3 into GSCs and that tight regulation of H3T3 phosphorylation is required for male germline activity.

When fate follows age: unequal centrosomes in asymmetric cell division

A strong correlation between centrosome age and fate has been reported in some stem cells and progenitors that divide asymmetrically. In some cases, such stereotyped centrosome behaviour is essential to endow stemness to only one of the two daughters, whereas in other cases causality is still uncertain. Here, we present the different cell types in which correlated centrosome age and fate has been documented, review current knowledge on the underlying molecular mechanisms and discuss possible functional implications of this process.

Multiple facets of histone variant H2AX: a DNA double-strand-break marker with several biological functions

In the last decade, many papers highlighted that the histone variant H2AX and its phosphorylation on Ser 139 (γH2AX) cannot be simply considered a specific DNA double-strand-break (DSB) marker with a role restricted to the DNA damage response, but rather as a ‘protagonist’ in different scenarios. This review will present and discuss an up-to-date view regarding the ‘non-canonical’ H2AX roles, focusing in particular on possible functional and structural parts in contexts different from the canonical DNA DSB response. We will present aspects concerning sex chromosome inactivation in male germ cells, X inactivation in female somatic cells and mitosis, but will also focus on the more recent studies regarding embryonic and neural stem cell development, asymmetric sister chromosome segregation in stem cells and cellular senescence maintenance. We will discuss whether in these new contexts there might be a relation with the canonical DNA DSB signalling function that could justify γH2AX formation. The authors will emphasize that, just as H2AX phosphorylation signals chromatin alteration and serves the canonical function of recruiting DSB repair factors, so the modification of H2AX in contexts other than the DNA damage response may contribute towards creating a specific chromatin structure frame allowing ‘non-canonical’ functions to be carried out in different cell types.

Asymmetric cell division and template DNA co-segregation in cancer stem cells

During tissue homeostasis, normal stem cells self-renew and repopulate the diverse cell types found within the tissue via a series of carefully controlled symmetric and asymmetric cell divisions (ACDs). The notion that solid tumors comprise a subset of cancer stem cells (CSCs) with dysregulated self-renewal and excessive symmetric cell divisions has led to numerous studies aimed to elucidate the mechanisms regulating ACD under steady-state conditions, during stem-cell expansion and in cancer. In this perspective, we focus on a type of asymmetry that can be established during ACD, called non-random co-segregation of template DNA, which has been identified across numerous species, cell types, and cancers. We discuss the role of p53 loss in maintaining self-renewal in both normal and malignant cells. We then review our current knowledge of the mechanisms underlying co-segregation of template DNA strands and the stem-cell pathways associated with it in normal and CSCs.

Cell adhesion geometry regulates non-random DNA segregation and asymmetric cell fates in mouse skeletal muscle stem cells

Cells of several metazoan species have been shown to non-randomly segregate their DNA such that older template DNA strands segregate to one daughter cell. The mechanisms that regulate this asymmetry remain undefined. Determinants of cell fate are polarized during mitosis and partitioned asymmetrically as the spindle pole orients during cell division. Chromatids align along the pole axis; therefore, it is unclear whether extrinsic cues that determine spindle pole position also promote non-random DNA segregation. To mimic the asymmetric divisions seen in the mouse skeletal stem cell niche, we used micropatterns coated with extracellular matrix in asymmetric and symmetric motifs. We show that the frequency of non-random DNA segregation and transcription factor asymmetry correlates with the shape of the motif and that these events can be uncoupled. Furthermore, regulation of DNA segregation by cell adhesion occurs within a defined time interval. Thus, cell adhesion cues have a major impact on determining both DNA segregation patterns and cell fates.

Stem cell decisions: a twist of fate or a niche market?

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Extrinsic and intrinsic cues that impact on stem cell biology.

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The importance to establish methods that allow to compare spindle orientation measurements.

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Mechanisms of centrosome segregation in asymmetrically dividing cells.

Establishing and maintaining cell fate in the right place at the right time is a key requirement for normal tissue maintenance. Stem cells are at the core of this process. Understanding how stem cells balance self-renewal and production of differentiating cells is key for understanding the defects that underpin many diseases. Both, external cues from the environment and cell intrinsic mechanisms can control the outcome of stem cell division. The role of the orientation of stem cell division has emerged as an important mechanism for specifying cell fate decisions. Although, the alignment of cell divisions can dependent on spatial cues from the environment, maintaining stemness is not always linked to positioning of stem cells in a particular microenvironment or `niche'. Alternate mechanisms that could contribute to cellular memory include differential segregation of centrosomes in asymmetrically dividing cells.

Asymmetric distribution of histones during Drosophila male germline stem cell asymmetric divisions

It has long been known that epigenetic changes are inheritable. However, except for DNA methylation, little is known about the molecular mechanisms of epigenetic inheritance. Many types of stem cells undergo asymmetric cell divisions to generate self-renewed stem cells and daughter cells committed for differentiation. Still, whether and how stem cells retain their epigenetic memory remain questions to be elucidated. During the asymmetric division of Drosophila male germline stem cell (GSC), our recent studies revealed that the preexisting histone 3 (H3) are selectively segregated to the GSC, whereas newly synthesized H3 deposited during DNA replication are enriched in the differentiating daughter cell. We propose a two-step model to explain this asymmetric histone distribution. First, prior to mitosis, preexisting histones and newly synthesized histones are differentially distributed at two sets of sister chromatids. Next, during mitosis, the set of sister chromatids that mainly consist of preexisting histones are segregated to GSCs, while the other set of sister chromatids enriched with newly synthesized histones are partitioned to the daughter cell committed for differentiation. In this review, we apply current knowledge about epigenetic inheritance and asymmetric cell division to inform our discussion of potential molecular mechanisms and the cellular basis underlying this asymmetric histone distribution pattern. We will also discuss whether this phenomenon contributes to the maintenance of stem cell identity and resetting chromatin structure in the other daughter cell for differentiation.

Nonrandom sister chromatid segregation of sex chromosomes in Drosophila male germline stem cells

Sister chromatids are the product of DNA replication, which is assumed to be a very precise process. Therefore, sister chromatids should be exact copies of each other. However, reports have indicated that sister chromatids are segregated nonrandomly during cell division, suggesting that sister chromatids are not the same, although their DNA sequences are the same. Researchers have speculated that stem cells may retain template strands to avoid replication-induced mutations. An alternative proposal is that cells may segregate distinct epigenetic information carried on sister chromatids. Recently, we found that Drosophila male germline stem cells segregate sister chromatids of X and Y chromosomes with a strong bias. We discuss this finding in relation to existing models for nonrandom sister chromatid segregation.

Sorting DNA with asymmetry: a new player in gene regulation?

In recent years, our views on how DNA and genes are organised and regulated have evolved significantly. One example is provided by reports that single DNA strands in the double helix could carry distinct forms of information. That chromatids carrying old and nascently replicated DNA strands are recognised by the mitotic machinery, then segregated in a concerted way to distinct daughter cells after cell division is remarkable. Notably, this phenomenon in several cases has been associated with the cell fate choice of resulting daughter cells. Here, we review the evidence for asymmetric or template DNA strand segregation in mammals with a focus on skeletal muscle.

Nonrandom template segregation: a way to break the symmetry of stem cells

Whether DNA segregates in a biased way has been a subject of intense controversy and debate. Although highly provocative in its biological implications, if true, technical problems have limited researchers from drawing firm conclusions from the data. Elabd et al. (2013. J. Cell Biol. http://dx.doi.org/10.1083/jcb.201307110/DC1) now show a high frequency of nonrandom template segregation during differentiation of embryonic stem cells using rigorous experimentation and implicate the methyltransferase Dnmt3 as a key regulator of this process.

DNA asymmetry in stem cells - immortal or mortal?

The immortal strand hypothesis proposes that stem cells retain a template copy of genomic DNA (i.e. an 'immortal strand') to avoid replication-induced mutations. An alternative hypothesis suggests that certain cells segregate sister chromatids non-randomly to transmit distinct epigenetic information. However, this area of research has been highly controversial, with conflicting data even from the same cell types. Moreover, historically, the same term of 'non-random sister chromatid segregation' or 'biased sister chromatid segregation' has been used to indicate distinct biological processes, generating a confusion in the biological significance and potential mechanism of each phenomenon. Here, we discuss the models of non-random sister chromatid segregation, and we explore the strengths and limitations of the various techniques and experimental model systems used to study this question. We also describe our recent study on Drosophila male germline stem cells, where sister chromatids of X and Y chromosomes are segregated non-randomly during cell division. We aim to integrate the existing evidence to speculate on the underlying mechanisms and biological relevance of this long-standing observation on non-random sister chromatid segregation.

A unified model for left-right asymmetry? Comparison and synthesis of molecular models of embryonic laterality

Understanding how and when the left-right (LR) axis is first established is a fundamental question in developmental biology. A popular model is that the LR axis is established relatively late in embryogenesis, due to the movement of motile cilia and the resultant directed fluid flow during late gastrulation/early neurulation. Yet, a large body of evidence suggests that biophysical, molecular, and bioelectrical asymmetries exist much earlier in development, some as early as the first cell cleavage after fertilization. Alternative models of LR asymmetry have been proposed that accommodate these data, postulating that asymmetry is established due to a chiral cytoskeleton and/or the asymmetric segregation of chromatids. There are some similarities, and many differences, in how these various models postulate the origin and timing of symmetry breaking and amplification, and these events' linkage to the well-conserved subsequent asymmetric transcriptional cascades. This review examines experimental data that lend strong support to an early origin of LR asymmetry, yet are also consistent with later roles for cilia in the amplification of LR pathways. In this way, we propose that the various models of asymmetry can be unified: early events are needed to initiate LR asymmetry, and later events could be utilized by some species to maintain LR-biases. We also present an alternative hypothesis, which proposes that individual embryos stochastically choose one of several possible pathways with which to establish their LR axis. These two hypotheses are both tractable in appropriate model species; testing them to resolve open questions in the field of LR patterning will reveal interesting new biology of wide relevance to developmental, cell, and evolutionary biology.

Attaching to spindles before they form: do early incorrect chromosome-microtubule attachments promote meiotic segregation fidelity?

The proper partitioning of the genome during meiosis depends on the correct segregation of chromosomes. Errors in this process result in the production of aneuploid gametes, a major cause of birth defects and infertility in humans. In order to segregate properly in meiosis, homologous chromosome partners must attach to microtubules that emanate from opposites poles of the spindle. However, a recent study in yeast has shown that, remarkably, the initial attachments between microtubules and the chromosomes are usually incorrect, which would lead to catastrophic segregation errors, but they are nearly always corrected through the detachment and reattachment of the microtubules. Here we review the reasons for the initial incorrect attachments, which stem from the timing of their formation early in the spindle assembly process, and the fact that the microtubule organizers, called spindle pole bodies in yeast, are not equal. One spindle pole body is older and better able to produce microtubules that attach to the chromosomes. We draw parallels to recent findings in animal cells and suggest that these early microtubule attachments, while often incorrect, may serve an important role in spindle assembly, which, in the long-term, promotes high-fidelity chromosome segregation.

Biased DNA segregation in Drosophila male germline stem cells

The immortal strand hypothesis, which emerged four decades ago, proposes that certain cells retain a template copy of chromosomal DNA to protect against replication-induced mutations. As the interest in stem cells rose in recent years, researchers speculated that stem cells, which must maintain proliferative capacity throughout the life of the organism, may be the population that most needs the strong protection afforded by immortal strand segregation. Alternative hypotheses have also been proposed to explain observed non-random sister chromatid segregation. We recently found that Drosophila male germline stem cells segregate sister chromatids non-randomly, but such bias was limited to the sex chromosomes. Interestingly, the biased segregation does not lead to immortal strand segregation. We will discuss the implications of this observation and molecular mechanisms, which might be applicable to non-random sister chromatid segregation in other systems as well.

The mortal strand hypothesis: non-random chromosome inheritance and the biased segregation of damaged DNA

If a eukaryotic cell is to reproduce, it must duplicate its genetic information in the form of DNA, and faithfully segregate that information during a complex process of cell division. During this division process, the resulting cells inherit one, and only one, copy of each chromosome. Over thirty years ago, it was predicted that the segregation of sister chromosomes could occur non-randomly, such that a daughter cell would preferentially inherit one of the two sister chromosomes according to some characteristic of that chromosome's template DNA strand. Although this prediction has been confirmed in studies of various cell-types, we know little of both the mechanism by which the asymmetric inheritance occurs and the significance it has to cells. In this essay, we propose a new model of non-random chromosome segregation-the mortal strand hypothesis-and discuss tests of the model that will provide insight into the molecular choreography of this intriguing phenomenon.

DNA asymmetry and cell fate regulation in stem cells

The semi-conservative nature of DNA replication has suggested that identical DNA molecules within chromatids are inherited by daughter cells after cell division. Numerous reports of non-random DNA segregation in prokaryotes and eukaryotes suggest that this is not always the case, and that epigenetic marks on chromatids, if not the individual DNA strands themselves, could have distinct signatures. Their selective distribution to daughter cells provides a novel mechanism for gene and cell fate regulation by segregating chromatids asymmetrically. Here we highlight some examples and potential mechanisms that can regulate this process. We propose that cellular asymmetry is inherently present during each cell division, and that it provides an opportunity during each cell cycle for moderating cell fates.

Strand-seq: a unifying tool for studies of chromosome segregation

Non random segregation of sister chromatids has been implicated to help specify daughter cell fate (the Silent Sister Hypothesis [1]) or to protect the genome of long-lived stem cells (the Immortal Strand Hypothesis [2]). The idea that sister chromatids are non-randomly segregated into specific daughter cells is only marginally supported by data in sporadic and often contradictory studies. As a result, the field has moved forward rather slowly. The advent of being able to directly label and differentiate sister chromatids in vivo using fluorescence in situ hybridization [3] was a significant advance for such studies. However, this approach is limited by the need for large tracks of unidirectional repeats on chromosomes and the reliance on quantitative imaging of fluorescent probes and rigorous statistical analysis to discern between the two competing hypotheses. A novel method called Strand-seq which uses next-generation sequencing to assay sister chromatid inheritance patterns independently for each chromosome [4] offers a comprehensive approach to test for non-random segregation. In addition Strand-seq enables studies on the deposition of chromatin marks in relation to DNA replication. This method is expected to help unify the field by testing previous claims of non-random segregation in an unbiased way in many model systems in vitro and in vivo.

Chromosome-specific nonrandom sister chromatid segregation during stem-cell division

Adult stem cells undergo asymmetric cell division to self-renew and give rise to differentiated cells that comprise mature tissue¹. Sister chromatids may be distinguished and segregated non-randomly in asymmetrically dividing stem cells², although the underlying mechanism and the purpose it may serve remain elusive. We developed the CO-FISH (chromosome orientation fluorescence in situ hybridization) technique³ with single-chromosome resolution and show that sister chromatids of X and Y chromosomes, but not autosomes, are segregated non-randomly during asymmetric divisions of Drosophila male germline stem cells (GSCs). This provides the first direct evidence that two sister chromatids containing identical genetic information can be distinguished and segregated non-randomly during asymmetric stem cell divisions. We further show that the centrosome, SUN-KASH nuclear envelope proteins, and Dnmt2 are required for non-random sister chromatid segregation. Our data suggest that the information on X and Y chromosomes that enables non-random segregation is primed during gametogenesis in the parents. Moreover, we show that sister chromatid segregation is randomized in GSC overproliferation and dedifferentiated GSCs. We propose that non-random sister chromatid segregation may serve to transmit distinct information carried on two sister chromatids to the daughters of asymmetrically dividing stem cells.

Metabolic reprogramming as a novel regulator of skeletal muscle development and regeneration

Adult skeletal muscle contains a resident population of stem cells, termed satellite cells, that exist in a quiescent state. In response to an activating signal (such as physical trauma), satellite cells enter the cell cycle and undergo multiple rounds of proliferation, followed by differentiation, fusion, and maturation. Over the last 10-15 years, our understanding of the transcriptional regulation of this stem cell population has greatly expanded, but there remains a dearth of knowledge with regard to the initiating signal leading to these changes in transcription. The recent renewed interest in the metabolic regulation of both cancer and stem cells, combined with previous findings indicating that satellite cells preferentially colocalize with blood vessels, suggests that satellite cell function may be regulated by changes in cellular metabolism. This review aims to describe what is currently known about satellite cell metabolism during changes in cell fate, as well as to describe some of the exciting findings in other cell types and how these might relate to satellite cells.

Asymmetric stem cell division: precision for robustness

Asymmetric cell division (ACD) produces two daughter cells with distinct fates or characteristics. Many adult stem cells use ACD as a means of maintaining stem cell number and thus tissue homeostasis. Here, we review recent progress on ACD, discussing conservation between stem and non-stem cell systems, molecular mechanisms, and the biological meaning of ACD.

Centrosome asymmetry and inheritance during animal development

The centrosome is a subcellular organelle that is responsible for the majority of microtubule organization. Through this ability, the centrosome is involved in cell division, migration, and polarization. Recent studies have revealed intriguing asymmetries between mother and daughter centrioles as well as between mother and daughter centrosomes, and the involvement of such asymmetries in multiple cellular and developmental processes. This review aims to summarize recent discoveries on such asymmetries in centrioles/centrosomes and the potential implication of their inheritance patterns during cell division and development.

Asymmetric chromatid segregation in cardiac progenitor cells is enhanced by Pim-1 kinase

RATIONALE

Cardiac progenitor cells (CPCs) in the adult heart are used for cell-based treatment of myocardial damage, but factors determining stemness, self-renewal, and lineage commitment are poorly understood. Immortal DNA strands inherited through asymmetric chromatid segregation correlate with self-renewal of adult stem cells, but the capacity of CPCs for asymmetric segregation to retain immortal strands is unknown. Cardioprotective kinase Pim-1 increases asymmetric cell division in vivo, but the ability of Pim-1 to enhance asymmetric chromatid segregation is unknown.

OBJECTIVE

We aimed to demonstrate immortal strand segregation in CPCs and the enhancement of asymmetric chromatid distribution by Pim-1 kinase.

METHODS AND RESULTS

Asymmetric segregation is tracked by incorporation of bromodeoxyuridine. The CPC DNA was labeled for several generations and then blocked in second cytokinesis during chase to determine distribution of immortal versus newly synthesized strands. Intensity ratios of binucleated CPCs with bromodeoxyuridine of ≥70:30 between daughter nuclei indicative of asymmetric chromatid segregation occur with a frequency of 4.57, and asymmetric chromatid segregation is demonstrated at late mitotic phases. Asymmetric chromatid segregation is significantly enhanced by Pim-1 overexpression in CPCs (9.19 versus 4.79 in eGFP-expressing cells; P=0.006).

CONCLUSIONS

Asymmetric segregation of chromatids in CPCs is increased nearly two-fold with Pim-1 kinase overexpression, indicating that Pim-1 promotes self-renewal of stem cells.

A subpopulation of adult skeletal muscle stem cells retains all template DNA strands after cell division

Satellite cells are adult skeletal muscle stem cells that are quiescent and constitute a poorly defined heterogeneous population. Using transgenic Tg:Pax7-nGFP mice, we show that Pax7-nGFP(Hi) cells are less primed for commitment and have a lower metabolic status and delayed first mitosis compared to Pax7-nGFP(Lo) cells. Pax7-nGFP(Hi) can give rise to Pax7-nGFP(Lo) cells after serial transplantations. Proliferating Pax7-nGFP(Hi) cells exhibit lower metabolic activity, and the majority performs asymmetric DNA segregation during cell division, wherein daughter cells retaining template DNA strands express stem cell markers. Using chromosome orientation-fluorescence in situ hybridization, we demonstrate that all chromatids segregate asymmetrically, whereas Pax7-nGFP(Lo) cells perform random DNA segregation. Therefore, quiescent Pax7-nGFP(Hi) cells represent a reversible dormant stem cell state, and during muscle regeneration, Pax7-nGFP(Hi) cells generate distinct daughter cell fates by asymmetrically segregating template DNA strands to the stem cell. These findings provide major insights into the biology of stem cells that segregate DNA asymmetrically.

Myf5 haploinsufficiency reveals distinct cell fate potentials for adult skeletal muscle stem cells

Skeletal muscle stem cell fate in adult mice is regulated by crucial transcription factors, including the determination genes Myf5 and Myod. The precise role of Myf5 in regulating quiescent muscle stem cells has remained elusive. Here we show that most, but not all, quiescent satellite cells express Myf5 protein, but at varying levels, and that resident Myf5 heterozygous muscle stem cells are more primed for myogenic commitment compared with wild-type satellite cells. Paradoxically however, heterotypic transplantation of Myf5 heterozygous cells into regenerating muscles results in higher self-renewal capacity compared with wild-type stem cells, whereas myofibre regenerative capacity is not altered. By contrast, Pax7 haploinsufficiency does not show major modifications by transcriptome analysis. These observations provide a mechanism linking Myf5 levels to muscle stem cell heterogeneity and fate by exposing two distinct and opposing phenotypes associated with Myf5 haploinsufficiency. These findings have important implications for how stem cell fates can be modulated by crucial transcription factors while generating a pool of responsive heterogeneous cells.

Coordinating cell polarity with cell division in space and time

Decisions of when and where to divide are crucial for cell survival and fate, and for tissue organization and homeostasis. The temporal coordination of mitotic events during cell division is essential to ensure that each daughter cell receives one copy of the genome. The spatial coordination of these events is also crucial because the cytokinetic furrow must be aligned with the axis of chromosome segregation and, in asymmetrically dividing cells, the polarity axis. Several recent papers describe how cell shape and polarity are coordinated with cell division in single cells and tissues and begin to unravel the underlying molecular mechanisms, revealing common principles and molecular players. Here, we discuss how cells regulate the spatial and temporal coordination of cell polarity with cell division.

Drosophila neuroblasts retain the daughter centrosome

During asymmetric mitosis, both in male Drosophila germline stem cells and in mouse embryo neural progenitors, the mother centrosome is retained by the self-renewed cell; hence suggesting that mother centrosome inheritance might contribute to stemness. We test this hypothesis in Drosophila neuroblasts (NBs) tracing photo converted centrioles and a daughter-centriole-specific marker generated by cloning the Drosophila homologue of human Centrobin. Here we show that upon asymmetric mitosis, the mother centrosome is inherited by the differentiating daughter cell. Our results demonstrate maturation-dependent centrosome fate in Drosophila NBs and that the stemness properties of these cells are not linked to mother centrosome inheritance.

Asymmetric partitioning of centrosomes has been reported in Drosophila neuroblasts, but whether this type of division has implications for stem cell self-renewal is unclear. In this study, the authors show that the asymmetric division of the centrosomes correlates with the asymmetric fate of the cells and that the daughter centrosome is retained by the neuroblast.

Drosophila male germline stem cells do not asymmetrically segregate chromosome strands

Adult stem cells continuously supply differentiated cells throughout the life of organisms. This increases the risk of replicative senescence or neoplastic transformation due to mutations that accumulate over many rounds of DNA replication. The immortal strand hypothesis proposes that stem cells reduce the accumulation of replication-induced mutations by retaining the older template DNA strands. Other models have also been proposed in which stem cells asymmetrically segregate chromosome strands for other reasons, such as retention of epigenetic memories. Recently, the idea has emerged that the mother centrosome, which is stereotypically retained within some asymmetrically dividing stem cells, might be utilized as a means of asymmetrically segregating chromosome strands. We have tested this hypothesis in germline stem cells (GSCs) from Drosophila melanogaster testis, which undergo asymmetric divisions marked by the asymmetric segregation of centrosomes and the acquisition of distinct daughter cell fates (stem cell self-renewal versus differentiation). Using 5-bromo-2-deoxyuridine labeling combined with direct visualization of GSC-gonialblast (differentiating daughter) pairs, we directly scored the outcome of chromosome strand segregation. Our data show that, in male GSCs in the Drosophila testis, chromosome strands are not asymmetrically segregated, despite asymmetrically segregating centrosomes. Our data demonstrate that asymmetric centrosome segregation in stem cells does not necessarily lead to asymmetric chromosome strand segregation.

Far from solved: a perspective on what we know about early mechanisms of left-right asymmetry

Consistent laterality is a crucial aspect of embryonic development, physiology, and behavior. While strides have been made in understanding unilaterally expressed genes and the asymmetries of organogenesis, early mechanisms are still poorly understood. One popular model centers on the structure and function of motile cilia and subsequent chiral extracellular fluid flow during gastrulation. Alternative models focus on intracellular roles of the cytoskeleton in driving asymmetries of physiological signals or asymmetric chromatid segregation, at much earlier stages. All three models trace the origin of asymmetry back to the chirality of cytoskeletal organizing centers, but significant controversy exists about how this intracellular chirality is amplified onto cell fields. Analysis of specific predictions of each model and crucial recent data on new mutants suggest that ciliary function may not be a broadly conserved, initiating event in left-right patterning. Many questions about embryonic left-right asymmetry remain open, offering fascinating avenues for further research in cell, developmental, and evolutionary biology.

Discovery of the mitotic selective chromatid segregation phenomenon and its implications for vertebrate development

The asymmetric cell division process is required for cellular differentiation and embryonic development. Recent evidence obtained in Drosophila and C. elegans suggest that this process occurs by non-equivalent distribution of proteins or mRNA (intrinsic factors) to daughter cells, or by their differential exposure to cell extrinsic factors. In contrast, haploid fission yeast sister cells developmentally differ by inheriting sister chromatids that are differentiated by epigenetic means. Specifically, the act of DNA replication at the mating-type locus in yeast switches it's alternate alleles only in one specific member of chromosome 2 sister chromatids in nearly every chromosome replication cycle. To employ this kind of mechanism for cellular differentiation, strictly based on Watson-Crick structure of DNA in diploid organism, selective segregation mechanism is required to coordinate distribution of potentially differentiated sister chromatids to daughter cells. Genetic evidence to this postulate was fortuitously provided by the analysis of mitotic recombinants of chromosome 7 in mouse cells. Remarkably, the biased segregation occurs in some cell types but not in others and the process seems to be chromosome-specific. This review summarizes the discovery of selective chromatid segregation phenomenon and it suggests that such a process of Somatic Sister chromatid Imprinting and Selective chromatid Segregation (SSIS model) might explain development in eukaryotes, such as that of the body axis left-right visceral organs laterality specification in mice.