New dimensions of asymmetric division in vertebrates

Temporal and anteriorly positioned mitotic zones drive asymmetric microtubule patterns needed for Left-Right Organizer development

Cellular proliferation plays a crucial role in tissue development, including the development of the Left-Right Organizer (LRO), the transient organ essential for dictating the vertebrate LR body plan. Here we investigate cell redistribution mechanisms and the dominance of specific progenitor cells in LRO formation, addressing cell lineage and cell behavior questions. Using zebrafish as a model, we mapped all LRO (Kupffer's Vesicle, KV) mitotic events, revealing an FGF-dependent, anteriorly enriched mitotic pattern. Using a KV-specific fluorescent microtubule (MT) line, we found that mitotic events align their spindle along the KV's longest axis until the rosette developmental stage, where "spinning" spindles followed by exclusion from KV occur. Daughter cells that remain are linked by cytokinetic bridges, shaping anteriorly focused MT patterns that precede apical actin recruitment. Our findings underscore the importance of spatially regulated mitotic events in establishing MT and actin pattern formation essential for LRO development.

Endoplasmic Reticulum Exit Sites scale with somato-dendritic size in neurons

Nervous systems exhibit dramatic diversity in cell morphology and size. How neurons regulate their biosynthetic and secretory machinery to support such diversity is not well understood. Endoplasmic reticulum exit sites (ERESs) are essential for maintaining secretory flux, and are required for normal dendrite development, but how neurons of different size regulate secretory capacity remains unknown. In Caenorhabditis elegans, we find that the ERES number is strongly correlated with the size of a neuron's dendritic arbor. The elaborately branched sensory neuron, PVD, has especially high ERES numbers. Asymmetric cell division provides PVD with a large initial cell size critical for rapid establishment of PVD's high ERES number before neurite outgrowth, and these ERESs are maintained throughout development. Maintenance of ERES number requires the cell fate transcription factor MEC-3, C. elegans TOR (ceTOR/let-363), and nutrient availability, with mec-3 and ceTOR/let-363 mutant PVDs both displaying reductions in ERES number, soma size, and dendrite size. Notably, mec-3 mutant animals exhibit reduced expression of a ceTOR/let-363 reporter in PVD, and starvation reduces ERES number and somato-dendritic size in a manner genetically redundant with ceTOR/let-363 perturbation. Our data suggest that both asymmetric cell division and nutrient sensing pathways regulate secretory capacities to support elaborate dendritic arbors.

Transposon control as a checkpoint for tissue regeneration

Tissue regeneration requires precise temporal control of cellular processes such as inflammatory signaling, chromatin remodeling and proliferation. The combination of these processes forms a unique microenvironment permissive to the expression, and potential mobilization of, transposable elements (TEs). Here, we develop the hypothesis that TE activation creates a barrier to tissue repair that must be overcome to achieve successful regeneration. We discuss how uncontrolled TE activity may impede tissue restoration and review mechanisms by which TE activity may be controlled during regeneration. We posit that the diversification and co-evolution of TEs and host control mechanisms may contribute to the wide variation in regenerative competency across tissues and species.

A centriole's subdistal appendages: contributions to cell division, ciliogenesis and differentiation

The centrosome is a highly conserved structure composed of two centrioles surrounded by pericentriolar material. The mother, and inherently older, centriole has distal and subdistal appendages, whereas the daughter centriole is devoid of these appendage structures. Both appendages have been primarily linked to functions in cilia formation. However, subdistal appendages present with a variety of potential functions that include spindle placement, chromosome alignment, the final stage of cell division (abscission) and potentially cell differentiation. Subdistal appendages are particularly interesting in that they do not always display a conserved ninefold symmetry in appendage organization on the mother centriole across eukaryotic species, unlike distal appendages. In this review, we aim to differentiate both the morphology and role of the distal and subdistal appendages, with a particular focus on subdistal appendages.

Centrosomes in asymmetric cell division

Asymmetric cell division (ACD) is a strategy for achieving cell diversity. Research carried out over the last two decades has shown that in some cell types that divide asymmetrically, mother and daughter centrosomes are noticeably different from one another in structure, behaviour, and fate, and that robust ACD depends upon centrosome function. Here, I review the latest advances in this field with special emphasis on the complex structure-function relationship of centrosomes with regards to ACD and on mechanistic insight derived from cell types that divide symmetrically but is likely to be relevant in ACD. I also include a comment arguing for the need to investigate the centrosome cycle in other cell types that divide asymmetrically.

Orderly assembly underpinning built-in asymmetry in the yeast centrosome duplication cycle requires cyclin-dependent kinase

Asymmetric astral microtubule organization drives the polarized orientation of the S. cerevisiae mitotic spindle and primes the invariant inheritance of the old spindle pole body (SPB, the yeast centrosome) by the bud. This model has anticipated analogous centrosome asymmetries featured in self-renewing stem cell divisions. We previously implicated Spc72, the cytoplasmic receptor for the gamma-tubulin nucleation complex, as the most upstream determinant linking SPB age, functional asymmetry and fate. Here we used structured illumination microscopy and biochemical analysis to explore the asymmetric landscape of nucleation sites inherently built into the spindle pathway and under the control of cyclin-dependent kinase (CDK). We show that CDK enforces Spc72 asymmetric docking by phosphorylating Nud1/centriolin. Furthermore, CDK-imposed order in the construction of the new SPB promotes the correct balance of nucleation sites between the nuclear and cytoplasmic faces of the SPB. Together these contributions by CDK inherently link correct SPB morphogenesis, age and fate.

Asymmetric cell division and replicative aging: a new perspective from the spindle poles

Although cell division is usually portrayed as an equitable process by which a progenitor cell originates two identical daughter cells, there are multiple examples of asymmetric divisions that generate two cells that differ in their content, morphology and/or proliferative potential. The capacity of the cells to generate asymmetry during their division is of paramount biological relevance, playing essential roles during embryonic development, cellular regeneration and tissue morphogenesis. Problems with the proper establishment of asymmetry and polarity during cell division can give rise to cancer and neurodevelopmental disorders, as well as to also accelerate cellular aging. Interestingly, the microtubule organizing centers that orchestrate the formation of the mitotic spindle have been described among the cellular structures that can be differentially allocated during asymmetric cell divisions. This mini-review focuses on recent research from our group and others uncovering a role for the non-random distribution of the spindle-associated microtubule organizing centers in the differential distribution of aging factors during asymmetric mitoses and therefore in the maintenance of the replicative lifespan of the cells.

Circulating Tumor Cells as a Tool for Assessing Tumor Heterogeneity

Tumor heterogeneity is the major cause of failure in cancer prognosis and prediction. Accurately detecting heterogeneity for the development of biomarkers and the detection of the clones resistant to therapy is one of the main goals of contemporary medicine. Metastases belong to the natural history of cancer. The present review gives an overview on the origin of tumor heterogeneity. Recent progress has made it possible to isolate and characterize circulating tumor cells (CTCs), which are the drivers of the disease between the primary sites and metastatic foci. The most recent methods for characterizing CTCs are summarized and we discuss the power of CTC profiling for analyzing tumor heterogeneity in early and advanced diseases.

Chromosome misalignment is associated with PLK1 activity at cenexin-positive mitotic centrosomes

The mitotic kinase, polo-like kinase 1 (PLK1), facilitates the assembly of the two mitotic spindle poles, which are required for the formation of the microtubule-based spindle that ensures appropriate chromosome distribution into the two forming daughter cells. Spindle poles are asymmetric in composition. One spindle pole contains the oldest mitotic centriole, the mother centriole, where the majority of cenexin, the mother centriole appendage protein and PLK1 binding partner, resides. We hypothesized that PLK1 activity is greater at the cenexin-positive older spindle pole. Our studies found that PLK1 asymmetrically localizes between spindle poles under conditions of chromosome misalignment, and chromosomes tend to misalign toward the oldest spindle pole in a cenexin- and PLK1-dependent manner. During chromosome misalignment, PLK1 activity is increased specifically at the oldest spindle pole, and this increase in activity is lost in cenexin-depleted cells. We propose a model where PLK1 activity elevates in response to misaligned chromosomes at the oldest spindle pole during metaphase.

Viral Infection or IFN-α Alters Mitotic Spindle Orientation by Modulating Pericentrin Levels

Congenital microcephaly occurs in utero during Zika virus (ZIKV) infection. The single-gene disorder, Majewski osteodysplastic primordial dwarfism type II (MOPDII), also leads to microcephaly and is concomitant with a decrease in the centrosomal protein, pericentrin (PCNT). This protein is a known contributor of mitotic spindle misorientation and ultimately, microcephaly. Similar to MOPDII, either viral infection or interferon (IFN)-α exposure reduced PCNT levels at the mitotic spindle poles. We unexpectedly found that infection of cells with any one of a diverse set of viruses, such as ZIKV, dengue virus, cytomegalovirus, influenza A virus, or hepatitis B virus, or treatment of cells with the anti-viral cytokine, IFN-α, produced mitotic spindle misorientation. These findings demonstrate a related mechanism for the development of microcephaly in viral infection, the host's antiviral IFN response, and primordial dwarfism.

•

ZIKV infection resembles MOPDII depletion of the centrosomal protein PCNT

•

Viral infection of mitotic cells results in loss of PCNT and spindle misorientation

•

IFN-α exposure to mitotic cells causes spindle misorientation

•

Loss of IFNAR abrogates both viral and IFN-α-induced spindle misorientation