Apicobasal polarity and cell proliferation during development

Cell polarization and cell division are two fundamental cellular processes. The mechanisms that establish and maintain cell polarity and the mechanisms by which cells progress through the cell cycle are now fairly well understood following decades of experimental work. There is also increasing evidence that the polarization state of a cell affects its proliferative properties. The challenge now is to understand how these two phenomena are mechanistically connected. The aim of the present chapter is to provide an overview of the evidence of cross-talk between apicobasal polarity and proliferation, and the current state of knowledge of the precise mechanism by which this cross-talk is achieved.     

Establishment of polarity during organization of the acentrosomal plant cortical microtubule array

The plant cortical microtubule array is a unique acentrosomal array that is essential for plant morphogenesis. To understand how this array is organized, we exploited the microtubule (+)-end tracking activity of two Arabidopsis EB1 proteins in combination with FRAP (fluorescence recovery after photobleaching) experiments of GFP-tubulin to examine the relationship between cortical microtubule array organization and polarity. Significantly, our observations show that the majority of cortical microtubules in ordered arrays, within a particular cell, face the same direction in both Arabidopsis plants and cultured tobacco cells. We determined that this polar microtubule coalignment is at least partially due to a selective stabilization of microtubules, and not due to a change in microtubule polymerization rates. Finally, we show that polar microtubule coalignment occurs in conjunction with parallel grouping of cortical microtubules and that cortical array polarity is progressively enhanced during array organization. These observations reveal a novel aspect of plant cortical microtubule array organization and suggest that selective stabilization of dynamic cortical microtubules plays a predominant role in the self-organization of cortical arrays.     

Cell polarity and cell diversification during early mouse embryogenesis.

At the eight-cell stage of mouse development, the organization of blastomeres changes from radially symmetrical to polarized. This acquisition of cell polarity, followed by asymmetric divisions, leads to the formation of two phenotypically different cell types, which give rise to the first two cell lineages of the mouse blastocyst embryo, trophectoderm and the inner cell mass. Cell fate, controlled by positional information, is not irreversibly fixed during differentiation, providing the embryo with considerable developmental flexibility.     

Cell-to-cell transport of macromolecules during early plant development

Plant cells exchange developmental signals, distribute nutrients and ribonucleoprotein complexes through dynamic intercellular channels termed plasmodesmata (PD). Multidisciplinary investigations over the last decade have provided evidence that plasmodesmatal regulation is critical to various basic plant functions, such as development, host-pathogen interactions, and systemic RNA-silencing. This review highlights the cell-to-cell transport of micro- and macromolecules via PD during embryo and seedling growth.     

Lipid function in plant cell polarity

The establishment and maintenance of cell polarity play pivotal roles during plant development. During the past five years, proteins that are required for different aspects of plant cell polarity have been identified. However, the functions of lipids and their interactions with proteins that mediate polarity remained largely unaddressed. Recent genetic studies have discovered cell and tissue polarity mutants that have defects in sterol composition, glycosylphosphatidylinositol-anchored proteins, glycosylphosphatidylinositol biosynthesis and phospholipid signalling. Analyses of the affected gene products have provided a first glance at the roles of lipids in cell polarity signalling, as well as in the trafficking and anchoring of polar proteins.     

Control of cell proliferation during plant development

Knowledge of the control of cell division in eukaryotes has increased tremendously in recent years. The isolation and characterization of the major players from a number of systems and the study of their interactions have led to a comprehensive understanding of how the different components of the cell cycle apparatus are brought together and assembled in a fine-tuned machinery. Many parts of this machine are highly conserved in organisms as evolutionary distant as yeast and animals. Some key regulators of cell division have also been identified in higher plants and have been shown to be functional homologues of the yeast or animal proteins. Although still in its early days, investigations into the regulation of these molecules have provided some clues on how cell division is coupled to plant development.     

Establishing cell polarity in development

Polarity is a common feature of many different cell types, including the Caenorhabditis elegans zygote, the Drosophila oocyte and mammalian epithelial cells. The initial establishment of cell polarity depends on asymmetric cues that lead to reorganization of the cytoskeleton and polarized localization of several cortical proteins that act downstream of the polarization cues. The past year revealed that homologs of the C. elegans par (partitioning defective) genes are also essential for establishing polarity in Drosophila and vertebrate cells. There is growing evidence that the proteins encoded by these genes interact with key regulators of both the actin and the microtubule cytoskeletons.     

The establishment of hepatocyte cell surface polarity during fetal liver development

Antibodies to six glycoproteins present in different domains of the hepatocyte plasma membrane were used to study the establishment of cell surface polarity during rat fetal liver development. The proteins were immunoprecipitated from fetal liver homogenates between 14 and 21 days of gestation and quantified by immunoblotting. Aminopeptidase N, CE 9, and HA 321, which reside in the apical, basolateral, and lateral plasma membrane in the adult hepatocyte, respectively, were present in high concentrations at 14 days of gestation and remained high until birth. In contrast, two apical proteins (HA 4 and dipeptidyl peptidase IV) and two basolateral proteins (ASGP receptor and EGF receptor) were first detected between 16 and 18 days of gestation and increased linearly until birth. HA 4 was the only molecule for which the fetal and adult forms differed, with the former having a faster mobility on SDS-PAGE, due to differences in N-linked oligosaccharides. With two exceptions, the localization of the molecules from earliest detection was restricted to the same domain as that in the adult. At 15 days of gestation, HA 321 and a small portion of aminopeptidase were detected on the basolateral membrane. By 21 days both molecules had assumed their adult localization pattern. Our results indicate that the biogenesis of cell surface polarity is an early event, implying that the mechanisms for sorting plasma membrane molecules are functional very early in development. Furthermore, the different patterns of appearance of the six molecules, irrespective of domain, indicate that the biochemical composition of the cell surface changes dramatically during fetal liver development.     

Stem cell function during plant vascular development

While many regulatory mechanisms controlling the development and function of root and shoot apical meristems have been revealed, our knowledge of similar processes in lateral meristems, including the vascular cambium, is still limited. Our understanding of even the anatomy and development of lateral meristems (procambium or vascular cambium) is still relatively incomplete, let alone their genetic regulation. Research into this particular tissue type has been mostly hindered by a lack of suitable molecular markers, as well as the fact that thus far very few mutants affecting plant secondary development have been described. The development of suitable molecular markers is a high priority in order to help define the anatomy, especially the location and identity of cambial stem cells and the developmental phases and molecular regulatory mechanisms of the cambial zone. To date, most of the advances have been obtained by studying the role of the major plant hormones in vascular development. Thus far auxin, cytokinin, gibberellin and ethylene have been implicated in regulating the maintenance and activity of cambial stem cells; the most logical question in research would be how these hormones interact during the various phases of cambial development.     

Planar cell polarity regulators in asymmetric organogenesis during development and disease

The phenomenon of planar cell polarity is critically required for a myriad of morphogenetic processes in metazoan and is accurately controlled by several conserved modules. Six “core” proteins, including Frizzled, Flamingo (Celsr), Van Gogh (Vangl), Dishevelled, Prickle, and Diego (Ankrd6), are major components of the Wnt/planar cell polarity pathway. The Fat/Dchs protocadherins and the Scrib polarity complex also function to instruct cellular polarization. In vertebrates, all these pathways are essential for tissue and organ morphogenesis, such as neural tube closure, left–right symmetry breaking, heart and gut morphogenesis, lung and kidney branching, stereociliary bundle orientation, and proximal–distal limb elongation. Mutations in planar polarity genes are closely linked to various congenital diseases. Striking advances have been made in deciphering their contribution to the establishment of spatially oriented pattern in developing organs and the maintenance of tissue homeostasis. The challenge remains to clarify the complex interplay of different polarity pathways in organogenesis and the link of cell polarity to cell fate specification. Interdisciplinary approaches are also important to understand the roles of mechanical forces in coupling cellular polarization and differentiation. This review outlines current advances on planar polarity regulators in asymmetric organ formation, with the aim to identify questions that deserve further investigation.     

Remodeling the cell surface distribution of membrane proteins during the development of epithelial cell polarity

The development of polarized epithelial cells from unpolarized precursor cells follows induction of cell-cell contacts and requires resorting of proteins into different membrane domains. We show that in MDCK cells the distributions of two membrane proteins, Dg-1 and E-cadherin, become restricted to the basal-lateral membrane domain within 8 h of cell-cell contact. During this time, however, 60-80% of newly synthesized Dg-1 and E-cadherin is delivered directly to the forming apical membrane and then rapidly removed, while the remainder is delivered to the basal-lateral membrane and has a longer residence time. Direct delivery of greater than 95% of these proteins from the Golgi complex to the basal-lateral membrane occurs greater than 48 h later. In contrast, we show that two apical proteins are efficiently delivered and restricted to the apical cell surface within 2 h after cell-cell contact. These results provide insight into mechanisms involved in the development of epithelial cell surface polarity, and the establishment of protein sorting pathways in polarized cells.     

MicroRNA gene regulation cascades during early stages of plant development

MicroRNAs (miRNAs) regulate various developmental programs of plants. This review focuses on miRNA involvement in early events of plant development, such as seed germination, seedling development and the juvenile to adult phase transition. miR159 and miR160 are involved in the regulation of seed germination through their effects on the sensitivity of seeds to ABA. miR156 and miR172 play critical roles in the emergence of vegetative leaves at post-germinative stages, which is important for the transition to autotrophic growth. The phase transition from the juvenile to adult stage in both monocots and dicots is also regulated by miR156 and miR172. In these early developmental processes, there are miRNA gene regulation cascades where the miR156 pathway acts upstream of the miR172 pathway. Moreover, targets of miR156 and miR172 exert positive feedback on the expression of MIR genes that suppress themselves. The early events of plant development appear to be controlled by complex mechanisms involving sequential expression of different miRNA pathways and feedback loops among miRNAs and their target genes.     

Polarized sphingolipid transport from the subapical compartment changes during cell polarity development

The subapical compartment (SAC) plays an important role in the polarized transport of proteins and lipids. In hepatoma-derived HepG2 cells, fluorescent analogues of sphingomyelin and glucosylceramide are sorted in the SAC. Here, evidence is provided that shows that polarity development is regulated by a transient activation of endogenous protein kinase A and involves a transient activation of a specific membrane transport pathway, marked by the trafficking of the labeled sphingomyelin, from the SAC to the apical membrane. This protein kinase A-regulated pathway differs from the apical recycling pathway, which also traverses SAC. After reaching optimal polarity, the direction of the apically activated pathway switches to one in the basolateral direction, without affecting the apical recycling pathway.     

Establishment of animal-vegetal polarity during maturation in ascidian oocytes

Mature ascidian oocytes are arrested in metaphase of meiosis I (Met I) and display a pronounced animal-vegetal polarity: a small meiotic spindle lies beneath the animal pole, and two adjacent cortical and subcortical domains respectively rich in cortical endoplasmic reticulum and postplasmic/PEM RNAs (cER/mRNA domain) and mitochondria (myoplasm domain) line the equatorial and vegetal regions. Symmetry-breaking events triggered by the fertilizing sperm remodel this primary animal-vegetal (a-v) axis to establish the embryonic (D-V, A-P) axes. To understand how this radial a-v polarity of eggs is established, we have analyzed the distribution of mitochondria, mRNAs, microtubules and chromosomes in pre-vitellogenic, vitellogenic and post-vitellogenic Germinal Vesicle (GV) stage oocytes and in spontaneously maturing oocytes of the ascidian Ciona intestinalis. We show that myoplasm and postplasmic/PEM RNAs move into the oocyte periphery at the end of oogenesis and that polarization along the a-v axis occurs after maturation in several steps which take 3-4 h to be completed. First, the Germinal Vesicle breaks down, and a meiotic spindle forms in the center of the oocyte. Second, the meiotic spindle moves in an apparently random direction towards the cortex. Third, when the microtubular spindle and chromosomes arrive and rotate in the cortex (defining the animal pole), the subcortical myoplasm domain and cortical postplasmic/PEM RNAs are excluded from the animal pole region, thus concentrating in the vegetal hemisphere. The actin cytoskeleton is required for migration of the spindle and subsequent polarization, whereas these events occur normally in the absence of microtubules. Our observations set the stage for understanding the mechanisms governing primary axis establishment and meiotic maturation in ascidians.     

Programmed cell death during plant growth and development

This review describes programmed cell death as it signifies the terminal differentiation of cells in anthers, xylem, the suspensor and senescing leaves and petals. Also described are cell suicide programs initiated by stress (e.g., hypoxia-induced aerenchyma formation) and those that depend on communication between neighboring cells, as observed for incompatible pollen tubes, the suspensor and synergids in some species. Although certain elements of apoptosis are detectable during some plant programmed cell death processes, the participation of autolytic and perhaps autophagic mechanisms of cell killing during aerenchyma formation, tracheary element differentiation, suspensor degeneration and senescence support the conclusion that nonapoptotic programmed cell death pathways are essential to normal plant growth and development. Heterophagic elimination of dead cells, a prominent feature of animal apoptosis, is not evident in plants. Rather autolysis and autophagy appear to govern the elimination of cells during plant cell suicide.     

Heterosis of biomass production in Arabidopsis. Establishment during early development

Heterosis has been widely used in agriculture to increase yield and to broaden adaptability of hybrid varieties and is applied to an increasing number of crop species. We performed a systematic survey of the extent and degree of heterosis for dry biomass in 63 Arabidopsis accessions crossed to three reference lines (Col-0, C24, and Nd). We detected a high heritability (69%) for biomass production in Arabidopsis. Among the 169 crosses analyzed, 29 exhibited significant mid-parent-heterosis for shoot biomass. Furthermore, we analyzed two divergent accessions, C24 and Col-0, the F(1) hybrids of which were shown to exhibit hybrid vigor, in more detail. In the combination Col-0/C24, heterosis for biomass was enhanced at higher light intensities; we found 51% to 66% mid-parent-heterosis at low and intermediate light intensities (60 and 120 micromol m(-2) s(-1)), and 161% at high light intensity (240 micromol m(-2) s(-1)). While at the low and intermediate light intensities relative growth rates of the hybrids were higher only in the early developmental phase (0-15 d after sowing [DAS]), at high light intensity the hybrids showed increased relative growth rates over the entire vegetative phase (until 25 DAS). An important finding was the early onset of heterosis for biomass; in the cross Col-0/C24, differences between parental and hybrid lines in leaf size and dry shoot mass could be detected as early as 10 DAS. The widespread occurrence of heterosis in the model plant Arabidopsis opens the possibility to investigate the genetic basis of this phenomenon using the tools of genetical genomics.