Cell polarity

Cell polarity refers to spatial differences in the shape, structure, and function of cells. Almost all cell types exhibit some sort of polarity, which enables them to carry out specialized functions. Classical examples of polarized cells are described below, including epithelial cells with apical-basal polarity, neurons in which signals propagate in one direction from dendrites to axons, and migrating cells.

Examples of polarized cells

Epithelial cells

Epithelial cells adhere to one another through tight junctions, desmosomes and adherens junctions, forming sheets of cells that line the surface of the animal body and internal cavities (e.g., digestive tract and circulatory system). These cells have an apical-basal polarity defined by the apical membrane facing the outside surface of the body, or the lumen of internal cavities, and the basolateral membrane oriented away from the lumen. The basolateral membrane refers to both the lateral membrane where cell-cell junctions connect neighboring cells and to the basal membrane where cells are attached to the basement membrane, a thin sheet of extracellular matrix proteins that separates the epithelial sheet from underlying cells and connective tissue. Epithelial cells also exhibit planar cell polarity,in which specialized structures are orientated within the plane of the epithelial sheet. Some examples of planar cell polarity include the scales of fish being oriented in the same direction and similarly the feathers of birds, the fur of mammals, and the cuticular projections (sensory hairs, etc.) on the bodies and appendages of flies and other insects.^[1]

Neurons

A neuron receives signals from neighboring cells through branched, cellular extensions called dendrites. The neuron then propagates an electrical signal down a specialized axon extension to the synapse, where neurotransmitters are released to propagate the signal to another neuron or effector cell (e.g., muscle or gland). The polarity of the neuron thus facilitates the directional flow of information, which is required for communication between neurons and effector cells.^[2]

Migratory cells

Many cell types are capable of migration, such as leukocytes and fibroblasts, and in order for these cells to move in one direction, they must have a defined front and rear. At the front of the cell is the leading edge, which is often defined by a flat ruffling of the cell membrane called the lamellipodium or thin protrusions called filopodia. Here, actin polymerization in the direction of migration allows cells to extend the leading edge of the cell and to attach to the surface.^[3] At the rear of the cell, adhesions are disassembled and bundles of actin microfilaments, called stress fibers, contract and pull the trailing edge forward to keep up with the rest of the cell. Without this front-rear polarity, cells would be unable to coordinate directed migration.^[4]

Vertebrate development

See also Embryogenesis and Embryonic development

The bodies of vertebrate animals are asymmetric along three axes: superior-inferior (head to tail), dorsal-ventral (spine to stomach), and left-right (for example, our heart is on the left side of our body). These polarities arise within the developing embryo through a combination of several processes: 1) asymmetric cell division, in which two daughter cells receive different amounts of cellular material (e.g. mRNA, proteins), 2) asymmetric localization of specific proteins or RNAs within cells (which is often mediated by the cytoskeleton), 3) concentration gradients of secreted proteins across the embryo such as Wnt, Nodal, and Bone Morphogenic Proteins (BMPs), and 4) differential expression of membrane receptors and ligands that cause lateral inhibition, in which the receptor-expressing cell adopts one fate and its neighbors another.^[5][6]

In addition to defining asymmetric axes in the adult organism, cell polarity also regulates both individual and collective cell movements during embryonic development such as apical constriction, invagination, and epiboly. These movements are critical for shaping the embryo and creating the complex structures of the adult body.

Molecular basis

Cell polarity arises primarily through the localization of specific proteins to specific areas of the cell membrane. This localization requires both the recruitment of cytoplasmic proteins to the cell membrane and polarized vesicle transport along cytoskeletal filaments to deliver transmembrane proteins from the golgi apparatus. Many of the molecules responsible for regulating cell polarity are conserved across cell types and throughout metazoan species. Examples include the PAR complex (Cdc42, PAR3/ASIP, PAR6, atypical protein kinase C),^[7][8] Crumbs complex (Crb, PALS, PATJ, Lin7), and Scribble complex (Scrib, Dlg, Lgl).^[9] These polarity complexes are localized at the cytoplasmic side of the cell membrane, asymmetrically within cells. For example, in epithelial cells the PAR and Crumbs complexes are localized along the apical membrane and the Scribble complex along the lateral membrane.^[10] Together with a group of signaling molecules called Rho GTPases, these polarity complexes can regulate vesicle transport and also control the localization of cytoplasmic proteins primarily by regulating the phosphorylation of phospholipids called phosphoinositides. Phosphoinositides serve as docking sites for proteins at the cell membrane, and their state of phosphorylation determines which proteins can bind.^[11]

See also

• Epithelial polarity
• Cell migration
• Embryogenesis
• Embryonic development
• Asymmetric cell division

References

1. Wu, Jun; Mlodzik, Marek A. (29 June 2009). "A quest for the mechanism regulating global planar cell polarity of tissues". Trends in Cell Biology 19 (7): 295–305. doi:10.1016/j.tcb.2009.04.003.
2. Rasband, Matthew N. (August 2010). "The axon initial segment and the maintenance of neuronal polarity". Nature Reviews Neuroscience 11 (8): 552–562. doi:10.1038/nrn2852.
3. Ridley, Anne J.; Schwartz, Martin A.; Burridge, Keith; Firtel, Richard A.; Ginsberg, Mark H.; Borisy, Gary; Parsons, J. Thomas; Horwitz, Alan Rick (5 December 2003). "Cell Migration: Integrating Signals from Front to back". Science 302 (5651): 1704–1709. doi:10.1126/science.1092053. PMID 14657486.
4. Friedl, Peter; Wolf, Katarina (May 2003). "Tumour-cell invasion and migration: diversity and escape mechanisms". Nature Reviews Cancer 3 (5): 362–374. doi:10.1038/nrc1075.
5. Hashimoto, Masakazu; Hamada, Hiroshi (August 2010). "Translation of anterior-posterior polarity into left-right polarity in the mouse embryo". Current Opinion in Genetics & Development 20 (4): 433–437. doi:10.1016/j.gde.2010.04.002.
6. Johnston, Daniel St; Ahringer, Julie (28 May 2010). "Cell Polarity in Eggs and Epithelia: Parallels and Diversity". Cell 141 (5): 757–774. doi:10.1016/j.cell.2010.05.011. PMID 20510924.
7. Izumi Y, Hirose T, Tamai Y, Hirai S, Nagashima Y, Fujimoto T, Tabuse Y, Kemphues KJ and Ohno S. (October 1998). "An Atypical PKC Directly Associates and Colocalizes at the Epithelial Tight Junction with ASIP, a Mammalian Homologue of Caenorhabditis elegans Polarity Protein PAR-3". J Cell Biol 143 (1): 95–106. doi:10.1083/jcb.143.1.95. PMC 2132825. PMID 9763423.
8. Tabuse Y, Izumi Y, Piano F, Kemphues KJ, Miwa J and Ohno S. (September 1998). "Atypical protein kinase C cooperates with PAR-3 to establish embryonic polarity in Caenorhabditis elegans". Development 125 (18): 3607–3614. PMID 9716526.
9. Bryant, David M.; Mostov, Keith E. (November 2008). "From cells to organs: building polarized tissue". Nature Reviews Molecular Cell Biology 9 (11): 887–901. doi:10.1038/nrm2523.
10. Johnston, Daniel St; Ahringer, Julie (28 May 2010). "Cell Polarity in Eggs and Epithelia: Parallels and Diversity". Cell 141 (5): 757–774. doi:10.1016/j.cell.2010.05.011. PMID 20510924.
11. Orlando, Kelly; Guo, Wei (November 2009). "Organization and Dynamics in Cell Polarity". Cold Spring Harbor Perspectives in Biology 1 (5). doi:10.1101/cshperspect.a001321.