Symmetry Breaking And Cortical Rotation
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Symmetry breaking in biology is the process by which uniformity is broken, or the number of points to view invariance are reduced, to generate a more structured and improbable state. Symmetry breaking is the event where symmetry along a particular axis is lost to establish a polarity.
Polarity Polarity may refer to: Science *Electrical polarity, direction of electrical current *Polarity (mutual inductance), the relationship between components such as transformer windings *Polarity (projective geometry), in mathematics, a duality of orde ...
is a measure for a biological system to distinguish poles along an axis. This measure is important because it is the first step to building complexity. For example, during organismal development, one of the first steps for the embryo is to distinguish its dorsal-ventral axis. The symmetry-breaking event that occurs here will determine which end of this axis will be the ventral side, and which end will be the dorsal side. Once this distinction is made, then all the structures that are located along this axis can develop at the proper location. As an example, during human development, the embryo needs to establish where is ‘back’ and where is ‘front’ before complex structures, such as the spine and lungs, can develop in the right location (where the lungs are placed ‘in front’ of the spine). This relationship between symmetry breaking and complexity was articulated by P.W. Anderson. He speculated that increasing levels of broken symmetry in many-body systems correlates with increasing complexity and functional specialization. In a biological perspective, the more complex an organism is, the higher number of symmetry-breaking events can be found. The importance of symmetry breaking in biology is also reflected in the fact that it's found at all scales. Symmetry breaking can be found at the macromolecular level, at the subcellular level and even at the tissues and organ level. It's also interesting to note that most asymmetry on a higher scale is a reflection of symmetry breaking on a lower scale. Cells first need to establish a polarity through a symmetry-breaking event before tissues and organs themselves can be polar. For example, one model proposes that left-right body axis asymmetry in vertebrates is determined by asymmetry of
cilia The cilium (: cilia; ; in Medieval Latin and in anatomy, ''cilium'') is a short hair-like membrane protrusion from many types of eukaryotic cell. (Cilia are absent in bacteria and archaea.) The cilium has the shape of a slender threadlike proj ...
rotation during early development, which will produce a constant, unidirectional flow. However, there is also evidence that earlier asymmetries in
serotonin Serotonin (), also known as 5-hydroxytryptamine (5-HT), is a monoamine neurotransmitter with a wide range of functions in both the central nervous system (CNS) and also peripheral tissues. It is involved in mood, cognition, reward, learning, ...
distribution and
ion-channel Ion channels are pore-forming membrane proteins that allow ions to pass through the channel pore. Their functions include establishing a resting membrane potential, shaping action potentials and other electrical signals by gating the flow of i ...
mRNA In molecular biology, messenger ribonucleic acid (mRNA) is a single-stranded molecule of RNA that corresponds to the genetic sequence of a gene, and is read by a ribosome in the process of Protein biosynthesis, synthesizing a protein. mRNA is ...
and
protein Proteins are large biomolecules and macromolecules that comprise one or more long chains of amino acid residue (biochemistry), residues. Proteins perform a vast array of functions within organisms, including Enzyme catalysis, catalysing metab ...
localization occur in
zebrafish The zebrafish (''Danio rerio'') is a species of freshwater ray-finned fish belonging to the family Danionidae of the order Cypriniformes. Native to South Asia, it is a popular aquarium fish, frequently sold under the trade name zebra danio (an ...
,
chicken The chicken (''Gallus gallus domesticus'') is a domesticated subspecies of the red junglefowl (''Gallus gallus''), originally native to Southeast Asia. It was first domesticated around 8,000 years ago and is now one of the most common and w ...
and ''
Xenopus ''Xenopus'' () (Gk., ξενος, ''xenos'' = strange, πους, ''pous'' = foot, commonly known as the clawed frog) is a genus of highly aquatic frogs native to sub-Saharan Africa. Twenty species are currently described with ...
'' development, and similar to observations of intrinsic chirality generated by the cytoskeleton leading to organ and whole organism asymmetries in ''
Arabidopsis ''Arabidopsis'' (rockcress) is a genus in the family Brassicaceae. They are small flowering plants related to cabbage and mustard. This genus is of great interest since it contains thale cress (''Arabidopsis thaliana''), one of the model organ ...
'' this itself seems to be controlled from the macromolecular level by the cytoskeleton. There are several examples of symmetry breaking that are currently being studied. One of the most studied examples is the cortical rotation during ''Xenopus'' development, where this rotation acts as the symmetry-breaking event that determines the dorsal-ventral axis of the developing embryo. This example is discussed in more detail below.
Another example that involves symmetry breaking is the establishment of dendrites and axon during
neuron A neuron (American English), neurone (British English), or nerve cell, is an membrane potential#Cell excitability, excitable cell (biology), cell that fires electric signals called action potentials across a neural network (biology), neural net ...
development, and the PAR protein network in ''
C. elegans ''Caenorhabditis elegans'' () is a free-living transparent nematode about 1 mm in length that lives in temperate soil environments. It is the type species of its genus. The name is a blend of the Greek ''caeno-'' (recent), ''rhabditis'' ( ...
''. It is thought that a protein called shootin-1 determines which outgrowth in neurons eventually becomes the axon, at it does this by breaking symmetry and accumulating in only one outgrowth. The PAR protein network works under similar mechanisms, where the certain PAR proteins, which are initially homogenous throughout the cell, break their symmetry and are segregated to different ends of the zygote to establish a polarity during development.


Cortical rotation

Cortical rotation is a phenomenon that seems to be limited to ''Xenopus'' and few ancient
teleosts Teleostei (; Greek ''teleios'' "complete" + ''osteon'' "bone"), members of which are known as teleosts (), is, by far, the largest group of ray-finned fishes (class Actinopterygii), with 96% of all extant species of fish. The Teleostei, which i ...
, however the underlying mechanisms of cortical rotation have conserved elements that are found in other
chordate A chordate ( ) is a bilaterian animal belonging to the phylum Chordata ( ). All chordates possess, at some point during their larval or adult stages, five distinctive physical characteristics ( synapomorphies) that distinguish them from ot ...
s. A sperm can bind a ''Xenopus'' egg at any position of the pigmented animal hemisphere; however, once bound, this position then determines the dorsal side of the animal. The dorsal side of the
egg An egg is an organic vessel grown by an animal to carry a possibly fertilized egg cell (a zygote) and to incubate from it an embryo within the egg until the embryo has become an animal fetus that can survive on its own, at which point the ...
is always directly opposite the
sperm Sperm (: sperm or sperms) is the male reproductive Cell (biology), cell, or gamete, in anisogamous forms of sexual reproduction (forms in which there is a larger, female reproductive cell and a smaller, male one). Animals produce motile sperm ...
entry point. The sperm's
centriole In cell biology a centriole is a cylindrical organelle composed mainly of a protein called tubulin. Centrioles are found in most eukaryotic cells, but are not present in conifers ( Pinophyta), flowering plants ( angiosperms) and most fungi, an ...
acts as an organizing center for the egg's
microtubule Microtubules are polymers of tubulin that form part of the cytoskeleton and provide structure and shape to eukaryotic cells. Microtubules can be as long as 50 micrometres, as wide as 23 to 27 nanometer, nm and have an inner diameter bet ...
s, which transport the maternal dorsalizing factors, such as ''wnt11'' mRNA, ''wnt5a'' mRNA, and
Dishevelled Dishevelled (Dsh) is a family of proteins involved in canonical and non-canonical Wnt signalling pathways. Dsh (Dvl in mammals) is a cytoplasmic phosphoprotein that acts directly downstream of frizzled receptors. It takes its name from its initi ...
protein.


Molecular mechanisms

A series of experiments utilizing UV irradiation, cold temperature and pressure (all of which cause microtubule depolymerization) demonstrated that without polymerized microtubules, cortical rotation did not occur and resulted in a mutant ventral phenotype. Another study also revealed that mutant phenotype could be rescued (returned to normal) by physically turning the embryo, thus mimicking cortical rotation and demonstrating that microtubules were not the determinant of dorsal development. From this it was hypothesized that there were other elements within the embryo being moved during cortical rotation. To identify these elements, researchers looked for mRNA and protein that demonstrated localization to either the vegetal pole or the dorsal side of the embryo to find candidates. The early candidates for the determinant were β-catenin and disheveled (Dsh). When maternal β-catenin mRNA was degraded in the oocyte, the resulting embryo developed into mutant ventral phenotype and this could be rescued by injecting the fertilized egg with β-catenin mRNA. β-catenin is observed to be enriched in the dorsal side of the embryo following cortical rotation. The Dsh protein was fused to a GFP and tracked during cortical rotation, it was observed to be in vesicles that were couriered along microtubules to the dorsal side. This led researchers to look into other candidates of the Wnt pathway. ''Wnt 11'' was found to be located specifically at the vegetal pole prior to cortical rotation and is moved to the dorsal side where it activates the
wnt signaling pathway In cellular biology, the Wnt signaling pathways are a group of signal transduction pathways which begin with proteins that pass signals into a cell through cell surface receptors. The name Wnt, pronounced "wint", is a portmanteau created from the ...
. VegT, a T-box transcription factor, is localized to the vegetal cortex and upon cortical rotation is released in a gradient fashion into the embryo to regulate
mesoderm The mesoderm is the middle layer of the three germ layers that develops during gastrulation in the very early development of the embryo of most animals. The outer layer is the ectoderm, and the inner layer is the endoderm.Langman's Medical ...
development. VegT activates Wnt expression, so while not acted on or moved during cortical rotation, it is active in dorsal-ventral axis formation. The question still remains, how are these molecules being moved to the dorsal side? This is still not completely known, however evidence suggests that microtubule bundles within the cortex are interacting with kinesin (plus-end directed) motors to become organized into parallel arrays within the cortex and this motion of the motors is the cause of the rotation of the cortex. Also unclear is whether Wnt 11 is the main dorsal determinant or is β-catenin also required, as these two molecules have both been demonstrated to be necessary and sufficient for dorsal development. This along with all of the other factors are important for activating Nodal genes that propagate normal dorsoventral development.


References

{{Reflist Developmental biology Cell biology