Many structural, signaling, and adhesion molecules contain tandemly repeated amino acid motifs. The alpha-actinin/spectrin/dystrophin superfamily of F-actin-crosslinking proteins contains an array of triple alpha-helical motifs (spectrin repeats). The complete sequence of the novel beta-spectrin isoform ßHeavy-spectrin (ßH) is presented in this paper. The sequence of ßH supports the origin of alpha- and ß-spectrins from a common ancestor, and a novel model is presented for the origin of the spectrins from a homodimeric actin-crosslinking precursor. The pattern of similarity between the spectrin repeat units indicates that they have evolved by a series of nested, nonuniform duplications. Furthermore, the spectrins and dystrophins clearly have common ancestry, yet the repeat unit is a different length in each family. Together, these observations suggest a dynamic period of increase in repeat number accompanied by homogenization within each array by concerted evolution. However, today, there is greater similarity of homologous repeats between species than there is across repeats within species, suggesting that concerted evolution ceased some time before the arthropod/vertebrate split. A two-phase model is therefore suggested to describe the evolution of the spectrin repeat arrays in which an initial phase of concerted evolution is subsequently retarded as each new protein becomes constrained to a specific length and the repeats diverge at the DNA level. This evolutionary model has general applicability to the origins of the many other proteins that have tandemly repeated motifs (Thomas, 1997).

Morphogenesis transforms the C. elegans embryo from a ball of cells into a vermiform larva. During this transformation, the embryo increases fourfold in length; present data indicates this elongation results from contraction of the epidermal actin cytoskeleton. The cells of the epidermis originate on the dorsal surface of the embryo and migrate ventrally to enclose the embryo at the start of morphogenesis. Shortly after the epidermis encloses the embryo, the epidermal actin cytoskeleton reorganizes, forming an array of parallel actin fibers, and the embryo begins to elongate. These actin fibers are located just under the apical plasma membrane of the epidermis, oriented around the circumference of the embryo and perpendicular to its anterior-posterior axis. As elongation progresses, the actin fibers decrease in length and there is a corresponding decrease in width and increase in the length of the epidermal cells. Embryonic elongation is dependent on the actin cytoskeleton: treatment with cytochalasin D prevents elongation and causes elongating embryos to retract to almost their original length. Based on these observations, it is believed that contraction of the epidermal actin fibers generates the force that causes the change in epidermal cell shape and constricts the underlying cell layers, resulting in elongation of the embryo. Subsequent studies have lent support to the proposed role of the epidermal actin cytoskeleton in C. elegans morphogenesis. Recent work has shown that the hmp-1, hmp-2 and hmr-1 genes encode C. elegans homologs of alpha-catenin, beta-catenin and cadherin, respectively, and that all three are required to anchor the circumferential actin fibers to the epidermal adherens junctions. hmp-1 and hmp-2 mutant embryos begin elongation, but then retract back to their original length as the actin fibers in the dorsal epidermis detach from the adherens junctions. In hmr-1 mutants, detachment of epidermal actin fibers is also observed, although defects in epidermal enclosure of the embryo prevent elongation. In these mutants, the actin fibers continue to decrease in length after separation from the adherens junctions, supporting the model that during morphogenesis they provide the contractile force required for elongation of the C. elegans embryo. Although there is no direct evidence for the presence of myosin in the epidermal actin fibers, mutations in let-502, a C. elegans member of the family of Rho kinases, block normal elongation; this phenotype is suppressed by mutations in the mel-11 gene, which encodes a homolog of the regulatory subunit of smooth muscle myosin phosphatase. Together, these results point to a central role for the epidermal actin cytoskeleton in C. elegans morphogenesis (McKeown, 1998).

sma-1 encodes a homolog of betaH-spectrin, a novel spectrin isoform identified as the product of the Drosophila karst gene. Spectrin is a heterodimeric actin-binding protein, composed of alpha- and beta-spectrin subunits, that form a cytoskeletal network associated with the plasma membrane. betaH-spectrin contains additional spectrin repeats and an SH3 domain not present in conventional beta-spectrins, and shows non-conservation of an ankyrin-binding sequence present in beta-spectrin. In the cell, beta- and betaH-spectrins form non-overlapping regions of membrane skeleton; in epithelial cells, beta-spectrin is associated with the lateral plasma membrane and betaH-spectrin is found in the apical region of the cell (McKeown. 1998).

In sma-1 mutants, the extent of embryonic elongation is decreased and the resulting sma-1 larvae, although viable, are shorter than normal. sma-1 mutants elongate for the same length of time as wild-type embryos, but at a decreased rate. The sma-1 mutants that have been isolated vary in phenotypic severity, with the most severe alleles showing the greatest decrease in elongation rate. The sma-1 gene encodes a homolog of betaH-spectrin, a novel beta-spectrin isoform first identified in Drosophila. sma-1 RNA is expressed in epithelial tissues in the C. elegans embryo: in the embryonic epidermis at the start of morphogenesis and subsequently in the developing pharynx, intestine and excretory cell. In Drosophila, betaH-spectrin associates with the apical plasma membrane of epithelial cells; beta-spectrin is found at the lateral membrane. It is proposed that SMA-1 is a component of an apical membrane skeleton in the C. elegans embryonic epidermis that determines the rate of elongation during morphogenesis (McKeown, 1998).

The exc mutations of C. elegans alter the position and shape of the apical cytoskeleton in polarized epithelial cells. Mutants in exc-7 form small cysts throughout the tubular excretory canals that regulate organizmal osmolarity. The exc-7 gene, the closest nematode homolog to the neural RNA-binding protein ELAV, has been cloned. EXC-7 is expressed in the canal for a short time midway through embryogenesis. Cysts in exc-7 mutants do not develop until several hours later, beginning at the time of hatching. The first larval period is when the canal completes the majority of its outgrowth, and adds new apical cytoskeleton at a rapid rate. Ultrastructural studies show that exc-7 mutant defects resemble loss of small ßH-spectrin (encoded by sma-1) at the distal ends of the excretory canals. In addition, exc-7 mutants exhibit synergistic excretory canal defects with mutations in sma-1, and EXC-7 binds sma-1 mRNA. These data imply that EXC-7 protein may affect expression of sma-1 and other genes to effect proper development of the excretory canals (Fujita, 2003).

The increased defects in canal structure seen when exc-7 is mutated in a sma-1 null background indicate that the loss of EXC-7 function must affect other genes besides sma-1. Since exc-7 mutations also show similar synergistic effects with exc-3 mutations, EXC-7 may also affect expression of this (as yet uncloned) gene. Since the observed defects of exc-7 mutation are relatively mild, and the gene is expressed in many cells besides the excretory cell, EXC-7 likely affects expression of additional genes, as do the vertebrate ELAVs. Further studies on other genes causing synergistic effects with ELAV homolog mutations may identify other genes needed in large amounts during specific stages of C. elegans development that are regulated by EXC-7 or other of the nine nematode ELAV homologs (Fujita, 2003).

Mechanical control of the sense of touch by beta-spectrin

The ability to sense and respond to mechanical stimuli emanates from sensory neurons and is shared by most, if not all, animals. Exactly how such neurons receive and distribute mechanical signals during touch sensation remains mysterious. This study shows that sensation of mechanical forces depends on a continuous, pre-stressed spectrin cytoskeleton inside neurons. Mutations in the tetramerization domain of Caenorhabditis elegans beta-spectrin (UNC-70), an actin-membrane crosslinker, cause defects in sensory neuron morphology under compressive stress in moving animals. Through atomic force spectroscopy experiments on isolated neurons, in vivo laser axotomy and fluorescence resonance energy transfer imaging to measure force across single cells and molecules, this study shows that spectrin is held under constitutive tension in living animals, which contributes to elevated pre-stress in touch receptor neurons. Genetic manipulations that decrease such spectrin-dependent tension also selectively impair touch sensation, suggesting that such pre-tension is essential for efficient responses to external mechanical stimuli (Krieg, 2014).

Genetic defects in β-spectrin and tau sensitize C. elegans axons to movement-induced damage via torque-tension coupling

Our bodies are in constant motion and so are the neurons that invade each tissue. Motion-induced neuron deformation and damage are associated with several neurodegenerative conditions. This study investigated the question of how the neuronal cytoskeleton protects axons and dendrites from mechanical stress, exploiting mutations in UNC-70 (see Drosophila karst), PTL-1 (see Drosophila tau) and MEC-7 (see Drosophila β-tubulin) proteins in Caenorhabditis elegans. It was found that mechanical stress induces supercoils and plectonemes in the sensory axons of spectrin and tau double mutants. Biophysical measurements, super-resolution, and electron microscopy, as well as numerical simulations of neurons as discrete, elastic rods provide evidence that a balance of torque, tension, and elasticity stabilizes neurons against mechanical deformation. The study concludes that the spectrin and microtubule cytoskeletons work in combination to protect axons and dendrites from mechanical stress and propose that defects in β-spectrin and tau may sensitize neurons to damage (Krieg, 2017).

karst: Biological Overview | Regulation | Developmental Biology | Effects of Mutation | References

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