Caenorhabditis elegans beta-G spectrin is dispensable for establishment of epithelial polarity, but essential for muscular and neuronal function

Epithelial UNC-23 limits mechanical stress to maintain glia-neuron architecture in C. elegans

For an organ to maintain correct architecture and function, its diverse cellular components must coordinate their size and shape. Although cell-intrinsic mechanisms driving homotypic cell-cell coordination are known, it is unclear how cell shape is regulated across heterotypic cells. We find that epithelial cells maintain the shape of neighboring sense-organ glia-neuron units in adult Caenorhabditis elegans (C. elegans). Hsp co-chaperone UNC-23/BAG2 prevents epithelial cell shape from deforming, and its loss causes head epithelia to stretch aberrantly during animal movement. In the sense-organ glia, amphid sheath (AMsh), this causes progressive fibroblast growth factor receptor (FGFR)-dependent disruption of the glial apical cytoskeleton. Resultant glial cell shape alteration causes concomitant shape change in glia-associated neuron endings. Epithelial UNC-23 maintenance of glia-neuron shape is specific both spatially, within a defined anatomical zone, and temporally, in a developmentally critical period. As all molecular components uncovered are broadly conserved across central and peripheral nervous systems, we posit that epithelia may similarly regulate glia-neuron architecture cross-species.

Comparative Analysis of Muscle Atrophy During Spaceflight, Nutritional Deficiency and Disuse in the Nematode Caenorhabditis elegans

While spaceflight is becoming more common than before, the hazards spaceflight and space microgravity pose to the human body remain relatively unexplored. Astronauts experience muscle atrophy after spaceflight, but the exact reasons for this and solutions are unknown. Here, we take advantage of the nematode C. elegans to understand the effects of space microgravity on worm body wall muscle. We found that space microgravity induces muscle atrophy in C. elegans from two independent spaceflight missions. As a comparison to spaceflight-induced muscle atrophy, we assessed the effects of acute nutritional deprivation and muscle disuse on C. elegans muscle cells. We found that these two factors also induce muscle atrophy in the nematode. Finally, we identified clp-4, which encodes a calpain protease that promotes muscle atrophy. Mutants of clp-4 suppress starvation-induced muscle atrophy. Such comparative analyses of different factors causing muscle atrophy in C. elegans could provide a way to identify novel genetic factors regulating space microgravity-induced muscle atrophy.

Apical-basal polarity of the spectrin cytoskeleton in the C. elegans vulva

The C. elegans vulva is a polarized epithelial tube that has been studied extensively as a model for cell-cell signaling, cell fate specification, and tubulogenesis. Here we used endogenous fusions to show that the spectrin cytoskeleton is polarized in this organ, with conventional beta-spectrin ( UNC-70 ) found only at basolateral membranes and beta heavy spectrin ( SMA-1 ) found only at apical membranes. The sole alpha-spectrin ( SPC-1 ) is present at both locations but requires SMA-1 for its apical localization. Thus, beta spectrins are excellent markers for vulva cell membranes and polarity.

Asymmetric organelle positioning during epithelial polarization of C. elegans intestinal cells

While the epithelial cell cortex displays profound asymmetries in protein distribution and morphology along the apico-basal axis, the extent to which the cytoplasm is similarly polarized within epithelial cells remains relatively unexplored. We show that cytoplasmic organelles within C. elegans embryonic intestinal cells develop extensive apico-basal polarity at the time they establish cortical asymmetry. Nuclei and conventional endosomes, including early endosomes, late endosomes, and lysosomes, become polarized apically. Lysosome-related gut granules, yolk platelets, and lipid droplets become basally enriched. Removal of par-3 activity does not disrupt organelle positioning, indicating that cytoplasmic apico-basal asymmetry is independent of the PAR polarity pathway. Blocking the apical migration of nuclei leads to the apical positioning of gut granules and yolk platelets, whereas the asymmetric localization of conventional endosomes and lipid droplets is unaltered. This suggests that nuclear positioning organizes some, but not all, cytoplasmic asymmetries in this cell type. We show that gut granules become apically enriched when WHT-2 and WHT-7 function is disrupted, identifying a novel role for ABCG transporters in gut granule positioning during epithelial polarization. Analysis of WHT-2 and WHT-7 ATPase mutants is consistent with a WHT-2/WHT-7 heterodimer acting as a transporter in gut granule positioning. In wht-2(-) mutants, the polarized distribution of other organelles is not altered and gut granules do not take on characteristics of conventional endosomes that could have explained their apical mispositioning. During epithelial polarization wht-2(-) gut granules exhibit a loss of the Rab32/38 family member GLO-1 and ectopic expression of GLO-1 is sufficient to rescue the basal positioning of wht-2(-) and wht-7(-) gut granules. Furthermore, depletion of GLO-1 causes the mislocalization of the endolysosomal RAB-7 to gut granules and RAB-7 drives the apical mispositioning of gut granules when GLO-1, WHT-2, or WHT-7 function is disrupted. We suggest that ABC transporters residing on gut granules can regulate Rab dynamics to control organelle positioning during epithelial polarization.

Plastin and spectrin cooperate to stabilize the actomyosin cortex during cytokinesis

Cytokinesis, the process that partitions the mother cell into two daughter cells, requires the assembly and constriction of an equatorial actomyosin network. Different types of non-motor F-actin crosslinkers localize to the network, but their functional contribution remains poorly understood. Here, we describe a synergy between the small rigid crosslinker plastin and the large flexible crosslinker spectrin in the C. elegans one-cell embryo. In contrast to single inhibitions, co-inhibition of plastin and the βH-spectrin (SMA-1) results in cytokinesis failure due to progressive disorganization and eventual collapse of the equatorial actomyosin network. Cortical localization dynamics of non-muscle myosin II in co-inhibited embryos mimic those observed after drug-induced F-actin depolymerization, suggesting that the combined action of plastin and spectrin stabilizes F-actin in the contractile ring. An in silico model predicts that spectrin is more efficient than plastin at stabilizing the ring and that ring formation is relatively insensitive to βH-spectrin length, which is confirmed in vivo with a sma-1 mutant that lacks 11 of its 29 spectrin repeats. Our findings provide the first evidence that spectrin contributes to cytokinesis and highlight the importance of crosslinker interplay for actomyosin network integrity.

Repulsive guidance molecule acts in axon branching in Caenorhabditis elegans

Repulsive guidance molecules (RGMs) are evolutionarily conserved proteins implicated in repulsive axon guidance. Here we report the function of the Caenorhabditis elegans ortholog DRAG-1 in axon branching. The axons of hermaphrodite-specific neurons (HSNs) extend dorsal branches at the region abutting the vulval muscles. The drag-1 mutants exhibited defects in HSN axon branching in addition to a small body size phenotype. DRAG-1 expression in the hypodermal cells was required for the branching of the axons. Although DRAG-1 is normally expressed in the ventral hypodermis excepting the vulval region, its ectopic expression in vulval precursor cells was sufficient to induce the branching. The C-terminal glycosylphosphatidylinositol anchor of DRAG-1 was important for its function, suggesting that DRAG-1 should be anchored to the cell surface. Genetic analyses suggested that the membrane receptor UNC-40 acts in the same pathway with DRAG-1 in HSN branching. We propose that DRAG-1 expressed in the ventral hypodermis signals via the UNC-40 receptor expressed in HSNs to elicit branching activity of HSN axons.

An asymmetric mechanical code ciphers curvature-dependent proprioceptor activity

A repetitive gait cycle is an archetypical component within the behavioral repertoire of many animals including humans. It originates from mechanical feedback within proprioceptors to adjust the motor program during locomotion and thus leads to a periodic orbit in a low-dimensional space. Here, we investigate the mechanics, molecules, and neurons responsible for proprioception in Caenorhabditis elegans to gain insight into how mechanosensation shapes the orbital trajectory to a well-defined limit cycle. We used genome editing, force spectroscopy, and multiscale modeling and found that alternating tension and compression with the spectrin network of a single proprioceptor encodes body posture and informs TRP-4/NOMPC and TWK-16/TREK2 homologs of mechanosensitive ion channels during locomotion. In contrast to a widely accepted model of proprioceptive “stretch” reception, we found that proprioceptors activated locally under compressive stresses in-vivo and in-vitro and propose that this property leads to compartmentalized activity within long axons delimited by curvature-dependent mechanical stresses.

Muscle and epidermal contributions of the structural protein β-spectrin promote hypergravity-induced motor neuron axon defects in C. elegans

Biology is adapted to Earth's gravity force, and the long-term effects of varying gravity on the development of animals is unclear. Previously, we reported that high gravity, called hypergravity, increases defects in the development of motor neuron axons in the nematode Caenorhabditis elegans. Here, we show that a mutation in the unc-70 gene that encodes the cytoskeletal β-spectrin protein suppresses hypergravity-induced axon defects. UNC-70 expression is required in both muscle and epidermis to promote the axon defects in high gravity. We reveal that the location of axon defects is correlated to the size of the muscle cell that the axon traverses. We also show that mutations that compromise key proteins of hemidesmosomal structures suppress hypergravity-induced axon defects. These hemidesmosomal structures play a crucial role in coupling mechanical force between the muscle, epidermis and the external cuticle. We speculate a model in which the rigid organization of muscle, epidermal and cuticular layers under high gravity pressure compresses the narrow axon migration pathways in the extracellular matrix hindering proper axon pathfinding of motor neurons.

Deficiency of αII-spectrin affects endothelial cell-matrix contact and migration leading to impairment of angiogenesis in vitro

Background

Precise coordination of cytoskeletal components and dynamic control of cell adhesion and migration are required for crucial cell processes such as differentiation and morphogenesis. We investigated the potential involvement of αII-spectrin, a ubiquitous scaffolding element of the membrane skeleton, in the adhesion and angiogenesis mechanism.

Methods

The cell models were primary human umbilical vein endothelial cells (HUVECs) and a human dermal microvascular endothelial cell line (HMEC-1). After siRNA- and shRNA-mediated knockdown of αII-spectrin, we assessed its expression and that of its partners and adhesion proteins using western blotting. The phenotypes of the control and spectrin-depleted cells were examined using immunofluorescence and video microscopy. Capillary tube formation was assessed using the thick gel Matrigel matrix-based method and a microscope equipped with a thermostatic chamber and a Nikon Biostation System camera.

Results

Knockdown of αII-spectrin leads to: modified cell shape; actin cytoskeleton organization with the presence of peripheral actin patches; and decreased formation of stress fibers. Spectrin deficiency affects cell adhesion on laminin and fibronectin and cell motility. This included modification of the localization of adhesion molecules, such as αVβ3- and α5-integrins, and organization of adhesion structures, such as focal points. Deficiency of αII-spectrin can also affect the complex mechanism of in vitro capillary tube formation, as demonstrated in a model of angiogenesis. Live imaging revealed that impairment of capillary tube assembly was mainly associated with a significant decrease in cell projection length and stability. αII-spectrin depletion is also associated with significantly decreased expression of three proteins involved in capillary tube formation and assembly: VE-cadherin, MCAM and β3-integrin.

Conclusion

Our data confirm the role of αII-spectrin in the control of cell adhesion and spreading. Moreover, our findings further support the participation of αII-spectrin in capillary tube formation in vitro through control of adhesion molecules, such as integrins. This indicates a new function of αII-spectrin in angiogenesis.

Axonal Spectrins: Nanoscale Organization, Functional Domains and Spectrinopathies

Spectrin cytoskeletons are found in all metazoan cells, and their physical interactions between actin and ankyrins establish a meshwork that provides cellular structural integrity. With advanced super-resolution microscopy, the intricate spatial organization and associated functional properties of these cytoskeletons can now be analyzed with unprecedented clarity. Long neuronal processes like peripheral sensory and motor axons may be subject to intense mechanical forces including bending, stretching, and torsion. The spectrin-based cytoskeleton is essential to protect axons against these mechanical stresses. Additionally, spectrins are critical for the assembly and maintenance of axonal excitable domains including the axon initial segment and the nodes of Ranvier (NoR). These sites facilitate rapid and efficient action potential initiation and propagation in the nervous system. Recent studies revealed that pathogenic spectrin variants and diseases that protealyze and breakdown spectrins are associated with congenital neurological disorders and nervous system injury. Here, we review recent studies of spectrins in the nervous system and focus on their functions in axonal health and disease.

Spectrin regulates cell contractility through production and maintenance of actin bundles in the Caenorhabditis elegans spermatheca

Disruption to the contractility of cells, including smooth muscle cells of the cardiovascular system and myoepithelial cells of the glandular epithelium, contributes to the pathophysiology of contractile tissue diseases, including asthma, hypertension, and primary Sjögren's syndrome. Cell contractility is determined by myosin activity and actomyosin network organization and is mediated by hundreds of protein-protein interactions, many directly involving actin. Here we use a candidate RNA interference screen of more than 100 Caenorhabditis elegans genes with predicted actin-binding and regulatory domains to identify genes that contribute to the contractility of the somatic gonad. We identify the spectrin cytoskeleton composed of SPC-1/α-spectrin, UNC-70/β-spectrin, and SMA-1/β heavy-spectrin as required for contractility and actin organization in the myoepithelial cells of the C. elegans spermatheca. We use imaging of fixed and live animals as well as tissue- and developmental-stage-specific disruption of the spectrin cytoskeleton to show that spectrin regulates the production of prominent central actin bundles and is required for maintenance of central actin bundles throughout successive rounds of stretch and contraction. We conclude that the spectrin cytoskeleton contributes to spermathecal contractility by promoting maintenance of the robust actomyosin bundles that drive contraction.

Genetic Analysis of Nuclear Migration and Anchorage to Study LINC Complexes During Development of Caenorhabditis elegans

Studying nuclear positioning in developing tissues of the model nematode Caenorhabditis elegans greatly contributed to the discovery of SUN and KASH proteins and the formation of the LINC model. Such studies continue to make important contributions into both how LINC complexes are regulated and how defects in LINC components disrupt normal development. The methods described explain how to observe and quantify the following: nuclear migration in embryonic dorsal hypodermal cells, nuclear migration through constricted spaces in larval P cells, nuclear positioning in the embryonic intestinal primordia, and nuclear anchorage in syncytial hypodermal cells. These methods will allow others to employ nuclear positioning in C. elegans as a model to further explore LINC complex regulation and function.

Regulation of actin-Spectrin cytoskeleton by ICA69 at the Drosophila neuromuscular junction

Bin-Amphiphysin-Rvs (BAR) domain containing proteins with their membrane deforming properties have emerged as key players in shaping up neuronal morphology and regulating cytoskeletal dynamics. However, the in vivo contexts in which BAR-domain proteins integrate membrane dynamics with cytoskeletal rearrangements remain poorly understood. Recently, we identified islet cell autoantigen 69 kDa as one of the N-BAR-domain containing proteins which regulate synaptic development and organization at the Drosophila neuromuscular junction. ICA69 genetically functions downstream of Rab2 to regulate synapse morphology. We found that ICA69 alters Spectrin level at the Drosophila NMJ, and redistributes actin regulatory proteins in cultured cells suggesting that ICA69 may regulate NMJ organization by regulating actin-Spectrin cytoskeleton. We propose a model in which ICA69 genetically interact with components of actin regulatory proteins for cytoskeleton dynamics to regulate NMJ development and synapse organization.

The interplay of stiffness and force anisotropies drives embryo elongation

The morphogenesis of tissues, like the deformation of an object, results from the interplay between their material properties and the mechanical forces exerted on them. The importance of mechanical forces in influencing cell behaviour is widely recognized, whereas the importance of tissue material properties, in particular stiffness, has received much less attention. Using Caenorhabditis elegans as a model, we examine how both aspects contribute to embryonic elongation. Measuring the opening shape of the epidermal actin cortex after laser nano-ablation, we assess the spatiotemporal changes of actomyosin-dependent force and stiffness along the antero-posterior and dorso-ventral axis. Experimental data and analytical modelling show that myosin-II-dependent force anisotropy within the lateral epidermis, and stiffness anisotropy within the fiber-reinforced dorso-ventral epidermis are critical in driving embryonic elongation. Together, our results establish a quantitative link between cortical tension, material properties and morphogenesis of an entire embryo.

DOI:http://dx.doi.org/10.7554/eLife.23866.001

Animals come in all shapes and size, from ants to elephants. In all cases, the tissues and organs in the animal’s body acquire their shape as the animal develops. Cells in developing tissues squeeze themselves or push and pull on one another, and the resulting forces generate the final shape. This process is called morphogenesis and it is often studied in a worm called Caenorhabditis elegans. This worm’s simplicity makes it easy to work with in the laboratory. Yet processes that occur in C. elegans also take place in other animals, including humans, and so the discoveries made using this worm can have far-reaching implications.

As they develop, the embryos of C. elegans transform from a bean-shaped cluster of cells into the characteristic long shape of a worm, with the head at one end and the tail at the other. The force required to power this elongation is provided by the outer layer of cells of the embryo, known as the epidermis. In these cells, motor-like proteins called myosins pull against a mesh-like scaffold within the cell called the actin cytoskeleton; this pulling is thought to squeeze the embryo all around and cause it to grow longer.

Six strips of cells, running from the head to the tail, make up the epidermis of a C. elegans embryo. Myosin is mostly active in two strips of cells that run along the two sides of the embryo. In the strips above and below these strips (in other words, those on the upper and lower sides of the worm), the myosins are much less active. However, it is not fully understood how this distribution of myosin causes worms to elongate only along the head-to-tail axis.

Vuong-Brender et al. have now mapped the forces exerted in the cells of the worm’s epidermis. The experiments show that, in the strips of cells on the sides of the embryo, myosin’s activity causes the epidermis to constrict around the embryo, akin to a boa constrictor tightening around its prey. At the same time, the actin filaments in the other strips form rigid bundles oriented along the circumference that stiffen the cells in these strips. This prevents the constriction from causing the embryo to inflate at the top and bottom strips. As such, the only direction the embryo can expand is along the axis that runs from its head to its tail.

Together, these findings suggest that a combination of oriented force and stiffness ensure that the embryo only elongates along the head-to-tail axis. The next step is to understand how this orientation and the coordination between cells are controlled at the molecular level.

DOI:http://dx.doi.org/10.7554/eLife.23866.002

Proteomic identification of non-erythrocytic alpha-spectrin-1 down-regulation in the pre-optic area of neonatally estradiol-17β treated female adult rats

It is well established that sexually dimorphic brain regions, which are critical for reproductive physiology and behavior, are organized by steroid hormones during the first 2 weeks after birth in the rodents. In our recent observation, neonatal exposure to estradiol-17β (E2) in the female rat revealed increase in cyclooxygenase 2 (COX-2) level, sexually dimorphic nucleus (SDN)-pre-optic area (POA) size and down-regulation of synaptogenesis related genes in POA in the adult stage. In the present study, using the same animal model, the protein profile of control and neonatally E2-treated POA was compared by 1D-SDS-PAGE, and the protein that shows a change in abundance was identified by LC-MS/MS analysis. Results indicated that there was a single protein band, which was down-regulation in E2-treated POA and it was identified as spectrin alpha chain, non-erythrocytic 1 (SPTAN1). Consistently, the down-regulation of SPTAN1 expression was also confirmed by reverse transcription polymerase chain reaction (RT-PCR) and Western blot analysis. The SPTAN1 was identified as a cytoskeletal protein that is involved in stabilization of the plasma membrane and organizes intracellular organelles, and it has been implicated in cellular functions including DNA repair and cell cycle regulation. The evidence shows that any mutation in spectrins causes impairment of synaptogenesis and other neurological disorders. Also, protein-protein interaction analysis of SPTAN1 revealed a strong association with proteins such as kirrel, actinin, alpha 4 (ACTN4) and vinculin (VCL) which are implicated in sexual behavior, masculinization and defeminization. Our results indicate that SPTAN1 expression in the developing rat brain is sexually dimorphic, and we suggest that this gene may mediate E2-17β-induced masculinization and defeminization, and disrupted reproductive function in the adult stage.

A Caenorhabditis elegans Homologue of LYST Functions in Endosome and Lysosome-Related Organelle Biogenesis

LYST-1 is a Caenorhabditis elegans BEACH domain containing protein (BDCP) homologous to LYST and NBEAL2, BDCPs controlling organelle biogenesis that are implicated in human disease. Unlike the three other BDCPs encoded by C. elegans, mutations in lyst-1 lead to smaller lysosome-related organelles (LROs), smaller lysosomes, increased numbers of LROs and decreased numbers of early endosomes. lyst-1(-) mutations do not obviously disrupt protein trafficking to lysosomes or LROs, however, the formation of gut granules is diminished.

FBN-1, a fibrillin-related protein, is required for resistance of the epidermis to mechanical deformation during C. elegans embryogenesis

During development, biomechanical forces contour the body and provide shape to internal organs. Using genetic and molecular approaches in combination with a FRET-based tension sensor, we characterized a pulling force exerted by the elongating pharynx (foregut) on the anterior epidermis during C. elegans embryogenesis. Resistance of the epidermis to this force and to actomyosin-based circumferential constricting forces is mediated by FBN-1, a ZP domain protein related to vertebrate fibrillins. fbn-1 was required specifically within the epidermis and FBN-1 was expressed in epidermal cells and secreted to the apical surface as a putative component of the embryonic sheath. Tiling array studies indicated that fbn-1 mRNA processing requires the conserved alternative splicing factor MEC-8/RBPMS. The conserved SYM-3/FAM102A and SYM-4/WDR44 proteins, which are linked to protein trafficking, function as additional components of this network. Our studies demonstrate the importance of the apical extracellular matrix in preventing mechanical deformation of the epidermis during development.

DOI:http://dx.doi.org/10.7554/eLife.06565.001

For an animal embryo to develop, its cells must organize themselves into tissues and organs. For example, skin and the lining of internal organs—such as the lungs and gut—are made from cells called epithelial cells, which are tightly linked to form flat sheets.

In a microscopic worm called Caenorhabditis elegans, the outermost layer of epithelial cells (called the epidermis) forms over the surface of the embryo early on in embryonic development. Shortly afterwards, the embryonic epidermis experiences powerful contractions along the surface of the embryo. The force generated by these contractions converts the embryo from an oval shape to a roughly cylindrical form. These contractions also squeeze the internal tissues and organs, which correspondingly elongate along with the epidermis.

It has been known for decades that such ‘mechanical’ forces are important for the normal development of embryos. However, it remains poorly understood how these forces generate tissues and organs of the proper shape—partly because it is difficult to measure forces in living embryos. It is also not clear how the mechanical properties of specific tissues are controlled.

Now, Kelley, Yochem, Krieg et al. have analyzed the development of C. elegans' embryos and discovered a novel mechanical interplay between the feeding organ (called the pharynx) and the worm's epidermis. The experiments involved studying several mutant worms that perturb epidermal contractions and disrupt the attachment of the pharynx to the epidermis. These studies suggested that the pharynx exerts a strong inward pulling force on the epidermis during development. Using recently developed methods, Kelley, Yochem, Krieg et al. then measured mechanical forces within intact worm embryos and demonstrated that greater forces were experienced in cells that were being pulled by the pharynx.

Kelley, Yochem, Krieg et al. further analyzed how the epidermis normally resists this pulling force from the pharynx and implicated a protein called FBN-1. This worm protein is structurally related to a human protein that is affected in people with a disorder called Marfan Syndrome. Worm embryos without the FBN-1 protein become severely deformed because they are unable to withstand mechanical forces at the epidermis. FBN-1 is normally synthesized and then transported to the outside of the worm embryo by epidermal cells, where it is thought to assemble into a meshwork of long fibers. This provides a strong scaffold that attaches to the epidermis to prevent the epidermis from undergoing excessive deformation while it experiences mechanical forces.

The work of Kelley, Yochem, Krieg et al. provides an opportunity to understand how FBN-1 and other fiber-forming proteins are produced and transported to the cell surface. Moreover, these findings may have implications for human diseases and birth defects that result from an inability of tissues to respond appropriately to mechanical forces.

DOI:http://dx.doi.org/10.7554/eLife.06565.002

β-Spectrin regulates the hippo signaling pathway and modulates the basal actin network

Emerging evidence suggests functional regulation of the Hippo pathway by the actin cytoskeleton, although the detailed molecular mechanism remains incomplete. In a genetic screen, we identified a requirement for β-Spectrin in the posterior follicle cells for the oocyte repolarization process during Drosophila mid-oogenesis. β-spectrin mutations lead to loss of Hippo signaling activity in the follicle cells. A similar reduction of Hippo signaling activity was observed after β-Spectrin knockdown in mammalian cells. We further demonstrated that β-spectrin mutations disrupt the basal actin network in follicle cells. The abnormal stress fiber-like actin structure on the basal side of follicle cells provides a likely link between the β-spectrin mutations and the loss of the Hippo signaling activity phenotype.

Mechanical control of the sense of touch by β-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. Here, we show that sensation of mechanical forces depends on a continuous, pre-stressed spectrin cytoskeleton inside neurons. Mutations in the tetramerization domain of Caenorhabditis elegans β-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, we show 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.

UNC-40/DCC, SAX-3/Robo, and VAB-1/Eph polarize F-actin during embryonic morphogenesis by regulating the WAVE/SCAR actin nucleation complex

Many cells in a developing embryo, including neurons and their axons and growth cones, must integrate multiple guidance cues to undergo directed growth and migration. The UNC-6/netrin, SLT-1/slit, and VAB-2/Ephrin guidance cues, and their receptors, UNC-40/DCC, SAX-3/Robo, and VAB-1/Eph, are known to be major regulators of cellular growth and migration. One important area of research is identifying the molecules that interpret this guidance information downstream of the guidance receptors to reorganize the actin cytoskeleton. However, how guidance cues regulate the actin cytoskeleton is not well understood. We report here that UNC-40/DCC, SAX-3/Robo, and VAB-1/Eph differentially regulate the abundance and subcellular localization of the WAVE/SCAR actin nucleation complex and its activator, Rac1/CED-10, in the Caenorhabditis elegans embryonic epidermis. Loss of any of these three pathways results in embryos that fail embryonic morphogenesis. Similar defects in epidermal enclosure have been observed when CED-10/Rac1 or the WAVE/SCAR actin nucleation complex are missing during embryonic development in C. elegans. Genetic and molecular experiments demonstrate that in fact, these three axonal guidance proteins differentially regulate the levels and membrane enrichment of the WAVE/SCAR complex and its activator, Rac1/CED-10, in the epidermis. Live imaging of filamentous actin (F-actin) in embryos developing in the absence of individual guidance receptors shows that high levels of F-actin are not essential for polarized cell migrations, but that properly polarized distribution of F-actin is essential. These results suggest that proper membrane recruitment and activation of CED-10/Rac1 and of WAVE/SCAR by signals at the plasma membrane result in polarized F-actin that permits directed movements and suggest how multiple guidance cues can result in distinct changes in actin nucleation during morphogenesis.

In silico prediction and analysis of Caenorhabditis EF-hand containing proteins

Calcium (Ca⁺²) is a ubiquitous messenger in eukaryotes including Caenorhabditis. Ca⁺²-mediated signalling processes are usually carried out through well characterized proteins like calmodulin (CaM) and other Ca⁺² binding proteins (CaBP). These proteins interact with different targets and activate it by bringing conformational changes. Majority of the EF-hand proteins in Caenorhabditis contain Ca⁺² binding motifs. Here, we have performed homology modelling of CaM-like proteins using the crystal structure of Drosophila melanogaster CaM as a template. Molecular docking was applied to explore the binding mechanism of CaM-like proteins and IQ1 motif which is a ∼25 residues and conform to the consensus sequence (I, L, V)QXXXRXXXX(R,K) to serve as a binding site for different EF hand proteins. We made an attempt to identify all the EF-hand (a helix-loop-helix structure characterized by a 12 residues loop sequence involved in metal coordination) containing proteins and their Ca⁺² binding affinity in Caenorhabditis by analysing the complete genome sequence. Docking studies revealed that F165, F169, L29, E33, F44, L57, M61, M96, M97, M108, G65, V115, F93, N104, E144 of CaM-like protein is involved in the interaction with IQ1 motif. A maximum of 170 EF-hand proteins and 39 non-EF-hand proteins with Ca⁺²/metal binding motif were identified. Diverse proteins including enzyme, transcription, translation and large number of unknown proteins have one or more putative EF-hands. Phylogenetic analysis revealed seven major classes/groups that contain some families of proteins. Various domains that we identified in the EF-hand proteins (uncharacterized) would help in elucidating their functions. It is the first report of its kind where calcium binding loop sequences of EF-hand proteins were analyzed to decipher their calcium affinities. Variation in Ca⁺²-binding affinity of EF-hand CaBP could be further used to study the behaviour of these proteins. Our analyses postulated that Ca⁺² is likely to be key player in Caenorhabditis cell signalling.

β-III spectrin is critical for development of purkinje cell dendritic tree and spine morphogenesis

Mutations in the gene encoding β-III spectrin give rise to spinocerebellar ataxia type 5, a neurodegenerative disease characterized by progressive thinning of the molecular layer, loss of Purkinje cells and increasing motor deficits. A mouse lacking full-length β-III spectrin (β-III⁻/⁻) displays a similar phenotype. In vitro and in vivo analyses of Purkinje cells lacking β-III spectrin, reveal a critical role for β-III spectrin in Purkinje cell morphological development. Disruption of the normally well ordered dendritic arborization occurs in Purkinje cells from β-III⁻/⁻ mice, specifically showing a loss of monoplanar organization, smaller average dendritic diameter and reduced densities of Purkinje cell spines and synapses. Early morphological defects appear to affect distribution of dendritic, but not axonal, proteins. This study confirms that thinning of the molecular layer associated with disease pathogenesis is a consequence of Purkinje cell dendritic degeneration, as Purkinje cells from 8-month-old β-III⁻/⁻ mice have drastically reduced dendritic volumes, surface areas and total dendritic lengths compared with 5- to 6-week-old β-III⁻/⁻ mice. These findings highlight a critical role of β-III spectrin in dendritic biology and are consistent with an early developmental defect in β-III⁻/⁻ mice, with abnormal Purkinje cell dendritic morphology potentially underlying disease pathogenesis.

Cell organization, growth, and neural and cardiac development require αII-spectrin

Spectrin α2 (αII-spectrin) is a scaffolding protein encoded by the Spna2 gene and constitutively expressed in most tissues. Exon trapping of Spna2 in C57BL/6 mice allowed targeted disruption of αII-spectrin. Heterozygous animals displayed no phenotype by 2 years of age. Homozygous deletion of Spna2 was embryonic lethal at embryonic day 12.5 to 16.5 with retarded intrauterine growth, and craniofacial, neural tube and cardiac anomalies. The loss of αII-spectrin did not alter the levels of αI- or βI-spectrin, or the transcriptional levels of any β-spectrin or any ankyrin, but secondarily reduced by about 80% the steady state protein levels of βII- and βIII-spectrin. Residual βII- and βIII-spectrin and ankyrins B and G were concentrated at the apical membrane of bronchial and renal epithelial cells, without impacting cell morphology. Neuroepithelial cells in the developing brain were more concentrated and more proliferative in the ventricular zone than normal; axon formation was also impaired. Embryonic fibroblasts cultured on fibronectin from E14.5 (Spna2(-/-)) animals displayed impaired growth and spreading, a spiky morphology, and sparse lamellipodia without cortical actin. These data indicate that the spectrin-ankyrin scaffold is crucial in vertebrates for cell spreading, tissue patterning and organ development, particularly in the developing brain and heart, but is not required for cell viability.

Disruption of spectrin-like cytoskeleton in differentiating keratinocytes by PKCδ activation is associated with phosphorylated adducin

Spectrin is a central component of the cytoskeletal protein network in a variety of erythroid and non-erythroid cells. In keratinocytes, this protein has been shown to be pericytoplasmic and plasma membrane associated, but its characteristics and function have not been established in these cells. Here we demonstrate that spectrin increases dramatically in amount and is assembled into the cytoskeleton during differentiation in mouse and human keratinocytes. The spectrin-like cytoskeleton was predominantly organized in the granular and cornified layers of the epidermis and disrupted by actin filament inhibitors, but not by anti-mitotic drugs. When the cytoskeleton was disrupted PKCδ was activated by phosphorylation on Thr505. Specific inhibition of PKCδ^(Thr505) activation with rottlerin prevented disruption of the spectrin-like cytoskeleton and the associated morphological changes that accompany differentiation. Rottlerin also inhibited specific phosphorylation of the PKCδ substrate adducin, a cytoskeletal protein. Furthermore, knock-down of endogenous adducin affected not only expression of adducin, but also spectrin and PKCδ, and severely disrupted organization of the spectrin-like cytoskeleton and cytoskeletal distribution of both adducin and PKCδ. These results demonstrate that organization of a spectrin-like cytoskeleton is associated with keratinocytes differentiation, and disruption of this cytoskeleton is mediated by either PKCδ^(Thr505) phosphorylation associated with phosphorylated adducin or due to reduction of endogenous adducin, which normally connects and stabilizes the spectrin-actin complex.

Neural integrity is maintained by dystrophin in C. elegans

The dystrophin protein complex, an important regulator of muscle membrane integrity, also maintains neural organization through interactions with the L1CAM family member SAX-7.

The dystrophin protein complex (DPC), composed of dystrophin and associated proteins, is essential for maintaining muscle membrane integrity. The link between mutations in dystrophin and the devastating muscle failure of Duchenne’s muscular dystrophy (DMD) has been well established. Less well appreciated are the accompanying cognitive impairment and neuropsychiatric disorders also presented in many DMD patients, which suggest a wider role for dystrophin in membrane–cytoskeleton function. This study provides genetic evidence of a novel role for DYS-1/dystrophin in maintaining neural organization in Caenorhabditis elegans. This neuronal function is distinct from the established role of DYS-1/dystrophin in maintaining muscle integrity and regulating locomotion. SAX-7, an L1 cell adhesion molecule (CAM) homologue, and STN-2/γ-syntrophin also function to maintain neural integrity in C. elegans. This study provides biochemical data that show that SAX-7 associates with DYS-1 in an STN-2/γ-syntrophin–dependent manner. These results reveal a recruitment of L1CAMs to the DPC to ensure neural integrity is maintained.

Rac1 modulation of the apical domain is negatively regulated by β (Heavy)-spectrin

Epithelial polarity and morphogenesis require the careful coordination of signaling and cytoskeletal elements. In this paper, we describe multiple genetic interactions between the apical cytoskeletal protein β(H) and Rac1 signaling in Drosophila: activation of Rac1 signaling by expression of the exchange factor Trio, is strongly enhanced by reducing β(H) levels, and such reductions in β(H) levels alone are shown to cause an increase in GTP-Rac1 levels. In contrast, co-expression of a C-terminal fragment of β(H) (βH33) suppresses the Trio expression phenotype. In addition, sustained expression of βH33 alone in the eye induces a strong dominant phenotype that is similar to the expression of dominant negative Rac1(N17), and this phenotype is also suppressed by the co-expression of Trio or by knockdown of RacGAP50C. We further demonstrate that a loss-of-function allele in pak, a Rac1 effector and negative regulator of β(H)' dominantly suppresses larval lethality arising loss-of-function karst (β(H)) alleles. Furthermore, expression of constitutively active Pak(myr) in the larval salivary gland induces expansion of the apical membrane and destabilization of the apical polarity determinants Crumbs and aPKC. These effects resemble a Rac1 activation phenotype and are suppressed by βH33. Together, our data suggest that apical proteins including β(H) are negatively regulated by Rac1 activation, but that Rac1 signaling is also suppressed by β(H) through its C-terminal domain. Such a system would be bistable with either Rac1 or β(H) predominant. We suggest a model for apical domain maintenance wherein Rac1 down-regulation of β(H) (via Pak) is opposed by β(H)-mediated down-regulation of Rac1 signaling.

The neurexin superfamily of Caenorhabditis elegans

The neurexin superfamily is a group of transmembrane molecules mediating cell-cell contacts and generating specialized membranous domains in polarized epithelial and nerves cells. We describe here the domain organization and expression of the entire, core neurexin superfamily in the nematode Caenorhabditis elegans, which is composed of three family members. One of the superfamily members, nrx-1, is an ortholog of vertebrate neurexin, the other two, itx-1 and nlr-1, are orthologs of the Caspr subfamily of neurexin-like genes. Based on reporter gene analysis, we find that nrx-1 is exclusively expressed in most if not all cells of the nervous system and localizes to presynaptic specializations. itx-1 and nrx-1 reporter genes are expressed in non-overlapping patterns within and outside the nervous system. ITX-1 protein co-localizes with β-G-spectrin to a subapical domain within intestinal cells. These studies provide a starting point for further functional analysis of this family of proteins.

Control of cortical rigidity by the cytoskeleton: emerging roles for septins

The cortex is the outermost region of the cell, comprising all of the elements from the plasma membrane to the cortical actin cytoskeleton that cooperate to maintain the cell's shape and topology. In eukaryotes without cell walls, this cortex governs the contact between their plasma membranes and the environment and thereby influences cell shape, motility, and signaling. It is therefore of considerable interest to understand how cells control their cortices, both globally and with respect to small subdomains. Here we review the current understanding of this control, including the regulation of cell shape by balances of outward hydrostatic pressure and cortical tension. The actomyosin cytoskeleton is the canonical regulator of cortical rigidity and indeed many would consider the cortex to comprise the actin cortex nearly exclusively. However, this actomyosin array is intimately linked to the membrane, for example via ERM and PIP2 proteins. Additionally, the lipid membrane likely undergoes rigidification by other players, such as Bin-Amphiphysin-Rvs proteins. Recent data also indicates that the septin cytoskeleton may play a formidable and more direct role in stabilization of membranes, particularly in contexts where cells receive limited external stabilization from their environments. Here, we review how septins may play this role, drawing on their physical form, their ability to directly bind and modify membranes and actomyosin, and their interactions with vesicular machinery. Deficiencies and alterations in the nature of the septin cytoskeleton may thus be relevant in multiple disease settings.

The spectrin-ankyrin-4.1-adducin membrane skeleton: adapting eukaryotic cells to the demands of animal life

The cells in animals face unique demands beyond those encountered by their unicellular eukaryotic ancestors. For example, the forces engendered by the movement of animals places stresses on membranes of a different nature than those confronting free-living cells. The integration of cells into tissues, as well as the integration of tissue function into whole animal physiology, requires specialisation of membrane domains and the formation of signalling complexes. With the evolution of mammals, the specialisation of cell types has been taken to an extreme with the advent of the non-nucleated mammalian red blood cell. These and other adaptations to animal life seem to require four proteins--spectrin, ankyrin, 4.1 and adducin--which emerged during eumetazoan evolution. Spectrin, an actin cross-linking protein, was probably the earliest of these, with ankyrin, adducin and 4.1 only appearing as tissues evolved. The interaction of spectrin with ankyrin is probably a prerequisite for the formation of tissues; only with the advent of vertebrates did 4.1 acquires the ability to bind spectrin and actin. The latter activity seems to allow the spectrin complex to regulate the cell surface accumulation of a wide variety of proteins. Functionally, the spectrin-ankyrin-4.1-adducin complex is implicated in the formation of apical and basolateral domains, in aspects of membrane trafficking, in assembly of certain signalling and cell adhesion complexes and in providing stability to otherwise mechanically fragile cell membranes. Defects in this complex are manifest in a variety of hereditary diseases, including deafness, cardiac arrhythmia, spinocerebellar ataxia, as well as hereditary haemolytic anaemias. Some of these proteins also function as tumor suppressors. The spectrin-ankyrin-4.1-adducin complex represents a remarkable system that underpins animal life; it has been adapted to many different functions at different times during animal evolution.

Characterization and expression of a heart-selective alternatively spliced variant of alpha II-spectrin, cardi+, during development in the rat

Spectrin is a large, flexible protein that stabilizes membranes and organizes proteins and lipids into microdomains in intracellular organelles and at the plasma membrane. Alternative splicing occurs in spectrins, but it is not yet clear if these small variations in structure alter spectrin's functions. Three alternative splice sites have been identified previously for alpha II-spectrin. Here we describe a new alternative splice site, a 21-amino acid sequence in the 21st spectrin repeat that is only expressed in significant amounts in cardiac muscle (GenBank GQ502182). The insert, which we term alpha II-cardi+, results in an insertion within the high affinity nucleation site for binding of alpha-spectrins to beta-spectrins. To assess the developmental regulation of the alpha II-cardi+ isoform, we used qRT-PCR and quantitative immunoblotting methods to measure the levels of this form and the alpha II-cardi- form in the cardiac muscles of rats, from embryonic day 16 (E16) through adulthood. The alpha II-cardi+ isoform constituted approximately 26% of the total alpha II-spectrin in E16 hearts but decreased to approximately 6% of the total after 3 weeks of age. We used long-range RT-PCR and Southern blot hybridization to examine possible linkage of the alpha II-cardi+ alternatively spliced sequence with alternatively spliced sequences of alpha II-spectrin that had been previously reported. We identified two new isoforms of alpha II-spectrin containing the cardi+ insert. These were named alpha II Sigma 9 and alpha II Sigma 10 in accordance with the spectrin naming conventions. In vitro studies of recombinant alpha II-spectrin polypeptides representing the two splice variants of alpha II-spectrin, alpha II-cardi+ and alpha II-cardi-, revealed that the alpha II-cardi+ subunit has lower affinity for the complementary site in repeats 1-4 of betaII-spectrin, with a K(D) value of approximately 1 nM, as measured by surface plasmon resonance (SPR). In addition, the alpha II-cardi+ form showed 1.8-fold lower levels of binding to its site on beta II-spectrin than the alpha II-cardi- form, both by SPR and blot overlay. This suggests that the 21-amino acid insert prevented some of the alpha II-cardi+ form from interacting with beta II-spectrin. Fusion proteins expressing the alpha II-cardi+ sequence within the two terminal spectrin repeats of alpha II-spectrin were insoluble in solution and aggregated in neonatal myocytes, consistent with the possibility that this insert removes a significant portion of the protein from the population that can bind beta subunits. Neonatal rat cardiomyocytes infected with adenovirus encoding GFP-fusion proteins of repeats 18-21 of alpha II-spectrin with the cardi+ insert formed many new processes. These processes were only rarely seen in myocytes expressing the fusion protein lacking the insert or in controls expressing only GFP. Our results suggest that the embryonic mammalian heart expresses a significant amount of alpha II-spectrin with a reduced avidity for beta-spectrin and the ability to promote myocyte growth.

Evolution of spectrin function in cytoskeletal and membrane networks

Spectrin is a cytoskeletal protein thought to have descended from an alpha-actinin-like ancestor. It emerged during evolution of animals to promote integration of cells into tissues by assembling signalling and cell adhesion complexes, by enhancing the mechanical stability of membranes and by promoting assembly of specialized membrane domains. Spectrin functions as an (alphabeta([H]))(2) tetramer that cross-links transmembrane proteins, membrane lipids and the actin cytoskeleton, either directly or via adaptor proteins such as ankyrin and 4.1. In the present paper, I review recent findings on the origins and adaptations in this system. (i) The genome of the choanoflagellate Monosiga brevicollis encodes alpha-, beta- and beta(Heavy)-spectrin, indicating that spectrins evolved in the immediate unicellular precursors of animals. (ii) Ankyrin and 4.1 are not encoded in that genome, indicating that spectrin gained function during subsequent animal evolution. (iii) Protein 4.1 gained a spectrin-binding activity in the evolution of vertebrates. (iv) Interaction of chicken or mammal beta-spectrin with PtdInsP(2) can be regulated by differential mRNA splicing, which can eliminate the PH (pleckstrin homology) domain in betaI- or betaII-spectrins; in the case of mammalian betaII-spectrin, the alternative C-terminal region encodes a phosphorylation site that regulates interaction with alpha-spectrin. (v) In mammalian evolution, the single pre-existing alpha-spectrin gene was duplicated, and one of the resulting pair (alphaI) neo-functionalized for rapid make-and-break of tetramers. I hypothesize that the elasticity of mammalian non-nucleated erythrocytes depends on the dynamic rearrangement of spectrin dimers/tetramers under the shearing forces experienced in circulation.

Tropomodulin1 is required for membrane skeleton organization and hexagonal geometry of fiber cells in the mouse lens

Hexagonal packing geometry is a hallmark of close-packed epithelial cells in metazoans. Here, we used fiber cells of the vertebrate eye lens as a model system to determine how the membrane skeleton controls hexagonal packing of post-mitotic cells. The membrane skeleton consists of spectrin tetramers linked to actin filaments (F-actin), which are capped by tropomodulin1 (Tmod1) and stabilized by tropomyosin (TM). In mouse lenses lacking Tmod1, initial fiber cell morphogenesis is normal, but fiber cell hexagonal shapes and packing geometry are not maintained as fiber cells mature. Absence of Tmod1 leads to decreased gammaTM levels, loss of F-actin from membranes, and disrupted distribution of beta2-spectrin along fiber cell membranes. Regular interlocking membrane protrusions on fiber cells are replaced by irregularly spaced and misshapen protrusions. We conclude that Tmod1 and gammaTM regulation of F-actin stability on fiber cell membranes is critical for the long-range connectivity of the spectrin-actin network, which functions to maintain regular fiber cell hexagonal morphology and packing geometry.

An ankyrin-based mechanism for functional organization of dystrophin and dystroglycan

beta-dystroglycan (DG) and the dystrophin-glycoprotein complex (DGC) are localized at costameres and neuromuscular junctions in the sarcolemma of skeletal muscle. We present evidence for an ankyrin-based mechanism for sarcolemmal localization of dystrophin and beta-DG. Dystrophin binds ankyrin-B and ankyrin-G, while beta-DG binds ankyrin-G. Dystrophin and beta-DG require ankyrin-G for retention at costameres but not delivery to the sarcolemma. Dystrophin and beta-DG remain intracellular in ankyrin-B-depleted muscle, where beta-DG accumulates in a juxta-TGN compartment. The neuromuscular junction requires ankyrin-B for localization of dystrophin/utrophin and beta-DG and for maintenance of its postnatal morphology. A Becker muscular dystrophy mutation reduces ankyrin binding and impairs sarcolemmal localization of dystrophin-Dp71. Ankyrin-B also binds to dynactin-4, a dynactin subunit. Dynactin-4 and a subset of microtubules disappear from sarcolemmal sites in ankyrin-B-depleted muscle. Ankyrin-B thus is an adaptor required for sarcolemmal localization of dystrophin, as well as dynactin-4.

The assembly and maintenance of epithelial junctions in C. elegans

The epithelial tissues of the C. elegans embryo provide a "minimalist" system for examining phylogenetically conserved proteins that function in epithelial polarity and cell-cell adhesion in a multicellular organism. In this review, we provide an overview of three major molecular complexes at the apical surface of epithelial cells in the C. elegans embryo: the cadherin-catenin complex, the more basal DLG-1/AJM-1 complex, and the apical membrane domain, which shares similarities with the subapical complex in Drosophila and the PAR/aPKC complex in vertebrates. We discuss how the assembly of these complexes contributes to epithelial polarity and adhesion, proteins that act as effectors and/or regulators of each subdomain, and how these complexes functionally interact during embryonic morphogenesis. Although much remains to be clarified, significant progress has been made in recent years to clarify the role of these protein complexes in epithelial morphogenesis, and suggests that C. elegans will continue to be a fruitful system in which to elucidate functional roles for these proteins in a living embryo.

Axons break in animals lacking beta-spectrin

Axons and dendrites can withstand acute mechanical strain despite their small diameter. In this study, we demonstrate that β-spectrin is required for the physical integrity of neuronal processes in the nematode Caenorhabditis elegans. Axons in β-spectrin mutants spontaneously break. Breakage is caused by acute strain generated by movement because breakage can be prevented by paralyzing the mutant animals. After breaking, the neuron attempts to regenerate by initiating a new growth cone; this second round of axon extension is error prone compared with initial outgrowth. Because spectrin is a major target of calpain proteolysis, it is possible that some neurodegenerative disorders may involve the cleavage of spectrin followed by the breakage of neural processes.

Thermal stabilities of brain spectrin and the constituent repeats of subunits

The different genes that encode mammalian spectrins give rise to proteins differing in their apparent stiffness. To explore this, we have compared the thermal stabilities of the structural repeats of brain spectrin subunits (alphaII and betaII) with those of erythrocyte spectrin (alphaI and betaI). The unfolding transition midpoints (T(m)) of the 36 alphaII- and betaII-spectrin repeats extend between 24 and 82 degrees C, with an average higher by some 10 degrees C than that of the alphaI- and betaI-spectrin repeats. This difference is reflected in the T(m) values of the intact brain and erythrocyte spectrins. Two of three tandem-repeat constructs from brain spectrin exhibited strong cooperative coupling, with elevation of the T(m) of the less stable partner corresponding to coupling free energies of approximately -4.4 and -3.5 kcal/mol. The third tandem-repeat construct, by contrast, exhibited negligible cooperativity. Tandem-repeat mutants, in which a part of the "linker" helix that connects the two domains was replaced with a corresponding helical segment from erythroid spectrin, showed only minor perturbation of the thermal melting profiles, without breakdown of cooperativity. Thus, the linker regions, which tolerate few point mutations without loss of cooperative function, have evidently evolved to permit conformational coupling in specified regions. The greater structural stability of the repeats in alphaII- and betaII-spectrin may account, at least in part, for the higher rigidity of brain compared to erythrocyte spectrin.

Ankyrin-G and β2-Spectrin Collaborate in Biogenesis of Lateral Membrane of Human Bronchial Epithelial Cells

Ankyrins are a family of adapter proteins required for localization of membrane proteins to diverse specialized membrane domains including axon initial segments, specialized sites at the transverse tubule/sarcoplasmic reticulum in cardiomyocytes, and lateral membrane domains of epithelial cells. Little is currently known regarding the molecular basis for specific roles of different ankyrin isoforms. In this study, we systematically generated alanine mutants of clusters of charged residues in the spectrin-binding domains of both ankyrin-B and -G. The corresponding mutants were evaluated for activity in either restoration of abnormal localization of the inositol trisphosphate receptor in the sarcoplasmic reticulum in mutant mouse cardiomyocytes deficient in ankyrin-B or in prevention of loss of lateral membrane in human bronchial epithelial cells depleted of ankyrin-G by small interfering RNA. Interestingly, ankyrin-B and -G share two homologous sites that result in loss of function in both systems, suggesting that common molecular interactions underlie diverse roles of these isoforms. Ankyrins G and B also exhibit differences; mutations affecting spectrin binding had no effect on ankyrin-B function but did abolish activity of ankyrin-G in restoring lateral membrane biogenesis. Depletion of beta(2)-spectrin by small interfering RNA phenocopied depletion of ankyrin-G and resulted in a failure to form new lateral membrane in interphase and mitotic cells. These results demonstrate that ankyrin-G and beta(2)-spectrin are functional partners in biogenesis of the lateral membrane of epithelial cells.

A postsynaptic spectrin scaffold defines active zone size, spacing, and efficacy at the Drosophila neuromuscular junction

Synaptic connections are established with characteristic, cell type–specific size and spacing. In this study, we document a role for the postsynaptic Spectrin skeleton in this process. We use transgenic double-stranded RNA to selectively eliminate α-Spectrin, β-Spectrin, or Ankyrin. In the absence of postsynaptic α- or β-Spectrin, active zone size is increased and spacing is perturbed. In addition, subsynaptic muscle membranes are significantly altered. However, despite these changes, the subdivision of the synapse into active zone and periactive zone domains remains intact, both pre- and postsynaptically. Functionally, altered active zone dimensions correlate with an increase in quantal size without a change in presynaptic vesicle size. Mechanistically, β-Spectrin is required for the localization of α-Spectrin and Ankyrin to the postsynaptic membrane. Although Ankyrin is not required for the localization of the Spectrin skeleton to the neuromuscular junction, it contributes to Spectrin-mediated synapse development. We propose a model in which a postsynaptic Spectrin–actin lattice acts as an organizing scaffold upon which pre- and postsynaptic development are arranged.

beta-Spectrin functions independently of Ankyrin to regulate the establishment and maintenance of axon connections in the Drosophila embryonic CNS

alpha- and beta-Spectrin are major components of a submembrane cytoskeletal network connecting actin filaments to integral plasma membrane proteins. Besides its structural role in red blood cells, the Spectrin network is thought to function in non-erythroid cells during protein targeting and membrane domain formation. Here, we demonstrate that beta-Spectrin is required in neurons for proper midline axon guidance in the Drosophila embryonic CNS. In beta-spectrin mutants many axons inappropriately cross the CNS midline, suggesting a role for beta-Spectrin in midline repulsion. Surprisingly, neither the Ankyrin-binding nor the pleckstrin homology (PH) domains of beta-Spectrin are required for accurate guidance decisions. alpha-Spectrin is dependent upon beta-Spectrin for its normal subcellular localization and/or maintenance, whereas alpha-spectrin mutants exhibit a redistribution of beta-Spectrin to the axon scaffold. beta-spectrin mutants show specific dose-dependent genetic interactions with the midline repellent slit and its neuronal receptor roundabout (robo), but not with other guidance molecules. The results suggest that beta-Spectrin contributes to midline repulsion through the regulation of Slit-Robo pathway components. We propose that the Spectrin network is playing a role independently of Ankyrin in the establishment and/or maintenance of specialized membrane domains containing guidance molecules that ensure the fidelity of axon repulsion at the midline.

Phosphorylation of a threonine unique to the short C-terminal isoform of betaII-spectrin links regulation of alpha-beta spectrin interaction to neuritogenesis

Spectrin tetramers are cytoskeletal proteins required in the formation of complex animal tissues. Mammalian alphaII- and betaII-spectrin subunits form dimers that associate head to head with high affinity to form tetramers, but it is not known if this interaction is regulated. We show here that the short C-terminal splice variant of betaII-spectrin (betaIISigma2) is a substrate for phosphorylation. In vitro, protein kinase CK2 phosphorylates Ser-2110 and Thr-2159; protein kinase A phosphorylates Thr-2159. Antiphospho-Thr-2159 peptide antibody detected phosphorylated betaIISigma2 in Cos-1 cells. Immunoreactivity was increased in Cos-1 cells by treatment with forskolin, indicating that phosphorylation is promoted by elevated cAMP. The effect of forskolin was counteracted by the cAMP-dependent kinase inhibitor, H89. In vitro, protein kinase A phosphorylation of an active fragment of betaIISigma2 greatly reduced its interaction with alphaII-spectrin at the tetramerization site. Mutation of Thr-2159 to alanine eliminated inhibition by phosphorylation. Among the processes that require spectrin in mammals is the formation of neurites (incipient nerve axons). We tested the relationship of spectrin phosphorylation to neuritogenesis by transfecting the neuronal cell line, PC12, with enhanced green fluorescent protein-coupled fragments of betaIISigma2-spectrin predicted to act as inhibitors of spectrin tetramer formation. Both wild-type and T2159E mutant fragments allowed normal neuritogenesis in PC12 cells in response to nerve growth factor. The mutant T2159A inhibited neuritogenesis. Because the T2159A mutant represents a high affinity inhibitor of tetramer formation, we conclude that tetramers are requisite for neuritogenesis. Furthermore, because both the T2159E mutant and the wild-type allow neuritogenesis, we conclude that the short C-terminal betaII-spectrin is phosphorylated during this process.

Conformational Stabilities of the Structural Repeats of Erythroid Spectrin and Their Functional Implications

The two polypeptide chains of the erythroid spectrin heterodimer contain between them 36 structural repeating modules, which can function as independently folding units. We have expressed all 36 and determined their thermal stabilities. These vary widely, with unfolding transition mid-points (T(m)) ranging from 21 to 72 degrees C. Eight of the isolated repeats are largely unfolded at physiological temperature. Constructs comprising two or more adjacent repeats show inter-repeat coupling with coupling free energies of several kcal mol(-1). Constructs comprising five successive repeats from the beta-chain displayed cooperativity and strong temperature dependence in forced unfolding by atomic force microscopy. Analysis of aligned sequences and molecular modeling suggests that high stability is conferred by large hydrophobic side chains at position e of the heptad hydrophobic repeats in the first helix of the three-helix bundle that makes up each repeat. This inference was borne out by the properties of mutants in which the critical residues have been replaced. The marginal stability of the tertiary structure at several points in the spectrin chains is moderated by energetic coupling with adjoining structural elements but may be expected to permit adaptation of the membrane to the large distortions that the red cell experiences in the circulation.

The hedgehog-related gene wrt-5 is essential for hypodermal development in Caenorhabditis elegans

The Caenorhabditis elegans genome encodes a series of hedgehog-related genes, which are thought to have evolved and diverged from an ancestral Hh gene. They are classified into several families based on their N-terminal domains. Here, we analyze the expression and function of a member of the warthog gene family, wrt-5, that lacks the Hint/Hog domain. wrt-5 is expressed in seam cells, the pharynx, pharyngeal-intestinal valve cells, neurons, neuronal support cells, the excretory cell, and the reproductive system. WRT-5 protein is secreted into the extracellular space during embryogenesis. Furthermore, during larval development, WRT-5 protein is secreted into the pharyngeal lumen and the pharyngeal expression changes in a cyclical manner in phase with the molting cycle. Deletion mutations in wrt-5 cause embryonic lethality, which are temperature sensitive and more severe at 15 degrees C than at 25 degrees C. Animals that hatch exhibit variable abnormal morphology, for example, bagging worms, blistering, molting defects, or Roller phenotypes. We examined hypodermal cell junctions using the AJM-1Colon, two colonsGFP marker in the wrt-5 mutant background and observed cell boundary abnormalities in the arrested embryos. AJM-1Colon, two colonsGFP protein is also misplaced in pharyngeal muscle cells in the absence of WRT-5. In conclusion, we show that wrt-5 is an essential gene that - despite its lack of a Hint domain - has multiple functions in C. elegans and is implicated in cell shape integrity.

Mammalian alpha I-spectrin is a neofunctionalized polypeptide adapted to small highly deformable erythrocytes

Mammalian red blood cells, unlike those of other vertebrates, must withstand the rigors of circulation in the absence of new protein synthesis. Key to this is plasma membrane elasticity deriving from the protein spectrin, which forms a network on the cytoplasmic face. Spectrin is a tetramer (alphabeta)(2), made up of alphabeta dimers linked head to head. We show here that one component of erythrocyte spectrin, alphaI, is encoded by a gene unique to mammals. Phylogenetic analysis suggests that the other alpha-spectrin gene (alphaII) common to all vertebrates was duplicated after the emergence of amphibia, and that the resulting alphaI gene was preserved only in mammals. The activities of alphaI and alphaII spectrins differ in the context of the human red cell membrane. An alphaI-spectrin fragment containing the site of head-to-head interaction with the beta-chain binds more weakly than the corresponding alphaII fragment to this site. The latter competes so strongly with endogenous alphaI as to cause destabilization of membranes at 100-fold lower concentration than the alphaI fragment. The efficacies of alphaI/alphaII chimeras indicate that the partial structural repeat, which binds to the complementary beta-spectrin element, and the adjacent complete repeat together determine the strength of the dimer-dimer interaction on the membrane. Alignment of all available alpha-spectrin N-terminal sequences reveals three blocks of sequence unique to alphaI. Furthermore, human alphaII-spectrin is closer to fruitfly alpha-spectrin than to human alphaI-spectrin, consistent with adaptation of alphaI to new functions. We conclude that alphaI-spectrin represents a neofunctionalized spectrin adapted to the rapid make and break of tetramers.

AlphaII-spectrin interacts with Tes and EVL, two actin-binding proteins located at cell contacts

The spectrin-based membrane skeleton, a multi-protein scaffold attached to diverse cellular membranes, is presumed to be involved in the stabilization of membranes, the establishment of membrane domains as well as in vesicle trafficking and nuclear functions. Spectrin tetramers made of alpha- and beta-subunits are linked to actin microfilaments, forming a network that binds a multitude of proteins. The most prevalent alpha-spectrin subunit in non-erythroid cells, alphaII-spectrin, contains two particular spectrin repeats in its central region, alpha9 and alpha10, which host an Src homology 3 domain, a tissue-specific spliced sequence of 20 residues, a calmodulin-binding site and major cleavage sites for caspases and calpains. Using yeast two-hybrid screening of kidney libraries, we identified two partners of the alpha9-alpha10 repeats: the potential tumour suppressor Tes, an actin-binding protein mainly located at focal adhesions; and EVL (Ena/vasodilator-stimulated phosphoprotein-like protein), another actin-binding protein, equally recruited at focal adhesions. Interactions between spectrin and overexpressed Tes and EVL were confirmed by co-immunoprecipitation. In vitro studies showed that the interaction between Tes and spectrin is mediated by a LIM (Lin-11, Isl-1 and Mec3) domain of Tes and by the alpha10 repeat of alphaII-spectrin whereas EVL interacts with the Src homology 3 domain located within the alpha9 repeat. Moreover, we describe an in vitro interaction between Tes and EVL, and a co-localization of these two proteins at focal adhesions. These interactions between alphaII-spectrin, Tes and EVL indicate new functions for spectrin in actin dynamics and focal adhesions.