Actin nucleation: nucleation-promoting factors are not all equal

Collapsin Response Mediator Protein 1 (CRMP1) Is Required for High-Frequency Hearing

Collapsin response mediator protein 1 (CRMP1), also known as dihydropyrimidinase-related protein 1, participates in cytoskeleton remodeling during axonal guidance and neuronal migration. In cochlear hair cells, the assembly and maintenance of the cytoskeleton is of great interest because it is crucial for the morphogenesis and maintenance of hair cells. Previous RNA sequencing analysis found that Crmp1 is highly expressed in cochlear hair cells. However, the expression profile and functions of CRMP1 in the inner ear remain unknown. In this study, the expression and localization of CRMP1 in hair cells was investigated using immunostaining, and was shown to be highly expressed in both outer and inner hair cells. Next, the stereocilia morphology of Crmp1-deficient mice was characterized. Abolishing CRMP1 did not affect the morphogenesis of hair cells. Interestingly, scanning electron microscopy detected hair cell loss at the basal cochlear region, an area responsible for high-frequency auditory perception, in Crmp1-deficient mice. Correspondingly, an auditory brainstem response test showed that mice lacking CRMP1 had progressive hearing loss at high frequencies. In summary, these data suggest that CRMP1 is required for high-frequency auditory perception.

Sequential inactivation of Rho GTPases and Lim kinase by Pseudomonas aeruginosa toxins ExoS and ExoT leads to endothelial monolayer breakdown

Pseudomonas aeruginosa is a major human opportunistic pathogen and one of the most important causal agents of bacteremia. For non-blood-borne infection, bacterial dissemination requires the crossing of the vascular endothelium, the main barrier between blood and the surrounding tissues. Here, we investigated the effects of P. aeruginosa type 3 secretion effectors, namely ExoS, ExoT, and ExoY, on regulators of actin cytoskeleton dynamics in primary endothelial cells. ExoS and ExoT similarly affected the Lim kinase-cofilin pathway, thereby promoting actin filament severing. Cofilin activation was also observed in a mouse model of P. aeruginosa-induced acute pneumonia. Rho, Rac, and Cdc42 GTPases were sequentially inactivated, leading to inhibition of membrane ruffling, filopodia, and stress fiber collapse, and focal adhesion disruption. At the end of the process, ExoS and ExoT produced a dramatic retraction in all primary endothelial cell types tested and thus a rupture of the endothelial monolayer. ExoY alone had no effect in this context. Cell retraction could be counteracted by overexpression of actin cytoskeleton regulators. In addition, our data suggest that moesin is neither a direct exotoxin target nor an important player in this process. We conclude that any action leading to inhibition of actin filament breakdown will improve the barrier function of the endothelium during P. aeruginosa infection.

Tyrosine kinase signaling in kidney glomerular podocytes

During the last decade, several key molecules have been identified as essential components for the filtration barrier function of kidney glomerular podocytes. Mutations in genes encoding these molecules severely impair the podocyte architecture in the affected patients, leading to the development of proteinuria. Extensive investigations have been performed on the function of these molecules, which highlights the importance of tyrosine kinase signaling in the podocytes. An Src family tyrosine kinase, Fyn, plays a major role in this signaling pathway. Here, we review the current understanding of this important signal transduction system and its role in the development and the maintenance of podocytes.

N-WASP has the ability to compensate for the loss of WASP in macrophage podosome formation and chemotaxis

Wiskott-Aldrich syndrome protein (WASP) and its homologue neural-WASP (N-WASP) are nucleation promoting factors that integrate receptor signaling with actin cytoskeleton rearrangement. While hematopoietic cells express both WASP and N-WASP, WASP deficiency results in altered cell morphology, loss of podosomes and defective chemotaxis. It was determined that cells from a mouse derived monocyte/macrophage cell line and primary cells of myeloid lineage expressed approximately 15-fold higher levels of WASP relative to N-WASP. To test whether N-WASP can compensate for the loss of WASP and restore actin cytoskeleton integrity, N-WASP was overexpressed in macrophages, in which endogenous WASP expression was reduced by short hairpin RNA (shWASP cells). Many of the defects associated with the loss of WASP, such as podosome-dependent matrix degradation and chemotaxis were corrected when N-WASP was expressed at equimolar level to that of the wild-type WASP. Furthermore, the ability of N-WASP to partially compensate for the loss of WASP may be physiologically relevant since activated murine WASP-deficient peritoneal macrophages, which show enhanced N-WASP expression, also show an increase in matrix degradation. Our study suggests that expression levels of WASP and N-WASP may influence their roles in actin cytoskeleton rearrangement and shed light to the complex intertwining roles WASP and N-WASP play in macrophages.

SipC multimerization promotes actin nucleation and contributes to Salmonella-induced inflammation

Actin nucleation is the rate-limiting step in actin assembly and is regulated by actin-binding proteins and signal transduction molecules. Salmonella enterica serovar Typhimurium exploits actin dynamics by reorganizing the host actin cytoskeleton to facilitate its own uptake. SipC is a Salmonella actin-binding protein that nucleates actin filament formation in vitro. The molecular mechanism by which SipC nucleates actin is not known. We show here that SipC(199-409) forms multimers to promote actin nucleation. We found that wild-type SipC(199-409) forms dimers and multimers while SipC(199-409)#1, a nucleation mutant, is less efficient in dimer and multimer formation. Biochemical analysis suggested that SipC(199-409) might form parallel dimers in an extended conformation. Furthermore, a mutant Salmonella strain that was defective in forming the SipC multimer and deficient in actin nucleation failed to cause severe colitis in a mouse model. These results allow us to present a model in which SipC forms multimers to promote actin nucleation.

A dynamic podosome-like structure of epithelial cells

Focal contacts and hemidesmosomes are cell-matrix adhesion structures of cultured epithelial cells. While focal contacts link the extracellular matrix to microfilaments, hemidesmosomes make connections with intermediate filaments. We have analyzed hemidesmosome assembly in 804G carcinoma cells. Our data show that hemidesmosomes are organized around a core of actin filaments that appears early during cell adhesion. These actin structures look similar to podosomes described in cells of mesenchymal origin. These podosome-like structures are distinct from focal contacts and specifically contain Arp3 (Arp2/3 complex), cortactin, dynamin, gelsolin, N-WASP, VASP, Grb2 and src-like kinase(s). The integrin alpha3beta1 is localized circularly around F-actin cores and co-distributes with paxillin, vinculin, and zyxin. We also show that the maintenance of the actin core and hemidesmosomes is dependent on actin polymerization, src-family kinases, and Grb2, but not on microtubules. Video microscopy analysis reveals that assembly of hemidesmosomes is preceded by recruitment of beta4 integrin subunit to the actin core before its positioning at hemidesmosomes. When 804G cells are induced to migrate, actin cores as well as hemidesmosomes disappear and beta4 integrin subunit becomes co-localized with dynamic actin at leading edges. We show that podosome-like structures are not unique to cells of mesenchymal origin, but also appear in epithelial cells, where they seem to be related to basement membrane adhesion.

Orchestration of synaptic plasticity through AKAP signaling complexes

Significant progress has been made toward understanding the mechanisms by which organisms learn from experiences and how those experiences are translated into memories. Advances in molecular, electrophysiological and genetic technologies have permitted great strides in identifying biochemical and structural changes that occur at synapses during processes that are thought to underlie learning and memory. Cellular events that generate the second messenger cyclic AMP (cAMP) and activate protein kinase A (PKA) have been linked to synaptic plasticity and long-term memory. In this review we will focus on the role of PKA in synaptic plasticity and discuss how the compartmentalization of PKA through its association with A-Kinase Anchoring Proteins (AKAPs) affect PKA function in this process.

Inducible clustering of membrane-targeted SH3 domains of the adaptor protein Nck triggers localized actin polymerization

BACKGROUND

SH2/SH3 adaptor proteins play a critical role in tyrosine kinase signaling pathways, regulating essential cell functions by increasing the local concentration or altering the subcellular localization of downstream effectors. The SH2 domain of the Nck adaptor can bind tyrosine-phosphorylated proteins, while its SH3 domains can modulate actin polymerization by interacting with effectors such as WASp/Scar family proteins. Although several studies have implicated Nck in regulating actin polymerization, its role in living cells is not well understood.

RESULTS

We used an antibody-based system to experimentally modulate the local concentration of Nck SH3 domains on the plasma membrane of living cells. Clustering of fusion proteins containing all three Nck SH3 domains induced localized polymerization of actin, including the formation of actin tails and spots, accompanied by general cytoskeletal rearrangements. All three Nck SH3 domains were required, as clustering of individual SH3 domains or a combination of the two N-terminal Nck SH3 domains failed to promote significant local polymerization of actin in vivo. Changes in actin dynamics induced by Nck SH3 domain clustering required the recruitment of N-WASp, but not WAVE1, and were unaffected by downregulation of Cdc42.

CONCLUSIONS

We show that high local concentrations of Nck SH3 domains are sufficient to stimulate localized, Cdc42-independent actin polymerization in living cells. This study provides strong evidence of a pivotal role for Nck in directly coupling ligand-induced tyrosine phosphorylation at the plasma membrane to localized changes in organization of the actin cytoskeleton through a signaling pathway that requires N-WASp.

Characterization of the WAVE1 knock-out mouse: implications for CNS development

Developing neurons must respond to a wide range of extracellular signals during the process of brain morphogenesis. One mechanism through which immature neurons respond to such signals is by altering cellular actin dynamics. A recently discovered link between extracellular signaling events and the actin cytoskeleton is the WASP/WAVE (Wiscott-Aldrich Syndrome protein/WASP-family verprolin-homologous protein) family of proteins. Through a direct interaction with the Arp2/3 (actin-related protein) complex, this family functions to regulate the actin cytoskeleton by mediating signals from cdc42 as well as other small GTPases. To evaluate the role of WASP/WAVE proteins in the process of neuronal morphogenesis, we used a retroviral gene trap to generate a line of mice bearing a disruption in the WAVE1 gene. Using a heterologous reporter gene, we found that WAVE1 expression becomes increasingly restricted to the CNS over the course of development. Homozygous disruption of the WAVE1 gene results in postnatal lethality. In addition, these animals have severe limb weakness, a resting tremor, and notable neuroanatomical malformations without overt histopathology of peripheral organs. We did not detect any alterations in neuronal morphology in vivo or the ability of embryonic neurons to form processes in vitro. Our data indicate that WAVE1, although important for the general development of the CNS, is not essential for the formation and extension of neuritic processes.

Characterization of Arp2/3 complex in chicken tissues

Arp2/3 protein complex consists of seven subunits (Arp2, Arp3, p41-Arc, p34-Arc, p21-Arc, p20-Arc and p16-Arc) in apparent 1:1 stoichiometry. This complex has been shown to promote the formation of Y-branch structures of F-actin in cultured cells. We generated specific antibodies against chicken Arp2, Arp3, and p34-Arc to analyze the distribution of these subunits in chicken tissues. In whole samples of brain and gizzard, antibodies against each recombinant protein reacted with single bands of predicted molecular mass based on their cDNA sequences of the antigens. Anti-p34-Arc antibody detected at least two neighboring spots in 2D-PAGE, which might suggest the existence of isoforms or modified forms. Arp2/3 complex bound to an F-actin affinity column from gizzard extract. However, Arp2/3 complex did not tightly bind major actin cytoskeleton because the complex was extracted easily when gizzard smooth muscle was homogenized in PBS. Immunoblot analysis of various tissues revealed that the amounts of Arp2/3 subunits were lower in striated muscle than in non-muscle and smooth muscle tissues. Amounts and ratio of the three subunits varied in tissues, as estimated by quantitative immunoblotting. With immunofluorescence microscopy, we also observed localization of Arp3 and p34-Arc in frozen sections of gizzard with different staining patterns around blood vessels. These results suggest that the Arp2/3 complex exists also in places where rapid actin polymerization does not occur, and that a part of the subunits may exist in different forms from the complex containing the seven subunits in some tissues.