Formins

Genome-Wide Characterization and Analysis of the FH Gene Family in Medicago truncatula Under Abiotic Stresses

BACKGROUND

The formin family proteins play an important role in guiding the assembly and nucleation of linear actin and can promote the formation of actin filaments independently of the Arp2/3 complex. As a key protein that regulates the cytoskeleton and cell morphological structure, the formin gene family has been widely studied in plants such as Arabidopsis thaliana and rice.

METHODS

In this study, we conducted comprehensive analyses, including phylogenetic tree construction, conserved motif identification, co-expression network analysis, and transcriptome data mining.

RESULTS

A total of 18 MtFH gene family members were identified, and the distribution of these genes on chromosomes was not uniform. The phylogenetic tree divided the FH proteins of the four species into two major subgroups (Clade I and Clade II). Notably, Medicago truncatula and soybean exhibited closer phylogenetic relationships. The analysis of cis-acting elements revealed the potential regulatory role of the MtFH gene in light response, hormone response, and stress response. GO enrichment analysis again demonstrated the importance of FH for reactions such as actin nucleation. Expression profiling revealed that MtFH genes displayed significant transcriptional responsiveness to cold, drought, and salt stress conditions. And there was a temporal complementary relationship between the expression of some genes under stress. The protein interaction network indicated an interaction relationship between MtFH protein and profilin, etc. In addition, 22 miRNAs were screened as potential regulators of the MtFH gene at the post-transcriptional level.

CONCLUSIONS

In general, this study provides a basis for deepening the understanding of the physiological function of the MtFH gene and provides a reference gene for stress resistance breeding in agricultural production.

Formin 3 stabilizes the cytoskeleton of Drosophila tendon cells, thus enabling them to resist muscle tensile forces

The cytoskeleton of Drosophila tendon cells features specialized F-actin and microtubule arrays that endow these cells with resistance to the tensile forces exerted by the attached muscles. In a forward genetic screen for mutants with neuromuscular junction and muscle morphology phenotypes in larvae, we identified formin 3 (form3) as a crucial component for stabilizing these cytoskeletal arrays under muscle tension. form3 mutants exhibit severely stretched tendon cells in contact with directly attached larval body wall muscles, leading to muscle retraction and rounding. Both the actomyosin and microtubule arrays are expanded likewise in these mutants and can separate laterally in extreme cases. Analysis of a natively HA-tagged, functional version of Form3 reveals that Form3 is distributed along the length of these cytoskeletal arrays. Based on our findings and existing data on vertebrate and Caenorhabditis elegans orthologs of form3, we propose that the primary function of Form3 in this context is to co-bundle actin filaments and microtubules, thus maximizing the rigidity of these cytoskeletal structures against muscle tensile forces.

Electrical Forces Regulate Single-Cell Wound Healing

Damage to the cell membrane can be life threatening for single-celled organisms. Several mechanisms of single-cell wound healing occur and aspects of these are regulated by electrical forces.

Cytoskeletal dynamics of gamete nuclear migration in flowering plants, animals, and yeast

Gamete nuclear migration is a critical process during fertilization in flowering plants, yet its molecular mechanisms remain poorly understood. Recent studies have highlighted the essential role of cytoskeletal elements, particularly F-actin, in directing sperm nuclear migration, which differ from the microtubule-driven migration in animals. We summarize the process of sperm nuclear migration in plants and the involvement of Class XI myosin XI-G in Arabidopsis, along with the ROP8-SCAR2 pathway's ARP2/3-independent mechanism for F-actin nucleation. We also provide a comparative overview of examples from sea urchins, C. elegans, mice and yeast contrasting these mechanisms with those in plants. Finally, we outline possible future research directions related to sperm nuclear migration in plants. This review highlights the need for further exploration of pre- and post-fertilization processes, emphasizing their importance in plant cytoskeleton biology and the coordinated development of seeds.

Regulation of formin INF2 and its alteration in INF2-linked inherited disorders

Formins are proteins that catalyze the formation of linear filaments made of actin. INF2, a formin, is crucial for correct vesicular transport, microtubule stability and mitochondrial division. Its activity is regulated by a complex of cyclase-associated protein and lysine-acetylated G-actin (KAc-actin), which helps INF2 adopt an inactive conformation through the association of its N-terminal diaphanous inhibitory domain (DID) with its C-terminal diaphanous autoinhibitory domain. INF2 activation can occur through calmodulin binding, KAc-actin deacetylation, G-actin binding, or association with the Cdc42 GTPase. Mutations in the INF2 DID are linked to focal segmental glomerulosclerosis (FSGS), affecting podocytes, and Charcot-Marie-Tooth disease, which affects Schwann cells and leads to axonal loss. At least 80 pathogenic DID variants of INF2 have been identified, with potential for many more. These mutations disrupt INF2 regulation, leading to excessive actin polymerization. This in turn causes altered intracellular trafficking, abnormal mitochondrial dynamics, and profound transcriptional reprogramming via the MRTF/SRF complex, resulting in mitotic abnormalities and p53-mediated cell death. This sequence of events could be responsible for progressive podocyte loss during glomerular degeneration in FSGS patients. Pharmacological targeting of INF2 or actin polymerization could offer the therapeutic potential to halt the progression of FSGS and improve outcomes for patients with INF2-linked disease.

Light-guided actin polymerization drives directed motility in protocells

Motility is a hallmark of life's dynamic processes, enabling cells to actively chase prey, repair wounds, and shape organs. Recreating these intricate behaviors using well-defined molecules remains a major challenge at the intersection of biology, physics, and molecular engineering. Although the polymerization force of the actin cytoskeleton is characterized as a primary driver of cell motility, recapitulating this process in protocellular systems has proven elusive. The difficulty lies in the daunting task of distilling key components from motile cells and integrating them into model membranes in a physiologically relevant manner. To address this, we developed a method to optically control actin polymerization with high spatiotemporal precision within cell-mimetic lipid vesicles known as giant unilamellar vesicles (GUVs). Within these active protocells, the reorganization of actin networks triggered outward membrane extensions as well as the unidirectional movement of GUVs at speeds of up to 0.43 μm/min, comparable to typical adherent mammalian cells. Notably, our findings reveal a synergistic interplay between branched and linear actin forms in promoting membrane protrusions, highlighting the cooperative nature of these cytoskeletal elements. This approach offers a powerful platform for unraveling the intricacies of cell migration, designing synthetic cells with active morphodynamics, and advancing bioengineering applications, such as self-propelled delivery systems and autonomous tissue-like materials.

Methylation and phosphorylation of formin homology domain proteins (Fhod1 and Fhod3) by protein arginine methyltransferase 7 (PRMT7) and Rho kinase (ROCK1)

Protein post-translational modifications (PTMs) can regulate biological processes by altering an amino acid's bulkiness, charge, and hydrogen bonding interactions. Common modifications include phosphorylation, methylation, acetylation, and ubiquitylation. Although a primary focus of studying PTMs is understanding the effects of a single amino acid modification, the possibility of additional modifications increases the complexity. For example, substrate recognition motifs for arginine methyltransferases and some serine/threonine kinases overlap, leading to potential enzymatic crosstalk. In this study we have shown that the human family of formin homology domain-containing proteins (Fhods) contain a substrate recognition motif specific for human protein arginine methyltransferase 7 (PRMT7). In particular, PRMT7 methylates two arginine residues in the diaphanous autoinhibitory domain (DAD) of the family of Fhod proteins: R1588 and/or R1590 of Fhod3 isoform 4. Additionally, we confirmed that S1589 and S1595 in the DAD domain of Fhod3 can be phosphorylated by Rho/ROCK1 kinase. Significantly, we have determined that if S1589 is phosphorylated then PRMT7 cannot subsequently methylate R1588 or R1590. In contrast, if R1588 or R1590 of Fhod3 is methylated then ROCK1 phosphorylation activity is only slightly affected. Finally, we show that the interaction of the N-terminal DID domain can also inhibit the methylation of the DAD domain. Taken together these results suggest that the family of Fhod proteins, potential in vivo substrates for PRMT7, might be regulated by a combination of methylation and phosphorylation.

Actin filament dynamics at barbed ends: New structures, new insights

The dynamic actin cytoskeleton contributes to many critical biological processes by providing the structural support underlying the morphology of most cells, facilitating intracellular transport, and generating forces required for cell motility and division. To execute many of these functions, actin monomers polymerize into polarized filaments that display different structural and biochemical properties at each end. Filament dynamics are regulated by diverse regulatory proteins which collaborate to dictate rates of elongation and disassembly, particularly at the fast-growing barbed (plus) end. This review highlights the biochemical mechanisms of six barbed end regulatory proteins: formin, profilin, capping protein, IQGAP1, cyclase-associated protein, and twinfilin. We discuss how individual proteins influence actin dynamics and how several intriguing complex assemblies influence the polymerization fate of actin filaments. Understanding these mechanisms offers insights into how actin is regulated in essential cell processes and dysregulated in disease.

Revealing Different Pathways for Influenza A Virus To Reach Microtubules after Endocytosis by Quantum Dot-Based Single-Virus Tracking

Actin- and microtubule (MT)-based transport systems are essential for intracellular transport. During influenza A virus (IAV) infection, MTs provide long tracks for virus trafficking toward the nucleus. However, the role of the actin cytoskeleton in IAV entry and especially the transit process is still ambiguous. Here, by using quantum dot-based single-virus tracking, it was revealed that the actin cytoskeleton was crucial for the virus entry via clathrin-mediated endocytosis (CME). After entry via CME, the virus reached MTs through three different pathways: the virus (1) was driven by myosin VI to move along actin filaments to reach MTs (AF); (2) was propelled by actin tails assembled by an Arp2/3-dependent mechanism to reach MTs (AT); and (3) directly reached MTs without experiencing actin-related movement (NA). Therefore, the NA pathway was the main one and the fastest for the virus to reach MTs. The AT pathway was activated only when plenty of viruses entered the cell. The viruses transported by the AF and AT pathways shared similar moving velocities, durations, and displacements. This study comprehensively visualized the role of the actin cytoskeleton in IAV entry and transport, revealing different pathways for IAV to reach MTs after entry. The results are of great significance for globally understanding IAV infection and the cellular endocytic transport pathway.

Mechanisms of actin filament severing and elongation by formins

Humans express 15 formins that play crucial roles in actin-based processes, including cytokinesis, cell motility and mechanotransduction1,2. However, the lack of structures bound to the actin filament (F-actin) has been a major impediment to understanding formin function. Whereas formins are known for their ability to nucleate and elongate F-actin3-7, some formins can additionally depolymerize, sever or bundle F-actin. Two mammalian formins, inverted formin 2 (INF2) and diaphanous 1 (DIA1, encoded by DIAPH1), exemplify this diversity. INF2 shows potent severing activity but elongates weakly8-11 whereas DIA1 has potent elongation activity but does not sever4,8. Using cryo-electron microscopy (cryo-EM) we show five structural states of INF2 and two of DIA1 bound to the middle and barbed end of F-actin. INF2 and DIA1 bind differently to these sites, consistent with their distinct activities. The formin-homology 2 and Wiskott-Aldrich syndrome protein-homology 2 (FH2 and WH2, respectively) domains of INF2 are positioned to sever F-actin, whereas DIA1 appears unsuited for severing. These structures also show how profilin-actin is delivered to the fast-growing barbed end, and how this is followed by a transition of the incoming monomer into the F-actin conformation and the release of profilin. Combined, the seven structures presented here provide step-by-step visualization of the mechanisms of F-actin severing and elongation by formins.

Single-cell transcriptome profiling highlights the importance of telocyte, kallikrein genes, and alternative splicing in mouse testes aging

Advancing healthcare for elderly men requires a deeper understanding of testicular aging processes. In this study, we conducted transcriptomic profiling of 43,323 testicular single cells from young and old mice, shedding light on 1032 telocytes-an underexplored testicular cell type in previous research. Our study unveiled 916 age-related differentially expressed genes (age-DEGs), with telocytes emerging as the cell type harboring the highest count of age-DEGs. Of particular interest, four genes (Klk1b21, Klk1b22, Klk1b24, Klk1b27) from the Kallikrein family, specifically expressed in Leydig cells, displayed down-regulation in aged testes. Moreover, cell-type-level splicing analyses unveiled 1838 age-related alternative splicing (AS) events. While we confirmed the presence of more age-DEGs in somatic cells compared to germ cells, unexpectedly, more age-related AS events were identified in germ cells. Further experimental validation highlighted 4930555F03Rik, a non-coding RNA gene exhibiting significant age-related AS changes. Our study represents the first age-related single-cell transcriptomic investigation of testicular telocytes and Kallikrein genes in Leydig cells, as well as the first delineation of cell-type-level AS dynamics during testicular aging in mice.

The DIAPH3 linker specifies a β-actin network that maintains RhoA and Myosin-II at the cytokinetic furrow

Cytokinesis is the final step of the cell division cycle that leads to the formation of two new cells. Successful cytokinesis requires significant remodelling of the plasma membrane by spatially distinct β- and γ-actin networks. These networks are generated by the formin family of actin nucleators, DIAPH3 and DIAPH1 respectively. Here we show that β- and γ-actin perform specialized and non-redundant roles in cytokinesis and cannot substitute for one another. Expression of hybrid DIAPH1 and DIAPH3 proteins with altered actin isoform specificity relocalized cytokinetic actin isoform networks within the cell, causing cytokinetic failure. Consistent with this we show that β-actin networks, but not γ-actin networks, are required for the maintenance of non-muscle myosin II and RhoA at the cytokinetic furrow. These data suggest that independent and spatially distinct actin isoform networks form scaffolds of unique interactors that facilitate localized biochemical activities to ensure successful cell division.

Dehydration-responsive cytoskeleton proteome of rice reveals reprograming of key molecular pathways to mediate metabolic adaptation and cell survival

The plant cytoskeletal proteins play a key role that control cytoskeleton dynamics, contributing to crucial biological processes such as cell wall morphogenesis, stomatal conductance and abscisic acid accumulation in repercussion to water-deficit stress or dehydration. Yet, it is still completely unknown which specific biochemical processes and regulatory mechanisms the cytoskeleton uses to drive dehydration tolerance. To better understand the role of cytoskeleton, we developed the dehydration-responsive cytoskeletal proteome map of a resilient rice cultivar. Initially, four-week-old rice plants were exposed to progressive dehydration, and the magnitude of dehydration-induced compensatory physiological responses was monitored in terms of physicochemical indices. The organelle fractionation in conjunction with label-free quantitative proteome analysis led to the identification of 955 dehydration-responsive cytoskeletal proteins (DRCPs). To our knowledge, this is the first report of a stress-responsive plant cytoskeletal proteome, representing the largest inventory of cytoskeleton and cytoskeleton-associated proteins. The DRCPs were apparently involved in a wide array of intra-cellular molecules transportation, organelles positioning, cytoskeleton organization followed by different metabolic processes including amino acid metabolism. These findings presented open a unique view on global regulation of plant cytoskeletal proteome is intimately linked to cellular metabolic rewiring of adaptive responses, and potentially confer dehydration tolerance, especially in rice, and other crop species, in general.

A trade-off in evolution: the adaptive landscape of spiders without venom glands

BACKGROUND

Venom glands play a key role in the predation and defense strategies of almost all spider groups. However, the spider family Uloboridae lacks venom glands and has evolved an adaptive strategy: they excessively wrap their prey directly with spider silk instead of paralyzing it first with toxins. This shift in survival strategy is very fascinating, but the genetic underpinnings behind it are poorly understood.

RESULTS

Spanning multiple spider groups, we conducted multiomics analyses on Octonoba sinensis and described the adaptive evolution of the Uloboridae family at the genome level. We observed the coding genes of myosin and twitchin in muscles are under positive selection, energy metabolism functions are enhanced, and gene families related to tracheal development and tissue mechanical strength are expanded or emerged, all of which are related to the unique anatomical structure and predatory behavior of spiders in the family Uloboridae. In addition, we also scanned the elements that are absent or under relaxed purifying selection, as well as toxin gene homologs in the genomes of 2 species in this family. The results show that the absence of regions and regions under relaxed selection in these spiders' genomes are concentrated in areas related to development and neurosystem. The search for toxin homologs reveals possible gene function shift between toxins and nontoxins and confirms that there are no reliable toxin genes in the genome of this group.

CONCLUSIONS

This study demonstrates the trade-off between different predation strategies in spiders, using either chemical or physical strategy, and provides insights into the possible mechanism underlying this trade-off. Venomless spiders need to mobilize multiple developmental and metabolic pathways related to motor function and limb mechanical strength to cover the decline in adaptability caused by the absence of venom glands.

Formin tails act as a switch, inhibiting or enhancing processive actin elongation

Formins are large, multidomain proteins that nucleate new actin filaments and accelerate elongation through a processive interaction with the barbed ends of filaments. Their actin assembly activity is generally attributed to their eponymous formin homology (FH) 1 and 2 domains; however, evidence is mounting that regions outside of the FH1FH2 stretch also tune actin assembly. Here, we explore the underlying contributions of the tail domain, which spans the sequence between the FH2 domain and the C terminus of formins. Tails vary in length from ∼0 to >200 residues and contain a number of recognizable motifs. The most common and well-studied motif is the ∼15-residue-long diaphanous autoregulatory domain. This domain mediates all or nothing regulation of actin assembly through an intramolecular interaction with the diaphanous inhibitory domain in the N-terminal half of the protein. Multiple reports demonstrate that the tail can enhance both nucleation and processivity. In this study, we provide a high-resolution view of the alternative splicing encompassing the tail in the formin homology domain (Fhod) family of formins during development. While four distinct tails are predicted, we found significant levels of only two of these. We characterized the biochemical effects of the different tails. Surprisingly, the two highly expressed Fhod-tails inhibit processive elongation and diminish nucleation, while a third supports activity. These findings demonstrate a new mechanism of modulating actin assembly by formins and support a model in which splice variants are specialized to build distinct actin structures during development.

Actin polymerization and depolymerization in developing vertebrates

Development is a complex process that occurs throughout the life cycle. F-actin, a major component of the cytoskeleton, is essential for the morphogenesis of tissues and organs during development. F-actin is formed by the polymerization of G-actin, and the dynamic balance of polymerization and depolymerization ensures proper cellular function. Disruption of this balance results in various abnormalities and defects or even embryonic lethality. Here, we reviewed recent findings on the structure of G-actin and F-actin and the polymerization of G-actin to F-actin. We also focused on the functions of actin isoforms and the underlying mechanisms of actin polymerization/depolymerization in cellular and organic morphogenesis during development. This information will extend our understanding of the role of actin polymerization in the physiologic or pathologic processes during development and may open new avenues for developing therapeutics for embryonic developmental abnormalities or tissue regeneration.

Investigating Mammalian Formins with SMIFH2 Fifteen Years in: Novel Targets and Unexpected Biology

The mammalian formin family comprises fifteen multi-domain proteins that regulate actin dynamics and microtubules in vitro and in cells. Evolutionarily conserved formin homology (FH) 1 and 2 domains allow formins to locally modulate the cell cytoskeleton. Formins are involved in several developmental and homeostatic processes, as well as human diseases. However, functional redundancy has long hampered studies of individual formins with genetic loss-of-function approaches and prevents the rapid inhibition of formin activities in cells. The discovery of small molecule inhibitor of formin homology 2 domains (SMIFH2) in 2009 was a disruptive change that provided a powerful chemical tool to explore formins' functions across biological scales. Here, I critically discuss the characterization of SMIFH2 as a pan-formin inhibitor, as well as growing evidence of unexpected off-target effects. By collating the literature and information hidden in public repositories, outstanding controversies and fundamental open questions about the substrates and mechanism of action of SMIFH2 emerge. Whenever possible, I propose explanations for these discrepancies and roadmaps to address the paramount open questions. Furthermore, I suggest that SMIFH2 be reclassified as a multi-target inhibitor for its appealing activities on proteins involved in pathological formin-dependent processes. Notwithstanding all drawbacks and limitations, SMIFH2 will continue to prove useful in studying formins in health and disease in the years to come.

FHODs: Nuclear tethered formins for nuclear mechanotransduction

In this review, we discuss FHOD formins with a focus on recent studies that reveal a new role for them as critical links for nuclear mechanotransduction. The FHOD family in vertebrates comprises two structurally related proteins, FHOD1 and FHOD3. Their similar biochemical properties suggest overlapping and redundant functions. FHOD1 is widely expressed, FHOD3 less so, with highest expression in skeletal (FHOD1) and cardiac (FHOD3) muscle where specific splice isoforms are expressed. Unlike other formins, FHODs have strong F-actin bundling activity and relatively weak actin polymerization activity. These activities are regulated by phosphorylation by ROCK and Src kinases; bundling is additionally regulated by ERK1/2 kinases. FHODs are unique among formins in their association with the nuclear envelope through direct, high affinity binding to the outer nuclear membrane proteins nesprin-1G and nesprin-2G. Recent crystallographic structures reveal an interaction between a conserved motif in one of the spectrin repeats (SRs) of nesprin-1G/2G and a site adjacent to the regulatory domain in the amino terminus of FHODs. Nesprins are components of the LINC (linker of nucleoskeleton and cytoskeleton) complex that spans both nuclear membranes and mediates bidirectional transmission of mechanical forces between the nucleus and the cytoskeleton. FHODs interact near the actin-binding calponin homology (CH) domains of nesprin-1G/2G enabling a branched connection to actin filaments that presumably strengthens the interaction. At the cellular level, the tethering of FHODs to the outer nuclear membrane mechanically couples perinuclear actin arrays to the nucleus to move and position it in fibroblasts, cardiomyocytes, and potentially other cells. FHODs also function in adhesion maturation during cell migration and in the generation of sarcomeres, activities distant from the nucleus but that are still influenced by it. Human genetic studies have identified multiple FHOD3 variants linked to dilated and hypertrophic cardiomyopathies, with many mutations mapping to "hot spots" in FHOD3 domains. We discuss how FHOD1/3's role in reinforcing the LINC complex and connecting to perinuclear actin contributes to functions of mechanically active tissues such as striated muscle.

Purification of human β- and γ-actin from budding yeast

Biochemical studies of human actin and its binding partners rely heavily on abundant and easily purified α-actin from skeletal muscle. Therefore, muscle actin has been used to evaluate and determine the activities of most actin regulatory proteins but there is an underlying concern that these proteins perform differently from actin present in non-muscle cells. To provide easily accessible and relatively abundant sources of human β- or γ-actin (i.e. cytoplasmic actins), we developed Saccharomyces cerevisiae strains that express each as their sole source of actin. Both β- or γ-actin purified in this system polymerize and interact with various binding partners, including profilin, mDia1 (formin), fascin and thymosin-β4 (Tβ4). Notably, Tβ4 and profilin bind to β- or γ-actin with higher affinity than to α-actin, emphasizing the value of testing actin ligands with specific actin isoforms. These reagents will make specific isoforms of actin more accessible for future studies on actin regulation.

Formin-like 2 promotes angiogenesis and metastasis of colorectal cancer by regulating the EGFL6/CKAP4/ERK axis

Increasing evidence indicates that angiogenesis plays a pivotal role in tumor progression. Formin-like 2 (FMNL2) is well-known for promoting metastasis; however, the molecular mechanisms by which FMNL2 promotes angiogenesis in colorectal cancer (CRC) remain unclear. Here, we found that FMNL2 promotes angiogenesis and metastasis of CRC in vitro and in vivo. The GDB/FH3 domain of FMNL2 directly interacts with epidermal growth factor-like protein 6 (EGFL6). Formin-like 2 promotes EGFL6 paracrine signaling by exosomes to regulate angiogenesis in CRC. Cytoskeleton associated protein 4 (CKAP4) is a downstream target of EGFL6 and is involved in CRC angiogenesis. Epidermal growth factor-like protein 6 binds to the N-terminus of CKAP4 to promote the migration of HUVECs by activating the ERK/MMP pathway. These findings suggest that FMNL2 promotes the migration of HUVECs and enhances angiogenesis and tumorigenesis in CRC by regulating the EGFL6/CKAP4/ERK axis. Therefore, the EGFL6/CKAP4/ERK axis could be a candidate therapeutic target for CRC treatment.

A census of actin-associated proteins in humans

Actin filaments help in maintaining the cell structure and coordinating cellular movements and cargo transport within the cell. Actin participates in the interaction with several proteins and also with itself to form the helical filamentous actin (F-actin). Actin-binding proteins (ABPs) and actin-associated proteins (AAPs) coordinate the actin filament assembly and processing, regulate the flux between globular G-actin and F-actin in the cell, and help maintain the cellular structure and integrity. We have used protein-protein interaction data available through multiple sources (STRING, BioGRID, mentha, and a few others), functional annotation, and classical actin-binding domains to identify actin-binding and actin-associated proteins in the human proteome. Here, we report 2482 AAPs and present an analysis of their structural and sequential domains, functions, evolutionary conservation, cellular localization, abundance, and tissue-specific expression patterns. This analysis provides a base for the characterization of proteins involved in actin dynamics and turnover in the cell.

Cytoskeletal and Cytoskeleton-Associated Proteins: Key Regulators of Cancer Stem Cell Properties

Cancer stem cells (CSCs) are a subpopulation of cancer cells possessing stemness characteristics that are closely associated with tumor proliferation, recurrence and resistance to therapy. Recent studies have shown that different cytoskeletal components and remodeling processes have a profound impact on the behavior of CSCs. In this review, we outline the different cytoskeletal components regulating the properties of CSCs and discuss current and ongoing therapeutic strategies targeting the cytoskeleton. Given the many challenges currently faced in targeted cancer therapy, a deeper comprehension of the molecular events involved in the interaction of the cytoskeleton and CSCs will help us identify more effective therapeutic strategies to eliminate CSCs and ultimately improve patient survival.

DAAM mediates the assembly of long-lived, treadmilling stress fibers in collectively migrating epithelial cells in Drosophila

Stress fibers (SFs) are actomyosin bundles commonly found in individually migrating cells in culture. However, whether and how cells use SFs to migrate in vivo or collectively is largely unknown. Studying the collective migration of the follicular epithelial cells in Drosophila, we found that the SFs in these cells show a novel treadmilling behavior that allows them to persist as the cells migrate over multiple cell lengths. Treadmilling SFs grow at their fronts by adding new integrin-based adhesions and actomyosin segments over time. This causes the SFs to have many internal adhesions along their lengths, instead of adhesions only at the ends. The front-forming adhesions remain stationary relative to the substrate and typically disassemble as the cell rear approaches. By contrast, a different type of adhesion forms at the SF's terminus that slides with the cell's trailing edge as the actomyosin ahead of it shortens. We further show that SF treadmilling depends on cell movement and identify a developmental switch in the formins that mediate SF assembly, with Dishevelled-associated activator of morphogenesis acting during migratory stages and Diaphanous acting during postmigratory stages. We propose that treadmilling SFs keep each cell on a linear trajectory, thereby promoting the collective motility required for epithelial migration.

Actin in 2021

Robert Insall introduces the cytoskeleton special issue and summarises some recent changes in our view of actin function and regulation.