PI(3,4)P2-mediated membrane tubulation promotes integrin trafficking and invasive cell migration

IFITM2 Modulates Endocytosis Maintaining Neural Stem Cells in Developing Neocortex

Brain development is orchestrated by a complex interplay of genetic and environmental signals, with endocytosis serving as a pivotal process in integrating extracellular cues. However, the specific role of endocytosis in neurogenesis remains unclear. We uncover a critical function of the interferon-induced transmembrane protein, IFITM2, essential for endocytic processes in radial glial cells (RGCs). IFITM2 is highly expressed near the ventricular surface in the developing brain. Loss of IFITM2 impairs endosome formation and disrupts RGC maintenance. Mechanistically, we confirmed that the YXXø endocytic motif on IFITM2 is essential for its subcellular localization, with mutations in this motif reducing endocytic vesicles. Additionally, the K82 and K87 residues of IFITM2 interact with phosphoinositides to promote endocytic vesicle formation. Polarized localization of phosphatidylinositol 3,4-bisphosphate (PI(3,4)P2) on the ventricular side suggests its role in vesicle formation. IFITM2 deficiency also leads to reduced phosphorylation of AKT and GSK3β. These findings highlight the essential role of IFITM2 in regulating endocytosis in RGCs, which is critical for maintaining neural stem cells and proper brain development, offering new insights into the connection between cellular signaling and neurogenesis in both mouse and human models.

Phosphoinositide signaling at the cytoskeleton in the regulation of cell dynamics

The cytoskeleton, composed of microfilaments, intermediate filaments, and microtubules, provides the structural basis for cellular functions such as motility and adhesion. Equally crucial, phosphoinositide (PIPn) signaling is a critical regulator of these processes and other biological activities, though its precise impact on cytoskeletal dynamics has yet to be systematically investigated. This review explores the complex interplay between PIPn signaling and the cytoskeleton, detailing how PIPn modulates the dynamics of actin, intermediate filaments, and microtubules to shape cellular behavior. Dysregulation of PIPn signaling is implicated in various diseases, including cancer, highlighting promising therapeutic opportunities through targeted modulation of these pathways. Future research should aim to elucidate the intricate molecular interactions and broader cellular responses to PIPn signaling perturbations, particularly in disease contexts, to devise effective strategies for restoring cytoskeletal integrity.

Glycocalyx-induced formation of membrane tubes

Tubular membrane structures are ubiquitous in cells and in the membranes of intracellular organelles such as the Golgi complex and the endoplasmic reticulum. Tubulation plays essential roles in numerous biological processes, including filopodia growth, trafficking, ion transport, and cellular motility. Understanding the fundamental mechanism of the formation of membrane tubes is thus an important problem in the fields of biology and biophysics. Although extensive studies have shown that tubes can be formed due to localized forces acting on the membrane or by the spontaneous curvature induced by membrane-bound proteins, little is known about how membrane tubes are induced by glycocalyx, a sugar-rich layer at the cell surface. In this work, we develop a biophysical model that combines polymer physics theory and the Canham-Helfrich membrane theory to investigate how the glycocalyx generates cylindrical tubular protrusions on the cell membrane. Our results show that the glycocalyx alone can induce the formation of tubular membrane structures. This tube formation involves a first-order shape transition without any externally applied force or other curvature-inducing mechanisms. We also find there exist critical values of glycocalyx grafting density and glycopolymer length needed to induce the formation of tubular structures. The presence of a vertical actin force, line tension, and spontaneous curvature reduce this critical grafting density and length of polymer that triggers the formation of membrane tube, which suggests that the glycocalyx makes tube formation energetically more favorable when combined with an actin force, line tension, and spontaneous curvature.

PI(3,4,5)P3-mediated Cdc42 activation regulates macrophage podosome assembly

Podosomes are adhesion structures with densely-polymerized F-actin. While PI(3,4,5)P3 and Cdc42-GTP are known factors to trigger WASP-mediated actin polymerization at the macrophage podosome, their causal mechanism to activate WASP remains unclear. Here, we demonstrate that spatially elevated Cdc42-GTP is a downstream effector of local PI(3,4,5)P3 production at the podosome. We further examine the expression and distribution of 19 Cdc42 guanine exchange factors (GEFs) and identify VAV1 as the key PI(3,4,5)P3-dependent Cdc42 GEF. VAV1 is spatially enriched at the macrophage podosome, and the association of VAV1 with the membrane plays a critical role in upregulating its GEF activity. Reintroduction of wildtype VAV1, rather than the PI(3,4,5)P3-binding deficient or catalytically dead mutants restores the matrix degradation and chemotactic migration of VAV1-knockdown macrophage. Thus, the biogenesis of PI(3,4,5)P3 acts as an upstream signal to locally recruit VAV1 and in turn triggers the guanine nucleotide exchange of Cdc42. Elevated levels of Cdc42-GTP then promote WASP-mediated podosome assembly and macrophage chemotaxis.

Phosphatidylinositol promoted the proliferation and invasion of pituitary adenoma cells by regulating POU1F1 expression

Invasiveness of pituitary adenoma is the main cause of its poor prognosis, mechanism of which remains largely unknown. In this study, the differential proteins between invasive and non-invasive pituitary tumors (IPA and NIPA) were identified by TMT labeled quantitative proteomics. The differential metabolites in venous bloods from patients with IPA and NIPA were analyzed by untargeted metabolomics. Proteomic data showed that the top five up-regulated proteins were AD021, C2orf15, PLCXD3, HIST3H2BB and POU1F1, and the top five down-regulated proteins were AIPL1, CALB2, GLUD2, SLC4A10 and GTF2I. Metabolomic data showed that phosphatidylinositol (PI) was most remarkably up-regulated and melibiose was most obviously down-regulated. Further investigation demonstrated that PI stimulation increased the expression of PITPNM1, POU1F1, C2orf15 and LDHA as well as the phosphorylation of AKT and ERK, and promoted the proliferation, migration and invasion of GH3 cells, which were blocked by PITPNM1knockdown. Inhibiting AKT phosphorylation reduced the expression of POU1F1, C2orf15 and LDHA in PI-stimulated cells while activating AKT increased their expression in PITPNM1-silencing cells, which was similar to the function of ERK. POU1F1 silence suppressed the expression of LDHA and C2orf15. Luciferase report assay and ChIP assay demonstrated that POU1F1 positively regulated the transcription of LDHA and C2orf15. In addition, PI propelled the metastasis of GH3 cells in vivo, and elevated the expression of PITPNM1, POU1F1, C2orf15 and LDHA. These results suggested that elevated serum PI might contribute to the proliferation and invasion of pituitary adenoma by regulating the expression of PITPNM1/AKT/ERK/POU1F1 axis.

Glycocalyx-induced formation of membrane tubes

Tubular membrane structures are ubiquitous in cells and in the membranes of intracellular organelles such as the Golgi complex and the endoplasmic reticulum. Tubulation plays essential roles in numerous biological processes, including filopodia growth, trafficking, ion transport, and cellular motility. Understanding the fundamental mechanism of the formation of membrane tubes is thus an important problem in the fields of biology and biophysics. Though extensive studies have shown that tubes can be formed due to localized forces acting on the membrane or by the curvature induced by membrane-bound proteins, little is known about how membrane tubes are induced by glycocalyx, a sugar-rich layer at the cell surface. In this work, we develop a biophysical model that combines polymer physics theory and the Canham-Helfrich membrane theory to investigate how the glycocalyx generates cylindrical tubular protrusions on the cell membrane. Our results show that the glycocalyx alone can induce the formation of tubular membrane structures. This tube formation involves a first-order shape transition without any externally applied force or other curvature-inducing mechanisms. We also find that critical values of glycocalyx grafting density and glycopolymer length are needed to induce the formation of tubular structures. The presence of vertical actin force, line tension, and spontaneous curvature reduces the critical grafting density and length of polymer that triggers the formation of membrane tube, which suggests that the glycocalyx makes tube formation energetically more favorable when combined with an actin force, line tension, and spontaneous curvature.

Significance Statement

In many cells, the existence of glycocalyx, a thick layer of polymer meshwork comprising proteins and complex sugar chains coating the outside of the cell membrane, regulates the formation of membrane tubes. Here, we propose a theoretical model that combines polymer physics theory and the Canham-Helfrich membrane theory to study the formation of cylindrical tubular protrusions induced by the glycocalyx. Our findings indicate that glycocalyx plays an important role in the formation of membrane tubes. We find that there exists critical grafting density and length of polymer that triggers the formation of membrane tubes, and the glycocalyx-induced tube formation is facilitated when combined with actin forces, line tension, and spontaneous curvature. Our theoretical model has implications for understanding how biological membranes may form tubular structures.

Phosphatidylinositol 4-Kinase III Alpha Governs Cytoskeletal Organization for Invasiveness of Liver Cancer Cells

BACKGROUND & AIMS

High expression of phosphatidylinositol 4-kinase III alpha (PI4KIIIα) correlates with poor survival rates in patients with hepatocellular carcinoma. In addition, hepatitis C virus (HCV) infections activate PI4KIIIα and contribute to hepatocellular carcinoma progression. We aimed at mechanistically understanding the impact of PI4KIIIα on the progression of liver cancer and the potential contribution of HCV in this process.

METHODS

Several hepatic cell culture and mouse models were used to study the functional importance of PI4KIIIα on liver pathogenesis. Antibody arrays, gene silencing, and PI4KIIIα-specific inhibitor were applied to identify the involved signaling pathways. The contribution of HCV was examined by using HCV infection or overexpression of its nonstructural protein.

RESULTS

High PI4KIIIα expression and/or activity induced cytoskeletal rearrangements via increased phosphorylation of paxillin and cofilin. This led to morphologic alterations and higher migratory and invasive properties of liver cancer cells. We further identified the liver-specific lipid kinase phosphatidylinositol 3-kinase C2 domain-containing subunit gamma (PIK3C2γ) working downstream of PI4KIIIα in regulation of the cytoskeleton. PIK3C2γ generates plasma membrane phosphatidylinositol 3,4-bisphosphate-enriched, invadopodia-like structures that regulate cytoskeletal reorganization by promoting Akt2 phosphorylation.

CONCLUSIONS

PI4KIIIα regulates cytoskeleton organization via PIK3C2γ/Akt2/paxillin-cofilin to favor migration and invasion of liver cancer cells. These findings provide mechanistic insight into the contribution of PI4KIIIα and HCV to the progression of liver cancer and identify promising targets for therapeutic intervention.

Phosphatidylinositol 3,4-bisphosphate: Out of the shadows and into the spotlight

Phosphoinositide 3-kinases regulate many cellular functions, including migration, growth, proliferation, and cell survival. Early studies equated the inhibition of Class I PI3Ks with loss of; phosphatidylinositol 3,4,5-trisphosphate (PIP3), but over time, it was realised that these; treatments also depleted phosphatidylinositol 3,4-bisphosphate (PI(3,4)P2). In recent years, the; use of better tools and an improved understanding of its metabolism have allowed for the; identification of specific roles of PI(3,4)P2. This includes the production of PI(3,4)P2 and the; activation of its effector Akt2 in response to growth factor signalling. In contrast, a lysosomal pool of PI(3,4)P2 is a negative regulator of mTORC1 during growth factor deprivation. A growing body of literature also demonstrates that PI(3,4)P2 controls many dynamic plasmalemmal processes. The significance of PI(3,4)P2 in cell biology is increasingly evident.

Dynamics of membrane tubulation coupled with fission by a two-component module

Membrane tubulation coupled with fission (MTCF) is a widespread phenomenon but mechanisms for their coordination remain unclear, partly because of the lack of assays to monitor dynamics of membrane tubulation and subsequent fission. Using polymer cushioned bilayer islands, we analyze the membrane tubulator Bridging Integrator 1 (BIN1) mixed with the fission catalyst dynamin2 (Dyn2). Our results reveal this mixture to constitute a minimal two-component module that demonstrates MTCF. MTCF is an emergent property and arises because BIN1 facilitates recruitment but inhibits membrane binding of Dyn2 in a dose-dependent manner. MTCF is therefore apparent only at high Dyn2 to BIN1 ratios. Because of their mutual involvement in T-tubules biogenesis, mutations in BIN1 and Dyn2 are associated with centronuclear myopathies and our analysis links the pathology with aberrant MTCF. Together, our results establish cushioned bilayer islands as a facile template for the analysis of membrane tubulation and inform of mechanisms that coordinate MTCF.

Membrane Tubulation with a Biomembrane Force Probe

Tubulation is a common cellular process involving the formation of membrane tubes ranging from 50 nm to 1 µm in diameter. These tubes facilitate intercompartmental connections, material transport within cells and content exchange between cells. The high curvature of these tubes makes them specific targets for proteins that sense local geometry. In vitro, similar tubes have been created by pulling on the membranes of giant unilamellar vesicles. Optical tweezers and micromanipulation are typically used in these experiments, involving the manipulation of a GUV with a micropipette and a streptavidin-coated bead trapped in optical tweezers. The interaction forms streptavidin/biotin bonds, leading to tube formation. Here, we propose a cost-effective alternative using only micromanipulation techniques, replacing optical tweezers with a Biomembrane Force Probe (BFP). The BFP, employing a biotinylated erythrocyte as a nanospring, allows for the controlled measurement of forces ranging from 1 pN to 1 nN. The BFP has been widely used to study molecular interactions in cellular processes, extending beyond its original purpose. We outline the experimental setup, tube formation and characterization of tube dimensions and energetics, and discuss the advantages and limitations of this approach in studying membrane tubulation.

Regulation of Cell Adhesion and Migration via Microtubule Cytoskeleton Organization, Cell Polarity, and Phosphoinositide Signaling

The capacity for cancer cells to metastasize to distant organs depends on their ability to execute the carefully choreographed processes of cell adhesion and migration. As most human cancers are of epithelial origin (carcinoma), the transcriptional downregulation of adherent/tight junction proteins (e.g., E-cadherin, Claudin and Occludin) with the concomitant gain of adhesive and migratory phenotypes has been extensively studied. Most research and reviews on cell adhesion and migration focus on the actin cytoskeleton and its reorganization. However, metastasizing cancer cells undergo the extensive reorganization of their cytoskeletal system, specifically in originating/nucleation sites of microtubules and their orientation (e.g., from non-centrosomal to centrosomal microtubule organizing centers). The precise mechanisms by which the spatial and temporal reorganization of microtubules are linked functionally with the acquisition of an adhesive and migratory phenotype as epithelial cells reversibly transition into mesenchymal cells during metastasis remains poorly understood. In this Special Issue of "Molecular Mechanisms Underlying Cell Adhesion and Migration", we highlight cell adhesion and migration from the perspectives of microtubule cytoskeletal reorganization, cell polarity and phosphoinositide signaling.

cis-B7:CD28 interactions at invaginated synaptic membranes provide CD28 co-stimulation and promote CD8+ T cell function and anti-tumor immunity

B7 ligands (CD80 and CD86), expressed by professional antigen-presenting cells (APCs), activate the main co-stimulatory receptor CD28 on T cells in trans. However, in peripheral tissues, APCs expressing B7 ligands are relatively scarce. This raises the questions of whether and how CD28 co-stimulation occurs in peripheral tissues. Here, we report that CD8+ T cells displayed B7 ligands that interacted with CD28 in cis at membrane invaginations of the immunological synapse as a result of membrane remodeling driven by phosphoinositide-3-kinase (PI3K) and sorting-nexin-9 (SNX9). cis-B7:CD28 interactions triggered CD28 signaling through protein kinase C theta (PKCθ) and promoted CD8+ T cell survival, migration, and cytokine production. In mouse tumor models, loss of T cell-intrinsic cis-B7:CD28 interactions decreased intratumoral T cells and accelerated tumor growth. Thus, B7 ligands on CD8+ T cells can evoke cell-autonomous CD28 co-stimulation in cis in peripheral tissues, suggesting cis-signaling as a general mechanism for boosting T cell functionality.

SWIP mediates retromer-independent membrane recruitment of the WASH complex

The pentameric WASH complex facilitates endosomal protein sorting by activating Arp2/3, which in turn leads to the formation of F-actin patches specifically on the endosomal surface. It is generally accepted that WASH complex attaches to the endosomal membrane via the interaction of its subunit FAM21 with the retromer subunit VPS35. However, we observe the WASH complex and F-actin present on endosomes even in the absence of VPS35. We show that the WASH complex binds to the endosomal surface in both a retromer-dependent and a retromer-independent manner. The retromer-independent membrane anchor is directly mediated by the subunit SWIP. Furthermore, SWIP can interact with a number of phosphoinositide species. Of those, our data suggest that the interaction with phosphatidylinositol-3,5-bisphosphate (PI(3,5)P2 ) is crucial to the endosomal binding of SWIP. Overall, this study reveals a new role of the WASH complex subunit SWIP and highlights the WASH complex as an independent, self-sufficient trafficking regulator.

Small molecules targeting endocytic uptake and recycling pathways

Over the past years a growing number of studies highlighted the pivotal role of intracellular trafficking in cell physiology. Among the distinct transport itineraries connecting the endocytic system, both internalization (endocytosis) and recycling (endocytic recycling) pathways were found fundamental to ensure cellular sensing, cell-to-cell communication, cellular division, and collective cell migration in tissue specific-contexts. Consistently, the dysregulation of endocytic trafficking pathways is correlated with several human diseases including both cancers and neurodegeneration. Aimed at suppress specific intracellular trafficking routes involved in disease onset and progression, huge efforts have been made to identify small molecule inhibitors with suitable pharmacological properties for in vivo administration. Here, we review most used drugs and recently discovered small molecules able to block endocytosis and endocytic recycling pathways. We characterize such pharmacological inhibitors by emphasizing their target specificity, molecular affinity, biological activity and efficacy in both in vitro and in vivo experimental models.

Endosomal sorting sorted - motors, adaptors and lessons from in vitro and cellular studies

Motor proteins are key players in exerting spatiotemporal control over the intracellular location of membrane-bound compartments, including endosomes containing cargo. In this Review, we focus on how motors and their cargo adaptors regulate positioning of cargoes from the earliest stages of endocytosis and through the two main intracellular itineraries: (1) degradation at the lysosome or (2) recycling back to the plasma membrane. In vitro and cellular (in vivo) studies on cargo transport thus far have typically focussed independently on either the motor proteins and adaptors, or membrane trafficking. Here, we will discuss recent studies to highlight what is known about the regulation of endosomal vesicle positioning and transport by motors and cargo adaptors. We also emphasise that in vitro and cellular studies are often performed at different scales, from single molecules to whole organelles, with the aim to provide a perspective on the unified principles of motor-driven cargo trafficking in living cells that can be learned from these differing scales.

Integrin receptor trafficking in health and disease

Integrins are a family of 24 different heterodimers that are indispensable for multicellular life. Cell polarity, adhesion and migration are controlled by integrins delivered to the cell surface which in turn is regulated by the exo- and endocytic trafficking of integrins. The deep integration between trafficking and cell signaling determines the spatial and temporal output from any biochemical cue. Integrin trafficking plays a key role in development and many pathological conditions, especially cancer. Several novel regulators of integrin traffic have been discovered in recent times, including a novel class of integrin carrying vesicles, the intracellular nanovesicles (INVs). The tight regulation of trafficking pathways by cell signaling, where kinases phosphorylate key small GTPases in the trafficking pathway enable coordination of cell response to the extracellular milieu. Integrin heterodimer expression and trafficking differ in different tissues and contexts. In this Chapter, we discuss recent studies on integrin trafficking and its contribution to normal physiological and pathophysiological states.

G-protein coupled receptor 34 regulates the proliferation and growth of LS174T cells through differential expression of PI3K subunits and PTEN

PURPOSE

G-protein coupled receptor (GPR 34) has been found to play important roles in some cancers and regulates the proliferation, apoptosis, and migration of these cancer cells. However, the mechanisms underlying how GPR34 functions to regulate growth and proliferation of colorectal cancer cells remains to be clarified.

METHODS

We employed stable GPR34 knockdown LS174T cell models, GPR34 Mab blocking, a CCK-8 kit, and a colony formation assay to characterize the effect of GPR34 on the proliferation of LS174T in vitro and xenograft tumor growth in vivo. The mRNA level of GPR34 was detected by RT-PCR in tumor tissues and adjacent normal tissues from 34 CRC patients.

RESULTS

Based on RT-PCR results, GPR34 exhibited high level in tumor samples compared with adjacent normal samples. Increased expression of GPR34 is more associated with poor prognosis of CRC as shown in The Cancer Genome Atlas (TCGA) dataset by Kaplan-Meier survival analysis. Furthermore, we showed that GPR34 knockdown inhibited the proliferation of LS174T colon cancer cells and related xenograft tumor growth. Searching for the distinct molecular mechanism, we identified several contributors to proliferation of LS174T colon cancer cells: PI3K subunits/PTEN, PDK1/AKT, and Src/Raf/Ras/ERK. GPR34 knockdown inhibited the proliferation of LS174T cells by upregulating expression of PTEN, and downregulating expression of PI3K subunits p110-beta.

CONCLUSION

Our findings provide direct evidence that GPR34 regulates the proliferation of LS174T cells and the growth of LS174T tumor xenografts by regulating different pathways. High expression of GPR34 mRNA could then be used to predict poor prognosis of CRC.

Phosphoinositide Conversion Inactivates R-RAS and Drives Metastases in Breast Cancer

Breast cancer is the most prevalent cancer and a major cause of death in women worldwide. Although early diagnosis and therapeutic intervention significantly improve patient survival rate, metastasis still accounts for most deaths. Here it is reported that, in a cohort of more than 2000 patients with breast cancer, overexpression of PI3KC2α occurs in 52% of cases and correlates with high tumor grade as well as increased probability of distant metastatic events, irrespective of the subtype. Mechanistically, it is demonstrated that PI3KC2α synthetizes a pool of PI(3,4)P2 at focal adhesions that lowers their stability and directs breast cancer cell migration, invasion, and metastasis. PI(3,4)P2 locally produced by PI3KC2α at focal adhesions recruits the Ras GTPase activating protein 3 (RASA3), which inactivates R-RAS, leading to increased focal adhesion turnover, migration, and invasion both in vitro and in vivo. Proof-of-concept is eventually provided that inhibiting PI3KC2α or lowering RASA3 activity at focal adhesions significantly reduces the metastatic burden in PI3KC2α-overexpressing breast cancer, thereby suggesting a novel strategy for anti-breast cancer therapy.

AKTIP interacts with ESCRT I and is needed for the recruitment of ESCRT III subunits to the midbody

To complete mitosis, the bridge that links the two daughter cells needs to be cleaved. This step is carried out by the endosomal sorting complex required for transport (ESCRT) machinery. AKTIP, a protein discovered to be associated with telomeres and the nuclear membrane in interphase cells, shares sequence similarities with the ESCRT I component TSG101. Here we present evidence that during mitosis AKTIP is part of the ESCRT machinery at the midbody. AKTIP interacts with the ESCRT I subunit VPS28 and forms a circular supra-structure at the midbody, in close proximity with TSG101 and VPS28 and adjacent to the members of the ESCRT III module CHMP2A, CHMP4B and IST1. Mechanistically, the recruitment of AKTIP is dependent on MKLP1 and independent of CEP55. AKTIP and TSG101 are needed together for the recruitment of the ESCRT III subunit CHMP4B and in parallel for the recruitment of IST1. Alone, the reduction of AKTIP impinges on IST1 and causes multinucleation. Our data altogether reveal that AKTIP is a component of the ESCRT I module and functions in the recruitment of ESCRT III components required for abscission.

To complete cell division, the bridge that links the two daughter cells needs to be cleaved. This step is carried out by a machinery named “endosomal sorting complex required for transport” (ESCRT). The dissection of this machinery is important in basic biology and for investigating diseases in which cell division is altered. AKTIP, a factor discovered to be needed for chromosome integrity, shares similarities with a component of the ESCRT machinery named TSG101. Here we present evidence that AKTIP is part of the ESCRT machinery, as TSG101. More specifically, we show that AKTIP physically interacts with members of the ESCRT machinery and forms a characteristic circular structure at the center of the bridge linking the daughter cells. We also show that the reduction of AKTIP levels causes defects in the assembly of the ESCRT machinery and in cell division. In future work, it will be interesting to investigate the association of AKTIP with cancer, because in tumorigenesis cell division is altered and since an implication in cancer has been described for TSG101 and other ESCRT factors.