Internalization-competent influenza hemagglutinin mutants form complexes with clathrin-deficient multivalent AP-2 oligomers in live cells

Viral Interactions with Adaptor-Protein Complexes: A Ubiquitous Trait among Viral Species

Numerous viruses hijack cellular protein trafficking pathways to mediate cell entry or to rearrange membrane structures thereby promoting viral replication and antagonizing the immune response. Adaptor protein complexes (AP), which mediate protein sorting in endocytic and secretory transport pathways, are one of the conserved viral targets with many viruses possessing AP-interacting motifs. We present here different mechanisms of viral interference with AP complexes and the functional consequences that allow for efficient viral propagation and evasion of host immune defense. The ubiquity of this phenomenon is evidenced by the fact that there are representatives for AP interference in all major viral families, covered in this review. The best described examples are interactions of human immunodeficiency virus and human herpesviruses with AP complexes. Several other viruses, like Ebola, Nipah, and SARS-CoV-2, are pointed out as high priority disease-causative agents supporting the need for deeper understanding of virus-AP interplay which can be exploited in the design of novel antiviral therapies.

Epsin15 Homology Domains: Role in the Pathogenesis of Pulmonary Arterial Hypertension

Intersectin-1s (ITSN) deficiency and expression of a biologically active ITSN fragment, result of granzyme B cleavage under inflammatory conditions associated with pulmonary arterial hypertension (PAH), are characteristics of lung tissue of human and animal models of PAH. Recently, we have shown that this ITSN fragment comprising two Epsin15 homology domains (EH_ITSN) triggers endothelial cell (EC) proliferation and the plexiform arteriopathy in PAH. Limited evidence also indicates that the EH domains of endocytic proteins such as ITSN, upregulate compensatory endocytic pathways in cells with impaired vesicular trafficking. Thus, we sought to investigate whether the EH_ITSN may be involved in this compensatory mechanism for improving the EC endocytic dysfunction induced by ITSN deficiency and possibly contribute to PAH pathogenesis. We used stably-transfected human pulmonary artery ECs expressing the Myc-EH_ITSN (EC_EH-ITSN) and ITSN knockout heterozygous mice (K0^ITSN+/-) transduced with the Myc-EH_ITSN, in conjunction with functional assays: the biotin assay for caveolae internalization and 8 nm gold (Au)- and dinitrophenylated (DNP)-albumin perfusion of murine lung microvasculature. Pulmonary artery ECs of PAH patients (EC_PAH), ITSN knockdown ECs (EC_KD-ITSN), the monocrotaline (MCT)-induced mouse and rat models of PAH, as well as untreated animals, served as controls. ELISA via streptavidin-HRP or anti-DNP antibody (Ab), applied on ECs and lung lysates indicated greater than 30% increase in biotin internalization in EC_EH-ITSN compared to EC_Ctrl. Despite their endocytic deficiency, EC_PAH internalized biotin similar to EC_Ctrl which is twofold higher compared to EC_KD-ITSN. Moreover, the lung microvascular bed of Myc-EH_ITSN-transduced mice and MCT-treated animals showed greater than twofold increase in DNP-BSA transendothelial transport, all compared to untreated controls. Electron microscopy (EM) revealed the increased occurrence of non-conventional endocytic/transcytotic structures (i.e., caveolae clusters, tubulo-vesicular and enlarged endocytic structures, membranous rings), usually underrepresented. Most of these structures were labeled by Au-BSA, consistent with their involvement in the transendothelial transport. Furthermore, ITSN deficiency and EH_ITSN expression alter the subcellular localization of the EH-binding protein 1 (EHBP1) and cortical actin organization, altogether supporting the increase occurrence/trafficking of the alternative endocytic structures. Thus, the EH_ITSN by shifting the physiological vesicular (caveolae) transport toward the alternative endocytic pathways is a significant contributor to the dysfunctional molecular phenotype of EC_PAH.

Differential regulation of translation and endocytosis of alternatively spliced forms of the type II bone morphogenetic protein (BMP) receptor

The cytoplasmic extension of the long-form isoform of BMPRII, unique among TGF-β superfamily receptors, is found to regulate the translation of BMPRII and its clathrin-mediated endocytosis. Both processes reduce its cell surface levels. The higher expression of BMPRII-SF at the plasma membrane results in enhanced activation of Smad signaling.

The expression and function of transforming growth factor-β superfamily receptors are regulated by multiple molecular mechanisms. The type II BMP receptor (BMPRII) is expressed as two alternatively spliced forms, a long and a short form (BMPRII-LF and –SF, respectively), which differ by an ∼500 amino acid C-terminal extension, unique among TGF-β superfamily receptors. Whereas this extension was proposed to modulate BMPRII signaling output, its contribution to the regulation of receptor expression was not addressed. To map regulatory determinants of BMPRII expression, we compared synthesis, degradation, distribution, and endocytic trafficking of BMPRII isoforms and mutants. We identified translational regulation of BMPRII expression and the contribution of a 3’ terminal coding sequence to this process. BMPRII-LF and -SF differed also in their steady-state levels, kinetics of degradation, intracellular distribution, and internalization rates. A single dileucine signal in the C-terminal extension of BMPRII-LF accounted for its faster clathrin-mediated endocytosis relative to BMPRII-SF, accompanied by mildly faster degradation. Higher expression of BMPRII-SF at the plasma membrane resulted in enhanced activation of Smad signaling, stressing the potential importance of the multilayered regulation of BMPRII expression at the plasma membrane.

Deletions in the highly polymorphic region (HPR) of infectious salmon anaemia virus HPR0 haemagglutinin-esterase enhance viral fusion and influence the interaction with the fusion protein

Since the discovery of a non-virulent infectious salmon anaemia virus (ISAV) HPR0 variant, many studies have speculated on the functional role of deletions within the highly polymorphic region (HPR) of genomic segment 6, which codes for the haemagglutinin-esterase (HE) protein. To address this issue, mutant HE proteins with deletions in their HPR were generated from the Scottish HPR0 template (NWM10) and fusion-inducing activity was measured using lipid (octadecyl rhodamine B) and content mixing assays (firefly luciferase). Segment six HPR was found to have a strong influence on ISAV fusion, and deletions in this near-membrane region predominantly increased the fusion-inducing ability of the resulting HE proteins. The position and length of the HPR deletions were not significant factors, suggesting that they may affect fusion non-specifically. In comparison, the amino acid composition of the associated fusion (F) protein was a more crucial criterion. Antibody co-patching and confocal fluorescence demonstrated that the HE and F proteins were highly co-localized, forming defined clusters on the cell surface post-transfection. The binding of erythrocyte ghosts on the attachment protein caused a reduction in the percentage of co-localization, suggesting that ISAV fusion might be triggered through physical separation of the F and HE proteins. In this process, HPR deletion appeared to modulate and reduce the strength of interaction between the two glycoproteins, causing more F protein to be released and activated. This work provides a first insight into the mechanism of virulence acquisition through HPR deletion, with fusion enhancement acting as a major contributing factor.

Different domains regulate homomeric and heteromeric complex formation among type I and type II transforming growth factor-beta receptors

Transforming growth factor-beta (TGF-beta) binds to and signals via two serine-threonine kinase receptors, type I (TbetaRI) and type II (TbetaRII). The oligomerization of TGF-beta receptors modulates ligand binding and receptor trafficking and may contribute to signal diversification. However, numerous features of the molecular domains that determine the homo- and hetero-oligomerization of full-length receptors at the cell surface and the mode of these interactions remain unclear. Here, we address these questions through computerized immunofluorescence co-patching and patch/fluorescence recovery after photobleaching measurements of different combinations of epitope-tagged receptors and their mutants in live cells. We show that TbetaRI and TbetaRII are present on the plasma membrane both as monomers and homo- and hetero-oligomers. The homodimerization of TbetaRII depends on a cytoplasmic juxtamembrane region (amino acid residues 200-220). In contrast, the cytoplasmic domain of TbetaRI is dispensable for its homodimerization. TbetaRI.TbetaRII hetero-oligomerization depends on the cytoplasmic domain of TbetaRI and on a C-terminal region of TbetaRII (residues 419-565). TGF-beta1 elevates TbetaRII homodimerization to some degree and strongly enhances TbetaRI.TbetaRII heteromeric complex formation. Both ligand-induced effects depend on the region encompassed between residues 200-242 of TbetaRII. Furthermore, the kinase activity of TbetaRI is also necessary for the latter effect. All forms of the homo- and hetero-oligomers, whether constitutively present on the membrane or formed upon TGF-beta1 stimulation, were stable in the time-scale of our patch/FRAP measurements. We suggest that the different forms of receptor oligomerization may serve as a basis for the heterogeneity of TGF-beta signaling responses.

Ligand-independent homomeric and heteromeric complexes between interleukin-2 or -9 receptor subunits and the gamma chain

Signaling via interleukin-2 (IL-2) and interleukin-9 receptors (IL-2R and IL-9R) involves heteromeric interactions between specific interleukin receptor subunits, which bind Janus kinase 1 (JAK1) and the JAK3 binding common gamma chain (gamma c). The potential existence and roles of homomeric and heteromeric complexes before ligand binding and their modulation by ligand and JAK3 are unclear. Using computerized antibody-mediated immunofluorescence co-patching of epitope-tagged receptors at the surface of live cells, we demonstrate that IL-2Rbeta, IL-9Ralpha, and gamma c each display a significant fraction of ligand-independent homomeric complexes (24-28% co-patching), whereas control co-patching levels with unrelated receptors are very low (7%). Heteromeric complex formation of IL2-Rbeta or IL-9Ralpha with gamma c is also observed in the absence of ligand (15-30%). Ligand binding increases this hetero-oligomerization 2-fold but does not affect homo-oligomerization. Co-expression of IL-2Ralpha does not affect the hetero-oligomerization of IL-2Rbeta and gamma c. Recruitment of gamma c into heterocomplexes is partly at the expense of its homo-oligomerization, suggesting that a functional role of the latter may be to keep the receptors inactive in the absence of ligand. At the same time, the preformed complexes between gamma c and IL-2Rbeta or IL-9Ralpha promote signaling by the JAK3 A572V mutant without ligand, supporting a pathophysiological role for the constitutive oligomerization in triggering ligand-independent activation of JAK3 (and perhaps other JAK mutants) mutants identified in several human cancers.

Host-cell-dependent role of actin cytoskeleton during the replication of a human strain of influenza A virus

This study was aimed at investigating the possible involvement of the actin cytoskeleton in the modulation of host permissiveness to A/NWS/33 human influenza virus infection in two mammalian (MDCK and LLC-MK2) cell lines in vitro. During the early stages of infection, no appreciable association between incoming NWS/33 virions and cortical actin was detectable in the permissive MDCK model by confocal microscopy, while extensive colocalization and a slower infection progression were observed in LLC-MK2 cells. In the latter model, we also demonstrated the inability of the virus to carry out multiple replication cycles, irrespective of the presence of cleaved HA subunits in the released virions. Treatment with the actin-depolymerizing agent cytochalasin D significantly increased the infection efficiency in LLC-MK2 cells, while a detrimental effect was observed in the MDCK cell line. Our data suggest a selective role of the actin network in inducing a restriction to influenza virus replication, mostly depending on its molecular organization, the host cell type and virus replication phase.

Differently anchored influenza hemagglutinin mutants display distinct interaction dynamics with mutual rafts

Lipid rafts play important roles in cellular functions through concentrating or sequestering membrane proteins. This requires proteins to differ in the stability of their interactions with lipid rafts. However, knowledge of the dynamics of membrane protein-raft interactions is lacking. We employed FRAP to measure in live cells the lateral diffusion of influenza hemagglutinin (HA) proteins that differ in raft association. This approach can detect weak interactions with rafts not detectable by biochemical methods. Wild-type (wt) HA and glycosylphosphatidylinositol (GPI)-anchored HA (BHA-PI) diffused slower than a nonraft HA mutant, but became equal to the latter after cholesterol depletion. When antigenically distinct BHA-PI and wt HA were coexpressed, aggregation of BHA-PI into immobile patches reduced wt HA diffusion rate, suggesting transient interactions with BHA-PI raft patches. Conversely, patching wt HA reduced the mobile fraction of BHA-PI, indicating stable interactions with wt HA patches. Thus, the anchoring mode determines protein-raft interaction dynamics. GPI-anchored and transmembrane proteins can share the same rafts, and different proteins can interact stably or transiently with the same raft domains.

Transforming growth factor-beta receptors interact with AP2 by direct binding to beta2 subunit

Transforming growth factor-beta (TGF-beta) superfamily members regulate a wide range of biological processes by binding to two transmembrane serine/threonine kinase receptors, type I and type II. We have previously shown that the internalization of these receptors is inhibited by K(+) depletion, cytosol acidification, or hypertonic medium, suggesting the involvement of clathrin-coated pits. However, the involvement of the clathrin-associated adaptor complex AP2 and the identity of the AP2 subunit that binds the receptors were not known. Herein, we have studied these issues by combining studies on intact cells with in vitro assays. Using fluorescence photobleaching recovery to measure the lateral mobility of the receptors on live cells (untreated or treated to alter their coated pit structure), we demonstrated that their mobility is restricted by interactions with coated pits. These interactions were transient and mediated through the receptors' cytoplasmic tails. To measure direct binding of the receptors to specific AP2 subunits, we used yeast two-hybrid screens and in vitro biochemical assays. In contrast to most other plasma membrane receptors that bind to AP2 via the mu2 subunit, AP2/TGF-beta receptor binding was mediated by a direct interaction between the beta2-adaptin N-terminal trunk domain and the cytoplasmic tails of the receptors; no binding was observed to the mu2, alpha, or sigma2 subunits of AP2 or to mu1 of AP1. The data uniquely demonstrate both in vivo and in vitro the ability of beta2-adaptin to directly couple TGF-beta receptors to AP2 and to clathrin-coated pits, providing the first in vivo evidence for interactions of a transmembrane receptor with beta2-adaptin.

G protein-coupled receptor/arrestin3 modulation of the endocytic machinery

Nonvisual arrestins (arr) modulate G protein-coupled receptor (GPCR) desensitization and internalization and bind to both clathrin (CL) and AP-2 components of the endocytic coated pit (CP). This raises the possibility that endocytosis of some GPCRs may be a consequence of arr-induced de novo CP formation. To directly test this hypothesis, we examined the behavior of green fluorescent protein (GFP)-arr3 in live cells expressing beta2-adrenergic receptors and fluorescent CL. After agonist stimulation, the diffuse GFP-arr3 signal rapidly became punctate and colocalized virtually completely with preexisting CP spots, demonstrating that activated complexes accumulate in previously formed CPs rather than nucleating new CP formation. After arr3 recruitment, CP appeared larger: electron microscopy analysis revealed an increase in both CP number and in the occurrence of clustered CPs. Mutant arr3 proteins with impaired binding to CL or AP-2 displayed reduced recruitment to CPs, but were still capable of inducing CP clustering. In contrast, though constitutively present in CPs, the COOH-terminal moiety of arr3, which contains CP binding sites but lacks receptor binding, did not induce CP clustering. Together, these results indicate that recruitment of functional arr3-GPCR complexes to CP is necessary to induce clustering. Latrunculin B or 16 degrees C blocked CP rearrangements without affecting arr3 recruitment to CP. These results and earlier studies suggest that discrete CP zones exist on cell surfaces, each capable of supporting adjacent CPs, and that the cortical actin membrane skeleton is intimately involved with both the maintenance of existing CPs and the generation of new structures.