Smad7 attenuates TGF-β-mediated aging-related hypofunction of submandibular glands

Submandibular glands have essential functions in taste, mastication, swallowing, and digestion. Submandibular gland hypofunction is prevalent in the elderly, impairing the patients’ quality of life. Current clinical treatment strategies have not decelerated or reversed the pathological process of submandibular gland hypofunction. Therefore, novel restoration strategies should be explored. However, studies on the mechanism of aging-related submandibular gland hypofunction remain very limited. The role of the TGF-β/Smad pathway in fibrosis has been studied in other organs. Therefore, this study aimed to elucidate the role of TGF-β/Smad signaling in the aging-related submandibular gland hypofunction. The results showed that Smad7 knockout in mice decreased the salivary flow rate. H&E, Masson trichrome, and immunohistochemistry staining of MCP-1 and α-SMA showed that Smad7 knockout in mice resulted in lymphocytic infiltration, acinar cell atrophy, and interstitial fibrosis. The Western blotting of collagen I and III also confirmed extensive fibrosis. We then found that Smad7 depletion resulted in the TGF-β-mediated fibrosis via mir-21, mir-29, and np_5318, and NFκB-driven inflammation activation. This study confirmed the inhibitory role of Smad7 in the aging-related submandibular gland hypofunction. Therefore, it provided a promising treatment target for aging-related dysfunction and sialadenitis of submandibular gland.     

Integrin Cross Talk: Activation of Lymphocyte Function-associated Antigen-1 on Human T Cells Alters α4β1- and α5β1-mediated Function

A regulated order of adhesion events directs leukocytes from the vascular compartment into injured tissues in response to inflammatory stimuli. We show that on human T cells, the interaction of the β2 integrin leucocyte function–associated antigen-1 (LFA-1) with its ligand intercellular adhesion molecule-1 (ICAM-1) will decrease adhesion mediated by α4β1 and, to a lesser extent, α5β1. Similar inhibition is also seen when T cells are exposed to mAb 24, which stabilizes LFA-1 in an active state after triggering integrin function through divalent cation Mg²⁺, PdBu, or T cell receptor/ CD3 complex (TCR/CD3) cross-linking. Such cross talk decreases α4β1 integrin–mediated binding of T cells to fibronectin and vascular cell adhesion molecule-1 (VCAM-1). In contrast, ligand occupancy or prolonged activation of β1 integrin has no effect on LFA-1 adhesion to ICAM-1. We also show that T cell migration across fibronectin, unlike adhesion, is mediated solely by α5β1, and is increased when the α4β1-mediated component of fibronectin adhesion is decreased either by cross talk or the use of α4-blocking mAb. The ability of mAb 24 Fab′ fragments to induce cross talk without cross-linking LFA-1 suggests signal transduction through the active integrin. These data provide the first direct evidence for cross talk between LFA-1 and β1 integrins on T cells. Together, these findings imply that activation of LFA-1 on the extravasating T cell will decrease the binding to VCAM-1 while enhancing the subsequent migration on fibronectin. This sequence of events provides a further level of complexity to the coordination of T cell integrins, whose sequential but overlapping roles are essential for transmigration.     

Direct and Regulated Interaction of Integrin αEβ7 with E-Cadherin

The cadherins are a family of homophilic adhesion molecules that play a vital role in the formation of cellular junctions and in tissue morphogenesis. Members of the integrin family are also involved in cell to cell adhesion, but bind heterophilically to immunoglobulin superfamily molecules such as intracellular adhesion molecule (ICAM)–1, vascular cell adhesion molecule (VCAM)–1, or mucosal addressin cell adhesion molecule (MadCAM)–1. Recently, an interaction between epithelial (E-) cadherin and the mucosal lymphocyte integrin, α_Eβ₇, has been proposed. Here, we demonstrate that a human E-cadherin–Fc fusion protein binds directly to soluble recombinant α_Eβ₇, and to α_Eβ₇ solubilized from intraepithelial T lymphocytes. Furthermore, intraepithelial lymphocytes or transfected JY′ cells expressing the α_Eβ₇ integrin adhere strongly to purified E-cadherin–Fc coated on plastic, and the adhesion can be inhibited by antibodies to α_Eβ₇ or E-cadherin.

The binding of α_Eβ₇ integrin to cadherins is selective since cell adhesion to P-cadherin–Fc through α_Eβ₇ requires >100-fold more fusion protein than to E-cadherin–Fc. Although the structure of the α_E-chain is unique among integrins, the avidity of α_Eβ₇ for E-cadherin can be regulated by divalent cations or phorbol myristate acetate. Cross-linking of the T cell receptor complex on intraepithelial lymphocytes increases the avidity of α_Eβ₇ for E-cadherin, and may provide a mechanism for the adherence and activation of lymphocytes within the epithelium in the presence of specific foreign antigen. Thus, despite its dissimilarity to known integrin ligands, the specific molecular interaction demonstrated here indicates that E-cadherin is a direct counter receptor for the α_Eβ₇ integrin.

     

Hepsin regulates TGFβ signaling via fibronectin proteolysis

Transforming growth factor‐beta (TGFβ) is a multifunctional cytokine with a well‐established role in mammary gland development and both oncogenic and tumor‐suppressive functions. The extracellular matrix (ECM) indirectly regulates TGFβ activity by acting as a storage compartment of latent‐TGFβ, but how TGFβ is released from the ECM via proteolytic mechanisms remains largely unknown. In this study, we demonstrate that hepsin, a type II transmembrane protease overexpressed in 70% of breast tumors, promotes canonical TGFβ signaling through the release of latent‐TGFβ from the ECM storage compartment. Mammary glands in hepsin CRISPR knockout mice showed reduced TGFβ signaling and increased epithelial branching, accompanied by increased levels of fibronectin and latent‐TGFβ1, while overexpression of hepsin in mammary tumors increased TGFβ signaling. Cell‐free and cell‐based experiments showed that hepsin is capable of direct proteolytic cleavage of fibronectin but not latent‐TGFβ and, importantly, that the ability of hepsin to activate TGFβ signaling is dependent on fibronectin. Altogether, this study demonstrates a role for hepsin as a regulator of the TGFβ pathway in the mammary gland via a novel mechanism involving proteolytic downmodulation of fibronectin.     

Cross Talk between β1 and αV Integrins: β1 Affects β3 mRNA Stability

There is increasing evidence that a fine-tuned integrin cross talk can generate a high degree of specificity in cell adhesion, suggesting that spatially and temporally coordinated expression and activation of integrins are more important for regulated cell adhesive functions than the intrinsic specificity of individual receptors. However, little is known concerning the molecular mechanisms of integrin cross talk. With the use of beta(1)-null GD25 cells ectopically expressing the beta(1)A integrin subunit, we provide evidence for the existence of a cross talk between beta(1) and alpha(V) integrins that affects the ratio of alpha(V)beta(3) and alpha(V)beta(5) integrin cell surface levels. In particular, we demonstrate that a down-regulation of alpha(V)beta(3) and an up-regulation of alpha(V)beta(5) occur as a consequence of beta(1)A expression. Moreover, with the use of GD25 cells expressing the integrin isoforms beta(1)B and beta(1)D, as well as two beta(1) cytoplasmic domain deletion mutants lacking either the entire cytoplasmic domain (beta(1)TR) or only its "variable" region (beta(1)COM), we show that the effects of beta(1) over alpha(V) integrins take place irrespective of the type of beta(1) isoform, but require the presence of the "common" region of the beta(1) cytoplasmic domain. In an attempt to establish the regulatory mechanism(s) whereby beta(1) integrins exert their trans-acting functions, we have found that the down-regulation of alpha(V)beta(3) is due to a decreased beta(3) subunit mRNA stability, whereas the up-regulation of alpha(V)beta(5) is mainly due to translational or posttranslational events. These findings provide the first evidence for an integrin cross talk based on the regulation of mRNA stability.     

Activation of Distinct α5β1-mediated Signaling Pathways by Fibronectin's Cell Adhesion and Matrix Assembly Domains

The interaction of cells with fibronectin generates a series of complex signaling events that serve to regulate several aspects of cell behavior, including growth, differentiation, adhesion, and motility. The formation of a fibronectin matrix is a dynamic, cell-mediated process that involves both ligation of the α₅β₁ integrin with the Arg-Gly-Asp (RGD) sequence in fibronectin and binding of the amino terminus of fibronectin to cell surface receptors, termed “matrix assembly sites,” which mediate the assembly of soluble fibronectin into insoluble fibrils. Our data demonstrate that the amino-terminal type I repeats of fibronectin bind to the α₅β₁ integrin and support cell adhesion. Furthermore, the amino terminus of fibronectin modulates actin assembly, focal contact formation, tyrosine kinase activity, and cell migration. Amino-terminal fibronectin fragments and RGD peptides were able to cross-compete for binding to the α₅β₁ integrin, suggesting that these two domains of fibronectin cannot bind to the α₅β₁ integrin simultaneously. Cell adhesion to the amino-terminal domain of fibronectin was enhanced by cytochalasin D, suggesting that the ligand specificity of the α₅β₁ integrin is regulated by the cytoskeleton. These data suggest a new paradigm for integrin-mediated signaling, where distinct regions within one ligand can modulate outside-in signaling through the same integrin.     

Mechanistic basis of post-treatment control of SIV after anti-α4β7 antibody therapy

Treating macaques with an anti-α4β7 antibody under the umbrella of combination antiretroviral therapy (cART) during early SIV infection can lead to viral remission, with viral loads maintained at < 50 SIV RNA copies/ml after removal of all treatment in a subset of animals. Depletion of CD8⁺ lymphocytes in controllers resulted in transient recrudescence of viremia, suggesting that the combination of cART and anti-α4β7 antibody treatment led to a state where ongoing immune responses kept the virus undetectable in the absence of treatment. A previous mathematical model of HIV infection and cART incorporates immune effector cell responses and exhibits the property of two different viral load set-points. While the lower set-point could correspond to the attainment of long-term viral remission, attaining the higher set-point may be the result of viral rebound. Here we expand that model to include possible mechanisms of action of an anti-α4β7 antibody operating in these treated animals. We show that the model can fit the longitudinal viral load data from both IgG control and anti-α4β7 antibody treated macaques, suggesting explanations for the viral control associated with cART and an anti-α4β7 antibody treatment. This effective perturbation to the virus-host interaction can also explain observations in other nonhuman primate experiments in which cART and immunotherapy have led to post-treatment control or resetting of the viral load set-point. Interestingly, because the viral kinetics in the various treated animals differed—some animals exhibited large fluctuations in viral load after cART cessation—the model suggests that anti-α4β7 treatment could act by different primary mechanisms in different animals and still lead to post-treatment viral control. This outcome is nonetheless in accordance with a model with two stable viral load set-points, in which therapy can perturb the system from one set-point to a lower one through different biological mechanisms.     

hMutSα forms an ATP-dependent complex with hMutLα and hMutLβ on DNA

The DNA binding properties of hMutSα and hMutLα and complex formation of hMutSα with hMutLα and hMutLβ were investigated using binding experiments on magnetic bead-coupled DNA substrates with nuclear extracts as well as purified proteins. hMutSα binding to homoduplex DNA was disrupted by lower NaCl concentrations than hMutSα binding to a mismatch. ATP markedly reduced the salt resistance of hMutSα binding but hMutSα still retained affinity for heteroduplexes. hMutSα formed a complex with hMutLα and hMutLβ on DNA in the presence of ATP. This complex only formed on 81mer and not 32mer DNA substrates. Complex formation was enhanced by a mismatch in the DNA substrate, and hMutLα and hMutLβ were shown to enter the complex at different ATP concentrations. Purified hMutLα showed an intrinsic affinity for DNA, with a preference for single-stranded over double-stranded DNA.     

The α5β1 Integrin Mediates Elimination of Amyloid-β Peptide and Protects Against Apoptosis

The amyloid-β peptide (Aβ) can mediate cell attachment by binding to β1 integrins through an arg-his-asp sequence. We show here that the α5β1 integrin, a fibronectin receptor, is an efficient binder of Aβ, and mediates cell attachment to nonfibrillar Aβ. Cells engineered to express α5β1 internalized and degraded more added Aβ1-40 than did α5β1-negative control cells. Deposition of an insoluble Aβ1-40 matrix around the α5β1-expressing cells was reduced, and the cells showed less apoptosis than the control cells. Thus, the α5β1 integrin may protect against Aβ deposition and toxicity, which is a course of Alzheimer's disease lesions.     

Ca2+ administration prevents α-synuclein proteotoxicity by stimulating calcineurin-dependent lysosomal proteolysis

The capacity of a cell to maintain proteostasis progressively declines during aging. Virtually all age-associated neurodegenerative disorders associated with aggregation of neurotoxic proteins are linked to defects in the cellular proteostasis network, including insufficient lysosomal hydrolysis. Here, we report that proteotoxicity in yeast and Drosophila models for Parkinson’s disease can be prevented by increasing the bioavailability of Ca²⁺, which adjusts intracellular Ca²⁺ handling and boosts lysosomal proteolysis. Heterologous expression of human α-synuclein (αSyn), a protein critically linked to Parkinson’s disease, selectively increases total cellular Ca²⁺ content, while the levels of manganese and iron remain unchanged. Disrupted Ca²⁺ homeostasis results in inhibition of the lysosomal protease cathepsin D and triggers premature cellular and organismal death. External administration of Ca²⁺ reduces αSyn oligomerization, stimulates cathepsin D activity and in consequence restores survival, which critically depends on the Ca²⁺/calmodulin-dependent phosphatase calcineurin. In flies, increasing the availability of Ca²⁺ discloses a neuroprotective role of αSyn upon manganese overload. In sum, we establish a molecular interplay between cathepsin D and calcineurin that can be activated by Ca²⁺ administration to counteract αSyn proteotoxicity.     

VLA-2 (α2β1) Integrin Promotes Rotavirus Entry into Cells but Is Not Necessary for Rotavirus Attachment

In an attempt to identify the rotavirus receptor, we tested 46 cell lines of different species and tissue origins for susceptibility to infection by three N-acetyl-neuraminic (sialic) acid (SA)-dependent and five SA-independent rotavirus strains. Susceptibility to SA-dependent or SA-independent rotavirus infection varied depending on the cell line tested and the multiplicity of infection (MOI) used. Cells of renal or intestinal origin and transformed cell lines derived from breast, stomach, bone, or lung were all susceptible to rotavirus infection, indicating a wider host tissue range than previously appreciated. Chinese hamster ovary (CHO), baby hamster kidney (BHK-21), guinea pig colon (GPC-16), rat small intestine (Rie1), and mouse duodenum (MODE-K) cells were found to support only limited rotavirus replication even at MOIs of 100 or 500, but delivery of rotavirus particles into the cytoplasm by lipofection resulted in efficient rotavirus replication. The rotavirus cell attachment protein, the outer capsid spike protein VP4, contains the sequence GDE(A) recognized by the VLA-2 (alpha2beta1) integrin, and to test if VLA-2 is involved in rotavirus attachment and entry, we measured infection in CHO cells that lack VLA-2 and CHO cells transfected with the human alpha2 subunit (CHOalpha2) or with both the human alpha2 and beta1 subunits (CHOalpha2beta1) of VLA-2. Infection by SA-dependent or SA-independent rotavirus strains was 2- to 10-fold more productive in VLA-2-expressing CHO cells than in parental CHO cells, and the increased susceptibility to infection was blocked with anti-VLA-2 antibody. However, the levels of binding of rotavirus to CHO, CHOalpha2, and CHOalpha2beta1 cells were equivalent and were not increased over binding to susceptible monkey kidney (MA104) cells or human colonic adenocarcinoma (Caco-2, HT-29, and T-84) cells, and binding was not blocked by antibody to the human alpha2 subunit. Although the VLA-2 integrin promotes rotavirus infection in CHO cells, it is clear that the VLA-2 integrin alone is not responsible for rotavirus cell attachment and entry. Therefore, VLA-2 is not involved in the initial attachment of rotavirus to cells but may play a role at a postattachment level.     

The RGD-binding integrins αvβ6 and αvβ8 are receptors for mouse adenovirus-1 and -3 infection

Mammalian adenoviruses (AdVs) comprise more than ~350 types including over 100 human (HAdVs) and just three mouse AdVs (MAdVs). While most HAdVs initiate infection by high affinity/avidity binding of their fiber knob (FK) protein to either coxsackievirus AdV receptor (CAR), CD46 or desmoglein (DSG)-2, MAdV-1 (M1) infection requires arginine-glycine-aspartate (RGD) binding integrins. To identify the receptors mediating MAdV infection we generated five novel reporter viruses for MAdV-1/-2/-3 (M1, M2, M3) transducing permissive murine (m) CMT-93 cells, but not B16 mouse melanoma cells expressing mCAR, human (h) CD46 or hDSG-2. Recombinant M1 or M3 FKs cross-blocked M1 and M3 but not M2 infections. Profiling of murine and human cells expressing RGD-binding integrins suggested that αvβ6 and αvβ8 heterodimers are associated with M1 and M3 infections. Ectopic expression of mβ6 in B16 cells strongly enhanced M1 and M3 binding, infection, and progeny production comparable with mαvβ6-positive CMT-93 cells, whereas mβ8 expressing cells were more permissive to M1 than M3. Anti-integrin antibodies potently blocked M1 and M3 binding and infection of CMT-93 cells and hαvβ8-positive M000216 cells. Soluble integrin αvβ6, and synthetic peptides containing the RGDLXXL sequence derived from FK-M1, FK-M3 and foot and mouth disease virus coat protein strongly interfered with M1/M3 infections, in agreement with high affinity interactions of FK-M1/FK-M3 with αvβ6/αvβ8, determined by surface plasmon resonance measurements. Molecular docking simulations of ternary complexes revealed a bent conformation of RGDLXXL-containing FK-M3 peptides on the subunit interface of αvβ6/β8, where the distal leucine residue dips into a hydrophobic pocket of β6/8, the arginine residue ionically engages αv aspartate215, and the aspartate residue coordinates a divalent cation in αvβ6/β8. Together, the RGDLXXL-bearing FKs are part of an essential mechanism for M1/M3 infection engaging murine and human αvβ6/8 integrins. These integrins are highly conserved in other mammals, and may favour cross-species virus transmission.     

A stapled chromogranin A-derived peptide homes in on tumors that express αvβ6 or αvβ8 integrins

Rationale: The αvβ6- and αvβ8-integrins, two cell-adhesion receptors upregulated in many tumors and involved in the activation of the latency associated peptide (LAP)/TGFβ complex, represent potential targets for tumor imaging and therapy. We investigated the tumor-homing properties of a chromogranin A-derived peptide containing an RGDL motif followed by a chemically stapled alpha-helix (called “5a”), which selectively recognizes the LAP/TGFβ complex-binding site of αvβ6 and αvβ8.Methods: Peptide 5a was labeled with IRDye 800CW (a near-infrared fluorescent dye) or with ¹⁸F-NOTA (a label for positron emission tomography (PET)); the integrin-binding properties of free peptide and conjugates were then investigated using purified αvβ6/αvβ8 integrins and various αvβ6/αvβ8 single - or double-positive cancer cells; tumor-homing, biodistribution and imaging properties of the conjugates were investigated in subcutaneous and orthotopic αvβ6-positive carcinomas of the pancreas, and in mice bearing subcutaneous αvβ8-positive prostate tumors.Results: In vitro studies showed that 5a can bind both integrins with high affinity and inhibits cell-mediated TGFβ activation. The 5a-IRDye and 5a-NOTA conjugates could bind purified αvβ6/αvβ8 integrins with no loss of affinity compared to free peptide, and selectively recognized various αvβ6/αvβ8 single- or double-positive cancer cells, including cells from pancreatic carcinoma, melanoma, oral mucosa, bladder and prostate cancer. In vivo static and dynamic optical near-infrared and PET/CT imaging and biodistribution studies, performed in mice with subcutaneous and orthotopic αvβ6-positive carcinomas of the pancreas, showed high target-specific uptake of fluorescence- and radio-labeled peptide by tumors and low non-specific uptake in other organs and tissues, except for excretory organs. Significant target-specific uptake of fluorescence-labeled peptide was also observed in mice bearing αvβ8-positive prostate tumors.Conclusions: The results indicate that 5a can home to αvβ6- and/or αvβ8-positive tumors, suggesting that this peptide can be exploited as a ligand for delivering imaging or anticancer agents to αvβ6/αvβ8 single- or double-positive tumors, or as a tumor-homing inhibitor of these TGFβ activators.     

A model for how Gβγ couples Gα to GPCR

Representing ∼5% of the human genome, G-protein-coupled receptors (GPCRs) are a primary target for drug discovery; however, the molecular details of how they couple to heterotrimeric G protein subunits are incompletely understood. Here, I propose a hypothetical initial docking model for the encounter between GPCR and Gβγ that is defined by transient interactions between the cytosolic surface of the GPCR and the prenyl moiety and the tripeptide motif, asparagine–proline–phenylalanine (NPF), in the C-terminus of the Gγ subunit. Analysis of class A GPCRs reveals a conserved NPF binding site formed by the interaction of the TM1 and H8. Functional studies using differentially prenylated proteins and peptides further suggest that the intracellular hydrophobic core of the GPCR is a prenyl binding site. Upon binding TM1 and H8 of GPCRs, the propensity of the C-terminal region of Gγ to convert into an α helix allows it to extend into the hydrophobic core of the GPCR, facilitating the GPCR active state. Conservation of the NPF motif in Gγ isoforms and interacting residues in TM1 and H8 suggest that this is a general mechanism of GPCR–G protein signaling. Analysis of the rhodopsin dimer also suggests that Gγ–rhodopsin interactions may facilitate GPCR dimer transactivation.     

Microbial 7α-Hydroxylation of 3-Ketobisnorcholenol

The transformation of 22-hydroxy-23,24-bisnorchol-4-en-3-one to 7α-22-dihydroxy-23,24-bisnorchol-4-en-3-one by Botryodiploida theobromae, Lasiodiplodia theobromae, and various Botryosphaeria strains is described. Factors affecting the reaction were incubation temperature, sonication of the substrate, and addition of 2,2′-dipyridyl, extra carbohydrate, and Amberlite XAD-7. The enzyme responsible for the reaction appeared to be very specific and was not characteristic of all members of the genera listed above.