Pharmacokinetic Drug-Drug Interaction With Coadministration of Cannabidiol and Everolimus in a Phase 1 Healthy Volunteer Trial.

When highly purified cannabidiol (CBD; Epidiolex) and the mammalian target of rapamycin inhibitor everolimus are used concomitantly in the treatment of tuberous sclerosis complex, there is evidence of a pharmacokinetic (PK) interaction, leading to increased everolimus systemic exposure. We evaluated the effect of steady-state CBD exposure following multiple clinically relevant CBD doses on everolimus PK in healthy adult participants in a single-center, fixed-sequence, open-label, phase 1 study. All participants received oral everolimus 5 mg on day 1, followed by a 7-day washout. On days 9-17, participants received CBD (100 mg/mL oral solution) at 12.5 mg/kg in the morning and evening. On the morning of day 13, participants also received a single dose of oral everolimus 5 mg. Medications were taken 30 or 45 minutes (morning or evening dose) after starting a standardized meal. Maximum concentration and area under the concentration-time curve (AUC) from time of dosing to the last measurable concentration and extrapolated to infinity, of everolimus in whole blood were estimated using noncompartmental analysis, with geometric mean ratios and 90% confidence intervals for the ratios of everolimus dosed with CBD to everolimus dosed alone. A single dose of everolimus 5 mg was well tolerated when administered with multiple doses of CBD. Log-transformed everolimus maximum concentration, AUC from time of dosing to the last measurable concentration, and AUC extrapolated to infinity values increased by ≈2.5-fold, and everolimus half-life remained largely unchanged in the presence of steady-state CBD relative to everolimus dosed alone. Everolimus blood concentration monitoring should be strongly advised with appropriate dose reduction when coadministered with CBD.

Tensin3 interaction with talin drives the formation of fibronectin-associated fibrillar adhesions.

The formation of healthy tissue involves continuous remodeling of the extracellular matrix (ECM). Whilst it is known that this requires integrin-associated cell-ECM adhesion sites (CMAs) and actomyosin-mediated forces, the underlying mechanisms remain unclear. Here, we examine how tensin3 contributes to the formation of fibrillar adhesions (FBs) and fibronectin fibrillogenesis. Using BioID mass spectrometry and a mitochondrial targeting assay, we establish that tensin3 associates with the mechanosensors such as talin and vinculin. We show that the talin R11 rod domain binds directly to a helical motif within the central intrinsically disordered region (IDR) of tensin3, whilst vinculin binds indirectly to tensin3 via talin. Using CRISPR knock-out cells in combination with defined tensin3 mutations, we show (i) that tensin3 is critical for the formation of α5ß1-integrin FBs and for fibronectin fibrillogenesis, and (ii) the talin/tensin3 interaction drives this process, with vinculin acting to potentiate it.

A Phase 1 Open-Label, Fixed-Sequence Pharmacokinetic Drug Interaction Trial to Investigate the Effect of Cannabidiol on the CYP1A2 Probe Caffeine in Healthy Subjects.

This pharmacokinetic (PK) drug-interaction trial investigated the effects of repeated dosing of a plant-derived pharmaceutical formulation of highly purified cannabidiol (CBD; Epidiolex in the United States and Epidyolex in Europe; 100 mg/mL oral solution) on caffeine clearance via modulation of cytochrome P450 (CYP) 1A2 activity in healthy adults. In this phase 1 open-label, fixed-sequence trial, all subjects received a single 200 mg caffeine dose and placebo on day 1. Subjects then titrated CBD from 250 mg once daily to 750 mg twice daily between days 3 and 11 and took 750 mg CBD twice daily between days 12 and 27. On day 26, subjects received a single 200-mg caffeine dose with their morning CBD dose. Plasma concentrations of caffeine and its CYP1A2-mediated metabolite, paraxanthine, were determined on days 1 and 26 and PK parameters derived using noncompartmental analysis. Safety was monitored throughout. Sixteen subjects enrolled, and 9 completed treatment. When caffeine was administered with steady-state CBD, caffeine exposure increased by 15% for Cmax and 95% for AUC0-∞ , tmax increased from 1.5 to 3.0 hours, and t1/2 increased from 5.4 to 10.9 hours compared with caffeine administered with placebo. Under the same conditions, paraxanthine exposure decreased by 22% for Cmax and increased by 18% for AUC0-∞ , tmax increased from 8.0 to 14.0 hours, and t1/2 increased from 7.2 to 13.7 hours. Overall, there were no unexpected adverse events; diarrhea was most common, and 6 subjects discontinued because of elevated liver transaminases. These data suggest that CBD is an inhibitor of CYP1A2.

Clinical implications of trials investigating drug-drug interactions between cannabidiol and enzyme inducers or inhibitors or common antiseizure drugs.

Highly purified cannabidiol (CBD) has demonstrated efficacy with an acceptable safety profile in patients with Lennox-Gastaut syndrome or Dravet syndrome in randomized, double-blind, add-on, controlled phase 3 trials. It is important to consider the possibility of drug-drug interactions (DDIs). Here, we review six trials of CBD (Epidiolex/Epidyolex; 100 mg/mL oral solution) in healthy volunteers or patients with epilepsy, which investigated potential interactions between CBD and enzymes involved in drug metabolism of common antiseizure drugs (ASDs). CBD did not affect CYP3A4 activity. Induction of CYP3A4 and CYP2C19 led to small reductions in exposure to CBD and its major metabolites. Inhibition of CYP3A4 activity did not affect CBD exposure and caused small increases in exposure to CBD metabolites. Inhibition of CYP2C19 activity led to a small increase in exposure to CBD and small decreases in exposure to CBD metabolites. One potentially clinically important DDI was identified: combination of CBD and clobazam (CLB) did not affect CBD or CLB exposure, but increased exposure to major metabolites of both compounds. Reduction of CLB dose may be considered if adverse reactions known to occur with CLB are experienced when it is coadministered with CBD. There was a small increase of exposure to stiripentol (STP) when coadministered with CBD. STP had no effect on CBD exposure but led to minor decreases in exposure to CBD metabolites. Combination of CBD and valproate (VPA) did not cause clinically important changes in the pharmacokinetics of either drug, or 2-propyl-4-pentenoic acid. Concomitant VPA caused small increases in exposure to CBD metabolites. Dose adjustments are not likely to be necessary when CBD is combined with STP or VPA. The safety results from these trials were consistent with the known safety profile of CBD. These trials indicate an overall low potential for DDIs between CBD and other ASDs, except for CLB.

A Phase II Randomized Trial to Explore the Potential for Pharmacokinetic Drug-Drug Interactions with Stiripentol or Valproate when Combined with Cannabidiol in Patients with Epilepsy.

BACKGROUND: In recent randomized, placebo-controlled, phase III trials, highly purified cannabidiol demonstrated efficacy with an acceptable safety profile in patients with Lennox-Gastaut syndrome or Dravet syndrome. It is anticipated that antiepileptic drugs such as stiripentol and valproate will be administered concomitantly with cannabidiol. OBJECTIVES: This trial evaluated the effect of cannabidiol on steady-state pharmacokinetics of stiripentol or valproate in patients with epilepsy, and the safety and tolerability of cannabidiol. METHODS: This phase II, two-arm, parallel-group, double-blind, randomized, placebo-controlled trial recruited male and female patients with epilepsy aged 16-55 years. Patients receiving a stable dose of stiripentol or valproate were randomized 4:1 to receive concomitant double-blind cannabidiol or placebo. Patients received plant-derived, highly purified cannabidiol medicine (Epidiolex® in the USA; Epidyolex® in the EU; 100 mg/mL oral solution) at a dose of 20 mg/kg/day from day 12 to 26, following a 10-day dose-escalation period. Blood samples for pharmacokinetic evaluations were collected on days 1 and 26 before stiripentol/valproate dosing and up to 12 h postdose. Treatment-emergent adverse events (AEs) were recorded. RESULTS: In total, 35 patients were recruited to the stiripentol arm (n = 14) or the valproate arm (n = 21). Both the safety and the pharmacokinetic populations of the stiripentol arm comprised 14 patients (2 placebo; 12 cannabidiol). The safety population of the valproate arm comprised 20 patients (4 placebo; 16 cannabidiol; one withdrew before receiving treatment); the pharmacokinetic population comprised 15 patients (3 placebo; 12 cannabidiol). Concomitant cannabidiol led to a small increase in stiripentol exposure (17% increase in maximum observed plasma concentration [Cmax]; 30% increase in area under the concentration-time curve over the dosing interval [AUCtau]). Concomitant cannabidiol also had little effect on valproate exposure (13% decrease in Cmax; 17% decrease in AUCtau) or its metabolite, 2-propyl-4-pentenoic acid (4-ene-VPA) (23% decrease in Cmax; 30% decrease in AUCtau). All changes in exposure are expressed as the dose-normalized geometric mean (CV%) day 26 to day 1 ratio. The most common AE was diarrhea; most AEs were mild. Two patients discontinued cannabidiol because of serious AEs (rash [n = 1] in the stiripentol arm; hypertransaminasemia [n = 1] in the valproate arm). A separate in vitro study investigated the bidirectional effect of cannabidiol, or its metabolite 7-carboxy-cannabidiol, on valproate plasma protein binding; no change in plasma protein binding was observed for either compound. CONCLUSIONS: The clinical relevance of the increase in stiripentol exposure is unknown; patients receiving cannabidiol and stiripentol concomitantly should be monitored for adverse reactions as individual patient responses may vary. Coadministration of cannabidiol did not affect the pharmacokinetics of valproate or its metabolite, 4-ene-VPA, in adult patients with epilepsy. Safety results were consistent with the known safety profile of cannabidiol at a dose of 20 mg/kg/day. Clinicaltrials.gov: NCT02607891.

A phase 1, randomized, pharmacokinetic trial of the effect of different meal compositions, whole milk, and alcohol on cannabidiol exposure and safety in healthy subjects.

OBJECTIVE: The pharmacokinetics (PK) and safety of single oral 750-mg doses of a plant-derived pharmaceutical formulation of highly purified cannabidiol (CBD; Epidiolex in the USA and Epidyolex in Europe; 100-mg/mL oral solution) were assessed in healthy adults following a high-fat/calorie meal (n = 15), a low-fat/calorie meal (n = 14), whole milk (n = 15), or alcohol (n = 14), relative to the fasted state (n = 29). METHODS: Blood samples were collected until 96 hours postdose in each period and evaluated by liquid chromatography and tandem mass spectrometry. PK parameters (maximum observed plasma concentration [Cmax ], area under the plasma concentration-time curve from time zero to the last observed quantifiable concentration, area under the concentration-time curve from time zero to infinity [AUC0-∞ ], and time to maximum plasma concentration [tmax ]) of CBD and its major metabolites were derived using noncompartmental analysis. RESULTS: CBD exposure increased by 3.8-fold for AUC0-∞ and 5.2-fold for Cmax when CBD was administered with a high-fat/calorie meal versus fasted. To a lesser extent, a low-fat/calorie meal enhanced CBD exposure versus fasted with a 2.7-fold increase in AUC0-∞ and a 3.8-fold increase in Cmax . Similarly, when dosed with whole milk, CBD exposure increased versus fasted by 2.4-fold for AUC0-∞ and 3.1-fold for Cmax . Modest elevations in CBD exposure occurred when it was dosed with alcohol: 1.6-fold for AUC0-∞ and 1.9-fold for Cmax . No clinically relevant effect of any test condition on CBD tmax or t½ versus the fasted state was apparent. The same trend was seen for the CBD metabolites, except that 7-carboxy-cannabidiol tmax was considerably longer when CBD was administered with alcohol (14 vs 4 hours fasted). Inter- and intrasubject variability in PK parameters was moderate to high during the trial. SIGNIFICANCE: CBD and metabolite exposures were most affected by a high-fat/calorie meal. CBD exposures also increased with a low-fat/calorie meal, whole milk, or alcohol, but to a lesser extent. CBD was tolerated, and there were no severe or serious adverse events during the trial.

Relief of talin autoinhibition triggers a force-independent association with vinculin.

Talin, vinculin, and paxillin are core components of the dynamic link between integrins and actomyosin. Here, we study the mechanisms that mediate their activation and association using a mitochondrial-targeting assay, structure-based mutants, and advanced microscopy. As expected, full-length vinculin and talin are autoinhibited and do not interact with each other. However, contrary to previous models that propose a critical role for forces driving talin-vinculin association, our data show that force-independent relief of autoinhibition is sufficient to mediate their tight interaction. We also found that paxillin can bind to both talin and vinculin when either is inactive. Further experiments demonstrated that adhesions containing paxillin and vinculin can form without talin following integrin activation. However, these are largely deficient in exerting traction forces to the matrix. Our observations lead to a model whereby paxillin contributes to talin and vinculin recruitment into nascent adhesions. Activation of the talin-vinculin axis subsequently leads to the engagement with the traction force machinery and focal adhesion maturation.

Talin regulates integrin ß1-dependent and -independent cell functions in ureteric bud development.

Kidney collecting system development requires integrin-dependent cell-extracellular matrix interactions. Integrins are heterodimeric transmembrane receptors consisting of α and ß subunits; crucial integrins in the kidney collecting system express the ß1 subunit. The ß1 cytoplasmic tail has two NPxY motifs that mediate functions by binding to cytoplasmic signaling and scaffolding molecules. Talins, scaffolding proteins that bind to the membrane proximal NPxY motif, are proposed to activate integrins and to link them to the actin cytoskeleton. We have defined the role of talin binding to the ß1 proximal NPxY motif in the developing kidney collecting system in mice that selectively express a Y-to-A mutation in this motif. The mice developed a hypoplastic dysplastic collecting system. Collecting duct cells expressing this mutation had moderate abnormalities in cell adhesion, migration, proliferation and growth factor-dependent signaling. In contrast, mice lacking talins in the developing ureteric bud developed kidney agenesis and collecting duct cells had severe cytoskeletal, adhesion and polarity defects. Thus, talins are essential for kidney collecting duct development through mechanisms that extend beyond those requiring binding to the ß1 integrin subunit NPxY motif.

Loss of mouse cardiomyocyte talin-1 and talin-2 leads to ß-1 integrin reduction, costameric instability, and dilated cardiomyopathy.

Continuous contraction-relaxation cycles of the heart require strong and stable connections of cardiac myocytes (CMs) with the extracellular matrix (ECM) to preserve sarcolemmal integrity. CM attachment to the ECM is mediated by integrin complexes localized at the muscle adhesion sites termed costameres. The ubiquitously expressed cytoskeletal protein talin (Tln) is a component of muscle costameres that links integrins ultimately to the sarcomere. There are two talin genes, Tln1 and Tln2. Here, we tested the function of these two Tln forms in myocardium where Tln2 is the dominant isoform in postnatal CMs. Surprisingly, global deletion of Tln2 in mice caused no structural or functional changes in heart, presumably because CM Tln1 became up-regulated. Tln2 loss increased integrin activation, although levels of the muscle-specific ß1D-integrin isoform were reduced by 50%. With this result, we produced mice that had simultaneous loss of both CM Tln1 and Tln2 and found that cardiac dysfunction occurred by 4 wk with 100% mortality by 6 mo. ß1D integrin and other costameric proteins were lost from the CMs, and membrane integrity was compromised. Given that integrin protein reduction occurred with Tln loss, rescue of the phenotype was attempted through transgenic integrin overexpression, but this could not restore WT CM integrin levels nor improve heart function. Our results show that CM Tln2 is essential for proper ß1D-integrin expression and that Tln1 can substitute for Tln2 in preserving heart function, but that loss of all Tln forms from the heart-muscle cell leads to myocyte instability and a dilated cardiomyopathy.

LD Motif Recognition by Talin: Structure of the Talin-DLC1 Complex.

Cell migration requires coordination between integrin-mediated cell adhesion to the extracellular matrix and force applied to adhesion sites. Talin plays a key role in coupling integrin receptors to the actomyosin contractile machinery, while deleted in liver cancer 1 (DLC1) is a Rho GAP that binds talin and regulates Rho, and therefore actomyosin contractility. We show that the LD motif of DLC1 forms a helix that binds to the four-helix bundle of the talin R8 domain in a canonical triple-helix arrangement. We demonstrate that the same R8 surface interacts with the paxillin LD1 and LD2 motifs. We identify key charged residues that stabilize the R8 interactions with LD motifs and demonstrate their importance in vitro and in cells. Our results suggest a network of competitive interactions in adhesion complexes that involve LD motifs, and identify mutations that can be used to analyze the biological roles of specific protein-protein interactions in cell migration.

Metabolite profiling of the multiple tyrosine kinase inhibitor lenvatinib: a cross-species comparison.

Lenvatinib is an oral, multiple receptor tyrosine kinase inhibitor. Preclinical drug metabolism studies showed unique metabolic pathways for lenvatinib in monkeys and rats. A human mass balance study demonstrated that lenvatinib related material is mainly excreted via feces with a small fraction as unchanged parent drug, but little is reported about its metabolic fate. The objective of the current study was to further elucidate the metabolic pathways of lenvatinib in humans and to compare these results to the metabolism in rats and monkeys. To this end, we used plasma, urine and feces collected in a human mass balance study after a single 24 mg (100 µCi) oral dose of (14)C-lenvatinib. Metabolites of (14)C-lenvatinib were identified using liquid chromatography (high resolution) mass spectrometry with off-line radioactivity detection. Close to 50 lenvatinib-related compounds were detected. In humans, unchanged lenvatinib accounted for 97 % of the radioactivity in plasma, and comprised 0.38 and 2.5 % of the administered dose excreted in urine and feces, respectively. The primary biotransformation pathways of lenvatinib were hydrolysis, oxidation and hydroxylation, N-oxidation, dealkylation and glucuronidation. Various combinations of these conversions with modifications, including hydrolysis, gluthathione/cysteine conjugation, intramolecular rearrangement and dimerization, were observed. Some metabolites seem to be unique to the investigated species (human, rat, monkey). Because all lenvatinib metabolites in human plasma were at very low levels compared to lenvatinib, only lenvatinib is expected to contribute to the pharmacological effects in humans.

Safety and pharmacokinetic profile of rufinamide in pediatric patients aged less than 4 years with Lennox-Gastaut syndrome: An interim analysis from a multicenter, randomized, active-controlled, open-label study.

OBJECTIVE: A good knowledge of safety and age group-specific pharmacokinetics (PK) of antiepileptic drugs (AEDs) in young pediatric patients is of great importance in clinical practice. This paper presents 6-month interim safety and PK from an ongoing 2-year open-label study (Study 303) of adjunctive rufinamide treatment in pediatric subjects ≥ 1 to < 4 years with inadequately controlled epilepsies of the Lennox-Gastaut syndrome (LGS) spectrum. METHODS: Subjects (N = 37) were randomized to either rufinamide or any other approved AED chosen by the investigator as adjunctive therapy to the subject's existing regimen of 1-3 AEDs. RESULTS: Interim safety results showed that treatment-emergent adverse events (TEAEs) were similar between the rufinamide (22 [88.0%]) and any-other-AED group (9 [81.8%]), with most events considered mild or moderate. A population PK analysis was conducted including plasma rufinamide concentrations from Study 303 and two other study populations of LGS subjects ≥ 4 years. The rufinamide PK profile was dose independent. The apparent clearance (CL/F) estimated from the PK model was 2.19 L/h; it was found to increase significantly as a function of body weight. Coadministration of valproic acid significantly decreased rufinamide CL/F. CL/F was not significantly affected by other concomitant AEDs, age, gender, race, hepatic function, or renal function. No adjustments to body weight-based rufinamide dosing in subjects ≥ 1 to < 4 years are necessary. SIGNIFICANCE: Rufinamide was safe and well tolerated in these pediatric subjects. Results from the interim analysis demonstrate that rufinamide's safety and PK profile is comparable in subjects ≥ 1 to < 4 and ≥ 4 years with LGS. CLINICAL TRIAL REGISTRATION: Study 303 (clinicaltrials.gov: NCT01405053).

Vinculin controls talin engagement with the actomyosin machinery.

The link between extracellular-matrix-bound integrins and intracellular F-actin is essential for cell spreading and migration. Here, we demonstrate how the actin-binding proteins talin and vinculin cooperate to provide this link. By expressing structure-based talin mutants in talin null cells, we show that while the C-terminal actin-binding site (ABS3) in talin is required for adhesion complex assembly, the central ABS2 is essential for focal adhesion (FA) maturation. Thus, although ABS2 mutants support cell spreading, the cells lack FAs, fail to polarize and exert reduced force on the surrounding matrix. ABS2 is inhibited by the preceding mechanosensitive vinculin-binding R3 domain, and deletion of R2R3 or expression of constitutively active vinculin generates stable force-independent FAs, although cell polarity is compromised. Our data suggest a model whereby force acting on integrin-talin complexes via ABS3 promotes R3 unfolding and vinculin binding, activating ABS2 and locking talin into an actin-binding configuration that stabilizes FAs.

Talin determines the nanoscale architecture of focal adhesions.

Insight into how molecular machines perform their biological functions depends on knowledge of the spatial organization of the components, their connectivity, geometry, and organizational hierarchy. However, these parameters are difficult to determine in multicomponent assemblies such as integrin-based focal adhesions (FAs). We have previously applied 3D superresolution fluorescence microscopy to probe the spatial organization of major FA components, observing a nanoscale stratification of proteins between integrins and the actin cytoskeleton. Here we combine superresolution imaging techniques with a protein engineering approach to investigate how such nanoscale architecture arises. We demonstrate that talin plays a key structural role in regulating the nanoscale architecture of FAs, akin to a molecular ruler. Talin diagonally spans the FA core, with its N terminus at the membrane and C terminus demarcating the FA/stress fiber interface. In contrast, vinculin is found to be dispensable for specification of FA nanoscale architecture. Recombinant analogs of talin with modified lengths recapitulated its polarized orientation but altered the FA/stress fiber interface in a linear manner, consistent with its modular structure, and implicating the integrin-talin-actin complex as the primary mechanical linkage in FAs. Talin was found to be ∼97 nm in length and oriented at ∼15° relative to the plasma membrane. Our results identify talin as the primary determinant of FA nanoscale organization and suggest how multiple cellular forces may be integrated at adhesion sites.

Ligand-Occupied Integrin Internalization Links Nutrient Signaling to Invasive Migration.

Integrin trafficking is key to cell migration, but little is known about the spatiotemporal organization of integrin endocytosis. Here, we show that α5ß1 integrin undergoes tensin-dependent centripetal movement from the cell periphery to populate adhesions located under the nucleus. From here, ligand-engaged α5ß1 integrins are internalized under control of the Arf subfamily GTPase, Arf4, and are trafficked to nearby late endosomes/lysosomes. Suppression of centripetal movement or Arf4-dependent endocytosis disrupts flow of ligand-bound integrins to late endosomes/lysosomes and their degradation within this compartment. Arf4-dependent integrin internalization is required for proper lysosome positioning and for recruitment and activation of mTOR at this cellular subcompartment. Furthermore, nutrient depletion promotes subnuclear accumulation and endocytosis of ligand-engaged α5ß1 integrins via inhibition of mTORC1. This two-way regulatory interaction between mTORC1 and integrin trafficking in combination with data describing a role for tensin in invasive cell migration indicate interesting links between nutrient signaling and metastasis.

Podocyte-associated talin1 is critical for glomerular filtration barrier maintenance.

Podocytes are specialized actin-rich epithelial cells that line the kidney glomerular filtration barrier. The interface between the podocyte and the glomerular basement membrane requires integrins, and defects in either α3 or ß1 integrin, or the α3ß1 ligand laminin result in nephrotic syndrome in murine models. The large cytoskeletal protein talin1 is not only pivotal for integrin activation, but also directly links integrins to the actin cytoskeleton. Here, we found that mice lacking talin1 specifically in podocytes display severe proteinuria, foot process effacement, and kidney failure. Loss of talin1 in podocytes caused only a modest reduction in ß1 integrin activation, podocyte cell adhesion, and cell spreading; however, the actin cytoskeleton of podocytes was profoundly altered by the loss of talin1. Evaluation of murine models of glomerular injury and patients with nephrotic syndrome revealed that calpain-induced talin1 cleavage in podocytes might promote pathogenesis of nephrotic syndrome. Furthermore, pharmacologic inhibition of calpain activity following glomerular injury substantially reduced talin1 cleavage, albuminuria, and foot process effacement. Collectively, these findings indicate that podocyte talin1 is critical for maintaining the integrity of the glomerular filtration barrier and provide insight into the pathogenesis of nephrotic syndrome.

Talins and kindlins: partners in integrin-mediated adhesion.

Integrin receptors provide a dynamic, tightly-regulated link between the extracellular matrix (or cellular counter-receptors) and intracellular cytoskeletal and signalling networks, enabling cells to sense and respond to their chemical and physical environment. Talins and kindlins, two families of FERM-domain proteins, bind the cytoplasmic tail of integrins, recruit cytoskeletal and signalling proteins involved in mechanotransduction and synergize to activate integrin binding to extracellular ligands. New data reveal the domain structure of full-length talin, provide insights into talin-mediated integrin activation and show that RIAM recruits talin to the plasma membrane, whereas vinculin stabilizes talin in cell-matrix junctions. How kindlins act is less well-defined, but disease-causing mutations show that kindlins are also essential for integrin activation, adhesion, cell spreading and signalling.

Structural studies on full-length talin1 reveal a compact auto-inhibited dimer: implications for talin activation.

Talin is a large adaptor protein that activates integrins and couples them to cytoskeletal actin. Talin contains an N-terminal FERM (band 4.1, ezrin, radixin, moesin) domain (the head) linked to a flexible rod comprised of 13 amphipathic helical bundles (R1-R13) that terminate in a C-terminal helix (DD) that forms an anti-parallel dimer. We derived a three-dimensional structural model of full-length talin at a resolution of approximately 2.5nm using EM reconstruction of full-length talin and the known shapes of the individual domains and inter-domain angles as derived from small angle X-ray scattering. Talin adopts a compact conformation consistent with a dimer in which the two talin rods form a donut-shaped structure, with the two talin heads packed side by side occupying the hole at the center of this donut. In this configuration, the integrin binding site in the head domain and the actin-binding site at the carboxy-terminus of the rod are masked, implying that talin must unravel before it can support integrin activation and engage the actin cytoskeleton.

RIAM and vinculin binding to talin are mutually exclusive and regulate adhesion assembly and turnover.

Talin activates integrins, couples them to F-actin, and recruits vinculin to focal adhesions (FAs). Here, we report the structural characterization of the talin rod: 13 helical bundles (R1-R13) organized into a compact cluster of four-helix bundles (R2-R4) within a linear chain of five-helix bundles. Nine of the bundles contain vinculin-binding sites (VBS); R2R3 are atypical, with each containing two VBS. Talin R2R3 also binds synergistically to RIAM, a Rap1 effector involved in integrin activation. Biochemical and structural data show that vinculin and RIAM binding to R2R3 is mutually exclusive. Moreover, vinculin binding requires domain unfolding, whereas RIAM binds the folded R2R3 double domain. In cells, RIAM is enriched in nascent adhesions at the leading edge whereas vinculin is enriched in FAs. We propose a model in which RIAM binding to R2R3 initially recruits talin to membranes where it activates integrins. As talin engages F-actin, force exerted on R2R3 disrupts RIAM binding and exposes the VBS, which recruit vinculin to stabilize the complex.

Talin1 has unique expression versus talin 2 in the heart and modifies the hypertrophic response to pressure overload.

Integrins are adhesive, signaling, and mechanotransduction proteins. Talin (Tln) activates integrins and links it to the actin cytoskeleton. Vertebrates contain two talin genes, tln1 and tln2. How Tln1 and Tln2 function in cardiac myocytes (CMs) is unknown. Tln1 and Tln2 expression were evaluated in the normal embryonic and adult mouse heart as well as in control and failing human adult myocardium. Tln1 function was then tested in the basal and mechanically stressed myocardium after cardiomyocyte-specific excision of the Tln1 gene. During embryogenesis, both Tln forms are highly expressed in CMs, but in the mature heart Tln2 becomes the main Tln isoform, localizing to the costameres. Tln1 expression is minimal in the adult CM. With pharmacological and mechanical stress causing hypertrophy, Tln1 is up-regulated in CMs and is specifically detected at costameres, suggesting its importance in the compensatory response to CM stress. In human failing heart, CM Tln1 also increases compared with control samples from normal functioning myocardium. To directly test Tln1 function in CMs, we generated CM-specific Tln1 knock-out mice (Tln1cKO). Tln1cKO mice showed normal basal cardiac structure and function but when subjected to pressure overload showed blunted hypertrophy, less fibrosis, and improved cardiac function versus controls. Acute responses of ERK1/2, p38, Akt, and glycogen synthase kinase 3 after mechanical stress were strongly blunted in Tln1cKO mice. Given these results, we conclude that Tln1 and Tln2 have distinct functions in the myocardium. Our data show that reduction of CM Tln1 expression can lead to improved cardiac remodeling following pressure overload.