Propofol neurotoxicity is mediated by p75 neurotrophin receptor activation

BACKGROUND

Propofol exposure to neurons during synaptogenesis results in apoptosis, leading to cognitive dysfunction in adulthood. Previous work from our laboratory showed that isoflurane neurotoxicity occurs through p75 neurotrophin receptor (p75(NTR)) and subsequent cytoskeleton depolymerization. Given that isoflurane and propofol both suppress neuronal activity, we hypothesized that propofol also induces apoptosis in developing neurons through p75(NTR).

METHODS

Days in vitro 5-7 neurons were exposed to propofol (3 μM) for 6 h and apoptosis was assessed by cleaved caspase-3 (Cl-Csp3) immunoblot and immunofluorescence microscopy. Primary neurons from p75(NTR-/-) mice or wild-type neurons were treated with propofol, with or without pretreatment with TAT-Pep5 (10 μM, 15 min), a specific p75(NTR) inhibitor. P75(NTR-/-) neurons were transfected for 72 h with a lentiviral vector containing the synapsin-driven p75(NTR) gene (Syn-p75(NTR)) or control vector (Syn-green fluorescent protein) before propofol. To confirm our in vitro findings, wild-type mice and p75(NTR-/-) mice (PND5) were pretreated with either TAT-Pep5 or TAT-ctrl followed by propofol for 6 h.

RESULTS

Neurons exposed to propofol showed a significant increase in Cl-Csp3, an effect attenuated by TAT-Pep5 and hydroxyfasudil. Apoptosis was significantly attenuated in p75(NTR-/-) neurons. In p75(NTR-/-) neurons transfected with Syn-p75(NTR), propofol significantly increased Cl-Csp3 in comparison with Syn-green fluorescent protein-transfected p75(NTR-/-) neurons. Wild-type mice exposed to propofol exhibited increased Cl-Csp3 in the hippocampus, an effect attenuated by TAT-Pep5. By contrast, propofol did not induce apoptosis in p75(NTR-/-) mice.

CONCLUSION

These results demonstrate that propofol induces apoptosis in developing neurons in vivo and in vitro and implicate a role for p75(NTR) and the downstream effector RhoA kinase.

Neuronal deletion of caspase 8 protects against brain injury in mouse models of controlled cortical impact and kainic acid-induced excitotoxicity

Background

Acute brain injury is an important health problem. Given the critical position of caspase 8 at the crossroads of cell death pathways, we generated a new viable mouse line (Ncasp8^−/−), in which the gene encoding caspase 8 was selectively deleted in neurons by cre-lox system.

Methodology/Principal Findings

Caspase 8 deletion reduced rates of neuronal cell death in primary neuronal cultures and in whole brain organotypic coronal slice cultures prepared from 4 and 8 month old mice and cultivated up to 14 days in vitro. Treatments of cultures with recombinant murine TNFα (100 ng/ml) or TRAIL (250 ng/mL) plus cyclohexamide significantly protected neurons against cell death induced by these apoptosis-inducing ligands. A protective role of caspase 8 deletion in vivo was also demonstrated using a controlled cortical impact (CCI) model of traumatic brain injury (TBI) and seizure-induced brain injury caused by kainic acid (KA). Morphometric analyses were performed using digital imaging in conjunction with image analysis algorithms. By employing virtual images of hundreds of brain sections, we were able to perform quantitative morphometry of histological and immunohistochemical staining data in an unbiased manner. In the TBI model, homozygous deletion of caspase 8 resulted in reduced lesion volumes, improved post-injury motor performance, superior learning and memory retention, decreased apoptosis, diminished proteolytic processing of caspases and caspase substrates, and less neuronal degeneration, compared to wild type, homozygous cre, and caspase 8-floxed control mice. In the KA model, Ncasp8^−/− mice demonstrated superior survival, reduced seizure severity, less apoptosis, and reduced caspase 3 processing. Uninjured aged knockout mice showed improved learning and memory, implicating a possible role for caspase 8 in cognitive decline with aging.

Conclusions

Neuron-specific deletion of caspase 8 reduces brain damage and improves post-traumatic functional outcomes, suggesting an important role for this caspase in pathophysiology of acute brain trauma.

Cardiac-specific overexpression of caveolin-3 attenuates cardiac hypertrophy and increases natriuretic peptide expression and signaling

OBJECTIVES

We hypothesized that cardiac myocyte-specific overexpression of caveolin-3 (Cav-3), a muscle-specific caveolin, would alter natriuretic peptide signaling and attenuate cardiac hypertrophy.

BACKGROUND

Natriuretic peptides modulate cardiac hypertrophy and are potential therapeutic options for patients with heart failure. Caveolae, microdomains in the plasma membrane that contain caveolin proteins and natriuretic peptide receptors, have been implicated in cardiac hypertrophy and natriuretic peptide localization.

METHODS

We generated transgenic mice with cardiac myocyte-specific overexpression of caveolin-3 (Cav-3 OE) and also used an adenoviral construct to increase Cav-3 in cardiac myocytes.

RESULTS

The Cav-3 OE mice subjected to transverse aortic constriction had increased survival, reduced cardiac hypertrophy, and maintenance of cardiac function compared with control mice. In left ventricle at baseline, messenger ribonucleic acid for atrial natriuretic peptide (ANP) and brain natriuretic peptide (BNP) were increased 7- and 3-fold, respectively, in Cav-3 OE mice compared with control subjects and were accompanied by increased protein expression for ANP and BNP. In addition, ventricles from Cav-3 OE mice had greater cyclic guanosine monophosphate levels, less nuclear factor of activated T-cell nuclear translocation, and more nuclear Akt phosphorylation than ventricles from control subjects. Cardiac myocytes incubated with Cav-3 adenovirus showed increased expression of Cav-3, ANP, and Akt phosphorylation. Incubation with methyl-β-cyclodextrin, which disrupts caveolae, or with wortmannin, a PI3K inhibitor, blocked the increase in ANP expression.

CONCLUSIONS

These results imply that cardiac myocyte-specific Cav-3 OE is a novel strategy to enhance natriuretic peptide expression, attenuate hypertrophy, and possibly exploit the therapeutic benefits of natriuretic peptides in cardiac hypertrophy and heart failure.

Neuron-targeted caveolin-1 protein enhances signaling and promotes arborization of primary neurons

Decreased expression of prosurvival and progrowth-stimulatory pathways, in addition to an environment that inhibits neuronal growth, contribute to the limited regenerative capacity in the central nervous system following injury or neurodegeneration. Membrane/lipid rafts, plasmalemmal microdomains enriched in cholesterol, sphingolipids, and the protein caveolin (Cav) are essential for synaptic development/stabilization and neuronal signaling. Cav-1 concentrates glutamate and neurotrophin receptors and prosurvival kinases and regulates cAMP formation. Here, we show that primary neurons that express a synapsin-driven Cav-1 vector (SynCav1) have increased raft formation, neurotransmitter and neurotrophin receptor expression, NMDA- and BDNF-mediated prosurvival kinase activation, agonist-stimulated cAMP formation, and dendritic growth. Moreover, expression of SynCav1 in Cav-1 KO neurons restores NMDA- and BDNF-mediated signaling and enhances dendritic growth. The enhanced dendritic growth occurred even in the presence of inhibitory cytokines (TNFα, IL-1β) and myelin-associated glycoproteins (MAG, Nogo). Targeting of Cav-1 to neurons thus enhances prosurvival and progrowth signaling and may be a novel means to repair the injured and neurodegenerative brain.

Role of decoy molecules in neuronal ischemic preconditioning

AIMS

Decoy receptors bind with TNF related apoptosis inducing ligands (TRAIL) but do not contain the cytoplasmic domains necessary to transduce apoptotic signals. We hypothesized that decoy receptors may confer neuronal protection against lethal ischemia after ischemic preconditioning (IPC).

MAIN METHOD

Mixed cortical neurons were exposed to IPC one day prior to TRAIL treatment or lethal ischemia.

KEY FINDINGS

IPC increased decoy receptor but reduced death receptor expression compared to lethal ischemia. IPC-induced increase in decoy receptor expression was reduced by prior treatment with CAPE, a nuclear factor-kappa B inhibitor (NFκB).

SIGNIFICANCE

Expression of decoy molecules, dependent on NFκB, may mediate neuronal survival induced by IPC.

A novel mitochondrial outer membrane protein, MOMA-1, that affects cristae morphology in Caenorhabditis elegans

Relatively constant diameters of Caenorhabditis elegans mitochondria and their cristae are disrupted by mutations in a novel mitochondrial outer membrane protein, MOMA-1, and by mutations in a mitofilin homologue, which is anchored in the inner membrane. Genetic data suggest that these proteins act in the same pathway but localize to different membranes.

 Three proteins with similar effects on mitochondrial morphology were identified in an RNA interference (RNAi) screen for mitochondrial abnormalities in Caenorhabditis elegans. One of these is the novel mitochondrial outer membrane protein MOMA-1. The second is the CHCHD3 homologue, CHCH-3, a small intermembrane space protein that may act as a chaperone. The third is a mitofilin homologue, IMMT-1. Mitofilins are inner membrane proteins that control the shapes of cristae. RNAi or mutations in each of these genes change the relatively constant diameters of mitochondria into highly variable diameters, ranging from thin tubes to localized swellings. Neither growth nor brood size of the moma-1, chch-3, or immt-1 single mutants is affected, suggesting that their metabolic functions are normal. However, growth of moma-1 or immt-1 mutants on chch-3(RNAi) leads to withered gonads, a lack of mitochondrial staining, and a dramatic reduction in fecundity, while moma-1; immt-1 double mutants are indistinguishable from single mutants. Mutations in moma-1 and immt-1 also have similar effects on cristae morphology. We conclude that MOMA-1 and IMMT-1 act in the same pathway. It is likely that the observed effects on mitochondrial diameter are an indirect effect of disrupting cristae morphology.

Isoflurane neurotoxicity is mediated by p75NTR-RhoA activation and actin depolymerization

BACKGROUND

The mechanisms by which isoflurane injured the developing brain are not clear. Recent work has demonstrated that it is mediated in part by activation of p75 neurotrophin receptor. This receptor activates RhoA, a small guanosine triphosphatase that can depolymerize actin. It is therefore conceivable that inhibition of RhoA or prevention of cytoskeletal depolymerization might attenuate isoflurane neurotoxicity. This study was conducted to test these hypotheses using primary cultured neurons and hippocampal slice cultures from neonatal mouse pups.

METHODS

Primary neuron cultures (days in vitro, 4-7) and hippocampal slice cultures from postnatal day 4-7 mice were exposed to 1.4% isoflurane (4 h). Neurons were pretreated with TAT-Pep5, an intracellular inhibitor of p75 neurotrophin receptor, the cytoskeletal stabilizer jasplakinolide, or their corresponding vehicles. Hippocampal slice cultures were pretreated with TAT-Pep5 before isoflurane exposure. RhoA activation was evaluated by immunoblot. Cytoskeletal depolymerization and apoptosis were evaluated with immunofluorescence microscopy using drebrin and cleaved caspase-3 staining, respectively.

RESULTS

RhoA activation was increased after 30 and 120 min of isoflurane exposure in neurons; TAT-Pep5 (10 μm) decreased isoflurane-mediated RhoA activation at both time intervals. Isoflurane decreased drebrin immunofluorescence and enhanced cleaved caspase-3 in neurons, effects that were attenuated by pretreatment with either jasplakinolide (1 μm) or TAT-Pep5. TAT-Pep5 attenuated the isoflurane-mediated decrease in phalloidin immunofluorescence. TAT-Pep5 significantly attenuated isoflurane-mediated loss of drebrin immunofluorescence in hippocampal slices.

CONCLUSIONS

Isoflurane results in RhoA activation, cytoskeletal depolymerization, and apoptosis. Inhibition of RhoA activation or prevention of downstream actin depolymerization significantly attenuated isoflurane-mediated neurotoxicity in developing neurons.

Loss of caveolin-1 accelerates neurodegeneration and aging

BACKGROUND

The aged brain exhibits a loss in gray matter and a decrease in spines and synaptic densities that may represent a sequela for neurodegenerative diseases such as Alzheimer's. Membrane/lipid rafts (MLR), discrete regions of the plasmalemma enriched in cholesterol, glycosphingolipids, and sphingomyelin, are essential for the development and stabilization of synapses. Caveolin-1 (Cav-1), a cholesterol binding protein organizes synaptic signaling components within MLR. It is unknown whether loss of synapses is dependent on an age-related loss of Cav-1 expression and whether this has implications for neurodegenerative diseases such as Alzheimer's disease.

METHODOLOGY/PRINCIPAL FINDINGS

We analyzed brains from young (Yg, 3-6 months), middle age (Md, 12 months), aged (Ag, >18 months), and young Cav-1 KO mice and show that localization of PSD-95, NR2A, NR2B, TrkBR, AMPAR, and Cav-1 to MLR is decreased in aged hippocampi. Young Cav-1 KO mice showed signs of premature neuronal aging and degeneration. Hippocampi synaptosomes from Cav-1 KO mice showed reduced PSD-95, NR2A, NR2B, and Cav-1, an inability to be protected against cerebral ischemia-reperfusion injury compared to young WT mice, increased Aβ, P-Tau, and astrogliosis, decreased cerebrovascular volume compared to young WT mice. As with aged hippocampi, Cav-1 KO brains showed significantly reduced synapses. Neuron-targeted re-expression of Cav-1 in Cav-1 KO neurons in vitro decreased Aβ expression.

CONCLUSIONS

Therefore, Cav-1 represents a novel control point for healthy neuronal aging and loss of Cav-1 represents a non-mutational model for Alzheimer's disease.

Dark chocolate receptors: epicatechin-induced cardiac protection is dependent on delta-opioid receptor stimulation

Epicatechin, a flavonoid, is a well-known antioxidant linked to a variety of protective effects in both humans and animals. In particular, its role in protection against cardiovascular disease has been demonstrated by epidemiologic studies. Low-dose epicatechin, which does not have significant antioxidant activity, is also protective; however, the mechanism by which low-dose epicatechin induces this effect is unknown. Our laboratory tested the hypothesis that low-dose epicatechin mediates cardiac protection via opioid receptor activation. C57BL/6 mice were randomly assigned to 1 of 10 groups: control, epicatechin, naloxone (nonselective opioid receptor antagonist), epicatechin + naloxone, naltrindole (δ-specific opioid receptor antagonist), epicatechin + naltrindole, norbinaltorphimine (nor-BNI, κ-specific opioid receptor antagonist), epicatechin + nor-BNI, 5-hydroxydecanoic acid [5-HD, ATP-sensitive potassium channel antagonist], and epicatechin + 5-HD. Epicatechin (1 mg/kg) or other inhibitors (5 mg/kg) were administered by oral gavage or intraperitoneal injection, respectively, daily for 10 days. Mice were subjected to 30 min coronary artery occlusion followed by 2 h of reperfusion, and infarct size was determined via planimetry. Whole heart homogenates were assayed for downstream opioid receptor signaling targets. Infarct size was significantly reduced in epicatechin- and epicatechin + nor-BNI-treated mice compared with control mice. This protection was blocked by naloxone, naltrindole, and 5-HD. Epicatechin and epicatechin + nor-BNI increased the phosphorylation of Src, Akt, and IκBα, while simultaneously decreasing the expression of c-Jun NH(2)-terminal kinase and caspase-activated DNase. All signaling effects are consistent with opioid receptor stimulation and subsequent cardiac protection. Naloxone, naltrindole, and 5-HD attenuated these effects. In conclusion, epicatechin acts via opioid receptors and more specifically through the δ-opioid receptor to produce cardiac protection from ischemia-reperfusion injury.

Inhibition of p75 neurotrophin receptor attenuates isoflurane-mediated neuronal apoptosis in the neonatal central nervous system

BACKGROUND

Exposure to anesthetics during synaptogenesis results in apoptosis and subsequent cognitive dysfunction in adulthood. Probrain-derived neurotrophic factor (proBDNF) is involved in synaptogenesis and can induce neuronal apoptosis via p75 neurotrophic receptors (p75). proBDNF is cleaved into mature BDNF (mBDNF) by plasmin, a protease converted from plasminogen by tissue plasminogen activator (tPA) that is released with neuronal activity; mBDNF supports survival and stabilizes synapses through tropomyosin receptor kinase B. The authors hypothesized that anesthetics suppress tPA release from neurons, enhance p75 signaling, and reduce synapses, resulting in apoptosis.

METHODS

Primary neurons (DIV5) and postnatal day 5-7 (PND5-7) mice were exposed to isoflurane (1.4%, 4 h) in 5% CO2, 95% air. Apoptosis was assessed by cleaved caspase-3 (Cl-Csp3) immunoblot and immunofluorescence microscopy. Dendritic spine changes were evaluated with the neuronal spine marker, drebrin. Changes in synapses in PND5-7 mouse hippocampi were assessed by electron microscopy. Primary neurons were exposed to tPA, plasmin, or pharmacologic inhibitors of p75 (Fc-p75 or TAT-Pep5) 15 min before isoflurane. TAT-Pep5 was administered by intraperitoneal injection to PND5-7 mice 15 min before isoflurane.

RESULTS

Exposure of neurons in vitro (DIV5) to isoflurane decreased tPA in the culture medium, reduced drebrin expression (marker of dendritic filopodial spines), and enhanced Cl-Csp3. tPA, plasmin, or TAT-Pep5 stabilized dendritic filopodial spines and decreased Cl-Csp3 in neurons. TAT-Pep5 blocked isoflurane-mediated increase in Cl-Csp3 and reduced synapses in PND5-7 mouse hippocampi.

CONCLUSION

tPA, plasmin, or p75 inhibition blocked isoflurane-mediated reduction in dendritic filopodial spines and neuronal apoptosis in vitro. Isoflurane reduced synapses and enhanced Cl-Csp3 in the hippocampus of PND5-7 mice, the latter effect being mitigated by p75 inhibition in vivo. These data support the hypothesis that isoflurane neurotoxicity in the developing rodent brain is mediated by reduced synaptic tPA release and enhanced proBDNF/p75-mediated apoptosis.

Cardiac-specific overexpression of caveolin-3 induces endogenous cardiac protection by mimicking ischemic preconditioning

BACKGROUND

Caveolae, lipid-rich microdomains of the sarcolemma, localize and enrich cardiac-protective signaling molecules. Caveolin-3 (Cav-3), the dominant isoform in cardiac myocytes, is a determinant of caveolar formation. We hypothesized that cardiac myocyte-specific overexpression of Cav-3 would enhance the formation of caveolae and augment cardiac protection in vivo.

METHODS AND RESULTS

Ischemic preconditioning in vivo increased the formation of caveolae. Adenovirus for Cav-3 increased caveolar formation and phosphorylation of survival kinases in cardiac myocytes. A transgenic mouse with cardiac myocyte-specific overexpression of Cav-3 (Cav-3 OE) showed enhanced formation of caveolae on the sarcolemma. Cav-3 OE mice subjected to ischemia/reperfusion injury had a significantly reduced infarct size relative to transgene-negative mice. Endogenous cardiac protection in Cav-3 OE mice was similar to wild-type mice undergoing ischemic preconditioning; no increased protection was observed in preconditioned Cav-3 OE mice. Cav-3 knockout mice did not show endogenous protection and showed no protection in response to ischemic preconditioning. Cav-3 OE mouse hearts had increased basal Akt and glycogen synthase kinase-3beta phosphorylation comparable to wild-type mice exposed to ischemic preconditioning. Wortmannin, a phosphoinositide 3-kinase inhibitor, attenuated basal phosphorylation of Akt and glycogen synthase kinase-3beta and blocked cardiac protection in Cav-3 OE mice. Cav-3 OE mice had improved functional recovery and reduced apoptosis at 24 hours of reperfusion.

CONCLUSIONS

Expression of caveolin-3 is both necessary and sufficient for cardiac protection, a conclusion that unites long-standing ultrastructural and molecular observations in the ischemic heart. The present results indicate that increased expression of caveolins, apparently via actions that depend on phosphoinositide 3-kinase, has the potential to protect hearts exposed to ischemia/reperfusion injury.

Anesthetics and brain protection

PURPOSE OF REVIEW

There is a considerable risk of cerebral ischemia during anesthesia and surgery. Anesthetic agents have been shown to have a profound effect on the pathophysiology of cerebral ischemia. The present review provides a brief historical review and details new information about the anesthetic effects on the ischemic brain.

RECENT FINDINGS

Although anesthetics have been shown to reduce ischemic cerebral injury, the durability of this neuroprotection has been questioned. Recent data indicate that, under the right circumstances, anesthetic neuroprotection can be sustained for at least 2-4 weeks; the durability of this protection is dependent upon the experimental model, control of physiologic parameters and the assurance of the adequacy of reperfusion. In addition, volatile anesthetics have been shown to accelerate postischemic neurogenesis; this suggests that anesthetics may enhance the endogenous reparative processes in the injured brain.

SUMMARY

The available data indicate that anesthetics can provide long-term durable protection against ischemic injury that is mild to moderate in severity. Experimental data do not provide support for the premise that anesthetics reduce injury when the ischemic injury is severe.

Caveolin-1 expression is essential for N-methyl-D-aspartate receptor-mediated Src and extracellular signal-regulated kinase 1/2 activation and protection of primary neurons from ischemic cell death

N-Methyl-D-aspartate (NMDA) receptor (NMDAR) activation and downstream signaling are important for neuronal function. Activation of prosurvival Src family kinases and extracellular signal-regulated kinase (ERK) 1/2 is initiated by NMDAR activation, but the cellular organization of these kinases in relation to NMDARs is not entirely clear. We hypothesized that caveolin-1 scaffolds and coordinates protein complexes involved in NMDAR signaling and that this organization is necessary for neuronal preconditioning, whereby NMDAR activation protects neurons from subsequent ischemic cell death. We found that sublethal ischemia (SLI) or preconditioning via NMDA treatment of primary cortical neurons from neonatal rats or mice increases expression of phosphorylated (P) caveolin-1, P-Src, and P-ERK1/2. The NMDAR antagonist, MK801, or the Src inhibitor, PP2, attenuated SLI-induced preconditioning. NMDAR2B distributed to buoyant fractions and heavy fractions, partially colocalized with caveolin-1 and the membrane raft marker, cholera toxin B. Cultures of primary neurons treated with caveolin-1 small interfering RNA or from caveolin-1(-/-) mice lacked the NMDA-mediated increase in P-Src and P-ERK, as well as SLI- and NMDA-induced preconditioning. Adenovirally mediated expression of caveolin-1 in neurons from caveolin-1(-/-) mice restored NMDA-mediated enhancement of P-Src and P-ERK1/2, redistributed NMDAR2B to buoyant fractions, and enhanced NMDAR2B localization to membrane rafts. We conclude that caveolin-1, perhaps via its ability to scaffold key signaling components, is essential for NMDAR localization to neuronal membrane rafts, NMDAR/Src tyrosine kinase family/ERK signaling, and protection of neurons from ischemic injury and cell death.

Increased smooth muscle cell expression of caveolin-1 and caveolae contribute to the pathophysiology of idiopathic pulmonary arterial hypertension

Vasoconstriction and vascular medial hypertrophy, resulting from increased intracellular [Ca2+] in pulmonary artery smooth muscle cells (PASMC), contribute to elevated vascular resistance in patients with idiopathic pulmonary arterial hypertension (IPAH). Caveolae, microdomains within the plasma membrane, contain the protein caveolin, which binds certain signaling molecules. We tested the hypothesis that PASMC from IPAH patients express more caveolin-1 (Cav-1) and caveolae, which contribute to increased capacitative Ca2+ entry (CCE) and DNA synthesis. Immunohistochemistry showed increased expression of Cav-1 in smooth muscle cells but not endothelial cells of pulmonary arteries from patients with IPAH. Subcellular fractionation and electron microscopy confirmed the increase in Cav-1 and caveolae expression in IPAH-PASMC. Treatment of IPAH-PASMC with agents that deplete membrane cholesterol (methyl-beta-cyclodextrin or lovastatin) disrupted caveolae, attenuated CCE, and inhibited DNA synthesis of IPAH-PASMC. Increasing Cav-1 expression of normal PASMC with a Cav-1-encoding adenovirus increased caveolae formation, CCE, and DNA synthesis; treatment of IPAH-PASMC with siRNA targeted to Cav-1 produced the opposite effects. Treatments that down-regulate caveolin/caveolae expression, including cholesterol-lowering drugs, reversed the increased CCE and DNA synthesis in IPAH-PASMC. Increased caveolin and caveolae expression thus contribute to IPAH-PASMC pathophysiology. The close relationship between caveolin/caveolae expression and altered cell physiology in IPAH contrast with previous results obtained in various animal models, including caveolin-knockout mice, thus emphasizing unique features of the human disease. The results imply that disruption of caveolae in PASMC may provide a novel therapeutic approach to attenuate disease manifestations of IPAH.

Mechanisms of cardiac protection from ischemia/reperfusion injury: a role for caveolae and caveolin-1

Caveolae, small invaginations in the plasma membrane, contain caveolins (Cav) that scaffold signaling molecules including the tyrosine kinase Src. We tested the hypothesis that cardiac protection involves a caveolin-dependent mechanism. We used in vitro and in vivo models of ischemia-reperfusion injury, electron microscopy (EM), transgenic mice, and biochemical assays to address this hypothesis. We found that Cav-1 mRNA and protein were expressed in mouse adult cardiac myocytes (ACM). The volatile anesthetic, isoflurane, protected ACM from hypoxia-induced cell death and increased sarcolemmal caveolae. Hearts of wild-type (WT) mice showed rapid phosphorylation of Src and Cav-1 after isoflurane and ischemic preconditioning. The Src inhibitor PP2 reduced phosphorylation of Src (Y416) and Cav-1 in the heart and abolished isoflurane-induced cardiac protection in WT mice. Infarct size (percent area at risk) was reduced by isoflurane in WT (30.5+/-4 vs. 44.2+/-3, n=7, P<0.05) but not Cav-1(-/-) mice (46.6+/-5 vs. 41.7+/-3, n=7). Cav-1(-/-) mice exposed to isoflurane showed significant alterations in Src phosphorylation and recruitment of C-terminal Src kinase, a negative regulator of Src, when compared to WT mice. The results indicate that isoflurane modifies cardiac myocyte sarcolemmal membrane structure and composition and that activation of Src and phosphorylation of Cav-1 contribute to cardiac protection. Accordingly, therapies targeted to post-translational modification of Src and Cav-1 may provide a novel approach for such protection.

Do caveolins regulate cells by actions outside of caveolae?

Caveolae (caveolin-containing lipid rafts) are plasma membrane domains that scaffold and organize a variety of important proteins in eukaryotic cells. Recent work shows that caveolins can act independently of caveolae, both in cells that lack caveolae (e.g. neurons and leukocytes) and in non-caveolar regions of cells that have caveolae (e.g. cardiac myocytes and fibroblasts). Phosphorylation of caveolins can influence the scaffolding of protein partners, and caveolins appear to participate in the protection and trafficking of proteins to and from the plasma membrane. Together, these results suggest that, despite their name, caveolins should now be thought of as proteins that scaffold signaling and other proteins in both caveolar and non-caveolar regions.

Microtubules and actin microfilaments regulate lipid raft/caveolae localization of adenylyl cyclase signaling components

Microtubules and actin filaments regulate plasma membrane topography, but their role in compartmentation of caveolae-resident signaling components, in particular G protein-coupled receptors (GPCR) and their stimulation of cAMP production, has not been defined. We hypothesized that the microtubular and actin cytoskeletons influence the expression and function of lipid rafts/caveolae, thereby regulating the distribution of GPCR signaling components that promote cAMP formation. Depolymerization of microtubules with colchicine (Colch) or actin microfilaments with cytochalasin D (CD) dramatically reduced the amount of caveolin-3 in buoyant (sucrose density) fractions of adult rat cardiac myocytes. Colch or CD treatment led to the exclusion of caveolin-1, caveolin-2, beta1-adrenergic receptors (beta1-AR), beta2-AR, Galpha(s), and adenylyl cyclase (AC)5/6 from buoyant fractions, decreasing AC5/6 and tyrosine-phosphorylated caveolin-1 in caveolin-1 immunoprecipitates but in parallel increased isoproterenol (beta-AR agonist)-stimulated cAMP production. Incubation with Colch decreased co-localization (by immunofluorescence microscopy) of caveolin-3 and alpha-tubulin; both Colch and CD decreased co-localization of caveolin-3 and filamin (an F-actin cross-linking protein), decreased phosphorylation of caveolin-1, Src, and p38 MAPK, and reduced the number of caveolae/mum of sarcolemma (determined by electron microscopy). Treatment of S49 T-lymphoma cells (which possess lipid rafts but lack caveolae) with CD or Colch redistributed a lipid raft marker (linker for activation of T cells (LAT)) and Galpha(s) from lipid raft domains. We conclude that microtubules and actin filaments restrict cAMP formation by regulating the localization and interaction of GPCR-G(s)-AC in lipid rafts/caveolae.

Focal adhesions in (myo)fibroblasts scaffold adenylyl cyclase with phosphorylated caveolin

Fibroblast-myofibroblast transformation, a critical event for enhanced extracellular matrix deposition, involves formation of an actin stress fiber contractile apparatus that radiates from focal adhesions (FA) in the plasma membrane. Activation of adenylyl cyclase (AC, i.e. increases in cAMP) negatively regulates such transformation. Caveolae and their resident protein caveolins scaffold signaling molecules, including AC isoforms, whereas phosphorylated caveolin-1 (phospho-cav-1) may localize at FA. Here, we used adult rat cardiac fibroblasts to examine distribution and expression of AC, phospho-cav-1, and FA proteins to define mechanisms that link increases in cAMP to caveolin-1 phosphorylation, actin/FA assembly, and fibroblast-myofibroblast transformation. Sucrose density gradient centrifugation, immunoblot, and immunohistochemical analysis revealed that, unlike cav-1, phospho-cav-1 enriches in membrane fractions that express FA proteins and localize at the ends of actin stress fibers. We detected AC in both cav-1 and phospho-cav-1 immunoprecipitates, but FA kinase (FAK), phospho-FAK (FAK Tyr-397), paxillin, and vinculin were detected only in phospho-cav-1 immunoprecipitates. Treatment with the AC activator forskolin or a cAMP analog increased cav-1 phosphorylation but decreased FAK Tyr-397 phosphorylation in a cAMP-dependent protein kinase-dependent manner. These events preceded actin cytoskeletal disruption, an effect that was blocked by small interfering RNA knock-down of cav-1. Inhibition of protein tyrosine phosphatase 1B abrogated cAMP-mediated disruption of actin cytoskeleton, cav-1 phosphorylation, and FAK Tyr-397 dephosphorylation. The data thus define a novel organization of signaling molecules that regulate fibroblasts: scaffolding of AC by phospho-cav-1 at FA sites in a caveolae-free microdomain along with components that mediate inhibition of actin/FA assembly and fibroblast-myofibroblast transformation via increases in cAMP.

Isoflurane produces sustained cardiac protection after ischemia-reperfusion injury in mice

BACKGROUND

Isoflurane reduces myocardial ischemia-reperfusion injury within hours to days of reperfusion. Whether isoflurane produces sustained cardiac protection has never been examined. The authors studied isoflurane-induced cardiac protection in the intact mouse after 2 h and 2 weeks of reperfusion and determined the dependence of this protection on adenosine triphosphate-dependent potassium channels and the relevance of this protection to myocardial function and apoptosis.

METHODS

Mice were randomly assigned to receive oxygen or isoflurane for 30 min with 15 min of washout. Some mice received mitochondrial (5-hydroxydecanoic acid) or sarcolemmal (HMR-1098) adenosine triphosphate-dependent potassium channel blockers with or without isoflurane. Mice were then subjected to a 30-min coronary artery occlusion followed by 2 h or 2 weeks of reperfusion. Infarct size was determined at 2 h and 2 weeks of reperfusion. Cardiac function and apoptosis were determined 2 weeks after reperfusion.

RESULTS

Isoflurane did not change hemodynamics. Isoflurane reduced infarct size after reperfusion when compared with the control groups (27.7 +/- 6.3 vs. 41.7 +/- 6.4% at 2 h and 19.6 +/- 5.9 vs. 28.8 +/- 9.0% at 2 weeks). Previous administration of 5-hydroxydecanoic acid, but not HMR-1098, abolished isoflurane-induced cardiac protection. At 2 weeks, left ventricular end-diastolic diameter was decreased significantly and end-systolic pressure and maximum and minimum dP/dt were improved by isoflurane. Isoflurane-treated mice subjected to ischemia and 2 weeks of reperfusion showed less expression of proapoptotic genes, significantly decreased expression of cleaved caspase-3, and significantly decreased deoxynucleotidyl transferase-mediated deoxyuridine triphosphate nick end labeling-positive nuclei compared with the control group.

CONCLUSIONS

Cardiac protection induced by isoflurane against necrotic and apoptotic cell death is associated with an acute memory period that is sustained and functionally relevant 2 weeks after ischemia-reperfusion injury in mice in vivo.

Protection of adult rat cardiac myocytes from ischemic cell death: role of caveolar microdomains and delta-opioid receptors

The role of caveolae, membrane microenvironments enriched in signaling molecules, in myocardial ischemia is poorly defined. In the current study, we used cardiac myocytes prepared from adult rats to test the hypothesis that opioid receptors (OR), which are capable of producing cardiac protection in vivo, promote cardiac protection in cardiac myocytes in a caveolae-dependent manner. We determined protein expression and localization of delta-OR (DOR) using coimmunohistochemistry, caveolar fractionation, and immunoprecipitations. DOR colocalized in fractions with caveolin-3 (Cav-3), a structural component of caveolae in muscle cells, and could be immunoprecipitated by a Cav-3 antibody. Immunohistochemistry confirmed plasma membrane colocalization of DOR with Cav-3. Cardiac myocytes were subjected to simulated ischemia (2 h) or an ischemic preconditioning (IPC) protocol (10 min ischemia, 30 min recovery, 2 h ischemia) in the presence and absence of methyl-beta-cyclodextrin (MbetaCD, 2 mM), which binds cholesterol and disrupts caveolae. We also assessed the cardiac protective effects of SNC-121 (SNC), a selective DOR agonist, on cardiac myocytes with or without MbetaCD and MbetaCD preloaded with cholesterol. Ischemia, simulated by mineral oil layering to inhibit gas exchange, promoted cardiac myocyte cell death (trypan blue staining), a response blunted by SNC (37 +/- 3 vs. 59 +/- 3% dead cells in the presence and absence of 1 muM SNC, respectively, P < 0.01) or by use of the IPC protocol (35 +/- 4 vs. 62 +/- 3% dead cells, P < 0.01). MbetaCD treatment, which disrupted caveolae (as detected by electron microscopy), fully attenuated the protective effects of IPC or SNC, resulting in cell death comparable to that of the ischemic group. By contrast, SNC-induced protection was not abrogated in cells incubated with cholesterol-saturated MbetaCD, which maintained caveolae structure and function. These findings suggest a key role for caveolae, perhaps through enrichment of signaling molecules, in contributing to protection of cardiac myocytes from ischemic damage.

Caveolae and lipid rafts: G protein-coupled receptor signaling microdomains in cardiac myocytes

A growing body of data indicates that multiple signal transduction events in the heart occur via plasma membrane receptors located in signaling microdomains. Lipid rafts, enriched in cholesterol and sphingolipids, form one such microdomain along with a subset of lipid rafts, caveolae, enriched in the protein caveolin. In the heart, a key caveolin is caveolin-3, whose scaffolding domain is thought to serve as an anchor for other proteins. In spite of the original morphologic definition of caveolae ("little caves"), most work related to their role in compartmenting signal transduction molecules has involved subcellular fractionation or immunoprecipitation with anti-caveolin antibodies. Use of such approaches has documented that several G protein-coupled receptors (GPCR), and their cognate heterotrimeric G proteins and effectors, localize to lipid rafts/caveolae in neonatal cardiac myocytes. Our recent findings support the view that adult cardiac myocytes appear to have different patterns of localization of such components compared to neonatal myocytes and cardiac fibroblasts. Such results imply the existence of multiple subcellular microdomains for GPCR-mediated signal transduction in cardiac myocytes, in particular adult myocytes, and raise a major unanswered question: what are the precise mechanism(s) that determine co-localization of GPCR and post-receptor components with lipid rafts/caveolae in cardiac myocytes and other cell types?

Compartmentation of G-protein-coupled receptors and their signalling components in lipid rafts and caveolae

G-protein-coupled receptors (GPCRs) and post-GPCR signalling components are expressed at low overall abundance in plasma membranes, yet they evoke rapid, high-fidelity responses. Considerable evidence suggests that GPCR signalling components are organized together in membrane microdomains, in particular lipid rafts, enriched in cholesterol and sphingolipids, and caveolae, a subset of lipid rafts that also possess the protein caveolin, whose scaffolding domain may serve as an anchor for signalling components. Caveolae were originally identified based on their morphological appearance but their role in compartmentation of GPCR signalling has been primarily studied by biochemical techniques, such as subcellular fractionation and immunoprecipitation. Our recent studies obtained using both microscopic and biochemical methods with adult cardiac myocytes show expression of caveolin not only in surface sarcolemmal domains but also at, or close to, internal regions located at transverse tubules/sarcoplasmic reticulum. Other results show co-localization in lipid rafts/caveolae of AC (adenylyl cyclase), in particular AC6, certain GPCRs, G-proteins and eNOS (endothelial nitric oxide synthase; NOS3), which generates NO, a modulator of AC6. Existence of multiple caveolin-rich microdomains and their expression of multiple modulators of signalling strengthen the evidence that caveolins and lipid rafts/caveolae organize and regulate GPCR signal transduction in eukaryotic cells.

G-protein-coupled Receptor Signaling Components Localize in Both Sarcolemmal and Intracellular Caveolin-3-associated Microdomains in Adult Cardiac Myocytes

This study tests the hypothesis that G-protein-coupled receptor (GPCR) signaling components involved in the regulation of adenylyl cyclase (AC) localize with caveolin (Cav), a protein marker for caveolae, in both cell-surface and intracellular membrane regions. Using sucrose density fractionation of adult cardiac myocytes, we detected Cav-3 in both buoyant membrane fractions (BF) and heavy/non-buoyant fractions (HF); beta2-adrenergic receptors (AR) in BF; and AC5/6, beta1-AR, M4-muscarinic acetylcholine receptors (mAChR), mu-opioid receptors, and Galpha(s) in both BF and HF. In contrast, M2-mAChR, Galpha(i3), and Galpha(i2) were found only in HF. Immunofluorescence microscopy showed co-localization of Cav-3 with AC5/6, Galpha(s), beta2-AR, and mu-opioid receptors in both sarcolemmal and intracellular membranes, whereas M2-mAChR were detected only intracellularly. Immunofluorescence of adult heart revealed a distribution of Cav-3 identical to that in isolated adult cardiac myocytes. Upon immunoelectron microscopy, Cav-3 co-localized with AC5/6 and Galpha(s) in sarcolemmal and intracellular vesicles, the latter closely allied with T-tubules. Cav-3 immunoprecipitates possessed components that were necessary and sufficient for GPCR agonist-promoted stimulation and inhibition of cAMP formation. The distribution of GPCR, G-proteins, and AC with Cav-3 in both sarcolemmal and intracellular T-tubule-associated regions indicates the existence of multiple Cav-3-localized cellular microdomains for signaling by hormones and drugs in the heart.

Localization of adenylyl cyclase isoforms and G protein-coupled receptors in vascular smooth muscle cells: expression in caveolin-rich and noncaveolin domains

A number of different agonists activate G protein-coupled receptors to stimulate adenylyl cyclase (AC), increase cAMP formation, and promote relaxation in vascular smooth muscle. To more fully understand this stimulation of AC, we assessed the expression, regulation, and compartmentation of AC isoforms in rat aortic smooth muscle cells (RASMC). Reverse transcription-polymerase chain reaction detected expression of AC3, AC5, and AC6 mRNA, whereas immunoblot analysis indicated expression of AC3 and AC5/6 protein primarily in caveolin-rich membrane (cav) fractions relative to noncaveolin (noncav) fractions. Beta(1)-adrenergic receptors (AR), beta(2)AR, and G(s) were detected in both cav and noncav fractions, whereas the prostanoid receptors EP(2)R and EP(4)R were excluded from cav fractions. We used an adenoviral construct to increase AC6 expression. Overexpressed AC6 localized only in noncav fractions. Two-fold overexpression of AC6 caused enhancement of forskolin-, isoproterenol- and prostaglandin E(2)-stimulated cAMP formation but no changes in basal levels of cAMP. At higher levels of AC6 overexpression, basal and adenosine receptor-stimulated cAMP levels were increased. Stimulation of cAMP levels by agents that increase Ca(2+) in native cells was consistent with the expression of AC3, but overexpression of AC6, which is inhibited by Ca(2+), blunted the Ca(2+)-stimulable cAMP response. These data indicate that: 1) RASMC express multiple AC isoforms that localize in both caveolin-rich and noncaveolin domains, 2) expression of AC6 in non-caveolin-rich membranes can increase basal levels of cAMP and response to several stimulatory agonists, and 3) Ca(2+)-mediated regulation of cAMP formation depends upon expression of different AC isoforms in RASMC. Compartmentation of GPCRs and AC is different in cardiomyocytes than in RASMC, indicating that targeting of these components to caveolin-rich membranes can be cell-specific. Moreover, our results imply that the colocalization of GPCRs and the AC isoforms they activate need not occur in caveolin-rich fractions.

Intracoronary delivery of adenovirus encoding adenylyl cyclase VI increases left ventricular function and cAMP-generating capacity

BACKGROUND

We tested the hypothesis that intracoronary injection of a recombinant adenovirus encoding adenylyl cyclase type VI (AC(VI)) would increase cardiac function in pigs.

METHODS AND RESULTS

Left ventricular (LV) dP/dt and cardiac output in response to isoproterenol and NKH477 stimulation were assessed in normal pigs before and 12 days after intracoronary delivery of histamine followed by intracoronary delivery of an adenovirus encoding lacZ (control) or AC(VI) (1.4x10(12) vp). Animals that had received AC(VI) gene transfer showed increases in peak LV dP/dt (average increase of 1267+/-807 mm Hg/s; P=0.0002) and cardiac output (average increase of 39+/-20 mL. kg(-1). min(-1); P<0.0001); control animals showed no changes. Increased LV dP/dt was evident 6 days after gene transfer and persisted for at least 57 days. Basal heart rate, blood pressure, and LV dP/dt were unchanged, despite changes in cardiac responsiveness to catecholamine stimulation. Twenty-three hour ECG recordings showed no change in mean heart rate or ectopic beats and no arrhythmias. LV homogenates from animals receiving AC(VI) gene transfer showed increased AC(VI) protein content (P=0.0007) and stimulated cAMP production (P=0.0006), confirming transgene expression and function; basal LV AC activity was unchanged. Increased cAMP-generating capacity persisted for at least 18 weeks (P<0.0002).

CONCLUSIONS

Intracoronary injection of a recombinant adenovirus encoding AC provides enduring increases in cardiac function.