Caveolin regulation of neuronal intracellular signaling

Physiological and pathological roles of caveolins in the central nervous system

Caveolins are a family of transmembrane proteins located in caveolae, small lipid raft invaginations of the plasma membrane. The roles of caveolin-enriched lipid rafts are diverse, and include mechano-protection, lipid homeostasis, metabolism, transport, and cell signaling. Caveolin-1 (Cav-1) and other caveolins were described in endothelial cells and later in other cell types of the central nervous system (CNS), including neurons, astrocytes, oligodendrocytes, microglia, and pericytes. This pancellular presence of caveolins demands a better understanding of their functional roles in each cell type. In this review we describe the various functions of Cav-1 in the cells of normal and pathological brains. Several emerging preclinical findings suggest that Cav-1 could represent a potential therapeutic target in brain disorders.

Are There Lipid Membrane-Domain Subtypes in Neurons with Different Roles in Calcium Signaling?

Lipid membrane nanodomains or lipid rafts are 10-200 nm diameter size cholesterol- and sphingolipid-enriched domains of the plasma membrane, gathering many proteins with different roles. Isolation and characterization of plasma membrane proteins by differential centrifugation and proteomic studies have revealed a remarkable diversity of proteins in these domains. The limited size of the lipid membrane nanodomain challenges the simple possibility that all of them can coexist within the same lipid membrane domain. As caveolin-1, flotillin isoforms and gangliosides are currently used as neuronal lipid membrane nanodomain markers, we first analyzed the structural features of these components forming nanodomains at the plasma membrane since they are relevant for building supramolecular complexes constituted by these molecular signatures. Among the proteins associated with neuronal lipid membrane nanodomains, there are a large number of proteins that play major roles in calcium signaling, such as ionotropic and metabotropic receptors for neurotransmitters, calcium channels, and calcium pumps. This review highlights a large variation between the calcium signaling proteins that have been reported to be associated with isolated caveolin-1 and flotillin-lipid membrane nanodomains. Since these calcium signaling proteins are scattered in different locations of the neuronal plasma membrane, i.e., in presynapses, postsynapses, axonal or dendritic trees, or in the neuronal soma, our analysis suggests that different lipid membrane-domain subtypes should exist in neurons. Furthermore, we conclude that classification of lipid membrane domains by their content in calcium signaling proteins sheds light on the roles of these domains for neuronal activities that are dependent upon the intracellular calcium concentration. Some examples described in this review include the synaptic and metabolic activity, secretion of neurotransmitters and neuromodulators, neuronal excitability (long-term potentiation and long-term depression), axonal and dendritic growth but also neuronal cell survival and death.

Unraveling the Cave: A Seventy-Year Journey into the Caveolar Network, Cellular Signaling, and Human Disease

In the mid-1950s, a groundbreaking discovery revealed the fascinating presence of caveolae, referred to as flask-shaped invaginations of the plasma membrane, sparking renewed excitement in the field of cell biology. Caveolae are small, flask-shaped invaginations in the cell membrane that play crucial roles in diverse cellular processes, including endocytosis, lipid homeostasis, and signal transduction. The structural stability and functionality of these specialized membrane microdomains are attributed to the coordinated activity of scaffolding proteins, including caveolins and cavins. While caveolae and caveolins have been long appreciated for their integral roles in cellular physiology, the accumulating scientific evidence throughout the years reaffirms their association with a broad spectrum of human disorders. This review article aims to offer a thorough account of the historical advancements in caveolae research, spanning from their initial discovery to the recognition of caveolin family proteins and their intricate contributions to cellular functions. Furthermore, it will examine the consequences of a dysfunctional caveolar network in the development of human diseases.

Integration and Spatial Organization of Signaling by G Protein-Coupled Receptor Homo- and Heterodimers

Information flow from a source to a receiver becomes informative when the recipient can process the signal into a meaningful form. Information exchange and interpretation is essential in biology and understanding how cells integrate signals from a variety of information-coding molecules into complex orchestrated responses is a major challenge for modern cell biology. In complex organisms, cell to cell communication occurs mostly through neurotransmitters and hormones, and receptors are responsible for signal recognition at the membrane level and information transduction inside the cell. The G protein-coupled receptors (GPCRs) are the largest family of membrane receptors, with nearly 800 genes coding for these proteins. The recognition that GPCRs may physically interact with each other has led to the hypothesis that their dimeric state can provide the framework for temporal coincidence in signaling pathways. Furthermore, the formation of GPCRs higher order oligomers provides the structural basis for organizing distinct cell compartments along the plasma membrane where confined increases in second messengers may be perceived and discriminated. Here, we summarize evidence that supports these conjectures, fostering new ideas about the physiological role played by receptor homo- and hetero-oligomerization in cell biology.

Age-related increase in caveolin-1 expression facilitates cell-to-cell transmission of α-synuclein in neurons

Parkinson's disease (PD) is the second most prevalent neurodegenerative disease, with aging being considered the greatest risk factor for developing PD. Caveolin-1 (Cav-1) is known to participate in the aging process. Recent evidence indicates that prion-like propagation of misfolded α-synuclein (α-syn) released from neurons to neighboring neurons plays an important role in PD progression. In the present study, we demonstrated that cav-1 expression in the brain increased with age, and considerably increased in the brain of A53T α-syn transgenic mice. Cav-1 overexpression facilitated the uptake of α-syn into neurons and formation of additional Lewy body-like inclusion bodies, phosphorylation of cav-1 at tyrosine 14 was found to be crucial for this process. This study demonstrates the relationship between age and α-syn spread and will facilitate our understanding of the molecular mechanism of the cell-to-cell transmission of α-syn.

Dopamine D2 Long Receptors Are Critical for Caveolae-Mediated α-Synuclein Uptake in Cultured Dopaminergic Neurons

α-synuclein accumulation into dopaminergic neurons is a pathological hallmark of Parkinson's disease. We previously demonstrated that fatty acid-binding protein 3 (FABP3) is critical for α-synuclein uptake and propagation to accumulate in dopaminergic neurons. FABP3 is abundant in dopaminergic neurons and interacts with dopamine D2 receptors, specifically the long type (D_2L). Here, we investigated the importance of dopamine D_2L receptors in the uptake of α-synuclein monomers and their fibrils. We employed mesencephalic neurons derived from dopamine D_2L ^-/-, dopamine D2 receptor null (D2 null), FABP3^-/-, and wild type C57BL6 mice, and analyzed the uptake ability of fluorescence-conjugated α-synuclein monomers and fibrils. We found that D_2L receptors are co-localized with FABP3. Immunocytochemistry revealed that TH⁺ D2L^-/- or D2 null neurons do not take up α-synuclein monomers. The deletion of α-synuclein C-terminus completely abolished the uptake to dopamine neurons. Likewise, dynasore, a dynamin inhibitor, and caveolin-1 knockdown also abolished the uptake. D_2L and FABP3 were also critical for α-synuclein fibrils uptake. D_2L and accumulated α-synuclein fibrils were well co-localized. These data indicate that dopamine D_2L with a caveola structure coupled with FABP3 is critical for α-synuclein uptake by dopaminergic neurons, suggesting a novel pathogenic mechanism of synucleinopathies, including Parkinson's disease.

Acoustic Waves in Axonal Membrane and Caveolins are the New Targets for Pain Treatment with High Frequency Ultrasound

Reciprocal interaction between electrical and mechanical waves observed in axonal membrane during its excitation leads to a paradigm shift in pain research making the uncoupling of electro-mechanical signals an interesting target in pain treatment. This uncoupling can be realized either through direct disturbance of the mechanical surface waves in axonal membrane or through shifting of the thermodynamic state of this membrane far from its phase transition point. Both effects can be effectively realized through application of the very high frequency ultrasound waves. Additional target for application of ultrasound in pain treatment is the caveolin-1, which is abundantly present in Schwann cells as well as in the non-axonal tissues. Both targets demonstrate frequency-dependent reactions, thus making a very high frequency ultrasound a promising treatment modality in pain treatment.

Caveolin 1 is required for axonal outgrowth of motor neurons and affects Xenopus neuromuscular development

Caveolins are essential structural proteins driving the formation of caveolae, specialized invaginations of the plasma membrane. Loss of Caveolin-1 (Cav1) function in mice causes distinct neurological phenotypes leading to impaired motor control, however, the underlying developmental mechanisms are largely unknown. In this study we find that loss-of-function of Xenopus Cav1 results in a striking swimming defect characterized by paralysis of the morphants. High-resolution imaging of muscle cells revealed aberrant sarcomeric structures with disorganized actin fibers. As cav1 is expressed in motor neurons, but not in muscle cells, the muscular abnormalities are likely a consequence of neuronal defects. Indeed, targeting cav1 Morpholino oligonucleotides to neural tissue, but not muscle tissue, disrupts axonal outgrowth of motor neurons and causes swimming defects. Furthermore, inhibition of voltage-gated sodium channels mimicked the Cav1 loss-of-function phenotype. In addition, analyzing axonal morphology we detect that Cav1 loss-of-function causes excessive filopodia and lamellipodia formation. Using rescue experiments, we show that the Cav1 Y14 phosphorylation site is essential and identify a role of RhoA, Rac1, and Cdc42 signaling in this process. Taken together, these results suggest a previously unrecognized function of Cav1 in muscle development by supporting axonal outgrowth of motor neurons.

Caveolin-1 regulates medium spiny neuron structural and functional plasticity

RATIONALE

Caveolin-1 (CAV1) is a structural protein critical for spatial organization of neuronal signaling molecules. Whether CAV1 is required for long-lasting neuronal plasticity remains unknown.

OBJECTIVE AND METHODS

We sought to examine the effects of CAV1 knockout (KO) on functional plasticity and hypothesized that CAV1 deficiency would impact drug-induced long-term plasticity in the nucleus accumbens (NAc). We first examined cell morphology of NAc medium spiny neurons in a striatal/cortical co-culture system before moving in vivo to study effects of CAV1 KO on cocaine-induced plasticity. Whole-cell patch-clamp recordings were performed to determine effects of chronic cocaine (15 mg/kg) on medium spiny neuron excitability. To test for deficits in behavioral plasticity, we examined the effect of CAV1 KO on locomotor sensitization.

RESULTS

Disruption of CAV1 expression leads to baseline differences in medium spiny neuron (MSN) structural morphology, such that MSNs derived from CAV1 KO animals have increased dendritic arborization when cultured with cortical neurons. The effect was dependent on phospholipase C and cell-type intrinsic loss of CAV1. Slice recordings of nucleus accumbens shell MSNs revealed that CAV1 deficiency produces a loss of neuronal plasticity. Specifically, cocaine-induced firing rate depression was absent in CAV1 KO animals, whereas baseline electrophysiological properties were similar. This was reflected by a loss of cocaine-mediated behavioral sensitization in CAV1 KO animals, with unaffected baseline locomotor responsiveness.

CONCLUSIONS

This study highlights a critical role for nucleus accumbens CAV1 in plasticity related to the administration of drugs of abuse.

Calcium Oscillation Frequency Is a Potential Functional Complex Physiological Relevance Indicator for a Neuroblastoma-Based 3D Culture Model

In vitro screening for drugs that affect neural function in vivo is still primitive. It primarily relies on single cellular responses from 2D monolayer cultures that have been shown to be exaggerations of the in vivo response. For the 3D model to be physiologically relevant, it should express characteristics that not only differentiate it from 2D but also closely emulate those seen in vivo. These complex physiologically relevant (CPR) outcomes can serve as a standard for determining how close a 3D culture is to its native tissue or which out of a given number of 3D platforms is better suited for a given application. In this study, Fluo-4-based calcium fluorescence imaging was performed followed by automated image data processing to quantify the calcium oscillation frequency of SHSY5Y cells cultured in 2D and 3D formats. It was found that the calcium oscillation frequency is upregulated in traditional 2D cultures while it was comparable to in vivo in spheroid and microporous polymer scaffold-based 3D models, suggesting calcium oscillation frequency as a potential functional CPR indicator for neural cultures.

Involvement of caveolin-1 in neurovascular unit remodeling after stroke: Effects on neovascularization and astrogliosis

Complex cellular and molecular events occur in the neurovascular unit after stroke, such as blood-brain barrier (BBB) dysfunction and inflammation that contribute to neuronal death, neurological deterioration and mortality. Caveolin-1 (Cav-1) has distinct physiological functions such as caveolae formation associated with endocytosis and transcytosis as well as in signaling pathways. Cav-1 has been proposed to be involved in BBB dysfunction after brain injury; however, its precise role is poorly understood. The goal of this study was to characterize the expression and effect of Cav-1 deletion on outcome in the first week in a transient Middle Cerebral Artery Occlusion stroke model. We found increased Cav-1 expression in new blood vessels in the lesion and in reactive astrocytes in the peri-lesion areas. In Cav-1 KO mice, the lesion volume was larger and the behavioral outcome worse than in WT mice. Cav-1 KO mice exhibited reduced neovascularization and modified astrogliosis, without formation of a proper glial scar around the lesion at three days post injury, coinciding with aggravated outcomes. Altogether, these results point towards a potential protective role of endogenous Cav-1 in the first days after ischemia by promoting neovascularization, astrogliosis and scar formation.

Cultured hippocampal neurons of dystrophic mdx mice respond differently from those of wild type mice to an acute treatment with corticosterone

Duchenne muscular dystrophy is a lethal genetic disease characterised by progressive degeneration of skeletal muscles induced by deficiency of dystrophin, a cytoskeletal protein expressed in myocytes and in certain neuron populations. The severity of the neurological disorder varies in humans and animal models owing to dysfunction in numerous brain areas, including the hippocampus. Cyclic treatments with high-dose glucocorticoids remain a major pharmacological approach for treating the disease; however, elevated systemic levels of either stress-induced or exogenously administered anti-inflammatory molecules dramatically affect hippocampal activity. In this study, we analysed and compared the response of hippocampal neurons isolated from wild-type and dystrophic mdx mice to acute administration of corticosterone in vitro, without the influence of other glucocorticoid-regulated processes. Our results showed that in neurons of mdx mice, both the genomic and intracellular signalling-mediated responses to corticosterone were affected compared to those in wild-type animals, evoking the characteristic response to detrimental chronic glucocorticoid exposure. Responsiveness to glucocorticoids is, therefore, another function of hippocampal neurons possibly affected by deficiency of Dp427 since embryonic development. Knowing the pivotal role of hippocampus in stress hormone signalling, attention should be paid to the effects that prolonged glucocorticoid treatments may have on this and other brain areas of DMD patients.

Synaptotagmin-11 regulates the functions of caveolae and responds to mechanical stimuli in astrocytes

Caveolae play crucial roles in intracellular membrane trafficking and mechanosensation. In this study, we report that synaptotagmin-11 (Syt11), a synaptotagmin isoform associated with Parkinson's disease and schizophrenia, regulates both caveolae-mediated endocytosis and the caveolar response to mechanical stimuli in astrocytes. Syt11-knockout (KO) accelerated caveolae-mediated endocytosis. Interestingly, the caveolar structures on the cell surface were markedly fewer in the absence of Syt11. Caveolar disassembly in response to hypoosmotic stimuli and astrocyte swelling were both impaired in Syt11-KO astrocytes. Live imaging revealed that Syt11 left caveolar structures before cavin1 during hypoosmotic stress and returned earlier than cavin1 after isoosmotic recovery. Chronic hypoosmotic stress led to proteasome-mediated Syt11 degradation. In addition, Syt11-KO increased the turnover of cavin1 and EH domain-containing protein 2 (EHD2), accompanied by compromised membrane integrity, suggesting a mechanoprotective role of Syt11. Direct interactions between Syt11 and cavin1 and EHD2, but not caveolin-1, are found. Altogether, we propose that Syt11 stabilizes caveolar structures on the cell surface of astrocytes and regulates caveolar functions under physiological and pathological conditions through cavin1 and EHD2.

Co-Encapsulation and Co-Delivery of Peptide Drugs via Polymeric Nanoparticles

Combination therapy is a promising form of treatment. In particular, co-treatment of P3 and QBP1 has been shown to enhance therapeutic effect in vivo in treating polyglutamine diseases. These peptide drugs, however, face challenges in clinical administration due to poor stability, inability to reach intracellular targets, and lack of method to co-deliver both drugs. Here we demonstrate two methods of co-encapsulating the peptide drugs via polymer poly(ethylene glycol)-block-polycaprolactone (PEG-b-PCL) based nanoparticles. Nanoparticles made by double emulsion were 100⁻200 nm in diameter, with drug encapsulation efficiency of around 30%. Nanoparticles made by nanoprecipitation with lipid 1-palmitoyl-2-oleoyl-sn-glycero-3-phospho-(1'-rac-glycerol) (POPG) were around 250⁻300 nm in diameter, with encapsulation efficiency of 85⁻100%. Particles made with both formulations showed cellular uptake when decorated with a mixture of peptide ligands that facilitate endocytosis. In vitro assay showed that nanoparticles could deliver bioactive peptides and encapsulation by double emulsion were found to be more effective in rescuing cells from polyglutamine-induced toxicity.

The expression of select genes necessary for membrane-associated estrogen receptor signaling differ by sex in adult rat hippocampus

17β-estradiol can rapidly modulate neuron function via membrane estrogen receptors (ERs) in a sex-specific manner. For example, female rat hippocampal neurons express palmitoylated versions of ERα and ERβ that associate with the plasma membrane. These membrane-associated ERs are organized by caveolin proteins into functional signaling microdomains with metabotropic glutamate receptors (mGluRs). ER/mGluR signaling mediates several sex-specific estradiol actions on hippocampal neuron function. An important unanswered question regards the mechanism by which sex-specific membrane-associated ER signaling is generated, especially since it has been previously demonstrated that mGluR action is not sex-specific. One possibility is that the genes necessary for the ER membrane complex are differentially expressed between males and females, including genes that encode ERα and β, caveolin 1 and 3, and/or the palmitoylacyltransferases DHHC-7 and -21. Thus we used qPCR to test the hypothesis that these genes show sex differences in expression in neonatal and adult rat hippocampus. As an additional control we tested the expression of the 20 other DHHC palmitoylacyltransferases with no known connections to ER. In neonatal hippocampus, no sex differences were detected in gene expression. In adult hippocampus, the genes that encode caveolin 1 and DHHC-7 showed decreased expression in females compared to males. Thus, select genes differ by sex at specific developmental stages, arguing for a more nuanced model than simple widespread perinatal emergence of sex differences in all genes enabling sex-specific estradiol action. These findings enable the generation of new hypotheses regarding the mechanisms by which sex differences in membrane-associated ER signaling are programmed.

Palmitoylation of caveolin-1 is regulated by the same DHHC acyltransferases that modify steroid hormone receptors

Palmitoylation is a reversible post-translational addition of a 16-carbon lipid chain involved in trafficking and compartmentalizing target proteins. It is important for many cellular functions, including signaling via membrane-localized estrogen receptors (ERs). Within the nervous system, palmitoylation of ERα is necessary for membrane surface localization and mediation of downstream signaling through the activation of metabotropic glutamate receptors (mGluRs). Substitution of the single palmitoylation site on ERα prevents its physical association with the integral membrane protein caveolin-1 (CAV1), required for the formation of the ER/mGluR signaling complex. Interestingly, siRNA knockdown of either of two palmitoyl acyltransferases, zinc finger DHHC type-containing 7 (DHHC7) or DHHC21, also eliminates this signaling mechanism. Because ERα has only one palmitoylation site, we hypothesized that one of these DHHCs palmitoylates CAV1. We investigated this possibility by using an acyl-biotin exchange assay in HEK293 cells in conjunction with DHHC overexpression and found that DHHC7 increases CAV1 palmitoylation. Substitution of the palmitoylation sites on CAV1 eliminated this effect but did not disrupt the ability of the DHHC enzyme to associate with CAV1. In contrast, siRNA-mediated knockdown of DHHC7 alone was not sufficient to decrease CAV1 palmitoylation but rather required simultaneous knockdown of DHHC21. These findings provide additional information about the overall influence of palmitoylation on the membrane-initiated estrogen signaling pathway and highlight the importance of considering the influence of palmitoylation on other CAV1-dependent processes.

Essential role of ATP6AP2 enrichment in caveolae/lipid raft microdomains for the induction of neuronal differentiation of stem cells

Background

The subcellular distribution of prorenin receptor and adaptor protein ATP6AP2 may affect neurogenesis. In this study, we hypothesized that ATP6AP2 expression and subcellular relocalization from caveolae/lipid raft microdomains (CLR-Ms) to intracellular sites may correlate with neuronal differentiation (Neu-Dif) of adipose-derived mesenchymal stem cells (ADSCs).

Methods

Human ADSCs isolated from 24 healthy donors and 24 patients with neurological disorders (ND) were cultured and induced for Neu-Dif. The mechanism of action of ATP6AP2 and the impact of its localization within the plasma membrane (particularly CLR-Ms) and intracellular sites on several pathways (mitogen-activated protein kinase, Wnt(s) signaling and others) and intracellular calcium and exosome release were evaluated. The impact of CLR-Ms on ATP6AP2 or vice versa was determined by pharmacological disruption of CLR-Ms or siATP6AP2 assays.

Results

In patients with ND, loss of ATP6AP2 from CLR-Ms correlated with an inhibition of Neu-Dif and signaling. However, its relocalization in CLR-Ms was positively correlated to induction of Neu-Dif in healthy subjects. An apparent switch from canonical to noncanonical Wnt signaling as well as from caveolin to flotillin occurs concurrently with the increases of ATP6AP2 expression during neurogenesis. Stimulation by renin activates ERK/JNK/CREB/c-Jun but failed to induce β-catenin. Wnt5a enhanced the renin-induced JNK responsiveness. Gα proteins crosslink ATP6AP2 to caveolin where a switch from Gαi to Gαq is necessary for Neu-Dif. In ATP6AP2-enriched CLR-Ms, the release of exosomes was induced dependently from the intracellular Ca²⁺ and Gαq. Pharmacological disruption of CLR-M formation/stability impairs both ATP6AP2 localization and Neu-Dif in addition to reducing exosome release, indicating an essential role of ATP6AP2 enrichment in CLR-Ms for the induction of Neu-Dif. The mechanism is dependent on CLR-M dynamics, particularly the membrane fluidity. Knockdown of ATP6AP2 inhibited Neu-Dif but increased astrocytic-Dif, depleted ATP6AP2/flotillin/Gαq but accumulated caveolin/Gαi in CLR-Ms, and blocked the activation of JNK/ERK/c-Jun/CREB/exosome release. siATP6AP2 cells treated with sphingomyelinase/methyl-β-cyclodextrin reversed the levels of caveolin/flotillin in CLR-Ms but did not induce Neu-Dif, indicating the crucial relocalization of ATP6AP2 in CLR-Ms for neurogenesis. Treatment of ND-derived cells with nSMase showed reversibility in ATP6AP2 abundance in CLR-Ms and enhanced Neu-Dif.

Conclusions

This study gives evidence of the determinant role of CLR-M ATP6AP2 localization for neuronal and oligodendrocyte differentiation involving mechanisms of switches from Gαi/caveolin/canonical to Gαq/flotillin/PCP, the ERK/JNK pathway and Ca²⁺-dependent release of exosomes and as a potential target of drug therapy for neurodegenerative disorders.

Electronic supplementary material

The online version of this article (10.1186/s13287-018-0862-9) contains supplementary material, which is available to authorized users.

Membrane estrogen receptor signaling impacts the reward circuitry of the female brain to influence motivated behaviors

Within the adult female, estrogen signaling is well-described as an integral component of the physiologically significant hypothalamic-pituitary-gonadal axis. In rodents, the timing of ovulation is intrinsically entwined with the display of sexual receptivity. For decades, the importance of estradiol activating intracellular estrogen receptors within the hypothalamus and midbrain/spinal cord lordosis circuits has been appreciated. These signaling pathways primarily account for the ability of the female to reproduce. Yet, often overlooked is that the desire to reproduce is also tightly regulated by estrogen receptor signaling. This lack of emphasis can be attributed to an absence of nuclear estrogen receptors in brain regions associated with reward, such as the nucleus accumbens, which are associated with motivated behaviors. This review outlines how membrane-localized estrogen receptors affect metabotropic glutamate receptor signaling within the rodent nucleus accumbens. In addition, we discuss how, as estrogens drive increased motivation for reproduction, they also produce the untoward side effect of heightening female vulnerability to drug addiction.

Biphasic Regulation of Caveolin-1 Gene Expression by Fluoxetine in Astrocytes: Opposite Effects of PI3K/AKT and MAPK/ERK Signaling Pathways on c-fos

Previously, we reported that fluoxetine acts on 5-HT_2B receptor and induces epidermal growth factor receptor (EGFR) transactivation in astrocytes. Recently, we have found that chronic treatment with fluoxetine regulates Caveolin-1 (Cav-1)/PTEN/PI3K/AKT/glycogen synthase kinase 3β (GSK-3β) signaling pathway and glycogen content in primary cultures of astrocytes with bi-phasic concentration dependence. At low concentrations fluoxetine down-regulates Cav-1 gene expression, decreases membrane content of PTEN, increases PI3K activity and increases phosphorylation of GSK-3β and increases its activity; at high concentrations fluoxetine acts on PTEN/PI3K/AKT/GSK-3β in an inverse fashion. Here, we present the data indicating that acute treatment with fluoxetine at lower concentrations down-regulates c-Fos gene expression via PI3K/AKT signaling pathway; in contrast at higher concentrations fluoxetine up-regulates c-Fos gene expression via MAPK/extracellular-regulated kinase (ERK) signaling pathway. However, acute treatment with fluoxetine has no effect on Cav-1 protein content. Similarly, chronic effects of fluoxetine on Cav-1 gene expression are suppressed by inhibitor of PI3K at lower concentrations, but by inhibitor of MAPK at higher concentrations, indicating that the mechanism underlying bi-phasic regulation of Cav-1 gene expression by fluoxetine is opposing effects of PI3K/AKT and MAPK/ERK signal pathways on c-Fos gene expression. The effects of fluoxetine on Cav-1 gene expression at both lower and higher concentrations are abolished by AG1478, an inhibitor of EGFR, indicating the involvement of 5-HT_2B receptor induced EGFR transactivation as we reported previously. However, PP1, an inhibitor of Src only abolished the effect by lower concentrations, suggesting the relevance of Src with PI3K/AKT signal pathway during activation of EGFR.

Estradiol Membrane-Initiated Signaling in the Brain Mediates Reproduction

Over the past few years our understanding of estrogen signaling in the brain has expanded rapidly. Estrogens are synthesized in the periphery and in the brain, acting on multiple receptors to regulate gene transcription, neural function, and behavior. Various estrogen-sensitive signaling pathways often operate in concert within the same cell, increasing the complexity of the system. In females, estrogen concentrations fluctuate over the estrous/menstrual cycle, dynamically modulating estrogen receptor (ER) expression, activity, and trafficking. These dynamic changes influence multiple behaviors but are particularly important for reproduction. Using the female rodent model, we review our current understanding of estradiol signaling in the regulation of sexual receptivity.

Nerve Growth Factor Signaling from Membrane Microdomains to the Nucleus: Differential Regulation by Caveolins

Membrane microdomains or "lipid rafts" have emerged as essential functional modules of the cell, critical for the regulation of growth factor receptor-mediated responses. Herein we describe the dichotomy between caveolin-1 and caveolin-2, structural and regulatory components of microdomains, in modulating proliferation and differentiation. Caveolin-2 potentiates while caveolin-1 inhibits nerve growth factor (NGF) signaling and subsequent cell differentiation. Caveolin-2 does not appear to impair NGF receptor trafficking but elicits prolonged and stronger activation of MAPK (mitogen-activated protein kinase), Rsk2 (ribosomal protein S6 kinase 2), and CREB (cAMP response element binding protein). In contrast, caveolin-1 does not alter initiation of the NGF signaling pathway activation; rather, it acts, at least in part, by sequestering the cognate receptors, TrkA and p75NTR, at the plasma membrane, together with the phosphorylated form of the downstream effector Rsk2, which ultimately prevents CREB phosphorylation. The non-phosphorylatable caveolin-1 serine 80 mutant (S80V), no longer inhibits TrkA trafficking or subsequent CREB phosphorylation. MC192, a monoclonal antibody towards p75NTR that does not block NGF binding, prevents exit of both NGF receptors (TrkA and p75NTR) from lipid rafts. The results presented herein underline the role of caveolin and receptor signaling complex interplay in the context of neuronal development and tumorigenesis.

Age-Related Modulations of AQP4 and Caveolin-1 in the Hippocampus Predispose the Toxic Effect of Phoneutria nigriventer Spider Venom

We have previously demonstrated that Phoneutria nigriventer venom (PNV) causes blood–brain barrier (BBB) breakdown, swelling of astrocytes end-feet and fluid permeation into brain interstitium in rats. Caveolae and water channels respond to BBB alterations by co-participation in shear stress response and edema formation/resolution. Herein, we showed post-natal developmental-related changes of two BBB-associated transporter proteins: the endothelial caveolin-1 (Cav-1), the major scaffolding protein from caveolae frame, and the astroglial aquaporin-4 (AQP4), the main water channel protein expressed in astrocytic peri-vascular end-feet processes, in the hippocampus of rats intraperitoneally-administered PNV. Western blotting protein levels; immunohistochemistry (IHC) protein distribution in CA1, CA2, and CA3 subfields; and gene expression by Real Time-Polymerase Chain Reaction (qPCR) were assessed in post-natal Day 14 (P14) and 8–10-week-old rats over critical periods of envenomation. The intensity and duration of the toxic manifestations indicate P14 neonate rats more vulnerable to PNV than adults. Histologically, the capillaries of P14 and 8–10-week-old rats treated with PNV showed perivascular edema, while controls did not. The intensity of the toxic manifestations in P14 decreases temporally (2 > 5 > 24 h), while inversely the expression of AQP4 and Cav-1 peaked at 24 h when clinically PNV-treated animals do not differ from saline controls. IHC of AQP4 revealed that hippocampal CA1 showed the least expression at 2 h when toxic manifestation was maximal. Subfield IHC quantification revealed that in P14 rats Cav-1 peaked at 24 h when toxic manifestations were absent, whereas in 8–10-week-old rats Cav-1 peaked at 2 h when toxic signs were highest, and progressively attenuated such increases until 24 h, remaining though significantly above baseline. Considering astrocyte-endothelial physical and functional interactions, we hypothesize that age-related modulations of AQP4 and Cav-1 might be linked both to changes in functional properties of astrocytes during post-natal development and in the BBB breakdown induced by the venom of P. nigriventer.

Overexpression of caveolin-1 attenuates brain edema by inhibiting tight junction degradation

Cerebral edema from the disruption of the blood-brain barrier (BBB) after cerebral ischemia is a major cause of morbidity and mortality as well as a common event in patients with stroke. Caveolins (Cavs) are thought to regulate BBB functions. Here, we report for the first time that Cav-1 overexpression (OE) decreased brain edema from BBB disruption following ischemic insult. Edema volumes and Cav-1 expression levels were measured following photothrombosis and middle cerebral artery occlusion (MCAO). Endothelial cells that were transduced with a Cav-1 lentiviral expression vector were transplanted into rats. BBB permeability was quantified with Evans blue extravasation. Edema volume was determined from measures of the extravasation area, brain water content, and average fluorescence intensity after Cy5.5 injections. Tight junction (TJ) protein expression was measured with immunoblotting. Cav-1 expression levels and vasogenic brain edema correlated strongly after ischemic insult. Cav-1 expression and BBB disruption peaked 3 d after the MCAO. In addition, intravenous administration of endothelial cells expressing Cav-1 effectively increased the Cav-1 levels 3 d after the MCAO ischemic insult. Importantly, Cav-1 OE ameliorated the vasogenic edema by inhibiting the degradation of TJ protein expression in the acute phase of ischemic stroke. These results suggested that Cav-1 OE protected the integrity of the BBB mainly by preventing the degradation of TJ proteins in rats. These findings need to be confirmed in a clinical setting in human subjects.

Almost a century of prion protein(s): From pathology to physiology, and back to pathology

Prions are one of the few pathogens whose name is renowned at all population levels, after the dramatic years pervaded by the fear of eating prion-infected food. If now this, somehow irrational, scare of bovine meat inexorably transmitting devastating brain disorders is largely subdued, several prion-related issues are still unsolved, precluding the design of therapeutic approaches that could slow, if not halt, prion diseases. One unsolved issue is, for example, the role of the prion protein (PrP^C), whole conformational misfolding originates the prion but whose physiologic reason d'etre in neurons, and in cells at large, remains enigmatic. Preceded by a historical outline, the present review will discuss the functional pleiotropicity ascribed to PrP^C, and whether this aspect could fall, at least in part, into a more concise framework. It will also be devoted to radically different perspectives for PrP^C, which have been recently brought to the attention of the scientific world with unexpected force. Finally, it will discuss the possible reasons allowing an evolutionary conserved and benign protein, as PrP^C is, to turn into a high affinity receptor for pathologic misfolded oligomers, and to transmit their toxic message into neurons.

DJ-1 deficiency impairs glutamate uptake into astrocytes via the regulation of flotillin-1 and caveolin-1 expression

Parkinson’s disease (PD) is a common chronic and progressive neurodegenerative disorder. Although the cause of PD is still poorly understood, mutations in many genes including SNCA, parkin, PINK1, LRRK2, and DJ-1 have been identified in the familial forms of PD. It was recently proposed that alterations in lipid rafts may cause the neurodegeneration shown in PD. Here, we observe that DJ-1 deficiency decreased the expression of flotillin-1 (flot-1) and caveolin-1 (cav-1), the main protein components of lipid rafts, in primary astrocytes and MEF cells. As a mechanism, DJ-1 regulated flot-1 stability by direct interaction, however, decreased cav-1 expression may not be a direct effect of DJ-1, but rather as a result of decreased flot-1 expression. Dysregulation of flot-1 and cav-1 by DJ-1 deficiency caused an alteration in the cellular cholesterol level, membrane fluidity, and alteration in lipid rafts-dependent endocytosis. Moreover, DJ-1 deficiency impaired glutamate uptake into astrocytes, a major function of astrocytes in the maintenance of CNS homeostasis, by altering EAAT2 expression. This study will be helpful to understand the role of DJ-1 in the pathogenesis of PD, and the modulation of lipid rafts through the regulation of flot-1 or cav-1 may be a novel therapeutic target for PD.

Regulation of Caveolin-1 Expression Determines Early Brain Edema After Experimental Focal Cerebral Ischemia

Background and Purpose—

Most patients with cerebral infarction die of brain edema because of the breakdown of the blood–brain barrier (BBB) in ischemic tissue. Caveolins (a group of proteins) are key modulators of vascular permeability; however, a direct role of caveolin-1 (Cav-1) in the regulation of BBB permeability during ischemic injury has yet to be identified.

Methods—

Cav-1 expression was measured by immunoblotting after photothrombotic ischemia. A direct functional role of Cav-1 in cerebral edema and BBB permeability during cerebral ischemia was investigated by genetic manipulation (gene disruption and re-expression) of Cav-1 protein expression in mice.

Results—

There was a significant correlation between the extent of BBB disruption and the Cav-1 expression. In Cav-1–deficient (Cav-1 −/− ) mice, the extent of BBB disruption after cerebral ischemia was increased compared with wild-type (Cav-1 +/+ ) mice, whereas the increase in cerebral edema volume was ameliorated by lentiviral-mediated re-expression of Cav-1. Furthermore, Cav-1 −/− mice had significantly higher degradation of tight junction proteins and proteolytic activity of matrix metalloproteinase than Cav-1 +/+ mice. Conversely, re-expression of Cav-1 in Cav-1 −/− mice restored tight junction protein expression and reduced matrix metalloproteinase proteolytic activity.

Conclusions—

These results indicate that Cav-1 is a critical determinant of BBB permeability. Strategies for regulating Cav-1 represent a novel therapeutic approach to controlling BBB disruption and subsequent neurological deterioration during cerebral ischemia.

Are Synchronized Changes in Connexin-43 and Caveolin-3 a Bystander Effect in a Phoneutria nigriventer Venom Model of Blood-Brain Barrier Breakdown?

Upregulation of caveolin-3 (Cav-3) or connexin-43 (Cx43) in astrocytes has been associated with important brain pathologies. We used Phoneutria nigriventer spider venom (PNV), which induces blood-brain barrier breakdown in rats, in order to investigate Cav-3 and Cx43 expression in the cerebellum over critical periods of rat envenomation. By immunofluorescence, western blotting (WB), and transmission electron microscopy (TEM), we assessed changes at 1, 2, 5, 24, and 72 h post-venom. WB showed immediate increases in Cav-3 and Cx43 at 1 h (interval of greatest manifestations of envenomation) that persisted at 5 h (when there were signs of recovery) and peaked at 24 h when no signs of envenomation were detectable. At 2 and 72 h, Cav-3 was downregulated and Cx43 had returned to baseline. PNV markedly intensified Cx43 in molecular, Purkinje and granular layers and Cav-3 in astrocytes whose colocalization to increased GFAP suggests interaction between reactive astrogliosis and Cav-3 upregulation. TEM showed swollen perivascular astrocytic end-feet and synaptic contact alterations that had generally resolved by 72 h. It is uncertain whether such PNV-induced synchronized changes are an interactive effect between Cav-3 and Cx43, or a bystander effect. Evidences indicate that Cav-3 downregulation coupled to Cx43 return to baseline at 72 h when no signs of envenomation were visible, suggesting homeostasis reestablishment. This experimental model is relevant to studying mechanisms involved in neurological disorders associated with Cav-3 overexpression.

Caveolin-1 is a checkpoint regulator in hypoxia-induced astrocyte apoptosis via Ras/Raf/ERK pathway

Astrocytes, the most numerous cells in the human brain, play a central role in the metabolic homeostasis following hypoxic injury. Caveolin-1 (Cav-1), a transmembrane scaffolding protein, has been shown to converge prosurvival signaling in the central nerve system. The present study aimed to investigate the role of Cav-1 in the hypoxia-induced astrocyte injury. We also examined how Cav-1 alleviates apoptotic astrocyte death. To this end, primary astrocytes were exposed to oxygen-glucose deprivation (OGD) for 6 h and a subsequent 24-h reoxygenation to mimic hypoxic injury. OGD significantly reduced Cav-1 expression. Downregulation of Cav-1 using Cav-1 small interfering RNA dramatically worsened astrocyte cell damage and impaired cellular glutamate uptake after OGD, whereas overexpression of Cav-1 with Cav-1 scaffolding domain peptide attenuated OGD-induced cell apoptosis. Mechanistically, the expressions of Ras-GTP, phospho-Raf, and phospho-ERK were sequestered in Cav-1 small interfering RNA-treated astrocytes, yet were stimulated after supplementation with caveolin peptide. MEK/ERK inhibitor U0126 remarkably blocked the Cav-1-induced counteraction against apoptosis following hypoxia, indicating Ras/Raf/ERK pathway is required for the Cav-1's prosurvival role. Together, these findings support Cav-1 as a checkpoint for the in hypoxia-induced astrocyte apoptosis and warrant further studies targeting Cav-1 to treat hypoxic-ischemic brain injury.

Membrane Trafficking in Neuronal Development: Ins and Outs of Neural Connectivity

During development, neurons progress through rapid yet stereotypical shape changes to achieve proper neuronal connectivity. This morphological progression requires carefully orchestrated plasma membrane expansion, insertion of membrane components including receptors for extracellular cues into the plasma membrane and removal and trafficking of membrane materials and proteins to specific locations. This review outlines the cellular machinery of membrane trafficking that play an integral role in neuronal cell shape change and function from initial neurite formation to pathway navigation and synaptogenesis.

Caveolin-1 Sensitivity of Excitatory Amino Acid Transporters EAAT1, EAAT2, EAAT3, and EAAT4

Excitatory amino acid transporters EAAT1 (SLC1A3), EAAT2 (SLC1A2), EAAT3 (SLC1A1), and EAAT4 (SLC1A6) serve to clear L-glutamate from the synaptic cleft and are thus important for the limitation of neuronal excitation. EAAT3 has previously been shown to form complexes with caveolin-1, a major component of caveolae, which participate in the regulation of transport proteins. The present study explored the impact of caveolin-1 on electrogenic transport by excitatory amino acid transporter isoforms EAAT1-4. To this end cRNA encoding EAAT1, EAAT2, EAAT3, or EAAT4 was injected into Xenopus oocytes without or with additional injection of cRNA encoding caveolin-1. The L-glutamate (2 mM)-induced inward current (I Glu) was taken as a measure of glutamate transport. As a result, I Glu was observed in EAAT1-, EAAT2-, EAAT3-, or EAAT4-expressing oocytes but not in water-injected oocytes, and was significantly decreased by coexpression of caveolin-1. Caveolin-1 decreased significantly the maximal transport rate. Treatment of EAATs-expressing oocytes with brefeldin A (5 µM) was followed by a decrease in conductance, which was similar in oocytes expressing EAAT together with caveolin-1 as in oocytes expressing EAAT1-4 alone. Thus, caveolin-1 apparently does not accelerate transporter protein retrieval from the cell membrane. In conclusion, caveolin-1 is a powerful negative regulator of the excitatory glutamate transporters EAAT1, EAAT2, EAAT3, and EAAT4.

Loss of parkin promotes lipid rafts-dependent endocytosis through accumulating caveolin-1: implications for Parkinson's disease

Background

Parkinson’s disease (PD) is characterized by progressive loss of midbrain dopaminergic neurons, resulting in motor dysfunctions. While most PD is sporadic in nature, a significant subset can be linked to either autosomal dominant or recessive mutations. PARK2, encoding the E3 ubiquitin ligase, parkin, is the most frequently mutated gene in autosomal recessive early onset PD. It has recently been reported that PD-associated gene products such as PINK1, α-synuclein, LRRK2, and DJ-1, as well as parkin associate with lipid rafts, suggesting that the dysfunction of these proteins in lipid rafts may be a causal factor of PD. Therefore here, we examined the relationship between lipid rafts-related proteins and parkin.

Results

We identified caveolin-1 (cav-1), which is one of the major constituents of lipid rafts at the plasma membrane, as a substrate of parkin. Loss of parkin function was found to disrupt the ubiquitination and degradation of cav-1, resulting in elevated cav-1 protein level in cells. Moreover, the total cholesterol level and membrane fluidity was altered by parkin deficiency, causing dysregulation of lipid rafts-dependent endocytosis. Further, cell-to-cell transmission of α-synuclein was facilitated by parkin deficiency.

Conclusions

Our results demonstrate that alterations in lipid rafts by the loss of parkin via cav-1 may be a causal factor of PD, and cav-1 may be a novel therapeutic target for PD.

Clinical, cytogenetic and molecular study of a case of ring chromosome 10

Ring chromosome 10 is a rare cytogenetic finding. Only a few cases with molecular cytogenetic definition have been reported. We report here on a child with a ring chromosome 10, which is associated with prenatal and postnatal growth retardation, microcephaly, dysmorphic features, hypotonia, heart defect, severe pes equinovarus, and bronchial asthma. The chromosomal aberration was defined by chromosome microarray analysis, which revealed two deletions at 10pter (3.68 Mb) and 10qter (4.26 Mb). The clinical features are very similar to those reported in other clinical cases with ring chromosome 10, excluding bronchial asthma, which has not been previously reported in individuals with ring chromosome 10.

Caveolae: molecular insights and therapeutic targets for stroke

INTRODUCTION

Caveolae are specialized plasma membrane micro-invaginations of most mammalian cell types. The organization and function of caveolae are carried out by their coat proteins, caveolins and adaptor proteins, cavins. Caveolae/caveolins physically interact with membrane-associated signaling molecules and function in cholesterol incorporation, signaling transduction and macromolecular transport/permeability.

AREAS COVERED

Recent investigations have implicated a check-and-balance role of caveolae in the pathophysiology of cerebral ischemia. Caveolin knockout mice displayed exacerbated ischemic injury, whereas caveolin peptide exerted remarkable protection against ischemia/reperfusion injury. This review attempts to provide a comprehensive synopsis of how caveolae/caveolins modulate blood-brain barrier permeability, pro-survival signaling, angiogenesis and neuroinflammation, and how this may contribute to a better understanding of the participation of caveolae in ischemic cascade. The role of caveolin in the preconditioning-induced tolerance against ischemia is also discussed.

EXPERT OPINION

Caveolae represent a novel target for cerebral ischemia. It remains open how to manipulate caveolin expression in a practical way to recapitulate the beneficial therapeutic outcomes. Caveolin peptides and associated antagomirs may be efficacious and deserve further investigations for their potential benefits for stroke.

Luminescent vesicular nanointerface: a highly selective and sensitive "turn-on" sensor for guanosine triphosphate

A novel amphiphilic Tb(3+) complex (TbL(3+)(I)) consisting of a +3 charged head and a hydrophobic alkyl chain has been developed. It spontaneously self-assembles in water and forms stable vesicles at neutral pH. TbL(3+)(I) has no aromatic groups (functioning as an antenna), and its intrinsic luminescence is thus minimized. These features lead to the self-assembling TbL(3+)(I) receptor molecules demonstrating an increased luminescence intensity upon binding of nucleotides. Upon addition of guanosine triphosphate (GTP), the luminescence from Tb(3+) was notably promoted (127-fold), as the light energy absorbed by the guanine group of GTP was efficiently transferred to the Tb(3+) center. In the case of guanosine diphosphate (GDP) and guanosine monophosphate (GMP), respectively, 78-fold and 43-fold increases in luminescence intensity were observed. This enhancement was less significant than that observed for GTP, due to fewer negative charges on GDP and GMP. No other nucleotides or the tested nonphosphorylated nucleosides affected the luminescence intensity to any notable extent. In marked contrast, all tested nucleotides, including guanine nucleotides, barely promoted the luminescence of molecularly dispersed receptors, TbL(3+)(II), indicating that the confinement and organization of molecules in a nanointerface play vital roles in improving the performance of a sensing system. This Tb(3+) complex nanointerface is successfully used for monitoring the GTP-to-GDP conversion.

PrP(C) signalling in neurons: from basics to clinical challenges

The cellular prion protein PrP(C) was identified over twenty-five years ago as the normal counterpart of the scrapie prion protein PrP(Sc), itself the main if not the sole component of the infectious agent at the root of Transmissible Spongiform Encephalopathies (TSEs). PrP(C) is a ubiquitous cell surface protein, abundantly expressed in neurons, which constitute the targets of PrP(Sc)-mediated toxicity. Converging evidence have highlighted that neuronal, GPI-anchored PrP(C) is absolutely required for prion-induced neuropathogenesis, which warrants investigating into the normal function exerted by PrP(C) in a neuronal context. It is now well-established that PrP(C) can serve as a cell signalling molecule, able to mobilize transduction cascades in response to interactions with partners. This function endows PrP(C) with the capacity to participate in multiple neuronal processes, ranging from survival to synaptic plasticity. A diverse array of data have allowed to shed light on how this function is corrupted by PrP(Sc). Recently, amyloid Aβ oligomers, whose accumulation is associated with Alzheimer's disease (AD), were shown to similarly instigate toxic events by deviating PrP(C)-mediated signalling. Here, we provide an overview of the various signal transduction cascades ascribed to PrP(C) in neurons, summarize how their subversion by PrP(Sc) or Aβ oligomers contributes to TSE or AD neuropathogenesis and discuss the ensuing clinical implications.

Traumatic brain injury enhances neuroinflammation and lesion volume in caveolin deficient mice

Background

Traumatic brain injury (TBI) enhances pro-inflammatory responses, neuronal loss and long-term behavioral deficits. Caveolins (Cavs) are regulators of neuronal and glial survival signaling. Previously we showed that astrocyte and microglial activation is increased in Cav-1 knock-out (KO) mice and that Cav-1 and Cav-3 modulate microglial morphology. We hypothesized that Cavs may regulate cytokine production after TBI.

Methods

Controlled cortical impact (CCI) model of TBI (3 m/second; 1.0 mm depth; parietal cortex) was performed on wild-type (WT; C57Bl/6), Cav-1 KO, and Cav-3 KO mice. Histology and immunofluorescence microscopy (lesion volume, glia activation), behavioral tests (open field, balance beam, wire grip, T-maze), electrophysiology, electron paramagnetic resonance, membrane fractionation, and multiplex assays were performed. Data were analyzed by unpaired t tests or analysis of variance (ANOVA) with post-hoc Bonferroni’s multiple comparison.

Results

CCI increased cortical and hippocampal injury and decreased expression of MLR-localized synaptic proteins (24 hours), enhanced NADPH oxidase (Nox) activity (24 hours and 1 week), enhanced polysynaptic responses (1 week), and caused hippocampal-dependent learning deficits (3 months). CCI increased brain lesion volume in both Cav-3 and Cav-1 KO mice after 24 hours (P < 0.0001, n = 4; one-way ANOVA). Multiplex array revealed a significant increase in expression of IL-1β, IL-9, IL-10, KC (keratinocyte chemoattractant), and monocyte chemoattractant protein 1 (MCP-1) in ipsilateral hemisphere and IL-9, IL-10, IL-17, and macrophage inflammatory protein 1 alpha (MIP-1α) in contralateral hemisphere of WT mice after 4 hours. CCI increased IL-2, IL-6, KC and MCP-1 in ipsilateral and IL-6, IL-9, IL-17 and KC in contralateral hemispheres in Cav-1 KO and increased all 10 cytokines/chemokines in both hemispheres except for IL-17 (ipsilateral) and MIP-1α (contralateral) in Cav-3 KO (versus WT CCI). Cav-3 KO CCI showed increased IL-1β, IL-9, KC, MCP-1, MIP-1α, and granulocyte-macrophage colony-stimulating factor in ipsilateral and IL-1β, IL-2, IL-9, IL-10, and IL-17 in contralateral hemispheres (P = 0.0005, n = 6; two-way ANOVA) compared to Cav-1 KO CCI.

Conclusion

CCI caused astrocyte and microglial activation and hippocampal neuronal injury. Cav-1 and Cav-3 KO exhibited enhanced lesion volume and cytokine/chemokine production after CCI. These findings suggest that Cav isoforms may regulate neuroinflammatory responses and neuroprotection following TBI.

The long coiled-coil protein NECC2 is associated to caveolae and modulates NGF/TrkA signaling in PC12 cells [corrected]

TrkA-mediated NGF signaling in PC12 cells has been shown to be compartimentalized in specialized microdomains of the plasma membrane, the caveolae, which are organized by scaffold proteins including the member of the caveolin family of proteins, caveolin-1. Here, we characterize the intracellular distribution as well as the biochemical and functional properties of the neuroendocrine long coiled-coil protein 2 (NECC2), a novel long coiled-coil protein selectively expressed in neuroendocrine tissues that contains a predicted caveolin-binding domain and displays structural characteristics of a scaffolding factor. NECC2 distributes in caveolae, wherein it colocalizes with the TrkA receptor, and behaves as a caveolae-associated protein in neuroendocrine PC12 cells. In addition, stimulation of PC12 cells with nerve growth factor (NGF) increased the expression and regulated the distribution of NECC2. Interestingly, knockdown as well as overexpression of NECC2 resulted in a reduction of NGF-induced phosphorylation of the TrkA downstream effector extracellular signal-regulated kinases 1 and 2 (ERK1/ERK2) but not of Akt. Altogether, our results identify NECC2 as a novel component of caveolae in PC12 cells and support the contribution of this protein in the maintenance of TrkA-mediated NGF signaling.

Interaction of membrane/lipid rafts with the cytoskeleton: impact on signaling and function: membrane/lipid rafts, mediators of cytoskeletal arrangement and cell signaling

The plasma membrane in eukaryotic cells contains microdomains that are enriched in certain glycosphingolipids, gangliosides, and sterols (such as cholesterol) to form membrane/lipid rafts (MLR). These regions exist as caveolae, morphologically observable flask-like invaginations, or as a less easily detectable planar form. MLR are scaffolds for many molecular entities, including signaling receptors and ion channels that communicate extracellular stimuli to the intracellular milieu. Much evidence indicates that this organization and/or the clustering of MLR into more active signaling platforms depends upon interactions with and dynamic rearrangement of the cytoskeleton. Several cytoskeletal components and binding partners, as well as enzymes that regulate the cytoskeleton, localize to MLR and help regulate lateral diffusion of membrane proteins and lipids in response to extracellular events (e.g., receptor activation, shear stress, electrical conductance, and nutrient demand). MLR regulate cellular polarity, adherence to the extracellular matrix, signaling events (including ones that affect growth and migration), and are sites of cellular entry of certain pathogens, toxins and nanoparticles. The dynamic interaction between MLR and the underlying cytoskeleton thus regulates many facets of the function of eukaryotic cells and their adaptation to changing environments. Here, we review general features of MLR and caveolae and their role in several aspects of cellular function, including polarity of endothelial and epithelial cells, cell migration, mechanotransduction, lymphocyte activation, neuronal growth and signaling, and a variety of disease settings. This article is part of a Special Issue entitled: Reciprocal influences between cell cytoskeleton and membrane channels, receptors and transporters. Guest Editor: Jean Claude Hervé.

Caveolin isoform switching as a molecular, structural, and metabolic regulator of microglia

Microglia are ramified cells that serve as central nervous system (CNS) guardians, capable of proliferation, migration, and generation of inflammatory cytokines. In non-pathological states, these cells exhibit ramified morphology with processes intermingling with neurons and astrocytes. Under pathological conditions, they acquire a rounded amoeboid morphology and proliferative and migratory capabilities. Such morphological changes require cytoskeleton rearrangements. The molecular control points for polymerization states of microtubules and actin are still under investigation. Caveolins (Cavs), membrane/lipid raft proteins, are expressed in inflammatory cells, yet the role of caveolin isoforms in microglia physiology is debatable. We propose that caveolins provide a necessary control point in the regulation of cytoskeletal dynamics, and thus investigated a role for caveolins in microglia biology. We detected mRNA and protein for both Cav-1 and Cav-3. Cav-1 protein was significantly less and localized to plasmalemma (PM) and cytoplasmic vesicles (CVs) in the microglial inactive state, while the active (amoeboid-shaped) microglia exhibited increased Cav-1 expression. In contrast, Cav-3 was highly expressed in the inactive state and localized with cellular processes and perinuclear regions and was detected in active amoeboid microglia. Pharmacological manipulation of the cytoskeleton in the active or non-active state altered caveolin expression. Additionally, increased Cav-1 expression also increased mitochondrial respiration, suggesting possible regulatory roles in cell metabolism necessary to facilitate the morphological changes. The present findings strongly suggest that regulation of microglial morphology and activity are in part due to caveolin isoforms, providing promising novel therapeutic targets in CNS injury or disease.

The cell biology of prion-like spread of protein aggregates: mechanisms and implication in neurodegeneration

The misfolding and aggregation of specific proteins is a common hallmark of many neurodegenerative disorders, including highly prevalent illnesses such as Alzheimer's and Parkinson's diseases, as well as rarer disorders such as Huntington's and prion diseases. Among these, only prion diseases are ‘infectious’. By seeding misfolding of the PrPC (normal conformer prion protein) into PrPSc (abnormal disease-specific conformation of prion protein), prions spread from the periphery of the body to the central nervous system and can also be transmitted between individuals of the same or different species. However, recent exciting data suggest that the transmissibility of misfolded proteins within the brain is a property that goes way beyond the rare prion diseases. Evidence indicates that non-prion aggregates [tau, α-syn (α-synuclein), Aβ (amyloid-β) and Htt (huntingtin) aggregates] can also move between cells and seed the misfolding of their normal conformers. These findings have enormous implications. On the one hand they question the therapeutical use of transplants, and on the other they indicate that it may be possible to bring these diseases to an early arrest by preventing cell-to-cell transmission. To better understand the prion-like spread of these protein aggregates it is essential to identify the underlying cellular and molecular factors. In the present review we analyse and discuss the evidence supporting prion-like spreading of amyloidogenic proteins, especially focusing on the cellular and molecular mechanisms and their significance.

Caveolins: targeting pro-survival signaling in the heart and brain

The present review discusses intracellular signaling moieties specific to membrane lipid rafts (MLRs) and the scaffolding proteins caveolin and introduces current data promoting their potential role in the treatment of pathologies of the heart and brain. MLRs are discreet microdomains of the plasma membrane enriched in gylcosphingolipids and cholesterol that concentrate and localize signaling molecules. Caveolin proteins are necessary for the formation of MLRs, and are responsible for coordinating signaling events by scaffolding and enriching numerous signaling moieties in close proximity. Specifically in the heart and brain, caveolins are necessary for the cytoprotective phenomenon termed ischemic and anesthetic preconditioning. Targeted overexpression of caveolin in the heart and brain leads to induction of multiple pro-survival and pro-growth signaling pathways; thus, caveolins represent a potential novel therapeutic target for cardiac and neurological pathologies.

Transient scrotal hyperthermia induces lipid droplet accumulation and reveals a different ADFP expression pattern between the testes and liver in mice

BACKGROUND

In most mammals, the testes provide a stable environment for spermatogenesis, which depends on a lower temperature than the core body temperature. It has been reported that mild testicular heating safely and reversibly suppresses spermatogenesis, and is under consideration for its potential application as a male contraceptive. Previously, we focused on the molecular mechanism of germ cell apoptosis and anti-apoptotic factors induced by heat treatment in humans and mice. However, the recovery process remains under investigation.

RESULTS

In this study, we found that lipid droplets in mouse testes are dramatically increased after a brief period of scrotal hyperthermia, and gradually dissipate following temperature normalization. Analysis of the human testis proteome revealed nine proteins associated with lipid droplets. Two of them, ADFP (also known as ADRP and PLIN2) and TIP47 (also known as PLIN3) may participate in acute lipid droplet formation in mammalian testes. We show that Adfp expression is upregulated after scrotal heat treatment in mice. Surprisingly, we find Adfp lacking its 5'-UTR is observed in Adfp(Δ1/Δ1) mouse testes, but is not detectable in liver.

CONCLUSIONS

These results reveal testis Adfp transcriptional regulation is tissue-specific, and is associated with lipid droplet accumulation induced by heat. The results also indicate that the testes could retain functional proteins through testes-specific transcriptional regulation.

Membrane-sensitive conformational states of helix 8 in the metabotropic Glu2 receptor, a class C GPCR

The recent elucidation of the X-ray structure of several class A GPCRs clearly indicates that the amphipathic helix 8 (H8) is a conserved structural domain in most crystallized GPCRs. Very little is known about the presence and the possible role of an analogous H8 domain in the distantly related class C GPCRs. In this study, we investigated the structural properties for the H8 domain of the mGluR2 receptor, a class C GPCR, by applying extended molecular dynamics simulations. Our study indicates that the amphipathic H8 adopts membrane-sensitive conformational states, which depend on the membrane composition. Cholesterol-rich membranes stabilize the helical structure of H8 whereas cholesterol-depleted membranes induce a disruption of H8. The observed link between membrane cholesterol levels and H8 conformational states suggests that H8 behaves as a sensor of cholesterol concentration.

CAV1 siRNA reduces membrane estrogen receptor-α levels and attenuates sexual receptivity

Although classic estrogen receptors (ER) have been proposed to mediate estradiol signaling, it has been relatively recently that mechanisms of trafficking these receptors have been elucidated. ERα is palmitoylated and associates with caveolin proteins to be targeted to the cell membrane. Caveolins are scaffold proteins that not only traffic ERα to the membrane but also are involved in establishing metabotropic glutamate receptor interactions that are necessary for activating G protein signaling. To demonstrate the role of caveolin proteins in regulating an estradiol-dependent behavior, sexual receptivity, we used small interfering RNA to knock down caveolin-1 (CAV1) expression in the arcuate nucleus of the hypothalamus. In CAV1 knockdown rats, membrane, but not intracellular levels of ERα, were significantly reduced. As expected, estrogenic stimulation of the arcuate nucleus of the hypothalamus to medial preoptic nucleus projection was abrogated in CAV1 knockdown rats, indicating that the membrane-initiated activation of this circuit was compromised. Moreover, estradiol-induced lordosis behavior that is dependent on activation of μ-opioid receptors in the medial preoptic nucleus was also significantly reduced. Thus, CAV1-mediated ERα trafficking to the cell membrane is required for estradiol activation of circuits underlying female sexual receptivity.

Expression of caveolin in trabecular meshwork cells and its possible implication in pathogenesis of primary open angle glaucoma

PURPOSE

Primary open-angle glaucoma (POAG), which is the most common form of glaucoma, has been associated with a heterogeneous genetic component. A genome-wide association study has identified a common sequence variant at 7q31 (rs4236601 [A]) near the caveolin genes in patients with POAG. Caveolins are a family of integral membrane proteins which participate in many cellular processes, including vesicular transport, cholesterol homeostasis, signal transduction, cell adhesion and migration. The goal of this study was to investigate the expression and regulation of caveolin 1 (CAV-1) and caveolin 2 (CAV-2) in normal and glaucoma trabecular meshwork (TM) cells.

METHODS

CAV-1 and CAV-2 protein expression was quantified by immunoblot analysis using lysates isolated from primary and immortalized TM cells or TM tissue dissected from normal and POAG eyes. The localization of caveolins in TM cells was assessed by immunofluorescent microscopy. CAV-1 and CAV-2 protein expression was also investigated in TM cells at various time points after subjecting the cells to known glaucomatous insults like dexamethasone (DEX) and tumor growth factor beta2 (TGF-β2) treatment. Phosphorylation of CAV-1 at tyrosine 14 in normal and glaucoma TM cell lines was evaluated using a specific monoclonal antibody (Ab). The 5' upstream region of the CAV-1 gene was amplified and the sequence variant rs4236601 (A/G polymorphic site) and several putative transcription factor-binding sites were modified by in vitro mutagenesis. The effect of nucleotide sequence modifications in the CAV-1 upstream region on gene expression was assayed in a luciferase-based system in TM and non-TM cells.

RESULTS

CAV-1 and CAV-2 are expressed in TM cells, with localization to the cytoplasm and perinuclear region. DEX increased CAV-1 expression in immortalized glaucoma TM cells by 2.8±0.1 (n=3) fold at 24 h and 2.5±0.1 (n=3) fold at 48 h, compared to 1.3±0.06 (n=3) fold at 24 and 48 h in immortalized normal TM cells. Phosphorylation of CAV-1 at Tyr14 was reduced by 3.2±0.15 (n=3) fold in glaucomatous TM cells when compared to normal TM cells. In POAG and normal TM tissue, CAV-1 expression was found to be uniform. CAV-2, on the other hand, was variable in independent normal and glaucoma TM tissue. Substitution of a G for an A at base pair -2,388 upstream of the start codon of CAV-1, corresponding to the minor allele rs4236601 [A], increased transcriptional activity in TM and non-TM cells when compared to the native sequence. Deletion analysis of putative transcription factor binding sites in the CAV-1 promoter region caused cell-specific effects on gene expression.

CONCLUSIONS

CAV-1 and CAV-2 are expressed in normal and glaucoma tissue and TM cell lines. Phosphorylation of Tyr14 in CAV-1 and transcriptional regulation of CAV-1 expression may have a role in glaucomatous alterations in TM cells.