Phosphofructokinase muscle-specific isoform requires caveolin-3 expression for plasma membrane recruitment and caveolar targeting: implications for the pathogenesis of caveolin-related muscle diseases

Hitting the Sweet Spot: How Glucose Metabolism Is Orchestrated in Space and Time by Phosphofructokinase-1

Glycolysis is the central metabolic pathway across all kingdoms of life. Intensive research efforts have been devoted to understanding the tightly orchestrated processes of converting glucose into energy in health and disease. Our review highlights the advances in knowledge of how metabolic and gene networks are integrated through the precise spatiotemporal compartmentalization of rate-limiting enzymes. We provide an overview of technically innovative approaches that have been applied to study phosphofructokinase-1 (PFK1), which represents the fate-determining step of oxidative glucose metabolism. Specifically, we discuss fast-acting chemical biology and optogenetic tools that have delineated new links between metabolite fluxes and transcriptional reprogramming, which operate together to enact tissue-specific processes. Finally, we discuss how recent paradigm-shifting insights into the fundamental basis of glycolytic regulatory control have shed light on the mechanisms of tumorigenesis and could provide insight into new therapeutic vulnerabilities in cancer.

Relocation of phosphofructokinases within epithelial cells is a novel event preceding breast cancer recurrence that accurately predicts patient outcomes

Although recurrent cancers are often aggressive, little is known about the intracellular events required for cancer recurrences. Due to this lack of mechanistic information, there is no test to predict cancer recurrences or non-recurrences during early stages of disease. In this retrospective study, we use ductal carcinoma in situ (DCIS) of the breast as a framework to better understand the mechanism of cancer recurrences using patient outcomes as the physiological observable. Conventional pathology slides were labeled with anti-phosphofructokinase type L (PFKL) and anti-phosphofructokinase/fructose-2,6-bisphosphatase type 4 (PFKFB4) reagents. PFKL and PFKFB4 were found in ductal epithelial cell nucleoli from DCIS samples of women who did not experience a cancer recurrence. In contrast, PFKL and PFKFB4 may be found near the plasma membrane in samples from patients who will develop recurrent cancer. Using machine learning to predict patient outcomes, holdout studies of individual patient micrographs for the three biomarkers PFKL, PFKFB4, and phosphorylated GLUT1 demonstrated 38.6% true negatives, 49.5% true positives, 11.9% false positives and 0% false negatives (N=101). A sub-population of recurrent samples demonstrated PFKL, PFKFB4, and phosphorylated glucose transporter 1 accumulation at the apical surface of epithelial cells, suggesting that carbohydrates can be harvested from the ducts' luminal spaces as an energy source. We suggest that PFK isotype patterns are metabolic switches representing key mechanistic steps of recurrences. Furthermore, PFK enzyme patterns within epithelial cells contribute to an accurate diagnostic test to classify DCIS patients as high or low recurrence risk.

Caveolin-3 promotes glycometabolism, growth and proliferation in muscle cells

Objective

Caveolin-3 (CAV3) protein is known to be expressed specifically in various myocytes, but its physiological function remains unclear. CAV3, located at the cell membrane, may promote the sensitivity of the Akt signaling pathway, which is closely related to glucose metabolism and to cell growth and proliferation.

Methods

The CAV3 gene was stably transfected into C2C12 muscle cells, and the effects were evaluated by biochemical assays, WB and confocal microscopy for the observation of cellular glucose metabolism, growth and proliferation, and the effect of CAV3 on the Akt signaling pathway with no insulin stimulation.

Results

After C2C12 cells were transfected with the mouse CAV3 gene, which increased CAV3 expression, the abundance of the CAV3 and GLUT4 proteins on the cell membrane increased, but the total GLUT4 protein content of the cell was unchanged. Glucose uptake was increased, and this did not affect the glycogen synthesis, but the cell surface area and cell proliferation increased. While there were significant increases in p-Akt and p-p70s6K, which is a downstream component of Akt signaling, the level of GSK3β protein, another component of Akt signaling did not change.

Conclusions

The muscle, CAV3 protein can activate Akt signaling, increase GLUT4 protein localization in the cell membrane, increase glucose uptake, and promote myocyte growth and proliferation. CAV3 protein has a physiological role in glycometabolism, growth and proliferation, independent of insulin stimulation.

Caveolae, caveolin, and cavins: potential targets for the treatment of cardiac disease

Caveolae are omega-shaped membrane invaginations present in essentially all cell types of the cardiovascular system, including endothelial cells, smooth muscle cells, macrophages, cardiac myocytes, and fibroblasts. Numerous functions have been ascribed to this omega-shaped structure. Caveolae are enriched with different signaling molecules and ion channel regulatory proteins and function both in protein trafficking and signal transduction in these cell types. Caveolins are the structural proteins that are necessary for the formation of caveola membrane domains. Mechanistically, caveolins interact with a variety of downstream signaling molecules, as, for example, Src-family tyrosine kinase, p42/44 mitogen-activated protein (MAP) kinase, and endothelial nitric oxide synthase (eNOS) and hold the signal transducers in the inactive condition until activated with proper stimulus. Caveolae are gradually acquiring increasing attention as cellular organelles contributing to the pathogenesis of several structural and functional processes including cardiac hypertrophy, atherosclerosis, and heart failure. At present, very little is known about the role of caveolae in cardiac function and dysfunction, although recent studies with caveolin knock-out mouse have shown that caveolae and caveolins play a pivotal role in various human pathobiological conditions. This review will discuss the possible role and mechanism of action of caveolae and caveolins in different cardiac diseases.

Fiber Types in Mammalian Skeletal Muscles

Mammalian skeletal muscle comprises different fiber types, whose identity is first established during embryonic development by intrinsic myogenic control mechanisms and is later modulated by neural and hormonal factors. The relative proportion of the different fiber types varies strikingly between species, and in humans shows significant variability between individuals. Myosin heavy chain isoforms, whose complete inventory and expression pattern are now available, provide a useful marker for fiber types, both for the four major forms present in trunk and limb muscles and the minor forms present in head and neck muscles. However, muscle fiber diversity involves all functional muscle cell compartments, including membrane excitation, excitation-contraction coupling, contractile machinery, cytoskeleton scaffold, and energy supply systems. Variations within each compartment are limited by the need of matching fiber type properties between different compartments. Nerve activity is a major control mechanism of the fiber type profile, and multiple signaling pathways are implicated in activity-dependent changes of muscle fibers. The characterization of these pathways is raising increasing interest in clinical medicine, given the potentially beneficial effects of muscle fiber type switching in the prevention and treatment of metabolic diseases.

Lipid raft in cardiac health and disease

Lipid rafts are sphingolipid and cholesterol rich micro-domains of the plasma membrane that coordinate and regulate varieties of signaling processes. Lipid rafts are also present in cardiac myocytes and are enriched in signaling molecules and ion channel regulatory proteins. Lipid rafts are receiving increasing attention as cellular organelles contributing to the pathogenesis of several structural and functional processes including cardiac hypertrophy and heart failure. At present, very little is known about the role of lipid rafts in cardiac function and dysfunction. This review will discuss the possible role of lipid rafts in cardiac health and disease.

Microgenomic analysis in skeletal muscle: expression signatures of individual fast and slow myofibers

Background

Skeletal muscle is a complex, versatile tissue composed of a variety of functionally diverse fiber types. Although the biochemical, structural and functional properties of myofibers have been the subject of intense investigation for the last decades, understanding molecular processes regulating fiber type diversity is still complicated by the heterogeneity of cell types present in the whole muscle organ.

Methodology/Principal Findings

We have produced a first catalogue of genes expressed in mouse slow-oxidative (type 1) and fast-glycolytic (type 2B) fibers through transcriptome analysis at the single fiber level (microgenomics). Individual fibers were obtained from murine soleus and EDL muscles and initially classified by myosin heavy chain isoform content. Gene expression profiling on high density DNA oligonucleotide microarrays showed that both qualitative and quantitative improvements were achieved, compared to results with standard muscle homogenate. First, myofiber profiles were virtually free from non-muscle transcriptional activity. Second, thousands of muscle-specific genes were identified, leading to a better definition of gene signatures in the two fiber types as well as the detection of metabolic and signaling pathways that are differentially activated in specific fiber types. Several regulatory proteins showed preferential expression in slow myofibers. Discriminant analysis revealed novel genes that could be useful for fiber type functional classification.

Conclusions/Significance

As gene expression analyses at the single fiber level significantly increased the resolution power, this innovative approach would allow a better understanding of the adaptive transcriptomic transitions occurring in myofibers under physiological and pathological conditions.

Alterations of excitation-contraction coupling and excitation coupled Ca(2+) entry in human myotubes carrying CAV3 mutations linked to rippling muscle

Rippling muscle disease is caused by mutations in the gene encoding caveolin-3 (CAV3), the muscle-specific isoform of the scaffolding protein caveolin, a protein involved in the formation of caveolae. In healthy muscle, caveolin-3 is responsible for the formation of caveolae, which are highly organized sarcolemmal clusters influencing early muscle differentiation, signalling and Ca²⁺ homeostasis. In the present study we examined Ca²⁺ homeostasis and excitation–contraction (E-C) coupling in cultured myotubes derived from two patients with Rippling muscle disease with severe reduction in caveolin-3 expression; one patient harboured the heterozygous c.84C>A mutation while the other patient harbored a homozygous splice-site mutation (c.102+ 2T>C) affecting the splice donor site of intron 1 of the CAV3 gene. Our results show that cells from control and rippling muscle disease patients had similar resting [Ca²⁺]_i and 4-chloro-m-cresol-induced Ca²⁺ release but reduced KCl-induced Ca²⁺ influx. Detailed analysis of the voltage-dependence of Ca²⁺ transients revealed a significant shift of Ca²⁺ release activation to higher depolarization levels in CAV3 mutated cells. High resolution immunofluorescence analysis by Total Internal Fluorescence microscopy supports the hypothesis that loss of caveolin-3 leads to microscopic disarrays in the colocalization of the voltage-sensing dihydropyridine receptor and the ryanodine receptor, thereby reducing the efficiency of excitation–contraction coupling. Hum Mutat 32:309–317, 2011. © 2011 Wiley-Liss, Inc.

Reversible high affinity inhibition of phosphofructokinase-1 by acyl-CoA: a mechanism integrating glycolytic flux with lipid metabolism

The enzyme phosphofructokinase-1 (PFK-1) catalyzes the first committed step of glycolysis and is regulated by a complex array of allosteric effectors that integrate glycolytic flux with cellular bioenergetics. Here, we demonstrate the direct, potent, and reversible inhibition of purified rabbit muscle PFK-1 by low micromolar concentrations of long chain fatty acyl-CoAs (apparent Ki∼1 μM). In sharp contrast, short chain acyl-CoAs, palmitoylcarnitine, and palmitic acid in the presence of CoASH were without effect. Remarkably, MgAMP and MgADP but not MgATP protected PFK-1 against inhibition by palmitoyl-CoA indicating that acyl-CoAs regulate PFK-1 activity in concert with cellular high energy phosphate status. Furthermore, incubation of PFK-1 with [1-(14)C]palmitoyl-CoA resulted in robust acylation of the enzyme that was reversible by incubation with acyl-protein thioesterase-1 (APT1). Importantly, APT1 reversed palmitoyl-CoA-mediated inhibition of PFK-1 activity. Mass spectrometric analyses of palmitoylated PFK-1 revealed four sites of acylation, including Cys-114, Cys-170, Cys-351, and Cys-577. PFK-1 in both skeletal muscle extracts and in purified form was inhibited by S-hexadecyl-CoA, a nonhydrolyzable palmitoyl-CoA analog, demonstrating that covalent acylation of PFK-1 was not required for inhibition. Tryptic footprinting suggested that S-hexadecyl-CoA induced a conformational change in PFK-1. Both palmitoyl-CoA and S-hexadecyl-CoA increased the association of PFK-1 with Ca2+/calmodulin, which attenuated the binding of palmitoylated PFK-1 to membrane vesicles. Collectively, these results demonstrate that fatty acyl-CoA modulates phosphofructokinase activity through both covalent and noncovalent interactions to regulate glycolytic flux and enzyme membrane localization via the branch point metabolic node that mediates lipid flux through anabolic and catabolic pathways.

Differential effects of myopathy-associated caveolin-3 mutants on growth factor signaling

Caveolin-3 is an important scaffold protein of cholesterol-rich caveolae. Mutations of caveolin-3 cause hereditary myopathies that comprise remarkably different pathologies. Growth factor signaling plays an important role in muscle physiology; it is influenced by caveolins and cholesterol-rich rafts and might thus be affected by caveolin-3 dysfunction. Prompted by the observation of a marked chronic peripheral neuropathy in a patient suffering from rippling muscle disease due to the R26Q caveolin-3 mutation and because TrkA is expressed by neuronal cells and skeletal muscle fibers, we performed a detailed comparative study on the effect of pathogenic caveolin-3 mutants on the signaling and trafficking of the TrkA nerve growth factor receptor and, for comparison, of the epidermal growth factor receptor. We found that the R26Q mutant slightly and the P28L strongly reduced nerve growth factor signaling in TrkA-transfected cells. Surface biotinylation experiments revealed that the R26Q caveolin-3 mutation markedly reduced the internalization of TrkA, whereas the P28L did not. Moreover, P28L expression led to increased, whereas R26Q expression decreased, epidermal growth factor signaling. Taken together, we found differential effects of the R26Q and P28L caveolin-3 mutants on growth factor signaling. Our findings are of clinical interest because they might help explain the remarkable differences in the degree of muscle lesions caused by caveolin-3 mutations and also the co-occurrence of peripheral neuropathy in the R26Q caveolinopathy case presented.

Caveolinopathies: from the biology of caveolin-3 to human diseases

In muscle tissue the protein caveolin-3 forms caveolae--flask-shaped invaginations localized on the cytoplasmic surface of the sarcolemmal membrane. Caveolae have a key role in the maintenance of plasma membrane integrity and in the processes of vesicular trafficking and signal transduction. Mutations in the caveolin-3 gene lead to skeletal muscle pathology through multiple pathogenetic mechanisms. Indeed, caveolin-3 deficiency is associated to sarcolemmal membrane alterations, disorganization of skeletal muscle T-tubule network and disruption of distinct cell-signaling pathways. To date, there have been 30 caveolin-3 mutations identified in the human population. Caveolin-3 defects lead to four distinct skeletal muscle disease phenotypes: limb girdle muscular dystrophy, rippling muscle disease, distal myopathy, and hyperCKemia. In addition, one caveolin-3 mutant has been described in a case of hypertrophic cardiomyopathy. Many patients show an overlap of these symptoms and the same mutation can be linked to different clinical phenotypes. This variability can be related to additional genetic or environmental factors. This review will address caveolin-3 biological functions in muscle cells and will describe the muscle and heart disease phenotypes associated with caveolin-3 mutations.

Caveolin-3 regulates myostatin signaling. Mini-review

Caveolins, components of the uncoated invaginations of plasma membrane, regulate signal transduction and vesicular trafflicking. Loss of caveolin-3, resulting from dominant negative mutations of caveolin-3 causes autosomal dominant limb-girdle muscular dystrophy (LGMD) 1C and autosomal dominant rippling muscle disease (AD-RMD). Myostatin, a member of the muscle-specific transforming growth factor (TGF)-beta superfamily, negatively regulates skeletal muscle volume. Herein we review caveolin-3 suppressing of activation of type I myostatin receptor, thereby inhibiting subsequent intracellular signaling. In addition, a mouse model of LGMD1C has shown atrophic myopathy with enhanced myostatin signaling. Myostatin inhibition ameliorates muscular phenotype in the model mouse, accompanied by normalized myostatin signaling. Enhanced myostatin signaling by caveolin-3 mutation in human may contribute to the pathogenesis of LGMD1C. Therefore, myostatin inhibition therapy may be a promising treatment for patients with LGMD1C. More recent studies concerning regulation of TGF-beta superfamily signaling by caveolins have provided new insights into the pathogenesis of several human diseases.

Caveolinopathy--new mutations and additional symptoms

Mutations in the caveolin-3 gene (CAV3) can lead to a broad spectrum of clinical phenotypes. Phenotypes that have so far been associated with primary caveolin-3 deficiency include limb girdle muscular dystrophy, rippling muscle disease, distal myopathy and hyperCKaemia. This is the first report describing the clinical, pathological and genetic features of patients with caveolinopathy from the UK. Ten patients (six families) were identified via the National Commissioning Group (NCG) service for patients with limb girdle muscle dystrophy in Newcastle. Myalgia was the most prominent symptom in our cohort of patients and for 50% it was the reason for referral. Muscle weakness was only found in 60% of the patients, whereas rippling muscle movement was present in 80%. One of the patients reported episodes of myoglobinuria and another one episodes of hypoglycaemia. Five different mutations were identified, two of which were novel and three that had previously been described. Caveolinopathy needs to be considered as a differential diagnosis in a range of clinical situations, including in patients who do not have any weakness. Indeed, rippling muscles are a more frequent symptom than weakness, and can be detected in childhood. Presentation with myalgia is common and management of it as well as of myoglobinuria and hypoglycaemia may have a major impact on the patients' quality of life.

Caveolin-3 T78M and T78K missense mutations lead to different phenotypes in vivo and in vitro

Caveolins are the principal protein components of caveolae, invaginations of the plasma membrane involved in cell signaling and trafficking. Caveolin-3 (Cav-3) is the muscle-specific isoform of the caveolin family and mutations in the CAV3 gene lead to a large group of neuromuscular disorders. In unrelated patients, we identified two distinct CAV3 mutations involving the same codon 78. Patient 1, affected by dilated cardiomyopathy and limb girdle muscular dystrophy (LGMD)-1C, shows an autosomal recessive mutation converting threonine to methionine (T78M). Patient 2, affected by isolated familiar hyperCKemia, shows an autosomal dominant mutation converting threonine to lysine (T78K). Cav-3 wild type (WT) and Cav-3 mutations were transiently transfected into Cos-7 cells. Cav-3 WT and Cav-3 T78M mutant localized at the plasma membrane, whereas Cav-3 T78K was retained in a perinuclear compartment. Cav-3 T78K expression was decreased by 87% when compared with Cav-3 WT, whereas Cav-3 T78M protein levels were unchanged. To evaluate whether Cav-3 T78K and Cav-3 T78M mutants behaved with a dominant negative pattern, Cos-7 cells were cotransfected with green fluorescent protein (GFP)-Cav-3 WT in combination with either mutant or WT Cav-3. When cotransfected with Cav-3 WT or Cav-3 T78M, GFP-Cav-3 WT was localized at the plasma membrane, as expected. However, when cotransfected with Cav-3 T78K, GFP-Cav-3 WT was retained in a perinuclear compartment, and its protein levels were reduced by 60%, suggesting a dominant negative action. Accordingly, Cav-3 protein levels in muscles from a biopsy of patient 2 (T78K mutation) were reduced by 80%. In conclusion, CAV3 T78M and T78K mutations lead to distinct disorders showing different clinical features and inheritance, and displaying distinct phenotypes in vitro.

Cell biology of membrane trafficking in human disease

Understanding the molecular and cellular mechanisms underlying membrane traffic pathways is crucial to the treatment and cure of human disease. Various human diseases caused by changes in cellular homeostasis arise through a single gene mutation(s) resulting in compromised membrane trafficking. Many pathogenic agents such as viruses, bacteria, or parasites have evolved mechanisms to subvert the host cell response to infection, or have hijacked cellular mechanisms to proliferate and ensure pathogen survival. Understanding the consequence of genetic mutations or pathogenic infection on membrane traffic has also enabled greater understanding of the interactions between organisms and the surrounding environment. This review focuses on human genetic defects and molecular mechanisms that underlie eukaryote exocytosis and endocytosis and current and future prospects for alleviation of a variety of human diseases.

Differential expression of caveolin-3 in mouse smooth muscle cells in vivo

Expression of caveolin-1 and -3 in mouse smooth muscle cells in vivo was examined by immunohistochemistry. Caveolin-1 was detected in almost all smooth muscles examined, except for the pupillary dilator muscle, whereas caveolin-3 was present only in smooth muscles of some specific tissues. In the eye, the pupillary sphincter muscle was intensely positive for caveolin-3, whereas the ciliary muscle and pupillary dilator muscle were negative. In the gastrointestinal tract, caveolin-3 was detected in the inner circular layer, but not in the outer longitudinal layer. Vascular smooth muscle cells of the resistance-sized artery in the uterus and corpus cavernosum were intensely positive for caveolin-3, whereas those of the aorta were only weakly positive and those of the vena cava were negative. Caveolin-3 was also detected in smooth muscle cells of the urinary bladder, ureter, prostatic vas deferens, and seminal vesicle. The different levels of caveolin-3 expression among various smooth muscle tissues were confirmed by Western blot analysis. Even within the same muscle, the relative expression levels of caveolin-1 and -3 were variable among neighboring cells, suggesting distinct fine regulation of expression of these two caveolins. Moreover, even in the same cell, caveolin-1 and -3 showed different distributions. These results indicate that the two caveolins form distinct caveolae in smooth muscles, and that caveolin-1 and -3 serve different functions. Their differential expression may therefore be related to the functional diversity of smooth muscles.

Caveolins in vascular smooth muscle: form organizing function

Caveolae are becoming increasingly recognized as an important organizational structure for a variety of signal and energy-transducing systems in vascular smooth muscle (VSM). In this review, we discuss the emerging role of the caveolins in organizing and modulating the basic functions of smooth muscle: contraction, growth/proliferation, and the energetic support systems that support these functions. With clear alterations in cell metabolism and function in VSM with altered caveolin-1 (Cav-1) protein expression and with cardiovascular abnormalities associated with Cav-1 null mice, the caveolin family of proteins may play an important role in the function and dysfunction of VSM.

Overexpression of caveolin-1 results in increased plasma membrane targeting of glycolytic enzymes: The structural basis for a membrane associated metabolic compartment

Although membrane-associated glycolysis has been observed in a variety of cell types, the mechanism of localization of glycolytic enzymes to the plasma membrane is not known. We hypothesized that caveolin-1 (CAV-1) serves as a scaffolding protein for glycolytic enzymes and may play a role in the organization of cell metabolism. To test this hypothesis, we over-expressed CAV-1 in cultured A7r5 (rat aorta vascular smooth muscle; VSM) cells. Confocal immunofluorescence microscopy was used to study the distribution of phosphofructokinase (PFK) and CAV-1 in the transfected cells. Areas of interest (AOI) were analyzed in a central Z-plane across the cell transversing the perinuclear region. To quantify any shift in PFK localization resulting from CAV-1 over-expression, we calculated a periphery to center (PC) index by taking the average of the two outer AOIs from each membrane region and dividing by the central one or two AOIs. We found the PC index to be 1.92 +/- 0.57 (mean +/- SEM, N = 8) for transfected cells and 0.59 +/- 0.05 (mean +/- SEM, N = 11) for control cells. Colocalization analysis demonstrated that the percentage of PFK associated with CAV-1 increased in transfected cells compared to control cells. The localization of aldolase (ALD) was also shifted towards the plasma membrane (and colocalized with PFK) in CAV-1 over-expressing cells. These results demonstrate that CAV-1 creates binding sites for PFK and ALD that may be of higher affinity than those binding sites localized in the cytoplasm. We conclude that CAV-1 functions as a scaffolding protein for PFK, ALD and perhaps other glycolytic enzymes, either through direct interaction or accessory proteins, thus contributing to compartmented metabolism in vascular smooth muscle.

Getting rid of caveolins: phenotypes of caveolin-deficient animals

The elucidation of the role of caveolae has been the topic of many investigations which were greatly enhanced after the discovery of caveolin, the protein marker of these flask-shaped plasma membrane invaginations. The generation of mice deficient in the various caveolin genes (cav-1, cav-2 and cav-3) has provided physiological models to unravel the role of caveolins or caveolae at the whole organism level. Remarkably, despite the essential role of caveolins in caveolae biogenesis, all knockout mice are viable and fertile. However, lack of caveolae or caveolins leads to a wide range of phenotypes including muscle, pulmonary or lipid disorders, suggesting their implication in many cellular processes. The aim of this review is to give a broad overview of the phenotypes described for the caveolin-deficient mice and to link them to the numerous functions so far assigned to caveolins/caveolae.

Caveolin-1 functions as a scaffolding protein for phosphofructokinase in the metabolic organization of vascular smooth muscle

Using confocal microscopy, we have demonstrated a similar distribution of phosphofructokinase (PFK) with caveolin-1 (CAV-1) mainly at the periphery (membrane) in freshly isolated vascular smooth muscle (VSM) cells and in cultured A7r5 VSM cells. Co-immunoprecipitation analysis validated the interaction between the proteins. To further test the hypothesis that PFK and CAV-1 are colocalized, we used small interfering RNA (siRNA) to downregulate CAV-1 expression and disrupt the protein-protein interactions between PFK and CAV-1. Transfection of cultured A7r5 cells with CAV-1 siRNA resulted in a decreased level of immunoreactive CAV-1 and a consequent shift in the distribution of PFK with less localization of PFK to the periphery of the cells and increased immunoreactivity at the perinuclear region as compared to control. Analysis of the average PFK intensity across cultured A7r5 cells demonstrated a higher central:peripheral intensity ratio (CPI ratio) in siRNA-treated cells than in the control. These results validate the possible role of CAV-1 as a scaffolding protein for PFK as evidenced by the significant redistribution of PFK after CAV-1 downregulation. We therefore conclude that CAV-1 may function as a scaffolding protein for PFK and that this contributes to the compartmentation of glycolysis from other metabolic pathways in VSM.

Expression of caveolin-1 in lymphocytes induces caveolae formation and recruitment of phosphofructokinase to the plasma membrane

Compartmentation of carbohydrate metabolism has been shown in a wide range of tissues including reports of one compartment of glycolysis associated with the plasma membrane of cells. However, only in the erythrocyte has the physical basis for plasma membrane-associated glycolytic pathway been established. We have previously found that phosphofructokinase (PFK) appeared to colocalize with the fairly ubiquitous plasma membrane protein caveolin-1 (CAV-1), consistent with a role for CAV-1 as an anchor for glycolysis to the plasma membrane. To test the hypothesis that CAV-1 functions as a scaffolding protein for PFK, we transfected human lymphocytes (a cell without CAV-1 expression) with human CAV-1 cDNA. We demonstrate that expression of CAV-1 in lymphocytes results in the formation of caveolae at the plasma membrane and affects the subcellular localization of PFK by recruiting PFK to the plasma membrane. Targeting of PFK by CAV-1 also was validated by the significant colocalization between the proteins after transfection, which resulted in a correlation of 0.97 +/- 0.004 between the two fluorophores. This finding is significant in as much as it illustrates the CAV-1 feasibility of generating binding sites for glycolytic enzymes on the plasma membrane. We therefore conclude that CAV-1 functions as a scaffolding protein for PFK and that this may contribute to the elucidation of the basis for carbohydrate compartmentation to the plasma membrane in a wide variety of cell types.

Caveolae-associated signalling in smooth muscle

Caveolae are flask-shaped invaginations in the membrane that depend on the contents of cholesterol and on the structural protein caveolin. The organisation of caveolae in parallel strands between dense bands in smooth muscle is arguably unique. It is increasingly recognised, bolstered in large part by recent studies in caveolae deficient animals, that caveolae sequester and regulate a variety of signalling intermediaries. The role of caveolae in smooth muscle signal transduction, as inferred from studies on transgenic animals and in vitro approaches, is the topic of the current review. Both G-protein coupled receptors and tyrosine kinase receptors are believed to cluster in caveolae, and the exciting possibility that caveolae provide a platform for interactions between the sarcoplasmic reticulum and plasmalemmal ion channels is emerging. Moreover, messengers involved in Ca2+ sensitization of myosin phosphorylation and contraction may depend on caveolae or caveolin. Caveolae thus appear to constitute an important signalling domain that plays a role not only in regulation of smooth muscle tone, but also in proliferation, such as seen in neointima formation and atherosclerosis.

Role of Caveolae and Caveolins in Health and Disease

Although they were discovered more than 50 years ago, caveolae have remained enigmatic plasmalemmal organelles. With their characteristic “flasklike” shape and virtually ubiquitous tissue distribution, these interesting structures have been implicated in a wide range of cellular functions. Similar to clathrin-coated pits, caveolae function as macromolecular vesicular transporters, while their unique lipid composition classifies them as plasma membrane lipid rafts, structures enriched in a variety of signaling molecules. The caveolin proteins (caveolin-1, -2, and -3) serve as the structural components of caveolae, while also functioning as scaffolding proteins, capable of recruiting numerous signaling molecules to caveolae, as well as regulating their activity. That so many signaling molecules and signaling cascades are regulated by an interaction with the caveolins provides a paradigm by which numerous disease processes may be affected by ablation or mutation of these proteins. Indeed, studies in caveolin-deficient mice have implicated these structures in a host of human diseases, including diabetes, cancer, cardiovascular disease, atherosclerosis, pulmonary fibrosis, and a variety of degenerative muscular dystrophies. In this review, we provide an in depth summary regarding the mechanisms by which caveolae and caveolins participate in human disease processes.

The Caveolin genes: from cell biology to medicine

Caveolae are vesicular organelles (50-100-nm in diameter) that are particularly abundant in cells of the cardiovascular system, including endothelial cells, smooth muscle cells, macrophages, cardiac myocytes and fibroblasts. In these cell types, caveolae function both in protein trafficking and signal transduction, as well as in cholesterol homeostasis. Caveolins are the structural proteins that are both necessary and sufficient for the formation of caveolae membrane domains. Caveolins 1 and 2 are co-expressed in most cell types, while the expression of caveolin-3 is muscle-specific. Thus, endothelial cells and fibroblasts are rich in caveolins 1 and 2, while cardiac myocytes and skeletal muscle fibers express caveolin-3. In contrast, smooth muscle cells express all three caveolins (Cav-1, -2, and -3). Mechanistically, caveolins interact with a variety of downstream signaling molecules, including Src-family tyrosine kinases, p42/44 mitogen activated protein (MAP) kinase, and endothelial nitric oxide synthase (eNOS), and hold these signal transducers in the inactive conformation until activation by an appropriate stimulus. In many ways, caveolins serve both to compartmentalize and regulate signaling. Recent studies using caveolin-deficient mouse models dramatically show that caveolae and caveolins play a prominent role in various human patho-biological conditions, especially those related to the cardiovascular system. These disease phenotypes include: atherosclerosis, cardiac hypertrophy, cardiomyopathy, pulmonary hypertension, and neointimal hyperplasia (smooth muscle cell proliferation). In addition, caveolins play a significant role in other disease phenotypes, such as cancer, diabetes, bladder dysfunction, and muscular dystrophy, as we discuss in this review. Thus, caveolin-deficient mice will serve as important new animal models to dissect the intricate role of caveolae and caveolins in the pathogenesis of human diseases.