Site-specific microtubule-associated protein 4 dephosphorylation causes microtubule network densification in pressure overload cardiac hypertrophy

Polarized microtubule remodeling transforms the morphology of reactive microglia and drives cytokine release

Microglial reactivity is a pathological hallmark in many neurodegenerative diseases. During stimulation, microglia undergo complex morphological changes, including loss of their characteristic ramified morphology, which is routinely used to detect and quantify inflammation in the brain. However, the underlying molecular mechanisms and the relation between microglial morphology and their pathophysiological function are unknown. Here, proteomic profiling of lipopolysaccharide (LPS)-reactive microglia identifies microtubule remodeling pathways as an early factor that drives the morphological change and subsequently controls cytokine responses. We find that LPS-reactive microglia reorganize their microtubules to form a stable and centrosomally-anchored array to facilitate efficient cytokine trafficking and release. We identify cyclin-dependent kinase 1 (Cdk-1) as a critical upstream regulator of microtubule remodeling and morphological change in-vitro and in-situ. Cdk-1 inhibition also rescues tau and amyloid fibril-induced morphology changes. These results demonstrate a critical role for microtubule dynamics and reorganization in microglial reactivity and modulating cytokine-mediated inflammatory responses.

Extracellular stiffness induces contractile dysfunction in adult cardiomyocytes via cell-autonomous and microtubule-dependent mechanisms

The mechanical environment of the myocardium has a potent effect on cardiomyocyte form and function, yet an understanding of the cardiomyocyte responses to extracellular stiffening remains incomplete. We therefore employed a cell culture substrate with tunable stiffness to define the cardiomyocyte responses to clinically relevant stiffness increments in the absence of cell-cell interactions. When cultured on substrates magnetically actuated to mimic the stiffness of diseased myocardium, isolated rat adult cardiomyocytes exhibited a time-dependent reduction of sarcomere shortening, characterized by slowed contraction and relaxation velocity, and alterations of the calcium transient. Cardiomyocytes cultured on stiff substrates developed increases in viscoelasticity and microtubule detyrosination in association with early increases in the α-tubulin detyrosinating enzyme vasohibin-2 (Vash2). We found that knockdown of Vash2 was sufficient to preserve contractile performance as well as calcium transient properties in the presence of extracellular substrate stiffening. Orthogonal prevention of detyrosination by overexpression of tubulin tyrosine ligase (TTL) was also able to preserve contractility and calcium homeostasis. These data demonstrate that a pathologic increment of extracellular stiffness induces early, cell-autonomous remodeling of adult cardiomyocytes that is dependent on detyrosination of α-tubulin.

Role of Nuclear Lamin A/C in the Regulation of Nav1.5 Channel and Microtubules: Lesson From the Pathogenic Lamin A/C Variant Q517X

In this work, we studied an lmna nonsense mutation encoding for the C-terminally truncated Lamin A/C (LMNA) variant Q517X, which was described in patients affected by a severe arrhythmogenic cardiomyopathy with history of sudden death. We found that LMNA Q517X stably expressed in HL-1 cardiomyocytes abnormally aggregates at the nuclear envelope and within the nucleoplasm. Whole-cell patch clamp experiments showed that LMNA Q517X-expressing cardiomyocytes generated action potentials with reduced amplitude, overshoot, upstroke velocity and diastolic potential compared with LMNA WT-expressing cardiomyocytes. Moreover, the unique features of these cardiomyocytes were 1) hyper-polymerized tubulin network, 2) upregulated acetylated α-tubulin, and 3) cell surface Nav1.5 downregulation. These findings pointed the light on the role of tubulin and Nav1.5 channel in the abnormal electrical properties of LMNA Q517X-expressing cardiomyocytes. When expressed in HEK293 with Nav1.5 and its β1 subunit, LMNA Q517X reduced the peak Na⁺ current (I_Na) up to 63% with a shift toward positive potentials in the activation curve of the channel. Of note, both AP properties in cardiomyocytes and Nav1.5 kinetics in HEK293 cells were rescued in LMNA Q517X-expressing cells upon treatment with colchicine, an FDA-approved inhibitor of tubulin assembly. In conclusion, LMNA Q517X expression is associated with hyper-polymerization and hyper-acetylation of tubulin network with concomitant downregulation of Nav1.5 cell expression and activity, thus revealing 1) new mechanisms by which LMNA may regulate channels at the cell surface in cardiomyocytes and 2) new pathomechanisms and therapeutic targets in cardiac laminopathies.

Targeting Microtubules for the Treatment of Heart Disease

Heart disease remains the leading cause of morbidity and mortality worldwide. With the advancement of modern technology, the role(s) of microtubules in the pathogenesis of heart disease has become increasingly apparent, though currently there are limited treatments targeting microtubule-relevant mechanisms. Here, we review the functions of microtubules in the cardiovascular system and their specific adaptive and pathological phenotypes in cardiac disorders. We further explore the use of microtubule-targeting drugs and highlight promising druggable therapeutic targets for the future treatment of heart diseases.

The microtubule cytoskeleton in cardiac mechanics and heart failure

The microtubule network of cardiac muscle cells has unique architectural and biophysical features to accommodate the demands of the working heart. Advances in live-cell imaging and in deciphering the 'tubulin code' have shone new light on this cytoskeletal network and its role in heart failure. Microtubule-based transport orchestrates the growth and maintenance of the contractile apparatus through spatiotemporal control of translation, while also organizing the specialized membrane systems required for excitation-contraction coupling. To withstand the high mechanical loads of the working heart, microtubules are post-translationally modified and physically reinforced. In response to stress to the myocardium, the microtubule network remodels, typically through densification, post-translational modification and stabilization. Under these conditions, physically reinforced microtubules resist the motion of the cardiomyocyte and increase myocardial stiffness. Accordingly, modified microtubules have emerged as a therapeutic target for reducing stiffness in heart failure. In this Review, we discuss the latest evidence on the contribution of microtubules to cardiac mechanics, the drivers of microtubule network remodelling in cardiac pathologies and the therapeutic potential of targeting cardiac microtubules in acquired heart diseases.

Cardiomyocyte Microtubules: Control of Mechanics, Transport, and Remodeling

Microtubules are essential cytoskeletal elements found in all eukaryotic cells. The structure and composition of microtubules regulate their function, and the dynamic remodeling of the network by posttranslational modifications and microtubule-associated proteins generates diverse populations of microtubules adapted for various contexts. In the cardiomyocyte, the microtubules must accommodate the unique challenges faced by a highly contractile, rigidly structured, and long-lasting cell. Through their canonical trafficking role and positioning of mRNA, proteins, and organelles, microtubules regulate essential cardiomyocyte functions such as electrical activity, calcium handling, protein translation, and growth. In a more specialized role, posttranslationally modified microtubules form load-bearing structures that regulate myocyte mechanics and mechanotransduction. Modified microtubules proliferate in cardiovascular diseases, creating stabilized resistive elements that impede cardiomyocyte contractility and contribute to contractile dysfunction. In this review, we highlight the most exciting new concepts emerging from recent studies into canonical and noncanonical roles of cardiomyocyte microtubules.

The detyrosination/re-tyrosination cycle of tubulin and its role and dysfunction in neurons and cardiomyocytes

Among the variety of post-translational modifications to which microtubules are subjected, the detyrosination/re-tyrosination cycle is specific to tubulin. It is conserved by evolution and characterized by the enzymatic removal and re-addition of a gene-encoded tyrosine residue at the C-terminus of α-tubulin. Detyrosinated tubulin can be further converted to Δ2-tubulin by the removal of an additional C-terminal glutamate residue. Detyrosinated and Δ2-tubulin are carried by stable microtubules whereas tyrosinated microtubules are present on dynamic polymers. The cycle regulates trafficking of many cargo transporting molecular motors and is linked to the microtubule dynamics via regulation of microtubule interactions with specific cellular effectors such as kinesin-13. Here, we give an historical overview of the general features discovered for the cycle. We highlight the recent progress toward structure and functioning of the enzymes that keep the levels of tyrosinated and detyrosinated tubulin in cells, the long-known tubulin tyrosine ligase and the recently discovered vasohibin-SVBP complexes. We further describe how the cycle controls microtubule functions in healthy neurons and cardiomyocytes and how deregulations of the cycle are involved in dysfunctions of these highly differentiated cells, leading to neurodegeneration and heart failure in humans.

Kv7 channel trafficking by the microtubule network in vascular smooth muscle

In arterial smooth muscle cells, changes in availability of integral membrane proteins influence the regulation of blood flow and blood pressure, which is critical for human health. However, the mechanisms that coordinate the trafficking and membrane expression of specific receptors and ion channels in vascular smooth muscle are poorly understood. In the vasculature, very little is known about microtubules, which form a road network upon which proteins can be transported to and from the cell membrane. This review article summarizes the impact of the microtubule network on arterial contractility, highlighting the importance of the network, with an emphasis on our recent findings regarding the trafficking of the voltage‐dependent Kv7 channels.

MARK4 controls ischaemic heart failure through microtubule detyrosination

Myocardial infarction is a major cause of premature death in adults. Compromised cardiac function after myocardial infarction leads to chronic heart failure with systemic health complications and a high mortality rate¹. Effective therapeutic strategies are needed to improve the recovery of cardiac function after myocardial infarction. More specifically, there is a major unmet need for a new class of drugs that can improve cardiomyocyte contractility, because inotropic therapies that are currently available have been associated with high morbidity and mortality in patients with systolic heart failure^2,3 or have shown a very modest reduction of risk of heart failure⁴. Microtubule detyrosination is emerging as an important mechanism for the regulation of cardiomyocyte contractility⁵. Here we show that deficiency of microtubule-affinity regulating kinase 4 (MARK4) substantially limits the reduction in the left ventricular ejection fraction after acute myocardial infarction in mice, without affecting infarct size or cardiac remodelling. Mechanistically, we provide evidence that MARK4 regulates cardiomyocyte contractility by promoting phosphorylation of microtubule-associated protein 4 (MAP4), which facilitates the access of vasohibin 2 (VASH2)-a tubulin carboxypeptidase-to microtubules for the detyrosination of α-tubulin. Our results show how the detyrosination of microtubules in cardiomyocytes is finely tuned by MARK4 to regulate cardiac inotropy, and identify MARK4 as a promising therapeutic target for improving cardiac function after myocardial infarction.

MAP4 as a New Candidate in Cardiovascular Disease

Microtubule and mitochondrial dysfunction have been implicated in the pathogenesis of cardiovascular diseases (CVDs), including cardiac hypertrophy, fibrosis, heart failure, and hypoxic/ischemic related heart dysfunction. Microtubule dynamics instability leads to disrupted cell homeostasis and cell shape, decreased cell survival, and aberrant cell division and cell cycle, while mitochondrial dysfunction contributes to abnormal metabolism and calcium flux, increased cell death, oxidative stress, and inflammation, both of which causing cell and tissue dysfunction followed by CVDs. A cytosolic skeleton protein, microtubule-associated protein 4 (MAP4), belonging to the family of microtubule-associated proteins (MAPs), is widely expressed in non-neural cells and possesses an important role in microtubule dynamics. Increased MAP4 phosphorylation results in microtubule instability. In addition, MAP4 also expresses in mitochondria and reveals a crucial role in maintaining mitochondrial homeostasis. Phosphorylated MAP4 promotes mitochondrial apoptosis, followed by cardiac injury. The aim of the present review is to highlight the novel role of MAP4 as a potential candidate in multiple cardiovascular pathologies.

Cardiac microtubules in health and heart disease

Cardiomyocytes are large (∼40,000 µm3), rod-shaped muscle cells that provide the working force behind each heartbeat. These highly structured cells are packed with dense cytoskeletal networks that can be divided into two groups—the contractile (i.e. sarcomeric) cytoskeleton that consists of filamentous actin-myosin arrays organized into myofibrils, and the non-sarcomeric cytoskeleton, which is composed of β- and γ-actin, microtubules, and intermediate filaments. Together, microtubules and intermediate filaments form a cross-linked scaffold, and these networks are responsible for the delivery of intracellular cargo, the transmission of mechanical signals, the shaping of membrane systems, and the organization of myofibrils and organelles. Microtubules are extensively altered as part of both adaptive and pathological cardiac remodeling, which has diverse ramifications for the structure and function of the cardiomyocyte. In heart failure, the proliferation and post-translational modification of the microtubule network is linked to a number of maladaptive processes, including the mechanical impediment of cardiomyocyte contraction and relaxation. This raises the possibility that reversing microtubule alterations could improve cardiac performance, yet therapeutic efforts will strongly benefit from a deeper understanding of basic microtubule biology in the heart. The aim of this review is to summarize the known physiological roles of the cardiomyocyte microtubule network, the consequences of its pathological remodeling, and to highlight the open and intriguing questions regarding cardiac microtubules.

Impact statement

Advancements in cell biological and biophysical approaches and super-resolution imaging have greatly broadened our view of tubulin biology over the last decade. In the heart, microtubules and microtubule-based transport help to organize and maintain key structures within the cardiomyocyte, including the sarcomere, intercalated disc, protein clearance machinery and transverse-tubule and sarcoplasmic reticulum membranes. It has become increasingly clear that post translational regulation of microtubules is a key determinant of their sub-cellular functionality. Alterations in microtubule network density, stability, and post-translational modifications are hallmarks of pathological cardiac remodeling, and modified microtubules can directly impede cardiomyocyte contractile function in various forms of heart disease. This review summarizes the functional roles and multi-leveled regulation of the cardiac microtubule cytoskeleton and highlights how refined experimental techniques are shedding mechanistic clarity on the regionally specified roles of microtubules in cardiac physiology and pathophysiology.

AMPK and cardiac remodelling

Cardiac remodelling is generally accepted as a critical process in the progression of heart failure. Myocyte hypertrophy, inflammatory responses and cardiac fibrosis are the main pathological changes associated with cardiac remodelling. AMP-activated protein kinase (AMPK) is known as an energy sensor and a regulator of cardiac metabolism under normal and ischaemic conditions. Additionally, AMPK has been shown to play roles in cardiac remodelling extending well beyond metabolic regulation. In this review, we discuss the currently defined roles of AMPK in cardiac remodelling and summarize the effects of AMPK on cardiac hypertrophy, inflammatory responses and fibrosis and the molecular mechanisms underlying these effects. In addition, we discuss some pharmacological activators of AMPK that are promising treatments for cardiac remodelling.

A Kinase-Independent Function of c-Src Mediates p130Cas Phosphorylation at the Serine-639 Site in Pressure Overloaded Myocardium

Early work in pressure overloaded (PO) myocardium shows that integrins mediate focal adhesion complex formation by recruiting the adaptor protein p130Cas (Cas) and nonreceptor tyrosine kinase c-Src. To explore c-Src role in Cas-associated changes during PO, we used a feline right ventricular in vivo PO model and a three-dimensional (3D) collagen-embedded adult cardiomyocyte in vitro model that utilizes a Gly-Arg-Gly-Asp-Ser (RGD) peptide for integrin stimulation. Cas showed slow electrophoretic mobility (band-shifting), recruitment to the cytoskeleton, and tyrosine phosphorylation at 165, 249, and 410 sites in both 48 h PO myocardium and 1 h RGD-stimulated cardiomyocytes. Adenoviral mediated expression of kinase inactive (negative) c-Src mutant with intact scaffold domains (KN-Src) in cardiomyocytes did not block the RGD stimulated changes in Cas. Furthermore, expression of KN-Src or kinase active c-Src mutant with intact scaffold function (A-Src) in two-dimensionally (2D) cultured cardiomyocytes was sufficient to cause Cas band-shifting, although tyrosine phosphorylation required A-Src. These data indicate that c-Src's adaptor function, but not its kinase function, is required for a serine/threonine specific phosphorylation(s) responsible for Cas band-shifting. To explore this possibility, Chinese hamster ovary cells that stably express Cas were infected with either β-gal or KN-Src adenoviruses and used for Cas immunoprecipitation combined with mass spectrometry analysis. In the KN-Src expressing cells, Cas showed phosphorylation at the serine-639 (human numbering) site. A polyclonal antibody raised against phospho-serine-639 detected Cas phosphorylation in 24-48 h PO myocardium. Our studies indicate that c-Src's adaptor function mediates serine-639 phosphorylation of Cas during integrin activation in PO myocardium.

The impact of cardiac ischemia/reperfusion on the mitochondria-cytoskeleton interactions

Cardiac ischemia-reperfusion (IR) injury compromises mitochondrial oxidative phosphorylation (OxPhos) and compartmentalized intracellular energy transfer via the phosphocreatine/creatine kinase (CK) network. The restriction of ATP/ADP diffusion at the level of the mitochondrial outer membrane (MOM) is an essential element of compartmentalized energy transfer. In adult cardiomyocytes, the MOM permeability to ADP is regulated by the interaction of voltage-dependent anion channel with cytoskeletal proteins, particularly with β tubulin II. The IR-injury alters the expression and the intracellular arrangement of cytoskeletal proteins. The objective of the present study was to investigate the impact of IR on the intracellular arrangement of β tubulin II and its effect on the regulation of mitochondrial respiration. Perfused rat hearts were subjected to total ischemia (for 20min (I20) and 45min (I45)) or to ischemia followed by 30min of reperfusion (I20R and I45R groups). High resolution respirometry and fluorescent confocal microscopy were used to study respiration, β tubulin II and mitochondrial arrangements in cardiac fibers. The results of these experiments evidence a heterogeneous response of mitochondria to IR-induced damage. Moreover, the intracellular rearrangement of β tubulin II, which in the control group colocalized with mitochondria, was associated with increased apparent affinity of OxPhos for ADP, decreased regulation of respiration by creatine without altering mitochondrial CK activity and the ratio between octameric to dimeric isoenzymes. The results of this study allow us to highlight changes of mitochondrial interactions with cytoskeleton as one of the possible mechanisms underlying cardiac IR injury.

Protein post-translational modifications and misfolding: new concepts in heart failure

A new concept in the field of heart-failure (HF) research points to a role of misfolded proteins, forming preamyloid oligomers (PAOs), in cardiac toxicity. This is largely based on few studies reporting the presence of PAOs, similar to those observed in neurodegenerative diseases, in experimental and human HF. As the majority of proteinopathies are sporadic in nature, protein post-translational modifications (PTMs) likely play a major role in this growing class of diseases. In fact, PTMs are known regulators of protein folding and of the formation of amyloid species in well-established proteinopathies. Proteomics has been instrumental in identifying both chemical and enzymatic PTMs, with a potential impact on protein mis-/folding. Here we provide the basics on how proteins fold along with a few examples of PTMs known to modulate protein misfolding and aggregation, with particular focus on the heart. Due to its innovative content and the growing awareness of the toxicity of misfolded proteins, an "Alzheimer's theory of HF" is timely. Moreover, the continuous innovations in proteomic technologies will help pinpoint PTMs that could contribute to the process. This nuptial between biology and technology could greatly assist in identifying biomarkers with increased specificity as well as more effective therapies.

LKB1 loss in melanoma disrupts directional migration toward extracellular matrix cues

The LKB1 kinase regulates directional migration in response to extracellular matrix gradients and may inhibit invasive motility by sensing inhibitory matrix cues.

Somatic inactivation of the serine/threonine kinase gene STK11/LKB1/PAR-4 occurs in a variety of cancers, including ∼10% of melanoma. However, how the loss of LKB1 activity facilitates melanoma invasion and metastasis remains poorly understood. In LKB1-null cells derived from an autochthonous murine model of melanoma with activated Kras and Lkb1 loss and matched reconstituted controls, we have investigated the mechanism by which LKB1 loss increases melanoma invasive motility. Using a microfluidic gradient chamber system and time-lapse microscopy, in this paper, we uncover a new function for LKB1 as a directional migration sensor of gradients of extracellular matrix (haptotaxis) but not soluble growth factor cues (chemotaxis). Systematic perturbation of known LKB1 effectors demonstrated that this response does not require canonical adenosine monophosphate–activated protein kinase (AMPK) activity but instead requires the activity of the AMPK-related microtubule affinity-regulating kinase (MARK)/PAR-1 family kinases. Inhibition of the LKB1–MARK pathway facilitated invasive motility, suggesting that loss of the ability to sense inhibitory matrix cues may promote melanoma invasion.

The p38/mitogen-activated protein kinase pathway is implicated in lipopolysaccharide-induced microtubule depolymerization via up-regulation of microtubule-associated protein 4 phosphorylation in human vascular endothelium

BACKGROUND

Microtubules (MTs) play an important role in lipopolysaccharide (LPS)-induced overexpression of inflammatory cytokines and vascular barrier dysfunction; however, the mechanisms behind MT dynamics changes in the vascular endothelium under septic conditions are still not well understood.

METHODS

Human umbilical vein endothelial cells (HUVECs) stimulated with LPS were pretreated with or without the specific p38/mitogen-activated protein kinase (MAPK) inhibitor, SB203580. p38/MAPK cascade-induced signaling events and proteins expression were investigated by Western blotting assay. The interaction between p38/MAPK and microtubule-associated protein 4 (MAP4) was examined by immunoprecipitation. Furthermore, the effects of agonists on LPS-induced MT disruption and alteration of acetylated alpha-tubulin (Acet-tubulin) were analyzed by double-immunofluorescent assay and Western blotting analysis.

RESULTS

In the present study, our results indicated that LPS induced MT depolymerization, but the effects of LPS could be reversed in endothelial cells pretreated with taxol. Furthermore, phosphor-p38 and MAP4 interacted to form a complex after exposure to LPS. LPS-induced MAP4 phosphorylation was greatly suppressed by SB203580, suggesting that activation of p38/MAPK signaling affected MAP4 phosphorylation linked to MT acetylation after stimulation with LPS.

CONCLUSION

The present study demonstrated that the p38/MAPK signaling pathway might disrupt MT dynamics via phosphorylation of MAP4 in vascular endothelial cells challenged by LPS. Our findings provide novel insights into the pathogenic mechanism of MT disassembly and consider new targets for therapeutic intervention under sepsis or septic shock conditions.

Regulation of microtubule-based transport by MAP4

Microtubule (MT)-based transport of organelles driven by the opposing MT motors kinesins and dynein is tightly regulated in cells, but the underlying molecular mechanisms remain largely unknown. Here we tested the regulation of MT transport by the ubiquitous protein MAP4 using Xenopus melanophores as an experimental system. In these cells, pigment granules (melanosomes) move along MTs to the cell center (aggregation) or to the periphery (dispersion) by means of cytoplasmic dynein and kinesin-2, respectively. We found that aggregation signals induced phosphorylation of threonine residues in the MT-binding domain of the Xenopus MAP4 (XMAP4), thus decreasing binding of this protein to MTs. Overexpression of XMAP4 inhibited pigment aggregation by shortening dynein-dependent MT runs of melanosomes, whereas removal of XMAP4 from MTs reduced the length of kinesin-2-dependent runs and suppressed pigment dispersion. We hypothesize that binding of XMAP4 to MTs negatively regulates dynein-dependent movement of melanosomes and positively regulates kinesin-2-based movement. Phosphorylation during pigment aggregation reduces binding of XMAP4 to MTs, thus increasing dynein-dependent and decreasing kinesin-2-dependent motility of melanosomes, which stimulates their accumulation in the cell center, whereas dephosphorylation of XMAP4 during dispersion has an opposite effect.

AMPK attenuates microtubule proliferation in cardiac hypertrophy

Cell hypertrophy requires increased protein synthesis and expansion of the cytoskeletal networks that support cell enlargement. AMPK limits anabolic processes, such as protein synthesis, when energy supply is insufficient, but its role in cytoskeletal remodeling is not known. Here, we examined the influence of AMPK in cytoskeletal remodeling during cardiomyocyte hypertrophy, a clinically relevant condition in which cardiomyocytes enlarge but do not divide. In neonatal cardiomyocytes, activation of AMPK with 5-aminoimidazole carboxamide ribonucleotide (AICAR) or expression of constitutively active AMPK (CA-AMPK) attenuated cell area increase by hypertrophic stimuli (phenylephrine). AMPK activation had little effect on intermediate filaments or myofilaments but dramatically reduced microtubule stability, as measured by detyrosinated tubulin levels and cytoskeletal tubulin accumulation. Importantly, low-level AMPK activation limited cell expansion and microtubule growth independent of mTORC1 or protein synthesis repression, identifying a new mechanism by which AMPK regulates cell growth. Mechanistically, AICAR treatment increased Ser-915 phosphorylation of microtubule-associated protein 4 (MAP4), which reduces affinity for tubulin and prevents stabilization of microtubules (MTs). RNAi knockdown of MAP4 confirmed its critical role in cardiomyocyte MT stabilization. In support of a pathophysiological role for AMPK regulation of cardiac microtubules, AMPK α2 KO mice exposed to pressure overload (transverse aortic constriction; TAC) demonstrated reduced MAP4 phosphorylation and increased microtubule accumulation that correlated with the severity of contractile dysfunction. Together, our data identify the microtubule cytoskeleton as a sensitive target of AMPK activity, and the data suggest a novel role for AMPK in limiting accumulation and densification of microtubules that occurs in response to hypertrophic stress.

X-ROS signaling in the heart and skeletal muscle: stretch-dependent local ROS regulates [Ca²⁺]i

X-ROS signaling is a novel redox signaling pathway that links mechanical stress to changes in [Ca(2+)]i. This pathway is activated rapidly and locally within a muscle cell under physiological conditions, but can also contribute to Ca(2+)-dependent arrhythmia in the heart and to the dystrophic phenotype in the heart and skeletal muscle. Upon physiologic cellular stretch, microtubules serve as mechanotransducers to activate NADPH oxidase 2 in the transverse tubules and sarcolemmal membranes to produce reactive oxygen species (ROS). In the heart, the ROS acts locally to activate ryanodine receptor Ca(2+) release channels in the junctional sarcoplasmic reticulum, increasing the Ca(2+) spark rate and "tuning" excitation-contraction coupling. In the skeletal muscle, where Ca(2+) sparks are not normally observed, the X-ROS signaling process is muted. However in muscular dystrophies, such as Duchenne Muscular Dystrophy and dysferlinopathy, X-ROS signaling operates at a high level and contributes to myopathy. Importantly, Ca(2+) permeable stretch-activated channels are activated by X-ROS and contribute to skeletal muscle pathology. Here we review X-ROS signaling and mechanotransduction in striated muscle, and highlight important questions to drive future work on stretch-dependent signaling. We conclude that X-ROS provides an exciting mechanism for the mechanical control of redox and Ca(2+) signaling, but much work is needed to establish its contribution to physiologic and pathophysiologic processes in diverse cell systems.

Time course of right ventricular pressure-overload induced myocardial fibrosis: relationship to changes in fibroblast postsynthetic procollagen processing

Myocardial fibrillar collagen is considered an important determinant of increased ventricular stiffness in pressure-overload (PO)-induced cardiac hypertrophy. Chronic PO was created in feline right ventricles (RV) by pulmonary artery banding (PAB) to define the time course of changes in fibrillar collagen content after PO using a nonrodent model and to determine whether this time course was dependent on changes in fibroblast function. Total, soluble, and insoluble collagen (hydroxyproline), collagen volume fraction (CVF), and RV end-diastolic pressure were assessed 2 days and 1, 2, 4, and 10 wk following PAB. Fibroblast function was assessed by quantitating the product of postsynthetic processing, insoluble collagen, and levels of SPARC (secreted protein acidic and rich in cysteine), a protein that affects procollagen processing. RV hypertrophic growth was complete 2 wk after PAB. Changes in RV collagen content did not follow the same time course. Two weeks after PAB, there were elevations in total collagen (control RV: 8.84 ± 1.03 mg/g vs. 2-wk PAB: 11.50 ± 0.78 mg/g); however, increased insoluble fibrillar collagen, as measured by CVF, was not detected until 4 wk after PAB (control RV CVF: 1.39 ± 0.25% vs. 4-wk PAB: 4.18 ± 0.87%). RV end-diastolic pressure was unchanged at 2 wk, but increased until 4 wk after PAB. RV fibroblasts isolated after 2-wk PAB had no changes in either insoluble collagen or SPARC expression; however, increases in insoluble collagen and in levels of SPARC were detected in RV fibroblasts from 4-wk PAB. Therefore, the time course of PO-induced RV hypertrophy differs significantly from myocardial fibrosis and diastolic dysfunction. These temporal differences appear dependent on changes in fibroblast function.

Tianma modulates blood vessel tonicity

Tianma is a traditional Chinese medicine (TCM) often used for the treatment of hypertension and heart diseases. To elucidate the function of tianma at the molecular level, we investigated the effect of tianma on vascular functions and aortic protein metabolism. We found that long-term treatment with tianma (~2.5g/kg/day for three months) in one-year-old rats could enhance acetylcholine (ACh)-induced vasorelaxation in endothelium-intact thoracic aortic rings against both KCl (80 mM)- and phenylephrine (PE)-induced contraction. By using the iTRAQ (isobaric tag for relative and absolute quantification) technique, we confirmed from the functional data at the proteome level that tianma treatment down-regulated the expressions of contractile proteins (e.g. Acta2) and other related structural proteins (e.g. desmin), and up-regulated the expressions of extracellular matrix (ECM) glycoproteins (e.g. Fbln5) and anti-thrombotic proteins (e.g. Anxa2) in aortic tissue. By inductive reasoning, tianma could perform its vasodilatory effect not only by inhibiting vascular smooth muscle contraction, but also by enhancing blood vessel elasticity and stabilizing the arterial structure. Thus, tianma might become a novel therapeutic herbal medicine for cardiovascular diseases by regulating the aortic proteome metabolism.

Cytoskeletal role in protection of the failing heart by β-adrenergic blockade

Formation of a dense microtubule network that impedes cardiac contraction and intracellular transport occurs in severe pressure overload hypertrophy. This process is highly dynamic, since microtubule depolymerization causes striking improvement in contractile function. A molecular etiology for this cytoskeletal alteration has been defined in terms of type 1 and type 2A phosphatase-dependent site-specific dephosphorylation of the predominant myocardial microtubule-associated protein (MAP)4, which then decorates and stabilizes microtubules. This persistent phosphatase activation is dependent upon ongoing upstream activity of p21-activated kinase-1, or Pak1. Because cardiac β-adrenergic activity is markedly and continuously increased in decompensated hypertrophy, and because β-adrenergic activation of cardiac Pak1 and phosphatases has been demonstrated, we asked here whether the highly maladaptive cardiac microtubule phenotype seen in pathological hypertrophy is based on β-adrenergic overdrive and thus could be reversed by β-adrenergic blockade. The data in this study, which were designed to answer this question, show that such is the case; that is, β(1)- (but not β(2)-) adrenergic input activates this pathway, which consists of Pak1 activation, increased phosphatase activity, MAP4 dephosphorylation, and thus the stabilization of a dense microtubule network. These data were gathered in a feline model of severe right ventricular (RV) pressure overload hypertrophy in response to tight pulmonary artery banding (PAB) in which a stable, twofold increase in RV mass is reached by 2 wk after pressure overloading. After 2 wk of hypertrophy induction, these PAB cats during the following 2 wk either had no further treatment or had β-adrenergic blockade. The pathological microtubule phenotype and the severe RV cellular contractile dysfunction otherwise seen in this model of RV hypertrophy (PAB No Treatment) was reversed in the treated (PAB β-Blockade) cats. Thus these data provide both a specific etiology and a specific remedy for the abnormal microtubule network found in some forms of pathological cardiac hypertrophy.

Role of ERK1/2 activation in microtubule stabilization and glucose transport in cardiomyocytes

We previously demonstrated that microtubule disruption impairs stimulation of glucose uptake in cardiomyocytes and that 9-cis retinoic acid (9cRA) treatment preserved both microtubule integrity and stimulated glucose transport. Herein we investigated whether 1) activation of the extracellular signal-regulated kinases (ERK1/2) is responsible for microtubule destabilization and 2) ERK1/2 inactivation may explain the positive effects of 9cRA on glucose uptake and microtubule stabilization. Adult rat cardiomyocytes in primary culture showed increased basal ERK1/2 phosphorylation. Cardiomyocytes exposed to inhibitors of the ERK1/2 kinase mitogen/extracellular signal-regulated kinase (MEK) 1/2 had preserved microtubular scaffold, including microtubule-organizing centers (MTOC), together with increased insulin and metabolic stress-stimulated glucose transport as well as signaling, thus replicating the effects of 9cRA treatment. Although 9cRA treatment did not significantly reduce global ERK1/2 activation, it markedly reduced perinuclear-activated ERK1/2 at the location of MTOC. 9cRA also triggered relocation of the ERK1/2 phosphatase mitogen-activated protein kinase phosphatase-3 from the cytosol to the nucleus. These results indicate that, in cardiomyocytes, microtubule destabilization, leading to impaired stimulation of glucose transport, is mediated by ERK1/2 activation, impacting on the MTOC. 9cRA acid restores stimulated glucose transport indirectly through compartmentalized inactivation of ERK1/2.

Development and application of a phosphoproteomic method using electrostatic repulsion-hydrophilic interaction chromatography (ERLIC), IMAC, and LC-MS/MS analysis to study Marek's Disease Virus infection

Marek's Disease (MD) is an avian neoplastic disease caused by Marek's Disease Virus (MDV). The mechanism of virus transition between the lytic and latent cycle is still being investigated; however, post-translational modifications, especially phosphorylation, have been thought to play an important role. Previously, our group has used strong cation exchange chromatography in conjunction with reversed-phase liquid chromatography-tandem mass spectrometry (LC-MS/MS) to study the changes in global proteomic expression upon MDV infection (Ramaroson , M. F.; Ruby, J.; Goshe, M. B.; Liu , H.-C. S. J. Proteome Res. 2008, 7, 4346-4358). Here, we extend our study by developing an effective separation and enrichment approach to investigate the changes occurring in the phosphoproteome using electrostatic repulsion-hydrophilic interaction chromatography (ERLIC) to fractionate peptides from chicken embryo fibroblast (CEF) digests and incorporating a subsequent IMAC enrichment step to selectively target phosphorylated peptides for LC-MS/MS analysis. To monitor the multidimensional separation between mock- and MDV-infected CEF samples, a casein phosphopeptide mixture was used as an internal standard. With LC-MS/MS analysis alone, no CEF phosphopeptides were detected, while with ERLIC fractionation only 1.2% of all identified peptides were phosphorylated. However, the incorporation of IMAC enrichment with ERLIC fractionation provided a 50-fold increase in the percentage of identified phosphopeptides. Overall, a total of 581 unique phosphopeptides were identified (p < 0.05) with those of the MDV-infected CEF sample containing nearly twice as many as the mock-infected control of which 11% were unique to MDV proteins. The changes in the phosphoproteome are discussed including the role that microtubule-associated proteins may play in MDV infection mechanisms.

Basis for MAP4 dephosphorylation-related microtubule network densification in pressure overload cardiac hypertrophy

Increased activity of Ser/Thr protein phosphatases types 1 (PP1) and 2A (PP2A) during maladaptive cardiac hypertrophy contributes to cardiac dysfunction and eventual failure, partly through effects on calcium metabolism. A second maladaptive feature of pressure overload cardiac hypertrophy that instead leads to heart failure by interfering with cardiac contraction and intracellular transport is a dense microtubule network stabilized by decoration with microtubule-associated protein 4 (MAP4). In an earlier study we showed that the major determinant of MAP4-microtubule affinity, and thus microtubule network density and stability, is site-specific MAP4 dephosphorylation at Ser-924 and to a lesser extent at Ser-1056; this was found to be prominent in hypertrophied myocardium. Therefore, in seeking the etiology of this MAP4 dephosphorylation, we looked here at PP2A and PP1, as well as the upstream p21-activated kinase 1, in maladaptive pressure overload cardiac hypertrophy. The activity of each was increased persistently during maladaptive hypertrophy, and overexpression of PP2A or PP1 in normal hearts reproduced both the microtubule network phenotype and the dephosphorylation of MAP4 Ser-924 and Ser-1056 seen in hypertrophy. Given the major microtubule-based abnormalities of contractile and transport function in maladaptive hypertrophy, these findings constitute a second important mechanism for phosphatase-dependent pathology in the hypertrophied and failing heart.