Plectin scaffolds recruit energy-controlling AMP-activated protein kinase (AMPK) in differentiated myofibres

Mechanics of cell sheets: plectin as an integrator of cytoskeletal networks

Epithelia are multicellular sheets that form barriers defining the internal and external environments. The constant stresses acting at this interface require that epithelial sheets are mechanically robust and provide a selective barrier to the hostile exterior. These properties are mediated by cellular junctions which are physically linked with heavily crosslinked cytoskeletal networks. Such hardwiring is facilitated by plakins, a family of giant modular proteins which serve as 'molecular bridges' between different cytoskeletal filaments and multiprotein adhesion complexes. Dysfunction of cytoskeletal crosslinking compromises epithelial biomechanics and structural integrity. Subsequent loss of barrier function leads to disturbed tissue homeostasis and pathological consequences such as skin blistering or intestinal inflammation. In this article, we highlight the importance of the cytolinker protein plectin for the functional organization of epithelial cytoskeletal networks. In particular, we focus on the ability of plectin to act as an integrator of the epithelial cytoarchitecture that defines the biomechanics of the whole tissue. Finally, we also discuss the role of cytoskeletal crosslinking in emerging aspects of epithelial mechanobiology that are critical for the maintenance of epithelial homeostasis.

Desmin and Plectin Recruitment to the Nucleus and Nuclei Orientation Are Lost in Emery-Dreifuss Muscular Dystrophy Myoblasts Subjected to Mechanical Stimulation

In muscle cells subjected to mechanical stimulation, LINC complex and cytoskeletal proteins are basic to preserve cellular architecture and maintain nuclei orientation and positioning. In this context, the role of lamin A/C remains mostly elusive. This study demonstrates that in human myoblasts subjected to mechanical stretching, lamin A/C recruits desmin and plectin to the nuclear periphery, allowing a proper spatial orientation of the nuclei. Interestingly, in Emery-Dreifuss Muscular Dystrophy (EDMD2) myoblasts exposed to mechanical stretching, the recruitment of desmin and plectin to the nucleus and nuclear orientation were impaired, suggesting that a functional lamin A/C is crucial for the response to mechanical strain. While describing a new mechanism of action headed by lamin A/C, these findings show a structural alteration that could be involved in the onset of the muscle defects observed in muscular laminopathies.

AMPK Localization: A Key to Differential Energy Regulation

As the central node between nutrition signaling input and the metabolic pathway, AMP-activated protein kinase (AMPK) is tightly regulated to maintain energy homeostasis. Subcellular compartmentalization of AMPK is one of the critical regulations that enables AMPK to access proper targets and generate appropriate responses to specific perturbations and different levels of stress. One of the characterized localization mechanisms is RanGTPase-driven CRM1 that recognizes the nuclear export sequence (NES) on the α subunit to translocate AMPK into the cytoplasm. Nuclear localization putatively employs RanGTPase-driven importin that might recognize the nuclear localization signal (NLS) present on the AMPKα2 kinase domain. Nucleo-cytoplasmic shuttling of AMPK is influenced by multiple factors, such as starvation, exercise, heat shock, oxidant, cell density, and circadian rhythm. Tissue-specific localization, which distributes AMPK trimers with different combinations, has also been shown to be vital in maintaining tissue-specific metabolism. Tissue-specific and subcellular distribution of AMPK might be attributed to differences in the expression of the subunit, the stabilization by protein regulators, tissue activity, and the localization of AMPK activators. Considering the importance of AMPK localization in coordinating signaling and metabolism, further research is due to fully elucidate the largely unknown complex mechanism underlying this regulation.

Plectin in the Central Nervous System and a Putative Role in Brain Astrocytes

Plectin, a high-molecular-mass cytolinker, is abundantly expressed in the central nervous system (CNS). Currently, a limited amount of data about plectin in the CNS prevents us from seeing the complete picture of how plectin affects the functioning of the CNS as a whole. Yet, by analogy to its role in other tissues, it is anticipated that, in the CNS, plectin also functions as the key cytoskeleton interlinking molecule. Thus, it is likely involved in signalling processes, thereby affecting numerous fundamental functions in the brain and spinal cord. Versatile direct and indirect interactions of plectin with cytoskeletal filaments and enzymes in the cells of the CNS in normal physiological and in pathologic conditions remain to be fully addressed. Several pathologies of the CNS related to plectin have been discovered in patients with plectinopathies. However, in view of plectin as an integrator of a cohesive mesh of cellular proteins, it is important that the role of plectin is also considered in other CNS pathologies. This review summarizes the current knowledge of plectin in the CNS, focusing on plectin isoforms that have been detected in the CNS, along with its expression profile and distribution alongside diverse cytoskeleton filaments in CNS cell types. Considering that the bidirectional communication between neurons and glial cells, especially astrocytes, is crucial for proper functioning of the CNS, we place particular emphasis on the known roles of plectin in neurons, and we propose possible roles of plectin in astrocytes.

Soluble (Pro)Renin Receptor as a Negative Regulator of NCC (Na+-Cl- Cotransporter) Activity

[Figure: see text].

Plectin-Mediated Intermediate Filament Functions: Why Isoforms Matter

This essay focuses on the role of plectin and its various isoforms in mediating intermediate filament (IF) network functions. It is based on previous studies that provided comprehensive evidence for a concept where plectin acts as an IF recruiter, and plectin-mediated IF networking and anchoring are key elements in IF function execution. Here, plectin’s global role as modulator of IF functionality is viewed from different perspectives, including the mechanical stabilization of IF networks and their docking platforms, contribution to cellular viscoelasticity and mechanotransduction, compartmentalization and control of the actomyosin machinery, connections to the microtubule system, and mechanisms and specificity of isoform targeting. Arguments for IF networks and plectin acting as mutually dependent partners are also given. Lastly, a working model is presented that describes a unifying mechanism underlying how plectin–IF networks mechanically control and propagate actomyosin-generated forces, affect microtubule dynamics, and contribute to mechanotransduction.

Plectin regulates Wnt signaling mediated-skeletal muscle development by interacting with Dishevelled-2 and antagonizing autophagy

Skeletal muscle is the most abundant tissue in the human and animal body, loss of its function can lead to muscle aging and various myogenic diseases. The skeletal muscle development is a complex and tightly regulated process, which is driven by a variety of many factors, signaling pathways and regulatory mechanisms. Plectin (Plec), a cytolinker protein, is ubiquitously expressed in various tissues such as skin, muscle, plasma membrane, and most types of cells. Although known isoforms of Plec is well-characterized in muscle dystrophy, very little is known on the function of Plec in the skeletal muscle development. Here, we found that Plec plays a vital role in promoting C2C12 myoblasts differentiation and proliferation, but inhibits their apoptosis. Also, Plec regulates the expression of atrophy-related genes (atrogin-1 and muRF-1) to rescue muscle atrophy. Furthermore, we have demonstrated that Plec binds to Dishevelled-2 (Dvl-2) and forms a protein complex, which is then activate the canonical Wnt signaling. We also observed that Plec resists ubiquitination by stabilizing Dvl-2 and reduces the level of LC3-labeled Dvl-2 and antagonizes the autophagy system. In conclusion, our findings suggest that Plec regulates canonical Wnt signaling mediated skeletal development by stabilizing Dvl-2 and downregulating the cellular autophagic degradation system.

Quantitative In Vivo Proteomics of Metformin Response in Liver Reveals AMPK-Dependent and -Independent Signaling Networks

Metformin is the front-line treatment for type 2 diabetes worldwide. It acts via effects on glucose and lipid metabolism in metabolic tissues, leading to enhanced insulin sensitivity. Despite significant effort, the molecular basis for metformin response remains poorly understood, with a limited number of specific biochemical pathways studied to date. To broaden our understanding of hepatic metformin response, we combine phospho-protein enrichment in tissue from genetically engineered mice with a quantitative proteomics platform to enable the discovery and quantification of basophilic kinase substrates in vivo. We define proteins whose binding to 14-3-3 are acutely regulated by metformin treatment and/or loss of the serine/threonine kinase, LKB1. Inducible binding of 250 proteins following metformin treatment is observed, 44% of which proteins bind in a manner requiring LKB1. Beyond AMPK, metformin activates protein kinase D and MAPKAPK2 in an LKB1-independent manner, revealing additional kinases that may mediate aspects of metformin response. Deeper analysis uncovered substrates of AMPK in endocytosis and calcium homeostasis.

Expression of Pork Plectin during Postmortem Aging

The current study investigated the distribution and degradation of pork plectin during postmortem aging. Longissimus thoracis (LT) muscles from 12 pig carcasses were vacuum-packaged and aged at 4 °C for 0 h, 6 h, 12 h, 1 day, 3 days, 7 days, and 13 days. Immunofluorescence analysis showed that pork plectin was distributed in a honeycomb-like pattern in the cross section and a regularly striated pattern in the longitudinal section. However, plectin was found preferentially expressed in fibers that were stained with high anti-fast-MyHC. Double immunostaining revealed the colocalization of plectin and desmin in the cytoplasm and beneath the sarcolemma. Western blot analysis showed that the amount of intact plectin was rapidly reduced during the early postmortem aging (P < 0.05) and almost disappeared at day 3. The degraded 240 kDa plectin accumulated fast and was further cleaved after 3 days of aging (P < 0.05). The plectin degradation could be significantly blocked by calpain inhibitor MDL-28170 rather than caspase-3 inhibitor Ac-DEVD-CHO (P < 0.05). Double immunostaining of μ-calpain and plectin showed a large amount of overlap at 0 h and 3 days of postmortem. Accordingly, these findings showed that plectin was preferentially expressed in fast muscle fiber and regularly distributed along with desmin at the strategic cellular sites. Plectin suffered a prominent and prompt degradation during postmortem aging, which might be attributed to μ-calpain.

Plectin controls biliary tree architecture and stability in cholestasis

BACKGROUND & AIMS

Plectin, a highly versatile cytolinker protein, controls intermediate filament cytoarchitecture and cellular stress response. In the present study, we investigate the role of plectin in the liver under basal conditions and in experimental cholestasis.

METHODS

We generated liver-specific plectin knockout (Ple^Δalb) mice and analyzed them using two cholestatic liver injury models: bile duct ligation (BDL) and 3,5-diethoxycarbonyl-1,4-dihydrocollidine (DDC) feeding. Primary hepatocytes and a cholangiocyte cell line were used to address the impact of plectin on keratin filament organization and stability in vitro.

RESULTS

Plectin deficiency in hepatocytes and biliary epithelial cells led to aberrant keratin filament network organization, biliary tree malformations, and collapse of bile ducts and ductules. Further, plectin ablation significantly aggravated biliary damage upon cholestatic challenge. Coincidently, we observed a significant expansion of A6-positive progenitor cells in Ple^Δalb livers. After BDL, plectin-deficient bile ducts were prominently dilated with more frequent ruptures corresponding to an increased number of bile infarcts. In addition, more abundant keratin aggregates indicated less stable keratin filaments in Ple^Δalb hepatocytes. A transmission electron microscopy analysis revealed a compromised tight junction formation in plectin-deficient biliary epithelial cells. In addition, protein profiling showed increased expression of the adherens junction protein E-Cadherin, and inefficient upregulation of the desmosomal protein desmoplakin in response to BDL. In vitro analyses revealed a higher susceptibility of plectin-deficient keratin networks to stress-induced collapse, paralleled by elevated activation of p38 MAP kinase.

CONCLUSION

Our study shows that by maintaining proper keratin network cytoarchitecture and biliary epithelial stability, plectin plays a critical role in protecting the liver from stress elicited by cholestasis.

LAY SUMMARY

Plectin is a cytolinker protein capable of interconnecting all three cytoskeletal filament systems and linking them to plasma membrane-bound junctional complexes. In liver, the plectin-controlled cytoskeleton mechanically stabilizes epithelial cells and provides them with the capacity to adapt to increased bile pressure under cholestasis.

Mammalian Plakins, Giant Cytolinkers: Versatile Biological Functions and Roles in Cancer

Cancer is a highly lethal disease that is characterized by aberrant cell proliferation, migration, and adhesion, which are closely related to the dynamic changes of cytoskeletons and cytoskeletal-adhesion. These will further result in cell invasion and metastasis. Plakins are a family of giant cytolinkers that connect cytoskeletal elements with each other and to junctional complexes. With various isoforms composed of different domain structures, mammalian plakins are broadly expressed in numerous tissues. They play critical roles in many cellular processes, including cell proliferation, migration, adhesion, and signaling transduction. As these cellular processes are key steps in cancer development, mammalian plakins have in recent years attracted more and more attention for their potential roles in cancer. Current evidence shows the importance of mammalian plakins in various human cancers and demonstrates mammalian plakins as potential biomarkers for cancer. Here, we introduce the basic characteristics of mammalian plakins, review the recent advances in understanding their biological functions, and highlight their roles in human cancers, based on studies performed by us and others. This will provide researchers with a comprehensive understanding of mammalian plakins, new insights into the development of cancer, and novel targets for cancer diagnosis and therapy.

Plectin is a novel regulator for apical extrusion of RasV12-transformed cells

Several lines of evidence have revealed that newly emerging transformed cells are often eliminated from the epithelium, though the underlying molecular mechanisms of this cancer preventive phenomenon still remain elusive. In this study, using mammalian cell culture systems we have identified plectin, a versatile cytoskeletal linker protein, as a novel regulator for apical extrusion of RasV12-transformed cells. Plectin is accumulated in RasV12 cells when they are surrounded by normal epithelial cells. Similarly, cytoskeletal proteins tubulin, keratin, and Epithelial Protein Lost In Neoplasm (EPLIN) are also accumulated in the transformed cells surrounded by normal cells. Knockdown or functional disruption of one of these molecules diminishes the accumulation of the others, indicating that the accumulation process of the individual protein mutually depends on each other. Furthermore, plectin-knockdown attenuates caveolin-1 (Cav-1) enrichment and PKA activity in RasV12 cells and profoundly suppresses the apical extrusion. These results indicate that the plectin-microtubules-EPLIN complex positively regulates apical elimination of RasV12-transformed cells from the epithelium in a coordinated fashion. Further development of this study would open a new avenue for cancer preventive medicine.

AMP-activated protein kinase: a cellular energy sensor that comes in 12 flavours

The AMP-activated protein kinase (AMPK) is a sensor of cellular energy status that is expressed in essentially all eukaryotic cells, suggesting that it arose during early eukaryotic evolution. It occurs universally as heterotrimeric complexes containing catalytic α subunits and regulatory β and γ subunits. Although Drosophila melanogaster contains single genes encoding each subunit, in mammals, each subunit exists as multiple isoforms encoded by distinct genes, giving rise to up to 12 heterotrimeric combinations. The multiple isoforms of each subunit are 2R-ohnologues generated by the two rounds of whole genome duplication that occurred at the evolutionary origin of the vertebrates. Although the differential roles of these isoform combinations remain only partly understood, there are indications that they may have different subcellular locations, different inputs and outputs, and different functions. The multiple isoforms are of particular interest with respect to the roles of AMPK in cancer because the genes encoding some isoforms, such as PRKAA1 and PRKAB2 (encoding α1 and β2), are quite frequently amplified in tumour cells, whereas the genes encoding others, such as PRKAA2 (encoding α2), tend to be mutated, which, in some but not all cases, may result in a loss of function. Thus, although AMPK acts downstream of the tumour suppressor liver kinase B1, and some of its isoform combinations may act as tumour suppressors that restrain the growth and proliferation of tumour cells, other isoform combinations may paradoxically act as oncogenes, perhaps by aiding the survival of tumour cells undergoing environmental stresses such as hypoxia or nutrient deprivation.

Liver Kinase B1 complex acts as a novel modifier of myofilament function and localizes to the Z-disk in cardiac myocytes

Contractile perturbations downstream of Ca(2+) binding to troponin C, the so-called sarcomere-controlled mechanisms, represent the earliest indicators of energy remodeling in the diseased heart [1]. Central to cellular energy "sensing" is the adenosine monophosphate-activated kinase (AMPK) pathway, which is known to directly target myofilament proteins and alter contractility [2-6]. We previously showed that the upstream AMPK kinase, LKB1/MO25/STRAD, impacts myofilament function independently of AMPK [5]. Therefore, we hypothesized that the LKB1 complex associated with myofilament proteins and that alterations in energy signaling modulated targeting or localization of the LKB1 complex to the myofilament. Using an integrated strategy of myofilament mechanics, immunoblot analysis, co-immunoprecipitation, mass spectroscopy, and immunofluorescence, we showed that 1) LKB1 and MO25 associated with myofibrillar proteins, 2) cellular energy stress re-distributed AMPK/LKB1 complex proteins within the sarcomere, and 3) the LKB1 complex localized to the Z-Disk and interacted with cytoskeletal and energy-regulating proteins, including vinculin and ATP Synthase (Complex V). These data represent a novel role for LKB1 complex proteins in myofilament function and myocellular "energy" sensing in the heart.

Plectin isoform 1-dependent nuclear docking of desmin networks affects myonuclear architecture and expression of mechanotransducers

Plectin is a highly versatile cytoskeletal protein that acts as a mechanical linker between intermediate filament (IF) networks and various cellular structures. The protein is crucial for myofiber integrity. Its deficiency leads to severe pathological changes in skeletal muscle fibers of patients suffering from epidermolysis bullosa simplex with muscular dystrophy (EBS-MD). Skeletal muscle fibers express four major isoforms of plectin which are distinguished solely by alternative, relatively short, first exon-encoded N-terminal sequences. Each one of these isoforms is localized to a different subcellular compartment and plays a specific role in maintaining integrity and proper function(s) of myofibers. The unique role of individual isoforms is supported by distinct phenotypes of isoform-specific knockout mice and recently discovered mutations in first coding exons of plectin that lead to distinct, tissue-specific, pathological abnormalities in humans. In this study, we demonstrate that the lack of plectin isoform 1 (P1) in myofibers of mice leads to alterations of nuclear morphology, similar to those observed in various forms of MD. We show that P1-mediated targeting of desmin IFs to myonuclei is essential for maintenance of their typically spheroidal architecture as well as their proper positioning and movement along the myofiber. Furthermore, we show that P1 deficiency affects chromatin modifications and the expression of genes involved in various cellular functions, including signaling pathways mediating mechanotransduction. Mechanistically, P1 is shown to specifically interact with the myonuclear membrane-associated (BAR domain-containing) protein endophilin B. Our results open a new perspective on cytoskeleton-nuclear crosstalk via specific cytolinker proteins.

PT-1 selectively activates AMPK-γ1 complexes in mouse skeletal muscle, but activates all three γ subunit complexes in cultured human cells by inhibiting the respiratory chain

AMP-activated protein kinase (AMPK) occurs as heterotrimeric complexes in which a catalytic subunit (α1/α2) is bound to one of two β subunits (β1/β2) and one of three γ subunits (γ1/γ2/γ3). The ability to selectively activate specific isoforms would be a useful research tool and a promising strategy to combat diseases such as cancer and Type 2 diabetes. We report that the AMPK activator PT-1 selectively increased the activity of γ1- but not γ3-containing complexes in incubated mouse muscle. PT-1 increased the AMPK-dependent phosphorylation of the autophagy-regulating kinase ULK1 (unc-51-like autophagy-activating kinase 1) on Ser555, but not proposed AMPK-γ3 substrates such as Ser231 on TBC1 (tre-2/USP6, BUB2, cdc16) domain family, member 1 (TBC1D1) or Ser212 on acetyl-CoA carboxylase subunit 2 (ACC2), nor did it stimulate glucose transport. Surprisingly, however, in human embryonic kidney (HEK) 293 cells expressing human γ1, γ2 or γ3, PT-1 activated all three complexes equally. We were unable to reproduce previous findings suggesting that PT-1 activates AMPK by direct binding between the kinase and auto-inhibitory domains (AIDs) of the α subunit. We show instead that PT-1 activates AMPK indirectly by inhibiting the respiratory chain and increasing cellular AMP:ATP and/or ADP:ATP ratios. Consistent with this mechanism, PT-1 failed to activate AMPK in HEK293 cells expressing an AMP-insensitive R299G mutant of AMPK-γ1. We propose that the failure of PT-1 to activate γ3-containing complexes in muscle is not an intrinsic feature of such complexes, but is because PT-1 does not increase cellular AMP:ATP ratios in the specific subcellular compartment(s) in which γ3 complexes are located.

Networking and anchoring through plectin: a key to IF functionality and mechanotransduction

Intermediate filaments (IFs) are involved in multiple cellular processes that are essential for the maintenance of cell and tissue integrity as well as response and adaption to stress. Mainly through pathological manifestations in patients and the analysis of genetic mouse models, it became evident that cytolinker proteins of the plakin protein family are essential for many of the functions ascribed to IFs. As discussed in this review, one of them, plectin, affects the assembly properties, interaction potential, compartmentalization, and linkage properties of IFs, making it to a key player for IF functionality. The far reaching consequences of IFs not being well-connected for skin and muscular integrity, migration, and mechanotransduction are highlighted.

Determining the mechanical properties of plectin in mouse myoblasts and keratinocytes

Plectin is the prototype of an intermediate filament (IF)-based cytolinker protein. It affects cells mechanically by interlinking and anchoring cytoskeletal filaments and acts as scaffolding and docking platform for signaling proteins to control cytoskeleton dynamics. The most common disease caused by mutations in the human plectin gene, epidermolysis bullosa simplex with muscular dystrophy (EBS-MD), is characterized by severe skin blistering and progressive muscular dystrophy. Therefore, we compared the biomechanical properties and the response to mechanical stress of murine plectin-deficient myoblasts and keratinocytes with wild-type cells. Using a cell stretching device, plectin-deficient myoblasts exhibited lower mechanical vulnerability upon external stress compared to wild-type cells, which we attributed to lower cellular pre-stress. Contrary to myoblasts, wild-type and plectin-deficient keratinocytes showed no significant differences. In magnetic tweezer measurements using fibronectin-coated paramagnetic beads, the stiffness of keratinocytes was higher than of myoblasts. Interestingly, cell stiffness, adhesion strength, and cytoskeletal dynamics were strikingly altered in plectin-deficient compared to wild-type myoblasts, whereas smaller differences were observed between plectin-deficient and wild-type keratinocytes, indicating that plectin might be more important for stabilizing cytoskeletal structures in myoblasts than in keratinocytes. Traction forces strongly correlated with the stiffness of plectin-deficient and wild-type myoblasts and keratinocytes. Contrary to that cell motility was comparable in plectin-deficient and wild-type myoblasts, but was significantly increased in plectin-deficient compared to wild-type keratinocytes. Thus, we postulate that the lack of plectin has divergent implications on biomechanical properties depending on the respective cell type.

•

The intermediate filament-associated protein plectin has divergent biomechanical implications depending on cell/tissue type.

•

In plectin^−/− myoblasts, cell vulnerability, stiffness, strain and binding strength are lower than in wild-type cells.

•

Plectin^−/− keratinocytes exhibit higher cell stiffness, binding strength, strain and velocity than wild-type cells.

AMP-activated protein kinase regulates intraocular pressure, extracellular matrix, and cytoskeleton in trabecular meshwork

PURPOSE

In this study, we investigate how adenosine monophosphate-activated protein kinase (AMPK) affects extracellular matrix (ECM) and cellular tone in the trabecular meshwork (TM), and examine how deletion of its catalytic α2 subunit affects IOP and aqueous humor clearance in mice.

METHODS

Human TM tissue was examined for expression of AMPKα1 and AMPKα2, genomically distinct isoforms of the AMPK catalytic subunit. Primary cultured human TM cells were treated for 24 hours with the AMPK activator 5-amino-1-β-Dffff-ribofuranosyl-imidazole-4-carboxamide (AICAR), under basal or TGF-β2 stimulatory conditions. Conditioned media (CM) was probed for secreted protein acidic and rich in cysteine (SPARC), thrombospondin-1 (TSP-1), and ECM proteins, and cells were stained for F-actin. Cells underwent adenoviral infection with a dominant negative AMPKα subunit (ad.DN.AMPKα) and were similarly analyzed. Intraocular pressure, central corneal thickness (CCT), and aqueous clearance were measured in AMPKα2-null and wild-type (WT) mice.

RESULTS

Both AMPKα1 and AMPKα2 are expressed in TM. AICAR activated AMPKα and suppressed the expression of various ECM proteins under basal and TGF-β2 stimulatory conditions. AICAR decreased F-actin staining and increased the phospho-total RhoA ratio (Ser188). Transforming growth factor-β2 transiently dephosphorylated AMPKα. Infection with ad.DN.AMPKα upregulated various ECM proteins, decreased the phospho-total RhoA ratio, and increased F-actin staining. AMPKα2-null mice exhibited 6% higher IOP and decreased aqueous clearance compared with WT mice, without significant differences in CCT or angle morphology.

CONCLUSIONS

Collectively, our data identify AMPK as a critical regulator of ECM homeostasis and cytoskeletal arrangement in the TM. Mice that are AMPKα2-null exhibit higher IOPs and decreased aqueous clearance than their WT counterparts.

Plakins, a versatile family of cytolinkers: roles in skin integrity and in human diseases

The plakin family consists of giant proteins involved in the cross-linking and organization of the cytoskeleton and adhesion complexes. They further modulate several fundamental biological processes, such as cell adhesion, migration, and polarization or signaling pathways. Inherited and acquired defects of plakins in humans and in animal models potentially lead to dramatic manifestations in the skin, striated muscles, and/or nervous system. These observations unequivocally demonstrate the key role of plakins in the maintenance of tissue integrity. Here we review the characteristics of the mammalian plakin members BPAG1 (bullous pemphigoid antigen 1), desmoplakin, plectin, envoplakin, epiplakin, MACF1 (microtubule-actin cross-linking factor 1), and periplakin, highlighting their role in skin homeostasis and diseases.

Localisation of AMPK γ subunits in cardiac and skeletal muscles

The trimeric protein AMP-activated protein kinase (AMPK) is an important sensor of energetic status and cellular stress, and mutations in genes encoding two of the regulatory γ subunits cause inherited disorders of either cardiac or skeletal muscle. AMPKγ2 mutations cause hypertrophic cardiomyopathy with glycogen deposition and conduction abnormalities; mutations in AMPKγ3 result in increased skeletal muscle glycogen. In order to gain further insight into the roles of the different γ subunits in muscle and into possible disease mechanisms, we localised the γ2 and γ3 subunits, along with the more abundant γ1 subunit, by immunofluorescence in cardiomyocytes and skeletal muscle fibres. The predominant cardiac γ2 variant, γ2-3B, gave a striated pattern in cardiomyocytes, aligning with the Z-disk but with punctate staining similar to T-tubule (L-type Ca²⁺ channel) and sarcoplasmic reticulum (SERCA2) markers. In skeletal muscle fibres AMPKγ3 localises to the I band, presenting a uniform staining that flanks the Z-disk, also coinciding with the position of Ca²⁺ influx in these muscles. The localisation of γ2-3B- and γ3-containing AMPK suggests that these trimers may have similar functions in the different muscles. AMPK containing γ2-3B was detected in oxidative skeletal muscles which had low expression of γ3, confirming that these two regulatory subunits may be co-ordinately regulated in response to metabolic requirements. Compartmentalisation of AMPK complexes is most likely dependent on the regulatory γ subunit and this differential localisation may direct substrate selection and specify particular functional roles.

Plectin-intermediate filament partnership in skin, skeletal muscle, and peripheral nerve

Plectin is a large, 500-kDa, intermediate filament (IF)-associated protein. It acts as a cytoskeletal crosslinker and signaling scaffold, affecting mechanical as well as dynamic properties of the cytoskeleton. As a member of the plakin family of cytolinker proteins, plectin has a multidomain structure that is responsible for its vast binding portfolio. It not only binds to all types of IFs, actin filaments and microtubules, but also to transmembrane receptors, proteins of the subplasma membrane protein skeleton, components of the nuclear envelope, and several kinases with known roles in migration, proliferation, and energy metabolism of cells. Due to alternative splicing, plectin is expressed as various isoforms with differing N-terminal heads that dictate their differential subcellular targeting. Through specific interactions with other proteins at their target sites and their ability to bind to all types of IFs, plectin molecules provide strategically located IF anchorage sites within the cytoplasm of cells. In this review, we will present an overview of the structural features and functional properties of plectin and discuss recent progress in defining the role of its isoforms in stress-prone tissues and the implicated diseases, with focus on skin, skeletal muscle, and Schwann cells of peripheral nerve.

Linking cytoarchitecture to metabolism: sarcolemma-associated plectin affects glucose uptake by destabilizing microtubule networks in mdx myofibers

BACKGROUND

Duchenne muscular dystrophy (DMD) is one of the most frequent forms of muscular disorders. It is caused by the absence of dystrophin, a core component of the sarcolemma-associated junctional complex that links the cytoskeleton to the extracellular matrix. We showed previously that plectin 1f (P1f), one of the major muscle-expressed isoforms of the cytoskeletal linker protein plectin, accumulates at the sarcolemma of DMD patients as well as of mdx mice, a widely studied animal model for DMD.Based on plectin's dual role as structural protein and scaffolding platform for signaling molecules, we speculated that the dystrophic phenotype observed after loss of dystrophin was caused, at least to some extent, by excess plectin. Thus, we hypothesized that elimination of plectin expression in mdx skeletal muscle, while probably resulting in an overall more severe phenotype, may lead to a partial phenotype rescue. In particular, we wanted to assess whether excess sarcolemmal plectin contributes to the dysregulation of sugar metabolism in mdx myofibers.

METHODS

We generated plectin/dystrophin double deficient (dKO) mice by breeding mdx with conditional striated muscle-restricted plectin knockout (cKO) mice. The phenotype of these mice was comparatively analyzed with that of mdx, cKO, and wild-type mice, focusing on structural integrity and dysregulation of glucose metabolism.

RESULTS

We show that the accumulation of plectin at the sarcolemma of mdx muscle fibers hardly compensated for their loss of structural integrity. Instead, it led to an additional metabolic deficit by impairing glucose uptake. While dKO mice suffered from an overall more severe form of muscular dystrophy compared to mdx or plectin-deficient mice, sarcolemmal integrity as well as glucose uptake of their myofibers were restored to normal levels upon ablation of plectin. Furthermore, microtubule (MT) networks in intact dKO myofibers, including subsarcolemmal areas, were found to be more robust than those in mdx mice. Finally, myotubes differentiated from P1f-overexpressing myoblasts showed an impairment of glucose transporter 4 translocation and a destabilization of MT networks.

CONCLUSIONS

Based on these results we propose that sarcolemma-associated plectin acts as an antagonist of MT network formation in myofibers, thereby hindering vesicle-mediated (MT-dependent) transport of glucose transporter 4. This novel role of plectin throws a bridge between extra-sarcomeric cytoarchitecture and metabolism of muscle fibers. Our study thus provides new insights into pathomechanisms of plectinopathies and muscular dystrophies in general.

Intermediate filament-associated cytolinker plectin 1c destabilizes microtubules in keratinocytes

The transition of microtubules (MTs) from an assembled to a disassembled state plays an essential role in several cellular functions. While MT dynamics are often linked to those of actin filaments, little is known about whether intermediate filaments (IFs) have an influence on MT dynamics. We show here that plectin 1c (P1c), one of the multiple isoforms of the IF-associated cytolinker protein plectin, acts as an MT destabilizer. We found that MTs in P1c-deficient (P1c(-/-)) keratinocytes are more resistant toward nocodazole-induced disassembly and display increased acetylation. In addition, live imaging of MTs in P1c(-/-), as well as in plectin-null, cells revealed decreased MT dynamics. Increased MT stability due to P1c deficiency led to changes in cell shape, increased velocity but loss of directionality of migration, smaller-sized focal adhesions, higher glucose uptake, and mitotic spindle aberrations combined with reduced growth rates of cells. On the basis of ex vivo and in vitro experimental approaches, we suggest a mechanism for MT destabilization in which isoform-specific binding of P1c to MTs antagonizes the MT-stabilizing and assembly-promoting function of MT-associated proteins through an inhibitory function exerted by plectin's SH3 domain. Our results open new perspectives on cytolinker-coordinated IF-MT interaction and its physiological significance.

The many faces of plectin and plectinopathies: pathology and mechanisms

Plectin, a giant multifunctional cytolinker protein, plays a crucial role in stabilizing and orchestrating intermediate filament networks in cells. Mutations in the human plectin gene result in multiple diseases manifesting with muscular dystrophy, skin blistering, and signs of neuropathy. The most common disease caused by plectin deficiency is epidermolysis bullosa simplex (EBS)-MD, a rare autosomal-recessive skin blistering disorder with late-onset muscular dystrophy. EBS-MD patients and plectin-deficient mice display pathologic desmin-positive protein aggregates, degenerated myofibrils, and mitochondrial abnormalities, the hallmarks of myofibrillar myopathies. In addition to EBS-MD, plectin mutations have been shown to cause EBS-MD with a myasthenic syndrome, limb-girdle muscular dystrophy type 2Q, EBS with pyloric atresia, and EBS-Ogna. This review focuses on clinical and pathological manifestations of these plectinopathies. It addresses especially plectin's role in skeletal muscle, where a loss of muscle fiber integrity and profound changes of myofiber cytoarchitecture are observed in its absence. Furthermore, the highly complex genetic and molecular structure of plectin is discussed; a high number of differentially spliced exons give rise to a variety of different isoforms, which fulfill distinct functions in different cell types and tissues. Plectin's abilities to act as a dynamic organizer of intermediate filament networks and to interact with a multitude of different interaction partners are the basis for its function as a scaffolding platform for proteins involved in signaling. Finally, the article addresses a series of genetically manipulated mouse lines that were generated to serve as powerful models to study functional and molecular consequences of plectin gene defects.

Embryonic expression of AMPK γ subunits and the identification of a novel γ2 transcript variant in adult heart

AMP-activated protein kinase (AMPK), the key sensor and regulator of cellular energy status, is a heterotrimeric enzyme with multiple isoforms for each subunit (α1/α 2; β1/β2; γ1/γ2/γ3). Mutations in PRKAG2, which encodes the γ2 regulatory subunit, cause a cardiomyopathy characterized by hypertrophy and conduction abnormalities. The two reported PRKAG2 transcript variants, γ2-short and γ2-long (encoding 328 and 569 amino acids respectively), are both widely expressed in adult tissues. We show that both γ2 variants are also expressed during cardiogenesis in mouse embryos; expression of the γ3 isoform was also detected unexpectedly at this stage. As neither γ2 transcript is cardiac specific nor differentially expressed during embryogenesis, it is paradoxical that the disease is largely restricted to the heart. However, a recently annotated γ2 transcript, termed γ2-3B as transcription starts at an alternative exon 3b, has been identified; it is spliced in-frame to exon 4 thus generating a protein of 443 residues in mouse with the first 32 residues being unique. It is increasingly expressed in the developing mouse heart and quantitative PCR analysis established that γ2-3B is the major PRKAG2 transcript (~ 60%) in human heart. Antibody against the novel N-terminal sequence showed that γ2-3B is predominantly expressed in the heart where it is the most abundant γ2 protein. The abundance of γ2-3B and its tissue specificity indicate that γ2-3B may have non-redundant role in the heart and hence mediate the predominantly cardiac phenotype caused by PRKAG2 mutations.

Fused in sarcoma (FUS) interacts with the cytolinker protein plectin: implications for FUS subcellular localization and function

Fused in sarcoma (FUS) is a multifunctional protein involved in transcriptional control, pre-mRNA processing, RNA transport and translation. The domain structure of FUS reflects its functions in gene regulation and its ability to interact with other proteins, RNA and DNA. By use of a recombinant fragment of FUS in pull-down experiments followed by mass spectrometry analysis we have identified a novel interaction between the FUS N-terminal and the cytolinker plectin. An in situ proximity ligation assay confirmed that FUS-plectin interactions take place in the cytoplasm of cells. Furthermore, plectin deficient cells showed an altered subcellular localization of FUS and a deregulated expression of mRNAs bound to FUS. Our results show that plectin is important for normal FUS localization and function. Mutations involving FUS are causative factors in sarcomas and leukemias and also hereditary forms of amyotrophic lateral sclerosis and frontotemporal lobar degeneration. Plectin deficiency causes epidermolysis bullosa, a disease involving the skin and neuromuscular system. The novel FUS-plectin interaction offers new perspectives for understanding the role of FUS and plectin mutations in the pathogenesis of these diseases.

The RON-receptor regulates pancreatic cancer cell migration through phosphorylation-dependent breakdown of the hemidesmosome

The recepteur d'origine nantais (RON) receptor tyrosine kinase is overexpressed and stimulates invasive growth in pancreatic cancer cells, yet the mechanisms that underlie RON-mediated phenotypes remain poorly characterized. To better understand RON function in pancreatic cancer cells, we sought to identify novel RON interactants using multidimensional protein identification analysis. These studies revealed plectin, a large protein of the spectrin superfamily, to be a novel RON interactant. Plectin is a multifunctional protein that complexes with integrin-β4 (ITGB4) to form hemidesmosomes, serves as a scaffolding platform crucial to the function of numerous protein signaling pathways and was recently described as an overexpressed protein in pancreatic cancer (Bausch D et al., Clin Cancer Res 2010; Kelly et al., PLoS Med 2008;5:e85). In this study, we demonstrate that on exposure to its ligand, macrophage-stimulating protein, RON binds to plectin and ITGB4, which results in disruption of the plectin-ITGB4 interaction and enhanced cell migration, a phenotype that can be recapitulated by small hairpin ribosomal nucleic acid (shRNA)-mediated suppression of plectin expression. We demonstrate that disruption of plectin-ITGB4 is dependent on RON and phosphoinositide-3 (PI3) kinase, but not mitogen-activated protein kinase (MEK), activity. Thus, in pancreatic cancer cells, plectin and ITGB4 form hemidesmosomes which serve to anchor cells to the extracellular matrix (ECM) and restrain migration. The current study defines a novel interaction between RON and plectin, provides new insight into RON-mediated migration and further supports efforts to target RON kinase activity in pancreatic cancer.

Subunit composition of AMPK trimers present in the cytokinetic apparatus: Implications for drug target identification

AMP-activated protein kinase has been shown to be a key regulator of energy homeostasis; it has also been identified as a tumor suppressor and is required for correct cell division and chromosome segregation during mitosis. The enzyme is a heterotrimer, with each subunit having more than one isoform, each encoded by a separate gene (two α, two β and three γ isoforms). In human endothelial cells, the activated kinase subunit of AMPK in the cytokinetic apparatus is α2, the minority α subunit, which co-localizes with β2 and γ2. This is the first demonstration of a trimeric complex of AMPK containing the γ2 regulatory subunit becoming selectively activated and being linked to mitotic processes. We also show that α1 and γ1, the predominant AMPK subunits, are almost exclusively localized in the cytoskeleton, while α2 and γ2 are present in all subcellular fractions, including the nuclei. These data suggest that pharmacological interventions targeted to specific AMPK subunit isoforms have the potential to modify selective functions of AMPK.

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.

Adhesion molecule periplakin is involved in cellular movement and attachment in pharyngeal squamous cancer cells

Background

We previously reported that periplakin (PPL) is downregulated in human esophageal cancer tissues compared to the adjacent non-cancer epithelium. Thus PPL could be a useful marker for detection of early esophageal cancer and evaluation of tumor progression, but largely remains unknown in this field. To investigate PPL involvement in carcinogenesis, tumor progression, cellular movement or attachment activity, siRNAs against PPL were transfected into pharyngeal squamous cancer cell lines and their effects on cellular behaviours were examined.

Results

PPL knockdown appeared to decrease tumor cell growth together with G2/M phase accumulation in cells attached to a culture dish. However, the extent of cell growth suppression, evaluated by the number of cells attached to the culture dish, was too distinctive to be explained only by cell cycle delay. Importantly, PPL knockdown suppressed cellular movement and attachment to the culture dish accompanied by decreased pAktSer473 phosphorylation. Additionally, LY294002, a PI3K inhibitor that dephosphorylates pAktSer473, significantly suppressed D562 cell migration. Thus PPL potentially engages in cellular movement al least partly via the PI3K/Akt axis.

Conclusions

PPL knockdown is related to reduced cellular movement and attachment activity in association with PI3K/Akt axis suppression, rather than malignant progression in pharyngeal cancer cells.

Plectin isoforms as organizers of intermediate filament cytoarchitecture

Intermediate filaments (IFs) form cytoplamic and nuclear networks that provide cells with mechanical strength. Perturbation of this structural support causes cell and tissue fragility and accounts for a number of human genetic diseases. In recent years, important additional roles, nonmechanical in nature, were ascribed to IFs, including regulation of signaling pathways that control survival and growth of the cells, and vectorial processes such as protein targeting in polarized cellular settings. The cytolinker protein plectin anchors IF networks to junctional complexes, the nuclear envelope and cytoplasmic organelles and it mediates their cross talk with the actin and tubulin cytoskeleton. These functions empower plectin to wield significant influence over IF network cytoarchitecture. Moreover, the unusual diversity of plectin isoforms with different N termini and a common IF-binding (C-terminal) domain enables these isoforms to specifically associate with and thereby bridge IF networks to distinct cellular structures. Here we review the evidence for IF cytoarchitecture being controlled by specific plectin isoforms in different cell systems, including fibroblasts, endothelial cells, lens fibers, lymphocytes, myocytes, keratinocytes, neurons and astrocytes, and discuss what impact the absence of these isoforms has on IF cytoarchitecture-dependent cellular functions.

A dystroglycan/plectin scaffold mediates mechanical pathway bifurcation in lung epithelial cells

In alveolar epithelial cells (AECs), the membrane-anchored proteoglycan dystroglycan (DG) is a mechanoreceptor that transmits mechanical stretch forces to activate independently the ERK1/2 and the adenosine 5'-monophosphate-activated protein kinase (AMPK) signaling cascades in a process called pathway bifurcation. We tested the hypothesis that the cytoskeleton cross-linker plectin, known to bind both DG and AMPK in muscle cells, acts as a scaffold to regulate DG-mediated mechanical stimulation and pathway bifurcation. We demonstrate that plectin and DG form a complex in AECs and that this complex interacts with ERK1/2 and AMPK. Plectin knockdown reduces DG interaction with AMPK but not with ERK1/2. Despite this, mechanoactivation of both signaling pathways is significantly attenuated in AECs deficient in plectin. Thus, DG has the dual role of mechanical receptor and scaffold for ERK1/2, whereas plectin acts as a scaffold for AMPK signaling but is also required for DG-mediated ERK1/2 activation. We conclude that the DG-plectin complex plays a central role in transmitting mechanical stress from the extracellular matrix to the cytoplasm.

The making of hemidesmosome structures in vivo

Hemidesmosomes are evolutionarily conserved attachment complexes linked to intermediate filaments that connect epithelial cells to the extracellular matrix. They provide tissue integrity and resistance to mechanical forces. Alterations in hemidesmosome structures are responsible for skin blistering, carcinoma invasion, and wound-healing defects. Valuable information about hemidesmosome assembly and disassembly has been obtained from in vitro cell culture studies. However, how these processes take place in vivo still remains elusive. Here, we discuss recent data about the formation and reorganization of hemidesmosomes in several in vivo model systems, particularly zebrafish and Caenorhabditis elegans, focusing on various factors affecting their dynamics. Mechanisms found in different organisms reveal that hemidesmosome formation and maintenance in vivo are carefully controlled by ECM protein folding, ECM-receptor expression and trafficking, and by post-translational modification of hemidesmosome components. These findings validate and extend the in vitro studies, and shed light on our understanding about hemidesmosomes across species.

AMPK inhibition in health and disease

All living organisms depend on dynamic mechanisms that repeatedly reassess the status of amassed energy, in order to adapt energy supply to demand. The AMP-activated protein kinase (AMPK) alphabetagamma heterotrimer has emerged as an important integrator of signals managing energy balance. Control of AMPK activity involves allosteric AMP and ATP regulation, auto-inhibitory features and phosphorylation of its catalytic (alpha) and regulatory (beta and gamma) subunits. AMPK has a prominent role not only as a peripheral sensor but also in the central nervous system as a multifunctional metabolic regulator. AMPK represents an ideal second messenger for reporting cellular energy state. For this reason, activated AMPK acts as a protective response to energy stress in numerous systems. However, AMPK inhibition also actively participates in the control of whole body energy homeostasis. In this review, we discuss recent findings that support the role and function of AMPK inhibition under physiological and pathological states.

Plectin gene defects lead to various forms of epidermolysis bullosa simplex

Plectin is an important organizer of the keratin filament cytoskeleton in basal keratinocytes. It is essential for anchoring these filaments to the extracellular matrix via hemidesmosomal integrins. Loss of plectin or incorrect function of the protein due to mutations in its gene can lead to various forms of the skin blistering disease, epidermolysis bullosa simplex. Severity and subtype of the disease is dependent on the specific mutation and can be associated with (late-onset) muscular dystrophy or pyloric atresia. Mouse models mimicking the human phenotypes allow detailed study of plectin function.

Ion channel regulation by AMPK: the route of hypoxia-response coupling in thecarotid body and pulmonary artery

Vital homeostatic mechanisms monitor O2 supply and adjust respiratory and circulatory function to meet demand. The pulmonary arteries and carotid bodies are key systems in this respect. Hypoxic pulmonary vasoconstriction (HPV) aids ventilation-perfusion matching in the lung by diverting blood flow from areas with an O2 deficit to those rich in O2, while a fall in arterial pO2 increases sensory afferent discharge from the carotid body to elicit corrective changes in breathing patterns. We discuss here the new concept that hypoxia, by inhibiting oxidative phosphorylation, activates AMP-activated protein kinase (AMPK) leading to consequent phosphorylation of target proteins, such as ion channels, which initiate pulmonary artery constriction and carotid body activation. Consistent with this view, AMPK knockout mice exhibit an impaired ventilatory response to hypoxia. Thus, AMPK may be sufficient and necessary for hypoxia-response coupling and may regulate O2 and thereby energy (ATP) supply at the whole body as well as the cellular level.

Targeted inactivation of a developmentally regulated neural plectin isoform (plectin 1c) in mice leads to reduced motor nerve conduction velocity

Cytolinker proteins stabilize cells mechanically, regulate cytoskeleton dynamics, and provide scaffolds for signaling molecules. For plectin, the prototype of these proteins, an unusual diversity of isoforms has been reported, which show distinct expression patterns, subcellular localizations, and functions. Plectin has been shown to have important functions in skin and muscle, but little is known about its role in neural cells. To address this issue, we generated two knock-out mouse lines, one which was selectively lacking plectin 1c (P1c), the major isoform expressed in neural cells, and another in which plectin was conditionally deleted in neuronal precursor cells. Using isoform-specific antibodies, we found P1c to be expressed late in development and to associate with postsynaptic dendrites of central nervous system neurons, motorneurons of spinal cord, sciatic nerve axons, and Schwann cells. Motor nerve conduction velocity was found significantly reduced in sciatic nerve from P1c-deficient as well as from conditional knock-out mice. This defect was traceable to an increased number of motor nerve fibers with small cross-sectional areas; the thicknesses of axons and of myelin sheaths were unaffected. This is the first report demonstrating an important role of plectin in a major nerve function.

AMPK activator, AICAR, inhibits palmitate-induced apoptosis in osteoblast

Osteoblast apoptosis reduces bone mineral density. Apoptosis can be induced in a variety of cells by palmitate, which is one of the most common saturated fatty acids in dietary fat. The AMPK activator, AICAR, has been shown to inhibit palmitate-induced apoptosis. However, the role of palmitate in osteoblast apoptosis is currently unknown. This study examined whether palmitate could induce apoptosis in osteoblasts, and if so, whether AICAR could alleviate palmitate-induced apoptosis. Palmitate reduced cell survival and induced apoptosis in a dose- and time-dependent manner in human fetal osteoblasts (hFOB) 1.19. While the long-chain acyl-CoA synthetase inhibitor, triacsin C, inhibited palmitate-induced apoptosis, anti-oxidants and ceramide synthesis inhibitors did not attenuate the apoptosis. AICAR prevented palmitate-induced apoptosis and the inhibition of AICAR-mediated increase in fatty acid oxidation by etomoxir did not affect the prevention of apoptosis by AICAR. Constitutively-active AMPK also inhibited palmitate-induced apoptosis. Treatment with an AMPK inhibitor (compound C) and a dominant-negative AMPK adenovirus suppressed the inhibitory effect of AICAR on apoptosis. Palmitate impaired the activation of ERK by fetal bovine serum, which was blocked by AICAR. Moreover, AICAR increased ERK activation, and ERK inhibitors, PD98059 and U0126, as well as a dominant-negative MEK1, abolished the inhibitory effect of AICAR on palmitate-induced apoptosis. AICAR also inhibited palmitate-induced apoptosis in osteoblastic differentiated cells from human bone marrow, which was accompanied by recovered ERK activity. These results suggest that palmitate induces apoptosis in osteoblasts through the impaired activation of ERK, and the activation of AMPK inhibits palmitate-induced apoptosis by activating ERK.

Plectin isoform 1b mediates mitochondrion-intermediate filament network linkage and controls organelle shape

Plectin is a versatile intermediate filament (IF)-bound cytolinker protein with a variety of differentially spliced isoforms accounting for its multiple functions. One particular isoform, plectin 1b (P1b), remains associated with mitochondria after biochemical fractionation of fibroblasts and cells expressing exogenous P1b. Here, we determined that P1b is inserted into the outer mitochondrial membrane with the exon 1b-encoded N-terminal sequence serving as a mitochondrial targeting and anchoring signal. To study P1b-related mitochondrial functions, we generated mice that selectively lack this isoform but express all others. In primary fibroblasts and myoblasts derived from these mice, we observe a substantial elongation of mitochondrial networks, whereas other mitochondrial properties remain largely unaffected. Normal morphology of mitochondria could be restored by isoform-specific overexpression of P1b in P1b-deficient as well as plectin-null cells. We propose a model where P1b both forms a mitochondrial signaling platform and affects organelle shape and network formation by tethering mitochondria to IFs.

Stretch-induced activation of AMP kinase in the lung requires dystroglycan

Lung cells are exposed to cyclic stretch during normal respiration and during positive pressure mechanical ventilation administered to support gas exchange. Dystroglycan is a ubiquitously expressed matrix receptor that is required for normal basement membrane formation during embryogenesis and for maintaining the function of skeletal muscle myocytes and neurons where it links cells to matrix. We previously reported that equibiaxial stretch of primary alveolar epithelial cells activated the MAP kinase pathway ERK1/2 through a mechanism that required an interaction between dystroglycan and matrix. We determined whether this mechanism of mechanotransduction activates other signaling cascades in lung epithelium. Exposure of rat epithelial alveolar type II cells (AEC) to cyclic mechanical stretch resulted in activation of 5' AMP-activated protein kinase (AMPK). This response was not affected by pretreatment of AEC with the ERK inhibitor PD98059 but was inhibited by knockdown in dystroglycan expression. Moreover, production of reactive oxygen species was enhanced in mechanically stimulated AEC in which dystroglycan was knocked down. This enhancement was reversed by treatment of AEC with an AMPK activator. Activation of AMPK was also observed in lung homogenates from mice after 15 minutes of noninjurious mechanical ventilation. Furthermore, knockdown of dystroglycan in the lungs of mice using an adenovirus encoding a dystroglycan shRNA prevented the stretch-induced activation of AMPK. These results suggest that exposure to cyclic stretch activates the metabolic sensing pathway AMPK in the lung epithelium and supports a novel role for dystroglycan in this mechanotransduction.

Anchoring junctions as drug targets: role in contraceptive development

In multicellular organisms, cell-cell interactions are mediated in part by cell junctions, which underlie tissue architecture. Throughout spermatogenesis, for instance, preleptotene leptotene spermatocytes residing in the basal compartment of the seminiferous epithelium must traverse the blood-testis barrier to enter the adluminal compartment for continued development. At the same time, germ cells must also remain attached to Sertoli cells, and numerous studies have reported extensive restructuring at the Sertoli-Sertoli and Sertoli-germ cell interface during germ cell movement across the seminiferous epithelium. Furthermore, the proteins and signaling cascades that regulate adhesion between testicular cells have been largely delineated. These findings have unveiled a number of potential "druggable" targets that can be used to induce premature release of germ cells from the seminiferous epithelium, resulting in transient infertility. Herein, we discuss a novel approach with the aim of developing a nonhormonal male contraceptive for future human use, one that involves perturbing adhesion between Sertoli and germ cells in the testis.

Targeted nanoparticles for imaging incipient pancreatic ductal adenocarcinoma

BACKGROUND

Pancreatic ductal adenocarcinoma (PDAC) carries an extremely poor prognosis, typically presenting with metastasis at the time of diagnosis and exhibiting profound resistance to existing therapies. The development of molecular markers and imaging probes for incipient PDAC would enable earlier detection and guide the development of interventive therapies. Here we sought to identify novel molecular markers and to test their potential as targeted imaging agents.

METHODS AND FINDINGS

Here, a phage display approach was used in a mouse model of PDAC to screen for peptides that specifically bind to cell surface antigens on PDAC cells. These screens yielded a motif that distinguishes PDAC cells from normal pancreatic duct cells in vitro, which, upon proteomics analysis, identified plectin-1 as a novel biomarker of PDAC. To assess their utility for in vivo imaging, the plectin-1 targeted peptides (PTP) were conjugated to magnetofluorescent nanoparticles. In conjunction with intravital confocal microscopy and MRI, these nanoparticles enabled detection of small PDAC and precursor lesions in engineered mouse models.

CONCLUSIONS

Our approach exploited a well-defined model of PDAC, enabling rapid identification and validation of PTP. The developed specific imaging probe, along with the discovery of plectin-1 as a novel biomarker, may have clinical utility in the diagnosis and management of PDAC in humans.

Plectin regulates the signaling and trafficking of the HIV-1 co-receptor CXCR4 and plays a role in HIV-1 infection

The CXC chemokine CXCL12 and its cognate receptor CXCR4 play an important role in inflammation, human immunodeficiency virus (HIV) infection and cancer metastasis. The signal transduction and intracellular trafficking of CXCR4 are involved in these functions, but the underlying mechanisms remain incompletely understood. In the present study, we demonstrated that the CXCR4 formed a complex with the cytolinker protein plectin in a ligand-dependent manner in HEK293 cells stably expressing CXCR4. The glutathione-S-transferase (GST)-CXCR4 C-terminal fusion proteins co-precipitated with the full-length and the N-terminal fragments of plectin isoform 1 but not with the N-terminal deletion mutants of plectin isoform 1, thereby suggesting an interaction between the N-terminus of plectin and the C-terminus of CXCR4. This interaction was confirmed by confocal microscopic reconstructions showing co-distribution of these two proteins in the internal vesicles after ligand-induced internalization of CXCR4 in HEK293 cells stably expressing CXCR4. Knockdown of plectin with RNA interference (RNAi) significantly inhibited ligand-dependent CXCR4 internalization and attenuated CXCR4-mediated intracellular calcium mobilization and activation of extracellular signal regulated kinase 1/2 (ERK1/2). CXCL12-induced chemotaxis of HEK293 cells stably expressing CXCR4 and of Jurkat T cells was inhibited by the plectin RNAi. Moreover, CXCR4 tropic HIV-1 infection in MAGI (HeLa-CD4-LTR-Gal) cells was inhibited by the RNAi of plectin. Thus, plectin appears to interact with CXCR4 and plays an important role in CXCR4 signaling and trafficking and HIV-1 infection.

Muscular integrity--a matter of interlinking distinct structures via plectin

Myocytes are characterized by the presence of highly specialized cytoskeletal structures that are part of regularly spaced functional units distributed over long distances. In this chapter we discuss previously published evidence as well as novel data showing that the proper positioning and architecture of Z-disks and of sarcolemma-associated costameric structures are largely dependent on the cytolinker protein plectin and its associated intermediate filament (desmin) cytoskeleton. Deficiency in either plectin or desmin lead to muscular dystrophies of similar pathology. However, while in the absence of plectin, desmin networks collapse and form aggregates, when desmin is missing, plectin retains its typical localization. This suggests that plectin recruits and anchors desmin filaments to both Z-disks and costameres and thus is a key element for maintaining and reinforcing myocyte cytoarchitecture. We hypothesize that as an essential link of the Z-disk-costamere axis, plectin is likely to play also a crucial role in myofiber signaling.

Dual Role of alpha6beta4 integrin in epidermal tumor growth: tumor-suppressive versus tumor-promoting function

An increased expression of the integrin alpha6beta4 is correlated with a poor prognosis in patients with squamous cell carcinomas. However, little is known about the role of alpha6beta4 in the early stages of tumor development. We have isolated cells from mouse skin (mouse tumor-initiating cells [mTICs]) that are deficient in both p53 and Smad4 and carry conditional alleles of the beta4 gene (Itgb4). The mTICs display many features of multipotent epidermal stem cells and produce well-differentiated tumors after subcutaneous injection into nude mice. Deletion of Itgb4 led to enhanced tumor growth, indicating that alpha6beta4 mediates a tumor-suppressive effect. Reconstitution experiments with beta4-chimeras showed that this effect is not dependent on ligation of alpha6beta4 to laminin-5, but on the recruitment by this integrin of the cytoskeletal linker protein plectin to the plasma membrane. Depletion of plectin, like that of beta4, led to increased tumor growth. In contrast, when mTICs had been further transformed with oncogenic Ras, alpha6beta4 stimulated tumor growth, as previously observed in human squamous neoplasms. Expression of different effector-loop mutants of Ras(V12) suggests that this effect depends on a strong activation of the Erk pathway. Together, these data show that depending on the mutations involved, alpha6beta4 can either mediate an adhesion-independent tumor-suppressive effect or act as a tumor promotor.

Cytolinker cross-talk: periplakin N-terminus interacts with plectin to regulate keratin organisation and epithelial migration

Periplakin is a cytoskeletal linker protein that participates in the assembly of epidermal cell cornified envelope and regulates keratin organisation in simple epithelial cells. We have generated a stably transfected MCF-7 subclone expressing HA-tagged periplakin N-terminus to identify molecular interactions of periplakin. Co-immunoprecipitation with anti-HA antibodies and mass spectrometry identified a 500-kDa periplakin-interacting protein as plectin, another plakin family member. Plectin-periplakin interaction was confirmed by immunoblotting of complexes immunoprecipitated by either anti-HA or anti-plectin antibodies. Transient transfections of periplakin deletion constructs indicated that first 133 amino acid residues of the N-terminus are sufficient for co-localisation with plectin at MCF-7 cell borders. Immunofluorescence analysis demonstrated that periplakin and plectin isoforms 1, 1f and 1k co-localise at cell borders of MCF-7 epithelia and that plectin-1f and 1k co-localise with periplakin in suprabasal epidermis. Ablation of plectin by siRNA in HaCaT keratinocytes resulted in aggregation of periplakin to small clusters. Scratch-wounded MCF-7 epithelia expressing periplakin N-terminus showed accelerated keratin re-organisation that was inhibited by siRNA knock-down of plectin. Finally, ablation of either periplakin or plectin, or both proteins simultaneously, impaired migration of MCF-7 epithelial sheets. Thus, we have identified a novel functional co-localisation between two plakin cytolinker proteins.

AMP-activated protein kinase and chemotransduction in the carotid body

AMP-activated protein kinase (AMPK) is a key component of a kinase cascade that regulates energy balance at the cellular level. Our recent research has raised the possibility that AMPK may also function to couple hypoxic inhibition of mitochondrial oxidative phosphorylation to O(2)-sensitive K(+) channel inhibition and hence underpin carotid body type I cell excitation. Thus, in addition to maintaining the cellular energy state AMPK may act as the primary metabolic sensor and effector of hypoxic chemotransduction in type I cells. These findings provide a unifying link between two previously separate theories pertaining to O(2)-sensing in the carotid body, namely the 'membrane hypothesis' and the 'mitochondrial hypothesis'. Furthermore, our data suggest that in addition to its effects at the cellular level the AMPK signalling cascade can mediate vital physiological mechanisms essential for meeting the metabolic needs of the whole organism.