Inhibition of G protein-coupled receptor trafficking in neuroblastoma cells by MAP 4 decoration of microtubules

MAP4 mechanism that stabilizes mitochondrial permeability transition in hypoxia: microtubule enhancement and DYNLT1 interaction with VDAC1

Mitochondrial membrane permeability has received considerable attention recently because of its key role in apoptosis and necrosis induced by physiological events such as hypoxia. The manner in which mitochondria interact with other molecules to regulate mitochondrial permeability and cell destiny remains elusive. Previously we verified that hypoxia-induced phosphorylation of microtubule-associated protein 4 (MAP4) could lead to microtubules (MTs) disruption. In this study, we established the hypoxic (1% O₂) cell models of rat cardiomyocytes, H9c2 and HeLa cells to further test MAP4 function. We demonstrated that increase in the pool of MAP4 could promote the stabilization of MT networks by increasing the synthesis and polymerization of tubulin in hypoxia. Results showed MAP4 overexpression could enhance cell viability and ATP content under hypoxic conditions. Subsequently we employed a yeast two-hybrid system to tag a protein interacting with mitochondria, dynein light chain Tctex-type 1 (DYNLT1), by hVDAC1 bait. We confirmed that DYNLT1 had protein-protein interactions with voltage-dependent anion channel 1 (VDAC1) using co-immunoprecipitation; and immunofluorescence technique showed that DYNLT1 was closely associated with MTs and VDAC1. Furthermore, DYNLT1 interactions with MAP4 were explored using a knockdown technique. We thus propose two possible mechanisms triggered by MAP4: (1) stabilization of MT networks, (2) DYNLT1 modulation, which is connected with VDAC1, and inhibition of hypoxia-induced mitochondrial permeabilization.

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.

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

In severe pressure overload-induced cardiac hypertrophy, a dense, stabilized microtubule network forms that interferes with cardiocyte contraction and microtubule-based transport. This is associated with persistent transcriptional up-regulation of cardiac alpha- and beta-tubulin and microtubule-stabilizing microtubule-associated protein 4 (MAP4). There is also extensive microtubule decoration by MAP4, suggesting greater MAP4 affinity for microtubules. Because the major determinant of this affinity is site-specific MAP4 dephosphorylation, we characterized this in hypertrophied myocardium and then assessed the functional significance of each dephosphorylation site found by mimicking it in normal cardiocytes. We first isolated MAP4 from normal and pressure overload-hypertrophied feline myocardium; volume-overloaded myocardium, which has an equal degree and duration of hypertrophy but normal functional and cytoskeletal properties, served as a control for any nonspecific growth-related effects. After cloning cDNA-encoding feline MAP4 and obtaining its deduced amino acid sequence, we characterized by mass spectrometry any site-specific MAP4 dephosphorylation. Solely in pressure overload-hypertrophied myocardium, we identified striking MAP4 dephosphorylation at Ser-472 in the MAP4 N-terminal projection domain and at Ser-924 and Ser-1056 in the assembly-promoting region of the C-terminal microtubule-binding domain. Site-directed mutagenesis of MAP4 cDNA was then used to switch each serine to non-phosphorylatable alanine. Wild-type and mutated cDNAs were used to construct adenoviruses; microtubule network density, stability, and MAP4 decoration were assessed in normal cardiocytes following an equivalent level of MAP4 expression. The Ser-924 --> Ala MAP4 mutant produced a microtubule phenotype indistinguishable from that seen in pressure overload hypertrophy, such that Ser-924 MAP4 dephosphorylation during pressure overload hypertrophy may be central to this cytoskeletal abnormality.

Microtubule-dependent distribution of mRNA in adult cardiocytes

Synthesis of myofibrillar proteins in the diffusion-restricted adult cardiocyte requires microtubule-based active transport of mRNAs as part of messenger ribonucleoprotein particles (mRNPs) to translation sites adjacent to nascent myofibrils. This is especially important for compensatory hypertrophy in response to hemodynamic overloading. The hypothesis tested here is that excessive microtubule decoration by microtubule-associated protein 4 (MAP4) after cardiac pressure overloading could disrupt mRNP transport and thus hypertrophic growth. MAP4-overexpressing and pressure-overload hypertrophied adult feline cardiocytes were infected with an adenovirus encoding zipcode-binding protein 1-enhanced yellow fluorescent protein fusion protein, which is incorporated into mRNPs, to allow imaging of these particles. Speed and distance of particle movement were measured via time-lapse microscopy. Microtubule depolymerization was used to study microtubule-based transport and distribution of mRNPs. Protein synthesis was assessed as radioautographic incorporation of [3H]phenylalanine. After microtubule depolymerization, mRNPs persist only perinuclearly and apparent mRNP production and protein synthesis decrease. Reestablishing microtubules restores mRNP production and transport as well as protein synthesis. MAP4 overdecoration of microtubules via adenovirus infection in vitro or following pressure overloading in vivo reduces the speed and average distance of mRNP movement. Thus cardiocyte microtubules are required for mRNP transport and structural protein synthesis, and MAP4 decoration of microtubules, whether directly imposed or accompanying pressure-overload hypertrophy, causes disruption of mRNP transport and protein synthesis. The dense, highly MAP4-decorated microtubule network seen in severe pressure-overload hypertrophy both may cause contractile dysfunction and, perhaps even more importantly, may prevent a fully compensatory growth response to hemodynamic overloading.

Cytoskeletal networks and the regulation of cardiac contractility: microtubules, hypertrophy, and cardiac dysfunction

The cytoskeleton as classically defined for eukaryotic cells consists of three systems of protein filaments: the microtubules, the intermediate filaments, and the microfilaments. In mature striated muscle such as the heart of the adult mammal, these three types of cytoskeletal filaments are superimposed spatially on the myofilaments, a specialized system of contractile protein filaments. Each of these systems of protein filaments has the potential to respond in an adaptive or maladaptive manner during load-induced hypertrophic cardiac growth. However, the extent to which such hypertrophy is compensatory is also critically dependent on the type of hemodynamic overload that serves as the hypertrophic stimulus. Thus cardiac hypertrophy is not intrinsically maladaptive; rather, it is the nature of the inducing load rather than hypertrophy itself that is responsible, through effects on structural and/or regulatory proteins, for the frequent deterioration of initially compensatory hypertrophy into the congestive heart failure state. As one example reviewed here of this load specificity of maladaptation, increased microtubule network density is a persistent feature of severely pressure-overloaded, hypertrophied, and failing myocardium that imposes a primarily viscous load on active myofilaments during contraction.

Inhibition of beta-adrenergic receptor trafficking in adult cardiocytes by MAP4 decoration of microtubules

Decreased beta-adrenergic receptor (beta-AR) number occurs both in animal models of cardiac hypertrophy and failure and in patients. beta-AR recycling is an important mechanism for the beta-AR resensitization that maintains a normal complement of cell surface beta-ARs. We have shown that 1) in severe pressure overload cardiac hypertrophy, there is extensive microtubule-associated protein 4 (MAP4) decoration of a dense microtubule network; and 2) MAP4 microtubule decoration inhibits muscarinic acetylcholine receptor recycling in neuroblastoma cells. We asked here whether MAP4 microtubule decoration inhibits beta-AR recycling in adult cardiocytes. [(3)H]CGP-12177 was used as a beta-AR ligand, and feline cardiocytes were isolated and infected with adenovirus containing MAP4 (AdMAP4) or beta-galactosidase (Adbeta-gal) cDNA. MAP4 decorated the microtubules extensively only in AdMAP4 cardiocytes. beta-AR agonist exposure reduced cell surface beta-AR number comparably in AdMAP4 and Adbeta-gal cardiocytes; however, after agonist withdrawal, the cell surface beta-AR number recovered to 78.4 +/- 2.9% of the pretreatment value in Adbeta-gal cardiocytes but only to 56.8 +/- 1.4% in AdMAP4 cardiocytes (P < 0.01). This result was confirmed in cardiocytes isolated from transgenic mice having cardiac-restricted MAP4 overexpression. In functional terms of cAMP generation, beta-AR agonist responsiveness of AdMAP4 cells was 47% less than that of Adbeta-gal cells. We conclude that MAP4 microtubule decoration interferes with beta-AR recycling and that this may be one mechanism for beta-AR downregulation in heart failure.

Rho Kinase Mediates Cold-Induced Constriction of Cutaneous Arteries

Cold-induced vasoconstriction in cutaneous blood vessels is mediated in part by increased activity of vascular smooth muscle α2-adrenoceptors (VSM α2-ARs). In mouse cutaneous arteries, α2C-ARs are normally silent at 37°C but mediate cold-induced augmentation of α2-AR responsiveness. In transfected HEK293 cells, this functional rescue is mediated by cold-induced translocation of α2C-ARs from the Golgi to the plasma membrane. Experiments were performed to determine the role of Rho/Rho kinase signaling in this process. Inhibition of Rho kinase (fasudil, Y27632 or H-1152) did not affect constriction of isolated mouse tail arteries to the α2-AR agonist UK 14 304 at 37°C but dramatically reduced the augmented responses to the agonist at 28°C. After Rho kinase inhibition, cooling no longer increased constriction evoked by α2-AR stimulation. Cooling (to 28°C) activated Rho in VSM cells and increased the calcium sensitivity of constriction in α toxin-permeabilized arteries. Stimulation of α2-ARs in VSM cells had no effect on Rho activity or calcium sensitivity at 37°C or 28°C. In HEK293 cells transfected with α2C-ARs, cooling (to 28°C) stimulated the translocation of α2C-ARs to the plasma membrane and this effect was prevented by inhibition of Rho kinase, using fasudil or RNA interference. Consistent with inhibition of the spatial rescue of α2C-ARs, fasudil inhibited α2-AR–mediated mobilization of calcium in tail arteries at 28°C but not 37°C. Therefore, cold-induced activation of Rho/Rho kinase can mediate cold-induced constriction in cutaneous arteries by enabling translocation of α2C-ARs to the plasma membrane and by increasing the calcium sensitivity of the contractile process.