Involvement of myosin II in dopamine-induced reorganization of the lactotroph cell's actin cytoskeleton

Characterization of Stimulatory Action on Voltage-Gated Na+ Currents Caused by Omecamtiv Mecarbil, Known to Be a Myosin Activator

Omecamtiv mecarbil (OM, CK-1827452) is recognized as an activator of myosin and has been demonstrated to be beneficial for the treatment of systolic heart failure. However, the mechanisms by which this compound interacts with ionic currents in electrically excitable cells remain largely unknown. The objective of this study was to investigate the effects of OM on ionic currents in GH3 pituitary cells and Neuro-2a neuroblastoma cells. In GH₃ cells, whole-cell current recordings showed that the addition of OM had different potencies in stimulating the transient (I_Na(T)) and late components (I_Na(L)) of the voltage-gated Na⁺ current (I_Na) with different potencies in GH₃ cells. The EC₅₀ value required to observe the stimulatory effect of this compound on I_Na(T) or I_Na(L) in GH₃ cells was found to be 15.8 and 2.3 µM, respectively. Exposure to OM did not affect the current versus voltage relationship of I_Na(T). However, the steady-state inactivation curve of the current was observed to shift towards a depolarized potential of approximately 11 mV, with no changes in the slope factor of the curve. The addition of OM resulted in an increase in the decaying time constant during the cumulative inhibition of I_Na(T) in response to pulse-train depolarizing stimuli. Furthermore, the presence of OM led to a shortening of the recovery time constant in the slow inactivation of I_Na(T). Adding OM also resulted in an augmentation of the strength of the window Na⁺ current, which was evoked by a short ascending ramp voltage. However, the OM exposure had little to no effect on the magnitude of L-type Ca²⁺ currents in GH₃ cells. On the other hand, the delayed-rectifier K⁺ currents in GH₃ cells were observed to be mildly suppressed in its presence. Neuro-2a cells also showed a susceptibility to the differential stimulation of I_Na(T) or I_Na(L) upon the addition of OM. Molecular analysis revealed potential interactions between the OM molecule and hNa_V1.7 channels. Overall, the direct stimulation of I_Na(T) and I_Na(L) by OM is assumed to not be mediated by an interaction with myosin, and this has potential implications for its pharmacological or therapeutic actions occurring in vivo.

Myosin motor proteins are involved in the final stages of the secretory pathways

In eukaryotes, the final steps in both the regulated and constitutive secretory pathways can be divided into four distinct stages: (i) the 'approach' of secretory vesicles/granules to the PM (plasma membrane), (ii) the 'docking' of these vesicles/granules at the membrane itself, (iii) the 'priming' of the secretory vesicles/granules for the fusion process, and, finally, (iv) the 'fusion' of vesicular/granular membranes with the PM to permit content release from the cell. Recent work indicates that non-muscle myosin II and the unconventional myosin motor proteins in classes 1c/1e, Va and VI are specifically involved in these final stages of secretion. In the present review, we examine the roles of these myosins in these stages of the secretory pathway and the implications of their roles for an enhanced understanding of secretion in general.

Myosin IIB deficiency in embryonic fibroblasts affects regulators and core members of the par polarity complex

Wild-type (WT) and myosin heavy chain IIB null [MHCIIB (-/-)] embryonic fibroblasts were used as an experimental model to assess the role of the isoform B of myosin II (MII) in the regulation of the cell shape and intrinsic polarity. Genetic ablation of MHCIIB causes a persistent albeit, unstable protrusive activity in embryonic fibroblasts (Lo et al. in Nonmuscle myosin IIB is involved in the guidance of fibroblast migration. Mol Biol Cell 15:982-989, 2004). Here, we show that MHCIIB-deficient fibroblasts are characterized by a sustained guanine nucleotide exchange factor (GEF)-dependent activation of the small GTPase Rac-1 that is responsible for the continual lamellipodium formation. Moreover, we observed a sustained PKC-ζ activation and an increased association of cortactin with the plasma membrane in the MHCIIB (-/-) cells that were also dependent on GEF-mediated Rac-1 activation. Rac-1 activation and its downstream effects were induced in WT fibroblasts by inhibiting MII ATPase and crosslinking activities, suggesting that an altered actin-MII interaction favours Rac-1 activation, regardless of the MII isoform implicated. In addition, we found MIIB isoform-specific effects that were independent of Rac-1 activation. MHCIIA interacts with cortactin whereas MHCIIB does not. By contrast, MHCIIB interacts with Lgl1, a member of the Scribble/Dlg/Lgl polarity complex, whereas MHCIIA does not. MHCIIB (-/-) fibroblasts exhibited deregulated endogenous levels of the Par polarity complex members, Par3 and Par6. Together, the data show that MHCIIB deficiency causes imbalances in signalling pathways that are responsible for cell polarity determination. The results suggest that these pathways are targets of MIIB in the regulation of the cell's shape and polarity.

Rapid dopaminergic modulation of the fish hypothalamic transcriptome and proteome

Background

Dopamine (DA) is a major neurotransmitter playing an important role in the regulation of vertebrate reproduction. We developed a novel method for the comparison of transcriptomic and proteomic data obtained from in vivo experiments designed to study the neuroendocrine actions of DA.

Methods and Findings

Female goldfish were injected (i.p.) with DA agonists (D1-specific; SKF 38393, or D2-specific; LY 171555) and sacrificed after 5 h. Serum LH levels were reduced by 57% and 75% by SKF 38393 and LY 171555, respectively, indicating that the treatments produced physiologically relevant responses in vivo. Bioinformatic strategies and a ray-finned fish database were established for microarray and iTRAQ proteomic analysis of the hypothalamus, revealing a total of 3088 mRNAs and 42 proteins as being differentially regulated by the treatments. Twenty one proteins and mRNAs corresponding to these proteins appeared on both lists. Many of the mRNAs and proteins affected by the treatments were grouped into the Gene Ontology categorizations of protein complex, signal transduction, response to stimulus, and regulation of cellular processes. There was a 57% and 14% directional agreement between the differentially-regulated mRNAs and proteins for SKF 38393 and LY 171555, respectively.

Conclusions

The results demonstrate the applicability of advanced high-throughput genomic and proteomic analyses in an amendable well-studied teleost model species whose genome has yet to be sequenced. We demonstrate that DA rapidly regulates multiple hypothalamic pathways and processes that are also known to be involved in pathologies of the central nervous system.

Isoform B of myosin II heavy chain mediates actomyosin contractility during TNFalpha-induced apoptosis

Cells that are treated long-term with TNFalpha or short-term with TGFalpha together with cycloheximide (CHX) undergo apoptosis. Cell shrinkage and detachment during apoptosis is dependent on actomyosin contractility. Myosin II heavy chain (MHCII) isoforms have shared and distinct functions. Here, we investigated whether the involvement of MHCII isoforms A and B (MHCIIA and MHCIIB, respectively) in cell shrinkage and detachment differs during apoptosis. We show that TNFalpha induces caspase-dependent MHCIIA degradation, whereas MHCIIB levels and association with the cytoskeleton remained virtually unchanged in TtT/GF cells and NIH 3T3 fibroblasts. MHCIIA proteolysis also occurred in fibroblasts that lack MHCIIB when treated with TNFalpha and CHX together. The absence of MHCIIB did not affect cell death rate. However, MHCIIB-/- cells showed more resistance to TNFalpha-induced actin disassembly, cell shrinkage and detachment than wild-type fibroblasts, indicating the participation of MHCIIB in these events. Moreover, inhibition of atypical PKCzeta, which targets MHCIIB but not MHCIIA, blocked TNFalpha-induced shrinkage and detachment in TtT/GF cells and wild-type fibroblasts, but the inhibitory effect was significantly reduced in MHCIIB-/- fibroblasts. TNFalpha treatment increased cytoskeleton-associated myosin light chain (MLC) phosphorylation but did not induce actin cleavage. In conclusion, our results demonstrate that MHCIIB, together with MLC phosphorylation and actin, constitute the actomyosin cytoskeleton that mediates contractility during apoptosis.

Role of myosin regulatory light chain and Rac1 in the migration of polyamine-depleted intestinal epithelial cells

We have previously shown that polyamine depletion decreased migration, Rac activation, and protein serine threonine phosphatase 2A activity. We have also shown that polyamine depletion increased cortical F-actin and decreased lamellipodia and stress fibers. In this study, we used staurosporine (STS), a potent, cell-permeable, and broad-spectrum serine/threonine kinase inhibitor, and studied migration. STS concentrations above 100 nM induced apoptosis. However, in polyamine-depleted cells, a lower concentration of STS (5 nM) increased attachment, spreading, Rac1 activation, and, subsequently, migration without causing apoptosis. STS-induced migration was completely prevented by a Rac1 inhibitor (NSC-23766) and dominant negative Rac1. These results imply that STS restores migration in polyamine-depleted cells through Rac1. The most important finding in this study was that polyamine depletion increased the association of phosphorylated myosin regulatory light chain (pThr(18)/Ser(19)-MRLC) at the cell periphery, which colocalized with thick cortical F-actin. Localization of pThr(18)- and pSer(19)-MRLC was found with stress fibers and nuclei, respectively. STS decreased the phosphorylation of cellular and peripheral pThr(18)-MRLC without any effect on nuclear pSer(19)-MRLC, dissolved thick cortical F-actin, and increased lamellipodia and stress fiber formation in polyamine-depleted cells. In control and polyamine-depleted cells, focal adhesion kinase (FAK) colocalized with stress fibers and the actin cortex, respectively. STS reorganized FAK, paxillin, and the cytoskeleton. These results suggest that polyamine depletion prevents the dephosphorylation of MRLC and thereby prevents the dynamic reorganization of the actin cytoskeleton and decreases lamellipodia formation resulting in the inhibition of migration.

Localization of myosin and actin in the pelage and whisker hair follicles of rat

The combined effects of myosin II and actin enable muscle and nonmuscle cells to generate forces required for muscle contraction, cell division, cell migration, cellular morphological changes, the maintenance of cellular tension and polarity, and so on. However, except for the case of muscle contraction, the details are poorly understood. We focus on nonmuscle myosin and actin in the formation and maintenance of hair and skin, which include highly active processes in mammalian life with respect to the cellular proliferation, differentiation, and movement. The localization of nonmuscle myosin II and actin in neonatal rat dorsal skin, mystacial pad, hair follicles, and vibrissal follicles was studied by immunohistochemical technique to provide the basis for the elucidation of the roles of these proteins. Specificities of the antibodies were verified by using samples from the relevant tissues and subjecting them to immunoblotting test prior to morphological analyses. The myosin and actin were abundant and colocalized in the spinous and granular layers but scarce in the basal layer of the dorsal and mystacial epidermis. In hair and vibrissal follicles, nonmuscle myosin and actin were colocalized in the outer root sheath and some hair matrix cells adjoining dermal papillae. In contrast, most areas of the inner root sheath and hair matrix appeared to comprise very small amounts of myosin and actin. Hair shaft may comprise significant myosin during the course of its keratinization. These results suggest that the actin-myosin system plays a part in cell movement, differentiation, protection and other key functions of skin and hair cells.

Myosin light chain kinase regulates synaptic plasticity and fear learning in the lateral amygdala

Learning and memory depend on signaling molecules that affect synaptic efficacy. The cytoskeleton has been implicated in regulating synaptic transmission but its role in learning and memory is poorly understood. Fear learning depends on plasticity in the lateral nucleus of the amygdala. We therefore examined whether the cytoskeletal-regulatory protein, myosin light chain kinase, might contribute to fear learning in the rat lateral amygdala. Microinjection of ML-7, a specific inhibitor of myosin light chain kinase, into the lateral nucleus of the amygdala before fear conditioning, but not immediately afterward, enhanced both short-term memory and long-term memory, suggesting that myosin light chain kinase is involved specifically in memory acquisition rather than in posttraining consolidation of memory. Myosin light chain kinase inhibitor had no effect on memory retrieval. Furthermore, ML-7 had no effect on behavior when the training stimuli were presented in a non-associative manner. Anatomical studies showed that myosin light chain kinase is present in cells throughout lateral nucleus of the amygdala and is localized to dendritic shafts and spines that are postsynaptic to the projections from the auditory thalamus to lateral nucleus of the amygdala, a pathway specifically implicated in fear learning. Inhibition of myosin light chain kinase enhanced long-term potentiation, a physiological model of learning, in the auditory thalamic pathway to the lateral nucleus of the amygdala. When ML-7 was applied without associative tetanic stimulation it had no effect on synaptic responses in lateral nucleus of the amygdala. Thus, myosin light chain kinase activity in lateral nucleus of the amygdala appears to normally suppress synaptic plasticity in the circuits underlying fear learning, suggesting that myosin light chain kinase may help prevent the acquisition of irrelevant fears. Impairment of this mechanism could contribute to pathological fear learning.

Development and application of a brain-specific cDNA microarray for effect evaluation of neuro-active pharmaceuticals in zebrafish (Danio rerio)

The environmental fate and ecotoxicological effect of pharmaceuticals are poorly understood, and standardized tests to detect and evaluate their potential effects in the environment are not available. We developed a zebrafish brain-specific microarray containing 682 neurologically relevant cDNA-fragments. To investigate the applicability of this microarray for studying neurotoxic modes-of-action and impact assessment of neuro-active pharmaceuticals in zebrafish, chlorpromazine was used as a model compound. After exposure to chlorpromazine (75 microg/L) for 2, 4, 14 and 28 days or control treatment RNA was extracted from brains of males and females. Fluorescently labeled cDNA was prepared and hybridized to the custom microarray. In total, 56 genes were differentially expressed in brains of male and/or female zebrafish, of which most genes were down-regulated. A clear difference in response to chlorpromazine exposure between males and females was observed with exposure time as well as in functional classes of affected genes. The presented study is one of the first reports on molecular effects of human neuro-active pharmaceuticals in aquatic non-target organisms. This new genomic tool successfully detected gene expression effects of exposure to chlorpromazine in the brain of zebrafish. Reported gene expression effects are found to be consistent with literature data for other laboratory animals.