<i>Rho's role in cell volume: sensing, strutting, or signaling?</i>  Focus on “Hyperosmotic stress activates Rho: differential involvement in  Rho kinase-dependent MLC phosphorylation and NKCC activation”

Eggermont, Jan

doi:10.1152/ajpcell.00214.2003

Cited by 12 publications

(6 citation statements)

References 21 publications

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“…The increased expression may be directly related to the marked enlargement in MK size after endomitosis because actin/myosin activation is strongly implicated in cell volume response, during cell swelling and shrinkage. 38,39 To rapidly adapt to the cell volume augmentation and to keep a normal shape, MKs probably need to enhance actin/myosin activity to provide more contractility. The parallel increase between actin and phospho-MLC2 suggests that actin/ myosin contractility may be needed to keep a normal cell shape during polyploidization and to impair premature PPT formation.…”

Section: Discussionmentioning

confidence: 99%

Proplatelet formation is regulated by the Rho/ROCK pathway

Chang

Aurade

Larbret

et al. 2007

Blood

156

184

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Platelets are released by megakaryocytes (MKs) via cytoplasmic extensions called proplatelets, which require profound changes in the microtubule and actin organization. Here, we provide evidence that the Rho/ROCK pathway, a well-known regulator of actin cytoskeleton, acts as a negative regulator of proplatelet formation (PPF). Rho is expressed at a high level during the entire MK differentiation including human CD34 ؉ cells. Thrombopoietin stimulates its activity but at a higher extent in immature than in mature MKs. Overexpression of a dominantnegative or a spontaneously active RhoA leads to an increase or a decrease in PPF indicating that Rho activation inhibits PPF. This inhibitory effect is mediated through the main Rho effector, Rho kinase (ROCK), the inhibition of which also increases PPF. Furthermore, inhibition of Rho or ROCK in MKs leads to a decrease in myosin light chain 2 (MLC2) phosphorylation, which is required for myosin contractility. Interestingly, inhibition of the MLC kinase also decreases MLC2 phosphorylation while increasing PPF. Taken together, our results suggest that MLC2 phosphorylation is regulated by both ROCK and MLC kinase and plays an important role in platelet biogenesis by controlling PPF and fragmentation. IntroductionMegakaryocytes (MKs) are the highly specialized precursor cells that lead to platelet production. MK differentiation is a continuous process characterized by sequential steps. 1 First, MKs increase their ploidy via endomitosis and begin to increase their size. 2 Then, the synthesis of storage organelles is enhanced, as well as the synthesis of plasma membrane to form the demarcation membranes. This cytoplasmic maturation is associated with a marked increase in the MK size. Finally, mature MKs release platelets probably through cytoplasmic fragmentation at the tips of long and thin extensions called proplatelets (PPTs) that contain all the platelet organelles. 3,4 The mechanisms controlling proplatelet formation (PPF) are still incompletely understood. However, PPF is associated with remarkable morphologic changes that require a profound reorganization of the cytoskeleton. 5 Increasing evidence indicates that PPTs arise from the unfolding of demarcation membranes. The microtubule cytoskeleton provides the sliding power to unfold demarcation membranes and thus to induce the pseudopodial elongations corresponding to PPTs. 6 In addition, microtubules permit the organelle transport in the PPTs and maintain the platelet discoid shape. [7][8][9][10] Although not studied in detail, the actin cytoskeleton may also participate in PPF because cytoplasmic polymerized actin is associated with demarcation membranes and actin is highly aggregated in cultured MKs when PPF occurs. 11 In addition, a crucial role of the actin cytoskeleton has been reported in platelet functions since it regulates platelet shape in unstimulated and activated platelets. 12 Evidence suggests that actin cytoskeleton may play important roles during PPT formation at 2 different stages: (1) at early stages,...

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Section: Discussionmentioning

confidence: 99%

Proplatelet formation is regulated by the Rho/ROCK pathway

Chang

Aurade

Larbret

et al. 2007

Blood

156

184

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“…Since Rho is activated by hypertonic stress (41,42), we examined the possibility that Rho kinase activation promotes PIP5KI␤ dephosphorylation. This is not the case because the Rho kinase inhibitor, Y27843, did not block PIP5KI␤ dephosphorylation (Fig.…”

Section: Treatmentmentioning

confidence: 99%

Hypertonic Stress Increases Phosphatidylinositol 4,5-Bisphosphate Levels by Activating PIP5KIβ

Yamamoto¹,

Chen²,

Wang

et al. 2006

Journal of Biological Chemistry

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2 and actin cytoskeletal responses. PIP5KI␤ was recruited to membranes and was activated by hypertonic stress through Ser/ Thr dephosphorylation. Calyculin A, a protein phosphatase 1 inhibitor, blocked the hypertonicity-induced PIP5KI␤ dephosphorylation/activation as well as PIP 2 increase in cells. Urea, which raises osmolarity without inducing cell shrinkage, did not promote dephosphorylation nor increase PIP 2 levels. Disruption or stabilization of the actin cytoskeleton, or inhibition of the Rho kinase, did not block the PIP 2 increase nor PIP5KI␤ dephosphorylation. Therefore, PIP5KI␤ is dephosphorylated in a volume-dependent manner by a calyculin A-sensitive protein phosphatase, which is activated upstream of actin remodeling and independently of Rho kinase activation. Our results establish a cause-and-effect relation between PIP5KI␤ dephosphorylation, lipid kinase activation, and PIP 2 increase in cells. This PIP 2 increase can orchestrate multiple downstream responses, including the reorganization of the actin cytoskeleton.All cells experience fluctuations in osmolarity. Unicellular organisms and plants continuously confront osmotic challenges in their environment. In higher animals, the kidney and the gastrointestinal system are routinely exposed to severe osmotic fluctuation, while the majority of cells in other organs are protected from large tonicity changes. Nevertheless, these other organs are also confronted with transient osmolarity variations due to changes in the transmembrane transport of solutes or shifts in the balance between low molecular weight precursors and their macromolecular products. Recently, there has been a renewed interest in understanding the mechanism of hypertonic response in the clinical arena (1), due to the discovery that treatments using hypertonic resuscitation in experimental models of trauma, hemorrhagic shock, sepsis, and burn injury are more beneficial than conventional isotonic resuscitation (2, 3). While the fundamental mechanism for such protection is not completely understood, the actin cytoskeleton, which is reorganized during hypertonic stress, has been implicated (3, 4).Actin remodeling as well as many of the other hyperosmotic responses are evolutionary conserved. These include large shifts in phosphoinositide metabolism, activation of the mitogen-activated protein and tyrosine kinase pathways, volume regulation and the reprogramming of gene transcription (3, 5). Phosphatidylinositol 4-phosphate (PI4P) 4 and phosphatidylinositol 4,5-bisphosphate (PIP 2 ) levels increase dramatically in mammalian cardiac muscle and tissue culture cells that were exposed to hypertonic sucrose or NaCl (6). Other phosphoinositides, including phosphatidylinositol 3,5-bisphosphate (7, 8), phosphatidylinositol 3,4-bisphosphate, and phosphatidylinositol 3,4,5-trisphosphate are increased in some other types of cells as well (9).Hyperosmotic stress acutely induces cell shrinkage, which is subsequently corrected by volume regulation. The response cascade can be classified into four mecha...

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“…A multitude of cellular signal transduction components, which are activated/inactivated by hypoosmotic cell swelling and hyperosmotic cell shrinkage, respectively, has been characterized (Eggermont, 2003;Lambert, 2003;Weiergr€ aber and H€ aussinger, 2000;Gual et al, 2002;Reinehr et al, 2002a;Feranchak et al, 2003;H€ aussinger et al, 2003;Sanguinetti et al, 2003;vom Dahl et al, 2003). There is also growing knowledge about the integration of ''osmosignalling'' in the overall context of signal transduction, which is activated by hormones (e.g.…”

Section: Osmosensing and Osmosignallingmentioning

confidence: 99%