The cellular microenvironment and cytoskeletal actin dynamics in liver fibrogenesis

Hepatic stellate cells(HSCs)are the primary effector cells in liver fibrosis.In the normal liver,HSCs serve as the primary vitamin A storage cells in the body and retain a“quiescent”phenotype.However,after liver injury,they transdifferentiate to an“activated”myofibroblast-like phenotype,which is associated with dramatic upregulation of smooth muscle specific actin and extracellular matrix proteins.The result is a fibrotic,stiff,and dysfunctional liver.Therefore,understanding the molecular mechanisms that govern HSC function is essential for the development of anti-fibrotic medications.The actin cytoskeleton has emerged as a key component of the fibrogenic response in wound healing.Recent data indicate that the cytoskeleton receives signals from the cellular microenvironment and translates them to cellular function—in particular,increased type I collagen expression.Dynamic in nature,the actin cytoskeleton continuously polymerizes and depolymerizes in response to changes in the cellular microenvironment.In this viewpoint,we discuss the recent developments underlying cytoskeletal actin dynamics in liver fibrosis,including how the cellular microenvironment affects HSC function and the molecular mechanisms that regulate the actininduced increase in collagen expression typical of activated HSCs.

Dynamics of perinuclear actin ring regulating nuclear morphology

Cells are capable of sensing and responding to the extracellular mechanical microenvironment via the actin skeleton.In vivo,tissues are frequently subject to mechanical forces,such as the rapid and significant shear flow encountered by vascular endothelial cells.However,the investigations about the transient response of intracellular actin networks under these intense external mechanical forces,their intrinsic mechanisms,and potential implications are very limited.Here,we observe that when cells are subject to the shear flow,an actin ring structure could be rapidly assembled at the periphery of the nucleus.To gain insights into the mechanism underlying this perinuclear actin ring assembly,we develop a computational model of actin dynamics.We demonstrate that this perinuclear actin ring assembly is triggered by the depolymerization of cortical actin,Arp2/3-dependent actin filament polymerization,and myosin-mediated actin network contraction.Furthermore,we discover that the compressive stress generated by the perinuclear actin ring could lead to a reduction in the nuclear spreading area,an increase in the nuclear height,and a decrease in the nuclear volume.The present model thus explains the mechanism of the perinuclear actin ring assembly under external mechanical forces and suggests that the spontaneous contraction of this actin structure can significantly impact nuclear morphology.

Regulation of Actin Dynamics in Pollen Tubes:Control of Actin Polymer Level

Actin cytoskeleton undergoes rapid reorganization in response to internal and external cues. How the dynamics of actin cytoskeleton are regulated, and how its dynamics relate to its function are fundamental questions in plant cell biology. The pollen tube is a well characterized actin-based cell morphogenesis in plants. One of the striking features of actin cytoskeleton characterized in the pollen tube is its surprisingly low level of actin polymer. This special phenomenon might relate to the function of actin cytoskeleton in pollen tubes. Understanding the molecular mechanism underlying this special phenomenon requires careful analysis of actin-binding proteins that modulate actin dynamics directly. Recent biochemical and biophysical analyses of several highly conserved plant actin-binding proteins reveal unusual and unexpected properties, which emphasizes the importance of carefully analyzing their action mechanism and cellular activity. In this review, we highlight an actin monomer sequestering protein, a barbed end capping protein and an F-actin severing and dynamizing protein in plant. We propose that these proteins function in harmony to regulate actin dynamics and maintain the low level of actin polymer in pollen tubes.

Actin Dynamics Regulates Voltage-Dependent Calcium-Permeable Channels of the Vicia faba Guard Cell Plasma Membrane

Free cytosolic Ca^2＋ （[Ca^2＋]cyt） is an ubiquitous second messenger in plant cell signaling, and [Ca^2＋]cyt elevation is associated with Ca^2＋-permeable channels in the plasma membrane and endomembranes regulated by a wide range of stimuli. However, knowledge regarding Ca^2＋ channels and their regulation remains limited in planta. A type of voltage- dependent Ca^2＋-permeable channel was identified and characterized for the Vicia faba L. guard cell plasma membrane by using patch-clamp techniques. These channels are permeable to both Ba^2＋ and Ca^2＋, and their activities can be inhibited by micromolar Gd^3＋. The unitary conductance and the reversal potential of the channels depend on the Ca^2＋ or Ba^2＋ gradients across the plasma membrane. The inward whole-cell Ca^2＋ （Ba^2＋） current, as well as the unitary current amplitude and NPo of the single Ca^2＋ channel, increase along with the membrane hyperpolarization. Pharmacological experiments suggest that actin dynamics may serve as an upstream regulator of this type of calcium channel of the guard cell plasma membrane. Cytochalasin D, an actin polymerization blocker, activated the NPo of these channels at the single channel level and increased the current amplitude at the whole-cell level. But these channel activations and current increments could be restrained by pretreatment with an F-actin stabilizer, phalloidin. The potential physiological significance of this regulatory mechanism is also discussed.

14-3-3 λ protein interacts with ADF1 to regulate actin cytoskeleton dynamics in Arabidopsis

Actin cytoskeleton dynamics is critical for variety of cellular events including cell elongation, division and morphogenesis, and is tightly regulated by numerous groups of actin binding proteins. However it is not well understood how these actin binding proteins are modulated in a physiological condition by their interaction proteins. In this study, we describe that Arabidopsis 14-3-3 λ protein interacted with actin depolymerizing factor 1(ADF1) in plant to regulate F-actin stability and dynamics. Loss of 14-3-3 λin Arabidopsis resulted in longer etiolated hypocotyls in dark and changed actin cytoskeleton architecture in hypocotyl cells. Overexpression of ADF1 repressed 14-3-3 λ mutant hypocotyl elongation and actin dynamic phenotype. In addition, the phosphorylation level of ADF1 was increased and the subcellular localization of ADF1 was altered in 14-3-3 λ mutant. Consistent with these observations, the actin filaments were more stable in 14-3-3 λ mutant. Our results indicate that 14-3-3 λ protein mediates F-actin dynamics possibly through inhibiting ADF1 phosphorylation in vivo.

Organizational Innovation of Apical Actin Filaments Drives Rapid Pollen Tube Growth and Turning

Polarized tip growth is a fundamental cellular process in many eukaryotes. In this study, we examined the dynamic restructuring of the actin cytoskeleton and its relationship to vesicle transport during pollen tip growth in Arabidopsis. We found that actin filaments originating from the apical membrane form a specialized structure consisting of longitudinally aligned actin bundles at the cortex and inner cytoplasmic fila- ments with a distinct distribution. Using actin-based pharmacological treatments and genetic mutants in combination with FRAP （fluorescence recovery after photobleaching） technology to visualize the transport of vesicles within the growth domain of pollen tubes, we demonstrated that cortical actin filaments facilitate tip-ward vesicle transport. We also discovered that the inner apical actin filaments prevent backward movement of vesicles, thus ensuring that sufficient vesicles accumulate at the pollen tube tip to support the rapid growth of the pollen tube. The combinatorial effect of cortical and internal apical actin filaments perfectly explains the generation of the inverted ＂V＂ cone-shaped vesicle distribution pattern at the pollen tube tip. When pollen tubes turn, apical actin filaments at the facing side undergo depolymerization and repolymerization to reorient the apical actin structure toward the new growth direction. This actin restructuring precedes vesicle accumulation and changes in tube morphology. Thus, our study provides new insights into the functional relationship between actin dynamics and vesicle transport during rapid and directional pollen tube growth.