The spectrin skeleton of newly-invaginated plasma membrane

Rapid Reversible Osmoregulation of Cytoplasmic Biomolecular Condensates of Human Interferon-α-Induced Antiviral MxA GTPase

We previously discovered that exogenously expressed GFP-tagged cytoplasmic human myxovirus resistance protein (MxA), a major antiviral effector of Type I and III interferons (IFNs) against several RNA- and DNA-containing viruses, existed in the cytoplasm in phase-separated membraneless biomolecular condensates of varying sizes and shapes with osmotically regulated disassembly and reassembly. In this study we investigated whether cytoplasmic IFN-α-induced endogenous human MxA structures were also biomolecular condensates, displayed hypotonic osmoregulation and the mechanisms involved. Both IFN-α-induced endogenous MxA and exogenously expressed GFP-MxA formed cytoplasmic condensates in A549 lung and Huh7 hepatoma cells which rapidly disassembled within 1-2 min when cells were exposed to 1,6-hexanediol or to hypotonic buffer (~40-50 mOsm). Both reassembled into new structures within 1-2 min of shifting cells to isotonic culture medium (~330 mOsm). Strikingly, MxA condensates in cells continuously exposed to culture medium of moderate hypotonicity (in the range one-fourth, one-third or one-half isotonicity; range 90-175 mOsm) first rapidly disassembled within 1-3 min, and then, in most cells, spontaneously reassembled 7-15 min later into new structures. This spontaneous reassembly was inhibited by 2-deoxyglucose (thus, was ATP-dependent) and by dynasore (thus, required membrane internalization). Indeed, condensate reassembly was preceded by crowding of the cytosolic space by large vacuole-like dilations (VLDs) derived from internalized plasma membrane. Remarkably, the antiviral activity of GFP-MxA against vesicular stomatitis virus survived hypoosmolar disassembly and subsequent reassembly. The data highlight the exquisite osmosensitivity of MxA condensates, and the preservation of antiviral activity in the face of hypotonic stress.

Cytotoxic Swelling of Sick Excitable Cells - Impaired Ion Homeostasis and Membrane Tension Homeostasis in Muscle and Neuron

When they become simultaneously leaky to both Na⁺ and Cl^-, excitable cells are vulnerable to potentially lethal cytotoxic swelling. Swelling ensues in spite of an isosmotic milieu because the entering ions add osmolytes to the cytoplasm's high concentration of impermeant anionic osmolytes. An influx of osmotically-obliged water is unavoidable. A cell that cannot stanch at least one the leaks will succumb to death by Donnan effect. "Sick excitable cells" are those injured through ischemia, trauma, inflammation, hyperactivity, genetically-impaired membrane skeletons and other insults, all of which foster bleb-damage to regions of the plasma membrane. Nav channels resident in damaged membrane exhibit left-shifted kinetics; the corresponding Nav window conductance constitutes a Na⁺-leak. In cortical neurons, sustained depolarization to ∼-20mV elicits a sustained lethal gCl. Underlying V_rest in skeletal muscle is a constitutively active gCl; not surprisingly therefore, dystrophic muscle fibers, which are prone to bleb damage and which exhibit Nav-leak and Na⁺-overload, are prone to cytotoxic swelling. To restore viability in cytotoxically swelling neurons and muscle, the imperative of fully functional ion homeostasis is well-recognized. However, as emphasized here, in a healthy excitable cell, fully functional membrane tension homeostasis is also imperative. ATPase-pumps keep plasma membrane batteries charged, and ATPase-motor proteins maintain membrane tone. In sick excitable cells, neither condition prevails.

The invagination of excess surface area by shrinking neurons

Over most of their surface, neurons are surrounded by a narrow extracellular gap across which they make adhesive cell-cell contacts. Thus constrained, how do they regulate their geometry when osmotically perturbed? Specifically, are there any interesting consequences of local osmosis in such conditions? Using confocal imaging of shrinking neurons in culture, we observe water exiting into the cell-substratum gap. This water efflux generates a hydrostatic pressure that, at discrete (low adhesion) sites, causes the neuron's excess plasma membrane to invaginate, thus compensating for shrinkage with a pseudo-intracellular volume. To identify the minimal requirements of the process, a compartment/flux model was constructed. It comprises, essentially, a large liposome adhering in a labyrinthine fashion to a substratum. The model predicts that invaginations form at the cell-substratum interface under the influence of local osmosis, provided that adhesion across the gap is neither too tight nor too loose. Local osmosis in the central nervous system, in contrast to epithelia, is usually considered a mishap, not a physiological opportunity. We postulate, however, that local osmotic forces acting in conjunction with confined extracellular spaces could be harnessed in service of surface area, shape, and volume regulation when intense neural activity alters a neuron's osmotic balance.

Changes in mechanosensitive channel gating following mechanical stimulation in skeletal muscle myotubes from the mdx mouse

We studied the effects of membrane stretch and voltage on the gating of single mechanosensitive (MS) channels in myotubes from dystrophin-deficient mdx mice. In earlier studies of MS channels in mdx myotubes, we found a novel class of stretch-inactivated channels. In the present experiments, we used a gentle suction protocol to determine whether seal formation damaged the membrane and altered MS channel gating, since dystrophin-deficiency is known to be associated with an increased susceptibility to mechanically induced damage. In some recordings from mdx myotubes, MS channel open probability gradually increased to levels approaching unity following seal formation. In these recordings, channels remained open for the duration of the recording. In other recordings, MS channel open probability remained low after seal formation and applying weak suction evoked conventional stretch-activated gating. Applying strong suction or very positive voltages, however, caused some channels to enter a high open probability gating mode. The shift to a high open probability gating mode coincided with the appearance of stretch-inactivated gating. These findings suggested that mechanical stimulation altered the mechanical properties of the patch causing some MS channels to enter a novel gating mode. In support of this idea, stretch-activated and stretch-inactivated channels were not detected in the same membrane patch and channel inactivation occurred at lower pressures than activation (P(1/2,) = -13 and -26.5 mmHg, respectively). Other experiments showed that stretch-inactivated gating was not due to a simple loss of MS channel activity from a non-random process such as vesiculation or bleb formation: channel inactivation by suction was readily reversible, stable over tens of minutes, and followed the predictions of the binomial theorem for independent, randomly gating channels. In addition, the voltage-dependent gating of stretch-inactivated channels was similar to that of stretch-activated channels. The results show that MS channels in dystrophin-deficient muscle exist in two distinct gating modes and that mechanical stimuli cause an irreversible conversion between modes. We discuss possible mechanisms for the changes in MS channel gating in relation to the known cytoskeletal abnormalities of mdx muscle and its possible implications for the pathogenesis of Duchenne dystrophy.

Modulation of membrane traffic by mechanical stimuli

All cells experience and respond to mechanical stimuli, such as changes in plasma membrane tension, shear stress, hydrostatic pressure, and compression. This review is an examination of the changes in membrane traffic that occur in response to mechanical forces. The plasma membrane has an associated tension that modulates both exocytosis and endocytosis. As membrane tension increases, exocytosis is stimulated, which acts to decrease membrane tension. In contrast, increased membrane tension slows endocytosis, whereas decreased tension stimulates internalization. In most cases, secretion is stimulated by external mechanical stimuli. However, in some cells mechanical forces block secretion. External stimuli also enhance membrane and fluid endocytosis in several cell types. Transduction of mechanical stimuli into changes in exocytosis/endocytosis may involve the cytoskeleton, stretch-activated channels, integrins, phospholipases, tyrosine kinases, and cAMP.