Responses of neurons to extreme osmomechanical stress

Evaluating the Monro-Kellie Doctrine: Contralateral Hemisphere Shrinkage in Intracerebral Hemorrhage Patients

Intracerebral hemorrhage (ICH) along with aggravating factors, such as edema, can raise intracranial pressure (ICP) to pathological levels. Diversion of some cerebrospinal fluid (CSF) and venous blood out of the cranium can limit ICP rises while maintaining cerebral perfusion pressure. Brain tissue itself is widely considered immutable in volume but prone to distortion (e.g., midline shift). However, distal brain regions shrink acutely following ICH in rodents. Tissue contraction arises from cell shrinkage and increased packing density. This "tissue compliance" is hypothesized to be an additional mechanism to limit ICP rises. Here, we examined whether and by how much parenchyma volume reduction occurs in ICH patients. We conducted a retrospective analysis on computed tomography (CT) scans of 96 ICH patients (average age 63.8 years old, 55% male) with an average hematoma volume of 32.4 and 35.3 mL at the first and second scan (separated by ~ 23 h), respectively. Hematoma growth (any absolute increase) occurred in 44% of patients, with a minimal but significant growth of the hematoma of 2.9 mL on average across all patients (p = 0.028). As hypothesized, the contralateral hemisphere volume was significantly reduced by 12.7 mL (p < 0.0001) between scans. This was unrelated to midline shift (R2 = 0.012, p = 0.21), which averaged 2.3 mm. These findings suggest that distal parenchymal shrinkage may be a major compliance mechanism after ICH; the implications for ICP and brain function merit further study.

Active shrinkage protects neurons following axonal transection

Trauma, vascular events, or neurodegenerative processes can lead to axonal injury and eventual transection (axotomy). Neurons can survive axotomy, yet the underlying mechanisms are not fully understood. Excessive water entry into injured neurons poses a particular risk due to swelling and subsequent death. Using in vitro and in vivo neurotrauma model systems based on laser transection and surgical nerve cut, we demonstrated that axotomy triggers actomyosin contraction coupled with calpain activity. As a consequence, neurons shrink acutely to force water out through aquaporin channels preventing swelling and bursting. Inhibiting shrinkage increased the probability of neuronal cell death by about 3-fold. These studies reveal a previously unrecognized cytoprotective response mechanism to neurotrauma and offer a fresh perspective on pathophysiological processes in the nervous system.

Intracranial pressure for clinicians: it is not just a number

BACKGROUND

Invasive intracranial pressure (ICP) monitoring is a standard practice in severe brain injury cases, where it allows to derive cerebral perfusion pressure (CPP); ICP-tracing can also provide additional information about intracranial dynamics, forecast episodes of intracranial hypertension and set targets for a tailored therapy to prevent secondary brain injury. Nevertheless, controversies about the advantages of an ICP clinical management are still debated.

FINDINGS

This article reviews recent research on ICP to improve the understanding of the topic and uncover the hidden information in this signal that may be useful in clinical practice. Parameters derived from time-domain as well as frequency domain analysis include compensatory reserve, autoregulation estimation, pulse waveform analysis, and behavior of ICP in time. The possibility to predict the outcome and apply a tailored therapy using a personalised perfusion pressure target is also described.

CONCLUSIONS

ICP is a crucial signal to monitor in severely brain injured patients; a bedside computer can empower standard monitoring giving new metrics that may aid in clinical management, establish a personalized therapy, and help to predict the outcome. Continuous collaboration between engineers and clinicians and application of new technologies to healthcare, is vital to improve the accuracy of current metrics and progress towards better care with individualized dynamic targets.

Endocytosis and exocytosis protect cells against severe membrane tension variations

The ability of cells to regulate their shape and volume is critical for many cell functions. How endocytosis and exocytosis, as important ways of membrane trafficking, affect cellular volume regulation is still unclear. Here, we develop a theoretical framework to study the dynamics of cell volume, endocytosis, and exocytosis in response to osmotic shocks and mechanical loadings. This model can not only explain observed dynamics of endocytosis and exocytosis during osmotic shocks but also predict the dynamics of endocytosis and exocytosis during cell compressions. We find that a hypotonic shock stimulates exocytosis, while a hypertonic shock stimulates endocytosis; and exocytosis in turn allows cells to have a dramatic change in cell volume but a small change in membrane tension during hyposmotic swelling, protecting cells from rupture under high tension. In addition, we find that cell compressions with various loading speeds induce three distinct dynamic modes of endocytosis and exocytosis. Finally, we show that increasing endocytosis and exocytosis rates reduce the changes in cell volume and membrane tension under fast cell compression, whereas they enhance the changes in cell volume and membrane tension under slow cell compression. Together, our findings reveal critical roles of endocytosis and exocytosis in regulating cell volume and membrane tension.

An electrodiffusive neuron-extracellular-glia model for exploring the genesis of slow potentials in the brain

Within the computational neuroscience community, there has been a focus on simulating the electrical activity of neurons, while other components of brain tissue, such as glia cells and the extracellular space, are often neglected. Standard models of extracellular potentials are based on a combination of multicompartmental models describing neural electrodynamics and volume conductor theory. Such models cannot be used to simulate the slow components of extracellular potentials, which depend on ion concentration dynamics, and the effect that this has on extracellular diffusion potentials and glial buffering currents. We here present the electrodiffusive neuron-extracellular-glia (edNEG) model, which we believe is the first model to combine compartmental neuron modeling with an electrodiffusive framework for intra- and extracellular ion concentration dynamics in a local piece of neuro-glial brain tissue. The edNEG model (i) keeps track of all intraneuronal, intraglial, and extracellular ion concentrations and electrical potentials, (ii) accounts for action potentials and dendritic calcium spikes in neurons, (iii) contains a neuronal and glial homeostatic machinery that gives physiologically realistic ion concentration dynamics, (iv) accounts for electrodiffusive transmembrane, intracellular, and extracellular ionic movements, and (v) accounts for glial and neuronal swelling caused by osmotic transmembrane pressure gradients. The edNEG model accounts for the concentration-dependent effects on ECS potentials that the standard models neglect. Using the edNEG model, we analyze these effects by splitting the extracellular potential into three components: one due to neural sink/source configurations, one due to glial sink/source configurations, and one due to extracellular diffusive currents. Through a series of simulations, we analyze the roles played by the various components and how they interact in generating the total slow potential. We conclude that the three components are of comparable magnitude and that the stimulus conditions determine which of the components that dominate.

An update to the Monro-Kellie doctrine to reflect tissue compliance after severe ischemic and hemorrhagic stroke

High intracranial pressure (ICP) can impede cerebral blood flow resulting in secondary injury or death following severe stroke. Compensatory mechanisms include reduced cerebral blood and cerebrospinal fluid volumes, but these often fail to prevent raised ICP. Serendipitous observations in intracerebral hemorrhage (ICH) suggest that neurons far removed from a hematoma may shrink as an ICP compliance mechanism. Here, we sought to critically test this observation. We tracked the timing of distal tissue shrinkage (e.g. CA1) after collagenase-induced striatal ICH in rat; cell volume and density alterations (42% volume reduction, 34% density increase; p < 0.0001) were highest day one post-stroke, and rebounded over a week across brain regions. Similar effects were seen in the filament model of middle cerebral artery occlusion (22% volume reduction, 22% density increase; p ≤ 0.007), but not with the Vannucci-Rice model of hypoxic-ischemic encephalopathy (2.5% volume increase, 14% density increase; p ≥ 0.05). Concerningly, this 'tissue compliance' appears to cause sub-lethal damage, as revealed by electron microscopy after ICH. Our data challenge the long-held assumption that 'healthy' brain tissue outside the injured area maintains its volume. Given the magnitude of these effects, we posit that 'tissue compliance' is an important mechanism invoked after severe strokes.

Optical Tweezers Exploring Neuroscience

Over the past decade, optical tweezers (OT) have been increasingly used in neuroscience for studies of molecules and neuronal dynamics, as well as for the study of model organisms as a whole. Compared to other areas of biology, it has taken much longer for OT to become an established tool in neuroscience. This is, in part, due to the complexity of the brain and the inherent difficulties in trapping individual molecules or manipulating cells located deep within biological tissue. Recent advances in OT, as well as parallel developments in imaging and adaptive optics, have significantly extended the capabilities of OT. In this review, we describe how OT became an established tool in neuroscience and we elaborate on possible future directions for the field. Rather than covering all applications of OT to neurons or related proteins and molecules, we focus our discussions on studies that provide crucial information to neuroscience, such as neuron dynamics, growth, and communication, as these studies have revealed meaningful information and provide direction for the field into the future.

Cell surface topography controls phagocytosis and cell spreading: The membrane reservoir in neutrophils

Neutrophils exhibit rapid cell spreading and phagocytosis, both requiring a large apparent increase in the cell surface area. The wrinkled surface topography of these cells may provide the membrane reservoir for this. Here, the effects of manipulation of the neutrophil cell surface topography on phagocytosis and cell spreading were established. Chemical expansion of the plasma membrane or osmotic swelling had no effects. However, osmotic shrinking of neutrophils inhibited both cell spreading and phagocytosis. Triggering a Ca²⁺ signal in osmotically shrunk cells (by IP₃ uncaging) evoked tubular blebs instead of full cell spreading. Phagocytosis was halted at the phagocytic cup stage by osmotic shrinking induced after the phagocytic Ca²⁺ signalling. Restoration of isotonicity was able to restore complete phagocytosis. These data thus provide evidence that the wrinkled neutrophil surface topography provides the membrane reservoir to increase the available cell surface area for phagocytosis and spreading by neutrophils.

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.

Inhibition of Regulatory Volume Decrease Enhances the Cytocidal Effect of Hypotonic Shock in Hepatocellular Carcinoma

Background: Hypotonic shock induces cytocidal effects through cell rupture, and cancer therapy based on this mechanism has been clinically administered to hepatocellular carcinoma patients. We herein investigated the effectiveness of hypotonic shock combined with the inhibition of regulatory volume decrease as cancer therapy for hepatocellular carcinoma.

Methods: Morphological changes in human hepatocellular carcinoma cell lines were observed under a differential interference contrast microscope connected to a high-speed digital video camera. Cell volume changes under hypotonic shock with or without chloride, potassium, or water channel blockers were observed using a high-resolution flow cytometer. In order to investigate cytocidal effects, the number of surviving cells was compared after exposure to hypotonic solution with and without each channel blocker (re-incubation experiment).

Results: Video recordings showed that cells exposed to distilled water rapidly swelled and then ruptured. Cell volume measurements revealed regulatory volume decrease under mild hypotonic shock, whereas severe hypotonic shock increased the number of broken fragments as a result of cell rupture. Moreover, regulatory volume decrease was inhibited in cells treated with each channel blocker. Re-incubation experiments showed the cytocidal effects of hypotonic shock in cells exposed to hypotonic solution, and additional treatments with each channel blocker enhanced these effects.

Conclusion: The inhibition of regulatory volume decrease with chloride, potassium, or water channel blockers may enhance the cytocidal effects of hypotonic shock in hepatocellular carcinoma. Hypotonic shock combined with the inhibition of regulatory volume decrease was a more effective therapy than hypotonic shock alone.

Force spectroscopy measurements show that cortical neurons exposed to excitotoxic agonists stiffen before showing evidence of bleb damage

In ischemic and traumatic brain injury, hyperactivated glutamate (N-methyl-D-aspartic acid, NMDA) and sodium (Nav) channels trigger excitotoxic neuron death. Na⁺, Ca⁺⁺ and H₂O influx into affected neurons elicits swelling (increased cell volume) and pathological blebbing (disassociation of the plasma membrane’s bilayer from its spectrin-actomyosin matrix). Though usually conflated in injured tissue, cell swelling and blebbing are distinct processes. Around an injury core, salvageable neurons could be mildly swollen without yet having suffered the bleb-type membrane damage that, by rendering channels leaky and pumps dysfunctional, exacerbates the excitotoxic positive feedback spiral. Recognizing when neuronal inflation signifies non-lethal osmotic swelling versus blebbing should further efforts to salvage injury-penumbra neurons. To assess whether the mechanical properties of osmotically-swollen versus excitotoxically-blebbing neurons might be cytomechanically distinguishable, we measured cortical neuron elasticity (gauged via atomic force microscopy (AFM)-based force spectroscopy) upon brief exposure to hypotonicity or to excitotoxic agonists (glutamate and Nav channel activators, NMDA and veratridine). Though unperturbed by solution exchange per se, elasticity increased abruptly with hypotonicity, with NMDA and with veratridine. Neurons then invariably softened towards or below the pre-treatment level, sometimes starting before the washout. The initial channel-mediated stiffening bespeaks an abrupt elevation of hydrostatic pressure linked to NMDA or Nav channel-mediated ion/H₂O fluxes, together with increased [Ca⁺⁺]_int-mediated submembrane actomyosin contractility. The subsequent softening to below-control levels is consistent with the onset of a lethal level of bleb damage. These findings indicate that dissection/identification of molecular events during the excitotoxic transition from stiff/swollen to soft/blebbing is warranted and should be feasible.

The control of brain mitochondrial energization by cytosolic calcium: the mitochondrial gas pedal

This review focuses on problems of the intracellular regulation of mitochondrial function in the brain via the (i) supply of mitochondria with ADP by means of ADP shuttles and channels and (ii) the Ca(2+) control of mitochondrial substrate supply. The permeability of the mitochondrial outer membrane for adenine nucleotides is low. Therefore rate dependent concentration gradients exist between the mitochondrial intermembrane space and the cytosol. The existence of dynamic ADP gradients is an important precondition for the functioning of ADP shuttles, for example CrP-shuttle. Cr at mM concentrations instead of ADP diffuses from the cytosol through the porin pores into the intermembrane space. The CrP-shuttle isoenzymes work in different directions which requires different metabolite concentrations mainly caused by dynamic ADP compartmentation. The ADP shuttle mechanisms alone cannot explain the load dependent changes in mitochondrial energization, and a complete model of mitochondrial regulation have to account the Ca(2+) -dependent substrate supply too. According to the old paradigmatic view, Ca(2+) (cyt) taken up by the mitochondrial Ca(2+) uniporter activates dehydrogenases within the matrix. However, recently it was found that Ca(2+) (cyt) at low nM concentrations exclusively activates the state 3 respiration via aralar, the mitochondrial glutamate/aspartate carrier. At higher Ca(2+) (cyt) (> 500 nM), brain mitochondria take up Ca(2+) for activation of substrate oxidation rates. Since brain mitochondrial pyruvate oxidation is only slightly influenced by Ca(2+) (cyt) , it was proposed that the cytosolic formation of pyruvate from its precursors is tightly controlled by the Ca(2+) dependent malate/aspartate shuttle. At low (50-100 nM) Ca(2+) (cyt) the pyruvate formation is suppressed, providing a substrate limitation control in neurons. This so called "gas pedal" mechanism explains why the energy metabolism of neurons in the nucleus suprachiasmaticus could be down-regulated at night but activated at day as a basis for the circadian changes in Ca(2+) (cyt) . It also could explain the energetic disadvantages caused by altered Ca(2+) (cyt) at mitochondrial diseases and neurodegeneration.

On the role of the difference in surface tensions involved in the allosteric regulation of NHE-1 induced by low to mild osmotic pressure, membrane tension and lipid asymmetry

The sodium-proton exchanger 1 (NHE-1) is a membrane transporter that exchanges Na(+) for H(+) ion across the membrane of eukaryotic cells. It is cooperatively activated by intracellular protons, and this allosteric regulation is modulated by the biophysical properties of the plasma membrane and related lipid environment. Consequently, NHE-1 is a mechanosensitive transporter that responds to osmotic pressure, and changes in membrane composition. The purpose of this study was to develop the relationship between membrane surface tension, and the allosteric balance of a mechanosensitive transporter such as NHE-1. In eukaryotes, the asymmetric composition of membrane leaflets results in a difference in surface tensions that is involved in the creation of a reservoir of intracellular vesicles and membrane buds contributing to buffer mechanical constraints. Therefore, we took this phenomenon into account in this study and developed a set of relations between the mean surface tension, membrane asymmetry, fluid phase endocytosis and the allosteric equilibrium constant of the transporter. We then used the experimental data published on the effects of osmotic pressure and membrane modification on the NHE-1 allosteric constant to fit these equations. We show here that NHE-1 mechanosensitivity is more based on its high sensitivity towards the asymmetry between the bilayer leaflets compared to mean global membrane tension. This compliance to membrane asymmetry is physiologically relevant as with their slower transport rates than ion channels, transporters cannot respond as high pressure-high conductance fast-gating emergency valves.

The role of the cytoskeleton in volume regulation and beading transitions in PC12 neurites

We present investigations on volume regulation and beading shape transitions in PC12 neurites, conducted using a flow-chamber technique. By disrupting the cell cytoskeleton with specific drugs, we investigate the role of its individual components in the volume regulation response. We find that microtubule disruption increases both swelling rate and maximum volume attained, but does not affect the ability of the neurite to recover its initial volume. In addition, investigation of axonal beading-also known as pearling instability-provides additional clues on the mechanical state of the neurite. We conclude that volume recovery is driven by passive diffusion of osmolites, and propose that the initial swelling phase is mechanically slowed down by microtubules. Our experiments provide a framework to investigate the role of cytoskeletal mechanics in volume homeostasis.

Visualizing dynamic cytoplasmic forces with a compliance-matched FRET sensor

Mechanical forces are ubiquitous modulators of cell activity but little is known about the mechanical stresses in the cell. Genetically encoded FRET-based force sensors now allow the measurement of local stress in specific host proteins in vivo in real time. For a minimally invasive probe, we designed one with a mechanical compliance matching that of many common cytoskeleton proteins. sstFRET is a cassette composed of Venus and Cerulean linked by a spectrin repeat. The stress sensitivity of the probe was measured in solution using DNA springs to push the donor and acceptor apart with 5-7 pN and this produced large changes in FRET. To measure cytoskeletal stress in vivo we inserted sstFRET into α-actinin and expressed it in HEK and BAEC cells. Time-lapse imaging showed the presence of stress gradients in time and space, often uncorrelated with obvious changes in cell shape. The gradients could be rapidly relaxed by thrombin-induced cell contraction associated with inhibition of myosin II. The tension in actinin fluctuated rapidly (scale of seconds) illustrating a cytoskeleton in dynamic equilibrium. Stress in the cytoskeleton can be driven by macroscopic stresses applied to the cell. Using sstFRET as a tool to measure internal stress, we tested the prediction that osmotic pressure increases cytoskeletal stress. As predicted, hypotonic swelling increased the tension in actinin, confirming the model derived from AFM. Anisotonic stress also produced a novel transient (~2 minutes) decrease in stress upon exposure to a hypotonic challenge, matched by a transient increase with hypertonic stress. This suggests that, at rest, the stress axis of actinin is not parallel to the stress axis of actin and that swelling can reorient actinin to lie more parallel where it can absorb a larger fraction of the total stress. Protein stress sensors are opening new perspectives in cell biology.

Stretch-activated ion channels: what are they?

Mechanosensitive ion channels (MSCs) exist in all cells, but mechanosensitivity is a phenotype not a genotype. Specialized mechanoreceptors such as the hair cells of the cochlea require elaborate mechanical impedance matching to couple the channels to the external stress. In contrast, MSCs in nonspecialized cells appear activated by stress in the bilayer local to the channel--within about three lipids. Local mechanical stress can be produced by far-field tension, amphipaths, phase separations, the cytoskeleton, the extracellular matrix, and the adhesion energy between the membrane and a patch pipette. Understanding MSC function requires under standing the stimulus.

Membrane trauma and Na+ leak from Nav1.6 channels

During brain trauma, white matter experiences shear and stretch forces that, without severing axons, nevertheless trigger their secondary degeneration. In central nervous system (CNS) trauma models, voltage-gated sodium channel (Nav) blockers are neuroprotective. This, plus the rapid tetrodotoxin-sensitive Ca2+ overload of stretch-traumatized axons, points to "leaky" Nav channels as a pivotal early lesion in brain trauma. Direct effects of mechanical trauma on neuronal Nav channels have not, however, been tested. Here, we monitor immediate responses of recombinant neuronal Nav channels to stretch, using patch-clamp and Na+-dye approaches. Trauma constituted either bleb-inducing aspiration of cell-attached oocyte patches or abrupt uniaxial stretch of cells on an extensible substrate. Nav1.6 channel transient current displayed irreversible hyperpolarizing shifts of steady-state inactivation [availability(V)] and of activation [g(V)] and, thus, of window current. Left shift increased progressively with trauma intensity. For moderately intense patch trauma, a approximately 20-mV hyperpolarizing shift was registered. Nav1.6 voltage sensors evidently see lower energy barriers posttrauma, probably because of the different bilayer mechanics of blebbed versus intact membrane. Na+ dye-loaded human embryonic kidney (HEK) cells stably transfected with alphaNav1.6 were subjected to traumatic brain injury-like stretch. Cytoplasmic Na+ levels abruptly increased and the trauma-induced influx had a significant tetrodotoxin-sensitive component. Nav1.6 channel responses to cell and membrane trauma are therefore consistent with the hypothesis that mechanically induced Nav channel leak is a primary lesion in traumatic brain injury. Nav1.6 is the CNS node of Ranvier Nav isoform. When, during head trauma, nodes experienced bleb-inducing membrane damage of varying intensities, nodal Nav1.6 channels should immediately "leak" over a broadly left-smeared window current range.

Osmotic stress and muscle tissue volume response of a freshwater bivalve

The freshwater bivalve, Corbicula fluminea, when submitted to hyperosmotic solutions, behaves as a hyperosmoconformer; we have observed an increase in osmolality and ions in its extracellular fluid. Osmotic and ionic changes in its watery environment represent a challenge for the tissues of this mollusk. Thus we evaluated, in vitro, muscle tissue volume variations (based on wet weight change) under anisosmotic salines, as well the possible regulatory mechanisms involved in the processes. This tissue did not exhibit complete volume regulation under anisosmotic saline solutions, but showed less variation than would be predicted by Van't Hoff's law, and tissue volume remained essentially stable throughout 90 min of exposure. To minimize tissue swelling in hyposmotic situations, C. fluminea muscle mobilizes organic osmolytes (ninhydrin positive substances) and inorganic ions (K(+) and Cl(-)). While under hyperosmotic stimulus, apparently only inorganic osmolytes (Na(+) and Cl(-)) are mobilized by the tissue. Our results indicate ionic accumulation by the Na(+)-K(+)-2Cl(-) cotransporter and the Na(+)/H(+) coupled to Cl(-)/HCO(3)(-) exchangers. Exposure of the muscle tissue to Ca(2+)-free anisosmotic saline did not result in a detectable inhibition of the mechanisms described above. The Ca(2+) gradient that derives from the absence of this ion, even apparently enhances the regulatory mechanisms. These responses of this freshwater mollusk in hyperosmotic solutions, and the muscle tissue under anisosmotic (hypo and hyperosmotic) saline solutions, have not been previously characterized in the manner and approach as reported here. Specifically, we analyze both organic and inorganic osmolytes mobilized under hyposmotic stress, and can infer the participation of Na(+) and Cl(-) pathways stimulated by hyperosmotic stress. From the perspective gained in this study, tissue volume responses may be used as models for toxicological investigations.

Dynamic effects of Hg2+-induced changes in cell volume

Using a microfluidic volume sensor, we studied the dynamic effects of Hg2+ on hypotonic stress-induced volume changes in CHO cells. A hypotonic challenge to control cells caused them to swell but did not evoke a significant regulatory volume decrease (RVD). Treatment with 100 muM HgCl2 caused a substantial increase in the steady-state volume following osmotic stress. Continuous hypotonic challenge following a single 10-min exposure to HgCl2 produced a biphasic volume increase with a steady-state volume 100% larger than control cells. Repeated hypotonic challenges to cells exposed once to Hg2+ resulted in a sequential approach to the same steady-state volume. Stimulation after reaching steady state caused a reduction in peak cell volume. Repeated stimulation was different than continuous stimulation resulting in a more rapid approach to steady state. Substituting extracellular Na+ with impermeant NMDG+ in the hypotonic solution produced a rapid RVD-like volume decrease and eliminated the Hg2+-induced excess swelling. The volume decrease in the presence of Hg2+ was inhibited by tetraethylammonium and 4,4'-diisothiocyanatostilbene-2,2'-disulfonic acid disodium, blockers of K+ and Cl(-) channels, respectively, suggesting that part of the Hg2+ effect was increasing NaCl influx over KCl efflux. The presence of multiple phases of steady-state volume and their sensitivity to the stimulation history suggests that factors beyond solute fluxes, such as modification of mechanical stress within the cytoskeleton also plays a role in the response to hypotonic stress.

Membrane reserves and hypotonic cell swelling

To accommodate expanding volume (V) during hyposmotic swelling, animal cells change their shape and increase surface area (SA) by drawing extra membrane from surface and intracellular reserves. The relative contributions of these processes, sources and extent of membrane reserves are not well defined. In this study, the SA and V of single substrate-attached A549, 16HBE14o(-), CHO and NIH 3T3 cells were evaluated by reconstructing cell three-dimensional topology based on conventional light microscopic images acquired simultaneously from two perpendicular directions. The size of SA reserves was determined by swelling cells in extreme 98% hypotonic (approximately 6 mOsm) solution until membrane rupture; all cell types examined demonstrated surprisingly large membrane reserves and could increase their SA 3.6 +/- 0.2-fold and V 10.7 +/- 1.5-fold. Blocking exocytosis (by N-ethylmaleimide or 10 degrees C) reduced SA and V increases of A549 cells to 1.7 +/- 0.3-fold and 4.4 +/- 0.9-fold, respectively. Interestingly, blocking exocytosis did not affect SA and V changes during moderate swelling in 50% hypotonicity. Thus, mammalian cells accommodate moderate (<2-fold) V increases mainly by shape changes and by drawing membrane from preexisting surface reserves, while significant endomembrane insertion is observed only during extreme swelling. Large membrane reserves may provide a simple mechanism to maintain membrane tension below the lytic level during various cellular processes or acute mechanical perturbations and may explain the difficulty in activating mechanogated channels in mammalian cells.

Properties and Mechanism of the Mechanosensitive Ion Channel Inhibitor GsMTx4, a Therapeutic Peptide Derived from Tarantula Venom

Mechanosensitive ion channels (MSCs) are found in all types of cells ranging from Escherichia coli to morning glories to humans. They seem to fall into two families: those in specialized receptors, such as the hair cells of the cochlea, and those in cells not clearly differentiated for sensory duty. The physiological function of the channels in nonspecialized cells has not been demonstrated, although their activity has been demonstrated innumerable times in vitro. The only specific reagent to block MSCs isGsMTx4, a 4-kDa peptide isolated from tarantula venom. Despite being isolated from venom, it is nontoxic to mice. GsMTx4 is specific for an MSC subtype, the nonselective cation channels that may be members of the transient receptor potential (TRP) family. GsMTx4 acts as a gating modifier, increasing the energy of the open state relative to the closed state. The mirror image D enantiomer of GsMTx4 is equally active, so mode of action is not via the traditional lock and key model. GsMTx4 probably acts in the boundary lipid of the channel by changing local curvature and mechanically stressing the channel toward the closed state. Despite the lack of definitive physiological data on the function of the cationic MSCs, GsMTx4 may prove useful as a drug or lead compound that can affect physiological processes. These processes would be those driven by mechanical stress, such as blood vessel autoregulation, stress-induced contraction of smooth muscle, and Ca(2+) loading in muscular dystrophy.

Modeling the effect of stretch and plasma membrane tension on Na+-K+-ATPase activity in alveolar epithelial cells

While a number of whole cell mechanical models have been proposed, few, if any, have focused on the relationship among plasma membrane tension, plasma membrane unfolding, and plasma membrane expansion and relaxation via lipid insertion. The goal of this communication is to develop such a model to better understand how plasma membrane tension, which we propose stimulates Na+-K+-ATPase activity but possibly also causes cell injury, may be generated in alveolar epithelial cells during mechanical ventilation. Assuming basic relationships between plasma membrane unfolding and tension and lipid insertion as the result of tension, we have captured plasma membrane mechanical responses observed in alveolar epithelial cells: fast deformation during fast cyclic stretch, slower, time-dependent deformation via lipid insertion during tonic stretch, and cell recovery after release from stretch. The model estimates plasma membrane tension and predicts Na+-K+-ATPase activation for a specified cell deformation time course. Model parameters were fit to plasma membrane tension, whole cell capacitance, and plasma membrane area data collected from the literature for osmotically swollen and shrunken cells. Predictions of membrane tension and stretch-stimulated Na+-K+-ATPase activity were validated with measurements from previous studies. As a proof of concept, we demonstrate experimentally that tonic stretch and consequent plasma membrane recruitment can be exploited to condition cells against subsequent cyclic stretch and hence mitigate stretch-induced responses, including stretch-induced cell death and stretch-induced modulation of Na+-K+-ATPase activity. Finally, the model was exercised to evaluate plasma membrane tension and potential Na+-K+-ATPase stimulation for an assortment of traditional and novel ventilation techniques.

The mechanism of extracellular stimulation of nerve cells on an electrolyte-oxide-semiconductor capacitor

Extracellular excitation of neurons is applied in studies of cultured networks and brain tissue, as well as in neuroprosthetics. We elucidate its mechanism in an electrophysiological approach by comparing voltage-clamp and current-clamp recordings of individual neurons on an insulated planar electrode. Noninvasive stimulation of neurons from pedal ganglia of Lymnaea stagnalis is achieved by defined voltage ramps applied to an electrolyte/HfO2/silicon capacitor. Effects on the smaller attached cell membrane and the larger free membrane are distinguished in a two-domain-stimulation model. Under current-clamp, we study the polarization that is induced for closed ion channels. Under voltage-clamp, we determine the capacitive gating of ion channels in the attached membrane by falling voltage ramps and for comparison also the gating of all channels by conventional variation of the intracellular voltage. Neuronal excitation is elicited under current-clamp by two mechanisms: Rising voltage ramps depolarize the free membrane such that an action potential is triggered. Falling voltage ramps depolarize the attached membrane such that local ion currents are activated that depolarize the free membrane and trigger an action potential. The electrophysiological analysis of extracellular stimulation in the simple model system is a basis for its systematic optimization in neuronal networks and brain tissue.

Swelling-activated pathways in human T-lymphocytes studied by cell volumetry and electrorotation

Small organic solutes, including sugar derivatives, amino acids, etc., contribute significantly to the osmoregulation of mammalian cells. The present study explores the mechanisms of swelling-activated membrane permeability for electrolytes and neutral carbohydrates in Jurkat cells. Electrorotation was used to analyze the relationship between the hypotonically induced changes in the electrically accessible surface area of the plasma membrane (probed by the capacitance) and its permeability to the monomeric sugar alcohol sorbitol, the disaccharide trehalose, and electrolyte. Time-resolved capacitance and volumetric measurements were performed in parallel using media of different osmolalities containing either sorbitol or trehalose as the major solute. Under mild hypotonic stress in 200 mOsm sorbitol or trehalose solutions, the cells accomplished regulatory volume decrease by releasing cytosolic electrolytes presumably through pathways activated by the swelling-mediated retraction of microvilli. This is suggested by a rapid decrease of the area-specific membrane capacitance C(m) (microF/cm2). The cell membrane was impermeable to both carbohydrates in 200 mOsm media. Whereas trehalose permeability remained also very poor in 100 mOsm medium, extreme swelling of cells in a strongly hypotonic solution (100 mOsm) led to a dramatic increase in sorbitol permeability as evidenced by regulatory volume decrease inhibition. The different osmotic thresholds for activation of electrolyte release and sorbitol influx suggest the involvement of separate swelling-activated pathways. Whereas the electrolyte efflux seemed to utilize pathways preexisting in the plasma membrane, putative sorbitol channels might be inserted into the membrane from cytosolic vesicles via swelling-mediated exocytosis, as indicated by a substantial increase in the whole-cell capacitance C(C) (pF) in strongly hypotonic solutions.

Plasma Membrane Surface Increases with Tonic Stretch of Alveolar Epithelial Cells

Cyclic stretch stimulates numerous responses in alveolar epithelial cells--some beneficial, some injurious--often through mechanosensitive membrane-associated proteins such as stretch-activated ion channels. Tonic stretch, in contrast, stimulates only some of these responses. In this study, we hypothesized that the plasma membranes of alveolar epithelial cells expand during tonic stretch, not only through cell surface unfolding, but also through recruitment of additional phospholipids. Such plasma membrane expansion would reduce membrane tension and decrease stimulation of mechanosensitive membrane proteins. Primary rat alveolar epithelial cells were isolated, cultured for 48 h, and stretched between 3 and 40% change in basal membrane surface area. Gross changes in total cell surface area were obtained from stacks of thin fluorescent confocal micrographs; fine changes in plasma membrane area were measured via whole cell capacitance. A 1:1 correspondence linked changes in basal and total cell surface area, implying that cell surface area change is dominated by stretch of the attached basal surface. We also found that plasma membrane increased proportionally with surface area within 5 min of tonic stretch, showing that, given time to occur, plasma membrane expansion via lipid recruitment preponderates the changes in cell surface shape and size demanded by stretching the cell. Similarly, in cells tonically stretched 10 min to allow lipid insertion and then returned to an unstretched state, reabsorption of excess lipid occurred within 5 min. Finally, we found that lipid insertion induced by tonic stretch was unaffected by F-actin disassembly, ATP depletion, and calcium deprivation.

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.

Gap junctions do not underlie changes in whole-cell conductance in anoxic turtle brain

An acute reduction in cell membrane permeability could provide an effective strategy to prolong anoxic survival. A previous study has shown that in the western painted turtle whole-cell neuronal conductance (G(w)) decreases during anoxia, which may be mediated by the activation of adenosine A(1) receptors and calcium. Reduction in G(w) is thought to be the result of ion channel closure, but closure of gap junctions could also be responsible for this phenomenon. In our study, antibody staining of connexin 32 and 43 (Cx32 and Cx43) suggested the presence of gap junctional components in the turtle cortex. To examine if gap junctions were involved in the previously measured anoxic decrease in G(w), neuronal connectivity was assessed through the measurement of whole-cell capacitance (C(w)). Turtle cortical sheets were perfused with normoxic (95%O(2)/5%CO(2)), anoxic (95%N(2)/5%CO(2)), high calcium (4 mM) and adenosine (200 microm) artificial cerebral spinal fluid (aCSF). No significant change in C(w) was observed under any of the above conditions. However, during hypo-osmotic aCSF perfusion C(w) decreased significantly, with the lowest value of 50+/-10.4 pF (P<0.05) occurring at 30 min. To visualize changes in gap junction permeability lucifer yellow was loaded into turtle neurons during normoxic, anoxic, 0 calcium, hypo-osmotic, cold shock, (+)-isoproterenol, nitric oxide donor S-nitoso-acetyl penicillamine, and 8-bromo-guanosine 3',5'-cyclic monophosphate aCSF perfusion. Dye propagation was only observed in 3 of 20 cold shock experiments (4 degrees C). We conclude that gap junctions are not involved in the acute reduction in G(w) previously observed during anoxia and that our results support the hypothesis that ion channel arrest is involved.

Cytochalasin D attenuates the desensitisation of pressure-stimulated vesicle fusion in guard cell protoplasts

Fusion of vesicular membranes with the plasma membrane during pressure-driven swelling of guard cell protoplasts was studied using patch clamp capacitance measurements. Hydrostatic pressure pulses were applied via the patch pipette and resulted in an immediate and linear increase in membrane capacitance, a parameter proportional to the surface area. In any given protoplast, pressure-stimulated increases in membrane capacitance could be provoked repetitively. However, the rate of rise in capacitance upon the same strength of stimulation decreased exponentially with time (tau = 4 min) for subsequent pressure stimuli. This process was the result of a desensitisation of the plasma membrane to mechanical forces. Incubation of guard cell protoplasts in cytochalasin D, which depolymerises actin filaments, nearly abolished this desensitisation process. These results suggest that membrane stretch initiates a reactive process that may fortify or stabilise the plasma membrane of guard cell protoplasts.

Molecular basis of mechanotransduction in living cells

The simplest cell-like structure, the lipid bilayer vesicle, can respond to mechanical deformation by elastic membrane dilation/thinning and curvature changes. When a protein is inserted in the lipid bilayer, an energetic cost may arise because of hydrophobic mismatch between the protein and bilayer. Localized changes in bilayer thickness and curvature may compensate for this mismatch. The peptides alamethicin and gramicidin and the bacterial membrane protein MscL form mechanically gated (MG) channels when inserted in lipid bilayers. Their mechanosensitivity may arise because channel opening is associated with a change in the protein's membrane-occupied area, its hydrophobic mismatch with the bilayer, excluded water volume, or a combination of these effects. As a consequence, bilayer dilation/thinning or changes in local membrane curvature may shift the equilibrium between channel conformations. Recent evidence indicates that MG channels in specific animal cell types (e.g., Xenopus oocytes) are also gated directly by bilayer tension. However, animal cells lack the rigid cell wall that protects bacteria and plants cells from excessive expansion of their bilayer. Instead, a cortical cytoskeleton (CSK) provides a structural framework that allows the animal cell to maintain a stable excess membrane area (i.e., for its volume occupied by a sphere) in the form of membrane folds, ruffles, and microvilli. This excess membrane provides an immediate membrane reserve that may protect the bilayer from sudden changes in bilayer tension. Contractile elements within the CSK may locally slacken or tighten bilayer tension to regulate mechanosensitivity, whereas membrane blebbing and tight seal patch formation, by using up membrane reserves, may increase membrane mechanosensitivity. In specific cases, extracellular and/or CSK proteins (i.e., tethers) may transmit mechanical forces to the process (e.g., hair cell MG channels, MS intracellular Ca(2+) release, and transmitter release) without increasing tension in the lipid bilayer.

Excitotoxic cell death dependent on inhibitory receptor activation

Although excitotoxic cell death is usually considered a Ca(2+)-dependent process, in certain neuronal systems there is strong evidence that excitotoxic cell death is independent of Ca2+ and is instead remarkably dependent on extracellular Cl-. We have shown (in isolated chick embryo retina) that at least some of the lethal Cl- entry is through GABA and glycine receptors. Here we show that when all the GABA and glycine receptors are blocked by using an appropriate cocktail of inhibitors, agonist-induced excitotoxic cell death can be completely prevented. To determine if ligand-gated Cl- channels contribute to excitotoxic cell death in other neurons, we examined KA-induced cell death in cultured rat cerebellar granule cells. GABA receptor blockade with either a competitive or noncompetitive antagonist provides complete neuroprotection. KA stimulates Cl- uptake by the granule cells, and this is blocked by the GABA antagonists. Granule cell cultures take up [3H]GABA and release it in response to KA treatment. A subpopulation of neurons in the cultures is shown to have GAD and high concentrations of GABA, and this presumably is the source of the GABA that leads to receptor activation and lethal Cl- entry. Finally, we show that retinal cell death due to 1 h of simulated ischemia (combined oxygen and glucose deprivation) is completely prevented by blocking the inhibitory receptors. These results indicate that, paradoxically, excitotoxic cell death is completely dependent on activation of inhibitory receptors, in at least some neuronal systems, and this pathological process may contribute to disease.

Membrane capacitance of cortical neurons and glia during sleep oscillations and spike-wave seizures

Dual intracellular recordings in vivo were used to disclose relationships between cortical neurons and glia during spontaneous slow (<1 Hz) sleep oscillations and spike-wave (SW) seizures in cat. Glial cells displayed a slow membrane potential oscillation (<1 Hz), in close synchrony with cortical neurons. In glia, each cycle of this oscillation was made of a round depolarizing potential of 1.5-3 mV. The depolarizing slope corresponded to a steady depolarization and sustained synaptic activity in neurons (duration, 0.5-0.8 s). The repolarization of the glial membrane (duration, 0.5-0.8 s) coincided with neuronal hyperpolarization, associated with disfacilitation, and suppressed synaptic activity in cortical networks. SW seizures in glial cells displayed phasic events, synchronized with neuronal paroxysmal potentials, superimposed on a plateau of depolarization, that lasted for the duration of the seizure. Measurements of the neuronal membrane capacitance during slow oscillating patterns showed small fluctuations around the resting values in relation to the phases of the slow oscillation. In contrast, the glial capacitance displayed a small-amplitude oscillation of 1-2 Hz, independent of phasic sleep and seizure activity. Additionally, in both cell types, SW seizures were associated with a modulatory, slower oscillation ( approximately 0.2 Hz) and a persistent increase of capacitance, developing in parallel with the progression of the seizure. These capacitance variations were dependent on the severity of the seizure and the distance between the presumed seizure focus and the recording site. We suggest that the capacitance variations may reflect changes in the membrane surface area (swelling) and/or of the interglial communication via gap junctions, which may affect the synchronization and propagation of paroxysmal activities.

Activation of mechanosensitive currents in traumatized membrane

Mechanosensitive (MS) channels, ones whose open probability varies with membrane tension in patch recordings, are diverse and ubiquitous, yet many are remarkably insensitive to mechanical stimuli in situ. Failure to elicit mechanocurrents from cells with abundant MS channels suggests that, in situ, the channels are protected from mechanical stimuli. To establish what conditions affect MS channel gating, we monitored Lymnaea neuron stretch-activated K (SAK) channels in cell-attached patches after diverse treatments. Mechanosensitivity was gauged by rapidity of onset and extent of channel activation during a step pressure applied to a "naive" patch. The following treatments enhanced mechanosensitivity: actin depolymerization (cytochalasin B), N-ethylmaleimide, an inhibitor of ATPases including myosin, elevated Ca (using A-23187), and osmotic swelling (acutely and after 24 h). Osmotic shrinking decreased mechanosensitivity. A unifying interpretation is that traumatized cortical cytoskeleton cannot prevent transmission of mechanical stimuli to plasma membrane channels. Mechanoprotection and capricious mechanosensitivity are impediments to cloning efforts with MS channels. We demonstrate a potpourri of endogenous MS currents from L-M(TK-) fibroblasts; others had reported these cells to be MS current null and hence to be suitable for expressing putative MS channels.

Membrane tension in swelling and shrinking molluscan neurons

When neurons undergo dramatic shape and volume changes, how is surface area adjusted appropriately? The membrane tension hypothesis-namely that high tensions favor recruitment of membrane to the surface whereas low tensions favor retrieval-provides a simple conceptual framework for surface area homeostasis. With membrane tension and area in a feedback loop, tension extremes may be averted even during excessive mechanical load variations. We tested this by measuring apparent membrane tension of swelling and shrinking Lymnaea neurons. With hypotonic medium (50%), tension that was calculated from membrane tether forces increased from 0.04 to as much as 0.4 mN/m, although at steady state, swollen-cell tension (0. 12 mN/m) exceeded controls only threefold. On reshrinking in isotonic medium, tension reduced to 0.02 mN/m, and at the substratum, membrane invaginated, creating transient vacuole-like dilations. Swelling increased membrane tension with or without BAPTA chelating cytoplasmic Ca2+, but with BAPTA, unmeasurably large (although not lytic) tension surges occurred in approximately two-thirds of neurons. Furthermore, in unarborized neurons voltage-clamped by perforated-patch in 50% medium, membrane capacitance increased 8%, which is indicative of increasing membrane area. The relatively damped swelling-tension responses of Lymnaea neurons (no BAPTA) were consistent with feedback regulation. BAPTA did not alter resting membrane tension, but the large surges during swelling of BAPTA-loaded neurons demonstrated that 50% medium was inherently treacherous and that tension regulation was impaired by subnormal cytoplasmic [Ca2+]. However, neurons did survive tension surges in the absence of Ca2+ signaling. The mechanism to avoid high-tension rupture may be the direct tension-driven recruitment of membrane stores.

Cellular control lies in the balance of forces

Mechanical tension generated within the cytoskeleton of living cells is emerging as a critical regulator of biological function in diverse situations ranging from the control of chromosome movement to the morphogenesis of the vertebrate brain. In this article, we review recent advances that have been made in terms of understanding how cells generate, transmit and sense mechanical tension, as well as how they use these forces to control their shape and behavior. An integrated view of cell regulation that incorporates mechanics and structure as well as chemistry is beginning to emerge.

Neuronal swelling and surface area regulation: elevated intracellular calcium is not a requirement

Neurons are mechanically robust. During prolonged swelling, molluscan neurons can triple their apparent membrane area. They gain surface area and capacitance independent of extracellular Ca concentration ([Ca]e), but it is unknown if an increase in intracellular Ca concentration ([Ca]i) is necessary. If Ca for stimulating exocytosis is unnecessary, it is possible that swelling-induced membrane tension changes directly trigger surface area readjustments. If, however, Ca-mediated but not tension-mediated membrane recruitment is responsible for surface area increases, swelling neurons should sustain elevated levels of [Ca]i. The purpose of this investigation is to determine if the [Ca]i in swelling neurons attains levels high enough to promote exocytosis and if any such increase is required. Lymnaea neurons were loaded with the Ca concentration indicator fura 2. Calibration was performed in situ using 4-bromo-A-23187 and Ca-ethylene glycol-bis(beta-aminoethyl ether)-N,N,N',N'-tetraacetic acid (EGTA), with free Ca concentration ranging from 0 to 5 microM. Swelling perturbations (medium osmolarity reduced to 25% for 5 min) were done at either a standard [Ca]e or very low [Ca]e level (0.9 mM or 0.13 microM, respectively). In neither case did the [Ca]i increase to levels that drive exocytosis. We also monitored osmomechanically driven membrane dynamics [swelling, then formation and reversal of vacuole-like dilations (VLDs)] with the [Ca]i clamped below 40 nM via 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid (BAPTA). [Ca]i did not change with swelling, and VLD behavior was unaffected, consistent with tension-driven, [Ca]i-independent surface area adjustments. In addition, neurons with [Ca]i clamped at 0.1 microM via an ionophore could produce VLDs. We conclude that, under mechanical stress, neuronal membranes are compliant by virtue of surface area regulatory adjustments that operate independent of [Ca]i. The findings support the hypothesis that plasma membrane area is regulated in part by membrane tension.

Cytoplasmic mechanisms of axonal and dendritic growth in neurons

The structural mechanisms responsible for the gradual elaboration of the cytoplasmic elongation of neurons are reviewed. In addition to discussing recent work, important older work is included to inform newcomers to the field how the current perspective arose. The highly specialized axon and the less exaggerated dendrite both result from the advance of the motile growth cone. In the area of physiology, studies in the last decade have directly confirmed the classic model of the growth cone pulling forward and the axon elongating from this tension. Particularly in the case of the axon, cytoplasmic elongation is closely linked to the formation of an axial microtubule bundle from behind the advancing growth cone. Substantial progress has been made in understanding the expression of microtubule-associated proteins during neuronal differentiation to stiffen and stabilize axonal microtubules, providing specialized structural support. Studies of membrane organelle transport along the axonal microtubules produced an explosion of knowledge about ATPase molecules serving as motors driving material along microtubule rails. However, most aspects of the cytoplasmic mechanisms responsible for neurogenesis remain poorly understood. There is little agreement on mechanisms for the addition of new plasma membrane or the addition of new cytoskeletal filaments in the growing axon. Also poorly understood are the mechanisms that couple the promiscuous motility of the growth cone to the addition of cytoplasmic elements.

Osmotic dilution stimulates axonal outgrowth by making axons more sensitive to tension

Mechanical tension is a potent stimulator of axonal growth rate, which is also stimulated by osmotic dilution. We wished to determine the relationship, if any, between osmotic stimulation and tensile regulation of axonal growth. We used calibrated glass needles to apply constant force to elongate axons of cultured chick sensory neurons. We find that a neurite being pulled at a constant force will grow 50-300% faster following a 50% dilution of inorganic ions in the culture medium. That is, osmotic dilution appears to cause axons to increase their sensitivity to applied tensions. Experimental interventions suggest that this effect is not mediated by dilution of extracellular calcium, or to osmotic stimulation of adenylate cyclase, or to osmotic stimulation of mechanosensitive ion channels. Rather, experiments measuring the static tension normally borne by neurites suggest a direct mechanical effect on the cytoskeletal proteins of the neurite shaft. Our results are consistent with a formal thermodynamic model for axonal growth in which removing a compressive load on axonal microtubules promotes their assembly, thus promoting axonal elongation.