Osmotically Driven Shape Transformations in Axons

Abstract

We report a cylindrical-peristaltic shape transformation in axons exposed to a controlled osmotic perturbation. The peristaltic shape relaxes and the axon recovers its original geometry within minutes. We show that the shape instability depends critically on swelling rate and that volume and membrane area regulation are responsible for the shape relaxation. We propose that volume regulation occurs via leakage of ions driven by elastic pressure, and analyse the peristaltic shape dynamics taking into account the internal structure of the axon. The results obtained provide a framework for understanding peristaltic shape dynamics in nerve fibers occurring in vivo. PACS numbers: 47.20.Dr, 87.19.La Axons and dendrites, collectively known as neurites, are thin, tubular extensions produced by neuronal cells [1]. Structurally, they consist of an outer lipid membrane sheath bound to a core made up of an elastic network of highly cross-linked biopolymers known as the cytoskeleton. Neurites are known to lose their normal cylindrical geometry and become peristaltically modulated-a process commonly known as beading-under a wide range of situations. These include neuro-degenerative diseases like Alzheimer's [2], brain trauma [3], stretch injuries to nerves [4] and in vitro as well as in vivo application of neurotoxins or drugs [5], among others. In stretch injuries, tension in the axon is responsible for beading. In the other examples, disruption of cytoskeletal integrity appears to be a common feature.Peristaltic modes have been studied in tubular membranes and gels under tension [6,7,8]. This "pearling instability" is driven by surface tension, as in the Rayleigh-Plateau instability of liquid columns [9]. In liquid columns, peristaltic perturbations with wavelengths larger than the circumference grow as they reduce surface area at constant volume. Unlike liquid columns, cylindrical vesicles/gels are unstable only beyond a critical tension, when the gain in surface energy due to a reduction in area overcomes the elastic energy for deforming the membrane/gel. Similar arguments can explain peristaltic modes observed in stretched nerves [10] and cell protrusions exposed to toxins [11]. In all the above mentioned biological examples the beaded state persists and no recovery has been observed.We report a novel, reversible, peristaltic instability observed in chick-embryo axons and in PC12 neurites [12] after a sudden dilution of the external medium, as shown in Fig. 1. The transition to the peristaltic state occurs at a threshold membrane tension, induced by osmotic swelling. However, unlike previous reports on beading, the osmotically induced instability relaxes and the neurite recovers its original shape within minutes. We show that the relaxation is due both to cell volume and membrane surface regulation. Remarkably, the shape stability depends critically on the swelling rate. We model the pearling/beading instability taking into account the gel-like dynamics of the cytoskeleton and the membranecytoskeleton coupling, and fin...

Neurite beading is sufficient to decrease the apparent diffusion coefficient after ischemic stroke

“…Therefore, the changes in the extracellular diffusivity were caused solely by changes in morphology, although the extracellular fraction is known to decrease (24). Nonetheless, the current model shows that a 5-10% shift of bulk water from the extracellular to the intracellular compartment (13,24) would be sufficient to induce neurite beading, causing a substantial decrease in intracellular MD and thereby decreasing overall MD by nearly one-half. Unlike previous models that depict axonal swelling as balloon-like expansion (25,26), the conservation of surface area more realistically mimics the biophysical properties of neurite swelling.…”

“…Cultured dorsal-root axons bead within 1 min after hyposmotic changes to the bathing medium (13). In contrast, acute brain preparations have shown that dendrites do not exhibit beading after osmotic challenge, but they do so quite rapidly after oxygen-glucose deprivation (37).…”

“…Specifically, neurites exhibit focal enlargements separated by constrictions (6), or beading, in response to osmotic or ischemic conditions both in vivo and in vitro. Beading is governed by the biophysical properties of lipid membranes subjected to tension and hydrostatic pressure and has been described extensively in tubular membranes (7-9), stretched nerve fibers (10, 11), and neurons in vitro (12)(13)(14). Importantly, in vivo two-photon microscopy has revealed that beading occurs within minutes of an ischemic event in the brain and resolves on reperfusion (figure 5 in ref.…”

Synapse-Level Determination of Action Potential Duration by K + Channel Clustering in Axons

“…As a second approach, we experimentally imposed an increase in GR between boutons and shafts and measured for an accompanying change in AP width. Perfusion of a hyposmotic extracellular solution induced rapid (∼30 s) enlargements of axonal structures (Babila et al, 1990; Pullarkat et al, 2006) with a greater volume change in boutons relative to shafts (Figure 2E and 2F). Despite an apparent GR increase, AP width was unaffected (Figure 2G; 1.07 ± 0.04% of control, n = 8, p = 0.12; paired t-test).…”

Model for Curvature-Driven Pearling Instability in Membranes

“…Tension-driven pearling induced by laser tweezers [5], bilayer asymmetry [6], polymer anchorage [7], or osmotic perturbations [8] has been reported, as well as budding and tubulation induced by polymer anchorage [9].…”