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

Abstract

Diffusion-weighted MRI (DWI) is a sensitive and reliable marker of cerebral ischemia. Within minutes of an ischemic event in the brain, the microscopic motion of water molecules measured with DWI, termed the apparent diffusion coefficient (ADC), decreases within the infarcted region. However, although the change is related to cell swelling, the precise pathological mechanism remains elusive. We show that focal enlargement and constriction, or beading, in axons and dendrites are sufficient to substantially decrease ADC. We first derived a biophysical model of neurite beading, and we show that the beaded morphology allows a larger volume to be encompassed within an equivalent surface area and is, therefore, a consequence of osmotic imbalance after ischemia. The DWI experiment simulated within the model revealed that intracellular ADC decreased by 79% in beaded neurites compared with the unbeaded form. To validate the model experimentally, excised rat sciatic nerves were subjected to stretching, which induced beading but did not cause a bulk shift of water into the axon (i.e., swelling). Beading-induced changes in cell-membrane morphology were sufficient to significantly hinder water mobility and thereby decrease ADC, and the experimental measurements were in excellent agreement with the simulated values. This is a demonstration that neurite beading accurately captures the diffusion changes measured in vivo. The results significantly advance the specificity of DWI in ischemia and other acute neurological injuries and will greatly aid the development of treatment strategies to monitor and repair damaged brain in both clinical and experimental settings.

Fig. 1.

Geometries and physical parameters of beading neurites. (A) The beaded cylinders are continuous, have smooth transitions between the enlargements and constrictions, and each of the displayed contours has identical surface area and length. (B) At all beading amplitudes, the surface area (SA) and length (L) were equivalent to the original cylinder of a given radius and bead separation. The resulting shape transformation yielded an increase in volume (V) as beading amplitude increased compared with the original cylinder. To meet the condition of conserved SA and L, the average radius (R_av) and bead separation (g) were varied as amplitude increased. Lines represent the analytical calculations of the physical parameters, and points indicate the equivalent parameters computed from the generated geometrical surfaces.

Fig. 2.

Monte Carlo random-walk simulation of intracellular diffusion properties in beaded geometries. Intracellular diffusion was highly restricted parallel to the main axis (ADC_‖) with increasing beading amplitude. In contrast, diffusion perpendicular to the main axis (ADC_⊥) exhibited only a minimal increase and only at the largest beading amplitudes. MD was reduced with increasing beading amplitude, whereas FA was decreased at the largest amplitudes. The separation between beads (g) had only a marginal effect on the diffusion properties at the diffusion-weighting values used in the current study.

Fig. 3.

Simulated diffusion properties of intra- and extracellular compartments in packed geometries of packed beading neurites. (A) Beading cylinders were packed in a hexagonal pattern. (B) The upper limit of the packing geometry was set at 0.79, and the local maximum was set at a beading amplitude of 0.73. Beading decreased ADC_‖ in both the intra- and extracellular compartments, whereas ADC_⊥ was substantially decreased only in the extracellular compartment. The decreases in MD were consequences of increased tortuosity occurring in both compartments.

Fig. 4.

Diffusion in excised, beaded sciatic nerve fibers. (A) Compared with the axons of control sciatic nerves, nerves fixed under tension display extensive beading of the axonal membrane. (B) Measurements of diffusion in the fixed nerves show a restriction of water mobility parallel to the axons but no change in the perpendicular direction. ADC_‖ was significantly decreased, whereas ADC_⊥ was unaffected. (C) Simulations performed with parameters and geometries mimicking the ex vivo situation were consistent with the ex vivo measurements.