Brain oedema in focal ischaemia: molecular pathophysiology and theoretical implications

Abstract

Focal cerebral ischaemia and post-ischaemic reperfusion cause cerebral capillary dysfunction, resulting in oedema formation and haemorrhagic conversion. There are substantial gaps in understanding the pathophysiology, especially regarding early molecular participants. Here, we review physiological and molecular mechanisms involved. We reaffirm the central role of Starling's principle, which states that oedema formation is determined by the driving force and the capillary "permeability pore". We emphasise that the movement of fluids is largely driven without new expenditure of energy by the ischaemic brain. We organise the progressive changes in osmotic and hydrostatic conductivity of abnormal capillaries into three phases: formation of ionic oedema, formation of vasogenic oedema, and catastrophic failure with haemorrhagic conversion. We suggest a new theory suggesting that ischaemia-induced capillary dysfunction can be attributed to de novo synthesis of a specific ensemble of proteins that determine osmotic and hydraulic conductivity in Starling's equation, and whose expression is driven by a distinct transcriptional program.

Figure 1. Brain swelling after middle cerebral artery occlusion

Left: photograph of coronal section of rat head following middle cerebral artery occlusion; post-mortem perfusion with Evans blue and India ink shows regions with persistent circulation (darker areas, left) versus regions without appreciable circulation (pink area, right); white line from the superior sagittal sinus to the clivus indicates the midline, showing extensive shift due to massive swelling of the involved hemisphere. Right: intraoperative photograph showing massive brain swelling causing herniation of the brain out of the skull following decompressive craniectomy.

Figure 2. Starling's equation, classically stated as J_v = K_f[(P_c – P_i) – (π_c – π_i)], describes capillary permeability under normal and pathological conditions

Formulated in 1896 by the British physiologist Ernest Starling, the Starling equation describes the role of hydrostatic and osmotic forces in the movement of fluid across capillary endothelial cells. According to the equation, the movement of fluid depends on five variables: capillary hydrostatic pressure (P_c), interstitial hydrostatic pressure (P_i), capillary osmotic pressure (π_c), interstitial osmotic pressure (π_i), and a filtration coefficient (K_f). Here, two distinct “filtration” coefficients, the hydraulic conductivity (K_H), and the osmotic conductivity (K_O), are used to describe the situation in brain capillaries. The equation gives the net filtration or net fluid movement (J_v), with outward force being positive, meaning that fluid will tend to leave the capillary. The filtration coefficients, K_H and K_O, determine oedema formation. Normally, values of K_O and K_H are small or close to zero, and no oedema forms. With ionic oedema, K_O ≥0 and K_H ≈0, with the change in K_O due to upregulation of Na+ flux pathways, such as the SUR1-regulated NC_Ca-ATP channel and possibly aquaporin (AQP) channels. With vasogenic oedema, K_O ≥0 and K_H ≥0, with the increase in K_H being due to upregulation of prothrombin, VEGF and MMP-9. Upregulation of various oedema-associated proteins can be attributed, at least partly, to activation of a transcriptional program involving AP-1, HIF-1, Sp-1 and NF-κB. Note that the driving forces for fluid movement are not generated by the ischaemic brain; rather, hydrostatic pressure, P, is generated by the heart, and osmotic pressure, π, arises from potential energy stored in electrochemical gradients established before onset of ischaemia.

Figure 3. SUR1, the regulatory subunit of the NC_Ca-ATP channel, is up-regulated in focal cerebral ischemia, as shown in rat tissue

Brain tissue from the core of an infarct 6 h after middle cerebral artery occlusion. Capillary (left) labelled for von Willebrand factor (green) and for SUR1 (red), next to a dying neuron with blebs (right) that labels strongly for SUR1 (red); nuclei labeled with DAPI (blue).

Figure 4. Cell blebbing after NaN₃-induced ATP depletion

Scanning electron micrographs of freshly isolated native reactive astrocytes from adult rat brain. Formaldehyde-glutaraldehyde fixation was initiated under control conditions (A), 5 min after exposure to 1 mM NaN₃ (B), and 25 min after exposure to 1 mM NaN (C). Bar, 12 μm. Reproduced with permission from Chen and Simard.

Figure 5. Schematic diagram illustrating various types of edema progressing to hemorrhagic conversion

Normally, Na⁺ concentrations in serum and in extracellular space are the same, and much higher than inside the neuron. Cytotoxic oedema of neurons is due to entry of Na⁺ into ischaemic neurons via pathways such as NC_Ca-ATP channels, depleting extracellular Na⁺ and thereby setting up a concentration gradient between intravascular and extracellular compartments. Ionic oedema results from cytotoxic oedema of endothelial cells, due to expression of cation channels on both the luminal and abluminal side, allowing Na⁺ from the intravascular compartment to traverse the capillary wall and replenish Na⁺ in the extracellular space. Vasogenic oedema results from degradation of tight junctions between endothelial cells, transforming capillaries into “fenestrated” capillaries that allow extravasation (outward filtration) of proteinatious fluid. Oncotic death of neuron is the ultimate consequence of cytotoxic edema. Oncotic death of endothelial cells results in complete loss of capillary integrity and in extravasation of blood—ie, haemorrhagic conversion.

Figure 6. Haemorrhagic conversion with petechial haemorrhage is associated with transcriptional upregulation of sulfonylurea receptor 1 (SUR1) in ischaemic CNS tissues

In situ hybridisation for SUR1 (purple) shows strong labelling with antisense probe in a microvessel surrounded by extravasated erythrocytes (red).

Figure 7. A distinct transcriptional program may account for sequential changes in ischaemia-induced changes in blood–brain permeability

The promoter regions of five genes (italicised) for proteins (in parentheses) involved in oedema formation, were analysed for potential consensus sequence binding sites for the transcription factors AP-1, Sp-1, HIF-1, and NF-κB. The predicted location of these putative binding sites on each promoter region is shown, with binding sites on the positive and negative strands indicated by upward and downward symbols, respectively.