Aquaporin 4: A Player in Cerebral Edema and Neuroinflammation

Andrew M Fukuda; Jerome Badaut

Disclosures

J Neuroinflammation. 2012;9(279) 

In This Article

Neuroinflammation and Edema in Brain Injury: Astrocyte AQP4

BBB Breakdown and Vasogenic Edema

AQP4 is one of the key players in edema formation and resolution[57,58] and increase in its expression is observed in reactive astrocytes after injury. Edema is frequently observed in brain injuries and is associated with BBB disruption.[57,59] Compromised BBB integrity leads to plasma protein leakage and extravascular fluid accumulation.[57] The breakdown of the BBB is a complex process partially caused by the activation of matrix metalloproteinases (MMPs), which is part of the neuroinflammatory response.[60–62] Pro-inflammatory cytokines such as IL-1β and TNFα has been shown to produce MMP-9 and MMP-3 in cultured astrocytes and microglia (reviewed in[62]). MMP-9 aggravates vasogenic edema development by degrading the basal lamina located between the astrocytic endfeet and endothelia.[62] Of particular interest is the link of MMP with AQP4; MMP-2 and MMP-9 are known to degrade agrin and MMP-3 degrades dystroglycan,[63] two proteins that have a critical role in the maintenance of the OAP.[64–67] So, when MMP are upregulated after a neuroinflammatory response, more AQP4-OAPs will be disorganized, leading to a possible disruption of the BBB and edema. Vasogenic edema development can further damage the endothelia by increased water volume and therefore increased hydrostatic pressure. Thus, if there is decreased BBB disruption, there will be less pro-inflammatory cytokines, MMPs, and edema (Figure 2).

Figure 2.

Schematic summary of a beneficial role AQP4 upregulation plays during the edema resolution phase. The upregulation of AQP4 causes increased water clearance from the tissue, which in turn causes decreased BBB disruption because of decreased pressure, and there is less neutrophil infiltration and decreased pro-inflammatory cytokines. This cause decreased MMP production,62 which possibly results in less destruction of the basal lamina and tight junctions, causes an even greater decrease of the BBB. In another pathway (dotted lines), the increased water clearance from the tissue and extracellular space causes changes in the osmotic pressure, changing the activation state of the stretch activated ion channels expressed in microglia,86–88 causing less microglial activation, thus causing decreased pro-inflammatory cytokine. The resulting decrease in BBB disruption/permeability leads to decreased vasogenic edema or better edema resolution. Finally, this scheme outlines the potential link between AQP4, edema and neuroinflammation.

Magnetic Resonance Imaging and AQP4

One useful modality in assessing injury severity and outcome in cerebral pathological conditions both in clinics and research is magnetic resonance imaging (MRI). Because MRI detects changes observed via the excitation of water molecules, the presence of the water channel protein, AQP4 in astrocytes suggests a possible involvement in MRI. Diffusion-weighted magnetic resonance imaging (DWI) is widely used as a diagnostic tool in clinical and research settings to assess edematous damage after various brain pathologies from ischemic stroke to various neuroinflammatory diseases.[57,68–71] The apparent diffusion coefficient (ADC) is obtained from DWI and is used to evaluate cerebral changes in clinical and experimental models. A decrease in the ADC is classically associated with a decrease in the extracellular space during cell swelling after brain injury.[70] More recently, ADC changes have been hypothesized to be linked with the level of expression of AQP4. Several experiments have shown increases in AQP4 expression and increased ADC[71] and decreased AQP4 expression with decreased ADC.[58,72] Of note, Tourdias et al.[2] have recently shown in a model of focal inflammation that AQP4 upregulation was associated with early edema formation via increased ADC, peak BBB disruption, and increased pro-inflammatory cytokine secretion. Diffusion tensor imaging (DTI) takes into account the non-uniform directionality of water flow (anisotropy) in the brain. This anisotropy has mainly been attributed to myelinated neuronal axons in the white matter tract, but recent evidence has hinted towards the possible role of astrocytes and glial scars in DTI signal changes after traumatic brain injury.[73] In fact, increased anisotropy was correlated with reactive astrocytes and not with axonal changes in the perilesional cortex.[73] This idea is supported by a study showing a correlation with changes in DTI signals associated with hypertrophic astrocytes and increase of AQP4.[74] As AQP4 expression changes after brain injury in astrocytes and microglia, it is rational to think that MRI may be a useful tool to evaluate the evolution of the neuroinflammatory process, especially in conjunction with AQP4 and edema.

Edema Resolution in Inflammatory Conditions and AQP4

In focal brain inflammation, AQP4 expression is upregulated during the edema resolution phase at 2 to 14 days post injury.[2] However the exact role of this increase in AQP4 is still a matter of discussion. In brain injection of L-α-lysophosphatidylcholine, a significant increase in AQP4 expression was observed at the edema resolution phase (7, 14, and 20 days post injection) compared to the edema build-up phase (1 and 3 days post injection). In this model, the edema resolution phase was defined as a return to baseline for ADC values and a lower IL-1β mRNA level, compared to the edema build-up phase.[2] Interestingly, a similar observation was made in a model of juvenile traumatic brain injury with upregulation of AQP4 observed during the edema resolution period when ADC is returning to normal.[41] These data suggest that the presence of AQP4 plays a positive role during edema resolution by facilitating water extravasations from the brain parenchyma to liquid compartments including CSF and blood vessels (Figure 2).

In stroke pathophysiology, animals with a pre-existing inflammatory condition had aggravated stroke outcomes as seen by more edema and BBB damage at 24 h after injury compared to groups with no pre-existing inflammation in the periphery.[75] Pre-existing systemic inflammation induced a surge in the levels of IL-1 in the ischemic cerebral cortex.[75] Interestingly, increase in IL-1α expression bordering dilated blood vessels in the ipsilateral cortex was observed, signifying a possible direct role of pro-inflammatory cytokines on edema formation.[75] Because IL-1β has been shown to induce AQP4 in astrocytes,[76,77] and blocking either AQP4 through gene deletion[78] or IL-1β through anti-IL-1β antibody[79] was seen to decrease edema, AQP4 may be a possible target for systemic inflammation leading to increased edema. In a mouse model of atherosclerosis (APOE−/− mice under high fat diet), development of chronic inflammation due to adhesion of a large number of T cells and macrophages in the vasculature,[80] as well as microglial activation in the brain is observed.[81] These mice upon aging showed BBB leakage and higher astrogliosis associated with increased AQP4.[4] These changes may contribute to a worse outcome in aged atherosclerotic patients who suffer an ischemic stroke because of higher risk of edema.

AQP4 and Microglial Activation After Injury

There are recent interesting data concerning the relationship between AQP4 and microglial activation. A link between neuroinflammation and AQP4 was described using the AQP4−/− mice that are more susceptible to seizures (decreased seizure latency and increased seizure severity) compared to WT 1 month after TBI and associated with a decrease in neuroinflammatory processes.[82] This difference is related to the neuroinflammatory response showing less astrogliosis and increased microglial activation in AQP4−/− compared to WT mice. Minocycline injection in AQP4−/− inhibited the increase in microglia and also mitigated the severity of the post-traumatic seizure.[82] Similar observations were reported in a model of cryoinjury with increased microglia and reduced astrogliosis in AQP4−/− mice compared to WT at 7 and 14 days post injury. In this model the authors reported a decrease in the lesion volume and lower neuronal loss in AQP4−/− mice compared to WT at 1 day after injury, and the opposite result at 7 and 14 days.[83] Similarly, in adult rats, intravenous minocycline administration after TBI[84] and subarachnoid hemorrhage[85] resulted in less BBB disruption associated with decreased MMP9 and AQP4 at 1 day post injury. In our lab, we have also observed that treatment with small interference RNA (siRNA) targeted against AQP4 (siAQP4) after juvenile TBI showed a decrease in AQP4 associated with less BBB disruption, edema, more NeuN positive cells, and better behavior outcomes compared to the control group at 3 days post injury (unpublished data). As shown in the adult model, we have also noticed a significant increase of activated microglia cells and decreased astrogliosis around the lesion at 3 days post injury in siAQP4-treated rats compared to controls (unpublished data). All together, these data underline that changes in AQP4 expression are associated with changes in astrogliosis and microglia activation in acute brain injury (Figure 3). Astrogliosis may require the presence of AQP4 to facilitate the water movement necessary for the migration[29,40] and hypertrophy. However, the mechanism behind the decrease of the AQP4 and activation of microglia is less obvious and still unknown. One possible mechanism behind the changes observed in post-traumatic or ischemic microglia activation and cytokine release in response to AQP4 downregulation or inhibition may be partly due to the presence of stretch-activated Cl channels expressed in microglia.[86,87] Stretch-activated/swelling-activated Cl channels are activated by osmotic stress.[88] It has been observed that the activation of these channels contributes the maintenance of the non-activated (ramified) phenotype of microglia.[86] Because AQP4 is responsible for water transport, inhibition of AQP4 either through genetic deletion or siRNA will alter the osmotic stress within the extracellular space surrounding the microglia, changing the activation status of the swelling activated chloride channels, resulting in microglial activation (Figures 2 and 3). Another possibility lies in the cross-talk that occurs between astrogliosis and microglial activation.[34] It is possible that the decreased extent of injury-induced reactive astrogliosis as a result of knocking down AQP4 caused increased microglial activity.

Figure 3.

AQP4 distribution in the astrocyte in normal cortex and after brain injury. (A) Confocal picture of AQP4 immunostaining (red, arrow heads) in normal brain shows the presence of the water channel protein on the astrocyte endfoot (GFAP staining, green, arrow) in contact to the blood vessels in the cortex. (B) Confocal pictures of the AQP4 immunostaining (red) on reactive astrocytes revealed with GFAP immunolabelling (green) in the cortex after traumatic brain injury. The presence of the AQP4 staining is not only localized on the endfeet in contact to the blood vessels but also distributed in all astrocyte processes (arrows). Scale bar 10 μm.

In summary, the presence of AQP4 seems to play a detrimental role acutely, but at a later phase starting from around 7 days post injury for at least 1 month, AQP4 may play a beneficial role that seems to be involved with inhibiting activation of microglia and promoting edema resolution.

processing....