Knockout of the P2Y6 Receptor Prevents Peri-Infarct Neuronal Loss after Transient, Focal Ischemia in Mouse Brain

After stroke, there is a delayed neuronal loss in brain areas surrounding the infarct, which may in part be mediated by microglial phagocytosis of stressed neurons. Microglial phagocytosis of stressed or damaged neurons can be mediated by UDP released from stressed neurons activating the P2Y₆ receptor on microglia, inducing microglial phagocytosis of such neurons. We show evidence here from a small trial that the knockout of the P2Y₆ receptor, required for microglial phagocytosis of neurons, prevents the delayed neuronal loss after transient, focal brain ischemia induced by endothelin-1 injection in mice. Wild-type mice had neuronal loss and neuronal nuclear material within microglia in peri-infarct areas. P2Y₆ receptor knockout mice had no significant neuronal loss in peri-infarct brain areas seven days after brain ischemia. Thus, delayed neuronal loss after stroke may in part be mediated by microglial phagocytosis of stressed neurons, and the P2Y₆ receptor is a potential treatment target to prevent peri-infarct neuronal loss.

Neuronal Loss after Stroke Due to Microglial Phagocytosis of Stressed Neurons

After stroke, there is a rapid necrosis of all cells in the infarct, followed by a delayed loss of neurons both in brain areas surrounding the infarct, known as 'selective neuronal loss', and in brain areas remote from, but connected to, the infarct, known as 'secondary neurodegeneration'. Here we review evidence indicating that this delayed loss of neurons after stroke is mediated by the microglial phagocytosis of stressed neurons. After a stroke, neurons are stressed by ongoing ischemia, excitotoxicity and/or inflammation and are known to: (i) release "find-me" signals such as ATP, (ii) expose "eat-me" signals such as phosphatidylserine, and (iii) bind to opsonins, such as complement components C1q and C3b, inducing microglia to phagocytose such neurons. Blocking these factors on neurons, or their phagocytic receptors on microglia, can prevent delayed neuronal loss and behavioral deficits in rodent models of ischemic stroke. Phagocytic receptors on microglia may be attractive treatment targets to prevent delayed neuronal loss after stroke due to the microglial phagocytosis of stressed neurons.

Combination of Single- and Paired-Pulse Somatosensory Evoked Potentials in Ischemic Monitoring: Preliminary Investigation in Carotid Endarterectomy

Introduction

Severe ischemia induces cerebral excitability imbalance before completion of infarct. To investigate the clinical availability of this imbalance with ischemic monitoring, paired-pulse somatosensory evoked potentials (SEPs) were performed in conjunction with conventional SEPs during carotid endarterectomy.

Methods

For carotid endarterectomy patients with hemodynamic deficits of the middle cerebral artery area (n = 34), the excitability imbalances (Q) were measured by paired-pulse SEPs, wherein the second response (A₂) was divided by the first (A₁; Q = A₂/A₁). Regional cerebral saturation (rSO₂) was also measured. Occlusion was performed twice using shunting.

Results

Each carotid occlusion induced a significant decrease in mean A₁ and rSO₂, and an increase in mean Q values (p < 0.001), which returned to the baseline level after occlusion. While neuronal imbalances were mostly transient, persistently increased Q values were observed in four cases (11.8%), all indicating postoperative abnormalities in diffusion-weighted magnetic resonance imaging (100%). Meanwhile, A₁ detected the postoperative abnormality in only one case (25%). Preoperative Q values at the time of surgery were significantly higher in symptomatic patients having the upper limb deficits than those without (p < 0.01), indicating persistent or permanent imbalances.

Conclusion

Paired-pulse SEPs reliably identified transient, persistent or permanent neuronal imbalances, depending on the ischemic severity. These preliminary results indicated that paired-pulse SEPs, in combination with conventional SEPs (A₁), may offer better ischemic monitoring.

Regional Expression of Act-MMP3 Contributes to the Selective Loss of Neurons in Ganglion Cell Layers following Acute Retinal ischemia/Reperfusion Injury

Purpose: Evidences suggest that during ischemia/reperfusion events, neuronal loss in ganglion cell layers (GCLs) occurs initially in the peripheral retinae followed by the central. However, which key molecule or factor mediates this selective loss needs elucidation. In the present study, we detected the regional expression of active matrix metalloproteinase 3 (Act-MMP3) in the central and peripheral rat retinae following acute retinal ischemia/reperfusion (RI/R) injury and explored the effects and mechanisms of this regional expression on the selective neuronal loss in GCLs.Methods: QPCR and Western Blotting were used to detect the expression of Act-MMP3 in the central part and peripheral part of the adult rat retinae. Immunofluorescence and double immunofluorescence were used to assess the number of NeuN-positive cells in the GCLs and Iba-1+CD 68-positive cells were determined. Additionally, the Linear-regression analysis was performed to test the correlation between the ODV of Act-MMP3 and the neuronal loss in the GCLs/Iba-1+CD 68 positive cells in retinae.Results: An evident up-regulation of active matrix metalloproteinase 3 (Act-MMP3) in peripheral retinae preceded to that in central region following acute RI/R. We found Act-MMP3 up-regulation to be associated with the selective neuronal loss in GCLs (central: r = 0.7566, p < .0001, r² = 0.5724; peripheral: r = 0.8241, p < .0001, r² = 0.6792). Suppressing Act-MMP3 ameliorated the selective neuronal loss in GCLs following acute RI/R. Furthermore, the activation of microglia in the peripheral retinae also preceded to that in the central and was found to be correlated with the regional expression of Act-MMP3 (Central: r = 0.8540, p < .0001, r² = 0.7294; Peripheral: r = 0.7820, p < .0001, r² = 0.6116). Suppressing Act-MMP3 ameliorated the microglia regional activation following acute RI/R.Conclusion: The regional expression of Act-MMP3 in the rat retinae may contribute to the selective neuronal loss in GCLs and microglia regional activation in acute RI/R.

Assessing the Effects of Cytoprotectants on Selective Neuronal Loss, Sensorimotor Deficit and Microglial Activation after Temporary Middle Cerebral Occlusion

Although early reperfusion after stroke salvages the still-viable ischemic tissue, peri-infarct selective neuronal loss (SNL) can cause sensorimotor deficits (SMD). We designed a longitudinal protocol to assess the effects of cytoprotectants on SMD, microglial activation (MA) and SNL, and specifically tested whether the KCa3.1-blocker TRAM-34 would prevent SNL. Spontaneously hypertensive rats underwent 15 min middle-cerebral artery occlusion and were randomized into control or treatment group, which received TRAM-34 intraperitoneally for 4 weeks starting 12 h after reperfusion. SMD was assessed longitudinally using the sticky-label test. MA was quantified at day 14 using in vivo [¹¹C]-PK111195 positron emission tomography (PET), and again across the same regions-of-interest template by immunofluorescence together with SNL at day 28. SMD recovered significantly faster in the treated group (p = 0.004). On PET, MA was present in 5/6 rats in each group, with no significant between-group difference. On immunofluorescence, both SNL and MA were present in 5/6 control rats and 4/6 TRAM-34 rats, with a non-significantly lower degree of MA but a significantly (p = 0.009) lower degree of SNL in the treated group. These findings document the utility of our longitudinal protocol and suggest that TRAM-34 reduces SNL and hastens behavioural recovery without marked MA blocking at the assessed time-points.

Mapping the dynamics of brain perfusion using functional ultrasound in a rat model of transient middle cerebral artery occlusion

Following middle cerebral artery occlusion, tissue outcome ranges from normal to infarcted depending on depth and duration of hypoperfusion as well as occurrence and efficiency of reperfusion. However, the precise time course of these changes in relation to tissue and behavioral outcome remains unsettled. To address these issues, a three-dimensional wide field-of-view and real-time quantitative functional imaging technique able to map perfusion in the rodent brain would be desirable. Here, we applied functional ultrasound imaging, a novel approach to map relative cerebral blood volume without contrast agent, in a rat model of brief proximal transient middle cerebral artery occlusion to assess perfusion in penetrating arterioles and venules acutely and over six days thanks to a thinned-skull preparation. Functional ultrasound imaging efficiently mapped the acute changes in relative cerebral blood volume during occlusion and following reperfusion with high spatial resolution (100 µm), notably documenting marked focal decreases during occlusion, and was able to chart the fine dynamics of tissue reperfusion (rate: one frame/5 s) in the individual rat. No behavioral and only mild post-mortem immunofluorescence changes were observed. Our study suggests functional ultrasound is a particularly well-adapted imaging technique to study cerebral perfusion in acute experimental stroke longitudinally from the hyper-acute up to the chronic stage in the same subject.

Normobaric hyperoxia markedly reduces brain damage and sensorimotor deficits following brief focal ischaemia

'True' transient ischaemic attacks are characterized not only clinically, but also radiologically by a lack of corresponding changes on magnetic resonance imaging. During a transient ischaemic attack it is assumed that the affected tissue is penumbral but rescued by early spontaneous reperfusion. There is, however, evidence from rodent studies that even brief focal ischaemia not resulting in tissue infarction can cause extensive selective neuronal loss associated with long-lasting sensorimotor impairment but normal magnetic resonance imaging. Selective neuronal loss might therefore contribute to the increasingly recognized cognitive impairment occurring in patients with transient ischaemic attacks. It is therefore relevant to consider treatments to reduce brain damage occurring with transient ischaemic attacks. As penumbral neurons are threatened by markedly constrained oxygen delivery, improving the latter by increasing arterial O2 content would seem logical. Despite only small increases in arterial O2 content, normobaric oxygen therapy experimentally induces significant increases in penumbral O2 pressure and by such may maintain the penumbra alive until reperfusion. Nevertheless, the effects of normobaric oxygen therapy on infarct volume in rodent models have been conflicting, although duration of occlusion appeared an important factor. Likewise, in the single randomized trial published to date, early-administered normobaric oxygen therapy had no significant effect on clinical outcome despite reduced diffusion-weighted imaging lesion growth during therapy. Here we tested the hypothesis that normobaric oxygen therapy prevents both selective neuronal loss and sensorimotor deficits in a rodent model mimicking true transient ischaemic attack. Normobaric oxygen therapy was applied from the onset and until completion of 15 min distal middle cerebral artery occlusion in spontaneously hypertensive rats, a strain representative of the transient ischaemic attack-prone population. Whereas normoxic controls showed normal magnetic resonance imaging but extensive cortical selective neuronal loss associated with microglial activation (present both at Day 14 in vivo and at Day 28 post-mortem) and marked and long-lasting sensorimotor deficits, normobaric oxygen therapy completely prevented sensorimotor deficit (P < 0.02) and near-completely Day 28 selective neuronal loss (P < 0.005). Microglial activation was substantially reduced at Day 14 and completely prevented at Day 28 (P = 0.002). Our findings document that normobaric oxygen therapy administered during ischaemia nearly completely prevents the neuronal death, microglial inflammation and sensorimotor impairment that characterize this rodent true transient ischaemic attack model. Taken together with the available literature, normobaric oxygen therapy appears a promising therapy for short-lasting ischaemia, and is attractive clinically as it could be started at home in at-risk patients or in the ambulance in subjects suspected of transient ischaemic attack/early stroke. It may also be a straightforward adjunct to reperfusion therapies, and help prevent subtle brain damage potentially contributing to long-term cognitive and sensorimotor impairment in at-risk populations.