Multi-modal characterisation of the neocortical clip model of focal cerebral ischaemia by MRI, behaviour and immunohistochemistry

Mapping the acute time course of immune cell infiltration into an ECM hydrogel in a rat model of stroke using 19F MRI

Extracellular matrix (ECM) hydrogel implantation into a stroke-induced tissue cavity invokes a robust cellular immune response. However, the spatio-temporal dynamics of immune cell infiltration into peri-infarct brain tissues versus the ECM-bioscaffold remain poorly understood. We here tagged peripheral immune cells using perfluorocarbon (PFC) nanoemulsions that afford their visualization by ¹⁹F magnetic resonance imaging (MRI). Prior to ECM hydrogel implantation, only blood vessels could be detected using ¹⁹F MRI. Using "time-lapse" ¹⁹F MRI, we established the infiltration of immune cells into the peri-infarct area occurs 5-6 h post-ECM implantation. Immune cells also infiltrated through the stump of the MCA, as well as a hydrogel bridge that formed between the tissue cavity and the burr hole in the skull. Tissue-based migration into the bioscaffold was observed between 9 and 12 h with a peak signal measured between 12 and 18 h post-implantation. Fluorescence-activated cell sorting of circulating immune cells revealed that 9% of cells were labeled with PFC nanoemulsions, of which the vast majority were neutrophils (40%) or monocytes (48%). Histology at 24 h post-implantation, in contrast, indicated that macrophages (35%) were more numerous in the peri-infarct area than neutrophils (11%), whereas the vast majority of immune cells within the ECM hydrogel were neutrophils (66%). Only a small fraction (12%) of immune cells did not contain PFC nanoemulsions, indicating a low type II error for ¹⁹F MRI. ¹⁹F MRI hence provides a unique tool to improve our understanding of the spatio-temporal dynamics of immune cells invading bioscaffolds and effecting biodegradation.

Post-Stroke Timing of ECM Hydrogel Implantation Affects Biodegradation and Tissue Restoration

Extracellular matrix (ECM) hydrogel promotes tissue regeneration in lesion cavities after stroke. However, a bioscaffold's regenerative potential needs to be considered in the context of the evolving pathological environment caused by a stroke. To evaluate this key issue in rats, ECM hydrogel was delivered to the lesion core/cavity at 7-, 14-, 28-, and 90-days post-stroke. Due to a lack of tissue cavitation 7-days post-stroke, implantation of ECM hydrogel did not achieve a sufficient volume and distribution to warrant comparison with the other time points. Biodegradation of ECM hydrogel implanted 14- and 28-days post-stroke were efficiently (80%) degraded by 14-days post-bioscaffold implantation, whereas implantation 90-days post-stroke revealed only a 60% decrease. Macrophage invasion was robust at 14- and 28-days post-stroke but reduced in the 90-days post-stroke condition. The pro-inflammation (M1) and pro-repair (M2) phenotype ratios were equivalent at all time points, suggesting that the pathological environment determines macrophage invasion, whereas ECM hydrogel defines their polarization. Neural cells (neural progenitors, neurons, astrocytes, oligodendrocytes) were found at all time points, but a 90-days post-stroke implantation resulted in reduced densities of mature phenotypes. Brain tissue restoration is therefore dependent on an efficient delivery of a bioscaffold to a tissue cavity, with 28-days post-stroke producing the most efficient biodegradation and tissue regeneration, whereas by 90-days post-stroke, these effects are significantly reduced. Improving our understanding of how the pathological environment influences biodegradation and the tissue restoration process is hence essential to devise engineering strategies that could extend the therapeutic window for bioscaffolds to repair the damaged brain.

ECM hydrogel improves the delivery of PEG microsphere-encapsulated neural stem cells and endothelial cells into tissue cavities caused by stroke

Intracerebral implantation of neural stem cells (NSCs) to treat stroke remains an inefficient process with <5% of injected cells being retained. To improve the retention and distribution of NSCs after a stroke, we investigated the utility of NSCs' encapsulation in polyethylene glycol (PEG) microspheres. We first characterized the impact of the physical properties of different syringes and needles, as well as ejection speed, upon delivery of microspheres to the stroke injured rat brain. A 20 G needle size at a 10 μL/min flow rate achieved the most efficient microsphere ejection. Secondly, we optimized the delivery vehicles for in vivo implantation of PEG microspheres. The suspension of microspheres in extracellular matrix (ECM) hydrogel showed superior retention and distribution in a cortical stroke caused by photothrombosis, as well as in a striatal and cortical cavity ensuing middle cerebral artery occlusion (MCAo). Thirdly, NSCs or NSCs + endothelial cells (ECs) encapsulated into biodegradable microspheres were implanted into a large stroke cavity. Cells in microspheres exhibited a high viability, survived freezing and transport. Implantation of 110 cells/microsphere suspended in ECM hydrogel produced a highly efficient delivery that resulted in the widespread distribution of NSCs in the tissue cavity and damaged peri-infarct tissues. Co-delivery of ECs enhanced the in vivo survival and distribution of ∼1.1 million NSCs. The delivery of NSCs and ECs can be dramatically improved using microsphere encapsulation combined with suspension in ECM hydrogel. These biomaterial innovations are essential to advance clinical efforts to improve the treatment of stroke using intracerebral cell therapy.

A systematic optimization of 19F MR image acquisition to detect macrophage invasion into an ECM hydrogel implanted in the stroke-damaged brain

¹⁹F-MR imaging of perfluorocarbon (PFC)-labeled macrophages can provide a unique insight into their participation and spatio-temporal dynamics of inflammatory events, such as the biodegradation of an extracellular matrix (ECM) hydrogel implanted into a stroke cavity. To determine the most efficient acquisition strategy for ¹⁹F-MR imaging, five commonly used sequences were optimized using a design of experiment (DoE) approach and compared based on their signal-to-noise ratio (SNR). The fast imaging with steady-state precession (FISP) sequence produced the most efficient detection of a ¹⁹F signal followed by the rapid acquisition with relaxation enhancement (RARE) sequence. The multi-slice multi-echo (MSME), fast low angle shot (FLASH), and zero echo time (ZTE) sequences were significantly less efficient. Imaging parameters (matrix/voxel size; slice thickness, number of averages) determined the accuracy (i.e. trueness and precision) of object identification by reducing partial volume effects, as determined by analysis of the point spread function (PSF). A 96 × 96 matrix size (0.35 mm³) produced the lowest limit of detection (LOD) for RARE (2.85 mM PFPE; 119 mM ¹⁹F) and FISP (0.43 mM PFPE; 18.1 mM ¹⁹F), with an SNR of 2 as the detection threshold. Imaging of a brain phantom with PFC-labeled macrophages invading an ECM hydrogel further illustrated the impact of these parameter changes. The systematic optimization of sequence and imaging parameters provides the framework for an accurate visualization of ¹⁹F-labeled macrophage distribution and density in the brain. This will enhance our understanding of the contribution of periphery-derived macrophages in bioscaffold degradation and its role in brain tissue regeneration.

Biodegradation of ECM hydrogel promotes endogenous brain tissue restoration in a rat model of stroke

The brain is considered to have a limited capacity to repair damaged tissue and no regenerative capacity following injury. Tissue lost after a stroke is therefore not spontaneously replaced. Extracellular matrix (ECM)-based hydrogels implanted into the stroke cavity can attract endogenous cells. These hydrogels can be formulated at different protein concentrations that govern their rheological and inductive properties. We evaluated histologically 0, 3, 4 and 8 mg/mL of porcine-derived urinary bladder matrix (UBM)-ECM hydrogel concentrations implanted in a 14-day old stroke cavity. Less concentrated hydrogels (3 and 4 mg/mL) were efficiently degraded with a 95% decrease in volume by 90 days, whereas only 32% of the more concentrated and stiffer hydrogel (8 mg/mL) was resorbed. Macrophage infiltration and density within the bioscaffold progressively increased in the less concentrated hydrogels and decreased in the 8 mg/mL hydrogels. The less concentrated hydrogels showed a robust invasion of endothelial cells with neovascularization. No neovascularization occurred with the stiffer hydrogel. Invasion of neural cells increased with time in all hydrogel concentrations. Differentiation of neural progenitors into mature neurons with axonal projections was evident, as well as a robust invasion of oligodendrocytes. However, relatively few astrocytes were present in the ECM hydrogel, although some were present in the newly forming tissue between degrading scaffold patches. Implantation of an ECM hydrogel partially induced neural tissue restoration, but a more complete understanding is required to evaluate its potential therapeutic application. STATEMENT OF SIGNIFICANCE: Extracellular matrix hydrogel promotes tissue regeneration in many peripheral soft tissues. However, the brain has generally been considered to lack the potential for tissue regeneration. We here demonstrate that tissue regeneration in the brain can be achieved using implantation of ECM hydrogel into a tissue cavity. A structure-function relationship is key to promote tissue regeneration in the brain. Specifically, weaker hydrogels that were retained in the cavity underwent an efficient biodegradation within 14 days post-implantation to promote a tissue restoration within the lesion cavity. In contrast, stiffer ECM hydrogel only underwent minor biodegradation and did not lead to a tissue restoration. Inductive hydrogels weaker than brain tissue provide the appropriate condition to promote an endogenous regenerative response that restores tissue in a cavity. This approach offers new avenues for the future treatment of chronic tissue damage caused by stroke and other acute brain injuries.

Long-term retention of ECM hydrogel after implantation into a sub-acute stroke cavity reduces lesion volume

Salvaging or functional replacement of damaged tissue caused by stroke in the brain remains a major therapeutic challenge. In situ gelation and retention of a hydrogel bioscaffold composed of 8mg/mL extracellular matrix (ECM) can induce a robust invasion of cells within 24h and potentially promote a structural remodeling to replace lost tissue. Herein, we demonstrate a long-term retention of ECM hydrogel within the lesion cavity. A decrease of approximately 32% of ECM volume is observed over 12weeks. Lesion volume, as measured by magnetic resonance imaging and histology, was reduced by 28%, but a battery of behavioral tests (bilateral asymmetry test; footfault; rotameter) did not reveal a therapeutic or detrimental effect of the hydrogel. Glial scarring and peri-infarct astrocytosis were equivalent between untreated and treated animals, potentially indicating that permeation into host tissue is required to exert therapeutic effects. These results reveal a marked difference of biodegradation of ECM hydrogel in the stroke-damaged brain compared to peripheral soft tissue repair. Further exploration of these structure-function relationships is required to achieve a structural remodeling of the implanted hydrogel, as seen in peripheral tissues, to replace lost tissue and promote behavioral recovery.

STATEMENT OF SIGNIFICANCE

In situ gelation of ECM is essential for its retention within a tissue cavity. The brain is a unique environment with restricted access that necessitates image-guided delivery through a thin needle to access tissue cavities caused by stroke, as well as other conditions, such as traumatic brain injury or glioma resection. Knowledge about a brain tissue response to implanted hydrogels remains limited, especially in terms of long-term effects and potential impact on behavioral function. We here address the long-term retention of hydrogel within the brain environment, its impact on behavioral function, as well as its ability to reduce further tissue deformation caused by stroke. This study highlights considerable differences in the brain's long-term response to an ECM hydrogel compared to peripheral soft tissue. It underlines the importance of understanding the effect of the structural presence of a hydrogel within a cavity upon host brain tissue and behavioral function. As demonstrated herein, ECM hydrogel can fill a cavity long-term to reduce further progression of the cavity, while potentially serving as a reservoir for local drug or cell delivery.

Diamagnetic chemical exchange saturation transfer (diaCEST) affords magnetic resonance imaging of extracellular matrix hydrogel implantation in a rat model of stroke

Extracellular matrix (ECM) is widely used as an inductive biological scaffold to repair soft tissue after injury by promoting functional site-appropriate remodeling of the implanted material. However, there is a lack of non-invasive analysis methods to monitor the remodeling characteristics of the ECM material after implantation and its biodegradation over time. We describe the use of diamagnetic chemical exchange saturation transfer (CEST) magnetic resonance imaging to monitor the distribution of an ECM hydrogel after intracerebral implantation into a stroke cavity. In vitro imaging indicated a robust concentration-dependent detection of the ECM precursor and hydrogel at 1.8 and 3.6 ppm, which broadly corresponded to chondroitin sulfate and fibronectin. This detection was robust to changes in pH and improved at 37 °C. In vivo implantation of ECM hydrogel into the stroke cavity in a rat model corresponded macroscopically to the distribution of biomaterial as indicated by histology, but mismatches were also evident. Indeed, CEST imaging detected an endogenous "increased deposition". To account for this endogenous activity, pre-implantation images were subtracted from post-implantation images to yield a selective visualization of hydrogel distribution in the stroke cavity and its evolution over 7 days. The CEST detection of ECM returned to baseline within 3 days due to a decrease in fibronectin and chondroitin sulfate in the hydrogel. The distribution of ECM hydrogel within the stroke cavity is hence feasible in vivo, but further advances are required to warrant a selective long-term monitoring in the context of biodegradation.

Quantification of MRI and MRS characteristics changes in a rat model at different stage of cerebral ischemia

BACKGROUND

A better understanding the mechanisms of cerebral ischemia is important both for diagnosis and treatment.

OBJECTIVE

The study aimed to quantify several characteristics of magnetic resonance imaging (MRI) and magnetic resonance spectroscopy (MRS) to indicate the brain tissue changes at different stage of cerebral ischemia in rats.

METHODS

In the present study, a rat model of cerebral ischemia was established by middle cerebral artery occlusion (MCAO) in the left hemisphere. MRI and MRS were performed on 15 Sprague Dawley rats 4 H, 24 H, and 1 W after MCAO. Apparent diffusion coefficient (ADC), relative ADC including FNR, PNR, PNF, and metabolite ratio NCC were proposed to reflect the changes of water diffusion and metabolism in brain tissue.

RESULTS

ADCs of focal zone and penumbra zone from 1 W group were significantly larger than those from 4H group, respectively (both p < 0.05). PNR and PNF of 24H and 1 W groups were significantly less than 4H group (all p < 0.01). NCCs of focal zone and penumbra zone were significantly less than the normal zone within 4H, 24H, and 1 W groups, respectively (both p < 0.01). While NCCs of penumbra zone from 24H and 1 W groups were significantly larger than 4H group (both p < 0.01).

CONCLUSION

We conclude that combination of MRI and MRS characteristics can provide significant indicators for ischemic damage at different stage of cerebral ischemia in a rat model.

ECM hydrogel for the treatment of stroke: Characterization of the host cell infiltrate

Brain tissue loss following stroke is irreversible with current treatment modalities. The use of an acellular extracellular matrix (ECM), formulated to produce a hydrogel in situ within the cavity formed by a stroke, was investigated as a method to replace necrotic debris and promote the infiltration of host brain cells. Based on magnetic resonance imaging measurements of lesion location and volume, different concentrations of ECM (0, 1, 2, 3, 4, 8 mg/mL) were injected at a volume equal to that of the cavity (14 days post-stroke). Retention of ECM within the cavity occurred at concentrations >3 mg/mL. A significant cell infiltration into the ECM material in the lesion cavity occurred with an average of ∼36,000 cells in the 8 mg/mL concentration within 24 h. An infiltration of cells with distances of >1500 μm into the ECM hydrogel was observed, but the majority of cells were at the tissue/hydrogel boundary. Cells were typically of a microglia, macrophage, or neural and oligodendrocyte progenitor phenotype. At the 8 mg/mL concentration, ∼60% of infiltrating cells were brain-derived phenotypes and 30% being infiltrating peripheral macrophages, polarizing toward an M2-like anti-inflammatory phenotype. These results suggest that an 8 mg/mL ECM concentration promotes a significant acute endogenous repair response that could potentially be exploited to treat stroke.

Concentration-dependent rheological properties of ECM hydrogel for intracerebral delivery to a stroke cavity

Biomaterials composed of mammalian extracellular matrix (ECM) promote constructive tissue remodeling with minimal scar tissue formation in many anatomical sites. However, the optimal shape and form of ECM scaffold for each clinical application can vary markedly. ECM hydrogels have been shown to promote chemotaxis and differentiation of neuronal stem cells, but minimally invasive delivery of such scaffold materials to the central nervous system (CNS) would require an injectable form. These ECM materials can be manufactured to exist in fluid phase at room temperature, while forming hydrogels at body temperature in a concentration-dependent fashion. Implantation into the lesion cavity after a stroke could hence provide a means to support endogenous repair mechanisms. Herein, we characterize the rheological properties of an ECM hydrogel composed of urinary bladder matrix (UBM) that influence its delivery and in vivo interaction with host tissue. There was a notable concentration-dependence in viscosity, stiffness, and elasticity; all characteristics important for minimally invasive intracerebral delivery. An efficient MRI-guided injection with drainage of fluid from the cavity is described to assess in situ hydrogel formation and ECM retention at different concentrations (0, 1, 2, 3, 4, and 8mg/mL). Only ECM concentrations >3mg/mL gelled within the stroke cavity. Lower concentrations were not retained within the cavity, but extensive permeation of the liquid phase ECM into the peri-infarct area was evident. The concentration of ECM hydrogel is hence an important factor affecting gelation, host-biomaterial interface, as well intra-lesion distribution.

STATEMENT OF SIGNIFICANCE

Extracellular matrix (ECM) hydrogel promotes constructive tissue remodeling in many tissues. Minimally invasive delivery of such scaffold materials to the central nervous system (CNS) would require an injectable form that exists in fluid phase at room temperature, while forming hydrogels at body temperature in a concentration-dependent fashion. We here report the rheological characterization of an injectable ECM hydrogel and its concentration-dependent delivery into a lesion cavity formed after a stroke based on MRI-guidance. The concentration of ECM determined its retention within the cavity or permeation into tissue and hence influenced its interaction with the host brain. This study demonstrates the importance of understanding the structure-function relationship of biomaterials to guide particular clinical applications.

What is the Optimal Duration of Middle-Cerebral Artery Occlusion Consistently Resulting in Isolated Cortical Selective Neuronal Loss in the Spontaneously Hypertensive Rat?

Introduction and objectives: Selective neuronal loss (SNL) in the reperfused penumbra may impact clinical recovery and is thus important to investigate. Brief proximal middle cerebral artery occlusion (MCAo) results in predominantly striatal SNL, yet cortical damage is more relevant given its behavioral implications and that thrombolytic therapy mainly rescues the cortex. Distal temporary MCAo (tMCAo) does target the cortex, but the optimal occlusion duration that results in isolated SNL has not been determined. In the present study, we assessed different distal tMCAo durations looking for consistently pure SNL.

Methods: Microclip distal tMCAo (md-tMCAo) was performed in ~6-month old male spontaneously hypertensive rats (SHRs). We previously reported that 45 min md-tMCAo in SHRs results in pan-necrosis in the majority of subjects. Accordingly, three shorter MCAo durations were investigated here in decremental succession, namely 30, 22, and 15 min (n = 3, 3, and 7 subjects, respectively). Recanalization was confirmed by MR angiography just prior to brain collection at 28 days and T2-weighted MRI was obtained for characterization of ischemic lesions. NeuN, OX42, and GFAP immunohistochemistry appraised changes in neurons, microglia, and astrocytes, respectively. Ischemic lesions were categorized into three main types: (1) pan-necrosis; (2) partial infarction; and (3) SNL.

Results: Pan-necrosis or partial infarction was present in all 30 min and 22 min subjects, but not in the 15 min group (p < 0.001), in which isolated cortical SNL was consistently present. MRI revealed characteristic hyperintense abnormalities in all rats with pan-necrosis or partial infarction, but no change in any 15 min subject.

Conclusion: We found that 15 min distal MCAo consistently resulted in pure cortical SNL, whereas durations equal or longer than 22 min consistently resulted in infarcts. This model may be of use to study the pathophysiology of cortical SNL and its prevention by appropriate interventions.

Glycosaminoglycans and glycomimetics in the central nervous system

With recent advances in the construction of synthetic glycans, selective targeting of the extracellular matrix (ECM) as a potential treatment for a wide range of diseases has become increasingly popular. The use of compounds that mimic the structure or bioactive function of carbohydrate structures has been termed glycomimetics. These compounds are mostly synthetic glycans or glycan-binding constructs which manipulate cellular interactions. Glycosaminoglycans (GAGs) are major components of the ECM and exist as a diverse array of differentially sulphated disaccharide units. In the central nervous system (CNS), they are expressed by both neurons and glia and are crucial for brain development and brain homeostasis. The inherent diversity of GAGs make them an essential biological tool for regulating a complex range of cellular processes such as plasticity, cell interactions and inflammation. They are also involved in the pathologies of various neurological disorders, such as glial scar formation and psychiatric illnesses. It is this diversity of functions and potential for selective interventions which makes GAGs a tempting target. In this review, we shall describe the molecular make-up of GAGs and their incorporation into the ECM of the CNS. We shall highlight the different glycomimetic strategies that are currently being used in the nervous system. Finally, we shall discuss some possible targets in neurological disorders that may be addressed using glycomimetics.

Cortical selective neuronal loss, impaired behavior, and normal magnetic resonance imaging in a new rat model of true transient ischemic attacks

BACKGROUND AND PURPOSE

New-definition transient ischemic attacks (TIAs) are frequent but difficult to diagnose because magnetic resonance imaging (MRI)-negative by definition. However, hidden underlying cell damage might be present and account for the reported long-lasting cognitive impairment after TIAs. Most prior rodent models of true TIA targeted the striatum or have not been fully characterized. Here we present the MRI, behavioral, and quantitative cell changes characterizing a new rodent model of true TIA targeting the more behaviorally relevant cerebral cortex.

METHODS

Fifteen-minute distal middle cerebral artery occlusion was performed in 29 spontaneously hypertensive rats allowed to survive for 7 to 60 days. Behavior was assessed serially using both global neurological and fine sensorimotor tests. Diffusion- and T2-weighted MRI was obtained 20 min postreperfusion and again 7 to 60 days later, and then changes in neurons and microglia were quantified across the middle cerebral artery territory using immunohistochemistry.

RESULTS

No MRI changes or pan-necrosis were observed at any time point, but patchy cortical selective neuronal loss affected 28/29 rats, regardless of survival interval, together with topographically congruent microglial activation that gradually declined over time. The Neuroscore was unchanged, but there was marked contralateral sensorimotor impairment, still recovering by day 28.

CONCLUSIONS

Our new rodent model mimicking true cortical TIA is characterized by normal MRI, but consistent cortical selective neuronal loss and microglial activation and long-lasting sensorimotor deficits. By causing selective neuronal loss, TIAs and silent microemboli might affect neuronal reserve, thereby increasing long-term cognitive impairment risk. Selective neuronal loss and microglial activation might represent novel therapeutic targets that could be detectable in vivo after TIAs using appropriate imaging tracers.

Selective neuronal loss in ischemic stroke and cerebrovascular disease

As a sequel of brain ischemia, selective neuronal loss (SNL)-as opposed to pannecrosis (i.e. infarction)-is attracting growing interest, particularly because it is now detectable in vivo. In acute stroke, SNL may affect the salvaged penumbra and hamper functional recovery following reperfusion. Rodent occlusion models can generate SNL predominantly in the striatum or cortex, showing that it can affect behavior for weeks despite normal magnetic resonance imaging. In humans, SNL in the salvaged penumbra has been documented in vivo mainly using positron emission tomography and (11)C-flumazenil, a neuronal tracer validated against immunohistochemistry in rodent stroke models. Cortical SNL has also been documented using this approach in chronic carotid disease in association with misery perfusion and behavioral deficits, suggesting that it can result from chronic or unstable hemodynamic compromise. Given these consequences, SNL may constitute a novel therapeutic target. Selective neuronal loss may also develop at sites remote from infarcts, representing secondary 'exofocal' phenomena akin to degeneration, potentially related to poststroke behavioral or mood impairments again amenable to therapy. Further work should aim to better characterize the time course, behavioral consequences-including the impact on neurological recovery and contribution to vascular cognitive impairment-association with possible causal processes such as microglial activation, and preventability of SNL.

3D reconstruction of 2D fluorescence histology images and registration with in vivo MR images: application in a rodent stroke model

To validate and add value to non-invasive imaging techniques, the corresponding histology is required to establish biological correlates. We present an efficient, semi-automated image-processing pipeline that uses immunohistochemically stained sections to reconstruct a 3D brain volume from 2D histological images before registering these with the corresponding 3D in vivo magnetic resonance images (MRI). A multistep registration procedure that first aligns the "global" volume by using the centre of mass and then applies a rigid and affine alignment based on signal intensities is described. This technique was applied to a training set of three rat brain volumes before being validated on three normal brains. Application of the approach to register "abnormal" images from a rat model of stroke allowed the neurobiological correlates of the variations in the hyper-intense MRI signal intensity caused by infarction to be investigated. For evaluation, the corresponding anatomical landmarks in MR and histology were defined to measure the registration accuracy. A registration error of 0.249 mm (approximately one in-plane voxel dimension) was evident in healthy rat brains and of 0.323 mm in a rodent model of stroke. The proposed reconstruction and registration pipeline allowed for the precise analysis of non-invasive MRI and corresponding microstructural histological features in 3D. We were thus able to interrogate histology to deduce the cause of MRI signal variations in the lesion cavity and the peri-infarct area.

A comparison of automated anatomical-behavioural mapping methods in a rodent model of stroke

•

The first application of voxel lesion symptom mapping (VLSM) in rodents.

•

A comparison of VLSM with tensor based morphometry (TBM) methods in a stroke model.

•

Comparison of automated techniques with manual measurements and model power calculations.

•

Analysis of both local and non-local lesion effects.

•

Correlation of structural change with behaviour.

Neurological damage, due to conditions such as stroke, results in a complex pattern of structural changes and significant behavioural dysfunctions; the automated analysis of magnetic resonance imaging (MRI) and discovery of structural–behavioural correlates associated with these disorders remains challenging. Voxel lesion symptom mapping (VLSM) has been used to associate behaviour with lesion location in MRI, but this analysis requires the definition of lesion masks on each subject and does not exploit the rich structural information in the images. Tensor-based morphometry (TBM) has been used to perform voxel-wise structural analyses over the entire brain; however, a combination of lesion hyper-intensities and subtle structural remodelling away from the lesion might confound the interpretation of TBM. In this study, we compared and contrasted these techniques in a rodent model of stroke (n = 58) to assess the efficacy of these techniques in a challenging pre-clinical application. The results from the automated techniques were compared using manually derived region-of-interest measures of the lesion, cortex, striatum, ventricle and hippocampus, and considered against model power calculations. The automated TBM techniques successfully detect both lesion and non-lesion effects, consistent with manual measurements. These techniques do not require manual segmentation to the same extent as VLSM and should be considered part of the toolkit for the unbiased analysis of pre-clinical imaging-based studies.

Immunolocalization of Kisspeptin Associated with Amyloid-β Deposits in the Pons of an Alzheimer's Disease Patient

The pons region of the Alzheimer's disease (AD) brain is one of the last to show amyloid-β (Aβ) deposits and has been suggested to contain neuroprotective compounds. Kisspeptin (KP) is a hormone that activates the hypothalamic-pituitary-gonadal axis and has been suggested to be neuroprotective against Aβ toxicity. The localization of KP, plus the established endogenous neuroprotective compounds corticotropin releasing hormone (CRH) and catalase, in tissue sections from the pons region of a male AD subject has been determined in relation to Aβ deposits. Results showed Aβ deposits also stained with KP, CRH, and catalase antibodies. At high magnification the staining of deposits was either KP or catalase positive, and there was only a limited area of the deposits with KP-catalase colocalization. The CRH does not bind Aβ, whilst both KP and catalase can bind Aβ, suggesting that colocalization in Aβ deposits is not restricted to compounds that directly bind Aβ. The neuroprotective actions of KP, CRH, and catalase were confirmed in vitro, and fibrillar Aβ preparations were shown to stimulate the release of KP in vitro. In conclusion, neuroprotective KP, CRH, and catalase all colocalize with Aβ plaque-like deposits in the pons region from a male AD subject.

Neuroprotective effect of agmatine in rats with transient cerebral ischemia using MR imaging and histopathologic evaluation

PURPOSE

This study aimed to further investigate the effects of agmatine on brain edema in the rats with middle cerebral artery occlusion (MCAO) injury using magnetic resonance imaging (MRI) monitoring and biochemical and histopathologic evaluation.

MATERIALS AND METHODS

Following surgical induction of MCAO for 90min, agmatine was injected 5min after beginning of reperfusion and again once daily for the next 3 post-operative days. The events during ischemia and reperfusion were investigated by T2-weighted images (T2WI), serial diffusion-weighted images (DWI), calculated apparent diffusion coefficient (ADC) maps and contrast-enhanced T1-weighted images (CE-T1WI) during 3h-72h in a 1.5T Siemens MAGNETON Avanto Scanner. Lesion volumes were analyzed in a blinded and randomized manner. Triphenyltetrazolium chloride (TTC), Nissl, and Evans Blue stainings were performed at the corresponding sections.

RESULTS

Increased lesion volumes derived from T2WI, DWI, ADC, CE-T1WI, and TTC all were noted at 3h and peaked at 24h-48h after MCAO injury. TTC-derived infarct volumes were not significantly different from the T2WI, DWI-, and CE-T1WI-derived lesion volumes at the last imaging time (72h) point except for significantly smaller ADC lesions in the MCAO model (P<0.05). Volumetric calculation based on TTC-derived infarct also correlated significantly stronger to volumetric calculation based on last imaging time point derived on T2WI, DWI or CE-T1WI than ADC (P<0.05). At the last imaging time point, a significant increase in Evans Blue extravasation and a significant decrease in Nissl-positive cells numbers were noted in the vehicle-treated MCAO injured animals. The lesion volumes derived from T2WI, DWI, CE-T1WI, and Evans blue extravasation as well as the reduced numbers of Nissl-positive cells were all significantly attenuated in the agmatine-treated rats compared with the control ischemia rats (P<0.05).

CONCLUSION

Our results suggest that agmatine has neuroprotective effects against brain edema on a reperfusion model after transient cerebral ischemia.

Spatial and temporal MRI profile of ischemic tissue after the acute stages of a permanent mouse model of stroke

OBJECT

To characterize the progression of injured tissue resulting from a permanent focal cerebral ischemia after the acute phase, Magnetic Resonance Imaging (MRI) monitoring was performed on adult male C57BL/6J mice in the subacute stages, and correlated to histological analyses.

MATERIAL AND METHODS

Lesions were induced by electrocoagulation of the middle cerebral artery. Serial MRI measurements and weighted-images (T2, T1, T2* and Diffusion Tensor Imaging) were performed on a 9.4T scanner. Histological data (Cresyl-Violet staining and laminin-, Iba1- and GFAP-immunostainings) were obtained 1 and 2 weeks after the stroke.

RESULTS

Two days after stroke, tissues assumed to correspond to the infarct core, were detected as a hyperintensity signal area in T2-weighted images. One week later, low-intensity signal areas appeared. Longitudinal MRI study showed that these areas remained present over the following week, and was mainly linked to a drop of the T2 relaxation time value in the corresponding tissues. Correlation with histological data and immuno-histochemistry showed that these areas corresponded to microglial cells.

CONCLUSION

The present data provide, for the first time detailed MRI parameters of microglial cells dynamics, allowing its non-invasive monitoring during the chronic stages of a stroke. This could be particularly interesting in regards to emerging anti-inflammatory stroke therapies.

A comparison of four PET tracers for brain hypoxia mapping in a rodent model of stroke

INTRODUCTION

Severe brain hypoxia in the territory of the occluded artery is a key feature of ischemic stroke. This region can be imaged using positron emission tomography (PET) and the standard hypoxia radiotracer (18)F-fluoromisonidazole ((18)F-FMISO). However, the utility of (18)F-FMISO is limited by its slow accumulation in the lesion. Therefore, this study investigated three hypoxia-sensitive radiotracers, namely the nitroimidazole (18)F-fluoroazomycin arabinoside ((18)F-FAZA) and two (64)Cu bis(thiosemicarbazone) complexes ((64)Cu-ATSM and (64)Cu-ATSE), expected to have improved pharmacokinetic profiles relative to (18)F-FMISO, in a rodent model of ischemic stroke.

METHODS

In anaesthetised Wistar rats, the distal middle cerebral artery was permanently occluded by electrocoagulation, the radiotracers administered intravenously and animals PET scanned for up to 3hours, followed by T2-weighted magnetic resonance imaging to map the infarct.

RESULTS

As expected, late and prominent (18)F-FMISO retention was observed despite lower tracer delivery into the affected region. Time-activity curves revealed that both (64)Cu-ATSM and (64)Cu-ATSE showed rapid entry and efflux from the brain, but did not show significant accumulation in the lesion. (18)F-FAZA showed limited brain penetration, and accumulation in the lesion was inconsistent, low and as slow as (18)F-FMISO.

CONCLUSIONS

This study suggests further development of these radiotracers as hypoxia markers for ischemic stroke may not be warranted.

Characterizing infarction and selective neuronal loss following temporary focal cerebral ischemia in the rat: a multi-modality imaging study

BACKGROUND AND PURPOSE

Current models dictate that, depending on occurrence of early reperfusion, the ischemic penumbra either undergoes or escapes infarction (i.e., "pan-necrosis"). However, tissue outcome following temporary middle-cerebral artery occlusion (tMCAo) in rodents can also include selective neuronal loss (SNL), which even if subtle may impede functional recovery. In order to explore the pathophysiology of ischemic stroke, determine potential therapeutic targets and monitor effects of therapy, in vivo imaging surrogates of these varied histopathological outcomes applicable in the clinical setting would be useful. Although hyperintense signal on T(2)-weighted MRI in the chronic post-stroke stage is considered a reliable surrogate of tissue infarction, SNL is not associated with T(2)W abnormal signal. In the clinical setting, the neuron-specific PET ligand (11)C-flumazenil (FMZ) has been used to identify both pan-necrosis and peri-infarct SNL, but this inference has not been histopathological confirmed so far. Here we investigated the late tissue sequelae of tMCAo in the rodent using in vivo T(2)W MRI and FMZ-PET against post mortem immunohistochemistry as gold standard.

METHODS

Adult spontaneously hypertensive rats (SHRs) underwent 45 min distal-clip middle-cerebral artery occlusion and, 28 days later, FMZ-PET and T(2)W-MRI, immediately followed by immunohistochemistry for neuronal loss (NeuN), activated microglia and astrocytosis. Based on standard histopathological definitions, ischemic lesions were classified into pan-necrosis, partial infarction or SNL. NeuN changes and FMZ binding across the whole hemisphere were quantified in the same set of 44 regions-of-interest according to previously validated protocols; linear regressions between these two measures were carried out both within and across subjects.

RESULTS

Both cortical pan-necrosis/partial infarction and SNL were present in all rats except one, where SNL was isolated and extensive. Infarction/partial infarction, but not SNL, was associated with T(2)W hyperintense signals and cortical atrophy. In contrast, FMZ binding was decreased in all types of lesions including SNL, in proportion with NeuN staining intensity both within (p<0.05 to <0.001) and across (p<0.001) subjects, including the subject that showed pure SNL (p=0.01).

CONCLUSION

This novel study revealed three main facts: i) long-term histopathological cortical changes following 45 min tMCAo in SHRs included all three of SNL, partial infarction and frank infarction; ii) T2W MRI showed conspicuous high signal lesions for complete or partial infarction, but no changes for SNL; and iii) FMZ-PET was sensitive to all three types of tMCAo-induced histopathological changes, including isolated SNL, suggesting it is a valid surrogate for the histological sequelae of focal cerebral ischemia. In addition, the finding of almost universal completed cortical infarction at 28 days differed from our previous findings at 14-day survival using the same model and rat strain, where SNL was the almost exclusive outcome, possibly representing delayed infarct maturation. Prospective studies are needed to investigate this interesting possibility.

Training the brain to survive stroke

Background

Presently, little can be done to repair brain tissue after stroke damage. We hypothesized that the mammalian brain has an intrinsic capacity to adapt to low oxygen which would improve outcome from a reversible hypoxic/ischemic episode. Acclimation to chronic hypoxia causes increased capillarity and tissue oxygen levels which may improve the capacity to survive ischemia. Identification of these adaptations will lead to protocols which high risk groups could use to improve recovery and reduce costs.

Methods and Findings

Rats were exposed to hypoxia (3 weeks living at ½ an atmosphere). After acclimation, capillary density was measured morphometrically and was increased by 30% in the cortex. Novel implantable oxygen sensors showed that partial pressure of oxygen in the brain was increased by 40% in the normal cortex. Infarcts were induced in brain with 1 h reversible middle cerebral artery occlusions. After ischemia (48 h) behavioural scores were improved and T2 weighted MRI lesion volumes were reduced by 52% in acclimated groups. There was a reduction in inflammation indicated by reduced lymphocytes (by 27–33%), and ED1 positive cells (by 35–45%).

Conclusions

It is possible to stimulate a natural adaptive mechanism in the brain which will reduce damage and improve outcome for a given ischemic event. Since these adaptations occur after factors such as HIF-1α have returned to baseline, protection is likely related more to morphological changes such as angiogenesis. Such pre-conditioning, perhaps with exercise or pharmaceuticals, would not necessarily reduce the incidence of stroke, but the severity of damage could be reduced by 50%.

A comparison of VLSM and VBM in a cohort of patients with post-stroke aphasia

Studies attempting to map post-stroke cognitive or motor symptoms to lesion location have been available in the literature for over 150 years. In the last two decades, two computational techniques have been developed to identify the lesion sites associated with behavioural impairments. Voxel Based Morphometry (VBM) has now been used extensively for this purpose in many different patient populations. More recently, Voxel-based Lesion Symptom Mapping (VLSM) was developed specifically for the purpose of identifying lesion–symptom relationships in stroke patients, and has been used extensively to study, among others functions, language, motor abilities and attention. However, no studies have compared the results of these two techniques so far. In this study we compared VLSM and VBM in a cohort of 20 patients with chronic post-stroke aphasia. Comparison of the two techniques showed overlap in regions previously found to be relevant for the tasks used, suggesting that using both techniques and looking for overlaps between them can increase the reliability of the results obtained. However, overall VBM and VLSM provided only partially concordant results and the differences between the two techniques are discussed.

Repetitive hypoxia extends endogenous neurovascular protection for stroke

OBJECTIVE

Brief systemic hypoxia protects the rodent brain from subsequent ischemic injury, although the protection wanes within days. We hypothesized that the duration of ischemic tolerance could be extended from days to months by repeated intermittent hypoxia of varying magnitude and duration.

METHODS

Infarction volumes following a 60-minute transient middle cerebral artery occlusion were determined in adult male mice 2 days through 8 weeks after completion of a 2-week repetitive hypoxic preconditioning (RHP) protocol. Separate cohorts were studied for the protective effects of RHP on postischemic and cytokine-induced cerebrovascular inflammation, and for potential deleterious effects of the RHP stimulus itself.

RESULTS

RHP protection against transient focal stroke persisted for 8 weeks. Leukocyte adherence to cortical venules was attenuated in response to stroke, as well as following tumor necrosis factor-α administration, indicating that reductions in postischemic inflammation were not secondary to smaller infarct volumes. RHP reduced poststroke leukocyte diapedesis concomitant with a long-lasting downregulation of endothelial adhesion molecule mRNAs, and also reduced postischemic blood--brain barrier permeability to endogenous immunoglobulin G. RHP was without effect on hippocampal CA1 pyramidal cell viability, only transiently elevated hematocrit, and did not affect the magnitude of cerebral blood flow during and after ischemia.

INTERPRETATION

Taken together, our findings reveal a novel form of epigenetic neurovascular plasticity characterized by a prominent anti-inflammatory phenotype that provides protection against stroke many weeks longer than previously established windows of preconditioning-induced tolerance. Translating these endogenous protective mechanisms into therapeutics could afford sustained periods of cerebroprotection in subpopulations of individuals at identified risk for stroke.

Morphological and neuro-behavioral parallels in the rat model of stroke

Middle cerebral artery occlusion (MCAO) is widely used as a rat model of focal brain ischemia. Evaluation of brain damage often includes the morphological analysis of the injury area, MRI, and various scales which depend on functional tests, commonly known as neurological severity score (NSS). We determined the optimal number of NSS tests and assessed their capacity for non-invasive evaluation of brain ischemic injury in the rat MCAO model. 275 male Sprague-Dawley rats were randomly divided into five groups, given either permanent (p) MCAO or transient (t) MCAO using an uncoated 4-0 monofilament catheter or a silicone-coated monofilament. The rats' neurological status was examined before and at 1 and 24h following MCAO. The size of brain injury was then measured histologically and the extent of right cerebral hemisphere edema was calculated. We established a correlation between these tests and morphological data for brain injury. Adjusted R(2) of the prediction of total histology score was 0.7. The Hosmer-Lemeshow p-value of this model was 0.812 for total brain histology. For the brain edema the adjusted R(2) of the prediction model was 0.48. The Hosmer-Lemeshow p-value of this model was 0.558 for brain edema. Our methods of estimating infarct size produces reliable and well correlated results at 24h and demonstrates to be an easy and quick way to assess infarct size soon after ischemic injury has occurred. The described method for neurological assessment could ultimately aid in assessing various treatment modalities in the early hours following stroke.

An enriched environment improves sensorimotor function post-ischemic stroke

OBJECTIVE

An enriched environment (EE) refers to conditions that facilitate or enhance sensory, cognitive, motor, and social stimulation relative to standard (laboratory) conditions. Despite numerous published studies investigating this concept in animal stroke models, there is still debate around its efficacy. The authors performed a systematic review and meta-analysis to determine the efficacy of an EE on neurobehavioral scores, learning, infarct size, and mortality in animal models of ischemic stroke.

METHODS

Systematic review of controlled studies of the use of an EE in experimental stroke was conducted. Data extracted were analyzed using weighted mean difference meta-analysis. For pooled tests of neurobehavioral scores, a random effects standardized method was used.

RESULTS

Animals recovering in an EE poststroke had mean neurobehavioral scores 0.9 standard deviations (95% confidence interval [CI] = 0.5-1.3; P < .001) above the mean scores of animals recovering in standard conditions and showed a trend toward improvement in learning (25.1% improvement; 95% CI = 3.7-46.6; P = .02). There was no significant increase in death. Animals exposed to an EE had 8.0% larger infarcts than control animals (95% CI = 1.8-14.1; P = .015).

CONCLUSIONS

The results indicate significant improvements in sensorimotor function with EE poststroke but suggest a small increase in infarct volume. Clarification of the underlying mechanisms requires further study but should not overshadow the observed functional improvements and their application to clinical trials during stroke rehabilitation.

Use of magnetic resonance imaging to predict outcome after stroke: a review of experimental and clinical evidence

Despite promising results in preclinical stroke research, translation of experimental data into clinical therapy has been difficult. One reason is the heterogeneity of the disease with outcomes ranging from complete recovery to continued decline. A successful treatment in one situation may be ineffective, or even harmful, in another. To overcome this, treatment must be tailored according to the individual based on identification of the risk of damage and estimation of potential recovery. Neuroimaging, particularly magnetic resonance imaging (MRI), could be the tool for a rapid comprehensive assessment in acute stroke with the potential to guide treatment decisions for a better clinical outcome. This review describes current MRI techniques used to characterize stroke in a preclinical research setting, as well as in the clinic. Furthermore, we will discuss current developments and the future potential of neuroimaging for stroke outcome prediction.

Non-invasive evaluation of nigrostriatal neuropathology in a proteasome inhibitor rodent model of Parkinson's disease

Background

Predominantly, magnetic resonance imaging (MRI) studies in animal models of Parkinson's disease (PD) have focused on alterations in T₂ water ¹H relaxation or ¹H MR spectroscopy (MRS), whilst potential morphological changes and their relationship to histological or behavioural outcomes have not been appropriately addressed. Therefore, in this study we have utilised MRI to scan in vivo brains from rodents bearing a nigrostriatal lesion induced by intranigral injection of the proteasome inhibitor lactacystin.

Results

Lactacystin induced parkinsonian-like behaviour, characterised by impaired contralateral forelimb grip strength and increased contralateral circling in response to apomorphine. T₂-weighted MRI, 3-weeks post-lesion, revealed significant morphological changes in PD-relevant brain areas, including the striatum and ventral midbrain in addition to a decrease in T₂ water ¹H relaxation in the substantia nigra (SN), but not the striatum. Post-mortem histological analyses revealed extensive dopaminergic neuronal degeneration and α-synuclein aggregation in the SN. However, extensive neuronal loss could also be observed in extra-nigral areas, suggesting non-specific toxicity of lactacystin. Iron accumulation could also be observed throughout the midbrain reflecting changes in T₂. Importantly, morphological, but not T₂ relaxivity changes, were significantly associated with both behavioural and histological outcomes in this model.

Conclusions

A pattern of morphological changes in lactacystin-lesioned animals has been identified, as well as alterations in nigral T₂ relaxivity. The significant relationship of morphological changes with behavioural and histological outcomes in this model raises the possibility that these may be useful non-invasive surrogate markers of nigrostriatal degeneration in vivo.

Neither in vivo MRI nor behavioural assessment indicate therapeutic efficacy for a novel 5HT(1A) agonist in rat models of ischaemic stroke

Background

5HT_1A agonists have previously been shown to promote recovery in animal models of stroke using ex vivo outcome measures which have raised the hopes for a potential clinical implementation. The purpose of this study was to evaluate the potential neuroprotective properties of a novel 5HT_1A agonist DU123015 in 2 different models of transient focal ischaemic stroke of varying severities using both in vivo neuroimaging and behavioural techniques as primary outcome measures. For these studies, the NMDA receptor antagonist MK-801 was also utilized as a positive control to further assess the effectiveness of the stroke models and techniques used.

Results

In contrast to MK-801, no significant therapeutic effect of DU123015 on lesion volume in either the distal MCAo or intraluminal thread model of stroke was found. MK-801 significantly reduced lesion volume in both models; the mild distal MCAo condition (60 min ischaemia) and the intraluminal thread model, although it had no significant impact upon the lesion size in the severe distal MCAo condition (120 min ischaemia). These therapeutic effects on lesion size were mirrored on a behavioural test for sensory neglect and neurological deficit score in the intraluminal thread model.

Conclusion

This study highlights the need for a thorough experimental design to test novel neuroprotective compounds in experimental stroke investigations incorporating: a positive reference compound, different models of focal ischaemia, varying the duration of ischaemia, and objective in vivo assessments within a single study. This procedure will help us to minimise the translation of less efficacious compounds.

Long-term survival and serial assessment of stroke damage and recovery - practical and methodological considerations

Impairments caused by stroke remain the main cause for adult disability. Despite a vigorous research effort, only 1 thrombolytic treatment has been approved in acute stroke (<3h). The limitations of preclinical studies and how these can be overcome have been the subject of various guidelines. However, often these guidelines focus on the acute stroke setting and omit long-term outcome measures, such as behaviour and neuroimaging. The considerations and practicalities of including the serial assessment of these approaches and their significance to establish therapeutic efficacy are discussed here.

A chronic 1 year assessment of MRI contrast agent-labelled neural stem cell transplants in stroke

Non-invasive identification of transplanted neural stem cells in vivo by pre-labelling with contrast agents may play an important role in the translation of cell therapy to the clinic. Understanding the impact of these labels on the cells' ability to repair is therefore vital. In rats with middle cerebral artery occlusion (MCAo), a model of stroke, the transhemispheric migration of MHP36 cells labelled with the bimodal contrast agent GRID was detected on magnetic resonance images (MRI) up to 4 weeks following transplantation. However, compared to MHP36 cells labelled with the red fluorescent dye PKH26, GRID-labelled transplants did not significantly improve behaviour, and performance was akin to non-treated animals. Likewise, the evolution of anatomical damage as assessed by serial, T(2)-weighted MRI over 1 year indicated that GRID-labelled transplants resulted in a slight increase in lesion size compared to MCAo-only animals, whereas the same, PKH26-labelled cells significantly decreased lesion size by 35%. Although GRID labelling allows the in vivo identification of transplanted cells up to 1 month after transplantation, it is likely that some is gradually degraded inside cells. The translation of cellular imaging therefore does not only require the in vitro assessment of contrast agents on cellular functions, but also requires the chronic, in vivo assessment of the label on the stem cells' ability to repair in preclinical models of neurological disease.