A comparison of manual and semi-automated methods in the assessment of axonal injury

Preclinical stroke research--advantages and disadvantages of the most common rodent models of focal ischaemia

This review describes the most commonly used rodent models and outcome measures in preclinical stroke research and discusses their strengths and limitations. Most models involve permanent or transient middle cerebral artery occlusion with therapeutic agents tested for their ability to reduce stroke-induced infarcts and improve neurological deficits. Many drugs have demonstrated preclinical efficacy but, other than thrombolytics, which restore blood flow, none have demonstrated efficacy in clinical trials. This failure to translate efficacy from bench to bedside is discussed alongside achievable steps to improve the ability of preclinical research to predict clinical efficacy: (i) Improvements in study quality and reporting. Study design must include randomization, blinding and predefined inclusion/exclusion criteria, and journal editors have the power to ensure statements on these and mortality data are included in preclinical publications. (ii) Negative and neutral studies must be published to enable preclinical meta-analyses and systematic reviews to more accurately predict drug efficacy in man. (iii) Preclinical groups should work within networks and agree on standardized procedures for assessing final infarct and functional outcome. This will improve research quality, timeliness and translational capacity. (iv) Greater uptake and improvements in non-invasive diagnostic imaging to detect and study potentially salvageable penumbral tissue, the target for acute neuroprotection. Drug effects on penumbra lifespan studied serially, followed by assessment of behavioural outcome and infarct within in the same animal group, will increase the power to detect drug efficacy preclinically. Similar progress in detecting drug efficacy clinically will follow from patient recruitment into acute stroke trials based on evidence of remaining penumbra.

Quantitative magnetization transfer imaging in postmortem multiple sclerosis brain

PURPOSE

To investigate the relationship of myelin content, axonal density, and gliosis with the fraction of macromolecular protons (fB) and T2 relaxation of the macromolecular pool (T2B) acquired using quantitative magnetization transfer (qMT) MRI in postmortem brains of subjects with multiple sclerosis (MS).

MATERIALS AND METHODS

fB and T2B were acquired in unfixed postmortem brain slices of 20 subjects with MS. The myelin content, axonal count, and severity of gliosis were all quantified histologically. t-Tests and multiple regression were used for analysis.

RESULTS

MR indices obtained in unfixed postmortem MS brains were consistent with in vivo values reported in the literature. A significant correlation was detected between Tr(myelin) (inversely proportional to myelin content) and 1) fB (r = -0.80, P < 0.001) and 2) axonal count (r = -0.79, P < 0.001). fB differed between 1) normal-appearing white matter (NAWM) and remyelinated WM lesions (rWMLs) (mean: fB 6.9 [SD 2] vs. 4.0 [1.8], P = 0.01), and 2) rWMLs and demyelinated WMLs (mean: 4.2 [2.2] vs. 2.5 [1.3], P = 0.016). No association was detected between T2B and any of the histological measures.

CONCLUSION

fB in MS WM is dependent on myelin content and may be a tool to monitor patients with this condition.

Diffusion tensor imaging of post mortem multiple sclerosis brain

Magnetic resonance imaging (MRI) is being used to probe the central nervous system (CNS) of patients with multiple sclerosis (MS), a chronic demyelinating disease. Conventional T₂-weighted MRI (cMRI) largely fails to predict the degree of patients' disability. This shortcoming may be due to poor specificity of cMRI for clinically relevant pathology. Diffusion tensor imaging (DTI) has shown promise to be more specific for MS pathology. In this study we investigated the association between histological indices of myelin content, axonal count and gliosis, and two measures of DTI (mean diffusivity [MD] and fractional anisotropy [FA]), in unfixed post mortem MS brain using a 1.5-T MR system. Both MD and FA were significantly lower in post mortem MS brain compared to published data acquired in vivo. However, the differences of MD and FA described in vivo between white matter lesions (WMLs) and normal-appearing white matter (NAWM) were retained in this study of post mortem brain: average MD in WMLs was 0.35 × 10^− 3 mm²/s (SD, 0.09) versus 0.22 (0.04) in NAWM; FA was 0.22 (0.06) in WMLs versus 0.38 (0.13) in NAWM. Correlations were detected between myelin content (Tr_myelin) and (i) FA (r = − 0.79, p < 0.001), (ii) MD (r = 0.68, p < 0.001), and (iii) axonal count (r = − 0.81, p < 0.001). Multiple regression suggested that these correlations largely explain the apparent association of axonal count with (i) FA (r = 0.70, p < 0.001) and (ii) MD (r = − 0.66, p < 0.001). In conclusion, this study suggests that FA and MD are affected by myelin content and – to a lesser degree – axonal count in post mortem MS brain.

Stereotactic co-registration of magnetic resonance imaging and histopathology in post-mortem multiple sclerosis brain

A number of groups have examined the pathological substrate of signal changes on magnetic resonance imaging (MRI) in post-mortem (PM) brain of patients with multiple sclerosis (MS). Such studies will benefit from using a standardized method to reliably co-register regions of interest on MRI and tissue specimens. We investigated the usefulness of a stereotactic navigation system for this purpose. We also addressed the sensitivity of different standard MRI sequences with regard to lesion conspicuity in PM MS brain. Post-mortem brains of eight patients with MS were studied. Formalin-fixed coronal slices were placed in the head frame of a stereotactic system. Proton density-, T2-weighted and fast fluid-attenuated inversion recovery (FLAIR) scans were obtained and visually matched with scans that had been previously obtained on the same, but fresh, specimens. Guided by the stereotactic target points, the dissection of the fixed specimens was performed. After processing the blocks for embedding in paraffin, sections were stained with haematoxylin-eosin and Luxol fast blue. T2-weighted MRI of fixed brain revealed 24 areas suspected to be MS lesions, all of which were confirmed histologically. Three of these lesions were not visible on macroscopic inspection. There were 14 additional hyperintensities on T2-weighted or FLAIR MRI of the fresh specimens, five of which did not correlate to MS lesions histologically. Stereotactic navigation is a useful approach to co-register MRI and histopathology in PM brain of MS patients and may improve the precision of MRI-guided sampling of tissue specimens. Standard T2-weighted MRI appeared to be the single most useful approach for lesion detection in fresh and fixed specimens.

Magnetization transfer ratio and myelin in postmortem multiple sclerosis brain

Several quantitative magnetic resonance (MR) measures are used to investigate multiple sclerosis (MS) in vivo. Precise quantitative investigation of the histopathological correlates of such measures has, to date, been limited. This study investigates the relationship of quantitative measures of myelin content, axonal density, and gliosis with quantitative MR measures in postmortem (PM) MS tissue. MR imaging (MRI) was performed on a 1.5T scanner and T1-relaxation time (T1-RT) and magnetization transfer ratio (MTR) maps were acquired in fresh PM brain of 20 MS subjects. Myelin content, axonal counts, and the extent of gliosis all were quantified using morphometric and digital imaging techniques. MRI and pathological data were in most cases coregistered using stereotactic navigation. Using multiple regression analysis, we detected significant correlations between myelin content (Tr(myelin)) and MTR (r = -0.84, p < 0.001) and myelin content and axonal count (-0.80, p < 0.001); MTR correlated with T1-RT (r = -0.79, p < 0.001). No association was detected between the extent of gliosis and either MR measure. MTR was significantly higher in remyelinated than demyelinated lesions (means: 30.0 [standard deviation, 2.9] vs 23.8 [standard deviation, 4.3], p = 0.008). In conclusion, MTR is affected by myelin content in MS white matter.

Simple morphometry of axonal swellings cannot be used in isolation for dating lesions after traumatic brain injury

Disruption of fast axonal transport as a result of traumatic brain injury is characterized by the accumulation of beta-amyloid precursor protein (APP) in axonal swellings. A recent report has suggested a correlation between the size of axonal swellings and survival time up to about 85 h after blunt head injury. The authors of the report concluded that this correlation, in conjunction with other evidence, might be useful in forensic science for timing injuries. To test this hypothesis we have used image analysis software to measure a number of different morphological parameters of axonal swellings. Paraffin sections from 63 cases of fatal head injury were stained with an antibody raised against the N-terminus of APP and counterstained with haematoxylin. Three different measurements were made of the APP-immunoreactive axonal swellings from the corpus callosum: (i) minimum and (ii) maximum Feret diameters, and (iii) area. Linear regression revealed a significant correlation between survival time and the minimum Feret diameter (p < 0.0001) and the area (p < 0.001) of axonal swellings. Our findings are in agreement with the previous study in that there is a significant correlation between axonal swelling size and survival time. However, we would suggest that the large variability in swelling size within individual cases and the heterogeneity of the original trauma seriously compromise the utility of such information in the timing of lesions.

New method for the quantitative assessment of axonal damage in focal cerebral ischemia

Quantification of damage in both grey and white matter is required for comprehensive assessment of neuroprotective drug efficacy. Although methods for quantification of neuronal perikaryal damage after ischemia are well established, assessment of axonal damage has been limited. This article describes a new method for quantitation of axonal injury after middle cerebral artery (MCA) occlusion in rats and its application to the study of the antioxidant ebselen. The methodology is based on immunohistochemical detection of amyloid precursor protein (APP) accumulation in deformed, swollen axons in zones of ischemia. Sixty-five axon-rich sites throughout the MCA territory are assessed for the presence (scored 1) or absence (scored 0) of accumulated APP in axonal swellings. Scores for individual sites are summated in predefined neuroanatomic regions (e.g. corpus callosum), stereotaxic levels, or for a total hemisphere APP score. Both intra-rater and inter-rater reproducibility were high (r = 0.87 and 0.80, respectively). Ebselen (1 mg kg(-1) hr(-1), intravenously) significantly reduced the volume of neuronal perikaryal damage (24%, P < 0.01) and axonal damage (total APP score reduced from 27 [23.9 to 35.1, 95% CI] to 21.5 [18.2 to 23.3], P = 0.002 with ebselen treatment). In conclusion, a robust and generally applicable method is described for assessing pathologic features in myelinated fiber tracts that is sensitive for detection of drug effects on axonal damage.

Axonal injury in cerebral malaria

Impairment of consciousness and other signs of cerebral dysfunction are common complications of severe Plasmodium falciparum malaria. Although the majority of patients make a complete recovery a significant minority, particularly children, have sequelae. The pathological process by which P. falciparum malaria induces severe but usually reversible neurological complications has not been elucidated. Impairment of transport within nerve fibers could induce neurological dysfunction and may have the potential either to resolve or to progress to irreversible damage. Beta-amyloid precursor protein (beta-APP) immunocytochemistry, quantified using digital image analysis, was used to detect defects in axonal transport in brain sections from 54 Vietnamese cases with P. falciparum malaria. The frequency and extent of beta-APP staining were more severe in patients with cerebral malaria than in those with no clinical cerebral involvement. Beta-APP staining was often associated with hemorrhages and areas of demyelination, suggesting that multiple processes may be involved in neuronal injury. The age of focal axonal damage, as determined by the extent of the associated microglial response, varied considerably within tissue sections from individual patients. These findings suggest that axons are vulnerable to a broad range of cerebral insults that occur during P. falciparum malaria infection. Disruption in axonal transport may represent a final common pathway leading to neurological dysfunction in cerebral malaria.

Axonal injury is accentuated in the caudal corpus callosum of head-injured patients

Amyloid precursor protein (APP) accumulation is a sensitive marker for the axonal damage that is commonly seen in the brain as the result of head injury. This form of damage is particularly associated with midline structures such as the corpus callosum, although it is not clear whether some areas are more susceptible than others. The aim of this study was to determine if there was a differential distribution of axonal injury throughout the corpus callosum after head injury in an unselected group of cases. Coronal tissue sections from eight cases were taken at different levels through the corpus callosum, including the genu, body, and splenium. The sections were immunostained with an antibody to APP, and the amount of axonal damage at the different levels was quantified using computer image analysis to build up a rostro-caudal profile for each case. The profiles revealed a significantly higher APP load in caudal parts of the corpus callosum. This supports previous nonquantitative reports in the literature and has important implications in terms of choosing where tissue should be sampled to maximize the chance of detecting axonal injury post mortem.