White matter lesions defined by diffusion tensor imaging in older adults

Abstract

Objective: The cellular and molecular mechanisms underlying magnetic resonance imaging-defined white matter (WM) changes associated with age-related cognitive decline remain poorly defined. We tested the hypothesis that WM lesions in older adults, defined by diffusion tensor imaging (DTI), arise in the setting of vascular brain injury (VBI) and are characterized by increased free radical injury and aberrant oligodendrocyte lineage (OL) cell response to injury.

Methods: We undertook a multimodal analysis of prefrontal cortex (PFC) WM from 25 autopsies derived from a population-based cohort where VBI and Alzheimer disease (AD) frequently coincide. Ex vivo high field strength DTI measurements of fractional anisotropy (FA), apparent diffusion coefficient, and axial and radial (D(⊥) ) diffusivity were measured at high magnetic field strength (11.7T) and analyzed relative to quantitative in vivo biomarkers of free radical injury, an OL-specific marker Olig2, and histologic evaluation of hyaluronan (HA), an inhibitor of OL maturation.

Results: Coincident AD and VBI showed significant association with lower FA and a robust relationship between decreasing FA and increasing D(⊥) . Free radical injury to docosahexaenoate and adrenate in PFC WM was significantly elevated in cases with VBI independent of AD, and was inversely correlated with FA. Similarly, increased density of Olig2-immunoreactive cells in PFC WM was significantly associated with VBI independent of AD and colocalized with regions enriched in HA.

Interpretation: DTI-defined PFC WM lesions in older individuals are characterized by free radical injury to myelin and neuroaxonal elements that coincides with pronounced expansion of the pool of OL cells in HA-rich regions.

Diagram 1

Diagram that shows a proposed relationship among biochemical, cellular, and imaging endpoints of white matter (WM) injury and it consequences on cognitive function in the context of other common co-morbid diseases that target primarily gray matter (GM). Abbreviations: Vascular Brain Injury (VBI), oligodendrocyte lineage (OL), hyaluronan (HA), fractional anisotropy (FA), apparent diffusion coefficient (ADC), radial diffusivity (D_⊥), and axial diffusivity (D_∥).

Figure 1

The burden of Alzheimer's disease (AD) and vascular brain injury (VBI) is represented for each of the twenty-five individuals that met criteria for our study by Braak stage for neurofibrillary tangles (range 0 to 6) and the sum of the number of cerebral microinfarcts plus the number of lacunar infarcts. No individual in this group had cerebral cortical Lewy body disease. *Individuals diagnosed with dementia by DSM-IVR criteria within two years of death. ^Cases with especially low fractional anisotropy.

Figure 2. Diffusion-based MRI contrast of right PFC WM from humans

Fixed tissue samples were imaged in a standard orientation (A) presented as 3-D model or (B) T₂-weighted images that classify image voxels (volumetric pixels) as GM or WM. (C-D) Individual slices from 3-D of fractional anisotropy (FA) and apparent diffusion coefficient (ADC) parameter maps from Low AD/No VBI case and High AD/VBI case are shown. We observed increased ADC and decreased FA in WM from this most severely damaged case from the High AD/VBI group. (E) Scatter plot of D_∥ vs. D_⊥ or all twenty-five cases regardless of pathologic classification with (F) best-fit line calculated while excluding the two cases with extremely low FA. (G-H) Mean ± SEM FA and ADC values for the four pathologic groups. Two way ANOVA showed that FA (G) significantly varied with presence or absence of VBI (P < 0.05) but not with ADC (H); there was no significant interaction between VBI and AD for FA or ADC. Multiple comparison-corrected post tests showed that relationship to FA resided with the High AD/VBI group (*P < 0.05).

Figure 3

Indices of free radical injury in human right pre-frontal cortical (PFC) white matter (WM). Samples immediately adjacent to those used to MRI measures were flash frozen in liquid nitrogen and stored at -80°C until extracted and analyzed with stable isotope dilution assay using gas chromatography/mass spectrometry and selective ion monitoring for the isoprostanoids F₄-Neuroprostanes (NeuroPs) and F₂-Adrenoprostanes (AdrenoPs). (A). Scatter plot with best-fit line for isoprostanoids (ng/g) vs. FA for all twenty-five PFC WM samples: F₄-NeuroP (r = -0.80, P < 0.0001), and F₂-AdrenoP (r = -0.84, P < 0.0001). (B and C). Stratification of isoprostanoid data (ng/g) by four pathologically determined disease groups. Two-way ANOVA for F₄-NeuroPs (B) had P < 0.01 for VBI and for F₂-AdrenoPs (C) had P < 0.05 for VBI; both isoprostanoids had P > 0.05 for AD or interaction. Multiple comparison-corrected post tests had *P < 0.05 for both F₄-NeuroPs and F₂-AdrenoPs in groups with VBI regardless of Low or High AD.

Figure 4

Density of cells with Olig2-immunoreactive nuclei (Olig2+) in pre-frontal cortical (PFC) samples. (A and B) Within white matter (WM), the number of Olig2+ cells varied from cases with low density (A) to cases with an apparent reactive response that had high Olig2 + cell density (B). (C) Olig2+ cell density in WM and overlying gray matter (GM) from the twenty-five PFC tissue blocks that underwent MRI; two-way ANOVA P < 0.001 for cortical GM vs. WM, P < 0.01 for presence or absence of vascular brain injury (VBI), and P < 0.05 for interaction between these terms. Multiple comparison-corrected post tests had **P < 0.01 for WM no VBI vs. WM with VBI. (D) Scatter plot of DTI measures vs. Olig2+ cell density in twenty-three PFC WM samples with less extreme injury and corresponding best-fit line for FA (solid line, P < 0.05) and ADC (dashed line, P < 0.05). The linear relationship between Olig2+ cell density was strongest with D_⊥ (P < 0.01). (E) Stratification of Olig2+ cell density into the four pathologic groups. Two-way ANOVA had P < 0.05 for VBI but not AD. Multiple comparison-corrected post tests had *P < 0.05 for VBI vs. no VBI in both the Low and High AD groups.

Figure 5

Photomicrographs of double immunohistochemistry in PFC WM. (A): Representative high HA area with intense HABP staining. (B): Representative low HA area with weak HABP staining. Cells with Olig2+ nuclei (arrows) are shown in both areas. (C) Paired t test for Olig2+ cell density in high vs. low HA areas for the twenty-five PFC WM samples (P < 0.0001).