Enhanced Adult Neurogenesis Increases Brain Stiffness: <i>In Vivo</i> Magnetic Resonance Elastography in a Mouse Model of Dopamine Depletion

<div><p>The mechanical network of the brain is a major contributor to neural health and has been recognized by in vivo magnetic resonance elastography (MRE) to be highly responsive to diseases. However, until now only brain softening was observed and no mechanism was known that reverses the common decrement of neural elasticity during aging or disease. We used MRE in the 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine hydrochloride (MPTP) mouse model for dopaminergic neurodegeneration as observed in Parkinson’s disease (PD) to study the mechanical response of the brain on adult hippocampal neurogenesis as a robust correlate of neuronal plasticity in healthy and injured brain. We observed a steep transient rise in elasticity within the hippocampal region of up to over 50% six days after MPTP treatment correlating with increased neuronal density in the dentate gyrus, which could not be detected in healthy controls. Our results provide the first indication that new neurons reactively generated following neurodegeneration substantially contribute to the mechanical scaffold of the brain. Diagnostic neuroimaging may thus target on regions of the brain displaying symptomatically elevated elasticity values for the detection of neuronal plasticity following neurodegeneration.</p></div

Results of cell counts in the DG.

<p>Fluorescence- and DAB-stained brain sections of MPTP-treated and CTR mice at the different time-points of MRE assessment, showing the total number of a) GCL/SGZ cells (DAPI), b) newborn cells (BrdU+), c) new precursor cells (BrdU+/Nestin/GFP+), d) new neuronal cells (BrdU+/NeuN+), and f) microglia and macrophages (Iba1+) in the GCL/SGZ, ML and hilus (mean±SEM). *p<0.05, **p<0.01.</p

Representative confocal images of doublelabeled BrdU+/Nestin/GFP+ and BrdU+/NeuN+ cells.

<p>Mouse brain sections (40 μm) fluorescently stained with BrdU (red), NeuN (blue) and Nestin/GFP (green), showing the granular cell layer of the dentate gyrus with a Nestin/GFP-expressing precursor cell that is also positive for BrdU (a, 200×magnification and b, 630×magnification; arrow) and BrdU-positive cells coexpressing the neuronal marker NeuN (c, 400×magnification and d, 630×magnification; arrows).</p

Variation of MRE parameters due to MPTP in the hippocampus of mice and results of cell counts in the DG.

<p>MPTP induced a transient increase of brain elasticity and viscosity (a, b and c) at 6 dpi, while the phase angle φ (d) remained unchanged (mean±SEM). Histologically, a transient MPTP-induced increase of new precursor cells (BrdU+/Nestin/GFP+) at 3 dpi (e) and of new neurons (BrdU+/NeuN+) at 6 dpi (f) as percentage of all newborn cells (BrdU+) was found (mean±SEM). *p<0.05, **p<0.01.</p

Representative images of the magnitude MRE signal, shear waves and the magnitude complex modulus (|G*|) in a mouse.

<p>The green line demarcates the chosen region of interest in the hippocampus.</p

Results of MRE measurement and histological cell count in the hippocampus.

<p>MPTP induced a transient increase of elasticity (a) and viscosity (b) in the hippocampus at 6dpi (mean±SEM, n(-3,3,6,10,14,18dpi = 5). Quantification of DAPI-stained cells showed an elevated amount at 6dpi (mean±SEM, n(-3,3,6,10,14,18dpi) = 2, no statistical analysis). * vs. -3dpi, **p≤0.01, ***p≤0.001.</p

Results of MRE measurements and histological cell counts in the substantia nigra.

<p>MPTP induced a significant reduction in MRE elasticity (a) and viscosity (b) in the substantia nigra (mean±SEM, n(-3,3,6,10,14,18dpi) = 5). DAB-stained brain sections showed an immediate significant drop in TH+ dopaminergic neurons in the substantia nigra after MPTP treatment (c) (mean±SEM, n(-3dpi) = 4, n(3,6,10,14,18dpi) = 5). DAPI-stained cell amount was decreased by MPTP (d) (mean±SEM, n(-3,3,6,10,14,18dpi) = 2, no statistical analysis). Initially, the amount of Iba1+ microglia and macrophages was significantly raised after MPTP treatment, but ceased over time (e) (mean±SEM, n(-3dpi) = 4, n(3,6,10,14,18dpi) = 5). * vs. -3dpi, *p≤0.05, **p≤0.01, ***p≤0.001. ^# vs. 3dpi, ^# p≤0.05.</p

Results of MRE measurement and histological cell count in the midbrain.

<p>MPTP induced a significant increase of MRE elasticity (a) and viscosity (b) in the midbrain (mean±SEM, n(-3,3,6,10,14,18dpi = 5). DAPI-stained brain sections showed a reduction following MPTP-treatment (c) (mean±SEM, n(-3,3,6,10,14,18dpi) = 2, no statistical analysis). * vs. -3dpi, *p≤0.05, **p≤0.01.</p

Representative images of DAB–stained brain slices showing the substantia nigra.

<p>TH+ cells at baseline at -3dpi (a) and directly after MPTP treatment at 3dpi (b) at 50x magnification, indicating a severe loss of dopaminergic neurons induced by MPTP. Iba-1+ at -3dpi (c) and 3dpi (d) at 50x magnification with detail in 200x magnification (scale bar 100 μm), showing a reactive increase in the number of microglia and macrophages in the substantia nigra immediately after MPTP treatment.</p

Representative images of MRI signal and complex modulus map of G' and G''.

<p>Regions of interest: substantia nigra (red line), midbrain (blue line) and hippocampus (yellow line) were marked in T1w-MRI.</p