Cortical network dysfunction caused by a subtle defect of myelination

Abstract

Subtle white matter abnormalities have emerged as a hallmark of brain alterations in magnetic resonance imaging or upon autopsy of mentally ill subjects. However, it is unknown whether such reduction of white matter and myelin contributes to any disease-relevant phenotype or simply constitutes an epiphenomenon, possibly even treatment-related. Here, we have re-analyzed Mbp heterozygous mice, the unaffected parental strain of shiverer, a classical neurological mutant. Between 2 and 20 months of age, Mbp(+/-) versus Mbp(+/+) littermates were deeply phenotyped by combining extensive behavioral/cognitive testing with MRI, 1H-MR spectroscopy, electron microscopy, and molecular techniques. Surprisingly, Mbp-dependent myelination was significantly reduced in the prefrontal cortex. We also noticed a mild but progressive hypomyelination of the prefrontal corpus callosum and low-grade inflammation. While most behavioral functions were preserved, Mbp(+/-) mice exhibited defects of sensorimotor gating, as evidenced by reduced prepulse-inhibition, and a late-onset catatonia phenotype. Thus, subtle but primary abnormalities of CNS myelin can be the cause of a persistent cortical network dysfunction including catatonia, features typical of neuropsychiatric conditions. GLIA 2016;64:2025-2040.

Keywords: Cnp mutant; MRI; catatonia; electron microscopy; myelin basic protein (Mbp) mutant; prepulse inhibition.

Figure 1

Myelin gene expression (mRNA, protein) in prefrontal cortex and brainstem of Mbp^+/‐ versus Mbp^+/+ mice at 3 different ages. A: Mbp/Mbp, B: Cnp/Cnp, C: Plp/Plp. N = 4 per group; 2‐sided t‐tests, mean ± SEM presented.

Figure 2

Hypomyelination phenotype in the corpus callosum detected by magnetic resonance imaging (MRI) and electron microscopy (EM). A: Sagittal MRI pictures illustrating subtle white matter reduction in CC (corpus callosum) and CB (cerebellum) of Mbp^+/‐ versus Mbp^+/+ mice. B: Quantification of corpus callosum MRI data of 6 and 18 months old Mbp^+/‐ versus Mbp^+/+, mice demonstrating reduction in volume, as well as in MT_SAT and MTR, and increase in T1 relaxation, together consistent with a discrete hypomyelination phenotype in Mbp^+/‐ versus Mbp^+/+ mice. N numbers are given in the bars; 2‐sided t‐tests, mean ± SEM presented. C: EM data of 20 month‐old mice, showing in Mbp^+/‐ mice borderline reduced overall myelin sheath thickness, which reaches significance in small caliber axons. No changes are seen in % of non‐myelinated axons, inner tongue size, or mean axon calibers. A = axon diameter; M = myelin + axon diameter; T = inner tongue + axon diameter; t‐ratio = A/T; g‐ratio = A/A+(M‐T); N = 3 per genotype; 2‐sided t‐tests, mean ± SEM presented.

Figure 3

Low‐grade inflammatory phenotype in the PFC/frontal corpus callosum of Mbp^+/‐ mice revealed by magnetic resonance spectroscopy (MRS), microglia counts and TNF‐alpha mRNA. A: Myo‐inositol enhancement in Mbp^+/‐ mice, prominent upon aging; B, C: Iba1+ cell increase in 14 month‐old Mbp^+/‐ mice; D: Representative section scanned for Iba1+ cell counting; E: Elevated TNF‐alpha mRNA expression in 18 month‐old Mbp^+/‐ versus Mbp^+/+ mice. N numbers given in the bars; 2‐sided t‐tests, mean ± SEM presented. F‐G: Single myelin gene‐dose reductions (Mbp, Cnp) can result in strikingly similar low‐grade inflammation phenotypes: Note the highly comparable findings in aging Cnp ^+/‐ mice: Increase in myo‐inositol (MRS), and in Iba1+ cells in the frontal corpus callosum. N numbers given in the bars; 2‐sided t‐tests, mean ± SEM presented.[TQ3]

Figure 4

Behavioral abnormalities of Mbp^+/‐ mice reveal a mental disease‐relevant phenotype. A: Prepulse inhibition of the startle response (PPI), experimental design. B: PPI and C: startle response reduction in Mbp^+/‐ mice. D: Catatonia in old Mbp^+/‐ mice, quantification of time (latency) on the bar; E: Illustrating photograph with typical catatonic posture. N numbers given in the bars; 2‐sided t‐tests, mean ± SEM presented.