Significance: This study assessed the effects of myelin decompaction on motor behavior and brain-wide structural and functional connectivity, and the effect of nitric oxide synthase inhibition byN-nitro-l-arginine methyl ester (L-NAME) on these imaging measures. We report that inducible oligodendrocyte-specific inactivation of theNf1gene, which causes myelin decompaction, results in reduced initial motor coordination. Using diffusion-based magnetic resonance imaging (MRI), we show reduced myelin integrity, and using functional MRI, we show reduced functional connectivity in awake passive mice. L-NAME administration results in rescue of the pathology at the mesoscopic level, as measured using imaging procedures that can be directly applied to humans to study treatment efficacy in clinical trials.

Neurofibromatosis type 1 (NF1) is caused by mutations in the NF1 gene. The clinical presentation of NF1 includes diverse neurological issues in pediatric and adult patients, ranging from learning disabilities, motor skill issues, and attention deficit disorder, to increased risk of depression and dementia. Preclinical research suggests that abnormal neuronal signaling mediates spatial learning and attention issues in NF1; however, drugs that improve phenotypes in models show inconclusive results in clinical trials, highlighting the need for a better understanding of NF1 pathophysiology and broader therapeutic options. Most NF1 patients show abnormalities in their brain white matter (WM) and myelin, and links with NF1 neuropathophysiology have been suggested; however, no current data can clearly support or refute this idea. We reported that myelin-targeted Nf1 mutation impacts oligodendrocyte signaling, myelin ultrastructure, WM connectivity, and sensory–motor behaviors in mice; however, any impact on learning and memory remains unknown. Here, we adapted a voluntary running test—the complex wheel (CW; a wheel with unevenly spaced rungs)—to delineate fine motor skill learning curves following induction of an Nf1 mutation in pre-existing myelinating cells (pNf1 mice). We found that pNf1 mutant females experience delayed or impaired learning in the CW, while proper learning in pNf1 males is predominantly disrupted; these phenotypes add complexity to the gender-dependent learning differences in the mouse strain used. No broad differences in memory of acquired CW skills were detected in any gender, but gene-dose effects were observed at the studied time points. Finally, nitric oxide signaling regulation differentially impacted learning in wild type (WT)/pNf1, male/female mice. Our results provide evidence for fine motor skill learning issues upon induction of an Nf1 mutation in mature myelinating cells. Together with previous connectivity, cellular, and molecular analyses, these results diversify the potential treatments for neurological issues in NF1.

Myelination of axons by oligodendrocytes (OLs) greatly improves information processing in the brain; consequently, compromised myelin negatively impacts brain function. Young-adult Costello Syndrome (CS) patients present with learning disabilities and regression of psychomotor skills in correlation with abnormal brain white matter, delayed myelination, and dysmyelination. This suggests the involvement of OLs and myelin in abnormal brain physiology during and beyond development; however, the neuropathophysiology of CS remains poorly understood. Replacing the endogenous HRas gene with the CS-causing HRasG12V mutant gene in adult OLs (pHRasG/+ mice) induces ultrastructural myelin abnormalities; nonetheless, any functional impact is unknown. Here we show robust learning - but not memory - deficits in pHRasG/+ males and modest, delayed learning issues in females. Learning phenotypes are transient, likely involving compensatory responses from the OL lineage and microglia. Diffusion-weighted magnetic resonance imaging reveals region- and sex-dependent compromised myelin microstructure, coinciding with the peak of learning issues. Remarkably, pharmacological control of nitric oxide synthase 2 restores learning in male pHRasG/+ mice. Our study supports the notion that abnormal nitric oxide signaling in mature OLs affects learning, suggests a chronic impact of myelin-driven mechanisms in CS neuropathology, and proposes molecular targets with therapeutic potential for CS.

Neurofibromin gene ( NF1 ) mutation causes Neurofibromatosis type 1 (NF1), a disorder in which brain white matter deficits identified by neuroimaging are common, yet of unknown cellular etiology. In mice, Nf1 loss in adult oligodendrocyte causes myelin decompaction, and increases oligodendrocyte nitric oxide (NO) levels. Nitric oxide synthase (NOS) inhibitors rescue this pathology. Whether oligodendrocyte pathology is sufficient to affect brainwide structure and account for NF1 imaging findings is unknown. Here, we show that Nf1 gene inactivation in adult oligodendrocytes (Plp- Nf1 ^fl/+ mice) results in a motor coordination deficit. Magnetic resonance imaging in awake mice shows that fractional anisotropy is reduced in Plp- Nf1 ^fl/+ corpus callosum and that interhemispheric functional connectivity in motor cortex is also reduced, consistent with disrupted myelin integrity. Further, NOS-specific inhibition rescued both measures. These results demonstrate that oligodendrocyte defects account for aspects of brain dysfunction in NF1, which can be identified by neuroimaging and ameliorated by NOS inhibition. Significance statement: This study assesses the effects of myelin decompaction on motor behavior and brain-wide structural and functional connectivity, and the effect of nitric oxide synthase (NOS) inhibition by N-nitro-L-arginine methyl ester (L-NAME) on these imaging measures. We report that inducible oligodendrocyte-specific inactivation of the Nf1 gene, which causes myelin decompaction, results in reduced motor coordination. Using diffusion-based MRI we show reduced myelin integrity and using functional MRI we show reduced functional connectivity in awake passive mice. L-NAME administration results in rescue of the pathology at the mesoscopic level using imaging procedures that can be directly applied to humans to study treatment efficacy in clinical trials.