Significance: Pathogenic mutations in subunits of RNA polymerase (Pol) III cause a prevalent autosomal recessive form of leukodystrophy. However, understanding of the mechanisms of pathogenesis, including how ubiquitously expressed Pol III mutations affect primarily the central nervous system, has been limited by the absence of an animal model of the disease. We show that conditional knockin of pathogenicPolr3amutations in the Olig2 lineage in mice results in growth, neurobehavioral, and hypomyelination phenotypes, reflecting a subset of clinical features of Pol III-related leukodystrophy patients. Myelination defects in the mice identify neural-specific roles for Pol III transcription. The phenotypes of Pol III-related leukodystrophic mice enable genetic and pharmacological approaches aimed at mitigating the consequences of this disease in humans.

The Src homology 3 (SH3) domain of Fyn binds to a conserved PXXP motif on microtubule-associated protein-2. Co-transfections into COS7 cells and in vitro kinase assays performed with Fyn and wild-type, or mutant MAP-2c, determined that Fyn phosphorylated MAP-2c on tyrosine 67. The phosphorylation generated a consensus sequence for the binding of the SH2 domain of Grb2 (pYSN). Pull-down assays with SH2-Grb2 from human fetal brain homogenates, and co-immunoprecipitation of Grb2 and MAP-2 confirmed the interaction in vivo, and demonstrated that MAP-2c is tyrosine-phosphorylated in human fetal brain. Filter overlay assays confirmed that the SH2 domain of Grb2 binds to human MAP-2c following incubation with active Fyn. Enzyme-linked immunosorbent assays confirmed the interaction between the SH2 domain of Grb2 and a tyrosine-phosphorylated MAP-2 peptide spanning the pY⁶⁷SN motif. Thus, MAP-2c can directly recruit multiple signaling proteins important for central nervous system development.

During inflammatory, demyelinating diseases such as multiple sclerosis (MS), inflammation and axonal damage are prevalent early in the course. Axonal damage includes swelling, defects in transport, and failure to clear damaged intracellular proteins, all of which affect recovery and compromise neuronal integrity. The clearance of damaged cell components is important to maintain normal turnover and restore homeostasis. In this study, we used mass spectrometry to identify insoluble proteins within high-speed/mercaptoethanol/sarcosyl-insoluble pellets from purified white matter plaques isolated from the brains of individuals with relapsing–remitting MS (RRMS). We determined that the transmembrane protein 106B (TMEM106B), normally lysosome-associated, is insoluble in RRMS plaques relative to normal-appearing white matter from individuals with Alzheimer’s disease and non-neurologic controls. Relative to wild-type mice, hypomorphic mice with a reduction in TMEM106B have increased axonal damage and lipid droplet accumulation in the spinal cord following myelin-oligodendrocyte-glycoprotein-induced experimental autoimmune encephalomyelitis. Additionally, the corpora callosa from cuprizone-challenged hypomorphic mice fail to clear lipid droplets efficiently during remyelination, suggesting that when TMEM106B is compromised, protein and lipid clearance by the lysosome is delayed. As TMEM106B contains putative lipid- and LC3-binding sites, further exploration of these sites is warranted.

Cytoplasmic dynein heavy chain (DYNC1H1) is a multi-subunit protein complex that provides motor force for movement of cargo on microtubules and traffics them back to the soma. In humans, mutations along the DYNC1H1 gene result in intellectual disabilities, cognitive delays, and neurologic and motor deficits. The aim of the study was to generate a mouse model to a newly identified de novo heterozygous DYNC1H1 mutation, within a functional ATPase domain (c9052C > T(P3018S)), identified in a child with motor deficits, and intellectual disabilities. P3018S heterozygous (HET) knockin mice are viable; homozygotes are lethal. Metabolic and EchoMRI™ testing show that HET mice have a higher metabolic rate, are more active, and have less body fat compared to wildtype mice. Neurobehavioral studies show that HET mice perform worse when traversing elevated balance beams, and on the negative geotaxis test. Immunofluorescent staining shows neuronal migration abnormalities in the dorsal and lateral neocortex with heterotopia in layer I. Neuron-subtype specific transcription factors CUX1 and CTGF identified neurons from layers II/III and VI respectively in cortical layer I, and abnormal pyramidal neurons with MAP2+ dendrites projecting downward from the pial surface. The HET mice are a good model for the motor deficits seen in the child, and highlights the importance of cytoplasmic dynein in the maintenance of cortical function and dendritic orientation relative to the pial surface. Our results are discussed in the context of other dynein mutant mice and in relation to clinical presentation in humans with DYNC1H1 mutations.

During inflammatory, demyelinating diseases such as multiple sclerosis (MS) axonal damage is prevalent early in the disease course. Axonal damage includes swellings, defects in transport, and failure to clear damaged intracellular proteins, all of which affect recovery and compromise the integrity of neurons and remyelination. Autophagy and the clearance of damaged cell components by the proteasome are important for the maintenance of normal cellular turnover; and the restoration of cellular homeostasis. The gradual accumulation of insoluble proteins in the brain is known to impair recovery from several neurodegenerative diseases. In this study, we used mass spectrometry to identify insoluble proteins within high-speed, mercaptoethanol/sarcosyl-insoluble pellets from purified white matter plaques isolated from the brains of individuals with MS. We determined that insoluble transmembrane protein106B (TMEM106B), expressed in neurons and glia and normally lysosomal-associated, is increased in MS plaques relative to normal-appearing white matter from individuals with Alzheimer’s disease and non-neurologic controls. We found that decreased TMEM106B protein in mice results in significant axonal damage and lipid droplet accumulation in the spinal cord following chronic myelin oligodendrocyte glycoprotein-induced experimental autoimmune encephalomyelitis. When TMEM106B ^t/t mice were treated with cuprizone to experimentally induce demyelination, a significant increase in lipid deposition was observed in the corpus callosum of TMEM106B ^t/t mice post-cuprizone withdrawal. Our study shows that the brain and spinal cord from challenged TMEM106B ^t/t mice accumulate OilRedO+/Perilipin2+ lipid droplets. We postulate that increased insolubility of TMEM106B in MS plaques limits debris clearance by the lysosome which over time contributes to failed remyelination and axonal defects. Abbreviated Transmembrane protein106B (TMEM106B), a lysosome-associated protein, is significantly less soluble in multiple sclerosis plaques than in white matter controls. Decreased TMEM106B produces significant axonal damage and lipid accumulation in mouse models of demyelinating diseases. TMEM106B insolubility and likely loss of function may limit lysosome transport and contribute to CNS pathology.