Significance: Truncating mutation of chromodomain helicase DNA-binding protein 8 ( CHD8 ) represents one of the strongest known risk factors for autism spectrum disorder (ASD). We mimicked the effects of such heterozygous loss-of-function mutations in neural progenitor cells and integrated RNA sequencing with genome-wide delineation of CHD8 binding. Our results reveal that the molecular mechanism by which CHD8 alters neurodevelopmental pathways may involve both direct and indirect effects, the latter involving down-regulation following CHD8 suppression. We also find that chd8 suppression in zebrafish results in macrocephaly, consistent with observations in patients harboring loss-of-function mutations. We show that reduced expression of CHD8 impacts a variety of other functionally distinct ASD-associated genes, suggesting that the diverse functions of ASD risk factors may constitute multiple means of triggering a smaller number of final common pathways.

The mesencephalic dopaminergic (mDA) cell system is composed of two major groups of projecting cells in the substantia nigra (SN) (A9 neurons) and the ventral tegmental area (VTA) (A10 cells). A9 neurons form the nigrostriatal pathway and are involved in regulating voluntary movements and postural reflexes. Their selective degeneration leads to Parkinson's disease. Here, we report that gene expression analysis of A9 dopaminergic neurons (DA) identifies transcripts for α- and β-chains of hemoglobin (Hb). Globin immunoreactivity decorates the majority of A9 DA, a subpopulation of cortical and hippocampal astrocytes and mature oligodendrocytes. This pattern of expression was confirmed in different mouse strains and in rat and human. We show that Hb is expressed in the SN of human postmortem brain. By microarray analysis of dopaminergic cell lines overexpressing α- and β-globin chains, changes in genes involved in O ₂ homeostasis and oxidative phopshorylation were observed, linking Hb expression to mitochondrial function. Our data suggest that the most famed oxygen-carrying globin is not exclusively restricted to the blood, but it may play a role in the normal physiology of the brain and neurodegenerative diseases.

The Huntington's disease (HD) induced pluripotent stem cell (iPSC) consortium describe the combined use of differentiated patient-derived iPSCs and systems biology to discover underlying mechanisms in HD. They identify neurodevelopmental deficits in HD cells that can be corrected in cells and in vivo with a small molecule. Neural cultures derived from Huntington's disease (HD) patient-derived induced pluripotent stem cells were used for 'omics' analyses to identify mechanisms underlying neurodegeneration. RNA-seq analysis identified genes in glutamate and GABA signaling, axonal guidance and calcium influx whose expression was decreased in HD cultures. One-third of gene changes were in pathways regulating neuronal development and maturation. When mapped to stages of mouse striatal development, the profiles aligned with earlier embryonic stages of neuronal differentiation. We observed a strong correlation between HD-related histone marks, gene expression and unique peak profiles associated with dysregulated genes, suggesting a coordinated epigenetic program. Treatment with isoxazole-9, which targets key dysregulated pathways, led to amelioration of expanded polyglutamine repeat-associated phenotypes in neural cells and of cognitive impairment and synaptic pathology in HD model R6/2 mice. These data suggest that mutant huntingtin impairs neurodevelopmental pathways that could disrupt synaptic homeostasis and increase vulnerability to the pathologic consequence of expanded polyglutamine repeats over time.

In patients with Huntington’s disease (HD), the protein huntingtin (Htt) has an expanded polyglutamine (poly-Q) tract. HD results in early loss of medium spiny neurons in the striatum, which impairs motor and cognitive functions. Identifying the physiological role and molecular functions of Htt may yield insight into HD pathogenesis. We found that Htt promotes signaling by mTORC1 [mechanistic target of rapamycin (mTOR) complex 1] and that this signaling is potentiated by poly-Q–expanded Htt. Knocking out Htt in mouse embryonic stem cells or human embryonic kidney cells attenuated amino acid–induced mTORC1 activity, whereas overexpressing wild-type or poly-Q–expanded Htt in striatal neuronal cells increased basal mTOR activity. Striatal cells expressing endogenous poly-Q–expanded Htt showed an increase in the number and size of mTOR puncta on the perinuclear regions compared to cells expressing wild-type Htt. Pull-down experiments indicated that amino acids stimulated the interaction of Htt and the guanosine triphosphatase (GTPase) Rheb (a protein that stimulates mTOR activity), and that Htt forms a ternary complex with Rheb and mTOR. Pharmacologically inhibiting PI3K (phosphatidylinositol 3-kinase) or knocking down Rheb abrogated mTORC1 activity induced by expression of a poly-Q–expanded amino-terminal Htt fragment. Moreover, striatum-specific deletion of TSC1, encoding tuberous sclerosis 1, a negative regulator of mTORC1, accelerated the onset of motor coordination abnormalities and caused premature death in an HD mouse model. Together, our findings demonstrate that mutant Htt contributes to the pathogenesis of HD by enhancing mTORC1 activity.