Autism spectrum disorder (ASD) is a neurodevelopmental disorder with a high prevalence (∼2% of the global population) (Schaaf and Zoghbi, 2011, Taylor et al., 2013). Recent exome-sequencing studies of individuals with ASD have identified many mutations, with the gene encoding the chromatin remodeler CHD8 (chromodomain helicase DNA-binding protein 8) being the most frequent site of such mutations (Neale et al., 2012, O’Roak et al., 2012a, O’Roak et al., 2012b, Talkowski et al., 2012). Many studies of CHD8 have thus focused on its role in neurogenesis and brain development (Cotney et al., 2015, Sugathan et al., 2014). CHD8 is expressed not only in neuronal tissues but also in many other organs, where it may also play an important developmental role (Ishihara et al., 2006, Nishiyama et al., 2009). Indeed, individuals with CHD8 mutations manifest not only ASD but also macrocephaly, distinct facial characteristics, gastrointestinal complaints, and a tall and slender habitus. Although psychological symptoms, macrocephaly, and gastrointestinal complaints resulting from CHD8 haploinsufficiency are likely attributable to defective neuronal development and associated responses, the mechanism by which CHD8 mutations give rise to slenderness has remained unclear (Bernier et al., 2014, Katayama et al., 2016). Such slenderness is an unexpected characteristic, given that ASD patients in general tend to be obese as a result of food selectivity, gastrointestinal symptoms, reduced physical activity, and medication use (Curtin et al., 2010, Curtin et al., 2014, Zheng et al., 2017). These observations suggest that CHD8 might also contribute to regulation of metabolism or adipogenesis.
CHD8 is a member of the CHD family of enzymes that belong to the SNF2 superfamily of ATP-dependent chromatin remodelers and are defined by the presence of two chromodomains: a helicase/ATPase domain and a DNA-binding domain (Hall and Georgel, 2007, Marfella and Imbalzano, 2007). Several members of this family play important roles during development and are implicated in various human diseases (Ho and Crabtree, 2010). For example, CHD1 is thought to contribute to the pluripotency of embryonic stem cells by maintaining an open chromatin state (Gaspar-Maia et al., 2009). Similarly, CHD5 is a key regulator of neurogenesis and plays a dual role in the activation of neuronal genes and the repression of Polycomb target genes (Egan et al., 2013). The CHD5 gene resides within a region of human chromosome 1p36 that shows loss of heterozygosity in individuals at high risk for the development of neuroblastoma (Bagchi et al., 2007). CHD7 is also required for neurogenesis and neural crest formation as a result of its direct binding to promoter or enhancer regions of fate-controlling transcription factor genes and consequent opening of chromatin structure (Bajpai et al., 2010, Feng et al., 2013). De novo mutations of human CHD7 give rise to CHARGE syndrome, a condition characterized by malformation of various organs (Vissers et al., 2004).
CHD8 was originally identified as a negative regulator of the Wnt/β-catenin signaling pathway (Nishiyama et al., 2012, Sakamoto et al., 2000). CHD8 generates two alternatively spliced transcripts that encode a 280-kDa full-length protein (CHD8L) or a 110-kDa protein (CHD8S) that contains only the NH2-terminal chromodomain (Ishihara et al., 2006, Nishiyama et al., 2009). Mutations identified in individuals with ASD are distributed throughout the CHD8 locus, with some expected to result in the loss of both CHD8 isoforms and others to affect only CHD8L. To recapitulate this situation, we generated two independent lines of mutant mice deficient in both CHD8 isoforms (ΔSL) or only CHD8L (ΔL), and we found that both heterozygous mutant strains manifest ASD-like behavioral characteristics (Katayama et al., 2016). CHD8 was also shown to regulate the expression of ASD-related neurodevelopmental genes in neurons, especially those targeted by RE1-silencing transcription factor (REST), which suppresses the transcription of many neuronal genes during development.
Adipogenesis is the strictly controlled cellular process by which preadipocytes differentiate into mature adipocytes. The expression of preadipogenic transcription factors such as CCAAT/enhancer-binding protein β (C/EBPβ) and C/EBPδ is induced during the early phase of adipogenesis and is followed by that of C/EBPα and peroxisome-proliferator-activated receptor γ (PPARγ), both of which are master regulators of adipogenesis. C/EBPα and PPARγ activate the expression of multiple adipogenic genes whose products mediate the final maturation process (Siersbæk et al., 2012). In response to adipogenic stimulation, chromatin structure undergoes substantial remodeling, resulting in the activation of a complex network of these transcription factors (Rosen and MacDougald, 2006). The mechanistic basis of such chromatin remodeling during adipogenesis has remained largely unknown, however.
We now show that CHD8 is essential for adipogenesis and the development of white adipose tissue (WAT). We found that CHD8 cooperates with C/EBPβ to regulate transactivation of the genes for C/EBPα and PPARγ during adipogenesis. Generation of mice in which Chd8 is deleted specifically in mesenchymal stem cells revealed that these animals have a markedly reduced adipose tissue mass. Our results thus indicate that CHD8 plays an essential role not only in neuronal development but also in adipogenesis.