Selective Roles of Normal and Mutant Huntingtin in Neural Induction and Early Neurogenesis

Abstract

Huntington's disease (HD) is a neurodegenerative disorder caused by abnormal polyglutamine expansion in the amino-terminal end of the huntingtin protein (Htt) and characterized by progressive striatal and cortical pathology. Previous reports have shown that Htt is essential for embryogenesis, and a recent study by our group revealed that the pathogenic form of Htt (mHtt) causes impairments in multiple stages of striatal development. In this study, we have examined whether HD-associated striatal developmental deficits are reflective of earlier maturational alterations occurring at the time of neurulation by assessing differential roles of Htt and mHtt during neural induction and early neurogenesis using an in vitro mouse embryonic stem cell (ESC) clonal assay system. We demonstrated that the loss of Htt in ESCs (KO ESCs) severely disrupts the specification of primitive and definitive neural stem cells (pNSCs, dNSCs, respectively) during the process of neural induction. In addition, clonally derived KO pNSCs and dNSCs displayed impaired proliferative potential, enhanced cell death and altered multi-lineage potential. Conversely, as observed in HD knock-in ESCs (Q111 ESCs), mHtt enhanced the number and size of pNSC clones, which exhibited enhanced proliferative potential and precocious neuronal differentiation. The transition from Q111 pNSCs to fibroblast growth factor 2 (FGF2)-responsive dNSCs was marked by potentiation in the number of dNSCs and altered proliferative potential. The multi-lineage potential of Q111 dNSCs was also enhanced with precocious neurogenesis and oligodendrocyte progenitor elaboration. The generation of Q111 epidermal growth factor (EGF)-responsive dNSCs was also compromised, whereas their multi-lineage potential was unaltered. These abnormalities in neural induction were associated with differential alterations in the expression profiles of Notch, Hes1 and Hes5. These cumulative observations indicate that Htt is required for multiple stages of neural induction, whereas mHtt enhances this process and promotes precocious neurogenesis and oligodendrocyte progenitor cell elaboration.

Selective expression of mutant huntingtin during development recapitulates characteristic features of Huntington’s disease

“…We and other groups have previously shown impairments in early embryonic development and stem cell-mediated striatal neurogenesis in HD mouse and cellular models (1,(25)(26)(27). These impairments may exert long-term disease-modifying effects (27), including selective neuronal vulnerability to degeneration.…”

“…To examine motor coordination, we measured the number of slips of the mice using the balance beam test. Compared with CTL mice, both 3-and 9-mo-old Q97 CRE and Q97 mice displayed higher numbers of slips [F (2,26) = 3.92, P = 0.032 and F (2,28) = 39.4, P < 0.0001, respectively; Fig. 3C].…”

Postnatal and adult consequences of loss of huntingtin during development: Implications for Huntington's disease

“…Whether HD developmental impairments associated with late life disease hallmarks are a consequence of gain or loss of function mechanisms or a compendium of both pathological processes is unknown. Interestingly, our previous in vitro studies have shown that developmental deficits associated with wild-type Huntingtin (Htt) ablation parallel many of those observed in specimens carrying mHtt (Nguyen et al, 2013a; Nguyen et al, 2013b), which supports a loss of function mechanism. In addition, other studies have shown that loss of huntingtin, via impairments of mitotic spindle orientation, affects neural stem cell (NSC) cycle progression and corresponding cell fate specification potential, further supporting an underlining loss-of-function mechanism (Godin et al, 2010).…”

“…In line with these findings, we and other groups have reported significant developmental abnormalities in striatal and cortical neurogenesis in HD mice (Molero et al, 2009; Molina-Calavita et al, 2014). Further, our in vitro studies using an embryonic stem cell line carrying mutant huntingtin (mHtt) demonstrates that this protein impairs the specification and maturation of organ-specific cellular lineages, as well as those of region-specific neural cellular subtypes (Nguyen et al, 2013a; Nguyen et al, 2013b). Accordingly, recent transcriptomic studies have reported that genes with key developmental and neural functions are preferentially impacted in specimens carrying mHtt (Achour et al, 2015; Jin et al, 2012; Labadorf et al, 2015; Ng et al, 2013).…”

Huntington’s Disease: Mechanisms of Pathogenesis and Therapeutic Strategies

“…Moreover, recent studies reveal that huntingtin plays a crucial role in neurogenesis. In fact, huntingtin was shown to be required for the maintenance of the lineage potential of primitive neuronal stem cells during the process of neural induction (Nguyen et al 2013). Furthermore, huntingtin has a crucial role in neurulation controlling homotypic interactions between neuroepithelial cells (Lo Sardo et al 2012).…”