Derivation of enkephalinergic medium spiny neurons from mouse embryonic stem cells
Medium spiny neurons (MSNs) play an important role in locomotion. Counterbalance between two MSN subtypes, enkephalin-positive and substance P-positive MSNs, is crucial for maintaining normal movement. Preferential degeneration of enkephalinergic MSNs in early stage Huntington’s disease (HD) contributes to abnormal involuntary movement called chorea. The reasons for this selective vulnerability are unknown. In vitro differentiation of pluripotent stem cells (PSCs) to neuronal cells is considered a potential approach for modelling neurodegenerative disorders including HD. Generation of PSC-derived enkephalinergic MSNs would be an ideal tool for dissecting their preferential degeneration. However, an enkephalinergic phenotype has never been reported in PSC-derived MSNs. We, therefore, have generated a mouse embryonic stem cell (mESC) reporter line that expresses enhanced yellow fluorescent protein (EYFP) when the cells are committed to an enkephalinergic fate. Characterisation of this mESC line via chimaera generation showed that all EYFP-positive cells were also enkephalin-positive. We have then optimised an enkephalinergic neuronal differentiation protocol using this ESC line. Interestingly, we found that a combination of Wnt inhibitor Dickkopf-related protein 1 (DKK1), sonic hedgehog (Shh) and brain-derived neurotrophic factor (BDNF), commonly used in addition to basal medium for deriving MSNs from PSCs, had a detrimental effect on enkephalin expression. Absence of these three factors, surprisingly, did not reduce the potential of ESCs to become MSNs nor did it affect the electrophysiological properties of ESC-derived MSNs. Further investigation revealed that Pre-pro-enkephalin is down-regulated in the presence of exogenous DKK1 and/or Shh but not in the presence of BDNF. We, therefore, propose that addition of exogenous DKK1 and Shh is unfavourable to derive enkephalinergic MSNs from mouse ESCs. These findings could be used to derive enkephalinergic MSNs in vitro allowing the disease in a dish approach for HD modelling.