FoxP1 marks medium spiny neurons from precursors to maturity and is required for their differentiation

M Holeiter,A.E Rosser,N.N Vinh,C Scherf,S.V Precious,M.V Taylor,R Jeyasingham,R.C Stickland,V Pekarik,C.M Kelly,J Glasbey,L Jones,A.E Reddington

doi:10.1016/j.expneurol.2016.05.002

Abstract

Identifying the steps involved in striatal development is important both for understanding the striatum in health and disease, and for generating protocols to differentiate striatal neurons for regenerative medicine. The most prominent neuronal subtype in the adult striatum is the medium spiny projection neuron (MSN), which constitutes more than 85% of all striatal neurons and classically expresses DARPP-32. Through a microarray study of genes expressed in the whole ganglionic eminence (WGE: the developing striatum) in the mouse, we identified the gene encoding the transcription factor Forkhead box protein P1 (FoxP1) as the most highly up-regulated gene, thus providing unbiased evidence for the association of FoxP1 with MSN development. We also describe the expression of FoxP1 in the human fetal brain over equivalent gestational stages. FoxP1 expression persisted through into adulthood in the mouse brain, where it co-localised with all striatal DARPP-32 positive projection neurons and a small population of DARPP-32 negative cells. There was no co-localisation of FoxP1 with any interneuron markers. FoxP1 was detectable in primary fetal striatal cells following dissection, culture, and transplantation into the adult lesioned striatum, demonstrating its utility as an MSN marker for transplantation studies. Furthermore, DARPP-32 expression was absent from FoxP1 knock-out mouse WGE differentiated in vitro, suggesting that FoxP1 is important for the development of DARPP-32-positive MSNs. In summary, we show that FoxP1 labels MSN precursors prior to the expression of DARPP-32 during normal development, and in addition suggest that FoxP1 labels a sub-population of MSNs that are not co-labelled by DARPP-32. We demonstrate the utility of FoxP1 to label MSNs in vitro and following neural transplantation, and show that FoxP1 is required for DARPP-32 positive MSN differentiation in vitro.

Highlights

Medium spiny neurons (MSNs), which comprise approximately 85% of striatal neurons in the mouse, are dysfunctional in a number of neurological conditions
Forkhead box protein P1 (FoxP1) expression is highly up-regulated during striatal neurogenesis and is maintained to adulthood where it is expressed in MSNs
We present evidence that FoxP1 is a valuable marker of both undifferentiated and mature MSNs and, significantly, that FoxP1 is suitable to recognise these cells in transplant paradigms

Summary

Introduction

Medium spiny neurons (MSNs), which comprise approximately 85% of striatal neurons in the mouse, are dysfunctional in a number of neurological conditions. In Huntington's disease (HD) MSNs become impaired and degenerate over a period of several decades Replacement of damaged striatal cells and reconstruction of striatal circuitry is being actively explored as a therapeutic strategy in this condition (Kelly et al, 2009). Both animal studies and pilot human transplantation trials have demonstrated functional benefit of this approach (Rosser et al, 2011). DARPP32 is not expressed in MSN precursors and is unreliably detected in culture, perhaps due to insufficient maturity of the emerging post-mitotic MSNs, and is a poor marker of developing MSNs

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