Retinoic acid influences neuronal migration from the ganglionic eminence to the cerebral cortex

Retinol (vitamin A) is an important nutrient with indispensable input into the control of embryonic development and homeostasis (Ross et al. 2000; Clagett-Dame and DeLuca 2002). Retinol is converted to retinoic acid (RA) through sequential oxidative steps: the reversible oxidation of retinol to retinaldehyde can be catalysed by a multitude of retinol dehydrogenases, several of which are widely expressed in the forebrain. The irreversible oxidation of retinaldehyde to RA, however, is primarily mediated by three retinaldehyde dehydrogenases (RALDHs), RALDH1, RALDH2 and RALDH3, whose expressions are restricted to a few sites (Smith et al. 2001; Duester 2008). RA treatment is estimated to regulate expression of several thousand genes by two-fold or more based on in vitro studies on human cell lines (Cawley et al. 2004), but where and when endogenous RA influences gene expression in vivo depends on the cellular and developmental context and is less understood. The best predictors for sites of RA actions in vivo are local peaks in RA levels, as they are generated at local RALDH expression sites (McCaffery and Drager 1994). Being a small amphipathic lipid, RA can rapidly diffuse out of RALDH-positive cells and travel extended distances through tissues (McCaffery et al. 2006; Duester 2008). Because for technical reasons RA cannot be directly visualized in the tissue, its concentration at a particular site can only be inferred from the known topographies of RALDHs in the neighbourhood, as well as from RA measurements in dissected tissue regions. Whereas at some embryonic sites, including the eye and face, RA synthesis is very high, RALDH expression in the forebrain is exceptionally sparse, and almost 90% of total RA content in the adult brain is supplied by the circulation (Kurlandsky et al. 1995; Luo et al. 2004). The meninges surrounding the brain express RALDH2, which generates high RA levels at the brain surface, and a unique region of very high neuronal RA synthesis is a transient neuroepithelial structure in the ventral telencephalon of the embryo, the ganglionic eminence (GE), and its descendant in the mature brain, the basal ganglia of the forebrain (McCaffery and Drager 1994). The GE is the birthplace of GABAergic interneurons for the cerebral cortex as well as of cells in the striatum and the olfactory bulb (Marin et al. 2000; Wichterle et al. 2001; Stenman et al. 2003; Wonders and Anderson 2006; Chedotal and Rijli 2009; Huang 2009; Nobrega-Pereira and Marin 2009; Rakic 2009). There are conflicting reports about whether RA is necessary for DARPP-32 expression in striatal cells (Waclaw et al. 2004; Molotkova et al. 2007), a protein required for dopaminergic transmission, but RA is believed to be instrumental for inducing a network of dopaminergic signal transduction in the GE (Wang and Liu 2005). In particular, the dopamine D2-receptor is under overriding control by RA (Farooqui 1994; Samad et al. 1997; Krezel et al. 1998; Valdenaire et al. 1998), and deficiency of RALDH3 expression in the GE leads to a reduction of this receptor in the basal ganglia (Molotkova et al. 2007). Dopamine receptors are expressed in the embryonic GE before their canonical role in neurotransmission is apparent, where they modulate several developmental processes including cell proliferation and differentiation (Popolo et al. 2004). We have previously described a function of the D2 receptor in cell migration: activation of the D2 receptor inhibits the tangential migration of GABAergic neurons from the GE into the cortex (Crandall et al. 2007). Because transcription of the D2 receptor is regulated by RA, and because the GE is enriched in RA, we asked whether RA can influence cell migration into the cortex. In a slice culture system of embryonic mouse forebrains we found that exogenous RA potently inhibits cell migration into the cortex via a mechanism that is completely reversed by a D2 dopamine receptor antagonist. Abolishing endogenous RA signalling by addition of a pan-RA receptor antagonist to the cultures, however, similarly reduced cell migration into the cortex, indicating that a minimum level of endogenous RA is necessary for cortical migration. To ask which of the two opposite RA effects observed in the slice cultures is likely the one that patho-physiologically prevails, we counted GABAergic interneurons in the cortices of adult mice with three different types of vitamin A deficiency syndromes: in all three mouse models we detected significant reductions of some GABAergic populations.