Differential expression of RARbeta isoforms in the mouse striatum during development: a gradient of RARbeta2 expression along the rostrocaudal axis

Parcellation of the striatal complex into dorsal and ventral districts

The striatal complex of basal ganglia comprises two functionally distinct districts. The dorsal district controls motor and cognitive functions. The ventral district regulates the limbic function of motivation, reward, and emotion. The dorsoventral parcellation of the striatum also is of clinical importance as differential striatal pathophysiologies occur in Huntington's disease, Parkinson's disease, and drug addiction disorders. Despite these striking neurobiologic contrasts, it is largely unknown how the dorsal and ventral divisions of the striatum are set up. Here, we demonstrate that interactions between the two key transcription factors Nolz-1 and Dlx1/2 control the migratory paths of striatal neurons to the dorsal or ventral striatum. Moreover, these same transcription factors control the cell identity of striatal projection neurons in both the dorsal and the ventral striata including the D1-direct and D2-indirect pathways. We show that Nolz-1, through the I12b enhancer, represses Dlx1/2, allowing normal migration of striatal neurons to dorsal and ventral locations. We demonstrate that deletion, up-regulation, and down-regulation of Nolz-1 and Dlx1/2 can produce a striatal phenotype characterized by a withered dorsal striatum and an enlarged ventral striatum and that we can rescue this phenotype by manipulating the interactions between Nolz-1 and Dlx1/2 transcription factors. Our study indicates that the two-tier system of striatal complex is built by coupling of cell-type identity and migration and suggests that the fundamental basis for divisions of the striatum known to be differentially vulnerable at maturity is already encoded by the time embryonic striatal neurons begin their migrations into developing striata.

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

The ganglionic eminence contributes cells to several forebrain structures including the cerebral cortex, for which it provides GABAergic interneurons. Migration of neuronal precursors from the retinoic-acid rich embryonic ganglionic eminence to the cerebral cortex is known to be regulated by several factors, but retinoic acid has not been previously implicated. We found retinoic acid to potently inhibit cell migration in slice preparations of embryonic mouse forebrains, which was reversed by an antagonist of the dopamine-D(2) receptor, whose gene is transcriptionally regulated by retinoic acid. Histone-deacetylase inhibitors, which amplify nuclear receptor-mediated transcription, potentiated the inhibitory effect of retinoic acid. Surprisingly, when retinoic acid signalling was completely blocked with a pan-retinoic acid receptor antagonist, this also decreased cell migration into the cortex, implying that a minimal level of endogenous retinoic acid is necessary for tangential migration. Given these opposing effects of retinoic acid in vitro, the in vivo contribution of retinoic acid to migration was tested by counting GABAergic interneurons in cortices of adult mice with experimental reductions in retinoic acid signalling: a range of perturbations resulted in significant reductions in the numerical density of some GABAergic interneuron subpopulations. These observations suggest functions of retinoic acid in interneuron diversity and organization of cortical excitatory-inhibitory balance.

Basal Ganglia disorders associated with imbalances in the striatal striosome and matrix compartments

The striatum is composed principally of GABAergic, medium spiny striatal projection neurons (MSNs) that can be categorized based on their gene expression, electrophysiological profiles, and input–output circuits. Major subdivisions of MSN populations include (1) those in ventromedial and dorsolateral striatal regions, (2) those giving rise to the direct and indirect pathways, and (3) those that lie in the striosome and matrix compartments. The first two classificatory schemes have enabled advances in understanding of how basal ganglia circuits contribute to disease. However, despite the large number of molecules that are differentially expressed in the striosomes or the extra-striosomal matrix, and the evidence that these compartments have different input–output connections, our understanding of how this compartmentalization contributes to striatal function is still not clear. A broad view is that the matrix contains the direct and indirect pathway MSNs that form parts of sensorimotor and associative circuits, whereas striosomes contain MSNs that receive input from parts of limbic cortex and project directly or indirectly to the dopamine-containing neurons of the substantia nigra, pars compacta. Striosomes are widely distributed within the striatum and are thought to exert global, as well as local, influences on striatal processing by exchanging information with the surrounding matrix, including through interneurons that send processes into both compartments. It has been suggested that striosomes exert and maintain limbic control over behaviors driven by surrounding sensorimotor and associative parts of the striatal matrix. Consistent with this possibility, imbalances between striosome and matrix functions have been reported in relation to neurological disorders, including Huntington’s disease, L-DOPA-induced dyskinesias, dystonia, and drug addiction. Here, we consider how signaling imbalances between the striosomes and matrix might relate to symptomatology in these disorders.

Phosphatidylinositide 3-kinase and protein kinase C zeta mediate retinoic acid induction of DARPP-32 in medium size spiny neurons in vitro

Mature striatal medium size spiny neurons express the dopamine and cAMP-regulated phosphoprotein, 32 kDa (DARPP-32), but little is known about the mechanisms regulating its levels, or the specification of fully differentiated neuronal subtypes. Cell extrinsic molecules that increase DARPP-32 mRNA and/or protein levels include retinoic acid (RA), brain-derived neurotrophic factor, and estrogen (E(2)). We now demonstrate that RA regulates DARPP-32 mRNA and protein in primary striatal neuronal cultures. Furthermore, DARPP-32 induction by RA in vitro requires phosphatidylinositide 3-kinase, but is independent of tropomyosin-related kinase B, cyclin-dependent kinase 5, and protein kinase B. Using pharmacologic inhibitors of various isoforms of protein kinase C (PKC), we also demonstrate that DARPP-32 induction by RA in vitro is dependent on PKC zeta (PKCzeta). Thus, the signal transduction pathways mediated by RA are very different than those mediating DARPP-32 induction by brain-derived neurotrophic factor. These data support the presence of multiple signal transduction pathways mediating expression of DARPP-32 in vitro, including a novel, important pathway via which phosphatidylinositide 3-kinase regulates the contribution of PKCzeta.

Modular patterning of structure and function of the striatum by retinoid receptor signaling

Retinoid signaling plays a crucial role in patterning rhombomeres in the hindbrain and motor neurons in the spinal cord during development. A fundamentally interesting question is whether retinoids can pattern functional organization in the forebrain that generates a high order of cognitive behavior. The striatum contains a compartmental structure of striosome (or "patch") and intervening matrix. How this highly complex mosaic design is patterned by the genetic programs during development remains elusive. We report a developmental mechanism by which retinoid receptor signaling controls compartmental formation in the striatum. We analyzed RARbeta(-/-) mutant mice and found a selective loss of striosomal compartmentalization in the rostral mutant striatum. The loss of RARbeta signaling in the mutant mice resulted in reduction of cyclin E2, a cell cycle protein regulating transition from G(1) to S phase, and also reduction of the proneural gene Mash1, which led to defective neurogenesis of late-born striosomal cells. Importantly, during striatal neurogenesis, endogenous levels of retinoic acid were spatiotemporally regulated such that transduction of high levels of retinoic acid through RARbeta selectively expanded the population of late-born striosomal progenitors, which evolved into a highly elaborate compartment in the rostral striatum. RARbeta(-/-) mutant mice, which lacked such enlarged compartment, displayed complex alternations of dopamine agonist-induced stereotypic motor behavior, including exaggeration of head bobbing movement and reduction of rearing activity. RARbeta signaling thus plays a crucial role in setting up striatal compartments that may engage in neural circuits of psychomotor control.

Genetic patterning of the mammalian telencephalon by morphogenetic molecules and transcription factors

Patterning centers that produce gradients of morphogenetic molecules, including fibroblast growth factor (FGF), bone morphogenetic proteins (BMP), Wnt, Sonic hedgehog (Shh), and retinoic acid (RA), are located in telencephalic anlage during early stages of development. Genetic evidence based on loss-of-function and gain-of-function studies indicate that they are involved in regional specification of the dorsal, ventral, and lateral telencephalon. For patterning of the dorsal telencephalon, FGF8 controls the anteroposterior patterning, while BMP and Wnt molecules regulate the mediolateral patterning. Shh and retinoic acid regulate patterning of the ventral and the lateral telencephalon. The regionalization of telencephalon is accompanied by expression of region-specific codes of transcription factors, which in turn regulate different phases of neuronal development to generate different cell types in each brain region. Therefore, bioactive signals of morphogenetic molecules are translated into transcription factor codes for regional specification, which subsequently leads to neurogenesis of the diversity of cell types in different regions of the telencephalon.

RARβ isoform-specific regulation of DARPP-32 gene expression: an ectopic expression study in the developing rat telencephalon

Dopamine and adenosine 3':5'-monophosphate-regulated phosphoprotein (DARPP-32) is a key molecule for dopamine neurotransmission. The molecular mechanisms underlying the regulation of DARPP-32 in the developing brain remains elusive. Previous studies have shown that retinoids are capable of inducing DARPP-32 in striatal cell culture, suggesting that retinoids are candidate molecules for controlling DARPP-32 expression. In the present study, we first studied the expression profiles of retinoid receptors and their associated co-factors in the developing rat telencephalon by RT-PCR. The results showed that among the retinoid receptors, RARbeta and RXRgamma were nearly selectively expressed in the developing striatum. By contrast, the retinoid receptors associated transcriptional co-factors, including the co-repressors of N-CoR and SMRT, and the co-activators of SRC-1 and P/CAF, were ubiquitously expressed in the developing telencephalon. In light of the previous findings that DARPP-32 was inducible by retinoids in striatal culture, but not in cortical culture, we hypothesized that the striatum-selective RARbeta and RXRgamma may mediate DARPP-32 induction by retinoids. To test this hypothesis, we used the gain-of-function approach to ectopically express RARbeta and RXRgamma in the developing cerebral cortex that lacked these two retinoid receptors. Ectopic expression of RARbeta1, but not RXRgamma1, up-regulated DARPP-32 in the cortical explant culture. Notably, DARPP-32 was up-regulated only by the RARbeta1 isoform, but not by other RARbeta isoforms. Our study suggests that RARbeta signaling may regulate DARPP-32 gene expression by an isoform-specific mechanism in developing telencephalic neurons. The molecular diversity of RARbeta isoforms may underlie parts of the complex gene regulation by retinoids during neural development.