ABSTRACT: Striatal medium spiny neurons (MSN) are critically involved in motor control, and their degeneration is a principal component of Huntingtonâs disease. We find that the transcription factor Ctip2 (also known as Bcl11b) is central to MSN differentiation and striatal development. Within the striatum, it is expressed by all MSN, while it is excluded from essentially all striatal interneurons. In the absence of Ctip2, MSN do not fully differentiate, as demonstrated by dramatically reduced expression of a large number of MSN markers, including DARPP-32, FOXP1, Chrm4, Reelin, MOR1, GluR1, and Plexin-D1. Furthermore, MSN fail to aggregate into patches, resulting in severely disrupted patch-matrix organization within the striatum. Finally, heterotopic cellular aggregates invade the Ctip2-/- striatum suggesting a failure by MSN to repel these cells in the absence of Ctip2. In order to investigate the molecular mechanisms that underlie Ctip2-dependent differentiation of MSN and that underlie the patch-matrix disorganization in the mutant striatum, we directly compared gene expression between wild type and mutant striatum at P0. Because CTIP2-expressing MSN constitute 90-95% of the neurons within the striatum, we reasoned that we should be able to detect changes in medium spiny neuron gene expression in Ctip2 null mutants. We microdissected out small regions of striatum at matched locations in wild type and Ctip2-/- mutant littermates at P0 and investigated gene expression with Affymetrix microarrays. We selected the 153 most significant genes and further analyzed them to identify a smaller set of genes of potentially high biological relevance. In order to verify the microarray data and define the distribution of the identified genes in the striatum, we performed in situ hybridization or immunohistochemistry for 12 selected genes: Plexin-D1, Ngef, Nectin-3, Kcnip2, Pcp4L1, Neto1, Basonuclin 2, Fidgetin, Semaphorin 3e, Secretagogin, Unc5d, and Neurotensin. We find that all these genes are either specifically downregulated (Plexin-D1, Ngef, Nectin-3 Kcnip2, Pcp4L1, Neto1), or upregulated (Basonuclin 2, Fidgetin, Semaphorin 3e, Secretagogin, Unc5d, Neurotensin), in the Ctip2-/- striatum, confirming and extending the microarray results. Together, these data indicate that Ctip2 is a critical regulator of MSN differentiation, striatal patch development, and the establishment of the cellular architecture of the striatum. Experiment Overall Design: Matched regions of striatum from wild type and Ctip2-/- mice were obtained via 500 µm diameter punch biopsies performed in the center of the developing striatum in acutely sectioned 300 µm coronal slices of the brain at postnatal day 0 (P0). Sections were matched rostro-caudally between wild type and null mutant tissue, and fiduciary landmarks were used to assure reproducible microdissection of comparable regions. RNA was extracted using the StrataPrep Total RNA Mini Kit (Stratagene, La Jolla, CA), and RNA quality was assayed using a bioanalyzer (Agilent Technologies, Paola Alto, CA). To ensure reproducibility and biological significance, microarrays were performed with RNA samples from three independent wild type, one heterozygote, and four Ctip2-/- mice (biological replicates). Microarray data were normalized using the RMA function within Bioconductor (Irizarry et al., 2003). Statistical significance of gene expression differences between wild type and knockout was determined using Statistical Analysis of Micrarrays (SAM) (Tusher et al., 2001). Using a SAM d-score cutoff of > 2 or < -2, we selected the 153 most significant genes and further analyzed them to identify a smaller set of genes of potentially high biological relevance.
