The Embryonic Specification of Interneurons
The Fishell Laboratory in interested in the genetic and developmental origins of inhibitory interneuron diversity. For the past decade we have examined the contribution of genetic determinants that while initiated within the proliferative zones become further elaborated and refined as interneurons integrate into distinct cortical and subcortical brain regions. Virtually all cortical interneurons arise from two transient developmental structures, the Medial and Caudal Ganglionic Eminences (MGE and CGE, respectively). Upon becoming postmitotic, the activation of particular genetic determinants, including Nkx2.1 (Butt et al., 2008) and Prox1 (Miyoshi et al, submitted), establishes seven to ten functionally relevant cardinal classes. Understanding the specifics of the genetic programs that create these cardinal classes represents one of the fundamental focuses of our laboratory. We expect to tackle this through two complementary approaches. First we have a long time interest in the extrinsic signals that specify the ganglionic eminences (Machold et al., 2003). Our recent work has implicated non-canonical Wnt-signaling as central to the selection of a PV- versus SST-expressing interneuron identity (Mckenzie et al., in preparation). We hope by further understanding the actions of extrinsic signals the connection between environment and initial specification will become clear. Complementing this are our efforts to determine intrinsic genetic programs, which we have explores through the complement of genome-wide analysis (microarray analysis and RNA-Seq) of specific interneuron subtypes and loss of function analysis. Recently we have extended this analysis to a modular gain of function approach (Au et al., 2013). Together, these methods are quickly providing us with insights as to the developmental genetic programs underlying interneuron subtype specification.
Example I - Most CGE-derived interneurons are labeled by EGFP (green) expressed from the Prox1 locus. Many CGE-derived RELN-positive (red) cells are found in the layer I. Blue (SST+) cells are derived from the MGE. (Image & Description provided by Dr. Goichi Miyoshi)
Example II - Sparse labeling of developing interneurons at E18.5. Early born interneurons from the MGE occupy deep layers and project their axons broadly even before birth. (Image & Description provided by Dr. Rob Machold)
Example III - Nkx2.1 fate map. (Image & Description provided by Dr. Rob Machold)
Example IV - Top, Schematic illustrating our strategy to specifically target gene expression to the medial ganglionic eminence via in utero electroporation of cre-dependent plasmids into Nkx2.1BACCre mice. Bottom left, Coronal section through an E14.5 brain that was electroporated at E12.5, demonstrating the restriction of GFP expression to progenitors within the MGE and precursors migrating through the LGE to the cortex. Bottom right, Coronal section through a P21 brain that was electroporated at E12.5, depicting many GFP+ interneurons in the cortex and striatum. (Image & Description provided by Dr. Tim Petros)
Machold R, Hayashi S, Rutlin M, Muzumdar MD, Nery S, Corbin JG, Gritli-Linde A, Dellovade T, Porter JA, Rubin LL, Dudek H, McMahon AP, Fishell G. Sonic hedgehog is required for progenitor cell maintenance in telencephalic stem cell niches. Neuron. 2003 Sep 11;39(6):937-50.
Butt SJ, Fuccillo M, Nery S, Noctor S, Kriegstein A, Corbin JG, Fishell G. The temporal and spatial origins of cortical interneurons predicts their physiological subtype. Neuron. 2005 Nov 23;48(4):591-604.
Miyoshi G, Butt SJ, Takebayashi H, Fishell G. Physiologically distinct temporal cohorts of cortical interneurons arise from telencephalic Olig2-expressing precursors. J Neurosci. 2007 Jul 18;27(29):7786-98.
Butt SJ, Sousa VH, Fuccillo MV, Hjerling-Leffler J, Miyoshi G, Kimura S, Fishell G. The requirement of Nkx2-1 in the temporal specification of cortical interneuron subtypes. Neuron. 2008 Sep 11;59(5):722-32.
Batista-Brito R, Fishell G. The developmental integration of cortical interneurons into a functional network. Curr Top Dev Biol. 2009;87:81-118.
Batista-Brito R, Rossignol E, Hjerling-Leffler J, Denaxa M, Wegner M, Lefebvre V, Pachnis V, Fishell G. The cell-intrinsic requirement of Sox6 for cortical interneuron development. Neuron. 2009 Aug 27;63(4):466-81.
Miyoshi G, Hjerling-Leffler J, Karayannis T, Sousa VH, Butt SJ, Battiste J, Johnson JE, Machold RP, Fishell G. Genetic fate mapping reveals that the caudal ganglionic eminence produces a large and diverse population of superficial cortical interneurons. J Neurosci. 2010 Feb 3;30(5):1582-94.
Au E, Ahmed T, Karayannis T, Biswas S, Gan L, Fishell G. A modular gain-of-function approach to generate cortical interneuron subtypes from ES cells. Neuron. 2013 Dec 4;80(5):1145-58.
Miyoshi G, Machold RP, Fishell G. Specification of GABAergic Neocortical Interneurons. Cortical Development: Neural Diversity and Neocortical Organization (Chapter 5, R. Kageyama, ed.). 2013;DOI 10.1007/978-4-431-54496-8_5, Springer Book Chapter.