Shubha Tole

Lhx2 Selector Activity Specifies Cortical Identity and Suppresses Hippocampal Organizer Fate

The earliest step in creating the cerebral cortex is the specification of neuroepithelium to a cortical fate. Using mouse genetic mosaics and timed inactivations, we demonstrated that Lhx2 acts as a classic selector gene and essential intrinsic determinant of cortical identity. Lhx2 selector activity is restricted to an early critical period when stem cells comprise the cortical neuroepithelium, where it acts cell-autonomously to specify cortical identity and suppress alternative fates in a spatially dependent manner. Laterally, Lhx2 null cells adopt antihem identity, whereas medially they become cortical hem cells, which can induce and organize ectopic hippocampal fields. In addition to providing functional evidence for Lhx2 selector activity, these findings show that the cortical hem is a hippocampal organizer.

Dynamic spatiotemporal expression of LIM genes and cofactors in the embryonic and postnatal cerebral cortex.

LIM‐Homeodomain (LIM‐HD) genes encode a family of transcription factors known to be involved in development and patterning in several systems. Previously, we have shown that LIM‐HD gene Lhx2 is required for the formation of a crucial boundary in the dorsal telencephalon (Bulchand et al. [2001] Mech Dev 100:165–175). To further explore the role of LIM‐HD genes as well as the broader LIM gene family in dorsal telencephalic development, we examined the expression pattern of the members of this gene family and their cofactors in the developing mouse cerebral cortex. Transcription factor activity of the LIM‐HD proteins requires the formation of a tetrameric complex consisting of two LIM‐HD molecules linked by a dimer of cofactor (Clim) molecules. LIM‐only (Lmo) proteins can interfere with this process by competing for the cofactors. LIM‐HD protein function, thus, can be modulated by the presence of the appropriate Clim or Lmo molecules. At least 13 LIM‐HD, 4 Lmo, and 2 Clim genes have been identified in the mouse. Several of these genes exhibit complex spatiotemporal patterns spanning different stages of cortical development, from embryonic to postnatal ages. Noteworthy features of the expression patterns include delineation of boundaries within the developing cortex, up‐ or down‐regulation during formation of selected cortical layers, and a striking complementarity of expression of several members consistent with specific functions in cortical development. Significantly, in some cases, Lmo or Clim gene expression is robust where no LIM‐HD gene expression is detectable. These results suggest multiple and distinct roles for LIM‐HD, Lmo, and Clim genes in cortical development, and also support a LIM‐HD–independent role for some Lmo and Clim members. Development Dynamics, 2003.

Selective Requirement of Pax6, But Not Emx2, in the Specification and Development of Several Nuclei of the Amygdaloid Complex

The amygdaloid complex is a group of nuclei that are thought to originate from multiple sites of the dorsal and ventral telencephalic neuroepithelium. The mechanisms that regulate their development are essentially unknown. We studied the role of Pax6 and Emx2, two transcription factors that regulate regional specification and growth of the telencephalon, in the morphogenesis of the amygdaloid complex. We used a set of specific marker genes that identify distinct amygdaloid nuclei to analyze Pax6/Small eye and Emx2 knock-out mutant mouse brains. We found that there is a selective requirement for Pax6, but not Emx2, in the formation a subset of nuclei within the amygdaloid complex. Specifically, structures that were not previously considered to be developmentally linked, the nucleus of the lateral olfactory tract and the lateral, basolateral, and basomedial nuclei, all appear to have a common requirement for Pax6. Together, our findings provide new insights into the origins and mechanisms underlying the development of the amygdaloid complex.

Expression of FGF receptors 1, 2, 3 in the embryonic and postnatal mouse brain compared with Pdgfralpha, Olig2 and Plp/dm20: implications for oligodendrocyte development.

Fibroblast growth factors (FGF) receptors FgfR1, FgfR2 and FgfR3 are differentially regulated during oligodendrocyte (OL) maturation in vitro: FgfR3 is expressed by OL progenitors whereas FgfR2 is expressed by differentiated OLs[Mol Cell Neurosci 1996;7:263–275], and we have recently shown that FgfR3 is required for the timely differentiation of OLs in vivo [J Neurosci 2003;23:883–894].Here we have used in situhybridization to investigate the expression patterns of FgfR1–3 and compare them to the putative OL progenitor markers Olig2, Pdgfrαand Plp/dm20 as a function of development in vivo, in particular at sites of OL specification, migration or differentiation in the mouse forebrain and cerebellum. We show that at early stages FgfR1–3 expression overlaps with that of Olig2 in the embryonic ventricular zone of the lateral and medial ganglionic eminences. Further, a scattered population of cells expressing FgfR3 (but not FgfR1 or FgfR2) in the ventral telencephalon appear to arise from the ventricular zone, and at later stages are found more dorsally in the cortex, in an overall pattern similar to Olig2 and/or Pdgfrα. Postnatal expression of FgfR2 increases with age, more prominently in specific regions, including the cortical and cerebellar white matter and optic nerve. Thus, the differential expression pattern of FgfR2 and FgfR3 observed in vivo suggests that their expression is developmentally regulated in a manner consistent with the pattern of their expression in culture. These data provide further insights into role of FgfRs in OL development, and they emphasize that these receptors are positioned both spatially and temporally to impact OL generation in vivo.

A stream of cells migrating from the caudal telencephalon reveals a link between the amygdala and neocortex

The amygdaloid complex consists of diverse nuclei that belong to distinct functional systems, yet many issues about its development are poorly understood. Here, we identify a stream of migrating cells that form specific amygdaloid nuclei in mice. In utero electroporation showed that this caudal amygdaloid stream (CAS) originated in a unique domain at the caudal telencephalic pole that is contiguous with the dorsal pallium, which was previously thought to generate only neocortical cells. The CAS and the neocortex share mechanisms for specification (transcription factors Tbr1, Lhx2 and Emx1/2) and migration (reelin and Cdk5). Reelin, a critical cue for migration in the neocortex, and Cdk5, which is specifically required for migration along radial glia in the neocortex, were both selectively required for the normal migration of the CAS, but not for that of other amygdaloid nuclei. This is first evidence of a dorsal pallial contribution to the amygdala, demonstrating a developmental and mechanistic link between the amygdala and the neocortex.

LIM Genes Parcellate the Embryonic Amygdala and Regulate Its Development

The mechanisms that regulate the development of the amygdaloid complex are as yet poorly understood. Here, we show that in the absence of the LIM-homeodomain (LIM-HD) gene Lhx2, a particular amygdaloid nucleus, the nucleus of the lateral olfactory tract (nLOT), is selectively disrupted. LIM family members are well suited for multiple roles in the development of complex structures because they participate in regulatory interactions that permit a diversity of function. To investigate the possible role for other LIM-HD genes as well as LIM-only (Lmo) genes in the developing amygdala, we examined their expression in the embryo. We show that amygdaloid nuclei upregulate distinct patterns of LIM gene expression from embryonic stages. This supports the hypothesis that LIM genes may participate in the mechanisms that control the development of the amygdala. The disruption of the nLOT in the Lhx2 mutant is the first evidence of a role for LIM-HD genes in the development of the amygdaloid complex. The combinatorial expression patterns of LIM genes suggest a comprehensive mechanism for patterning this structure.

Recruitment of the Sonic hedgehog signalling cascade in electroconvulsive seizure-mediated regulation of adult rat hippocampal neurogenesis

Electroconvulsive seizure (ECS) induces structural remodelling in the adult mammalian brain, including an increase in adult hippocampal neurogenesis. The molecular mechanisms that underlie this increase in the proliferation of adult hippocampal progenitors are at present not well understood. We hypothesized that ECS may recruit the Sonic hedgehog (Shh) pathway to mediate its effects on adult hippocampal neurogenesis, as Shh is known to enhance the proliferation of neuronal progenitors and is expressed in the adult basal forebrain, a region that sends robust projections to the hippocampus. Here we demonstrate that the ECS‐induced increase in proliferation of adult hippocampal progenitors was completely blocked in animals treated with cyclopamine, a pharmacological inhibitor of Shh signalling. Our results suggest that both acute and chronic ECS enhance Shh signalling in the adult hippocampus, as we observed a robust upregulation of Patched (Ptc) mRNA, a component of the Shh receptor complex and a downstream transcriptional target of Shh signalling. This increase was rapid and restricted to the dentate gyrus, where the adult hippocampal progenitors reside. In addition, both acute and chronic ECS decreased Smoothened (Smo) mRNA, the other component of the Shh receptor complex, selectively within the dentate gyrus. However, ECS did not appear to influence Shh expression within the basal forebrain, the site from which it has been suggested to be anterogradely transported to the hippocampus. Together, our findings demonstrate that ECS regulates the Shh signalling cascade and indicate that the Shh pathway may be an important mechanism through which ECS enhances adult hippocampal neurogenesis.

Specific Glial Populations Regulate Hippocampal Morphogenesis

The hippocampus plays an integral role in spatial navigation, learning and memory, and is a major site for adult neurogenesis. Critical to these functions is the proper organization of the hippocampus during development. Radial glia are known to regulate hippocampal formation, but their precise function in this process is yet to be defined. We find that in Nuclear Factor I b (Nfib)-deficient mice, a subpopulation of glia from the ammonic neuroepithelium of the hippocampus fail to develop. This results in severe morphological defects, including a failure of the hippocampal fissure, and subsequently the dentate gyrus, to form. As in wild-type mice, immature nestin-positive glia, which encompass all types of radial glia, populate the hippocampus in Nfib-deficient mice at embryonic day 15. However, these fail to mature into GLAST- and GFAP-positive glia, and the supragranular glial bundle is absent. In contrast, the fimbrial glial bundle forms, but alone is insufficient for proper hippocampal morphogenesis. Dentate granule neurons are present in the mutant hippocampus but their migration is aberrant, likely resulting from the lack of the complete radial glial scaffold usually provided by both glial bundles. These data demonstrate a role for Nfib in hippocampal fissure and dentate gyrus formation, and that distinct glial bundles are critical for correct hippocampal morphogenesis.

Kinesin-5, a mitotic microtubule-associated motor protein, modulates neuronal migration

Kinesin-5 is traditionally considered a mitotic motor protein. This article presents evidence that kinesin-5 is also critically influential in the process of neuronal migration, wherein terminally postmitotic neurons undergo orderly movement from their sites of birth to their final destinations.

Transcription factor Lhx2 is necessary and sufficient to suppress astrogliogenesis and promote neurogenesis in the developing hippocampus

The sequential production of neurons and astrocytes from neuroepithelial precursors is a fundamental feature of central nervous system development. We report that LIM-homeodomain (LIM-HD) transcription factor Lhx2 regulates this transition in the developing hippocampus. Disrupting Lhx2 function in the embryonic hippocampus by in utero electroporation and in organotypic slice culture caused the premature production of astrocytes at stages when neurons are normally generated. Lhx2 function is therefore necessary to suppress astrogliogenesis during the neurogenic period. Furthermore, Lhx2 overexpression was sufficient to suppress astrogliogenesis and prolong the neurogenic period. We provide evidence that Lhx2 overexpression can counteract the instructive astrogliogenic effect of Notch activation. Lhx2 overexpression was also able to override and suppress the activation of the GFAP promoter by Nfia, a Notch-regulated transcription factor that is required for gliogenesis. Thus, Lhx2 appears to act as a “brake” on Notch/Nfia-mediated astrogliogenesis. This critical role for Lhx2 is spatially restricted to the hippocampus, because loss of Lhx2 function in the neocortex did not result in premature astrogliogenesis at the expense of neurogenesis. Our results therefore place Lhx2 as a central regulator of the neuron-glia cell fate decision in the hippocampus and reveal a striking regional specificity of this fundamental function within the dorsal telencephalon.