Transcription factors in glutamatergic neurogenesis: Conserved programs in neocortex, cerebellum, and adult hippocampus

Abstract

Glutamatergic, pyramidal-projection neurons are produced in the embryonic cerebral cortex by a series of genetically programmed fate choices, implemented in large part by developmental transcription factors. Our work has focused on Pax6, Tbr2/Eomes, NeuroD, and Tbr1, which are expressed sequentially during the neurogenesis of pyramidal-projection neurons. Recently, we have found that the same transcription factors are expressed, in the same order, during glutamatergic neurogenesis in the adult dentate gyrus, and (with modifications) in the developing cerebellum. While the precise functional significance of this transcription factor expression sequence is unknown, its common appearance in embryonic and adult neurogenesis, and in different brain regions, suggests it is part of a conserved genetic program that specifies general properties of glutamatergic neurons in these regions. Subtypes of glutamatergic neurons (e.g., layer-specific fates in the cortex) are further determined by combinations of transcription factors, superimposed on general sequential programs. These new perspectives on neurogenesis add to the conceptual framework for strategies to engineer neural stem cells for the repair of specific brain circuits.

Introduction

To understand the differentiation of cortical neuron types, it is essential to investigate the molecular regulation of cortical neuron development. A growing body of evidence suggests that most essential features of cortical neurons, and differences between them, are programmed genetically during development (see review by Guillemot et al., 2006). The gene expression profiles that define neuronal properties are broadly controlled by transcription factors, which activate or repress the transcription of a multitude of downstream genes. Our lab has focused on a small group of transcription factors involved in neurogenesis and laminar fate specification of pyramidal-projection neurons in the neocortex. In this minireview, we will summarize our research on a transcription factor expression sequence that occurs in the developing neocortex, as well as the adult hippocampus and developing cerebellum. Our findings suggest that these transcription factors are part of a conserved program for neurogenesis of glutamatergic neurons in the cerebral cortex and cerebellum.

To interpret how programs of transcription factor expression regulate neurogenesis, it is important to have a clear picture of the overall process. Kempermann (2002) has defined neurogenesis as “a series of developmental events (no matter if during embryogenesis or in adulthood) leading to a new neuron.” This definition encapsulates three important principles: first, neurogenesis is a series of events, which occur in orderly sequence; second, these events are developmental in nature, and generally involve transitions from proliferation to differentiation, accompanied by progressive restriction of fate potential; and third, these events are similar in embryogenesis or adulthood (Espósito et al., 2005), presumably including underlying molecular mechanisms.

Numerous studies have demonstrated that the specification of general neuronal identity is inextricably linked with the specification of other aspects of cell fate, such as regional identity and neurotransmitter fate (reviewed by Sur and Rubenstein, 2005). Some phenotypes, such as regional identity and neurotransmitter fate, are determined even before general neuronal fate (Schuurmans and Guillemot, 2002, Schuurmans et al., 2004). Other phenotypes, such as laminar fate and axon projections, are specified concurrently or subsequent to neuronal identity (McConnell, 1995, Hevner et al., 2001, Haubensak et al., 2004, Chen et al., 2005, Molyneaux et al., 2005). Individual transcription factors typically regulate multiple aspects of neuronal identity (reviewed by Hevner, 2006). Pax6, for example, controls phenotypes as diverse as glutamatergic fate (Schuurmans et al., 2004, Kroll and O’Leary, 2005) and area identity (Bishop et al., 2000), among others. Whereas different aspects of neuronal identity may be specified at different times, all may continue to unfold through later stages. For example, transporters and other enzymes linked to glutamatergic or GABAergic fate may not be expressed until several days, or even weeks after the glutamatergic versus GABAergic fate choice has been determined, and well after later-determined fates such as laminar position have been specified (Boulland et al., 2004).

Previous studies have emphasized the importance of transcription factor “combinatorial codes” for specifying neuronal identity in regions as diverse as the cerebral cortex, the spinal cord and the retina (Shirasaki and Pfaff, 2002, Arlotta et al., 2005, Wang and Harris, 2005). However, neuronal fate choices are not made in temporal isolation, but instead depend on conditions established previously through a series of patterning and regional identity choices. In the developing spinal cord, for example, Shh-mediated dorsoventral patterning leads to the establishment of progenitor pools, which express different transcription factor profiles and produce distinct sets of neurons (Shirasaki and Pfaff, 2002). Later in dorsal spinal cord development, GABAergic and glutamatergic fates are determined by the sequential expression of Lbx1, necessary for GABAergic fate, followed by Tlx3, which antagonizes Lbx1 and defines the glutamatergic phenotype (Cheng et al., 2005).

In this minireview, we highlight sequential expression of transcription factors in the cerebral cortex and the cerebellum. Our studies suggest that Pax6, Tbr2/Eomes (referred to hereafter as Tbr2), NeuroD, and Tbr1 are expressed sequentially during glutamatergic neurogenesis in each region (with some modifications in the cerebellum), and in adult as well as embryonic neurogenesis. These similarities suggest that these transcription factors may play conserved roles in a general program of glutamatergic neurogenesis. On the other hand, different molecular contexts in the cerebral cortex and cerebellum, and in the embryo and adult, probably modulate the functions of each transcription factor, thereby helping to sculpt the specific properties of glutamatergic neurons in each region and age. In the developing forebrain, Neurogenin2, a basic helix–loop–helix (bHLH) transcription factor gene, regulates glutamatergic differentiation of early-born neurons and expression of Tbr1 and Tbr2 mRNA (Schuurmans et al., 2004; reviewed by Guillemot et al., 2006). A different bHLH transcription factor gene, Math1, regulates glutamatergic neuron development and Tbr1 expression in the cerebellum (Wang et al., 2005). Recently, we have found that Math1 is also necessary for Tbr2 and Pax6 expression in the cerebellum (our unpublished observations).

Molecules that define the context for neurogenesis include not only transcription factors, but also importantly, environmental factors in the proliferative niche (Desai and McConnell, 2000, Gaiano and Fishell, 2002, Lledo et al., 2006). The interaction of sequential and combinatorial programs with molecular context defines the identity of neuronal types in neocortical development (reviewed by Götz and Huttner, 2005).