Her6 regulates the neurogenetic gradient and neuronal identity in the thalamus

Abstract

During vertebrate brain development, the onset of neuronal differentiation is under strict temporal control. In the mammalian thalamus and other brain regions, neurogenesis is regulated also in a spatially progressive manner referred to as a neurogenetic gradient, the underlying mechanism of which is unknown. Here we describe the existence of a neurogenetic gradient in the zebrafish thalamus and show that the progression of neurogenesis is controlled by dynamic expression of the bHLH repressor her6. Members of the Hes/Her family are known to regulate proneural genes, such as Neurogenin and Ascl. Here we find that Her6 determines not only the onset of neurogenesis but also the identity of thalamic neurons, marked by proneural and neurotransmitter gene expression: loss of Her6 leads to premature Neurogenin1-mediated genesis of glutamatergic (excitatory) neurons, whereas maintenance of Her6 leads to Ascl1-mediated production of GABAergic (inhibitory) neurons. Thus, the presence or absence of a single upstream regulator of proneural gene expression, Her6, leads to the establishment of discrete neuronal domains in the thalamus.diencephalon ͉ Hes1 ͉ mash1 ͉ ngn1 ͉ zona limitans intrathalamica N eurogenesis in the developing vertebrate CNS is regulated with a high degree of temporal and spatial precision, with stereotypic patterns of neuronal differentiation and extensive neuronal migration (1, 2). Dynamic patterns of mitotically active neuronal precursors, known as 'neurogenetic gradients' (3) have been described in several brain regions, including the neocortex (4, 5), the dorsal midbrain colliculi (6), and the dopaminergic region of the ventral midbrain (7). In the mammalian diencephalon, in particular the thalamus (formerly known as dorsal thalamus), two main neurogenetic gradients have been described: from posterior to anterior and from lateral to medial (8,9). In rodents, all thalamic neurons are generated in about 6 days, and the orthogonal gradients of glutamatergic neurogenesis sweep across the boundaries of future nuclei. The underlying molecular mechanisms responsible for generating the neurogenetic gradients of the thalamus are unknown.The major constituent of the thalamus is a population of excitatory neurons generated in the caudal thalamus (cTh), whereas a minor population of inhibitory neurons is generated in the rostral thalamus (rTh). The latter is thought to give rise to the reticular nucleus and the ventral lateral geniculate nucleus, including the intergeniculate leaflet (10). During development, this rostro-caudal partitioning is seen in the expression domains of proneural basic helix-loop-helix (bHLH) genes: the achaete-scute-like complex genes (Ascl formerly known as Mash in mouse and Zash in zebrafish) mark the GABAergic rTh and the prethalamus (PTh) and the Neurogenin genes (Neurog, formerly known as Ngn) mark the glutamatergic cTh (2,11,12). Several lines of evidence show that these genes function as determinants of transmitter phenotype: in the mouse telencephalon, Neurog1/2 are required to ...

Transcriptional regulatory mechanisms underlying the GABAergic neuron fate in different diencephalic prosomeres

“…Development of ectopic GABAergic neurons in P2/thalamus of conditional Otx2 mutants correlates with upregulation of Ascl1 expression (Puelles et al, 2006). Also, changes in ascl1 expression due to modulation of Her6 or Ngn1 activity correlate with changes in GABAergic neurogenesis in zebrafish (Scholpp et al, 2009). Finally, inactivation of Ascl1 has been reported to lead to loss of GABAergic neurons in the posterior diencephalon (P2 and P1) (Miyoshi et al, 2004).…”

“…A recent study offers a possible explanation for the different effects of Ascl1 inactivation in P1 and pTh-R. In zebrafish, the basic helixloop-helix (bHLH) transcription factor Her6 represses ngn2 (neurog3 -Zebrafish Information Network) expression affecting the balance between the proneural genes ascl1 and ngn2 and allowing GABAergic neurogenesis (Scholpp et al, 2009). In mouse, as Her6 is expressed in rostral P2 but not in P1, loss of the Ngn2 repressor Ascl1 might lead to Ngn2 upregulation in P1 but might not be sufficient to induce it in pTh-R. Another bHLHOrange (bHLH-O) transcription factor Helt, an Ngn repressor in the embryonic midbrain (Nakatani et al, 2007), is also expressed in the diencephalon and is required for normal GABAergic neuron development in the posterior pretectum (Guimera et al, 2006a;Guimera et al, 2006b).…”

Spatial and temporal requirements for sonic hedgehog in the regulation of thalamic interneuron identity

“…In addition to these early patterning events, Shh signaling is also required to specify neuronal subtypes within thalamic nuclei. The prevailing model stipulates that graded Shh signaling from the zli is necessary and sufficient to promote distinct classes of thalamic progenitor subtypes (HashimotoTorii et al, 2003;Szabó et al, 2009;Vue et al, 2009;Scholpp et al, 2009). The pTH-R domain, which develops closest to the zli, is dependent on the highest concentration of Shh, whereas the rostral population of pTH-C progenitors, developing several cell diameters away from the zli, is dependent on a lower concentration of Shh.…”

Molecular Pathways Controlling Development of Thalamus and Hypothalamus: From Neural Specification to Circuit Formation

“…This zonation of proneural gene expression is followed by posterior-to-anterior differentiation of glutamatergic relay neurons from the Ngn1-positive precursors in the caudal relay thalamus and of GABAergic inhibitory neurons from the Ascl1-expressing precursors in the reticular nucleus of the rostral thalamus. Therefore, the dynamic expression of Her6 defines neurotransmitter fates within the thalamus (Scholpp et al, 2009), and release of Shh determines the time point at which proneural gene expression, and subsequent neuronal differentiation, initiates within the developing thalamus.…”

“…The prosomeric model identifies three major compartments within the developing diencephalon (p1, p2 and p3, moving posterior to anterior) along the longitudinal axis. A hallmark of this model is the clear boundary that separates p2 and p3, termed the zona limitans intrathalamica (Zli), which includes the mid-diencephalic organizer (MDO) (Scholpp et al, 2009). This structure releases several secreted factors essential for patterning the developing thalamus, including Sonic hedgehog (Shh), and members of the Wnt and fibroblast growth factor (Fgf) family (Fig.…”