Johan Ericson

Specification of neuronal fates in the ventral neural tube.

The generation of distinct classes of neurons at defined positions is a fundamental step in the development of the vertebrate central nervous system. Recent work has begun to reveal the extracellular signals and transcriptional mediators that direct the pattern of generation of distinct neuronal subtypes in the neural tube. This work has provided a framework to understand the patterning of the ventral neural tube and is permitting molecular analyses of the assembly of functional neuronal circuits.

Groucho-Mediated Transcriptional Repression Establishes Progenitor Cell Pattern and Neuronal Fate in the Ventral Neural Tube

The pattern of neuronal specification in the ventral neural tube is controlled by homeodomain transcription factors expressed by neural progenitor cells, but no general logic has emerged to explain how these proteins determine neuronal fate. We show that most of these homeodomain proteins possess a conserved eh1 motif that mediates the recruitment of Gro/TLE corepressors. The eh1 motif underlies the function of these proteins as repressors during neural patterning in vivo. Inhibition of Gro/TLE-mediated repression in vivo results in a deregulation of cell pattern in the neural tube. These results imply that the pattern of neurogenesis in the neural tube is achieved through the spatially controlled repression of transcriptional repressors-a derepression strategy of neuronal fate specification.

Arrays of ultraconserved non-coding regions span the loci of key developmental genes in vertebrate genomes

BackgroundEvolutionarily conserved sequences within or adjoining orthologous genes often serve as critical cis-regulatory regions. Recent studies have identified long, non-coding genomic regions that are perfectly conserved between human and mouse, termed ultra-conserved regions (UCRs). Here, we focus on UCRs that cluster around genes involved in early vertebrate development; genes conserved over 450 million years of vertebrate evolution.ResultsBased on a high resolution detection procedure, our UCR set enables novel insights into vertebrate genome organization and regulation of developmentally important genes. We find that the genomic positions of deeply conserved UCRs are strongly associated with the locations of genes encoding key regulators of development, with particularly strong positional correlation to transcription factor-encoding genes. Of particular importance is the observation that most UCRs are clustered into arrays that span hundreds of kilobases around their presumptive target genes. Such a hallmark signature is present around several uncharacterized human genes predicted to encode developmentally important DNA-binding proteins.ConclusionThe genomic organization of UCRs, combined with previous findings, suggests that UCRs act as essential long-range modulators of gene expression. The exceptional sequence conservation and clustered structure suggests that UCR-mediated molecular events involve greater complexity than traditional DNA binding by transcription factors. The high-resolution UCR collection presented here provides a wealth of target sequences for future experimental studies to determine the nature of the biochemical mechanisms involved in the preservation of arrays of nearly identical non-coding sequences over the course of vertebrate evolution.

Multiple dorsoventral origins of oligodendrocyte generation in the spinal cord and hindbrain.

Studies have indicated that oligodendrocytes in the spinal cord originate from a ventral progenitor domain defined by expression of the oligodendrocyte-determining bHLH proteins Olig1 and Olig2. Here, we provide evidence that progenitors in the dorsal spinal cord and hindbrain also produce oligodendrocytes and that the specification of these cells may result from a dorsal evasion of BMP signaling over time. Moreover, we show that the generation of ventral oligodendrocytes in the spinal cord depends on Nkx6.1 and Nkx6.2 function, while these homeodomain proteins in the anterior hindbrain instead suppress oligodendrocyte specification. The opposing roles for Nkx6 proteins in the spinal cord and hindbrain, in turn, appear to reflect that oligodendrocytes are produced by distinct ventral progenitor domains at these axial levels. Based on these findings, we propose that oligodendrocytes derive from several distinct positional origins and that the activation of Olig1/2 at different positions is controlled by distinct genetic programs.

Different Levels of Repressor Activity Assign Redundant and Specific Roles to Nkx6 Genes in Motor Neuron and Interneuron Specification

Specification of neuronal fate in the vertebrate central nervous system depends on the profile of transcription factor expression by neural progenitor cells, but the precise roles of such factors in neurogenesis remain poorly characterized. Two closely related transcriptional repressors, Nkx6.2 and Nkx6.1, are expressed by progenitors in overlapping domains of the ventral spinal cord. We provide genetic evidence that differences in the level of repressor activity of these homeodomain proteins underlies the diversification of interneuron subtypes, and provides a fail-safe mechanism during motor neuron generation. A reduction in Nkx6 activity further permits V0 neurons to be generated from progenitors that lack homeodomain proteins normally required for their generation, providing direct evidence for a model in which progenitor homeodomain proteins direct specific cell fates by actively suppressing the expression of transcription factors that direct alternative fates.

Lmx1b is essential for the development of serotonergic neurons

The specification and differentiation of serotonergic (5-HT) neurons require both extrinsic signaling molecules and intrinsic transcription factors to work in concert or in cascade. Here we identify the genetic cascades that control the specification and differentiation of 5-HT neurons in mice. A major determinant in the cascades is an LIM homeodomain-containing gene, Lmx1b, which is required for the development of all 5-HT neurons in the central nervous system. Our results suggest that, during development of 5-HT neurons, Lmx1b is a critical intermediate factor that couples Nkx2-2–mediated early specification with Pet1-mediated terminal differentiation. Moreover, our data indicate that genetic cascades controlling the caudal and rostral 5-HT neurons are distinct, despite their shared components.

Interaction with factor inhibiting HIF-1 defines an additional mode of cross-coupling between the Notch and hypoxia signaling pathways

Cells adapt to hypoxia by a cellular response, where hypoxia-inducible factor 1α (HIF-1α) becomes stabilized and directly activates transcription of downstream genes. In addition to this “canonical” response, certain aspects of the pathway require integration with Notch signaling, i.e., HIF-1α can interact with the Notch intracellular domain (ICD) to augment the Notch downstream response. In this work, we demonstrate an additional level of complexity in this cross-talk: factor-inhibiting HIF-1 (FIH-1) regulates not only HIF activity, but also the Notch signaling output and, in addition, plays a role in how Notch signaling modulates the hypoxic response. We show that FIH-1 hydroxylates Notch ICD at two residues (N1945 and N2012) that are critical for the function of Notch ICD as a transactivator within cells and during neurogenesis and myogenesis in vivo. FIH-1 negatively regulates Notch activity and accelerates myogenic differentiation. In its modulation of the hypoxic response, Notch ICD enhances recruitment of HIF-1α to its target promoters and derepresses HIF-1α function. Addition of FIH-1, which has a higher affinity for Notch ICD than for HIF-1α, abrogates the derepression, suggesting that Notch ICD sequesters FIH-1 away from HIF-1α. In conclusion, the data reveal posttranslational modification of the activated form of the Notch receptor and an intricate mode of cross-coupling between the Notch and hypoxia signaling pathways.

Efficient production of mesencephalic dopamine neurons by Lmx1a expression in embryonic stem cells

Signaling factors involved in CNS development have been used to control the differentiation of embryonic stem cells (ESCs) into mesencephalic dopamine (mesDA) neurons, but tend to generate a limited yield of desired cell type. Here we show that forced expression of Lmx1a, a transcription factor functioning as a determinant of mesDA neurons during embryogenesis, effectively can promote the generation of mesDA neurons from mouse and human ESCs. Under permissive culture conditions, 75%–95% of mouse ESC-derived neurons express molecular and physiological properties characteristic of bona fide mesDA neurons. Similar to primary mesDA neurons, these cells integrate and innervate the striatum of 6-hydroxy dopamine lesioned neonatal rats. Thus, the enriched generation of functional mesDA neurons by forced expression of Lmx1a may be of future importance in cell replacement therapy of Parkinson disease.

Tbx20 is essential for cardiac chamber differentiation and repression of Tbx2

Tbx20, a member of the T-box family of transcriptional regulators, shows evolutionary conserved expression in the developing heart. In the mouse, Tbx20 is expressed in the cardiac crescent, then in the endocardium and myocardium of the linear and looped heart tube before it is restricted to the atrioventricular canal and outflow tract in the multi-chambered heart. Here, we show that Tbx20 is required for progression from the linear heart tube to a multi-chambered heart. Mice carrying a targeted mutation of Tbx20 show early embryonic lethality due to hemodynamic failure. A linear heart tube with normal anteroposterior patterning is established in the mutant. The tube does not elongate, indicating a defect in recruitment of mesenchyme from the secondary heart field, even though markers of the secondary heart field are not affected. Furthermore, dorsoventral patterning of the tube, formation of working myocardium, looping, and further differentiation and morphogenesis fail. Instead, Tbx2, Bmp2 and vinexin α (Sh3d4), genes normally restricted to regions of primary myocardium and lining endocardium, are ectopically expressed in the linear heart tube of Tbx20 mutant embryos. Because Tbx2 is both necessary and sufficient to repress chamber differentiation (Christoffels et al., 2004a; Harrelson et al., 2004), Tbx20 may ensure progression to a multi-chambered heart by repressing Tbx2 in the myocardial precursor cells of the linear heart tube destined to form the chambers.

NKX6 transcription factor activity is required for α- andβ -cell development in the pancreas

In diabetic individuals, the imbalance in glucose homeostasis is caused by loss or dysfunction of insulin-secreting β-cells of the pancreatic islets. As successful generation of insulin-producing cells in vitro could constitute a cure for diabetes, recent studies have explored the molecular program that underlies β-cell formation. From these studies, the homeodomain transcription factor NKX6.1 has proven to be a key player. In Nkx6.1 mutants, β-cell numbers are selectively reduced, while other islet cell types develop normally. However, the molecular events downstream of NKX6.1, as well as the molecular pathways that ensure residualβ -cell formation in the absence of NKX6.1 are largely unknown. Here, we show that the Nkx6.1 paralog, Nkx6.2, is expressed during pancreas development and partially compensates for NKX6.1 function. Surprisingly, our analysis of Nkx6 compound mutant mice revealed a previously unrecognized requirement for NKX6 activity in α-cell formation. This finding suggests a more general role for NKX6 factors in endocrine cell differentiation than formerly suggested. Similar to NKX6 factors, the transcription factor MYT1 has recently been shown to regulateα - as well as β-cell development. We demonstrate that expression of Myt1 depends on overall Nkx6 gene dose, and therefore identify Myt1 as a possible downstream target of Nkx6 genes in the endocrine differentiation pathway.