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Sonic hedgehog is a chemoattractant for midbrain dopaminergic axons.

Hammond R, Blaess S, Abeliovich A - PLoS ONE (2009)

Bottom Line: Finally, in mice in which Shh signaling is inactivated during late neuronal development, the most medial dopaminergic projections are deficient.In addition to the role of Shh in the induction of mDN precursors, Shh plays an important role in dopaminergic axon pathfinding to rostral target tissues.Furthermore, Shh signaling is involved in determining the structural diversity of these dopaminergic projections.

View Article: PubMed Central - PubMed

Affiliation: Department of Pathology, Center for Neurobiology and Behavior and Taub Institute, Columbia University College of Physicians and Surgeons, New York, New York, United States of America.

ABSTRACT
Midbrain dopaminergic axons project from the substantia nigra (SN) and the ventral tegmental area (VTA) to rostral target tissues, including the striatum, pallidum, and hypothalamus. The axons from the medially located VTA project primarily to more medial target tissues in the forebrain, whereas the more lateral SN axons project to lateral targets including the dorsolateral striatum. This structural diversity underlies the distinct functions of these pathways. Although a number of guidance cues have been implicated in the formation of the distinct axonal projections of the SN and VTA, the molecular basis of their diversity remains unclear. Here we investigate the molecular basis of structural diversity in mDN axonal projections. We find that Sonic Hedgehog (Shh) is expressed at a choice point in the course of the rostral dopaminergic projections. Furthermore, in midbrain explants, dopaminergic projections are attracted to a Shh source. Finally, in mice in which Shh signaling is inactivated during late neuronal development, the most medial dopaminergic projections are deficient. In addition to the role of Shh in the induction of mDN precursors, Shh plays an important role in dopaminergic axon pathfinding to rostral target tissues. Furthermore, Shh signaling is involved in determining the structural diversity of these dopaminergic projections.

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Dopaminergic projection alterations in Smoothened conditional mutant mice do not appear to be a consequence of target tissue defects.(A) At the choice point for rostrally coursing mDN axonal projections (as in Figure 3) the most medial fibers course ventrally (arrowheads in lower left panel) to the embryonic pallidum, subthalamus, and hypothalamus, as indicated by immunostaining with an antibody specific for Nkx2.1. These fibers are deficient in the Nestin-Smo cko mice at E12.5 (arrowheads in upper left panel). (B) Analysis of gene expression by in situ hybridization in the midbrain and target tissues of wild-type and Nestin-Smo cko mice at E12.5. Expression of the target tissue markers Hnf3β (Foxa2; i–ii), Shh (iii–iv), and Islet-1 ( Isl1;v–vi) appear unaltered in the mutant mice, as determined in horizontal sections. Plane of section is indicated in D. (C) Analysis of Netrin1 gene expression by in situ hybridization in the hypothalmus of wild-type and Nestin-Smo cko mice at E12.5. Expression of the chemoattractant Netrin-1 is unaltered in the mutant mice. Arrows indicate ventral midline/floor plate region where Netrin expression is maximal. Arrowheads indicate weak Netrin expression in the thalamus. Plane of section is indicated in D. (D) Schematic of sagittal section of midbrain at E12.5. Scale bars: (A) 100 µm, (B+C) 200 µm.
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pone-0007007-g004: Dopaminergic projection alterations in Smoothened conditional mutant mice do not appear to be a consequence of target tissue defects.(A) At the choice point for rostrally coursing mDN axonal projections (as in Figure 3) the most medial fibers course ventrally (arrowheads in lower left panel) to the embryonic pallidum, subthalamus, and hypothalamus, as indicated by immunostaining with an antibody specific for Nkx2.1. These fibers are deficient in the Nestin-Smo cko mice at E12.5 (arrowheads in upper left panel). (B) Analysis of gene expression by in situ hybridization in the midbrain and target tissues of wild-type and Nestin-Smo cko mice at E12.5. Expression of the target tissue markers Hnf3β (Foxa2; i–ii), Shh (iii–iv), and Islet-1 ( Isl1;v–vi) appear unaltered in the mutant mice, as determined in horizontal sections. Plane of section is indicated in D. (C) Analysis of Netrin1 gene expression by in situ hybridization in the hypothalmus of wild-type and Nestin-Smo cko mice at E12.5. Expression of the chemoattractant Netrin-1 is unaltered in the mutant mice. Arrows indicate ventral midline/floor plate region where Netrin expression is maximal. Arrowheads indicate weak Netrin expression in the thalamus. Plane of section is indicated in D. (D) Schematic of sagittal section of midbrain at E12.5. Scale bars: (A) 100 µm, (B+C) 200 µm.

Mentions: To investigate the function of Shh in mDN axonal pathfinding in the intact CNS, we took advantage of conditional mutant mice in which the Shh receptor Smo was inactivated by a transgenic Cre recombinase regulated by the Nestin promoter (Nestin-Smo cko mice). In these mice Cre-mediated recombination leads to complete inactivation of the Smo allele in the central nervous system by E11.5 [11], [14]. Analysis of lateral dopaminergic fibers that project rostrally to the striatum in the Nestin-Smo cko mice at E13.5 revealed normal projections (Figures 3B–3C). In contrast, the medial dopaminergic projections that normally course ventromedially were defective in these mice (Figure 3D, 3E). This was particularly evident in the medial fibers that course most ventrally to extrastriatal targets including the pallidum and the subthalamus, as confirmed by immunostaining for the pallidum precursor marker Nkx2.1 (Figure 4A). Similar to the defects observed at E13.5 in Nestin-Smo cko mice, the most medial projections remained defective at a later developmental time point, E15.5 (data not shown). These findings suggest a role for Shh signaling in the ventral targeting of medial dopaminergic axonal projections.


Sonic hedgehog is a chemoattractant for midbrain dopaminergic axons.

Hammond R, Blaess S, Abeliovich A - PLoS ONE (2009)

Dopaminergic projection alterations in Smoothened conditional mutant mice do not appear to be a consequence of target tissue defects.(A) At the choice point for rostrally coursing mDN axonal projections (as in Figure 3) the most medial fibers course ventrally (arrowheads in lower left panel) to the embryonic pallidum, subthalamus, and hypothalamus, as indicated by immunostaining with an antibody specific for Nkx2.1. These fibers are deficient in the Nestin-Smo cko mice at E12.5 (arrowheads in upper left panel). (B) Analysis of gene expression by in situ hybridization in the midbrain and target tissues of wild-type and Nestin-Smo cko mice at E12.5. Expression of the target tissue markers Hnf3β (Foxa2; i–ii), Shh (iii–iv), and Islet-1 ( Isl1;v–vi) appear unaltered in the mutant mice, as determined in horizontal sections. Plane of section is indicated in D. (C) Analysis of Netrin1 gene expression by in situ hybridization in the hypothalmus of wild-type and Nestin-Smo cko mice at E12.5. Expression of the chemoattractant Netrin-1 is unaltered in the mutant mice. Arrows indicate ventral midline/floor plate region where Netrin expression is maximal. Arrowheads indicate weak Netrin expression in the thalamus. Plane of section is indicated in D. (D) Schematic of sagittal section of midbrain at E12.5. Scale bars: (A) 100 µm, (B+C) 200 µm.
© Copyright Policy
Related In: Results  -  Collection

Show All Figures
getmorefigures.php?uid=PMC2742719&req=5

pone-0007007-g004: Dopaminergic projection alterations in Smoothened conditional mutant mice do not appear to be a consequence of target tissue defects.(A) At the choice point for rostrally coursing mDN axonal projections (as in Figure 3) the most medial fibers course ventrally (arrowheads in lower left panel) to the embryonic pallidum, subthalamus, and hypothalamus, as indicated by immunostaining with an antibody specific for Nkx2.1. These fibers are deficient in the Nestin-Smo cko mice at E12.5 (arrowheads in upper left panel). (B) Analysis of gene expression by in situ hybridization in the midbrain and target tissues of wild-type and Nestin-Smo cko mice at E12.5. Expression of the target tissue markers Hnf3β (Foxa2; i–ii), Shh (iii–iv), and Islet-1 ( Isl1;v–vi) appear unaltered in the mutant mice, as determined in horizontal sections. Plane of section is indicated in D. (C) Analysis of Netrin1 gene expression by in situ hybridization in the hypothalmus of wild-type and Nestin-Smo cko mice at E12.5. Expression of the chemoattractant Netrin-1 is unaltered in the mutant mice. Arrows indicate ventral midline/floor plate region where Netrin expression is maximal. Arrowheads indicate weak Netrin expression in the thalamus. Plane of section is indicated in D. (D) Schematic of sagittal section of midbrain at E12.5. Scale bars: (A) 100 µm, (B+C) 200 µm.
Mentions: To investigate the function of Shh in mDN axonal pathfinding in the intact CNS, we took advantage of conditional mutant mice in which the Shh receptor Smo was inactivated by a transgenic Cre recombinase regulated by the Nestin promoter (Nestin-Smo cko mice). In these mice Cre-mediated recombination leads to complete inactivation of the Smo allele in the central nervous system by E11.5 [11], [14]. Analysis of lateral dopaminergic fibers that project rostrally to the striatum in the Nestin-Smo cko mice at E13.5 revealed normal projections (Figures 3B–3C). In contrast, the medial dopaminergic projections that normally course ventromedially were defective in these mice (Figure 3D, 3E). This was particularly evident in the medial fibers that course most ventrally to extrastriatal targets including the pallidum and the subthalamus, as confirmed by immunostaining for the pallidum precursor marker Nkx2.1 (Figure 4A). Similar to the defects observed at E13.5 in Nestin-Smo cko mice, the most medial projections remained defective at a later developmental time point, E15.5 (data not shown). These findings suggest a role for Shh signaling in the ventral targeting of medial dopaminergic axonal projections.

Bottom Line: Finally, in mice in which Shh signaling is inactivated during late neuronal development, the most medial dopaminergic projections are deficient.In addition to the role of Shh in the induction of mDN precursors, Shh plays an important role in dopaminergic axon pathfinding to rostral target tissues.Furthermore, Shh signaling is involved in determining the structural diversity of these dopaminergic projections.

View Article: PubMed Central - PubMed

Affiliation: Department of Pathology, Center for Neurobiology and Behavior and Taub Institute, Columbia University College of Physicians and Surgeons, New York, New York, United States of America.

ABSTRACT
Midbrain dopaminergic axons project from the substantia nigra (SN) and the ventral tegmental area (VTA) to rostral target tissues, including the striatum, pallidum, and hypothalamus. The axons from the medially located VTA project primarily to more medial target tissues in the forebrain, whereas the more lateral SN axons project to lateral targets including the dorsolateral striatum. This structural diversity underlies the distinct functions of these pathways. Although a number of guidance cues have been implicated in the formation of the distinct axonal projections of the SN and VTA, the molecular basis of their diversity remains unclear. Here we investigate the molecular basis of structural diversity in mDN axonal projections. We find that Sonic Hedgehog (Shh) is expressed at a choice point in the course of the rostral dopaminergic projections. Furthermore, in midbrain explants, dopaminergic projections are attracted to a Shh source. Finally, in mice in which Shh signaling is inactivated during late neuronal development, the most medial dopaminergic projections are deficient. In addition to the role of Shh in the induction of mDN precursors, Shh plays an important role in dopaminergic axon pathfinding to rostral target tissues. Furthermore, Shh signaling is involved in determining the structural diversity of these dopaminergic projections.

Show MeSH