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Tsc2-Rheb signaling regulates EphA-mediated axon guidance.

Nie D, Di Nardo A, Han JM, Baharanyi H, Kramvis I, Huynh T, Dabora S, Codeluppi S, Pandolfi PP, Pasquale EB, Sahin M - Nat. Neurosci. (2010)

Bottom Line: We also found that EphA receptor activation by ephrin-A ligands in neurons led to inhibition of extracellular signal-regulated kinase 1/2 (ERK1/2) activity and decreased inhibition of Tsc2 by ERK1/2.Furthermore, Tsc2 deficiency and hyperactive Rheb constitutively activated mTOR and inhibited ephrin-induced growth cone collapse.Our results indicate that TSC2-Rheb-mTOR signaling cooperates with the ephrin-Eph receptor system to control axon guidance in the visual system.

View Article: PubMed Central - PubMed

Affiliation: The F.M. Kirby Neurobiology Center, Department of Neurology, Children's Hospital Boston, Harvard Medical School, Boston, Massachusetts, USA.

ABSTRACT
Tuberous sclerosis complex is a disease caused by mutations in the TSC1 or TSC2 genes, which encode a protein complex that inhibits mTOR kinase signaling by inactivating the Rheb GTPase. Activation of mTOR promotes the formation of benign tumors in various organs and the mechanisms underlying the neurological symptoms of the disease remain largely unknown. We found that Tsc2 haploinsufficiency in mice caused aberrant retinogeniculate projections that suggest defects in EphA receptor-dependent axon guidance. We also found that EphA receptor activation by ephrin-A ligands in neurons led to inhibition of extracellular signal-regulated kinase 1/2 (ERK1/2) activity and decreased inhibition of Tsc2 by ERK1/2. Thus, ephrin stimulation inactivates the mTOR pathway by enhancing Tsc2 activity. Furthermore, Tsc2 deficiency and hyperactive Rheb constitutively activated mTOR and inhibited ephrin-induced growth cone collapse. Our results indicate that TSC2-Rheb-mTOR signaling cooperates with the ephrin-Eph receptor system to control axon guidance in the visual system.

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Tsc2+/− mice have defects in topographic mapping of contralateral projections in dLGN(a) Representative series of fluorescent micrographs showing termination territories in contralateral dLGN from DiI-labeled ventro-temporal RGCs of Tsc2+/+ versus Tsc2+/− littermates at P19. Anterior is to the left and posterior to the right in each series of sections. Dashed lines represent the borders of dLGN. Arrows indicate DiI labeled RGC termination zones. Insets represent tracings of the flat-mounted retina and injection site for each mouse. OT: optic tract; D: dorsal; V: ventral; T: temporal; N: nasal; and L: lateral. (b) Schematic drawings and colored dots show the position and area of DiI labeled termination in each corresponding dLGN section from wild-type (in red) and Tsc2+/− (in green) littermates. In Tsc2+/−, the lower border of the termination extended erroneously into the area that is innervated by the ventral-nasal RGCs in wild-type mice8. Scale bar represents 100 µm. (c) Quantification of the percentages of DM to VL extension to length of dLGN along the same axis. Data are represented as mean ± SEM (19.1 ± 2.1% in wild-type, n = 4 mice; and 50.3% ± 9.7 % in Tsc2+/− littermates, n = 6 mice. **: P < 0.01 by t-test).
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Figure 3: Tsc2+/− mice have defects in topographic mapping of contralateral projections in dLGN(a) Representative series of fluorescent micrographs showing termination territories in contralateral dLGN from DiI-labeled ventro-temporal RGCs of Tsc2+/+ versus Tsc2+/− littermates at P19. Anterior is to the left and posterior to the right in each series of sections. Dashed lines represent the borders of dLGN. Arrows indicate DiI labeled RGC termination zones. Insets represent tracings of the flat-mounted retina and injection site for each mouse. OT: optic tract; D: dorsal; V: ventral; T: temporal; N: nasal; and L: lateral. (b) Schematic drawings and colored dots show the position and area of DiI labeled termination in each corresponding dLGN section from wild-type (in red) and Tsc2+/− (in green) littermates. In Tsc2+/−, the lower border of the termination extended erroneously into the area that is innervated by the ventral-nasal RGCs in wild-type mice8. Scale bar represents 100 µm. (c) Quantification of the percentages of DM to VL extension to length of dLGN along the same axis. Data are represented as mean ± SEM (19.1 ± 2.1% in wild-type, n = 4 mice; and 50.3% ± 9.7 % in Tsc2+/− littermates, n = 6 mice. **: P < 0.01 by t-test).

Mentions: Since the ventral-lateral shift and the exuberance of the ipsilateral retinogeniculate projections in Tsc2+/− mice resemble the defects observed in ephrin-A-deficient mice21, we examined in more detail the topography of the RGC termination zones in the dLGN. We selectively labeled subsets of RGC axons with focal DiI-crystal placement in the ventral-temporal quarter of the retina. Single termination zones restricted to the dorsal-medial end of the dLGN were observed in wild-type mice, as expected (Fig. 3a). In contrast, the termination zones in Tsc2+/− littermates showed a ventral-lateral shift and sometimes more than one distinct termination zone (Fig. 3b). Quantification of the RGC termination zones along the dorsal-medial to ventral-lateral axis of the dLGN demonstrated a significantly larger extension of the Tsc2+/− RGC projections along the DM-VL axis compared to controls (Fig. 3c). Overall, the mapping defects observed in the Tsc2+/− ventral-temporal projections mimic those described in ephrin-A knockout mice, suggesting attenuation of EphA receptor signaling in Tsc2-deficient RGCs.


Tsc2-Rheb signaling regulates EphA-mediated axon guidance.

Nie D, Di Nardo A, Han JM, Baharanyi H, Kramvis I, Huynh T, Dabora S, Codeluppi S, Pandolfi PP, Pasquale EB, Sahin M - Nat. Neurosci. (2010)

Tsc2+/− mice have defects in topographic mapping of contralateral projections in dLGN(a) Representative series of fluorescent micrographs showing termination territories in contralateral dLGN from DiI-labeled ventro-temporal RGCs of Tsc2+/+ versus Tsc2+/− littermates at P19. Anterior is to the left and posterior to the right in each series of sections. Dashed lines represent the borders of dLGN. Arrows indicate DiI labeled RGC termination zones. Insets represent tracings of the flat-mounted retina and injection site for each mouse. OT: optic tract; D: dorsal; V: ventral; T: temporal; N: nasal; and L: lateral. (b) Schematic drawings and colored dots show the position and area of DiI labeled termination in each corresponding dLGN section from wild-type (in red) and Tsc2+/− (in green) littermates. In Tsc2+/−, the lower border of the termination extended erroneously into the area that is innervated by the ventral-nasal RGCs in wild-type mice8. Scale bar represents 100 µm. (c) Quantification of the percentages of DM to VL extension to length of dLGN along the same axis. Data are represented as mean ± SEM (19.1 ± 2.1% in wild-type, n = 4 mice; and 50.3% ± 9.7 % in Tsc2+/− littermates, n = 6 mice. **: P < 0.01 by t-test).
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Figure 3: Tsc2+/− mice have defects in topographic mapping of contralateral projections in dLGN(a) Representative series of fluorescent micrographs showing termination territories in contralateral dLGN from DiI-labeled ventro-temporal RGCs of Tsc2+/+ versus Tsc2+/− littermates at P19. Anterior is to the left and posterior to the right in each series of sections. Dashed lines represent the borders of dLGN. Arrows indicate DiI labeled RGC termination zones. Insets represent tracings of the flat-mounted retina and injection site for each mouse. OT: optic tract; D: dorsal; V: ventral; T: temporal; N: nasal; and L: lateral. (b) Schematic drawings and colored dots show the position and area of DiI labeled termination in each corresponding dLGN section from wild-type (in red) and Tsc2+/− (in green) littermates. In Tsc2+/−, the lower border of the termination extended erroneously into the area that is innervated by the ventral-nasal RGCs in wild-type mice8. Scale bar represents 100 µm. (c) Quantification of the percentages of DM to VL extension to length of dLGN along the same axis. Data are represented as mean ± SEM (19.1 ± 2.1% in wild-type, n = 4 mice; and 50.3% ± 9.7 % in Tsc2+/− littermates, n = 6 mice. **: P < 0.01 by t-test).
Mentions: Since the ventral-lateral shift and the exuberance of the ipsilateral retinogeniculate projections in Tsc2+/− mice resemble the defects observed in ephrin-A-deficient mice21, we examined in more detail the topography of the RGC termination zones in the dLGN. We selectively labeled subsets of RGC axons with focal DiI-crystal placement in the ventral-temporal quarter of the retina. Single termination zones restricted to the dorsal-medial end of the dLGN were observed in wild-type mice, as expected (Fig. 3a). In contrast, the termination zones in Tsc2+/− littermates showed a ventral-lateral shift and sometimes more than one distinct termination zone (Fig. 3b). Quantification of the RGC termination zones along the dorsal-medial to ventral-lateral axis of the dLGN demonstrated a significantly larger extension of the Tsc2+/− RGC projections along the DM-VL axis compared to controls (Fig. 3c). Overall, the mapping defects observed in the Tsc2+/− ventral-temporal projections mimic those described in ephrin-A knockout mice, suggesting attenuation of EphA receptor signaling in Tsc2-deficient RGCs.

Bottom Line: We also found that EphA receptor activation by ephrin-A ligands in neurons led to inhibition of extracellular signal-regulated kinase 1/2 (ERK1/2) activity and decreased inhibition of Tsc2 by ERK1/2.Furthermore, Tsc2 deficiency and hyperactive Rheb constitutively activated mTOR and inhibited ephrin-induced growth cone collapse.Our results indicate that TSC2-Rheb-mTOR signaling cooperates with the ephrin-Eph receptor system to control axon guidance in the visual system.

View Article: PubMed Central - PubMed

Affiliation: The F.M. Kirby Neurobiology Center, Department of Neurology, Children's Hospital Boston, Harvard Medical School, Boston, Massachusetts, USA.

ABSTRACT
Tuberous sclerosis complex is a disease caused by mutations in the TSC1 or TSC2 genes, which encode a protein complex that inhibits mTOR kinase signaling by inactivating the Rheb GTPase. Activation of mTOR promotes the formation of benign tumors in various organs and the mechanisms underlying the neurological symptoms of the disease remain largely unknown. We found that Tsc2 haploinsufficiency in mice caused aberrant retinogeniculate projections that suggest defects in EphA receptor-dependent axon guidance. We also found that EphA receptor activation by ephrin-A ligands in neurons led to inhibition of extracellular signal-regulated kinase 1/2 (ERK1/2) activity and decreased inhibition of Tsc2 by ERK1/2. Thus, ephrin stimulation inactivates the mTOR pathway by enhancing Tsc2 activity. Furthermore, Tsc2 deficiency and hyperactive Rheb constitutively activated mTOR and inhibited ephrin-induced growth cone collapse. Our results indicate that TSC2-Rheb-mTOR signaling cooperates with the ephrin-Eph receptor system to control axon guidance in the visual system.

Show MeSH
Related in: MedlinePlus