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TIF-IA-dependent regulation of ribosome synthesis in drosophila muscle is required to maintain systemic insulin signaling and larval growth.

Ghosh A, Rideout EJ, Grewal SS - PLoS Genet. (2014)

Bottom Line: When we mimic this decrease in muscle ribosome synthesis using RNAi-mediated knockdown of TIF-IA, we observe delayed larval development and reduced body growth.This reduction in growth is caused by lowered systemic insulin signaling via two endocrine responses: reduced expression of Drosophila insulin-like peptides (dILPs) from the brain and increased expression of Imp-L2-a secreted factor that binds and inhibits dILP activity-from muscle.Finally, we show that activation of TOR specifically in muscle can increase overall body size and this effect requires TIF-IA function.

View Article: PubMed Central - PubMed

Affiliation: Department of Biochemistry and Molecular Biology, and Clark H. Smith Brain Tumour Centre, Southern Alberta Cancer Research Institute, University of Calgary, Health Research Innovation Center, Calgary, Alberta, Canada.

ABSTRACT
The conserved TOR kinase signaling network links nutrient availability to cell, tissue and body growth in animals. One important growth-regulatory target of TOR signaling is ribosome biogenesis. Studies in yeast and mammalian cell culture have described how TOR controls rRNA synthesis-a limiting step in ribosome biogenesis-via the RNA Polymerase I transcription factor TIF-IA. However, the contribution of TOR-dependent ribosome synthesis to tissue and body growth in animals is less clear. Here we show in Drosophila larvae that ribosome synthesis in muscle is required non-autonomously to maintain normal body growth and development. We find that amino acid starvation and TOR inhibition lead to reduced levels of TIF-IA, and decreased rRNA synthesis in larval muscle. When we mimic this decrease in muscle ribosome synthesis using RNAi-mediated knockdown of TIF-IA, we observe delayed larval development and reduced body growth. This reduction in growth is caused by lowered systemic insulin signaling via two endocrine responses: reduced expression of Drosophila insulin-like peptides (dILPs) from the brain and increased expression of Imp-L2-a secreted factor that binds and inhibits dILP activity-from muscle. We also observed that maintaining TIF-IA levels in muscle could partially reverse the starvation-mediated suppression of systemic insulin signaling. Finally, we show that activation of TOR specifically in muscle can increase overall body size and this effect requires TIF-IA function. These data suggest that muscle ribosome synthesis functions as a nutrient-dependent checkpoint for overall body growth: in nutrient rich conditions, TOR is required to maintain levels of TIF-IA and ribosome synthesis to promote high levels of systemic insulin, but under conditions of starvation stress, reduced muscle ribosome synthesis triggers an endocrine response that limits systemic insulin signaling to restrict growth and maintain homeostasis.

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TIF-IA overexpression in muscle can partially reverse the effects of starvation on FOXO-dependent genes.(A) Data present mean +/− SEM values from qPCR analysis of InR mRNA levels in fed and starved larvae of dMef2>+ and dMef2>TIF-IA animals. Starvation increased InR mRNA levels, compared to fed controls (*P<0.0001, One-way ANOVA and Tukey's post test). Overexpression of TIF-IA in muscle significantly suppressed this starvation-mediated InR induction (*P<0.0001, One-way ANOVA and Tukey's post test). Data normalized to β tubulin mRNA. (B) Data present mean +/− SEM values from qPCR analysis of 4EBP mRNA levels in fed and starved larvae of dMef2>+ and dMef2>TIF-IA animals. Starvation increased 4EBP mRNA levels, compared to fed controls (*P = 0.0003, One-way ANOVA and Tukey's post test). Overexpression of TIF-IA in partially suppressed the starvation mediated 4EBP mRNA induction, although not to a statistically significant level (P = 0.125, One-way ANOVA and Tukey's post test). Data normalized to β tubulin mRNA. All error bars indicate SEM.
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pgen-1004750-g006: TIF-IA overexpression in muscle can partially reverse the effects of starvation on FOXO-dependent genes.(A) Data present mean +/− SEM values from qPCR analysis of InR mRNA levels in fed and starved larvae of dMef2>+ and dMef2>TIF-IA animals. Starvation increased InR mRNA levels, compared to fed controls (*P<0.0001, One-way ANOVA and Tukey's post test). Overexpression of TIF-IA in muscle significantly suppressed this starvation-mediated InR induction (*P<0.0001, One-way ANOVA and Tukey's post test). Data normalized to β tubulin mRNA. (B) Data present mean +/− SEM values from qPCR analysis of 4EBP mRNA levels in fed and starved larvae of dMef2>+ and dMef2>TIF-IA animals. Starvation increased 4EBP mRNA levels, compared to fed controls (*P = 0.0003, One-way ANOVA and Tukey's post test). Overexpression of TIF-IA in partially suppressed the starvation mediated 4EBP mRNA induction, although not to a statistically significant level (P = 0.125, One-way ANOVA and Tukey's post test). Data normalized to β tubulin mRNA. All error bars indicate SEM.

Mentions: Our data suggest that TIF-IA function in muscle is required to maintain systemic insulin signaling in fed animals. We next examined whether TIF-IA-mediated ribosome synthesis in muscle may provide one mechanism to couple nutrient availability to systemic insulin signaling. We overexpressed a UAS-TIF-IA transgene in muscle (dMef2>TIF-IA) and observed a very slight, but statistically significant acceleration in development compared to (dMef2>+) larvae (Figure S6A), although final body size was not affected (Figure S6B). Similar effects were observed with a second, independent UAS-TIF-IA transgene (Figure S6C, S6D). We then examined effects of muscle TIF-IA overexpression in starved animals. When larvae are deprived of dietary protein, insulin signaling is reduced leading to upregulated levels of FOXO target genes such as 4EBP and InR, an effect we observed here following 24 hr starvation. However, when we overexpressed TIF-IA in muscle (dMef2>TIF-IA), the starvation-induced increase in both 4EBP and InR mRNA was partially reversed compared to control (dMef2>+) larvae (Figure 6A, B). This result suggests that TIF-IA function in muscle can, in part, couple nutrient availability to systemic insulin signaling.


TIF-IA-dependent regulation of ribosome synthesis in drosophila muscle is required to maintain systemic insulin signaling and larval growth.

Ghosh A, Rideout EJ, Grewal SS - PLoS Genet. (2014)

TIF-IA overexpression in muscle can partially reverse the effects of starvation on FOXO-dependent genes.(A) Data present mean +/− SEM values from qPCR analysis of InR mRNA levels in fed and starved larvae of dMef2>+ and dMef2>TIF-IA animals. Starvation increased InR mRNA levels, compared to fed controls (*P<0.0001, One-way ANOVA and Tukey's post test). Overexpression of TIF-IA in muscle significantly suppressed this starvation-mediated InR induction (*P<0.0001, One-way ANOVA and Tukey's post test). Data normalized to β tubulin mRNA. (B) Data present mean +/− SEM values from qPCR analysis of 4EBP mRNA levels in fed and starved larvae of dMef2>+ and dMef2>TIF-IA animals. Starvation increased 4EBP mRNA levels, compared to fed controls (*P = 0.0003, One-way ANOVA and Tukey's post test). Overexpression of TIF-IA in partially suppressed the starvation mediated 4EBP mRNA induction, although not to a statistically significant level (P = 0.125, One-way ANOVA and Tukey's post test). Data normalized to β tubulin mRNA. All error bars indicate SEM.
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getmorefigures.php?uid=PMC4214618&req=5

pgen-1004750-g006: TIF-IA overexpression in muscle can partially reverse the effects of starvation on FOXO-dependent genes.(A) Data present mean +/− SEM values from qPCR analysis of InR mRNA levels in fed and starved larvae of dMef2>+ and dMef2>TIF-IA animals. Starvation increased InR mRNA levels, compared to fed controls (*P<0.0001, One-way ANOVA and Tukey's post test). Overexpression of TIF-IA in muscle significantly suppressed this starvation-mediated InR induction (*P<0.0001, One-way ANOVA and Tukey's post test). Data normalized to β tubulin mRNA. (B) Data present mean +/− SEM values from qPCR analysis of 4EBP mRNA levels in fed and starved larvae of dMef2>+ and dMef2>TIF-IA animals. Starvation increased 4EBP mRNA levels, compared to fed controls (*P = 0.0003, One-way ANOVA and Tukey's post test). Overexpression of TIF-IA in partially suppressed the starvation mediated 4EBP mRNA induction, although not to a statistically significant level (P = 0.125, One-way ANOVA and Tukey's post test). Data normalized to β tubulin mRNA. All error bars indicate SEM.
Mentions: Our data suggest that TIF-IA function in muscle is required to maintain systemic insulin signaling in fed animals. We next examined whether TIF-IA-mediated ribosome synthesis in muscle may provide one mechanism to couple nutrient availability to systemic insulin signaling. We overexpressed a UAS-TIF-IA transgene in muscle (dMef2>TIF-IA) and observed a very slight, but statistically significant acceleration in development compared to (dMef2>+) larvae (Figure S6A), although final body size was not affected (Figure S6B). Similar effects were observed with a second, independent UAS-TIF-IA transgene (Figure S6C, S6D). We then examined effects of muscle TIF-IA overexpression in starved animals. When larvae are deprived of dietary protein, insulin signaling is reduced leading to upregulated levels of FOXO target genes such as 4EBP and InR, an effect we observed here following 24 hr starvation. However, when we overexpressed TIF-IA in muscle (dMef2>TIF-IA), the starvation-induced increase in both 4EBP and InR mRNA was partially reversed compared to control (dMef2>+) larvae (Figure 6A, B). This result suggests that TIF-IA function in muscle can, in part, couple nutrient availability to systemic insulin signaling.

Bottom Line: When we mimic this decrease in muscle ribosome synthesis using RNAi-mediated knockdown of TIF-IA, we observe delayed larval development and reduced body growth.This reduction in growth is caused by lowered systemic insulin signaling via two endocrine responses: reduced expression of Drosophila insulin-like peptides (dILPs) from the brain and increased expression of Imp-L2-a secreted factor that binds and inhibits dILP activity-from muscle.Finally, we show that activation of TOR specifically in muscle can increase overall body size and this effect requires TIF-IA function.

View Article: PubMed Central - PubMed

Affiliation: Department of Biochemistry and Molecular Biology, and Clark H. Smith Brain Tumour Centre, Southern Alberta Cancer Research Institute, University of Calgary, Health Research Innovation Center, Calgary, Alberta, Canada.

ABSTRACT
The conserved TOR kinase signaling network links nutrient availability to cell, tissue and body growth in animals. One important growth-regulatory target of TOR signaling is ribosome biogenesis. Studies in yeast and mammalian cell culture have described how TOR controls rRNA synthesis-a limiting step in ribosome biogenesis-via the RNA Polymerase I transcription factor TIF-IA. However, the contribution of TOR-dependent ribosome synthesis to tissue and body growth in animals is less clear. Here we show in Drosophila larvae that ribosome synthesis in muscle is required non-autonomously to maintain normal body growth and development. We find that amino acid starvation and TOR inhibition lead to reduced levels of TIF-IA, and decreased rRNA synthesis in larval muscle. When we mimic this decrease in muscle ribosome synthesis using RNAi-mediated knockdown of TIF-IA, we observe delayed larval development and reduced body growth. This reduction in growth is caused by lowered systemic insulin signaling via two endocrine responses: reduced expression of Drosophila insulin-like peptides (dILPs) from the brain and increased expression of Imp-L2-a secreted factor that binds and inhibits dILP activity-from muscle. We also observed that maintaining TIF-IA levels in muscle could partially reverse the starvation-mediated suppression of systemic insulin signaling. Finally, we show that activation of TOR specifically in muscle can increase overall body size and this effect requires TIF-IA function. These data suggest that muscle ribosome synthesis functions as a nutrient-dependent checkpoint for overall body growth: in nutrient rich conditions, TOR is required to maintain levels of TIF-IA and ribosome synthesis to promote high levels of systemic insulin, but under conditions of starvation stress, reduced muscle ribosome synthesis triggers an endocrine response that limits systemic insulin signaling to restrict growth and maintain homeostasis.

Show MeSH
Related in: MedlinePlus