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Impaired Mitochondrial Energy Production Causes Light-Induced Photoreceptor Degeneration Independent of Oxidative Stress.

Jaiswal M, Haelterman NA, Sandoval H, Xiong B, Donti T, Kalsotra A, Yamamoto S, Cooper TA, Graham BH, Bellen HJ - PLoS Biol. (2015)

Bottom Line: Its function is essential to maintain membrane depolarization of the photoreceptors upon repetitive light exposure, and an impaired phototransduction cascade in ppr mutants results in excessive Rhodopsin1 endocytosis.Moreover, loss of ppr results in a reduction in mitochondrial RNAs, reduced electron transport chain activity, and reduced ATP levels.We propose that the reduced ATP level in ppr mutants underlies the phototransduction defect, leading to increased Rhodopsin1 endocytosis during light exposure, causing photoreceptor degeneration independent of oxidative stress.

View Article: PubMed Central - PubMed

Affiliation: Department of Molecular and Human Genetics, Baylor College of Medicine (BCM), Houston, Texas, United States of America; Howard Hughes Medical Institute, BCM, Houston, Texas, United States of America.

ABSTRACT
Two insults often underlie a variety of eye diseases including glaucoma, optic atrophy, and retinal degeneration--defects in mitochondrial function and aberrant Rhodopsin trafficking. Although mitochondrial defects are often associated with oxidative stress, they have not been linked to Rhodopsin trafficking. In an unbiased forward genetic screen designed to isolate mutations that cause photoreceptor degeneration, we identified mutations in a nuclear-encoded mitochondrial gene, ppr, a homolog of human LRPPRC. We found that ppr is required for protection against light-induced degeneration. Its function is essential to maintain membrane depolarization of the photoreceptors upon repetitive light exposure, and an impaired phototransduction cascade in ppr mutants results in excessive Rhodopsin1 endocytosis. Moreover, loss of ppr results in a reduction in mitochondrial RNAs, reduced electron transport chain activity, and reduced ATP levels. Oxidative stress, however, is not induced. We propose that the reduced ATP level in ppr mutants underlies the phototransduction defect, leading to increased Rhodopsin1 endocytosis during light exposure, causing photoreceptor degeneration independent of oxidative stress. This hypothesis is bolstered by characterization of two other genes isolated in the screen, pyruvate dehydrogenase and citrate synthase. Their loss also causes a light-induced degeneration, excessive Rhodopsin1 endocytosis and reduced ATP without concurrent oxidative stress, unlike many other mutations in mitochondrial genes that are associated with elevated oxidative stress and light-independent photoreceptor demise.

No MeSH data available.


Related in: MedlinePlus

Light-induced degeneration due to loss of CS and Pyruvate dehydrogenase.(A) Quantification of relative ERG amplitudes from control, kdn16A, and pdha21A eye clones. Flies were reared in the dark for two (blue) or seven days (red) or in constant light for seven days (green). (B) Quantification of relative ERG amplitudes measured during repetitive light stimuli (1 sec light and 1.5 sec dark, as shown in Fig 3A) from control, kdn16A, and pdha21A eye clones. Error bars represent ± SEM; Student's t test (p-value: ***<0.001). (C–H) Whole mount Rh1 (red) immunostaining in control, kdn16A, and pdha21A mutant PRs. Rhabdomeres are marked by Phalloidin and Actin (green). The flies used in this experiment were 2–3 d old and reared in the dark (C, E, G) or exposed to ~24 h of light (D, F, H).
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pbio.1002197.g009: Light-induced degeneration due to loss of CS and Pyruvate dehydrogenase.(A) Quantification of relative ERG amplitudes from control, kdn16A, and pdha21A eye clones. Flies were reared in the dark for two (blue) or seven days (red) or in constant light for seven days (green). (B) Quantification of relative ERG amplitudes measured during repetitive light stimuli (1 sec light and 1.5 sec dark, as shown in Fig 3A) from control, kdn16A, and pdha21A eye clones. Error bars represent ± SEM; Student's t test (p-value: ***<0.001). (C–H) Whole mount Rh1 (red) immunostaining in control, kdn16A, and pdha21A mutant PRs. Rhabdomeres are marked by Phalloidin and Actin (green). The flies used in this experiment were 2–3 d old and reared in the dark (C, E, G) or exposed to ~24 h of light (D, F, H).

Mentions: Based on our findings, loss of ppr causes reduced ATP production but does not alter steady state ROS levels. However, ppr deficiency causes a severe loss of ERG responses and Rh1 accumulation upon repetitive light stimulation as well as a progressive light-induced PR degeneration. In the genetic screen that permitted the isolation of ppr, we identified mutations in numerous genes whose proteins are targeted to mitochondria [20]. To assess if mutations in genes that have been shown to affect ATP production display similar phenotypes, we evaluated an embryonic lethal allele of knockdown (knd16A) [21], which encodes a homolog of CS, and CG7010 (pdha21A, G170E) [20], which encodes the E1 subunit of Pyruvate dehydrogenase. Loss of CS impairs the tricarboxylic acid (TCA) cycle and hence NADH and ATP production [82,83], whereas Pyruvate dehydrogenase converts pyruvate to acetyl-CoA and mediates entry of glycolytic products into the TCA cycle [84]. Mutant clones in the eyes of knd and pdha show normal primary ERG amplitudes in young flies and flies aged in complete darkness, similar to ppr mutant PRs (Fig 9A). However, a seven-day exposure to light nearly abolishes ERG amplitudes in these mutants, whereas wild type control PRs are barely affected. Hence, loss of kdn or pdha causes a severe light-induced degeneration. In addition, both mutants fail to sustain the ERG amplitude upon repetitive stimulation in young animals (Fig 9B), similar to the phenotypes associated with the loss of ppr (Fig 3A). These observations suggest that perturbations of oxidative metabolism leading to loss of ATP production in both mutants underlie these phenotypes.


Impaired Mitochondrial Energy Production Causes Light-Induced Photoreceptor Degeneration Independent of Oxidative Stress.

Jaiswal M, Haelterman NA, Sandoval H, Xiong B, Donti T, Kalsotra A, Yamamoto S, Cooper TA, Graham BH, Bellen HJ - PLoS Biol. (2015)

Light-induced degeneration due to loss of CS and Pyruvate dehydrogenase.(A) Quantification of relative ERG amplitudes from control, kdn16A, and pdha21A eye clones. Flies were reared in the dark for two (blue) or seven days (red) or in constant light for seven days (green). (B) Quantification of relative ERG amplitudes measured during repetitive light stimuli (1 sec light and 1.5 sec dark, as shown in Fig 3A) from control, kdn16A, and pdha21A eye clones. Error bars represent ± SEM; Student's t test (p-value: ***<0.001). (C–H) Whole mount Rh1 (red) immunostaining in control, kdn16A, and pdha21A mutant PRs. Rhabdomeres are marked by Phalloidin and Actin (green). The flies used in this experiment were 2–3 d old and reared in the dark (C, E, G) or exposed to ~24 h of light (D, F, H).
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Related In: Results  -  Collection

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Show All Figures
getmorefigures.php?uid=PMC4503542&req=5

pbio.1002197.g009: Light-induced degeneration due to loss of CS and Pyruvate dehydrogenase.(A) Quantification of relative ERG amplitudes from control, kdn16A, and pdha21A eye clones. Flies were reared in the dark for two (blue) or seven days (red) or in constant light for seven days (green). (B) Quantification of relative ERG amplitudes measured during repetitive light stimuli (1 sec light and 1.5 sec dark, as shown in Fig 3A) from control, kdn16A, and pdha21A eye clones. Error bars represent ± SEM; Student's t test (p-value: ***<0.001). (C–H) Whole mount Rh1 (red) immunostaining in control, kdn16A, and pdha21A mutant PRs. Rhabdomeres are marked by Phalloidin and Actin (green). The flies used in this experiment were 2–3 d old and reared in the dark (C, E, G) or exposed to ~24 h of light (D, F, H).
Mentions: Based on our findings, loss of ppr causes reduced ATP production but does not alter steady state ROS levels. However, ppr deficiency causes a severe loss of ERG responses and Rh1 accumulation upon repetitive light stimulation as well as a progressive light-induced PR degeneration. In the genetic screen that permitted the isolation of ppr, we identified mutations in numerous genes whose proteins are targeted to mitochondria [20]. To assess if mutations in genes that have been shown to affect ATP production display similar phenotypes, we evaluated an embryonic lethal allele of knockdown (knd16A) [21], which encodes a homolog of CS, and CG7010 (pdha21A, G170E) [20], which encodes the E1 subunit of Pyruvate dehydrogenase. Loss of CS impairs the tricarboxylic acid (TCA) cycle and hence NADH and ATP production [82,83], whereas Pyruvate dehydrogenase converts pyruvate to acetyl-CoA and mediates entry of glycolytic products into the TCA cycle [84]. Mutant clones in the eyes of knd and pdha show normal primary ERG amplitudes in young flies and flies aged in complete darkness, similar to ppr mutant PRs (Fig 9A). However, a seven-day exposure to light nearly abolishes ERG amplitudes in these mutants, whereas wild type control PRs are barely affected. Hence, loss of kdn or pdha causes a severe light-induced degeneration. In addition, both mutants fail to sustain the ERG amplitude upon repetitive stimulation in young animals (Fig 9B), similar to the phenotypes associated with the loss of ppr (Fig 3A). These observations suggest that perturbations of oxidative metabolism leading to loss of ATP production in both mutants underlie these phenotypes.

Bottom Line: Its function is essential to maintain membrane depolarization of the photoreceptors upon repetitive light exposure, and an impaired phototransduction cascade in ppr mutants results in excessive Rhodopsin1 endocytosis.Moreover, loss of ppr results in a reduction in mitochondrial RNAs, reduced electron transport chain activity, and reduced ATP levels.We propose that the reduced ATP level in ppr mutants underlies the phototransduction defect, leading to increased Rhodopsin1 endocytosis during light exposure, causing photoreceptor degeneration independent of oxidative stress.

View Article: PubMed Central - PubMed

Affiliation: Department of Molecular and Human Genetics, Baylor College of Medicine (BCM), Houston, Texas, United States of America; Howard Hughes Medical Institute, BCM, Houston, Texas, United States of America.

ABSTRACT
Two insults often underlie a variety of eye diseases including glaucoma, optic atrophy, and retinal degeneration--defects in mitochondrial function and aberrant Rhodopsin trafficking. Although mitochondrial defects are often associated with oxidative stress, they have not been linked to Rhodopsin trafficking. In an unbiased forward genetic screen designed to isolate mutations that cause photoreceptor degeneration, we identified mutations in a nuclear-encoded mitochondrial gene, ppr, a homolog of human LRPPRC. We found that ppr is required for protection against light-induced degeneration. Its function is essential to maintain membrane depolarization of the photoreceptors upon repetitive light exposure, and an impaired phototransduction cascade in ppr mutants results in excessive Rhodopsin1 endocytosis. Moreover, loss of ppr results in a reduction in mitochondrial RNAs, reduced electron transport chain activity, and reduced ATP levels. Oxidative stress, however, is not induced. We propose that the reduced ATP level in ppr mutants underlies the phototransduction defect, leading to increased Rhodopsin1 endocytosis during light exposure, causing photoreceptor degeneration independent of oxidative stress. This hypothesis is bolstered by characterization of two other genes isolated in the screen, pyruvate dehydrogenase and citrate synthase. Their loss also causes a light-induced degeneration, excessive Rhodopsin1 endocytosis and reduced ATP without concurrent oxidative stress, unlike many other mutations in mitochondrial genes that are associated with elevated oxidative stress and light-independent photoreceptor demise.

No MeSH data available.


Related in: MedlinePlus