Effect of selectively knocking down key metabolic genes in Müller glia on photoreceptor health

Abstract

The importance of Müller glia for retinal homeostasis suggests that they may have vulnerabilities that lead to retinal disease. Here, we studied the effect of selectively knocking down key metabolic genes in Müller glia on photoreceptor health. Immunostaining indicated that murine Müller glia expressed insulin receptor (IR), hexokinase 2 (HK2) and phosphoglycerate dehydrogenase (PHGDH) but very little pyruvate dehydrogenase E1 alpha 1 (PDH‐E1α) and lactate dehydrogenase A (LDH‐A). We crossed Müller glial cell‐CreER (MC‐CreER) mice with transgenic mice carrying a floxed IR, HK2, PDH‐E1α, LDH‐A, or PHGDH gene to study the effect of selectively knocking down key metabolic genes in Müller glia cells on retinal health. Selectively knocking down IR, HK2, or PHGDH led to photoreceptor degeneration and reduced electroretinographic responses. Supplementing exogenous l‐serine prevented photoreceptor degeneration and improved retinal function in MC‐PHGDH knockdown mice. We unexpectedly found that the levels of retinal serine and glycine were not reduced but, on the contrary, highly increased in MC‐PHGDH knockdown mice. Moreover, dietary serine supplementation, while rescuing the retinal phenotypes caused by genetic deletion of PHGDH in Müller glial cells, restored retinal serine and glycine homeostasis probably through regulation of serine transport. No retinal abnormalities were observed in MC‐CreER mice crossed with PDH‐E1α‐ or LDH‐A‐floxed mice despite Cre expression. Our findings suggest that Müller glia do not complete glycolysis but use glucose to produce serine to support photoreceptors. Supplementation with exogenous serine is effective in preventing photoreceptor degeneration caused by PHGDH deficiency in Müller glia.

Glucose uptake by GLUT1 in photoreceptors is essential for outer segment renewal and rod photoreceptor survival

“…The impairment of visual function in Ret∆Glut1 mice could result from a lack of glucose uptake into photoreceptor cells directly. Conversely, it could also result secondarily from a lack of glucose transport into MGCs, as they can generate metabolites from glucose to support photoreceptor function 10,50,63,64 . To distinguish these possibilities, we investigated the functional and structural impact of Slc2a1 deletion in rods by crossing Glut1 flox/flox mice with transgenic mice having tamoxifen‐inducible Cre expressed under the control of the rod Pde6g promoter ( RodΔGlut1 ).…”

“…Conversely, it could also result secondarily from a lack of glucose transport into MGCs, as they can generate metabolites from glucose to support photoreceptor function. 10,50,63,64 To distinguish these possibilities, we investigated the functional and structural impact of Slc2a1 deletion in rods by crossing Glut1 flox/flox mice with transgenic mice having tamoxifen-inducible Cre expressed under the control of the rod Pde6g promoter (RodΔGlut1). To confirm the deletion of GLUT1 from rods, we performed in situ hybridization and immunohistochemistry in control and RodΔGlut1 retinas 1 month after tamoxifen induction (Figure 8A,B).…”

Hepatocyte-Specific Phgdh-Deficient Mice Culminate in Mild Obesity, Insulin Resistance, and Enhanced Vulnerability to Protein Starvation

“…Mutations in PHGDH have been identified as the cause of a rare neurodegenerative degenerative retinal disease, macular telangiectasia type 2 (MacTel), which results in photoreceptor degeneration in humans [ 43 ]. Shen et al recently reported that the Müller cell-specific deletion of the Phgdh gene in mice caused photoreceptor degeneration but did not cause a reduction in the retinal Ser levels, rather, it markedly increased amino acid levels [ 44 ]. This may be due to the enhanced expression of Phgdh in microglia and macrophages in the retina, which were recruited and activated by photoreceptor degeneration, and most likely compensated for Ser in the retina.…”

Patterning and Development of Photoreceptors in the Human Retina

“…When phosphoglycerate dehydrogenase was inhibited in cultured macular Müller glia, they were more susceptible to oxidative stress ( Zhang et al, 2019 ). Additionally, Müller glia have recently been shown in mice to play a role in serine biosynthesis, which can prevent photoreceptor degeneration observed in phosphoglycerate dehydrogenase deficiencies ( Shen et al, 2021 ). These experiments suggest that Müller glia play a critical role in the health and maintenance of the photoreceptors of the human macula.…”