Catalytic isoforms of AMP-activated protein kinase differentially regulate IMPDH activity and photoreceptor neuron function
Tae Jun Lee, … , Andrea Santeford, Rajendra S. Apte
Tae Jun Lee, … , Andrea Santeford, Rajendra S. Apte
Published January 16, 2024
Citation Information: JCI Insight. 2024;9(4):e173707. https://doi.org/10.1172/jci.insight.173707.
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Research Article Metabolism Ophthalmology

Catalytic isoforms of AMP-activated protein kinase differentially regulate IMPDH activity and photoreceptor neuron function

Abstract

AMP-activated protein kinase (AMPK) plays a crucial role in maintaining ATP homeostasis in photoreceptor neurons. AMPK is a heterotrimeric protein consisting of α, β, and γ subunits. The independent functions of the 2 isoforms of the catalytic α subunit, PRKAA1 and PRKAA2, are uncharacterized in specialized neurons, such as photoreceptors. Here, we demonstrate in mice that rod photoreceptors lacking PRKAA2, but not PRKAA1, showed altered levels of cGMP, GTP, and ATP, suggesting isoform-specific regulation of photoreceptor metabolism. Furthermore, PRKAA2-deficient mice displayed visual functional deficits on electroretinography and photoreceptor outer segment structural abnormalities on transmission electron microscopy consistent with neuronal dysfunction, but not neurodegeneration. Phosphoproteomics identified inosine monophosphate dehydrogenase (IMPDH) as a molecular driver of PRKAA2-specific photoreceptor dysfunction, and inhibition of IMPDH improved visual function in Prkaa2 rod photoreceptor–knockout mice. These findings highlight a therapeutically targetable PRKAA2 isoform–specific function of AMPK in regulating photoreceptor metabolism and function through a potentially previously uncharacterized mechanism affecting IMPDH activity.

Authors

Tae Jun Lee, Yo Sasaki, Philip A. Ruzycki, Norimitsu Ban, Joseph B. Lin, Hung-Ting Wu, Andrea Santeford, Rajendra S. Apte

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Figure 1

Both catalytic isoforms of AMPK are appreciably expressed in the retina with consistent expression profiles across human and mouse retinas.

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Both catalytic isoforms of AMPK are appreciably expressed in the retina ...
(A–C) Gene expression of Prkaa1 and Prkaa2 (counts per million mapped reads) across different mouse tissues. Box plots show interquartile range, median (line), and minimum and maximum (whiskers). (A) Hepatocytes (n = 4) of mice favor Prkaa2 over Prkaa1 expression while (B) macrophages (n = 8) of mice favor Prkaa1 over Prkaa2 expression. (C) Mouse retinas demonstrate appreciable expression levels of Prkaa1 and Prkaa2 (n = 6). (D and E) In situ hybridization of wild-type retina sections confirmed expression of (D) Prkaa1 and (E) Prkaa2, seen in magenta dots within the outer nuclear layer (red arrows), suggesting expression of both isoforms in rod photoreceptors. RGC, retinal ganglion cells. (F) Scatterplot of Prkaa1 and Prkaa2 expression profiles of mouse retina cell types from single-cell RNA-sequencing data. The cell types show appreciable Prkaa1 and Prkaa2 expression. The rod photoreceptor cluster (red arrow) shows roughly a 2-fold expression of Prkaa2 over Prkaa1. (G) Scatterplot of PRKAA1 and PRKAA2 expression profiles of human retina cell types. The cell types show appreciable PRKAA1 and PRKAA2 expression. The rod photoreceptor cluster (red arrow) shows roughly a 3-fold expression of Prkaa2 over Prkaa1. (H) Scatterplot of Prkaa1 and Prkaa2 expression profiles of mouse brain cell types. Most cell types in the mouse brain demonstrate appreciable expression of Prkaa1 and Prkaa2. (I) Scatterplot of PRKAA1 and PRKAA2 expression profiles of human PBMC types. These cell types do not appreciably express PRKAA2, unlike the central nervous system tissues. (J) Waterfall graph depicting the expression ratios of both catalytic isoforms across mouse retina, human retina, mouse brain, and human PBMCs. The majority of neuronal cell types express the α2 isoform over α1 whereas immune cells overwhelmingly express the α1 isoform over α2. Scale bar: 100 μm; insets: 20× original magnification.