Abstract
Protein kinase A (PKA) signaling is essential for numerous processes but the subcellular localization of specific PKA regulatory (R) and catalytic (C) subunits has yet to be explored comprehensively. Additionally, the localization of the Cβ subunit has never been spatially mapped in any tissue even though ∼50% of PKA signaling in neuronal tissues is thought to be mediated by Cβ. Here we used human retina with its highly specialized neurons as a window into PKA signaling in the brain and characterized localization of PKA Cα, Cβ, RIIα, and RIIβ subunits. We found that each subunit presented a distinct localization pattern. Cα and Cβ were localized in all cell layers (photoreceptors, interneurons, retinal ganglion cells), while RIIα and RIIβ were selectively enriched in photoreceptor cells where both showed distinct patterns of co-localization with Cα but not Cβ. Only Cα was observed in photoreceptor outer segments and at the base of the connecting cilium. Cβ in turn, was highly enriched in mitochondria and was especially prominent in the ellipsoid of cone cells. Further investigation of Cβ using RNA BaseScope technology showed that two Cβ splice variants (Cβ4 and Cβ4ab) likely code for the mitochondrial Cβ proteins. Overall, our data indicates that PKA Cα, Cβ, RIIα, and RIIβ subunits are differentially localized and are likely functionally non-redundant in the human retina. Furthermore, Cβ is potentially important for mitochondrial-associated neurodegenerative diseases previously linked to PKA dysfunction.
Highlights
Vision is the most appreciated of the five senses, and age- and disease-related loss of visual perception has devastating impacts on quality of life
Tight regulation of intracellular cAMP levels is critical in the modulation of photoreceptor light adaptation
Current dogma emphasizes the importance of subcellular localization of A Kinase Anchoring proteins (AKAPs)-bound R subunits in the functional diversity of Protein kinase A (PKA) signaling (Pawson and Scott, 2010), little attention has been paid to the subunit diversity of the C subunits, especially in the brain, the only tissue where many of the Cβ splice variants are expressed at high levels
Summary
Vision is the most appreciated of the five senses, and age- and disease-related loss of visual perception has devastating impacts on quality of life. Restricted localization of these holoenzymes, mediated primarily by scaffold proteins referred to as A Kinase Anchoring proteins (AKAPs), provides an important layer of specificity in PKA signaling (Michel and Scott, 2002) while the unique cell-type specific and subcellular of RIβ and RIIβ in brain supports their functional non-redundancy (Ilouz et al, 2017). Current dogma emphasizes the importance of subcellular localization of AKAP-bound R subunits in the functional diversity of PKA signaling (Pawson and Scott, 2010), little attention has been paid to the subunit diversity of the C subunits, especially in the brain, the only tissue where many of the Cβ splice variants are expressed at high levels. Without spatial information on endogenous protein localization, it is difficult to hypothesize how these two C subunits might differentially influence regulation of specific neuronal processes and to provide evidence for their functional non-redundancy
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