M-Type Pyruvate Kinase Isoforms and Lactate Dehydrogenase A in the Mammalian Retina: Metabolic Implications

Purpose: Like cancer cells, photoreceptor cells produce lactate aerobically, requiring lactate dehydrogenase A (LDH-A). Cancer cells also use glycolytic intermediates for biosynthesis. The molecular switch controlling glycolytic flow is thought to be an isoenzyme of pyruvate kinase (PKM2). Here, we determined the expression and localization of PKM2 and LDH-A in mammalian retina and make comparisons with the brain.

Methods: Single- and double-labeling immunohistochemistry for PKM2, pyruvate kinase M1 (PKM1), and LDH-A were performed using retinal sections from C57BL/6 mice, Sprague-Dawley rats, rabbits, marmosets, and humans. Pyruvate kinase M1 and PKM2 mRNA and protein expression levels were quantified in rodent retina and brain by using qPCR and immunoblotting. The quaternary forms of PKM2 in rat retina were also determined.

Results: Pyruvate kinase M2 was present in some glial cells and rod and cone photoreceptors in the retina of all species but was exclusively localized to glia in the brain. Pyruvate kinase M1 was confined to neurons in the retina and brain. Lactate dehydrogenase A was principally found in photoreceptors and inner portion of the avascular rabbit retina. Western blotting and qPCR confirmed high levels of PKM2 and LDH-A in the retina. There was a 6- to 9-fold greater expression of PKM2 mRNA in the rodent retina than in the brain. Both the dimeric (inactive, biosynthesis-driving form) and the active tetrameric (glycolytic-driving) forms of PKM2 were present in retina but not in brain.

Conclusions: Mammalian photoreceptors contain dimeric and tetrameric PKM2 and LDH-A. This is consistent with the ability to switch between energy production and biosynthesis like a proliferating tissue, possibly due to demands of opsin synthesis.