Pyruvate kinase M2 regulates photoreceptor structure, function, and viability

Abstract

Pyruvate kinase M2 (PKM2) is a glycolytic enzyme that is expressed in cancer cells. Its role in tumor metabolism is not definitively established, but investigators have suggested that regulation of PKM2 activity can cause accumulation of glycolytic intermediates and increase flux through the pentose phosphate pathway. Recent evidence suggests that PKM2 also may have non-metabolic functions, including as a transcriptional co-activator in gene regulation. We reported previously that PKM2 is abundant in photoreceptor cells in mouse retinas. In the present study, we conditionally deleted PKM2 (rod-cre PKM2-KO) in rod photoreceptors and found that the absence of PKM2 causes increased expression of PKM1 in rods. Analysis of metabolic flux from U-13C glucose shows that rod-cre PKM2-KO retinas accumulate glycolytic intermediates, consistent with an overall reduction in the amount of pyruvate kinase activity. Rod-cre PKM2-KO mice also have an increased NADPH availability could favor lipid synthesis, but we found no difference in phospholipid synthesis between rod-cre PKM2 KO and PKM2-positive controls. As rod-cre PKM2-KO mice aged, we observed a significant loss of rod function, reduced thickness of the photoreceptor outer segment layer, and reduced expression of photoreceptor proteins, including PDE6β. The rod-cre PKM2-KO retinas showed greater TUNEL staining than wild-type retinas, indicating a slow retinal degeneration. In vitro analysis showed that PKM2 can regulate transcriptional activity from the PDE6β promoter in vitro. Our findings indicate that both the metabolic and transcriptional regulatory functions of PKM2 may contribute to photoreceptor structure, function, and viability.

Small molecule activation of metabolic enzyme pyruvate kinase muscle isozyme 2, PKM2, provides photoreceptor neuroprotection

“…21–24 PKM2 is expressed by photoreceptors in the retina while PKM1 expression is confined to the inner retina, and ML-265 shows a high degree of selectivity for PKM2 over other isoforms, PKM1, PKR, and PKL. 5,10,12,13,15,17,20–22 Therefore, ML-265 represents an ideal lead molecule to explore the neuroprotective effects of pharmacologically activating PKM2 in photoreceptors. In line with previous reports, ML-265 displayed potent activation of recombinant human PKM2 with a maximum activation of 249 ± 14% (best-fit value ± std.…”

“…21 However, conditional knockout of Pkm2 in photoreceptors (Rho-Cre: Pkm2 f/f ) resulted in upregulation of the constitutively active PKM1 isoform and demonstrated mild degenerative changes in the outer retina and its function over time. 12,15 ML-265-mediated PKM2 activation mimics a PKM2-to-PKM1 isoform shift, which could cause similar degenerative changes in the retina. 22 Therefore, a repeat-dose tolerability study was undertaken to determine if prolonged pharmacologic activation of PKM2 would result in anatomic and/or functional changes in the retina.…”

“…In these retinas, relative to control retinas where both rod Pkm2 alleles are present (Rho-Cre: Pkm2 + / + ), there was a statistically significant increase in 2-PG and a trend towards increased PEP, consistent with previous metabolic flux studies. 12…”

“…PKM2 localizes primarily to the outer retina while PKM1 expression is mostly confined to the inner retina. 12,15,20 The loss of Pkm2 in photoreceptors led to the compensatory expression of Pkm1 and upregulation of genes involved in glucose metabolism. When these animals were subjected to acute outer retinal stress produced by experimental retinal detachment, they displayed decreased photoreceptor apoptosis and improved survival.…”

PKM2 ablation enhanced retinal function and survival in a preclinical model of retinitis pigmentosa

“…[47][48][49] One important consideration is that Rajala et al have shown in vitro evidence that PKM2 may regulate transcriptional activity through the PDE6β promoter. 24 This finding suggests that PKM2 knockout may have promoted photoreceptor health by working directly with Pde6β, which would indicate that the beneficial effects are specific to Pde6β deficiency. As such, we highlight the need to use a different RP model to evaluate whether the rescue phenotypes from PKM2 ablation are in fact a result of its role in a gene non-specific metabolic pathway.…”