Ppp1r3d deficiency preferentially inhibits neuronal and cardiac Lafora body formation in a mouse model of the fatal epilepsy Lafora disease

Mammalian glycogen chain lengths are subject to complex regulation, including by seven proteins (protein phosphatase‐1 regulatory subunit 3, PPP1R3A through PPP1R3G) that target protein phosphatase‐1 (PP1) to glycogen to activate the glycogen chain‐elongating enzyme glycogen synthase and inactivate the chain‐shortening glycogen phosphorylase. Lafora disease is a fatal neurodegenerative epilepsy caused by aggregates of long‐chained, and as a result insoluble, glycogen, termed Lafora bodies (LBs). We previously eliminated PPP1R3C from a Lafora disease mouse model and studied the effect on LB formation. In the present work, we eliminate and study the effect of absent PPP1R3D. In the interim, brain cell type levels of all PPP1R3 genes have been published, and brain cell type localization of LBs clarified. Integrating these data we find that PPP1R3C is the major isoform in most tissues including brain. In the brain, PPP1R3C is expressed at 15‐fold higher levels than PPP1R3D in astrocytes, the cell type where most LBs form. PPP1R3C deficiency eliminates ~90% of brain LBs. PPP1R3D is quantitatively a minor isoform, but possesses unique MAPK, CaMK2 and 14‐3‐3 binding domains and appears to have an important functional niche in murine neurons and cardiomyocytes. In neurons, it is expressed equally to PPP1R3C, and its deficiency eliminates ~50% of neuronal LBs. In heart, it is expressed at 25% of PPP1R3C where its deficiency eliminates ~90% of LBs. This work studies the role of a second (PPP1R3D) of seven PP1 subunits that regulate the structure of glycogen, toward better understanding of brain glycogen metabolism generally, and in Lafora disease. Glycogen metabolism is tightly controlled. For example, proteins PPP1R3A through PPP1R3G target the phosphatase PP1 to activate glycogen synthase. Lafora disease results from glycogen with overlong branches that accumulates into Lafora bodies, predominantly in astrocytes but also in neurons. Ppp1r3c (R5) knockout rescues Lafora disease. We show that Ppp1r3d (R6) knockout is much less effective. In mouse, R5 predominates in astrocytes, whereas R5 and R6 are expressed equally in neurons. Our work confirms the role of disordered astrocytic glycogen metabolism in the pathogenesis of Lafora disease. It also highlights the specialization in glycogen metabolism regulation in different brain cell types.