The Laforin-Malin Complex Negatively Regulates Glycogen Synthesis by Modulating Cellular Glucose Uptake via Glucose Transporters

Abstract

Lafora disease (LD), an inherited and fatal neurodegenerative disorder, is characterized by increased cellular glycogen content and the formation of abnormally branched glycogen inclusions, called Lafora bodies, in the affected tissues, including neurons. Therefore, laforin phosphatase and malin ubiquitin E3 ligase, the two proteins that are defective in LD, are thought to regulate glycogen synthesis through an unknown mechanism, the defects in which are likely to underlie some of the symptoms of LD. We show here that laforin's subcellular localization is dependent on the cellular glycogen content and that the stability of laforin is determined by the cellular ATP level, the activity of 5=-AMP-activated protein kinase, and the affinity of malin toward laforin. By using cell and animal models, we further show that the laforin-malin complex regulates cellular glucose uptake by modulating the subcellular localization of glucose transporters; loss of malin or laforin resulted in an increased abundance of glucose transporters in the plasma membrane and therefore excessive glucose uptake. Loss of laforin or malin, however, did not affect glycogen catabolism. Thus, the excessive cellular glucose level appears to be the primary trigger for the abnormally higher levels of cellular glycogen seen in LD. Glucose is an essential metabolite in living systems. However, the regulatory roles of glucose in cellular physiological pathways and the mechanisms by which cells respond to changes in the intracellular levels of glucose are not fully understood (26). Dysregulation in these processes is thought to underlie the pathology of a few disorders that are associated with cytoplasmic glycogen inclusions (50). One such disorder is Lafora disease (LD), a heritable and fatal neurodegenerative disorder characterized by progressive myoclonus epilepsy and other neurological deficits, including ataxia and dementia (17,41). A hallmark of LD is the presence of Lafora bodies-insoluble and abnormally branched intracellular glycogen inclusions called polyglucosan-in neurons, muscle, liver, and other tissues (16,17,51,52). LD is caused by defects in the gene EPM2A, which encodes a dual-specificity protein phosphatase named laforin, or the NHLRC1 gene, which encodes an E3 ubiquitin ligase named malin (6,15,20,32). Laforin harbors a carbohydrate-binding domain (CBD) that binds to glycogen and Lafora bodies, both in vitro and in vivo (5,18,49). Thus, a role for laforin in carbohydrate metabolism and in the disposition of Lafora bodies was proposed (5, 18, 49). Besides Lafora bodies, glycogen content has also been found at higher levels in animals that were deficient for laforin or malin (11,43). Intriguingly, the glycogen reserve in LD animal models shows a higher phosphate content (11, 43), and laforin has been shown to dephosphorylate glycogen (43,44). A recent report suggested that glycogen phosphorylation possibly represents an error in a catalytic step in glycogen synthesis and that its removal by laforin could be a damage control mechanism (...

Loss of neuronatin promotes “browning” of primary mouse adipocytes while reducing Glut1-mediated glucose disposal

“…Notably, a failure of these proteins to be tightly regulated can result in a variety of glycogen storage diseases (7, 30, 31). For example, malin (NHLRC1/EPM2B gene) is an E3 ubiquitin ligase that, together with the phosphatase laforin (EPM2A gene), helps target GS and glycogen debranching enzyme for proteasomal degradation (30).…”

“…For example, malin (NHLRC1/EPM2B gene) is an E3 ubiquitin ligase that, together with the phosphatase laforin (EPM2A gene), helps target GS and glycogen debranching enzyme for proteasomal degradation (30). siRNA-mediated knockdown of malin resulted in increased glucose uptake and more glucose transporters being located to the plasma membrane (31). Sharma et al (30) recently reported that neuronatin was a substrate of malin and that malin was responsible for regulating the degradation of neuronatin in PC12 cells.…”

Deciphering the role of malin in the lafora progressive myoclonus epilepsy

“…Ganesh and co-workers, using overexpression studies of malin fused to GFP indicated that malin is located at the endoplasmic reticulum and that upon treatment of the cells with the proteasome inhibitor MG132, malin forms perinuclear aggregates that are also immunoreactive against ubiquitin, ubiquitin-conjugating enzymes, chaperones and proteasome subunits [29]. The same group showed later that malin predominantly localizes to the nucleus and this localization does not change upon subjecting the cells to heat shock or glucose starvation ([30], [31]). Cheng et al .…”

“…AMPK phosphorylates laforin at residue Ser25 and this modification increases the interaction between laforin and malin [38]. Conditions that trigger the activation of AMPK such as glucose starvation, improve the interaction between laforin and malin ([15], [31]). These results predict that AMPK activation would lead to lower levels of R5/PTG and glycogen synthase, known substrates of the laforin-malin complex.…”

Low glucose transporter SLC2A5-inhibited human normal adjacent lung adenocarcinoma cytoplasmic pro-B cell development mechanism network

“…Glucose deprivation induces Akt-dependent synthesis and incorporation of GLUT1 into the plasma membrane of 3T3-L1 adipocytes [26]. Glycogen synthesis is negatively regulated by the laforin-malin complex through modulating cellular glucose uptake via glucose transporters [27]. In summary, SLC2A5-inhibited different molecular Pearson mutual-positive-correlation networks were constructed in human normal adjacent tissues by the overlapping molecules of GRNInfer with Pearson.…”