Endocytosis of the glutamate transporter 1 is regulated by laforin and malin: Implications in Lafora disease

Lafora disease (LD) is a fatal rare type of progressive myoclonus epilepsy that appears during early adolescence. The disease is caused by mutations in EPM2A or EPM2B genes, which encode laforin, a glucan phosphatase, and malin, an E3‐ubiquitin ligase, respectively. Although the exact roles of laforin and malin are still not well understood, it is known that they work as a complex in which laforin recruits targets that will be ubiquitinated by malin. Recently, we suggested that the type of epilepsy that accompanies LD could be due to deficiencies in the function of the astrocytic glutamate transporter GLT‐1. We described that astrocytes from LD mouse models presented decreased levels of GLT‐1 at the plasma membrane, leading to increased levels of glutamate in the brain parenchyma. In this work, we present evidence indicating that in the absence of a functional laforin/malin complex (as in LD cellular models) there is an alteration in the ubiquitination of GLT‐1, which could be the cause of the reduction in the levels of GLT‐1 at the plasma membrane. On the contrary, overexpression of the laforin/malin complex promotes the retention of GLT‐1 at the plasma membrane. This retention may be due to the direct ubiquitination of GLT‐1 and/or to an opposite effect of this complex on the dynamics of the Nedd4.2‐mediated endocytosis of the transporter. This work, therefore, presents new pieces of evidence on the regulation of GLT‐1 by the laforin/malin complex, highlighting its value as a therapeutic target for the amelioration of the type of epilepsy that accompanies LD. Main Points Laforin/malin complex ubiquitinates GLT‐1 glutamate transporter. Laforin/malin complex modulates the endocytosis of GLT‐1 negatively. Laforin/malin complex opposes the effect of Nedd4.2 on GLT‐1, by interacting and ubiquitinating α‐ and β‐arrestins.