Final Report Summary - MGS AND DISEASE (Analysis of the pathological implications of the abnormal accumulation of glycogen in specific cell types.)
Glucose is stored in specialised tissues (mainly liver and muscle) in the form of a branched polymer called glycogen. Glycogen synthase (GS) is the only enzyme able to catalyse this polymerisation, and its two isoforms (liver glycogen synthase (LGS) and muscle glycogen synthase (MGS)) are expressed principally in liver and in muscle, respectively. However, MGS is also expressed in all other tissues, regardless of their limited capacity to produce glycogen. MGS activity is tightly controlled by several mechanisms. Previous results from the receiving group showed that the demise of these mechanisms in neurons causes apoptosis and may be the cause of Lafora disease (LD). The working hypothesis of this project was that deregulation of MGS protein levels and / or activity is causative of several pathologies. Therefore, using a state-of-the-art genetic approach, we aimed to dissect the consequences of the loss of MGS regulation in several cell types and tissues. All these questions have been addressed in vitro, using cell culture and diverse cell biology techniques, and in vivo, using newly available conditional mouse and fly models.
In accordance with our specific research objectives, we first confirmed that glycogen accumulation causes neuronal death in vitro and in vivo, and we identified the specific proteases (caspases) mediating glycogen-induced apoptosis. Secondly, we studied whether aberrant accumulation of glycogen is responsible for some degenerative conditions. A mouse model of the progressive myoclonus epilepsy of Lafora type (LD), which lacks the malin (Nhlrc1) gene, was generated and characterised. We observed a progressive accumulation of glycogen-containing aggregates in various tissues. This accumulation was paralleled by a progressive loss of certain neuronal types in the hippocampus and an increased susceptibility to epileptogenic drugs. Furthermore, we generated mouse and fly models in which glycogen synthesis was enhanced in neurons. Both animal models underwent glycogen-induced neuronal cell death, accompanied by a shortening of lifespan and functional decline. Moreover, we initiated the study of abnormal glycogen accumulation in the form of polyglucosan bodies, called Corpora amilacea (CA), during normal aging. We are currently evaluating whether the occurrence of CA in the aging brain is a protective, indolent, or damage-causing reaction to metabolic stress. Third, we evaluated the impact of forced glycogen accumulation in tissues other than the nervous type. Using adenoviral transduction of constitutively active mutants of GS (active-GS) to the liver of rats or transgenic mice expressing active-GS or PTG (a GS activator), we concluded that enhanced glycogen synthesis in the liver improved glucose homeostasis and ameliorated diabetes. Forth and last, we worked to establish whether GS has a moonlighting function. It is believed that neurons do not synthesise glycogen; however, they express the potentially harmful enzyme GS. Thus the question remains as to why neurons expend energy in expressing such a dangerous protein. Our results suggest the regular presence of glycogen in neurons, where it would play physiological functions. Additionally, in the absence of glycogen, GS translocates to the nucleus where it binds RNA. We have identified several residues in the GS sequence that determine glycogen binding and nuclear shuttling, thus facilitating the subsequent study of the nuclear functions of this enzyme. In summary, our results support the notion that glycogen metabolism is active and functional in tissues other than liver and muscle. Enhanced glycogen accumulation has beneficial effects in some tissues (i.e. the liver) but harmful ones in others (i.e. the brain). The results obtained provide further insight into many unresolved human health conditions and will contribute to identifying potential therapeutic strategies.
Regarding the training objectives of this Marie Curie Career Development action, with the aim to become a skilled, independent research manager, either as a group leader or as scientific manager of research institutions, the fellow joined a reference group in glycogen metabolism research, in an institute recently awarded a National Distinction of Research Excellence (Severo Ochoa Award 2011). At the scientific-technical level, this project was a highly enriching experience for both the fellow and the receiving group, since the background of the fellow in an unrelated field of research facilitated the exchange of knowledge and technologies. At the managerial level, during the implementation of the project the fellow has faced several new challenges:
(a) coordinating a group of young and experienced researchers;
(b) managing collaborations and resources; interacting with several administrative departments;
(c) reporting to funding bodies; participating in the redaction of scientific manuscripts;
(d) writing funding applications;
(e) supervising the group finances; communicating science to other scientists and to the public in general; and
(f) interacting with technology transfer officers, among others. Thus, this project provided the fellow with excellent scientific and managerial training opportunities to further his goal to become an independent scientific manager.
In accordance with our specific research objectives, we first confirmed that glycogen accumulation causes neuronal death in vitro and in vivo, and we identified the specific proteases (caspases) mediating glycogen-induced apoptosis. Secondly, we studied whether aberrant accumulation of glycogen is responsible for some degenerative conditions. A mouse model of the progressive myoclonus epilepsy of Lafora type (LD), which lacks the malin (Nhlrc1) gene, was generated and characterised. We observed a progressive accumulation of glycogen-containing aggregates in various tissues. This accumulation was paralleled by a progressive loss of certain neuronal types in the hippocampus and an increased susceptibility to epileptogenic drugs. Furthermore, we generated mouse and fly models in which glycogen synthesis was enhanced in neurons. Both animal models underwent glycogen-induced neuronal cell death, accompanied by a shortening of lifespan and functional decline. Moreover, we initiated the study of abnormal glycogen accumulation in the form of polyglucosan bodies, called Corpora amilacea (CA), during normal aging. We are currently evaluating whether the occurrence of CA in the aging brain is a protective, indolent, or damage-causing reaction to metabolic stress. Third, we evaluated the impact of forced glycogen accumulation in tissues other than the nervous type. Using adenoviral transduction of constitutively active mutants of GS (active-GS) to the liver of rats or transgenic mice expressing active-GS or PTG (a GS activator), we concluded that enhanced glycogen synthesis in the liver improved glucose homeostasis and ameliorated diabetes. Forth and last, we worked to establish whether GS has a moonlighting function. It is believed that neurons do not synthesise glycogen; however, they express the potentially harmful enzyme GS. Thus the question remains as to why neurons expend energy in expressing such a dangerous protein. Our results suggest the regular presence of glycogen in neurons, where it would play physiological functions. Additionally, in the absence of glycogen, GS translocates to the nucleus where it binds RNA. We have identified several residues in the GS sequence that determine glycogen binding and nuclear shuttling, thus facilitating the subsequent study of the nuclear functions of this enzyme. In summary, our results support the notion that glycogen metabolism is active and functional in tissues other than liver and muscle. Enhanced glycogen accumulation has beneficial effects in some tissues (i.e. the liver) but harmful ones in others (i.e. the brain). The results obtained provide further insight into many unresolved human health conditions and will contribute to identifying potential therapeutic strategies.
Regarding the training objectives of this Marie Curie Career Development action, with the aim to become a skilled, independent research manager, either as a group leader or as scientific manager of research institutions, the fellow joined a reference group in glycogen metabolism research, in an institute recently awarded a National Distinction of Research Excellence (Severo Ochoa Award 2011). At the scientific-technical level, this project was a highly enriching experience for both the fellow and the receiving group, since the background of the fellow in an unrelated field of research facilitated the exchange of knowledge and technologies. At the managerial level, during the implementation of the project the fellow has faced several new challenges:
(a) coordinating a group of young and experienced researchers;
(b) managing collaborations and resources; interacting with several administrative departments;
(c) reporting to funding bodies; participating in the redaction of scientific manuscripts;
(d) writing funding applications;
(e) supervising the group finances; communicating science to other scientists and to the public in general; and
(f) interacting with technology transfer officers, among others. Thus, this project provided the fellow with excellent scientific and managerial training opportunities to further his goal to become an independent scientific manager.