- Elucidating the Functions of the TSC1 and TSC2 Genes
- Understanding the Causes of TSC-Related Epilepsy
Tuberous sclerosis is an inherited disease caused by mutations in the TSC1 or TSC2 genes. Approximately 80% of people with tuberous sclerosis suffer from epileptic seizures, which are often resistant to anticonvulsive therapy. Other manifestations of the disorder, including kidney disease and tumors on vital organs, also can be severe, yet there are no treatments specific to tuberous sclerosis. A complete understanding of the functions of the TSC1 protein, hamartin, and the TSC2 protein, tuberin, is required for the development of targeted therapeutics. Dr. Elizabeth Henske, a recipient of a Department of Defense Tuberous Sclerosis Complex Research Program fiscal year 2002 Idea Development Award, is investigating hamartin and tuberin function in the yeast Schizosaccharomyces pombe. Yeast model systems provide powerful and convenient means to examine important cellular and molecular processes, as many pathways are similar in yeast and mammals and experiments can be performed more rapidly than in mice or other mammalian systems. Dr. Henske determined that the loss of either TSC1 or TSC2 in yeast resulted in decreased uptake of arginine, an amino acid used for the synthesis of proteins, polyamines (molecules required for cell growth), and nitric oxide (a signaling molecule involved in many cellular functions). Expression of putative amino acid and polyamine transporters also was lower in mutant yeast than in normal control yeast. Moreover, deficiency in either TSC1 or TSC2 caused decreases in intracellular levels of glutamate and other amino acids. This phenotype was rescued by Pas1, a G1 cyclin, suggesting a link between nutrient availability and cell cycle progression. These results indicate that hamartin and tuberin play critical roles in amino acid sensing, uptake, and metabolism and that tuberous sclerosis symptoms may be linked to defects in those key cellular functions. Importantly, diminished glutamate uptake from the synapses in the brains of TSC1-deficient mice is believed to contribute to seizure development, suggesting that the yeast model may provide a novel system for the study of tuberous sclerosis-related epilepsy and for preclinical screening of new therapeutics that may ameliorate seizures in individuals with the disease.
van Slegtenhorst M, Carr E, Stoyanova R, Kruger WD,and Henske EP. 2004. Tsc1+ and tsc2+ regulate arginine uptake and metabolism in Schizosaccharomyces pombe. Journal of Biological Chemistry 279:12706-12713.
van Slegtenhorst M, Mustafa A, and Henske EP. 2005. Pas1, a G1 cyclin, regulates amino acid uptake and rescues a delay in G1 arrest in Tsc1 and Tsc2 mutants in Schizosaccharomyces pombe. Human Molecular Genetics 14 (19): 2851-2858.
Epilepsy is one of the most devastating complications of tuberous sclerosis complex (TSC). Approximately 80% of children who have TSC develop epileptic seizures, which are often severe and refractory to available treatments. Seizures and other manifestations of TSC have been attributed to malformed areas in the brain, known as cortical tubers, that are believed to arise during embryonic development. Dr. David Gutmann and colleagues of the Washington University School of Medicine have determined new mechanisms by which TSC gene defects in the brain result in seizures. Understanding such cellular and molecular mechanisms of seizures is necessary for developing new tailored therapies.
With funding from a Department of Defense Fiscal Year 2002 Tuberous Sclerosis Complex Research Program Idea Development Award, Dr. Gutmann and colleagues used mouse models in which the Tsc1 gene was inactivated in the astrocyte class of neuroglial cells to study TSC-related epilepsy. These mice exhibit enhanced astrocyte proliferation, abnormal neuronal organization, and seizures. The investigators found abnormal expression of neuroglial differentiation markers in Tsc1-deficient astrocytes. Similar gene expression patterns were observed in cortical tubers and subependymal giant cell astrocytoma tumors from TSC patients, suggesting that both types of lesions arise from similar neuroglial progenitor cells and that Tsc1 gene inactivation in humans leads to aberrant progenitor cell differentiation. Tsc1 loss in mouse astrocytes also was associated with increased activity of the Rheb/mammalian target of rapamycin (mTOR)/p70S6 kinase (S6K) pathway, supporting recent evidence that the TSC1/TSC2 protein complex may regulate Rheb and S6K. In addition, they found that an important astrocyte adhesion molecule, adhesion molecule in glia (AMOG), also regulates the mTOR signaling pathway, but independent of TSC.
Gutmann and colleagues examined the molecular mechanisms that might underlie increased neuronal excitability and lead to seizures. They showed that Tsc1-deficient astrocytes had decreased expression of two weak inward rectifier potassium currents and exhibited diminished potassium current in functional assays. These astrocyte potassium current defects were not reversed by rapamycin, an inhibitor of mTOR. Dr. Gutmann and his team believe that the observed abnormalities in astrocyte potassium uptake could potentially lead to excessive synaptic stimulation in neurons, resulting in hyperexcitability and seizures.
Dr. Gutmann's research team has (1) discovered several genetic and cellular abnormalities that result from astrocyte-specific inactivation of Tsc1, (2) demonstrated that there is a role for astrocyte potassium homeostasis in influencing seizures in mouse models of TSC, and (3) developed a novel concept that the astrocyte may be centrally involved in the pathogenesis of neurological complications of TSC, including epilepsy. Based on these findings, innovative therapies for epilepsy could potentially target astrocytes.
Ess KC, Uhlmann EJ, Li W, Li H, DeClue JE, Crino PB, and Gutmann DH. 2004. Expression profiling in tuberous sclerosis complex (TSC) knockout mouse astrocytes to characterize human TSC brain pathology. Glia 46:28-40.
Uhlmann EJ, Li W, Scheidenhelm D, Gau CL, Tamanoi F, and Gutmann DH. 2004. Loss of tuberous sclerosis complex 1 (Tsc1) expression results in increased Rheb/S6K pathway signaling important for astrocyte cell size regulation. Glia 47:180-188.
Ess KC, Kamp KA, Tu BP, and Gutmann DH. 2005. Developmental origin of subependymal giant cell astrocytoma in tuberous sclerosis complex. Neurology 64:1446-1449.
Scheidenheim DK, Cresswell J, Haipek CA, Fleming TP, Mercer RW, and Gutmann DH. 2005. Akt-dependent cell size regulation by the adhesion molecule on glia (AMOG) occurs independently of phosphotidylinositol 3-kinase and Rheb signaling. Molecular and Cellular Biology 25:3151-3162.
Jansen LA, Uhlmann EJ, Crino PB, Gutmann DH, and Wong M. 2005. Epileptogenesis and reduced inward rectifier potassium current in tuberous sclerosis complex-1 deficient astrocytes. Epilepsia (in press).