Current Thimerosal Mercury Autism Research

Research

SafeMinds editorial comment on new findings on rapamycin treatment for tuberous sclerosis and possibly autism, and the possible link to mercury.

Several recent studies (Ehninger 2008, Meikle 2008, Hofbauer 2008) have suggested that rapamycin, an immunosuppressive drug, can ameliorate symptoms in tuberous sclerosis complex (TSC). Many individuals with TSC have autistic features. In most but not all cases, TSC arises from one of two genetic abnormalities that lead to abnormal production of a protein complex. The target of this complex inhibits a signaling pathway called “mammalian target of rapacycin” or mTOR. mTOR is a central controller of cell growth, size, survival and proliferation.

One mechanism of mTOR acts through effects on mitochondrial function (see, for example, D’Souza 2007, Cunningham 2007, Floyd 2007, Nobukuni 2007). The role of mitochondria in autism has been the subject of much discussion and scientific activity lately. SafeMinds has provided analyses showing how mercury, including thimerosal, disrupts mitochondrial function. mTOR is involved in the control of mitochondrial oxidative activities. Excerpting from Floyd et al in Molecular Biology of the Cell (2007):

“A direct role for mTOR signaling in enhancing mitochondrial oxidative phosphorylation has also recently been described…as well as a role for mTOR in regulating the production of mitochondrial-derived reactive oxygen species (ROS)…Mitochondrial carrier proteins link metabolic pathways in mitochondria and the cytosol by transporting nucleotides, metabolites, and cofactors through the otherwise impermeable inner mitochondrial membrane. They are required for the generation of energy; amino acid synthesis and degradation; intramitochondrial DNA, RNA, and protein synthesis; and other fundamental cellular functions… [The] expression [of the mitochondrial carrier] is dependant on activation of the PI-3 kinase and mTOR pathways... “

Rapamycin is an mTOR inhibitor that has been used in organ transplants to prevent rejection and is a focus of cancer research. The National Cancer Institute defines it as follows (http://www.cancer.gov/templates/db_alpha.aspx?CdrID=285947):

A drug used to prevent the rejection of organ and bone marrow transplants by the body. Rapamycin is an antibiotic that blocks a protein involved in cell division and inhibits the growth and function of certain T cells of the immune system involved in the body's rejection of foreign tissues and organs. It is a type of immunosuppressant and a type of serine/threonine kinase inhibitor. Rapamycin is now called sirolimus.

Wyeth manufactures rapamycin under the brand name Rapamune.

Further reading:

Nature. 2007 Nov 29;450(7170):736-40.

mTOR controls mitochondrial oxidative function through a YY1-PGC-1alpha transcriptional complex.

Cunningham JT, Rodgers JT, Arlow DH, Vazquez F, Mootha VK, Puigserver P.

Dana-Farber Cancer Institute and Department of Cell Biology, Harvard Medical School, Boston, Massachusetts 02115, USA.

Transcriptional complexes that contain peroxisome-proliferator-activated receptor coactivator (PGC)-1alpha control mitochondrial oxidative function to maintain energy homeostasis in response to nutrient and hormonal signals. An important component in the energy and nutrient pathways is mammalian target of rapamycin (mTOR), a kinase that regulates cell growth, size and survival. However, it is unknown whether and how mTOR controls mitochondrial oxidative activities. Here we show that mTOR is necessary for the maintenance of mitochondrial oxidative function. In skeletal muscle tissues and cells, the mTOR inhibitor rapamycin decreased the gene expression of the mitochondrial transcriptional regulators PGC-1alpha, oestrogen-related receptor alpha and nuclear respiratory factors, resulting in a decrease in mitochondrial gene expression and oxygen consumption. Using computational genomics, we identified the transcription factor yin-yang 1 (YY1) as a common target of mTOR and PGC-1alpha. Knockdown of YY1 caused a significant decrease in mitochondrial gene expression and in respiration, and YY1 was required for rapamycin-dependent repression of those genes. Moreover, mTOR and raptor interacted with YY1, and inhibition of mTOR resulted in a failure of YY1 to interact with and be coactivated by PGC-1alpha. We have therefore identified a mechanism by which a nutrient sensor (mTOR) balances energy metabolism by means of the transcriptional control of mitochondrial oxidative function. These results have important implications for our understanding of how these pathways might be altered in metabolic diseases and cancer.

Mitochondrion. 2007 Dec;7(6):374-85. Epub 2007 Aug 16.

Convergence of multiple signaling pathways is required to coordinately up-regulate mtDNA and mitochondrial biogenesis during T cell activation.

D'Souza AD, Parikh N, Kaech SM, Shadel GS.

Department of Pathology, Yale University School of Medicine, 310 Cedar Street, P.O. Box 208023, New Haven, CT 06520-8023, USA.

The quantity and activity of mitochondria vary dramatically in tissues and are modulated in response to changing cellular energy demands and environmental factors. The amount of mitochondrial DNA (mtDNA), which encodes essential subunits of the oxidative phosphorylation complexes required for cellular ATP production, is also tightly regulated, but by largely unknown mechanisms. Using murine T cells as a model system, we have addressed how specific signaling pathways influence mitochondrial biogenesis and mtDNA copy number. T cell receptor (TCR) activation results in a large increase in mitochondrial mass and membrane potential and a corresponding amplification of mtDNA, consistent with a vital role for mitochondrial function for growth and proliferation of these cells. Independent activation of protein kinase C (via PMA) or calcium-related pathways (via ionomycin) had differential and sub-maximal effects on these mitochondrial parameters, as did activation of naïve T cells with proliferative cytokines. Thus, the robust mitochondrial biogenesis response observed upon TCR activation requires synergy of multiple downstream signaling pathways. One such pathway involves AMP-activated protein kinase (AMPK), which we show has an unprecedented role in negatively regulating mitochondrial biogenesis that is mammalian target of rapamycin (mTOR)-dependent. That is, inhibition of AMPK after TCR signaling commences results in excessive, but uncoordinated mitochondrial proliferation. Thus mitochondrial biogenesis is not under control of a single master regulatory circuit, but rather requires the convergence of multiple signaling pathways with distinct downstream consequences on the organelle's structure, composition, and function.

Nat Med. 2008 Jun 22. [Epub ahead of print]

Reversal of learning deficits in a Tsc2(+/-) mouse model of tuberous sclerosis.

Ehninger D, Han S, Shilyansky C, Zhou Y, Li W, Kwiatkowski DJ, Ramesh V, Silva AJ.

Departments of Neurobiology, Psychiatry & Biobehavioral Sciences, Psychology and the Brain Research Institute, University of California, Los Angeles, 695 Charles E. Young Drive South, Los Angeles, California 90095, USA.

Tuberous sclerosis is a single-gene disorder caused by heterozygous mutations in the TSC1 (9q34) or TSC2 (16p13.3) gene and is frequently associated with mental retardation, autism and epilepsy. Even individuals with tuberous sclerosis and a normal intelligence quotient (approximately 50%) are commonly affected with specific neuropsychological problems, including long-term and working memory deficits. Here we report that mice with a heterozygous, inactivating mutation in the Tsc2 gene (Tsc2(+/-) mice) show deficits in learning and memory. Cognitive deficits in Tsc2(+/-) mice emerged in the absence of neuropathology and seizures, demonstrating that other disease mechanisms are involved. We show that hyperactive hippocampal mammalian target of rapamycin (mTOR) signaling led to abnormal long-term potentiation in the CA1 region of the hippocampus and consequently to deficits in hippocampal-dependent learning. These deficits included impairments in two spatial learning tasks and in contextual discrimination. Notably, we show that a brief treatment with the mTOR inhibitor rapamycin in adult mice rescues not only the synaptic plasticity, but also the behavioral deficits in this animal model of tuberous sclerosis. The results presented here reveal a biological basis for some of the cognitive deficits associated with tuberous sclerosis, and they show that treatment with mTOR antagonists ameliorates cognitive dysfunction in a mouse model of this disorder.

Mol Biol Cell. 2007 Sep;18(9):3545-55. Epub 2007 Jun 27.

The insulin-like growth factor-I-mTOR signaling pathway induces the mitochondrial pyrimidine nucleotide carrier to promote cell growth.

Floyd S, Favre C, Lasorsa FM, Leahy M, Trigiante G, Stroebel P, Marx A, Loughran G, O'Callaghan K, Marobbio CM, Slotboom DJ, Kunji ER, Palmieri F, O'Connor R.

Cell Biology Laboratory, Department of Biochemistry, BioSciences Institute, National University of Ireland, Cork, Ireland.

The insulin/insulin-like growth factor (IGF) signaling pathway to mTOR is essential for the survival and growth of normal cells and also contributes to the genesis and progression of cancer. This signaling pathway is linked with regulation of mitochondrial function, but how is incompletely understood. Here we show that IGF-I and insulin induce rapid transcription of the mitochondrial pyrimidine nucleotide carrier PNC1, which shares significant identity with the essential yeast mitochondrial carrier Rim2p. PNC1 expression is dependent on PI-3 kinase and mTOR activity and is higher in transformed fibroblasts, cancer cell lines, and primary prostate cancers than in normal tissues. Overexpression of PNC1 enhances cell size, whereas suppression of PNC1 expression causes reduced cell size and retarded cell cycle progression and proliferation. Cells with reduced PNC1 expression have reduced mitochondrial UTP levels, but while mitochondrial membrane potential and cellular ATP are not altered, cellular ROS levels are increased. Overall the data indicate that PNC1 is a target of the IGF-I/mTOR pathway that is essential for mitochondrial activity in regulating cell growth and proliferation.

Br J Dermatol. 2008 Jun 11. [Epub ahead of print]

The mTOR inhibitor rapamycin significantly improves facial angiofibroma lesions in a patient with tuberous sclerosis.

Hofbauer GF, Marcollo-Pini A, Corsenca A, Kistler AD, French LE, Wüthrich RP, Serra AL.

Department of Dermatology, University Hospital, CH-8091, Zurich, Switzerland.

Tuberous sclerosis complex (TSC) is an autosomal dominant disorder with an incidence of approximately one in 6000. It arises from a genetic abnormality involving either the TSC1 gene on chromosome 9 or the TSC2 gene on chromosome 16. The protein product of TSC1 is hamartin and that of TSC2 is tuberin. In cells, hamartin and tuberin form a complex which inhibits the mammalian target of rapamycin (mTOR), a central controller of cell growth and proliferation. Angiofibroma affects 70-80% of patients with TSC, typically on the face. We report a patient with TSC with recurrent life-threatening haemorrhage from both kidneys due to extensive angiomyolipoma formation leading to bilateral nephrectomy and renal transplantation. Immunosuppressive treatment with rapamycin, a specific mTOR inhibitor, initiated because of renal transplantation, reduced facial angiofibroma dramatically.

J Neurosci. 2008 May 21;28(21):5422-32.

Response of a neuronal model of tuberous sclerosis to mammalian target of rapamycin (mTOR) inhibitors: effects on mTORC1 and Akt signaling lead to improved survival and function.

Meikle L, Pollizzi K, Egnor A, Kramvis I, Lane H, Sahin M, Kwiatkowski DJ.

Division of Translational Medicine, Department of Medicine, Brigham and Women's Hospital, Children's Hospital, Harvard Medical School, Boston, Massachusetts 02115, USA.

Tuberous sclerosis (TSC) is a hamartoma syndrome attributable to mutations in either TSC1 or TSC2 in which brain involvement causes epilepsy, mental retardation, and autism. We have reported recently (Meikle et al., 2007) a mouse neuronal model of TSC in which Tsc1 is ablated in most neurons during cortical development. We have tested rapamycin and RAD001 [40-O-(2-hydroxyethyl)-rapamycin], both mammalian target of rapamycin mTORC1 inhibitors, as potential therapeutic agents in this model. Median survival is improved from 33 d to more than 100 d; behavior, phenotype, and weight gain are all also markedly improved. There is brain penetration of both drugs, with accumulation over time with repetitive treatment, and effective reduction of levels of phospho-S6, a downstream target of mTORC1. In addition, there is restoration of phospho-Akt and phospho-glycogen synthase kinase 3 levels in the treated mice, consistent with restoration of Akt function. Neurofilament abnormalities, myelination, and cell enlargement are all improved by the treatment. However, dysplastic neuronal features persist, and there are only modest changes in dendritic spine density and length. Strikingly, mice treated with rapamycin or RAD001 for 23 d only (postnatal days 7-30) displayed a persistent improvement in phenotype, with median survival of 78 d. In summary, rapamycin/RAD001 are highly effective therapies for this neuronal model of TSC, with benefit apparently attributable to effects on mTORC1 and Akt signaling and, consequently, cell size and myelination. Although caution is appropriate, the results suggest the possibility that rapamycin/RAD001 may have benefit in the treatment of TSC brain disease, including infantile spasms.

Curr Opin Cell Biol. 2007 Apr;19(2):135-41. Epub 2007 Feb 23.

hvps34, an ancient player, enters a growing game: mTOR Complex1/S6K1 signaling.

Nobukuni T, Kozma SC, Thomas G.

Genome Research Institute, University of Cincinnati, 2180 E Galbraith Rd, Cincinnati, OH 45237, USA.

Recent studies have shown that the nutrient input to the mTOR Complex1/S6K1 signaling pathway is mediated by class 3 PI3K or hVps34, the oldest member of the PI3K family. Moreover, studies to date would suggest that during the evolution of multicellular organisms this ancient branch of the pathway was merged with the growth-factor-hormone-controlled class 1 PI3K pathway at the level of mTOR Complex1 to control the development and growth of the organism. However, hVps34 also plays a role in the regulation of macroautophagy - the mechanism by which cells generate nutrients, such as amino acids, through the degradation of intracellular complexes, including mitochondria and ribosomes. These functions of hVps34 initially appear contradictory, since increased mTOR Complex1 activation is triggered by increased amino acid levels, while autophagy is triggered when cells are faced with amino acid deprivation.