Rapamycin Rescues TDP-43 Mislocalization and the Associated Low Molecular Mass Neurofilament Instability

132

TDP-43 (TAR DNA-binding protein) (1) is a highly conserved and abundantly expressed protein among the mammals and invertebrates (2, 3). The protein is located mainly in the nucleus as punctuated structures designated T-bodies (4), but it is also distributed in the cytoplasm as discrete RNA granules (5). Multiple cellular functions have been implicated to be regulated by TDP-43, which consist of but are not restricted to transcriptional repression, RNA processing including splicing, biogenesis of miRNAs, and even translation (reviewed in Refs. 6 -8). Some of these regulations have been shown to be mediated through the two RNA recognition motifs (RRM1 and RRM2) and the C-terminal glycine-rich, prion-like GQN domain (6, 7). Mutational studies have revealed that the RNA recognition motif 1 (RRM1) 3 domain contributes much of the nucleic acid binding activity, whereas the function of RRM2 is still unclear and appears to play a lesser role in RNA binding (7). Studies have also established that an intact GQN domain, which recruits additional heterogeneous ribonucleoprotein (hnRNPs), is also necessary for the splicing activity of TDP-43 (2, 9).More recently, TDP-43 has been identified as the major pathological signature protein of patients with the frontotemporal lobar degeneration with ubiquitin-positive inclusions and amyotrophic lateral sclerosis (10, 11). In particular, the metabolism of TDP-43 in the diseased cells of frontotemporal lobar degeneration with ubiquitin-positive inclusions and amyotrophic lateral sclerosis are misregulated, resulting in the generation of polyubiquitinated, phosphorylated, and caspase-3-cleaved species concentrated in the ubiquitinated inclusions (UBIs) in the cytoplasm and, to a much lesser extent, in the nucleus. Often accompanied with formation of the cytoplasmic UBIs is depletion of the nuclear TDP-43 (10, 11) (reviewed in Ref. 12). A causative role of TDP-43 in the pathogenesis of these diseases is substantiated genetically by the identification of Ͼ30 different mutations in its C-terminal GQN domain (reviewed in Ref. 13). The molecular basis of misregulation of the TDP-43 metabolism remains to be examined. Also unclear are the exact roles of TDP-43 in the pathogenesis of diseases with TDP-43(ϩ)UBIs. Both loss-of-function of TDP-43 and gain-of-cytotoxicity, e.g. as caused by the TDP-43(ϩ)UBIs, have been suggested to facilitate the disease initiation as well as propagation (reviewed in Refs. 6,7,12,14).Regarding the scenario of pathogenesis by loss-of-function of TDP-43, it is interesting to note that knock-out of TDP-43 expression in mice and/or Drosophila could result in defects of early embryonic developments, impairment of the motor function, and decrease of learning ability (15-19). These studies, among others, have evidenced the importance of TDP-43 in development and/or neuronal function, but they do not provide clues to the role of this protein in TDP-43 proteinopathies on the molecular level. With respect to the metabolism of TDP-43, it is interesting to note that the homeo...

Section: Discussionmentioning

confidence: 99%

Regulation of Autophagy by Neuropathological Protein TDP-43

Bose¹,

Huang²,

Shen

2011

132

“…Cell culture methods were adapted from Ref. 32. Briefly, cells were maintained in Dulbecco's modified Eagle's medium with 10% fetal bovine serum at 37°C with a humidified environment (5% CO 2 ϩ 95% atmosphere).…”

Section: Methodsmentioning

confidence: 99%

Molecular Interplay between Mammalian Target of Rapamycin (mTOR), Amyloid-β, and Tau

Caccamo

Majumder

Richardson

et al. 2010

Self Cite

775

659

Accumulation of amyloid-␤ (A␤) and Tau is an invariant feature of Alzheimer disease (AD). The upstream role of A␤ accumulation in the disease pathogenesis is widely accepted, and there is strong evidence showing that A␤ accumulation causes cognitive impairments. However, the molecular mechanisms linking A␤ to cognitive decline remain to be elucidated. Here we show that the buildup of A␤ increases the mammalian target of rapamycin (mTOR) signaling, whereas decreasing mTOR signaling reduces A␤ levels, thereby highlighting an interrelation between mTOR signaling and A␤. The mTOR pathway plays a central role in controlling protein homeostasis and hence, neuronal functions; indeed mTOR signaling regulates different forms of learning and memory. Using an animal model of AD, we show that pharmacologically restoring mTOR signaling with rapamycin rescues cognitive deficits and ameliorates A␤ and Tau pathology by increasing autophagy. Indeed, we further show that autophagy induction is necessary for the rapamycinmediated reduction in A␤ levels. The results presented here provide a molecular basis for the A␤-induced cognitive deficits and, moreover, show that rapamycin, an FDA approved drug, improves learning and memory and reduces A␤ and Tau pathology. Neurofibrillary tangles (NFTs)2 and amyloid plaques represent the two major hallmark neuropathological lesions of AD (1). NFTs are intraneuronal inclusions that are mainly formed of the hyperphosphorylated microtubule-binding protein Tau (2-5). In contrast, amyloid plaques accumulate extracellularly and are mainly composed of a peptide called amyloid-␤ (A␤) (6, 7). Although the key role of A␤ accumulation in the pathogenesis of AD is widely accepted, the molecular pathways by which A␤ accumulation leads to cognitive decline and Tau pathology remain to be elucidated.The mammalian target of rapamycin (mTOR) is a conserved Ser/Thr kinase that forms two multiprotein complexes known as mTOR complex (mTORC) 1 and 2 (8). mTORC1 controls cellular homeostasis, and its activity is inhibited by rapamycin; in contrast mTORC2 is insensitive to rapamycin and controls cellular shape by modulating actin function (8, 9). By regulating both protein synthesis and degradation, mTOR plays a key role in controlling protein homeostasis and hence brain function; indeed, mTOR activity has been directly linked to learning and memory (10 -13). Additionally, genetic and pharmacological reduction of mTOR activity has been shown to increase the lifespan in different organisms including yeast, Drosophila, and mice (14 -19).mTOR is an inhibitor of macroautophagy, which is a conserved intracellular system designed for the degradation of long-lived proteins and organelles in lysosomes (20 -22). Cumulative evidence suggests that an age-dependent decrease in the autophagy/lysosome system may account for the accumulation of abnormal proteins during aging (23). Macroautophagy (herein referred to as autophagy) is induced when an isolation membrane is generated surrounding cytosolic components, forming an autopha...

“…47. Densitometric analysis was conducted using the ImageJ software from the National Institutes of Health.…”

Section: Methodsmentioning

confidence: 99%

Naturally Secreted Amyloid-β Increases Mammalian Target of Rapamycin (mTOR) Activity via a PRAS40-mediated Mechanism

Caccamo¹,

Maldonado²,

Majumder³

et al. 2011

Self Cite

159

132

Reducing the mammalian target of rapamycin (mTOR) activity increases lifespan and health span in a variety of organisms. Alterations in protein homeostasis and mTOR activity and signaling have been reported in several neurodegenerative disorders, including Alzheimer disease (AD); however, the causes of such deregulations remain elusive. Here, we show that mTOR activity and signaling are increased in cell lines stably transfected with mutant amyloid precursor protein (APP) and in brains of 3xTg-AD mice, an animal model of AD. In addition, we show that in the 3xTg-AD mice, mTOR activity can be reduced to wild type levels by genetically preventing A␤ accumulation. Similarly, intrahippocampal injections of an anti-A␤ antibody reduced A␤ levels and normalized mTOR activity, indicating that high A␤ levels are necessary for mTOR hyperactivity in 3xTg-AD mice. We also show that the intrahippocampal injection of naturally secreted A␤ is sufficient to increase mTOR signaling in the brains of wild type mice. The mechanism behind the A␤-induced mTOR hyperactivity is mediated by the proline-rich Akt substrate 40 (PRAS40) as we show that the activation of PRAS40 plays a key role in the A␤-induced mTOR hyperactivity. Taken together, our data show that A␤ accumulation, which has been suggested to be the culprit of AD pathogenesis, causes mTOR hyperactivity by regulating PRAS40 phosphorylation. These data further indicate that the mTOR pathway is one of the pathways by which A␤ exerts its toxicity and further support the idea that reducing mTOR signaling in AD may be a valid therapeutic approach.Amyloid plaques and neurofibrillary tangles are hallmark neuropathological lesions of Alzheimer disease (AD), 3 the most common form of neurodegenerative disorder (1). Neurofibrillary tangles are intracellular inclusions formed of hyperphosphorylated Tau (2-4). Plaques are extracellular inclusions mainly formed of a small peptide called amyloid-␤ (A␤) (5, 6). Clinically, AD is characterized by profound memory loss and cognitive dysfunction (7). Growing evidence is converging on soluble A␤ as a mediator of early cognitive decline in AD (8, 9). Although the molecular mechanisms underlying A␤-induced cognitive decline remain elusive, soluble A␤ oligomers have been shown to alter signal transduction pathways that are key for learning and memory, suggesting that alterations in such pathways may underlie the onset of cognitive decline in AD (10).The mammalian target of rapamycin (mTOR) is a conserved Ser/Thr kinase that forms two multiprotein complexes known as mTOR complex (mTORC) 1 and 2 (11). mTORC1 controls protein homeostasis; its activity is inhibited by rapamycin, and it contains mTOR, raptor, proline-rich Akt substrate 40 kDa (PRAS40), and mLT8. mTORC2, which is insensitive to rapamycin, controls cellular shape by modulating actin function and contains mTOR, rictor, mLST8, and hSIN (11, 12). In mTORC1, raptor binds to mTOR substrates and is necessary for mTOR activity (13). PRAS40 is another mTOR regulatory protein, which inhibits mTOR...