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Molecules to Behaviors

Regulation of mTORC1 Signaling in Brain Function and Disease 

mTOR is a multifunctional kinase involved in embryonic development, cancer, and diabetes. Its role and regulation in nervous system physiology and disease, however, remains less understood. This is a major problem, because the malfunction of mTOR activity (either high or low) has been linked to a variety of brain dysfunctions, such as epilepsy, mental retardation, Huntington’s disease (HD) and Parkinson’s disease (PD), which affect a specific set of neuronal populations in the brain. A detailed understanding of how mTOR is regulated and what role it plays in selective brain regions is important for the development of better intervention strategies. We will focus on the striatum—a region of the brain that comprises more than 95% inhibitory medium spiny neurons (MSN), and which plays an important role in motor, cognitive, psychiatric, and reward behaviors.

MSN dysfunctions can lead to the motor abnormalities seen in HD and PD, but the molecular mechanisms are less understood. Blocking mTOR with rapamycin affords protection against the pathological and behavioral symptoms associated with HD and PD in mouse models. Despite this evidence, whether or how mTOR is regulated in a striatal-specific manner remains unclear. We focus on the striatum, a region that control, cognitive, psychiatric, and motor functions.

Based on the previous work, new evidence, indicated above, we propose a model (See Fig. 1) that Rhes, with its SUMO E3 ligase domain, binds to mTOR and regulates its stability through SUMOylation. In the presence of mTORC1 stimuli, such as amino acids, RasGRP1 adds GTP to Rhes to activate mTORC1 in a wortmannin-sensitive manner (Fig. 1). Preliminary data supports the fact that MAPK, which is strongly induced by RasGRP1, is not involved in AA-mTORC1 activity. We will investigate the role of other mTORC1 stimuli (growth factor, dopamine), RHEB, Rag, and Vps34, the role of SUMOylation, and the role of mTOR in striatal behavior, to further strengthen the RasGRP1-Rhes-mTOR circuitry in striatal biology/disease.



Fig 1

Fig. 1. Model of how Rhes GTPase activates mTORC1 in striatum (details in the text).

Mechanisms of Striatal Vulnerability in Huntington’s Disease.

Huntingtin (Htt) is a ubiquitously expressed protein whose polyglutamine tract is encoded by the HTT CAG repeats expansion (mHtt) that causes Huntington disease (HD). HD results in early loss of medium spiny neurons in the striatum affecting motor and cognitive functions, yet the physiological role for huntingtin or its mechanisms for HD pathogenesis remain unclear. The current working model is focused on genetic modulation of mTORC1 signaling selectively in the striatum and how that signaling influences HD-related behavior and striatal pathology. But a bigger question arises, does mHtt–mTORC1-mediated cellular dysfunction also occurs in parts of the brain other than striatum and in peripheral tissue? Or if it is only restricted to striatum, what mechanisms contribute to this striatal selectivity? Because Htt, like mTOR, is ubiquitously expressed, it is conceivable that Htt-mTORC1 signaling would operate in brain and in peripheral tissue, albeit with different potency. Yet, the factors that contribute to tissue-specific enhancement of mTORC1 activity and how this might play a role in eliciting tissue-selective neuronal damage remains less clear. We predict striatal GTPase-Rhes might provide molecular explanation for a tissue-specific damage in HD, as proposed in the unified model for striatal damage in HD (Fig. 2). This model is consistent with, existing literature underlying the molecular mechanisms for striatal degeneration, involving dysregulated vesicular movements, mTOR trafficking and autophagy pathway by mHtt in HD.

   Figure 2   

Fig. 2. Unified model for striatal damage in HD. Prediction of this model is striatal GTPase-Rhes induces the SUMOylation of mHtt, which then binds to vesicles and helps docking of mTOR onto lysosomes to bring mTOR in close contact with its strong regulator Rheb. Consistent with this model, Rhes KO mice are protected from HD-related behavioral deficits and striatal atrophy.

Targeting Alzheimer’s Disease-Associated Signaling for therapeutics.

The precise molecular and cellular events responsible for AD remain unclear.BACE1 is a rate-limiting step in the APP metabolism. BACE1 cleaves APP at the NH2-terminus to release a soluble NH2-terminal fragment, APPsβ, and a 12-kD COOH terminal fragment, C99, which is further cleaved by γ-secretase to produce Aβ (Vassar, 1999;Sinha, 1999;Kandalepas and Vassar, 2012;Kuhn, 2012). Genetic deletion of BACE1 confirmed its indispensable role in the age-related decline of cognitive functions in normal aging and in AD (Fukumoto, 2004;Laird, 2005;Singer, 2005). Moreover, BACE1 is considered an important drug target in AD, due to its mandatory role in the generation of Aβ, which is a hallmark of both sporadic, as well as familial, AD pathology (Vassar and Kandalepas, 2011). But, the biology of BACE1 remains enigmatic. Recently, we have found that Rheb, anubiquitously expressed GTPase, binds to BACE1 and regulates it stability and activity, as measured by Aβ generation, a study that formed our central hypothesis is that Rheb regulates AD-related biochemical and behavioral changes through modulating the levels of BACE1–Ab pathway, but the mechanisms are unclear (Fig. 3). 

Figure 3

Fig. 3.
 Based on the published and preliminary work, we predict that Rheb might play a major role in AD-related abnormalities, through BACE1 regulation and tau phosphorylation, but the mechanisms are unclear.

Mechanisms of Protein Translation

Protein translation is vital to life. Several years of research indicate that it is orchestrated in multiple steps by the coordinated actions of diverse proteins. The initiation step, mediated by an array of proteins called initiation factors, is one of the best-characterized mechanisms in protein production. Two major kinases, the mammalian target of rapamycin (mTOR), and the eukaryotic translation initiation factor-2 alpha (eIF2a) kinase family, target initiation factors to modulate translation, albeit in an opposite manner [see reviews, (1,2)].  While the mTOR phosphorylates 4EBP1, an inhibitor of initiation factor eIF4E, to promote translation, the eIF2a kinase phosphorylates eIF2a, an essential component of the pre-initiation complex, to block translation. Despite these advances, how the mTOR and eIF2a kinase pathways are regulated (Is there any “molecular switch” that controls these kinases?) remains unknown. The lack of such knowledge is an important problem, because, without it, we cannot reach a fundamental understanding of how cells decide when to turn “on” or turn “off” protein translation. Based on our recent work, we predict that Rheb GTPase might act as a “on/off” of protein translation (Fig. 4), but the mechanisms are unclear.

  Figure 4


Fig. 4Rheb in the homeostasis of protein translation. Rheb GTPase is a master regulator of the mTOR pathway, which promotes protein translation. We found that Rheb physiologically promotes the phosphorylation of eIF2a, which is a blocker of translation. Our objectives, based on the Preliminary Data, are to identify the mechanism, biological basis, of Rheb-mTOR vs. Rheb-PERK (ER-kinase) signaling. Our hypothesis is that Rheb acts as a molecular sensor to regulate the homeostasis of protein translation through mTOR and PERK, but the mechanisms are unknown.