Short-Term and Long-Term Sensitization Differentially Alters the Composition of an Anterograde Transport Complex in Aplysia.

Long-term memory formation requires anterograde transport of proteins from the soma of a neuron to its distal synaptic terminals. This allows new synaptic connections to be grown and existing ones remodeled. However, we do not yet know which proteins are transported to synapses in response to activity and temporal regulation. Here, using quantitative mass spectrometry, we have profiled anterograde protein cargos of a learning-regulated molecular motor protein kinesin [Aplysia kinesin heavy chain 1 (ApKHC1)] following short-term sensitization (STS) and long-term sensitization (LTS) in Aplysia californica Our results reveal enrichment of specific proteins associated with ApKHC1 following both STS and LTS, as well as temporal changes within 1 and 3 h of LTS training. A significant number of proteins enriched in the ApKHC1 complex participate in synaptic function, and, while some are ubiquitously enriched across training conditions, a few are enriched in response to specific training. For instance, factors aiding new synapse formation, such as synaptotagmin-1, dynamin-1, and calmodulin, are differentially enriched in anterograde complexes 1 h after LTS but are depleted 3 h after LTS. Proteins including gelsolin-like protein 2 and sec23A/sec24A, which function in actin filament stabilization and vesicle transport, respectively, are enriched in cargos 3 h after LTS. These results establish that the composition of anterograde transport complexes undergo experience-dependent specific changes and illuminate dynamic changes in the communication between soma and synapse during learning.

Molecular motor protein KIF5C mediates structural plasticity and long-term memory by constraining local translation.

Synaptic structural plasticity, key to long-term memory storage, requires translation of localized RNAs delivered by long-distance transport from the neuronal cell body. Mechanisms and regulation of this system remain elusive. Here, we explore the roles of KIF5C and KIF3A, two members of kinesin superfamily of molecular motors (Kifs), and find that loss of function of either kinesin decreases dendritic arborization and spine density whereas gain of function of KIF5C enhances it. KIF5C function is a rate-determining component of local translation and is associated with ∼650 RNAs, including EIF3G, a regulator of translation initiation, and plasticity-associated RNAs. Loss of function of KIF5C in dorsal hippocampal CA1 neurons constrains both spatial and contextual fear memory, whereas gain of function specifically enhances spatial memory and extinction of contextual fear. KIF5C-mediated long-distance transport of local translation substrates proves a key mechanism underlying structural plasticity and memory.

Activity-regulated synaptic targeting of lncRNA ADEPTR mediates structural plasticity by localizing Sptn1 and AnkB in dendrites.

Activity-dependent structural plasticity at the synapse requires specific changes in the neuronal transcriptome. While much is known about the role of coding elements in this process, the role of the long noncoding transcriptome remains elusive. Here, we report the discovery of an intronic long noncoding RNA (lncRNA)-termed ADEPTR-that is up-regulated and synaptically transported in a cAMP/PKA-dependent manner in hippocampal neurons, independently of its protein-coding host gene. Loss of ADEPTR function suppresses activity-dependent changes in synaptic transmission and structural plasticity of dendritic spines. Mechanistically, dendritic localization of ADEPTR is mediated by molecular motor protein Kif2A. ADEPTR physically binds to actin-scaffolding regulators ankyrin (AnkB) and spectrin (Sptn1) via a conserved sequence and is required for their dendritic localization. Together, this study demonstrates how activity-dependent synaptic targeting of an lncRNA mediates structural plasticity at the synapse.

Double Duty: Mitotic Kinesins and Their Post-Mitotic Functions in Neurons.

Neurons, regarded as post-mitotic cells, are characterized by their extensive dendritic and axonal arborization. This unique architecture imposes challenges to how to supply materials required at distal neuronal components. Kinesins are molecular motor proteins that mediate the active delivery of cellular materials along the microtubule cytoskeleton for facilitating the local biochemical and structural changes at the synapse. Recent studies have made intriguing observations that some kinesins that function during neuronal mitosis also have a critical role in post-mitotic neurons. However, we know very little about the function and regulation of such kinesins. Here, we summarize the known cellular and biochemical functions of mitotic kinesins in post-mitotic neurons.

Molecular Motor KIF3B Acts as a Key Regulator of Dendritic Architecture in Cortical Neurons.

Neurons require a well-coordinated intercellular transport system to maintain their normal cellular function and morphology. The kinesin family of proteins (KIFs) fills this role by regulating the transport of a diverse array of cargos in post-mitotic cells. On the other hand, in mitotic cells, KIFs facilitate the fidelity of the cellular division machinery. Though certain mitotic KIFs function in post-mitotic neurons, little is known about them. We studied the role of a mitotic KIF (KIF3B) in neuronal architecture. We find that the RNAi mediated knockdown of KIF3B in primary cortical neurons resulted in an increase in spine density; the number of thin and mushroom spines; and dendritic branching. Consistent with the change in spine density, we observed a specific increase in the distribution of the excitatory post-synaptic protein, PSD-95 in KIF3B knockdown neurons. Interestingly, overexpression of KIF3B produced a reduction in spine density, in particular mushroom spines, and a decrease in dendritic branching. These studies suggest that KIF3B is a key determinant of cortical neuron morphology and that it functions as an inhibitory constraint on structural plasticity, further illuminating the significance of mitotic KIFs in post-mitotic neurons.

RasGRP1 is a causal factor in the development of l-DOPA-induced dyskinesia in Parkinson's disease.

The therapeutic effects of l-3,4-dihydroxyphenylalanine (l-DOPA) in patients with Parkinson's disease (PD) severely diminishes with the onset of abnormal involuntary movement, l-DOPA-induced dyskinesia (LID). However, the molecular mechanisms that promote LID remain unclear. Here, we demonstrated that RasGRP1 [(guanine nucleotide exchange factor (GEF)] controls the development of LID. l-DOPA treatment rapidly up-regulated RasGRP1 in the striatum of mouse and macaque model of PD. The lack of RasGRP1 in mice (RasGRP1-/- ) dramatically diminished LID without interfering with the therapeutic effects of l-DOPA. Besides acting as a GEF for Ras homolog enriched in the brain (Rheb), the activator of the mammalian target of rapamycin kinase (mTOR), RasGRP1 promotes l-DOPA-induced extracellular signal-regulated kinase (ERK) and the mTOR signaling in the striatum. High-resolution tandem mass spectrometry analysis revealed multiple RasGRP1 downstream targets linked to LID vulnerability. Collectively, the study demonstrated that RasGRP1 is a critical striatal regulator of LID.

Correction to: Endoplasmic Reticulum Stress Plays a Key Role in Rotenone-Induced Apoptotic Death of Neurons.

The original version of this article unfortunately contained a mistake.

Synapse Formation Activates a Transcriptional Program for Persistent Enhancement in the Bi-directional Transport of Mitochondria.

Mechanisms that regulate the bi-directional transport of mitochondria in neurons for maintaining functional synaptic connections are poorly understood. Here, we show that in the pre-synaptic sensory neurons of the Aplysia gill withdrawal reflex, the formation of functional synapses leads to persistent enhancement in the flux of bi-directional mitochondrial transport. In the absence of a functional synapse, activation of cAMP signaling is sufficient to enhance bi-directional transport in sensory neurons. Furthermore, persistent enhancement in transport does not depend on NMDA and AMPA receptor signaling nor signaling from the post-synaptic neuronal cell body, but it is dependent on transcription and protein synthesis in the pre-synaptic neuron. We identified ∼4,000 differentially enriched transcripts in pre-synaptic neurons, suggesting a long-term change in the transcriptional program produced by synapse formation. These results provide insights into the regulation of bi-directional mitochondrial transport for synapse maintenance.

Kinesin Family of Proteins Kif11 and Kif21B Act as Inhibitory Constraints of Excitatory Synaptic Transmission Through Distinct Mechanisms.

Despite our understanding of the functions of the kinesin family of motor proteins (Kifs) in neurons, their specific roles in neuronal communication are less understood. To address this, by carrying out RNAi-mediated loss of function studies, we assessed the necessity of 18 Kifs in excitatory synaptic transmission in mouse primary hippocampal neurons prepared from both sexes. Our measurements of excitatory post-synaptic currents (EPSCs) have identified 7 Kifs that were found to be not critical and 11 Kifs that are essential for synaptic transmission by impacting either frequency or amplitude or both components of EPSCs. Intriguingly we found that knockdown of mitotic Kif4A and Kif11 and post-mitotic Kif21B resulted in an increase in EPSCs suggesting that they function as inhibitory constraints on synaptic transmission. Furthermore, Kifs (11, 21B, 13B) with distinct effects on synaptic transmission are expressed in the same hippocampal neuron. Mechanistically, unlike Kif21B, Kif11 requires the activity of pre-synaptic NMDARs. In addition, we find that Kif11 knockdown enhanced dendritic arborization, synapse number, expression of synaptic vesicle proteins synaptophysin and active zone protein Piccolo. Moreover, expression of Piccolo constrained Kif11 function in synaptic transmission. Together these results suggest that neurons are able to utilize specific Kifs as tools for calibrating synaptic function. These studies bring novel insights into the biology of Kifs and functioning of neural circuits.

Long noncoding RNA GM12371 acts as a transcriptional regulator of synapse function.

Despite the growing evidence suggesting that long noncoding RNAs (lncRNAs) are critical regulators of several biological processes, their functions in the nervous system remain elusive. We have identified an lncRNA, GM12371, in hippocampal neurons that is enriched in the nucleus and necessary for synaptic communication, synapse density, synapse morphology, and dendritic tree complexity. Mechanistically, GM12371 regulates the expression of several genes involved in neuronal development and differentiation, as well as expression of specific lncRNAs and their cognate mRNA targets. Furthermore, we find that cAMP-PKA signaling up-regulates the expression of GM12371 and that its expression is essential for the activity-dependent changes in synaptic transmission in hippocampal neurons. Taken together, our data establish a key role for GM12371 in regulating synapse function.

Encoding of contextual fear memory requires de novo proteins in the prelimbic cortex.

BACKGROUND: Despite our understanding of the significance of the prefrontal cortex in the consolidation of long-term memories (LTM), its role in the encoding of LTM remains elusive. Here we investigated the role of new protein synthesis in the mouse medial prefrontal cortex (mPFC) in encoding contextual fear memory. METHODS: Because a change in the association of mRNAs to polyribosomes is an indicator of new protein synthesis, we assessed the changes in polyribosome-associated mRNAs in the mPFC following contextual fear conditioning (CFC) in the mouse. Differential gene expression in mPFC was identified by polyribosome profiling (n = 18). The role of new protein synthesis in mPFC was determined by focal inhibition of protein synthesis (n = 131) and by intra-prelimbic cortex manipulation (n = 56) of Homer 3, a candidate identified from polyribosome profiling. RESULTS: We identified several mRNAs that are differentially and temporally recruited to polyribosomes in the mPFC following CFC. Inhibition of protein synthesis in the prelimbic (PL), but not in the anterior cingulate cortex (ACC) region of the mPFC immediately after CFC disrupted encoding of contextual fear memory. Intriguingly, inhibition of new protein synthesis in the PL 6 hours after CFC did not impair encoding. Furthermore, expression of Homer 3, an mRNA enriched in polyribosomes following CFC, in the PL constrained encoding of contextual fear memory. CONCLUSIONS: Our studies identify several molecular substrates of new protein synthesis in the mPFC and establish that encoding of contextual fear memories require new protein synthesis in PL subregion of mPFC.

RasGRP1 promotes amphetamine-induced motor behavior through a Rhes interaction network ("Rhesactome") in the striatum.

The striatum of the brain coordinates motor function. Dopamine-related drugs may be therapeutic to patients with striatal neurodegeneration, such as Huntington's disease (HD) and Parkinson's disease (PD), but these drugs have unwanted side effects. In addition to stimulating the release of norepinephrine, amphetamines, which are used for narcolepsy and attention-deficit/hyperactivity disorder (ADHD), trigger dopamine release in the striatum. The guanosine triphosphatase Ras homolog enriched in the striatum (Rhes) inhibits dopaminergic signaling in the striatum, is implicated in HD and L-dopa-induced dyskinesia, and has a role in striatal motor control. We found that the guanine nucleotide exchange factor RasGRP1 inhibited Rhes-mediated control of striatal motor activity in mice. RasGRP1 stabilized Rhes, increasing its synaptic accumulation in the striatum. Whereas partially Rhes-deficient (Rhes+/-) mice had an enhanced locomotor response to amphetamine, this phenotype was attenuated by coincident depletion of RasGRP1. By proteomic analysis of striatal lysates from Rhes-heterozygous mice with wild-type or partial or complete knockout of Rasgrp1, we identified a diverse set of Rhes-interacting proteins, the "Rhesactome," and determined that RasGRP1 affected the composition of the amphetamine-induced Rhesactome, which included PDE2A (phosphodiesterase 2A; a protein associated with major depressive disorder), LRRC7 (leucine-rich repeat-containing 7; a protein associated with bipolar disorder and ADHD), and DLG2 (discs large homolog 2; a protein associated with chronic pain). Thus, this Rhes network provides insight into striatal effects of amphetamine and may aid the development of strategies to treat various neurological and psychological disorders associated with the striatal dysfunction.

Endoplasmic Reticulum Stress Plays a Key Role in Rotenone-Induced Apoptotic Death of Neurons.

Rotenone, a pesticide, causes neurotoxicity via the mitochondrial complex-I inhibition. The present study was conducted to evaluate the role of endoplasmic reticulum (ER) stress in rotenone-induced neuronal death. Cell viability, cytotoxicity, reactive oxygen species (ROS) generation, nitrite level, mitochondrial membrane potential (MMP), and DNA damage were assessed in rotenone-treated neuro-2A cells. Protein levels of ER stress markers glucose regulated protein 78 (GRP78), growth arrest- and DNA damage-inducible gene 153 (GADD153), and phosphorylation of eukaryotic translation initiation factor 2 subunit α (eIF2-α) were estimated to assess the ER stress. To confirm the apoptotic death of neurons, mRNA levels of caspase-9, caspase-12 and caspase-3 were estimated. Further, to confirm the involvement of ER stress, neuro-2A cells were pretreated with ER stress inhibitor salubrinal. Co-treatment of antioxidant melatonin was also given to assess the role of oxidative stress in rotenone-induced apoptosis. Rotenone (0.1, 0.5, and 1 µM) treatment to neurons caused significantly decreased cell viability, increased cytotoxicity, increased ROS generation, increased expression of GRP78 and GADD, DNA damage and activation of caspase-12 and caspase-3 which were significantly attenuated by pretreatment of salubrinal (25 µM). Rotenone-induced dephosphorylation of eIF2α was also inhibited with salubrinal treatment. However, pretreatment of salubrinal did not affect the rotenone-induced increased nitrite levels, decreased MMP and caspase-9 activation. Co-treatment of antioxidant melatonin (1 mM) did not offer attenuation against rotenone-induced increased expression of caspase-9, caspase-12 and caspase-3. In conclusion, results indicated that ER stress plays a key role in rotenone-induced neuronal death, rather than oxidative stress. Graphical Abstract Pictorial presentation showed the involvement of endoplasmic reticulum (ER) stress, increased reactive oxygen species (ROS), nitrite level, decreased mitochondrial membrane potential (MMP), caspase activation and DNA damage in neuronal cells after rotenone treatment. ER stress inhibitor-salubrinal showed significant attenuation against most of the rotenone-induced adverse effects reflecting its key involvement in rotenone-induced neuronal death.

Mechanism of Oxidative Stress and Synapse Dysfunction in the Pathogenesis of Alzheimer's Disease: Understanding the Therapeutics Strategies.

Synapses are formed by interneuronal connections that permit a neuronal cell to pass an electrical or chemical signal to another cell. This passage usually gets damaged or lost in most of the neurodegenerative diseases. It is widely believed that the synaptic dysfunction and synapse loss contribute to the cognitive deficits in patients with Alzheimer's disease (AD). Although pathological hallmarks of AD are senile plaques, neurofibrillary tangles, and neuronal degeneration which are associated with increased oxidative stress, synaptic loss is an early event in the pathogenesis of AD. The involvement of major kinases such as mitogen-activated protein kinase (MAPK), extracellular receptor kinase (ERK), calmodulin-dependent protein kinase (CaMKII), glycogen synthase-3ß (GSK-3ß), cAMP response element-binding protein (CREB), and calcineurin is dynamically associated with oxidative stress-mediated abnormal hyperphosphorylation of tau and suggests that alteration of these kinases could exclusively be involved in the pathogenesis of AD. N-methyl-D-aspartate (NMDA) receptor (NMDAR) activation and beta amyloid (Aß) toxicity alter the synapse function, which is also associated with protein phosphatase (PP) inhibition and tau hyperphosphorylation (two main events of AD). However, the involvement of oxidative stress in synapse dysfunction is poorly understood. Oxidative stress and free radical generation in the brain along with excitotoxicity leads to neuronal cell death. It is inferred from several studies that excitotoxicity, free radical generation, and altered synaptic function encouraged by oxidative stress are associated with AD pathology. NMDARs maintain neuronal excitability, Ca(2+) influx, and memory formation through mechanisms of synaptic plasticity. Recently, we have reported the mechanism of the synapse redox stress associated with NMDARs altered expression. We suggest that oxidative stress mediated through NMDAR and their interaction with other molecules might be a driving force for tau hyperphosphorylation and synapse dysfunction. Thus, understanding the oxidative stress mechanism and degenerating synapses is crucial for the development of therapeutic strategies designed to prevent AD pathogenesis.

Ectopic expression of the striatal-enriched GTPase Rhes elicits cerebellar degeneration and an ataxia phenotype in Huntington's disease.

Huntington's disease (HD) is caused by an expansion of glutamine repeats in the huntingtin protein (mHtt) that invokes early and prominent damage of the striatum, a region that controls motor behaviors. Despite its ubiquitous expression, why certain brain regions, such as the cerebellum, are relatively spared from neuronal loss by mHtt remains unclear. Previously, we implicated the striatal-enriched GTPase, Rhes (Ras homolog enriched in the striatum), which binds and SUMOylates mHtt and increases its solubility and cellular cytotoxicity, as the cause for striatal toxicity in HD. Here, we report that Rhes deletion in HD mice (N171-82Q), which express the N-terminal fragment of human Htt with 82 glutamines (Rhes(-/-)/N171-82Q), display markedly reduced HD-related behavioral deficits, and absence of lateral ventricle dilatation (secondary to striatal atrophy), compared to control HD mice (N171-82Q). To further validate the role of GTPase Rhes in HD, we tested whether ectopic Rhes expression would elicit a pathology in a brain region normally less affected in HD. Remarkably, ectopic expression of Rhes in the cerebellum of N171-82Q mice, during the asymptomatic period led to an exacerbation of motor deficits, including loss of balance and motor incoordination with ataxia-like features, not apparent in control-injected N171-82Q mice or Rhes injected wild-type mice. Pathological and biochemical analysis of Rhes-injected N171-82Q mice revealed a cerebellar lesion with marked loss of Purkinje neuron layer parvalbumin-immunoreactivity, induction of caspase 3 activation, and enhanced soluble forms of mHtt. Similarly reintroducing Rhes into the striatum of Rhes deleted Rhes(-/-)Hdh(150Q/150Q) knock-in mice, elicited a progressive HD-associated rotarod deficit. Overall, these studies establish that Rhes plays a pivotal role in vivo for the selective toxicity of mHtt in HD.

Rheb Inhibits Protein Synthesis by Activating the PERK-eIF2α Signaling Cascade.

Rheb, a ubiquitous small GTPase, is well known to bind and activate mTOR, which augments protein synthesis. Inhibition of protein synthesis is also physiologically regulated. Thus, with cell stress, the unfolded protein response system leads to phosphorylation of the initiation factor eIF2α and arrest of protein synthesis. We now demonstrate a major role for Rheb in inhibiting protein synthesis by enhancing the phosphorylation of eIF2α by protein kinase-like ER kinase (PERK). Interplay between the stimulatory and inhibitory roles of Rheb may enable cells to modulate protein synthesis in response to varying environmental stresses.

Huntingtin promotes mTORC1 signaling in the pathogenesis of Huntington's disease.

In patients with Huntington's disease (HD), the protein huntingtin (Htt) has an expanded polyglutamine (poly-Q) tract. HD results in early loss of medium spiny neurons in the striatum, which impairs motor and cognitive functions. Identifying the physiological role and molecular functions of Htt may yield insight into HD pathogenesis. We found that Htt promotes signaling by mTORC1 [mechanistic target of rapamycin (mTOR) complex 1] and that this signaling is potentiated by poly-Q-expanded Htt. Knocking out Htt in mouse embryonic stem cells or human embryonic kidney cells attenuated amino acid-induced mTORC1 activity, whereas overexpressing wild-type or poly-Q-expanded Htt in striatal neuronal cells increased basal mTOR activity. Striatal cells expressing endogenous poly-Q-expanded Htt showed an increase in the number and size of mTOR puncta on the perinuclear regions compared to cells expressing wild-type Htt. Pull-down experiments indicated that amino acids stimulated the interaction of Htt and the guanosine triphosphatase (GTPase) Rheb (a protein that stimulates mTOR activity), and that Htt forms a ternary complex with Rheb and mTOR. Pharmacologically inhibiting PI3K (phosphatidylinositol 3-kinase) or knocking down Rheb abrogated mTORC1 activity induced by expression of a poly-Q-expanded amino-terminal Htt fragment. Moreover, striatum-specific deletion of TSC1, encoding tuberous sclerosis 1, a negative regulator of mTORC1, accelerated the onset of motor coordination abnormalities and caused premature death in an HD mouse model. Together, our findings demonstrate that mutant Htt contributes to the pathogenesis of HD by enhancing mTORC1 activity.

Mechanism of synapse redox stress in Okadaic acid (ICV) induced memory impairment: Role of NMDA receptor.

The N-methyl-D-aspartate (NMDA) receptor is a subtype of ionotropic glutamate receptor that is involved in synaptic mechanisms of learning and memory, and mediates excitotoxic neuronal injury. In this study, we tested the hypothesis that NMDA receptor subunit gene expression is altered in cortex and hippocampus of OKA induced memory impairment. Therefore in the present study, we checked the effect of OKA (ICV) on NMDA receptor regulation and synapse function. The memory function anomalies and synaptosomal calcium ion (Ca(2+)) level were increased in OKA treated rats brain; which was further protected by MK801 (0.05mg/kg. i.p) treatment daily for 13days. To elucidate the involvement of NMDA receptor, we estimated NR1, NR2A and NR2B (subunits) expression in rat brain. Results showed that expression of NR1 and NR2B were significantly increased, but expression of NR2A had no significant change in OKA treated rat brain. We also observed decrease in synapsin-1 mRNA and protein expression which indicates synapse dysfunction. In addition, we detected an increase in MDA and nitrite levels and a decrease in GSH level in synapse preparation which indicates synapse altered redox stress. Moreover, neuronal loss was also confirmed by nissl staining in periventricular cortex and hippocampus. Altered level of oxidative stress markers along with neuronal loss confirmed neurotoxicity. Further, MK801 treatment restored the level of NR1, NR2B and synapsin-1 expression, and protected from neuronal loss and synapse redox stress. In conclusion, Okadaic acid (OKA) induced expression of NR1 and NR2B deteriorates synapse function in rat brain which was confirmed by the neuroprotective effect of MK801.

Molecular and cellular mechanism of okadaic acid (OKA)-induced neurotoxicity: a novel tool for Alzheimer's disease therapeutic application.

Okadaic acid (OKA), a polyether C38 fatty acid toxin extracted from a black sponge Hallichondria okadaii, is a potent and selective inhibitor of protein phosphatase, PP1 and PP2A. OKA has been proved to be a powerful probe for studying the various regulatory mechanisms and neurotoxicity. Because of its property to inhibit phosphatase activity, OKA is associated with protein phosphorylation; it is implicated in hyperphosphorylation of tau and in later stages causes Alzhiemer's disease (AD)-like pathology. AD is a progressive neurodegenerative disorder, pathologically characterized by extracellular amyloid beta (Aß) plaques and intracellular neurofibrillary tangles (NFTs). The density of tau tangles in AD pathology is associated with cognitive dysfunction. Recent studies have highlighted the importance of serine/threonine protein phosphatases in many processes including apoptosis and neurotoxicity. Although OKA causes neurotoxicity by various pathways, the exact mechanism is still not clear. The activation of major kinases, such as Ser/Thr, MAPK, ERK, PKA, JNK, PKC, CaMKII, Calpain, and GSK3ß, in neurons is associated with AD pathology. These kinases, associated with abnormal hyperphosphorylation of tau, suggest that the cascade of these kinases could exclusively be involved in the pathogenesis of AD. The activity of serine/threonine protein phosphatases needs extensive study as these enzymes are potential targets for novel therapeutics with applications in many diseases including cancer, inflammatory diseases, and neurodegeneration. There is a need to pay ample attention on MAPK kinase pathways in AD, and OKA can be a better tool to study cellular and molecular mechanism for AD pathology. This review elucidates the regulatory mechanism of PP2A and MAPK kinase and their possible mechanisms involved in OKA-induced apoptosis, neurotoxicity, and AD-like pathology.

Rheb GTPase regulates ß-secretase levels and amyloid ß generation.

The ß-site amyloid precursor protein (APP)-cleaving enzyme 1 (ß-secretase, BACE1) initiates amyloidogenic processing of APP to generate amyloid ß (Aß), which is a hallmark of Alzheimer disease (AD) pathology. Cerebral levels of BACE1 are elevated in individuals with AD, but the molecular mechanisms are not completely understood. We demonstrate that Rheb GTPase (Ras homolog enriched in brain), which induces mammalian target of rapamycin (mTOR) activity, is a physiological regulator of BACE1 stability and activity. Rheb overexpression depletes BACE1 protein levels and reduces Aß generation, whereas the RNAi knockdown of endogenous Rheb promotes BACE1 accumulation, and this effect by Rheb is independent of its mTOR signaling. Moreover, GTP-bound Rheb interacts with BACE1 and degrades it through proteasomal and lysosomal pathways. Finally, we demonstrate that Rheb levels are down-regulated in the AD brain, which is consistent with an increased BACE1 expression. Altogether, our study defines Rheb as a novel physiological regulator of BACE1 levels and Aß generation, and the Rheb-BACE1 circuitry may have a role in brain biology and disease.