AAV transduction of dopamine neurons with constitutively active Rheb protects from neurodegeneration and mediates axon regrowth

Expression mediated by three partial sequences of the human tyrosine hydroxylase promoter in vivo

The use of viral vectors to transfect postmitotic neurons has provided an important research tool, and it offers promise for treatment of neurologic disease. The utility of vectors is enhanced by the use of selective promoters that permit control of the cellular site of expression. One potential clinical application is in the neurorestorative treatment of Parkinson’s disease by the induction of new axon growth. However, many of the genes with an ability to restore axons have oncogenic potential. Therefore, clinical safety would be enhanced by restriction of expression to neurons affected by the disease, particularly dopamine neurons. To achieve this goal we have evaluated in vivo three partial sequences of the promoter for human tyrosine hydroxylase, the rate limiting enzyme in catecholamine synthesis. All sequences induced expression in dopamine neurons. None of them induced expression in glia or in nondopaminergic neurons in striatum or cortex. We conclude that these sequences have potential use for targeting dopamine neurons in research and clinical applications.

Prothrombin Kringle-2: A Potential Inflammatory Pathogen in the Parkinsonian Dopaminergic System

Although accumulating evidence suggests that microglia-mediated neuroinflammation may be crucial for the initiation and progression of Parkinson's disease (PD), and that the control of neuroinflammation may be a useful strategy for preventing the degeneration of nigrostriatal dopaminergic (DA) projections in the adult brain, it is still unclear what kinds of endogenous biomolecules initiate microglial activation, consequently resulting in neurodegeneration. Recently, we reported that the increase in the levels of prothrombin kringle-2 (pKr-2), which is a domain of prothrombin that is generated by active thrombin, can lead to disruption of the nigrostriatal DA projection. This disruption is mediated by neurotoxic inflammatory events via the induction of microglial Toll-like receptor 4 (TLR4) in vivo , thereby resulting in less neurotoxicity in TLR4-deficient mice. Moreover, inhibition of microglial activation following minocycline treatment, which has anti-inflammatory activity, protects DA neurons from pKr-2-induced neurotoxicity in the substantia nigra (SN) in vivo. We also found that the levels of pKr-2 and microglial TLR4 were significantly increased in the SN of PD patients compared to those of age-matched controls. These observations suggest that there may be a correlation between pKr-2 and microglial TLR4 in the initiation and progression of PD, and that inhibition of pKr-2-induced microglial activation may be protective against the degeneration of the nigrostriatal DA system in vivo. To describe the significance of pKr-2 overexpression, which may have a role in the pathogenesis of PD, we have reviewed the mechanisms of pKr-2-induced microglial activation, which results in neurodegeneration in the SN of the adult brain.

Control of Granule Cell Dispersion by Natural Materials Such as Eugenol and Naringin: A Potential Therapeutic Strategy Against Temporal Lobe Epilepsy

The hippocampus is an important brain area where abnormal morphological characteristics are often observed in patients with temporal lobe epilepsy (TLE), typically showing the loss of the principal neurons in the CA1 and CA3 areas of the hippocampus. TLE is frequently associated with widening of the granule cell layer of the dentate gyrus (DG), termed granule cell dispersion (GCD), in the hippocampus, suggesting that the control of GCD with protection of hippocampal neurons may be useful for preventing and inhibiting epileptic seizures. We previously reported that eugenol (EUG), which is an essential component of medicinal herbs and has anticonvulsant activity, is beneficial for treating epilepsy through its ability to inhibit GCD via suppression of mammalian target of rapamycin complex 1 (mTORC1) activation in the hippocampal DG in a kainic acid (KA)-treated mouse model of epilepsy in vivo. In addition, we reported that naringin, a bioflavonoid in citrus fruits, could exert beneficial effects, such as antiautophagic stress and antineuroinflammation, in the KA mouse model of epilepsy, even though it was unclear whether naringin might also attenuate the seizure-induced morphological changes of GCD in the DG. Similar to the effects of EUG, we recently observed that naringin treatment significantly reduced KA-induced GCD and mTORC1 activation, which are both involved in epileptic seizures, in the hippocampus of mouse brain. Therefore, these observations suggest that the utilization of natural materials, which have beneficial properties such as inhibition of GCD formation and protection of hippocampal neurons, may be useful in developing a novel therapeutic agent against TLE.

Expressing Constitutively Active Rheb in Adult Dorsal Root Ganglion Neurons Enhances the Integration of Sensory Axons that Regenerate Across a Chondroitinase-Treated Dorsal Root Entry Zone Following Dorsal Root Crush

While the peripheral branch of dorsal root ganglion neurons (DRG) can successfully regenerate after injury, lesioned central branch axons fail to regrow across the dorsal root entry zone (DREZ), the interface between the dorsal root and the spinal cord. This lack of regeneration is due to the limited regenerative capacity of adult sensory axons and the growth-inhibitory environment at the DREZ, which is similar to that found in the glial scar after a central nervous system (CNS) injury. We hypothesized that transduction of adult DRG neurons using adeno-associated virus (AAV) to express a constitutively-active form of the GTPase Rheb (caRheb) will increase their intrinsic growth potential after a dorsal root crush. Additionally, we posited that if we combined that approach with digestion of upregulated chondroitin sulfate proteoglycans (CSPG) at the DREZ with chondroitinase ABC (ChABC), we would promote regeneration of sensory axons across the DREZ into the spinal cord. We first assessed if this strategy promotes neuritic growth in an in vitro model of the glial scar containing CSPG. ChABC allowed for some regeneration across the once potently inhibitory substrate. Combining ChABC treatment with expression of caRheb in DRG significantly improved this growth. We then determined if this combination strategy also enhanced regeneration through the DREZ after dorsal root crush in adult rats in vivo. After unilaterally crushing C4-T1 dorsal roots, we injected AAV5-caRheb or AAV5-GFP into the ipsilateral C5-C8 DRGs. ChABC or PBS was injected into the ipsilateral dorsal horn at C5-C8 to digest CSPG, for a total of four animal groups (caRheb + ChABC, caRheb + PBS, GFP + ChABC, GFP + PBS). Regeneration was rarely observed in PBS-treated animals, whereas short-distance regrowth across the DREZ was observed in ChABC-treated animals. No difference in axon number or length between the ChABC groups was observed, which may be related to intraganglionic inflammation induced by the injection. ChABC-mediated regeneration is functional, as stimulation of ipsilateral median and ulnar nerves induced neuronal c-Fos expression in deafferented dorsal horn in both ChABC groups. Interestingly, caRheb + ChABC animals had significantly more c-Fos⁺ nuclei indicating that caRheb expression in DRGs promoted functional synaptogenesis of their axons that regenerated beyond a ChABC-treated DREZ.

Combined Nurr1 and Foxa2 roles in the therapy of Parkinson's disease

Use of the physiological mechanisms promoting midbrain DA (mDA) neuron survival seems an appropriate option for developing treatments for Parkinson's disease (PD). mDA neurons are specifically marked by expression of the transcription factors Nurr1 and Foxa2. We show herein that Nurr1 and Foxa2 interact to protect mDA neurons against various toxic insults, but their expression is lost during aging and degenerative processes. In addition to their proposed cell-autonomous actions in mDA neurons, forced expression of these factors in neighboring glia synergistically protects degenerating mDA neurons in a paracrine mode. As a consequence of these bimodal actions, adeno-associated virus (AAV)-mediated gene delivery of Nurr1 and Foxa2 in a PD mouse model markedly protected mDA neurons and motor behaviors associated with nigrostriatal DA neurotransmission. The effects of the combined gene delivery were dramatic, highly reproducible, and sustained for at least 1 year, suggesting that expression of these factors is a promising approach in PD therapy.

Expressing Constitutively Active Rheb in Adult Neurons after a Complete Spinal Cord Injury Enhances Axonal Regeneration beyond a Chondroitinase-Treated Glial Scar

After a spinal cord injury (SCI), CNS axons fail to regenerate, resulting in permanent deficits. This is due to: (1) the presence of inhibitory molecules, e.g., chondroitin sulfate proteoglycans (CSPG), in the glial scar at the lesion; and (2) the diminished growth capacity of adult neurons. We sought to determine whether expressing a constitutively active form of the GTPase Rheb (caRheb) in adult neurons after a complete SCI in rats improves intrinsic growth potential to result in axon regeneration out of a growth-supportive peripheral nerve grafted (PNG) into the SCI cavity. We also hypothesized that treating the glial scar with chondroitinase ABC (ChABC), which digests CSPG, would further allow caRheb-transduced neurons to extend axons across the distal graft interface. We found that targeting this pathway at a clinically relevant post-SCI time point improves both sprouting and regeneration of axons. CaRheb increased the number of axons, but not the number of neurons, that projected into the PNG, indicative of augmented sprouting. We also saw that caRheb enhanced sprouting far rostral to the injury. CaRheb not only increased growth rostral and into the graft, it also resulted in significantly more regrowth of axons across a ChABC-treated scar into caudal spinal cord. CaRheb(+) neurons had higher levels of growth-associated-43, suggestive of a newly identified mechanism for mTOR-mediated enhancement of regeneration. Thus, we demonstrate for the first time that simultaneously addressing intrinsic and scar-associated, extrinsic impediments to regeneration results in significant regrowth beyond an extremely challenging, complete SCI site.

SIGNIFICANCE STATEMENT

After spinal cord injury (SCI), CNS axons fail to regenerate, resulting in permanent deficits. This is due to the diminished growth capacity of adult neurons and the presence of inhibitory molecules in the scar at the lesion. We sought to simultaneously counter both of these obstacles to achieve more robust regeneration after complete SCI. We transduced neurons postinjury to express a constitutively active Rheb to enhance their intrinsic growth potential, transplanted a growth supporting peripheral nerve graft into the lesion cavity, and enzymatically modulated the inhibitory glial scar distal to the graft. We demonstrate, for the first time, that simultaneously addressing neuron-related, intrinsic deficits in axon regrowth and extrinsic, scar-associated impediments to regeneration results in significant regeneration after SCI.

mTOR in Brain Physiology and Pathologies

TOR (target of rapamycin) and its mammalian ortholog mTOR have been discovered in an effort to understand the mechanisms of action of the immunosuppressant drug rapamycin extracted from a bacterium of the Easter Island (Rapa Nui) soil. mTOR is a serine/threonine kinase found in two functionally distinct complexes, mTORC1 and mTORC2, which are differentially regulated by a great number of nutrients such as glucose and amino acids, energy (oxygen and ATP/AMP content), growth factors, hormones, and neurotransmitters. mTOR controls many basic cellular functions such as protein synthesis, energy metabolism, cell size, lipid metabolism, autophagy, mitochondria, and lysosome biogenesis. In addition, mTOR-controlled signaling pathways regulate many integrated physiological functions of the nervous system including neuronal development, synaptic plasticity, memory storage, and cognition. Thus it is not surprising that deregulation of mTOR signaling is associated with many neurological and psychiatric disorders. Preclinical and preliminary clinical studies indicate that inhibition of mTORC1 can be beneficial for some pathological conditions such as epilepsy, cognitive impairment, and brain tumors, whereas stimulation of mTORC1 (direct or indirect) can be beneficial for other pathologies such as depression or axonal growth and regeneration.

Induction of microglial toll-like receptor 4 by prothrombin kringle-2: a potential pathogenic mechanism in Parkinson's disease

Microglia-mediated neuroinflammation may play an important role in the initiation and progression of dopaminergic (DA) neurodegeneration in Parkinson’s disease (PD), and toll-like receptor 4 (TLR4) is essential for the activation of microglia in the adult brain. However, it is still unclear whether patients with PD exhibit an increase in TLR4 expression in the brain, and whether there is a correlation between the levels of prothrombin kringle-2 (pKr-2) and microglial TLR4. In the present study, we first observed that the levels of pKr-2 and microglial TLR4 were increased in the substantia nigra (SN) of patients with PD. In rat and mouse brains, intranigral injection of pKr-2, which is not directly toxic to neurons, led to the disruption of nigrostriatal DA projections. Moreover, microglial TLR4 was upregulated in the rat SN and in cultures of the BV-2 microglial cell line after pKr-2 treatment. In TLR4-deficient mice, pKr-2-induced microglial activation was suppressed compared with wild-type mice, resulting in attenuated neurotoxicity. Therefore, our results suggest that pKr-2 may be a pathogenic factor in PD, and that the inhibition of pKr-2-induced microglial TLR4 may be protective against degeneration of the nigrostriatal DA system in vivo.

Neuroprotective effects of piperine on the 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine-induced Parkinson's disease mouse model

Parkinson's disease (PD) is second only to Alzheimer's disease as the most common and debilitating age-associated neurodegenerative disorder. Currently, no therapy has been shown to unequivocally retard or arrest the progression of the disease. The aim of the present study was to investigate the protective effect of piperine on the 1-methyl-4-phenyl-1,2,3,6‑tetrahydropyridine (MPTP)-induced Parkinson's mouse model. For MPTP treatment, the animals received repeated intraperitoneal injections (i.p.) of MPTP (30 mg/kg) solution for 7 days. Piperine (10 mg/kg) was administered orally for 15 days including 8 days of pretreatment. Motor behavior analysis was conducted with the rotarod test. The Morris water maze (MWM) was used to assess the cognitive learning ability of the mice. A histological examination was subsequently conducted. The results ddemonstrate that piperine treatment attenuated MPTP-induced deficits in motor coordination and cognitive functioning. Piperine also prevented MPTP-induced decreases in the number of tyrosine hydroxylase-positive cells in the substantia nigra. Additionally, piperine reduced the number of activated microglia, expression of cytokine IL-1β, and oxidative stress following MPTP treatment. An anti-apoptotic property of piperine was identified by maintaining the balance of Bcl-2/Bax. In conclusion, the results show that piperine exerts a protective effect on dopaminergic neurons via antioxidant, anti-apoptotic, and anti-inflammatory mechanisms in an MPTP-induced mouse model of PD. Thus, piperine is a potential therapeutic treatment for PD.

An early axonopathy in a hLRRK2(R1441G) transgenic model of Parkinson disease

Mutations in the gene for LRRK2 are the most common cause of familial Parkinson's disease (PD) and patients with these mutations manifest clinical features that are indistinguishable from those of the more common sporadic form. Thus, investigations of disease mechanisms based on disease-causing LRRK2 mutations can be expected to shed light on the more common sporadic form as well as the inherited form. We have shown that as human BAC transgenic hLRRK2(R1441G) mice age, they exhibit two abnormalities in the nigrostriatal dopaminergic system: an axonopathy and a diminished number of dendrites in the substantia nigra (SN). To better understand disease mechanisms it is useful to determine where in the affected neural system the pathology first begins. We therefore examined the nigrostriatal dopaminergic system in young mice to determine the initial site of pathology. Brains from hLRRK2(R1441G) and littermate control mice at 2-4months of age were examined by immunohistochemistry, anterograde fluorescent axon labeling and ultrastructural analysis. SN neurons, their projecting axons and the striatal terminal fields were assessed. The first identifiable abnormality in this system is an axonopathy characterized by giant polymorphic axon spheroids, the presence of intra-axonal autophagic vacuoles and intra-axonal myelin invagination. An initial involvement of axons has also been reported for other genetic models of PD. These observations support the concept that axons are involved early in the course of the disease. We suggest that effective neuroprotective approaches will be aimed at preventing axonal degeneration.

Activation of CNTF/CNTFRα signaling pathway by hRheb(S16H) transduction of dopaminergic neurons in vivo

Ciliary neurotrophic factor (CNTF) is one of representative neurotrophic factors for the survival of dopaminergic neurons. Its effects are primarily mediated via CNTF receptor α (CNTFRα). It is still unclear whether the levels of CNTFRα change in the substantia nigra of Parkinson's disease (PD) patients, but CNTF expression shows the remarkable decrease in dopaminergic neurons in the substantia nigra pars compacta (SNpc), suggesting that the support of CNTF/CNTFRα signaling pathway may be a useful neuroprotective strategy for the nigrostriatal dopaminergic projection in the adult brain. Here, we report that transduction of rat SNpc dopaminergic neurons by adeno-associated virus with a gene encoding human ras homolog enriched in brain (hRheb), with an S16H mutation [hRheb(S16H)], significantly upregulated the levels of both CNTF and CNTFRα in dopaminergic neurons. Moreover, the hRheb(S16H)-activated CNTF/CNTFRα signaling pathway was protective against 1-methyl-4-phenylpyridinium-induced neurotoxicity in the nigrostriatal dopaminergic projections. These results suggest that activation of CNTF/CNTFRα signaling pathway by specific gene delivery such as hRheb(S16H) may have therapeutic potential in the treatment of PD.

Reinstating aberrant mTORC1 activity in Huntington's disease mice improves disease phenotypes

Huntington's disease (HD) is caused by a polyglutamine tract expansion in huntingtin (HTT). Despite HTTs ubiquitous expression, there is early and robust vulnerability in striatum, the cause of which is poorly understood. Here, we provide evidence that impaired striatal mTORC1 activity underlies varied metabolic and degenerative phenotypes in HD brain and show that introducing the constitutively active form of the mTORC1 regulator, Rheb, into HD mouse brain, alleviates mitochondrial dysfunction, aberrant cholesterol homeostasis, striatal atrophy, impaired dopamine signaling, and increases autophagy. We also find that the expression of Rhes, a striatum-enriched mTOR activator, is reduced in HD patient and mouse brain and that exogenous addition of Rhes alleviates motor deficits and improves brain pathology in HD mice. Our combined work indicates that impaired Rhes/mTORC1 activity in HD brain may underlie the notable striatal susceptibility and thus presents a promising therapeutic target for HD therapy.

In vivo AAV1 transduction with hRheb(S16H) protects hippocampal neurons by BDNF production

Recent evidence has shown that Ras homolog enriched in brain (Rheb) is dysregulated in Alzheimer's disease (AD) brains. However, it is still unclear whether Rheb activation contributes to the survival and protection of hippocampal neurons in the adult brain. To assess the effects of active Rheb in hippocampal neurons in vivo, we transfected neurons in the cornu ammonis 1 (CA1) region in normal adult rats with an adeno-associated virus containing the constitutively active human Rheb (hRheb(S16H)) and evaluated the effects on thrombin-induced neurotoxicity. Transduction with hRheb(S16H) significantly induced neurotrophic effects in hippocampal neurons through activation of mammalian target of rapamycin complex 1 (mTORC1) without side effects such as long-term potentiation impairment and seizures from the alteration of cytoarchitecture, and the expression of hRheb(S16H) prevented thrombin-induced neurodegeneration in vivo, an effect that was diminished by treatment with specific neutralizing antibodies against brain-derived neurotrophic factor (BDNF). In addition, our results showed that the basal mTORC1 activity might be insufficient to mediate the level of BDNF expression, but hRheb(S16H)-activated mTORC1 stimulated BDNF production in hippocampal neurons. These results suggest that viral vector transduction with hRheb(S16H) may have therapeutic value in the treatment of neurodegenerative diseases such as AD.

Roles of Rheb(S16H) in substantia nigra pars compacta dopaminergic neurons in vivo

Although there are ongoing intensive research efforts, no effective pharmacological therapies for Parkinson's disease (PD) have been developed thus far. However, with the development of efficient gene delivery systems, gene therapy for PD has become a focus of research and increasing evidence suggests that continuous production of neurotrophic factors play a significant role in the functional restoration of the nigrostriatal dopaminergic (DA) system. Our recent study reported that the transduction of DA neurons with ras homolog enriched in brain, which has an S16H mutation [Rheb(S16H)], protected the nigrostriatal DA projection in a neurotoxin model of PD in vivo. In addition, Rheb(S16H) expression significantly increased the levels of glial cell line-derived neurotrophic factor and brain-derived neurotrophic factor, which contributed to the neuroprotective effects of Rheb(S16H) in DA neurons in the adult brain, indicating that the activation of the signaling pathways involved in cell survival by a specific gene delivery, such as Rheb(S16H) to adult neurons, may be a useful strategy to protect neural systems in the adult brain. In the present study, a brief overview of our recent studies is provided, which demonstrates the neuroprotective mechanisms of Rheb(S16H) on the nigrostriatal DA projection in the adult brain.

Nobiletin protects dopaminergic neurons in the 1-methyl-4-phenylpyridinium-treated rat model of Parkinson's disease

This study investigated the effect of nobiletin, a flavonoid found in citrus fruits, on the degeneration of dopaminergic (DA) neurons in a neurotoxin model of Parkinson's disease (PD). 1-Methyl-4-phenylpyridinium (MPP(+)) was unilaterally injected into the median forebrain bundle of rat brains (to generate a neurotoxin model of PD) with or without daily intraperitoneal injection of nobiletin. Our results showed that nobiletin treatment at 10 mg/kg bw, but not at 1 or 20 mg/kg bw, significantly protected DA neurons in the substantia nigra (SN) of MPP(+)-treated rats. In parallel to the neuroprotection, nobiletin treatment at 10 mg/kg inhibited microglial activation and preserved the expression of the glial cell line-derived neurotrophic factor, which is a therapeutic agent against PD, in the SN. These results suggest that the proper supplementation with nobiletin may protect against the neurodegeneration involved in PD.

Effects of naringin, a flavanone glycoside in grapefruits and citrus fruits, on the nigrostriatal dopaminergic projection in the adult brain

Recently, we have demonstrated the ability of naringin, a well-known flavanone glycoside of grapefruits and citrus fruits, to prevent neurodegeneration in a neurotoxin model of Parkinson's disease. Intraperitoneal injection of naringin protected the nigrostriatal dopaminergic projection by increasing glial cell line-derived neurotrophic factor expression and decreasing the level of tumor necrosis factor-alpha in dopaminergic neurons and microglia, respectively. These results suggest that naringin can impart to the adult dopaminergic neurons the ability to produce glial cell line-derived neurotrophic factor against Parkinson's disease with anti-inflammatory effects. Based on these results, we would like to describe an important perspective on its possibility as a therapeutic agent for Parkinson's disease.

Naringin: a protector of the nigrostriatal dopaminergic projection

Parkinson's disease is the second most common neurodegenerative disorder characterized by the progressive degeneration of dopaminergic neurons and a biochemical reduction of striatal dopamine levels. Despite the lack of fully understanding of the etiology of Parkinson's disease, accumulating evidences suggest that Parkinson's disease may be caused by the insufficient support of neurotrophic factors, and by microglial activation, resident immune cells in the brain. Naringin, a major flavonone glycoside in grapefruits and citrus fruits, is considered as a protective agent against neurodegenerative diseases because it can induce not only anti-oxidant effects but also neuroprotective effects by the activation of anti-apoptotic pathways and the induction of neurotrophic factors such as brain-derived neurotrophic factor and vascular endothelial growth factor. We have recently reported that naringin has neuroprotective effects in a neurotoxin model of Parkinson's disease. Our observations show that intraperitoneal injection of naringin induces increases in glial cell line-derived neurotrophic factor expression and mammalian target of rapamycin complex 1 activity in dopaminergic neurons of rat brains with anti-inflammatory effects. Moreover, the production of glial cell line-derived neurotrophic factor by naringin treatment contributes to the protection of the nigrostriatal dopaminergic projection in a neurotoxin model of Parkinson's disease. Although the effects of naringin on the nigrostriatal dopaminergic system in human brains are largely unknown, these results suggest that naringin may be a beneficial natural product for the prevention of dopaminergic degeneration in the adult brain.

Targeting GTPases in Parkinson's disease: comparison to the historic path of kinase drug discovery and perspectives

Neurological diseases have placed heavy social and financial burdens on modern society. As the life expectancy of humans is extended, neurological diseases, such as Parkinson’s disease, have become increasingly common among senior populations. Although the enigmas of Parkinson’s diseases await resolution, more vivid pictures on the cause, progression, and control of the illness are emerging after years of research. On the molecular level, GTPases are implicated in the etiology of Parkinson’s disease and are rational pharmaceutical targets for their control. However, targeting individual GTPases, which belong to a superfamily of proteins containing multiple members with a conserved guanine nucleotide binding domain, has proven to be challenging. In contrast, pharmaceutical pursuit of inhibition of kinases, which constitute another superfamily of proteins with more than 500 members, has been fairly successful. We reviewed the breakthroughs in the history of kinase drug discovery to provide guidance for the GTPase field. We summarize recent progress made in the regulation of GTPase activity. We also present an efficient and cost effective approach to drug screening, which uses multiplex flow cytometry and mixture-based positional scanning libraries. These methods allow simultaneous measurements of both the activity and the selectivity of the screened library. Several GTPase activator clusters were identified which showed selectivity against different GTPase subfamilies. While the clusters need to be further deconvoluted to identify individual active compounds, the method described here and the structure information gathered create a foundation for further developments to build upon.

Induction of GDNF and BDNF by hRheb(S16H) transduction of SNpc neurons: neuroprotective mechanisms of hRheb(S16H) in a model of Parkinson's disease

The transduction of dopaminergic (DA) neurons with human ras homolog enriched in brain, which has a S16H mutation [hRheb(S16H)] protects the nigrostriatal DA projection in the 6-hydroxydopamine (6-OHDA)-treated animal model of Parkinson's disease (PD). However, it is still unclear whether the expression of active hRheb induces the production of neurotrophic factors such as glial cell line-derived neurotrophic factor (GDNF) and brain-derived neurotrophic factor (BDNF), which are involved in neuroprotection, in mature neurons. Here, we show that transduction of nigral DA neurons with hRheb(S16H) significantly increases the levels of phospho-cyclic adenosine monophosphate (cAMP) response element-binding protein (p-CREB), GDNF, and BDNF in neurons, which are attenuated by rapamycin, a specific inhibitor of mammalian target of rapamycin complex 1 (mTORC1). Moreover, treatment with specific neutralizing antibodies for GDNF and BDNF reduced the protective effects of hRheb(S16H) against 1-methyl-4-phenylpyridinium (MPP(+))-induced neurotoxicity. These results show that activation of hRheb/mTORC1 signaling pathway could impart to DA neurons the important ability to continuously produce GDNF and BDNF as therapeutic agents against PD.

Recent progress in the study of the Rheb family GTPases

In this review we highlight recent progress in the study of Rheb family GTPases. Structural studies using X-ray crystallography and NMR have given us insight into unique features of this GTPase. Combined with mutagenesis studies, these works have expanded our understanding of residues that affect Rheb GTP/GDP bound ratios, effector protein interactions, and stimulation of mTORC1 signaling. Analysis of cancer genome databases has revealed that several human carcinomas contain activating mutations of the protein. Rheb's role in activating mTORC1 signaling at the lysosome in response to stimuli has been further elucidated. Rheb has also been suggested to play roles in other cellular pathways including mitophagy and peroxisomal ROS response. A number of studies in mice have demonstrated the importance of Rheb in development, as well as in a variety of functions including cardiac protection and myelination. We conclude with a discussion of future prospects in the study of Rheb family GTPases.

Silibinin attenuates MPP⁺-induced neurotoxicity in the substantia nigra in vivo

Parkinson's disease (PD) is characterized by degeneration of the nigrostriatal dopaminergic (DA) pathway. The cause of neuronal death in PD is largely unknown, but it is becoming clear that inflammation plays a significant role in the pathophysiology of PD. Silibinin is a major flavonoid in milk thistle which has an anti-inflammatory activity. We investigated whether silibinin could have neuroprotective effects on DA neurons in the 1-methyl-4-phenylpyridinium ion (MPP(+))-treated animal model of PD in vivo. To address this question, animals received intraperitoneal (i.p.) injections 10, 50, or 100 mg/kg of silibinin, starting 1 day before MPP(+) injection and continued daily until 6 days post-lesion for tyrosine hydroxylase (TH) staining, or until 1 hour prior to the MPP(+) injection to examine the expression levels of inflammatory proteins. Finally, their brains were harvested at the indicated time points for the analyses. Silibinin treatment with 10 mg/kg had no significantly neuroprotective effects in the substantia nigra (SN). However, 50 and 100 mg/kg of silibinin ameliorated the MPP(+)-induced neurotoxicity in the SN in a dose-dependent manner, and the increased levels of inflammatory molecules such as tumor necrosis factor-alpha (TNF-α), interleukin-1 beta (IL-1β) and inducible nitric oxide synthase (iNOS) by MPP(+) treatment were attenuated by treatment with 100 mg/kg of silibinin. These results indicate that silibinin could be a useful and beneficial natural product offering promise for the prevention of DA neuronal degeneration involved in PD.

Rheb GTPase regulates β-secretase levels and amyloid β generation

The β-site amyloid precursor protein (APP)-cleaving enzyme 1 (β-secretase, BACE1) initiates amyloidogenic processing of APP to generate amyloid β (Aβ), 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 Aβ 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 Aβ generation, and the Rheb-BACE1 circuitry may have a role in brain biology and disease.

Quantitative morphological comparison of axon-targeting strategies for gene therapies directed to the nigro-striatal projection

Cellular targeting of mRNAs and proteins to axons is essential for axon growth during development and is likely to be important for adult maintenance as well. Given the importance and potency of these axon-targeting motifs to the biology of axons, it seems possible that they can be used in the design of transgenes that are intended to enhance axon growth or maintenance, so as to improve potency and minimize off-target effects. To investigate this possibility, it is first essential to assess known motifs for their efficacy. We have therefore evaluated four axon-targeting motifs, using adeno-associated viral vector-mediated gene delivery in the nigro-striatal dopaminergic system, a projection that is predominantly affected in Parkinson's disease. We have tested two mRNA axonal zipcodes, the 3' untranslated region (UTR) of β-actin and 3' UTR of tau, and two axonal-targeting protein motifs, the palmitoylation signal sequence in GAP-43 and the last 15 amino acids in the amyloid precursor protein, to direct the expression of the fluorescent protein Tomato in axons. These sequences, fused to Tomato, were able to target its expression to dopaminergic axons. Based on quantification of Tomato-positive axons, and the density of striatal innervation, we conclude that the C-terminal of the amyloid precursor protein is the most effective axon-targeting motif.

(ADP-ribose) polymerase 1 and AMP-activated protein kinase mediate progressive dopaminergic neuronal degeneration in a mouse model of Parkinson's disease

Genetic and epidemiologic evidence suggests that cellular energy homeostasis is critically associated with Parkinson's disease (PD) pathogenesis. Here we demonstrated that genetic deletion of Poly (ADP-ribose) polymerase 1 completely blocked 6-hydroxydopamine-induced dopaminergic neurodegeneration and related PD-like symptoms. Hyperactivation of PARP-1 depleted ATP pools in dopaminergic (DA) neurons, thereby activating AMP-activated protein kinase (AMPK). Further, blockade of AMPK activation by viral infection with dominant-negative AMPK strongly inhibited DA neuronal atrophy with moderate suppression of nuclear translocation of apoptosis-inhibiting factor (AIF), whereas overactivation of AMPK conversely strengthened the 6-OHDA-induced DA neuronal degeneration. Collectively, these results suggest that manipulation of PARP-1 and AMPK signaling is an effective therapeutic approach to prevent PD-related DA neurodegeneration.

Widespread spinal cord transduction by intrathecal injection of rAAV delivers efficacious RNAi therapy for amyotrophic lateral sclerosis

Amyotrophic lateral sclerosis (ALS) causes motor neuron degeneration and paralysis. No treatment can significantly slow or arrest the disease progression. Mutations in the SOD1 gene cause a subset of familial ALS by a gain of toxicity. In principle, these cases could be treated with RNAi that destroys the mutant mRNA, thereby abolishing the toxic protein. However, no system is available to efficiently deliver the RNAi therapy. Recombinant adenoassociated virus (rAAV) is a promising vehicle due to its long-lasting gene expression and low toxicity. However, ALS afflicts broad areas of the central nervous system (CNS). A lack of practical means to spread rAAV broadly has hindered its application in treatment of ALS. To overcome this barrier, we injected several rAAV serotypes into the cerebrospinal fluid. We found that some rAAV serotypes such as rAAVrh10 and rAAV9 transduced cells throughout the length of the spinal cord following a single intrathecal injection and in the broad forebrain following a single injection into the third ventricle. Furthermore, a single intrathecal injection of rAAVrh10 robustly transduced motor neurons throughout the spinal cord in a non-human primate. These results suggested a therapeutic potential of this vector for ALS. To test this, we injected a rAAVrh10 vector that expressed an artificial miRNA targeting SOD1 into the SOD1G93A mice. This treatment knocked down the mutant SOD1 expression and slowed the disease progression. Our results demonstrate the potential of rAAVs for delivering gene therapy to treat ALS and other diseases that afflict broad areas of the CNS.

Purkinje cell axonal anatomy: quantifying morphometric changes in essential tremor versus control brains

Growing clinical, neuro-imaging and post-mortem data have implicated the cerebellum as playing an important role in the pathogenesis of essential tremor. Aside from a modest reduction of Purkinje cells in some post-mortem studies, Purkinje cell axonal swellings (torpedoes) are present to a greater degree in essential tremor cases than controls. Yet a detailed study of more subtle morphometric changes in the Purkinje cell axonal compartment has not been undertaken. We performed a detailed morphological analysis of the Purkinje cell axonal compartment in 49 essential tremor and 39 control brains, using calbindin D28k immunohistochemistry on 100-µm cerebellar cortical vibratome tissue sections. Changes in axonal shape [thickened axonal profiles (P = 0.006), torpedoes (P = 0.038)] and changes in axonal connectivity [axonal recurrent collaterals (P < 0.001), axonal branching (P < 0.001), terminal axonal sprouting (P < 0.001)] were all present to an increased degree in essential tremor cases versus controls. The changes in shape and connectivity were significantly correlated [e.g. correlation between thickened axonal profiles and recurrent collaterals (r = 0.405, P < 0.001)] and were correlated with tremor duration among essential tremor cases with age of onset >40 years. In essential tremor cases, thickened axonal profiles, axonal recurrent collaterals and branched axons were 3- to 5-fold more frequently seen on the axons of Purkinje cells with torpedoes versus Purkinje cells without torpedoes. We document a range of changes in the Purkinje cell axonal compartment in essential tremor. Several of these are likely to be compensatory changes in response to Purkinje cell injury, thus illustrating an important feature of Purkinje cells, which is that they are relatively resistant to damage and capable of mobilizing a broad range of axonal responses to injury. The extent to which this plasticity of the Purkinje cell axon is partially neuroprotective or ultimately ineffective at slowing further cellular changes and cell death deserves further study in essential tremor.

Enhancing the utility of adeno-associated virus gene transfer through inducible tissue-specific expression

The ability to regulate both the timing and specificity of gene expression mediated by viral vectors will be important in maximizing its utility. We describe the development of an adeno-associated virus (AAV)-based vector with tissue-specific gene regulation, using the ARGENT dimerizer-inducible system. This two-vector system based on AAV serotype 9 consists of one vector encoding a combination of reporter genes from which expression is directed by a ubiquitous, inducible promoter and a second vector encoding transcription factor domains under the control of either a heart- or liver-specific promoter, which are activated with a small molecule. Administration of the vectors via either systemic or intrapericardial injection demonstrated that the vector system is capable of mediating gene expression that is tissue specific, regulatable, and reproducible over induction cycles. Somatic gene transfer in vivo is being considered in therapeutic applications, although its most substantial value will be in basic applications such as target validation and development of animal models.

Functional diversity

There is increasing evidence to support a gene economy model that is fully based on the principles of evolution in which a limited number of proteins does not necessarily reflect a finite number of biochemical processes. The concept of 'gene sharing' proposes that a single protein can have alternate functions that are typically attributed to other proteins. GAPDH appears to play this role quite well in that it exhibits more than one function. GAPDH represents the prototype for this new paradigm of protein multi-functionality. The chapter discusses the diverse functions of GAPDH among three broad categories: cell structure, gene expression and signal transduction. Protein function is curiously re-specified given the cell's unique needs. GAPDH provides the cell with the means of linking metabolic activity to various cellular processes. While interpretations may often lead to GAPDH's role in meeting focal energy demands, this chapter discusses several other very distinct GAPDH functions (i.e. membrane fusogenic properties) that are quite different from its ability to catalyze oxidative phosphorylation of the triose, glyceraldehyde 3-phosphate. It is suggested that a single protein participates in multiple processes in the structural organization of the cell, controls the transmission of genetic information (i.e. GAPDH's involvement may not be finite) and mediates intracellular signaling.

Inhibition of rho kinase enhances survival of dopaminergic neurons and attenuates axonal loss in a mouse model of Parkinson's disease

Axonal degeneration is one of the earliest features of Parkinson's disease pathology, which is followed by neuronal death in the substantia nigra and other parts of the brain. Inhibition of axonal degeneration combined with cellular neuroprotection therefore seem key to targeting an early stage in Parkinson's disease progression. Based on our previous studies in traumatic and neurodegenerative disease models, we have identified rho kinase as a molecular target that can be manipulated to disinhibit axonal regeneration and improve survival of lesioned central nervous system neurons. In this study, we examined the neuroprotective potential of pharmacological rho kinase inhibition mediated by fasudil in the in vitro 1-methyl-4-phenylpyridinium cell culture model and in the subchronic in vivo 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine mouse model of Parkinson's disease. Application of fasudil resulted in a significant attenuation of dopaminergic cell loss in both paradigms. Furthermore, dopaminergic terminals were preserved as demonstrated by analysis of neurite network in vitro, striatal fibre density and by neurochemical analysis of the levels of dopamine and its metabolites in the striatum. Behavioural tests demonstrated a clear improvement in motor performance after fasudil treatment. The Akt survival pathway was identified as an important molecular mediator for neuroprotective effects of rho kinase inhibition in our paradigm. We conclude that inhibition of rho kinase using the clinically approved small molecule inhibitor fasudil may be a promising new therapeutic strategy for Parkinson's disease.