Two-fold elevation of endogenous GDNF levels in mice improves motor coordination without causing side-effects

Serum glial cell line-derived neurotrophic factor: a potential biomarker for white matter alteration in Parkinson's disease with mild cognitive impairment

Objective

Mild cognitive impairment (MCI) is a common non-motor manifestation of Parkinson's disease, commonly referred to as PD-MCI. However, there is a lack of comprehensive data regarding the role of glial cell line-derived neurotrophic factor (GDNF) and cerebral white matter damage in the pathogenesis of PD-MCI. The objective of this study is to investigate the association between alterations in GDNF levels and cerebral white matter damage in individuals diagnosed with PD-MCI, as well as to explore their potential involvement in cognitive progression.

Methods

Neuropsychological assessments were conducted on 105 patients with Parkinson's disease and 45 healthy volunteers to examine various cognitive domains. An enzyme-linked immunosorbent assay (ELISA) was employed to measure serum levels of GDNF. Additionally, all participants underwent 3.0T magnetic resonance imaging (MRI) to acquire diffusion tensor images (DTI), and a voxel-based analysis (VBA) approach was utilized to compare the fractional anisotropy (FA) values of white matter in the brain.

Results

There was a significant correlation between the right corpus callosum, right cingulate gyrus, and the Digit Span Backward Test (DSB-T) as well as the Trail Making Test A (TMT-A), both of which assess attention and working memory functions. The left internal capsule exhibited a significant correlation with the Trail Making Test B (TMT-B) and the Clock Drawing Test (CDT), which evaluate executive function. Additionally, the right cingulate gyrus showed a significant association with scores on the Auditory Verbal Learning Test-HuaShan (AVLT-H), assessing memory function. Abnormal fiber structures that demonstrated significant correlations with serum GDNF levels included the left internal capsule, left corticospinal tract, right corpus callosum, and right cingulate gyrus.

Conclusion

The decrease in serum GDNF levels among PD-MCI patients exhibiting impairments in attention and working memory function was significantly correlated with alterations in the corpus callosum (knee) and posterior cingulate gyrus. Furthermore, the reduction of serum GDNF levels in PD-MCI patients with impaired executive function is associated with changes in the internal capsule (forelimb) projection fibers. Additionally, the decline of serum GDNF levels in PD-MCI patients experiencing memory function impairment is related to alterations in the right cingulate gyrus.

Delivery of CDNF by AAV-mediated gene transfer protects dopamine neurons and regulates ER stress and inflammation in an acute MPTP mouse model of Parkinson's disease

Cerebral dopamine neurotrophic factor (CDNF) and its close structural relative, mesencephalic astrocyte-derived neurotrophic factor (MANF), are proteins with neurotrophic properties. CDNF protects and restores the function of dopamine (DA) neurons in rodent and non-human primate (NHP) toxin models of Parkinson's disease (PD) and therefore shows promise as a drug candidate for disease-modifying treatment of PD. Moreover, CDNF was found to be safe and to have some therapeutic effects on PD patients in phase 1/2 clinical trials. However, the mechanism underlying the neurotrophic activity of CDNF is unknown. In this study, we delivered human CDNF (hCDNF) to the brain using an adeno-associated viral (AAV) vector and demonstrated the neurotrophic effect of AAV-hCDNF in an acute 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) mouse model of PD. AAV-hCDNF resulted in the expression of hCDNF in the striatum (STR) and substantia nigra (SN), and no toxic effects on the nigrostriatal pathway were observed. Intrastriatal injection of AAV-hCDNF reduced motor impairment and partially alleviated gait dysfunction in the acute MPTP mouse model. In addition, gene therapy with AAV-hCDNF had significant neuroprotective effects on the nigrostriatal pathway and decreased the levels of interleukin 1beta (IL-1β) and complement 3 (C3) in glial cells in the acute MPTP mouse model. Moreover, AAV-hCDNF reduced C/EBP homologous protein (CHOP) and glucose regulatory protein 78 (GRP78) expression in astroglia. These results suggest that the neuroprotective effects of CDNF may be mediated at least in part through the regulation of neuroinflammation and the UPR pathway in a mouse MPTP model of PD in vivo.

MicroRNA-Mediated Suppression of Glial Cell Line-Derived Neurotrophic Factor Expression Is Modulated by a Schizophrenia-Associated Non-Coding Polymorphism

Plasma levels of glial cell line-derived neurotrophic factor (GDNF), a pivotal regulator of differentiation and survival of dopaminergic neurons, are reportedly decreased in schizophrenia. To explore the involvement of GDNF in the pathogenesis of the disease, a case-control association analysis was performed between five non-coding single nucleotide polymorphisms (SNP) across the GDNF gene and schizophrenia. Of them, the 'G' allele of the rs11111 SNP located in the 3' untranslated region (3'-UTR) of the gene was found to associate with schizophrenia. In silico analysis revealed that the rs11111 'G' allele might create binding sites for three microRNA (miRNA) species. To explore the significance of this polymorphism, transient co-transfection assays were performed in human embryonic kidney 293T (HEK293T) cells with a luciferase reporter construct harboring either the 'A' or 'G' allele of the 3'-UTR of GDNF in combination with the hsa-miR-1185-1-3p pre-miRNA. It was demonstrated that in the presence of the rs11111 'G' (but not the 'A') allele, hsa-miR-1185-2-3p repressed luciferase activity in a dose-dependent manner. Deletion of the miRNA binding site or its substitution with the complementary sequence abrogated the modulatory effect. Our results imply that the rs11111 'G' allele occurring more frequently in patients with schizophrenia might downregulate GDNF expression in a miRNA-dependent fashion.

Refinement of IntelliCage protocols for complex cognitive tasks through replacement of drinking restrictions by incentive-disincentive paradigms

The IntelliCage allows automated testing of cognitive abilities of mice in a social home cage environment without handling by human experimenters. Restricted water access in combination with protocols in which only correct responses give access to water is a reliable learning motivator for hippocampus-dependent tasks assessing spatial memory and executive function. However, water restriction may negatively impact on animal welfare, especially in poor learners. To better comply with the 3R principles, we previously tested protocols in which water was freely available but additional access to sweetened water could be obtained by learning a task rule. While this purely appetitive motivation worked for simple tasks, too many mice lost interest in the sweet reward during more difficult hippocampus-dependent tasks. In the present study, we tested a battery of increasingly difficult spatial tasks in which water was still available without learning the task rule, but rendered less attractive either by adding bitter tasting quinine or by increasing the amount of work to obtain it. As in previous protocols, learning of the task rule provided access to water sweetened with saccharin. The two approaches of dual motivation were tested in two cohorts of female C57BL/6 N mice. Compared to purely appetitive motivation, both novel protocols strongly improved task engagement and increased task performance. Importantly, neither of the added disincentives had an adverse impact on liquid consumption, health status or body weight of the animals. Our results show that it is possible to refine test protocols in the IntelliCage so that they challenge cognitive functions without restricting access to water.

Finding an Optimal Level of GDNF Overexpression: Insights from Dopamine Cycling

The application of glial cell line-derive neurotrophic factor (GDNF) to cell cultures and animal models has demonstrated positive effects upon dopaminergic neuronal survival and development, function, restoration, and protection. On this basis, recombinant GDNF protein has been trialled in the treatment of late-stage human Parkinson's disease patients with only limited success that is likely due to a lack of viable receptor targets in an advanced state of neurodegeneration. The latest research points to more refined approaches of modulating GDNF signalling and an optimal quantity and spatial regulation of GDNF can be extrapolated using regulation of dopamine as a proxy measure. The basic research literature on dopaminergic effects of GDNF in animal models is reviewed, concluding that a twofold increase in natively expressing cells increases dopamine turnover and maximises neuroprotective and beneficial motor effects whilst minimising hyperdopaminergia and other side-effects. Methodological considerations for measurement of dopamine levels and neuroanatomical distinctions are made between populations of dopamine neurons and their respective effects upon movement and behaviour that will inform future research into this still-relevant growth factor.

In vivo modulation of endogenous gene expression via CRISPR/Cas9-mediated 3'UTR editing

The 3' untranslated regions (UTRs) modulate gene expression levels by regulating mRNA stability and translation. We previously showed that the replacement of the negative regulatory elements from the 3'UTR of glial cell line-derived neurotrophic factor (GDNF) resulted in increased endogenous GDNF expression while retaining its normal spatiotemporal expression pattern. Here, we have developed a methodology for the generation of in vivo hyper- and hypomorphic alleles via 3'UTR targeting using the CRISPR/Cas9 system. We demonstrate that CRISPR/Cas9-mediated excision of a long inhibitory sequence from Gdnf native 3'UTR in mouse zygotes increases the levels of endogenous GDNF with similar phenotypic alterations in embryonic kidney development as we described in GDNF constitutive and conditional hypermorphic mice. Furthermore, we show that CRISPR/Cas9-mediated targeting of 3'UTRs in vivo allows the modulation of the expression levels of two other morphogens, Gdf11 and Bdnf. Together, our work demonstrates the power of in vivo 3'UTR editing using the CRISPR/Cas9 system to create hyper- and hypomorphic alleles, suggesting wide applicability in studies on gene function and potentially, in gene therapy.

Increased Physiological GDNF Levels Have No Effect on Dopamine Neuron Protection and Restoration in a Proteasome Inhibition Mouse Model of Parkinson's Disease

Parkinson's disease (PD) is a progressive neurodegenerative disease that comprises a range of motor and nonmotor symptoms. Glial cell line-derived neurotrophic factor (GDNF) promotes the survival of dopamine neurons in vitro and in vivo, and intracranial delivery of GDNF has been tested in six clinical trials for treating PD. However, clinical trials with ectopic GDNF have yielded variable results, which could in part result from abnormal expression site and levels caused by ectopic overexpression. Therefore, an important open question is whether an increase in endogenous GDNF expression could be potent in reversing PD progression. Here, we tested the therapeutic potential of endogenous GDNF using mice in which endogenous GDNF can be conditionally upregulated specifically in cells that express GDNF naturally (conditional GDNF hypermorphic mice; Gdnf^cHyper ). We analyzed the impact of endogenous GDNF upregulation in both neuroprotection and neurorestoration procedures, and for both motor and nonmotor symptoms in the proteasome inhibitor lactacystin (LC) model of PD. Our results showed that upregulation of endogenous GDNF in the adult striatum is not protective in LC-induced PD model in mice. Since age is the largest risk factor for PD, we also analyzed the effect of deletion of endogenous GDNF in aged Gdnf conditional knock-out mice. We found that GDNF deletion does not increase susceptibility to LC-induced damage. We conclude that endogenous GDNF does not impact the outcome in the LC-induced proteasome inhibition mouse model of Parkinson's disease.

Bank Voles Show More Impulsivity in IntelliCage Learning Tasks than Wood Mice

Impulsivity is a personality trait of healthy individuals, but in extreme forms common in mental disorders. Previous behavioral testing of wild-caught bank voles and wood mice suggested impulsiveness in bank voles. Here, we compared behavioral performance of bank voles and wood mice in tests for response control in the IntelliCage. In the reaction time task, a test similar to the five-choice serial-reaction time task (5CSRTT), bank voles made more premature responses. Impulsivity in the reaction time task was associated with smaller medial habenular nucleus in bank voles. Additional tests revealed reduced behavioral flexibility in the self-paced flexibility task in bank voles, but equal spatial and reversal learning in the chaining/reversal task in both species. Expression of immediate early gene Arc after behavioral testing was low in medial prefrontal cortex, but high in hypothalamic supraoptic and paraventricular nucleus in bank voles. Wood mice showed the opposite pattern. Numbers of Arc-positive cells in the dorsal hippocampus were higher in bank voles than wood mice. Due to continuous behavioral testing (24/7), associations between behavioral performance and Arc were rare. Corticosterone measurements at the end of experiments suggested that IntelliCage testing did not elicit a stress response in these wild rodents. In summary, habenular size differences and altered activation of brain areas after testing might indicate differently balanced activations of cortico-limbic and cortico-hypothalamic circuits in bank voles compared to wood mice. Behavioral performance of bank voles suggest that these rodents could be a natural animal model for investigating impulsive and perseverative behaviors.

Gender differences in plasma glial cell line-derived neurotrophic factor levels of patients with bipolar disorder

BACKGROUND

The glial cell line-derived neurotrophic factor (GDNF) has an important role in neurons and is closely associated with psychiatric disorders. The development of bipolar disorder (BD) may differ between genders. Existing studies have shown that plasma GDNF levels are altered in patients with BD. In this study, we investigate whether the GDNF levels in patients with BD differ in terms of gender.

METHODS

Participants were divided into the BD group (n = 76, with 26 males and 50 females) and healthy control (HC) group (n = 89, with 35 males and 54 females). Plasma GDNF levels were detected via multifactor assay. Clinical symptoms of patients with BD were collected and assessed using the Hamilton Depression-17 Inventory, Hamilton Anxiety-17 Inventory, Young's Mania Rating Scale, and Brief Psychiatric Rating Scale.

RESULTS

The GDNF levels were significantly higher in all participants in the HC group (F = 4.262, p < 0.05) compared with those in the BD group. In the HC group, the males (t = 4.814, p < 0.001) presented significantly higher levels than the females. The plasma GDNF levels in males in the BD group (t = 3.022, p < 0.05) were significantly lower than those in males in the HC group.

CONCLUSION

Differences in plasma GDNF levels are associated with the gender of patients with BD.

A Newly Identified Monoterpenoid-Based Small Molecule Able to Support the Survival of Primary Cultured Dopamine Neurons and Alleviate MPTP-Induced Toxicity In Vivo

Parkinson's disease (PD) is the most common age-related movement disorder characterized by the progressive loss of nigrostriatal dopaminergic neurons. To date, PD treatment strategies are mostly based on dopamine replacement medicines, which can alleviate motor symptoms but do not slow down the progression of neurodegeneration. Thus, there is a need for disease-modifying PD therapies. The aim of this work was to evaluate the neuroprotective effects of the novel compound PA96 on dopamine neurons in vivo and in vitro, assess its ability to alleviate motor deficits in MPTP- and haloperidol-based PD models, as well as PK profile and BBB penetration. PA96 was synthesized from (1R,2R,6S)-3-methyl-6-(prop-1-en-2-yl) cyclohex-3-ene-1,2-diol (Prottremin) using the original three-step stereoselective procedure. We found that PA96: (1) supported the survival of cultured näive dopamine neurons; (2) supported the survival of MPP⁺-challenged dopamine neurons in vitro and in vivo; (3) had chemically appropriate properties (synthesis, solubility, etc.); (4) alleviated motor deficits in MPTP- and haloperidol-based models of PD; (5) penetrated the blood-brain barrier in vivo; and (6) was eliminated from the bloodstream relative rapidly. In conclusion, the present article demonstrates the identification of PA96 as a lead compound for the future development of this compound into a clinically used drug.

Generation of glial cell-derived neurotrophic factor (gdnf) morphants in zebrafish larvae by cerebroventricular microinjection of vivo morpholino

Dopaminergic neurons in the brain are an important source of dopamine, which is a crucial neurotransmitter for wellbeing, memory, reward, and motor control. Deficiency of dopamine due to advanced age and accumulative dopaminergic neuron defects can lead to movement disorders such as Parkinson's disease. Glial cell-derived neurotrophic factor (GDNF) is one of many factors involved in dopaminergic neuron development and/or survival. However, other endogenous GDNF functions in the brain await further investigation. Zebrafish is a well-established genetic model for neurodevelopment and neurodegeneration studies. Importantly, zebrafish shares approximately 70% functional orthologs with human genes including GDNF. To gain a better understanding on the precise functional role of gdnf in dopaminergic neurons, our laboratory devised a targeted knockdown of gdnf in the zebrafish larval brain using vivo morpholino. Here, detailed protocols on the generation of gdnf morphants using vivo morpholino are outlined. This method can be applied for targeting of genes in the brain to determine specific spatiotemporal gene function in situ.

Increased Endogenous GDNF in Mice Protects Against Age-Related Decline in Neuronal Cholinergic Markers

Gradual decline in cholinergic transmission and cognitive function occurs during normal aging, whereas pathological loss of cholinergic function is a hallmark of different types of dementia, including Alzheimer’s disease (AD), Lewy body dementia (LBD), and Parkinson’s disease dementia (PDD). Glial cell line-derived neurotrophic factor (GDNF) is known to modulate and enhance the dopamine system. However, how endogenous GDNF influences brain cholinergic transmission has remained elusive. In this study, we explored the effect of a twofold increase in endogenous GDNF (Gdnf hypermorphic mice, Gdnf^wt/hyper) on cholinergic markers and cognitive function upon aging. We found that Gdnf^wt/hyper mice resisted an overall age-associated decline in the cholinergic index observed in the brain of Gdnf^wt/wt animals. Biochemical analysis revealed that the level of nerve growth factor (NGF), which is important for survival and function of central cholinergic neurons, was significantly increased in several brain areas of old Gdnf^wt/hyper mice. Analysis of expression of genes involved in cholinergic transmission in the cortex and striatum confirmed modulation of cholinergic pathways by GDNF upon aging. In line with these findings, Gdnf^wt/hyper mice did not undergo an age-related decline in cognitive function in the Y-maze test, as observed in the wild type littermates. Our results identify endogenous GDNF as a potential modulator of cholinergic transmission and call for future studies on endogenous GDNF function in neurodegenerative disorders characterized by cognitive impairments, including AD, LBD, and PDD.

Elevated expression of endogenous glial cell line-derived neurotrophic factor impairs spatial memory performance and raises inhibitory tone in the hippocampus

Parvalbumin-positive interneurons (PV+) are a key component of inhibitory networks in the brain and are known to modulate memory and learning by shaping network activity. The mechanisms of PV+ neuron generation and maintenance are not fully understood, yet current evidence suggests that signalling via the glial cell line-derived neurotrophic factor (GDNF) receptor GFRα1 positively modulates the migration and differentiation of PV+ interneurons in the cortex. Whether GDNF also regulates PV+ cells in the hippocampus is currently unknown. In this study, we utilized a Gdnf "hypermorph" mouse model where GDNF is overexpressed from the native gene locus, providing greatly increased spatial and temporal specificity of protein expression over established models of ectopic expression. Gdnf^wt/hyper mice demonstrated impairments in long-term memory performance in the Morris water maze test and an increase in inhibitory tone in the hippocampus measured electrophysiologically in acute brain slice preparations. Increased PV+ cell number was confirmed immunohistochemically in the hippocampus and in discrete cortical areas and an increase in epileptic seizure threshold was observed in vivo. The data consolidate prior evidence for the actions of GDNF as a regulator of PV+ cell development in the cortex and demonstrate functional effects upon network excitability via modulation of functional GABAergic signalling and under epileptic challenge.

Do serum GDNF levels correlate with severity of Alzheimer's disease?

INTRODUCTION

A growing body of evidence that glial cell line-derived neurotrophic factor (GDNF) levels are probably involved in pathogenesis and disease course of Alzheimer's disease (AD) suggested that its blood levels could potentially be used as a biomarker of AD. The aim of this study was to compare serum GDNF levels in patients with AD and age-matched controls.

METHODS

Serum concentrations of GDNF were compared in 25 AD patients and 25 healthy volunteers using a double-antibody sandwich enzyme-linked immunosorbent assay (ELISA). Severity of the disease in AD patients was assessed using Functional Assessment Staging (FAST). Cognitive assessment of the patients was done using the Mini-Mental State Examination (MMSE).

RESULTS

Mean GDNF levels were found to be 2.45 ± 0.93 ng/ml in AD patients and 4.61 ± 3.39 ng/ml in age-matched controls. There was a statistically significant difference in GDNF serum levels in patients with AD compared to age-matched controls (p = 0.001). Moreover, GDNF serum levels were significantly correlated with disease severity (p < 0.001) and cognitive impairment (p < 0.001).

CONCLUSION

This study showed that serum levels of GDNF are significantly decreased in AD patients in comparison with age-matched controls, thus suggesting a potential role of GDNF as a disease biomarker. However, a comprehensive study of changes in serum levels of multiple neurotrophic factors reflective of different neurobiological pathways in large-scale population studies is recommended.

Three Pillars of Automated Home-Cage Phenotyping of Mice: Novel Findings, Refinement, and Reproducibility Based on Literature and Experience

Animal models of neurodegenerative and neuropsychiatric disorders require extensive behavioral phenotyping. Currently, this presents several caveats and the most important are: (i) rodents are nocturnal animals, but mostly tested during the light period; (ii) the conventional behavioral experiments take into consideration only a snapshot of a rich behavioral repertoire; and (iii) environmental factors, as well as experimenter influence, are often underestimated. Consequently, serious concerns have been expressed regarding the reproducibility of research findings on the one hand, and appropriate welfare of the animals (based on the principle of 3Rs-reduce, refine and replace) on the other hand. To address these problems and improve behavioral phenotyping in general, several solutions have been proposed and developed. Undisturbed, 24/7 home-cage monitoring (HCM) is gaining increased attention and popularity as demonstrating the potential to substitute or complement the conventional phenotyping methods by providing valuable data for identifying the behavioral patterns that may have been missed otherwise. In this review, we will briefly describe the different technologies used for HCM systems. Thereafter, based on our experience, we will focus on two systems, IntelliCage (NewBehavior AG and TSE-systems) and Digital Ventilated Cage (DVC®, Tecniplast)-how they have been developed and applied during recent years. Additionally, we will touch upon the importance of the environmental/experimenter artifacts and propose alternative suggestions for performing phenotyping experiments based on the published evidence. We will discuss how the integration of telemetry systems for deriving certain physiological parameters can help to complement the description of the animal model to offer better translation to human studies. Ultimately, we will discuss how such HCM data can be statistically interpreted and analyzed.

GDNF/RET signaling in dopamine neurons in vivo

The glial cell line-derived neurotrophic factor (GDNF) and its canonical receptor Ret can signal both in tandem and separately to exert many vital functions in the midbrain dopamine system. It is known that Ret has effects on maintenance, physiology, protection and regeneration in the midbrain dopamine system, with the physiological functions of GDNF still somewhat unclear. Notwithstanding, Ret ligands, such as GDNF, are considered as promising candidates for neuroprotection and/or regeneration in Parkinson's disease, although data from clinical trials are so far inconclusive. In this review, we discuss the current knowledge of GDNF/Ret signaling in the dopamine system in vivo as well as crosstalk with pathology-associated proteins and their signaling in mammals.

Activation of AMPK/aPKCζ/CREB pathway by metformin is associated with upregulation of GDNF and dopamine

Parkinson's disease (PD) is the second most common neurodegenerative disease, which is characterized by the progressive loss of dopaminergic neurons in the substantia nigra, leading to a decrease in striatal dopamine. There is no antiparkinsonian therapy that offers a true disease-modifying treatment till date and there is an urgent need for a safe and effective neuroprotective or neurorestorative therapy. Our previous study demonstrated that metformin upregulated dopamine in the mouse brain and provided significant neuroprotection in animal model of PD. Therefore, we designed this study to investigate the molecular mechanism underlying such pharmacological effect of metformin. Herein, we found that metformin enhanced the phosphorylation of tyrosine hydroxylase (TH) which was accompanied by increase in brain-derived neurotrophic factor (BDNF), glial cell line-derived neurotrophic factor (GDNF), and activation of their downstream signaling pathways in the mouse brain and SH-SY5Y cells. We further investigated the role of the neurotrophic factors in the activation of TH and observed that both BDNF and GDNF-induction were essential for metformin-induced TH activation. We found that the AMPK/aPKCζ/CREB pathway was essential for metformin-induced GDNF upregulation and TH activation. Thus, this study reveals the TH-activating property of metformin in the brain via induction of neurotrophic factors along with the signaling mechanism. These results potentiate the candidacy of metformin not only as a neuroprotective agent, but also as restorative therapy for the treatment of PD.

Chronic 2-Fold Elevation of Endogenous GDNF Levels Is Safe and Enhances Motor and Dopaminergic Function in Aged Mice

Glial cell line-derived neurotrophic factor (GDNF) supports function and survival of dopamine neurons that degenerate in Parkinson’s disease (PD). Ectopic delivery of GDNF in clinical trials to treat PD is safe but lacks significant therapeutic effect. In pre-clinical models, ectopic GDNF is effective but causes adverse effects, including downregulation of tyrosine hydroxylase, only a transient boost in dopamine metabolism, aberrant neuronal sprouting, and hyperactivity. Hindering development of GDNF mimetic increased signaling via GDNF receptor RET by activating mutations results in cancer. Safe and effective mode of action must be defined first in animal models to develop successful GDNF-based therapies. Previously we showed that about a 2-fold increase in endogenous GDNF expression is safe and results in increased motor and dopaminergic function and protection in a PD model in young animals. Recently, similar results were reported using a novel Gdnf mRNA-targeting strategy. Next, it is important to establish the safety of a long-term increase in endogenous GDNF expression. We report behavioral, dopamine system, and cancer analysis of five cohorts of aged mice with a 2-fold increase in endogenous GDNF. We found a sustained increase in dopamine levels, improvement in motor learning, and no side effects or cancer. These results support the rationale for further development of endogenous GDNF-based treatments and GDNF mimetic.

Upregulation of Neuronal Rheb(S16H) for Hippocampal Protection in the Adult Brain

Ras homolog protein enriched in brain (Rheb) is a key activator of mammalian target of rapamycin complex 1 (mTORC1). The activation of mTORC1 by Rheb is associated with various processes such as protein synthesis, neuronal growth, differentiation, axonal regeneration, energy homeostasis, autophagy, and amino acid uptake. In addition, Rheb-mTORC1 signaling plays a crucial role in preventing the neurodegeneration of hippocampal neurons in the adult brain. Increasing evidence suggests that the constitutive activation of Rheb has beneficial effects against neurodegenerative diseases such as Alzheimer's disease (AD) and Parkinson's disease (PD). Our recent studies revealed that adeno-associated virus serotype 1 (AAV1) transduction with Rheb(S16H), a constitutively active form of Rheb, exhibits neuroprotective properties through the induction of various neurotrophic factors, promoting neurotrophic interactions between neurons and astrocytes in the hippocampus of the adult brain. This review provides compelling evidence for the therapeutic potential of AAV1-Rheb(S16H) transduction in the hippocampus of the adult brain by exploring its neuroprotective effects and mechanisms.