Adeno-associated virus type 2 vector-mediated glial cell line-derived neurotrophic factor gene transfer induces neuroprotection and neuroregeneration in a ubiquitin-proteasome system impairment animal model of Parkinson's disease

From lab bench to hope: a review of gene therapies in clinical trials for Parkinson's disease and challenges

Parkinson's disease (PD) is a chronic neurological disorder that is identified by a characteristic combination of symptoms such as bradykinesia, resting tremor, rigidity, and postural instability. It is the second most common neurodegenerative disease after Alzheimer's disease and is characterized by the progressive loss of dopamine-producing neurons in the brain. Currently, available treatments for PD are symptomatic and do not prevent the disease pathology. There is growing interest in developing disease-modifying therapy that can reduce disease progression and improve patients' quality of life. One of the promising therapeutic approaches under evaluation is gene therapy utilizing a viral vector, adeno-associated virus (AAV), to deliver transgene of interest into the central nervous system (CNS). Preclinical studies in small animals and nonhuman primates model of PD have shown promising results utilizing the gene therapy that express glial cell line-derived neurotrophic factor (GDNF), cerebral dopamine neurotrophic factor (CDNF), aromatic L-amino acid decarboxylase (AADC), and glutamic acid decarboxylase (GAD). This study provides a comprehensive review of the current state of the above-mentioned gene therapies in various phases of clinical trials for PD treatment. We have highlighted the rationale for the gene-therapy approach and the findings from the preclinical and nonhuman primates studies, evaluating the therapeutic effect, dose safety, and tolerability. The challenges associated with gene therapy for heterogeneous neurodegenerative diseases, such as PD, have also been described. In conclusion, the review identifies the ongoing promising gene therapy approaches in clinical trials and provides hope for patients with PD.

Emerging Gene Therapies for Alzheimer's and Parkinson's Diseases: An Overview of Clinical Trials and Promising Candidates

Gene therapy as a disease-modifying therapeutic approach for neurodegenerative diseases, such as Alzheimer's disease (AD) and Parkinson's disease (PD), is a promising avenue. Promising results in the preclinical studies involving rodents and nonhuman primates utilizing gene therapy have led to multiple clinical trials evaluating various genes of interest for AD and PD. In AD, clinical trials are assessing gene therapy involving brain-derived neurotrophic factor (BDNF) and other targets such as apolipoprotein E2 (APOE2) and human telomerase reverse transcriptase (hTERT). In PD, clinical trials are evaluating gene therapy delivering neurotrophic factors, such as glial cell line-derived neurotrophic factor (GDNF). Additionally, gene therapy delivering enzymes aromatic L-amino acid decarboxylase (AADC) and glutamic acid decarboxylase (GAD) are also being evaluated for PD. All these trials primarily utilized adeno-associated virus (AAV) to deliver the above transgene of interest. This review summarizes the current clinical trials involving gene therapy for AD and PD. It also discusses the challenges and opportunities associated with the gene therapy approach in AD and PD and ongoing developments related to increasing the safety and efficacy of the gene therapy for long-term outcomes, which include evaluation of various serotypes and administration routes. This comprehensive review emphasizes translating preclinical findings into clinical trials, further directions, and the potential for this promising therapeutic approach to alleviate neurodegenerative disease.

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.

HDAC6-mediated Hsp90 deacetylation reduces aggregation and toxicity of the protein alpha-synuclein by regulating chaperone-mediated autophagy

Histone deacetylase 6 (HDAC6) has been shown to control major cell response pathways to the cytotoxic ubiquitinated aggregates in some protein aggregation diseases. However, it is not well known whether HDAC6 affects the aggregation process of α-synuclein (α-syn) in Parkinson's disease (PD). Previously, we demonstrated that HDAC6 inhibition exacerbated the nigrostriatal dopamine neurodegeneration and up-regulated α-syn oligomers in a heat shock protein 90 (Hsp90)-dependent manner in PD mouse model. Here, we further showed that HDAC6 overexpression partly improved the behavior deficits of the PD model and alleviated the nigrostriatal dopamine (DA) neurons injury. Furthermore, HDAC6 was found to regulate α-syn oligomers levels through activation of chaperone-mediated autophagy (CMA). During this process, Hsp90 deacetylation mediated the crosstalk between HDAC6 and lysosome-associated membrane protein type 2A. Liquid chromatography-tandem mass spectrometry and mutational analysis showed that acetylation status Hsp90 at the K489 site was a strong determinant for HDAC6-induced CMA activation, α-syn oligomers levels, and cell survival in the cell model of PD. Therefore, our findings uncovered the mechanism of HDAC6 in the PD model that HDAC6 regulated α-syn oligomers levels and DA neurons survival partly through modulating CMA, and Hsp90 deacetylation at the K489 site mediated the crosstalk between HDAC6 and CMA. HDAC6 and its downstream effectors appear as key modulators of the cytotoxic α-syn aggregates, which deserve further investigations to evaluate their values as potential therapeutic targets in PD.

Adeno-Associated Viral Vectors as Versatile Tools for Parkinson's Research, Both for Disease Modeling Purposes and for Therapeutic Uses

It is without any doubt that precision medicine therapeutic strategies targeting neurodegenerative disorders are currently witnessing the spectacular rise of newly designed approaches based on the use of viral vectors as Trojan horses for the controlled release of a given genetic payload. Among the different types of viral vectors, adeno-associated viruses (AAVs) rank as the ones most commonly used for the purposes of either disease modeling or for therapeutic strategies. Here, we reviewed the current literature dealing with the use of AAVs within the field of Parkinson’s disease with the aim to provide neuroscientists with the advice and background required when facing a choice on which AAV might be best suited for addressing a given experimental challenge. Accordingly, here we will be summarizing some insights on different AAV serotypes, and which would be the most appropriate AAV delivery route. Next, the use of AAVs for modeling synucleinopathies is highlighted, providing potential readers with a landscape view of ongoing pre-clinical and clinical initiatives pushing forward AAV-based therapeutic approaches for Parkinson’s disease and related synucleinopathies.

Adult Endogenous Dopaminergic Neuroregeneration Against Parkinson's Disease: Ideal Animal Models?

Parkinson's disease (PD) is the second most common neurodegenerative disease. The etiology of PD remains an enigma with no available disease modifying treatment or cure. Pharmacological compensation is the only quality of life improving treatments available. Endogenous dopaminergic neuroregeneration has recently been considered a plausible therapeutic strategy for PD. However, researchers have to first decipher the complexity of adult endogenous neuroregeneration. This raises the need of animal models to understand the underlying molecular basis. Mammalian models with highly conserved genetic homology might aid researchers to identify specific molecular mechanisms. However, the scarcity of adult neuroregeneration potential in mammals obfuscates such investigations. Nowadays, non-mammalian models are gaining popularity due to their explicit ability to neuroregenerate naturally without the need of external enhancements, yet these non-mammals have a much diverse gene homology that critical molecular signals might not be conserved across species. The present review highlights the advantages and disadvantages of both mammalian and non-mammalian animal models that can be essentially used to study the potential of endogenous DpN regeneration against PD.

Overexpression of α-Synuclein Reorganises Growth Factor Profile of Human Astrocytes

Misfolding and accumulation of aberrant α-synuclein in the brain is associated with the distinct class of neurodegenerative diseases known as α-synucleinopathies, which include Parkinson's disease, dementia with Lewy bodies and multiple system atrophy. Pathological changes in astrocytes contribute to all neurological disorders, and astrocytes are reported to possess α-synuclein inclusions in the context of α-synucleinopathies. Astrocytes are known to express and secrete numerous growth factors, which are fundamental for neuroprotection, synaptic connectivity and brain metabolism; changes in growth factor secretion may contribute to pathobiology of neurological disorders. Here we analysed the effect of α-synuclein overexpression in cultured human astrocytes on growth factor expression and release. For this purpose, the intracellular and secreted levels of 33 growth factors (GFs) and 8 growth factor receptors (GFRs) were analysed in cultured human astrocytes by chemiluminescence-based western/dot blot. Overexpression of human α-synuclein in cultured foetal human astrocytes significantly changes the profile of GF production and secretion. We found that human astrocytes express and secrete FGF2, FGF6, EGF, IGF1, AREG, IGFBP2, IGFBP4, VEGFD, PDGFs, KITLG, PGF, TGFB3 and NTF4. Overexpression of human α-synuclein significantly modified the profile of GF production and secretion, with particularly strong changes in EGF, PDGF, VEGF and their receptors as well as in IGF-related proteins. Bioinformatics analysis revealed possible interactions between α-synuclein and EGFR and GDNF, as well as with three GF receptors, EGFR, CSF1R and PDGFRB.

The Proteasome Inhibition Model of Parkinson's Disease

The pathological hallmarks of Parkinson's disease are the progressive loss of nigral dopaminergic neurons and the formation of intracellular inclusion bodies, termed Lewy bodies, in surviving neurons. Accumulation of proteins in large insoluble cytoplasmic aggregates has been proposed to result, partly, from a failure in the function of intracellular protein degradation pathways. Evidence in support for such a hypothesis emerged in the beginning of the years 2000 with studies demonstrating structural and functional deficits in the ubiquitin-proteasome pathway in post-mortem nigral tissue of patients with Parkinson's disease. These fundamental findings have inspired the development of a new generation of animal models based on the use of proteasome inhibitors to disturb protein homeostasis and trigger nigral dopaminergic neurodegeneration. In this review, we provide an updated overview of the current approaches in employing proteasome inhibitors to model Parkinson's disease, with particular emphasis on rodent studies. In addition, the mechanisms underlying proteasome inhibition-induced cell death and the validity criteria (construct, face and predictive validity) of the model will be critically discussed. Due to its distinct, but highly relevant mechanism of inducing neuronal death, the proteasome inhibition model represents a useful addition to the repertoire of toxin-based models of Parkinson's disease that might provide novel clues to unravel the complex pathogenesis of this disorder.

Combining NT3-overexpressing MSCs and PLGA microcarriers for brain tissue engineering: A potential tool for treatment of Parkinson's disease

Parkinson's disease (PD) is a progressive neurodegenerative disorder that characterized by destruction of substantia nigrostriatal pathway due to the loss of dopaminergic (DA) neurons. Regardless of substantial efforts for treatment of PD in recent years, an effective therapeutic strategy is still missing. In a multidisciplinary approach, bone marrow derived mesenchymal stem cells (BMSCs) are genetically engineered to overexpress neurotrophin-3 (nt-3 gene) that protect central nervous system tissues and stimulates neuronal-like differentiation of BMSCs. Poly(lactic-co-glycolic acid) (PLGA) microcarriers are designed as an injectable scaffold and synthesized via double emulsion method. The surface of PLGA microcarriers are functionalized by collagen as a bioadhesive agent for improved cell attachment. The results demonstrate effective overexpression of NT-3. The expression of tyrosine hydroxylase (TH) in transfected BMSCs reveal that NT-3 promotes the intracellular signaling pathway of DA neuron differentiation. It is also shown that transfected BMSCs are successfully attached to the surface of microcarriers. The presence of dopamine in peripheral media of cell/microcarrier complex reveals that BMSCs are successfully differentiated into dopaminergic neuron. Our approach that sustains presence of growth factor can be suggested as a novel complementary therapeutic strategy for treatment of Parkinson disease.

GDNF Overexpression from the Native Locus Reveals its Role in the Nigrostriatal Dopaminergic System Function

Degeneration of nigrostriatal dopaminergic system is the principal lesion in Parkinson’s disease. Because glial cell line-derived neurotrophic factor (GDNF) promotes survival of dopamine neurons in vitro and in vivo, intracranial delivery of GDNF has been attempted for Parkinson’s disease treatment but with variable success. For improving GDNF-based therapies, knowledge on physiological role of endogenous GDNF at the sites of its expression is important. However, due to limitations of existing genetic model systems, such knowledge is scarce. Here, we report that prevention of transcription of Gdnf 3’UTR in Gdnf endogenous locus yields GDNF hypermorphic mice with increased, but spatially unchanged GDNF expression, enabling analysis of postnatal GDNF function. We found that increased level of GDNF in the central nervous system increases the number of adult dopamine neurons in the substantia nigra pars compacta and the number of dopaminergic terminals in the dorsal striatum. At the functional level, GDNF levels increased striatal tissue dopamine levels and augmented striatal dopamine release and re-uptake. In a proteasome inhibitor lactacystin-induced model of Parkinson’s disease GDNF hypermorphic mice were protected from the reduction in striatal dopamine and failure of dopaminergic system function. Importantly, adverse phenotypic effects associated with spatially unregulated GDNF applications were not observed. Enhanced GDNF levels up-regulated striatal dopamine transporter activity by at least five fold resulting in enhanced susceptibility to 6-OHDA, a toxin transported into dopamine neurons by DAT. Further, we report how GDNF levels regulate kidney development and identify microRNAs miR-9, miR-96, miR-133, and miR-146a as negative regulators of GDNF expression via interaction with Gdnf 3’UTR in vitro. Our results reveal the role of GDNF in nigrostriatal dopamine system postnatal development and adult function, and highlight the importance of correct spatial expression of GDNF. Furthermore, our results suggest that 3’UTR targeting may constitute a useful tool in analyzing gene function.

Intracranial delivery of GDNF has been attempted for Parkinson’s disease (PD) treatment but with variable success. For improving GDNF-based therapies, knowledge on physiological role of endogenous GDNF at the sites of its expression is important. However, due to limitations of existing genetic model systems, such knowledge is scarce. Here, we utilize an innovative genetic approach by targeting the 3’UTR regulation of Gdnf in mice. Such animals express elevated levels of Gdnf exclusively in natively Gdnf-expressing cells, enabling dissection of endogenous GDNF functions in vivo. We show that endogenous GDNF regulates dopamine system development and function and protects mice in a rodent PD model without side effects associated with ectopic GDNF applications. Further, we report how GDNF levels regulate kidney development and identify microRNAs which control GDNF expression. Our study highlights the importance of correct spatial expression of GDNF and opens a novel approach to study gene function in mice.