Gene therapy for Parkinson's disease

BDNF-Regulated Modulation of Striatal Circuits and Implications for Parkinson's Disease and Dystonia

Neurotrophins, particularly brain-derived neurotrophic factor (BDNF), act as key regulators of neuronal development, survival, and plasticity. BDNF is necessary for neuronal and functional maintenance in the striatum and the substantia nigra, both structures involved in the pathogenesis of Parkinson's Disease (PD). Depletion of BDNF leads to striatal degeneration and defects in the dendritic arborization of striatal neurons. Activation of tropomyosin receptor kinase B (TrkB) by BDNF is necessary for the induction of long-term potentiation (LTP), a form of synaptic plasticity, in the hippocampus and striatum. PD is characterized by the degeneration of nigrostriatal neurons and altered striatal plasticity has been implicated in the pathophysiology of PD motor symptoms, leading to imbalances in the basal ganglia motor pathways. Given its essential role in promoting neuronal survival and meditating synaptic plasticity in the motor system, BDNF might have an important impact on the pathophysiology of neurodegenerative diseases, such as PD. In this review, we focus on the role of BDNF in corticostriatal plasticity in movement disorders, including PD and dystonia. We discuss the mechanisms of how dopaminergic input modulates BDNF/TrkB signaling at corticostriatal synapses and the involvement of these mechanisms in neuronal function and synaptic plasticity. Evidence for alterations of BDNF and TrkB in PD patients and animal models are reviewed, and the potential of BDNF to act as a therapeutic agent is highlighted. Advancing our understanding of these mechanisms could pave the way toward innovative therapeutic strategies aiming at restoring neuroplasticity and enhancing motor function in these diseases.

Transduction of Brain Neurons in Juvenile Chum Salmon (Oncorhynchus keta) with Recombinant Adeno-Associated Hippocampal Virus Injected into the Cerebellum during Long-Term Monitoring

Corpus cerebelli in juvenile chum salmon is a multiprojective region of the brain connected via afferent and efferent projections with the higher regions of the brainstem and synencephalon, as well as with multiprojection regions of the medulla oblongata and spinal cord. During the postembryonic development of the cerebellum in chum salmon, Oncorhynchus keta, the lateral part of the juvenile cerebellum gives rise to the caudomedial part of the definitive cerebellum, which is consistent with the data reported for zebrafish and mouse cerebellum. Thus, the topographic organization of the cerebellum and its efferents are similar between fish (chum salmon and zebrafish) and mammals, including mice and humans. The distributions of recombinant adeno-associated viral vectors (rAAVs) after an injection of the base vector into the cerebellum have shown highly specific patterns of transgene expression in bipolar neurons in the latero-caudal lobe of the juvenile chum tectum opticum. The distribution of rAAVs in the dorsal thalamus, epithalamus, nucleus rotundus, and pretectal complex indicates the targeted distribution of the transgene via the thalamo-cerebellar projections. The detection of GFP expression in the cells of the epiphysis and posterior tubercle of juvenile chum salmon is associated with the transgene’s distribution and with the cerebrospinal fluid flow, the brain ventricles and its outer surface. The direct delivery of the rAAV into the central nervous system by intracerebroventricular administration allows it to spread widely in the brain. Thus, the presence of special projection areas in the juvenile chum salmon cerebellum, as well as outside it, and the identification of the transgene’s expression in them confirm the potential ability of rAAVs to distribute in both intracerebellar and afferent and efferent extracerebellar projections of the cerebellum.

Next-Generation Gene Therapy for Parkinson's Disease Using Engineered Viral Vectors

Recent technological and conceptual advances have resulted in a plethora of exciting novel engineered adeno associated viral (AAV) vector variants. They all have unique characteristics and abilities. This review summarizes the development and their potential in treating Parkinson's disease (PD). Clinical trials in PD have shown over the last decade that AAV is a safe and suitable vector for gene therapy but that it also is a vehicle that can benefit significantly from improvement in specificity and potency. This review provides a concise collection of the state-of-the-art for synthetic capsids and their utility in PD. We also summarize what therapeutical strategies may become feasible with novel engineered vectors, including genome editing and neuronal rejuvenation.

cAMP Response Element Binding-Protein- and Phosphorylation-Dependent Regulation of Tyrosine Hydroxylase by PAK4: Implications for Dopamine Replacement Therapy

Parkinson's disease (PD) is characterized by a progressive loss of dopamine-producing neurons in the midbrain, which results in decreased dopamine levels accompanied by movement symptoms. Oral administration of l-3,4-dihydroxyphenylalanine (L-dopa), the precursor of dopamine, provides initial symptomatic relief, but abnormal involuntary movements develop later. A deeper understanding of the regulatory mechanisms underlying dopamine homeostasis is thus critically needed for the development of a successful treatment. Here, we show that p21-activated kinase 4 (PAK4) controls dopamine levels. Constitutively active PAK4 (caPAK4) stimulated transcription of tyrosine hydroxylase (TH) via the cAMP response element-binding protein (CREB) transcription factor. Moreover, caPAK4 increased the catalytic activity of TH through its phosphorylation of S40, which is essential for TH activation. Consistent with this result, in human midbrain tissues, we observed a strong correlation between phosphorylated PAK4S474, which represents PAK4 activity, and phosphorylated THS40, which reflects their enzymatic activity. Our findings suggest that targeting the PAK4 signaling pathways to restore dopamine levels may provide a new therapeutic approach in PD.

Sonoselective transfection of cerebral vasculature without blood-brain barrier disruption

Treatment of many pathologies of the brain could be improved markedly by the development of noninvasive therapeutic approaches that elicit robust, endothelial cell-selective gene expression in specific brain regions that are targeted under MR image guidance. While focused ultrasound (FUS) in conjunction with gas-filled microbubbles (MBs) has emerged as a noninvasive modality for MR image-guided gene delivery to the brain, it has been used exclusively to transiently disrupt the blood-brain barrier (BBB), which may induce a sterile inflammation response. Here, we introduce an MR image-guided FUS method that elicits endothelial-selective transfection of the cerebral vasculature (i.e., "sonoselective" transfection), without opening the BBB. We first determined that activating circulating, cationic plasmid-bearing MBs with pulsed low-pressure (0.1 MPa) 1.1-MHz FUS facilitates sonoselective gene delivery to the endothelium without MRI-detectable disruption of the BBB. The degree of endothelial selectivity varied inversely with the FUS pressure, with higher pressures (i.e., 0.3-MPa and 0.4-MPa FUS) consistently inducing BBB opening and extravascular transfection. Bulk RNA sequencing analyses revealed that the sonoselective low-pressure regimen does not up-regulate inflammatory or immune responses. Single-cell RNA sequencing indicated that the transcriptome of sonoselectively transfected brain endothelium was unaffected by the treatment. The approach developed here permits targeted gene delivery to blood vessels and could be used to promote angiogenesis, release endothelial cell-secreted factors to stimulate nerve regrowth, or recruit neural stem cells.

BDNF as a Promising Therapeutic Agent in Parkinson's Disease

Brain-derived neurotrophic factor (BDNF) promotes neuroprotection and neuroregeneration. In animal models of Parkinson’s disease (PD), BDNF enhances the survival of dopaminergic neurons, improves dopaminergic neurotransmission and motor performance. Pharmacological therapies of PD are symptom-targeting, and their effectiveness decreases with the progression of the disease; therefore, new therapeutical approaches are needed. Since, in both PD patients and animal PD models, decreased level of BDNF was found in the nigrostriatal pathway, it has been hypothesized that BDNF may serve as a therapeutic agent. Direct delivery of exogenous BDNF into the patient’s brain did not relieve the symptoms of disease, nor did attempts to enhance BDNF expression with gene therapy. Physical training was neuroprotective in animal models of PD. This effect is mediated, at least partly, by BDNF. Animal studies revealed that physical activity increases BDNF and tropomyosin receptor kinase B (TrkB) expression, leading to inhibition of neurodegeneration through induction of transcription factors and expression of genes related to neuronal proliferation, survival, and inflammatory response. This review focuses on the evidence that increasing BDNF level due to gene modulation or physical exercise has a neuroprotective effect and could be considered as adjunctive therapy in PD.

Repairing the Brain: Gene Therapy

In vivo gene therapy for neurodegenerative disorders has turned out to be a formidable challenge. It is a field not much older than twenty years, but we were many who would have predicted a much easier path towards the clinic using this treatment modality. For Parkinson’s disease patients, this has meant a frustrating wait, seeing many promising therapies being forgotten after a few pre-clinical proof-of-concept studies. The reasons for this are both scientific and economical. However, this is slowly but surely changing and over the next two decades we will see a very exciting development in this field. In a foreseeable future, gene therapy will be a very natural component of many clinical therapies, not least in Parkinson’s disease.

Targeted gene transfer to the brain via the delivery of brain-penetrating DNA nanoparticles with focused ultrasound

Gene therapy holds promise for the treatment of many pathologies of the central nervous system (CNS), including brain tumors and neurodegenerative diseases. However, the delivery of systemically administered gene carriers to the CNS is hindered by both the blood-brain barrier (BBB) and the nanoporous and electrostatically charged brain extracelluar matrix (ECM), which acts as a steric and adhesive barrier. We have previously shown that these physiological barriers may be overcome by, respectively, opening the BBB with MR image-guided focused ultrasound (FUS) and microbubbles and using highly compact "brain penetrating" nanoparticles (BPN) coated with a dense polyethylene glycol corona that prevents adhesion to ECM components. Here, we tested whether this combined approach could be utilized to deliver systemically administered DNA-bearing BPN (DNA-BPN) across the BBB and mediate localized, robust, and sustained transgene expression in the rat brain. Systemically administered DNA-BPN delivered through the BBB with FUS led to dose-dependent transgene expression only in the FUS-treated region that was evident as early as 24h post administration and lasted for at least 28days. In the FUS-treated region ~42% of all cells, including neurons and astrocytes, were transfected, while less than 6% were transfected in the contralateral non-FUS treated hemisphere. Importantly, this was achieved without any sign of toxicity or astrocyte activation. We conclude that the image-guided delivery of DNA-BPN with FUS and microbubbles constitutes a safe and non-invasive strategy for targeted gene therapy to the brain.

Drug and gene delivery across the blood-brain barrier with focused ultrasound

The blood-brain barrier (BBB) remains one of the most significant limitations to treatments of central nervous system (CNS) disorders including brain tumors, neurodegenerative diseases and psychiatric disorders. It is now well-established that focused ultrasound (FUS) in conjunction with contrast agent microbubbles may be used to non-invasively and temporarily disrupt the BBB, allowing localized delivery of systemically administered therapeutic agents as large as 100nm in size to the CNS. Importantly, recent technological advances now permit FUS application through the intact human skull, obviating the need for invasive and risky surgical procedures. When used in combination with magnetic resonance imaging, FUS may be applied precisely to pre-selected CNS targets. Indeed, FUS devices capable of sub-millimeter precision are currently in several clinical trials. FUS mediated BBB disruption has the potential to fundamentally change how CNS diseases are treated, unlocking potential for combinatorial treatments with nanotechnology, markedly increasing the efficacy of existing therapeutics that otherwise do not cross the BBB effectively, and permitting safe repeated treatments. This article comprehensively reviews recent studies on the targeted delivery of therapeutics into the CNS with FUS and offers perspectives on the future of this technology.

Fiber-modified adenovirus for central nervous system Parkinson's disease gene therapy

Gene-based therapies for neurological diseases continue to develop briskly. As disease mechanisms are elucidated, flexible gene delivery platforms incorporating transcriptional regulatory elements, therapeutic genes and targeted delivery are required for the safety and efficacy of these approaches. Adenovirus serotype 5 (Ad5)-based vectors can carry large genetic payloads to provide this flexibility, but do not transduce neuronal cells efficiently. To address this, we have developed a tropism-modified Ad5 vector with neuron-selective targeting properties for evaluation in models of Parkinson disease therapy. A panel of tropism-modified Ad5 vectors was screened for enhanced gene delivery in a neuroblastoma cell line model system. We used these observations to design and construct an unbiased Ad vector platform, consisting of an unmodified Ad5 and a tropism-modified Ad5 vector containing the fiber knob domain from canine Ad serotype 2 (Ad5-CGW-CK2). Delivery to the substantia nigra or striatum showed that this vector produced a neuronally-restricted pattern of gene expression. Many of the transduced neurons were from regions with afferent projections to the injection site, implicating that the vector binds the presynaptic terminal resulting in presynaptic transduction. We show that Ad5-CGW-CK2 can selectively transduce neurons in the brain and hypothesize that this modular platform is potentially adaptable to clinical use.

Imaging of human differentiated 3D neural aggregates using light sheet fluorescence microscopy

The development of three dimensional (3D) cell cultures represents a big step for the better understanding of cell behavior and disease in a more natural like environment, providing not only single but multiple cell type interactions in a complex 3D matrix, highly resembling physiological conditions. Light sheet fluorescence microscopy (LSFM) is becoming an excellent tool for fast imaging of such 3D biological structures. We demonstrate the potential of this technique for the imaging of human differentiated 3D neural aggregates in fixed and live samples, namely calcium imaging and cell death processes, showing the power of imaging modality compared with traditional microscopy. The combination of light sheet microscopy and 3D neural cultures will open the door to more challenging experiments involving drug testing at large scale as well as a better understanding of relevant biological processes in a more realistic environment.

Parkinson's disease gene therapy: success by design meets failure by efficacy

Over the past decade, nine gene therapy clinical trials for Parkinson's disease (PD) have been initiated and completed. Starting with considerable optimism at the initiation of each trial, none of the programs has yet borne sufficiently robust clinical efficacy or found a clear path toward regulatory approval. Despite the immediately disappointing nature of the efficacy outcomes in these trials, the clinical data garnered from the individual studies nonetheless represent tangible and significant progress for the gene therapy field. Collectively, the clinical trials demonstrate that we have overcome the major safety hurdles previously suppressing central nervous system (CNS) gene therapy, for none produced any evidence of untoward risk or harm after administration of various vector-delivery systems. More importantly, these studies also demonstrated controlled, highly persistent generation of biologically active proteins targeted to structures deep in the human brain. Therefore, a renewed, focused emphasis must be placed on advancing clinical efficacy by improving clinical trial design, patient selection and outcome measures, developing more predictive animal models to support clinical testing, carefully performing retrospective analyses, and most importantly moving forward—beyond our past limits.

A new approach to disease-modifying drug trials in Parkinson's disease

Translating new findings in the laboratory into therapies for patients is a slow and expensive process. The development of therapies for neurodegenerative diseases is further complicated by the difficulty in determining whether the drug truly retards the slow degenerative process or provides only symptomatic benefit. In this issue, Aviles-Olmos et al. describe a first in Parkinson's disease (PD) patient study using a drug previously approved for diabetes treatment. In addition to suggesting that the drug may indeed be disease modifying in PD, their innovative approach suggests there may be more rapid and inexpensive avenues for testing novel therapies in PD.

Functional neuroprotection and efficient regulation of GDNF using destabilizing domains in a rodent model of Parkinson's disease

Glial cell line-derived neurotrophic factor (GDNF) has great potential to treat Parkinson's disease (PD). However, constitutive expression of GDNF can over time lead to side effects. Therefore, it would be useful to regulate GDNF expression. Recently, a new gene inducible system using destabilizing domains (DD) from E. coli dihydrofolate reductase (DHFR) has been developed and characterized. The advantage of this novel DD is that it is regulated by trimethoprim (TMP), a well-characterized drug that crosses the blood-brain barrier and can therefore be used to regulate gene expression in the brain. We have adapted this system to regulate expression of GDNF. A C-terminal fusion of GDNF and a DD with an additional furin cleavage site was able to be efficiently regulated in vitro, properly processed and was able to bind to canonical GDNF receptors, inducing a signaling cascade response in target cells. In vivo characterization of the protein showed that it could be efficiently induced by TMP and it was only functional when gene expression was turned on. Further characterization in a rodent model of PD showed that the regulated GDNF protected neurons, improved motor behavior of animals and was efficiently regulated in a pathological setting.

Neurotrophin delivery using nanotechnology

Deficits or overexpression of neurotrophins cause neurodegenerative diseases and psychiatric disorders. These proteins are required for the maintenance of the function, plasticity and survival of neurons in the central (CNS) and peripheral nervous systems. Significant efforts have been devoted to developing therapeutic delivery systems that enable control of neurotrophin dosage in the brain. Here, we suggest that nanoparticulate carriers favoring targeted delivery in specific brain areas and minimizing biodistribution to the systemic circulation should be developed toward clinical benefits of neuroregeneration. We also provide examples of improved targeted neurotrophin delivery to localized areas in the CNS.

Resident adult neural stem cells in Parkinson's disease--the brain's own repair system?

One important pathological process in the brain of Parkinson disease (PD) patients is the degeneration of the dopaminergic neurons in the substantia nigra, which leads to a decline in striatal dopamine levels and motor dysfunction. A major clinical problem is that this degenerative process currently cannot be stopped or reversed. Expectations from the restorative capacity of neural stem cells (NSCs) are high, as these cells can potentially replace the degenerating neurons. The discovery of the presence of NSCs in the adult human brain has instigated research into the potential of these cells as a resource to promote brain repair in neurodegenerative diseases. Neural stem and progenitor cells reside in the subventricular zone (SVZ), which is closely situated to the striatum, which is affected in PD. Therefore, restoring the dopamine levels in the striatum of PD patients through stimulating endogenous NSCs in the nearby SVZ to migrate into the striatum and differentiate into dopaminergic neurons might thus be an attractive future therapeutic approach. We will review the reported changes in NSCs in the SVZ of PD animal models and PD patients, which are due to a lack of striatal dopamine. Furthermore, we will summarise the reports that describe efforts to stimulate NSCs to replace dopaminergic cells in the SN and restore striatal dopamine levels. In our opinion, mobilizing the endogenous SVZ NSCs to replenish striatal dopamine is an attractive approach to alleviate the motor symptoms in PD patients, without the ethical and immunological challenges of transplantation of NSCs and foetal brain tissue.

Gene therapy with AAV2-CDNF provides functional benefits in a rat model of Parkinson's disease

Cerebral dopamine neurotrophic factor (CDNF) protein has been shown to protect the nigrostriatal dopaminergic pathway when given as intrastriatal infusions in rat and mouse models of Parkinson's disease (PD). In this study, we assessed the neuroprotective effect of CDNF delivered with a recombinant adeno-associated viral (AAV) serotype 2 vector in a rat 6-hydroxydopamine (6-OHDA) model of PD. AAV2 vectors encoding CDNF, glial cell line-derived neurotrophic factor (GDNF), or green fluorescent protein were injected into the rat striatum. Protein expression analysis showed that our AAV2 vector efficiently delivered the neurotrophic factor genes into the brain and gave rise to a long-lasting expression of the proteins. Two weeks after AAV2 vector injection, 6-OHDA was injected into the rat striatum, creating a progressive degeneration of the nigrostriatal dopaminergic system. Treatment with AAV2-CDNF resulted in a marked decrease in amphetamine-induced ipsilateral rotations while it provided only partial protection of tyrosine hydroxylase (TH)-immunoreactive cells in the rat substantia nigra pars compacta and TH-reactive fibers in the striatum. Results from this study provide additional evidence that CDNF can be considered a potential treatment of Parkinson's disease.

From molecular to nanotechnology strategies for delivery of neurotrophins: emphasis on brain-derived neurotrophic factor (BDNF)

Neurodegenerative diseases represent a major public health problem, but beneficial clinical treatment with neurotrophic factors has not been established yet. The therapeutic use of neurotrophins has been restrained by their instability and rapid degradation in biological medium. A variety of strategies has been proposed for the administration of these leading therapeutic candidates, which are essential for the development, survival and function of human neurons. In this review, we describe the existing approaches for delivery of brain-derived neurotrophic factor (BDNF), which is the most abundant neurotrophin in the mammalian central nervous system (CNS). Biomimetic peptides of BDNF have emerged as a promising therapy against neurodegenerative disorders. Polymer-based carriers have provided sustained neurotrophin delivery, whereas lipid-based particles have contributed also to potentiation of the BDNF action. Nanotechnology offers new possibilities for the design of vehicles for neuroprotection and neuroregeneration. Recent developments in nanoscale carriers for encapsulation and transport of BDNF are highlighted.

Post-injury delivery of rAAV2-CNTF combined with short-term pharmacotherapy is neuroprotective and promotes extensive axonal regeneration after optic nerve trauma

Recombinant adeno-associated viral (rAAV) vectors expressing neurotrophic genes reduce neuronal death and promote axonal regeneration in central nervous system (CNS) injury models. Currently, however, use of rAAV to treat clinical neurotrauma is problematic because there is a delay in the onset of transgene expression. Using the adult rat retina and optic nerve (ON), we have tested whether rAAV gene therapy administered at the time of injury combined with short-term pharmacotherapy has synergistic effects that enhance neuronal survival and regeneration. The ON was transected and a 1.5 cm segment of autologous peripheral nerve (PN) was grafted onto the cut end. At this time, bicistronic rAAV2 encoding ciliary neurotrophic factor (CNTF) and green fluorescent protein (rAAV2-CNTF-GFP) was injected into the injured eye. To provide interim support for axotomized retinal ganglion cells (RGCs) during vector integration and therapeutic transgene expression, rCNTF protein and a cyclic adenosine monophosphate (cAMP) analogue (CPT-cAMP) were injected intravitreally 3 and 10 days postoperatively. For comparison, another rAAV2-CNTF-GFP group received two intravitreal saline injections 3 and 10 days after the PN-ON surgery. A further PN graft group received only postoperative intravitreal injections of rCNTF plus CPT-cAMP. After 4 weeks, regenerating RGCs were retrogradely labelled by applying fluorogold to the distal end of each PN graft. Compared to saline-injected animals, both RGC survival and axonal regrowth were significantly higher in the rCNTF and CPT-cAMP injected rAAV2-CNTF-GFP group; approximately one third of the RGC population survived axotomy, and 27% of these regrew an axon. These values were also higher than those obtained in rats that received only rCNTF plus CPT-cAMP injections. Therefore, we show for the first time that rAAV-mediated gene delivery at the time of, or just after, neurotrauma is most successful when combined with temporary post-injury trophic support, and is potentially a viable treatment strategy for patients after acute CNS injury.

Molecular chaperones and co-chaperones in Parkinson disease

Parkinson disease, a progressive neurodegenerative disorder, is caused by the pathological accumulation of proteins, including the ubiquitous presynaptic protein α-synuclein. Alterations in the metabolism of α-synuclein have clearly been linked to neurodegeneration, and early steps in the pathological sequence of this protein include the formation of oligomers, fibrils, and small aggregates. Targeting these early steps of oligomerization is one of the main therapeutic approaches in the quest to develop disease-modifying agents. Molecular chaperones, molecules that can mediate the proper folding and refolding of client proteins, are vital to cell function and survival and thus have been explored as potential therapeutic agents. Important to Parkinson disease, chaperones are capable of preventing α-synuclein misfolding, oligomerization, and aggregate formation as shown in vitro and in Parkinson disease animal models. Furthermore, chaperones and associated co-chaperones are closely linked to pathways of protein degradation, like the ubiquitin-proteasome system and autophagy, and are thus able to remove irreversibly misfolded proteins. In this review, we summarize the role of molecular chaperones in Parkinson disease models and discuss the importance of preserving protein homeostasis to prevent neurodegeneration. We also review the growing number of exciting studies that have targeted molecular chaperone function as a novel therapeutic approach.

Long-term gene therapy causes transgene-specific changes in the morphology of regenerating retinal ganglion cells

Recombinant adeno-associated viral (rAAV) vectors can be used to introduce neurotrophic genes into injured CNS neurons, promoting survival and axonal regeneration. Gene therapy holds much promise for the treatment of neurotrauma and neurodegenerative diseases; however, neurotrophic factors are known to alter dendritic architecture, and thus we set out to determine whether such transgenes also change the morphology of transduced neurons. We compared changes in dendritic morphology of regenerating adult rat retinal ganglion cells (RGCs) after long-term transduction with rAAV2 encoding: (i) green fluorescent protein (GFP), or (ii) bi-cistronic vectors encoding GFP and ciliary neurotrophic factor (CNTF), brain-derived neurotrophic factor (BDNF) or growth-associated protein-43 (GAP43). To enhance regeneration, rats received an autologous peripheral nerve graft onto the cut optic nerve of each rAAV2 injected eye. After 5–8 months, RGCs with regenerated axons were retrogradely labeled with fluorogold (FG). Live retinal wholemounts were prepared and GFP positive (transduced) or GFP negative (non-transduced) RGCs injected iontophoretically with 2% lucifer yellow. Dendritic morphology was analyzed using Neurolucida software. Significant changes in dendritic architecture were found, in both transduced and non-transduced populations. Multivariate analysis revealed that transgenic BDNF increased dendritic field area whereas GAP43 increased dendritic complexity. CNTF decreased complexity but only in a subset of RGCs. Sholl analysis showed changes in dendritic branching in rAAV2-BDNF-GFP and rAAV2-CNTF-GFP groups and the proportion of FG positive RGCs with aberrant morphology tripled in these groups compared to controls. RGCs in all transgene groups displayed abnormal stratification. Thus in addition to promoting cell survival and axonal regeneration, vector-mediated expression of neurotrophic factors has measurable, gene-specific effects on the morphology of injured adult neurons. Such changes will likely alter the functional properties of neurons and may need to be considered when designing vector-based protocols for the treatment of neurotrauma and neurodegeneration.

Neurotrophic factors and neurodegenerative diseases: a delivery issue

Neurotrophic factors (NTFs) represent one of the most stimulating challenge in neurodegenerative diseases, due to their potential in neurorestoring and neuroprotection. Despite the large number of proofs-of-concept and evidences of their activity, most of the clinical trials, mainly regarding Parkinson's disease and Alzheimer's disease, demonstrated several failures of the therapeutic intervention. A large number of researches were conducted on this hot topic of neuroscience, clearly evidencing the advantages of NTF approach, but evidencing the major limitations in its application. The inability in crossing the blood-brain barrier and the lack of selectivity actually represent some of the most highlighted limits of NTFs-based therapy. In this review, beside an overview of NTF activity versus the main neuropathological disorders, a summary of the most relevant approaches, from invasive to noninvasive strategies, applied for improving NTF delivery to the central nervous systems is critically considered and evaluated.

Parkinson's disease therapeutics: new developments and challenges since the introduction of levodopa

The demonstration that dopamine loss is the key pathological feature of Parkinson's disease (PD), and the subsequent introduction of levodopa have revolutionalized the field of PD therapeutics. This review will discuss the significant progress that has been made in the development of new pharmacological and surgical tools to treat PD motor symptoms since this major breakthrough in the 1960s. However, we will also highlight some of the challenges the field of PD therapeutics has been struggling with during the past decades. The lack of neuroprotective therapies and the limited treatment strategies for the nonmotor symptoms of the disease (ie, cognitive impairments, autonomic dysfunctions, psychiatric disorders, etc.) are among the most pressing issues to be addressed in the years to come. It appears that the combination of early PD nonmotor symptoms with imaging of the nigrostriatal dopaminergic system offers a promising path toward the identification of PD biomarkers, which, once characterized, will set the stage for efficient use of neuroprotective agents that could slow down and alter the course of the disease.

Harpagoside attenuates MPTP/MPP⁺ induced dopaminergic neurodegeneration and movement disorder via elevating glial cell line-derived neurotrophic factor

Parkinson's disease is a chronic neurodegenerative movement disorder characterized by the loss of dopaminergic neurons in the substantia nigra pars compacta. New therapeutic approaches aiming at delaying or reversing the neurodegenerative process are under active investigations. In this work, we found that harpagoside, an iridoid purified from the Chinese medicinal herb Scrophularia ningpoensis, could not only prevent but also rescue the dopaminergic neurodegeneration in MPTP/MPP(+) intoxication with promising efficacy. Firstly, in cultured mesencephalic neurons, harpagoside significantly attenuated the loss of TH-positive neuron numbers and the shortening of axonal length. Secondly, in a chronic MPTP mouse model, harpagoside dose-dependently improved the loco-motor ability (rotarod test), increased the TH-positive neuron numbers in the substantia nigra pars compacta (unbiased stereological counting) and increased the striatal DAT density ((125) I-FP-CIT autoradiography). Thirdly, harpagoside markedly elevated the GDNF mRNA and GDNF protein levels in MPTP/MPP(+) lesioned models. However, the protecting effect of harpagoside on the dopaminergic degeneration disappeared when the intrinsic GDNF action was blocked by either the Ret inhibitor PP1 or the neutralizing anti-GDNF antibody. Taken together, we conclude that harpagoside attenuates the dopaminergic neurodegeneration and movement disorder mainly through elevating glial cell line-derived neurotrophic factor.

A PET brain reporter gene system based on type 2 cannabinoid receptors

PET of gene expression in the brain may greatly facilitate neuroscience research and potential clinical implementation of gene or cell therapy of central nervous system diseases. To date, no adequate PET reporter system is available for the central nervous system because available tracers either do not cross the intact blood-brain barrier or have high background signals. Here we report the first, to our knowledge, PET reporter system for imaging gene expression in the intact brain.

METHODS

We selected the human type 2 cannabinoid receptor (hCB(2)) as a reporter because of its low basal expression in the brain. An inactive mutant (D80N) was chosen so as not to interfere with signal transduction. As a reporter probe we used the (11)C-labeled CB(2) ligand, (11)C-GW405833, which readily crosses the blood-brain barrier. Dual-modality imaging lentiviral and adeno-associated viral vectors encoding both hCB(2)(D80N) and firefly luciferase or enhanced green fluorescent protein were engineered and validated in cell culture. Next, hCB(2)(D80N) was locoregionally overexpressed in rat striatum by stereotactic injection of lentiviral and adeno-associated viral vectors.

RESULTS

Kinetic PET revealed specific and reversible CB(2) binding of (11)C-GW405833 in the transduced rat striatum. hCB(2) and firefly luciferase expression was followed until 9 mo and showed similar kinetics. Both hCB(2) expression and enhanced green fluorescent protein expression were confirmed by immunohistochemistry.

CONCLUSION

Dual-modality imaging viral vectors encoding hCB(2)(D80N) were engineered, and the reporter system was validated in different animal species. The results support the potential future clinical use of CB(2) as a PET reporter in the intact brain.

Cell transplantation and gene therapy in Parkinson's disease

Parkinson's disease is a progressive neurodegenerative disorder affecting, in part, dopaminergic motor neurons of the ventral midbrain and their terminal projections that course to the striatum. Symptomatic strategies focused on dopamine replacement have proven effective at remediating some motor symptoms during the course of disease but ultimately fail to deliver long-term disease modification and lose effectiveness due to the emergence of side effects. Several strategies have been experimentally tested as alternatives for Parkinson's disease, including direct cell replacement and gene transfer through viral vectors. Cellular transplantation of dopamine-secreting cells was hypothesized as a substitute for pharmacotherapy to directly provide dopamine, whereas gene therapy has primarily focused on restoration of dopamine synthesis or neuroprotection and restoration of spared host dopaminergic circuitry through trophic factors as a means to enhance sustained controlled dopamine transmission. This seems now to have been verified in numerous studies in rodents and nonhuman primates, which have shown that grafts of fetal dopamine neurons or gene transfer through viral vector delivery can lead to improvements in biochemical and behavioral indices of dopamine deficiency. However, in clinical studies, the improvements in parkinsonism have been rather modest and variable and have been plagued by graft-induced dyskinesias. New developments in stem-cell transplantation and induced patient-derived cells have opened the doors for the advancement of cell-based therapeutics. In addition, viral-vector-derived therapies have been developed preclinically with excellent safety and efficacy profiles, showing promise in clinical trials thus far. Further progress and optimization of these therapies will be necessary to ensure safety and efficacy before widespread clinical use is deemed appropriate.

New small molecules for the treatment of Parkinson's disease

IMPORTANCE OF THE FIELD

Parkinson's disease (PD) is characterized by a slowly ongoing neuronal death, which affects neurotransmitter metabolism and causes a wide variety of motor and non-motor features. Until now, therapy approaches have predominantly focused on motor behavior associated with dopamine substitution.

AREAS COVERED IN THIS REVIEW

This review aims to discuss putative reasons for recent failures of investigated treatment approaches, and to introduce currently tested and future compounds.

WHAT THE READER WILL GAIN

We will describe how development programs of novel molecules now additionally consider non-motor features of PD as promising targets in order to obtain regulatory approval. Regulatory authorities increasingly exert influence on trial designs, demanding therapeutic effects that are not always clinically feasible given the variety of manifestations of the disease entity known as PD.

TAKE HOME MESSAGE

In the past, research pitfalls have resulted in the failure of promising new compounds. Among the many reasons for this are massive placebo responses; the participation of too many investigators, with consequent wide variations of efficacy assessments; and a misconception of preclinical drug development, with models of PD that do not mimic its clinical nature. A few compounds are now being tested that have modes of action indirectly modulating the dopamine system; however, critical analysis of the preclinical and clinical research concept and drug approval is warranted to prevent further frustration in this field.

MicroRNA-regulated, systemically delivered rAAV9: a step closer to CNS-restricted transgene expression

Recombinant adeno-associated viruses (rAAVs) that can cross the blood-brain-barrier and achieve efficient and stable transvascular gene transfer to the central nervous system (CNS) hold significant promise for treating CNS disorders. However, following intravascular delivery, these vectors also target liver, heart, skeletal muscle, and other tissues, which may cause untoward effects. To circumvent this, we used tissue-specific, endogenous microRNAs (miRNAs) to repress rAAV expression outside the CNS, by engineering perfectly complementary miRNA-binding sites into the rAAV9 genome. This approach allowed simultaneous multi-tissue regulation and CNS-directed stable transgene expression without detectably perturbing the endogenous miRNA pathway. Regulation of rAAV expression by miRNA was primarily via site-specific cleavage of the transgene mRNA, generating specific 5' and 3' mRNA fragments. Our findings promise to facilitate the development of miRNA-regulated rAAV for CNS-targeted gene delivery and other applications.

Anterograde axonal transport of AAV2-GDNF in rat basal ganglia

We elucidated the effects of parkinsonian degeneration on trafficking of AAV2-GDNF in the nigro-striatum (nigro-ST) of unilaterally 6-hydroxydopamine (6-OHDA)-lesioned rats. Vector infused into striatum (ST) was transported to substantia nigra (SN), both pars compacta (SNc), and pars reticulata (SNr). In the lesioned hemisphere, glial cell line-derived neurotrophic factor (GDNF) immunoreactivity was only found in SNr consistent with elimination of SNc dopaminergic (DA) neurons by 6-OHDA. Further analysis showed that striatal delivery of AAV2-GDNF resulted in GDNF expression in globus pallidus (GP), entopeduncular nucleus (EPN), and subthalamic nucleus (STN) in both lesioned and unlesioned hemispheres. Injection of vector into SN, covering both SNc and SNr, resulted in striatal expression of GDNF in the unlesioned hemisphere but not in the lesioned hemisphere. No expression was seen in GP or EPN. We conclude that adeno-associated virus serotype 2 (AAV2) is transported throughout the nigro-ST exclusively by anterograde transport. This transport phenomenon directs GDNF expression throughout the basal ganglia in regions that are adversely affected in Parkinson's disease (PD) in addition to SNc. Delivery of vector to SN, however, does not direct expression of GDNF in ST, EPN, or GP. On this basis, we believe that striatal delivery of AAV2-GDNF is the preferred course of action for trophic rescue of DA function.

Healing genes in the nervous system

The evolution of gene therapy has led to the development of promising new therapeutic approaches. This NeuroView will introduce the variety of delivery vehicles currently available for gene therapy, a range of preclinical strategies for tackling major diseases of the nervous system, the clinical limitations, and ethical considerations.

Transduction of brain dopamine neurons by adenoviral vectors is modulated by CAR expression: rationale for tropism modified vectors in PD gene therapy

Background

Gene-based therapy is a new paradigm for the treatment of Parkinson disease (PD) and offers considerable promise for precise targeting and flexibility to impact multiple pathobiological processes for which small molecule agents are not available. Some success has been achieved utilizing adeno-associated virus for this approach, but it is likely that the characteristics of this vector system will ultimately create barriers to progress in clinical therapy. Adenovirus (Ad) vector overcomes limitations in payload size and targeting. The cellular tropism of Ad serotype 5 (Ad5)–based vectors is regulated by the Ad attachment protein binding to its primary cellular receptor, the coxsackie and adenovirus receptor (CAR). Many clinically relevant tissues are refractory to Ad5 infection due to negligible CAR levels but can be targeted by tropism-modified, CAR-independent forms of Ad. Our objective was to evaluate the role of CAR protein in transduction of dopamine (DA) neurons in vivo.

Methodology/Principal Findings

Ad5 was delivered to the substantia nigra (SN) in wild type (wt) and CAR transgenic animals. Cellular tropism was assessed by immunohistochemistry (IHC) in the SN and striatal terminals. CAR expression was assessed by western blot and IHC. We found in wt animals, Ad5 results in robust transgene expression in astrocytes and other non-neuronal cells but poor infection of DA neurons. In contrast, in transgenic animals, Ad5 infects SNc neurons resulting in expression of transduced protein in their striatal terminals. Western blot showed low CAR expression in the ventral midbrain of wt animals compared to transgenic animals. Interestingly, hCAR protein localizes with markers of post-synaptic structures, suggesting synapses are the point of entry into dopaminergic neurons in transgenic animals.

Conclusions/Significance

These findings demonstrate that CAR deficiency limits infection of wild type DA neurons by Ad5 and provide a rationale for the development of tropism-modified, CAR-independent Ad-vectors for use in gene therapy of human PD.

Neurotrophic growth factors for the treatment of amyotrophic lateral sclerosis: where do we stand?

Amyotrophic lateral sclerosis (ALS) is a devastating neurodegenerative disease that results in progressive loss of motoneurons, motor weakness and death within 3–5 years after disease onset. Therapeutic options remain limited despite substantial number of approaches that have been tested clinically. Many neurotrophic growth factors are known to promote the survival of neurons and foster regeneration in the central nervous system. Various neurotrophic factors have been investigated pre-clinically and clinically for the treatment of ALS. Although pre-clinical data appeared promising, no neurotrophic factors succeeded yet in a clinical phase III trial. In this review we discuss the rationale behind those factors, possible reasons for clinical failures, and argue for a renewal of hope in this powerful class of drugs for the treatment of ALS.