Address for correspondence: Prof. Girish Modi, Department of Neurology, Division of Neurosciences, University of the Witwatersrand, Johannesburg, South Africa. Voice: +27836012878; fax: +27114847576. email@example.com
Due to limitations posed by the restrictive blood–brain barrier, conventional drug delivery systems do not provide adequate cyto-architecture restoration and connection patterns that are essential for functional recovery in neurodegenerative disorders (NDs). Nanotechnology employs engineered materials or devices that interact with biological systems at a molecular level and could revolutionize the treatment of NDs by stimulating, responding to, and interacting with target sites to induce physiological responses while minimizing side effects. This review provides a concise discussion of the current applications of nano-enabled drug-delivery systems for the treatment of NDs, in particular Alzheimer's and Parkinson's diseases, and explores the future applications of nanotechnology in clinical neuroscience to develop innovative therapeutic modalities for the treatment of NDs.
Advances in biotechnology are expected to have a major impact in neurological research leading especially to the development of newer and more directed therapeutic modalities. Nanotechnology is at the core of these advances. Nanotechnology employs engineered polymeric materials to design devices with the smallest functional organization on the nanometer scale (1–100 nm) that are able to interact with biological systems at a molecular level. They may stimulate, respond to, and interact with target cells and tissues to induce desired physiological responses while simultaneously minimizing undesirable side effects. Importantly, nanotechnology may offer ways to manipulate complex biological systems with greater selectivity.
The term neurodegenerative disorders (NDs), encompasses conditions that are sporadic and/or familial and characterized by the persistent and progressive loss of neuronal subtypes. The most widely recognized are Alzheimer's disease (AD) and Parkinson's disease (PD), which are among the principal debilitating conditions of the current century.1–3
Approximately 24 million people worldwide suffer from dementia, of which 60% is due to AD.4 AD occurs in 1% of individuals aged 50 to 70 years and dramatically increases to 50% of those over 70 years. AD is typified clinically by learning and memory impairment and pathologically by gross cerebral atrophy, indicative of neuronal loss, with numerous extracellular neuritic amyloid plaques and intracellular neurofibrillary tangles found predominantly in the frontal and temporal lobes, including the hippocampus.5 The etiology remains elusive, and despite exhaustive searches for clues, the main risk and possible causative factor appears to be the relationship between AD and the Apo Ee4 allele of the Apo E gene family. The Apo Ee4 allele of the Apo E gene, located on chromosome 19, has been associated with an increased risk for and lower age of onset of AD. The role of Apo Ee4 in AD is thought to be related to the formation of amyloid plaques due to its function as a carrier for β-amyloid. Furthermore, it has been suggested that the failure of Apo Ee4 to bind to tau protein may lead to the failure of phosphorylation of tau and thereby the formation of neurofibrillary tangles. Interestingly, the Apo Ee2 gene appears to confer some protection against sporadic AD. Genetic factors that present as dominant mutations account for less than 1% of the few cases of familial early-onset AD. However, the ultimate cause of AD is unknown and current treatment remains symptomatic. Current treatments for cognitive impairment in AD are based on neurotransmitter or enzyme replacement/modulation, which provide symptomatic benefits and include acetylcholinesterase inhibitors,6 cholinesterase inhibitors,7 antioxidants,8 amyloid-β-targeted drugs, nerve growth factors, c-secretase inhibitors,9 and vaccines against β-amyloid.10 However, none of the available therapies appears to be able to cure AD or to attenuate disease progression.
PD is characterized by the sporadic degeneration of midbrain nigrostriatal dopaminergic neurons with resultant reduction in brain dopamine (DA) levels causing the characteristic motor symptoms of bradykinesia, rigidity, and resting tremor.11 DA is the neural transmitter responsible for transmitting the electrical signals required for normal physical motion. The deficiency of DA that typifies PD results in these abnormal movements. While the etiology of PD is not well known, its pathogenesis is thought to be a multifactorial cascade of deleterious factors. PD affects 1% of the population over the age of 65 years. Genetic factors are decidedly uncommon, with at most 5% of cases being familial or hereditary. A small subset of patients within the hereditary group appears to follow a pattern of autosomal dominant inheritance, although the majority of the hereditary cases do not exhibit a recognizable inheritance pattern. Currently, frontline therapy for PD is the oral administration of dopamine agonists such as levodopa. To complement the pharmacological treatment, deep brain stimulation and transplantation of fetal dopamine neurons have been explored.12 However, these approaches remain controversial.
Drug delivery to the brain remains the major challenge for the treatment of all NDs because of the numerous protective barriers surrounding the CNS. The bioactive agents that are currently approved by the U.S. Food and Drug Administration have demonstrated modest effects in modifying disease symptoms for relatively short periods in subsets of patients, and none has shown an effect on disease progression. One of the significant facts on neurotherapeutics is the constraint of the blood–brain barrier (BBB) and the drug release kinetics that cause peripheral side effects. Furthermore, contrary to common belief, NDs may be multisystemic in nature, and this presents numerous difficulties for the potential treatment of these disorders. In NDs the death of specific types of neurons is provoked by a cascade of multiple deleterious molecular and cellular events rather than a single pathogenic factor.
The advent of nanotechnology may provide a solution to overcome these diagnostic and neurotherapeutic challenges for AD and PD. Nanotechnology employs engineered materials or devices with the smallest functional organization on the nanometer scale (1–100 nm) that are able to interact with biological systems at the molecular level. Nanoparticles are able to penetrate the BBB of in vitro and in vivo models.13–16 Nanotechnology can therefore be used to develop diagnostic tools as well as nano-enabled delivery systems that can bypass the BBB in order to facilitate conventional and novel neurotherapeutic interventions such as drug therapy, gene therapy, and tissue regeneration.17,18 Nanotechnology is currently being used to refine the discovery of biomarkers, molecular diagnostics, drug discovery, and drug delivery, which could be applicable to the management of AD and PD.
The blood–brain barrier
Transport mechanisms at the BBB can be manipulated for cerebral drug targeting. Studies of kinetic flux have revealed a unidirectional, concentration-dependent movement of compounds across the BBB.19 The direction of flow was reported to be from the plasma to the brain, or visa versa. Thus the net flux is the difference between the two unidirectional flow rates and is a significant determinant for drugs reaching therapeutic concentrations within the CNS.27 Small lipophilic molecules pass easily from blood capillaries. Charge-bearing large or hydrophilic molecules require gated channels, ATP, proteins, and/or receptors to facilitate passage across the BBB. Circumvention of the BBB can be achieved through the systemic administration or implantation of nano-enabled drug delivery systems (step 1, Fig. 1) that have the ability to control and target the release of various bioactive agents (step 5, Fig. 1) used in the treatment of NDs.21–24
Various potential nanostructures employed for the treatment of neurodegenerative disorders
The majority of nanotechnological drug delivery systems for the treatment of NDs are in the form of polymeric nanoparticles. Polymeric nanoparticles are promising for the treatment of AD and PD as they can pass through tight cell junctions, cross the BBB, achieve a high drug-loading capacity, and be targeted toward the mutagenic proteins in AD and PD. Promising features of these nanosystems in targeted CNS drug delivery are that (1) their chemical properties can be easily modified to achieve organ-, tissue-, or cell-specific and selective drug delivery, (2) the targeted delivery of drugs can be controlled, (3) they increase the bioavailability and efficacy of incorporated drugs by masking the physicochemical characteristics and thus increase the transfer of drug across the BBB, (4) they protect incorporated drugs against enzymatic degradation, and (5) they have fewer side effects.
Polymeric nanoparticles, nanocapsules, and nanospheres
Polymeric nanoparticles and nanocapsules range from 10–1000 nm.25 They possess high drug-loading capacities, are able to protect the incorporated drug load against degradation, thus increasing the chances of drug reaching the brain. They are stable and can target the delivery of drugs to the CNS due to their surface properties that can be manipulated in order to evade recognition by macrophages of the reticuloendothelial system.26 Polymeric nanoparticles have been used for the CNS delivery of several drugs, including doxorubicin.27–30 Nanospheres are dense polymeric matrices in which drug is dispersed and are prepared by micro-emulsion polymerization.31 Nanospheres are nanoparticle systems constituted by a solid core with a dense polymeric matrix, whereas nanocapsules are formed by a thin polymeric envelope surrounding an oil-filled cavity.23,32–34
Polymeric nanogels and nanosuspensions
Nanogels are networks of cross-linked polymers that often combine ionic and nonionic polymeric chains and are prepared using an emulsification solvent evaporation approach.35,36 Nanogels swell in water and are able to incorporate molecules such as oligonucleotides, siRNA, DNA, proteins, and low-molecular-mass drugs. The drug-loading capacity is up to 40–60%.37 Vinogradov and coworkers37 have encapsulated oligonucleotides within a cross-linked nanogel for delivery across the BBB. In vivo studies suggested that the nanogel increased brain uptake of oligonucleotides while decreasing uptake in the liver and spleen. Drug-loaded nanosuspensions are crystalline drug particles stabilized by nonionic surfactants or mixtures of lipids.38,39 Major advantages of nanosuspensions include their simplicity, high drug-loading capacity, and applicability to numerous drugs for CNS delivery.25,39
Carbon nanotubes and nanofibers
Carbon nanotubes are being explored to improve chronic CNS electrical stimulation.40 Clinically, functional electrical stimulation implants are gaining momentum for the treatment of PD. A significant challenge with the development of recording or stimulating chronic CNS electrodes is device failure associated with the fibrotic response mediated by glial and immune cells.41 The development of compressed carbon nanofiber–based electrode arrays for CNS neuronal stimulation could be injected at sites of degeneration to provide both a physical substrate and the molecular signals needed to stimulate and support tissue healing in treating NDs.42 The mechanism involved during carbon nanotube neuronal stimulation may be explicated in terms of an in vitro neuronal circuit model that is cultured on nanotube substrates to affect single and multiple synaptic pathway stimulation via the carbon nanotube layers and neuronal–nanotube electrical coupling and adhesion that may facilitate population firing that is strengthened by the appearance of a fast Na+ current, taken to constitute an early sign of axonal differentiation. These interactions may also sustain unconventional electrical coupling, thus unveiling new approaches to the basic understanding of the CNS electrophysiology.
Polymeric nanomicelles have a core–shell architecture with a hydrophobic core and a shell of hydrophilic polymer blocks. The core can incorporate up to 20–30% w/w of hydrophobic drugs, thus preventing premature drug release and degradation. The shell stabilizes the nanomicelles and masks the drug from interactions with serum proteins and untargeted cells. Once the target cells are reached drug is released by diffusion. Polymeric nanomicelles are versatile and have been shown to efficiently deliver DNA molecules in vitro and in vivo although no successful study on their delivery to the CNS has been reported thus far.43–45
Nanoliposomes are vesicular structures composed of uni- or multilamellar lipid bilayers surrounding internal aqueous compartments.46 Relatively large quantities of drug can be incorporated into liposome aqueous compartments or within the lipid bilayers. Extended systemic circulation times can be accomplished with nanoliposomes with modified surfaces that reduce opsonization in plasma and decrease its recognition and removal by the liver and spleen.46,47 Evaluation of nanoliposomes for targeted CNS drug delivery has been studied for various applications.48–51
Nanosystems explored for advanced experimental treatment of Alzheimer's disease
N-butylcyanoacrylate nanoparticles for clioquinol delivery in AD
The quinoline derivative clioquinol (CQ) is a Cu2+/Zn2+ chelator known to solubilize β-amyloid plaques in vitro and inhibit β-amyloid accumulation in AD induced transgenic mice in vivo.52 (Fig. 7). CQ-encapsulated poly(butylcyanoacrylate) (PBCA) nanoparticles have been prepared as a vector for the in vivo brain imaging of β-amyloid senile plaques. Cherney and coworkers52 showed that CQ-loaded PBCA nanoparticles crossed the BBB at a higher threshold than native CQ and may be a promising prototype for the treatment of AD. Roney and colleagues53 have also prepared nanoparticles by various polymerization approaches and performed in vivo biodistribution studies in order to search for an appropriate candidate for future in vivo imaging of β-amyloid plaques. CQ was radio-iodinated and incorporated within PBCA nanoparticles for the in vivo biodistribution studies in wild-type mice. The nanoparticles were polymerized as per the modified procedure of Kreuter and coworkers23 and delivered to the mice intravenously. The nanoparticles were shown to successfully transport CQ across the BBB making it ideal for in vivo imaging.
Poly(butyl)cyanoacrylate nanoparticles in AD
PBCA nanoparticles have been used to deliver drugs to the CNS with a good degree of success.30,33,35 These particles are characteristically 250 nm in diameter and are loaded with drug either by incorporating the drug during the initial particle polymerization process or via absorption onto the surface of the preformed particle. The particles are subsequently coated with polysorbate 80 (Tween 80®) as depicted in Figure 8 . Following intravenous administration the surface of the particle becomes further coated with adsorbed plasma proteins, most prominently apolipoprotein E (Apo-E). It is proposed that the final particle is mistaken for low-density lipoprotein (LDL) particles by the cerebral endothelium and is internalized by the LDL uptake system.29,34 Transactivating-transduction (TAT) peptide may also be attached to the surface of both liposomes54 and nanoparticles. This modification facilitates internalization by cells.55
Thioflavin-T nanocapsules for β-amyloid detection in AD
Thioflavin-T (ThT) has been previously described as a probe for the detection of β-amyloid in senile plaques of AD.56 Hartig and colleagues57 delivered the ThT-loaded nanocapsules comprising PBCA into the brains of mice by direct intrahippocampal injection, and followed the photoconversion of ThT from the nanocapsules in fixed tissues, post injection (Fig. 9). The nanocapsules were prepared by emulsion polymerization of styrene in a water–ethanol mixture containing ThT. The brains were fixed 3 days postinjection, and the nanocapsules were localized by photoconversion of the ThT in a closed chamber enriched with oxygen. Light microscopy localized the photoconverted nanocapsules in the dentate gyrus, and vacuoles were found in the cytoplasm near the aggregated latex nanocapsules. Transmission electron microscopy verified the presence of the nanocapsules in microglia and neurons. Furthermore, confocal microscopy demonstrated that ThT was delivered from the nanocapsules. As a result, the authors suggested that ThT nanocapsules may be used to probe the synthesis of β-amyloid, which is an extracellular cleavage product of the amyloid precursor protein (APP).58 They have not delivered the nanocapsules to the brain through the systemic circulation. However, the chemical similarity of the nanocapsules to PBCA, which has been shown to cross the BBB after intravenous administration, suggests that this approach has potential as a β-amyloid detection method for AD.32
d-penicillamine nanoparticles for the treatment of AD
The concentration of metal ions in the brain increases with age, and this imparts lethal effects on the AD brain.59 An increase in the concentration of copper ions (Cu2+) initiates oxidative stress that generates toxic hydroxyl radicals that disrupt DNA and modify proteins and lipids.60 It is known that amyloid plaques contain elevated levels of Cu2+ and Zn2+ compared to the healthy brain.24 These findings have formed the basis of new therapeutic approaches for AD, which involves iron-chelating compounds with limited neurotoxicity. Iron-chelating compounds have been recently incorporated in the form of nanoparticles to facilitate BBB penetration.61 In an in vitro study of the chelation therapy for the possible treatment of AD, Cui and coworkers24 conjugated the Cu (I) chelator d-penicillamine to nanoparticles to reverse the metal-induced precipitation of the β-amyloid protein. Nanoparticles were prepared from micro-emulsion precursors. The sodium salts of 1,2-dioleoyl-sn-glycero-3phosphoethanolamine-N-[4-(p-maleimidophenyl) butyramide] (MPB-PE) or 1,2 dioleoyl-sn-glycero-3-phosphoethanolamine-N-[3-(2-pyridyldithio)-propionate] (PDP-PE) were added and the sulfhydryl moiety of d-penicillamine was coupled to the MPB-PE or PDP-PE nanoparticles. Cui and coworkers24 have shown that the nanoparticles together with the partially released d-penicillamine resolubilized plaques under reducing conditions. At equimolar concentrations, the resolubilization of β-amyloid was 80% with EDTA and 40% with d-penicillamine, but at higher concentrations of d-penicillamine, resolubilization was just as effective and they concluded the significant role of nanoparticles in the in vivo investigation of AD through Cu2+ chelation.
Gold nanoparticles employed to destroy β-amyloid plaques in AD
A recently identified approach to the treatment of AD has the potential to destroy β-amyloid fibrils and plaques contributing to mental decline through a type of “molecular surgery” involving the use of gold nanoparticles to halt or slow the progression of AD without harming healthy brain cells. Attempts have been made to attach gold nanoparticles to a group of β-amyloid fibrils, incubating the resulting mixture for several days, and then exposing it to weak microwave fields for several hours. The energy levels of the fields were found to be six times smaller than those of conventional cell phones and thus unlikely to harm healthy cells.62 The fibrils subsequently dissolved and remained dissolved for at least 1 week after being irradiated. This indicated that the treatment was not only effective at breaking up the fibrils but also reduced the tendency of the proteins to reaggregate. A similar approach holds promise for treating other neurodegenerative diseases that involve protein aggregation, including PD. The approach is similar to an experimental technique that uses metallic nanoparticles to label and destroy cancer cells.40
Self-assembling biomolecular nanostructures in AD
Self-assembly of biological molecules is a basic principle in the formation of complex biological structures. Numerous proteins and peptides have been emerging as nano-biomaterials due to their ability to self-assemble into nanoscale structures such as nanotubes, nanovesicles, helical ribbons, and three-dimensional fibrous scaffolds.63 For instance Stupp and colleagues64 managed to have paralyzed lab mice with spinal cord injuries regain the ability to walk using their hind limbs 6 weeks after an injection of a nanomaterial designed for the purpose. By injecting molecules that were designed to self-assemble into nanostructures in the spinal tissue, they were able to rescue and regenerate damaged neurons.64 Nanofibers are pivotal for stimulating the body into regenerating lost or damaged cells. In addition, they play a role in preventing the formation of scar tissue, which inhibits spinal cord healing. When the nanofibers form they can be immobilized in an area of tissue where it is necessary to activate a biological process, for example regenerating differentiated cells from stem cells. This has significant implications for the treatment of AD and PD, in which key brain cells stop functioning.63,64
Glycerol nucleic acid: A nano-based DNA chemical analogue in AD
DNA has being utilized to produce nano-sized elements for potential application in the treatment of AD. DNA is an ideal building block for nanotechnology due to its ability to self-assemble and bind into various shapes based on the natural chemical rules of attraction.65–71 Chaput72 has produced a self-assembled nanostructure comprising glycerol nucleic acid (GNA), a synthetic analog of DNA (Fig. 10). The only chemical difference between DNA and GNA is the polysaccharide molecule. GNA uses the 3-carbon polysaccharide glycerol rather than the 5-carbon deoxyribose used in DNA. The polysaccharide provides the chemical backbone for nucleic acid polymers, anchoring a phosphate molecule and nitrogenous base (Fig. 10). GNA nanostructures possess additional properties not found in DNA, including the ability to have anti-amyloid activity and form mirror image structures.67,71,72 GNA is an oligonucleotide. GNA can therefore be used to inhibit undesirable gene expression or to synthesize therapeutic proteins that can play a vital role in the in vivo gene delivery of AD treatments.
Inhibition of β-amyloid plaque formation using nanogels in AD
The formation of fibrils by β-amyloid is considered a key step in the pathology of AD. Inhibiting the aggregation of β-amyloid is a promising approach for AD therapy. Biocompatible nanogels have been developed comprised of polysaccharide pullulan backbones with hydrophobic cholesterol moieties (cholesterol-bearing pullulan, CHP) as artificial chaperones to inhibit the formation of β-amyloid (1–42) fibrils with marked amyloidgenic activity and cytotoxicity. The CHP-nanogels are able to incorporate up to 6–8 β-amyloid (1–42) molecules per particle and induce a change in the conformation of β-amyloid from a random coil to an alpha-helix- or β-sheet-rich structure. The structure is stable over 24 h (at 37°C), and the aggregation of β-amyloid (1–42) can be suppressed. Furthermore, the dissociation of the nanogel caused by the addition of methyl-β-cyclodextrin released monomeric β-amyloid molecules. Nanogels composed of amino-group-modified CHP (CHPNH2) with positive charges under physiological conditions have a greater inhibitory effect than CHP nanogels, suggesting the importance of electrostatic interactions between CHPNH2 and β-amyloid for inhibiting the formation of fibrils. In addition, CHPNH2 nanogels protected PC12 cells from β-amyloid toxicity. Ex vivo studies showed that biocompatible nanogels 20–30 nm in diameter can prevent aggregation of proteins associated with AD and inhibit amyloid fibers from forming.
Nanosystems explored for advanced experimental treatment of Parkinson's disease
Brain-targeted delivery of dopamine using nanosystems in PD
Various approaches of delivering dopamine (DA) to the brain with particular focus on the use of redox-based delivery systems for the targeted delivery and localized release of DA in the brain have been performed.73 Results have shown that DA can be successfully delivered into the brain, accompanied by localized release and metabolism, which allows the execution of appropriate pharmacological responses. These results open the possibility of treating a variety of NDs, since normally the BBB restricts the entry of polar compounds such as DA into the brain parenchyma.73
Convection-enhanced drug delivery in PD
The intravascular administration of neuroactives has been confounded by the BBB, which prohibits the entry of molecules based on size, lipid solubility, and ionic charge, thus limiting the entry of charged small molecules or larger compounds such as proteins, genes, or viral particles.74 Osmotic disruption of the BBB has been used clinically to temporarily infiltrate the BBB and allow a greater quantity and variety of intravascular neuroactives to access the brain parenchyma, including monoclonal antibodies and radio-immunoconjugates.75,76 Unfortunately, despite selective opening of unilateral or bilateral vertebrobasilar or carotid cerebrovascular distributions via the arterial approach, controlling the site of treatment through this technique remains complicated and limited to a major vascular distribution.74 In addition, repeated treatment adds to patient discomfort and potential morbidity, ultimately narrowing the clinical use of osmotic BBB disruption therapy. However, convection-enhanced delivery (CED) technology is able to deliver neuroactives at a larger and more consistent treatment volume than standard diffusion-based technologies.74 CED employs bulk flow of the neuroactives through the extracellular space of the tissue.77 Bulk flow distributes neuroactives homogeneously within a controlled brain volume, regardless of molecular size, with a steep concentration drop at the advancing margin of the bulk flow. This allows for the convection of viral particles (for gene therapy) and large macromolecules such as growth factors into the brain. The volume in which a neuroactive (such as a drug, virus, gene, or growth factor) distributes in the brain with CED is primarily a function of the infusion rate and specific tissue characteristics. CED has been modeled in animals and has recently been evaluated in human brain tumor trials.78–80 CED of neuroactives within the brain is becoming a more frequent experimental treatment option in the management of brain tumors, and more recently in Phase I trials for gene therapy in PD.74
Nonviral vectors for the safe and efficient delivery of genes in PD
Gene therapy for PD may become clinically relevant upon the development of viral and nonviral gene-transfer vectors. Viral vectors are able to deliver a gene to the nucleus of a cell and have it expressed through its integration into the genome or as an episomal vector.81 A critical concern for any form of gene therapy is the safety of the vector, as there is a risk of excessive immune response as well as insertional mutagenesis when viruses are used as transfection vectors. Actual death has occurred in human trials, leading to a halt in further use of viral vectors for gene transfection. In addition, the approach of using viral vectors suffers from inherent challenges in the pharmaceutical processing, scale-up, immunogenicity, and reversion of an engineered virus.82 A focus in nanotechnology is the development and use of nonviral vectors for the safe and efficient delivery of genes.83 The potential for the treatment of PD has advanced based on the ability to identify specific defective or absent genes responsible for PD. Specifically designed therapeutic genes, if successfully delivered into the appropriate cells, may provide a significant advancement in the therapy of PD.83In vivo gene delivery involves the use of genetic materials such as DNA, RNA, and oligonucleotides that are able to inhibit undesirable gene expression or synthesize therapeutic proteins.81,82 However, for effective gene therapy, a genetic payload must be delivered to the targeted cell or tissue and thereafter be transported to the nucleus of the cell to achieve expression.83 Amino-functionalized organically modified silica (ORMOSIL) nanoparticles as a nonviral vector have been shown to bind and protect plasmid DNA from enzymatic digestion and to effect cell transfection in vitro.84 ORMOSIL nanoparticles are able to overcome the limitations of “unmodified” silica nanoparticle. The presence of both hydrophobic and hydrophilic groups on the precursor alkoxy organosilane assists self-assembly of both normal and reverse micelles under appropriate conditions. ORMOSIL nanoparticles are prepared from oil-in-water micro-emulsions that avoid corrosive solvents and follow a complex purification process (Fig. 3). Organic groups can be further modified for the attachment of biodegradable targeting molecules that can also impart a degree of flexibility to the rigid silica matrix, enhancing the stability of the particles in aqueous systems.85
Nanoparticle-based gene therapy for PD
Yurek and coworkers86 have assessed the feasibility of employing novel technology to condense DNA plasmids into nanoparticles for delivery to the brain in order to halt or prevent neurodegeneration in an animal model of PD (Fig. 12). They explored a relatively new gene-therapy approach for the treatment of PD and established that the strategy has the potential to repair defective genes. Yurek and coworkers used transduction, a technique for expressing a particular gene in a cell by delivering DNA into the cell and making the cell synthesize the protein that corresponds to that DNA. Furthermore, by capitalizing on the fact that neurotrophic factors are neuroprotective, it is possible to utilize neurotrophic factors to revive dormant brain cells and assist them to produce DA, thereby prompting a dramatic improvement of symptoms in animal models.86 A study conducted by Kaplitt and During87 in advanced PD patients showed that there was lack of side effects related to gene therapy. In addition, there were statistically significant improvements from baseline in both clinical symptoms and abnormal brain metabolism as measured by tomography.87 Their study represents not only an encouraging first step in the development of a promising new approached to PD therapy, but also provides a platform to translate a variety of new gene-therapy agents into human clinical trials for numerous NDs.
Nanofibers as stem cell therapy in PD
Polymer-based biodegradable nanofibers have been engineered to prepare a scaffold that could potentially allow stem cells to repair damaged nerves rapidly and effectively.88 This was achieved by utilizing a combined process of electrospinning and chemical treatment to customize the nanofiber structure into a scaffold that can then be located within the body.88 The scaffold is injected into the body at the site that requires nerve regeneration. The stem cells can be imbedded into the scaffold externally or once the scaffold is implanted. The nerve cells adhere to the scaffold, which forms a bridge in the brain or spinal cord. As time progresses, the scaffold erodes and is naturally eliminated from the body, leaving the newly regenerated nerves intact.88 This approach may lead to a cure for PD. In another endeavor using stem cells for treating PD, Lindvall and Hagell89 revealed the genes that initiate and control the DA-producing nerve cells within the brain. They managed to develop embryonic stem cells into DA-producing nerve cells in chicken and mouse models (Fig. 13). However, a significant challenge was that they could not succeed in producing pure samples of the DA-producing cells as they also produced 10–20% of unwanted stem cells.
Nanorobots as stem cell therapy in PD
The potential of stem cell therapy for NDs has been demonstrated on implantation of different types of stem cells in animal models of PD.90 Transplantation of stem cells into the rat brain has demonstrated the reinnervation of striatal neurons and the partial recovery of motor deficit associated with DA deficiency.91 Similar results have been obtained after transplantation of fetal dopaminergic neurons in clinical trials. Thus, it is possible to employ various types of stem cells to generate dopaminergic neurons. Currently, the process of dopaminergic neuron differentiation from embryonic stem cells in vitro is most effective.92 Recent progress in human therapeutic cloning allows this approach of generating neurons to be more attractive.93 Approaches for therapy may include in vitro processing of stem cells before implantation, supporting and guiding the cells after implantation with the help of nanorobots, and the in vivo creation of molecular scaffolds for stimulating their growth in the correct direction.94 Nanorobots are controllable biomachines that algorithmically respond to stimuli and are capable of actuation, sensing, signaling, information processing, intelligence, and swarm behavior in order to interact and influence cells at the molecular level. Nanorobot actuators are also designed to be biocompatible and to have sufficient dimensions to transport and guide cells and other biomolecules. The potential for neural stem cells to establish appropriate long-distance axonal projection after region-specific differentiation has been demonstrated.95 Unfortunately, the adult brain when compared to the neonatal brain has unfavorable conditions for axon growth in the sense that growth does not occur in the correct direction. Thus the stimulation of new neuron growth along the surface of neurons in the zone of progressive degeneration is necessary.94 This therapeutic strategy may be possible after the development of technology for the controlled growth of neurons along the surface of target (dysfunctional) neurons with the help of nanorobots. It is evident that using dysfunctional neurons as a “niche for growth” is one way of ensuring accurate and safe regeneration of neuronal circuits. With gradual replacement of the dysfunctional or apoptotic neurons, new neurons will be integrated into the existing cellular structure and therefore be involved in the intended processes without any obvious mental degeneration.94,95
Carbon nanotube and nanowire biosensors in PD
Through the use of nanotechnology, wireless implantable biosensors that may help in treating patients with PD have been developed.96 The biosensors comprise carbon nanotubes and nanowires that are hollow, light weight, and chemically inert and have superior mechanical strength (Fig. 15). They are grown and organized into arrays that are combined to make nanochips similar to those found in electronic devices. Carbon nanochips are biocompatible and are not rejected by the human body as a foreign object. Once inside the body, nanochips perform several functions, such as sensing and monitoring the release of DA produced by the brain. The carbon nanotube–based biosensor that was developed by Li and colleagues96 records the loss of DA and stimulates activity between neurons and neurites (immature developing neurons). Thus, they generally monitor and control DA levels in the brain. In addition to sensing the release of DA and contributing to the growth of healthy DA-producing neurons, the biosensor also communicates with an organic, polymer-based sensor attached to an area of the body in which a tremor occurs. The signal from the implanted sensor can control and direct the motion of the area of the body on which the exterior sensor is attached.96 This exterior sensor can be easily placed under a wristwatch. Essentially, the implanted carbon nanotube–based sensor detects the sensor attached to the watch, controls the trembling, and directs the hand or prosthetic limb movement.96
Deep brain stimulation in PD
Deep brain stimulation (DBS) has been shown to be effective for PD, though with a few limitations.97 The limitations include the large size of current microelectrodes (∼1 mm diameter), lack of monitoring of local brain electrical activity and DA levels, and the open-loop nature of the stimulation.97 It has been demonstrated that reducing the size of the monitoring and stimulating electrodes to the nanoscale allows a remarkable improvement in both the monitoring (spatial resolution, temporal resolution, and sensitivity) and the stimulation.97 Carbon nanofiber nano-electrode technology (Fig. 16) offers the possibility of trimodal arrays for monitoring electrical activity, monitoring DA levels, and precise stimulation.98 DBS can then be guided by changes in brain electrical activity and/or DA (i.e., closed-loop DBS).99 Thus, there is a need for thoroughly understanding the basic manufacturing techniques of prototype nano-electrodes used in DBS, their electrical characteristics, and the electro-conductive polymers that can be used to optimize DBS in vivo. Such an approach may offer a generic electrical-neural interface for use in various NDs such as PD and AD.
Surgical intervention in the treatment of PD
Femtosecond laser systems, nanoneedles, and nanotweezers are currently emerging nanotechnologies that have the potential to revolutionize the practice of neurosurgery in PD.100 Surgery for PD was popularized in the mid-20th century before the advent of effective medical therapies.100 Early lesioning treatments contributed to the understanding of the functional anatomy of PD. Observations of the limitations and long-term complications of established pharmacological therapies for PD, together with major contributions from animal research to elucidate the roles of the basal ganglia in movement disorders, inspired a recent renaissance in neurosurgical interventions for PD, including DBS.100 The development of potentially restorative treatment modalities, such as gene therapy, neural transplantation, and nanotechnology, hold much promise for surgery, both therapeutically and in revealing further insights into the pathophysiology of PD.
Nanotechnology has proven to have great potential for providing neurotherapeutic modalities to limit and reverse the neuropathology of AD and PD by supporting and promoting functional regeneration of damaged neurons, providing neuroprotection, and facilitating the delivery of neuroactives such as drugs, genes, and cells across the BBB. It may contribute significantly toward the development of nano-enabled drug delivery systems for the treatment of NDs, taking advantage of the nanoscale structures of neural cells. Several novel approaches, inspired by recent advances in nanotechnology are already applicable to the treatment of AD and PD. Nanoscale classes of neuroactives will widen the scope of therapeutic action beyond merely modifying transmitter function to include stem cell and gene therapies that could offer a more selective mode of targeting. However, in order for nanotechnology applications directed toward NDs to be fully exploited, it would be important for neurosurgeons, neurologists, and neuroscientists to participate and contribute to the scientific process along with pharmaceutical scientists and biomedical engineers to develop technological advancements in conjunction with advancements in basic and clinical neuroscience.