with antisense oligonucleotides directed at the mutated site of APP exhibit reduced βamyloid plaque deposition.106 Recently, Modarresi et al. showed that the upregulation of BDNF (brain-derived neurotrophic factor) can be achieved in vivo by targeting BDNF-natural antisense transcript (BDNF-AS) with modified oligonucleotides called antagoNATs.98 BDNF-AS represses the transcription of BDNF by altering the chromatin landscape at the gene locus, resulting in decreased BDNF pro- tein levels. Oligonucleotides targeting BDNF-AS were admin- istered to mice through intracerebroventricular delivery, using osmotic mini-pumps. After 28 days of continuous infusion of the AntagoNAT, BDNF mRNA levels were increased across fore- brain regions as compared with control oligonucleotide treated mice. Importantly, the neurotrophin BDNF is a growth factor that enhances the survival, development, differentiation and function of neurons.112-114 Several neurodegenerative, neurode- velopmental and psychiatric disorders115-117 show impaired levels of neurotrophins, suggesting that the upregulation of BDNF can have beneficial effects on several disorders. It has long been known that BDNF levels are decreased in AD brain118 and it also seems that the ratio of BDNF sense to BDNF antisense tran- script concentration is markedly dysregulated in this disease.98 In vitro and in vivo models of neurodegenerative disease show that BDNF prevents neuronal degeneration.115,119 Thus, its upregula- tion mediated by AntagoNATs might represent a potential neu- roprotective agent useful to prevent neurodegeneration. Intracranial delivery of oligonucleotides in an outpatient set- ting has been tested in a phase II clinical trial for human glio- blastoma. Trabedersen®, an antisense oligonucleotide targeting TGF-2β, was infused intratumorally using convection-enhanced delivery (CED) via a subcutaneous port access system connected to an external pump to allow for outpatient treatment.96 The treatment was safe and showed significant benefits compared with standard chemotherapy. The advantages of ventricular infu- sion with an implanted pump includes accurate continuous accu- rate delivery120 and trapping of oligonucleotides inside the brain by the BBB.121 Intranasal delivery is a promising alternative to standard deliv- ery systems as it provides a practical, painless method for deliver- ing therapeutic agents to the brain (Fig. 3). Rats bearing brain tumors were treated intranasally with an oligonucleotide against Telomerase (TERT).122 The intranasally delivered treatment bypassed the blood-brain barrier, reached the tumor and inhib- ited tumor growth, leading to a prolonged lifespan in treated rats. This delivery approach appears to selectively kill tumor cells as toxic effects were not observed in normal brain tissues. These data support further research into the intranasal delivery of tumor-specific therapeutic agents for brain cancer patients. Finally, while intrathecal administration (IT) of drugs is well established in anesthesia and pain management, recent data sug- gest that oligonucleotides introduced IT can be distributed to the brain through CSF suggesting an additional route of administra- tion to treat CNS diseases. As discussed previously in this review, the onset of many dis- eases affecting the CNS occurs during development and treatment thus would ideally have to begin in the very early stages of the disease. Neverthess, Michalon et al. for example, have published encouraging results showing that the early and chronic inhibition of the glutamate receptor mGlu5 in young FXS mouse model can “correct” a broad range of complex behavioral and neuro- anatomical defects associated to that disease. Importantly, the treatment was started in these mice at an age of 4–5 weeks, when the brain development is anatomically complete but highly plas- tic and when all behavioral and anatomical FXS phenotypes are evident.123 These results may demonstrate the feasibility of treat- ments for developmental disorders, but there are a few important considerations in the design of therapies for these conditions. First, as in Michalon et al. study, treatment must be initiated early in the course of disease, which would generally imply very young pediatric patients. In addition, as most neurodevelopmen- tal disorders have a strong genetic component, they can rarely be completely cured and patients would therefore require long-term treatment. Because of this, therapies and delivery strategies such as those discussed in this article should be considered for poten- tial use over the course of many years. Biological carriers for RNA-targeting therapies. Viruses are nat- ural vehicles for the efficient transfer of genes into host cells. This characteristic makes viruses an attractive system for the delivery of therapeutic nucleic acids. Viral vectors that have recently been used in the laboratory and in clinical practice include adenovi- ruses, retroviruses and lentiviruses. Non-viral vectors have also been popular tools for gene delivery, because they are relatively safe and can be modified through the incorporation of ligands to target specific cell types.124 This method of delivery includes three major vectors: polymer-, lipid- and peptide-based systems. However, the levels of gene expression mediated by these vectors are low as compared with viral vectors. A new twist on conven- tional viral-delivery systems is the virosome. Using virosomes, vesicles can be made from reconstituted virion-like phospholipid bilayers and unlike conventional liposomes, they are able to carry various drugs including nucleic acids.125 Despite their low toxic- ity, in vivo application of this technology is sometimes limited by the risk of immunogenicity; to overcome this obstacle virosomes have been modified with polyethylene glycol (PEG). Carrier-based strategies are currently being developed using organisms such as bacteria that can naturally evade the host immune system to enter the target cell.126 Non-pathogenic bacte- ria such as Lactococcus lactis or Streptococcus gordonii could serve as vehicles for the production and delivery of biologically active proteins such as cytokines, enzymes or even siRNAs. To date, recombinant Salmonella strains have been engineered to deliver siRNAs to tumors to suppress tumor growth in vivo.127,128 In fact, Gram-negative bacteria such as Salmonella can colonize and grow within aerobic and anaerobic areas of tumors. The poten- tial to selectively colonize hypoxic areas of tumors that cannot be treated by chemotherapy presents an exciting opportunity for cancer therapeutics. Another strategy involves the use of the envelope from Gram- negative bacteria that is lysed with protein E,129 to produce a “bac- terial ghost”. Despite the absence of the cytoplasmic contents, the 866 rNA Biology volume 9 issue 6