Nonviral approaches for neuronal delivery of nucleic acids

Abstract

The delivery of therapeutic nucleic acids to neurons has the potential to treat neurological disease and spinal cord injury. While select viral vectors have shown promise as gene carriers to neurons, their potential as therapeutic agents is limited by their toxicity and immunogenicity, their broad tropism, and the cost of large-scale formulation. Nonviral vectors are an attractive alternative in that they offer improved safety profiles compared to viruses, are less expensive to produce, and can be targeted to specific neuronal subpopulations. However, most nonviral vectors suffer from significantly lower transfection efficiencies than neurotropic viruses, severely limiting their utility in neuron-targeted delivery applications. To realize the potential of nonviral delivery technology in neurons, vectors must be designed to overcome a series of extra- and intracellular barriers. In this article, we describe the challenges preventing successful nonviral delivery of nucleic acids to neurons and review strategies aimed at overcoming these challenges.

Fig. 1

Structures of common polymers and lipids used as nonviral gene vectors. Cationic polymers include poly (l-lysine) (PLL) and poly(ethylenimine) (PEI). 1,2-dioleoyl-3-trimethylammonium propane (DOTAP) is a cationic lipid and dioleoylphosphatidylethanolamine (DOPE) is a neutral “helper” lipid often included in cationic lipid formulations.

Fig. 2

A series of intracellular barriers is encountered by nonviral gene carriers in neurons. (A) Successful vehicles must be able to (1) associate with the neuronal plasma membrane, (2) undergo internalization, (3a) escape endosomes, and (4) deliver nucleic acids to the nucleus (a) or cytoplasm (b). (B) If vehicles arrive at the axon terminus, they must additionally undergo retrograde axonal transport (3b).

Fig. 3

The internalization of nonviral vectors can be accomplished specifically via receptor targeting (a) or nonspecifically through electrostatic attractions between positively charged gene carriers and negatively charged groups on the neuronal plasma membrane (b).

Fig. 4

Vehicles delivered to the distal ends of neurites must undergo active retrograde transport either inside of vesicles (a) or through direct interaction with microtubule-based motor proteins (b).

Fig. 5

Concept of a dynein-binding peptide. Synthetic gene carriers can be modified with dynein-binding peptides to mediate attachment to the dynein motor complex and active retrograde transport toward the neuronal nucleus.

Fig. 6

Representative administration routes for neuronal gene delivery in vivo: intraparenchymal injection (a), intrathecal injection (b), intramuscular injection (c).