Crossing the blood–brain-barrier with viral vectors
Introduction
The central nervous system (CNS) is a critical target for gene therapy for the treatment of genetic, developmental, and acquired diseases. While some conditions, such as cancer, Parkinson's disease, stroke, or epileptic seizure may benefit from highly localized gene transfer, others, primarily genetic neurodegenerative diseases, require global gene delivery. New approaches are being developed to address both of these gene therapy modalities. The extraordinarily abundant vasculature of the brain lends itself to a systemic delivery approach to access the CNS, in that no neuron is more than 8 μm from a capillary. Standing in the way of this entry route is the blood–brain-barrier (BBB), an assembly of brain vascular endothelial cells, astrocytic endfeet, and pericytes, forming tight junctions that prevent most molecules greater than 400 Da from entering the CNS via the blood circulation [1]. While the BBB in mice is somewhat permeable in neonates, it is fully formed at birth in humans, and for clinical relevance, this review will therefore focus on gene delivery in adult animal models.
Section snippets
Paracellular transport by disruption of tight junctions
There are two very general approaches to transporting a viral vector across the BBB. The first is receptor-mediated transport across brain vascular endothelial cells by transcytosis; the second is transient disruption of the BBB, allowing the vector to enter by paracellular transport into CNS interstitial spaces (Figure 1). This has conventionally been accomplished by osomotically shrinking the cells that make up the BBB, using intravenous (IV) administration of highly concentrated mannitol
Localized disruption of the BBB
A more concentrated and localized vector transduction pattern can be achieved by transient disruption of the BBB using magnetic resonance-guided focused ultrasound and IV administered microbubbles, followed by IV administration of AAV vector [11•, 12, 13]. This method of BBB permeabilization has been developed for the treatment of a wide range of disorders, and relies on the mechanical action of the lipid/gas microbubbles, in response to ultrasonic pressure waves, to disrupt the brain
Receptor-mediated transport via transcytosis
While paracellular transport through a compromised BBB has achieved some significant successes in animal models, the alternative strategy of exploiting receptor-mediated transcytosis to cross the BBB has shown a great deal more promise recently. These approaches have fallen into three general categories: using a viral vector that naturally crosses the BBB (archetypally, AAV9); modifying vectors to attach to a specific transcellular transport receptor; or, using directed evolution to create a
Re-targeting adenovirus vectors
Adenovirus (Ad) vectors have been modified to bind specifically to a wide range of cellular receptors, either using conjugated adaptor molecules or peptide fusions into the fiber, penton, or hexon proteins [18]. One such modification, directed specifically at transport across the BBB, was targeted to the LRP-1 receptor. In this case, the extracellular domain of the coxackie-adneovirus receptor (CAR), which binds to the Ad fiber, was fused to melanotransferrin, which is transported by LRP-1 [19
AAV serotype 9 for gene delivery across the BBB
Although engineering approaches offer the possibility of highly specialized vectors for CNS delivery in the future, the greatest progress in recent therapeutic applications has relied on the natural ability of AAV9 to cross the intact BBB through transcytosis. This unique feature has been demonstrated in adult mice and cats, and reproduced in non-human primates, though studies suggest that overall efficiency and neural cell targeting may be reduced in primates [21, 22, 23, 24, 25]. The
Directed evolution of AAV9 capsid to improve BBB transcytosis
The natural ability of AAV9 and related vectors to cross the BBB has made possible a great number of new therapeutic applications, which are beginning to be tested in clinical trials. However, further gains in efficiency or specificity are likely to be made through engineering or directed evolution. The versatility of the AAV capsid platform regarding interactions with the BBB was clearly demonstrated though modification of AAV2, which normally does not cross the BBB, or transduce cells of the
Conclusion
The field of gene therapy targeting the CNS through the BBB is still in its very early stages, and is likely to undergo rapid development within the next few years. If early results are any indication, this will be a very promising approach to gene therapy.
References and recommended reading
Papers of particular interest, published within the period of review, have been highlighted as:
• of special interest
•• of outstanding interest
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