The obstacle course to the inner retina: Hyaluronic acid-coated lipoplexes cross the vitreous but fail to overcome the inner limiting membrane

Considerable research over the last few years has revealed dysregulation of growth factors in various retinal diseases, such as glaucoma, diabetic retinopathy and photoreceptor degenerations. The use of messengerRNA (mRNA) to transiently overexpress a specific factor could compensate for this imbalance. However, a critical challenge of this approach lies in the ability to efficiently deliver mRNA molecules to the retinal target cells. In this study we found that intravitreal (IVT) injection is an attractive approach to deliver mRNA to the retina, providing two critical barriers can be overcome: the vitreous and the inner limiting membrane (ILM). We demonstrated that the vitreous is indeed a major hurdle in the delivery of the cationic mRNA-complexes to retinal cells, both in terms of vitreal mobility and cellular uptake. To improve their intravitreal mobility and avoid unwanted extracellular interactions, we evaluated the use of hyaluronic acid (HA) as an electrostatic coating strategy. This HA-coating provided the complexes with a negative surface charge, markedly enhancing their mobility in the vitreous humor, without reducing their cellular internalization and transfection efficiency. However, although this coating strategy allows the mRNA-complexes to successfully overcome the vitreal barrier, the majority of the particles accumulated at the ILM. This study therefore underscores the crucial barrier function of the ILM toward non-viral retinal gene delivery and the need to smartly design mRNA-carriers able to surmount the vitreous as well as the ILM.