siRNA Therapy for Glomerulonephritis
Authors: Jordan A. Kreidberg
Journal: Journal of the American Society of Nephrology (2010)

We are well into a new era in pharmacology in which small molecules, often kinase inhibitors, are used to target signal transduction pathways in cancer and other diseases. In addition to small molecules, antibody-based treatments have become a major part of the pharmacologic armamentarium. RNA interference (RNAi)-based treatments promise to be a third pillar in this new frontier. Indeed, several biotech firms have been established in the past decade to develop RNAi-based therapies. In this issue of JASN, Shimizu et al.1 bring RNAi-based therapies to bear on kidney disease by demonstrating that small interfering RNAs (siRNAs) to a mitogen-activated protein kinase (MAPK) prevent glomerular disease in a murine model of lupus nephritis. siRNAs are short, double-stranded RNAi molecules that contain complementary sequences to specific mRNAs encoding proteins. By interacting with mRNAs through these complementary sequences, they act similarly to naturally occurring microRNAs to cause degradation of the mRNA or to inhibit translation.2 Interruption of microRNA processing causes dramatic glomerulotubular injury.3–6 mRNA degradation is effected by argonaute catalytic proteins that are part of the RNA-induced silencing complex–expressing RNase activity.5,7 siRNAs have been used for several years in the laboratory setting, primarily in tissue culture experiments to knockdown the expression of desired genes. They can effectively reveal physiologic processes8–10 or modulate proinflammatory events in renal cells.11–14 Similar RNAi reagents known as short hairpin RNAs (shRNAs) have been also used in tissue culture or transgenic mice. The distinction between shRNAs and siRNAs is that siRNAs come as commercially available, ready-to-use, double-stranded RNA reagents, whereas shRNAs are typically expressed from a transfected plasmid or transgene and require intracellular processing to remove the hairpin, such that they then resemble siRNAs.15 Given the great success in using siRNA and shRNA in the laboratory setting, it is no surprise there are many efforts under way to use siRNAs as novel therapies for disease.15–19 See the July 25, 2009, issue of Advanced Drug Delivery Reviews for several recent reviews of this area of research. There are several hurdles to overcome in bringing RNAi-based therapies to the clinic. First, there is the issue of delivery to a specific location. This is best approached by localized delivery of siRNA molecules or DNA encoding shRNA. A second hurdle is the delivery system itself. Several delivery systems have been used to package and release siRNAs, including lipid-based systems, polycation particles, and others.16,18,19 For shRNAs, viral vectors and plasmid-based systems are under study.20 Finally, there is the challenge of identifying a target mRNA whose decreased abundance is likely to have a therapeutic effect while not otherwise inducing unwanted off-target effects that render the treatment unsafe or undesirable. Shimizu et al.1 approach this problem using siRNAs complexed with polyethylene glycol-poly(L-lysine)-polymers to form nanocarriers in which formation of the complex relies on electrostatic interactions between the positively charged poly(L-lysine) and negatively charged siRNA. First, they establish in vitro validation using cultured mesangial cells to knockdown expression of MAPK1, a member of the MAPK family also known as Erk2.21 The authors also perform essential validation of the localization of these particles to the kidney after intraperitoneal injection along with a demonstration of decreased expression of MAPK1 protein in glomeruli. They claim localization of the particles is in the mesangium, although the stainings shown seem to be broader than typically seen for mesangial cells and may include some endothelial cells as well. The authors then go on to show that suppression of MAPK1 expression decreases glomerular sclerosis in the model of lupus nephritis.22 TGF-β is a widely known mediator that increases expression of connective tissue genes during development and in inflammatory disease23 through NF-κB activation.24 TGF-β expression was remarkably decreased after the knockdown of MAPK1.1 Thus, the results from Shimizu et al.1 represent an important step in bringing RNAi therapy to the treatment of kidney disease. Many challenges, of course, remain. MAPKs act in a plethora of physiologic and pathophysiologic processes,21 and suppressing their function in humans over the long term may invite vast off-target effects, unless it is possible to lower specifically hyperexpression at the site of pathology while maintaining relatively normal levels of expression in other, unaffected tissues. It also will probably be necessary to use alternative delivery routes in humans that still allow targeting to the kidney. Even more desirable will be approaches thatlocalize targeting of RNAi molecules to the kidney or even to glomeruli or specific cell types therein. Nevertheless, the work of Shimizu et al.1 represents an important step forward in the use of RNAi-based therapies for kidney disease. Disclosures None.