Neuroinflammation is a defining feature of various neurodegenerative disorders, including Alzheimer’s disease (AD). Microglia, the primary immune cells in the brain, are at the heart of this phenomenon. These cells are not mere bystanders; they actively shape the progression of AD. Genome-wide association studies have linked numerous AD-risk genes to microglia, underlining their pivotal role in AD’s etiology. One such gene is the transcription factor PU.1, crucial for microglial function. Elevated levels of PU.1 are associated with an increased risk of AD, while its reduction appears to be protective.
The advent of RNA therapeutics, including short interfering RNA (siRNA), offers a promising avenue to modulate gene expression in microglia. However, the delivery of these therapeutic molecules into microglia presents significant challenges due to their large size, charge, and the presence of the blood-brain barrier (BBB). Recent strides in nanotechnology have led to the development of lipid nanoparticles (LNP) as effective RNA delivery vehicles. These LNPs can encapsulate RNA molecules, protecting them from degradation and facilitating their entry into cells. The study under discussion has meticulously examined several LNP formulations, ultimately identifying one – the MG-LNP – that efficiently delivers RNA to microglia-like cells derived from human stem cells, both in vitro and in animal models. The MG-LNP formulation stands out due to its minimal toxicity and heightened efficiency in delivering RNA to activated microglia. This selectivity is particularly relevant given the activated state of microglia in AD. Intraperitoneal and intracisternal injections of MG-LNP in animal models reveal a broad distribution, with a preference for the brain when administered directly into the cerebrospinal fluid.
In a new study published in Advanced Materials Journal by Dr. William Ralvenius, Dr. Jason Andresen, Dr. Margaret Huston, Dr. Jay Penney, Dr. Julia Maeve Bonner, Professor Owen Fenton, Professor Robert Langer, and Professor Li-Huei Tsai from Massachusetts Institute of Technology and University of North Carolina, the authors validated the role of PU.1 as a therapeutic target in AD. Using LNP-mediated siRNA delivery to knock down PU.1, a notable reduction in neuroinflammation was observed in mouse models. This effect was evident both in models of systemic inflammation induced by lipopolysaccharide (LPS) and in the CK-p25 mouse model that mimics chronic neuroinflammation seen in AD. The findings underscore the potential of targeting microglial genes, like PU.1, in AD treatment. The ability of MG-LNP to selectively deliver siRNA to microglia opens new doors in neuroinflammation-directed gene therapy, offering hope for more effective AD treatments.
The researchers conducted a series of detailed experiments to explore the efficacy of LNP-mediated RNA delivery in targeting microglia, specifically focusing on the PU.1 gene, which is associated with AD. To assess the effectiveness and cytotoxicity of various commercial transfection reagents in delivering siRNA to human induced pluripotent stem cell (iPSC)-derived microglia-like cells (iMGLs). They tested eight different transfection reagents. They used iMGLs as the in vitro model due to their relevance in studying AD-associated inflammation. They also measured cell viability and PU.1 mRNA levels post-treatment. To identify an effective LNP formulation for RNA delivery to iMGLs without causing cellular toxicity, they experimented with seven LNP formulations, varying the structure of ionizable lipid, phospholipid types, and lipid-to-RNA ratios. Firefly Luciferase mRNA was used to assess delivery efficacy. They measured luminescence as an indicator of successful mRNA delivery.
To test if MG-LNP formulation could efficiently deliver RNA in an inflammatory environment, they pretreated iMGLs with LPS to induce inflammation. They assessed the delivery efficiency of MG-LNP in both normal and LPS-treated iMGLs. To evaluate the in vivo delivery efficacy of LNPs, they administered LNPs intraperitoneally (i.p.) and intracisternally (i.c.st.) in mice, they analyzed luminescence in the liver and brain post-injection to assess RNA distribution. To examine the effectiveness of MG-LNP in delivering anti-PU.1 siRNA to iMGLs, they delivered anti-PU.1 siRNA using MG-LNP to iMGLs and measured the reduction in PU.1 mRNA levels. They assessed changes in gene expression of various PU.1 target genes. To test the effect of MG-LNP-delivered anti-PU.1 siRNA in mouse models of neuroinflammation, they used two mouse models: an LPS-induced systemic inflammation model and the CK-p25 model of neurodegeneration. They performed i.p. and i.c.st. injections of MG-LNP containing anti-PU.1 siRNA. They analyzed behavioral changes, immune cell infiltration in the liver, and neuroinflammatory markers in the brain.
The researchers presented here marks a significant advance in our understanding of the role of microglia in AD and the potential of RNA therapeutics. The development of the MG-LNP formulation represents a major step forward in RNA delivery technology, particularly in targeting microglia. This research not only sheds light on the complex mechanisms underpinning AD but also paves the way for innovative therapeutic strategies targeting neuroinflammation. As we move forward, the potential of MG-LNP and similar technologies in clinical settings must be explored. Further research is needed to optimize these delivery systems for human use and to fully understand their long-term effects. The fight against AD and other neurodegenerative diseases is poised for a new era, one where RNA therapeutics play a central role, offering new hope to millions affected by these debilitating conditions.
In conclusion, the comprehensive series of experiments demonstrated the potential of MG-LNP as an efficient vehicle for RNA delivery to microglia, particularly in the context of neuroinflammatory conditions like AD. The researchers’ approach combined in vitro and in vivo methodologies to validate the effectiveness of LNPs in both experimental settings, offering promising insights for future therapeutic applications in neurodegenerative diseases.
Ralvenius WT, Andresen JL, Huston MM, Penney J, Bonner JM, Fenton OS, Langer R, Tsai LH. Nanoparticle-Mediated Delivery of Anti-PU.1 siRNA via Localized Intracisternal Administration Reduces Neuroinflammation. Adv Mater. 2023:e2309225. doi: 10.1002/adma.202309225.