Second session of the Longevity Therapeutics summit. For our coverage of the opening session, see here.
Influencing Splicing Factor Expression to Target Senescence (Lorna Harries, Associate Professor in Molecular Genetics, University of Exeter Medical School)
We now know that the diseases of aging are underpinned by common mechanisms described by the “hallmarks of aging,” famously reviewed by Lopez-Otín et al. in 2013. These represent potential points of therapeutic intervention for the future.
One of the hallmarks of aging is senescence, and experiments in transgenic mice have provided an elegant illustration of how aging can be meaningfully treated by attacking a core ‘pivot point’ underlying the process.
Harries argues that her group has identified a new hallmark of aging, dysregulation of RNA processing. Via alternative splicing, the selective inclusion or exclusion of coding units called exons, 98% of genes make more than one mRNA product (an average of 3), and differential regulation of this process plays a critical role in the cellular response to stress.
The first evidence that splicing regulators might play a role in aging came from a large study of 698 Italian subjects from ages 30 to 104, which revealed that in peripheral blood, only seven of 1065 cellular pathways are robustly differentially regulated as a function of age—and these pathways are overwhelmingly associated with RNA processing.
The Harries group then showed that expression of splicing factors decreases in cellular senescence in diverse cell types. Intriguingly, they also found that splicing factor expression is associated with lifespan in mice, and that the differences in expression levels are already present when the mice are young.
Consistent with this, resveratrol derivatives that increase the activity or expression of splicing factors decreased the proportion of senescent cells in vitro. This appeared to be mediated by rejuvenation rather than senolysis, as they saw no evidence of apoptosis following the drug treatment, and the rate of proliferation seemed to increase. These effects were long-lasting and persisted even after the drugs were withdrawn. The treated cells had longer telomeres, possibly because some splicing factors have known roles in telomere maintenance.
Another class of drugs tested were donors of hydrogen sulfide, a signaling molecule that is both produced by and targeted to the mitochondria. Hydrogen sulfide rescued some aspects of the senescence phenotype, but did not induce an increase in the proliferation rate (although there was a slight increase in the rate of proliferation).
Conversely, knockdown of specific splicing factors induced senescence and abolished the senescence rescue response to both of the drugs.
This raises the question of why splicing factors are dysregulated during aging? CEll signaling pathways can be activated by DNA damage, inflammation dysregulated growth factors, and oxidative stress—all of which are classical drivers of aging. To address this issue, the Harries group treated the cells with inhibitors of cell signaling pathways and showed that this also reversed the senescence phenotype. Moreover, knockdown of ETV6 or FOXO1, genes known to influence lifespan, splicing factors were upregulated and the number of senescent cells decreased. Importantly, knockdown of both genes had no effect, implying the existence of a finely tuned feedback loop controlling the expression of splicing factors.
The Challenge of Developing Novel Senolytic Medicines (Marc Ramis-Castelltort, Co-Founder & CEO, Senolytic Therapeutics Inc)
Senolytx was founded in 2017 by a group of European scientists, including Dr. Manuel Serrano, with the goal of delivering drugs that target senescent cells. It is funded by Life Biosciences (“a company devoted to developing medicines for aging in its entirety. Each Daughter company was founded by the world’s leading scientists who work on the cutting-edge of aging research. Life Biosciences provides the capital, centralized management infrastructure and resources, acts as catalyst for ideas, insights and knowledge transfer, and provides advanced research facilities through its central lab in Cambridge, Mass.”).
The company is seeking to understand fibrosis in several different organ systems (lung, liver, kidney) as well as the role of senescence in oncology (particularly chemotherapy-induced senescence). The pipeline currently contains three drugs targeting IPF, renal fibrosis, and cancer.
In his talk, Dr. Ramis elected to focus on the company’s discovery platform, in part to help distinguish Senolytx from other enterprises in the space. He emphasized that the company has a clear focus on designing delivery systems to target senescent cells, and is commited to creating novel concepts in the senescence field.
First, as an example of a delivery system, he described the company’s nano-delivery platform, in which drugs are encapsulated into a silica matrix covered with a galacto-oligomer. The particles can enter both senescent or non-senescent cells, but in senescent cells, because of the high level of galactosidase expression, the are disassembled and thus deliver their payload in a specific manner.
As a proof of principle, Ramis presented that this approach works in vivo, e.g., in a mouse model of IPF in which the lungs are filled with senescent cells. Delivery of a cytotoxic payload to the senescent cells in fibrotic tissue resulted in recovery of lung function—an important finding in the context of this intractable disease.
He then shifted focus to the company’s second priority, novel concepts in the senescence field. In this context, Senolytx is pursuing a monoclonal antibody that binds a surface molecule upregulated in multiple forms of senescence, thereby promoting immune clearance of senescent cells.
A key new concept (to me) in both of these threads of the talk was the idea of using conventional cancer drugs to induce senescence in cancer cells, and then using the senolytic to wipe out the tumor.
Another new term was “senomorphic,” a medication that attenuates the senescence-associated secretory phenotype without cell killing.
A Novel Apoptotic Gene Therapy Approach for Systemic Senolysis (John Lewis, CSO, Oisin Biotechnologies)
The vision of Oisin Biotechnologies is, to eliminate age-related disease. To this end, they leverage their platform technology, which Lewis describes as a way to selectively kill cells “based on what they’re thinking” (i.e., their transcriptional activity).
What does the ideal senolytic therapeutic look like? It would utilize a strategy similar to those that have been successful in transgenic mice, is well tolerated at therapeutic doses in healthy aged individuals, and is suitable for repeated or prolonged dosing. Moreover, due to the diversity of diseases associated with senescence, the drugs would have broad bioavailability throughout the body.
In the Fusogenix Lipid Nanoparticle (LNP) approach, a non-integrating DNA plasmid regulated by a senescence-specific promoter (e.g., p16), drives a potent conditional suicide gene (e.g., iCasp9, a drug-inducible form of the apoptotic protease Caspase 9), which causes senescent cells to die from apoptosis.
However, in vivo gene delivery is challenging from the standpoint of drug development. Nucleic aid therapies need help to reach their targets, because without protection, the body’s natural defenses break them down. Moreover, getting to the outside of the cell is not sufficient; a drug must enter the cell in order to be effective. Traditional lipid-based nanoparticles have several limitations that make them inappropriate as delivery systems for senolytic compounds—so rather than using cationic lipids, Oisin is taking advantage of tricks we have learned from a special class of viral membrane fusion proteins called fusion-associated small transmembrane (FAST) proteins. These molecules form the basis for Oisin’s delivery platform.
Using this system in mice, the company has been able to achieve significant reductions in the proportion of p16-positive cells, a significant increase in bone density, and an extension of median lifespan (from 119 to 133 weeks) even when the intervention begins in very old mice (104 weeks at the beginning of the study). Combination therapy with a p53-based agent extends lifespan even further.
The path to the clinical will not be an aging indication, but cancer: p53 transcriptional targeting of solid tumors. A first-in-human safety trial should begin in Canada later this year.