A number of groups have demonstrated that selectively exposing aged rodent tissues to expression of the Yamanaka factors - OCT4, SOX2, KLF4, and MYC (collectively OSKM) - can induce restoration of more youthful epigenetic patterns and gene expression, accompanied by restoration of tissue function. The Yamanaka factors were first explored as a way to replicate the process by which adult germline cells become embryonic stem cells at the outset of embryogenesis, leading to the now well established capacity to produce what are known as induced pluripotent stem cells from somatic cell samples. Importantly, this process doesn't just lead over time to a radical alteration of cell fate, but also quite rapidly rejuvenates epigenetic regulation of gene expression.
As noted in today's open access paper, one can't just apply Yamanaka factors globally to obtain a good outcome. Some tissues react poorly. Thus researchers have focused initially on a few use cases in which it seems likely that there is a path to therapies at the end of the day, many of which are focused on neural tissue. That said, the work here involves direct injection of gene therapy vectors to specific areas of the brain, and thus is at the very least a lengthy delivery technology research and development program away from adoption. The primary challenge in the development of gene therapy is how to obtain selective delivery to specific areas of the body when direct injection is infeasible, expensive, or risky. There is no clear path ahead at this time for many of the relatively small and deeply internal tissues.
Cognitive rejuvenation in old rats by hippocampal OSKM gene therapy
At the molecular level, gene expression studies in aging rodents have documented significant changes in hippocampal genes related to cholesterol synthesis, inflammation, transcription factors, neurogenesis, and synaptic plasticity. In the hippocampus of female rats, 210 genes have been reported to be differentially expressed in aged individuals compared to their young counterparts, with the majority being downregulated. /p> Yamanaka genes, along with other pluripotency genes, possess high therapeutic potential for treating the aged central nervous system affected by various neurodegenerative diseases. Recent results revealed that the Yamanaka genes display a dual behavior when expressed continuously in vivo, being regenerative when delivered via viral vectors but highly toxic when expressed in transgenic mice. Thus, it has been reported that delivery of the OSK genes by intravitreally injecting a regulatable adeno-associated viral vector type 2 (AAV2) expressing the polycistron OSK can reverse vision deficits in an experimental model of glaucoma in mice as well as in 11 months old mice showing age-related vison impairment. Fifteen months of continuous expression of the OSK genes in retinal ganglion cells (RGCs) induced neither pathological changes nor proliferation of RGCs. Young- and middle-aged mice injected intravenously with OSK-AAV2 for 15 months did not exhibit any adverse side effects. In contrast, DOX-induced expression of OSK genes in mice transgenic for OSK resulted in rapid weight loss and death, likely due to severe dysplasia in the digestive system. Administering an adenovector to the hypothalamus of young female rats, which carries both the OSKM transcription factors and the green fluorescent protein (GFP) marker, has not only significantly decelerated the pace of reproductive aging but also tripled the fertility rates in 9-month-old females compared to those receiving a placebo vector. Notably, at 9 months of age, female rats are approaching the age of ovulatory cessation, which typically occurs at around 10 months. Inspired by the pioneering results achieved by a team employing OSK gene therapy in the retina of mice, we decided to conduct a medium-term 39-day OSKM gene therapy trial in another brain region: the hippocampus of aged rats. The main goal was to restore learning and spatial memory performance in this animal model. For comparison, we used control groups of similarly aged rats injected with a placebo adenovector. The Barnes maze test, used to assess cognitive performance, demonstrated enhanced cognitive abilities in old rats treated with OSKM compared to old control animals. In the treated old rats, there was a noticeable trend towards improved spatial memory relative to the old controls. Further, OSKM gene expression did not lead to any pathological alterations within the 39 days. Analysis of DNA methylation following OSKM treatment yielded three insights. First, epigenetic clocks for rats suggested a marginally significant epigenetic rejuvenation. Second, chromatin state analysis revealed that OSKM treatment rejuvenated the methylome of the hippocampus. Third, an epigenome-wide association analysis indicated that OSKM expression in the hippocampus of old rats partially reversed the age-related increase in methylation.
