Rabep2 Variants Reduce the Harm of Stroke through Increased Alternative Vasculature
The research I'll point out today examines one of the mechanisms by which the damage caused by similar strokes can vary from individual to individual. The researchers focus on the degree to which the vascular network of the brain grows to contain alternative routes to the same destination tissues, and identify a gene in mice that accounts for a fair amount of this difference between individuals. A stroke is the outcome of structural failure in a blood vessel in the brain, weakened by the molecular damage of aging, leading to loss of elasticity, and put under stress by the increased blood pressure of hypertension. The result is either blockage or rupture, disrupting the flow of blood to where it is needed. The largest effects result from the downstream loss of blood flow to a region of brain tissue, ischemia, followed by its sudden return. Most cells survive a short loss of oxygenation and nutrients, but then die in the reaction to a renewed influx of oxygenated blood, an effect known as reperfusion injury. The effects of similar strokes vary from individual to individual for many reasons. To pick one example among many, differences in the cellular reaction to reperfusion can be quite influential, and thus genetically engineered mice lacking the oxygen sensor PHD1 have been demonstrated to experience greatly reduced cell death following a stroke.
The vascular network incorporates some redundancy throughout the body, though nowhere near as much as one would like. Blockage of larger vessels is going to cause trauma: there is a given amount of blood flowing through that channel and it can't all fit through the alternative paths. In many cases enough can get by to keep tissues alive, however, and thus reduce the degree of ischemia. Some people have more of those additional redundant blood vessels than others, and thus suffer less in the event of stroke. Being more resistant to damage is better than being less resistant to damage if that is the only game in town, but aiming higher certainly should be the goal. No-one wants to be put in the position of suffering a stroke in the first place. Prevention is far better than cleaning up after the fact, especially given the high risk of death and permanent disability that accompanies stroke.
Thus consider the forms of rejuvenation therapy that can address the root causes of hypertension, blood vessel stiffness, and damage to blood vessel walls, such as that caused by atherosclerosis - these are the goals to work towards. Of the SENS rejuvenation research portfolio, breaking cross-links will most likely do the most address loss of elasticity, but if you look at the evidence almost all of the fundamental forms of cell and tissue damage that cause aging have some contribution to make. So mitochondrial repair of one form or another will reduce the flux of damaged lipids that contribute to atherosclerosis. Cleaning up the related metabolic waste such as 7-ketocholesterol found in atherosclerotic lesions will no doubt also help. From recent work there are hints that senescent cell clearance will also assist in turning back loss of tissue elasticity, and any approach that reduces chronic inflammation will also slow the decline of the vascular system.
Scientists identify "collateral vessel" gene that protects against stroke damage
Scientists have known that when an artery is blocked, the damage to tissues downstream is often limited because these tissues continue to be nourished by special "collateral" vessels that connect the tissue to other arteries. However, for reasons that haven't been understood, the number and size of these collateral vessels - and thus the protection they afford - can vary greatly from one individual to the next. Scientists have now implicated the Rabep2 gene as a major contributor to this variation in collateral vessel formation. Variants of this gene account for most of the differences in collateral vasculature among laboratory mice. Since humans and mice are more than 90 percent genetically similar, the human version of Rabep2 is likely to have a comparable function.
Through a series of experiments, researchers replaced a defective variant of the gene in a mouse strain with poor collaterals with a normal copy of the gene, resulting in the formation of abundant collateral vessels during embryonic development and much greater resistance to tissue injury and cell death when the mice were subjected to experimental stroke as adults. The scientists hope that one day doctors will be able to use a simple blood test to detect variants of the human form of the gene. This would help doctors quickly gauge the extent of collateral vessels in patients who experience heart attacks, strokes, peripheral artery disease, and occlusive disorders in other tissues. In principle the findings also could help lead to therapies that stimulate the formation of more collateral vessels in healthy people to reduce the severity of tissue injury in the event of a future arterial blockage, as well as in people who already have occlusions, thereby reducing damage and improving their recovery.
This comes nine years after researchers first observed that the extent of the collateral vasculature - and thus the damage after arterial occlusion - can differ greatly between different strains of lab mice, even though no differences in the rest of the circulatory systems were evident. They focused on collateral vessels in the brain, which are easier to image than in other tissues, and undertook experiments involving thousands of mice. By 2014, the group had narrowed the search to a small region on mouse chromosome 7. In the new study, the researchers set out to identify the particular gene in this region that might explain the differences in collateral vessel development. From the 28 protein-coding genes in the region, the scientists were able to exclude 13, after determining that mice lacking any of those genes didn't have more or fewer collaterals. Of the 15 remaining genes under suspicion, the team decided to focus on their top suspect, Rabep2. Little was known about this gene, but the scientists had previously found a Rabep2 variant in mouse strains with low collateral extent, whereas high-collateral strains had the normal version of the gene. The variant differs from the normal gene in only a single DNA "letter," but that change - because of its location - is predicted to impair the function of the resulting protein.
Using new CRISPR gene-editing technology, the team was able to test the effect of this Rabep2 variant. They replaced the DNA letter in normal Rabep2 that is present in the genomes of high-collateral mice with the suspect variant. The result: the mice formed many fewer collaterals during development and had much greater stroke damage as adults. And this shift was even greater when the gene was deleted entirely. Conversely, in mice from the low-collateral strain, replacing the variant gene with the normal one induced the animals to develop the abundant collateral vasculature present in the high-collateral strain. These beneficially "edited" mice were thus far more resistant to damage from stroke.
The extent (number and diameter) of collateral vessels varies widely and is a major determinant, along with arteriogenesis (collateral remodeling), of variation in severity of tissue injury after large artery occlusion. Differences in genetic background underlie the majority of the variation in collateral extent in mice, through alterations in collaterogenesis (embryonic collateral formation). In brain and other tissues, ≈80% of the variation in collateral extent among different mouse strains has been linked to a region on chromosome 7. We used additional CNG mapping and knockout mice to narrow the number of candidate genes. Subsequent inspection identified a nonsynonymous single nucleotide polymorphism between B6 and BC within Rabep2 (rs33080487). We then created B6 mice with the BC single nucleotide polymorphism at this locus plus 3 other lines for predicted alteration or knockout of Rabep2 using gene editing. The single amino acid change caused by rs33080487 accounted for the difference in collateral extent and infarct volume between B6 and BC mice. Mechanistically, variants of Rabep2 altered collaterogenesis during embryogenesis but had no effect on angiogenesis examined in vivo and in vitro. Rabep2 deficiency altered endosome trafficking known to be involved in VEGF-A / VEGFR2 signaling required for collaterogenesis.