Every several weeks, I notice publication of yet another article trumpeting another aspect of Alzheimer’s. Where once it was APOE-4, AB42, or SS31 (an antioxidant peptide), more recent work emphasizes arginine metabolism in the microglia. The good news is that research community has — ponderously and hesitantly — finally begun to shift the clinical focus […]
Alzheimer’s, Microglia, Mitochondria, and Arginine
Every several weeks, I notice publication of yet another article trumpeting another aspect of Alzheimer’s. Where once it was APOE-4, AB42, or SS31 (an antioxidant peptide), more recent work emphasizes arginine metabolism in the microglia. The good news is that research community has — ponderously and hesitantly — finally begun to shift the clinical focus from the neuron to the microglial cells, a shift that many of us have been pushing for almost two decades. Neuronal damage was always the more obvious pathology, at least under the optical microscope, but it was never the underlying cause of the cascade of damage that results in Alzheimer’s disease. Gradually, we have come to realize that the microglial cells, and often vascular changes, play an early role in starting the avalanche of this horribly tragic pathology.
And yet, even now, it is frustrating to watch how much of the research creeps along, staring myopically down at trivial and secondary problems. It’s not so much that we see the trees and ignore the forest, but that we see the specific lichen on the specific root of a specific type of tree, while missing the interactions and overall pathology that drives the entire forest. The recent focus on arginine is a case in point, but SS31 is a parallel example. In the case of arginine, we notice the microglia; in the case of SS31 we notice the mitochondria, but in both cases we fail to look harder and deeper and we fail to understand the broad processes that drive these changes.
Mitochondrial dysfunction within the microglia is a good example. The dysfunction is not seen in germ cells, nor in young somatic cells, but is prominent in aging somatic cells. How can a germ cell lineage, carrying a line of 1.5 billion year old mitochondria, have normal function, while a somatic cell, having undergone a few dozen divisions in a few dozen years, suddenly have a dysfunctional mitochondria that was doing well for the last few billion years? Actually, we know the answer to that. Not only is it due to changing gene expression within the cell nucleus, slowing the production of many key enzymes needed in the citric acid cycle within the mitochondria, but we know that when we reset this pattern of gene expression in the nucleus, the mitochondria resume normal function. While the aging cell makes less ATP and a higher proportion of ROS as the damaged mitochondrial enzymes permit electrons to “slip” down the chain, but these changes are entirely reversible when we reset telomere lengths within the nucleus.
Nor does it stop there. Just as the aging cell begins to have a lower ATP/ROS ratio, so too do the lipid membranes begin to leak those ROS species, so too do our scavenger enzymes (like SOD) fail to capture those escaped ROS species, and so too do our cells fail to rapidly recycle the molecules damaged by those ROS species. And in every case, these four issues can be traced directly back to the slower turnover induced by a changing pattern of gene expression within the nucleus, which is orchestrated by a gradual telomere loss. Such changes can be (and have been) reset in human cells, in tissues, and in animal models. So why not reset the microglial telomeres and cure Alzheimer’s?