Increasing Glucose Metabolism Improves Alzheimer’s Disease Symptoms
Post by Shireen Parimoo
The takeaway
The enzyme IDO1 in astrocytes is important for glucose metabolism in the brain. Inhibiting IDO1 in Alzheimer’s disease pathology restores glucose metabolism and rescues cognitive deficits such as spatial memory.
What's the science?
Alzheimer’s disease is characterized by deficits in learning and memory. In the brain, there is an accumulation of the protein amyloid beta as well as misfolding of the tau protein with increasing severity of the disease, eventually leading to neuron death. Astrocytes regulate glucose metabolism and provide lactate as energetic fuel to neurons in the brain. Glucose metabolism is known to decline in Alzheimer’s disease, but the mechanism through which this process is disrupted is unclear. Past research also shows that inflammatory stimuli – such as amyloid beta – tend to increase IDO1 activity. Thus, one possibility is that the enzyme IDO1, which helps convert the amino acid tryptophan to kynurenine in astrocytes, is involved. This week in Science, Minhas and colleagues investigated the genetic and physiological effects of IDO1 on the glucose metabolism process and on learning and memory in Alzheimer’s disease.
How did they do it?
The authors systematically examined the effects of Alzheimer’s pathology and the role of IDO1 in different stages of the glucose metabolism pathway in hippocampal astrocytes and neurons. First, they derived astrocytes from mouse hippocampi and human induced pluripotent stem cells. These included astrocytes derived from post-mortem brains of individuals with varying stages of late-onset dementia, with later stages being characterized by higher levels of amyloid and tau accumulation in the brain. The astrocytes were treated with oligomers of amyloid beta and tau in-vitro to simulate Alzheimer’s pathology and with PF068, which inhibits IDO1 activity. They then recorded changes in IDO1, tryptophan, and kynurenine levels in response to Alzheimer’s pathology and IDO1 inhibition. Additionally, they studied the downstream effects of IDO1 inhibition on glucose metabolism by measuring changes in the concentrations of intermediate metabolites and lactate. Next, the authors replicated the above experiments in astrocytes derived from mouse models of Alzheimer’s disease that overexpressed either the amyloid or tau protein. Finally, they examined the effects of IDO1 inhibition on object memory (novel object recognition) and spatial memory tasks (Morris water maze, Barnes maze) in the mouse models of Alzheimer’s disease.
What did they find?
Glucose metabolism was disrupted in Alzheimer’s pathology - the concentration of intermediate metabolites and lactate were reduced following amyloid and tau treatment. However, inhibiting IDO1 (or knocking it out) restored glucose metabolism. Similarly, kynurenine levels, which were elevated in Alzheimer’s pathology, returned to normal when IDO1 activity was inhibited. On the other hand, administering additional kynurenine disrupted the metabolic pathway in astrocytes treated with amyloid and tau oligomers in a dose-dependent manner. Similar effects were observed in mouse models of Alzheimer’s disease, with elevated kynurenine and lower lactate levels in hippocampal astrocytes. As before, inhibiting IDO1 lowered kynurenine levels and increased lactate production. Thus, these results show that the production of kynurenine via IDO1 activity is a crucial component of glucose metabolism in Alzheimer’s pathology.
Inhibiting IDO1 also rescued memory deficits and reduced the accumulation of amyloid beta in the subiculum near the hippocampus. Similarly, the deletion of IDO1 led to a reduction in kynurenine and increased lactate levels. In post-mortem human brains, kynurenine levels were higher in cases with more severe Alzheimer’s disease pathology. In astrocytes derived from individuals with late-onset dementia, there was a reduction in glucose metabolism while kynurenine levels were elevated. Inhibiting IDO1 in these astrocytes rescued both glucose metabolism and returned kynurenine to comparable levels to those without dementia. Altogether, these findings highlight IDO1 as a key enzyme in regulating glucose metabolism and consequently, cognition in both humans and in mouse models of Alzheimer’s disease.
What's the impact?
This study is the first to demonstrate the mechanism through which glucose metabolism is disrupted in Alzheimer’s disease and the important role that the IDO1 enzyme plays in this process. These findings have important applications for the development of treatments for pathologies that are characterized by protein aggregation in the brain.