When given to patients with AD, L-serine supplements could be driving abnormally increased serine levels in the brain even higher, potentially accelerating neuronal death, according to study author Xu Chen, PhD, of the University of California, San Diego, and colleagues.
This conclusion conflicts with a 2020 study by Juliette Le Douce, PhD, and colleagues, who reported that oral L-serine supplementation may act as a “ready-to-use therapy” for AD, based on their findings that patients with AD had low levels of PHGDH, an enzyme necessary for synthesizing serine, and AD-like mice had low levels of serine.
Dr. Sheng Zhong
Writing in Cell Metabolism, Dr. Chen and colleagues framed the present study, and their findings, in this context.
“In contrast to the work of Le Douce et al., here we report that PHGDH mRNA and protein levels are increased in the brains of two mouse models of AD and/or tauopathy, and are also progressively increased in human brains with no, early, and late AD pathology, as well as in people with no, asymptomatic, and symptomatic AD,” they wrote.
They suggested adjusting clinical recommendations for L-serine, the form of the amino acid commonly found in supplements. In the body, L-serine is converted to D-serine, which acts on the NMDA receptor (NMDAR).
‘Long-term use of D-serine contributes to neuronal death’ suggests research
“We feel oral L-serine as a ready-to-use therapy to AD warrants precaution,” Dr. Chen and colleagues wrote. “This is because despite being a cognitive enhancer, some [research] suggests that long-term use of D-serine contributes to neuronal death in AD through excitotoxicity. Furthermore, D-serine, as a co-agonist of NMDAR, would be expected to oppose NMDAR antagonists, which have proven clinical benefits in treating AD.”
According to principal author Sheng Zhong, PhD, of the University of California, San Diego, “Research is needed to test if targeting PHGDH can ameliorate cognitive decline in AD.”
Dr. Zhong also noted that the present findings support the “promise of using a specific RNA in blood as a biomarker for early detection of Alzheimer’s disease.” This approach is currently being validated at UCSD Shiley-Marcos Alzheimer’s Disease Research Center, he added.
Roles of PHGDH and serine in Alzheimer’s disease require further study
Commenting on both studies, Steve W. Barger, PhD, of the University of Arkansas for Medical Sciences, Little Rock, suggested that more work is needed to better understand the roles of PHGDH and serine in AD before clinical applications can be considered.
“In the end, these two studies fail to provide the clarity we need in designing evidence-based therapeutic hypotheses,” Dr. Barger said in an interview. “We still do not have a firm grasp on the role that D-serine plays in AD. Indeed, the evidence regarding even a single enzyme contributing to its levels is ambiguous.”
Dr. Barger, who has published extensively on the topic of neuronal death, with a particular focus on Alzheimer’s disease, noted that “determination of what happens to D-serine levels in AD has been of interest for decades,” but levels of the amino acid have been notoriously challenging to measure because “D-serine can disappear rapidly from the brain and its fluids after death.”
While Dr. Le Douce and colleagues did measure levels of serine in mice, Dr. Barger noted that the study by Dr. Chen and colleagues was conducted with more “quantitatively rigorous methods.” Even though Dr. Chen and colleagues “did not assay the levels of D-serine itself ... the implication of their findings is that PHGDH is poised to elevate this critical neurotransmitter,” leading to their conclusion that serine supplementation is “potentially dangerous.”
At this point, it may be too early to tell, according to Dr. Barger.
He suggested that conclusions drawn from PHGDH levels alone are “always limited,” and conclusions based on serine levels may be equally dubious, considering that the activities and effects of serine “are quite complex,” and may be influenced by other physiologic processes, including the effects of gut bacteria.
Instead, Dr. Barger suggested that changes in PHGDH and serine may be interpreted as signals coming from a more relevant process upstream: glucose metabolism.
“What we can say confidently is that the glucose metabolism that PHGDH connects to D-serine is most definitely a factor in AD,” he said. “Countless studies have documented what now appears to be a universal decline in glucose delivery to the cerebral cortex, even before frank dementia sets in.”
Dr. Barger noted that declining glucose delivery coincides with some of the earliest events in the development of AD, perhaps “linking accumulation of amyloid β-peptide to subsequent neurofibrillary tangles and tissue atrophy.”
Dr. Barger’s own work recently demonstrated that AD is associated with “an irregularity in the insertion of a specific glucose transporter (GLUT1) into the cell surface” of astrocytes.
“It could be more effective to direct therapeutic interventions at these events lying upstream of PHGDH or serine,” he concluded.
The study was partly supported by a Kreuger v. Wyeth research award. The investigators and Dr. Barger reported no conflicts of interest.