Glutamate (Glu) is the most abundant excitatory neurotransmitter, and gamma-aminobutyric acid (GABA) is the main inhibitory neurotransmitter in the mature central nervous system (CNS). A multicellular metabolic pathway known as the Glu/GABA/glutamine (Gln) cycle maintains the balance between these metabolites, which plays a critical role in several brain functions including learning and memory, development, pain, synaptogenesis, motor stimuli, and neuronal synaptic transmission. The nervous system-specific metabolite N-acetyl aspartate (NAA) is synthesized from aspartate and acetyl-coenzyme A in neurons and is present at exceptionally high concentrations in the brain. It is a direct precursor for the enzymatic synthesis of the neuron-specific dipeptide N-acetyl aspartyl-glutamate, the most concentrated neuropeptide in the human brain. In this study, the authors from the United States used brain proton magnetic resonance spectroscopy (MR spectroscopy) to investigate the neurometabolites that indicate neuroinflammation, neuronal damage, and glial dysfunction in patients with post-COVID syndrome and persistent neuropsychiatric symptoms.
More than three years after the global coronavirus disease 2019 (COVID-19) pandemic, it is clear that infection with severe acute respiratory syndrome coronavirus type-2 (SARS-CoV-2) can lead to a new disease called long-COVID or post-acute COVID syndrome which encompasses a wide range of organ dysfunction and clinical symptoms, including neurological.
MR spectroscopy provides information on various tissue metabolites, and it is a supplement to standard diagnostic MR imaging. Protons in different molecules have slightly different magnetic properties, and this difference enables MR spectroscopy to detect small molecules in the body. In addition to the water signal, the dominant peaks in the MR spectra of the brain are from choline (Cho), creatine (Cr), and NAA which appear in all spectra, and changes in these signals can often be linked to pathology. Lower concentrations of neurometabolites myo-inositol (MI) and Glu/Gln, can also be detected in brain tissue. MR spectroscopy is capable of revealing metabolic changes that precede pathological structural changes in the brain tissue. Decreased levels of total N-acetyl compounds and Glu/Gln are interpreted to represent irreversible neuronal dysfunction, injury, or loss in affected brain areas. Neuroinflammation and the proliferation of microglia are typically associated with increased glial marker MI. Concomitantly increased Cho compound levels and total Cr levels, due to their higher concentrations in glia than in neurons, are associated with increased MI levels.
The changes in the Glu/GABA/Gln balance have been linked to brain damage and several neurodegenerative diseases including Alzheimer’s, Parkinson’s, and neuroHIV. In neuro HIV, changes in Glu/GABA/Gln balance contribute to neuronal and glial dysfunction as well as to cognitive impairment found in HIV-associated neurocognitive disorder (HAND). MR spectroscopy showed that the most common neurometabolic changes in HIV-1-positive patients are decreased Cr, total N-acetyl compounds (NAA + N-acetyl aspartyl-glutamate) and Glu/Gln levels, and increased Cho and MI levels.
Brain Proton Magnetic Resonance Spectroscopy: Metabolic maps showing the ratio of MI and NAA
About the study
The study included 54 participants, 29 diagnosed with post-COVID syndrome and persistent neuropsychiatric symptoms, and 25 healthy controls matched by age, sex, and education. More healthy controls than post-COVID patients received COVID-19 vaccines. The mean age of post-COVID participants was 42.4 years.
Participants diagnosed with post-COVID syndrome had a documented diagnosis of COVID-19 ≥six weeks earlier and had at least one neuropsychiatric symptom that emerged after COVID-19. They were studied at a mean of 242 ± 156 days after the acute infection. Nine post-COVID participants were hospitalized and required supplemental oxygen and/or ventilation during COVID-19.
Healthy controls did not have prior COVID-19 diagnosis. They tested negative for SARS-CoV-2 on the polymerase chain reaction test within a week or had a negative rapid antigen test on the evaluation day. Exclusion criteria were: any significant neurologic or psychiatric disorder (eg, stroke, encephalitis from any cause except COVID-19, a neurodegenerative disorder, schizophrenia, uncontrolled major depression or anxiety disorder requiring medication before COVID-19, traumatic brain injury with loss of consciousness for more than one hour requiring hospitalization), severe substance use disorders except for tobacco or cannabis use, and any contraindication for magnetic resonance imaging (MRI).
All 54 participants were assessed by the National Institutes of Health Toolbox–Cognition Battery and Motor Battery. All patients with post-COVID syndrome completed a standardized symptom severity questionnaire.
To evaluate the concentrations of neurometabolites, all patients underwent brain proton MRS of the anterior cingulate cortex gray matter and frontal white matter. The authors stated that they have chosen the anterior cingulate cortex as the major node of the attention network required for all cognitive tasks. The frontal white matter was chosen because during viral neuroinfections, neuroinflammation is often found in this brain region (e.g., HIV, John Cunningham virus, and hepatitis C virus).
Results
In participants with post-COVID syndrome, the most frequent neuropsychiatric symptoms were difficulties with concentration (93%) and memory (79%), fatigue (86%), and depression or anxiety (68%). Despite the high prevalence of complaints regarding concentration and memory, their performance in all domains assessed by the NIH Toolbox–Cognition Battery was similar to that observed in healthy controls. Nonetheless, on the PROMIS surveys, individuals diagnosed with post-COVID syndrome showed a greater number of symptoms such as depression, fatigue, anxiety, and pain, and poorer global mental and physical health scores.
Importantly, post-COVID participants had poorer endurance on the Motor Battery tests, especially on the 2-minute endurance walk test, the 4-meter walk gait speed test, and the 9-hole pegboard dexterity test (test of dominant hand). A recent BOLD-functional MRI study, which investigated brain activation during a working memory task, has shown that participants with post-acute COVID and neuropsychiatric symptoms performed worse than healthy controls in the domains of endurance, locomotion, and dominant hand manual dexterity. https://discovermednews.com/patients-with-post-covid-syndrome-and-neuropsychiatric-symptoms-have-different-brain-activation-during-the-working-memory-task/
Brain neurometabolites in patients with post-COVID and persistent neuropsychiatric symptoms
Brain proton MR spectroscopy revealed changes in brain neurometabolites in post-COVID patients with persistent neuropsychiatric symptoms. The total N-acetyl compounds, Glu/Gln, and MI levels were lower in the frontal white matter of post-COVID participants, compared to healthy controls. Since most neurons (∼80%) in the frontal white matter are glutamatergic, the markedly decreased Glu/Gln levels suggest damage or loss of glutamatergic neurons. Importantly, increased MI, Cho compounds, and total Cr levels were found only in participants who were hospitalized during acute COVID-19 (concomitantly increased Cho compound levels and total Cr levels, due to their higher concentrations in glia than in neurons, are associated with increased MI levels).
These findings are consistent with brain MR spectroscopy results detected in HIV-1-positive patients that demonstrated decreased Cr, NAA, and Glu levels and increased Cho and MI levels, indicating neuroinflammation, microglial proliferation, and neuronal injury or loss. A decreased NAA and Glu/Gln levels were also observed in the cortical gray matter during early HIV-1 infection, indicating that HIV-1 causes neuronal and astroglial dysfunction within a short time after infection.
As neuroinflammation is typically associated with elevated MI levels, the authors assert that the lower MI levels observed in post-COVID patients who were not hospitalized represent glial dystrophy or dysfunction rather than neuroinflammation and glial activation. Glial dystrophy is considered to be an important contributory factor to persistent neuropsychiatric symptoms. In contrast, higher MI levels found in patients with post-COVID who were hospitalized during acute COVID-19 might indicate persistent neuroinflammation with glial activation, already reported in patients with severe COVID-19.
Since NAA, the major component of total N-acetyl compounds is synthesized in mitochondria, the authors suggested that a reduced level of total N-acetyl compounds could potentially result from mitochondrial dysfunction.
Lower total N-acetyl-compound levels in the frontal white matter predicted poorer performance on several cognitive measures in all participants. Lower N-acetyl-compound levels in the gray matter of the anterior cingulate cortex were associated with lower endurance on the 2-minute walk test. In HIV-1 infected individuals, a decreased Glu/Cr ratio was shown to correlate with worse performance on verbal recall, psychomotor speed, and reaction time.
Conclusion
In this study, decreased tNAA and Glu/Gln levels, increased Cho and tCr levels, and altered MI levels (decreased or increased depending on the severity of acute COVID-19), measured by brain proton MR spectroscopy, were found in the frontal brain regions of post-COVID patients with persistent neuropsychiatric symptoms. The observed changes in Glu/Gln balance, total N-acetyl-compound levels, and MI provided evidence of neuronal injury or loss, as well as of persistent neuroinflammation and glial dysfunction in the brains of post-COVID individuals, and are consistent with findings associated with brain damage in other neurodegenerative diseases, such as neuroHIV.
This article was published in the Journal of Infectious Diseases.
Journal Reference
Ernst T et al. Neuronal and Glial Metabolite Abnormalities in Participants With Persistent Neuropsychiatric Symptoms After COVID-19: A Brain Proton Magnetic Resonance Spectroscopy Study. The Journal of Infectious Diseases. 2023; jiad309 (Open Access). https://doi.org/10.1093/infdis/jiad309
