Reduced PLCbeta2 Protein in Taste Cells Explains Persistent Sweet and Bitter Loss After COVID-19

Taste cells normally replace themselves every two to four weeks. The fact that some patients still could not taste sweet, bitter, or umami flavors more than a year after clearing a COVID-19 infection pointed toward something deeper than cell death - something disrupting either the molecular machinery inside taste cells or the structural organization of taste buds themselves.

A study published in Chemical Senses by researchers from the University of Colorado Anschutz and two Swedish universities provides the first direct biological evidence for that disruption. The work examined actual tissue from patients rather than relying on symptom surveys or indirect markers - and found measurable molecular and structural abnormalities that align with the specific pattern of taste loss patients report.

Who Was Studied and What They Reported

The team enrolled 28 non-hospitalized patients who reported persistent taste disturbances more than one year after contracting COVID-19. The non-hospitalized designation matters: these were not patients with the most severe acute illness, which might involve systemic inflammation disrupting taste through multiple indirect pathways. They were people who had, by most clinical measures, recovered - yet continued to report abnormal taste.

Formal taste testing found that 8 of the 28 patients showed clearly abnormal scores. Eleven specifically reported loss of sweet, bitter, and umami taste. Salty and sour taste were largely preserved across the group - a distribution that gave the researchers an important clue about mechanism.

The Molecular Amplifier That Weakens

Taste perception is not uniform in how it works. Sweet, bitter, and umami flavors are detected by a class of receptor cells (Type II taste cells) that rely on a protein called PLCbeta2 as a signal amplifier. PLCbeta2 strengthens the electrical signal inside the taste cell before it travels to the nerve leading to the brain. Without it working at full capacity, the signal weakens even if the initial receptor-flavor interaction occurs normally.

Salty and sour tastes use different receptor mechanisms - ion channels rather than G-protein coupled receptors - and do not depend on PLCbeta2. That biochemical distinction predicted exactly the pattern of loss the patients reported: deficits concentrated in sweet, bitter, and umami while salt and sour perception held.

Biopsies from 20 of the study participants confirmed the molecular prediction: reduced levels of messenger RNA responsible for producing PLCbeta2 in taste receptor cells. The mRNA reduction indicates that the cells are not making enough of the protein, not that the protein is being destroyed after synthesis - a distinction that has implications for potential therapeutic strategies.

"PLCbeta2 acts like a molecular amplifier inside taste cells," said Thomas Finger, professor of cell and developmental biology at the University of Colorado Anschutz and corresponding author. "It strengthens the signal before it's transmitted to the brain. When levels are reduced, the taste signal weakens."

Structural Disorganization in Some Patients

Beyond the molecular finding, microscopic examination of taste bud tissue from some patients showed altered organization - cells arranged less regularly than in normal taste buds. The degree of structural disruption varied among individuals, which may explain why the clinical presentations also varied.

"Some subjects had normal-looking taste buds, while others showed structural disorganization," Finger said. "This suggests that both molecular and architectural changes may contribute to persistent taste dysfunction."

The coexistence of molecular and structural abnormalities raises questions about recovery timelines. Reduced mRNA expression could potentially normalize as viral effects on transcription fade. Structural disorganization in taste buds - which, despite their rapid turnover, depend on coordinated signaling to maintain normal architecture - may take longer or may require different conditions to resolve.

Honest Limits of a Small Study

This is a study of 28 patients, all non-hospitalized, from a self-selected group reporting persistent symptoms. The sample is not representative of all post-COVID taste disturbance; people who recovered quickly are not included. Biopsies were performed on only 20 of the 28. The study establishes that molecular and structural abnormalities exist in this patient group but cannot determine what fraction of all post-COVID taste loss involves the same mechanism.

Whether the molecular dysfunction can fully reverse, over what timescale, and whether any intervention could accelerate restoration of normal PLCbeta2 expression - these questions were outside the scope of the current work and have not yet been answered.