Protein Axis That Shields the Cornea Offers New Hope Against Dry Eye Pain

Dry eye disease affects hundreds of millions of people worldwide, and for many sufferers the condition is far more than a gritty inconvenience. When the tear film breaks down, the exposed cornea becomes inflamed, its delicate nerve endings are damaged, and patients can develop a chronic neuropathic pain that persists even when the eye appears to have healed. Existing therapies, from artificial tears to anti-inflammatory drops, address the symptoms only indirectly, and no approved treatment reliably restores the damaged corneal nerves or silences the resulting pain. A new study published in the Journal of Translational Medicine now points to a molecular partnership that may change that picture.

Researchers led by Jian Guo and Yihua Zhu of the First Affiliated Hospital of Fujian Medical University report that a protein called CALML5, a member of the calmodulin-like family of calcium-binding proteins, is sharply depleted in the corneas of mice with experimentally induced dry eye. When the team boosted CALML5 levels in the cornea using viral vector-mediated gene delivery, the animals recovered tear secretion, showed less corneal surface damage on fluorescence staining, and exhibited markedly reduced pain-related behaviors. The finding is striking because CALML5 has attracted little prior attention in the eye, having been studied mostly in the context of skin differentiation.

The team established dry eye disease in mice by surgically removing the lacrimal glands, a model that reproduces the loss of aqueous tear production and the secondary corneal nerve damage seen in severe human disease. They then measured tear output with Schirmer tests, tracked ocular discomfort with behavioral assays, and examined the corneas with histopathology, quantitative PCR, Western blotting and immunofluorescence. Together these assays allowed them to connect a single molecular change to measurable improvements in gland function, surface integrity and pain.

The mechanistic details are where the study becomes especially intriguing. In corneas overexpressing CALML5, the researchers found increased expression of beta III tubulin, a canonical marker of neuronal health and regeneration, alongside reduced levels of pain mediators including substance P, the P2X3 receptor and protein kinase C. At the same time, markers of neuroinflammation, Iba-1 in microglia-like cells and GFAP in glial cells, dropped. In other words, restoring CALML5 appeared to simultaneously encourage nerve repair, dampen pain signaling and quiet the inflammatory response that normally accompanies corneal injury.

CALML5, however, does not appear to act alone. The team identified protein phosphatase 4 catalytic subunit, or PPP4C, a serine-threonine phosphatase, as a second protein that is also downregulated in dry eye corneas and that physically interacts with CALML5. PPP4C is known in other systems as a regulator of stress signaling and inflammation, and the new data suggest the cornea is no exception. When the researchers delivered PPP4C into the diseased corneas, they observed the same therapeutic benefits as with CALML5, including restored tear secretion, improved corneal staining scores, reduced pain behaviors and lowered neuroinflammatory markers, and PPP4C overexpression also pushed CALML5 levels back up.

The decisive experiment came in the form of a rescue series. When the researchers knocked down CALML5 while overexpressing PPP4C, the benefits of PPP4C vanished: tear secretion did not recover, corneal integrity deteriorated, pain behaviors returned, nerve density fell, and pain marker and neuroinflammation levels climbed again. That loss-of-function result positions CALML5 as a likely downstream effector of PPP4C, meaning the phosphatase appears to do its protective work largely through this calcium-binding partner rather than through an independent route.

This arrangement, a phosphatase acting upstream of a calmodulin-like protein to protect peripheral nerves, is unusual in ocular research and offers a concrete framework for drug development. If PPP4C activity or CALML5 abundance can be enhanced in the corneal epithelium, whether by small molecules, gene therapy vectors or topical formulations that boost endogenous expression, clinicians might one day treat the root cause of dry eye pain rather than merely lubricating the surface. The authors suggest these two proteins are potential therapeutic targets for the disease, and the rescue data provide the causal evidence that a single molecular axis can influence both corneal healing and pain perception.

Several caveats temper the enthusiasm. The work was performed entirely in mice whose tear production was eliminated surgically, a severe model that does not capture the full heterogeneity of human dry eye, which often arises from inflammation, meibomian gland dysfunction, autoimmune disease or screen use rather than gland removal. Viral vector-mediated overexpression is also a laboratory tool, not a ready-made therapy, and translating it to human eyes would require answers to questions about delivery route, dosing, duration and safety. Still, the consistency of the phenotypes across tear secretion, staining, behavior and molecular markers gives the findings unusual internal coherence for an early mechanistic study.

The broader significance may lie in how the study reframes dry eye disease itself. Rather than viewing it as a simple deficiency of lubrication, the work supports an emerging view of dry eye as a neuroinflammatory disorder in which corneal nerve damage and glial activation perpetuate both the pain and the impaired tear reflex. The lacrimal functional unit, which links the corneal nerves, the brainstem and the tear glands, depends on intact sensory innervation, and therapies that regenerate nerves could in principle restore the entire loop. CALML5 and PPP4C now join a short list of molecular levers that appear capable of nudging that loop back toward health.

For patients enduring the burning, stinging and light sensitivity of chronic dry eye, the study is best understood as a promising early step rather than an imminent cure. But it demonstrates that a pair of previously underappreciated proteins can, when restored, protect the cornea, regrow its nerve supply and quiet the pain circuitry in a living animal. The research was supported by the Fujian Province Science and Technology Innovation Joint Fund, and the team’s next challenge will be showing that the PPP4C-CALML5 axis behaves the same way in human corneas and can be safely targeted with deliverable therapies. If it can, one of ophthalmology’s most stubborn sources of chronic pain may finally have a molecular handle.

Subject of Research: The PPP4C-CALML5 protein axis in corneal injury and neuropathic pain associated with dry eye disease

Article Title: PPP4C-CALML5 axis alleviates corneal injury and corneal pain in dry eye disease

Article References: Ren, X., Liu, S., Jing, D., Zhu, Y., & Guo, J. (2026). PPP4C-CALML5 axis alleviates corneal injury and corneal pain in dry eye disease. Journal of Translational Medicine. https://doi.org/10.1186/s12967-026-09007-y

Image Credits: AI Generated

DOI: 10.1186/s12967-026-09007-y

Keywords: dry eye disease, CALML5, PPP4C, corneal pain, corneal nerve damage, neuroinflammation, tear secretion, lacrimal gland, gene therapy, ophthalmology, neuropathic pain, Journal of Translational Medicine