Nerve Fibers Invade the Aging Lens, Revealing How Cataracts Take Hold

Cataracts remain the leading cause of blindness worldwide, and despite more than a century of surgical refinement, no one has been able to prevent the clouding of the lens that accompanies aging. Now, a large-scale single-cell study of human eye tissue has uncovered a surprise buried inside the lens capsule, the thin membrane that envelops the lens: as cataracts develop, nerve-like fibers begin to grow into a tissue that is supposed to contain no nerves at all. The finding, published in Advanced Science, suggests that aging does not simply wear the lens down passively but actively reshapes its molecular environment in ways that recruit neural signaling into the disease process.

The research team, based at the Eye Center of the Second Affiliated Hospital of Zhejiang University School of Medicine, analyzed anterior lens capsules from 554 patients undergoing cataract surgery between October 2022 and December 2024. The patients were divided into three clinically recognized groups using the Lens Opacities Classification System II: 185 with mild cataracts, 210 with severe cortical cataracts, and 159 with severe nuclear cataracts. Using single-nucleus RNA sequencing, the researchers profiled a staggering 230,838 lens epithelial cells, building one of the most comprehensive cellular atlases of the aging lens capsule assembled to date.

Lens epithelial cells are the only proliferative cells in the lens, and they are essential for maintaining its transparency. Sitting just beneath the anterior capsule, they continuously renew lens fibers and shield the organ from oxidative stress and external damage. When their function deteriorates with age, cataracts follow. But until now, scientists had lacked a detailed map of the distinct cell states within this microenvironment, and how those states shift as cataracts progress from mild clouding to severe opacification.

The atlas revealed seven functionally distinct subpopulations of lens epithelial cells, which the team named according to their dominant gene expression profiles: secretory, regulatory, protective, metabolic, homeostatic, fibrotic, and extracellular-matrix-regulatory cells. As disease advanced, the proportions of the protective, homeostatic, and fibrotic subpopulations steadily declined, while the extracellular-matrix-regulatory group expanded. This shifting balance suggests that specific cell states govern the transitions between mild and severe disease, and that preserving vulnerable subpopulations might one day slow or prevent progression.

The most striking discovery concerned the secretory subpopulation, designated Cluster 0. Gene enrichment analyses showed that neurotrophic signaling pathways, particularly the SLIT-ROBO pathway, which normally guides axonal growth and neural progenitor proliferation, were active across the cataract groups. Immunofluorescence staining then confirmed the presence of beta-III tubulin-positive, axon-like structures penetrating the capsular periphery of cataract patients. No such structures were detected in the transparent lens capsules of healthy organ donors. In severe nuclear cataracts, the density of these axon-like projections was roughly eight to sixteen times higher than in the mild and severe cortical groups, respectively.

To trace the origin of these signals, the team integrated their lens data with single-nucleus sequencing data from human sympathetic ganglia. The analysis identified a specific neuronal subcluster, Sympathetic_neurons2, as the most interactive neural population, and pinpointed a molecular axis at the heart of the communication: Cluster 0 cells express high levels of the neurotrophic factor NRG1, which pairs with the ERBB3 receptor on sympathetic neurons. Immunofluorescence confirmed that both NRG1 and ERBB3 were dramatically elevated in severe nuclear cataract capsules compared with normal tissue, with NRG1 expression increasing several hundredfold.

Within Cluster 0 itself, further subclustering revealed three cell states: S100A4-positive, SYN3-positive, and MT-CO1-positive lens epithelial cells. As cataracts progressed, the SYN3-positive population, associated with neurotrophic signaling, expanded, while the MT-CO1-positive population, tied to mitochondrial energy production, shrank. This pattern echoes earlier findings that the mitochondrial protein MT-CO1 is upregulated early in cataract development to sustain ATP production but collapses in advanced opacification, and that Synapsin III expression rises in aged lenses, potentially reflecting age-related stress responses.

The study also documented a breakdown in the mechanisms that normally restrain nerve growth. SEMA3 signaling, which repels axons and prevents them from invading inappropriate tissue regions, shifted from coordinated expression across multiple cell subpopulations in mild cataracts to dominance by single clusters in severe disease. The researchers propose that this loss of inhibitory control, combined with aging-driven activation of NRG signaling, permits aberrant axonal proliferation into the capsule. Meanwhile, distinct metabolic signatures separated the two severe subtypes: cortical cataracts were enriched for calcium transport pathways, a known driver of lens protein degradation, while nuclear cataracts showed activation of ferroptosis pathways, the iron-dependent cell death process increasingly implicated in lens clouding.

The clinical implications are tantalizing. Because the NRG1-ERBB3 axis appears to be a central driver of the neural ingrowth associated with cataract progression, the authors suggest that targeted therapies, such as NRG1 antibodies, could eventually modulate these neurotrophic pathways. Prior work has shown that ERBB-family signaling participates in lens fibrosis and epithelial-mesenchymal transition after cataract surgery, and that NRG1 signaling can suppress stress-induced cellular senescence in vascular tissue, lending plausibility to the idea that manipulating this axis could influence lens aging more broadly. Still, the researchers caution that their study has limits: single-nucleus sequencing requires dissociating tissue, severing the spatial relationships between cells, and the exact origin of the axon-like structures remains unresolved.

The team plans to address these gaps with spatial transcriptomics, which preserves tissue architecture while mapping gene expression, and with functional validation of axonal growth in organoid-based models. For now, the study reframes age-related cataracts as a disease of cellular heterogeneity and aberrant neuro-lens communication rather than a uniform degenerative process. With the global population aged 60 and over projected to grow by 1.4 billion by 2030, and cataract prevalence exceeding 92 percent in people over 80, understanding the molecular choreography of lens aging has never mattered more. If the neural invasion of the aging lens can be intercepted, the first genuine preventive treatment for the world’s leading cause of blindness may become a realistic goal.

Subject of Research: Single-nucleus sequencing of human age-related cataract lens capsules identifying neural ingrowth and cell subpopulations in lens aging

Article Title: Deconstruction of Human Age‐Related Cataract Capsules Defines Aging

Article References: Tang, Q., Tong, Z., Fan, C., Chen, S., Guo, J., Hu, J., Yao, K., Yin, Z., Chen, X., & Yu, Y. (2026). Deconstruction of Human Age‐Related Cataract Capsules Defines Aging. Advanced Science, 13(55), Article e76532. https://doi.org/10.1002/advs.76532

Image Credits: AI Generated

DOI: 10.1002/advs.76532

Keywords: age-related cataract, lens epithelial cells, single-nucleus RNA sequencing, NRG1-ERBB3 signaling, neural ingrowth, lens capsule, SLIT-ROBO pathway, ferroptosis, calcium signaling, mitochondrial dysfunction, cell atlas, sympathetic neurons