# Age-Dependent Alterations in Intestinal Barrier Function: Involvement of Microbiota and TLR4 Signaling

## TL;DR

This study shows how the gut and its microbes change with age, leading to inflammation and how targeting TLR4 could help maintain gut health.

## Contribution

The study reveals TLR4 as a key driver of age-related gut deterioration and identifies a lifespan-wide microbiota–host signaling axis.

## Key findings

- Microbial diversity and SCFA levels peak in adulthood, while inflammation increases with aging.
- TLR4 knockout mice show no age-associated gut decline, indicating TLR4's critical role in gut aging.
- Transcriptomics reveal biphasic PI3K/Akt activation in pups and old mice, suggesting metabolic and immune rewiring.

## Abstract

The intestinal barrier, a crucial defense system against pathogens and toxins, undergoes dynamic changes throughout a lifespan. While age-related gut decline and systemic inflammation are well-documented, a systematic understanding of how the gut microbiota and host immune signaling coordinately evolve from infancy to old age remains limited. This study comprehensively investigated these interactions across major developmental stages, including pups, adults, middle-aged, and old age in mouse models. It was found that microbial diversity and beneficial metabolites peak in adulthood, while inflammatory signals progressively increase with aging. Crucially, the immune receptor Toll-like receptor 4 (TLR4) was identified as a key driver of age-related gut deterioration. These findings reveal that aging disrupts the delicate host–microbe dialogue, leading to barrier dysfunction and chronic inflammation. Targeting this interaction, especially through TLR4 modulation, presents a promising strategy to support gut health and promote healthier aging.

The intestinal barrier undergoes profound changes with age, impacting local immunity and systemic health, yet the mechanisms coordinating immune and microbial dynamics across the lifespan remain incompletely understood. Toll-like receptor 4 (TLR4) serves as a key mediator of host–microbiota interactions. This study investigated age-related changes in barrier function and the role of TLR4 using C57BL/6J and TLR4 knockout (TLR4−/−) mice across key developmental stages: pups (postnatal day 9), adults (2–4 months), middle-aged (7–9 months), and old (16–19 months). Through a multi-layered approach integrating histology, microbiome profiling, short-chain fatty acid (SCFA) analysis, cytokine quantification, ex vivo functional assays, and transcriptomics, we identified a multi-phase process of intestinal remodeling. Pup-P9 mice exhibited immature colonic structure, a simple microbiota dominated by Firmicutes and Proteobacteria, and undetectable acetic acid level. Adults reached peak diversity and SCFA concentrations, marked by a rise in Bacteroidota and the emergence of Akkermansia. In middle and old age, pro-inflammatory cytokines (IL-1β, IL-6, and TNF-α) increased, Bacteroidota declined while Firmicutes, Actinobacteria, and Turicibacter expanded, and aged colons showed blunted ex vivo responses to IL-1β. This age-associated functional decline phenotype was absent in TLR4−/− mice, supporting the involvement of TLR4 signaling. Transcriptomics further revealed biphasic PI3K/Akt activation in both pups-P9 and old mice. Together, these findings suggest a systemic rewiring of host metabolic and immune signaling pathways in response to an aging microbiota, highlighting this dynamic, lifespan-wide microbiota–host signaling axis as a potential intervention target.