Ultrastructural morphological study as a tool for identifying early heart damage: the protective effects of a high-fiber diet against dysbiosis

Recent clinical and experimental findings in human microbiota research highlight that gut microbiota dysbiosis plays a role in the development and progression of breast cancer (BC), and they suggest dietary intervention as a potential strategy to synergize with anticancer therapies. Moreover, it’s well established the role of dysbiosis also in the onset of cardiovascular diseases (CVDs). Patients with BC, especially those with the HER2-positive subtype, are particularly vulnerable to cardiovascular events due to the adverse effects of cancer therapies on cardiac tissues. CVDs represent the most common and significant comorbidities in BC patients, with many post-treatment recurrences and cancer-specific deaths linked to cardiovascular events. We hypothesize that targeting dysbiosis through dietary interventions could benefit BC patients by providing cardioprotection in addition to anti-tumor effects. In this study, we used an in vivo HER2-positive BC mouse model to investigate the effects of gut microbiota homeostasis alteration, induced by administering the antibiotic vancomycin (VAN) in drinking water, on cardiac muscle tissue. Additionally, we explored whether a high-fiber diet (HFiber) could provide cardioprotection following antibiotic-induced dysbiosis. Light microscopy analysis of heart tissue structure showed no visible morphological changes in response to VAN treatment. However, digital transmission electron microscopy (TEM) analysis of cardiomyocyte ultrastructure revealed that, although the myofibril arrangement was preserved, mitochondria in the VAN group were significantly larger, more elongated, and often partially or entirely damaged compared to the untreated control group (CT). No significant ultrastructural alterations were observed in the HFiber group, suggesting potential protection against mitochondrial damage. Molecular characterization via real-time PCR indicated that antibiotic treatment upregulated genes associated with mitochondrial fusion (e.g., OPA1, DNM1, MFS1) and oxidative damage (e.g., SOD2), while showing a trend toward reduced expression of biogenesis-related genes (e.g., NRF1, PPRargc1). These findings are consistent with the observed mitochondrial enlargement in dysbiotic mice. Proteomic analysis by FT-Orbitrap highlighted alterations of the mitochondrial respiratory chain in dysbiotic mice and also an increase of interstitial collagen, not observed with the followed high-fiber diet administration. Luminex analysis of circulating inflammatory markers showed changes relevant to CVD pathogenesis. In conclusion: i) dysbiosis drived by VAN treatment induces early ultrastructural alterations in intermyofibrillar mitochondria of cardiomyocytes; ii) TEM analysis is effective in detecting ultrastructural damage in the absence of overt histological tissue changes, as confirmed by molecular and proteomic data; iii) a high-fiber diet may counteract the adverse cardiac effects triggered by antibiotic-induced dysbiosis, providing potential protective benefits for BC patients at risk for CVDs.