Stress urinary incontinence (SUI) is defined as "the complaint of involuntary loss of urine during exercise or physical activity (such as exercise), sneezing or coughing". It is the most common pelvic floor dysfunction disease in middle-aged and elderly women, seriously affecting patients' life quality. Erbium laser has been shown to improve SUI symptoms, but its therapeutic mechanism need to be further explored. Therefore, this study aims to explore the mechanisms of Smooth mode ER: YAG laser in the treatment for SUI.

Forty-six Age-matched female Sprague-Dawley rats(250-300g) were randomly divided into 3 groups, no intervention (Control, n=17), vaginal distention (VD, n=17) or VD with ER: YAG laser treatment (VD&ER , n=15). The latter two groups underwent vaginal distension (VD) to simulate postpartum injury to establish a rat model of SUI. The last group was treated with ER:YAG laser once on the third day after modelling. The treatment parameters were set as follows: ① laser energy: 2.0J/cm2; ② spot diameter: 7mm; ③ frequency: 1.4Hz; ④ number of light waves/times: 7. The three groups were tested for functional and histological performance on the eighth day after modelling. Functionally, the abdominal leak point pressure (aLPP) was used to evaluate urinary voiding function. Histologically, immunohistochemistry and immunofluorescence were used to evaluate the content of collagen I、collagen III、elastin and morphological changes of the anterior vaginal wall surrounding tissue. RNA-Seq and single-cell sequencing analysis were combined to explore the mechanisms of Smooth mode ER:YAG laser treatment for SUI.

There was no significant difference in aLPP between the Control and VD&ER, while the aLPP of  VD was significantly lower than Control and VD&ER [mean (SEM) Control 41.37 (1.65) cmH2O, VD 31.09 (1.86) cmH2O, VD&ER 38.74 (1.48) cmH2O; P < 0.05]. Histologically, immunohistochemistry showed that the content of collagen Ⅰ in the VD&ER was significantly higher than that in Control and VD [mean (SEM) Control 27.10 (4.58)%, VD 21.00 (4.22)%, VD&ER 46.80 (6.46)%; P < 0.05], and the content of collagen Ⅲ in the VD&ER was also significantly higher than that in VD [mean (SEM) Control 35.60 (2.51)%, VD 26.80 (4.24)%, VD&ER 40.50 (3.89)%; P < 0.05]. There was no difference in the content of elastin reflected by immunofluorescence among the 3 groups. The results of RNA-seq suggested that compared with the other 2 groups, the ECM-receptor interaction and PI3K-Akt signaling pathway in the treated tissue of VD&ER were significantly upregulated. In addition, the estrogen signaling pathway in the treated tissue of VD&ER was also significantly upregulated compared with VD. The preliminary results of single-cell sequencing showed that the steroid biosynthesis in epithelial cells was significantly upregulated and the ECM-receptor interaction in fibroblasts was also significantly stimulated.

The results show that Smooth mode ER: YAG laser treatment can significantly improve the symptom of SUI rats by improving the vaginal support for the urethra, which was promoted by the remodeling and proliferation of tissue around the anterior vaginal wall. As for the current sequencing results, it can be speculated that the mechanism of laser treatment may be the comprehensive effect of stimulated steroid biosynthesis in the epithelial cells as well as upregulated estrogen signaling pathway in the treated area. Besides, the laser may promote the synthesis and secretion of collagen in the fibroblasts. However, further studies, e.g. the estrogen concentration in the epithelial cells, the role of PI3K-Akt signaling pathway in Smooth-mode ER:YAG laser therapy and downstream analysis of single cell sequencing, should be made to verify these hypotheses.

Smooth mode ER:YAG laser can effectively improve SUI symptoms, which may play a therapeutic role through ECM-receptor interaction and Estrogen signaling pathway.