A recent investigation utilizing a prominent GLP-1 medication has shed light on its effects on the intricate microbial ecosystems within the gut. This research, conducted on murine subjects, meticulously charted the transient transformations occurring in bacterial populations, offering fresh perspectives on the durability of these shifts once the therapeutic agent is no longer administered.
For several decades, researchers in metabolic science have recognized a profound physiological connection between excess weight and its associated cardiovascular and metabolic ailments, alongside the subsequent alterations observed in the gut's bacterial composition. The introduction of GLP-1 receptor agonists (GLP-1 RAs) has significantly broadened the treatment options for these metabolic conditions. However, a comprehensive understanding of their influence on the makeup and functionality of gut bacteria remains limited.
While human clinical trials have affirmed the therapeutic effectiveness and safety profile of liraglutide, a specific GLP-1 RA, preclinical research has been scant regarding whether these agonists induce persistent structural changes in the gut microbiota. Additionally, previous studies on GLP-1 RAs have noted microbial shifts following their administration, but it was unclear if these changes were transient pharmacological responses that revert to normal after a drug withdrawal period, or if they endure post-treatment.
To address these microbial ambiguities, the current study focused on male C57BL/6J mice with diet-induced obesity. The goal was to longitudinally track changes in gut bacterial diversity and composition over a 21-day period. The experiment involved 24 mice, divided into three groups of eight: one group received a high-fat diet and daily subcutaneous liraglutide injections (0.2 mg/kg) for 14 days, followed by a 7-day drug washout (HFL). A second group, serving as a high-fat diet control (HFC), received daily phosphate-buffered saline (PBS) vehicle injections. The third group, a low-fat diet control (LFC), also received the same vehicle injections. Body weights were recorded daily, but food intake could not be precisely analyzed due to the high-fat-fed mice shredding their food.
Bacterial community dynamics were assessed by extracting DNA from fecal samples at three key intervals: at the study's commencement (Day 0), after treatment (Day 14), and following the washout period (Day 21). This DNA was then subjected to high-throughput Illumina paired-end sequencing, targeting the V4-V5 region of the bacterial 16S ribosomal RNA gene, which generated approximately 7.1 million high-quality reads. Subsequent bioinformatics analyses utilized non-metric multidimensional scaling (NMDS) to evaluate changes in community structure based on Bray-Curtis dissimilarities. Furthermore, a Similarity Percentages (SIMPER) analysis was employed to pinpoint distinct bacterial DNA sequence types, known as amplicon sequence variants (ASVs), that significantly contributed to the observed differences in the microbial community.
The findings indicated that liraglutide led to a decrease in body weight among the HFL mice, which became notably lighter than the HFC control group by Day 4, while the HFC group continued to gain weight. Following the discontinuation of liraglutide on Day 14, the HFL animals showed a gradual regain in body weight through Day 21, though they remained considerably lighter than the HFC controls. This demonstrated a partial reversal of the weight loss associated with the treatment after its withdrawal. Ecological examinations of the fecal bacterial community revealed distinct structural changes in the treated mice during the active treatment phase. For instance, NMDS ordination clearly showed a divergence in the bacterial community structure of HFL mice on Day 14 from their initial baseline (Day 0) and from both control groups, HFC and LFC, with statistical analyses confirming significant distinctions. However, by the end of the 7-day washout period on Day 21, the gut bacterial community of HFL mice began to revert towards its baseline composition and resembled that of the vehicle-treated HFC animals, although it remained markedly different from the LFC group. Shannon diversity and Chao1 species richness measurements corroborated these observations, also revealing a temporary reduction during active treatment. Shannon diversity only partially recovered, while Chao1 richness largely returned to its pre-treatment level after the washout. Furthermore, Bacteroidales-related sequences remained diminished at Day 21. The SIMPER analysis ultimately identified 21 primary ASVs responsible for 83% of the community variation within the HFL group. Nine of these ASVs, predominantly related to Lactobacillus gasseri, L. paragasseri, and L. johnsonii, along with three related to Leptogranulimonas caecicola, significantly increased during active treatment. Conversely, 12 ASVs linked to protein- and carbohydrate-fermenting bacteria, including genera such as Romboutsia, Faecalicatena, and Oscillibacter, showed a significant decrease by Day 14, from 11.49% to 2.61%, before partially rebounding to 8.54% post-washout on Day 21.
This study reveals that liraglutide induced swift and largely reversible alterations in the fecal bacterial community of male high-fat-fed mice. While these findings, derived exclusively from male mice, cannot be directly extrapolated to human populations, they suggest that liraglutide temporarily favored lactic acid bacteria associated with mucin and bile acids, while concurrently diminishing several fermentative bacterial groups. However, the study did not directly measure mucin, bile acids, bacterial metabolism, or the underlying mechanisms driving these changes, leaving open the question of whether similar effects manifest in humans. Additionally, because liraglutide was administered only to high-fat-fed mice, the research could not ascertain if the microbial response varied depending on the dietary regimen.