Animal models of gut microbiota-male reproductive interactions: methodological assessment, translational bottlenecks, and optimization strategies.

Idiopathic infertility accounts for 30%-40% of male infertility cases with etiology remaining largely unexplained.

• The gut-testis axis hypothesis has emerged as a promising direction for explaining idiopathic male infertility.
• Human cohort studies have revealed statistical associations between gut dysbiosis and impaired semen quality.
• The paper identifies a gap between observational associations in humans and mechanistic/causal evidence.

Existing animal models of the gut-testis axis, particularly those testing individual molecular pathways, employ exposure magnitudes, routes, and time scales that may not fully reflect the prolonged, low-intensity disturbances hypothesized in gut-driven reproductive injury.

• The authors identify 'a recurring pattern' in which model exposures do not match the 'chronic, low-grade nature of gut-derived insults' hypothesized to contribute to idiopathic infertility.
• Models were organized according to the tier of causal question they address and evaluated for internal, external, and construct validity.
• Whether current models adequately capture chronic, low-grade gut-derived insults remains 'insufficiently examined' according to the authors.

Collective animal model research has identified several signaling axes that, under defined experimental conditions, can measurably affect the testis.

• The authors describe these demonstrations as showing 'conditional biological capacity' of signaling pathways.
• The findings are described as demonstrations of pathway capacity rather than evidence of etiological contribution to human disease.
• Internal and external validity limitations of many models 'warrant caution' when interpreting these findings.

The authors propose a logical framework for evaluating causal evidence in gut-testis axis research anchored on three criteria: endogenous signal origin, pathomimetic exposure profile, and attributive exclusivity.

• The framework is designed to transition evaluation from establishing a pathway's biological capacity to assessing its etiological contribution.
• Endogenous signal origin requires that the signaling molecules originate from gut sources rather than exogenous administration.
• Pathomimetic exposure profile requires that exposure conditions mimic the chronic, low-grade nature of gut-derived disturbances.
• Attributive exclusivity requires distinguishing the contribution of the specific pathway from other confounding variables.

The authors propose a preliminary reporting framework termed Minimal Information for Gut-Testis Axis Rodent Studies (MIGTARS) to support more transparent and methodologically informative reporting.

• MIGTARS is described as a 'preliminary reporting framework' rather than a finalized standard.
• The framework aims to support 'more transparent and methodologically informative reporting' of gut-testis axis rodent studies.
• MIGTARS and the causal evaluation framework together are described as providing 'a methodological scaffold for moving the gut-testis field from pathway verification toward translational modeling.'

The authors identify a fundamental distinction between demonstrating a pathway's biological capacity and establishing its etiological contribution to human idiopathic infertility.

• The paper warns against interpreting 'demonstrations of conditional biological capacity as direct evidence that the same pathways carry etiological weight in human idiopathic infertility.'
• This distinction is framed as a translational bottleneck limiting the field's progress.
• Establishing mechanistic and causal evidence requires 'complementary approaches, including animal models' beyond observational human cohort studies.