Robust inference on effects attributable to mediators: A controlled-direct-effect-based approach for causal effect decomposition with multiple mediators.

Effect decomposition is a critical technique for mechanism investigation in settings with multiple causally ordered mediators. Causal mediation analysis is a standard method for effect decomposition, but the assumptions required for the identification process are extremely strong. Moreover, mediation analysis focuses on addressing mediating mechanisms rather than interacting mechanisms. Mediation and interaction for mediators both contribute to the occurrence of disease, and therefore unifying mediation and interaction in effect decomposition is important to causal mechanism investigation. By extending the framework of controlled direct effects, this study proposes the effect attributable to mediators (EAM) as a novel measure for effect decomposition. For policymaking, EAM represents how much an effect can be eliminated by setting mediators to certain values. From the perspective of mechanism investigation, EAM contains information about how much a particular mediator or set of mediators is involved in the causal mechanism through mediation, interaction, or both. EAM is more appropriate than the conventional path-specific effect for application in clinical or medical studies. The assumptions of EAM for identification are considerably weaker than those of causal mediation analysis. We develop a semiparametric estimator of EAM with robustness to model misspecification. The asymptotic property is fully realized. We applied EAM to assess the magnitude of the effect of hepatitis C virus infection on mortality, which was eliminated by controlling alanine aminotransferase and treating hepatocellular carcinoma.

LIMK-dependent actin polymerization in primary sensory neurons promotes the development of inflammatory heat hyperalgesia in rats.

Changes in the actin cytoskeleton in neurons are associated with synaptic plasticity and may also be involved in mechanisms of nociception. We found that the LIM motif-containing protein kinases (LIMKs), which regulate actin dynamics, promoted the development of inflammatory hyperalgesia (excessive sensitivity to painful stimuli). Pain is sensed by the primary sensory neurons of dorsal root ganglion (DRG). In rats injected with complete Freund's adjuvant (CFA), which induces inflammatory heat hyperalgesia, DRG neurons showed an increase in LIMK activity and in the phosphorylation and thus inhibition of the LIMK substrate cofilin, an actin-severing protein. Manipulations that reduced LIMK activity or abundance, prevented the phosphorylation of cofilin, or disrupted actin filaments in DRG neurons attenuated CFA-induced heat hyperalgesia. Inflammatory stimuli stimulated actin polymerization and enhanced the response of the cation channel TRPV1 (transient receptor potential V1) to capsaicin in DRG neurons, effects that were reversed by the knockdown of LIMK or preventing cofilin phosphorylation. Furthermore, inflammatory stimuli caused the serine phosphorylation of TRPV1, which was abolished by preventing cofilin phosphorylation in DRG neurons. We conclude that LIMK-dependent actin rearrangement in primary sensory neurons, leading to altered TRPV1 sensitivity, is involved in the development of inflammatory hyperalgesia.