Coping With Urban Habitats Via Glucocorticoid Regulation: Physiology, Behavior, and Life History in Stream Fishes

As environments become urbanized, tolerant species become more prevalent. The physiological, behavioral and life-history mechanisms associated with the success of such species in urbanized habitats are not well understood, especially in freshwater ecosystems. Here, we examined the glucocorticoid (GC) profiles, life-history traits, and behavior of two species of fish across a gradient of urbanization to understand coping capacity and associated trade-offs. We studied the tolerant live-bearing Western Mosquitofish (Gambusia affinis) for two years and the slightly less tolerant, egg-laying, Blacktail Shiner (Cyprinella venusta) for one year. We used a water-borne hormone method to examine baseline, stress-induced, and recovery cortisol release rates across six streams with differing degrees of urbanization. We also measured life-history traits related to reproduction, and for G. affinis, we measured shoaling behavior and individual activity in a novel arena. Both species showed a trend for reduced stress responsiveness in more urbanized streams, accompanied by higher reproductive output. Although not all populations fit this trend, these results suggest that GC suppression may be adaptive for coping with urban habitats. In G. affinis, GC recovery increased with urbanization, and individuals with the lowest stress response and highest recovery had the greatest reproductive allotment, suggesting that rapid return to baseline GC levels is also an important coping mechanism. In G. affinis, urban populations showed altered life-history trade-offs whereas behavioral traits did not vary systematically with urbanization. Thus, these tolerant species of fish may cope with anthropogenically modified streams by altering their GC profiles and life-history trade-offs. These results contribute to understanding the mechanisms driving species-specific adaptations and thereby community structure in freshwater systems associated with land-use converted areas

Rbfox-Splicing Factors Maintain Skeletal Muscle Mass by Regulating Calpain3 and Proteostasis

Summary: Maintenance of skeletal muscle mass requires a dynamic balance between protein synthesis and tightly controlled protein degradation by the calpain, autophagy-lysosome, and ubiquitin-proteasome systems (proteostasis). Several sensing and gene-regulatory mechanisms act together to maintain this balance in response to changing conditions. Here, we show that deletion of the highly conserved Rbfox1 and Rbfox2 alternative splicing regulators in adult mouse skeletal muscle causes rapid, severe loss of muscle mass. Rbfox deletion did not cause a reduction in global protein synthesis, but it led to altered splicing of hundreds of gene transcripts, including capn3, which produced an active form of calpain3 protease. Rbfox knockout also led to a reduction in autophagy flux, likely producing a compensatory increase in general protein degradation by the proteasome. Our results indicate that the Rbfox-splicing factors are essential for the maintenance of skeletal muscle mass and proteostasis. : Rbfox-splicing factors are highly conserved and expressed in brain, heart, and skeletal muscle. Singh et al. show that Rbfox proteins are essential for the maintenance of muscle mass and proteostasis, as Rbfox double knockout causes increased calpain and proteasome activity and reduced autophagy flux. Keywords: alternative splicing, proteostasis, calpain, autophagy, muscle biology, Rbfox, proteasom