Preparing to migrate: expression of androgen signaling molecules and insulin-like growth factor-1 in skeletal muscles of Gambel’s white-crowned sparrows
Migratory birds, including Gambel’s white-crowned sparrows (Zonotrichia leucophrys gambelii), exhibit profound modifications of skeletal muscles prior to migration, notably hypertrophy of the pectoralis muscle required for powered flight. Muscle growth may be influenced by anabolic effects of androgens; however, prior to spring departure, circulating androgens are low in sparrows. A seasonal increase in local androgen signaling may occur within muscle to promote remodeling. We measured morphological parameters, plasma and tissue levels of testosterone, as well as mRNA expression levels of androgen receptor, 5α-reductase (converts testosterone to 5α-dihydrotestosterone), and the androgen-dependent myotrophic factor insulin-like growth factor-1. We studied the pectoralis muscle as well as the gastrocnemius (leg) muscle of male sparrows across three stages on the wintering grounds: winter (February), pre-nuptial molt (March), and pre-departure (April). Testosterone levels were low, but detectable, in plasma and muscles at all three stages. Androgen receptor mRNA and 5α-reductase Type 1 mRNA increased at pre-departure, but did so in both muscles. Notably, mRNA levels of insulin-like growth factor-1, an androgen-dependent gene critical for muscle remodeling, increased at pre-departure in the pectoralis but decreased in the gastrocnemius. Taken together, these data suggest a site-specific molecular basis for muscle remodeling that may serve to enable long-distance flight.
Keywords5α-Reductase Androgen receptor Life-history stages Songbird Testosterone
Insulin-like growth factor-1, IGF-1
We thank E. Graves, Drs. Z. Németh, J. Krause, and J. Pérez for help with fieldwork, D. Comito and R. Van Ness for help with laboratory assays, and Drs. X. Chen and M. Rensel for technical advice. This study was supported by The National Institutes of Health T32 training grant (5T32HD007228) to DSP, National Science Foundation ARC-1147289 to MR, Canadian Institutes of Health Research Operating Grant MOP 133606 to KKS, and National Institutes of Health MH061994 to BAS. All authors contributed to the writing of the manuscript. DSP performed the molecular laboratory work and associated data analysis. CM performed T measurements and participated in data analysis. BAS and KKS participated in the design of the study and provided technical advice. MR conceived of the study, designed the study, coordinated the study, collected field data, participated in morphological data analysis. All authors gave final approval for publication. All applicable international, national, and University of California Davis Institutional Animal Care and Use Committee (IACUC—Protocol #17144) guidelines were followed and conducted under the scientific collecting permits of M. Ramenofsky issued by the California Department of Fish and Wildlife (# 11024) and US Fish and Wildlife Department (# MB11826A-5).
Compliance with ethical standards
Conflict of interest
Authors have no competing or conflict of interests.
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