Date of Award

2013

Degree Type

Thesis

Degree Name

Master of Science (MS)

Department

Fisheries, Animal and Veterinary Science

First Advisor

Marta Gomez-Chiarri

Abstract

Myogenesis in vertebrates is a complex and intricately regulated process, various mechanisms of which have been alternately conserved or lost throughout the course of vertebrate evolution. Mammalian myogenesis is highly regulated by myostatin, a member of the Transforming Growth Factor-β (TGF-β) family of cytokines, however, a similar role for myostatin in fish currently remains the subject of inquiry. Myostatin initiates signal transduction by means of the activin type IIb receptor, and shares considerable sequence and structural homology across vertebrate taxa, though its functional homology remains unclear. Two families of transgenic (TG) rainbow trout (RBT), one in which myostatin was inhibited (PD), and another in which myostatin and related TGF-β ligands were collectively inhibited (ActRIIB), were utilized to investigate the growth, morphology, muscle phenotype, and downstream genetic activity proceeding from myostatin/TGF-β inhibition. Additionally, the well established function of rbST in rendering increased somatogenesis was investigated in these two families; groups of TG and non-transgenic (NTG) fish received either a dosage of rbST (50 μg g-1 body weight) or placebo (sesame oil). Growth and muscularity in TG fish were hypothesized to surpass that observed in NTG fish in the context of myostatin inhibition. Administration of rbST resulted in accelerated growth and increased body size in both genotypes in the ActRIIB and PD families. Growth rate for fish in both families did not differ between genotypes for fish receiving identical treatments, suggesting that myostatin/TGF-β inhibition does not augment growth rate in RBT. Neither inhibitory mechanism increased body size in TG fish beyond that of NTG individuals, demonstrating that these effectors do not serve a predominant role in constraining absolute growth. While TG pro-domain fish maintained body size and conformation equivalent to that of NTG fish within both treatment groups, ActRIIB TG fish remained significantly shorter, with a significantly lesser body mass and higher condition factor than NTG fish, irrespective of treatment. The findings presented here suggest that one or more TGF-β molecules, excluding myostatin, that bind the activin type IIb receptor appear to be involved in axial patterning and growth potential. A role for a TGF-β molecule other than myostatin in restricting the capacity for hyperplasia is supported by the increased myofiber density of the epaxial musculature. Myostatin and related TGF-β molecules were found to impede expression of the myogenic regulatory factor MyoD, however, increased expression failed to enhance myogenesis. Myogenin expression was not influenced by the reduction in myostatin/TGF-β signaling. These data collectively suggest that myostatin and associated TGF-β molecules are not functioning to restrict muscle growth in basal teleosts as they are in mammals, although some TGF-β ligands may be involved in limiting myogenic proliferation. TGF-β molecules other than myostatin do appear to function to some extent in somite architecture and patterning, and may be necessary to maintain growth potential. Collectively, these findings support the proposal that myostatin has evolved differing capacities within the context of disparate vertebrate physiologies.

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