Seasonal temperature induced heart-collagen remodeling response in the rainbow darter (𝘌𝘵𝘩𝘦𝘰𝘴𝘵𝘰𝘮𝘢 𝘤𝘢𝘦𝘳𝘶𝘭𝘦𝘶𝘮)

Abstract

Acclimation to temperature changes in fish has been shown to prompt a cardiac remodeling response, with collagen protein playing a key role, although the mechanism of this response remains unclear. Currently, it is believed to be a seasonal adaptation to shifting temperatures, with studies indicating that microRNA-29b (miR-29b), an epigenetic non-coding RNA, targets collagen mRNA in the heart. To further explore these questions, this thesis characterizes the remodeling response in a wild population of rainbow darters (Etheostoma caeruleum) to examine seasonal effects in a natural environment, which moves beyond the current lab-only studies. Rainbow darter heart tissue was collected on-site at three season timepoints (Spring, Summer, Fall 2023) from the Grand River, Grand Valley, ON. Water temperature loggers were deployed to monitor the fluctuating river temperatures throughout the study. Gene expression of miR-29b, and the three collagen type I protein transcripts (col1a1, col1a2, & col1a3) was measured in heart tissue through RT-qPCR. Hearts were embedded for histological analysis to visualize heart morphology and quantify the collagen protein content through picro-sirius red staining. Additionally, western blot analysis was performed to measure collagen type I abundance for each season. Temperature loggers revealed substantial daily water temperature fluctuations, with differing fluctuation profiles dependent on season. Results from RT-qPCR revealed seasonal differences in expression of col1a1, and col1a2, and western blot revealed a season effect driving a trend in collagen type I protein differences, suggesting the presence of this remodeling response in a non-model species. Although only a sex-specific difference was found in miR-29b expression and no significant regression was found between miR-29b and any of the collagen type I mRNA, further investigation into its role in this remodeling in vivo will be required. The lack of a compact myocardium layer was determined from cardiac tissue sections, which previously had not been investigated in the rainbow darter and evidence of cardiac hypertrophy at acclimatization to colder temperatures was demonstrated by higher relative heart mass compared to body mass in spring and fall, compared to summer. Understanding the impacts of temperature fluctuations and extreme weather events on local fish populations is increasingly crucial. With limited studies of cardiac collagen remodeling in natural environments, it is important that there be continued focus on bringing elements of natural seasonality into the context of lab studies to further investigate this remodeling response as seasonal plasticity and flexibility. This thesis contributes to a more comprehensive understanding of seasonal effects in a natural environment.