Date of Award

Spring 4-12-2023

Embargo Period

4-12-2028

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Oral Health Sciences

College

College of Graduate Studies

First Advisor

Chad Novince

Second Advisor

Caroline Westwater

Third Advisor

Alexander Alekseyenko

Fourth Advisor

Stephen Duncan

Fifth Advisor

William Hill

Sixth Advisor

Don Rockey

Abstract

Problem: Antibiotic-induced shifts in the commensal gut microbiota can induce a dysbiotic relationship between the host and microbes. Antibiotic administration during critical windows of development and long-term antibiotic treatment impairs the gut microbiota’s ability to recover to a stable state. Systemic tetracyclines (minocycline, doxycycline) are prescribed for extended durations to treat adolescent acne. However, the impact of tetracyclines on metabolism, adiposity, and skeletal maturation is unclear.

Approach: Sex-matched specific-pathogen-free and germ-free mice were administered a clinically relevant minocycline or doxycycline or vehicle-control from age 6-12 weeks. Mice were euthanized at ages 12 and 18 weeks to determine immediate and sustained antibiotic treatment effects. 16S rDNA sequencing was performed in colonic contents. Skeletal properties and adiposity were evaluated by micro-CT and biomechanical analysis. Osteoblastogenesis/osteoclastogenesis were evaluated by histomorphometry and serum ELISA. qRT-PCR/RNA-seq/nCounter analysis were performed in ileums and livers. Serum bile acids were assessed by mass spectrometry. Osteoblasts from wild-type and FXR-null mice were stimulated with circulating therapeutic levels of minocycline or the circulating bile acid profile detected in specific-pathogen-free mice.

Results: Minocycline, but not doxycycline, impaired bone mass accrual and fracture resistance which was attributed to suppressed osteoblast function. Minocycline did not alter the skeletal phenotype in germ-free mice, which implies that minocycline effects on the skeleton are dependent on the microbiota. Minocycline dysregulated the intestinal FXR-FGF15 axis in specific-pathogen-free mice. The intestinal FXR-FGF15 axis is a gut-liver endocrine axis that supports bile acid homeostasis and host metabolism. Minocycline induced changes in the abundance of genera critical for bile acid metabolism. Minocycline decreased ileum FGF15, upregulated hepatic Cyp7a1, and increased circulating conjugated bile acids. Stimulating primary osteoblasts with the serum bile acid profile from minocycline-treated specific-pathogen-free mice recapitulated the suppressed osteogeneic phenotype found in vivo, which was mediated through attenuated FXR-signaling. Systemic tetracyclines dysregulated liver metabolism and increased adiposity in specific-pathogen-free mice.

Conclusion: This preclinical research reveals that prolonged antibiotic exposure during adolescence suppresses skeletal maturation and promotes adiposity. Further, this body of work introduces gut-liver bile acid metabolism as a candidate regulator of antibiotic-induced gut dysbiosis effects on the skeleton and adiposity.

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Copyright is held by the author. All rights reserved.

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