Date of Award
Spring 4-30-2025
Degree Type
Thesis
Degree Name
Master of Science in Integrative Biology
Department
Department of Molecular & Cellular Biology
Committee Chair/First Advisor
Brandon Carpenter
Second Advisor
Chris Cornelison, Scott Nowak, Joanna Wardwell-Ozgo
Abstract
Transcription-coupled histone methylation acquired at sperm and oocyte genes during gametogenesis helps establish distinct gamete cell fates. Like DNA, acquired histone methylation can be inherited between generations and must be maternally reprogrammed at fertilization to reset the epigenetic ground state of the zygote. During maternal reprogramming of histone methylation in C. elegans, H3K4me1/2 is removed by the H3K4 demethylase, SPR-5, and H3K9me1/2 is subsequently added by the histone methyltransferase, MET-2. Maternal reprogramming by SPR-5 and MET-2 is antagonized by the H3K36 methyltransferase, MES-4, which maintains H3K36me2/3 at germline genes to ensure proper germline gene expression in germ cells. In the absence of SPR-5; MET-2 maternal reprogramming MES-4 maintains H3K36me2/3 at germline genes in the soma, leading to somatic expression of germline genes and a variety of abnormal somatic and developmental defects. In addition to MES-4 germline genes, a recently identified germline transcription factor, LSL-1, is misexpressed in spr-5; met-2 mutants and may contribute to maintaining germline gene expression in the soma of these mutants. Here, we explore how misinherited histone methylation affects somatic tissues by examining muscle morphology and function in early, middle and late generation spr-5 and met-2 single mutants that inherit increasing levels of aberrant H3K4me2. Interestingly, met-2 mutants show lower motility levels compared to wildtype and spr-5 mutants at early generations, but this initial decrease in motility does not decline over generations, even as muscle sarcomeres become moderately disorganized. In spr-5; met-2 mutants, we find that muscle sarcomeres and motility are severely perturbed and that maternal knockdown of either mes-4 or lsl-1 in spr-5; met-2 mutants rescues these muscle defects. Despite the muscle defects that we observed in spr-5; met-2 mutants, muscle-specific gene expression is normal. After discovering that muscle-specific transcription is normal, we wanted to determine if germline-specific proteins are being expressed in spr-5; met-2 mutant muscle cells. We found that SPE-6, a germline protein localized to the inactivated chromatin of sperm, was aberrantly expressed in muscle cell nuclei, suggesting that germline genes are being translated into proteins in somatic tissues. Finally, we show that muscle cells overexpressing low levels of LSL-1 display sarcomere disorganization. Together, our findings provide a unique insight into how tissue-specific phenotypes arise when histone methylation is inappropriately inherited and support a model where germline-expressed proteins themselves may perturb somatic cell function without compromising the normal somatic transcription program.
Comments
I would like to thank Hiroshi Qadota, Sandra Nguyen, Penelope Rodriguez, Mattie Villhauer, Guy M. Benian and Brandon Carpenter for their help with data acquisition and analysis in this thesis