Stem cell maintenance in naked mole rats and other longevity mechanisms in rodents

Abstract

Thesis (Ph. D.)--University of Rochester. Department of Biology, 2018.This thesis investigated the stem cell biology of the longest-lived rodent the naked mole rat (Heterocephalus glaber, NMR), compared to the short-lived laboratory mouse. The NMR is one of only two eusocial mammals, shows extreme longevity with a maximum lifespan of 32 years and negligible age-associated degeneration. During aging, stem cells play a pivotal role in maintaining tissue homeostasis by replenishing damaged and dead cells with newly differentiated functional cells. Age-associated stem cell dysfunction is a driving force of aging. What stem cell capacity the NMR has and how the NMR maintains their stem cell pool is unknown. In this thesis, I comparatively studied the hematopoietic stem cell (HSC) properties in the NMR and the mouse. I employed BrdU label retention assays to label HSC and track the HSC turnover. I was able to show that the BrdU label-retaining cells were responsive to fluorouracil induced bone marrow injury. I have discovered that the NMR HSC localizes to a niche with extremely high levels of high molecular weight hyaluronic acid (HA), displays slow turnover and extreme quiescence, compared to the mouse HSC. I have demonstrated that the niche HA levels reversely relate to HSC reactive oxidative species (ROS) levels. In addition, overexpressing NMR HAS2 in mice reduces ROS in the HSC and expands the HSC pool by about 3-fold. I have also shown the inhibition of HA synthesis by 4-MU impaired hematopoiesis in NMRs. With these findings, I propose a model of HSC maintenance that highlights the key role of niche HA in maintaining the quiescent HSC pool. Additionally, I describe the NMR iPSC reprogramming done with extensive collaboration with Dr. Li Tan. We have found the NMR fibroblasts are resistant to both the mouse and the NMR defined factors induced reprogramming, either in primed or naïve culture conditions. We have screened factors enhancing reprogramming and found the large T antigen drastically increased iPSC reprogramming efficiency. We have found that the NMR fibroblasts have a more repressive Rb signal pathway and large T induces massive opening of more closed promoter regions in the NMR, compared to the mouse. These results suggest that NMR displays a more stable epigenome that resists iPSC reprogramming. In another study that I collaborated with Dr. Jorge Azpurua, we have discovered that the NMR shows about 10 times higher translation fidelity than the mouse. However, whether the translation fidelity correlates with species maximum lifespan is unknown. Thus, I examined translation fidelity in 17 rodent species with diverse maximum lifespans. I have found that the fidelity at the first and second codon positions strongly correlates with species maximum lifespan, and that correlation remains significant after phylogenetic correction. This finding suggests that translation fidelity plays a novel role in longevity. Lastly, I had been managing the NMR colonies during my study and developed strategies to improve the husbandry and breeding of captive NMR colonies. As breeding and keeping NMRs in captivity is challenging to researchers, the slow breeding and low survival of NMRs under laboratory condition limits the NMR research. I have optimized the colony setting which allows NMR colonies to settle down more rapidly and established different chambers for different functions. I have found that pairing young NMRs, but not younger than 2 years old, could result in higher successful rate of establishing new colonies. I have also successfully cross-fostered NMR pups in a foreign colony. All of these strategies will help researchers struggling with breeding NMRs in captivity