Mitochondria are a major source of cellular energy. Their numbers vary significantly between cell types and correlate with cell functionality as high energy requiring cells such as muscle and neurons tend to have more mitochondria and mitochondrial DNA (mtDNA). Thus, mitochondrial biogenesis requires careful regulation, however, the mechanisms employed by differentiating cells to acquire the precise number remains poorly understood. Oogenesis is perhaps the most dramatic example of a rapid increase in mitochondria, which are replicated over a 100-fold from that found in the germ cell precursor, and is absolutely crucial for embryonic development. The initiation of this burst in mitochondrial biogenesis and how the oocyte determines when to turn it off remains elusive. Mitochondria in the mature oocyte are also kept transcriptionally silent until fertilization via an unknown mechanism. Preliminary data in our lab, which focuses on how cells monitor and respond to mitochondrial dysfunction, suggests a role for the mitochondrial unfolded protein response (UPRmt) in mitochondrial biogenesis in the C. elegans germline. During mitochondrial dysfunction, cells in essence count the number of functioning mitochondria by monitoring the import efficiency of the transcription factor, ATFS-1. ATFS-1 has a mitochondrial and a nuclear localization signal and is normally imported into mitochondria and degraded. During mitochondrial stress, import capacity is impaired causing ATFS-1 to accumulate in the cytosol, which then traffics to the nucleus to induce the UPRmt, which includes mitochondrial chaperones and the mtDNA polymerase. Interestingly, the soma of worms lacking ATFS-1 develop normally but germlines are defective suggesting a role in oocyte biogenesis.
I aim to examine the role ATFS-1 in regulating mtDNA replication and suppression of mtDNA transcription during oogenesis. We propose a model where progenitor germ cells have a low number of mitochondria that is insufficient to import the constitutively expressed ATFS-1, resulting in ATFS-1 activation driving a mitochondrial biogenesis program. As the maturing oocyte produces more mitochondria and eventually reaches the optimal number for embryogenesis, import capacity increases to a point where ATFS-1 is imported and degraded. Using genetic approaches, I aim to test this model by altering ATFS-1 import efficiency in the germline by manipulating import capacity or the mitochondrial targeting sequence of ATFS-1 and determining mitochondrial number in the resultant oocytes. Subsequently, the effects of mitochondrial number on fertilization and subsequent embryogenesis will be examined. As we have previously shown that ATFS-1 can bind mtDNA and repress transcription, I will also examine the role of ATFS-1 in maintaining oocyte mitochondria in a low activity state until the initiation of embryogenesis. We anticipate these studies providing insight into pathologic conditions associated with mitochondrial dysfunction including infertility, cancers and neurodegenerative disease.
Abnormal mitochondrial function and variations in mitochondrial genome numbers are associated with several human diseases such as cancer, neurodegenerative diseases and general aging; emphasizing the importance of understanding regulatory mechanisms that maintain mitochondria. The goal of our proposed research is to investigate a new role for ATFS-1 in the regulation of mitochondrial duplication during oocyte production. Understanding mitochondrial abnormalities in the oocyte will provide insight on infertility, human development, insulin resistance and obesity.
