Mitochondria and Female Germline Stem Cells-A Mitochondrial DNA Perspective

Abstract

Mitochondria and mitochondrial DNA have important roles to play in development. In primordial germ cells, they progress from small numbers to populate the maturing oocyte with high numbers to support post-fertilization events. These processes take place under the control of significant changes in DNA methylation and other epigenetic modifiers, as well as changes to the DNA methylation status of the nuclear-encoded mitochondrial DNA replication factors. Consequently, the differentiating germ cell requires significant synchrony between the two genomes in order to ensure that they are fit for purpose. In this review, I examine these processes in the context of female germline stem cells that are isolated from the ovary and those derived from embryonic stem cells and reprogrammed somatic cells. Although our knowledge is limited in this respect, I provide predictions based on other cellular systems of what is expected and provide insight into how these cells could be used in clinical medicine.

Keywords: DNA methylation; egg precursor cells; embryonic stem cells; female germ cells; mitochondria; mitochondrial DNA; mitochondrial DNA copy number; mtDNA set point.

Figure 1

The multiple pathways available for the production of cellular ATP. In the cytoplasm of the cell, glycolysis generates four molecules of ATP for every two molecules of glucose invested. This is the pathway favored by fast replicating cells. In the mitochondrion, the processes of β-oxidation, the citric acid cycle and OXPHOS take place. OXPHOS is favored by cells with complex functions. Two molecules of ATP are generated by the citric acid cycle and 34 molecules are generated by OXPHOS (adapted from reference [20]).

Figure 2

The regulation of mtDNA copy number during development. In primordial germ cells, mtDNA is maintained at low levels. As oogenesis progresses, mtDNA copy number increases significantly and is then arrested at the metaphase II stage. A threshold (broken blue line) needs to be reached in order that oocytes mature and fertilize. Following fertilization, mtDNA copy number decreases through to the blastocyst stage. mtDNA replication is initiated in the trophectoderm, whilst the ICM continues to reduce mtDNA copy number. This enables the developing embryo to establish the mtDNA set point prior to differentiation. Following commitment to a specific lineage, cells then replicate their mtDNA in a cell-specific manner to enable them to perform their specialized functions through OXPHOS, as required. Furthermore, there are synchronous changes to DNA methylation and gene expression profiles throughout these processes. TET enzymes reduce parental DNA methylation through to the blastocyst stage whilst de novo DNA methylation, mediated by DNMT3a and DNMT3b, is initiated in the blastocyst. DNMT1 then maintains the newly established cell-specific DNA methylation profiles.

Figure 3

The use of female germline stem cells to overcome mtDNA disease. Female germline stem cells from a carrier of a mtDNA mutation or deletion can be depleted of their mtDNA and fused to an enucleated stem cell harboring unaffected (non-mutated or deleted) mtDNA from a donor source to generate a reconstructed female ‘cybrid’ germline stem cell. The cell can then be proliferated and cultured to the metaphase II stage in readiness for fertilization.

Figure 4

Germ cell differentiation. Stage-specific, synchronized interactions between the nuclear and mitochondrial genomes during germ cell differentiation and oogenesis are required to establish a viable and functional oocyte fit for fertilization (metaphase II oocyte).