Histological section of the fallopian tube. Image credit: Navid Golpour (CC BY 2.0)
Infertility in women can be caused by many factors, such as defects in the ovaries. An important part of the ovaries for fertility are internal structures called follicles, which house early forms of egg cells. A follicle grows and develops until the egg is finally released from the ovary into the fallopian tube, where the egg can then be fertilised. In the follicle, an egg is surrounded by other types of cells, such as granulosa cells. The egg and neighbouring cells must maintain healthy contacts with each other, otherwise the follicle can stop growing and developing, potentially causing infertility.
The development of a follicle depends on an array of proteins. For example, the transcription factor WT1 controls protein levels by activating other genes and their proteins and is produced in high numbers by granulosa cells at the beginning of follicle development. Although WT1 levels dip towards the later stages of follicle development, insufficient levels can lead to defects. So far, it has been unclear how levels of WT1in granulose cells are regulated.
Chen, Dong et al. studied mouse follicles to reveal more about the role of WT1 in follicle development. The researchers measured protein levels in mouse granulosa cells as the follicles developed, and discovered elevated levels of PRMT5, a protein needed for egg cells to form and survive in the follicles. Blocking granulosa cells from producing PRMT5 led to abnormal follicles and infertility in mice. Moreover, mice that had been engineered to lack PRMT5 developed abnormal follicles, where the egg and surrounding granulosa cells were not attached to each other, and the granulosa cells had low levels of WT1. Further experiments revealed that PRMT5 controlled WT1 levels by adding small molecules called methyl groups to another regulatory protein called HnRNPA1.
The addition of methyl groups to genes or their proteins is an important modification that takes place in many processes within a cell. Chen, Dong et al. reveal that this activity also plays a key role in maintaining healthy follicle development in mice, and that PRMT5 is necessary for controlling WT1. Identifying all of the intricate mechanism involved in regulating follicle development is important for finding ways to combat infertility.
