Modeling human infertility with pluripotent stem cells

Abstract

Human fertility is dependent upon the correct establishment and differentiation of the germline. This is because no other cell type in the body is capable of passing a genome and epigenome from parent to child. Terminally differentiated germline cells in the adult testis and ovary are called gametes. However, the initial specification of germline cells occurs in the embryo around the time of gastrulation. Most of our knowledge regarding the cell and molecular events that govern human germline specification involves extrapolating scientific principles from model organisms, most notably the mouse. However, recent work using next generation sequencing, gene editing and differentiation of germline cells from pluripotent stem cells has revealed that the core molecular mechanisms that regulate human germline development are different from rodents. Here, we will discuss the major molecular pathways required for human germline differentiation and how pluripotent stem cells have revolutionized our ability to study the earliest steps in human embryonic lineage specification in order to understand human fertility.

Figure 1. Time line of PGC development in humans

Early PGCs (green) are identified in the yolk sac followed by the hindgut and then ultimately the genital ridge. Once PGCs exit the hindgut and begin expressing VASA they are called “late PGCs”. Late PGCs colonize the genital ridges beginning at week 5. Advanced PGCs develop at the conclusion of the Carnegie stages from 60–77 days with the emergence of male and female-specific transcriptional programs. In humans development is sometimes referred to as gestation (G), which refers to time since last menstrual cycle. PF = post fertilization, E = embryonic day, PC = post-coitus. The timing of mouse and macaque (rhesus) PGC development is shown for comparison.

Figure 2. Major signaling pathways and transcription factors in mouse and human PGC specification

The mouse has been invaluable for identifying the signaling pathways required for PGC specification. The finding that NANOG can induce PGCLC formation independent from BMP4 was discovered using in vitro differentiation into epiblast-like cells followed by induction of PGCLCs. Although the information on human PGC development is limited, initial experiments using pluripotent stem cell differentiation indicate that the mechanisms of human PGC development are different from the mouse.

Figure 3. Using Human iPSCs to understand and treat human infertility

Human iPSCs can be used to generate stem cells from infertile patients where the disease is speculated to originate from the germline. Today, differentiation of the hiPSCs into PGCLCs could be used as a model to understand fertility. In the future, hiPSC technology and the differentiation of PGCLCs could be used to restore fertility to individuals who lost their germline due to injury. For example, this therapy could be used in young adults who have become infertile after chemotherapy or radiation therapy to treat cancer when all other options have been exhausted.