Population Council Research that makes a difference

Primitive stem cells, largely believed to be of mesenchymal origin and present in the interstitium of the testis at birth, are the points of origin for the postnatal Leydig cell lineage. The embryonic derivation of these stem cells is still unknown, but both mesodermal and ectodermal origins have been proposed. Leydig cells first become recognizable as progenitor Leydig cells by day 14 postpartum when, in contrast to stem cells, they start to express lineage-specific markers.

These include histochemical staining for 3β-hydroxysteroid dehydrogenase (3β-HSD) and androgens. Progenitor Leydig cells, which are small, spindle-shaped, and highly proliferative on day 14 in situ, enlarge, become round, and greatly reduce their rate of cell division over the next 14 days.

For reasons that remain unclear, progenitor Leydig cells do not differentiate directly into cells that secrete testosterone as their predominant steroid product, but exist for the next four weeks (days 28–56) as immature Leydig cells that secrete 5α-reduced androgens. Immature Leydig cells divide a final time before maturing into adult Leydig cells by day 56. Several hormones and growth factors known to affect Leydig cell development include luteinizing hormone (LH, a pituitary hormone) together with local paracrine and autocrine cytokines and growth factors. A reservoir of Leydig stem cells persists in the testis, supporting a slow turnover of the adult Leydig cell population, or repopulating Leydig cells during the regeneration that follows toxic insults.

The focus of these investigations continues to be elucidating the molecular mechanisms that underlie progenitor commitment to a fully steroidogenic-competent Leydig cell in the context of its microenvironment. Studies show that one lineage-specification factor for a testicular progenitor is growth hormone (GH), with effects on transcription of target genes directly mediated by its receptor and downstream signaling components.

Population Council researchers found that in the testicular interstitium, progenitor but not immature or adult Leydig cells express growth hormone receptor (GHR), a member of the class 1-cytokine receptor family. GH rapidly activates the tyrosine kinase JAK2 and phosphorylation of the transcription factor STAT5b in progenitor but not adult Leydig cells; significant p44/42 activation is not observed unless a second growth factor is present. Locally, several cytokines and prostaglandins regulate the hormone responsiveness of the progenitors and presumably their precursor stem cells.

Researchers have shown that hormone binding to GHR transcriptionally directs the commitment of progenitors to the Leydig cell lineage ex vivo. Significantly, GH induces the expression of the START-domain cholesterol transfer protein, StAR. GHR signaling also induces an array of growth factors and their receptors. These findings are consistent with a direct interaction of GH with GHR in progenitors, and one that should not occur in differentiated Leydig cells in situ. While it has long been appreciated that GH potentiates the effects of LH on the testis, the data predict that GH is one of the environmental factors that predisposes the progenitor cell to "commit" to differentiating to an immature Leydig cell. Taken together, these data suggest that final Leydig cell lineage commitment is made in the context of the local growth factors and the cell’s acquisition of responsiveness to LH.