Germ cell depletion in recipient testis has adverse effects on spermatogenesis in orthotopically transplanted testis pieces via retinoic acid insufficiency

Abstract

Germ cell depletion in recipient testes is indispensable for successful transplantation of spermatogonial stem cells. However, we found that such treatment had an adverse effect on spermatogenesis of orthotopically transplanted donor testis tissues. In the donor tissue, the frequency of stimulated by retinoic acid (RA) 8 (STRA8) expression was reduced in germ cells, suggesting that RA signalling indispensable for spermatogenesis was attenuated in germ cell-depleted recipient testes. In this context, germ cell depletion diminished expression of testicular Aldh1a2, which is responsible for testicular RA synthesis, while Cyp26b1, which is responsible for testicular RA metabolism, was still expressed even after germ cell depletion, suggesting an alteration of the RA synthesis/ metabolism ratio. These observations suggested that RA insufficiency was one of the causes of the defective donor spermatogenesis. Indeed, repetitive RA administrations significantly improved donor spermatogenesis to produce fertile offspring without any side effects. These findings may contribute to improving fertility preservation techniques for males, especially to prevent iatrogenic infertility induced by chemotherapy in prepubertal cancer patients. Spermatogonial stem cells (SSCs), a subset of undifferentiated spermatogonia, maintain their population by glial cell line-derived neurotrophic factor (GDNF)-mediated self-renewal 1. Undifferentiated spermatogonia are induced to differentiate into differentiating spermatogonia by retinoic acid (RA). This molecule also mediates the next differentiation step, namely differentiation of differentiating spermatogonia into spermatocytes 2,3. These processes require support from the germline niche consisting of Sertoli and Leydig cells in addition to the basement membrane 4,5. The former cell type was found by Enrico Sertoli as 'nurse cells' for germ cells, which produces GDNF and form a blood-testis barrier, both of which are indispensable for proper spermatogenesis 6. The latter cell type found by Franz Leydig synthesises androgens depending on luteinising hormone to regulate Sertoli cell functions 7. Recent reports have demonstrated that residential macrophages, peritubular myoid cells, and neighbouring lymphatic epithelial cells also have some effects on spermatogenic processes 8-11. Moreover, the hypothalamic-pituitary axis further controls Leydig cell functions via luteinising hormones to produce