Reproduction disrupts stem cell homeostasis in testes of aged male Drosophila via an induced microenvironment

Abstract

Stem cells rely on instructive cues from their environment. Alterations in microenvironments might contribute to tissue dysfunction and disease pathogenesis. Germline stem cells (GSCs) and cyst stem cells (CySC) in Drosophila testes are normally maintained in the apical area by the testicular hub. In this study, we found that reproduction leads to accumulation of early differentiating daughters of CySCs and GSCs in the testes of aged male flies, due to hyperactivation of Jun-N-terminal kinase (JNK) signaling to maintain self-renewal gene expression in the differentiating cyst cells. JNK activity is normally required to maintain CySCs in the apical niche. A muscle sheath surrounds the Drosophila testis to maintain its long coiled structure. Importantly, reproduction triggers accumulation of the tumor necrosis factor (TNF) Eiger in the testis muscle to activate JNK signaling via the TNF receptor Grindelwald in the cyst cells. Reducing Eiger activity in the testis muscle sheath suppressed reproduction-induced differentiation defects, but had little effect on testis homeostasis of unmated males. Our results reveal that reproduction in males provokes a dramatic shift in the testicular microenvironment, which impairs tissue homeostasis and spermatogenesis in the testes.

Upregulated TNF/Eiger signaling mediates stem cell recovery and tissue homeostasis during nutrient resupply in Drosophila testis

“…Previously we have shown that reproduction increases Egr levels in the testicular muscle to hyperactivate JNK signaling in somatic cyst cells 12 . Since JNK signaling is also activated in cyst cells in response to amino acid starvation 3 , we then asked if protein starvation induces Egr protein upregulation in testicular smooth muscles.…”

“…We found that Egr is required for the fast recovery of CySCs induced by protein resupply. CySCs are defined as the Zfh-1 positive cells immediately adjacent to GSCs 12 , 27 . In wild-type Oregon-R ( OR ) flies, the number of CySC increased dramatically from 11 to 22 during the first two days of refeeding (R1 and R2) (compare Fig.…”

“…CySC number continued to increase gradually each day from R2 to R5 in egr 1 adults. Egr acts on cyst lineage cells through the receptor Grnd 12 . Examination of CySC recovery in grnd null mutant ( grnd minos /Df ) showed that the recovery rate was very low at R2 compared to R5 (9%, Fig.…”

“…Previously we have found that TNF-JNK signaling is hyper-activated by reproduction in testes, leading to ectopic expression of the CySC self-renewal protein Zfh-1 12 . In this study, we show that prolonged protein starvation induces Egr upregulation in testis smooth muscle.…”

“…The smooth muscles lie immediately adjacent to cyst cells with a thin layer of extracellular matrix between them 11 . Recent studies have shown that testicular muscles produce tumor necrosis factor (TNF) to influence spermatogenesis 12 . Eiger (Egr) is the sole Drosophila TNF homologue.…”