Somatic ERK activation during transit amplification is essential for maintaining the synchrony of germline divisions in Drosophila testis

Abstract

Transit amplification (TA) of progenitor cells maintains tissue homeostasis by balancing proliferation and differentiation. In Drosophila testis, the germline proliferation is tightly regulated by factors present in both the germline and the neighbouring somatic cyst cells (SCCs). Although the exact mechanism is unclear, the epidermal growth factor receptor (EGFR) activation in SCCs has been reported to control spermatogonial divisions within a cyst, through downstream activations of Rac1-dependent pathways. Here, we report that somatic activation of the mitogen-activated protein kinase (Rolled/ERK) downstream of EGFR is required to synchronize the mitotic divisions and regulate the transition to meiosis. The process operates independently of the Bag-of-marble activity in the germline. Also, the integrity of the somatic cyst enclosure is inessential for this purpose. Together, these results suggest that synchronization of germ-cell divisions through somatic activation of distinct ERK-downstream targets independently regulates TA and subsequent differentiation of neighbouring germline cells.

Atypical septate junctions maintain the somatic enclosure around maturing spermatids and prevent premature sperm release inDrosophilatestis

“…A previous study in adult Drosophila testis reported that the expression of Cora and NrxIV in the SCCs is essential for forming a functional germ-soma permeability barrier during the spermatogonial stage, which is necessary for further germline differentiation (Fairchild et al, 2015). A subsequent report, however, showed that disruption of the permeability barrier by independent means did not always affect the spermatogonial division and differentiation (Gupta et al, 2018). However, as discussed earlier, SJ proteins also have roles other than serving as a diffusion barrier.…”

“…Subsequently, they are individualized, coiled and released into the seminal vesicle (SV) as mature sperm (Lindsley and Tokuyasu, 1980). The spermatogonial cysts become impermeable to the soluble dye, FITC-dextran, from an early stage of development (Fairchild et al, 2015; Gupta et al, 2018). Further, knockdown of NrxIV and Cora in SCCs permeabilizes the cysts and affects the spermatogonia to spermatocyte transition (Fairchild et al, 2015).…”

Atypical septate junctions maintain the somatic enclosure around maturing spermatids and prevent premature sperm release inDrosophilatestis

“…In Dlg1 homozygous mutant larvae, the Eyes-absent (Eya)-positive SCCs were reduced, and apoptosis was induced in the 16-cell spermatocyte cysts, indicating a role of Dlg in somatic differentiation, as well as for the survival and differentiation of germ cells (Papagiannouli and Mechler, 2009). Although the tj - Gal4 mediated knockdown of Dlg1 in the SCCs during the spermatogonial stages disrupted the cyst permeability barrier and arrested differentiation, it did not affect the transit amplifying divisions of the spermatogonia (Gupta et al, 2018). Hence, we conjectured that in addition to maintaining the barrier function, the somatic Dlg1 activity might specifically regulate the transition to the meiotic stages in the male germline.…”

“…In the testis, however, the SJ proteins are localized on both the germline and somatic cell membrane during the spermatogonial stages, and the somatic permeability barrier is established from the 4-cell stage. Loss of the SJ components-Dlg1, Nrx-IV, and Cora-from the germ-soma interface during this period disrupted the permeability barrier and affected subsequent differentiation to the spermatocyte stage (Fairchild et al, 2015; Gupta et al, 2018). A similar loss of permeability due to the knockdown of Armadillo/β-catenin, however, did not appear to affect the immediate differentiation to the spermatocyte stage (Gupta et al, 2018).…”

“…Loss of the SJ components-Dlg1, Nrx-IV, and Cora-from the germ-soma interface during this period disrupted the permeability barrier and affected subsequent differentiation to the spermatocyte stage (Fairchild et al, 2015; Gupta et al, 2018). A similar loss of permeability due to the knockdown of Armadillo/β-catenin, however, did not appear to affect the immediate differentiation to the spermatocyte stage (Gupta et al, 2018). Therefore, the SJ proteins Dlg1, Cora, and Nrx-IV are likely to regulate the germline differentiation independent of their role in establishing the barrier function.…”