Canonical and non‐canonical functions of STAT in germline stem cell maintenance

Abstract

Background Maintenance of the Drosophila male germline stem cells (GSCs) requires activation of the Janus kinase/signal transducer and activators of transcription (JAK/STAT) pathway by niche signals. The precise role of JAK/STAT signaling in GSC maintenance, however, remains incompletely understood. Results Here, we show that, GSC maintenance requires both canonical and non‐canonical JAK/STAT signaling, in which unphosphorylated STAT (uSTAT) maintains heterochromatin stability by binding to heterochromatin protein 1 (HP1). We found that GSC‐specific overexpressing STAT, or even the transcriptionally inactive mutant STAT, increases GSC number and partially rescues the GSC‐loss mutant phenotype due to reduced JAK activity. Furthermore, we found that both HP1 and STAT are transcriptional targets of the canonical JAK/STAT pathway in GSCs, and that GSCs exhibit higher heterochromatin content. Conclusions These results suggest that persistent JAK/STAT activation by niche signals leads to the accumulation of HP1 and uSTAT in GSCs, which promote heterochromatin formation important for maintaining GSC identity. Thus, the maintenance of Drosophila GSCs requires both canonical and non‐canonical STAT functions within GSCs for heterochromatin regulation.

Computational simulation of JAK/STAT signaling in somatic versus germline stem cells

“…10 In contrast, in GSCs with sustained Upd stimulation, this mutation led to a 2.5-fold elevated uSTAT level to 24.6 nM at steady state, despite a similar initial decrease (Figure 5B), consistent with the observed higher uSTAT levels heterochromatin levels in GSCs. 32 GSCs having higher levels of uSTAT, as detected by antibodies against total STAT, have been previously reported. [38][39][40] Alternatively, the hop Tum-l mutation could lead to constitutive activation of JAK regardless of Upd stimulation.…”

“…23,29,[38][39][40] It has been previously shown that both canonical and noncanonical functions of JAK/STAT signaling in GSCs are important for GSC maintenance. 32 Moreover, it has been shown that the central heterochromatin components such as HP1 and Su(var)3-9 methyltransferase are required for GSC maintenance 41 and that GSCs have higher levels of heterochromatin, which is regulated by the non-canonical STAT function. Further, both HP1 and STAT are among the transcription targets of canonical JAK/STAT signaling in GSCs.…”

“…This simulation is consistent with observations of high STAT levels in GSCs detected with antibodies specific for total STAT92E 23,29,[38][39][40] and the presence of high levels of heterochromatin important for GSC maintenance. 32,41 Thus, sustained activation of the JAK/STAT pathway, as occurs in GSCs, can lead to the accumulation of high levels of uSTAT, which favors heterochromatin formation.…”

“…31 Recent research has further highlighted the necessity of both canonical and noncanonical functions of STAT in GSC maintenance, in part, through regulating heterochromatin formation. 32 As the stem cell niche, hub cells at the testis tip region provide a continuous supply of Upd ligand, resulting in sustained JAK/STAT pathway stimulation in adjacent GSCs and CySCs. 22,33 In somatic tissues, however, JAK/STAT is activated only under certain physiological situations, by transient increases in ligands.…”

“…Paradoxically, in somatic tissues, transient activation of JAK/STAT signaling decreases heterochromatin, 10,16 whereas sustained activation of JAK/STAT signaling in GSCs increases heterochromatin formation. 32 This paradoxical observation raises the possibility that the JAK/STAT pathway may operate differently in somatic versus germline cells, possibly due to differences in pathway components or regulators. This study employed mathematical simulations to elucidate how activation of the JAK/STAT pathway in the germline vs somatic cells could result in different biological consequences.…”