Bidirectional transfer of a small membrane-impermeable molecule between the Caenorhabditis elegans intestine and germline

The extracellular signal-regulated kinase/mitogen-activated protein kinase (ERK/MAPK) is a positive regulator of cell proliferation often upregulated in cancer. Its Caenorhabditis elegans ortholog MPK-1 stimulates germline stem cell (GSC) proliferation nonautonomously from the intestine or somatic gonad. How MPK-1 can perform this task from either of these two tissues however remains unclear. We reasoned that somatic MPK-1 activity could lead to the generation of proproliferative small molecules that could transfer from the intestine and/or somatic gonad to the germline. Here, in support of this hypothesis, we demonstrate that a significant fraction of the small membrane-impermeable fluorescent molecule, 5-carboxyfluorescein, transfers to the germline after its microinjection in the animal's intestine. The larger part of this transfer targets oocytes and requires the germline receptor mediated endocytosis 2 (RME-2) yolk receptor. A minor quantity of the dye is however distributed independently from RME-2 and more widely in the animal, including the distal germline, gonadal sheath, coelomocytes, and hypodermis. We further show that the intestine-to-germline transfer efficiency of this RME-2 independent fraction does not vary together with GSC proliferation rates or MPK-1 activity. Therefore, if germline proliferation was influenced by small membrane-impermeable molecules generated in the intestine, it is unlikely that proliferation would be regulated at the level of molecule transfer rate. Finally, we show that conversely, a similar fraction of germline injected 5-carboxyfluorescein transfers to the intestine, demonstrating transfer bidirectionality. Altogether, our results establish the possibility of an intestine-to-germline signaling axis mediated by small membrane-impermeable molecules that could promote GSC proliferation cell nonautonomously downstream of MPK-1 activity.