Oscillatory recruitment of signaling proteins to cell tips promotes coordinated behavior during cell fusion

Abstract

Cell-cell communication is essential for coordinating physiological responses in multicellular organisms and is required for various developmental processes, including cell migration, differentiation, and fusion. To facilitate communication, functional differences are usually required between interacting cells, which can be established either genetically or developmentally. However, genetically identical cells in the same developmental state are also capable of communicating, but must avoid self-stimulation. We hypothesized that such cells must alternate their physiological state between signal sending and receiving to allow recognition and behavioral changes. To test this hypothesis, we studied cell communication in the filamentous fungus Neurospora crassa, a simple and experimentally amenable model system. In N. crassa, germinating asexual spores (germlings) of identical genotype chemotropically sense others in close proximity, show attraction-mediated directed growth, and ultimately undergo cell fusion. Here, we report that two proteins required for cell fusion, a MAP kinase (MAK-2) and a protein of unknown molecular function (SO), exhibit rapid oscillatory recruitment to the plasma membranes of interacting germlings undergoing chemotropic interactions via directed growth. Using an inhibitable MAK-2 variant, we show that MAK-2 kinase activity is required both for chemotropic interactions and for oscillation of MAK-2 and SO to opposing cell tips. Thus, N. crassa germlings undergoing chemotropic interactions rapidly alternate between two different physiological states, associated with signal delivery and response. Such spatiotemporal coordination of signaling allows genetically identical and developmentally equivalent cells to avoid self-stimulation and to coordinate their behavior to achieve the beneficial physiological outcome of cell fusion.chemotropism ͉ MAPK ͉ Neurospora

Gene expression associated with vegetative incompatibility in Amylostereum areolatum

“…However, its expression was up-regulated more strongly in the incompatible interaction. This is consistent with the fact that the MAPK pathway is known to regulate related processes such as hyphal fusion and autophagy (Codogno and Meijer, 2005;Lam et al, 2006;Thaiville et al, 2008;Fleissner et al, 2009;Gough, 2010).…”

Excitable behavior can explain the “ping‐pong” mode of communication between cells using the same chemoattractant

“…Remarkably, another essential protein for CAT fusion, with unknown function, soft (SO) [17], exhibited a complementary oscillatory pattern, with a maximum intensity in one cell synchronized with the maximum of MAK-2 in the other. Fleißner et al [20] suggested that the MAK-2 peaks correspond to the receiving of a signal while the SO maxima to the sending of it (see Fig. 1C).…”

“…Deletion of the gene encoding the MAP kinase MAK-2 resulted in cells unable to chemotropically home and fuse with other, even normal, wild-type, cells. To shed light on the cellular localization of this kinase, Fleißner et al [20] genetically tagged MAK-2 with green fluorescent protein (GFP). Unexpectedly, they found that in the CATs undergoing chemotropic homing, MAK-2 localized to the plasma membrane at the CAT tips periodically in time, reaching maximum intensity every 10-12 min (see Fig.…”

Tube fusion: Making connections in branched tubular networks

“…Thereby, each of the two cells not only stimulates directed growth of the partner, but also avoids self-stimulation by the signal [57]. It is conceivable that similar systems could be used for cell-cell recognition during organogenesis in multicellular organisms.…”

“…However, a chemokine signalling mechanism must avoid self-stimulation and therefore cannot readily explain mutual attraction, unless the two partner cells use different chemokines and complementary receptors. A fascinating solution to this problem was described in vegetative hyphal fusion in the Ascomycete Neurospora crassa [57]. Here, genetically identical cells, as in case of tracheal FCs, are able to sense each other in close proximity by communicating via chemoattractants.…”