Accumulation of specific sterol precursors targets a MAP kinase cascade mediating cell–cell recognition and fusion

Abstract

Sterols are vital components of eukaryotic cell membranes. Defects in sterol biosynthesis, which result in the accumulation of precursor molecules, are commonly associated with cellular disorders and disease. However, the effects of these sterol precursors on the metabolism, signaling, and behavior of cells are only poorly understood. In this study, we show that the accumulation of only ergosterol precursors with a conjugated double bond in their aliphatic side chain specifically disrupts cell-cell communication and fusion in the fungus Neurospora crassa. Genetically identical germinating spores of this fungus undergo cell-cell fusion, thereby forming a highly interconnected supracellular network during colony initiation. Before fusion, the cells use an unusual signaling mechanism that involves the coordinated and alternating switching between signal sending and receiving states of the two fusion partners. Accumulation of only ergosterol precursors with a conjugated double bond in their aliphatic side chain disrupts this coordinated cell-cell communication and suppresses cell fusion. These specific sterol precursors target a single ERK-like mitogen-activated protein (MAP) kinase (MAK-1)-signaling cascade, whereas a second MAP kinase pathway (MAK-2), which is also involved in cell fusion, is unaffected. These observations indicate that a minor specific change in sterol structure can exert a strong detrimental effect on a key signaling pathway of the cell, resulting in the absence of cell fusion.

Fungal Morphogenesis, from the Polarized Growth of Hyphae to Complex Reproduction and Infection Structures

“…These domains, termed "lipid rafts," contribute to the specific localization of proteins, such as GPIanchored and lipid-associated proteins, that play important roles in cell signaling and cell polarity (101)(102)(103). Filipin stains the tips of mating projections in S. cerevisiae (104), cell ends in S. pombe (105), germling tips in N. crassa (447), and hyphal tips in Candida albicans (106) and A. nidulans (107). In S. pombe, reentry into the cell cycle involves the de novo definition of growth zones via the reorganization of the SRD to the cell ends, accompanied by the recruitment of the cell growth machinery.…”

Molecular mechanisms of chemotropism and cell fusion in unicellular fungi

“…Interestingly, this ping-pong mechanism involves two MAPK pathways. As one cell accumulates the homolog of the yeast pheromoneresponsive MAPK, mitogen-activated protein kinase 2 (MAK-2), and its scaffold, HAM-5, the other accumulates the WD40 protein SO (SOFT), proposed to act as scaffold for the homolog of the yeast cell wall integrity MAPK MAK-1, which visibly accumulate only at fusion time (Fleissner et al, 2009b;Jonkers et al, 2014;Weichert et al, 2016). MAK-2 induces dispersion of SO, which in consequence oscillates in alternate phase.…”

“…The small GTPases CDC-42 and RAC-1, which are present and active at the growing tip, are required for chemotropic growth, MAK-2 recruitment and its oscillatory dynamics (Lichius et al, 2014). Sterol intermediates cause fusion defects by interfering with MAK-1 localization (Weichert et al, 2016). MAK-2 is also required for the final stages of cell fusion leading to cytoplasmic mixing (Serrano et al, 2018).…”

Phosphatidic acid produced by phospholipase D is required for hyphal cell-cell fusion and fungal-plant symbiosis

“…However, in Dpla-7 mutants, chemotropic growth was not observed, indicating that instead of a requirement of PA for membrane fusion, PA may actually be required for signaling during chemotropic growth. Interestingly, defects in ergosterol biosynthesis also result in cell-to-cell communication and cell fusion defects (Weichert et al, 2016). Nevertheless, exogenous addition of PA was able to restore hyphal fusion to the E. festucae DpldB strains.…”