Staurosporine-induced cell death in Neurospora crassa includes a well defined sequence of alterations in cytosolic calcium levels, comprising extracellular Ca(2+) influx and mobilization of Ca(2+) from internal stores. Here, we show that cells undergoing respiratory stress due to the lack of certain components of the mitochondrial complex I (like the 51kDa and 14kDa subunits) or the Ca(2+)-binding alternative NADPH dehydrogenase NDE-1 are hypersensitive to staurosporine and incapable of setting up a proper intracellular Ca(2+) response. Cells expressing mutant forms of NUO51 that mimic human metabolic diseases also presented Ca(2+) signaling deficiencies. Accumulation of reactive oxygen species is increased in cells lacking NDE-1 and seems to be required for Ca(2+) oscillations in response to staurosporine. Measurement of the mitochondrial levels of Ca(2+) further supported the involvement of these organelles in staurosporine-induced Ca(2+) signaling. In summary, our data indicate that staurosporine-induced fungal cell death involves a sophisticated response linking Ca(2+) dynamics and bioenergetics.
Alterations in the intracellular levels of calcium are a common response to cell
death stimuli in animals and fungi and, particularly, in the Neurospora
crassa response to staurosporine. We highlight the importance of
the extracellular availability of Ca2+ for this response. Limitation
of the ion in the culture medium further sensitizes cells to the drug and
results in increased accumulation of reactive oxygen species (ROS). Conversely,
an approximately 30-fold excess of external Ca2+ leads to increased
drug tolerance and lower ROS generation. In line with this, distinct
staurosporine-induced cytosolic Ca2+ signaling profiles were observed
in the absence or presence of excessive external Ca2+.
High-throughput RNA sequencing revealed that different concentrations of
extracellular Ca2+ define distinct transcriptional programs. Our
transcriptional profiling also pointed to two putative novel
Ca2+-binding proteins, encoded by the NCU08524 and NCU06607 genes,
and provides a reference dataset for future investigations on the role of
Ca2+ in fungal biology.
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