Cellular calcium levels influenced by NCA-2 impact circadian period determination in Neurospora

Abstract

Intracellular calcium signaling has been implicated in control of a variety of circadian processes in animals and plants but its role in microbial clocks has remained largely cryptic. To examine the role of intracellular Ca2+ in the Neurospora clock we screened knockouts of calcium transporter genes and identified a gene encoding a calcium exporter, nca-2, uniquely as having significant period effects. Loss of NCA-2 results in an increase in cytosolic calcium level, and this leads to hyper-phosphorylation of core clock components, FRQ and WC-1, and a short period as measured by both the core oscillator and overt clock. Genetic analyses showed that mutations in certain frq phospho-sites, and in Ca2+-calmodulin-dependent kinase (camk-2), are epistatic to nca-2 in controlling the pace of the oscillator. These data are consistent with a model in which elevated intracellular Ca+2 leads to increased activity of CAMK-2 leading to enhanced FRQ phosphorylation, accelerated closure of the circadian feedback loop, and a shortened circadian period length. At a mechanistic level some CAMKs undergo more auto-phosphorylations in {Delta}nca-2, consistent with high calcium in the {Delta}nca-2 mutant influencing the enzymatic activity of CAMKs. NCA-2 interacts with multiple proteins including CSP-6, a protein known to be required for circadian output. Most importantly, expression of nca-2 is circadian clock-controlled at both the transcriptional and translational levels, and this in combination with the period effects seen in strains lacking NCA-2, firmly places calcium signaling within the larger circadian system where it acts as both an input to and output from the core clock. ImportanceCircadian rhythms are based on cell-autonomous, auto-regulatory, feedback loops formed by interlocked positive and negative arms, and they regulate myriad molecular and cellular processes in most eukaryotes including fungi. Intracellular calcium signaling is also a process that impacts a broad range of biological events in most eukaryotes. Clues have suggested that calcium signaling can influence circadian oscillators through multiple pathways; however, mechanistic details have been lacking in microorganisms. Building on prior work describing calcium transporters in the fungus Neurospora, one such transporter, NCA-2, was identified as a regulator of circadian period length. Increased intracellular calcium levels caused by loss of NCA-2 results in over-activation of calcium-responsive protein kinases, in turn leading to a shortened circadian period length. Importantly, expression of NCA-2 is itself controlled by the molecular clock. In this way calcium signaling can be seen as providing both input to and output from the circadian system.