Molecular Mechanisms of Circadian Clocks and RNA Interference

Circadian Clock

Circadian clocks have been described in almost all organisms ranging in complexity from single cells to mammals and function to control daily rhythms in a variety of biochemical, cellular, physiological, and behavioral events. These rhythms have a period close to 24 hours (circadian) and persist in the absence of external time cues.

One of the most important characteristics of circadian rhythms is that they can be synchronized or entrained by environmental signals, the strongest of which are light and temperature. In humans and mammals, circadian clocks control events such as sleep-wake and activity cycles, body temperature cycles, endocrine functions, and gene expression.

Clinical consequences in humans including sleep disorders and depression can be observed when the clock malfunctions. The influence of a functional clock on temporal regulation is evident from the decreased performance of shift workers and the jet lag felt by long distance travelers.

Our lab is using filamentous fungus Neurospora crassa, one of the best studied model organisms for circadian clocks, to understand the molecular mechanisms of the circadian clock. In Neurospora, the circadian clock acts to control a variety of processes, and previous studies have shown that the Neurospora circadian clock is an auto-regulated negative feedback loop in which the frequency (frq) gene is an essential component.

My laboratory is using molecular, biochemical, and genetic approaches to answer three general questions: 1) What are the components of the input pathways to the clock and how do environmental signals entrain the clock; 2) What are the genes that make up the oscillator and how are they regulated to generate rhythms and 3) How does the clock control rhythmic output events?

In the long term, these studies will enable us to compare clock mechanisms of fungi with those of other eukaryotes and to help guide research in other organisms.

RNA Interference

The production of double-stranded RNA (dsRNA) is known to elicit RNA interference (RNAi) in most eukaryotes and interferon response in mammals. RNAi and related pathways are evolutionarily conserved gene silencing mechanisms that regulate gene expression, development, genome stability, and host-defense responses. The filamentous fungus Neurospora crassa, an organism that broadly employs gene silencing in regulation of gene expression, offers a unique and powerful system for understanding the RNAi pathway and its function in eukaryotes.

Using Neurospora as a model system, we have revealed the mechanism of the RISC activation process in the RNAi pathway. We also showed that dsRNA activates a novel signaling pathway to induce transcription of many genes in Neurospora, including most of the RNAi components, putative antiviral genes, and homologs of the interferon stimulated genes; this activation is analogous to the interferon response in mammals.

Our current research is focusing on the understanding of the regulation of RNAi components and on the involvement of RNAi pathway in various cellular processes.