Content area
Full Text
Output from the circadian clock controls rhythmic behavior through poorly understood mechanisms. In Drosophila, null mutations of the neurofibromatosis-1 (Nf1) gene produce abnormalities of circadian rhythms in locomotor activity. Mutant flies show normal oscillations of the clock genes period (per) and timeless (tim) and of their corresponding proteins, but altered oscillations and levels of a clock-controlled reporter. Mitogen-activated protein kinase (MAPK) activity is increased in Nf1 mutants, and the circadian phenotype is rescued by loss-of-function mutations in the Ras/MAPK pathway. Thus, Nf1 signals through Ras/MAPK in Drosophila. Immunohistochemical staining revealed a circadian oscillation of phospho-MAPK in the vicinity of nerve terminals containing pigment-dispersing factor (PDF), a secreted output from clock cells, suggesting a coupling of PDF to Ras/MAPK signaling.
The endogenous circadian pacemaker controls the daily oscillations of both cellular and behavioral processes and can be entrained to environmental cues such as light and maintain daily cycling in the absence of such cues. The molecular components of the circadian clock form a perpetually oscillating 24-hour feedback loop (1). The signaling mechanism that mediates output from these clock proteins to behavior is not known, although a secreted neuropeptide, PDF, may be a crucial output element in Drosophila (2).
We sought to identify other output signaling components by testing candidate molecules. One of these, the neurofibromatosis-1 (Nf1) gene product neurofibromin, is highly conserved between humans and flies, with sequence similarity throughout the length of the protein (3). In humans, Nf1 is a tumor suppressor. Neurofibromin...