Abstract
Circadian clocks consist of molecular negative feedback loops that coordinate physiological, neurological, and behavioral variables into “circa” 24-h rhythms. Rhythms in behavioral and other circadian outputs tend to weaken during aging, as evident in progressive disruptions of sleep-wake cycles in aging organisms. However, less is known about the molecular changes in the expression of clock genes and proteins that may lead to the weakening of circadian outputs. Western blot studies have demonstrated that the expression of the core clock protein PERIOD (PER) declines in the heads of aged Drosophila melanogaster flies. This age-related decline in PER does not occur in the central pacemaker neurons but has been demonstrated so far in retinal photoreceptors. Besides photoreceptors, clock proteins are also expressed in fly glia, which play important roles in neuronal homeostasis and are further categorized into subtypes based on morphology and function. While previous studies of mammalian glial cells have demonstrated the presence of functional clocks in astrocytes and microglia, it is not known which glial cell types in Drosophila express clock proteins and how their expression may change in aged individuals. Here, we conducted immunocytochemistry experiments to identify which glial subtypes express PER protein suggestive of functional circadian clocks. Glial cell subtypes that showed night-time accumulation and day-time absence in PER consistent with oscillations reported in the pacemaker neurons were selected to compare the level of PER protein between young and old flies. Our data demonstrate that some glial subtypes show rhythmic PER expression and the relative PER levels become dampened with advanced age. Identification of glial cell types that display age-related dampening of PER levels may help to understand the cellular changes that contribute to the loss of homeostasis in the aging brain.
Highlights
Many molecular, cellular, and physiological processes in most organisms are coordinated with the predictable changes of the 24-h solar day
Young and old males expressing nuclear GFP in specific glial cell subtypes were obtained by crossing w;UAS-GFP with nuclear localization signal (Bloomington Drosophila Stock Center stock 4775) males with females carrying GAL4 drivers expressing in the following glia types: perineurial glia, NP6293-GAL4 (Awasaki et al, 2008); subperineurial glia moody-GAL4 (Schwabe et al, 2005); cortex glia NP577-GAL4 and NP2222-GAL4; ensheathing glia NP6520GAL4 (Awasaki et al, 2008) and mz0709-GAL4 (Ito et al, 1995); astrocytes alrm-GAL4 (Doherty et al, 2009)
We investigated which glia subtypes express the circadian clock protein PER in a manner similar to that of the pacemaker neurons and whether the relative amounts of PER signal change with age
Summary
Cellular, and physiological processes in most organisms are coordinated with the predictable changes of the 24-h solar day. Circadian Clock in Drosophila Glia protein into the cell nuclei followed by its degradation is the fundamental feature of clock function (Hardin, 2011). The central pacemaker neurons driving behavioral rest/activity rhythms consist of a network of about 150 neurons in the Drosophila brain In addition to these central pacemaker neurons, mammals have intrinsic peripheral clocks in cells of fat tissues, kidneys, liver, and most other organs. Many of these peripheral tissues that express autonomous oscillators coordinate local tissue-specific processes. Which glial cell subtypes express the PERbased oscillator and what their roles may be in the timekeeping processes remain poorly understood
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