On Variations in the Level of PER in Glial Clocks of Drosophila Optic Lobe and Its Negative Regulation by PDF Signaling

Dietary acrylamide disrupts the functioning of the biological clock

Acrylamide (ACR) is a known carcinogen and neurotoxin. It is chronically consumed in carbohydrate-rich snacks processed at high temperatures. This calls for systematic research into the effects of ACR intake, best performed in an experimental model capable of detecting symptoms of its neurotoxicity at both high and low doses. Here, we study the influence of 10 µg/g (corresponding to the concentrations found in food products) and, for comparison, 60, 80 and 110 µg/g dietary ACR, on the fruit fly Drosophila melanogaster. We show that chronic administration of ACR affects lifespan, activity level and, most importantly, the daily and circadian pattern of locomotor activity of Drosophila. ACR-treated flies show well-defined and concentration-dependent symptoms of ACR neurotoxicity; a reduced anticipation of upcoming changes in light conditions and increased arrhythmicity in constant darkness. The results suggest that the rhythm-generating neural circuits of their circadian oscillator (biological clock) are sensitive to ACR even at low concentrations if the exposure time is sufficiently long. This makes the behavioural readout of the clock, the rhythm of locomotor activity, a useful tool for studying the adverse effects of ACR and probably other compounds.

The Role of Glia Clocks in the Regulation of Sleep in Drosophila melanogaster

In Drosophila melanogaster, the pacemaker located in the brain plays the main role in maintaining circadian rhythms; however, peripheral oscillators including glial cells, are also crucial components of the circadian network. In the present study, we investigated an impact of oscillators located in astrocyte-like glia, the chiasm giant glia of the optic lobe, epithelial and subperineurial glia on sleep of Drosophila males. We described that oscillators located in astrocyte-like glia and chiasm giant glia are necessary to maintain daily changes in clock neurons arborizations, while those located in epithelial glia regulate amplitude of these changes. Finally, we showed that communication between glia and neurons through tripartite synapses formed by epithelial glia and, in effect, neurotransmission regulation plays important role in wake-promoting during the day.SIGNIFICANCE STATEMENT Circadian clock or pacemaker regulates many aspects of animals' physiology and behavior. The pacemaker is located in the brain and is composed of neurons. However, there are also additional oscillators, called peripheral clocks, which synchronize the main clock. Despite the critical role of glia in the clock machinery, little is known which type of glia houses peripheral oscillators and how they affect neuronal clocks. This study using Drosophila shows that oscillators in specific glia types maintain awakeness during the day by regulating the daily plasticity of clock neurons.

Roles of peripheral clocks: lessons from the fly

To adapt to and anticipate rhythmic changes in the environment such as daily light-dark and temperature cycles, internal timekeeping mechanisms called biological clocks evolved in a diverse set of organisms, from unicellular bacteria to humans. These biological clocks play critical roles in organisms' fitness and survival by temporally aligning physiological and behavioral processes to the external cues. The central clock is located in a small subset of neurons in the brain and drives daily activity rhythms, whereas most peripheral tissues harbor their own clock systems, which generate metabolic and physiological rhythms. Since the discovery of Drosophila melanogaster clock mutants in the early 1970s, the fruit fly has become an extensively studied model organism to investigate the mechanism and functions of circadian clocks. In this review, we primarily focus on D. melanogaster to survey key discoveries and progresses made over the past two decades in our understanding of peripheral clocks. We discuss physiological roles and molecular mechanisms of peripheral clocks in several different peripheral tissues of the fly.

Mesencephalic Astrocyte-Derived Neurotrophic Factor Regulates Morphology of Pigment-Dispersing Factor-Positive Clock Neurons and Circadian Neuronal Plasticity in Drosophila melanogaster

Mesencephalic Astrocyte-derived Neurotrophic Factor (MANF) is one of a few neurotrophic factors described in Drosophila melanogaster (DmMANF) but its function is still poorly characterized. In the present study we found that DmMANF is expressed in different clusters of clock neurons. In particular, the PDF-positive large (l-LNv) and small (s-LNv) ventral lateral neurons, the CRYPTOCHROME-positive dorsal lateral neurons (LNd), the group 1 dorsal neurons posterior (DN1p) and different tim-positive cells in the fly's visual system. Importantly, DmMANF expression in the ventral lateral neurons is not controlled by the clock nor it affects its molecular mechanism. However, silencing DmMANF expression in clock neurons affects the rhythm of locomotor activity in light:dark and constant darkness conditions. Such phenotypes correlate with abnormal morphology of the dorsal projections of the s-LNv and with reduced arborizations of the l-LNv in the medulla of the optic lobe. Additionally, we show that DmMANF is important for normal morphology of the L2 interneurons in the visual system and for the circadian rhythm in the topology of their dendritic tree. Our results indicate that DmMANF is important not only for the development of neurites but also for maintaining circadian plasticity of neurons.

Glial expression of DmMANF is required for the regulation of activity, sleep and circadian rhythms in the visual system of Drosophila melanogaster

DmMANF, Drosophila melanogaster mesencephalic astrocyte-derived neurotrophic factor (DmMANF) is an evolutionarily conserved orthologue of mammalian MANF. This neurotrophic factor exerts many functions in the Drosophila brain, particularly those dependent on glial cells. As we found in our earlier study, downregulation of DmMANF in glia induces degeneration of glial cells in the first optic neuropil (lamina) where DmMANF abundance is especially high. In the present study, we observed that changes in the level of DmMANF in two types of glia, astrocyte-like glia (AlGl) and ensheathing glia (EnGl), affect activity and sleep of flies. Interestingly, a proper level of DmMANF in AlGl seems to be important in guiding processes of pigment dispersing factor (PDF)-expressing clock neurons. This is supported by our finding that DmMANF overexpression in AlGl leads to structural changes in the architecture of the PDF-positive arborization in the brain. Finally, we detected that DmMANF also affects rhythms in glia themselves, as circadian oscillations in expression of the catalytic α subunit of the sodium pump in the lamina epithelial glia were abolished after DmMANF silencing. DmMANF expressed in AlGl and EnGl seems to affect the activity of neurons leading to changes in behaviour.

Daily rewiring of a neural circuit generates a predictive model of environmental light

Behavioral responsiveness to external stimulation is shaped by context. We studied how sensory information can be contextualized, by examining light-evoked locomotor responsiveness of Drosophila relative to time of day. We found that light elicits an acute increase in locomotion (startle) that is modulated in a time-of-day-dependent manner: Startle is potentiated during the nighttime, when light is unexpected, but is suppressed during the daytime. The internal daytime-nighttime context is generated by two interconnected and functionally opposing populations of circadian neurons-LNvs generating the daytime state and DN1as generating the nighttime state. Switching between the two states requires daily remodeling of LNv and DN1a axons such that the maximum presynaptic area in one population coincides with the minimum in the other. We propose that a dynamic model of environmental light resides in the shifting connectivities of the LNv-DN1a circuit, which helps animals evaluate ongoing conditions and choose a behavioral response.

Astrocyte identity: evolutionary perspectives on astrocyte functions and heterogeneity

The development of new animal models, in vivo isolation approaches, and improvements in genome-wide RNA expression methods have greatly propelled molecular profiling of astrocytes and the characterization of astrocyte heterogeneity in the central nervous system (CNS). Several recent reviews have comprehensively discussed the molecular and functional diversity of mammalian astrocytes. In this brief review, we emphasize interspecies comparisons and an evolutionary perspective regarding the astro(glia) of vertebrates and invertebrates which are similar in form and function. This analysis has revealed conserved astrocyte transcriptomes in the fly, mouse and human. We also offer opinions about the pattern and origin of astrocyte heterogeneity in the CNS.

Different Levels of Expression of the Clock Protein PER and the Glial Marker REPO in Ensheathing and Astrocyte-Like Glia of the Distal Medulla of Drosophila Optic Lobe

Circadian plasticity of the visual system of Drosophila melanogaster depends on functioning of both the neuronal and glial oscillators. The clock function of the former is already quite well-recognized. The latter, however, is much less known and documented. In this study we focus on the glial oscillators that reside in the distal part of the second visual neuropil, medulla (dMnGl), in vicinity of the PIGMENT-DISPERSING FACTOR (PDF) releasing terminals of the circadian clock ventral Lateral Neurons (LNvs). We reveal the heterogeneity of the dMnGl, which express the clock protein PERIOD (PER) and the pan-glial marker REVERSED POLARITY (REPO) at higher (P1) or lower (P2) levels. We show that the cells with stronger expression of PER display also stronger expression of REPO, and that the number of REPO-P1 cells is bigger during the day than during the night. Using a combination of genetic markers and immunofluorescent labeling with anti PER and REPO Abs, we have established that the P1 and P2 cells can be associated with two different types of the dMnGl, the ensheathing (EnGl), and the astrocyte-like glia (ALGl). Surprisingly, the EnGl belong to the P1 cells, whereas the ALGl, previously reported to play the main role in the circadian rhythms, display the characteristics of the P2 cells (express very low level of PER and low level of REPO). Next to the EnGl and ALGl we have also observed another type of cells in the distal medulla that express PER and REPO, although at very low levels. Based on their morphology we have identified them as the T1 interneurons. Our study reveals the complexity of the distal medulla circadian network, which appears to consist of different types of glial and neuronal peripheral clocks, displaying molecular oscillations of higher (EnGl) and lower (ALGl and T1) amplitudes.