Novel algorithms for improved detection and analysis of fluorescent signal fluctuations.

Fluorescent dyes and genetically encoded fluorescence indicators (GEFI) are common tools for visualizing concentration changes of specific ions and messenger molecules during intra- as well as intercellular communication. Using advanced imaging technologies, fluorescence indicators are a prerequisite for the analysis of physiological molecular signaling. Automated detection and analysis of fluorescence signals require to overcome several challenges, including correct estimation of fluorescence fluctuations at basal concentrations of messenger molecules, detection, and extraction of events themselves as well as proper segmentation of neighboring events. Moreover, event detection algorithms need to be sensitive enough to accurately capture localized and low amplitude events exhibiting a limited spatial extent. Here, we present two algorithms (PBasE and CoRoDe) for accurate baseline estimation and automated detection and segmentation of fluorescence fluctuations.

Microglial morphology in the somatosensory cortex across lifespan. A quantitative study.

BACKGROUND: Microglia are long-lived cells that constantly monitor their microenvironment. To accomplish this task, they constantly change their morphology both in the short and long term under physiological conditions. This makes the process of quantifying physiological microglial morphology difficult. RESULTS: By using a semi-manual and a semi-automatic method to assess fine changes in cortical microglia morphology, we were able to quantify microglia changes in number, surveillance and branch tree starting from the fifth postnatal day to 2 years of life. We were able to identify a fluctuating behavior of most analyzed parameters characterized by a rapid cellular maturation, followed by a long period of relative stable morphology during the adult life with a final convergence to an aged phenotype. Detailed cellular arborization analysis revealed age-induced differences in microglia morphology, with mean branch length and the number of terminal processes changing constantly over time. CONCLUSIONS: Our study provides insight into microglia morphology changes across lifespan under physiological conditions. We were able to highlight, that due to the dynamic nature of microglia several morphological parameters are needed to establish the physiological state of these cells.

Elucidating regulators of astrocytic Ca2+ signaling via multi-threshold event detection (MTED).

Recent achievements in indicator optimization and imaging techniques promote the advancement of functional imaging to decipher complex signaling processes in living cells, such as Ca2+ activity patterns. Astrocytes are important regulators of the brain network and well known for their highly complex morphology and spontaneous Ca2+ activity. However, the astrocyte community is lacking standardized methods to analyze and interpret Ca2+ activity recordings, hindering global comparisons. Here, we present a biophysically-based analytical concept for deciphering the complex spatio-temporal changes of Ca2+ biosensor fluorescence for understanding the underlying signaling mechanisms. We developed a pixel-based multi-threshold event detection (MTED) analysis of multidimensional data, which accounts for signal strength as an additional signaling dimension and provides the experimenter with a comprehensive toolbox for a differentiated and in-depth characterization of fluorescence signals. MTED was validated by analyzing astrocytic Ca2+ activity across Ca2+ indicators, imaging setups, and model systems from primary cell culture to awake, head-fixed mice. We identified extended Ca2+ activity at 25°C compared to 37°C physiological body temperature and dissected how neuronal activity shapes long-lasting astrocytic Ca2+ activity. Our MTED strategy, as a parameter-free approach, is easily transferrable to other fluorescent indicators and biosensors and embraces the additional dimensionality of signaling activity strength. It will also advance the definition of standardized procedures and parameters to improve comparability of research data and reports.

Epigenetic control of region-specific transcriptional programs in mouse cerebellar and cortical astrocytes.

Astrocytes from the cerebral cortex (CTX) and cerebellum (CB) share basic molecular programs, but also form distinct spatial and functional subtypes. The regulatory epigenetic layers controlling such regional diversity have not been comprehensively investigated so far. Here, we present an integrated epigenome analysis of methylomes, open chromatin, and transcriptomes of astroglia populations isolated from the cortex or cerebellum of young adult mice. Besides a basic overall similarity in their epigenomic programs, cortical astrocytes and cerebellar astrocytes exhibit substantial differences in their overall open chromatin structure and in gene-specific DNA methylation. Regional epigenetic differences are linked to differences in transcriptional programs encompassing genes of region-specific transcription factor networks centered around Lhx2/Foxg1 in CTX astrocytes and the Zic/Irx families in CB astrocytes. The distinct epigenetic signatures around these transcription factor networks point to a complex interconnected and combinatorial regulation of region-specific transcriptomes. These findings suggest that key transcription factors, previously linked to temporal, regional, and spatial control of neurogenesis, also form combinatorial networks important for astrocytes. Our study provides a valuable resource for the molecular basis of regional astrocyte identity and physiology.

Serotonin receptor 4 regulates hippocampal astrocyte morphology and function.

Astrocytes are an important component of the multipartite synapse and crucial for proper neuronal network function. Although small GTPases of the Rho family are powerful regulators of cellular morphology, the signaling modules of Rho-mediated pathways in astrocytes remain enigmatic. Here we demonstrated that the serotonin receptor 4 (5-HT4 R) is expressed in hippocampal astrocytes, both in vitro and in vivo. Through fluorescence microscopy, we established that 5-HT4 R activation triggered RhoA activity via Gα13 -mediated signaling, which boosted filamentous actin assembly, leading to morphological changes in hippocampal astrocytes. We investigated the effects of these 5-HT4 R-mediated changes in mixed cultures and in acute slices, in which 5-HT4 R was expressed exclusively in astrocytes. In both systems, 5-HT4 R-RhoA signaling changed glutamatergic synaptic transmission: It increased the frequency of miniature excitatory postsynaptic currents (mEPSCs) in mixed cultures and reduced the paired-pulse-ratio (PPR) of field excitatory postsynaptic potentials (fEPSPs) in acute slices. Overall, our present findings demonstrate that astrocytic 5-HT4 R-Gα13 -RhoA signaling is a previously unrecognized molecular pathway involved in the functional regulation of excitatory synaptic circuits.

During Development NG2 Glial Cells of the Spinal Cord are Restricted to the Oligodendrocyte Lineage, but Generate Astrocytes upon Acute Injury.

NG2 glia are self-renewal cells widely populating the entire central nervous system (CNS). The differentiation potential of NG2 glia in the brain has been systematically studied. However, the fate of NG2 glia in the spinal cord during development and after injury is still unclear. Here, we took advantage of faithful expression of Cre in NG2-CreERT2 knock-in mice to demonstrate that spinal NG2 glia remain committed to the oligodendrocyte (OL) lineage and generate OLs, but not astrocytes or neurons, during development. However, we found significant age- and region dependent differences in differentiation into OLs. Embryonic or neonatal NG2 glia generated more than 90% of the white matter OLs, but only 50% (embryonic) or 75% (neonatal) of gray matter OLs. Such differences disappeared after myelin completion coinciding with a decrease in the differentiation rate. While we never detected the generation of astrocytes from NG2 glia during spinal cord development, we found a small portion of NG2 glia could generate astrocytes in adult spinal cord upon acute traumatic injury.

Microglia morphology in the physiological and diseased brain - from fixed tissue to in vivo conditions.

First mentioned almost 100 years ago, neuroresearch has linked microglia to the initiation and/or maintenance of most central nervous system pathologies. Since their discovery, we learned that the microglia immune status is often correlated directly to their morphology. However, only recently, have we realized that, in vivo microglia are extremely dynamic cells capable to respond within minutes. Therefore, the name "resting" microglia is replaced slowly with surveilling microglia. Even more recent, it has been shown that microglia change their morphology and/or immune status depending on the way the tissue is obtained, processed and imaged, making labeling microglia, based on their morphology alone, as active or surveying even more difficult. All these observations correlated with a better understanding of cellular and subcellular process that microglia undergo in pathological conditions, could have profound implications on the time window and/or targets used to ensure the best outcome, when treating brain disease.

The in situ morphology of microglia is highly sensitive to the mode of tissue fixation.

Microglia are known as the most motile cells in the central nervous system (CNS). It was shown in vivo that they permanently scan their direct microenvironment and react to pathological conditions within minutes. Many studies of brain pathologies use fixed brain tissue to investigate cellular changes. Unfortunately, due to technical reasons, the time span between the induction of the fixation procedure (start of the perfusion) and the finally-fixed tissue lasts several minutes, giving time to microglia to start reacting to the ischemic conditions due to perfusion start. Here, we investigated the microglial changes generated by the fixation itself in TgH(CX3CR1-EGFP) mice with fluorescent labelled microglia using confocal laser scanning microscopy (CLSM) of fixed brain tissue as well as two-photon laser scanning microscopy (2P-LSM) during the perfusion of a living animal. We revealed the impact of fixation and buffer parameters on cell morphology. The largest morphological differences compared to physiological in vivo branch arborization were observed when the directly dissected brain was immersed in paraformaldehyde fixation solution overnight, without prior fixative perfusion of the animal. But even perfusion with a fixative, followed by post-fixation leads to small changes in microglial process length and number and could not be prevented when compared to physiological in vivo microglia morphology acquired using in vivo 2P-LSM. Interestingly, perfusion with different buffers either oxygenated artificial cerebrospinal fluid or phosphate buffered saline prior to perfusion-fixation showed minor microglia changes in arborization and/or number of processes. Fixation methods influence microglia morphology. Therefore, to define microglia activation states immunohistochemical stainings or genetic labelling of the cells have to be included in addition to morphological analysis.