Biology subjects

Oertner, T. G.

Publications and source records attributed to Oertner, T. G..

18 recordsLinked to original sources

bioRxiv2026

Mouse dehydration as a consequence of water bottle design

Animal care technicians reported that accidental deaths of mice occur more frequently under certain weather conditions. This is surprising, given that modern mouse facilities are fully climate-controlled. Our investigation revealed that air bubbles become trapped in the drinking tube of water bottles and block the downward flow of water. Mice cannot drink from such bottles and become dehydrated. A coiled wire inside the drinking tube dislodges the air bubbles and ensures unrestricted access to water.

physiology↗

bioRxiv2026

Dissecting the molecular triggers of early and late long-term potentiation

The brain stores information by changing the strength of its synapses, a process that has at least two phases: Late long-term potentiation (L-LTP) is thought to result from the consolidation of early LTP (E-LTP), just as long-term memory requires the prior establishment of short-term memory. Recently, inhibitory avoidance experiments under CaMKII inhibition have challenged this notion, demonstrating long-term fear memory without measurable short-term memory. Here we use optogenetic activation and inhibition of CaMKII during induction of spike-timing-dependent potentiation (tLTP) to dissect the signaling pathways. While CaMKII activation in CA1 neurons was sufficient to induce E-LTP, growth of the postsynaptic density and spine neck expansion, we found that CaMKII-induced LTP does not give rise to L-LTP. Conversely, inhibition of CaMKII during tLTP induction prevented E-LTP, but FOS and L-LTP were still expressed, driven by CaMKK and PKM{zeta}. Thus, both long-term memory and L-LTP form in the absence of CaMKII activation.

neuroscience↗

bioRxiv2025

Mapping prefrontal afferents along development

The medial prefrontal cortex (mPFC) acts as a hub for cognitive, emotional, and social processes, integrating neuronal inputs from numerous brain regions. Knowledge on the processes underlying formation and myelination of prefrontal afferents is essential for a comprehensive understanding of the emergence, maturation, and refinement of mPFC-dependent behaviors. However, despite detailed investigation of long-range projections to the mPFC in the adult brain, little is known about their maturation. Here, we use retrograde labeling, light-sheet imaging, and automatic cell segmentation to quantify afferent projections to the mPFC from neonatal to adult age. We show that densities of ipsi- and contralateral prefrontal afferents change along development from a widespread bilateral distribution at neonatal age to a predominantly ipsilateral organization. Furthermore, myelination of interhemispheric prefrontal afferents starts only after the decline of neonatal contralateral projections and is followed by temporally elevated projection densities from limbic brain regions. Overall, these distinct developmental dynamics of prefrontal afferents might have major implications for the maturation of mPFC-dependent functional and behavioral outputs.

neuroscience↗

bioRxiv2025

Ultrastructural analysis of synapses after induction of spike-timing-dependent plasticity

Repeated sequential activation of connected neurons causes lasting changes in synaptic strength, a process known as spike-timing-dependent plasticity (STDP). Recently, sequential spike patterns have been induced without electrodes, using two spectrally separated channelrhodopsins. However, due to the difficulty of labeling and localizing the few connecting synapses between the stimulated pre and postsynaptic neurons ([~]1-5 per neuron pair), ultrastructural analysis after STDP has not been reported. Here, we optogenetically induce STDP at CA3-CA1 hippocampal synapses and identify stimulated boutons and spines in CA1 using transmission electron microscopy (TEM). Presynaptic CA3 neurons express vesicle-targeted horseradish peroxidase, cre recombinase and cre-dependent ChrimsonR, a red light activatable channelrhodopsin. Postsynaptic neurons express violet light activatable CheRiff and dAPEX2, an enhanced ascorbate peroxidase. In transmission electron microscopy, presynaptic boutons and postsynaptic spines are readily identifiable with well-preserved ultrastructural features. Our labeling strategy allows ultrastructural analysis of optogenetically manipulated neurons and their synapses.

neuroscience↗

bioRxiv2025

An expanded palette of bright and photostable organellar Ca2+ sensors

The use of fluorescent sensors for functional imaging has revolutionized the study of organellar Ca2+ signaling. However, understanding the dynamic interplay between intracellular Ca2+ sinks and sources has been hindered by the lack of bright, photostable, and multiplexed measurements in different organelles, limiting our ability to define how Ca2+ shapes cell physiology across fields of biology. Here we introduce a new toolkit of chemigenetic organellar Ca2+ indicators whose color is tunable by reconstituting their fluorescence with different exogenous rhodamine dye-ligands, which significantly expand the capacity for multiplexing organellar Ca2+ measurements. These sensors, which we named ER-HaloCaMP and Mito-HaloCaMP, are optimized to report Ca2+ dynamics in the endoplasmic reticulum (ER) and mitochondria of mammalian cells and neurons, and show significantly improved brightness, photostability and responsiveness when compared to current best-in-class alternatives. Using either red or far-red dye-ligands, both ER-HaloCaMP and Mito-HaloCaMP enable visualizing ER and mitochondrial Ca2+ dynamics in neuronal axons, a subcellular location that only contains a few ER tubules and small mitochondria, structural limitations that have impaired measurements with previous red sensors. To show the expanded multiplexing capacities of our toolkit, we measured interorganellar Ca2+ fluxes simultaneously in three different subcellular compartments in live cells, revealing that the amplitude of ER Ca2+ release controls the efficacy of ER-mitochondria Ca2+ coupling in a cooperative manner. Organellar HaloCaMPs enable also measuring Ca2+ dynamics in intact brain tissue from flies and rodents, demonstrating their versatility across biological models. Our new toolkit provides an expanded palette of bright, photostable and responsive organellar Ca2+ sensors, which will facilitate future studies of intracellular Ca2+ signaling across fields of biology in health and disease.

cell biology↗

bioRxiv2024

Epigenetic repression of cFos supports sequential formation of distinct spatial memories

Expression of the immediate early gene cFos modifies the epigenetic landscape of activated neurons with downstream effects on synaptic plasticity. The production of cFos is inhibited by a long-lived isoform of another Fos family gene, {Delta}FosB. It has been speculated that this negative feedback mechanism may be critical for protecting episodic memories from being overwritten by new information. Here, we investigate the influence of {Delta}FosB inhibition on cFos expression and memory. Hippocampal neurons in slice culture produce more cFos on the first day of stimulation compared to identical stimulation on the following day. This downregulation affects all hippocampal subfields and requires histone deacetylation. Overexpression of {Delta}FosB in individual pyramidal neurons effectively suppresses cFos, indicating that accumulation of {Delta}FosB is the causal mechanism. Water maze training of mice over several days leads to accumulation of {Delta}FosB in granule cells of the dentate gyrus, but not in CA3 and CA1. Because the dentate gyrus is thought to support pattern separation and cognitive flexibility, we hypothesized that inhibiting the expression of {Delta}FosB would affect reversal learning, i.e., the ability to successively learn new platform locations in the water maze. The results indicate that pharmacological HDAC inhibition, which prevents cFos repression, impairs reversal learning, while learning and memory of the initial platform location remain unaffected. Our study supports the hypothesis that epigenetic mechanisms tightly regulate cFos expression in individual granule cells to orchestrate the formation of time-stamped memories.

neuroscience↗

bioRxiv2023

Neuronal FOS reports synchronized activity of presynaptic neurons

The immediate early gene FOS is frequently used as a marker for highly active neurons. Implicit in this use is the assumption that there is a correlation between neuronal spiking and FOS expression. Here we use optogenetic stimulation of hippocampal neurons to investigate the relation between spike frequency and FOS expression, and report several surprising observations. First, FOS expression is cell-type specific, spiking CA2 pyramidal neurons rarely express FOS. Second, FOS has a U-shaped dependence on frequency: Spiking at 0.1 Hz is more effective than high frequency spiking (50 Hz) while intermediate frequencies do not induce FOS. Third, the pathway from spiking to FOS is not cell-autonomous. Instead, transmitter release and metabotropic glutamate receptor (mGluR) activation are required and, at 0.1 Hz, FOS is induced independently of CREB/calcineurin/MEK pathways. We propose that FOS does not primarily encode a neurons own spike frequency but indicates repeated participation in highly synchronized activity, e.g. sharp wave ripples.

neuroscience↗

bioRxiv2023

Optical Control of G-Actin with a Photoswitchable Latrunculin

Actin is one of the most abundant proteins in eukaryotic cells and a key component of the cytoskeleton. A range of small molecules have emerged that interfere with actin dynamics by either binding to polymeric F-actin or monomeric G-actin to stabilize or destabilize filaments or prevent their formation and growth, respectively. Amongst these, the latrunculins, which bind to G-actin and affect polymerization, are widely used as tools to investigate actin-dependent cellular processes. Here, we report a photoswitchable version of latrunculin, termed opto-latrunculin (OptoLat), which binds to G-actin in a light-dependent fashion and affords optical control over actin polymerization. OptoLat can be activated with 390 - 490 nm pulsed light and rapidly relaxes to the inactive form in the dark. Light activated OptoLat induced depolymerization of F-actin networks in oligodendrocytes and budding yeast, as shown by fluorescence microscopy. Subcellular control of actin dynamics in human cancer cell lines was demonstrated by live cell imaging. Light-activated OptoLat also reduced microglia surveillance in organotypic mouse brain slices while ramification was not affected. Incubation in the dark did not alter the structural and functional integrity of microglia. Together, our data demonstrate that OptoLat is a useful tool for the elucidation of G-actin dependent dynamic processes in cells and tissues.

pharmacology and toxicology↗

bioRxiv2021

Improved two-photon imaging of GPCR-based optogenetic neurotransmitter sensors using orthogonally polarized excitation

Fluorescent proteins such as GFP are best excited by light that is polarized parallel to the dipole axis of the fluorophore. In most cases, fluorescent proteins are randomly oriented, resulting in unbiased images even when polarized light is used for excitation, e.g. in two-photon microcopy. Here we reveal a surprisingly strong polarization sensitivity in a class of GPCR-based neurotransmitter sensors where the fluorophore is anchored on both ends. In tubular structures such as dendrites, this effect led to a complete loss of membrane signal in dendrites running parallel to the polarization direction of the excitation beam. Our data reveal a major problem for two-photon measurements of neurotransmitter concentration that has not been recognized by the neuroscience community. To remedy the sensitivity to dendritic orientation, we designed an optical device that generates interleaved pulse trains of orthogonal polarization, removing the orientation bias from images. The passive device, which we inserted in the beam path of an existing two-photon microscope, also removed the strong direction bias in second harmonic generation (SHG) images. We conclude that for optical measurements of transmitter concentration with GPCR-based sensors, orthogonally polarized excitation is essential.

neuroscience↗

bioRxiv2020

Freeze-frame imaging of dendritic calcium signals with TubuTag

The extensive dendritic arbor of neurons is thought to be actively involved in the processing of information. Dendrites contain a rich diversity of ligand- and voltage-activated ion channels as well as metabotropic receptors. In addition, they are capable of releasing calcium from intracellular stores. Under specific conditions, large neurons produce calcium spikes that are locally restricted to a dendritic section. To investigate calcium signaling in dendrites, we introduce TubuTag, a genetically encoded calcium sensor anchored to the cytoskeleton. TubuTag integrates cytoplasmic calcium signals by irreversible photoconversion from green to red fluorescence when illuminated with violet light. To image the mm-long dendritic tree of pyramidal neurons at subcellular resolution, we used a custom two-photon microscope with a large field of view. As the read-out of fluorescence can be performed several hours after photoconversion, TubuTag will help investigating dendritic signal integration and calcium homeostasis in large populations of neurons.

neuroscience↗

bioRxiv2020

cFos ensembles in the dentate gyrus rapidly segregate over time and do not form a stable map of space

Transgenic cFos reporter mice are used to identify and manipulate neurons that store contextual information during fear learning. It is not clear, however, how spatial information acquired over several training days is integrated in the hippocampus. Using a water maze task, we observed that cFos expression patterns in the dentate gyrus are temporally unstable and shift daily. Surprisingly, cFos patterns did not get more stable with increasing spatial memory precision. Despite the fact that cFos was no longer expressed, optogenetic inhibition of neurons that expressed cFos on the first training day affected performance days later. Triggered by training, {Delta}FosB accumulates and provides a negative feedback mechanism that makes the cFos ensemble in the dentate gyrus dependent on the history of activity. Shifting cFos expression to a different set of granule cells every day may aid the formation of episodic memories.

neuroscience↗

bioRxiv2020

The vesicular release probability sets the strength of individual Schaffer collateral synapses

Information processing in the brain is controlled by quantal release of neurotransmitters, a tightly regulated process. From ultrastructural analysis, it is known that presynaptic boutons along single axons differ in the number of vesicles docked at the active zone. It is not clear whether the probability of these vesicles to get released (pves) is homogenous or also varies between individual boutons. Here, we optically measure evoked transmitter release at individual Schaffer collateral synapses at different calcium concentrations, using the genetically encoded glutamate sensor iGluSnFR. Fitting a binomial model to measured response amplitude distributions allowed us to extract the quantal parameters N, pves, and q. We find that Schaffer collateral boutons typically release single vesicles under low pves conditions and switch to multivesicular release in high calcium saline. Analyzing the variability of quantal parameters, we conclude that the vesicular release probability rather than the number of readily releasable vesicles or their transmitter content determines the potency of individual boutons.

neuroscience↗

bioRxiv2020

Endoplasmic reticulum visits highly active spines and prevents runaway potentiation of synapses

In hippocampal pyramidal cells, a small subset of dendritic spines contain endoplasmic reticulum (ER). In large spines, ER frequently forms a spine apparatus, while smaller spines contain just a single tubule of smooth ER. Here we show that the ER visits dendritic spines in a non-random manner, targeting spines during periods of high synaptic activity. When we blocked ER motility using a dominant negative approach against myosin V, spine synapses became stronger compared to controls. We were not able to further potentiate these maxed-out synapses, but LTD was readily induced by low-frequency stimulation. We conclude that the brief ER visits to active spines have the important function of preventing runaway potentiation of individual spine synapses, keeping most of them at an intermediate strength level from which both LTP and LTD are possible.

neuroscience↗

bioRxiv2020

Optogenetic control of microglia membrane potential reveals signal transduction in chemotaxis

Microglia react to danger signals by rapid and targeted extension of cellular processes towards the source of the signal. This positive chemotactic response is accompanied by a hyperpolarization of the microglia membrane. Here we show that optogenetic depolarization of microglia has little effect on baseline motility, but significantly slows down the chemotactic response. Reducing the extracellular Ca2+ concentration mimics the effect of optogenetic depolarization. As the membrane potential sets the driving force for Ca2+ entry, hyperpolarization is an integral part of rapid stimulus-response coupling in microglia. Compared to typical excitable cells such as neurons, the sign of the activating response is inverted in microglia, leading to inhibition by depolarizing channelrhodopsins.

neuroscience↗

bioRxiv2019

Long vs short-term synaptic learning rules after optogenetic spike-timing-dependent plasticity

Spike-timing-dependent plasticity (STDP) is a candidate mechanism for information storage in the brain, but the whole-cell recordings required for the experimental induction of STDP are typically limited to one hour. This mismatch of time scales is a long-standing weakness in synaptic theories of memory. Here we use spectrally separated optogenetic stimulation to fire precisely timed action potentials (spikes) in CA3 and CA1 pyramidal cells. Twenty minutes after optogenetic induction of STDP (oSTDP), we observed timing-dependent depression (tLTD) and timing-dependent potentiation (tLTP), depending on the sequence of spiking. As oSTDP does not require electrodes, we could also assess the strength of these paired connections three days later. At this late time point, late tLTP was observed for both causal (CA3 before CA1) and anti-causal (CA1 before CA3) timing, but not for asynchronous activity patterns ({Delta}t = 50 ms). Blocking activity after induction of oSTDP prevented stable potentiation. Our results confirm that neurons wire together if they fire together, but suggest that synaptic depression after anti-causal activation (tLTD) is a transient phenomenon. HighlightsO_LIOptogenetic induction of spike-timing-dependent plasticity at Schaffer collateral synapses C_LIO_LICausal pairing induces potentiation whereas anti-causal pairing induces depression during patch-clamp recordings. C_LIO_LIThree days after optogenetic induction, the consequence of STDP is potentiation (tLTP) irrespective of spiking order. C_LIO_LILate tLTP requires ongoing activity in the days following oSTDP. C_LI Graphical Abstract O_FIG O_LINKSMALLFIG WIDTH=200 HEIGHT=138 SRC="FIGDIR/small/863365v4_ufig1.gif" ALT="Figure 1"> View larger version (19K): org.highwire.dtl.DTLVardef@1937ad5org.highwire.dtl.DTLVardef@16566f8org.highwire.dtl.DTLVardef@4b4a73org.highwire.dtl.DTLVardef@104fc97_HPS_FORMAT_FIGEXP M_FIG C_FIG

neuroscience↗

bioRxiv2019

Freeze-frame imaging of synaptic activity using SynTagMA

Information within the brain travels from neuron to neuron across synapses. At any given moment, only a few synapses within billions will be active and are thought to transmit key information about the environment, a behavior being executed or memory being recalled. Here we present SynTagMA, which marks active synapses within a ~2 s time window. Upon violet illumination, the genetically expressed tag converts from green to red fluorescence if bound to calcium. Targeted to presynaptic terminals, preSynTagMA allows discrimination between active and silent axons. Targeted to excitatory postsynapses, postSynTagMA creates a snapshot of synapses active just before photoconversion. To analyze large datasets, we developed an analysis program that automatically identifies and tracks the fluorescence of thousands of individual synapses in tissue. Together, these tools provide a high throughput method for repeatedly mapping active synapses in vitro and in vivo.

neuroscience↗

bioRxiv2018

Strong diffusional coupling between spines and dendrites promotes long-term potentiation of Schaffer collateral synapses

Dendritic spines on CA1 pyramidal cells are highly variable in size and shape. For some spine synapses, long and narrow spine necks provide strong diffusional and electrical isolation from the main dendrite, and it has been speculated that synapses on well isolated spines could be more plastic than synapses on spines with low resistance necks. Here we test this hypothesis by first measuring the diffusional resistance of spine necks, then pairing two-photon glutamate uncaging with bursts of back-propagating action potentials. Paired stimulation induced significant (155%) long-term potentiation (LTP) of synapses on weakly isolated spines, but no net functional change of synapses on highly isolated spines. No correlation was found between spine head volume and functional plasticity of the synapses. We conclude that contrary to our expectations, diffusional isolation makes potentiation of synapses more difficult. Our results support the concept that delivery of plasticity-related proteins from the dendrite into the spine is a limiting factor for LTP.

neuroscience↗

bioRxiv2018

The fate of hippocampal synapses depends on the sequence of plasticity-inducing events

Synapses change their strength in response to specific activity patterns. This functional plasticity is assumed to be the brains primary mechanism for information storage. We used optogenetic stimulation of rat hippocampal slice cultures to induce long-term potentiation (LTP), long-term depression (LTD), or both forms of plasticity in sequence. Two-photon imaging of spine calcium signals allowed us to identify stimulated synapses and to follow their fate for the next 7 days. We found that plasticity-inducing protocols affected the synapses chance for survival: LTP increased synaptic stability, LTD destabilized synapses, and the effect of the last stimulation protocol was dominant over earlier stimulations. Interestingly, most potentiated synapses were resistant to depression-inducing protocols delivered 24 hours later. Our findings suggest that activity-dependent changes in the transmission strength of individual synapses are transient, but have long-lasting consequences for synaptic lifetime.

neuroscience↗