1. Research Background

Mechanisms regulating neuronal excitability and thereby cortical network homeostasis have been recognized as critical for health maintenance and brain aging, while dysfunction of these mechanisms were found to be important factors in disease progression as described for psychiatric disorders and for age-related disorders like stroke. Insofar, analysis of molecular, cellular and systemic mechanisms regulating cortical network excitability are of critical importance for understanding brain function in health, disease and aging.

We have provided evidence that bioactive synaptic lipids like lysophosphatidic acid (LPA) regulate synaptic connectivity and function (Liu et al., 2016, Cheng et al., 2016, Vogt et al., 2017), E/I balance in cortical networks (Tüscher et al., 2024) and are important for successful brain aging (Fischer et al., 2021). Synaptic LPA is synthesized by the enzyme autotaxin (ATX) present at perisynaptic astrocytic processes (Thalman et al., 2018) where it converts lysophosphatidyl choline (LPC) into LPA. Interestingly, LPC is derived from the peripheral lipid metabolism and is actively transported into the CNS via Mfsd2a transporters. LPA effects are mediated by presynaptic LPA₂-receptors (LPA₂) while plasticity-related gene-1 (PRG-1/LPPR4), a regulatory molecule located at the postsynaptic density of excitatory synapses, controls synaptic LPA-levels by transmembrane transport into intracellular compartments (Trimbuch et al., 2009; Vogt et al., 2016). Hereby, PRG-1 regulates presynaptic LPA₂ signaling and controls the functional set-point at the glutamatergic junction. Alteration of this postsynaptic regulatory mechanism by PRG-1 deficiency  or loss-of-function mutations like the human SNP Prg-1^R345T led to increased glutamatergic release probabilities, higher neuronal excitability and consequently to a shift in cortical network E/I-balance towards higher excitability in mice and humans (Trimbuch et al., 2009; Vogt et al., 2016; Tüscher et al., 2024). Our previous data show that altered E/I-balance in the somatosensory cortex resulted in reduced processing of sensorimotor information and impaired motor control (Unichenko et al., 2016). Moreover, higher cortical network excitability reduced sensorimotor gating, in mice and men, which is a typical finding in psychiatric disorders (Thalman et al., 2018; Vogt et al., 2016). However, increased cortical network excitability resulted in spontaneous hyperlocomotion and increased stereotypic behavior in mice (Schneider et al., 2017), which were associated with dopaminergic signaling and are typical positive symptoms of psychiatric disorders. Interestingly, hyperlocomotion, stereotypic behavior as well as impaired sensorimotor gating, returned to wild type levels by disruption of synaptic lipid signaling (Schneider et al.,2017) or by inhibition of the LPA-synthesizing enzyme ATX in an animal model of schizophrenia (Thalman et al., 2018).

2. Research questions addressed by the group:

What is the role of peripheral lipids in body-brain regulation of cortical excitability?

We have shown on a molecular level that fasting-induced synaptic lipid changes in the cortex, shifted cortical E/I balance, and modulated foraging and food intake while not directly acting on hypothalamic neurons (Endle et al., 2022). Increased cortical E/I balance controls exploratory and food intake behavior in mice most probably in a top-down manner. This is in line with observations in humans (Tataranni et al., 1999) suggesting a strong interaction between cortical control centers and hypothalamic execution of food intake. Our preliminary data show that fasting-induced changes in lipid composition have a direct impact on neuronal excitability in the dorsomedial prefrontal cortex (dmPFC), which is part of the default mode network (DMN). In line, LPA-related cortical excitability modulated fasting-induced rebound hyperphagia as well as weight gain under high-fat diet (Endle et al., 2022). In a follow-up paper we have shown that a human loss-of function SNP Prg-1^R345T altering lipid regulation at glutamatergic synapses increased cortical excitability in mice and humans. Moreover, we have shown increased cortical excitability led to an disrupted fear processing, which was associated with reduced stress coping, and impaired memory formation in humans resulting in an intermediate phenotype for psychiatric disorders (Tüscher et al., 2024). In the next steps we aim to investigate the underlying neuronal circuits, which are activated peripheral lipids during fasting and regulate food intake via the hypothalamus.

How do bioactive lipids and cortical excitability affect age-related disorders and neurodegeneration?

Another research direction in the lab is devoted to deciphering the disease-related role of lipid-induced cortical excitability. Using an in vivo model of stroke, the MCAO (middle cerebral artery occlusion), we could show that MCA occlusion and reperfusion drastically increased concentrations of autotaxin (ATX), the LPA synthesizing molecule in the CSF of human stroke patients and in affected brain regions. Using specific ATX-inhibitors and genetically engineered ATX-deficient animals, we could significantly reduce brain infarct volume and improve stroke outcome (Bitar et al., 2022). Moreover, we could show that human stroke patients with a loss-of-function SNP (Prg-1^R345T) in synaptic lipid signaling leading to higher cortical excitability had a worse outcome when compared with control stroke subjects (Bitar et al., 2022). Finally, we could show that aged animals deficient for the presynaptic LPA2-receptor had a preserved juvenile cortical excitability and displayed better memory formation (Fischer et al., 2021). In another set of experiments we aim at understanding how LPA-related critical excitability when combined with genetic mutations of Tau pathology (i.e. with mice expressing P301L tau) may lead to AT8-positive tau phosphorylation in middle-aged mice (50 weeks old), which is not observed in P301L-expressing mice or in wild type mice (unpublished data). Current work in the lab combines in vivo microscopy, behavioral tracking and single-cell transcriptomics, and aims to understand the role of synaptic lipid signaling in neuronal and glial network interactions after acute ischemic stroke and reperfusion.

What is the role of metabolically regulated peripheral lipids on adult neurogenesis?

Bioactive lipids acting at glutamatergic synapses in the cortex are fueled by peripheral lipids released by adipose tissue and by the liver. In line, metabolic changes alter peripheral lipid levels affecting synaptic lipid production. Besides regulating excitatory transmission, bioactive lipids stimulate mitosis of neuronal stem cells and act as anti-apoptotic factors via LPA1 and LPA2-receptors (Kingsbury et al 2003). However, in how far bioactive synaptic lipids, which underly metabolic changes are able to influence adult neurogenesis is not known. Our data suggest, that increasing synaptic lipid levels stimulates adult neurogenesis increasing the number of surviving, circuit integrated newborn neurons in the hippocampal dentate gyrus.

3. Possible project(s):

Currently we have three main projects dealing with interrelated topics:

1. role of bioactive lipids in the regulation of inhibitory neuronal transmission
2. role of synaptic lipid signaling in the regulation of cortical excitability during exploration and foraging
3. role of bioactive lipid signaling in neuronal and glial network interactions after acute ischemic stroke and reperfusion

1. In brief, we have found that while excitatory transmission is mediated by presynaptic LPA₂-receptors present on excitatory terminals, inhibitory transmission seems to be regulated by LPA₁ receptors. Our results suggest that activation of the LPA₁ R decreased inhibitory transmission (unpublished data). This is a very interesting finding suggesting that one ligand, here lysophosphatidic acid (LPA) acts on two receptors, which both increase excitability either by increasing glutamatergic release probabilities (via the LPA₂ receptor) or by decreasing inhibition (via the LPA₁ receptor). Insofar, we are interested in characterization and analysis of the LPA₁ receptor induced signaling leading to reduction of synaptic inhibition. Since altered cortical excitability is a critical factor in brain aging (Fischer et al., 2021) and important for episodic memory formation (Tüscher et al., 2024), we will further investigate the effects of LPA₁ receptor deficiency in learning, memory formation and aging.

2. While we have shown that bioactive lipids regulate food intake after fasting, the exact pathways are still not exactly described (Endle et al., 2022). Bioactive lipids mainly act on cortical excitatory neurons while the effector neurons regulating food intake are located in the hypothalamus. Here, we aim at identifying bioactive lipid activated regions in the dorsomedial prefrontal cortex (dmPFC), which we have shown to be involved in fasting-related hyperphagia. Moreover, the dmPFC is interconnected to the hypothalamus. Characterization of lipid-activated dmPFC regions will be performed using TRAP-mice and mice expressing Cre under the arc-promotor, which we have established in our lab. Hereby, we will be able to characterize the molecular signature of these neurons as well as the neuronal projections of these neurons, which ultimately may activate hypothalamic regions involved in food intake.

3. We have shown that the LPA-synthesis enzyme autotaxin (ATX) is a critical factor in the early stroke development and determines long-term stroke outcome. Here, ATX inhibition - even when applied 180 min after stroke - reduced stroke volume and cell death, and improved neurologic score at 72h after stroke (Bitar et al., 2022). The role of bioactive lipids in neuronal glial network interactions as well as in changes in the neurovascular unit during the early stroke period are, however, not yet understood. Our data show that ATX displays highest levels in stroke patients in the first 24h after stroke suggesting that early effects of bioactive lipid signaling are critically determining further stroke progression. We therefore aim to investigate early ischemia and reperfusion effects on the neurovascular unit and to understand their role in the mutual glial and neuronal network interactions, which are critically determining disease progression in early stroke.

4. Applied Methods and model organisms:

In our projects, we combine ex vivo and in vivo electrophysiological studies with studies on animal behavior and molecular studies, and use murine primary neuronal cultures for biochemical investigations. Our range of methods includes mouse genetics, histology, cell biology, biochemistry, RNA-sequencing, mouse behavioral analysis, stereotactic brain surgeries, and in vivo one-photon microscopy in anesthetized and awake behaving mice.

5. Desirable skills and qualifications:

We are looking for enthusiastic candidates with a background in neurobiology and experience in at least one of the following areas: electrophysiology, in vivo imaging, behavioral tests. Willingness to work with mice is essential.

6. Key publications on the topics:

1. Liu X, Huai J, Endle H, Schlüter L, Fan, W, Li Y, Richers S, Yurugi H, Rajalingam K, Ji H… Vogt J (2016) PRG-1 regulates spine density via PP2A/ß1-integrin signaling. Developmental Cell 8;38(3):275-90
2. Cheng J, Sahani S, Hausrat TJ, Yang JW, Endle H, Liu X, Li Y, Böttche R, Radyushkin K, Hoerder-Suabedissen A… Vogt J. (2016) Precise somatotopic thalamocortical axon guidance depends on LPA-mediated PRG-2/Radixin signaling. Neuron 92(1):126-142.
3. Vogt J, Kirischuk S, Unichenko P, Schlüter L, Pelosi A, Cheng J, Yang JW,   Strauss U, Prokudin A,  Bharati Bs et al. (2017) Synaptic phospholipid signaling modulates synchronized cortical activity during brain development altering adult connectivity and memory. Cerebral Cortex 27(1):131-145
4. Tüscher O., Muthuraman M., Horstmann, JP… Vogt, J. (2024) Altered cortical synaptic lipid signaling leads to intermediate phenotypes of mental disorders Molecular Psychiatry 2024 May 28. doi: 10.1038/s41380-024-02598-2
5. Fischer C, Endle H… Vogt J*, Tegeder I* (2021) Prevention of age-associated neuronal hyperexcitability with improved learning and attention upon knockout or antagonism of LPAR2. Cellular and Molecular Life Sciences 78 (3): 1029 - 1050 *contributed equally as senior author
6. Thalman C, Horta G, Qiao L, Endle H, Tegeder I, Cheng H, Laube G, Sigrudsson T, Hauser MJ, Tenzer S…  Vogt J. (2018) Synaptic phospholipids as a new target for cortical hyperexcitability and E/I-balance in psychiatric disorders. Molecular Psychiatry 23:1699-1710
7. Trimbuch, T.*, Beed, P.*, Vogt, J.*, Schuchmann, S., Maier, N., Kintscher, M., Breustedt, J., Schuelke, M., Streu, N., Kieselmann, O., Brunk, I., Laube, G., Strauss, U. et al. (2009) Synaptic PRG-1 modulates excitatory transmission via lipid phosphate-mediated signaling. Cell 138:1222-1235
8. Vogt, J., Yang, J.W., Mobascher, A., Cheng, J., Li, Y., Liu, X., Baumgart, J., Thalman, C., Kirischuk, S., Unichenko, P., et al. (2016). Molecular cause and functional impact of altered synaptic lipid signaling due to a prg-1 gene SNP. EMBO Mol Med.8, 25-38
9. Schneider P, Petzold S, Sommer A, Nitsch R, Schwegler H, Vogt J*, Roskoden T*. (2018) Altered synaptic phospholipid signaling in PRG-1 deficient mice induces exploratory behavior and motor hyperactivity resembling psychiatric disorders. A “Forrest Gump” mouse? Behav Brain Res 336:1-7(*equal contributing last authors)
10. Unichenko P, Kirischuk S, Yang JW, Baumgart J, Roskoden T, Schneider P, Sommer A,Nitsch R, Vogt J*, Luhmann HJ* (2016) Plasticity-related gene 1 affects mouse barrel cortex function via strengthening of glutamatergic thalamocortical transmission. Cerebral Cortex26, 3260-3272 (*equal contributing last authors)
11. Endle H, Horta G, Stutz B, Muthuraman M… Horvath TL, Nitsch R & Vogt J (2022). AgRP neurons control feeding behaviour at cortical synapses via peripherally derived lysophospholipids. Nature Metabolism 4(6):683-692
12. Bitar L, Uphaus T, Thalman C, Muthuraman M, Gyr L, Ji H, Domingues M, Endle H, Groppa S, Steffen F, Koirala N, Fan W, Ibanez L, Heitsch I, Cruchaga C, Lee JM, Kloss F, Bittner S, Nitsch R, Zipp F, Vogt J. (2022) Inhibition of the enzyme autotaxin rebalances overexcitation in stroke. Science Translational Medicine 14 (641):eabk0135