Thomas Mittmann

Glucocorticoid receptors (GRs) are key mediators of how the stress hormone glucocorticoids (GCs) shape postnatal brain development and adaptive plasticity. Because GC signaling is critical during this period, postnatal GC concentrations are tightly regulated in the brain, whereas excessive levels of circulating GCs can disrupt developmental trajectories and increase the risk of psychiatric disorders later in life. GR function influences multiple neural cell types, but its cell-specific roles, particularly early in development, remain poorly understood. Oligodendrocyte precursor cells (OPCs), which generate myelinating oligodendrocytes and actively modulate neuronal networks, express GRs and can therefore respond to fluctuations in GC levels. Although excessive GC exposure during early life adversity has been linked to changes in OPC development, the physiological role of GR signaling specifically within OPCs remains unclear. To address this, we conditionally deleted GRs in postnatal OPCs in mice to investigate the role of physiological GC signaling in OPC proliferation and maturation, as well as in neuronal network activity and behavior. This deletion resulted in reduced oligodendrocyte and myelinated axon density in the hippocampus, sex-specific alterations in hippocampal activity and long-term potentiation following acute challenge, and impairments in memory formation in adulthood. Our findings reveal an OPC-specific role for GRs and suggest that physiological GR activity in the oligodendrocyte lineage contributes to normal hippocampal plasticity, learning, and memory.

Nerve injury causes an imbalance of glutamatergic excitation over GABAergic inhibition, contributing thereby to lasting neuropathic pain. Transgenic GAD67-GFP knock-in reporter mice were developed to visualize GABAergic interneurons. The knock-in into glutamate decarboxylase (GAD67) causes haploinsufficiency that manifest in low GABA levels. In this model, we studied if diminished GABA exacerbates neuropathic pain after nerve injury. Adolescent male and female GAD67-GFP knock-in mice were subjected to Spared Sciatic Nerve Injury (SNI). At baseline, nociception and thermal preferences were equal but after SNI, GAD67-GFP mice developed thermal allodynia which was not detected in wildtype littermates. At the electrophysiology level, SNI caused a partial decrease in the excitability in layer 2/3 pyramidal neurons in the projection-hemisphere in wildtype mice. This effect was exacerbated in GAD67-GFP, affecting both sides, and was accompanied with imbalance of field-potential (FP) amplitudes between projection and non-projection hemisphere, which did not occur in wildtype mice. The results suggest that GABA deficiency can be compensated to protect from hyperexcitability at baseline, but it cannot be further upscaled, ultimately leading to network hyperactivity after injury. Metabolomic studies confirmed the moderate loss of GABA in ipsi- and contralateral cortex and lumbar spinal cord of GAD67-GFP mice and failure to raise GABA in the ipsilateral dorsal horn after injury. Carnosine, cystathionine, and glutathione, three important anti-oxidative metabolites, were co-reduced with GABA suggesting that GABAergic activity and anti-oxidative capacity are interconnected and failure of this axis contributes to neuropathic “pain”.

Traumatic brain injury (TBI) can lead to long-lasting cognitive deficits in the human brain, with a considerable contribution of secondary morphological and functional sequela in cortical regions distant to the lesion site. In order to uncover the role of early functional alterations in parvalbumin-positive basket cells (PV-BCs), an interneuron population required to maintain inhibition of neocortical circuits, to this dysfunctional plasticity, we investigated anatomical and electrophysiological properties of PV-BCs in PV-IRES-Cre-tdTomato mice of both sexes 24 h after a cortical impact. These experiments revealed that the number of PV-BCs was moderately decreased around the cortical impact site, while their morphology was unaffected. Patch-clamp experiments demonstrated that TBI increased the input resistance of PV-BCs and the amplitude of hyperpolarization-activated inward currents ( I _h ). In addition, the maximal firing frequency upon depolarizing stimuli was decreased. The increase in I _h amplitude was paralleled by the appearance of somatic HCN channels in immunohistochemical staining and the occurrence of somatic I _h in nucleated patch recordings, suggesting that TBI induced a redistribution of HCN channels from a purely axonal to an additional somatodendritic expression. Pharmacological experiments showed that inhibition of axonal HCN-mediated currents impairs the maximal firing frequency of PV-BCs. Additional in silico simulations disclosed the general importance of axonal HCN channels to maintain high-frequency firing of PV-BCs by counteracting Na ⁺ -K ⁺ -pump associated hyperpolarizing currents. In summary, our results suggest that the early loss of PV-BCs and the TBI-induced distinct alterations in their electrophysiological properties can contribute to the establishment of disturbed network activity following TBI.

Vasoactive intestinal polypeptide-expressing inhibitory interneurons (VIP-INs) in the adult barrel cortex are crucial for mediating active whisking (AW) by disinhibiting pyramidal neurons. Past studies have investigated the development of VIP-IN network integration, focusing mainly on the excitatory network or the postsynaptic side of the inhibitory network. Hence, we aimed to explore the inhibitory network integration of VIP-INs, concentrating on the presynaptic side. We addressed this by investigating VIP-INs in three different age groups (postnatal day (P)8–P10, P14–P16, and P30–P36) in Vip-IRES-cre x tdTomato mice with whole-cell patch clamp recordings. By placing a stimulation electrode into L4 of the barrel field, we elicited electrically-evoked inhibitory postsynaptic currents (eIPSCs) in L2/3 VIP-INs following a high-frequency stimulation. We then analysed recorded eIPSCs by applying the binomial model of synaptic transmission. Our results show significant increases in both the number of readily-releasable vesicles and the presynaptic release probability between P9 and P15, suggesting that the inhibitory network integration is at least partially conducted via a presynaptic functional maturation. Despite an increase in the release probability, synaptic depression is decreased at P30–P36 due to an accelerated vesicle replenishment rate within the same time window. Lastly, asynchronous vesicle release decreases in favour of a stimulus-locked signal transmission by P30–P36. Our results suggest a maturation of the inhibitory projections towards a strong, precise, and stimulus-locked inhibition. This can be physiologically relevant to define the temporal precision of AW at the relevant frequencies.

Usher syndrome type 1, caused by pathogenic variants in the USH1C gene, leads to congenital deafness and progressive retinal degeneration resulting in vision loss. While auditory deficits can be compensated by cochlea implants and hearing aids, no treatment exists to prevent retinal degeneration. Here, we generated retinal organoids from induced pluripotent stem cells of two USH1C patients to elucidate the cellular and molecular mechanisms driving ocular pathogenesis. Single-cell RNA sequencing of both healthy and USH1C retinal organoids identified differential expression of genes related to phototransduction in photoreceptors, as well as alterations in cell adhesion and canonical Wnt signaling in Müller glia cells. Analysis of intercellular communication revealed an overall reduced signaling efficiency, particularly affecting Müller glia-mediated retinal adhesion processes. Morphological characterization of organoids confirmed transcriptome changes by showing degeneration of the outer limiting membrane and loss of adherens junction architecture. Moreover, photoreceptors revealed increased susceptibility to morphological and functional changes related to phototransduction. These results demonstrate that disruption of Müller glia signaling contributes to a loss of retinal integrity, providing novel insights into USH1C pathogenesis and offering targets for therapeutic interventions.

The metalloprotease meprin β is known for its multifunctional involvement in various physiological processes throughout the body including the brain. However, its broader functions within the brain besides amyloid β generation remain largely unexplored. To investigate this, we utilized a mouse model overexpressing meprin β in neurons within the cortex and hippocampus, regions crucial for learning and memory. Behavioral assessments, employing the Morris' Water Maze paradigm test, revealed impaired cognitive functions in animals overexpressing meprin β. Furthermore, electrophysiological recordings in hippocampal slices using multielectrode arrays showed an impaired long‐term potentiation (LTP) in meprin β‐overexpressing mice compared to wild‐type counterparts. Intriguingly, concomitant with the LTP impairment, we observed an increased neuronal excitability. These findings underline the complicated interplay between meprin β abundance and behavioral manifestations, suggesting a broader impact on neural circuit dynamics. To elucidate the molecular mechanisms underlying these observed deficits, western blotting analyses were conducted to address the expression of glutamatergic receptors. Neither the expression of the N‐methyl‐D‐aspartate (NMDA) nor the α‐amino‐3‐hydroxy‐5‐methyl‐4‐isoxazolepropionic acid (AMPA) receptor showed variation relative to each other. The application of N‐terminomics identified brevican as a proteolytic substrate of meprin β and thus a potential key mediator linking meprin β overexpression to the observed effects. Previous studies have reported that brevican knockout in animal models influences learning and memory. Our data demonstrate that meprin β modulates brevican expression, likely contributing to the effects we have observed in our mouse model. These results shed light on the broader functional significance of meprin β in neurological processes.

Traumatic brain injury (TBI) remains the leading cause of death and disability worldwide with no existing effective treatment. The early phase after TBI induction triggers numerous molecular cascades to regulate adaptive processes and cortical network activity. Kinases play a particularly prominent role in modifying peptide substrates, which include ion channels, receptors, transcription factors and inflammatory mediators. This study aimed to better understand the post-injury serine/threonine kinome; (1) Which kinases conduct phosphorylation-induced alterations of target peptides following unilateral TBI in mouse cortex? (2) How do these kinases effectuate pathological network hyperexcitability, which has detrimental long-term outcomes? We used a serine/threonine kinase assay at 4 h, 24 h and 72 h post-TBI to identify hyper-/hypo-active/phosphorylated kinases and peptides in the ipsilateral and contralateral cortical hemispheres relative to sham-operated controls. We pharmacologically mimicked the changes seen in ERK1/2 and PKC kinase activity, and using microelectrode array recordings we explored their significant electrophysiological implications on spontaneous and evoked cortical activity. We then used these findings to manipulate key kinase activity changes at 24 h post-TBI to rescue the hyperexcitability that is seen in the contralateral cortical network at this timepoint back to sham level. The contribution of specific downstream peptide target channel/receptor subunits was also shown. We conclude that volatile kinase activity has potent implications on cortical network activity after the injury and that these kinases and/or their peptide substrates should be more seriously considered as therapeutic targets for the clinical treatment of TBI.