Subtype-specific deletion of Dnmt1 in cortical interneurons reveals differential contributions to murine behavior

The functional integrity of cortical networks is essential for proper sensory processing, cognitive performance, and emotional regulation. Within these networks, inhibitory interneurons play a pivotal role in coordinating the precise timing of neuronal activity and in stabilizing cortical oscillations. Among these, parvalbumin- (PV) and somatostatin-expressing (SOM) interneurons are particularly important, each contributing in distinct ways to network stability and information processing. Numerous studies have demonstrated that disruptions in the development or function of these interneurons can lead to deficits in cortical processing. These impairments are increasingly recognized to contribute to numerous brain pathologies, including neuropsychiatric and neurodevelopmental disorders. Altered numbers or functional defects of PV and SOM interneurons were shown to be implicated in schizophrenia, mood disorders and anxiety, although their contributions to these pathophysiologies follow different mechanisms. Differences in interneuron numbers and their distribution can arise from disturbances of their developmental program, leading to defects of the excitation/inhibition-balance and network synchrony that can entail cognitive and emotional disturbances due to cortical network instability. Among the cognitive impairments are, inter alia, disruptions of learning and memory formation, which also constitute a common defect in many neuropsychiatric disorders. Moreover, disturbances in interneuron development are strongly associated with neuro-developmental disorders, highlighting the vulnerability of these cells during critical periods of brain maturation. However, neuropsychiatric and neurodevelopmental disorders are multifactorial, arising from complex interactions between genetic, environmental, and epigenetic factors. These multifaceted origins underscore the importance of understanding how epigenetic mechanisms, such as DNA methylation catalyzed by DNA methyltransferases, contribute to the regulation of interneuron function and, by extension, the development of neuropsychiatric conditions.Previous work from our research group has underscored the essential role of the DNA methyltransferase 1 (DNMT1) in the development and function of inhibitory interneurons fated for the murine cortex. DNMT1 exerts stage-specific and subtype-specific effects: In early-born interneuron populations, Dnmt1 deletion was shown to lead to profound developmental alterations, e.g., by subtype-specifically affecting either the cells’ migration or morphology and survival. For instance, in a mouse model investigating the effects of Dnmt1-deletion in post-mitotic SOM-positive cells, DNA methylation-dependent changes in interneuron migration during development manifest in an altered cortical architecture and functionality of adult mice. In contrast, PV expression-dependent Dnmt1-depletion primarily affects adult interneuron function in mice as the expression onset – and thus also the Dnmt1-deletion – occurs postnatally. In a mouse model with a GAD2-dependent Dnmt1-knockout – with GAD2 being broadly expressed in all subtypes of cortical inhibitory interneurons starting at the post-mitotic developmental stage – preliminary data suggest a potential fate switch with more interneurons expressing PV, although the overall number and distribution of cortical interneurons remained unaltered. Thus, this mouse model possibly combines developmental and functional effects of the Dnmt1-deletion in interneurons. In sum, these mouse lines highlight the importance of stage- and subtype-specific DNMT1-mediated epigenetic control and enable the investigation of different DNMT1-dependent effects on cellular and systemic scales. Furthermore, emerging evidence suggests that various epigenetic mechanisms, including DNA methylations, are modulated by sex, e.g., via interactions of epigenetic key players with sex hormones. Similarly, neuropathological phenotypes are subject to sex-specific variations, suggesting a potential role of sex-specific modulation of epigenetic processes for disease manifestation. However, the behavioral consequences of DNMT1 loss across distinct interneuron subtypes and sexes remain incompletely understood and warrant further investigation. To address this, several transgenic mouse models with targeted Dnmt1 deletion in GAD2-, PV-, or SOM-positive cortical interneurons were investigated for this thesis. These analyses revealed that Dnmt1 deletion in PV and SOM interneurons of male mice altered cortical processing of sensory stimuli. This was reflected by a reduced temporal precision and amplitude of cortical responses, alongside a significant reduction in gamma oscillation power. Interestingly, behavioral data propose that basic sensory perception remained unaffected in both optomotor response and sensory decision-making tasks. Instead, Dnmt1-deficiency was associated with signs of neuropsychiatric phenotypes. In mice with a PV cell-specific Dnmt1-deletion we observed behavioral alterations suggestive of increased apathy, anxiety-like behavior, and altered hedonic valuation – features consistent with a depression-like phenotype. To further explore the functional impact of DNMT1 in cortical interneurons, including the aspect of potential sex-dependent variation, we tested male and female mice in a standardized learning, memory, and relearning task using the Morris water maze (MWM). Dnmt1 deletion in GAD2-positive interneurons led to marked impairments in both learning and relearning, with particularly severe deficits observed in female mice. In contrast, PV-specific deletion did not affect basic learning but improved performance in relearning of male mice, suggesting enhanced cognitive flexibility. SOM-specific Dnmt1-knockout showed no sex-specific effects on learning, yet male mice also exhibited improved relearning performances. Collectively, these findings support a sex- and cell type-specific role of DNMT1 in shaping murine behavior. The observed alterations in cortical network activity and behavior suggest that DNMT1 expression in cortical interneurons represents a potential facilitator of pathological symptoms, opening up new research avenues for novel therapeutic strategies of neuropsychiatric disorders. Furthermore, our results emphasize the importance of considering sex-specific differences in future studies on epigenetic regulation in the brain.