Elisenda Sanz

Mitochondrial dysfunction is increasingly recognized as a trigger of innate immune responses, yet whether these responses contribute to neurodegeneration remains poorly understood. Here, we uncover a cell-type-specific antiviral program in vulnerable GABAergic neurons of Ndufs4-deficient mice and human brain organoids modeling Leigh syndrome. Despite robust type I interferon responses, genetic ablation of innate immune mediators Ifnar1 or Sting1 fails to ameliorate disease progression. Instead, we identify the double-stranded RNA sensor protein kinase R (PKR) as a critical mediator of mitochondrial neuropathology. PKR associates with mitochondrial RNA, while its genetic deletion or silencing delays neurological decline, attenuates neuroinflammation, and extends survival without restoring mitochondrial respiration. PKR deletion also normalizes JNK signaling without detectable global translational repression, suggesting a noncanonical contribution to disease. Together, our findings reveal PKR-mediated antiviral RNA sensing as a pathogenic mechanism in mitochondrial complex I deficiency and highlight PKR as a candidate target for mitochondrial encephalopathies.

Obesity and its associated metabolic disorders, including insulin resistance and type 2 diabetes, are major global health challenges. Early hypothalamic microinflammation is emerging as a key contributor to the onset of metabolic dysfunction, but its temporal dynamics, underlying mechanisms, and sex-specific differences remain unclear. Here, we examined the effects of short-term high-fat diet (HFD) exposure on hypothalamic microinflammation, glial activation, and insulin signaling in male and female mice. Males rapidly developed hypothalamic microinflammation, characterized by increased pro-inflammatory cytokines, reactive gliosis, and impaired insulin signaling, whereas females did not show these alterations. Interestingly, we also observed a sex-specific pattern in microglial M2 polarization: females maintained a sustained M2 response throughout the experimental period, while males exhibited only a transient peak that declined in the following days. These sex-specific differences in microglial dynamics and polarization may be linked to the c-Jun N-terminal kinase pathway, a classical mediator of inflammatory signaling. To further explore this, we analyzed mice lacking JNK3, the CNS-enriched isoform of JNK. Loss of JNK3 disrupted early microglial polarization, abolished the transient M2 response, and predisposed males to enhanced gliosis and partial hypothalamic insulin resistance, while females retained partial resilience. These findings indicate that JNK3 contributes to the regulation of microglial dynamics and polarization in response to metabolic stress. Overall, our study highlights a sex-dependent role of microglia and JNK in shaping early hypothalamic microinflammation and central insulin sensitivity, providing potential targets for intervention in obesity-associated metabolic disease.

Deficits in the mitochondrial energy–generating machinery cause mitochondrial disease, a group of untreatable and usually fatal disorders. Refractory epileptic events are a common neurological presentation of mitochondrial disease, including Leigh syndrome, a severe form of mitochondrial disease associated with epilepsy. However, the neuronal substrates and circuits for mitochondrial disease–induced epilepsy remain unclear. Here, using mouse models of Leigh syndrome that lack mitochondrial complex I subunit NDUFS4 in a constitutive or conditional manner, we demonstrated that mitochondrial dysfunction leads to a reduction of GABAergic neurons in the rostral external globus pallidus (GPe) and identified a specific affectation of pallidal Lhx6 –expressing inhibitory neurons contributing to altered GPe excitability. Our findings revealed that viral vector–mediated Ndufs4 reexpression in the GPe effectively prevented seizures and improved survival in the models. Additionally, we highlight the subthalamic nucleus (STN) as a critical structure in the neural circuit involved in mitochondrial epilepsy, as its inhibition effectively reduces epileptic events. Thus, we have identified a role for pallido-subthalamic projections in epilepsy development in the context of mitochondrial dysfunction. Our results suggest STN inhibition as a potential therapeutic intervention for refractory epilepsy in patients with mitochondrial disease, providing promising leads in the quest to identify effective treatments.

Mitochondria play essential metabolic roles and are increasingly understood to interact with other organelles, influencing cellular function and disease. Primary cilia, as sensory and signalling organelles, are crucial for neuronal communication and function. Emerging evidence suggests that mitochondria and primary cilia may interact to regulate cellular processes, as recently shown in brain cells such as astrocytes. Here, we investigated whether mitochondria also regulate primary cilia in neurons, focusing on molecular pathways linking both organelles and structural components within cilia. We employed a cross‐species, molecular pathway‐focused approach to explore connections between mitochondrial and ciliary pathways in neurons, revealing strong associations suggesting coordinated functionality. Furthermore, we found that viral‐induced downregulation of the mitochondrial fusion gene mitofusin 2 (Mfn2) in dopamine D1 receptor‐expressing medium spiny neurons (D1‐MSNs) of the nucleus accumbens (NAc) altered ciliary gene expression, with Crocc – the gene encoding rootletin – showing the most pronounced downregulation. This reduction in Crocc expression was linked to decreased levels of rootletin protein, a key structural component of the ciliary rootlet. Notably, viral‐mediated overexpression of rootletin restored ciliary complexity and elongation, without compromising neuronal adaptation to Mfn2 downregulation. Our findings provide novel evidence of a functional mitochondria–cilia interaction in neurons, specifically in striatal D1‐MSNs. These results reveal a previously unrecognized role of mitochondrial dynamics in regulating ciliary structure in neurons, with potential implications for neuropsychiatric and neurodegenerative disease mechanisms. image Key points: Mitochondria are cell structures known for producing energy but are also emerging as regulators of other cellular components, including primary cilia, antenna‐like structures involved in cell communication. Previous studies suggest that mitochondria may influence cilia structure and function, including in astrocytes. However, this has not been explored in neurons. This study shows that natural variation in mitochondrial molecular pathways correlates with primary cilia pathways in striatal medium spiny neurons in both rats and mice. Reducing expression of mitofusin 2 (Mfn2), a key mitochondrial protein involved in fusion and mitochondria–endoplasmic reticulum interactions, changes specific molecular ciliary pathways, notably including Crocc, a gene essential for cilia structure, and reduces the levels of its protein product, rootletin, which supports cilia integrity. Our findings reveal an important role for mitochondria in regulating ciliary structure in neurons, highlighting a potential pathway for mitochondrial regulation of neuronal signalling.

Summary: Interleukin-6 (IL-6) is a cytokine detected in the brains and peripheral fluids of both Alzheimer’s disease (AD) patients and mouse models, where it colocalizes with amyloid-beta (Aβ) levels and amyloid plaques. Interestingly, IL-6 deficiency ameliorates cognitive deficits and attenuates hippocampal neuroinflammation, whereas astrocyte-targeted IL-6 signaling via its soluble receptor accentuates pathological features in AD mouse models. This finding suggests that central IL-6 overexpression may actively drive disease manifestations. However, because IL-6 also signals through its classical membrane-bound receptor pathway, the overall impact of central IL-6 on Alzheimer’s disease pathophysiology is still not fully elucidated. To explore the contribution of central IL-6 overexpression in modulating AD-related mortality, metabolic, behavioral and neuroinflammatory changes in the hippocampus and cortex, we crossed a transgenic mouse model (Tg2576) of Aβ-driven amyloidosis with mice expressing IL-6 under the Glial Fibrillary Acidic Protein (GFAP) promoter, which predominantly targets astrocytes. Chronic IL-6 overexpression reduced inguinal white adiposity in both males and females and decreased body weight in females. Early behavioral alterations were also observed, along with increased cortical and hippocampal Aβ ₄₂ /Aβ ₄₀ ratios and gliosis in aged Tg2576 female and male mice. Interestingly, chronic IL-6 overexpression also decreased cortical and hippocampal periplaque astrocytosis and microgliosis, suggesting a heterogeneous response of astrocytes and microglia to IL-6 overexpression within the primary regions affected by this pathology. Finally, cortical transcriptomic profiling in Tg2576 mice revealed widespread changes in immune, synaptic, and stress response pathways in response to chronic IL-6 overexpression, with cortical neuroinflammatory and neurotransmission-associated gene networks showing sex-dependent differences. Our findings emphasize that chronic central-targeted IL-6 overexpression shapes the cortical and hippocampal molecular landscape underlying amyloidosis in both male and female Tg2576 mice. Thereby, we propose IL-6 as a potential target for future AD therapeutic strategies.

The medial extended amygdala (EAme) is part of the social behavior network and its subdivisions show expression of sex steroid receptors, which participate in the regulation of sexually dimorphic behaviors. However, EAme subdivisions are highly heterogeneous in terms of neuron subtypes, with different subpopulations being involved in regulation of different aspects of social and non-social behaviors. To further understand the role of the different EAme neurons and their contribution to sexual differences, here we studied one of its major subtypes of glutamatergic neurons, those derived from the telencephalon-opto-hypothalamic domain that coexpress Otp and Foxg1 genes during development. Our results showed that the vast majority of the Otp glutamatergic neurons of the medial amygdala and BSTM in both sexes express Ar , Esr1 (ERα) and Esr2 (ERβ ) mRNA. Moreover, the high percentage of receptors expression in the Otp neurons (between 93 and 100%) indicates that probably the majority of the Otp neurons of EAme are coexpressing the three receptors. In addition, Otp neurons of the posterodorsal medial amygdala have a larger soma and occupy more space in males than in females. These and other features of the Otp neurons regarding their expression of sex steroid receptors likely contribute to some of the sexually-dimorphic behaviors regulated by EAme.

Deficits in the mitochondrial energy-generating machinery cause mitochondrial disease (MD), a group of untreatable and usually fatal disorders. Among many severe symptoms, refractory epileptic events are a common neurological presentation of MD. However, the neuronal substrates and circuits for MD-induced epilepsy remain unclear. Here, using mouse models of mitochondrial epilepsy that lack mitochondrial complex I subunit NDUFS4 in a constitutive or conditional manner, we demonstrate that mitochondrial dysfunction leads to a reduction in the number of GABAergic neurons in the rostral external globus pallidus (GPe) and identify a specific affectation of pallidal Lhx6 -expressing inhibitory neurons. Our findings further reveal that viral vector-mediated Ndufs4 re-expression in the GPe effectively prevents seizures and improves the survival in the models. Additionally, we highlight the subthalamic nucleus (STN) as a critical structure in the neural circuit involved in mitochondrial epilepsy, as its inhibition effectively reduces epileptic events. Thus, we have identified a novel role for pallido-subthalamic projections in the development of epilepsy in the context of mitochondrial dysfunction. Our results suggest STN inhibition as a potential therapeutic intervention for refractory epilepsy in patients with MD providing new leads in the quest to identify novel and effective treatments.

Mutations in mitochondrial energy-producing genes lead to a heterogeneous group of untreatable disorders known as primary mitochondrial diseases (MD). Leigh syndrome (LS) is the most common pediatric MD and is characterized by progressive neuromuscular affectation and premature death. Here, we show that daily cannabidiol (CBD) administration significantly extends lifespan and ameliorates pathology in two LS mouse models, and improves cellular function in fibroblasts from LS patients. CBD delays motor decline and neurodegenerative signs, improves social deficits and breathing abnormalities, decreases thermally induced seizures, and improves neuropathology in affected brain regions. Mechanistically, we identify peroxisome proliferator-activated receptor gamma (PPARγ) as a key nuclear receptor mediating CBD’s beneficial effects, while also providing proof of dysregulated PPARγ expression and activity as a common feature in both mouse neurons and fibroblasts from LS patients. Taken together, our results provide the first evidence for CBD as a potential treatment for LS.

Peripheral neurons are heterogeneous and functionally diverse, but all share the capability to switch to a pro-regenerative state after nerve injury. Despite the assumption that the injury response is similar among neuronal subtypes, functional recovery may differ. Understanding the distinct intrinsic regenerative properties between neurons may help to improve the quality of regeneration, prioritizing the growth of axon subpopulations to their targets. Here, we present a comparative analysis of regeneration across four key peripheral neuron populations: motoneurons, proprioceptors, cutaneous mechanoreceptors, and nociceptors. Using Cre/Ai9 mice that allow fluorescent labeling of neuronal subtypes, we found that nociceptors showed the greater regeneration after a sciatic crush, followed by motoneurons, mechanoreceptors, and, finally, proprioceptors. By breeding these Cre mice with Ribotag mice, we isolated specific translatomes and defined the regenerative response of these neuronal subtypes after axotomy. Only 20% of the regulated genes were common, revealing a diverse response to injury among neurons, which was also supported by the differential influence of neurotrophins among neuron subtypes. Among differentially regulated genes, we proposed MED12 as a specific regulator of the regeneration of proprioceptors. Altogether, we demonstrate that the intrinsic regenerative capacity differs between peripheral neuron subtypes, opening the door to selectively modulate these responses.

Peripheral neurons are heterogeneous and functionally diverse, but all share the capability to switch to a pro-regenerative state after nerve injury. Despite the assumption that the injury response is similar among neuronal subtypes, functional recovery may differ. Understanding the distinct intrinsic regenerative properties between neurons may help to improve the quality of regeneration, prioritizing the growth of axon subpopulations to their targets. Here, we present a comparative analysis of regeneration across four key peripheral neuron populations: motoneurons, proprioceptors, cutaneous mechanoreceptors, and nociceptors. Using Cre/Ai9 mice that allow fluorescent labelling of neuronal subtypes, we found that nociceptors showed the greater regeneration after a sciatic crush, followed by motoneurons, mechanoreceptors and, finally, proprioceptors. By breeding these Cre mice with Ribotag mice, we isolated specific translatomes and defined the regenerative response of these neuronal subtypes after axotomy. Only 20% of the regulated genes were common, revealing a diverse response to injury among neurons, which was also supported by the differential influence of neurotrophins among neuron subtypes. Among differentially regulated genes, we proposed MED12 as a specific regulator of the regeneration of proprioceptors. Altogether, we demonstrate that the intrinsic regenerative capacity differs between peripheral neuron subtypes, opening the door to selectively modulate these responses.