Leigh syndrome (LS) is a progressive neurological disorder which often manifests within the first year of life and is characterized by the gradual loss of mental and movement abilities accompanied by epilepsy. LS has been associated with loss-of-function (LOF) mutations in genes that encode for proteins present in complex 1 of the electron transport chain. LOF mutations in one such gene, NADH dehydrogenase (ubiquinone) iron sulfur protein 4 (Ndufs4), are strongly associated with LS. Mice carrying a deletion of this gene exhibit symptoms similar to those found in humans, creating a relevant mouse model of LS. In this study, we investigated the effects of an Ndufs4 knockout on the neuronal excitability of both inhibitory and excitatory neurons located in different regions of the brain in LS mouse models. Two LS mouse models were generated by knocking out Ndufs4 in inhibitory or excitatory neurons utilizing LoxP/Cre technology. Mice carrying floxed alleles of Ndufs4 were crossed with VglutCre or GadCre driver mice. The progeny with excitatory or inhibitory neuron-specific Ndufs4 knockout and their control littermates obtained were perfused with phosphate buffered saline (PBS), then fixed with 4% paraformaldehyde (PFA). Brains from these mice were sliced and stained with c-Fos immunocytochemistry, then imaged to quantify neuronal activity. We hypothesize that neuronal excitability in both inhibitory and excitatory neurons will decrease after the Ndufs4 knockout, as mitochondrial defects would reduce the activity of both subsets of neurons. Findings from this study will potentially help understand the mechanisms for development of seizures in LS.

Leigh Syndrome (LS) is the most common pediatric mitochondrial disease, and it is associated with loss-of function mutations in genes that encode for proteins in Complex I of the electron transport chain. Mutations in a gene called NADH dehydrogenase (ubiquinone) iron sulfur protein 4 (NDUFS4) is linked with LS which results in many neurological symptoms and neurodegenerative biomarkers in afflicted patients. Prior studies have discovered that many neurodegenerative diseases are characterized by disturbances in circadian function, which can impact disease symptoms and worsen quality of life. However, it remains unknown if disruptions of circadian function are a characteristic phenotype of all mitochondrial diseases and Leigh Syndrome in particular. In this study, we investigated the integrity of the circadian rhythm in conditional knockout (KO)-models of LS. KO-models carried the mutation in either excitatory (glutamatergic) or inhibitory (GABAergic) neurons. We generated mice with Ndufs4 KO restricted to glutamatergic or GABAergic neurons using LoxP Cre technology. To examine circadian rhythm patterns, we placed each mouse in an individual cage with a running wheel and infrared (IR) sensor. Mice were maintained on a 12:12 hour light-dark schedule where the light period began at 7:00 AM. Mouse wheel activity and home cage locomotor activity were recorded and subsequently analyzed offline using ClockLab Analysis. Our initial results showed that Ndufs4 KO in excitatory neurons leads to severe disruption of circadian rhythms in which locomotive activity was not synchronized with a light-dark cycle, whereas Ndufs4 KO in inhibitory neurons had no detectable effect on circadian rhythm. These results reveal that disruptions in circadian function are present in Ndufs4-related LS, particularly due to excitatory neurons. A better understanding of circadian rhythm disruptions in LS can lead to further research on a molecular level to discover underlying characteristics of LS and become an identifier for the progression of LS.