Rationale:
SLC13A5 Epilepsy is a rare pediatric neurodevelopmental disorder characterized by severe seizures starting in the first days following birth and persisting throughout life. In addition to epilepsy, patients can have symptoms ranging from severe speech and language delays to dystonia. This disease is caused by autosomal recessive mutations in the SLC13A5 gene, which encodes for the sodium citrate transporter (NaCT), a plasma membrane protein highly expressed in the brain, liver, and teeth. Loss of NaCT function leads to decreased citrate within tissues and increased citrate outside of cells. How a lack of NaCT causes neurologic problems and seizures is unknown. One hypothesis is that excess extracellular citrate can drive abnormal brain activity by reducing inhibition of the excitatory NDMA receptor as citrate is a potent zinc chelator.
Methods:
This study used SLC13A5 knockout (KO) mice, which have increased neuronal excitability and propensity for epileptic seizures, although they do not show signs of overt behavioral impairments. Like patients, KO mice have increased citrate in both plasma and cerebral spinal fluid compared to wildtype (WT) mice. To investigate the role of extracellular citrate in driving SLC13A5 Epilepsy, KO and WT mice were given up to 6% citrate water
ad libitum starting
in utero or in adulthood and were followed for ~3 months post-supplementation. During supplementation animals were evaluated for weight gain, survival, behavior, EEG activity (wireless telemetry implants), seizure susceptibility (pentylenetetrazol), and citrate levels in the plasma and CSF (GC-MS). Citrate supplemented animals were tested in parallel with control KO and WT mice that were maintained on regular drinking water.
Results:
In both KO and WT mice, increased dietary citrate was well tolerated as evidenced by normal body weight gain and behavior as assessed by the accelerating rotarod and open field tests. To assess brain activity mice received wireless telemetry implants. Interestingly, of the citrate supplemented animals 50% of SLC13A5 KO mice died by five days post-surgery and 20% of WT mice died by seven days post-surgery, while there were no deaths in the animals on regular drinking water. The cause of deaths were unknown although one KO mouse was found seizing prior to death. EEG activity and seizure susceptibility of the surviving citrate-treated mice was similar to that of the non-supplemented mice. Analysis of plasma and CSF showed that increased dietary citrate increased extracellular citrate levels in both the blood and CSF of KO mice.
Conclusions:
In SLC13A5, KO mice increased dietary citrate further elevated extracellular citrate levels in both the plasma and CSF. Citrate supplementation did not affect weight gain or behavior of WT and KO mice, but it was associated with decreased survival following implant surgery that was significantly worsened in KO mice. While brain and epileptic activity of remaining citrate-supplemented mice was similar to non-supplemented mice, this could be due to selection/survival of the least affected and additional investigation is needed.
Funding: This project was supported by the TESS Research Foundation.