Enhanced Ca2+ influx by mTORC1 increases neuronal network activity in TSC neurons

TOKYO METROPOLITAN INSTITUTE OF MEDICAL SCIENCE

IMAGE: IN NORMAL NEURONS, ONLY CERTAIN PARTS ARE SPONTANEOUSLY ACTIVATED. LOSS OF FUNCTION OF MUTATIONS IN TSC1 OR TSC2 LEADS TO HYPERACTIVATION OF MTOR. HYPERACTIVATED MTOR INCREASES LTCC EXPRESSION AND CAUSES A LARGE CA2+ INFLUX UPON NEURONAL ACTIVATION IN TSC NEURONS. INCREASED CA2+ INFLUX CAUSES ABNORMAL AXONAL EXTENSIONS AND SUSTAINED ACTIVATION OF CREB. THESE PHYSIOLOGICAL CHANGES CAN TRIGGER EPILEPSY IN TSC.

CREDIT: TMIMS

Tuberous sclerosis complex (TSC) is an intractable disease characterized by benign tumors, called hamartomas, in the brain, lungs, and kidneys. Patients with TSC develop neurological symptoms, such as epilepsy, intellectual disability, and autism. Epilepsy is known to occur in as many as 80% of the patients with TSC; and is highly associated with intellectual disability and inferior quality of life of patients.

The causal genes for TSC are TSC1 and TSC2. Previous studies have shown that “loss-of-function” mutations in either TSC1 or TSC2 trigger hyperactivation of the mammalian target of rapamycin complex 1 (mTORC1), which is tightly associated with the onset of epilepsy. However, the precise mechanism by which mTORC1 causes epilepsy remains largely unknown.

In a new study published online on August 20th in the Journal of Neuroscience, Chihiro Hisatsune and his colleagues at the Tokyo Metropolitan Institute of Medical Science (TMiMS) reported mTORC1-dependent enhancement of Ca2+ influx via L-type Ca2+ channels (LTCCs) in TSC neurons, which could explain the occurrence of epilepsy.

The research group established human iPS cells lacking TSC2 using the CRISPR-Cas9 system, differentiated the TSC2-deficient iPSCs into cortical excitatory neurons, and examined their physiological properties in culture. Using Ca2+ imaging, they first showed that TSC2 neurons exhibit highly synchronized spontaneous neuronal activity with Ca2+ spikes. They also showed that TSC2 neurons had abnormally extended axons. Since both synchronized spontaneous neuronal activity and altered axonal extensions in TSC2 neurons were suppressed by an mTORC1 inhibitor, it seems that intimate neural networks with abnormally extended axons by mTOR contribute to the synchronized neuronal network activity of TSC2 neurons.

The research group further examined Ca2+ signaling in individual neurons, since intracellular Ca2+ concentration is known to have an intimate relationship with neuronal activity. They found that TSC2 neurons show increased Ca2+ influx via LTCCs upon membrane depolarization, as compared to wild-type neurons. Since an mTORC1 inhibitor suppressed this increase, the increased Ca2+ influx was mTORC1-dependent.

They further examined the effect of increased Ca2+ influx on the properties of TSC2 neurons. They showed that the enhanced Ca2+ influx significantly promoted axonal extensions in TSC2 neurons and triggered sustained activation of cAMP response element binding protein (CREB), a transcription factor that is important for synaptic plasticity.

In conclusion, this study suggests an important connection between sustained mTOR activation and elevated Ca2+ signaling via LTCC in human TSC neurons, which could cause epilepsy in TSC. Hisatsune says “the next important issue is to explore whether the neurons of patients with TSC have enhanced Ca2+ influx via LTCCs and altered LTCC levels in the brain”.