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

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 20^th 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”.

About the Tokyo Metropolitan Institute of Medical Science

The Tokyo Metropolitan Institute of Medical Science (TMIMS) is dedicated to advancing basic and medical research in order to improve human health and quality of life. Founded in 2011 through the consolidation of three medical institutes, TMIMS is funded by the Tokyo metropolitan government and supports basic research in molecular and cellular biology in areas including genome replication, protein degradation, and infectious and neurodegenerative diseases. TMIMS also supports the development of new technologies in areas such as genome editing, control of neural prostheses, and vaccine development, and clinical research in fields such as optimization of nursing care and development of new treatments for psychiatric, neurodegenerative and other diseases. By integrating top-down applied research with bottom-up basic research, a goal of TMIMS is to more efficiently translate basic research results into treatments beneficial for humankind. For more information about TMIMS, see www.igakuken.or.jp/english.