Abstract
Mutations in the Tre2/Bub2/Cdc16 (TBC)1 domain family member 24 (TBC1D24) gene are associated with a range of inherited neurological disorders, from drug-refractory lethal epileptic encephalopathy and DOORS syndrome (deafness, onychodystrophy, osteodystrophy, mental retardation, seizures) to non-syndromic hearing loss. TBC1D24 has been implicated in neuronal transmission and maturation, although the molecular function of the gene and the cause of the apparently complex disease spectrum remain unclear. Importantly, heterozygous TBC1D24 mutation carriers have also been reported with seizures, suggesting that haploinsufficiency for TBC1D24 is significant clinically. Here we have systematically investigated an allelic series of disease-associated mutations in neurons alongside a new mouse model to investigate the consequences of TBC1D24 haploinsufficiency to mammalian neurodevelopment and synaptic physiology. The cellular studies reveal that disease-causing mutations that disrupt either of the conserved protein domains in TBC1D24 are implicated in neuronal development and survival and are likely acting as loss-of-function alleles. We then further investigated TBC1D24 haploinsufficiency in vivo and demonstrate that TBC1D24 is also crucial for normal presynaptic function: genetic disruption of Tbc1d24 expression in the mouse leads to an impairment of endocytosis and an enlarged endosomal compartment in neurons with a decrease in spontaneous neurotransmission. These data reveal the essential role for TBC1D24 at the mammalian synapse and help to define common synaptic mechanisms that could underlie the varied effects of TBC1D24 mutations in neurological disease.
Highlights
The regulation of synaptic transmission is a key pathological mechanism in disease, with mutations in genes that mediate the synaptic vesicle (SV) cycle joining an ever-expanding group of neurological disorders recently termed as the synaptopathies [1,2]
These disorders include inherited forms of epilepsy caused by presynaptic proteins such as MUNC18–1 [encoded by syntaxin-binding protein 1 (STXBP1)] [3,4], Synapsin I (SYN1) [5,6], syntaxin 1B (STX1B) [7,8] and synaptosome-associated protein 25B (SNAP25B) [9], and there are clues from studies in model organisms that many more SV-associated genes will play a central role in diseases characterized by seizures and/or neurodevelopmental delay [10,11]
As the functional data available in mammalian systems to date have been limited to acute embryonic knockdown or overexpression studies in wild-type (WT) rodent tissues or cells, we chose to generate a CRISPR-Cas9-mediated TBC1D24 knockout neuronal cell line (Tbc1d24Δ/Δ) and studied the expression of an allelic series TBC1D24 mutants compared to WT TBC1D24
Summary
The regulation of synaptic transmission is a key pathological mechanism in disease, with mutations in genes that mediate the synaptic vesicle (SV) cycle joining an ever-expanding group of neurological disorders recently termed as the synaptopathies [1,2] These disorders include inherited forms of epilepsy caused by presynaptic proteins such as MUNC18–1 [encoded by syntaxin-binding protein 1 (STXBP1)] [3,4], Synapsin I (SYN1) [5,6], syntaxin 1B (STX1B) [7,8] and synaptosome-associated protein 25B (SNAP25B) [9], and there are clues from studies in model organisms that many more SV-associated genes will play a central role in diseases characterized by seizures and/or neurodevelopmental delay [10,11]. The effects of TBC1D24 disruption on synapse biology is yet to be studied in a mammalian genetic system
Sign-in/Register to view highlights & summary Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
