The epilepsy-associated protein TBC1D24 is required for normal development, survival and vesicle trafficking in mammalian neurons

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 critical 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.

The epilepsy and intellectual disability-associated protein TBC1D24 regulates the maintenance of excitatory synapses and animal behaviors

“…The Drosophila TBC1D24 orthologue Skywalker regulates neurotransmitter release at the presynaptic terminals [15,16]. The presynaptic function of TBC1D24 has recently been confirmed in mammalian neurons, in which enlarged endosomes are detected in the presynaptic terminals of TBC1D24 heterozygous knockout neurons that are coupled to reduced glutamate release [61]. On the other hand, we found that TBC1D24 is also localized at postsynaptic sites of excitatory synapses.…”

“…Milder phenotype in genetic knockout has also been observed regarding regulators of GTPases. For example, only knockdown but not knockout of the Rap-GEF Epac2 reduces spine density [59,60], while acute knockdown but not genetic knockout of Tbc1d24 results in neuronal migration defects [17,61]. Since TBC1D24 depletion by the homozygous F251L mutation occurs at the very beginning of development, it resembles the deficiency of gene product in knockout animals.…”

“…In this regard, it is noteworthy that Homer1 is present in endosomes and regulates AMPA receptor endocytosis [75,79]. Given the role of TBC1D24 in regulating endosomal function [61], it will be important to determine whether TBC1D24 directly affects Homer1-mediated endocytosis in dendritic spines in future studies.…”

“…Given the strong association of TBC1D24 mutations with ID, it is important to characterize how TBC1D24 deficiency in vivo affects animal behaviors including learning and memory. Despite the emergence of TBC1D24 transgenic mouse models recently [61,88], the behavioral changes associated with TBC1D24 gene mutations have not been studied, probably because of early lethality of the mice. The TBC1D24 F251L/F251L mice in our current study also suffer early death around three weeks after birth.…”

Refinement of 16p13.3 microdeletion syndrome from a case presentation of a girl with epilepsy and intellectual disability

“…Compound heterozygous or homozygous TBC1D24 mutations result in epileptic encephalopathy and DOORS syndrome (deafness, onychodystrophy, osteodystrophy, intellectual disability, and seizures) . TBC1D24 haploinsufficiency has been associated with epilepsy, and haploinsufficiency in mice resulted in neurite growth abnormalities …”