Alternative titles; symbols
HGNC Approved Gene Symbol: CPLX1
Cytogenetic location: 4p16.3 Genomic coordinates (GRCh38) : 4:784,957-826,129 (from NCBI)
| Location | Phenotype |
Phenotype MIM number |
Inheritance |
Phenotype mapping key |
|---|---|---|---|---|
| 4p16.3 | Developmental and epileptic encephalopathy 63 | 617976 | Autosomal recessive | 3 |
The highly specialized uptake and exocytosis system of synaptic vesicle traffic has been studied extensively as a model of membrane fusion. The fusion reaction begins with the assembly of a core complex consisting of the plasma membrane proteins syntaxin (e.g., STX1A; 186590) and SNAP25 (600322) and the synaptic vesicle protein synaptobrevin (e.g., VAMP2; 185881). The core complex then serves as a soluble NSF attachment protein (SNAP; see 603215) receptor, or SNARE. Binding of SNAP to SNARE leads to ATP-dependent binding of NSF (601633), which subsequently catalyzes disruption of the SNARE complex. Synaptotagmin (e.g., SYT1; 185605) triggers the final step in the fusion process, possibly via Ca(2+)-dependent interaction with syntaxin. Complexins, such as CPLX1, are soluble proteins that regulate SNARE function during membrane fusion (McMahon et al., 1995).
By immunoprecipitating the SNARE core complex from rat brain homogenates using syntaxin antibodies, followed by SDS-PAGE analysis, McMahon et al. (1995) identified 18- and 19-kD proteins distinct from synaptobrevin and other members of the complex. The authors designated the 18- and 19-kD proteins complexin-1 (CPLX1) and -2 (CPLX2; 605033), respectively. By screening rat brain, human temporal cortex, and human hippocampus cDNA libraries with degenerate oligonucleotides corresponding to complexin peptide sequences, McMahon et al. (1995) isolated cDNAs encoding rat and human CPLX1 and CPLX2. Sequence analysis determined that the rat Cplx1 and Cplx2 genes encode 134-amino acid, highly charged proteins that share 84% amino acid identity. Asp, glu, lys, and arg account for 44 to 47% of the complexin protein residues. The amino acid sequences, but not the nucleotide sequences, of mouse, rat, and human CPLX2 are 100% identical. Northern blot and Western blot analyses detected abundant expression of Cplx1 and Cplx2 in rat brain; low levels of Cplx1 were also detected in testis, and Cplx2 was detected at low levels in all tissues tested. Immunofluorescence microscopy demonstrated that complexins are largely colocalized with syntaxin and SNAP25, particularly at synapses.
Maximov et al. (2009) stated that the 134-amino acid CPX1 protein has an N-terminal domain, followed by an accessory alpha helix (residues 27 to 46), a central alpha helix (residues 47 to 70) that binds the SNARE complex, and a C-terminal domain.
Functional analysis by McMahon et al. (1995) showed that complexin binding to syntaxin increased substantially in the presence of SNAP25 and synaptobrevin, and that it was calcium independent.
Complexins, also called synaphins, are cytosolic proteins that preferentially bind to syntaxin within the SNARE complex. Studying squid and rat synaphin, Tokumaru et al. (2001) found that synaphin promotes SNAREs to form precomplexes that oligomerize into higher-order structures. A peptide from the central, syntaxin-binding domain of synaphin competitively inhibited these 2 proteins from interacting and prevented SNARE complexes from oligomerizing. Injection of this peptide into squid giant presynaptic terminals inhibited neurotransmitter release at a late prefusion step of synaptic vesicle exocytosis. Tokumaru et al. (2001) proposed that oligomerization of SNARE complexes into a higher-order structure creates a SNARE scaffold for efficient, regulated fusion of synaptic vesicles.
Reim et al. (2001) demonstrated that complexins are important regulators of transmitter release at a step immediately preceding vesicle fusion. Neurons from mice lacking complexins showed a dramatically reduced transmitter release efficiency due to decreased calcium sensitivity of the synaptic secretion process. Analyses of mutant mouse neurons demonstrated that complexins acted at or following the calcium-triggering step of fast synchronous transmitter release by regulating the exocytotic calcium sensor, its interaction with the core complex fusion machinery, or the efficiency of the fusion apparatus itself.
Giraudo et al. (2006) described what may represent a basic principle of the coupling mechanism between synaptic vesicle fusion and calcium: a reversible clamping protein, complexin, that can freeze the SNAREpin, an assembled fusion-competent intermediate, en route to fusion. When calcium binds to the calcium sensor synaptotagmin, the clamp would be released. SNARE proteins, and key regulators like synaptotagmin and complexin, can be ectopically expressed on the cell surface. Cells expressing such 'flipped' synaptic SNAREs fuse constitutively, but when complexin was coexpressed, fusion was blocked. Adding back calcium triggered fusion from this intermediate in the presence of synaptotagmin.
Using biochemical approaches, Tang et al. (2006) showed that SYT1 competed with CPLX1 for SNARE complex binding in a Ca(2+)-dependent manner. Excess CPLX1 blocked fast Ca(2+)-triggered release and exocytosis, but not slow asynchronous release. Tang et al. (2006) proposed that primed metastable vesicles form the substrate for SYT1, which triggers fast synchronous release upon Ca(2+) influx by binding to SNARE complexes, displacing CPLX1, and coupling the SNARE complex to phospholipids.
Fusion between vesicle (v) and target (t) membrane SNAREs is favored energetically and would occur spontaneously in the absence of inhibitory clamping factors, such as CPX1. Giraudo et al. (2009) noted that in the fused SNARE complex, the central helix of human CPX1 lies along the interface between v- and t-SNAREs, making numerous contacts with both. In contrast, the accessory helix of CPX1 lacks contacts with the SNARE proteins in the fused complex. Using a reconstituted fusion system, Giraudo et al. (2009) obtained evidence suggesting that the accessory helix of CPX1 functions as an on-off switch for SNARE complex fusion. In their model, the accessory helix assembles with the 3-helix t-SNARE as an alternative to the v-SNARE VAMP2, preventing SNARE zippering and membrane fusion. Activators, like synaptotagmin, would cause the accessory helix to switch out, permitting membrane fusion.
Maximov et al. (2009) showed that complexin both suppressed spontaneous fusion and activated fast Ca(2+)-evoked fusion in mouse neuronal synapses. Deletion analysis showed that the N-terminal 27 amino acids of complexin were required for fast Ca(2+)-activated fusion, and that the accessory helix was required for clamping of spontaneous fusion. Maximov et al. (2009) noted that the N-terminal complexin sequence lies near the point where SNARE complexes insert into the 2 fusing membranes. They proposed that complexin stabilizes SNARE complexes in a 'superprimed' position, and that synaptotagmin subsequently triggers complexin release, as well as initiating Ca(2+)-dependent phospholipid binding, membrane fusion, and neurotransmitter release. Maximov et al. (2009) concluded that synaptotagmin and complexin operate as interdependent clamp-activators of SNARE-dependent fusion.
In 2 sisters with developmental and epileptic encephalopathy-63 (DEE63; 617976), Karaca et al. (2015) identified a homozygous nonsense mutation in the CPLX1 gene (E108X; 605032.0001), consistent with a complete loss of function. The mutation, which was found by whole-exome sequencing and confirmed by Sanger sequencing, segregated with the disorder in the family. Functional studies of the variant and studies of patient cells were not performed, but Karaca et al. (2015) noted that the CPLX1 gene is involved in modulating presynaptic neurotransmitter release. The family was part of a cohort of 128 mostly consanguineous families with neurogenetic disorders, often including brain malformations, that underwent whole-exome sequencing.
In 3 patients from 2 unrelated consanguineous families with DEE63, Redler et al. (2017) identified homozygous mutations in the CPLX1 gene. One was a nonsense mutation (C105X; 605032.0002) and 1 was a missense mutation (L128M; 605302.0003). The mutations, which were found by exome sequencing and confirmed by Sanger sequencing, segregated with the disorder in the families. Functional studies of the variants and studies of patient cells were not performed, but the authors noted that homozygous Cplx1-null mice develop a neurologic disorder manifest as ataxia. The patients were part of a cohort of 311 patients with intellectual disability who underwent whole-exome sequencing.
Reim et al. (2001) created deletion mutations in the murine Cplx1 and Cplx2 genes and homozygous single and double mutants with their respective controls. They found that mice lacking Cplx2 showed no obvious phenotypic changes. In contrast, homozygous Cplx1 deletion mutants developed a strong ataxia, suffered from sporadic seizures, were unable to reproduce, and died within 2 to 4 months after birth. Although loss of Cplx1 was ultimately fatal, the fact that mice lacking either Cplx1 or Cplx2 lived for at least 2 months after birth indicated that Cplx1 and Cplx2 are partially redundant. They therefore generated double mutants lacking both Cplx isoforms. Homozygous Cplx1/2 double mutants died within a few hours after birth. To detect possible developmental changes or alterations in brain structure due to Cplx deletion mutations, Reim et al. (2001) analyzed morphologic characteristics of brains from homozygous adult Cplx1 and Cplx2 single mutants as well as newborn Cplx1/2 double mutants.
Glynn et al. (2005) found that Cplx1 -/- mice showed pronounced deficits in motor coordination and locomotion including abnormal gait, inability to run or swim, impaired rotarod performance, reduced neuromuscular strength, dystonia, and resting tremor. Although the abnormal motor phenotype dominated their overt symptoms, Cplx1 -/- mice also showed behavioral deficits in complex such as grooming, rearing on hindlimbs, and reduced exploration in several different paradigms. They also showed deficits in tasks reflecting emotional reactivity. They failed to habituate to confinement and showed a 'panic' response when exposed to water. Behavioral deficits of Cplx1 -/- mice reflected those predicted from the distribution of complexin I in the brain. Glynn et al. (2005) concluded that complexin I is essential not only for normal motor function in mice, but also for normal performance of other complex behaviors.
Drew et al. (2007) found that juvenile Cplx1-null mice showed no evidence of cognitive impairment. However, Cplx1-null mice failed in the social transmission of food preference task due to abnormal social interactions rather than due to cognitive impairments or increased anxiety. Cplx1-null mice also failed to demonstrate a preference for social novelty, and male Cplx1-null failed to show the aggressive behavior that is typical of wildtype males toward an intruder mouse. The results showed that, in addition to the severe motor and exploratory deficits already described, Cplx1-null mice have pronounced deficits in social behaviors.
In 2 sisters (patients BAB6167 and BAB6168), born of consanguineous parents (family HOU2319) with developmental and epileptic encephalopathy-63 (DEE63; 617976), Karaca et al. (2015) identified a homozygous c.322G-T transversion (c.322G-T, NM_006651) in the CPLX1 gene, resulting in a glu108-to-ter (E108X) substitution. The mutation, which was found by whole-exome sequencing and confirmed by Sanger sequencing, segregated with the disorder in the family. Functional studies of the variants and studies of patient cells were not performed, but Karaca et al. (2015) noted that the CPLX1 gene is involved in modulating presynaptic neurotransmitter release. The family was part of a cohort of 128 mostly consanguineous families with neurogenetic disorders, often including brain malformations, that underwent whole-exome sequencing.
In 2 sisters, born of consanguineous Lebanese parents, with developmental and epileptic encephalopathy-63 (DEE63; 617976), Redler et al. (2017) identified a homozygous c.315C-A transversion (c.315C-A, NM_006651.2) in exon 4 of the CPLX1 gene, resulting in a cys105-to-ter (C105X) substitution. The mutation, which was found by whole-exome sequencing and confirmed by Sanger sequencing, segregated with the disorder in the family. Functional studies of the variant and studies of patient cells were not performed. The patients had onset of intractable infantile spasms and myoclonic epilepsy in the first weeks to months of life.
In a boy (patient 3), born of consanguineous parents originating from the Turkmen population, with developmental and epileptic encephalopathy-63 (DEE63; 617976), Redler et al. (2017) identified a homozygous c.382C-A transversion (c.382C-A, NM_006651.2) in exon 4 of the CPLX1 gene, resulting in a leu128-to-met (L128M) substitution at a highly conserved residue in the C-terminal domain in a region important for synaptic vesicle binding. The mutation, which was found by whole-exome sequencing and confirmed by Sanger sequencing, segregated with the disorder in the family. The variant was present in heterozygous state in 6 of 33,197 individuals in the ExAC database. Functional studies of the variant and studies of patient cells were not performed. The patient had onset of refractory myoclonic seizures at 2 years of age.
Drew, C. J. G., Kyd, R. J., Morton, A. J. Complexin 1 knockout mice exhibit marked deficits in social behaviours but appear to be cognitively normal. Hum. Molec. Genet. 16: 2288-2305, 2007. [PubMed: 17652102] [Full Text: https://doi.org/10.1093/hmg/ddm181]
Giraudo, C. G., Eng, W. S., Melia, T. J., Rothman, J. E. A clamping mechanism involved in SNARE-dependent exocytosis. Science 313: 676-680, 2006. [PubMed: 16794037] [Full Text: https://doi.org/10.1126/science.1129450]
Giraudo, C. G., Garcia-Diaz, A., Eng, W. S., Chen, Y., Hendrickson, W. A., Melia, T. J., Rothman, J. E. Alternative zippering as an on-off switch for SNARE-mediated fusion. Science 323: 512-516, 2009. [PubMed: 19164750] [Full Text: https://doi.org/10.1126/science.1166500]
Glynn, D., Drew, C. J., Reim, K., Brose, N., Morton, A. J. Profound ataxia in complexin I knockout mice masks a complex phenotype that includes exploratory and habituation deficits. Hum. Molec. Genet. 14: 2369-2385, 2005. [PubMed: 16000319] [Full Text: https://doi.org/10.1093/hmg/ddi239]
Karaca, E., Harel, T., Pehlivan, D., Jhangiani, S. N., Gambin, T., Akdemir, Z. C., Gonzaga-Jauregui, C., Erdin, S., Bayram, Y., Campbell, I. M., Hunter, J. V., Atik, M. M., and 52 others. Genes that affect brain structure and function identified by rare variant analyses of mendelian neurologic disease. Neuron 88: 499-513, 2015. [PubMed: 26539891] [Full Text: https://doi.org/10.1016/j.neuron.2015.09.048]
Maximov, A., Tang, J., Yang, X., Pang, Z. P., Sudhof, T. C. Complexin controls the force transfer from SNARE complexes to membranes in fusion. Science 323: 516-521, 2009. [PubMed: 19164751] [Full Text: https://doi.org/10.1126/science.1166505]
McMahon, H. T., Missler, M., Li, C., Sudhof, T. C. Complexins: cytosolic proteins that regulate SNAP receptor function. Cell 83: 111-119, 1995. [PubMed: 7553862] [Full Text: https://doi.org/10.1016/0092-8674(95)90239-2]
Redler, S., Strom, T. M., Wieland, T., Cremer, K., Engels, H., Distelmaier, F., Schaper, J., Kuchler, A., Lemke, J. R., Jeschke, S., Schreyer, N., Sticht, H., Koch, M., Ludecke, H.-J., Wieczorek, D. Variants in CPLX1 in two families with autosomal-recessive severe infantile myoclonic epilepsy and ID. Europ. J. Hum. Genet. 25: 889-893, 2017. [PubMed: 28422131] [Full Text: https://doi.org/10.1038/ejhg.2017.52]
Reim, K., Mansour, M., Varoqueaux, F., McMahon, H. T., Sudhof, T. C., Brose, N., Rosenmund, C. Complexins regulate a late step in Ca(2+)-dependent neurotransmitter release. Cell 104: 71-81, 2001. [PubMed: 11163241] [Full Text: https://doi.org/10.1016/s0092-8674(01)00192-1]
Tang, J., Maximov, A., Shin, O.-H., Dai, H., Rizo, J., Sudhof, T. C. A complexin/synaptotagmin 1 switch controls fast synaptic vesicle exocytosis. Cell 126: 1175-1187, 2006. [PubMed: 16990140] [Full Text: https://doi.org/10.1016/j.cell.2006.08.030]
Tokumaru, H., Umayahara, K., Pellegrini, L. L., Ishizuka, T., Saisu, H., Betz, H., Augustine, G. J., Abe, T. SNARE complex oligomerization by synaphin/complexin is essential for synaptic vesicle exocytosis. Cell 104: 421-432, 2001. [PubMed: 11239399] [Full Text: https://doi.org/10.1016/s0092-8674(01)00229-x]