Gene name - hikaru genki
Cytological map position - 45C1--45C9
Function - secreted synaptic protein
Symbol - hig
Genetic map position - 2-
Classification - IG superfamily, RGD motif, complement binding domain
Cellular location - secreted
|Recent literature||Xiao, X., Zhang, R., Pang, X., Liang, G., Wang, P. and Cheng, G. (2015). A neuron-specific antiviral mechanism prevents lethal flaviviral infection of mosquitoes.. PLoS Pathog 11: e1004848. PubMed ID: 25915054
Mosquitoes are natural vectors for many etiologic agents of human viral diseases. Mosquito-borne flaviviruses can persistently infect the mosquito central nervous system without causing dramatic pathology or influencing the mosquito behavior and lifespan. The mechanism by which the mosquito nervous system resists flaviviral infection is still largely unknown. This study reports that an Aedes aegypti homologue of the neural factor Hikaru genki (AaHig) efficiently restricts flavivirus infection of the central nervous system. AaHig was predominantly expressed in the mosquito nervous system and localized to the plasma membrane of neural cells. Functional blockade of AaHig enhanced Dengue virus (DENV) and Japanese encephalitis virus (JEV), but not Sindbis virus (SINV), replication in mosquito heads and consequently caused neural apoptosis and a dramatic reduction in the mosquito lifespan. Consistently, delivery of recombinant AaHig to mosquitoes reduced viral infection. Furthermore, the membrane-localized AaHig directly interfaced with a highly conserved motif in the surface envelope proteins of DENV and JEV, and consequently interrupted endocytic viral entry into mosquito cells. Loss of either plasma membrane targeting or virion-binding ability rendered AaHig nonfunctional. Interestingly, Culex pipien pallens Hig also demonstrated a prominent anti-flavivirus activity, suggesting a functionally conserved function for Hig. These results demonstrate that an evolutionarily conserved antiviral mechanism prevents lethal flaviviral infection of the central nervous system in mosquitoes, and thus may facilitate flaviviral transmission in nature.
Flies mutant for hikaru genki show reduced activity levels; in addition to a reduction in locomotion, they also have lower fertility and reduced longevity. Mutants remain motionless most of the time; movements occur only slowly and occasionally. Mutants rarely fly and never jump, but they do carry out grooming behavior. When placed under a strong light, all this changes: they quickly respond and move vigorously. This phenotype gives the gene its name. In Japanese, hikaru genki means "light activated."
Hikaru genki protein is produced by neurons and is secreted from the presynaptic terminals into the spaces between presynaptic and postsynaptic terminals. HIG protein is found in organelles involved in secretion in the neuronal soma - the endoplasmic reticulum/Golgi apparatus, vesicles and nuclear membrane. Most striking is the observation that in the neuropils of the adult brain, large quantities of HIG protein are found in a number of discrete intercellular spaces bordered by cell membranes. These regions are identified as synaptic clefts; it is to these synaptic clefts that the protein localizes, in both the pupal and adult nervous system. Although HIG protein is initially localized in the cell bodies of the young pupal brain, later it accumulates in the neuropils of all brain regions and only a fraction of the cell bodies.
Localization in synaptic spaces of pupal neuropils temporally correlates with its functional requirement during a critical period that occurs in the middle stage of pupal formation, a period when a number of dendrite and axon growth cones meet to form synapses. Placing hig under heat shock regulation and subjecting pupal flies to heat shock rescues hig mutants. Such rescued flies are able to jump and fly and do not exhibit body tremors. HIG protein is detected in neuropils immediately after heat treatment, indicating that the protein is immediately transported to this region. Attempts to rescue mutants by expression in embryonic or larval stages fails. Expression too late in the larval period results in an association of HIG protein with cell bodies, indicating that the transport of HIG protein is inhibited at this stage. These results indicate that HIG protein is developmentally required at particular stages during pupariation for the formation of normal neural circuitry, and that protein distibution shows a stage-dependent regulation (Hoshino, 1996).
The distribution patterns of HIG protein suggest that only subsets of synapses are affected in the mutant. For example, hig is expressed in 10% of neurons in the embryonic CNS at stage 17 and HIG protein is observed in only a small proportion of neuromuscular junctions of muscle 8 in the third instar larvae. Although no unequivocal alterations have been detected in the synaptic morphology of the adult CNS, or in the electrophysiological properties of NMJs of third instar larvae muscle 8, some synapses do display altered electrophophysiological phenotypes in hig mutants. It is thought that HIG is involved as a signaling protein from the presynaptic neuron, resulting in the formation of proper connections in the synaptic cleft. Specific protein interactions in this process have not yet been characterized, but the involvement of HIG should point the way to a more detailed understanding of protein interaction in the formation of functional synaptic clefts (Hoshino, 1996).
Bases in 5' UTR - 316
Exons - 10 or more
Bases in 3' UTR - 790
The presence of a putative signal sequence (N-terminal) and the absence of a transmembrane region suggest that the four proteins generated by alternative splicing of mRNA are either secreted or membrane anchored. Two sequence features of HIG proteins suggest involvement in cell recognition. These proteins have both an RGD sequence and a domain exhibiting sequence similar to members of the immunoglobulin superfamily. RGD domains function in interaction with integrins (see Myospheroid). The RGD domain is in the central part of a large hydrophilic central domain. Besides the RGD domain, the central portion of the molecule contains a 25 amino acid sequence of unknown function whose presence or absence is determined by alternative splicing (Hoshino, 1993).
A single immunoglobulin superfamily domain exists between the center of the protein and the C-terminus. Invertebrates have a dozen or more cell surface proteins possessing domains resembling those found in vertebrate immunoglobulins. Examples in Drosophila include, Dlar, Fasciclin II, Fibroblast growth factor receptor 1, Frazzled, Hikaru genki, Neuroglian, and Semaphorin 2.
Alternative splicing is responsible for three or four C-terminal CB (complement binding) domains. Proteins with CB domains form a large superfamily that includes more than 15 proteins involved in the complement systems, as well as other proteins involved in blood coagulation, lymphocyte stimulation and oxygen transport. Most proteins with CB domains show binding activities to other proteins in their distinct functional contexts. It has been suggested that CB domains are involved in protein-protein interactions. HIG proteins may be the first of the CB protein family known to be expressed in the nervous system (Hoshino, 1993).
Synapse formation in the developing brain depends on the coordinated activity of synaptogenic proteins, some which have been implicated in a number of neurodevelopmental disorders. This study shows that the sushi repeat-containing domain protein X-linked 2 (SRPX2) gene encodes a protein that promotes synaptogenesis in the cerebral cortex. In humans, SRPX2 is an epilepsy- and language-associated gene that is a target of the foxhead box protein P2 (FoxP2) transcription factor. It was also shown that FoxP2 modulates synapse formation through regulating SRPX2 levels, and that SRPX2 reduction impairs development of ultrasonic vocalization in mice. The results suggest FoxP2 modulates the development of neural circuits through regulating synaptogenesis and that SRPX2 is a synaptogenic factor that plays a role in the pathogenesis of language disorders (Sia, 2013).
This study has shown that SRPX2 is a sushi domain protein involved in synapse formation. In invertebrates, sushi domain proteins have been shown to cluster AChRs at synapses in C. elegans, and the Drosophila homolog, Hikaru Genki, is localized to the nascent synaptic cleft. In vertebrates, sushi domain proteins are primarily studied as regulators of the classical complement cascade. The current results suggest that sushi domain proteins may also play roles in regulating synaptic development and organization in vertebrates. In addition, as genes of the classical complement cascade have been shown to regulate synapse elimination, it is speculated that SRPX2 may act through modulation of components of the complement cascade (Sia, 2013).
To date, FoxP2 is the only gene that has been shown to be involved in a human monogenic language disorder, although the cellular mechanisms involved remain obscure. Previous studies have suggested that FoxP2 may regulate neurite growth, dendritic morphology and synaptic physiology of basal ganglia neurons. This study shows that FoxP2 can regulate synaptogenesis of excitatory synapses in cortical neurons through SRXP2. While activity-regulated transcription factors have been shown to regulate synaptogenesis, developmental synapse formation can occur in the complete absence of activity, and it is unclear whether such synapse formation is also regulated by activity-independent transcription factors. This study shows that FoxP2 is an activity-independent transcription factor that regulates synaptogenesis through SRPX2. In conclusion, this study suggests that FoxP2 can affect the development of language-related neural circuitry through regulating synaptogenesis, and that SRPX2 may be involved in the pathogenesis of language disorders (Sia, 2013).
date revised: 5 December 2013
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