8.A.83 The Shisa Regulator of Short-term Neuronal Synaptic Plasticity (Shisa) Family
The Shisa family of single-TMS proteins is characterized by an N-terminal cysteine-rich domain and a proline-rich C-terminal region. Its founding member, Xenopus Shisa, promotes head development by antagonizing Wnt and FGF signaling. A mouse brain-specific Shisa protein CKAMP44 (Shisa9) was shown to play an important role in AMPA receptor desensitization, and the same proved to be true of Shisa7 (Han et al. 2019). Sequence similarity searches against protein, genome and EST databases allowed the study of the evolutionary origin and phylogenetic distribution of Shisa homologs. In addition to nine Shisa subfamilies in vertebrates, Pei and Grishin 2012 detected distantly related Shisa homologs that possess an N-terminal domain with six conserved cysteines. These Shisa-like proteins include FAM159 and KIAA1644 mainly from vertebrates, and members from various bilaterian invertebrates and Porifera, suggesting their presence in the last common ancestor of Metazoa.
Shisa-like genes have undergone large expansions in Branchiostoma floridae and Saccoglossus kowalevskii, and appear to have been lost in certain insects. Pattern-based searches against eukaryotic proteomes also uncovered several other families of predicted single-transmembrane proteins with a similar cysteine-rich domain (Shisa/Shisa-like, WBP1/VOPP1, CX, DUF2650, TMEM92, and CYYR1). They are collectively referred to as STMC6 proteins (single-transmembrane proteins with conserved 6 cysteines). STMC6 genes are widespread in Metazoa, with the human genome containing 17 members. Frequent occurrences of PY motifs in STMC6 proteins suggest that most of them could interact with WW-domain-containing proteins, such as the NEDD4 family E3 ubiquitin ligases, and could play critical roles in protein degradation and sorting. STMC6 proteins are likely transmembrane adaptors that regulate membrane proteins such as cell surface receptors (Pei and Grishin 2012).
Trafficking and biophysical properties of AMPA receptors (AMPARs) in the brain depend on interactions with associated proteins. Klaassen et al. 2016 identify Shisa6, a single transmembrane protein, as a stable and directly interacting bona fide AMPAR auxiliary subunit. Shisa6 is enriched at hippocampal postsynaptic membranes and co-localizes with AMPARs. The Shisa6 C-terminus harbours a PDZ domain ligand that binds to PSD-95, constraining mobility of AMPARs in the plasma membrane and confining them to postsynaptic densities. Shisa6 expressed in HEK293 cells alters GluA1- and GluA2-mediated currents by prolonging decay times and decreasing the extent of AMPAR desensitization, while slowing the rate of recovery from desensitization. Using gene deletion, it was shown that Shisa6 increases rise and decay times of hippocampal CA1 miniature excitatory postsynaptic currents (mEPSCs). Shisa6-containing AMPARs show prominent sustained currents, indicating protection from full desensitization. Accordingly, Shisa6 prevents synaptically trapped AMPARs from depression at high-frequency synaptic transmission (Klaassen et al. 2016). Shisa7 is a GABAA receptor auxiliary subunit controlling benzodiazepine actions (Han et al. 2019).
Shisa6 of 500 aas and 2 TMSs, one N-terminal and one at position 176 - 195. Interacts with AMPA receptors (TC# 1.A.10) at post synaptic sites and prevents desensitization (Klaassen et al. 2016).
Shisa6 of Homo sapiens
Shisa8 protein of 492 aas and 2 TMSs with a Class II PDZ domain. Regulates short term neuronal synaptic plasticity. Present in the olfactory bulb and cerebellum. Involved in prolongation of deactivation and an increase in desensitization of GluA2 without change in that of GluA1. It slowed recovery from desensitization, enhanced of glutamate potency, and reducted cyclothiazide potency (Kato and Witkin 2018).
Shisa8 of Homo sapiens
Shisa9, or CKAMP44 of 424 aas and 2 TMSs. Regulator of short-term neuronal synaptic plasticity in the dentate gyrus. It associates with AMPA receptors (ionotropic glutamate receptors; TC#1/A/10) in synaptic spines and promotes AMPA receptor desensitization at excitatory synapses. It is found in the dentate gyrus and olfactory bulb. It has a class II PDZ domain and is expressed in a majority of regions of the brain. It slows recovery from desensitization, enhances glutamate potency and reduces cyclothiazide potency and efficacy (Kato and Witkin 2018).
Shisa9 of Homo sapiens
Shisa7 of 538 aas and 2 TMSs. It is a AMPAR modulatory protein affecting channel kinetics of AMPARs, necessary for synaptic hippocampal plasticity and memory recall (Schmitz et al. 2017). Shisa7 controls receptor abundance at synapses and speeds up the channel deactivation kinetics. Shisa7 also potently enhances the action of diazepam, a classic benzodiazepine, on GABAARs. Genetic deletion of Shisa7 selectively impairs GABAergic transmission and diminishes the effects of diazepam in mice. Thus, Shisa7 regulates GABAAR trafficking, function, and pharmacology (Han et al. 2019).
Shisa7 of Homo sapiens
Shesa-1 (Shisa1) of 269 aas and 1 TMS. It is required for head formation during gastrulation. It functions as an inhibitor for the caudalizing signals wnt and fgf, but does not inhibit bmp, activin and nodal signaling in the head formating process. It induces retention of fzd8 in the endoplasmic reticulum and inhibits trafficking of fzd8 to the cell surface (Yamamoto et al. 2005).
Shisa1 of Xenopus laevis (African clawed frog)
Shisa-2 (Shisa2) of 288 aas and 2 TMSs, an N-terminal targetting signal, and a central TMS, common to other Shisa proteins. It plays an essential role in the maturation of presomitic mesoderm cells by individual attenuation of both Fgf and Wnt signaling. It inhibits both Wnt and Fgf signaling through the regulation of protein maturation and cell surface transport of their receptors within the endoplasmic reticulum (Nagano et al. 2006).
Shisa2 of Xenopus laevis (African clawed frog)