Role of the STRIPAK complex and the Hippo pathway in synaptic terminal formation

Role of the STRIPAK complex and the Hippo pathway in synaptic terminal formation

To transmit neural information from pre to postsynaptic neurons, the number, morphology, and function of the synapse have to be strictly regulated. Failures in synaptic formations are linked to neural disorders. For example, it is suggested that localization of N-methyl-D-aspartate (NMDA) receptors plays important roles in NMDA receptor-induced excitotoxicity, which is in turn thought to contribute to cell death associated with certain neurodegenerative diseases (Parsons and Raymond, 2014). Furthermore, synaptic overgrowth is observed inDrosophila dfmr1(homolog of mammalianFMR1) gene mutant, which causes the Fragile X syndrome (Zhang et al., 2001). In this perspective, we introduce the involvement of the striatin-interacting phosphatase and kinase (STRIPAK) complex in synapse formation.

Recent proteomic studies have identified the evolutionarily conserved STRIPAK complex that regulates various cellular processes including cell-cycle control and cell polarity (Figure 1A; Hwang and Pallas, 2014).e main component of STRIPAK complex is striatin, which belongs to the subfamily of regulatory B subunits of the protein phosphatase 2A (PP2A) complex. The A and C subunits of PP2A complex, CCM3, Mob3, Mst3, Mst4, Ysk1, Ccm3, Strip1, and Strip2 form the core mammalian STRIPAK complex together with striatin.is multi-component core complex is capable of assembling in a mutually exclusive manner with other accessory proteins depending on the function that it mediates (Hwang and Pallas, 2014). Although the role of STRIPAK complexes in cellular processes of multiple organisms has been extensively studied in recent years, little is known about the role of STRIPAK complex in synapse formation.us, while it was reported that theDrosophilamutant ofMob4(homolog of mammalianMob3) shows abnormal synaptic terminal development (Hwang and Pallas, 2014), no information is available regarding the underlying mechanism.

We have previously identified thatDrosophilaStrip (homolog of mammalian Strip1 and 2) is expressed at the synaptic sites and regulates axon elongation and dendrite branching in olfactory projection neurons (Sakuma et al., 2014). SinceDrosophilalarval neuromuscular junction (NMJ) is the established model to study synapse formation, we examined the localization of Strip at larval NMJ and found that Strip showed a punctate distribution at the presynaptic sites (Sakuma et al., 2016).DrosophilaNMJs are composed of chains of oval structures called boutons that contain multiple active zones (which serve as neurotransmitter release sites) (Figure 1B) (Menon et al., 2013). When Strip was knocked down specif i cally in the presynaptic motor neurons, the number of satellite boutons (small boutons that emanate from the normal boutons and are thought to arise from defects in synaptic growth), was increased (Figure 1C). Hence, we concluded that presynaptic Strip regulates bouton formation. Furthermore, we found that Strip not only regulates synapse formation but also affects synaptic function.is was indicated in our observation that the frequency of miniature excitatory junction potential was increased when Strip was knocked down in the presynaptic motor neurons.

Figure 1 Strip suppresses satellite bouton formation by regulating the activity of the Hippo pathway and the actin assembly/elongation factor Enabled.

To clarify the role of Strip in synapse formation, we focused on the Hippo pathway, which is known to be a major regulator of cell proliferation and cell death (Staley and Irvine, 2012).e Hippo pathway was investigated because a previous report suggested that theDrosophilaSTRIPAK complex serves as a negative regulator of Hippo (Ribeiro et al., 2010). UsingDro-sophilaS2 cells, we conf i rmed that Strip could form a complex with Hippo and inactivate it. We also found that overexpression of Hippo in the motor neurons led to the same phenotype in cells as seen with the knockdown of Strip. To control cell proliferation, Hippo forms the core Hippo kinase cassette with its downstream kinase, Warts (Staley and Irvine, 2012). We also found thatstripgenetically interacts withHippoandWartsto regulate synapse formation.

Based on our fi ndings, we hypothesize that the localization of Strip could be used as a marker for new bouton outgrowth (Figure 1D). In presence of Strip, the core Hippo kinase cassette is inactivated, which locally increases the expression of the active (un-phosphorylated) form of Enabled, which in turn prevents satellite bouton formation. On the other hand, in absence of Strip, the core Hippo kinase cassette is activated, which results in satellite bouton formation.

Neurons are usually regarded as post mitotic cells that do not proliferate further. It is surprising that Hippo, a wellknown regulator of cell proliferation, is involved in synapse formation.e Hippo pathway may be important for sensing the local environment around NMJ, since the size of muscles and axon termini of motor neurons dynamically change during synaptic terminal development. Many studies have shown that mutants for molecules implicated in endocytosis (Dynamin, Dap-160, Synaptotagmin,etc.) exhibit satellite boutons (Menon et al., 2013). We previously identified that Strip regulates the endocytic pathway and microtubule stability in the axon elongation and dendrite branching ofDrosophilaolfactory projection neurons (Sakuma et al., 2014). It is therefore possible that Strip and Hippo are involved in the endocytic pathway and microtubule stability in the synapse formation as well. Furthermore, several genes whose mutants exhibit the satellite bouton phenotype also regulate microtubule stability and contribute to neurodegenerative diseases (Carrillo et al., 2013).us, further investigations of the function of Strip and Hippo in synapse formation might provide new insights into the development of strategies for the prevention and treatment of neurodegenerative diseases. Furthermore, mechanistic insights we found here might be applicable to mammals, since both vertebrate central nervous system andDrosophilaNMJ are glutamatergic, and STRIPAK complex and Hippo pathway molecules are conserved among species.

This work was supported by grants from the Naito Foundation to T.C, and the Japan Society for the Promotion of Science to C.S and T.C.

Chisako Sakuma, Takahiro Chihara*

Department of Tropical Medicine, Center for Medical Entomology,e Jikei University School of Medicine, Tokyo, Japan (Sakuma C) Department of Biological Science, Graduate School of Science, Hiroshima University, Hiroshima, Japan (Chihara T)

*Correspondence to:Takahiro Chihara, Ph.D., tchihara@hiroshima-u.ac.jp.

Accepted:2017-03-12

orcid:0000-0001-9989-3619 (Takahiro Chihara)

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10.4103/1673-5374.205089

How to cite this article:Sakuma C, Chihara T (2017) Role of the STRIPAK complex and the Hippo pathway in synaptic terminal formation. Neural Regen Res 12(4):578-579.

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