The equilibrium between antagonistic signaling pathways determines the number of synapses in Drosophila

Abstract

The number of synapses is a major determinant of behavior and many neural diseases exhibit deviations in that number. However, how signaling pathways control this number is still poorly understood. Using the Drosophila larval neuromuscular junction, we show here a PI3K-dependent pathway for synaptogenesis which is functionally connected with other previously known elements including the Wit receptor, its ligand Gbb, and the MAPkinases cascade. Based on epistasis assays, we determined the functional hierarchy within the pathway. Wit seems to trigger signaling through PI3K, and Ras85D also contributes to the initiation of synaptogenesis. However, contrary to other signaling pathways, PI3K does not require Ras85D binding in the context of synaptogenesis. In addition to the MAPK cascade, Bsk/JNK undergoes regulation by Puc and Ras85D which results in a narrow range of activity of this kinase to determine normalcy of synapse number. The transcriptional readout of the synaptogenesis pathway involves the Fos/Jun complex and the repressor Cic. In addition, we identified an antagonistic pathway that uses the transcription factors Mad and Medea and the microRNA bantam to down-regulate key elements of the pro-synaptogenesis pathway. Like its counterpart, the anti-synaptogenesis signaling uses small GTPases and MAPKs including Ras64B, Ras-like-a, p38a and Licorne. Bantam downregulates the pro-synaptogenesis factors PI3K, Hiw, Ras85D and Bsk, but not AKT. AKT, however, can suppress Mad which, in conjunction with the reported suppression of Mad by Hiw, closes the mutual regulation between both pathways. Thus, the number of synapses seems to result from the balanced output from these two pathways.

Multiple genetic loci affect place learning and memory performance in Drosophila melanogaster

“…There were just four significantly differentially 93 and has also been shown to play a role in inhibiting synapse formation, as one part of two antagonistic signaling pathways that act together to produce the correct number of synapses. 94 In the Q5 interval of interest ( Figure 6E), just two genes are significantly differentially expressed, Odorant-binding protein 28a (Obp28a) and ). An RNAi screen specific to neurons identified ergic53 as a candidate gene involved in the perception of very high temperatures.…”

“…Mad is a transcription factor that regulates the expression of BMP response target genes 93 and has also been shown to play a role in inhibiting synapse formation, as one part of two antagonistic signaling pathways that act together to produce the correct number of synapses. 94 In the Q5 interval of interest ( Figure 6E), just two genes are significantly differentially expressed, Odorant-binding protein 28a (Obp28a) and…”

PI3K activation prevents Aβ42-induced synapse loss and favors insoluble amyloid deposits formation

“…Being PI3K a shared element in several signaling pathways (Acebes and Morales, 2012), we explored two possibilities, 1) the insulin receptor canonical PI3K/Akt pathway, which is known to elicit pro-survival and cell growth responses (Basu et al, 2012;Dyer et al, 2016), and 2) the recently revealed pro-synaptogenesis pathway (Jordán-Álvarez et al, 2017). To assay the effect of the classical pathway effector, we overexpressed Target of Rapamycin (mTOR), a downstream factor of PI3K, and we analyzed the synaptic signal in abdominal motor neurons of 15 days-old adults.…”

“…Class I phosphoinositide 3 kinase (PI3K) is involved in a signaling pathway that leads to synapse formation in Drosophila (Martín-Peña et al, 2006) and vertebrates (Cuesto et al, 2011). This synaptogenic pathway includes the type II BMP receptor Wit (wishfull thinking) and several MAPKs, but not mTOR or S6K, which are characteristic of other PI3K classical signaling pathways (Jordán-Álvarez et al, 2017). The noncanonical PI3K-mediated pathway for synaptogenesis is counterbalanced by a Mad/Medea regulated anti-synaptogenesis pathway (Jordán-Álvarez et al, 2017).…”

“…This synaptogenic pathway includes the type II BMP receptor Wit (wishfull thinking) and several MAPKs, but not mTOR or S6K, which are characteristic of other PI3K classical signaling pathways (Jordán-Álvarez et al, 2017). The noncanonical PI3K-mediated pathway for synaptogenesis is counterbalanced by a Mad/Medea regulated anti-synaptogenesis pathway (Jordán-Álvarez et al, 2017). In turn, inhibitors of the PI3K pathway, as GSK3-β, reduce the number of synapses (Cuesto et al, 2015).…”