Terminal Axonal Arborization and Synaptic Bouton Formation Critically Rely on Abp1 and the Arp2/3 Complex

Abstract

Neuronal network formation depends on properly timed and localized generation of presynaptic as well as postsynaptic structures. Although of utmost importance for understanding development and plasticity of the nervous system and neurodegenerative diseases, the molecular mechanisms that ensure the fine-control needed for coordinated establishment of pre- and postsynapses are still largely unknown. We show that the F-actin-binding protein Abp1 is prominently expressed in the Drosophila nervous system and reveal that Abp1 is an important regulator in shaping glutamatergic neuromuscular junctions (NMJs) of flies. STED microscopy shows that Abp1 accumulations can be found in close proximity of synaptic vesicles and at the cell cortex in nerve terminals. Abp1 knock-out larvae have locomotion defects and underdeveloped NMJs that are characterized by a reduced number of both type Ib synaptic boutons and branches of motornerve terminals. Abp1 is able to indirectly trigger Arp2/3 complex-mediated actin nucleation and interacts with both WASP and Scar. Consistently, Arp2 and Arp3 loss-of-function also resulted in impairments of bouton formation and arborization at NMJs, i.e. fully phenocopied abp1 knock-out. Interestingly, neuron- and muscle-specific rescue experiments revealed that synaptic bouton formation critically depends on presynaptic Abp1, whereas the NMJ branching defects can be compensated for by restoring Abp1 functions at either side. In line with this presynaptic importance of Abp1, also presynaptic Arp2 and Arp3 are crucial for the formation of type Ib synaptic boutons. Interestingly, presynaptic Abp1 functions in NMJ formation were fully dependent on the Arp2/3 complex, as revealed by suppression of Abp1-induced synaptic bouton formation and branching of axon terminals upon presynaptic Arp2 RNAi. These data reveal that Abp1 and Arp2/3 complex-mediated actin cytoskeletal dynamics drive both synaptic bouton formation and NMJ branching. Our data furthermore shed light on an intense bidirectional functional crosstalk between pre- and postsynapses during the development of synaptic contacts.

Cross-phenotype association tests uncover genes mediating nutrient response in Drosophila

“…Functional genomic tests revealed enrichment in association to starvation resistance among genes categories annotated in actin filament production (Additional file 1: Table S5). In the latter, top-scoring genes were Arp2 , Arpc2 , and SCAR (association p  = 0.0008, 0.001, and 0.003 respectively), which have roles in the development of neural processes and axonal defasciculation [52, 53]. Also emerging from functional genomic tests was an enrichment in association to starvation resistance among heterophilic cell-cell adhesion genes (Additional file 1: Table S5); these included Notch (association p  = 0.006), among whose many biological roles is that of axon guidance [54], and “beaten path” genes such as beat - Ib , beat - IIIb , and beat - Va (association p  = 0.00001, p  = 0.00008, p  = 0.0002 respectively), which are involved in motor neuron defasciculation [55].…”

Identification of genes involved in the differentiation of R7y and R7p photoreceptor cells inDrosophila

“…Imp encodes an mRNA binding protein that promotes and regulates transcript targeting, and plays a role in axonal remodeling, synaptogenesis and oogenesis (BOYLAN et al 2008;MEDIONI et al 2014). Arp3 encodes an actin related protein that is required for myoblast fusion and axonal arborization and synapse formation (RICHARDSON et al 2007;KOCH et al 2014).…”

The Strip-Hippo Pathway Regulates Synaptic Terminal Formation by Modulating Actin Organization at the Drosophila Neuromuscular Synapses

“…Ena prevents Arp2/3-induced branching (Bear and Gertler, 2009), suggesting that Ena inactivation activates Arp2/3 and results in satellite bouton formation, similar to Rac activation (Ball et al, 2010) or to loss of SCAR/WAVE components (Ball et al, 2010; Qurashi et al, 2007; Schenck et al, 2004; Zhao et al, 2013). It is reported that Arp2/3 is involved in bouton formation and axon terminal branching downstream of WAVE/SCAR complex in NMJ (Koch et al, 2014). Indeed, our findings support this hypothesis.…”

“…Amongst the many factors, elements of the actin cytoskeleton are key effectors of morphological change, functioning downstream of several cell surface receptors and signaling pathways (Long and Van Vactor, 2013). Of the two types of actin filaments (branched and linear), the activity of Arp2/3 complex, responsible for nucleation of branched F-actin, the first step of actin polymerization (Insall and Machesky, 2009), should be strictly regulated (Koch et al, 2014). Arp2/3 hyperactivation results in synaptic terminal overgrowth characterized by excess small boutons emanating from the main branch (Ball et al, 2010; Qurashi et al, 2007; Schenck et al, 2004; Zhao et al, 2013) that are termed satellite boutons (Dickman et al, 2006).…”