Restraint of presynaptic protein levels by Wnd/DLK signaling mediates synaptic defects associated with the kinesin-3 motor Unc-104.

The kinesin-3 family member Unc-104/KIF1A is required for axonal transport of many presynaptic components to synapses, and mutation of this gene results in synaptic dysfunction in mice, flies and worms. Our studies at the Drosophila neuromuscular junction indicate that many synaptic defects in unc-104-null mutants are mediated independently of Unc-104's transport function, via the Wallenda (Wnd)/DLK MAP kinase axonal damage signaling pathway. Wnd signaling becomes activated when Unc-104's function is disrupted, and leads to impairment of synaptic structure and function by restraining the expression level of active zone (AZ) and synaptic vesicle (SV) components. This action concomitantly suppresses the buildup of synaptic proteins in neuronal cell bodies, hence may play an adaptive role to stresses that impair axonal transport. Wnd signaling also becomes activated when pre-synaptic proteins are over-expressed, suggesting the existence of a feedback circuit to match synaptic protein levels to the transport capacity of the axon.

A high affinity RIM-binding protein/Aplip1 interaction prevents the formation of ectopic axonal active zones.

Synaptic vesicles (SVs) fuse at active zones (AZs) covered by a protein scaffold, at Drosophila synapses comprised of ELKS family member Bruchpilot (BRP) and RIM-binding protein (RBP). We here demonstrate axonal co-transport of BRP and RBP using intravital live imaging, with both proteins co-accumulating in axonal aggregates of several transport mutants. RBP, via its C-terminal Src-homology 3 (SH3) domains, binds Aplip1/JIP1, a transport adaptor involved in kinesin-dependent SV transport. We show in atomic detail that RBP C-terminal SH3 domains bind a proline-rich (PxxP) motif of Aplip1/JIP1 with submicromolar affinity. Pointmutating this PxxP motif provoked formation of ectopic AZ-like structures at axonal membranes. Direct interactions between AZ proteins and transport adaptors seem to provide complex avidity and shield synaptic interaction surfaces of pre-assembled scaffold protein transport complexes, thus, favouring physiological synaptic AZ assembly over premature assembly at axonal membranes.

Independent pathways downstream of the Wnd/DLK MAPKKK regulate synaptic structure, axonal transport, and injury signaling.

Mitogen-activated protein (MAP) kinase signaling cascades orchestrate diverse cellular activities with common molecular players. To achieve specific cellular outcomes in response to specific signals, scaffolding proteins play an important role. Here we investigate the role of the scaffolding protein JNK interacting protein-1 (JIP1) in neuronal signaling by a conserved axonal MAP kinase kinase kinase, known as Wallenda (Wnd) in Drosophila and dual leucine kinase (DLK) in vertebrates and Caenorhabditis elegans. Recent studies in multiple model organisms suggest that Wnd/DLK regulates both regenerative and degenerative responses to axonal injury. Here we report a new role for Wnd in regulating synaptic structure during development, which implies that Wnd is also active in uninjured neurons. This synaptic role of Wnd can be functionally separated from the role of Wnd in axonal regeneration and injury signaling by the requirement for the JIP1 scaffold and the p38b MAP kinase. JIP1 mediates the synaptic function of Wnd via p38, which is not required for injury signaling or new axonal growth after injury. Our results indicate that Wnd regulates multiple independent pathways in Drosophila motoneurons and that JIP1 scaffolds a specific downstream cascade required for the organization of presynaptic microtubules during synaptic development.

Activation of Hsp70 reduces neurotoxicity by promoting polyglutamine protein degradation.

We sought new strategies to reduce amounts of the polyglutamine androgen receptor (polyQ AR) and achieve benefits in models of spinobulbar muscular atrophy, a protein aggregation neurodegenerative disorder. Proteostasis of the polyQ AR is controlled by the heat shock protein 90 (Hsp90)- and Hsp70-based chaperone machinery, but mechanisms regulating the protein's turnover are incompletely understood. We demonstrate that overexpression of Hsp70 interacting protein (Hip), a co-chaperone that enhances binding of Hsp70 to its substrates, promotes client protein ubiquitination and polyQ AR clearance. Furthermore, we identify a small molecule that acts similarly to Hip by allosterically promoting Hsp70 binding to unfolded substrates. Like Hip, this synthetic co-chaperone enhances client protein ubiquitination and polyQ AR degradation. Both genetic and pharmacologic approaches targeting Hsp70 alleviate toxicity in a Drosophila model of spinobulbar muscular atrophy. These findings highlight the therapeutic potential of allosteric regulators of Hsp70 and provide new insights into the role of the chaperone machinery in protein quality control.

The ATP-sensitive potassium (KATP) channel-encoded dSUR gene is required for Drosophila heart function and is regulated by tinman.

The homeobox transcription factor Tinman plays an important role in the initiation of heart development. Later functions of Tinman, including the target genes involved in cardiac physiology, are less well studied. We focused on the dSUR gene, which encodes an ATP-binding cassette transmembrane protein that is expressed in the heart. Mammalian SUR genes are associated with K(ATP) (ATP-sensitive potassium) channels, which are involved in metabolic homeostasis. We provide experimental evidence that Tinman directly regulates dSUR expression in the developing heart. We identified a cis-regulatory element in the first intron of dSUR, which contains Tinman consensus binding sites and is sufficient for faithful dSUR expression in the fly's myocardium. Site-directed mutagenesis of this element shows that these Tinman sites are critical to dSUR expression, and further genetic manipulations suggest that the GATA transcription factor Pannier is synergistically involved in cardiac-restricted dSUR expression in vivo. Physiological analysis of dSUR knock-down flies supports the idea that dSUR plays a protective role against hypoxic stress and pacing-induced heart failure. Because dSUR expression dramatically decreases with age, it is likely to be a factor involved in the cardiac aging phenotype of Drosophila. dSUR provides a model for addressing how embryonic regulators of myocardial cell commitment can contribute to the establishment and maintenance of cardiac performance.

How functions in leg development during Drosophila metamorphosis.

The Drosophila how gene encodes a KH RNA binding protein with strong similarity to GLD-1 from nematodes and QK1 from mice. Here, we investigate the function of how during metamorphosis. We show that how RNA and protein are present in a variety of tissues, and phenotypic analyses of how mutants reveal multiple lethal phases and defects during metamorphosis. In addition to previously reported abnormalities in muscle and wing development, how mutants exhibit defects in leg development. how mutant leg imaginal discs undergo cell shape changes associated with elongation, but are oriented improperly, do not evert normally, and often remain incased in peripodial epithelium longer than normal. Consequently, how mutants exhibit short, crooked legs. Our findings suggest that how functions in interactions between imaginal epithelium, peripodial epithelium, and larval epidermal cells during imaginal disc eversion.

Gata factor Pannier is required to establish competence for heart progenitor formation.

Inductive signaling is of pivotal importance for developmental patterns to form. In Drosophila, the transfer of TGFbeta (Dpp) and Wnt (Wg) signaling information from the ectoderm to the underlying mesoderm induces cardiac-specific differentiation in the presence of Tinman, a mesoderm-specific homeobox transcription factor. We present evidence that the Gata transcription factor, Pannier, and its binding partner U-shaped, also a zinc-finger protein, cooperate in the process of heart development. Loss-of-function and germ layer-specific rescue experiments suggest that pannier provides an essential function in the mesoderm for initiation of cardiac-specific expression of tinman and for specification of the heart primordium. u-shaped also promotes heart development, but unlike pannier, only by maintaining tinman expression in the cardiogenic region. By contrast, pan-mesodermal overexpression of pannier ectopically expands tinman expression, whereas overexpression of u-shaped inhibits cardiogenesis. Both factors are also required for maintaining dpp expression after germ band retraction in the dorsal ectoderm. Thus, we propose that Pannier mediates as well as maintains the cardiogenic Dpp signal. In support, we find that manipulation of pannier activity in either germ layer affects cardiac specification, suggesting that its function is required in both the mesoderm and the ectoderm.