Generation and characterization of mice bearing null alleles of nradd/Nrh2.

The Neurotrophin receptor associated death domain gene (Nradd/Nrh2/Plaidd) is a type I transmembrane protein with a unique and short N-terminal extracellular domain and a transmembrane and intracellular domain that bears high similarity to the p75 neurotrophin receptor (p75NTR/Ngfr). Initial studies suggested that NRADD regulates neurotrophin signaling but very little is known about its physiological roles. We have generated and characterized NRADD conditional and germ-line null mouse lines. These mice are viable and fertile and dont show evident abnormalities. However, NRADD deletion results in an increase in the proportion of dorsal root ganglion neurons expressing p75NTR. The NRADD conditional and complete knockout mouse lines generated are new and useful tools to study the physiological roles of NRADD. Birth Defects Research (Part A) 106:605-612, 2016. © 2016 Wiley Periodicals, Inc.

Bcl-2-associated athanogene 5 (BAG5) regulates Parkin-dependent mitophagy and cell death.

As pathogenic Parkin mutations result in the defective clearance of damaged mitochondria, Parkin-dependent mitophagy is thought to be protective against the dopaminergic neurodegeneration observed in Parkinson's disease. Recent studies, however, have demonstrated that Parkin can promote cell death in the context of severe mitochondrial damage by degrading the pro-survival Bcl-2 family member, Mcl-1. Therefore, Parkin may act as a 'switch' that can shift the balance between protective or pro-death pathways depending on the degree of mitochondrial damage. Here, we report that the Parkin interacting protein, Bcl-2-associated athanogene 5 (BAG5), impairs mitophagy by suppressing Parkin recruitment to damaged mitochondria and reducing the movement of damaged mitochondria into the lysosomes. BAG5 also enhanced Parkin-mediated Mcl-1 degradation and cell death following severe mitochondrial insult. These results suggest that BAG5 may regulate the bi-modal activity of Parkin, promoting cell death by suppressing Parkin-dependent mitophagy and enhancing Parkin-mediated Mcl-1 degradation.

Mfn2 ubiquitination by PINK1/parkin gates the p97-dependent release of ER from mitochondria to drive mitophagy.

Despite their importance as signaling hubs, the function of mitochondria-ER contact sites in mitochondrial quality control pathways remains unexplored. Here we describe a mechanism by which Mfn2, a mitochondria-ER tether, gates the autophagic turnover of mitochondria by PINK1 and parkin. Mitochondria-ER appositions are destroyed during mitophagy, and reducing mitochondria-ER contacts increases the rate of mitochondrial degradation. Mechanistically, parkin/PINK1 catalyze a rapid burst of Mfn2 phosphoubiquitination to trigger p97-dependent disassembly of Mfn2 complexes from the outer mitochondrial membrane, dissociating mitochondria from the ER. We additionally demonstrate that a major portion of the facilitatory effect of p97 on mitophagy is epistatic to Mfn2 and promotes the availability of other parkin substrates such as VDAC1. Finally, we reconstitute the action of these factors on Mfn2 and VDAC1 ubiquitination in a cell-free assay. We show that mitochondria-ER tethering suppresses mitophagy and describe a parkin-/PINK1-dependent mechanism that regulates the destruction of mitochondria-ER contact sites.

HBpF-proBDNF: A New Tool for the Analysis of Pro-Brain Derived Neurotrophic Factor Receptor Signaling and Cell Biology.

Neurotrophins activate intracellular signaling pathways necessary for neuronal survival, growth and apoptosis. The most abundant neurotrophin in the adult brain, brain-derived neurotrophic factor (BDNF), is first synthesized as a proBDNF precursor and recent studies have demonstrated that proBDNF can be secreted and that it functions as a ligand for a receptor complex containing p75NTR and sortilin. Activation of proBDNF receptors mediates growth cone collapse, reduces synaptic activity, and facilitates developmental apoptosis of motoneurons but the precise signaling cascades have been difficult to discern. To address this, we have engineered, expressed and purified HBpF-proBDNF, an expression construct containing a 6X-HIS tag, a biotin acceptor peptide (BAP) sequence, a PreScission™ Protease cleavage site and a FLAG-tag attached to the N-terminal part of murine proBDNF. Intact HBpF-proBDNF has activities indistinguishable from its wild-type counterpart and can be used to purify proBDNF signaling complexes or to monitor proBDNF endocytosis and retrograde transport. HBpF-proBDNF will be useful for characterizing proBDNF signaling complexes and for deciphering the role of proBDNF in neuronal development, synapse function and neurodegenerative disease.

p75 neurotrophin receptor reduces ligand-induced Trk receptor ubiquitination and delays Trk receptor internalization and degradation.

Target-derived neurotrophins regulate neuronal survival and growth by interacting with cell-surface tyrosine kinase receptors. The p75 neurotrophin receptor (p75 NTR) is coexpressed with Trk receptors in long-range projection neurons, in which it facilitates neurotrophin binding to Trk and enhances Trk activity. Here, we show that TrkA and TrkB receptors undergo robust ligand-dependent ubiquitination that is dependent on activation of the endogenous Trk activity of the receptors. Coexpression of p75 NTR attenuated ubiquitination of TrkA and TrkB and delayed nerve growth factor-induced TrkA receptor internalization and receptor degradation. These results indicate that p75 NTR may prolong cell-surface Trk-dependent signalling events by negatively regulating receptor ubiquitination.

K252a and CEP1347 are neuroprotective compounds that inhibit mixed-lineage kinase-3 and induce activation of Akt and ERK.

K252a is best known as a Trk inhibitor, but is also a neuroprotective compound. CEP1347, a K252a derivative, retains neuroprotective properties, but does not inhibit TrkA. CEP1347 has recently been shown to directly inhibit MAPKKKs, including MLK3, but the effect of K252a on MAPKKKs remains unknown. K252a and CEP1347 not only prevent death, but also facilitate neurite outgrowth and maintenance, somal hypertrophy, and neurotransmitter synthesis. The biochemical basis for these trophic effects remains unknown. We have compared the effects of CEP1347 and K252a on MLK and JNK signaling and on neurotrophic pathways that support survival and growth. Our data show that K252a is a potent inhibitor of MLK3 activity in vivo and in vitro (IC(50) approximately 5 nm). However, we also found that K252a and CEP1347 activate Akt and ERK and show that blockade of phosphatidylinositol 3-kinase or MEK activity ablates the effect of K252a and CEP1347 on cell survival. Activation of Akt and ERK occurs through an MLK-independent pathway that may involve c-Src. Together, these data show that the neuroprotective and neurotrophic effects of K252a and CEP1347 involve activation of several neurotrophic signaling pathways.