Interaction of Mint2 with TrkA is involved in regulation of nerve growth factor-induced neurite outgrowth

Perinatal exposure to the immune-suppressant di-n-octyltin dichloride affects brain development in rats

Disruption of the immune system during embryonic brain development by environmental chemicals was proposed as a possible cause of neurodevelopmental disorders. We previously found adverse effects of di-n-octyltin dichloride (DOTC) on maternal and developing immune systems of rats in an extended one-generation reproductive toxicity study according to the OECD 443 test guideline. We hypothesize that the DOTC-induced changes in the immune system can affect neurodevelopment. Therefore, we used in-vivo MRI and PET imaging and genomics, in addition to behavioral testing and neuropathology as proposed in OECD test guideline 443, to investigate the effect of DOTC on structural and functional brain development. Male rats were exposed to DOTC (0, 3, 10, or 30 mg/kg of diet) from 2 weeks prior to mating of the F0-generation until sacrifice of F1-animals. The brains of rats, exposed to DOTC showed a transiently enlarged volume of specific brain regions (MRI), altered specific gravity, and transient hyper-metabolism ([18F]FDG PET). The alterations in brain development concurred with hyper-responsiveness in auditory startle response and slight hyperactivity in young adult animals. Genomics identified altered transcription of key regulators involved in neurodevelopment and neural function (e.g. Nrgrn, Shank3, Igf1r, Cck, Apba2, Foxp2); and regulators involved in cell size, cell proliferation, and organ development, especially immune system development and functioning (e.g. LOC679869, Itga11, Arhgap5, Cd47, Dlg1, Gas6, Cml5, Mef2c). The results suggest the involvement of immunotoxicity in the impairment of the nervous system by DOTC and support the hypothesis of a close connection between the immune and nervous systems in brain development.

Interrogation and validation of the interactome of neuronal Munc18-interacting Mint proteins with AlphaFold2

Munc18-interacting proteins (Mints) are multidomain adaptors that regulate neuronal membrane trafficking, signaling, and neurotransmission. Mint1 and Mint2 are highly expressed in the brain with overlapping roles in the regulation of synaptic vesicle fusion required for neurotransmitter release by interacting with the essential synaptic protein Munc18-1. Here, we have used AlphaFold2 to identify and then validate the mechanisms that underpin both the specific interactions of neuronal Mint proteins with Munc18-1 as well as their wider interactome. We found that a short acidic α-helical motif within Mint1 and Mint2 is necessary and sufficient for specific binding to Munc18-1 and binds a conserved surface on Munc18-1 domain3b. In Munc18-1/2 double knockout neurosecretory cells, mutation of the Mint-binding site reduces the ability of Munc18-1 to rescue exocytosis, and although Munc18-1 can interact with Mint and Sx1a (Syntaxin1a) proteins simultaneously in vitro, we find that they have mutually reduced affinities, suggesting an allosteric coupling between the proteins. Using AlphaFold2 to then examine the entire cellular network of putative Mint interactors provides a structural model for their assembly with a variety of known and novel regulatory and cargo proteins including ADP-ribosylation factor (ARF3/ARF4) small GTPases and the AP3 clathrin adaptor complex. Validation of Mint1 interaction with a new predicted binder TJAP1 (tight junction-associated protein 1) provides experimental support that AlphaFold2 can correctly predict interactions across such large-scale datasets. Overall, our data provide insights into the diversity of interactions mediated by the Mint family and show that Mints may help facilitate a key trigger point in SNARE (soluble N-ethylmaleimide-sensitive factor attachment receptor) complex assembly and vesicle fusion.

BmK NSPK, a Potent Potassium Channel Inhibitor from Scorpion Buthus martensii Karsch, Promotes Neurite Outgrowth via NGF/TrkA Signaling Pathway

Scorpion toxins represent a variety of tools to explore molecular mechanisms and cellular signaling pathways of many biological functions. These toxins are also promising lead compounds for developing treatments for many neurological diseases. In the current study, we purified a new scorpion toxin designated as BmK NSPK (Buthus martensii Karsch neurite-stimulating peptide targeting K_v channels) from the BmK venom. The primary structure was determined using Edman degradation. BmK NSPK directly inhibited outward K⁺ current without affecting sodium channel activities, depolarized membrane, and increased spontaneous calcium oscillation in spinal cord neurons (SCNs) at low nanomolar concentrations. BmK NSPK produced a nonmonotonic increase on the neurite extension that peaked at ~10 nM. Mechanistic studies demonstrated that BmK NSPK increased the release of nerve growth factor (NGF). The tyrosine kinases A (TrkA) receptor inhibitor, GW 441756, eliminated the BmK NSPK-induced neurite outgrowth. BmK NSPK also increased phosphorylation levels of protein kinase B (Akt) that is the downstream regulator of TrkA receptors. These data demonstrate that BmK NSPK is a new voltage-gated potassium (K_v) channel inhibitor that augments neurite extension via NGF/TrkA signaling pathway. K_v channels may represent molecular targets to modulate SCN development and regeneration and to develop the treatments for spinal cord injury.

Exploratory locomotion, a predictor of addiction vulnerability, is oligogenic in rats selected for this phenotype

Artificially selected model organisms can reveal hidden features of the genetic architecture of the complex disorders that they model. Addictions are disease phenotypes caused by different intermediate phenotypes and pathways and thereby are potentially highly polygenic. High responder (bHR) and low responder (bLR) rat lines have been selectively bred (b) for exploratory locomotion (EL), a behavioral phenotype correlated with novelty-seeking, impulsive response to reward, and vulnerability to addiction, and is inversely correlated with spontaneous anxiety and depression-like behaviors. The rapid response to selection indicates loci of large effect for EL. Using exome sequencing of HR and LR rats, we identified alleles in gene-coding regions that segregate between the two lines. Quantitative trait locus (QTL) analysis in F2 rats derived from a bHR × bLR intercross confirmed that these regions harbored genes affecting EL. The combined effects of the seven genome-wide significant QTLs accounted for approximately one-third of the total variance in EL, and two-thirds of the variance attributable to genetic factors, consistent with an oligogenic architecture of EL estimated both from the phenotypic distribution of F2 animals and rapid response to selection. Genetic association in humans linked APBA2, the ortholog of the gene at the center of the strongest QTL, with substance use disorders and related behavioral phenotypes. Our finding is also convergent with molecular and animal behavioral studies implicating Apba2 in locomotion. These results provide multilevel evidence for genes/loci influencing EL. They shed light on the genetic architecture of oligogenicity in animals artificially selected for a phenotype modeling a more complex disorder in humans.

Phosphorylation of multiple sites within an acidic region of Alcadein α is required for kinesin-1 association and Golgi exit of Alcadein α cargo

Alcadein a (Alca) is reported to function as a cargo receptor when associated with kinesin-1. Phosphorylation of three serine residues in the acidic region located between the two WD motifs of Alca is required for interaction with kinesin-1 and Golgi exit of Alca cargo.

Alcadein α (Alcα) is a major cargo of kinesin-1 that is subjected to anterograde transport in neuronal axons. Two tryptophan- and aspartic acid-containing (WD) motifs located in its cytoplasmic domain directly bind the tetratricopeptide repeat (TPR) motifs of the kinesin light chain (KLC), which activate kinesin-1 and recruit kinesin-1 to Alcα cargo. We found that phosphorylation of three serine residues in the acidic region located between the two WD motifs is required for interaction with KLC. Phosphorylation of these serine residues may alter the disordered structure of the acidic region to induce direct association with KLC. Replacement of these serines with Ala results in a mutant that is unable to bind kinesin-1, which impairs exit of Alcα cargo from the Golgi. Despite this deficiency, the compromised Alcα mutant was still transported, albeit improperly by vesicles following missorting of the Alcα mutant with amyloid β-protein precursor (APP) cargo. This suggests that APP partially compensates for defective Alcα in anterograde transport by providing an alternative cargo receptor for kinesin-1.

The Intersection of NGF/TrkA Signaling and Amyloid Precursor Protein Processing in Alzheimer's Disease Neuropathology

Dysfunction of nerve growth factor (NGF) and its high-affinity Tropomyosin receptor kinase A (TrkA) receptor has been suggested to contribute to the selective degeneration of basal forebrain cholinergic neurons (BFCN) associated with the progressive cognitive decline in Alzheimer's disease (AD). The aim of this review is to describe our progress in elucidating the molecular mechanisms underlying the dynamic interplay between NGF/TrkA signaling and amyloid precursor protein (APP) metabolism within the context of AD neuropathology. This is mainly based on the finding that TrkA receptor binding to APP depends on a minimal stretch of ~20 amino acids located in the juxtamembrane/extracellular domain of APP that carries the α- and β-secretase cleavage sites. Here, we provide evidence that: (i) NGF could be one of the “routing” proteins responsible for modulating the metabolism of APP from amyloidogenic towards non-amyloidogenic processing via binding to the TrkA receptor; (ii) the loss of NGF/TrkA signaling could be linked to sporadic AD contributing to the classical hallmarks of the neuropathology, such as synaptic loss, β-amyloid peptide (Aβ) deposition and tau abnormalities. These findings will hopefully help to design therapeutic strategies for AD treatment aimed at preserving cholinergic function and anti-amyloidogenic activity of the physiological NGF/TrkA pathway in the septo-hippocampal system.

Up-Regulated ATF4 Expression Increases Cell Sensitivity to Apoptosis in Response to Radiation

BACKGROUND/AIMS

Activating transcription factor 4 (ATF4) is a member of the activating transcription factor family which regulates the expression of genes involved in amino acid metabolism, redox homeostasis and ER stress responses. ATF4 is also over-expressed in human solid tumors, although its effect on responsiveness to radiation is largely unexplored.

METHODS

Real-time PCR was used to detect ATF4 mRNA levels in cells treated with different doses of 60Coγ radiation. Cell viability was assayed using a cell counting kit. The cell cycle was analyzed using flow cytometry, and cell apoptosis was assayed using Annexin V-PI double labeling. Small interfering RNA (siRNA) against ATF4 was transfected into ECV304 cells using Lipofectamine 2000. An ATF4 over-expression plasmid (p-ATF4-CGN) was transfected into HEK293 cells that endogenously expressed low levels of ATF4. The levels of intracellular reactive oxygen species (ROS) were measured using CM-H2DCFDA as a probe.

RESULTS

ATF4 mRNA and protein expression levels were higher after radiation and increased in a dose- and time-dependent manner in AHH1 lymphoblast cells (P < 0.05). An increase in ATF4 levels was also observed after radiation in primary murine spleen cells, human endothelial ECV304 cells, human liver LO2 cells, breast cancer MCF7 cells, and human hepatocellular carcinoma HEPG2 cells. No change was observed in human embryonic kidney 293 (HEK293) cells. Over-expressing ATF4 in HEK293 cells inhibited cell proliferation, increased cell apoptosis and significantly increased the proportion of cells in G1 phase. Conversely, when ATF4 expression was knocked down using siRNA in ECV304 cells, it protected the cells from radiation-induced apoptosis. These findings suggest that ATF4 may play a role in radiation-induced cell killing by inhibiting cell proliferation and promoting cell apoptosis.

CONCLUSIONS

In this study, we found that radiation up-regulated the expression of ATF4. We used ATF4 knockdown and over-expression systems to show that ATF4 may play a role in radiation-induced cellular apoptosis.

Association of TrkA and APP Is Promoted by NGF and Reduced by Cell Death-Promoting Agents

The amyloid precursor protein (APP) interacts with the tropomyosin receptor kinase A (TrkA) in normal rat, mouse, and human brain tissue but not in Alzheimer's disease (AD) brain tissue. However, it has not been reported whether the two proteins interact directly, and if so, which domains are involved. Clarifying these points will increase our understanding of the role and regulation of the TrkA/APP interaction in normal brain functioning as well as in AD. Here we addressed these questions using bimolecular fluorescence complementation (BiFC) and the proximity ligation assay (PLA). We demonstrated that exogenously expressed APP and TrkA associate through their juxtamembrane/transmembrane domains, to form a complex that localizes mainly to the plasma membrane, endoplasmic reticulum (ER) and Golgi. Formation of the complex was inhibited by p75NTR, ShcC and Mint-2. Importantly, we demonstrated that the association between endogenous APP and TrkA in primary septal neurons were modified by NGF, or by drugs that either inhibit ER-to-Golgi transport or perturb microtubules and microfilaments. Interestingly, several agents that induce cell death [amyloid β (Aβ)-peptide, staurosporine and rapamycin], albeit via different mechanisms, all caused dissociation of APP/TrkA complexes and increased production of C-terminal fragment (β-CTF) APP fragment. These findings open new perspectives for investigating the interplay between these proteins during neurodegeneration and AD.

Syntaxin 8 modulates the post-synthetic trafficking of the TrkA receptor and inflammatory pain transmission

Nerve growth factor (NGF) promotes the survival, maintenance, and neurite outgrowth of sensory and sympathetic neurons, and the effects are mediated by TrkA receptor signaling. Thus, the cell surface location of the TrkA receptor is crucial for NGF-mediated functions. However, the regulatory mechanism underlying TrkA cell surface levels remains incompletely understood. In this study, we identified syntaxin 8 (STX8), a Q-SNARE protein, as a novel TrkA-binding protein. Overexpression and knockdown studies showed that STX8 facilitates TrkA transport from the Golgi to the plasma membrane and regulates the surface levels of TrkA but not TrkB receptors. Furthermore, STX8 modulates downstream NGF-induced TrkA signaling and, consequently, the survival of NGF-dependent dorsal root ganglia neurons. Finally, knockdown of STX8 in rat dorsal root ganglia by recombinant adeno-associated virus serotype 6-mediated RNA interference led to analgesic effects on formalin-induced inflammatory pain. These findings demonstrate that STX8 is a modulator of TrkA cell surface levels and biological functions.

TROY interacts with Rho guanine nucleotide dissociation inhibitor α (RhoGDIα) to mediate Nogo-induced inhibition of neurite outgrowth

TROY can functionally substitute p75 to comprise the Nogo receptor complex, which transduces the inhibitory signal of myelin-associated inhibitory factors on axon regeneration following CNS injury. The inhibition of neurite extension relies on TROY-dependent RhoA activation, but how TROY activates RhoA remains unclear. Here, we firstly identified Rho guanine nucleotide dissociation inhibitor α (RhoGDIα) as a binding partner of TROY using GST pull-down combined with two-dimensional gel electrophoresis and mass spectra analysis. The interaction was further confirmed by coimmunoprecipitation in vitro and in vivo. Deletion mutagenesis revealed that two regions of the TROY intracellular domain (amino acids 234-256 and 321-350) were essential for the interaction with RhoGDIα. Secondly, TROY and RhoGDIα were coexpressed in postnatal dorsal root ganglion neurons, cortex neurons, and cerebellar granule neurons (CGNs). Thirdly, TROY/RhoGDIα association was potentiated by Nogo-66 and was independent of p75/RhoGDIα interaction. Fourthly, TROY/RhoGDIα interaction was still able to activate RhoA when p75 was deficient. Furthermore, RhoA activation was decreased dramatically when TROY was knocked down in p75-deficient CGNs cells. Finally, RhoGDIα overexpression abolished RhoA activation and following neurite outgrowth inhibition by Nogo-66 in both wild-type and p75-deficient CGNs. These results showed that the association of RhoGDIα with TROY contributed to TROY-dependent RhoA activation and neurite outgrowth inhibition after Nogo-66 stimulation.

X11/Mint genes control polarized localization of axonal membrane proteins in vivo

Mislocalization of axonal proteins can result in misassembly and/or miswiring of neural circuits, causing disease. To date, only a handful of genes that control polarized localization of axonal membrane proteins have been identified. Here we report that Drosophila X11/Mint proteins are required for targeting several proteins, including human amyloid precursor protein (APP) and Drosophila APP-like protein (APPL), to axonal membranes and for their exclusion from dendrites of the mushroom body in Drosophila, a brain structure involved in learning and memory. Axonal localization of APP is mediated by an endocytic motif, and loss of X11/Mint results in a dramatic increase in cell-surface levels of APPL, especially on dendrites. Mutations in genes required for endocytosis show similar mislocalization of these proteins to dendrites, and strongly enhance defects seen in X11/Mint mutants. These results suggest that X11/Mint-dependent endocytosis in dendrites may serve to promote the axonal localization of membrane proteins. Since X11/Mint binds to APP, and abnormal trafficking of APP contributes to Alzheimer's disease, deregulation of X11/Mint may be important for Alzheimer's disease pathogenesis.

Extracellular pH and neuronal depolarization serve as dynamic switches to rapidly mobilize trkA to the membrane of adult sensory neurons

Activation of the nerve growth factor (NGF) receptor trkA and tissue acidosis are critically linked to inflammation-associated nociceptor sensitization. This study explored how increased acidity is linked to sensory neuron sensitization to NGF. Adult Wistar rat primary sensory neurons grown at physiological pH 7.4, then either kept at pH 7.4 or challenged for 30 min in pH 6.5 medium, provided a model of acidosis. Nonpermeabilizing trkA immunofluorescence revealed a significant increase in trkA mobilization to the plasma membrane from intracellular stores in response to proton challenge. This was confirmed using a surface protein biotinylation assay and Brefeldin A disruption of the rough endoplasmic reticulum-Golgi-trans-Golgi network. Mobilization of trkA to the membrane at pH 6.5 was abolished in neurons treated with the acid-sensitive ion channel blocker, amiloride. While elevated levels of NGF-independent trkA phosphorylation occurred at pH 6.5 alone, the level of activation was significantly increased in response to NGF challenge. Exposure of sensory neurons to pH 6.5 medium also resulted in strong calcium (Ca(2+)) transients that were reversible upon reintroduction to physiological pH. The pH 6.5-induced mobilization of trkA to the membrane was Ca(2+) dependent, as BAPTA-AM Ca(2+) chelation abrogated the response. Interestingly, KCl-induced depolarization was sufficient to induce mobilization of trkA to the cell surface at pH 7.4, but did not augment the response to pH 6.5. In conclusion, increased mobilization of trkA to neuronal membranes in response to either acidosis or neuronal depolarization provides two novel mechanisms by which sensory neurons can rapidly sensitize to NGF and has important implications for inflammatory pain states.

MiR-21 plays an important role in radiation induced carcinogenesis in BALB/c mice by directly targeting the tumor suppressor gene Big-h3

Dysregulation of certain microRNAs (miRNAs) in cancer can promote tumorigenesis, metastasis and invasion. However, the functions and targets of only a few mammalian miRNAs are known. In particular, the miRNAs that participates in radiation induced carcinogenesis and the miRNAs that target the tumor suppressor gene Big-h3 remain undefined. Here in this study, using a radiation induced thymic lymphoma model in BALB/c mice, we found that the tumor suppressor gene Big-h3 is down-regulated and miR-21 is up-regulated in radiation induced thymic lymphoma tissue samples. We also found inverse correlations between Big-h3 protein and miR-21 expression level among different tissue samples. Furthermore, our data indicated that miR-21 could directly target Big-h3 in a 3′UTR dependent manner. Finally, we found that miR-21 could be induced by TGFβ, and miR-21 has both positive and negative effects in regulating TGFβ signaling. We conclude that miR-21 participates in radiation induced carcinogenesis and it regulates TGFβ signaling.

Peripheral somatosensation: a touch of genetics

The somatosensory system processes information that organisms 'feel': joint position, muscle stretch, pain, pressure, temperature, and touch. The system is composed of a diverse array of peripheral nerve endings specialized to detect these sensory modalities. Several recent discoveries have shed light on the genetic pathways that control specification and differentiation of these neurons, how they accurately innervate their central and peripheral targets, and the molecules that enable them to detect mechanical stimuli. Here, we review the cadre of genes that control these processes, focusing on mechanosensitive neurons and support cells of the skin that mediate different aspects of the sense of touch.

Deletions flanked by breakpoints 3 and 4 on 15q13 may contribute to abnormal phenotypes

Non-allelic homologous recombination (NAHR) between segmental duplications in proximal chromosome 15q breakpoint (BP) regions can lead to microdeletions and microduplications. Several individuals with deletions flanked by BP3 and BP4 on 15q13, immediately distal to, and not including the Prader-Willi/Angelman syndrome (PW/AS) critical region and proximal to the BP4-BP5 15q13.3 microdeletion syndrome region, have been reported; however, because the deletion has also been found in normal relatives, the significance of these alterations is unclear. We have identified six individuals with deletions limited to the BP3-BP4 interval and an additional four individuals with deletions of the BP3-BP5 interval from 34 046 samples submitted for clinical testing by microarray-based comparative genomic hybridization (aCGH). Of four individuals with BP3-BP4 deletions for whom parental testing was conducted, two were apparently de novo and two were maternally inherited. A comparison of clinical features, available for five individuals in our study (four with deletions within BP3-BP4 and one with a BP3-BP5 deletion), with those in the literature show common features of short stature and/or failure to thrive, microcephaly, hypotonia, and premature breast development in some individuals. Although the BP3-BP4 deletion does not yet demonstrate statistically significant enrichment in abnormal populations compared with control populations, the presence of common clinical features among probands and the presence of genes with roles in development and nervous system function in the deletion region suggest that this deletion may have a role in abnormal phenotypes in some individuals.

Control of TrkA-induced cell death by JNK activation and differential expression of TrkA upon DNA damage

TrkA, a receptor for nerve growth factor, plays a crucial role in neuronal cell growth and differentiation. However, overactivation of TrkA signaling leads to cell death in various cell types. TrkA-mediated cell death shows some similarities to DNA damage-induced cell death. In this study, we examined how TrkA-induced cell death is regulated upon DNA damage. Cytoplasmic expression of TrkA protein was differentially modulated during the camptothecin-induced DNA damage response in TrkA-expressing U2OS cells. TrkA-induced cell death was synergistically increased by DNA damage, but it was blocked in the presence of the JNK inhibitor SP600125. Overexpression of a 54-kDa JNK isoform (JNK1alpha2) aggravated TrkA-induced cell death and was associated with TrkA functional activation. These results suggest that TrkA shares a functional connection with other mediators in the DNA damage response via JNK signaling.

Expression and ultrastructural localization of Mint2 in the spinal cord of rats

Mint protein family, as adaptor molecules, contains three members, Mint1, Mint2 and Mint3. Although Mint3 is ubiquitously expressed, Mint1 and Mint2 have been reported to express specifically in neuron. Here we demonstrated Mint1 and Mint2 expression pattern in rat spinal cord. The protein level of Mint2 was found to be higher than that of Mint1 in rat spinal by western blot. In an attempt to know Mint2 distribution in the spinal cord of rat, in situ hybridization was carried out, Mint2 mRNA was showed to be ubiquitously distributed in cervical, thoracic and lumbar sections of rat spinal cord, and high intensive signal was detected in motor neurons. These were further confirmed by fluorescent immunohistochemistry, Mint2 was also found to exist throughout gray matter especially motor neurons where Mint2 was mainly located in perikaryon, however, Mint1 was showed to be relatively lower. By electron microscope, Mint2 was found to be mainly located in vesicles in perikaryon in motor neuron of lumbar section, and at the same time Mint2 was located in axons in myelin and presynaptic terminals. These data suggest that Mint2 may play more important role in spinal cord than the other two family members.

Quantitative assessment of neurite outgrowth in human embryonic stem cell-derived hN2 cells using automated high-content image analysis

Throughout development neurons undergo a number of morphological changes including neurite outgrowth from the cell body. Exposure to neurotoxic chemicals that interfere with this process may result in permanent deficits in nervous system function. Traditionally, rodent primary neural cultures and immortalized human and non-human clonal cell lines have been used to investigate the molecular mechanisms controlling neurite outgrowth and examine chemical effects on this process. The present study characterizes the molecular phenotype of hN2 human embryonic stem cell (hESC)-derived neural cells and uses automated high-content image analysis to measure neurite outgrowth in vitro. At 24h post-plating hN2 cells express a number of protein markers indicative of a neuronal phenotype, including: nestin, beta(III)-tubulin, microtubule-associated protein 2 (MAP2) and phosphorylated neurofilaments. Neurite outgrowth in hN2 cells proceeded rapidly, with a majority of cells extending one to three neurites by 48h in culture. In addition, concentration-dependent decreases in neurite outgrowth and ATP-content were observed following treatment of hN2 cells with either bisindolylmaleimide I, U0126, lithium chloride, sodium orthovanadate and brefeldin A, all of which have previously been shown to inhibit neurite outgrowth in primary rodent neural cultures. Overall, the molecular phenotype, rate of neurite outgrowth and sensitivity of hN2 cells to neurite outgrowth inhibitors were comparable to other in vitro models previously characterized in the literature. hN2 cells provide a model in which to investigate chemical effects on neurite outgrowth in a non-transformed human-derived cells and provide an alternative to the use of primary rodent neural cultures or immortalized clonal cell lines.