Growth-associated protein-43 is degraded via the ubiquitin-proteasome system

Hypoxic preconditioned MSCs-derived small extracellular vesicles for photoreceptor protection in retinal degeneration

Photoreceptor apoptosis is an important pathogenesis of retinal degeneration and a primary cause of vision loss with limited treatment methods. Mesenchymal stem/stromal cells-derived small extracellular vesicles (MSC-sEVs) have shown therapeutic value in various ocular disorders. Recent studies have revealed that hypoxic preconditioning can improve the effectiveness of MSC-sEVs in tissue regeneration. However, whether hypoxic preconditioned MSC-sEVs (Hyp-sEVs) exert superior effects on photoreceptor protection relative to normoxic conditioned MSC-sEVs (Nor-sEVs) remains unclear. Here, we reported that Hyp-sEVs further improved retinal structure, recovered retinal function, and suppressed photoreceptor apoptosis in N-methyl-N-nitrosourea (MNU)-induced mouse model compared with Nor-sEVs. Hyp-sEVs also exhibited enhanced anti-apoptotic roles in MNU-provoked 661 W cell injury in vitro. We then analyzed the protein profiles of Nor-sEVs and Hyp-sEVs by LC-MS/MS and found that growth-associated protein 43 (GAP43) was enriched in Hyp-sEVs. The knockdown of GAP43 abolished the retinal therapeutic effects of Hyp-sEVs. Mechanistically, hypoxic stimulation-induced hypoxia-inducible factor-1α (HIF-1α) activation was responsible for preventing tripartite motif-containing protein 25 (TRIM25)-mediated GAP43 ubiquitination and degradation, leading to the upregulation of GAP43 in Hyp-sEVs. Together, our findings uncover the efficacy and mechanism of Hyp-sEVs-based photoreceptor protection and highlight the potential of Hyp-sEVs as optimized therapeutics for retinal degeneration.

Identification of hub genes and potential molecular mechanisms of chickpea isoflavones on MCF-7 breast cancer cells by integrated bioinformatics analysis

Background

Chickpea isoflavones have been demonstrated to play an inhibitory role in breast cancer cells. In this study, we aimed to explore the mechanism of chickpea isoflavones inhibiting the formation and development of breast carcinoma through the integration of wet and dry experiments.

Methods

Chickpea isoflavones were added to the MCF-7 cells for 48 hours, and the subsequent morphological changes of cells were observed using an inverted microscope, while apoptosis was quantified by flow cytometry. The mRNA and LncRNA expression profiles were detected by RNA-sequencing (RNA-Seq) technology. The protein-protein interaction (PPI) network was constructed from the STRINGdb database. To identify the co-expressed long non-coding RNA and messenger RNA (lncRNA-mRNA) pairs, Pearson's correlation coefficients were calculated based on the expression value between every differentially expressed lncRNA and mRNA pair. The hub gene expression was verified by quantitative reverse transcription polymerase chain reaction (qRT-PCR), and survival analysis results were provided by The Human Protein Atlas website.

Results

Microscopic observation and flow cytometry results confirmed that chickpea isoflavones with a final concentration of 32.8 µg/mL could cause apoptosis of the MCF-7 cells. Transcriptome results showed that a total of 1,094 mRNAs and 378 lncRNAs were differentially expressed in isoflavone-treated cells. Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway enrichment revealed that inhibition of cell proliferation was mainly due to the up-regulation of genes in the apoptosis signaling pathway and the down-regulation of genes in mRNA splicing pathway. The co-expressed genes of the top 10 down-regulated lncRNAs were mainly heterogeneous nuclear ribonucleoproteins (HNRNP) family genes, which interacted with apoptosis-related genes through ubiquitin C (UBC). The abnormal expression of 11 hub genes (degree >10) of PPI networks were beneficial to improve the overall survival time of breast cancer patients.

Conclusions

Our results reveal a potential mechanism for chickpea isoflavones to inhibit MCF-7 breast cancer cell proliferation and provide a reference for the development of new anti-cancer drugs used in breast cancer.

hnRNP-Q1 represses nascent axon growth in cortical neurons by inhibiting Gap-43 mRNA translation

A novel posttranscriptional mechanism for regulating the neuronal protein GAP-43 is reported. The mRNA-binding protein hnRNP-Q1 represses Gap-43 mRNA translation by a mechanism involving a 5′ untranslated region G-quadruplex structure, which affects GAP-43 function, as demonstrated by a GAP-43–dependent increase in neurite length and number with hnRNP-Q1 knockdown.

Posttranscriptional regulation of gene expression by mRNA-binding proteins is critical for neuronal development and function. hnRNP-Q1 is an mRNA-binding protein that regulates mRNA processing events, including translational repression. hnRNP-Q1 is highly expressed in brain tissue, suggesting a function in regulating genes critical for neuronal development. In this study, we have identified Growth-associated protein 43 (Gap-43) mRNA as a novel target of hnRNP-Q1 and have demonstrated that hnRNP-Q1 represses Gap-43 mRNA translation and consequently GAP-43 function. GAP-43 is a neuronal protein that regulates actin dynamics in growth cones and facilitates axonal growth. Previous studies have identified factors that regulate Gap-43 mRNA stability and localization, but it remains unclear whether Gap-43 mRNA translation is also regulated. Our results reveal that hnRNP-Q1 knockdown increased nascent axon length, total neurite length, and neurite number in mouse embryonic cortical neurons and enhanced Neuro2a cell process extension; these phenotypes were rescued by GAP-43 knockdown. Additionally, we have identified a G-quadruplex structure in the 5′ untranslated region of Gap-43 mRNA that directly interacts with hnRNP-Q1 as a means to inhibit Gap-43 mRNA translation. Therefore hnRNP-Q1–mediated repression of Gap-43 mRNA translation provides an additional mechanism for regulating GAP-43 expression and function and may be critical for neuronal development.

2-Bromopalmitate reduces protein deacylation by inhibition of acyl-protein thioesterase enzymatic activities

S-acylation, the covalent attachment of palmitate and other fatty acids on cysteine residues, is a reversible post-translational modification that exerts diverse effects on protein functions. S-acylation is catalyzed by protein acyltransferases (PAT), while deacylation requires acyl-protein thioesterases (APT), with numerous inhibitors for these enzymes having already been developed and characterized. Among these inhibitors, the palmitate analog 2-brompalmitate (2-BP) is the most commonly used to inhibit palmitoylation in cells. Nevertheless, previous results from our laboratory have suggested that 2-BP could affect protein deacylation. Here, we further investigated in vivo and in vitro the effect of 2-BP on the acylation/deacylation protein machinery, with it being observed that 2-BP, in addition to inhibiting PAT activity in vivo, also perturbed the acylation cycle of GAP-43 at the level of depalmitoylation and consequently affected its kinetics of membrane association. Furthermore, 2-BP was able to inhibit in vitro the enzymatic activities of human APT1 and APT2, the only two thioesterases shown to mediate protein deacylation, through an uncompetitive mechanism of action. In fact, APT1 and APT2 hydrolyzed both the monomeric form as well as the micellar state of the substrate palmitoyl-CoA. On the basis of the obtained results, as APTs can mediate deacylation on membrane bound and unbound substrates, this suggests that the access of APTs to the membrane interface is not a necessary requisite for deacylation. Moreover, as the enzymatic activity of APTs was inhibited by 2-BP treatment, then the kinetics analysis of protein acylation using 2-BP should be carefully interpreted, as this drug also inhibits protein deacylation.

Opposite effects of acute ethanol exposure on GAP-43 and BDNF expression in the hippocampus versus the cerebellum of juvenile rats

The adolescent brain is particularly vulnerable to the effects of alcohol, with intoxications at this developmental age often producing long-lasting effects. The present study addresses the effects of a single acute ethanol exposure on growth-associated protein-43 (GAP-43) and brain-derived neurotrophic factor (BDNF) gene expression in neurons in the cerebellum and hippocampus of adolescent rats. Male postnatal day 23 (P23) Sprague-Dawley rats were exposed to ethanol vapors for 2h and after a recovery period of 2h, the cerebellum and hippocampus were harvested and samples were taken for blood alcohol concentration (BAC) determinations. We found that this exposure resulted in a mean BAC of 174 mg/dL, which resembles levels in human adolescents after binge drinking. Analyses of total RNA and protein by quantitative reverse transcription PCR and western blotting, respectively, revealed that this single ethanol exposure significantly decreased the levels of GAP-43 mRNA and protein in the cerebellum but increased the levels of mRNA and protein in the hippocampus. BDNF mRNA and protein levels were also increased in the hippocampus but not in the cerebellum of these animals. In situ hybridizations revealed that GAP-43 and BDNF mRNA levels were primarily increased by alcohol exposure in hippocampal dentate granule cells and CA3 neurons. Overall, the reported alterations in the expression of the plasticity-associated genes GAP-43 and BDNF in juvenile rats are consistent with the known deleterious effects of binge drinking on motor coordination and cognitive function.

Acyl-protein thioesterase 2 catalyzes the deacylation of peripheral membrane-associated GAP-43

An acylation/deacylation cycle is necessary to maintain the steady-state subcellular distribution and biological activity of S-acylated peripheral proteins. Despite the progress that has been made in identifying and characterizing palmitoyltransferases (PATs), much less is known about the thioesterases involved in protein deacylation. In this work, we investigated the deacylation of growth-associated protein-43 (GAP-43), a dually acylated protein at cysteine residues 3 and 4. Using fluorescent fusion constructs, we measured in vivo the rate of deacylation of GAP-43 and its single acylated mutants in Chinese hamster ovary (CHO)-K1 and human HeLa cells. Biochemical and live cell imaging experiments demonstrated that single acylated mutants were completely deacylated with similar kinetic in both cell types. By RT-PCR we observed that acyl-protein thioesterase 1 (APT-1), the only bona fide thioesterase shown to mediate deacylation in vivo, is expressed in HeLa cells, but not in CHO-K1 cells. However, APT-1 overexpression neither increased the deacylation rate of single acylated GAP-43 nor affected the steady-state subcellular distribution of dually acylated GAP-43 both in CHO-K1 and HeLa cells, indicating that GAP-43 deacylation is not mediated by APT-1. Accordingly, we performed a bioinformatic search to identify putative candidates with acyl-protein thioesterase activity. Among several candidates, we found that APT-2 is expressed both in CHO-K1 and HeLa cells and its overexpression increased the deacylation rate of single acylated GAP-43 and affected the steady-state localization of diacylated GAP-43 and H-Ras. Thus, the results demonstrate that APT-2 is the protein thioesterase involved in the acylation/deacylation cycle operating in GAP-43 subcellular distribution.

Molecular mechanisms, biological actions, and neuropharmacology of the growth-associated protein GAP-43

GAP-43 is an intracellular growth-associated protein that appears to assist neuronal pathfinding and branching during development and regeneration, and may contribute to presynaptic membrane changes in the adult, leading to the phenomena of neurotransmitter release, endocytosis and synaptic vesicle recycling, long-term potentiation, spatial memory formation, and learning. GAP-43 becomes bound via palmitoylation and the presence of three basic residues to membranes of the early secretory pathway. It is then sorted onto vesicles at the late secretory pathway for fast axonal transport to the growth cone or presynaptic plasma membrane. The palmitate chains do not serve as permanent membrane anchors for GAP-43, because at steady-state most of the GAP-43 in a cell is membrane-bound but is not palmitoylated. Filopodial extension and branching take place when GAP-43 is phosphorylated at Ser-41 by protein kinase C, and this occurs following neurotrophin binding and the activation of numerous small GTPases. GAP-43 has been proposed to cluster the acidic phospholipid phosphatidylinositol 4,5-bisphosphate in plasma membrane rafts. Following GAP-43 phosphorylation, this phospholipid is released to promote local actin filament-membrane attachment. The phosphorylation also releases GAP-43 from calmodulin. The released GAP-43 may then act as a lateral stabilizer of actin filaments. N-terminal fragments of GAP-43, containing 10-20 amino acids, will activate heterotrimeric G proteins, direct GAP-43 to the membrane and lipid rafts, and cause the formation of filopodia, possibly by causing a change in membrane tension. This review will focus on new information regarding GAP-43, including its binding to membranes and its incorporation into lipid rafts, its mechanism of action, and how it affects and is affected by extracellular agents.

Expression of PKC substrate proteins, GAP-43 and neurogranin, is downregulated by cAMP signaling and alterations in synaptic activity

Growth-associated protein 43 (GAP-43) and neurogranin are protein kinase C substrate proteins that are thought to play an important role in synaptic plasticity, but little is currently known about the mechanisms that may regulate their function at the synapse. In this study, we show that long-term elevation of intracellular cAMP levels in rat primary cortical cultures results in a persistent downregulation of GAP-43 and neurogranin, most likely at the transcriptional level. This effect may be at least partially mediated by protein kinase A, but is independent of protein kinase C activation. Moreover, it is mimicked and occluded by manipulations that alter the levels of spontaneous synaptic activity in primary cultures, such as bicuculline and tetrodotoxin. These data suggest that levels of GAP-43 and neurogranin are regulated by factors known to modulate synaptic strength, thus providing a potential mechanism by which protein kinase C signaling pathways and their substrates might contribute to synaptic function and/or plasticity.

M-calpain-mediated cleavage of GAP-43 near Ser41 is negatively regulated by protein kinase C, calmodulin and calpain-inhibiting fragment GAP-43-3

Neuronal protein GAP-43 performs multiple functions in axon guidance, synaptic plasticity and regulation of neuronal death and survival. However, the molecular mechanisms of its action in these processes are poorly understood. We have shown that in axon terminals GAP-43 is a substrate for calcium-activated cysteine protease m-calpain, which participates in repulsion of axonal growth cones and induction of neuronal death. In pre-synaptic terminals in vivo, in synaptosomes, and in vitro, m-calpain cleaved GAP-43 in a small region near Ser41, on either side of this residue. In contrast, micro-calpain cleaved GAP-43 in vitro at several other sites, besides Ser41. Phosphorylation of Ser41 by protein kinase C or GAP-43 binding to calmodulin strongly suppressed GAP-43 proteolysis by m-calpain. A GAP-43 fragment, lacking about forty N-terminal residues (named GAP-43-3), was produced by m-calpain-mediated cleavage of GAP-43 and inhibited m-calpain, but not micro-calpain. This fragment prevented complete cleavage of intact GAP-43 by m-calpain as a negative feedback. GAP-43-3 also blocked m-calpain activity against casein, a model calpain substrate. This implies that GAP-43-3, which is present in axon terminals in high amount, can play important role in regulation of m-calpain activity in neurons. We suggest that GAP-43-3 and another (N-terminal) GAP-43 fragment produced by m-calpain participate in modulation of neuronal response to repulsive and apoptotic signals.