Laminin-1 induces endocytosis of 67KDa laminin receptor and protects Neuroscreen-1 cells against death induced by serum withdrawal

Non-integrin laminin receptor (LamR) plays a role in axonal outgrowth from chicken DRG via modulating the Akt and Erk signaling

37/67 kDa laminin receptor (LamR)/ribosomal protein SA exhibits dual function as both a ribosomal protein and cell surface receptor for laminin. LamR influences critical cellular processes such as invasion, adhesion, and migration when acting as a receptor. Despite the acknowledged importance of LamR/67LR in various cellular processes, its contribution to the peripheral nervous system development is obscure. Thus, this study investigated the biological activity of LamR in peripheral axonal outgrowth in the presence of laminin-1 or Ile-Lys-Val-Ala-Val (IKVAV) peptide, whose important role in dorsal root ganglia (DRG) axonal outgrowth we recently showed. Unexpectedly, we did not observe LamR on the surface of DRG cells or in a conditioned medium, suggesting its intracellular action in the negative regulation of DRG axonal outgrowth. Using C-terminus LamR-targeting IgG, we demonstrated the role of LamR in that process, which is independent of the presence of Schwann cell precursors (SCPs) and is mediated by extracellular signal-regulated kinase (Erk) and Protein kinase B (Akt1/2/3) signaling pathways. Additionally, we show that the action of LamR towards laminin-1-dependent axonal outgrowth is unmasked only when the activity of integrin β1 is perturbed. We believe that modulation of LamR activity provides the basis for its use for inhibiting axon growth as a potential therapeutic agent for regulating abnormal or excessive neurite growth during neurodevelopmental diseases or pathological nerve regeneration.

PEDF-derived peptide protects against Amyloid-β toxicity in vitro and prevents retinal dysfunction in rats

Amyloid-beta (Aβ), a family of aggregation-prone and neurotoxic peptides, has been implicated in the pathophysiology of age-related macular degeneration (AMD). We have previously shown that oligomeric and fibrillar species of Aβ42 exerted retinal toxicity in rats, but while the consequences of exposure to amyloid were related to intracellular effects, the mechanism of Aβ42 internalization in the retina is not well characterized. In the brain, the 67 kDa laminin receptor (67LR) participates in Aβ-related neuronal cell death. A short peptide derived from pigment epithelium-derived factor (PEDF), formerly designated PEDF-335, was found to mitigate experimental models of ischemic retinopathy via targeting of 67LR. In the present study, we hypothesized that 67LR mediates the uptake of pathogenic Aβ42 assemblies in the retina, and that targeting of this receptor by PEDF-335 may limit the internalization of Aβ, thereby ameliorating its retinotoxicity. To test this assumption ARPE-19 cells in culture were incubated with PEDF-335 before treatment with fibrillar or oligomeric structures of Aβ42. Immunostaining confirmed that PEDF-335 treatment substantially prevented amyloid internalization into ARPE-19 cells and maintained their viability in the presence of toxic oligomeric and fibrillar Aβ42 entities in vitro. FRET competition assay was performed and confirmed the binding of PEDF-335 to 67LR in RPE-like cells. Wild-type rats were treated with intravitreal PEDF-335 in the experimental eye 2 days prior to administration of retinotoxic Aβ42 oligomers or fibrils to both eyes. Retinal function was assessed by electroretinography through 6 weeks post injection. The ERG responses in rats treated with oligomeric or fibrillar Aβ42 assemblies were near-normal in eyes previously treated with intravitreal PEDF-335, whereas those measured in the control eyes treated with injection of the Aβ42 assemblies alone showed pathologic attenuation of the retinal function through 6 weeks. The retinal presence of 67LR was determined ex vivo by immunostaining and western blotting. Retinal staining demonstrated the constitutional expression of 67LR mainly in the retinal nuclear layers. In the presence of Aβ42, the levels of 67LR were increased, although its retinal distribution remained largely unaltered. In contrast, no apparent differences in the retinal expression level of 67LR were noted following exposure to PEDF-335 alone, and its pattern of localization in the retina remained similarly concentrated primarily in the inner and outer nuclear layers. In summary, we found that PEDF-335 confers protection against Aβ42-mediated retinal toxicity, with significant effects noted in cells as well as in vivo in rats. The effects of PEDF-335 in the retina are potentially mediated via binding to 67LR and by at least partial inhibition of Aβ42 internalization. These results suggest that PEDF-335 may merit further consideration in the development of targeted inhibition of amyloid-related toxicity in the retina. More broadly, our observations provide evidence on the importance of extracellular versus intracellular Aβ42 in the retina and suggest concepts on the molecular mechanism of Aβ retinal pathogenicity.

Resveratrol and its metabolites elicit neuroprotection via high-affinity binding to the laminin receptor at low nanomolar concentrations

Resveratrol prevents various neurodegenerative diseases in animal models despite reaching only low nanomolar concentrations in the brain after oral administration. In this study, based on the quenching of intrinsic tryptophan fluorescence and molecular docking, we found that trans-resveratrol, its conjugates (glucuronide and sulfate), and dihydro-resveratrol (intestinal microbial metabolite) bind with high affinities (Kd, 0.2-2 nm) to the peptide G palindromic sequence (near glycosaminoglycan-binding motif) of the 67-kDa laminin receptor (67LR). Preconditioning with low concentrations (0.01-10 nm) of these polyphenols, especially resveratrol-glucuronide, protected neuronal cells from death induced by serum withdrawal via activation of cAMP-mediated signaling pathways. This protection was prevented by a 67LR-blocking antibody, suggesting a role for this cell-surface receptor in neuroprotection by resveratrol metabolites.

Flavonoid quercetin and its glucuronide and sulfate conjugates bind to 67-kDa laminin receptor and prevent neuronal cell death induced by serum starvation

Quercetin, a dietary flavonoid, has been shown to protect against various neurodegenerative diseases with mechanisms largely unknown. After oral administration, quercetin is rapidly conjugated, and the aglycone is not detectable in the plasma and brain. However, its glucuronide and sulfate conjugates are present only at low nanomolar concentrations in the brain. Since quercetin and its conjugates have limited antioxidant capability at low nanomolar concentrations, it is crucial to determine whether they induce neuroprotection by binding to high-affinity receptors. Previously we found that (-)-epigallocatechin-3-gallate (EGCG), a polyphenol from green tea, induces neuroprotection by binding to the 67-kDa laminin receptor (67LR). Therefore, in this study, we determined whether quercetin and its conjugates bind 67LR to induce neuroprotection and compared their ability with EGCG. Based on the quenching of intrinsic tryptophan fluorescence of peptide G (residues 161-180 in 67LR), we found quercetin, quercetin-3-O-glucuronide, and quercetin-3-O-sulfate bind to this peptide with a high affinity comparable to EGCG. Molecular docking using the crystal structure of 37-kDa laminin receptor precursor supported the high-affinity binding of all these ligands to the site corresponding to peptide G. A pretreatment with quercetin (1-1000 nM) did not effectively protect Neuroscreen-1 cells from death induced by serum starvation. Contrarily, a pretreatment with low concentrations (1-10 nM) of quercetin conjugates better protected these cells than quercetin and EGCG. The 67LR-blocking antibody substantially prevented neuroprotection by all these agents, suggesting the role of 67LR in this process. Collectively, these studies reveal that quercetin induces neuroprotection primarily through its conjugates via high affinity binding to 67LR.

Emerging roles of the cellular prion protein (PrPC) and 37/67 kDa laminin receptor (RPSA) interaction in cancer biology

The cellular prion protein (PrPC) is well-known for its involvement, under its pathogenic protease-resistant form (PrPSc), in a group of neurodegenerative diseases, known as prion diseases. PrPC is expressed in nervous system, as well as in other peripheral organs, and has been found overexpressed in several types of solid tumors. Notwithstanding, studies in recent years have disclosed an emerging role for PrPC in various cancer associated processes. PrPC has high binding affinity for 37/67 kDa laminin receptor (RPSA), a molecule that acts as a key player in tumorigenesis, affecting cell growth, adhesion, migration, invasion and cell death processes. Recently, we have characterized at cellular level, small molecules able to antagonize the direct PrPC binding to RPSA and their intracellular trafficking. These findings are very crucial considering that the main function of RPSA is to modulate key events in the metastasis cascade. Elucidation of the role played by PrPC/RPSA interaction in regulating tumor development, progression and response to treatment, represents a very promising challenge to gain pathogenetic information and discover novel specific biomarkers and/or therapeutic targets to be exploited in clinical settings. This review attempts to convey a detailed description of the complexity surrounding these multifaceted proteins from the perspective of cancer hallmarks, but with a specific focus on the role of their interaction in the control of proliferation, migration and invasion, genome instability and mutation, as well as resistance to cell death controlled by autophagic pathway.

cAMP-induced decrease in cell-surface laminin receptor and cellular prion protein attenuates amyloid-β uptake and amyloid-β-induced neuronal cell death

Previous studies have shown that amyloid-β oligomers (AβO) bind with high affinity to cellular prion protein (PrP^C ). The AβO-PrP^C complex binds to cell-surface co-receptors, including the laminin receptor (67LR). Our current studies revealed that in Neuroscreen-1 cells, 67LR is the major co-receptor involved in the cellular uptake of AβO and AβΟ-induced cell death. Both pharmacological (dibutyryl-cAMP, forskolin and rolipram) and physiological (pituitary adenylate cyclase-activating polypeptide) cAMP-elevating agents decreased cell-surface PrP^C and 67LR, thereby attenuating the uptake of AβO and the resultant neuronal cell death. These cAMP protective effects are dependent on protein kinase A, but not dependent on the exchange protein directly activated by cAMP. Conceivably, cAMP protects neuronal cells from AβO-induced cytotoxicity by decreasing cell-surface-associated PrP^C and 67LR.

An engineered CD81-based combinatorial library for selecting recombinant binders to cell surface proteins: Laminin binding CD81 enhances cellular uptake of extracellular vesicles

The research of extracellular vesicles (EVs) has boomed in the last decade, with the promise of them functioning as target-directed drug delivery vehicles, able to modulate proliferation, migration, differentiation, and other properties of the recipient cell that are vital for health of the host organism. To enhance the ability of their targeted delivery, we employed an intrinsically overrepresented protein, CD81, to serve for recognition of the desired target antigen. Yeast libraries displaying mutant variants of the large extracellular loop of CD81 have been selected for binders to human placental laminin as an example target. Their specific interaction with laminin was confirmed in a mammalian display system. Derived sequences were reformatted to full-length CD81 and expressed in EVs produced by HeLa cells. These EVs were examined for the presence of the recombinant protein and were shown to exhibit an enhanced uptake into laminin-secreting mammalian cell lines. For the best candidate, the specificity of antigen interaction was demonstrated with a competition experiment. To our knowledge, this is the first example of harnessing an EV membrane protein as mediator of de novo target antigen recognition via in vitro molecular evolution, opening horizons to a broad range of applications in various therapeutic settings.

Rpsa Signaling Regulates Cortical Neuronal Morphogenesis via Its Ligand, PEDF, and Plasma Membrane Interaction Partner, Itga6

Neuromorphological defects underlie neurodevelopmental disorders and functional defects. We identified a function for Rpsa in regulating neuromorphogenesis using in utero electroporation to knockdown Rpsa, resulting in apical dendrite misorientation, fewer/shorter extensions, and decreased spine density with altered spine morphology in upper neuronal layers and decreased arborization in upper/lower cortical layers. Rpsa knockdown disrupts multiple aspects of cortical development, including radial glial cell fiber morphology and neuronal layering. We investigated Rpsa's ligand, PEDF, and interacting partner on the plasma membrane, Itga6. Rpsa, PEDF, and Itga6 knockdown cause similar phenotypes, with Rpsa and Itga6 overexpression rescuing morphological defects in PEDF-deficient neurons in vivo. Additionally, Itga6 overexpression increases and stabilizes Rpsa expression on the plasma membrane. GCaMP6s was used to functionally analyze Rpsa knockdown via ex vivo calcium imaging. Rpsa-deficient neurons showed less fluctuation in fluorescence intensity, suggesting defective subthreshold calcium signaling. The Serpinf1 gene coding for PEDF is localized at chromosome 17p13.3, which is deleted in patients with the neurodevelopmental disorder Miller-Dieker syndrome. Our study identifies a role for Rpsa in early cortical development and for PEDF-Rpsa-Itga6 signaling in neuromorphogenesis, thus implicating these molecules in the etiology of neurodevelopmental disorders like Miller-Dieker syndrome and identifying them as potential therapeutics.

Caveolae/rafts protect human cerebral microvascular endothelial cells from Streptococcus suis serotype 2 α-enolase-mediated injury

Streptococcus suis serotype 2 (SS2) is an important zoonotic pathogen that causes meningitis. The ubiquitously expressed 40S ribosome protein SA (RPSA) is a multifunctional protein involved in the pathogenesis of multiple pathogens, especially those causing meningitis. However, the role of RPSA in SS2-induced meningitis is not clear. In this study, immunofluorescence staining revealed that SS2 infection promoted the intracellular transfer of RPSA to the surface of human cerebral microvascular endothelial cells (HCMECs). Moreover, SS2 infection promoted the accumulation of caveolin 1 (CAV1) and the formation of membrane bulges where RPSA enveloped CAV1 on the cell surface. SS2 infection also caused dynamic changes in the localization of RPSA and CAV1 on the cell surface which could be eliminated by disruption of caveolae/rafts by addition of methyl-β-cyclodextrin (MβCD). Co-immunoprecipitation analysis demonstrated that α-enolase (ENO), a key virulence factor of SS2, interacted with RPSA, and promoted the interaction between RPSA and CAV1. Immunofluorescence staining, western blotting and flow cytometry analyses showed that damaged caveolae/rafts significantly enhanced ENO adhesion to HCMECs, promoted the "destruction" of RPSA by ENO, and enhanced the toxic effect of ENO on HCMECs. Importantly, these effects could be relieved upon the addition of cholesterol. We conclude that caveolae/rafts weaken the toxic effect of SS2 ENO on RPSA-mediated events in HCMECs. Our study has led to better understanding of the roles of RPSA and caveolae/rafts upon SS2 infection, and a new pathological role for RPSA in infection.

Cyclic-AMP induces Nogo-A receptor NgR1 internalization and inhibits Nogo-A-mediated collapse of growth cone

The promotion of axonal regeneration is required for functional recovery from stroke and various neuronal injuries. However, axonal regeneration is inhibited by diverse axonal growth inhibitors, such as Nogo-A. Nogo-66, a C-terminal domain of Nogo-A, binds to the Nogo-A receptor 1 (NgR1) and induces the collapse of growth cones and inhibits neurite outgrowth. NgR1 is also a receptor for additional axonal growth inhibitors, suggesting it is an important target for the prevention of axonal growth inhibition. By using the indirect immunofluorescence method, we show for the first time that a cell-permeable cAMP analog (dibutyryl-cAMP) induced a rapid decrease in the cell surface expression of NgR1 in Neuroscreen-1 (NS-1) cells. The biotinylation method revealed that cAMP indeed induced internalization of NgR1 within minutes. Other intracellular cAMP-elevating agents, such as forskolin, which directly activates adenylyl cyclase, and rolipram, which inhibits cyclic nucleotide phosphodiesterase, also induced this process. This internalization was found to be reversible and influenced by intracellular levels of cAMP. Using selective activators and inhibitors of protein kinase A (PKA) and the exchange protein directly activated by cAMP (Epac), we found that NgR1 internalization is independent of PKA, but dependent on Epac. The decrease in cell surface expression of NgR1 desensitized NS-1 cells to Nogo-66-induced growth cone collapse. Therefore, it is likely that besides axonal growth inhibitors affecting neurons, neurons themselves also self-regulate their sensitivity to axonal growth inhibitors, as influenced by intracellular cAMP/Epac. This normal cellular regulatory mechanism may be pharmacologically exploited to overcome axonal growth inhibitors, and enhance functional recovery after stroke and neuronal injuries.

Novel anti-angiogenic PEDF-derived small peptides mitigate choroidal neovascularization

Abnormal migration and proliferation of endothelial cells (EC) drive neovascular retinopathies. While anti-VEGF treatment slows progression, pathology is often supported by decrease in intraocular pigment epithelium-derived factor (PEDF), an endogenous inhibitor of angiogenesis. A surface helical 34-mer peptide of PEDF, comprising this activity, is efficacious in animal models of neovascular retina disease but remains impractically large for therapeutic use. We sought smaller fragments within this sequence that mitigate choroidal neovascularization (CNV). Expecting rapid intravitreal (IVT) clearance, we also developed a method to reversibly attach peptides to nano-carriers for extended delivery. Synthetic fragments of 34-mer yielded smaller anti-angiogenic peptides, and N-terminal capping with dicarboxylic acids did not diminish activity. Charge restoration via substitution of an internal aspartate by asparagine improved potency, achieving low nM apoptotic response in VEGF-activated EC. Two optimized peptides (PEDF 335, 8-mer and PEDF 336, 9-mer) were tested in a mouse model of laser-induced CNV. IVT injection of either peptide, 2-5 days before laser treatment, gave significant CNV decrease at day +14 post laser treatment. The 8-mer also decreased CNV, when administered as eye drops. Also examined was a nanoparticle-conjugate (NPC) prodrug of the 9-mer, having positive zeta potential, expected to display longer intraocular residence. This NPC showed extended efficacy, even when injected 14 days before laser treatment. Neither inflammatory cells nor other histopathologic abnormalities were seen in rabbit eyes harvested 14 days following IVT injection of PEDF 336 (>200 μg). No rabbit or mouse eye irritation was observed over 12-17 days of PEDF 335 eye drops (10 mM). Viability was unaffected in 3 retinal and 2 choroidal cell types by PEDF 335 up to 100 μM, PEDF 336 (100 μM) gave slight growth inhibition only in choroidal EC. A small anti-angiogenic PEDF epitope (G-Y-D-L-Y-R-V) was identified, variants (adipic-Sar-Y-N-L-Y-R-V) mitigate CNV, with clinical potential in treating neovascular retinopathy. Their shared active motif, Y - - - R, is found in laminin (Ln) peptide YIGSR, which binds Ln receptor 67LR, a known high-affinity ligand of PEDF 34-mer.

Proteomic Study of a Parkinson's Disease Model of Undifferentiated SH-SY5Y Cells Induced by a Proteasome Inhibitor

Parkinson's disease (PD) is one of the most common nervous system degenerative diseases. However, the etiology of this disease remains elusive. Here, a proteasome inhibitor (PSI)-induced undifferentiated SH-SY5Y PD model was established to analyze protein alterations through proteomic study.

METHODS

Cultured undifferentiated SH-SY5Y cells were divided into a control group and a group treated with 2.5 µM PSI (PSI-treated group). An methyl thiazolyl tetrazolium (MTT) assay was applied to detect cell viability. Acridine orange/ethidium bromide (AO/EB), α-synuclein immunofluorescence and hematoxylin and eosin (H&E) staining were applied to evaluate apoptosis and cytoplasmic inclusions, respectively. The protein spots that were significantly changed were separated, analyzed by 2D gel electrophoresis and DIGE De Cyder software, and subsequently identified by MALDI-TOF mass spectrometry and database searching.

RESULTS

The results of the MTT assay showed that there was a time and dose dependent change in cell viability following incubation with PSI. After 24 h incubation, PSI resulted in early apoptosis, and cytoplasmic inclusions were found in the PSI-treated group through H&E staining and α-synuclein immunofluorescence. Thus, undifferentiated SH-SY5Y cells could be used as PD model following PSI-induced inhibition of proteasomal function. In total, 18 proteins were differentially expressed between the groups, 7 of which were up-regulated and 11 of which were down-regulated. Among them, 5 protein spots were identified as being involved in the ubiquitin proteasome pathway-induced PD process.

CONCLUSIONS

Mitochondrial heat shock protein 75 (MTHSP75), phosphoglycerate dehydrogenase (PHGDH), laminin binding protein (LBP), tyrosine 3/tryptophan 5-monooxygenase activation protein (14-3-3ε) and YWHAZ protein (14-3-3ζ) are involved in mitochondrial dysfunction, serine synthesis, amyloid clearance, apoptosis process and neuroprotection. These findings may provide new clues to deepen our understanding of PD pathogenesis.