Intracellular trafficking and secretion of mouse mesencephalic astrocyte-derived neurotrophic factor

Navigating the Landscape of MANF Research: A Scientometric Journey with CiteSpace Analysis

This study employs bibliometric analysis through CiteSpace to comprehensively evaluate the status and trends of MANF (mesencephalic astrocyte-derived neurotrophic factor) research spanning 25 years (1997-2022). It aims to fill the gap in objective and comprehensive reviews of MANF research. MANF-related studies were extracted from the Web of Science database. MANF publications were quantitatively and qualitatively analyzed for various factors by CiteSpace, including publication volume, journals, countries/regions, institutions, and authors. Keywords and references were visually analyzed to unveil research evolution and hotspot. Analysis of 353 MANF-related articles revealed escalating annual publications, indicating growing recognition of MANF's importance. High-impact journals such as the International Journal of Molecular Sciences and Journal of Biological Chemistry underscored MANF's interdisciplinary significance. Collaborative networks highlighted China and the USA's pivotal roles, while influential figures and partnerships drove understanding of MANF's mechanisms. Co-word analysis of MANF-related keywords exposed key evolutionary hotspots, encompassing neurotrophic effects, cytoprotective roles, MANF-related diseases, and the CDNF/MANF family. This progression from basic understanding to clinical potential showcased MANF's versatility from cellular protection to therapy. Bibliometric analysis reveals MANF's diverse research trends and pathways, from basics to clinical applications, driving medical progress. This comprehensive assessment enriches understanding and empowers researchers for dynamic evolution, advancing innovation, and benefiting patients. Bibliometric analysis of MANF research. The graphical abstract depicts the bibliometric analysis of MANF research, highlighting its aims, methods, and key results.

Effect of mesencephalic astrocyte-derived neurotrophic factor on the inflammatory response in human gingival fibroblasts cells

Mesencephalic astrocyte-derived neurotrophic factor (MANF) is a unique member of the neurotrophic factor family residing in the endoplasmic reticulum, where it functions as a stress response protein maintaining endoplasmic reticulum homeostasis, in addition to being secreted extracellularly as a neurotrophic factor to bind with receptors to initiate intracellular signal transduction pathways. Interestingly, MANF has shown an important protective role in the inflammatory response of many diseases. In neural stem cells, pancreatic β cells, and retinal cells, MANF can inhibit the inflammatory response, modulate the immune response, and promote tissue repair. However, the role of MANF in the periodontal inflammatory response remains unclear. In the present study, we used lipopolysaccharide (LPS) from Porphyromonas gingivalis (Pg) to establish a Pg-LPS-stimulated periodontal inflammatory model in human gingival fibroblasts cells (HGF-1) to investigate the role of MANF in vitro. We found that MANF could inhibit pro-inflammatory cytokine secretion, alleviate the endoplasmic reticulum stress response, promote cell survival, and inhibit cell apoptosis. Therefore, MANF might be a novel promising target for the treatment of periodontitis.

Upregulation of mesencephalic astrocyte-derived neurotrophic factor (MANF) expression offers protection against alcohol neurotoxicity

Alcohol exposure has detrimental effects on both the developing and mature brain. Endoplasmic reticulum (ER) stress is one of the mechanisms that contributes to alcohol-induced neuronal damages. Mesencephalic astrocyte-derived neurotrophic factor (MANF) is an ER stress-responsive protein and is neuroprotective in multiple neuronal injury and neurodegenerative disease models. MANF deficiency has been shown to exacerbate alcohol-induced ER stress and neurodegeneration. However, it is unknown whether MANF supplement is sufficient to protect against alcohol neurotoxicity. Alcohol alters MANF expression in the brain, but the mechanisms underlying alcohol modulation of MANF expression remain unclear. This study was designed to determine how alcohol alters MANF expression in neuronal cells and whether exogeneous MANF can alleviate alcohol neurotoxicity. We showed that alcohol increased MANF transcription and secretion without affecting MANF mRNA stability and protein degradation. ER stress was necessary for alcohol-induced MANF upregulation, as pharmacological inhibition of ER stress by 4-PBA diminished alcohol-induced MANF expression. In addition, the presence of ER stress response element II (ERSE-II) was required for alcohol-stimulated MANF transcription. Mutations or deletion of this sequence abolished alcohol-regulated transcriptional activity. We generated MANF knockout (KO) neuronal cells using CRISPR/Cas9. MANF KO cells exhibited increased unfolded protein response (UPR) and were more susceptible to alcohol-induced cell death. On the other hand, MANF upregulation by the addition of recombinant MANF protein or adenovirus gene transduction protected neuronal cells against alcohol-induced cell death. Further studies using early postnatal mouse pups demonstrated that enhanced MANF expression in the brain by intracerebroventricular (ICV) injection of MANF adeno-associated viruses ameliorated alcohol-induced cell death. Thus, alcohol increased MANF expression through inducing ER stress, which could be a protective response. Exogenous MANF was able to protect against alcohol-induced neurodegeneration.

CRELD2, endoplasmic reticulum stress, and human diseases

CRELD2, a member of the cysteine-rich epidermal growth factor-like domain (CRELD) protein family, is both an endoplasmic reticulum (ER)-resident protein and a secretory factor. The expression and secretion of CRELD2 are dramatically induced by ER stress. CRELD2 is ubiquitously expressed in multiple tissues at different levels, suggesting its crucial and diverse roles in different tissues. Recent studies suggest that CRELD2 is associated with cartilage/bone metabolism homeostasis and pathological conditions involving ER stress such as chronic liver diseases, cardiovascular diseases, kidney diseases, and cancer. Herein, we first summarize ER stress and then critically review recent advances in the knowledge of the characteristics and functions of CRELD2 in various human diseases. Furthermore, we highlight challenges and present future directions to elucidate the roles of CRELD2 in human health and disease.

Emerging trophic activities of mesencephalic astrocyte-derived neurotrophic factor in tissue repair and regeneration

Mesencephalic astrocyte-derived neurotrophic factor (MANF) is a soluble endoplasmic reticulum (ER) luminal protein and its expression and secretion can be induced by ER stress. Despite initially being classified as a neurotrophic factor, MANF has been demonstrated to have restorative and protective effects in many different cell types such as neurons, liver cells, retinal cells, cardiac myocytes, and pancreatic β cells. However, underlying molecular mechanisms are complex and remain incompletely understood. The aims of this review are to highlight the latest advances in the understanding of the trophic activities of MANF in tissue repair and regeneration as well as underlying molecular mechanisms. The structural motifs and immune modulation of MANF are also described. We therefore propose that MANF might be a promising therapeutic target for tissue repair.

Mesencephalic astrocyte-derived neurotrophic factor (MANF): Structure, functions and therapeutic potential

Mesencephalic astrocyte-derived neurotrophic factor (MANF) is a novel evolutionarily conserved protein present in both vertebrate and invertebrate species. MANF shows distinct structural and functional properties than the traditional neurotrophic factors (NTF). MANF is composed of an N-terminal saposin-like lipid-binding domain and a C-terminal SAF-A/B, Acinus and PIAS (SAP) domain connected by a short linker. The two well-described activities of MANF include (1) role as a neurotrophic factor that plays direct neuroprotective effects in the nervous system and (2) cell protective effects in the animal models of non-neuronal diseases, including retinal damage, diabetes mellitus, liver injury, myocardial infarction, nephrotic syndrome, etc. The main objective of the current review is to provide up-to-date insights regarding the structure of MANF, mechanisms regulating its expression and secretion, physiological functions in various tissues and organs, protective effects during aging, and potential clinical applications. Together, this review highlights the importance of MANF in reversing age-related dysfunction and geroprotection.

DLC1 inhibits colon adenocarcinoma cell migration by promoting secretion of the neurotrophic factor MANF

DLC1 (deleted in liver cancer-1) is downregulated or deleted in colorectal cancer (CRC) tissues and functions as a potent tumor suppressor, but the underlying molecular mechanism remains elusive. We found that the conditioned medium (CM) collected from DLC1-overexpressed SW1116 cells inhibited the migration of colon adenocarcinoma cells HCT116 and SW1116, but had no effect on proliferation, which suggested DLC1-mediated secretory components containing a specific inhibitor for colon adenocarcinoma cell migration. Analysis by mass spectrometry identified mesencephalic astrocyte-derived neurotrophic factor (MANF) as a candidate. More importantly, exogenous MANF significantly inhibited the migration of colon adenocarcinoma cells HCT116 and SW1116, but did not affect proliferation. Mechanistically, DLC1 reduced the retention of MANF in ER by competing the interaction between MANF and GRP78. Taken together, these data provided new insights into the suppressive effects of DLC1 on CRC, and revealed the potential of MANF in the treatment of CRC.

Potential Role of MANF, an ER Stress Responsive Neurotrophic Factor, in Protecting Against Alcohol Neurotoxicity

Alcohol exposure during pregnancy is harmful to the fetus and causes a wide range of long-lasting physiological and neurocognitive impairments, collectively referred to as fetal alcohol spectrum disorders (FASD). The neurobehavioral deficits observed in FASD result from structural and functional damages in the brain, with neurodegeneration being the most destructive consequence. Currently, there are no therapies for FASD. It is exigent to delineate the underlying mechanisms of alcohol neurotoxicity and develop an effective strategy of treatment. ER stress, caused by the accumulation of unfolded/misfolded proteins in the ER, is the hallmark of many neurodegenerative diseases, including alcohol-induced neurodegeneration. Mesencephalic astrocyte-derived neurotrophic factor (MANF) is a newly discovered endoplasmic reticulum (ER) stress responsive neurotrophic factor that regulates diverse neuronal functions. This review summarizes the recent findings revealing the effects of MANF on the CNS and its protective role against neurodegeneration. Particularly, we focus the role of MANF on alcohol-induced ER stress and neurodegeneration and discuss the therapeutic potential of MANF in treating alcohol neurotoxicity such as FASD.

Cerebral dopamine neurotrophic factor protects and repairs dopamine neurons by novel mechanism

Midbrain dopamine neurons deteriorate in Parkinson's disease (PD) that is a progressive neurodegenerative movement disorder. No cure is available that would stop the dopaminergic decline or restore function of injured neurons in PD. Neurotrophic factors (NTFs), e.g., glial cell line-derived neurotrophic factor (GDNF) are small, secreted proteins that promote neuron survival during mammalian development and regulate adult neuronal plasticity, and they are studied as potential therapeutic agents for the treatment of neurodegenerative diseases. However, results from clinical trials of GDNF and related NTF neurturin (NRTN) in PD have been modest so far. In this review, we focus on cerebral dopamine neurotrophic factor (CDNF), an unconventional neurotrophic protein. CDNF delivered to the brain parenchyma protects and restores dopamine neurons in animal models of PD. In a recent Phase I-II clinical trial CDNF was found safe and well tolerated. CDNF deletion in mice led to age-dependent functional changes in the brain dopaminergic system and loss of enteric neurons resulting in slower gastrointestinal motility. These defects in Cdnf^-/- mice intriguingly resemble deficiencies observed in early stage PD. Different from classical NTFs, CDNF can function both as an extracellular trophic factor and as an intracellular, endoplasmic reticulum (ER) luminal protein that protects neurons and other cell types against ER stress. Similarly to the homologous mesencephalic astrocyte-derived neurotrophic factor (MANF), CDNF is able to regulate ER stress-induced unfolded protein response (UPR) signaling and promote protein homeostasis in the ER. Since ER stress is thought to be one of the pathophysiological mechanisms contributing to the dopaminergic degeneration in PD, CDNF, and its small-molecule derivatives that are under development may provide useful tools for experimental medicine and future therapies for the treatment of PD and other neurodegenerative protein-misfolding diseases.

Mesencephalic astrocyte-derived neurotrophic factor restores blood-brain barrier integrity of aged mice after ischaemic stroke/reperfusion through anti-inflammation via TLR4/MyD88/NF-κB pathway

Ischaemic stroke remains a leading cause of disability and mortality worldwide and ageing-associated inflammation for the aged patients specifically leads to worse post-stroke blood-brain barrier (BBB) disruption than young subjects. Accordingly, suppression of excessive inflammation can alleviate BBB injury, which provides potential therapeutic treatment for ischaemic stroke of the aged. Prior studies revealed that mesencephalic astrocyte-derived neurotrophic factor (MANF) regulated inflammatory response and alleviated liver injury in ageing. However, it is unclear whether MANF confer similar benefit to BBB of aged mice suffered from ischaemic stroke. Transient cerebral ischaemia induced by middle cerebral artery occlusion (MCAO) was conducted in aged mice (18-20 months old). MANF was injected into the right lateral ventricle 2 h after MCAO. BBB integrity, tight junctional proteins, ultrastructure of microvessels, infarct volume, neurological scores, brain water content, pro-inflammatory cytokines and neutrophil infiltration rate were determined 72 h after MCAO. H₂O₂-induced senescent bEnd.3 cells were applied in the in vitro study to investigate the possible mechanism. First, we confirmed that ischaemic stroke/reperfusion in senescent condition promoted the over-expression of MANF on brain endothelial cells. Then, MANF supplement could suppress the pro-inflammatory factor production, restore BBB integrity and then alleviate infarct volume, neurological scores, brain water content and neutrophil infiltration rate. In addition, MANF maintained BBB integrity after ischaemic stroke of aged condition dependent on TLR4/MyD88/NF-κB pathway via intervention of pro-inflammatory factors production. In summary, the recognition of MANF in the process of BBB breakdown at aged condition may offer novel therapeutic approaches for ischaemic stroke.

MANF: A Novel Endoplasmic Reticulum Stress Response Protein-The Role in Neurological and Metabolic Disorders

The mesencephalic astrocyte-derived neurotrophic factor (MANF), also named as arginine-rich protein (ARP) or arginine-rich mutated in early-stage tumors (ARMET), is a novel evolutionary conserved protein related to unfolded protein response. Growing evidence suggests that MANF critically involves in many ER stress-related diseases with a protective effect. Here, we review the function of MANF based on its structure in neurological and metabolic disorders and summarize its potential applications in disease diagnosis and therapies.

The Pancreatic ß-cell Response to Secretory Demands and Adaption to Stress

Pancreatic β cells dedicate much of their protein translation capacity to producing insulin to maintain glucose homeostasis. In response to increased secretory demand, β cells can compensate by increasing insulin production capability even in the face of protracted peripheral insulin resistance. The ability to amplify insulin secretion in response to hyperglycemia is a critical facet of β-cell function, and the exact mechanisms by which this occurs have been studied for decades. To adapt to the constant and fast-changing demands for insulin production, β cells use the unfolded protein response of the endoplasmic reticulum. Failure of these compensatory mechanisms contributes to both type 1 and 2 diabetes. Additionally, studies in which β cells are "rested" by reducing endogenous insulin demand have shown promise as a therapeutic strategy that could be applied more broadly. Here, we review recent findings in β cells pertaining to the metabolic amplifying pathway, the unfolded protein response, and potential advances in therapeutics based on β-cell rest.

MANF Is Neuroprotective in Early Stages of EAE, and Elevated in Spinal White Matter by Treatment With Dexamethasone

Multiple sclerosis (MS) is a progressive autoimmune disease characterized by T-cell mediated demyelination in central nervous system (CNS). Experimental autoimmune encephalomyelitis (EAE) is a widely used in vivo disease model of MS. Glucocorticoids such as dexamethasone (dex) function as immunosuppressants and are commonly used to treat acute exacerbations of MS. Dex is also often used as a positive control in EAE studies, as it has been shown to promote motor behavior, inhibit immune cell infiltration into the CNS and regulate the activation of glial cell in EAE. This study further validated the effects of intravenously administrated dex by time-dependent fashion in EAE. Dex postponed clinical signs and motor defects in early stages of EAE. Histological analysis revealed that the degeneration of myelin and axons, as well as the infiltration of peripheral immune cells into the white matter of spinal cord was inhibited by dex in early stages of EAE. Additionally, dex-treatment delayed the neuroinflammatory activation of microglia and astrocytes. Furthermore, this study analyzed the expression of the neurotrophic factor mesencephalic astrocyte-derived neurotrophic factor (MANF) in EAE, and the effect of treatment with dex on MANF-expression. We show that in dex-treated EAE mice expression MANF increased within myelinated areas of spinal cord white matter. We also show that intravenous administration with hMANF in EAE mice improved clinical signs and motor behavior in the early stage of EAE. Our report gives insight to the progression of EAE by providing a time-dependent analysis. Moreover, this study investigates the link between MANF and the EAE model, and shows that MANF is a potential drug candidate for MS.

New Cardiomyokine Reduces Myocardial Ischemia/Reperfusion Injury by PI3K-AKT Pathway Via a Putative KDEL-Receptor Binding

Background CDNF (cerebral dopamine neurotrophic factor) belongs to a new family of neurotrophic factors that exert systemic beneficial effects beyond the brain. Little is known about the role of CDNF in the cardiac context. Herein we investigated the effects of CDNF under endoplasmic reticulum-stress conditions using cardiomyocytes (humans and mice) and isolated rat hearts, as well as in rats subjected to ischemia/reperfusion (I/R). Methods and Results We showed that CDNF is secreted by cardiomyocytes stressed by thapsigargin and by isolated hearts subjected to I/R. Recombinant CDNF (exoCDNF) protected human and mouse cardiomyocytes against endoplasmic reticulum stress and restored the calcium transient. In isolated hearts subjected to I/R, exoCDNF avoided mitochondrial impairment and reduced the infarct area to 19% when administered before ischemia and to 25% when administered at the beginning of reperfusion, compared with an infarct area of 42% in the untreated I/R group. This protection was completely abrogated by AKT (protein kinase B) inhibitor. Heptapeptides containing the KDEL sequence, which binds to the KDEL-R (KDEL receptor), abolished exoCDNF beneficial effects, suggesting the participation of KDEL-R in this cardioprotection. CDNF administered intraperitoneally to rats decreased the infarct area in an in vivo model of I/R (from an infarct area of ≈44% in the I/R group to an infarct area of ≈27%). Moreover, a shorter version of CDNF, which lacks the last 4 residues (CDNF-ΔKTEL) and thus allows CDNF binding to KDEL-R, presented no cardioprotective activity in isolated hearts. Conclusions This is the first study to propose CDNF as a new cardiomyokine that induces cardioprotection via KDEL receptor binding and PI3K/AKT activation.

Mesencephalic Astrocyte-Derived Neurotrophic Factor (MANF) Regulates Neurite Outgrowth Through the Activation of Akt/mTOR and Erk/mTOR Signaling Pathways

Neurite outgrowth is essential for brain development and the recovery of brain injury and neurodegenerative diseases. In this study, we examined the role of the neurotrophic factor MANF in regulating neurite outgrowth. We generated MANF knockout (KO) neuro2a (N2a) cell lines using clustered regularly interspaced short palindromic repeats (CRISPR)/Cas9 and demonstrated that MANF KO N2a cells failed to grow neurites in response to RA stimulation. Using MANF siRNA, this finding was confirmed in human SH-SY5Y neuronal cell line. Nevertheless, MANF overexpression by adenovirus transduction or addition of MANF into culture media facilitated the growth of longer neurites in RA-treated N2a cells. MANF deficiency resulted in inhibition of Akt, Erk, mTOR, and P70S6, and impaired protein synthesis. MANF overexpression on the other hand facilitated the growth of longer neurites by activating Akt, Erk, mTOR, and P70S6. Pharmacological blockade of Akt, Erk or mTOR eliminated the promoting effect of MANF on neurite outgrowth. These findings suggest that MANF positively regulated neurite outgrowth by activating Akt/mTOR and Erk/mTOR signaling pathways.

Trophic activities of endoplasmic reticulum proteins CDNF and MANF

Mesencephalic astrocyte-derived neurotrophic factor (MANF) and cerebral dopamine neurotrophic factor (CDNF) are endoplasmic reticulum (ER) luminal proteins that confer trophic activities in a wide range of tissues under diverse pathological conditions. Despite initially being classified as neurotrophic factors, neither protein structurally nor functionally resembles bona fide neurotrophic factors. Their highly homologous structures comprise a unique globular, saposin-like domain within the N-terminus joined by a flexible linker to a C-terminus containing a SAP-like domain, CXXC motif and an ER retention sequence. Neurotrophic factors exert effects by binding to cognate receptors in the plasma membrane; however, no cell surface receptors have been identified for MANF and CDNF. Both can act as unfolded protein response (UPR) genes that modulate the UPR and inflammatory processes. The trophic activity of MANF and CDNF extends beyond the central nervous system with MANF being crucial for the development of pancreatic β cells and both have trophic effects in a variety of diseases related to the liver, heart, skeletal tissue, kidney and peripheral nervous system. In this article, the unique features of MANF and CDNF, such as their structure and mechanisms of action related to ER stress and inflammation, will be reviewed. Recently identified interactions with lipids and membrane trafficking will also be described. Lastly, their function and therapeutic potential in different diseases including a recent clinical trial using CDNF to treat Parkinson's disease will be discussed. Collectively, this review will highlight MANF and CDNF as broad-acting trophic factors that regulate functions of the endoplasmic reticulum.

A soluble endoplasmic reticulum factor as regenerative therapy for Wolfram syndrome

Endoplasmic reticulum (ER) stress-mediated cell death is an emerging target for human chronic disorders, including neurodegeneration and diabetes. However, there is currently no treatment for preventing ER stress-mediated cell death. Here, we show that mesencephalic astrocyte-derived neurotrophic factor (MANF), a neurotrophic factor secreted from ER stressed cells, prevents ER stress-mediated β cell death and enhances β cell proliferation in cell and mouse models of Wolfram syndrome, a prototype of ER disorders. Our results indicate that molecular pathways regulated by MANF are promising therapeutic targets for regenerative therapy of ER stress-related disorders, including diabetes, retinal degeneration, neurodegeneration, and Wolfram syndrome.

Mesencephalic Astrocyte-Derived Neurotrophic Factor Is Upregulated with Therapeutic Fasting in Humans and Diet Fat Withdrawal in Obese Mice

Dietary restriction induces beneficial metabolic changes and prevents age-related deterioration. Mesencephalic astrocyte-derived neurotrophic factor (MANF) shows protective effects on cells in various models of degenerative diseases. Here we studied whether circulating concentrations of MANF are associated with fasting-induced positive effects. We quantified the levels of circulating MANF from 40 human subjects before and after therapeutic fasting. As measured by an enzyme-linked immunosorbent assay (ELISA), the mean concentration of plasma MANF increased after an average fasting of 15 days. Plasma MANF levels correlated inversely with adiponectin, a hormone that regulates metabolism, thus suggesting that MANF levels are related to metabolic homeostasis. To study the effects of dietary intervention on MANF concentrations in mice, we developed an ELISA for mouse MANF and verified its specificity using MANF knock-out (KO) tissue. A switch from high-fat to normal diet increased MANF levels and downregulated the expression of unfolded protein response (UPR) genes in the liver, indicating decreased endoplasmic reticulum (ER) stress. Liver MANF and serum adiponectin concentrations correlated inversely in mice. Our findings demonstrate that MANF expression and secretion increases with dietary intervention. The MANF correlation to adiponectin and its possible involvement in metabolic regulation and overall health warrants further studies.

Discovery of endoplasmic reticulum calcium stabilizers to rescue ER-stressed podocytes in nephrotic syndrome

Emerging evidence has established primary nephrotic syndrome (NS), including focal segmental glomerulosclerosis (FSGS), as a primary podocytopathy. Despite the underlying importance of podocyte endoplasmic reticulum (ER) stress in the pathogenesis of NS, no treatment currently targets the podocyte ER. In our monogenic podocyte ER stress-induced NS/FSGS mouse model, the podocyte type 2 ryanodine receptor (RyR2)/calcium release channel on the ER was phosphorylated, resulting in ER calcium leak and cytosolic calcium elevation. The altered intracellular calcium homeostasis led to activation of calcium-dependent cytosolic protease calpain 2 and cleavage of its important downstream substrates, including the apoptotic molecule procaspase 12 and podocyte cytoskeletal protein talin 1. Importantly, a chemical compound, K201, can block RyR2-Ser2808 phosphorylation-mediated ER calcium depletion and podocyte injury in ER-stressed podocytes, as well as inhibit albuminuria in our NS model. In addition, we discovered that mesencephalic astrocyte-derived neurotrophic factor (MANF) can revert defective RyR2-induced ER calcium leak, a bioactivity for this ER stress-responsive protein. Thus, podocyte RyR2 remodeling contributes to ER stress-induced podocyte injury. K201 and MANF could be promising therapies for the treatment of podocyte ER stress-induced NS/FSGS.

Mesencephalic astrocyte-derived neurotropic factor is an important factor in chondrocyte ER homeostasis

Mesencephalic astrocyte-derived neurotrophic factor (MANF) is an endoplasmic reticulum (ER) resident protein that can be secreted due to an imperfect KDEL motif. MANF plays a cytoprotective role in several soft tissues and is upregulated in conditions resulting from intracellular retention of mutant protein, including two skeletal diseases, metaphyseal chondrodysplasia, Schmid type (MCDS) and multiple epiphyseal dysplasia (MED). The role of MANF in skeletal tissue homeostasis is currently unknown. Interestingly, cartilage-specific deletion of Manf in a mouse model of MED resulted in increased disease severity, suggesting its upregulation may be chondroprotective. Treatment of MED chondrocytes with exogenous MANF led to a decrease in the cellular levels of BiP (GRP78), confirming MANF's potential to modulate ER stress responses. However, it did not alleviate the intracellular retention of mutant matrilin-3, suggesting that it is the intracellular MANF that is of importance in the pathobiology of skeletal dysplasias. The Col2Cre-driven deletion of Manf from mouse cartilage resulted in a chondrodysplasia-like phenotype. Interestingly, ablation of MANF in cartilage did not have extracellular consequences but led to an upregulation of several ER-resident chaperones including BiP. This apparent induction of ER stress in turn led to dysregulated chondrocyte apoptosis and decreased proliferation, resulting in reduced long bone growth. We have previously shown that ER stress is an underlying disease mechanism for several skeletal dysplasias. The cartilage-specific deletion of Manf described in this study phenocopies our previously published chondrodysplasia models, further confirming that ER stress itself is sufficient to disrupt skeletal growth and thus represents a potential therapeutic target.

MANF: A New Player in the Control of Energy Homeostasis, and Beyond

All human behaviors, including the control of energy homeostasis, are ultimately mediated by neuronal activities in the brain. Neurotrophic factors represent a protein family that plays important roles in regulating neuronal development, function, and survival. It has been well established that canonical neurotrophic factors, such as brain-derived neurotrophic factor (BDNF) and ciliary neurotrophic factor (CNTF), play important roles in the central regulation of energy homeostasis. Recently, a class of non-canonical neurotrophic factors, represented by mesencephalic astrocyte-derived neurotrophic factor (MANF), has been discovered. MANF is structurally and functionally distinct from those canonical neurotrophic factors, hence raising the issue of MANF being non-canonical. Nonetheless, emerging evidence suggests that MANF is critically involved in many neuronal activities. Here, we review our current understanding about the functions of MANF in the brain, with a primary focus on the control of energy homeostasis.

Mesencephalic astrocyte-derived neurotrophic factor (MANF), a new player in endoplasmic reticulum diseases: structure, biology, and therapeutic roles

Mesencephalic astrocyte-derived neurotrophic factor (MANF), a newly identified 18-kDa soluble protein, localizes to the luminal endoplasmic reticulum (ER), whose stress can stimulate MANF expression and secretion. In Drosophila and zebrafish, MANF regulates dopaminergic neuron development. In contrast, in mice, MANF deficiency leads to diabetes and activation of the unfolded protein response. Recent studies in rodent models have demonstrated that MANF mitigates diabetes, exerts neurotrophic function in neurodegenerative disease, protects cardiomyocytes and neurons in myocardial infarction and cerebral ischemia, respectively, and promotes immune cell phenotype switch from proinflammatory macrophages to prorepair anti-inflammatory macrophages. The cytoprotective mechanisms of MANF on ER stress are currently under active investigation. In addition, for the first time, we have discovered that MANF can potentially serve as a urinary ER stress biomarker in ER stress-mediated kidney disease. These studies have underscored the diagnostic and therapeutic importance of MANF in ER diseases.

Cerebral dopamine neurotrophic factor protects H9c2 cardiomyocytes from apoptosis

BACKGROUND

Cerebral dopamine neurotrophic factor (CDNF) has been studied in animal models of Parkinson's disease, where it was shown to repair and protect dopamine neurons. Alongside its neurotrophic activity, it can also localize in the endoplasmic reticulum (ER) acting as an ER stress response (ERSR) protein to maintain ER homeostasis. Since ER stress plays a major role in the development and progression of cardiovascular diseases, we investigated the role of CDNF in cardiomyocytes during ER stress.

MATERIAL AND METHODS

Initially, the expression of CDNF was tested by treating H9c2 cells with various concentrations of tunicamycin (TM) and performing reverse-transcriptase polymerase chain reaction and Western blotting. To evaluate the overexpression of CDNF in cardiomyocytes, H9c2 cells were transfected with pcDNA-CDNF and analyzed by Western blotting and indirect immunofluorescence microscopy. The effects of CDNF on cardiomyocytes during ER stress were analyzed with CCK-8 method and TUNEL staining using cells transfected with pcDNA-CDNF or pcDNA3.1. The percentage of TUNEL-positive cells was quantified as the apoptotic level.

RESULTS

Our results showed that CDNF protein expression can be induced by activation of ER stress in cultured cardiomyocytes. Moreover, overexpression of CDNF improved cell viability and protected cardiomyocytes from apoptosis induced by ER stress.

CONCLUSION

The findings presented here contribute toward identifying the physiological functions of CDNF in cardiovascular diseases.

Intrastriatally Infused Exogenous CDNF Is Endocytosed and Retrogradely Transported to Substantia Nigra

Cerebral dopamine neurotrophic factor (CDNF) protects the nigrostriatal dopaminergic (DA) neurons in rodent models of Parkinson’s disease and restores DA circuitry when delivered after these neurons have begun to degenerate. These DA neurons have been suggested to transport striatal CDNF retrogradely to the substantia nigra (SN). However, in cultured cells the binding and internalization of extracellular CDNF has not been reported. The first aim of this study was to examine the cellular localization and pharmacokinetic properties of recombinant human CDNF (rhCDNF) protein after its infusion into rat brain parenchyma. Second, we aimed to study whether the transport of rhCDNF from the striatum to the SN results from its retrograde transport via DA neurons or from its anterograde transport via striatal GABAergic projection neurons. We show that after intrastriatal infusion, rhCDNF diffuses rapidly and broadly, and is cleared with a half-life of 5.5 h. Confocal microscopy analysis of brain sections at 2 and 6 h after infusion of rhCDNF revealed its widespread unspecific internalization by cortical and striatal neurons, exhibiting different patterns of subcellular rhCDNF distribution. Electron microscopy analysis showed that rhCDNF is present inside the endosomes and multivesicular bodies. In addition, we present data that after intrastriatal infusion the rhCDNF found in the SN is almost exclusively localized to the DA neurons, thus showing that it is retrogradely transported.

Mesencephalic Astrocyte-Derived Neurotrophic Factor as a Urine Biomarker for Endoplasmic Reticulum Stress-Related Kidney Diseases

Endoplasmic reticulum (ER) stress and disrupted proteostasis contribute to the pathogenesis of a variety of glomerular and tubular diseases. Thus, it is imperative to develop noninvasive biomarkers for detecting ER stress in podocytes or tubular cells in the incipient stage of disease, when a kidney biopsy is not yet clinically indicated. Mesencephalic astrocyte-derived neurotrophic factor (MANF) localizes to the ER lumen and is secreted in response to ER stress in several cell types. Here, using mouse models of human nephrotic syndrome caused by mutant laminin β2 protein-induced podocyte ER stress and AKI triggered by tunicamycin- or ischemia-reperfusion-induced tubular ER stress, we examined MANF as a potential urine biomarker for detecting ER stress in podocytes or renal tubular cells. ER stress upregulated MANF expression in podocytes and tubular cells. Notably, urinary MANF excretion concurrent with podocyte or tubular cell ER stress preceded clinical or histologic manifestations of the corresponding disease. Thus, MANF can potentially serve as a urine diagnostic or prognostic biomarker in ER stress-related kidney diseases to help stratify disease risk, predict disease progression, monitor treatment response, and identify subgroups of patients who can be treated with ER stress modulators in a highly targeted manner.

Unconventional neurotrophic factors CDNF and MANF: Structure, physiological functions and therapeutic potential

Cerebral dopamine neurotrophic factor (CDNF) and mesencephalic astrocyte-derived neurotrophic factor (MANF) promote the survival of midbrain dopaminergic neurons which degenerate in Parkinson's disease (PD). However, CDNF and MANF are structurally and functionally clearly distinct from the classical, target-derived neurotrophic factors (NTFs) that are solely secreted proteins. In cells, CDNF and MANF localize in the endoplasmic reticulum (ER) and evidence suggests that MANF, and possibly CDNF, is important for the maintenance of ER homeostasis. MANF expression is particularly high in secretory tissues with extensive protein production and thus a high ER protein folding load. Deletion of MANF in mice results in a diabetic phenotype and the activation of unfolded protein response (UPR) in the pancreatic islets. However, information about the intracellular and extracellular mechanisms of MANF and CDNF action is still limited. Here we will discuss the structural motifs and physiological functions of CDNF and MANF as well as their therapeutic potential for the treatment of neurodegenerative diseases and diabetes. Currently available knockout models of MANF and CDNF in mice, zebrafish and fruit fly will increase information about the biology of these interesting proteins.

Elucidation of a novel phenformin derivative on glucose-deprived stress responses in HT-29 cells

Recently, we developed a variety of phenformin derivatives as selective antitumor agents. Based on previous findings, this study evaluated a promising compound, 2-(2-chlorophenyl)ethylbiguanide (2-Cl-Phen), on the basis of stress responses in the human colon cancer cell line HT-29 under a serum- and glucose-deprived condition. 2-Cl-Phen triggered morphological changes such as shrinkage and plasma membrane disintegration, as well as a decrease in mitochondrial activity and an increase in LDH leakage. To understand intracellular issues relating to 2-Cl-Phen, this study focused on the expression levels of ER stress-inducible genes and several oncogenic genes. Serum and glucose deprivation significantly induced a variety of ER stress-inducible genes, but a 12-h treatment of 2-Cl-Phen down-regulated expression of several ER stress-related genes, with the exception of GADD153. Interestingly, the expression levels of ATF6α, GRP78, MANF, and CRELD2 mRNA were almost completely decreased by 2-Cl-Phen. This study also observed that a 24-h treatment of 2-Cl-Phen attenuated the expression levels of GRP78, GADD153, and c-Myc protein. The decrease in c-Myc protein occurred before the fluctuation of GRP78 protein, while the expression of c-Myc mRNA showed little change with cotreatment of serum and glucose deprivation with 2-Cl-Phen. To further understand the 2-Cl-Phen-induced down-regulation of ATF6-related genes, this study investigated the stability of ATF6α and GRP78 proteins using NanoLuc-tagged constructs. The expression levels of NanoLuc-tagged ATF6α and GRP78 were significantly down-regulated by 2-Cl-Phen in the presence or absence of the translation inhibitor cycloheximide. Taken together, our novel phenformin derivative 2-Cl-Phen has the unique characteristic of diminishing tumor adaptive responses, especially the expression of ATF6-related genes, as well as that of c-Myc protein, in a transcriptional and posttranscriptional manner under a serum- and glucose-deprived condition. Further characterization of cytotoxic mechanisms related to phenformin derivatives may give new insights into developing additional promising anticancer agents.

Characterization of the Role of MANF in Regulating the Secretion of CRELD2

We recently demonstrated that the secretion of two novel endoplasmic reticulum (ER) stress-inducible proteins, cysteine-rich with epidermal growth factor (EGF)-like domains 2 (CRELD2) and mesencephalic astrocyte-derived neurotrophic factor (MANF), are oppositely regulated by the overexpression of 78 kDa glucose-regulated protein (GRP78). In the present study, we found that the co-transfection of CRELD2 and MANF remarkably enhanced the secretion of CRELD2 without affecting the expression level of GRP78. To identify the structural features of CRELD2 and MANF involved in this process, we generated several CRELD2 and MANF expression constructs. The deletion of the four C-terminal amino acids, either REDL in CRELD2 or RTDL in MANF, abolished the increased secretion of CRELD2 induced by the co-expression of MANF. The deleted mutation of MANF partially abolished the increased secretion of wild type CRELD2 (wtCRELD2) as a positive action of wild type MANF (wtMANF), even when we added the amino acid sequence RTDL at the C-terminus of each mutated MANF construct. Enhanced green fluorescent protein (EGFP), which was tagged with the signal peptide sequence at the N-terminus and four C-terminal amino acids (KEDL, REDL or RTDL), were retained intracellularly, but they did not enhance the secretion of wtCRELD2. Taken together, our data demonstrate that MANF is a factor in regulating the secretion of CRELD2 through four C-terminal amino acids, RTDL and REDL, and the fluctuation of intracellular MANF seems to potentiate the secretion of CRELD2.

A Comparative Analysis of the Molecular Features of MANF and CDNF

Cerebral dopamine neurotrophic factor (CDNF) is a paralogous protein of mesencephalic astrocyte-derived neurotrophic factor (MANF). Both proteins have been reported to show a common cytoprotective effect on dopaminergic neurons as a secretory protein containing the KDEL-like motif of the ER retrieval signal at the C-terminus, RTDL in MANF and [Q/K]TEL in CDNF among many species, although functions of paralogous proteins tend to differ from each other. In this study, we focused on post-translational regulations of their retention in the endoplasmic reticulum (ER) and secretion and performed comparative experiments on characterization of mouse MANF and mouse CDNF according to our previous report about biosynthesis and secretion of mouse MANF using a NanoLuc system. In this study, co-expression of glucose-regulated protein 78 kDa (GRP78), KDEL receptor 1 or mutant Sar1 into HEK293 cells similarly decreased MANF and CDNF secretion with some degree of variation. Next, we investigated whether CDNF affects the secretion of mouse cysteine-rich with EGF-like domains 2 (CRELD2) because mouse wild-type (wt) MANF but not its KDEL-like motif deleted mutant (ΔC_MANF) was found to promote the CRELD2 release from the transfected cells. Co-expressing CRELD2 with wt or ΔC CDNF, we found that CDNF and ΔC_MANF hardly elevated the CRELD2 secretion. We then investigated effects of the four or six C-terminal amino acids of MANF and CDNF on the CRELD2 secretion. As a result, co-transfection of mouse CDNF having the mouse MANF-type C-terminal amino acids (CDNF_RTDL and CDNF_SARTDL) increased the CRELD2 secretion to a small extent, but mouse CDNF having human CDNF-type ones (CDNF_KTEL and CDNF_HPKTEL) well increased the CRELD2 secretion. On the other hand, the replacement of C-terminal motifs of mouse MANF with those of mouse CDNF (MANF_QTEL and MANF_YPQTEL) enhanced the CRELD2 secretion, and the mouse MANF having human CDNF-type ones (MANF_KTEL and MANF_HPKTEL) dramatically potentiated the CRELD2 secretion. These results indicate that the secretion of mouse MANF and mouse CDNF is fundamentally regulated in the same manner and that the variation of four C-terminal amino acids in the MANF and CDNF among species might influence their intracellular functions. This finding could be a hint to identify physiological functions of MANF and CDNF.

Role of two sequence motifs of mesencephalic astrocyte-derived neurotrophic factor in its survival-promoting activity

Mesencephalic astrocyte-derived neurotrophic factor (MANF) is a prosurvival protein that protects the cells when applied intracellularly in vitro or extracellularly in vivo. Its protective mechanisms are poorly known. Here we studied the role of two short sequence motifs within the carboxy-(C) terminal domain of MANF in its neuroprotective activity: the CKGC sequence (a CXXC motif) that could be involved in redox reactions, and the C-terminal RTDL sequence, an endoplasmic reticulum (ER) retention signal. We mutated these motifs and analyzed the antiapoptotic effect and intracellular localization of these mutants of MANF when overexpressed in cultured sympathetic or sensory neurons. As an in vivo model for studying the effect of these mutants after their extracellular application, we used the rat model of cerebral ischemia. Even though we found no evidence for oxidoreductase activity of MANF, the mutation of CXXC motif completely abolished its protective effect, showing that this motif is crucial for both MANF's intracellular and extracellular activity. The RTDL motif was not needed for the neuroprotective activity of MANF after its extracellular application in the stroke model in vivo. However, in vitro the deletion of RTDL motif inactivated MANF in the sympathetic neurons where the mutant protein localized to Golgi, but not in the sensory neurons where the mutant localized to the ER, showing that intracellular MANF protects these peripheral neurons in vitro only when localized to the ER.

Key subdomains in the C-terminal of cerebral dopamine neurotrophic factor regulate the protein secretion

Cerebral dopamine neurotrophic factor (CDNF) is a novel evolutionary conserved neurotrophic factor (NTF) which can protect and repairs dopamine neurons in animal models of Parkinson's disease. The crystal structure of CDNF is dominated by eight α-helices and the 3-dimensional structure of CDNF consists of an N-terminal saposin-like domain and a C-terminal SAP-domain. In particular, the C-terminal domain contains two critical motifs, a CXXC motif and a putative ER retention signal sequences (KTEL) at the C-terminus, which indicate that CDNF may be involved in endoplasmic reticulum (ER) stress. In this study, we found that disrupting helix-7 in the C-terminal could significantly increase CDNF secretion. Moreover, we identified the 149aa-154aa is the key amino acids in helix-7. In all, these findings suggest that helix-7 in the C-terminal is important for CDNF secretion, which maybe potential affect CDNF function in ER stress.

Mesencephalic astrocyte-derived neurotrophic factor and cerebral dopamine neurotrophic factor: New endoplasmic reticulum stress response proteins

Mesencephalic astrocyte-derived neurotrophic factor (MANF) and cerebral dopamine neurotrophic factor (CDNF) are a novel evolutionary conserved neurotrophic factor (NTF) family. There are two distinct domains in MANF and CDNF 3-dimentional structure, N-terminal saposin-like domain and C-terminal SAP-domain, which suggest their unique mode of action. Although identified for their neurotrophic activity, recent studies have shown MANF and CDNF can protect cells during endoplasmic reticulum (ER) stress. This review summarizes the unique structure and related potential protective role for cells during ER stress of MANF and CDNF.

Characterization of the structural and functional determinants of MANF/CDNF in Drosophila in vivo model

Mammalian MANF and CDNF proteins are evolutionarily conserved neurotrophic factors that can protect and repair mammalian dopaminergic neurons in vivo. In Drosophila, the sole MANF protein (DmManf) is needed for the maintenance of dopaminergic neurites and dopamine levels. Although both secreted and intracellular roles for MANF and CDNF have been demonstrated, very little is known about the molecular mechanism of their action. Here, by using a transgenic rescue approach in the DmManf mutant background we show that only full-length MANF containing both the amino-terminal saposin-like and carboxy-terminal SAP-domains can rescue the larval lethality of the DmManf mutant. Independent N- or C-terminal domains of MANF, even when co-expressed together, fail to rescue. Deleting the signal peptide or mutating the CXXC motif in the C-terminal domain destroys the activity of full-length DmManf. Positively charged surface amino acids and the C-terminal endoplasmic reticulum retention signal are necessary for rescue of DmManf mutant lethality when DmManf is expressed in a restricted pattern. Furthermore, rescue experiments with non-ubiquitous expression reveals functional differences between the C-terminal domain of human MANF and CDNF. Finally, DmManf and its C-terminal domain rescue mammalian sympathetic neurons from toxin-induced apoptosis in vitro demonstrating functional similarity of the mammalian and fly proteins. Our study offers further insights into the functional conservation between invertebrate and mammalian MANF/CDNF proteins and reveals the importance of the C-terminal domain for MANF activity in vivo.

Armet/Manf and Creld2 are components of a specialized ER stress response provoked by inappropriate formation of disulphide bonds: implications for genetic skeletal diseases

Mutant matrilin-3 (V194D) forms non-native disulphide bonded aggregates in the rER of chondrocytes from cell and mouse models of multiple epiphyseal dysplasia (MED). Intracellular retention of mutant matrilin-3 causes endoplasmic reticulum (ER) stress and induces an unfolded protein response (UPR) including the upregulation of two genes recently implicated in ER stress: Armet and Creld2. Nothing is known about the role of Armet and Creld2 in human genetic diseases. In this study, we used a variety of cell and mouse models of chondrodysplasia to determine the genotype-specific expression profiles of Armet and Creld2. We also studied their interactions with various mutant proteins and investigated their potential roles as protein disulphide isomerases (PDIs). Armet and Creld2 were up-regulated in cell and/or mouse models of chondrodysplasias caused by mutations in Matn3 and Col10a1, but not Comp. Intriguingly, both Armet and Creld2 were also secreted into the ECM of these disease models following ER stress. Armet and Creld2 interacted with mutant matrilin-3, but not with COMP, thereby validating the genotype-specific expression. Substrate-trapping experiments confirmed Creld2 processed PDI-like activity, thus identifying a putative functional role. Finally, alanine substitution of the two terminal cysteine residues from the A-domain of V194D matrilin-3 prevented aggregation, promoted mutant protein secretion and reduced the levels of Armet and Creld2 in a cell culture model. We demonstrate that Armet and Creld2 are genotype-specific ER stress response proteins with substrate specificities, and that aggregation of mutant matrilin-3 is a key disease trigger in MED that could be exploited as a potential therapeutic target.

Mesencephalic astrocyte-derived neurotrophic factor (MANF) secretion and cell surface binding are modulated by KDEL receptors

Mesencephalic astrocyte-derived neurotrophic factor (MANF) is an endoplasmic reticulum (ER) stress-responsive protein with neuroprotective effects in animal models of neurodegeneration, but the underlying mechanism is not understood. We constructed a set of lentiviral vectors that contain or lack the highly conserved final four amino acids of MANF ("RTDL"), which resemble the canonical ER retention signal ("KDEL"), to study MANF regulation in neuroblastoma cells and rat primary cortical neurons. The RTDL sequence was required for both ER retention and secretory response to ER stress. Overexpression of KDEL receptor paralogs (KDELRs) differentially reduced MANF secretion but had no effect on MANF lacking RTDL. MANF binding to the plasma membrane also required the RTDL sequence and was inhibited with a peptide known to interact with KDELRs, suggesting MANF binds KDELRs at the surface. We detected surface localization of FLAG-tagged KDELRs, with levels increasing following ER stress. Our study provides new insight into the regulation of MANF trafficking and has implications for other secreted proteins containing a KDEL-like retention signal.