The potential of bone morphogenetic protein 2 as a neurotrophic factor for Parkinson's disease

Differentiation and regulation of CD4+ T cell subsets in Parkinson's disease

Parkinson's disease (PD) is the second most common neurodegenerative disease, and its hallmark pathological features are the loss of dopaminergic (DA) neurons in the midbrain substantia nigra pars compacta (SNpc) and the accumulation of alpha-synuclein (α-syn). It has been shown that the integrity of the blood-brain barrier (BBB) is damaged in PD patients, and a large number of infiltrating T cells and inflammatory cytokines have been detected in the cerebrospinal fluid (CSF) and brain parenchyma of PD patients and PD animal models, including significant change in the number and proportion of different CD4+ T cell subsets. This suggests that the neuroinflammatory response caused by CD4+ T cells is an important risk factor for the development of PD. Here, we systematically review the differentiation of CD4+ T cell subsets, and focus on describing the functions and mechanisms of different CD4+ T cell subsets and their secreted cytokines in PD. We also summarize the current immunotherapy targeting CD4+ T cells with a view to providing assistance in the diagnosis and treatment of PD.

Circulating plasma-derived extracellular vesicles expressing bone and kidney markers are associated with neurocognitive impairment in people living with HIV

Background

Although effective antiretroviral therapy (ART) has improved the life expectancy of people with HIV (PWH), the prevalence of milder forms of HIV-associated neurocognitive disorders (HAND) persist, and it is associated with systemic and neuro-inflammatory processes that could impact other organ systems. However, the complex signaling mechanisms between the bone-kidney systems and the brain in HAND remain unknown. Extracellular vesicles (EVs) play a potential role in inter-organ communication and are involved in regulating cell activity in distant tissues. In this study, we examined whether levels of EVs from bone-and kidney-related cells associate with cognitive dysfunction and explored the relationship between kidney-bone EV axis in PWH experiencing cognitive deficits.

Methods

EV subtypes were characterized in plasma from 61 PWH with either cognitive impairment (CI, n = 53) or normal cognition (NC, n = 8) based on the American Academy of Neurology criteria for HIV-associated dementia (HAD, n = 11), minor cognitive motor disorder (MCMD, n = 25) or asymptomatic neurocognitive impairment (ANI, n = 17) by spectral flow cytometry. EVs were profiled with markers reflecting bone and kidney cell origin. A support vector machine learning-based model was employed for analyses of EV phenotypes to predict the cognitive dysfunction.

Results

Plasma-EVs expressing osteocalcin, sclerostin, and nephrin were significantly higher in the cognitive impairment group compared to the normal cognition group. EVs bearing kidney cell markers correlated significantly with bone-derived EVs. A machine learning-based model, comprised of osteocalcin+, nephrin+, and CD24+ EVs predicted cognitive impairment in PWH on ART.

Conclusion

Our study reveals that neurocognitive impairment in PWH is associated with increased levels of plasma EVs enriched with the bone markers osteocalcin and sclerostin and the kidney marker nephrin, suggesting that these EV subtypes may be novel candidate biomarkers for disease-spanning neurocognitive dysfunction. Moreover, the relationship between bone-derived EVs with kidney-derived EVs may suggest their role in mediating inter-organ crosstalk in the pathogenesis of HIV-associated cognitive impairment.

Research progress of neural stem cells as a source of dopaminergic neurons for cell therapy in Parkinson's disease

Parkinson's disease (PD) is a neurodegenerative disease, the most characteristic pathological feature is the progressive loss of dopaminergic (DA) neurons in the substantia nigra pars compactus (SNpc) of the mesencephalon, along with reduced dopamine content in the striatum. Researchers have been searching for drugs and therapies to treat PD in decades. However, no approach could stop the progression of the disease, and even some of them caused adverse clinical side effects. PD has a well-defined lesion. Therefore, it is considered to be one of the most curable central nervous system diseases by cell replacement treatment. Fetal ventral mesencephalic tissue transplantation has been used to treat patients with PD and obtained positive treatment results. However, ethical issues, such as limited donor tissue, and side effects including graft-induced dyskinesias, limit its clinical applications. Neural stem cell (NSC) transplantation is a viable therapy choice because it possesses multipotency, self-renewal ability, and differentiation into DA neurons, which may substitute for lost DA neurons and slow down the neurodegenerative process in PD. Studies that investigated the delivery of NSCs by using animal models of PD revealed survival, migration, and even amelioration of behavioral deficits. Here, the research progress of NSCs or NSC-derived DA neurons in treating PD was reviewed, and the practicability of present manufacturing processes for clinical testing was considered. This review is expected to offer ideas for practical strategies to solve the present technical and biological problems related to the clinical application of NSCs in PD.

Brain-Derived Neurotrophic Factor - The Protective Agent Against Neurological Disorders

The burden of neurological illnesses on global health is significant. Our perception of the molecular and biological mechanisms underlying intellectual processing and behavior has significantly advanced over the last few decades, laying the groundwork for potential therapies for various neurodegenerative diseases. A growing body of literature reveals that most neurodegenerative diseases could be due to the gradual failure of neurons in the brain's neocortex, hippocampus, and various subcortical areas. Research on various experimental models has uncovered several gene components to understand the pathogenesis of neurodegenerative disorders. One among them is the brain-derived neurotrophic factor (BDNF), which performs several vital functions, enhancing synaptic plasticity and assisting in the emergence of long-term thoughts. The pathophysiology of some neurodegenerative diseases, including Alzheimer's, Parkinson's, Schizophrenia, and Huntington's, has been linked to BDNF. According to numerous research, high levels of BDNF are connected to a lower risk of developing a neurodegenerative disease. As a result, we want to concentrate on BDNF in this article and outline its protective role against neurological disorders.

Does the Blood-Brain Barrier Integrity Change in Regard to the Onset of Fetal Growth Restriction?

The aim of the study was to determine whether early-onset and late-onset fetal growth restriction (FGR) differentially affects the blood-brain barrier integrity. Furthermore, the purpose of the study was to investigate the relationship between the blood-brain barrier breakdown and neurological disorders in FGR newborns. To evaluate the serum tight junction (TJ) proteins and the placental TJ proteins expression, an ELISA method was used. A significant difference in serum OCLN concentrations was noticed in pregnancies complicated by the early-onset FGR, in relation to the intraventricular hemorrhage (IVH) occurrence in newborns. No significant differences in concentrations of the NR1 subunit of the N-methyl-d-aspartate receptor (NR1), nucleoside diphosphate kinase A (NME1), S100 calcium-binding protein B (S100B), occludin (OCLN), claudin-5 (CLN5), zonula occludens-1 (zo-1), the CLN5/zo-1 ratio, and the placental expression of OCLN, CLN5, claudin-4 (CLN4), zo-1 were noticed between groups. The early-onset FGR was associated with a higher release of NME1 into the maternal circulation in relation to the brain-sparing effect and premature delivery. Additionally, in late-onset FGR, the higher release of the S100B into the maternal serum in regard to fetal distress was observed. Furthermore, there was a higher release of zo-1 into the maternal circulation in relation to newborns' moderate acidosis in late-onset FGR. Blood-brain barrier disintegration is not dependent on pregnancy advancement at the time of FGR diagnosis. NME1 may serve as a biomarker useful in the prediction of fetal circulatory centralization and extremely low birth weight in pregnancies complicated by the early-onset FGR. Moreover, the serum zo-1 concentration may have prognostic value for moderate neonatal acidosis in late-onset FGR pregnancies.

Growth factors and molecular-driven plasticity in neurological systems

It has been almost 70 years since the discovery of nerve growth factor (NGF), a period of a dramatic evolution in our understanding of dynamic growth, regeneration, and rewiring of the nervous system. In 1953, the extraordinary finding that a protein found in mouse submandibular glands generated a halo of outgrowing axons has now redefined our concept of the nervous system connectome. Central and peripheral neurons and their axons or dendrites are no longer considered fixed or static "wiring." Exploiting this molecular-driven plasticity as a therapeutic approach has arrived in the clinic with a slate of new trials and ideas. Neural growth factors (GFs), soluble proteins that alter the behavior of neurons, have expanded in numbers and our understanding of the complexity of their signaling and interactions with other proteins has intensified. However, beyond these "extrinsic" determinants of neuron growth and function are the downstream pathways that impact neurons, ripe for translational development and potentially more important than individual growth factors that may trigger them. Persistent and ongoing nuances in clinical trial design in some of the most intractable and irreversible neurological conditions give hope for connecting new biological ideas with clinical benefits. This review is a targeted update on neural GFs, their signals, and new therapeutic ideas, selected from an expansive literature.

Ferritin confers protection against iron-mediated neurotoxicity and ferroptosis through iron chelating mechanisms in MPP+-induced MES23.5 dopaminergic cells

Ferritin is the main iron storage protein and plays an important role in maintaining iron homeostasis. In a previous study, we reported that apoferritin exerted a neuroprotective effect against MPTP by regulation of brain iron metabolism and ferroptosis. However, the precise cellular mechanisms of extracellular ferritin underlying this protection are not fully elucidated. Ferritin was reported to be localized in different intracellular compartments, cytoplasm or released outside cells. Here we demonstrated that the intracellular iron increased after iron treatment in primary cultured astrocytes. These iron-loaded astrocytes released more ferritin in order to buffer extracellular iron. Using co-culture system of primary cultured astrocytes and MES23.5 dopaminergic cells, we showed that ferritin released by astrocytes could enter MES23.5 dopaminergic cells. And primary cultured astrocytes protected MES23.5 dopaminergic cells against 1-methyl-4-phenylpyridinium ion (MPP⁺)-induced neurotoxicity and ferroptosis. In addition, we found that exogenous Apoferritin or Ferritin pretreatment could significantly inhibit MPP⁺-induced cell damage by restoring the cell viability and mitochondrial transmembrane potential (ΔΨm). Furthermore, exogenous Apoferritin and Ferritin might also protect MES23.5 dopaminergic cells against MPP⁺ by decreasing reactive oxygen species (ROS) and inhibiting the increase of the labile iron pool (LIP). This suggests that astrocytes increased ferritin release to respond to iron overload, which might inhibit iron-mediated oxidative damage and ferroptosis of dopamine (DA) neurons in Parkinson's disease (PD).

Growth differentiation factor 5: a neurotrophic factor with neuroprotective potential in Parkinson's disease

Parkinson's disease is the most common movement disorder worldwide, affecting over 6 million people. It is an age-related disease, occurring in 1% of people over the age of 60, and 3% of the population over 80 years. The disease is characterized by the progressive loss of midbrain dopaminergic neurons from the substantia nigra, and their axons, which innervate the striatum, resulting in the characteristic motor and non-motor symptoms of Parkinson's disease. This is paralleled by the intracellular accumulation of α-synuclein in several regions of the nervous system. Current therapies are solely symptomatic and do not stop or slow disease progression. One promising disease-modifying strategy to arrest the loss of dopaminergic neurons is the targeted delivery of neurotrophic factors to the substantia nigra or striatum, to protect the remaining dopaminergic neurons of the nigrostriatal pathway. However, clinical trials of two well-established neurotrophic factors, glial cell line-derived neurotrophic factor and neurturin, have failed to meet their primary end-points. This failure is thought to be at least partly due to the downregulation by α-synuclein of Ret, the common co-receptor of glial cell line-derived neurorophic factor and neurturin. Growth/differentiation factor 5 is a member of the bone morphogenetic protein family of neurotrophic factors, that signals through the Ret-independent canonical Smad signaling pathway. Here, we review the evidence for the neurotrophic potential of growth/differentiation factor 5 in in vitro and in vivo models of Parkinson's disease. We discuss new work on growth/differentiation factor 5's mechanisms of action, as well as data showing that viral delivery of growth/differentiation factor 5 to the substantia nigra is neuroprotective in the α-synuclein rat model of Parkinson's disease. These data highlight the potential for growth/differentiation factor 5 as a disease-modifying therapy for Parkinson's disease.

Quinacrine and Niclosamide Promote Neurite Growth in Midbrain Dopaminergic Neurons Through the Canonical BMP-Smad Pathway and Protect Against Neurotoxin and α-Synuclein-Induced Neurodegeneration

Parkinson's disease is a neurodegenerative disorder characterised by nigrostriatal dopaminergic degeneration, and intracellular α-synuclein aggregation. Current pharmacological treatments are solely symptomatic so there is a need to identify agents that can slow or stop dopaminergic degeneration. One proposed class of therapeutics are neurotrophic factors which promote the survival of nigrostriatal dopaminergic neurons. However, neurotrophic factors need to be delivered directly to the brain. An alternative approach may be to identify pharmacological agents which can reach the brain to stimulate neurotrophic factor expression and/or their signalling pathways in dopaminergic neurons. BMP2 is a neurotrophic factor that is expressed in the human substantia nigra; exogenous BMP2 administration protects against dopaminergic degeneration in in vitro models of PD. In this study, we investigated the neurotrophic potential of two FDA-approved drugs, quinacrine and niclosamide, that are modulators of BMP2 signalling. We report that quinacrine and niclosamide, like BMP2, significantly increased neurite length, as a readout of neurotrophic action, in SH-SY5Y cells and dopaminergic neurons in primary cultures of rat ventral mesencephalon. We also show that these effects of quinacrine and niclosamide require the activation of BMP-Smad signalling. Finally, we demonstrate that quinacrine and niclosamide are neuroprotective against degeneration induced by the neurotoxins, MPP+ and 6-OHDA, and by viral-mediated overexpression of α-synuclein in vitro. Collectively, this study identifies two drugs, that are safe for use in patients' to 'are approved for human use, that exert neurotrophic effects on dopaminergic neurons through modulation of BMP-Smad signalling. This rationalises the further study of drugs that target the BMP-Smad pathway as potential neuroprotective pharmacotherapy for Parkinson's disease.

Bone-to-Brain: A Round Trip in the Adaptation to Mechanical Stimuli

Besides the classical ones (support/protection, hematopoiesis, storage for calcium, and phosphate) multiple roles emerged for bone tissue, definitively making it an organ. Particularly, the endocrine function, and in more general terms, the capability to sense and integrate different stimuli and to send signals to other tissues, has highlighted the importance of bone in homeostasis. Bone is highly innervated and hosts all nervous system branches; bone cells are sensitive to most of neurotransmitters, neuropeptides, and neurohormones that directly affect their metabolic activity and sensitivity to mechanical stimuli. Indeed, bone is the principal mechanosensitive organ. Thanks to the mechanosensing resident cells, and particularly osteocytes, mechanical stimulation induces metabolic responses in bone forming (osteoblasts) and bone resorbing (osteoclasts) cells that allow the adaptation of the affected bony segment to the changing environment. Once stimulated, bone cells express and secrete, or liberate from the entrapping matrix, several mediators (osteokines) that induce responses on distant targets. Brain is a target of some of these mediator [e.g., osteocalcin, lipocalin2, sclerostin, Dickkopf-related protein 1 (Dkk1), and fibroblast growth factor 23], as most of them can cross the blood-brain barrier. For others, a role in brain has been hypothesized, but not yet demonstrated. As exercise effectively modifies the release and the circulating levels of these osteokines, it has been hypothesized that some of the beneficial effects of exercise on brain functions may be associated to such a bone-to-brain communication. This hypothesis hides an interesting clinical clue: may well-addressed physical activities support the treatment of neurodegenerative diseases, such as Alzheimer’s and Parkinson’s diseases?

Efficacy of short-term multidisciplinary intensive rehabilitation in patients with different Parkinson's disease motor subtypes: a prospective pilot study with 3-month follow-up

Parkinson's disease (PD) can be classified into three motor-based subtypes: postural instability/gait difficulty (PIGD), tremor dominant (TD), and indeterminate. The neuropathophysiological mechanisms of the three motor subtypes are different, which may lead to different responses to therapy. Sixty-nine patients with idiopathic Parkinson's disease (Hoehn-Yahr stage ≤ 3) were screened from 436 patients with Parkinsonism recruited through outpatient services and the internet. According to the Movement Disorder Society Unified Parkinson's Disease Rating Scale (MDS-UPDRS) TD/PIGD ratio, the patients were divided into PIGD (TD/PIGD ≤ 0.09; n = 36), TD (TD/PIGD ≥1.15; n = 19), and indeterminate (TD/PIGD = 0.90-1.15; n = 14) groups. All patients received 2 weeks of multidisciplinary intensive rehabilitation treatment (MIRT) during hospitalization, as well as a remote home rehabilitation health education class. Compared with the scores at admission, all patients showed significant improvements in their MDS-UPDRS III score, walking ability, balance, and posture control at discharge. Moreover, the MDS-UPDRS III score improvement was greater in the PIGD group than in the TD group. The follow-up data, collected for 3 months after discharge, showed that overall symptom improvement in each group was maintained for 1-3 months. Furthermore, there were no significant differences in the duration or grade effects of symptom improvement among the three groups. These findings suggest that 2 weeks of MIRT is effective for improving motor performance in all three motor subtypes. Patients in the PIGD group had a better response after hospitalization than those in the TD group. This study was approved by the Institutional Ethics Committee of Beijing Rehabilitation Hospital of Capital Medical University of China (approval No. 2018bkky022) on May 7, 2018 and registered with the Chinese Clinical Trial Registry (registration No. ChiCTR1900020771) on January 19, 2019.

Effects of bilateral subthalamic nucleus deep brain stimulation on motor symptoms in Parkinson's disease: a retrospective cohort study

Deep brain stimulation of the bilateral subthalamic nucleus (STN) is a therapeutic option for patients with Parkinson's disease (PD) in whom medical therapies have been ineffective. This retrospective cohort study analyzed the motor function of 27 patients with advanced PD, from the First Affiliated Hospital of Guangzhou Medical University, China, who received deep brain stimulation of the bilateral subthalamic nucleus and evaluated its therapeutic effects. The 10-year follow-up data of patients was analyzed in Qingyuan People's Hospital, Sixth Affiliated Hospital of Guangzhou Medical University, China. The follow-up data were divided into two categories based on patients during levodopa treatment (on-medication) and without levodopa treatment (off-medication). Compared with baseline, the motor function of on-medication PD patients improved after deep brain stimulation of the bilateral subthalamic nucleus. Even 2 years later, the motor function of off-medication PD patients had improved. On-medication PD patients exhibited better therapeutic effects over the 5 years than off-medication PD patients. On-medication patients' akinesia, speech, postural stability, gait, and cognitive function worsened only after 5 years. These results suggest that the motor function of patients with advanced PD benefitted from treatment with deep brain stimulation of the bilateral subthalamic nucleus over a period up to 5 years. The overall therapeutic effects were more pronounced when levodopa treatment was combined with deep brain stimulation of the bilateral subthalamic nucleus. This study was approved by Institutional Review Board of Qingyuan People's Hospital, The Sixth Affiliated Hospital of Guangzhou Medical University, China (approval No. QPH-IRB-A0140) on January 11, 2018.

Possible role of glial cell line-derived neurotrophic factor for predicting cognitive impairment in Parkinson's disease: a case-control study

Glial cell line-derived neurotrophic factor (GDNF) plays an important role in the protection of dopaminergic neurons, but there are few reports of the relationship between GDNF and its precursors (α-pro-GDNF and β-pro-GDNF) and cognitive impairment in Parkinson’s disease. This study aimed to investigate the relationship between the serum levels of GDNF and its precursors and cognitive impairment in Parkinson’s disease, and to assess their potential as a diagnostic marker. Fifty-three primary outpatients and hospitalized patients with Parkinson’s disease (23 men and 30 women) with an average age of 66.58 years were enrolled from the Affiliated Hospital of Xuzhou Medical University of China in this case-control study. The patients were divided into the Parkinson’s disease with cognitive impairment group (n = 27) and the Parkinson’s disease with normal cognitive function group (n = 26) based on their Mini-Mental State Examination, Montreal Cognitive Assessment, and Clinical Dementia Rating scores. In addition, 26 age- and sex-matched healthy subjects were included as the healthy control group. Results demonstrated that serum GDNF levels were significantly higher in the Parkinson’s disease with normal cognitive function group than in the other two groups. There were no significant differences in GDNF precursor levels among the three groups. Correlation analysis revealed that serum GDNF levels, GDNF/α-pro-GDNF ratios, and GDNF/β-pro-GDNF ratios were moderately or highly correlated with the Mini-Mental State Examination, Montreal Cognitive Assessment, and Clinical Dementia Rating scores. To explore the risk factors for cognitive impairment in patients with Parkinson’s disease, logistic regression analysis and stepwise linear regression analysis were performed. Both GDNF levels and Hoehn-Yahr stage were risk factors for cognitive impairment in Parkinson’s disease, and were the common influencing factors for cognitive scale scores. Neither α-pro-GDNF nor β-pro-GDNF was risk factors for cognitive impairment in Parkinson’s disease. A receiver operating characteristic curve of GDNF was generated to predict cognitive function in Parkinson’s disease (area under the curve = 0.859). This result indicates that the possibility that serum GDNF can correctly distinguish whether patients with Parkinson’s disease have cognitive impairment is 0.859. Together, these results suggest that serum GDNF may be an effective diagnostic marker for cognitive impairment in Parkinson’s disease. However, α-pro-GDNF and β-pro-GDNF are not useful for predicting cognitive impairment in this disease. This study was approved by Ethics Committee of the Affiliated Hospital of Xuzhou Medical University, China (approval No. XYFY2017-KL047-01) on November 30, 2017.

Caveolin-1 downregulation promotes the dopaminergic neuron-like differentiation of human adipose-derived mesenchymal stem cells

Previous studies have shown that caveolin-1 is involved in regulating the differentiation of mesenchymal stem cells. However, its role in the differentiation of human adipose mesenchymal stem cells into dopaminergic neurons remains unclear. The aim of this study was to investigate whether caveolin-1 regulates the differentiation of human adipose mesenchymal stem cells into dopaminergic-like neurons. We also examined whether the expression of caveolin-1 could be modulated by RNA interference technology to promote the differentiation of human adipose mesenchymal stem cells into dopaminergic-like neurons. The differentiation of human adipose mesenchymal stem cells into dopaminergic neurons was evaluated morphologically and by examining expression of the markers tyrosine hydroxylase, Lmx1a and Nurr1. The analyses revealed that during the differentiation of human adipose mesenchymal stem cells into dopaminergic neurons, the expression of caveolin-1 is decreased. Notably, the downregulation of caveolin-1 promoted the differentiation of human adipose mesenchymal stem cells into dopaminergic-like neurons, and it increased the expression of tyrosine hydroxylase, Lmx1a and Nurr1. Together, our findings suggest that caveolin-1 plays a negative regulatory role in the differentiation of dopaminergic-like neurons from stem cells, and it may therefore be a potential molecular target for strategies for regulating the differentiation of these cells. This study was approved by the Medical Ethics Committee of the First Affiliated Hospital of Dalian Medical University of China (approval No. PJ-KS-KY-2020-54) on March 7, 2017.

The neuroprotective action of lenalidomide on rotenone model of Parkinson's Disease: Neurotrophic and supportive actions in the substantia nigra pars compacta

Lenalidomide is a centrally active thalidomide analog that has potent anti-inflammatory and antiangiogenic activities. Currently, it is primarily used in the treatment of multiple myeloma and myelodysplastic syndromes. However, recent studies have revealed in addition to neuroprotection and neuromodulation of lenalidomide. Because of this combination of inflammation and neuro-immunogenic properties, lenalidomide is considered as a high potential compound for the treatment of neurodegenerative diseases. Despite intensive research during the last decade, the role of neurotrophic elements in the effect of lenalidomide is still not well understood. Therefore, in the current study, the effects of lenalidomide on neurodegeneration were investigated in a rotenone model of Parkinson's disease (PD) rat model. The PD rat model was generated by rotenone injection into the substantia nigra pars compacta (SNpc). After validation of the PD model, the rats were treated with lenalidomide (100 mg/kg) for 28 days. Our data shows that lenalidomide alleviated rotenone-induced motor impairments and deficits in dopamine-related behaviors and resulted in increased levels of tumor necrosis factor-α and calcium-binding protein B in the SNpc. Moreover, chronic lenalidomide treatment resulted increase in transforming growth factor immunoreactivity and brain derived neurotrophic factor expression in the SNPc. In addition, chronic treatment mitigated tyrosine hydroxylase expression prevented the rotenone-induced decrease in dopamine levels, and consequently a decrease in caspase-3/9 immunoreactivity. This thus shows that chronic lenalidomide treatment improves neuronal survival. Together with our data demonstrate that lenalidomide, in addition to its anti-inflammatory and immunomodulatory actions, is also capable of increasing neurotrophic factors in the SNpc, thereby preventing rotenone-induced motor impairments.

Repulsive guidance molecules lock growth differentiation factor 5 in an inhibitory complex

Repulsive guidance molecules (RGMs) are cell surface proteins that regulate the development and homeostasis of many tissues and organs, including the nervous, skeletal, and immune systems. They control fundamental biological processes, such as migration and differentiation by direct interaction with the Neogenin (NEO1) receptor and function as coreceptors for the bone morphogenetic protein (BMP)/growth differentiation factor (GDF) family. We determined crystal structures of all three human RGM family members in complex with GDF5, as well as the ternary NEO1-RGMB-GDF5 assembly. Surprisingly, we show that all three RGMs inhibit GDF5 signaling, which is in stark contrast to RGM-mediated enhancement of signaling observed for other BMPs, like BMP2. Despite their opposite effect on GDF5 signaling, RGMs occupy the BMP type 1 receptor binding site similar to the observed interactions in RGM-BMP2 complexes. In the NEO1-RGMB-GDF5 complex, RGMB physically bridges NEO1 and GDF5, suggesting cross-talk between the GDF5 and NEO1 signaling pathways. Our crystal structures, combined with structure-guided mutagenesis of RGMs and BMP ligands, binding studies, and cellular assays suggest that RGMs inhibit GDF5 signaling by competing with GDF5 type 1 receptors. While our crystal structure analysis and in vitro binding data initially pointed towards a simple competition mechanism between RGMs and type 1 receptors as a possible basis for RGM-mediated GDF5 inhibition, further experiments utilizing BMP2-mimicking GDF5 variants clearly indicate a more complex mechanism that explains how RGMs can act as a functionality-changing switch for two structurally and biochemically similar signaling molecules.