GDNF, neurturin, persephin and artemin are a class of secretory proteins with similar structure and function. They all have 7 conserved cysteine residues, with similar spatial structure, nucleotide sequence and amino acid sequence with high homology. They constitute a subfamily of the transforming growth factor beta (TGF-beta) superfamily, which are similar in physiological functions, receptors, signal transduction pathways, and so on.
GDNF is a small protein that potently promotes the survival of many types of neurons. It signals through GFRα receptors, particularly GFRα1. Persephin promotes the survival and growth of central dopaminergic and motor neurons, and is also involved in kidney development. Artemin promotes the survival and growth of various peripheral and central neurons, including sympathetic and dopaminergic neurons. Neurturin exerts a positive effect on the survival of a variety of neurons and possibly other cell types.
Members of GDNF family
Table 1. GDNF family related products
The glial cell derived neurotrophic factor (GDNF) is a neurotrophic factor isolated and purified from the conditioned medium of rat glial cell line B49 by Lin et al. (1993), and is named after that. The GDNF gene of rat and human was cloned by using the amino terminal sequence of purified GDNF as a probe. The human GDNF precursor protein is 211 amino acid residues (including 19 amino acids of signal peptide). After processing, it forms a secretory mature protein and has 134 amino acids. It is a glycosylated disulfide-linked homodimeric protein, and its molecular weight is 32~34kD, which is an alkaline protein.
Neurturin (NRTN) is a protein. Neurturin belongs to the glial cell-line derived neurotrophic factor (GDNF) family of neurotrophic factors, which regulate the survival and function of neurons. Neurturin acts as a growth factor places it in the TGF-beta (transforming growth factor) subfamily along with its homologs persephin, artemin, and GDNF. It is also considered as a trophic factor and is critical in the development and growth of neurons in the brain. Neurotrophic factors like neurotrophic factors have been tested in several clinical trials setting for the potential treatment of neurodegenerative diseases, specifically Parkinson’s disease.
The amino acid sequence of persephin (PSPN) has about 40% homology with NRTN and GDNF. It has not only seven conserved cysteines in the TGF-beta family, but also a conservative sequence only between NRTN and GDNF. It was found that the expression of PSPN in embryonic and adult rat tissues was extremely low, and the content of mRNA in each tissue was relatively high, which suggested that the regulation of mRNA shear processing may play an important role in regulating the expression of PSPN protein. The study of the PSPN function has just begun. It has been found in the experiment that it has a nutritive effect on the motor neurons and dopaminergic neurons, and it also promotes the development of the cultured ureteric bud. But unlike NRTN and GDNF, PSPN has no nutritional effects on sympathetic neurons, sensory neurons and intestinal neurons in the peripheral nervous system.
Artemin, also known as enovin or neublastin, is a protein that in humans is encoded by the ARTN gene. Artemin is a neurotrophic factor in the glial cell line-derived neurotrophic factor family of ligands which are a group of ligands within the TGF-beta superfamily of signaling molecules. GDNFs are unique in having neurotrophic properties and have potential use for gene therapy in neurodegenerative disease. Artemin has been shown in culture to support the survival of a number of peripheral neuron populations and at least one population of dopaminergic CNS neurons. Its role in the PNS and CNS is further substantiated by its expression pattern in the proximity of these neurons. This protein is a ligand for the RET receptor and uses GFR-alpha 3 as a coreceptor.
Figure 1. Tertiary structure of human artemin.
The most prominent feature of GDNF is its ability to support the survival of dopaminergic and motor neurons. GDNF gene encodes a highly conserved neurotrophic factor. The recombinant form of this protein was shown to promote the survival and differentiation of dopaminergic neurons in culture, and was able to prevent apoptosis of motor neurons induced by axotomy. The encoded protein is processed to a mature secreted form that exists as a homodimer. The mature form of the protein is a ligand for the product of the RET (rearranged during transfection) protooncogene. In addition to the transcript encoding GDNF, two additional alternative transcripts encoding distinct proteins, referred to as astrocyte-derived trophic factors, have also been described. Mutations in this gene may be associated with Hirschsprung's disease.
NRTN is encoded by the NRTN gene located on chromosome 19 in humans and has been shown to promote potent effects on the survival and function of developing and mature midbrain dopaminergic neurons (DA) in vitro. In vivo the direct administration of NRTN into substantia nigra of mice models also shows mature DA neuron protection. In addition, NRTN has also been shown to support the survival of several other neurons including sympathetic and sensory neurons of the dorsal root ganglia. Knockout mice have shown that NRTN does not appear to be essential for survival. However, there is evidence that growth of the gut, sensory and parasympathetic neurons in mice is delayed after the removal of NRTN receptors. NRTN has been shown to upregulate B1 (bradykinin) receptors in neurons of mice, indicating a possible influence on pain and inflammatory pathways. In addition, knockout mice have shown that an increased acetylcholine response is observed in the absence of NRTN .
PSPN can promote the survival of dopaminergic neurons in rat embryos and promote the high affinity uptake of dopamine. In vitro experiments showed that GDNF, NRTN and PSPN were effective in promoting the survival of dopaminergic neurons in the embryonic mesencephalon. PSPN could also promote the morphological differentiation of dopaminergic neurons, increase the cell body and extend the axon. PSPN can prevent the degeneration of dopaminergic neurons in the body. The injection of 6-hydroxydopamine (6-OHDA) in the striatum of the Parkinson model rats and the dopaminergic neuron degeneration in the striated body, such as pre-injection of PSPN into rats, can antagonize the damage caused by 6-hydroxyl dopamine and prevent the death of dopaminergic neurons. Studies have shown that PSPN can prevent degenerative changes of motor neurons in vivo and play a protective role in spinal motor neurons.
ARTN plays a role in the survival and differentiation of a variety of neurons. ARTN can promote the differentiation of the cultured mesencephalic dopaminergic neurons, and play an important role in the development and plasticity of dopaminergic neurons. In in vitro culture, the number of glomus cells in the carotid body increased and the cell growth was increased by ARTN. This indicates that ARTN plays a role in the function of the carotid body, and may improve the directional distribution of the nerve through the transport of the carotid body. ARTN can also play a role in the differentiation, survival and growth of sympathetic neurons. ARTN also has obvious effects on GABA neurons and serotonin activated neurons in the midbrain of rats. Although ARTN was not detected on the ventral side of the rat embryo, the in vitro cultured dopaminergic neurons test showed that ARTN could also promote the survival of dopaminergic neurons.
Role in disease
GDNF has regenerative properties for brain cells and shows the potential to treat Parkinson's disease - monkeys with an induced form of Parkinson's disease show less trembling when treated with the drug, and neuronal fibres grow in part of the human brain exposed to the drug. However, the progress of treatment is hampered by the problem of delivering the drug to brain cells through the blood-brain barrier in human.
The most studied are the role of neurotrophic factors in neurodegenerative disease like Parkinson’s disease and Huntington's disease, several of which have implicated neurturin’s role in rescuing neurons. However, these results have never been observed in humans. Hirschsprung's disease is an autosomal dominant genetic disorder characterized by complete absence of neuronal ganglion cells from the intestinal tract. Previous studies indicate the role of NRTN gene mutations in the disease. One study showed that the mutation in the NRTN gene was not enough to cause the onset of the disease, but when coupled with the mutation in the RET gene, both family members and individuals had disease. A more recent study showed that NRTN variants are present in individuals with Hirschsprung’s disease. However, RET associated mutations were not found, and in one variant, RET phosphorylation levels were reduced, which may have downstream effects on the proliferation and differentiation of neuronal crests. Also, high levels of expression of neurturin were found to be associated with nephroblastoma, indicating that growth factors may affect differentiation. Lastly, a study also associated neurturin deficiency in mice with keratoconjunctivitis and dry eye.
The expression of ARTN in human breast cancer can increase the degree of malignancy of tumor cells. It has been found in xenograft models that upregulation of ARTN expression levels will potentiate the malignant properties of highly proliferative, poorly differentiated and highly invasive breast cancer tumor cells. The higher expression level of ARTN was positively correlated with metastasis, recurrence and death of residual cancer after chemotherapy. It can be seen that ARTN is carcinogenic in human breast cancer cells, so inhibiting the physiological function of ARTN is a factor that should be considered in breast cancer targeted therapy.
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