GDNF prevents TGF-β-induced damage of the plasma membrane in cerebellar granule neurons by suppressing activation of p38-MAPK via the phosphatidylinositol 3-kinase pathway

Building better brains: the pleiotropic function of neurotrophic factors in postnatal cerebellar development

The cerebellum is a multifunctional brain region that controls diverse motor and non-motor behaviors. As a result, impairments in the cerebellar architecture and circuitry lead to a vast array of neuropsychiatric and neurodevelopmental disorders. Neurotrophins and neurotrophic growth factors play essential roles in the development as well as maintenance of the central and peripheral nervous system which is crucial for normal brain function. Their timely expression throughout embryonic and postnatal stages is important for promoting growth and survival of both neurons and glial cells. During postnatal development, the cerebellum undergoes changes in its cellular organization, which is regulated by a variety of molecular factors, including neurotrophic factors. Studies have shown that these factors and their receptors promote proper formation of the cerebellar cytoarchitecture as well as maintenance of the cerebellar circuits. In this review, we will summarize what is known on the neurotrophic factors' role in cerebellar postnatal development and how their dysregulation assists in developing various neurological disorders. Understanding the expression patterns and signaling mechanisms of these factors and their receptors is crucial for elucidating their function within the cerebellum and for developing therapeutic strategies for cerebellar-related disorders.

Glial-derived neurotrophic factor in human airway smooth muscle

Airway smooth muscle (ASM) cells modulate the local airway milieu via production of inflammatory mediators and growth factors including classical neurotrophins, such as brain-derived neurotrophic factor (BDNF). The glial cell-derived neurotrophic factor (GDNF) family of ligands (GFLs) are nonclassical neurotrophins and their role in the airway is barely understood. The major GFLs, GDNF and Neurturin (NRTN) bind to GDNF family receptor (GFR) α1 and α2 respectively that pair with Ret receptor to accomplish signaling. In this study, we found GDNF is expressed in human lung and increased in adult asthma, while human ASM expresses GDNF and its receptors. Accordingly, we used human ASM cells to test the hypothesis that ASM expression and autocrine signaling by GFLs regulate [Ca²⁺ ]_i . Serum-deprived ASM cells from non-asthmatics were exposed to 10 ng/ml GDNF or NRTN for 15 min (acute) or 24 h (chronic). In fura-2 loaded cells, acute GDNF or NRTN alone induced [Ca²⁺ ]_i responses, and further enhanced responses to 1 µM ACh or 10 µM histamine. Ret inhibitor (SPP86; 10 µM) or specific GDNF chelator GFRα1-Fc (1 µg/ml) showed roles of these receptors in GDNF effects. In contrast, NRTN did not enhance [Ca²⁺ ]_i response to histamine. Furthermore, conditioned media of nonasthmatic and asthmatic ASM cells showed GDNF secretion. SPP86, Ret inhibitor and GFRα1-Fc chelator markedly decreased [Ca²⁺ ]_i response compared with vehicle, highlighting autocrine effects of secreted GDNF. Chronic GDNF treatment increased histamine-induced myosin light chain phosphorylation. These novel data demonstrate GFLs particularly GDNF/GFRα1 influence ASM [Ca²⁺ ]_i and raise the possibility that GFLs are potential targets of airway hyperresponsiveness.

RET Regulates Human Medullary Thyroid Cancer Cell Proliferation through CDK5 and STAT3 Activation

Medullary thyroid cancer (MTC) is a neuroendocrine tumor that arises from the parafollicular C-cells, which produces the hormone calcitonin. RET is a transmembrane receptor protein-tyrosine kinase, which is highly expressed in MTC. Our previous studies reported that cyclin-dependent kinase 5 (CDK5) plays a crucial role in cancer progression, including MTC. However, the role of CDK5 in GDNF-induced RET signaling in medullary thyroid cancer proliferation remains unknown. Here, we investigated RET activation and its biochemically interaction with CDK5 in GDNF-induced medullary thyroid cancer proliferation. Our results demonstrated that GDNF stimulated RET phosphorylation and thus subsequently resulted in CDK5 activation by its phosphorylation. Activated CDK5 further caused STAT3 activation by its specific phosphorylation at Ser727. Moreover, we also found that GDNF treatment enhanced ERK1/2 and EGR1 activity, which is involved in p35 activation. Interestingly, we identified for the first time that CDK5 physically interacted with RET protein in MTC. Overall, our results provide a new mechanism for medullary thyroid cancer cell proliferation, suggesting that targeting CDK5 may be a promising therapeutic candidate for human medullary thyroid cancer in the near future.

Hydrogen gas post-conditioning attenuates early neuronal pyroptosis in a rat model of subarachnoid hemorrhage through the mitoKATP signaling pathway

Neuronal pyroptosis serves an important role in the progress of neurologic dysfunction following subarachnoid hemorrhage (SAH), which is predominantly caused by a ruptured aneurysm. Hydrogen gas has been previously reported to be an effective anti-inflammatory agent against ischemia-associated diseases by regulating mitochondrial function. The objective of the present study was to investigate the potential neuroprotective effects of hydrogen gas post-conditioning against neuronal pyroptosis after SAH, with specific focus on the mitochondrial ATP-sensitive K⁺ (mitoK_ATP) channels. Following SAH induction by endovascular perforation, rats were treated with inhalation of 2.9% hydrogen gas for 2 h post-perforation. Neurologic deficits, brain water content, reactive oxygen species (ROS) levels, neuronal pyroptosis, phosphorylation of ERK1/2, p38 MAPK and pyroptosis-associated proteins IL-1β and IL-18 were evaluated 24 h after perforation by a modified Garcia method, ratio of wet/dry weight, 2',7'-dichlorofluorescin diacetate, immunofluorescence and western blot assays, respectively. An inhibitor of the mitoK_ATP channel, 5-hydroxydecanoate sodium (5-HD), was used to assess the potential role of the mitoK_ATP-ERK1/2-p38 MAPK signal pathway. Hydrogen gas post-conditioning significantly alleviated brain edema and improved neurologic function, reduced ROS production and neuronal pyroptosis, suppressed the expression of IL-1β and IL-18 whilst upregulating ERK1/2 phosphorylation, but downregulated p38 MAPK activation 24 h post-SAH. These aforementioned effects neuroprotective were partially reversed by 5-HD treatment. Therefore, these observations suggest that post-conditioning with hydrogen gas ameliorated SAH-induced neuronal pyroptosis at least in part through the mitoK_ATP/ERK1/2/p38 MAPK signaling pathway.

P2 receptor interaction and signalling cascades in neuroprotection

Nucleotides can contribute to the survival of different glial and neuronal models at the nervous system via activation of purinergic P2X and P2Y receptors. Their activation counteracts different proapoptotic events, such as excitotoxicity, mitochondrial impairment, oxidative stress and DNA damage, which concur to elicit cell loss in different processes of neurodegeneration and brain injury. Thus, it is frequent to find that different neuroprotective mediators converge in the activation of the same intracellular survival pathways to protect cells from death. The present review focuses on the role of P2Y₁ and P2Y₁₃ metabotropic receptors, and P2X7 ionotropic receptors to regulate the balance between survival and apoptosis. In particular, we analyze the intracellular pathways involved in the signaling of these nucleotide receptors to elicit survival, including calcium/PLC, PI3K/Akt/GSK3, MAPK cascades, and the expression of antioxidant and antiapoptotic genes. This review emphasizes the novel contribution of nucleotide receptors to maintain cell homeostasis through the regulation of MAP kinases and phosphatases. Unraveling the different roles found for nucleotide receptors in different models and cellular contexts may be crucial to delineate future therapeutic applications based on targeting nucleotide receptors for neuroprotection.

The Non-Survival Effects of Glial Cell Line-Derived Neurotrophic Factor on Neural Cells

Glial cell line-derived neurotrophic factor (GDNF) was first characterized as a survival-promoting molecule for dopaminergic neurons (DANs). Afterwards, other cells were also discovered to respond to GDNF not only as a survival factor but also as a protein supporting other cellular functions, such as proliferation, differentiation, maturation, neurite outgrowth and other phenomena that have been less studied than survival and are now more extendedly described here in this review article. During development, GDNF favors the commitment of neural precursors towards dopaminergic, motor, enteric and adrenal neurons; in addition, it enhances the axonal growth of some of these neurons. GDNF also induces the acquisition of a dopaminergic phenotype by increasing the expression of Tyrosine Hydroxylase (TH), Nurr1 and other proteins that confer this identity and promote further dendritic and electrical maturation. In motor neurons (MNs), GDNF not only promotes proliferation and maturation but also participates in regenerating damaged axons and modulates the neuromuscular junction (NMJ) at both presynaptic and postsynaptic levels. Moreover, GDNF modulates the rate of neuroblastoma (NB) and glioblastoma cancer cell proliferation. Additionally, the presence or absence of GDNF has been correlated with conditions such as depression, pain, muscular soreness, etc. Although, the precise role of GDNF is unknown, it extends beyond a survival effect. The understanding of the complete range of properties of this trophic molecule will allow us to investigate its broad mechanisms of action to accelerate and/or improve therapies for the aforementioned pathological conditions.

Mitogen-activated protein kinases regulate expression of neuronal nitric oxide synthase and neurite outgrowth via non-classical retinoic acid receptor signaling in human neuroblastoma SH-SY5Y cells

We have previously shown that retinoic acid receptor (RAR) stimulation by an agonist Am80 recruits nitric oxide-dependent signaling via increased expression of neuronal nitric oxide synthase (nNOS) in rat midbrain slice cultures. Using neuroblastoma SH-SY5Y cells, here we investigated the mechanisms of RAR-induced nNOS expression, together with relationship between nNOS expression and neurite outgrowth. Am80 promoted neurite outgrowth, which was attenuated by inhibitors of phosphoinositide 3-kinase (PI3K; LY294002), c-Jun N-terminal kinase (JNK; SP600125) and p38 mitogen-activated protein kinase (p38 MAPK; SB203580). A selective nNOS inhibitor 3-bromo-nitroindazole also suppressed Am80-induced neurite outgrowth. Am80-induced increase in nNOS protein expression was attenuated by LY294002, SP600125 and SB203580, whereas increase in nNOS mRNA expression was attenuated only by LY294002. Am80-induced activation of JNK and p38 MAPK was blocked by LY294002, suggesting that these kinases acted downstream of PI3K. We also confirmed that DAX1, a nuclear receptor reported to regulate nNOS expression, was up-regulated in response to Am80. siRNA-mediated knockdown of DAX1 abrogated Am80-induced nNOS expression and neurite outgrowth. These results reveal for the first time that nNOS expression is crucial for RAR-mediated neurite outgrowth, and that non-genomic signaling such as JNK and p38 MAPK is involved in RAR-mediated nNOS expression.

Effects of glial cell line-derived neurotrophic factor on microRNA expression in a 6-hydroxydopamine-injured dopaminergic cell line

Parkinson's disease (PD) is the second most prevalent, progressive neurodegenerative disease and is characterized by the irreversible and selective loss of nigrostriatal dopaminergic (DA) neurons. Glial cell line-derived neurotrophic factor (GDNF), which is a potent protective factor for DA neurons, is considered a promising neuroprotective candidate for PD. microRNAs (miRNAs) have been shown to be involved in a number of neurodegenerative diseases. Both miRNAs and GDNF affect DA neuronal processes, but the molecular crosstalk between these molecules remains unclear. The present study aimed to evaluate whether GDNF modulates miRNA expression. We used microarray analysis and real-time polymerase chain reaction (RT-PCR) to investigate miRNAs expression in 6-hydroxydopamine (6-OHDA)-injured MN9D cells treated with GDNF for 30 min, 1 h, or 3 h. Our results showed that GDNF treatment led to differential expression of 143 miRNAs. To further identify mechanisms by which GDNF exerts its effects, we compared miRNAs and mRNAs microarray data at the 1-h time point. We found that various biological processes and pathways were regulated at the miRNA level following GDNF treatment. Collectively, these results provide evidence of the capacity of GDNF to influence miRNAs expression, suggesting a new mechanism of GDNF action.

Genes expressed in Atoh1 neuronal lineages arising from the r1/isthmus rhombic lip

During embryogenesis, the rhombic lip of the fourth ventricle is the germinal origin of a diverse collection of neuronal populations that ultimately reside in the brainstem and cerebellum. Rhombic lip neurogenesis requires the bHLH transcription factor Atoh1 (Math1), and commences shortly after neural tube closure (E9.5). Within the rhombomere 1-isthmus region, the rhombic lip first produces brainstem and deep cerebellar neurons (E9.5-E12), followed by granule cell precursors after E12. While Atoh1 function is essential for all of these populations to be specified, the downstream genetic programs that confer specific properties to early and late born Atoh1 lineages are not well characterized. We have performed a comparative microarray analysis of gene expression within early and later born cohorts of Atoh1 expressing neural precursors purified from E14.5 embryos using a transgenic labeling strategy. We identify novel transcription factors, cell surface molecules, and cell cycle regulators within each pool of Atoh1 lineages that likely contribute to their distinct developmental trajectories and cell fates. In particular, our analysis reveals new insights into the genetic programs that regulate the specification and proliferation of granule cell precursors, the putative cell of origin for the majority of medulloblastomas.

ERK and cell death: ERK1/2 in neuronal death

Extracellular signal-regulated kinase (ERK) is a versatile protein kinase that regulates many cellular functions. Growing evidence suggests that ERK1/2 plays a crucial role in promoting cell death in a variety of neuronal systems, including neurodegenerative diseases. It is believed that the magnitude and the duration of ERK1/2 activity determine its cellular function. In this review, we summarize recent evidence for a role of ERK1/2 in neuronal death. Furthermore, we discuss the mechanisms involved in ERK1/2 mediating neuronal death.