A Novel Neuroprotective Mechanism for Lithium That Prevents Association of the p75NTR-Sortilin Receptor Complex and Attenuates proNGF-Induced Neuronal Death In Vitro and In Vivo

Brain-Derived Neurotrophic Factor Dysregulation as an Essential Pathological Feature in Huntington's Disease: Mechanisms and Potential Therapeutics

Brain-derived neurotrophic factor (BDNF) is a major neurotrophin whose loss or interruption is well established to have numerous intersections with the pathogenesis of progressive neurological disorders. There is perhaps no greater example of disease pathogenesis resulting from the dysregulation of BDNF signaling than Huntington's disease (HD)-an inherited neurodegenerative disorder characterized by motor, psychiatric, and cognitive impairments associated with basal ganglia dysfunction and the ultimate death of striatal projection neurons. Investigation of the collection of mechanisms leading to BDNF loss in HD highlights this neurotrophin's importance to neuronal viability and calls attention to opportunities for therapeutic interventions. Using electronic database searches of existing and forthcoming research, we constructed a literature review with the overarching goal of exploring the diverse set of molecular events that trigger BDNF dysregulation within HD. We highlighted research that investigated these major mechanisms in preclinical models of HD and connected these studies to those evaluating similar endpoints in human HD subjects. We also included a special focus on the growing body of literature detailing key transcriptomic and epigenetic alterations that affect BDNF abundance in HD. Finally, we offer critical evaluation of proposed neurotrophin-directed therapies and assessed clinical trials seeking to correct BDNF expression in HD individuals.

Lithium and hormesis: Enhancement of adaptive responses and biological performance via hormetic mechanisms

Biomedical and consumer interest in the health-promoting properties of pure single entities of known or unknown chemical constituents and mixtures has never been greater. Since its "rediscovery" in the 1950s, lithium is an example of such a constituent that represents an array of scientific and public health challenges and medical potentials that may now be understood best when seen through the lens of the dose-response paradigm known as hormesis. The present paper represents the first review of the capacity of lithium to induce hormetic dose responses in a broad range of biological models, organ systems, and endpoints. Of significance is that the numerous hormetic findings occur with extensive concentration/dose response evaluations with the optimal dosing being similar across multiple organ systems. The particular focus of these hormetic dose-response findings was targeted to research with a broad spectrum of stem cell types and neuroprotective effects. These findings suggest that lithium may have critically valuable systemic effects with respect to those therapeutically treated with lithium as well as for exposures that may be achieved via dietary intervention.

In vivo functions of p75NTR: challenges and opportunities for an emerging therapeutic target

The p75 neurotrophin receptor (p75^NTR) functions at the molecular nexus of cell death, survival, and differentiation. In addition to its contribution to neurodegenerative diseases and nervous system injuries, recent studies have revealed unanticipated roles of p75^NTR in liver repair, fibrinolysis, lung fibrosis, muscle regeneration, and metabolism. Linking these various p75^NTR functions more precisely to specific mechanisms marks p75^NTR as an emerging candidate for therapeutic intervention in a wide range of disorders. Indeed, small molecule inhibitors of p75^NTR binding to neurotrophins have shown efficacy in models of Alzheimer's disease (AD) and neurodegeneration. Here, we outline recent advances in understanding p75^NTR pleiotropic functions in vivo, and propose an integrated view of p75^NTR and its challenges and opportunities as a pharmacological target.

Elevation of CSF Sortilin Following Subarachnoid Hemorrhage in Patients and Experimental Model Rats

Subarachnoid hemorrhage (SAH) can cause acute neuronal injury and chronic neurocognitive deficits; biomarkers reflecting its associated neuronal injury are of potential prognostic value. Sortilin, a member of the vacuolar protein sorting 10p (Vps10p) family, is enriched in neurons and is likely involved in neurodegenerative diseases. Here, we explored sortilin in the cerebrospinal fluid (CSF) as a potential biomarker for early neuronal injury after SAH. Sortilin levels in the CSF of SAH patients (n = 11) and controls (n = 6) were analyzed by immunoblot. SAH rats surviving 3-72 h (h) were evaluated neurologically, with their brain and CSF samples examined histologically and biochemically. Sortilin protein ~100 kDa was detected in the CSF from SAH patients only, with its levels correlated to Hunt-Hess scale. Rats in the SAH groups showed poorer Garcia score and beam balancing capability than sham controls. Sortilin ~100 kDa was detectable in the CSF of the SAH, but not sham, animals. Levels of sortilin ~100 kDa and fragments ~40 kDa in cortical lysates were elevated in the SAH relative to control rats. Levels of cortical glial fibrillary acidic protein (GFAP) were also elevated in the SAH rats. In immunohistochemistry, the pattern of sortilin labeling in the brain was largely comparable between the SAH and control rats, whereas an increased astrocytic GFAP immunolabeling was evident in the former. Together, these results suggest that SAH can cause an early and remarkable rise of sortilin products in CSF, likely reflecting neuronal change. Sortilin could be further explored as a potential biomarker in some brain disorders.

c-Jun N-terminal Kinase Mediates Ligand-independent p75NTR Signaling in Mesencephalic Cells Subjected to Oxidative Stress

The p75 neurotrophin receptor (p75^NTR) is a multifunctional protein that regulates cellular responses to pathological conditions in specific regions of the nervous system. Activation of p75^NTR in certain neuronal populations induces proteolytic processing of the receptor, thereby generating p75^NTR fragments that facilitate downstream signaling. Expression of p75^NTR has been reported in neurons of the ventral midbrain, but p75^NTR signaling mechanisms in such cells are poorly understood. Here, we used Lund Human Mesencephalic cells, a population of neuronal cells derived from the ventral mesencephalon, to evaluate the effects of oxidative stress on p75^NTR signaling. Subjection of the cells to oxidative stress resulted in decreased cell-surface localization of p75^NTR and intracellular accumulation of p75^NTR fragments. Oxidative stress-induced p75^NTR processing was reduced by pharmacological inhibition of metalloproteases or γ-secretase, but was unaltered by blockade of the ligand-binding domain of p75^NTR. Furthermore, inhibition of c-Jun N-terminal Kinase (JNK) decreased p75^NTR cleavage induced by oxidative damage. Altogether, these results support a mechanism of p75^NTR activation in which oxidative stress stimulates JNK signaling, thereby facilitating p75^NTR processing via a ligand-independent mechanism involving induction of metalloprotease and γ-secretase activity. These findings reveal a novel role for JNK in ligand-independent p75^NTR signaling, and, considering the susceptibility of mesencephalic neurons to oxidative damage associated with Parkinson's disease (PD), merit further investigation into the effects of p75^NTR on PD-related neurodegeneration.

Nerve growth factor in metabolic complications and Alzheimer's disease: Physiology and therapeutic potential

As the population ages, obesity and metabolic complications as well as neurological disorders are becoming more prevalent, with huge economic burdens on both societies and families. New therapeutics are urgently needed. Nerve growth factor (NGF), first discovered in 1950s, is a neurotrophic factor involved in regulating cell proliferation, growth, survival, and apoptosis in both central and peripheral nervous systems. NGF and its precursor, proNGF, bind to TrkA and p75 receptors and initiate protein phosphorylation cascades, resulting in changes of cellular functions, and are associated with obesity, diabetes and its complications, and Alzheimer's disease. In this article, we summarize changes in NGF levels in metabolic and neuronal disorders, the signal transduction initiated by NGF and proNGF, the physiological and pathophysiological relevance, and therapeutic potential in treating chronic metabolic diseases and cognitive decline.

Multifunctional compounds lithium chloride and methylene Blue attenuate the negative effects of diisopropylfluorophosphate on axonal transport in rat cortical neurons

Organophosphates (OPs) are valuable as pesticides in agriculture and for controlling deadly vector-borne illnesses; however, they are highly toxic and associated with many deleterious health effects in humans including long-term neurological impairments. Antidotal treatment regimens are available to combat the symptoms of acute OP toxicity, which result from the irreversible inhibition of acetylcholinesterase (AChE). However, there are no established treatments for the long-term neurological consequences of OP exposure. In addition to AChE, OPs can negatively affect multiple protein targets as well as biological processes such as axonal transport. Given the fundamental nature of axonal transport to neuronal health, we rationalized that this process might serve as a general focus area for novel therapeutic strategies against OP toxicity. In the studies described here, we employed a multi-target, phenotypic screening, and drug repurposing strategy for the evaluations of potential novel OP-treatments using a primary neuronal culture model and time-lapse live imaging microscopy. Two multi-target compounds, lithium chloride (LiCl) and methylene blue (MB), which are FDA-approved for other indications, were evaluated for their ability to prevent the negative effects of the OP, diisopropylfluorophosphate (DFP) on axonal transport. The results indicated that both LiCl and MB prevented DFP-induced impairments in anterograde and retrograde axonal transport velocities in a concentration dependent manner. While in vivo studies will be required to confirm our in vitro findings, these experiments support the potential of LiCl and MB as repurposed drugs for the treatment of the long-term neurological deficits associated with OP exposure (currently an unmet medical need).

The p75 Neurotrophin Receptor Facilitates TrkB Signaling and Function in Rat Hippocampal Neurons

Neurotrophins activate Trk receptor signaling to support neuronal survival and many aspects of neuronal function. Early studies demonstrated that TrkA formed a complex with the p75 neurotrophin receptor (p75^NTR), which increased the affinity and selectivity of NGF binding, however, whether interaction of p75^NTR with other Trk receptors performs a similar function to enhance ligand binding has not been demonstrated. We investigated the interaction of TrkB with full length p75^NTR in hippocampal neurons in response to BDNF and found that the association of these receptors occurs after ligand binding and requires phosphorylation of TrkB, indicating that formation of this receptor complex was not necessary for ligand binding. Moreover, the interaction of these receptors required internalization and localization to early endosomes. We found that association of TrkB with p75^NTR was necessary for optimal downstream signaling of the PI₃K-Akt pathway, but not the Erk pathway, in hippocampal neurons. The absence of p75^NTR impaired the ability of BDNF to rescue hippocampal neurons in a trophic deprivation model, suggesting that p75^NTR facilitates the ability of TrkB to activate specific pathways to promote neuronal survival.

RAGE/galectin-3 yields intraplaque calcification transformation via sortilin

AIMS

Macrocalcification and microcalcification present different clinical risks, but the regulatory of their formation was unclear. Therefore, this study explored the underlying mechanisms of macrocalcification and microcalcification in diabetes mellitus.

METHODS

Anterior tibial arteries of amputated diabetic feet were collected. According to the calcium content, patients were divided into less-calcification group and more-calcification group. And calcification morphology in plaques was observed. For further study, an in vivo mouse diabetic atherosclerosis model and an in vitro primary mouse aortic smooth muscle cell model were established. After the receptors for AGEs (RAGE) or galectin-3 were silenced, calcified nodule sizes and sortilin expression were determined. Scanning electron microscopy (SEM) was performed to detect the aggregation of matrix vesicles with the inhibition or promotion of sortilin.

RESULTS

Both macro- and microcalcification were found in human anterior tibial artery plaques. Macrocalcification formed after the silencing of RAGE, and microcalcification formed after the silencing of galectin-3. In the process of RAGE- or galcetin-3-induced calcification, sortilin played an important role downstream. SEM showed that sortilin promoted the aggregation of MVs in the early stage of calcification and formed larger calcified nodules.

CONCLUSION

RAGE downregulated sortilin and then transmitted microcalcification signals, whereas galectin-3 upregulated sortilin, which accelerated the aggregation of MVs in the early stage of calcification and mediated the formation of macrocalcifications, These data illustrate the progression of two calcification types and suggest sortilin as a potential target for early intervention of calcification and as an effective biomarker for the assessment of long-term clinical risk and prognosis.

Loss of TrkB Signaling Due to Status Epilepticus Induces a proBDNF-Dependent Cell Death

Neurotrophins (NTs) are secretory proteins that bind to target receptors and influence many cellular functions, such as cell survival and cell death in neurons. The mammalian NT brain-derived neurotrophic factor (matBDNF) is the C-terminal mature form released by cleavage from the proBDNF precursor. The binding of matBDNF to the tyrosine kinase receptor B (TrkB) activates different signaling cascades and leads to neuron survival and plasticity, while the interaction of proBDNF with the p75 NT receptor (p75NTR)/sortilin receptor complex has been highly involved in apoptosis. Many studies have demonstrated that prolonged seizures such as status epilepticus (SE) induce changes in the expression of NT, pro-NT, and their receptors. We have previously described that the blockage of both matBDNF and proBDNF signaling reduces neuronal death after SE in vivo (Unsain et al., 2008). We used an in vitro model as well as an in vivo model of SE to determine the specific role of TrkB and proBDNF signaling during neuronal cell death. We found that the matBDNF sequestering molecule TrkB-Fc induced an increase in neuronal death in both models of SE, and it also prevented a decrease in TrkB levels. Moreover, SE triggered the interaction between proBDNF and p75NTR, which was not altered by sequestering matBDNF. The intra-hippocampal administration of TrkB-Fc, combined with an antibody against proBDNF, prevented neuronal degeneration. In addition, we demonstrated that proBDNF binding to p75NTR exacerbates neuronal death when matBDNF signaling is impaired through TrkB. Our results indicated that both the mature and the precursor forms of BDNF may have opposite effects depending on the scenario in which they function and the signaling pathways they activate.

Effects of NaHS and hydroxylamine on the expressions of brain-derived neurotrophic factor and its receptors in rats after cardiac arrest and cardiopulmonary resuscitation

BACKGROUND

H₂S can also protect nerve cells. The objective of the study is to investigate the effects of hydrogen sulfide (H₂S) on the expressions of brain-derived neurotrophic factor (BDNF) and its receptors, tyrosine protein kinase B (TrkB) and p75 neurotrophin receptor (p75NTR), in brain tissues of rats with cardiac arrest and cardiopulmonary resuscitation (CA/CPR) following the restoration of spontaneous circulation (ROSC).

METHODS

Rats (n = 240) with CA/CPR were divided into three groups: Intervention (n = 80) that received sodium hydrosulfide (NaHS, 14 μmoL/kg·d) intervention after ROSC; Inhibition (n = 80) that received hydroxylamine (40 μmoL/kg·d) intervention after ROSC; and Control (n = 80) that received saline after ROSC. Kaplan-Meyer analysis was used to analyze the survival data. Quantitative real-time PCR (q-PCR), Western blot, immunohistochemistry and IODs (integrated optical density) were performed to determine the mRNA and protein expressions of BDNF, TrkB and p75NTR in rat brain tissues.

RESULTS

Survival rate of the three groups had significant difference (χ² = 28.376, p = 0.000). The Intervention group had the highest survival rate (82.5%), while the Inhibition group had the lowest survival rate (62.5%). The mRNA and protein levels of BDNF and TrkB in the Intervention group were significantly higher compared to the Control group (p < 0.05); while the mRNA and protein levels of BDNF and TrkB in the Inhibition group was significantly lower than the Control group (p < 0.05) on days 1, 3, and 7. However, the mRNA and protein levels of p75NTR in the Intervention group were significantly lower than the Control group (p < 0.05); while the mRNA and protein levels of p75NTR in the Inhibition group were significantly higher than the Control group (p < 0.05) on days 1, 3, and 7.

CONCLUSION

NaHS treatment increases the survival rate of rats after CA and ROSC by upregulating the expression and activation of BDNF and its receptor TrkB, and down-regulating p75NTR expression.