Evaluation of the Effect of Resistance Exercise and Donepezil on Some Neurotrophin-Induced Changes in Gene Expression and Trk Receptors in the Hippocampus of Rats with Alzheimer’s Disease

Objectives: This study aimed to evaluate the impact of resistance exercise and donepezil on some neurotrophins gene expression and Trk receptors in the hippocampus of rats with Alzheimer’s disease (AD). Methods: In this study, 32 male adult Wistar rats (mean weight: 230 - 280 g) were assigned into two groups of AD and control. The control and AD groups received normal saline and streptozotocin (STZ) through intraventricular injection, respectively. Then, six subgroups were considered: (1) control rest (Con); (2) control exercise (Con-Exe); (3) Alzheimer’s rest (Alz); (4) Alzheimer’s exercise (Alz-Exe); (5) Alzheimer’s donepezil (Alz-Don); and (6) Alzheimer’s donepezil-exercise (Alz-Don-Exe). Donepezil was fed daily at a dose of 1.5 mg/kg to the treated groups. The three subgroups of exercising rats received exercises for eight weeks (three times a week). Each day, the resting groups were managed to decrease stress impacts. Twenty-four hours after the last session of exercise by the eighth week, deep anesthesia was applied, and the rats' heads were severed. Results: Considering an error rate below 5% (P < 0.05) and a confidence of more than 95%, a significant difference was observed in BDNF, NT3, NGF, TrkA, and TrkB values between exercising and donepezil-exercise rats compared to AD group. These values were considerably greater for donepezil-exercising Alzheimer’s group. Besides, the donepezil group was not significantly different from the Alzheimer’s group. Conclusions: Although the use of donepezil alone did not significantly increase the expression of the studied genes, the concomitant use of the drug and resistance training significantly increased the expression levels.

Localization of Neurotrophin Specific Trk Receptors in Mechanosensory Systems of Killifish (Nothobranchius guentheri)

Neurotrophins (NTs) and their signal-transducing Trk receptors play a crucial role in the development and maintenance of specific neuronal subpopulations in nervous and sensory systems. NTs are supposed to regulate two sensory systems in fish, the inner ear and the lateral line system (LLS). The latter is one of the major mechanosensory systems in fish. Considering that annual fishes of the genus Nothobranchius, with their short life expectancy, have become a suitable model for aging studies and that the occurrence and distribution of neurotrophin Trk receptors have never been investigated in the inner ear and LLS of killifish (Nothobranchius guentheri), our study aimed to investigate the localization of neurotrophin-specific Trk receptors in mechanosensory systems of N. guentheri. For histological and immunohistochemical analysis, adult specimens of N. guentheri were processed using antibodies against Trk receptors and S100 protein. An intense immunoreaction for TrkA and TrkC was found in the sensory cells of the inner ear as well as in the hair cells of LLS. Moreover, also the neurons localized in the acoustic ganglia displayed a specific immunoreaction for all Trk receptors (TrkA, B, and C) analyzed. Taken together, our results demonstrate, for the first time, that neurotrophins and their specific receptors could play a pivotal role in the biology of the sensory cells of the inner ear and LLS of N. guentheri and might also be involved in the hair cells regeneration process in normal and aged conditions.

Identification of Novel Positive Allosteric Modulators of Neurotrophin Receptors for the Treatment of Cognitive Dysfunction

Alzheimer’s disease (AD) is the most common neurodegenerative disorder and results in severe neurodegeneration and progressive cognitive decline. Neurotrophins are growth factors involved in the development and survival of neurons, but also in underlying mechanisms for memory formation such as hippocampal long-term potentiation. Our aim was to identify small molecules with stimulatory effects on the signaling of two neurotrophins, the nerve growth factor (NGF) and the brain derived neurotrophic factor (BDNF). To identify molecules that could potentiate neurotrophin signaling, 25,000 molecules were screened, which led to the identification of the triazinetrione derivatives ACD855 (Ponazuril) and later on ACD856, as positive allosteric modulators of tropomyosin related kinase (Trk) receptors. ACD855 or ACD856 potentiated the cellular signaling of the neurotrophin receptors with EC50 values of 1.9 and 3.2 or 0.38 and 0.30 µM, respectively, for TrkA or TrkB. ACD855 increased acetylcholine levels in the hippocampus by 40% and facilitated long term potentiation in rat brain slices. The compounds acted as cognitive enhancers in a TrkB-dependent manner in several different behavioral models. Finally, the age-induced cognitive dysfunction in 18-month-old mice could be restored to the same level as found in 2-month-old mice after a single treatment of ACD856. We have identified a novel mechanism to modulate the activity of the Trk-receptors. The identification of the positive allosteric modulators of the Trk-receptors might have implications for the treatment of Alzheimer’s diseases and other diseases characterized by cognitive impairment.

Effects of Eight Weeks of Resistance Exercises on Expression of Neurotrophins and Trk Receptors in Alzheimer Model Male Wistar Rats

This study aims to evaluate the effects of eight weeks of resistance exercises on expression of neurotrophins and Trk receptors in Alzheimer model male Wistar rats. For this purpose, 32 mature male Wistar rats with mean weights of 230 to 280 g were chosen and divided into Alzheimer and Sham groups. Rats in the Sham group received normal saline while rats in the Alzheimer group received STZ via intraventricular injection. These rats were then divided into the following four subgroups: 1. Resting Sham, 2. Exercising Sham, 3. Resting Alzheimer, and 4. Exercising Alzheimer. The two exercising rat subgroups, exercised three times a week for a period of eight weeks. A weight was attached to their tails and they had to carry this weight on a 26-step ladder in each cycle. Resting groups were handled every day to minimize the effects of stress level. At the end of the 8th week and 24 hours after the last exercise session (to avoid the effects of the last exercise session), the rats were put under deep anesthesia and scarified by head separation. Hippocampus tissues were precisely extracted and samples were sent to laboratory for molecular and cellular tests. In order to investigate gene expression, Quantitative RT-PCR was used. Results of the tests for comparing the means of BDNF, NT3, NGF, TrkA, and TrkB in two rat groups showed that with error levels of less than 5% and confidence of over 95%, there is a significant difference in the amounts of BDNF, NT3, NGF, TrkA, and TrkB between exercising rats and resting rats. These amounts were much higher in exercising Alzheimer rats group. Eight weeks of resistance exercises increased the expression of BDNF, NT3, and NGF genes as well as TrkA and TrkB receptors in Alzheimer model Wistar rats.

Constitutively active TrkB kinase signalling reduces actin filopodia dynamics and cell migration

Trk receptors and gene fusions of NTRK are targets in precision oncology. Classical Trk signalling concepts fail to explain ligand-independent signalling of intracellular TrkB or NTRK fusion proteins. Here, we show that abundance of the intracellular domain of TrkB is sufficient for ligand-independent autophosphorylation. This constitutive TrkB signalling reduced actin filopodia dynamics, could phosphorylate FAK, and changed cell morphology. Mutating Y705 in the kinase domain of TrkB alone specifically blocked these pathways. Engineered intracellular kinase domain proteins and a cancer-related intracellular NTRK2-fusion protein (SQSTM1-NTRK2) also underwent constitutive activation. In migrating glioblastoma-like U87MG cells, self-active TrkB kinase reduced cell migration. Moreover, we found evidences for constitutively active, intracellular TrkB in tissue of human grade IV glioblastoma. Structural modelling of the kinase domain let us postulate that ‘release from cis-autoinhibition by abundance’ is sufficient for TrkB/FAK/Actin signalling via Y705. These constitutive signalling pathways could be fully blocked within minutes by clinically approved, anti-tumorigenic Trk inhibitors. In conclusion, our data provide an explanation and biological function for TrkB kinase domain signalling in the absence of a ligand.

A soluble derivative of PrPC activates cell-signaling and regulates cell physiology through LRP1 and the NMDA receptor

Cellular prion protein (PrPC) is a widely expressed glycosylphosphatidylinositol-anchored membrane protein. Scrapie prion protein is a misfolded and aggregated form of PrPC responsible for prion-induced neurodegenerative diseases. Understanding the function of the nonpathogenic PrPC monomer is an important objective. PrPC may be shed from the cell surface to generate soluble derivatives. Herein, we studied a recombinant derivative of PrPC (soluble cellular prion protein, S-PrP) that corresponds closely in sequence to a soluble form of PrPC shed from the cell surface by proteases in the A Disintegrin And Metalloprotease (ADAM) family. S-PrP activated cell-signaling in PC12 and N2a cells. TrkA was transactivated by Src family kinases and extracellular signal–regulated kinase 1/2 was activated downstream of Trk receptors. These cell-signaling events were dependent on the N-methyl-d-aspartate receptor (NMDA-R) and low-density lipoprotein receptor-related protein-1 (LRP1), which functioned as a cell-signaling receptor system in lipid rafts. Membrane-anchored PrPC and neural cell adhesion molecule were not required for S-PrP–initiated cell-signaling. S-PrP promoted PC12 cell neurite outgrowth. This response required the NMDA-R, LRP1, Src family kinases, and Trk receptors. In Schwann cells, S-PrP interacted with the LRP1/NMDA-R system to activate extracellular signal–regulated kinase 1/2 and promote cell migration. The effects of S-PrP on PC12 cell neurite outgrowth and Schwann cell migration were similar to those caused by other proteins that engage the LRP1/NMDA-R system, including activated α2-macroglobulin and tissue-type plasminogen activator. Collectively, these results demonstrate that shed forms of PrPC may exhibit important biological activities in the central nervous system and the peripheral nervous system by serving as ligands for the LRP1/NMDA-R system.

Recent advances in understanding context-dependent mechanisms controlling neurotrophin signaling and function

Complex mechanisms control the signaling of neurotrophins through p75NTR and Trk receptors, allowing cellular responses that are highly context dependent, particularly in the nervous system and particularly with regard to the neurotrophin brain-derived neurotrophic factor (BDNF). Recent reports describe a variety of sophisticated regulatory mechanisms that contribute to such functional flexibility. Mechanisms described include regulation of trafficking of alternative BDNF transcripts, regulation of post-translational processing and secretion of BDNF, engagement of co-receptors that influence localization and signaling of p75NTR and Trk receptors, and control of trafficking of receptors in the endocytic pathway and during anterograde and retrograde axonal transport.