BDNF-TrkB Signaling in Mitochondria: Implications for Neurodegenerative Diseases.

Brain-derived neurotrophic factor (BDNF) plays a pivotal role in neuronal development, synaptic plasticity, and overall neuronal health by binding to its receptor, tyrosine receptor kinase B (TrkB). This review delves into the intricate mechanisms through which BDNF-TrkB signaling influences mitochondrial function and potentially influences pathology in neurodegenerative diseases. This review highlights the BDNF-TrkB signaling pathway which regulates mitochondrial bioenergetics, biogenesis, and dynamics, mitochondrial processes vital for synaptic transmission and plasticity. Furthermore, we explore how the BDNF-TrkB-PKA signaling in the cytosol and in mitochondria affects mitochondrial transport and distribution and mitochondrial content, which is crucial for supporting the energy demands of synapses. The dysregulation of this signaling pathway is linked to various neurodegenerative diseases, including Alzheimer's and Parkinson's disease, which are characterized by mitochondrial dysfunction and reduced BDNF expression. By examining seminal studies that have characterized this signaling pathway in health and disease, the present review underscores the potential of enhancing BDNF-TrkB signaling to mitigate mitochondrial dysfunction in neurodegenerative diseases, offering insights into therapeutic strategies to enhance neuronal resilience and function.

Brain-derived neurotrophic factor protects neurons by stimulating mitochondrial function through protein kinase A.

Brain-derived neurotrophic factor (BDNF) stimulates dendrite outgrowth and synaptic plasticity by activating downstream protein kinase A (PKA) signaling. Recently, BDNF has been shown to modulate mitochondrial respiration in isolated brain mitochondria, suggesting that BDNF can modulate mitochondrial physiology. However, the molecular mechanisms by which BDNF stimulates mitochondrial function in neurons remain to be elucidated. In this study, we surmised that BDNF binds to the TrkB receptor and translocates to mitochondria to govern mitochondrial physiology in a PKA-dependent manner. Confocal microscopy and biochemical subcellular fractionation assays confirm the localization of the TrkB receptor in mitochondria. The translocation of the TrkB receptor to mitochondria was significantly enhanced upon treating primary cortical neurons with exogenous BDNF, leading to rapid PKA activation. Showing a direct role of BDNF in regulating mitochondrial structure/function, time-lapse confocal microscopy in primary cortical neurons showed that exogenous BDNF enhances mitochondrial fusion, anterograde mitochondrial trafficking, and mitochondrial content within dendrites, which led to increased basal and ATP-linked mitochondrial respiration and glycolysis as assessed by an XF24e metabolic analyzer. BDNF-mediated regulation of mitochondrial structure/function requires PKA activity as treating primary cortical neurons with a pharmacological inhibitor of PKA or transiently expressing constructs that target an inhibitor peptide of PKA (PKI) to the mitochondrion abrogated BDNF-mediated mitochondrial fusion and trafficking. Mechanistically, western/Phos-tag blots show that BDNF stimulates PKA-mediated phosphorylation of Drp1 and Miro-2 to promote mitochondrial fusion and elevate mitochondrial content in dendrites, respectively. Effects of BDNF on mitochondrial function were associated with increased resistance of neurons to oxidative stress and dendrite retraction induced by rotenone. Overall, this study revealed new mechanisms of BDNF-mediated neuroprotection, which entails enhancing mitochondrial health and function of neurons.

Multiplexing Seahorse XFe24 and ImageXpress® Nano Platforms for Comprehensive Evaluation of Mitochondrial Bioenergetic Profile and Neuronal Morphology.

The measurement of mitochondrial function has become imperative to understand and characterize diseases characterized by bioenergetic alterations. The advancement of automation and application of high-throughput technologies has propelled our understanding of biological complexity and facilitated drug discovery. Seahorse extracellular flux (XFe) technology measures changes in dissolved oxygen and proton concentration in cell culture media, providing kinetic measurements of oxidative phosphorylation and glycolytic metabolism. ImageXpress® Nano is an automated fluorescent microscope with the ability to perform high-content, fast, and robust imaging in multi-well formats. In this chapter, we present a comprehensive protocol to multiplex the Seahorse XFe24 analyzer with the ImageXpress® Nano high content imaging microscope to provide a comprehensive yet rigorous profile of bioenergetics and its correlation to neuronal function and morphology.

Mitochondrial PKA Is Neuroprotective in a Cell Culture Model of Alzheimer's Disease.

Alzheimer's disease (AD) is a neurodegenerative disease characterized by progressive memory loss and cognitive decline. In hippocampal neurons, the pathological features of AD include the accumulation of extracellular amyloid-beta peptide (Aß) accompanied by oxidative stress, mitochondrial dysfunction, and neuron loss. A decrease in neuroprotective Protein Kinase A (PKA) signaling contributes to mitochondrial fragmentation and neurodegeneration in AD. By associating with the protein scaffold Dual-Specificity Anchoring Protein 1 (D-AKAP1), PKA is targeted to mitochondria to promote mitochondrial fusion by phosphorylating the fission modulator dynamin-related protein 1 (Drp1). We hypothesized that (1) a decrease in the endogenous level of endogenous D-AKAP1 contributes to decreased PKA signaling in mitochondria and that (2) restoring PKA signaling in mitochondria can reverse neurodegeneration and mitochondrial fragmentation in neurons in AD models. Through immunohistochemistry, we showed that endogenous D-AKAP1, but not other mitochondrial proteins, is significantly reduced in primary neurons treated with Aß42 peptide (10µM, 24 h), and in the hippocampus and cortex from asymptomatic and symptomatic AD mice (5X-FAD). Transiently expressing wild-type, but not a PKA-binding deficient mutant of D-AKAP1, was able to reduce mitochondrial fission, dendrite retraction, and apoptosis in primary neurons treated with Aß42. Mechanistically, the protective effects of D-AKAP1/PKA are moderated through PKA-mediated phosphorylation of Drp1, as transiently expressing a PKA phosphomimetic mutant of Drp1 (Drp1-S656D) phenocopies D-AKAP1's ability to reduce Aß42-mediated apoptosis and mitochondrial fission. Overall, our data suggest that a loss of D-AKAP1/PKA contributes to mitochondrial pathology and neurodegeneration in an in vitro cell culture model of AD.