5-HT3 Receptors on Mitochondria Influence Mitochondrial Function

Mitochondria Express Functional Signaling Ligand-Binding Receptors that Regulate their Biological Responses - the Novel Role of Mitochondria as Stress-Response Sentinels

Evidence accumulated mitochondria, as the "powerplants of the cell," express several functional receptors for external ligands that modify their function and regulate cell biology. This review sheds new light on the role of these organelles in sensing external stimuli to facilitate energy production for cellular needs. This is possible because mitochondria express some receptors on their membranes that are responsible for their autonomous responses. This is not surprising given the widely accepted hypothesis that these intracellular organelles originated from prokaryotic ancestors that fused with eukaryotic cells during early evolution. It has been reported that mitochondria express functional estrogen, androgen, glucocorticoid, 5-hydroxytryptamine, melatonin, and cannabinoid receptors. What is intriguing is recent evidence showing that mitochondria could also be directly regulated by active mediators of intracellular complement (complosome) and intrinsic mediators of purinergic signaling. Accordingly, they express receptors for intracellular complement cleavage fragments (C5a and C3a) as well as for adenosine triphosphate (ATP), which, besides its crucial role in transferring energy in the cells, is also an important signaling molecule interacting with P2X7 receptor expressed not only on the cell surface but also on the mitochondria membrane. Based on this, intrinsic complosome and purinergic signaling mediators emerge as important cooperating regulators of reactive oxygen species (ROS) release from mitochondria and activators of intracellular pattern recognition receptor Nlrp3 inflammasome. This activation within the beneficial "hormetic zone response" regulates cell metabolism, proliferation, migration, and adaptation to the surrounding challenges of the microenvironment in a favorable way.

Serotonin effects on human iPSC-derived neural cell functions: from mitochondria to depression

Depression's link to serotonin dysregulation is well-known. The monoamine theory posits that depression results from impaired serotonin activity, leading to the development of antidepressants targeting serotonin levels. However, their limited efficacy suggests a more complex cause. Recent studies highlight mitochondria as key players in depression's pathophysiology. Mounting evidence indicates that mitochondrial dysfunction significantly correlates with major depressive disorder (MDD), underscoring its pivotal role in depression. Exploring the serotonin-mitochondrial connection, our study investigated the effects of chronic serotonin treatment on induced-pluripotent stem cell-derived astrocytes and neurons from healthy controls and two case study patients. One was a patient with antidepressant non-responding MDD ("Non-R") and another had a non-genetic mitochondrial disorder ("Mito"). The results revealed that serotonin altered the expression of genes related to mitochondrial function and dynamics in neurons and had an equalizing effect on calcium homeostasis in astrocytes, while ATP levels seemed increased. Serotonin significantly decreased cytosolic and mitochondrial calcium in neurons. Electrophysiological measurements evidenced that serotonin depolarized the resting membrane potential, increased both sodium and potassium current density and ultimately improved the overall excitability of neurons. Specifically, neurons from the Non-R patient appeared responsive to serotonin in vitro, which seemed to improve neurotransmission. While it is unclear how this translates to the systemic level and AD resistance mechanisms are not fully elucidated, our observations show that despite his treatment resistance, this patient's cortical neurons are responsive to serotonergic signals. In the Mito patient, evidence suggested that serotonin, by increasing excitability, exacerbated an existing hyperexcitability highlighting the importance of considering mitochondrial disorders in patients with MDD, and avoiding serotonin-increasing medication. Taken together, our findings suggested that serotonin positively affects calcium homeostasis in astrocytes and increases neuronal excitability. The latter effect must be considered carefully, as it could have beneficial or detrimental implications based on individual pathologies.

Lycopene alleviates BCG-induced depressive phenotypes in mice by disrupting 5-HT3 receptor - IDO1 interplay in the brain

The 5-HT3 receptor and indoleamine 2,3-dioxygenase 1 (IDO1) enzyme play a crucial role in the pathogenesis of depression as their activation reduces serotonin contents in the brain. Since molecular docking analysis revealed lycopene as a potent 5-HT3 receptor antagonist and IDO1 inhibitor, we hypothesized that lycopene might disrupt the interplay between the 5-HT3 receptor and IDO1 to mitigate depression. In mice, the depression-like phenotypes were induced by inoculating Bacillus Calmette-Guerin (BCG). Lycopene (intraperitoneal; i.p.) was administered alone or in combination with 5-HT3 receptor antagonist ondansetron (i.p.) or IDO1 inhibitor minocycline (i.p.), and the behavioral screening was performed by the sucrose preference test, open field test, tail suspension test, and splash test which are based on the different principles. Further, the brains were subjected to the biochemical analysis of serotonin and its precursor tryptophan by the HPLC. The results showed depression-like behavior in BCG-inoculated mice, which was reversed by lycopene administration. Moreover, prior treatment with ondansetron or minocycline potentiated the antidepressant action of lycopene. Minocycline pretreatment also enhanced the antidepressant effect of ondansetron indicating the regulation of IDO1 activity by 5-HT3 receptor-triggered signaling. Biochemical analysis of brain samples revealed a drastic reduction in the levels of tryptophan and serotonin in depressed animals, which were restored following treatment with lycopene and its combination with ondansetron or minocycline. Taken together, the data from molecular docking, behavioral experiments, and biochemical estimation suggest that lycopene might block the 5-HT3 receptor and consequently inhibit the activity of IDO1 to ameliorate BCG-induced depression in mice.

Serotonin regulation of mitochondria in kidney diseases

Serotonin, while conventionally recognized as a neurotransmitter in the CNS, has recently gained attention for its role in the kidney. Specifically, serotonin is not only synthesized in the kidney, but it also regulates glomerular function, vascular resistance, and mitochondrial homeostasis. Because of serotonin's importance to mitochondrial health, this review is focused on the role of serotonin and its receptors in mitochondrial function in the context of acute kidney injury, chronic kidney disease, and diabetic kidney disease, all of which are characterized by mitochondrial dysfunction and none of which has approved pharmacological treatments. Evidence indicates that activation of certain serotonin receptors can stimulate mitochondrial biogenesis (MB) and restore mitochondrial homeostasis, resulting in improved renal function. Serotonin receptor agonists that induce MB are therefore of interest as potential therapeutic strategies for renal injury and disease. SIGNIFICANCE STATEMENT: Mitochondrial dysfunction is associated with many human renal diseases such as acute kidney injury, chronic kidney disease, and diabetic kidney disease, which are associated with increased morbidity and mortality. Unfortunately, none of these pathologies has an FDA-approved pharmacological intervention, underscoring the urgency of identifying new therapeutics for such disorders. Studies show that induction of mitochondrial biogenesis via serotonin (5-hydroxytryptamine, 5-HT) receptors reduces kidney injury markers, restores mitochondrial and renal function after kidney injury, and decreases mortality, suggesting that targeting 5-HT receptors may be a promising therapeutic avenue for mitochondrial dysfunction in kidney diseases. While numerous reviews describe the importance of mitochondria and mitochondrial quality control mechanisms in kidney disease, the relevance of 5-HT receptor-mediated mitochondrial metabolic modulation in the kidney has yet to be thoroughly explored.

Metabolic control of induced pluripotency

Pluripotent stem cells of the mammalian epiblast and their cultured counterparts-embryonic stem cells (ESCs) and epiblast stem cells (EpiSCs)-have the capacity to differentiate in all cell types of adult organisms. An artificial process of reactivation of the pluripotency program in terminally differentiated cells was established in 2006, which allowed for the generation of induced pluripotent stem cells (iPSCs). This iPSC technology has become an invaluable tool in investigating the molecular mechanisms of human diseases and therapeutic drug development, and it also holds tremendous promise for iPSC applications in regenerative medicine. Since the process of induced reprogramming of differentiated cells to a pluripotent state was discovered, many questions about the molecular mechanisms involved in this process have been clarified. Studies conducted over the past 2 decades have established that metabolic pathways and retrograde mitochondrial signals are involved in the regulation of various aspects of stem cell biology, including differentiation, pluripotency acquisition, and maintenance. During the reprogramming process, cells undergo major transformations, progressing through three distinct stages that are regulated by different signaling pathways, transcription factor networks, and inputs from metabolic pathways. Among the main metabolic features of this process, representing a switch from the dominance of oxidative phosphorylation to aerobic glycolysis and anabolic processes, are many critical stage-specific metabolic signals that control the path of differentiated cells toward a pluripotent state. In this review, we discuss the achievements in the current understanding of the molecular mechanisms of processes controlled by metabolic pathways, and vice versa, during the reprogramming process.