An Examination of Cellular Stress and its Effects on Mitochondrial DNA

Mitochondria Overview

  • Living organisms require a constant energy resource.

  • The primary source of energy comes from mitochondria, referred to as the "powerhouse of the cell".

  • Mitochondria are found in the cytosol (the liquid medium within eukaryotic cells).

    • Eukaryotic cells possess a defined nucleus and a nuclear membrane.

  • Mitochondria are responsible for approximately 90% of energy production, which is stored in adenosine triphosphate (ATP).

  • Mitochondria also influence cellular and physiological functions through signaling molecules.

Supportive Terms and Definitions

  • Enzyme: A biological macromolecule, typically a protein, that acts as a biological catalyst, accelerating chemical reactions without being consumed.

  • ATP Synthase: An enzyme that catalyzes the formation of ATP from adenosine diphosphate (ADP) and inorganic phosphate (Pi).

Electron Transport Chain (ETC)

  • The ETC consists of a series of protein complexes where electrons are transported from donors to acceptors, coupling electron transfer with proton (H+) transfer across a membrane.

  • Many enzymes involved in the ETC are embedded within the mitochondrial membrane.

Mitochondrial Structure

  • Key structures of mitochondria as seen through an electron microscope include:

    • Inner Membrane

    • Intermembrane Space

    • Outer Membrane

    • Matrix and Cristae

    • Ribosomes

  • FOF1-ATPase - F1 Portion: Integral to ATP synthesis.

Mitochondrial DNA (mtDNA) and Oxidative Stress

  • Vulnerability to Oxidative Stress: Oxidative phosphorylation involves adding a phosphoryl group (PO3-) to organic molecules and links ATP synthesis with electron movement through the ETC and oxygen consumption, leading to reactive oxygen species (ROS) production.

  • Physical or psychological stress causes oxidative stress, leading to intracellular ROS build-up that directly affects mtDNA.

  • mtDNA is more susceptible to damage from oxidative stress than nuclear DNA.

Mitochondrial vs. Nuclear DNA

  • Each mitochondrion contains its own genome, known as mitochondrial DNA (mtDNA), which is:

    • Double-stranded nucleic acid encoding essential genes inherited maternally only.

    • Replicates independently of nuclear DNA and is structurally distinct.

    • Smaller than nuclear DNA with 37 genes, 13 responsible for mitochondrial respiration.

  • Unlike nuclear DNA, mtDNA is more vulnerable to oxidative stress.

Psychological Stress and Mitochondria

  • Connection between Psychological Stress and Mitochondrial DNA:

    • Psychometric measures (e.g., Beck Depression Inventory, Perceived Stress Scale) lack biological correlation reflecting stress levels.

    • Biological measures such as ccf-mtDNA (circulating cell-free mtDNA) may indicate stress levels and resource availability for functioning.

Implications of ccf-mtDNA

  • Elevated ccf-mtDNA levels correlate with a 4-8 times increased risk of mortality in critically ill patients.

  • Higher ccf-mtDNA levels have been observed in conditions like:

    • Cancer

    • Acute systemic inflammatory responses

    • Chronic inflammation

    • Diabetes

    • Sepsis

    • Physical trauma

    • Myocardial infarction

  • Significantly higher ccf-mtDNA in patients with Major Depressive Disorders (MDD) and in suicidal individuals, with persistent high levels in SSRI non-responders.

Effects of Chronic and Acute Psychological Stress

  • Chronic psychological stress additionally damages mitochondrial function, decreasing enzymatic activity of ETC complexes.

  • This dysregulation leads to:

    • Altered rates of oxygen consumption

    • Changes in intracellular content

    • Genetic sequence alterations

Research Studies on Stress Effects

  • Studies examining the effects of acute stress (e.g., using the Trier Social Stress Test) reveal a consistent increase in ccf-mtDNA:

    • Hummeel et al: 1.6-fold increase shortly after stress induction.

    • Trumpff et al: 2-3-fold increase noted within 30 minutes after stress exposure in a mixed gender sample.

    • Current Study Aim: Aimed to replicate findings in a female university student population.

mtDNA Vulnerability to Oxidative Stress

  • Studies indicate mtDNA is 10-200 times more vulnerable to mutations compared to nuclear DNA under oxidative stress conditions.

  • Cells with high-energy demands (e.g., myocytes, neurons) are particularly affected.

  • Accumulation of oxidative damage can overwhelm repair systems, leading to release of damaged mtDNA into circulation.

  • Quantities of ccf-mtDNA detected in blood plasma reflect stress levels.

Pro-inflammatory Responses Induced by ccf-mtDNA

  • Released mtDNA from damaged mitochondria initiates pro-inflammatory responses:

    • Involvement of neutrophils, pattern recognition receptors (PRRs) such as TLR (Toll-like receptor) and NLRs (NOD-like receptors), leading to immune responses.

Study Rationale and Methodology

  • The importance of mitochondrial health is highlighted in chronic psychological diseases.

  • Significant gaps exist in knowledge relating to other stress induction forms.

  • Study Design:

    • Single-arm trial replicating prior stress induction studies.

    • Participants: University students viewing distressing images from the International Affective Picture System (IAPS).

Exclusion and Inclusion Criteria

  • Exclusion:

    • Pregnant/lactating individuals, smokers, recent mental health problems, chronic/acute disabilities, certain medications, infections, and recent tattoos.

  • Inclusion:

    • Aged 20-30, residing in Canada, and good general health with no significant medical history.

Study Population and Assessment Procedures

  • The study occurs on weekdays, with specific time slots to minimize dietary and stress-related confounding factors.

  • Participants must abstain from certain substances pre-testing.

  • Emotional image stress task (IST) to be used, assessing physiological and psychological reaction to stress.

Blood Sampling and Analysis

  • Blood tests conducted before and after exposure to measure changes in ccf-mtDNA.

  • Serum extraction involves multiple centrifugation steps to isolate ccf-mtDNA efficiently, preventing platelet contamination that could affect measurement.

Statistical Analysis Plan

  • Descriptive statistics to analyze demographic data and stress-induced ccf-mtDNA changes.

  • Evaluation of group differences in baseline and post-stressor blood samples.

Results

  • Notable pre-post differences in nDNA, with insignificant changes in ccf-mtDNA.

  • Results suggest significant reductions in nDNA rather than increases in ccf-mtDNA, similar to trends seen in cancer detection studies.