Seminario 16 Patrones de herencia no clásicos 1 - Tomas Falzone

Introduction to Non-Classical Inheritance Patterns

Overview of the Seminar

• The seminar focuses on non-classical inheritance patterns, specifically mitochondrial inheritance and its implications.

• It is recommended to view both this seminar (16) and the next one (17), which will cover triplet expansion and imprinting.

Key Concepts in Mitochondrial Inheritance

• Understanding molecular mechanisms associated with mitochondrial entities is crucial for grasping how these patterns manifest within families.

• The concept of a threshold is introduced, indicating that certain conditions can lead to phenotypic expression of diseases linked to a mix of healthy and damaged mitochondria.

Historical Context and Genetic Structure

Advances in Mitochondrial Genetics

• The discussion traces back to the 1980s when sequencing advancements allowed for the first detailed understanding of mitochondrial DNA organization.

• Eukaryotic cells contain a nucleus with genetic information organized into chromosomes, alongside mitochondria that provide energy through their own distinct DNA.

Characteristics of Mitochondrial DNA

• Mitochondrial DNA (mtDNA) is circular and double-stranded, resembling bacterial plasmids, allowing replication independent from nuclear DNA.

• It encodes 37 genes: 13 proteins essential for mitochondrial function, along with 22 transfer RNA genes and ribosomal RNA genes necessary for protein synthesis within mitochondria.

Structural Features of Mitochondria

Membrane Structure and Functionality

• Mitochondria have a unique double membrane structure; the outer membrane differs chemically from the inner membrane, which features invaginations known as cristae that enhance energy production efficiency.

• This complex structure supports various cellular functions including ATP production through oxidative phosphorylation processes occurring in the inner membrane's folds.

Endosymbiotic Theory

Evolutionary Insights into Mitochondrial Origin

• The endosymbiotic theory posits that ancestral eukaryotic cells incorporated aerobic bacteria capable of energy production, leading to mutual benefits and higher energy levels conducive to multicellularity development.

Mitochondrial Structure and Function

Characteristics of Mitochondria

• Mitochondria have a double membrane structure, which is crucial for their function. The outer membrane allows the incorporation of bacteria into eukaryotic cells, leading to the formation of mitochondrial cristae that enhance energy production.

• The outer membrane contains pores that regulate the passage of certain proteins, while the inner membrane has lower permeability due to cardiolipin, facilitating ion transport and oxidative phosphorylation.

• The inner mitochondrial membrane houses ion transporters and proteins essential for the electron transport chain and ATP synthesis. The matrix contains various enzymes involved in metabolic processes like pyruvate dehydrogenase and beta-oxidation.

Genetic Aspects of Mitochondria

• Mitochondrial DNA (mtDNA) is circular and resembles prokaryotic DNA rather than nuclear DNA. It has a distinct genetic code with higher mutation rates and less developed repair mechanisms compared to nuclear DNA.

• Ribosomal structures in mitochondria differ from those in nuclear genomes, supporting the theory of their prokaryotic origin after being incorporated into eukaryotic cells.

Implications for Cellular Energy Dynamics

• The presence of prokaryotic-like structures in mitochondria leads to potential vulnerabilities; antibiotics targeting bacterial functions can also affect mitochondrial protein synthesis.

• Mitochondrial numbers within a cell are dynamic and can change based on energy demands through processes like fission (splitting one mitochondrion into two) or fusion (combining smaller mitochondria).

Recovery Mechanisms in Mitochondria

• Damaged mitochondria can undergo recovery through fusion events that allow them to incorporate new DNA and proteins, enhancing their structural integrity.

• When damage is irreparable, dysfunctional mitochondria are marked for degradation via mitophagy—a selective autophagic process that removes damaged organelles from the cell.

Summary of Key Features

• Key indicators supporting the endosymbiotic theory include naked circular mtDNA without histones, unique ribosomal characteristics, high mutation rates, and susceptibility to inhibitors affecting prokaryotes.

• Fusion-fission dynamics play a critical role in maintaining mitochondrial health by allowing adaptation to cellular energy needs while managing damaged components effectively.

Mitochondrial Function and Protein Synthesis

Autophagy and Mitochondrial Damage

• Autophagy is initiated around damaged mitochondria, which are marked for degradation. This process involves the formation of an autophagosome that fuses with lysosomes to degrade mitochondrial components.

Role of Mitochondria in Apoptosis

• Mitochondria play a crucial role in apoptosis through intrinsic signaling pathways, particularly via the release of cytochrome C from the intermembrane space.

Mitochondrial DNA and Protein Coding

• Mitochondrial DNA encodes only 13 proteins essential for mitochondrial function, while many other important proteins are encoded by nuclear DNA.

Protein Import into Mitochondria

• Proteins synthesized in free ribosomes must be imported into mitochondria. Chaperones like cytosolic HSP70 keep these proteins unfolded for translocation through mitochondrial membrane transporters (TOM).

Chaperone Functions and Protein Folding

• After passing through TOM, proteins require ATP-dependent chaperones (HSP60) within mitochondria to fold correctly for their specific functions in the matrix.

Protein Targeting Mechanisms

Pathways for Nuclear-Coded Proteins

• Different nuclear-coded proteins have distinct targeting pathways to mitochondria involving cytosolic chaperones and mitochondrial translocators.

Techniques for Studying Mitochondrial Proteins

• To study mitochondrial protein functionality, subcellular fractionation techniques can isolate these proteins for analysis using methods like Western blotting.

Subcellular Fractionation Process

Isolation of Cellular Components

• Subcellular fractionation separates cellular components based on density or weight. This involves homogenizing tissues or cells to break membranes and create a solution containing all cellular components.

Centrifugation Techniques

• Differential centrifugation allows separation of larger components (like nuclei) from smaller ones (like synaptic vesicles), enabling isolation of mitochondria from other cell parts.

Further Separation of Organelles

Mitochondrial Function and Energy Production

Mitochondrial Isolation and Analysis

• The process begins with obtaining mitochondria from a precipitate, allowing for various assays to measure membrane transport, protein activity, and energy production.

• Soft, non-ionic detergents can selectively disrupt the outer membrane of mitochondria, separating intermembrane space components from those in the matrix.

• Further breakdown of membranes allows for isolation of internal membrane components and matrix materials to study mitochondrial functionality related to energy production.

Key Functions of Mitochondria

• The primary function of mitochondria is ATP synthesis; however, they also play crucial roles in calcium storage, steroid hormone production, and apoptosis signaling.

• Mitochondria are involved in lipid metabolism and amino acid synthesis alongside their energetic functions.

Cellular Metabolism Overview

• A brief overview of cellular metabolism associated with carbohydrate catabolism is provided; understanding mechanisms rather than memorizing enzyme names is emphasized.

• Glycolysis converts glucose into pyruvate in the cytoplasm, which then enters the mitochondria for further processing through oxidative decarboxylation.

Energy Production Pathways

• Glycolysis yields two ATP molecules but is inefficient compared to more advanced energy production mechanisms developed through endosymbiosis.

• Coenzymes like NAD+ (oxidized form) and NADH (reduced form) are critical as early electron acceptors during metabolic cycles.

Krebs Cycle and Electron Transport Chain

• Pyruvate decarboxylation produces acetyl-CoA that feeds into the Krebs cycle; this cycle generates reduced forms of NADH and FADH2 essential for electron transport.

• Electrons transferred through complexes I-IV in the electron transport chain facilitate proton pumping into the intermembrane space, creating a proton gradient necessary for ATP synthesis.

ATP Synthesis Mechanism

• The generated proton gradient drives protons back through ATP synthase, synthesizing ATP from ADP and inorganic phosphate via mechanical rotation induced by proton flow.

• Disruption of this gradient or increased membrane permeability can hinder ATP production efficiency.

Mitochondrial Function and Genetic Inheritance

Mitochondrial Energy Production

• The mitochondria play a crucial role in ATP production, utilizing proton transfer to maintain a gradient essential for energy generation.

• Acetyl-CoA can be produced from fatty acids through beta-oxidation within the mitochondrial matrix, contributing to the Krebs cycle for enhanced energy output.

• Mutations in mitochondrial DNA can disrupt energy production; however, not all mitochondrial disorders are inherited maternally.

Genetic Inheritance Patterns

• Mitochondrial inheritance is maternal since only the oocyte contributes mitochondria to the offspring, while sperm contribute nuclear DNA.

• This unique inheritance pattern allows for tracing mutations through maternal lineage in genealogical studies.

Implications of Mitochondrial Mutations

• Male individuals with mitochondrial mutations do not pass these on to their children, whereas females can transmit them to all offspring.

• Mitochondrial DNA is polyploid, meaning multiple copies exist within each mitochondrion, complicating traditional concepts of dominance and recessivity.

Heteroplasmy and Its Effects

• Heteroplasmy refers to the presence of both normal and mutated mitochondrial DNA within a single cell, affecting cellular function based on the proportion of healthy versus dysfunctional mitochondria.

• Cells may exhibit homoplasmy (all normal or all dysfunctional mitochondria), but heteroplasmic cells show varying degrees of functionality depending on their mitochondrial composition.

Segregation Mechanisms in Cell Division

• During cell division, heteroplasmic cells may differentially segregate healthy and damaged mitochondria, impacting future generations' health.

• The "bottleneck effect" illustrates how random segregation during cell division can lead to significant variability in mitochondrial populations across cells.

Mitochondrial Dysfunction and Energy Thresholds

Observations of Tissue Specificity

• The discussion begins with the observation of specific tissues that are altered or dysfunctional when there is a coexistence of healthy and damaged mitochondria, as well as normal and dysfunctional mitochondria.

• Tissues with high energy demands show significant functional impairments due to their reliance on a specific number of functional mitochondria for ATP production.

Energy Threshold Indicators

• An energy threshold indicator is introduced, which helps determine the coexistence of cells with functional versus dysfunctional mitochondria capable of maintaining an energy level.

• If cells contain a low number of dysfunctional mitochondria, they can maintain the energy threshold; however, an increase in dysfunctional mitochondria leads to an inability to sustain this threshold, resulting in observable phenotypes.

Heteroplasmy and Clinical Phenotypes

• The concept of heteroplasmy is linked to the energy threshold effect, indicating that a high presence of dysfunctional mitochondria correlates with cellular phenotypes that may translate into clinical manifestations in patients.

Example: Leber's Hereditary Optic Neuropathy

• Leber's hereditary optic neuropathy (LHON) serves as an example where mitochondrial inheritance affects visual fields due to damage or alteration in the optic nerve.

• Neurons within the optic nerve have high energy demands due to their extensive projections from the optic nerve to the visual cortex. Mutations affecting mitochondrial protein coding can lead to dysfunctionality.

Maternal Transmission Patterns