Fundamentals Of Cell Biology Yr 1

Sports and exercise science

General cell structure

Muscle cell structure

• Multinucleated - located close to the plasma membrane

• Contain mitochondria

• Microfibrils ( Actin/Myosin)

• Sarcoplasmic reticulum

Myogenesis

How we build muscle from a single cell

• Embryonic precursor cells (Myoblasts)

• Fusion/ alignment of these cells

• Constant cycles of de/regeneration

Cross section of muscle

Longitudinal portion

Plasma membrane

Fluid Mosaic Model:

• Phospholipid bilayer - Hydrophilic head/Hydrophobic tails

• Integral proteins act as carriers or channels

• Selective permeability - H2O and O2 easy entry/ CO2 easy removal

Sarcolemma

• Properties of excitability - allows for rapid distribution of signal throughout the muscle

• Conduct electrical impulses during depolarisation

• T tubules begin here and enter the muscle fibre

ER

Endoplasmic Reticulum (SER, RER & SR (muscle only))

RER = Protein

SER = steroids/lipids/phospholipids - important for metabolism

SR ( Sarcoplasmic reticulum)

Critical for excitation-contraction coupling

Mitochondria

• The majority of ATP pathways

• Regulates fats/CHO to produce energy

• Double membrane (Cristae) - Increase SA for Electron Transport Chain

Cytoskeleton

80% of muscle cell space - 1% in other cells

• Maintain cell shape/ structure

• Microtubules - Substrate/organelle transport

• Microfilaments - Allow contraction ( bind to myosin)

• Intermediate filaments

Ribosomes

1. Bind to the start of mRNA - tRNA delivers 1st amino acid ( Initiation)

2. The ribosome moves along mRNA, creating peptide bonds between Amino acids delivered by tRNA (elongation)

3. Ribosome reaches stop codon - Polypeptide chain released ( Termination)

Nucleus

• Holds genetic info

• Regulates gene expression

• Allows the cell to respond to stimuli or damage

Myo-nuclei

• Cannot divide

• One cell has many nuclei - Supports adaptation of skeletal muscle

Muscle force generation / Contraction

• Action potential at NMJ - Ach released

• Ach → Sarcolemma → action potential, Propagated T tubules

• Ca2+ released from SR

• Ca2+ binds to troponin → exposing actin

Sliding filament theory

• Myosin + Actin = Crossbridge

• “Power stroke” - Pulls Actin to centre (Shortening Sarcomere)

• Cross-bridge broken by ATP→ Myosin

• Cocking myosin ( ATP hydrolysis → ADP =Pi

• Ca2+ returns to SR

• Tropomyosin block restored

Motor unit

1 motor neuron can innervate MANY muscle fibres

BUT, 1 muscle fibre can only be innervated by 1 motor neurone

Localisation

The location of a protein or organelle within a cell is important

• Central nuclei = muscle fibre repair

• GLUT 4 = Important glucose transporter protein - location dictates function

Muscle fibre types

Satellite cells

Located at the periphery of the fibre - fuse when necessary

• Satellite cells can divide

• Contribute to growth and repair - myogenesis

Muscular repair

Less severe than regeneration

Enable healthy maintenance of muscle

Damage through eccentric contractions

• inflammatory response/satellite cell activation = strength recovery

• Force can still be reduced for up to 7 days - Increased levels of satelite cells

Contribution of Satellite cells to Hypertrophy

• Nuclei can only govern a defined volume of cytoplasm

• Growth requires more nuclei

• Bigger muscles = more nuclei + available satellite cells

Myogenic Regulatory Factors

Stem cells

Cells that are capable of self-renewal, regulation & differentiation into specialised cell types

Embryonic stem cell

Pluripotent - Differentiate into many different cell types

Adult stem cell

Multipotent- Limited differentiation potential

Tissues that require stem cells

• Epithelial lining ( SI absorption)

• Endothelial (Lining of blood vessels - repair)

• Connective tissue ( Bone, tendons, cartilage)

Cell differentiation

Starts early & progressively narrows options of what a cell can become

Terminal differentiation

Cell that becomes highly specialised & can no longer adapt

Types of cell division

Hematopoietic stem cells (HSCs)

Multipotent

• Located in bone marrow & responsible for Blood cell production

• Hierarchy that dictates what cells they can become

Progenitor cells

Common myeloid progenitor ( CMP)

• Rise to RBCs, platelets, & Myeloid cells

Common lymphoid progenitor ( CLP)

• Rise to lymphocytes (B/T/ NK cells)

Erythropoiesis

Production of RBCs

• Controlled by EPO - Liver

• RBCs ( Erythrocytes) cannot grow, divide & have a limited lifespan

• reduced O2/RBCs = Increased Erythropoiesis

Colony Stimulating Factor (CSF)

Stimulate the function of differentiated blood cell colonies formed from progenitor cells

EG. EPO acts on the erythropoietin progenitor

Function

• Cell survival

• Late in the hierarchy to facilitate differentiation

• Early in the hierarchy to influence commitment

Colony Stimulating Factor (CSF) - Role in exercise

Maintain Blood cell function

• Exercise = physiological stressor ( Increased O2 demand)

• HSCs move into the bloodstream during exercise

Process is enhanced by endurance exercise (increased immune response) - Overtraining may reduce HSC activity

Volume & intensity = big factor to HSCs in blood - 70% = good intensity for mobilisation

Cell signalling

Process where cells respond to internal/External cues

• Important for growth, metabolism, homeostasis, & Skill adaptation

Basic processing of Cell signalling

1. Signal (ligand) - Molecule that starts communication

2. Receptor - Protein in/on a cell that detects a signal

3. Signal pathway - Cascade of molecular events leading to a response

4. Changes in gene expression, protein activity & metabolism

Types of Cell signalling

• Autocrine = Cell signals itself

• Paracrine = Signal near cells

• Endocrine = Signal travels a long distance through the bloodstream

• Direct = Signal through junctions / cell-cell interactions

Receptors

Single molecule - protein, AA or steroid

• Allows target cells to respond

• Protein on cell surface / Receptor inside cell ( Small/ hydrophobic diffusion across membrane)

Types of cell surface receptor

• Ion channel = changing ion/ excitability

• G protein = binds to GPCR to change function - triggers interaction

• Enzyme

2nd Messengers

Generated in large volumes following the 1st signal

• Spread by binding/ altering protein behaviour

• Acts as an on/off switch

• Affected by phosphorylation - phosphate group removed

Specificity ( Cell signalling)

Cells receive 100s of signals

A cell may:

• Respond to many signals

• Require extracellular signals to promote processes

• Express receptors/intracellular signalling pathways that respond to signals for cell regulation

Cell signalling & its role in Exercise

Exercise = increases metabolic demand

Different types of exercise use different signalling pathways

• AMPK

• CaMK

• PGC-1a

AMPK

Trigger = Cellular energy stress ( Low ATP/high AMP)

Function = Enhance glucose uptake & fatty acid oxidation - Restores energy balance

CaMK

Trigger = Increase Ca2+ during exercise

Function = Activate genes involved in mitochondrial biogenesis

PGC-1a

Central regulator for mitochondrial biogenesis

Role = Enhance aerobic adaptation by increasing mitochondrial density or efficiency

How change occurs from signalling pathways induced by exercise

mRNA & proteins

Related mRNA & synthesis involving mitochondria

1. Sensor proteins - detection

2. Signal pathways

3. Effector - Adaptation

Anaerobic signalling in skeletal muscle

• Results in protein synthesis/ Less breakdown of proteins

• Cell growth requires a positive protein balance

• Stimulated by exercise/ Amino Acids

MTOR1

Enhances protein synthesis/ Less breakdown of proteins

Via:

• Cap-dependent translation

• Translation elongation

• mRNA biogenesis

• Ribosome biogenesis

Upstream regulators of anaerobic signalling in skeletal muscle

• Insulin signalling

• Amino Acids

• Mechanical stimuli

AAs - activate Rag GTPases & recruit MTOR1

MS - PLD dependent increases in PA ( Binds to MTOR + signals)

MTOR1 inactivation

• Nutrient deficient - Lack of AAs

• Energy stress - AMPK activation

• Negative feedback of S6K1

• Pharmacological inhibition, redox stress

Competition of pathways

Fliiping does matter - Endurance better after strength

Innate immunity

Quick response to non- self antigens ( Barriers, inflammation , Phagocytosis)

Barriers preventing infection

• Anatomic - Skin/ Membranes

• Physiological - PH, Temp, chemical mediators

• Inflammation

• Endocytic/ phagocytic

Adaptive immunity

• Mediated by T cells & antibodies

• Recognises non-self antigens - activate relevant pathways

• APCs are key to T cell antigens

MHCs:

• Surfaces of APCs - express proteins

• T cells express T receptors

Relevant immune cells

• Dendritic cells - APCs - Induce immune response

• Macrophage - Phagocytosis/inflammation

• Neutrophil - guided by cytokine factors - 1st cell to arrive - Phagocytosis

• NK cells - Kill virus cells

Exercise effect on immune system

• Moderate/vigorous exercise for 60 mins = Increase mobilisation

Open window theory:

• Temporary decrease in immunity post-exercise

• Decreased respiratory infections in active individuals/ Increase in elite

Insulin action

• Stimulate glucose uptake into skeletal muscle & adipose

• Inhibits glucose production (Gluconeogenesis/ Glycogenesis) - Promotes liver storage

• Promotes fat storage - Enhanced Lipogenesis/Lipolysis

• Inhibits protein breakdown

• Increase blood flow and perfusion

• Appetite suppressant

Insuline synthesis pathway

Insulin/ exercise similarity

Insulin/ muscle contraction has similar pathways - GLUT 4 translocation

• Synergistic effect on tissue glucose uptake

Insulin signalling pathway

1. Insulin binds to receptors (IR)

2. Insulin activates the Tyrosine Kinase domain of the beta subunit of IR

3. Insulin receptor substrate proteins (IRS) become tyrosine phosphorylated

4. PI3K becomes activated and converts PIP2 to PIP3

5. PIP3 recruits Akt

6. Akt becomes phosphorylated on 2 different sites

7. Cross-talk between Akt and Rac1

8. Reorganisation of the actin skeleton

9. GLUT4 translocation to the cell membrane

Key nuclear receptor in Skeletal muscle

• PGC 1a - Regulates mitochondrial biogenesis

• Oestrogen receptors - Energy metabolism & Muscle endurance

• Androgen receptors - Muscle growth

Oestrogen recpetors

Mediate effects of oestrogen:

• Mitochondrial function

• Muscle repair

• Muscle strength/contraction

• Metabolic homeostasis

Androgen receptors

Binds androgen (Test)

• Ligand induces AR translocation to the nucleus

• Binds androgen response elements to DNA

• Regulates genes involved in muscle protein synthesis

• Dependent on muscle growth - Men have more than women

Nuclear receptor 1

Ligand = Steroid hormone

• In the cytoplasm, when inactive

• Bound to chaperone proteins

• Hormone binding to receptor = movement to the nucleus

• In nucleus = Bind to HREs on DNA - regulates transcription

Eg - Androgen receptor

Nuclear receptor 2

Ligand = nonsteroid hormone - Thyroid

• Nucleus, even when inactive

• No ligand = Bound to DNA with compressor proteins that suppress genes

• Ligand binds = Compressor released = Gene activation

Eg. Thyroid hormone receptor

Nuclear receptor 3

Ligand = Unknown/ respond to metabolism/Nutrients

• Similar to type 1 - do not require ligand

Eg - Rev-Erb

Nuclear receptor 4

Ligand = Unknown

• Do not bind to DNA

• Interacts with binding proteins/ Transcription factors

Eg - SF1