Apoptosis and the Immune System

  • Apoptosis is primarily triggered by the immune system cells.

    • Immune cells possess ligands that bind to death receptors on target cells.

  • Important pathways of apoptosis:

    • Intrinsic Pathway: Triggered by environmental toxins, chemicals, or radiation.

    • Extrinsic Pathway: Activated by the immune system.

  • The transition to mitochondrial involvement in apoptosis is consistent regardless of pathway origin.

  • Understanding of caspases:

    • Caspases are a family of cysteine proteases involved in apoptosis.

    • Grouping caspases into categories aids memorization.

  • Visual aids:

    • A blank diagram of apoptosis highlighting key proteins is available for reference.

Astrocytes and Central Nervous System (CNS)

  • Astrocytes are unique glial cells within the CNS.

    • They play an essential role and often receive less attention in discussions of neuroscience.

  • Research at Virginia Tech focuses on glial cell contributions, particularly in diseases affecting neurons.

    • The Olson Lab studies glial involvement in various neuronal diseases.

Tripartite Synapse

  • Structure of the tripartite synapse:

    • Composed of a presynaptic neuron, a postsynaptic neuron, and astrocytes surrounding them.

    • Astrocytes buffer the extracellular environment to ensure optimal synaptic function.

Morphology and Function of Astrocytes

  • Distinct morphology visible under the microscope; commonly stained using GFAP (Glial Fibrillary Acidic Protein).

    • GFAP is an intermediate filament protein upregulated in astrocytes.

  • Communication among astrocytes:

    • Astrocytes communicate via gap junctions, allowing ions and small molecules to pass between them.

  • Role of gap junctions:

    • Facilitate intercellular communication and maintain tissue integrity.

Gap Junctions and Cell Adhesion Molecules

  • Astrocytes and other cells use various cell adhesion molecules to connect and interact with the extracellular matrix.

    • Importance of understanding both internal structures and external connections.

  • Gap junctions allow for:

    • Direct communication between adjacent astrocytes via shared cytoplasmic content, supporting tissue function.

  • Characteristics of gap junctions:

    • Comprised of connexins, which form hemichannels (connexons).

    • Alignment of hemichannels from two adjacent cells forms an actual gap junction.

  • Connexin Structure:

    • Each connexon consists of six connexin proteins.

    • Hemichannels allow selective passage of ions, typically less than 1 kDa in size, such as potassium and calcium.

Role of Astrocytes in Ion Regulation

  • Astrocytes help regulate ion concentrations (e.g., calcium and potassium) in the brain:

    • They can 'mop up' excess ions to prevent toxic buildup.

    • Coordination of calcium signaling is crucial for cellular activities.

  • Importance of bidirectional communication facilitated by gap junctions:

    • Unlike chemical synapses, gap junctions allow both cells to communicate with no delay.

    • Synchronization of activity among groups of connected astrocytes leads to functional syncytiums, especially in cardiac and smooth muscle as well as CNS.

Cellular Junction Types

  • Tight junctions: Key for blood-brain barrier integrity.

    • This prevents unwanted substances from entering the brain.

    • Composed of occludin proteins, which can be variably expressed in different brain regions and under stress or infection.

  • Mutations in connexins can impair astrocytic function:

    • Example: Connexin 26 mutation leading to deafness.

  • Other types of cell-cell junctions include mechanical junctions that support structural integrity by connecting cytoskeletons of adjacent cells.

Functional Implications of Gap Junctions

  • Enabling sharing of hydrophilic molecules, second messengers, and metabolites among cells promotes efficient metabolic exchanges.

  • Importance of regulatory mechanisms:

    • Connexins can be modulated by various factors, including calcium levels and phosphorylation.

    • This allows cells to control permeability and communication dynamically.

Conclusion

  • Understanding astrocyte functions, including their roles in communication and metabolic support, is critical in the study of neuronal health and diseases.

    • Exploration of gap junctions, tight junctions, and cell adhesion molecules enhances comprehension of neurobiology.

  • Encouragement for students to stay informed about how these systems work together in health and disease contexts.