BIOL: Lecture Notes- Sept. 8: ECM
- ECM = fibrous network outside cells; provides support, adhesion, migration, regulation of cell behavior; strongly influences cell morphology and growth.
- Injury in the nervous system: astrocytes release inhibitory ECM proteins → inhibitory environment (glial scar) that can limit regeneration.
ECM components and effects on gene expression
- Laminin is important for promoting certain gene expressions; collagen alone or heparan sulfate alone have little effect.
- In hepatocytes, albumin mRNA levels depend on ECM composition; laminin-containing matrices yield higher albumin expression, with slight additional benefit when combined with other components.
- Overall ECM is a complex, multi-protein matrix; Laminin often a key component for signaling and support.
Proteoglycans and glycosaminoglycans (GAGs)
- Proteoglycans appear as diffuse clouds of varying isoforms across brain regions and development stages.
- Roles in axon pathfinding: some proteoglycans promote growth, others inhibit; response to proteoglycans can change during development.
- Structure: long core protein with attached GAG side chains; GAGs are the bristles on a bottle-brush.
- Functions: bind growth factors and nutrients; participate in the ECM network to modulate signaling and guidance.
Intercellular junctions: overview
- Intercellular junctions connect neighboring cells; key types include tight junctions, desmosomes, and gap junctions.
- Intracellular vs intercellular distinction: between cells (intercellular) vs inside a cell (intracellular).
Tight junctions (occluding junctions)
- Locate at the apical-most region between adjacent cells (e.g., epithelial and skin cells).
- Proteins pull membranes very close to each other to prevent leakage of extracellular fluid; create a seal between cells.
Desmosomes (anchoring junctions)
- Fasten cells together into strong sheets; not as tight as tight junctions.
- Link to intermediate filament cytoskeleton; involve cadherin family proteins that mediate cell–cell adhesion.
- Provide mechanical strength to tissues such as skin and heart.
Gap junctions
- Channels formed by connexins that bridge neighboring cells; allow passage of small molecules and ions.
- Each cell contributes half of the channel; enables direct cytoplasmic continuity.
- In neurons, gap junctions synchronize activity by spreading signals across connected cells.
Quick takeaway
- ECM composition (laminin, collagen, GAGs) shapes cell behavior and gene expression; proteoglycans modulate growth with developmental timing.
- Intercellular junctions create barriers (tight), mechanical adhesion (desmosomes), and electrical/chemical coupling (gap junctions).