17.1 Endocytic and secretory pathways

Endocytic Pathway

The endocytic pathway brings materials from outside the cell into its interior. Cells rely on this pathway for nutrient uptake, signal regulation, and immune defense. Different mechanisms handle different types of cargo, but they all converge on a shared set of intracellular compartments where materials get sorted, recycled, or degraded.

Types of Endocytosis

Phagocytosis engulfs large particles like bacteria and cell debris. The cell extends pseudopodia around the target, wrapping it in membrane and pulling it inside as a large vesicle called a phagosome.

Pinocytosis takes up fluids and dissolved solutes through smaller vesicles. Several subtypes exist:

• Macropinocytosis is non-specific. The membrane forms large ruffles that collapse back and trap extracellular fluid, bringing in whatever happens to be dissolved in it.
• Clathrin-mediated endocytosis is highly selective. Specific cargo molecules (like low-density lipoprotein or transferrin) bind receptors that concentrate in clathrin-coated pits on the membrane surface. This is the most well-characterized form of receptor-mediated endocytosis.
• Caveolae-mediated endocytosis uses flask-shaped invaginations coated with the protein caveolin. These structures are enriched in cholesterol and glycosphingolipids and internalize specific signaling molecules and lipids.

Endosomal Sorting and Degradation

Once a vesicle pinches off from the plasma membrane, it enters a series of compartments:

1. The vesicle sheds any coat proteins (e.g., clathrin) and fuses with an early endosome, the first sorting station. Here, cargo is triaged: some material is recycled back to the plasma membrane, while other cargo continues deeper into the pathway.
2. Cargo destined for degradation moves to the late endosome, which has a more acidic lumen (around pH 5.5). This drop in pH helps dissociate ligands from their receptors.
3. Late endosomes fuse with lysosomes, which contain hydrolytic enzymes that break down macromolecules into building blocks like amino acids, simple sugars, and nucleotides.

The progressive acidification from early endosome to lysosome is critical. It drives receptor-ligand dissociation and activates the hydrolases that do the actual degradation.

Receptor-Mediated Endocytosis (Step by Step)

1. A ligand (e.g., insulin, epidermal growth factor) binds its specific receptor on the cell surface.
2. Ligand-receptor complexes migrate laterally in the membrane and cluster in clathrin-coated pits, regions where the cytoplasmic face of the membrane is lined with a clathrin lattice and adaptor proteins like AP2.
3. The coated pit invaginates and, with the help of the GTPase dynamin, pinches off to form a clathrin-coated vesicle.
4. The clathrin coat disassembles (aided by the chaperone Hsc70), and the uncoated vesicle fuses with an early endosome.
5. Inside the early endosome, the acidic pH causes many ligands to release from their receptors. Receptors are typically sorted into recycling tubules and returned to the plasma membrane for reuse.
6. Released ligands remain in the endosomal lumen and are delivered to late endosomes and then lysosomes for enzymatic breakdown.

This process matters for three main reasons: it allows cells to selectively internalize essential nutrients (like cholesterol via LDL receptors), it regulates signaling by controlling how many receptors are available at the cell surface (receptor downregulation), and it contributes to immune function by facilitating uptake of antibody-antigen complexes.

Secretory Pathway

The secretory pathway moves newly synthesized proteins from the rough endoplasmic reticulum (RER) through the Golgi apparatus and out to the cell surface or other destinations. Two versions of this pathway exist, distinguished by whether secretion is continuous or triggered by a signal.

Constitutive vs. Regulated Secretory Pathways

Constitutive secretion operates continuously. Proteins are synthesized, processed, and shipped to the plasma membrane without being stored. This pathway delivers extracellular matrix components (collagen, fibronectin) and membrane proteins (receptors, ion channels) on an ongoing basis. Every cell performs constitutive secretion.

Regulated secretion stores finished products in dense-core secretory granules until an external signal triggers their release. Classic examples include insulin stored in pancreatic beta cell granules (released when blood glucose rises) and neurotransmitters like dopamine stored in synaptic vesicles (released upon nerve impulse).

Both pathways share the same early steps:

• Protein synthesis on RER-bound ribosomes
• Modification and packaging in the Golgi apparatus
• Transport of vesicles to the plasma membrane for exocytosis

The key difference is what happens after the trans-Golgi network (TGN). Constitutive cargo goes directly to the surface. Regulated cargo gets concentrated and stored in granules, waiting for a stimulus (often a rise in cytoplasmic $\text{Ca}^{2+}$) to trigger fusion with the plasma membrane.

Role of the Golgi in Vesicular Trafficking

The Golgi apparatus sits at the center of the secretory pathway, receiving, modifying, and sorting proteins before dispatching them to their final destinations.

Receiving cargo from the ER: Newly synthesized proteins leave the RER in COPII-coated vesicles, which bud from specialized ER exit sites and travel to the cis face of the Golgi. Retrograde transport (Golgi back to ER) uses COPI-coated vesicles to retrieve escaped ER-resident proteins.

Processing through the Golgi stack: The Golgi is organized into functionally distinct compartments:

• cis-Golgi network (CGN): Receives incoming cargo and begins modifications such as trimming of mannose residues from N-linked glycans.
• Medial cisternae: Continue glycan processing and add new sugar residues. Sulfation and phosphorylation also occur here.
• trans-Golgi network (TGN): Performs final modifications and, most importantly, sorts proteins into distinct transport vesicles based on sorting signals carried by the proteins themselves.

Sorting at the TGN determines where each protein ends up:

• Proteins tagged with mannose-6-phosphate (M6P) are recognized by M6P receptors and packaged into clathrin-coated vesicles headed for endosomes and ultimately lysosomes.
• Proteins lacking specific sorting signals enter the constitutive secretory pathway by default and are delivered to the plasma membrane.
• Proteins with regulated secretory signals are concentrated into secretory granules for stimulus-dependent release.

This sorting logic means the constitutive pathway is the default route. Diversion to lysosomes or to regulated secretory granules requires active sorting signals.