Comprehensive Study Notes on Proteostasis and Autophagy

Overview of Proteostasis and Autophagy

• Definition of Proteostasis: This refers to the dynamic regulation of the protein complement within a cell, maintaining a balance between biosynthetic and degradative pathways.
• Biosynthetic Pathways: These processes include everything from expression and folding to the trafficking of proteins.
• Counterpoint to Synthesis: Autophagy serves as the counterpoint to creation; it is part of the degradation pathway, alongside mechanisms such as proteasomal degradation (typically encountered in early study).
• Autophagy Definition: The word itself is derived from the meaning "self-eating." It involves the recycling of cellular constituents.
• Conservation: The process of autophagy is highly conserved among eukaryotes.
• Triggers for Autophagy:     * ER Stress: Management of proteostasis within the cell.     * Starvation: Limited nutrient availability within the cell triggers autophagic pathways to provide essential building blocks.

Biological Functions and Scale of Autophagy

• Nutrient Cycling: A basal level of autophagy occurs continuously to cycle nutrients. In low-nutrient conditions, the cell recycles its own internal contents to generate necessary building blocks.
• Cell Maintenance: Autophagy is essential for getting rid of damaged organelles or those that have reached the end of their life expectancy.
• Immune Response: It facilitates the clearance of bacteria and viruses.
• Neuroprotection: The pathway has the potential to clear damaging protein aggregates.
• Cell Differentiation and Developmental Remodeling:     * During a cell's lifetime, it may no longer require certain organelles.     * Case Study (Erythrocytes): Erythrocytes begin their development with a nucleus and mitochondria. While the nucleus is separated by a different process, the mitochondria are digested via autophagy to create the mature, organelle-free erythrocyte.

The Central Role of the Lysosome

• Cellular Context: The lysosome is central to all digestive processes in autophagy and sits at the junction of the endocytic and secretory pathways.
• Enzyme Transport and Maturation:     * Enzymes are produced in the Endoplasmic Reticulum ($ER$) and processed in the Golgi.     * During Golgi processing, enzymes are modified for transport to the endosome in clathrin-coated vesicles.     * Upon reaching the endosome, enzymes dissociate from their receptors.     * The maturation of the late endosome into a lysosome is characterized by an increasingly acidic environment.
• Activation Mechanism: Enzymes are transported in an inactive state to prevent damage. Activation is autocatalytic and occurs only when the environment reaches a sufficiently low $pH$.
• Acidification and Ion Balance:     * A $V$-type $ATPase$ proton pump uses $ATP$ as an energy source to pump protons ($H^+$) into the lysosomal lumen to drop the $pH$.     * This creates both a concentration gradient and a voltage gradient.     * To maintain voltage across the membrane, the cell employs the efflux of calcium ($Ca^{2+}$) or the influx of chlorine ($Cl^-$) ions.     * Acidity can also be contributed by the fusion of the late endosome and the lysosome.
• Range of Hydrolytic Enzymes: The lysosome contains nucleases, glycosidases, lipases, phospholipases, and phosphatases.

Chaperone-Mediated Autophagy (CMA)

• Mechanism: Soluble proteins are targeted directly to the lysosome and translocated across the membrane via specific channels.
• LAMP2 Receptor:     * The primary receptor for CMA is LAMP2A.     * There are three variants of LAMP2 created by splicing, but variant A is the one involved in chaperone-mediated autophagy.     * Structure: Includes a $C$-terminus that binds substrates in the cytosol, a transmembrane domain, and an $N$-terminus in the lysosomal lumen.
• Targeting Motifs (K for Q):     * Targeting is performed by the chaperone HSC70, which binds to K for Q motifs on proteins intended for degradation.     * Approximately $40\%$ of mammalian proteins contain these motifs.     * Motif Composition: Consists of two positively charged residues (Lysine, Arginine), two hydrophobic residues (Isoleucine, Phenylalanine, Valine, Leucine), one negatively charged residue, and a single Glutamine ($Q$) residue at the $C$ or $N$ terminus of the motif.     * Modifications: The range of targeted proteins is broadened by post-translational modifications. For example, the essential Glutamine residue can be replaced by an acetylated Lysine.
• The Translocation Process:     1. HSC70 identifies the motif and delivers the protein to the $C$-terminus of LAMP2A on the lysosomal surface.     2. LAMP2A forms a trimer with the assistance of lysosomal HSP90 and other co-chaperones on both the cytosolic and lysosomal sides.     3. This trimer forms the translocation channel.     4. Lysosomal HSC70 acts as a molecular motor to pull the protein into the lumen for digestion.     5. The trimer then dissociates.     6. Spent LAMP2 proteins move to lipid-rich regions of the lysosomal surface where they are degraded by metalloproteinases.

Regulation and Clinical Implications of CMA

• The MTORC2 Pathway: MTORC2 acts as a nutrient sensor to regulate the pathway.
• High Nutrient scenario (Inhibition):     * MTORC2 is active and activates AKT1 (a kinase).     * AKT1 phosphorylates GFAP (Glial Fibrillary Acidic Protein).     * Phosphorylated GFAP blocks LAMP2A activity, preventing chaperone binding and translocation, thereby inhibiting CMA.
• Low Nutrient scenario (Stimulation):     * MTORC2 is inactive, making AKT1 inactive.     * GFAP remains unphosphorylated and available to bind to LAMP2A.     * GFAP stabilizes the LAMP2A trimer, promoting and stimulating CMA.
• Neurodegenerative Disease:     * The protein Tau is a normal substrate for CMA.     * In Alzheimer's disease, dysfunction in CMA leads to the accumulation of Tau tangles.     * Alpha synuclein is another CMA substrate associated with Parkinson's disease; failures in this pathway lead to protein accumulation.
• Cancer: CMA is often upregulated in cancer. This allows tumor cells to persist by degrading pro-apoptotic signals that would otherwise cause the cell to die.

Microautophagy and Macroautophagy

• Microautophagy:     * Involves the direct movement of the lysosomal plasma membrane (invagination or protrusion) to capture relatively small cargo.     * There is overlap with CMA: many microautophagy substrates also contain K for Q motifs, and HSC70 is involved in targeting them to the lysosomal surface. However, it leads to membrane invagination rather than channel formation.     * The mechanisms of microautophagy are far less elucidated compared to other types.
• Macroautophagy:     * The major pathway for bulk and specific digestion. It can be non-specific (canonical) or highly targeted.     * Terminologies for Target Substrates:         * Xenophagy: Digestion of bacteria.         * Mitophagy: Digestion of mitochondria.         * Reticulophagy: Digestion of the ER.     * Process of Formation:         1. Isolation Membrane: A double-membrane structure begins to form.         2. Phagophore: As the double membrane grows and elongates, it is termed a phagophore.         3. Autophagosome: Once the target is fully encapsulated and the membrane joins, it is called an autophagosome.         4. Autolysosome: The autophagosome fuses with the lysosome to deliver enzymes for digestion.