M04.01.003 The Golgi apparatus

The Golgi apparatus is a membrane-bound organelle found in most eukaryotic cells. It plays a central role in the secretory pathway, modifying proteins and lipids received from the endoplasmic reticulum (ER), packaging them into vesicles, and directing them to their final intracellular or extracellular destinations.

Structure

The Golgi apparatus is composed of 5–8 flattened sacs (cisternae) arranged in a stack. Each cisterna contains specific enzymes that carry out stepwise modifications as molecules pass through. Functionally, it can be divided into five regions:

• Cis-Golgi network: Entry site; receives vesicles from the ER.
• Cis-Golgi: Processing of proteins begins.
• Medial-Golgi: Continued biochemical modifications.
• Trans-Golgi: Final stages of modification.
• Trans-Golgi network: Exit site; sorts and packages proteins and lipids into vesicles for delivery.

Functions

Protein Processing

• Modifies carbohydrate regions of glycoproteins.
• Adds, removes, or alters sugar residues (glycosylation).

Lipid Processing

• Adds phosphate or glycoprotein groups to lipids.
• Produces phospholipids for the plasma membrane.

Sorting, Budding, and Exocytosis

• Chemical labeling: Molecules are tagged for the correct destination.
• Lysosomal proteins: Tagged with mannose-6-phosphate for delivery to lysosomes.
• Secretory proteins: Packaged into vesicles for regulated exocytosis (requires ATP).
• Constitutive pathway: Vesicles deliver proteins and lipids continuously to the plasma membrane in all cells.

Clinical Relevance

Wilson’s Disease

• Inheritance: Autosomal recessive.
• Cause: Mutation in ATP7B gene → defective copper-transporting ATPase (normally located in the Golgi of hepatocytes and neurons).
• Normal function of ATP7B:
  • Incorporates copper into ceruloplasmin for systemic transport.
  • Excretes excess copper into bile.
• Pathophysiology: Loss of ATP7B function → impaired copper incorporation into ceruloplasmin + reduced biliary excretion → copper accumulation in liver, brain, eyes, and joints.

Clinical features:

• Hepatic: Jaundice, hepatomegaly, cirrhosis, ascites.
• Neurologic/Psychiatric: Tremor, dysarthria, dystonia, confusion, personality changes, psychosis.
• Ophthalmologic: Kayser–Fleischer rings (golden-brown corneal deposits).
• Musculoskeletal: Joint pain, arthritis.

Diagnosis:

• Low serum ceruloplasmin.
• Elevated urinary copper.
• Abnormal liver function tests.
• Liver biopsy if uncertain.

Management:

• Copper chelating agents (e.g., penicillamine).
• Zinc supplementation (reduces intestinal copper absorption).
• Liver transplantation in severe cases.

Learning Objective

Understand the structural organization and functions of the Golgi apparatus in protein and lipid processing, sorting, and secretion. Recognize its clinical relevance through Wilson’s disease, where defective copper handling in the Golgi leads to systemic copper overload with hepatic, neurologic, and ocular manifestations.