Part of the TeachMe Series

Golgi Apparatus

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Original Author(s): Hamish Patel
Last updated: 25th November 2020
Revisions: 23

Original Author(s): Hamish Patel
Last updated: 25th November 2020
Revisions: 23

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The golgi apparatus is a membrane bound organelle found in most cells. It is responsible for packaging proteins into vesicles prior to secretion and therefore plays a key role in the secretory pathway.

In this article we shall look at the structure and function of the golgi apparatus and its role in Wilson’s disease.

Structure

The golgi is made of 5-8 folds called cisternae. The cisternae contain specific enzymes creating five functional regions which modify proteins passing through them in a stereotypical way, as follows:

  1. Cis-Golgi network: faces the nucleus, forms a connection with the endoplasmic reticulum and is the entry point into the Golgi apparatus.
  2. Cis-Golgi: major processing area allowing biochemical modifications.
  3. Medial-Golgi: major processing area allowing biochemical modifications.
  4. Trans-Golgi: major processing area allowing biochemical modifications.
  5. Trans-Golgi network: exit point for vesicles budding off the Golgi surface, packages and sorts biochemicals into the vesicles according to their destination.
Shows the Golgi with the various cistern and vesicles entering and exiting the structure

Fig 1.0 – Structure of the golgi apparatus

Function

The golgi apparatus modifies proteins and lipids that it receives from the endoplasmic reticulum. These biochemicals leave the golgi by exocytosis before being delivered to different intracellular or extracellular targets.

  • Protein processing – carbohydrate regions of glycoproteins are altered by addition, removal or modification of carbohydrates.
  • Lipid processing – adds phosphate groups and glycoproteins to lipids from the endoplasmic reticulum (such as cholesterol) to create the phospholipids that make up the cell membrane.
Shows the layers of the lipid bilayer to demonstrate an area where processing in the Golgi is key to function

Fig 2.0 – The Phospholipid bilayer is made of components produced in the golgi

Sorting, Budding and Exocytosis

Biochemicals are chemically labelled in the golgi to ensure appropriate delivery to the correct destination. Once they bud off the trans-Golgi they will enter a specific pathway according to this signalling sequence.

Lysosomal proteins – such as enzymes are packaged into specific vesicles. These proteins are typically tagged with mannose-6-phosphate in the Golgi

Secretory proteins – such as hormones are packaged into secretory vesicles ready for exocytosis. This requires ATP as two negatively charged membranes need to fuse to allow their release. The membrane of the vesicle will form part of the cell membrane. This is only possible in Golgi of secretory cells.

Cell surface proteins – such as phospholipids enter the constitutive secretory pathway present in all cells.

Clinical Relevance – Wilson’s Disease

Wilson’s disease is an autosomal recessive disorder characterised by an abnormal accumulation of copper in the body. The liver is particularly susceptible to this accumulation.

Mutation of the ATP7B gene is implicated in the development of Wilson disease. The ATP7B gene codes for copper-transporting ATPase 2 (also known as Wilson disease protein or WDP), an enzyme located in the Golgi apparatus of many cells such as hepatocytes and neurones.

WDP normally allows copper to bind to caeruloplasmin, a protein which transports copper to various parts of the body. If copper levels are too high, WDP leaves the Golgi, allowing copper to be excreted in bile instead of transported around the body.

Patients experience symptoms and signs of liver failure, such as itching, abdominal swelling, fatigue, jaundice, ascites and Kayser-Fleischer rings (deposition of copper in the cornea).

They may also present with neuropsychiatric symptoms such as confusion, psychosis, personality changes and tremor. Copper may also accumulate in joints causing joint pain and swelling.

Shows a cornea with the classical yellow-brown Keyser-Fleischer rings seen in copper overload

Fig 3.0 – Kayser-Fleischer ring seen on the outside of the cornea

Diagnosis is based on abnormal liver function tests, serum copperlevels, urinary copper levels, and low serum caeruloplasmin levels. If the diagnosis is unclear a liver biopsy may be indicated.

Management is aimed at reducing copper levels using copper chelating agents such as penicillamine.