LYSOSOME

Lysosomes - a 'new look' from new research

Recent bioscience work on programmed cell death (apoptosis) and the endocytic pathway is indicating that our knowledge and perceptions about lysosomes are in need of a 'makeover'.

Historically lysosomes have been called 'suicide bags' and 'garbage disposal units'.
By bursting and releasing chemicals within the cell they were thought to bring about cell death and autolysis (a word hardly used now). These functions, once attributed to lysosomes, no longer apply.

Research has shown that programmed cell death and phagocytosis is responsible for the elimination of cells. This happens throughout the life of an organism, but a striking example is seen during metamorphosis of tadpole to frog. Research on the endocytic pathway is indicating that lysosomes are storage organelles for hydrolases and that these are held in an inactive form. Activation takes place when the lysosome fuses with a specific organelle to form a hybrid structure.

Lysosomes (lysosome: from the Greek: lysis; loosen and soma; body) are found in nearly all animal and plant cells. In plant cells vacuoles can carry out lysosomal functions. Lysosomes appear initially as spherical bodies about 50-70nm in diameter and are bounded by a single membrane. Several hundred lysosomes may be present in a single animal cell.

Recent work suggests that there are two types of lysosomes: secretory lysosomes and conventional ones.

Secretory lysosomes are found, although not exclusively, in different cells of the immune system, such as T lymphocytes, derived from the hemopoietic cell line.
Secretory lysosomes are a combination of conventional lysosomes and secretory granules. They differ from conventional lysosomes in that they contain the particular secretory product of the cell in which they reside. T lymphocytes for example contain secretory products (perforin and granzymes) that can attack both virus infected and tumour cells. Secretory lysosome 'combi cells' also contain the hydrolases, membrane proteins and have the pH regulating facility of conventional lysosomes. The latter facility maintains an acidic environment in which the secretory products are maintained in an inactive form.

The mature secretory lysosomes move within the cytoplasm to the plasma membrane. Here they are held in 'stand by mode' with potent 'warhead' secretions inactive but at the ready. When the T lymphocyte cell is perfectly focused on the target cell the secretion is 'fired' and environmental and chemical changes, including pH, activate the secretions before they lock on the target. This is all done with precise control of location and timing not only to maximise effect on the target but also to minimise collateral damage to friendly neighbouring cells.
Some conventional cells e.g. melanocytes and renal tubular cells can also carry out regulated secretion.

Genetically driven disorders of secretory lysosomes can lead to impaired platelet synthesis, a type of immunodeficiency and hypopigmentation.

Conventional lysosomes
- Arrivals, meetings, 'kiss and run', fusions and maturation models.

Arrivals and meetings
Lysosomes reside in the cell as re-usable organelles and when cell division takes place each daughter cell receives a number of lysosomes. How this number is increased has not yet been elucidated. It is thought that the reservoir of chemicals in the lysosome can be 'topped up' by supplies from the Golgi apparatus. The chemicals are manufactured in the endoplasmic reticulum, modified in the Golgi apparatus and transported to the lysosomes in vesicles (sealed droplets). Modification in the Golgi apparatus includes 'destination labelling' at a molecular level ensuring that the vesicle is delivered to a lysosome and not to the plasma membrane or elsewhere. The 'label' is returned to the Golgi apparatus for re-use.

Lysosomes contain about 50 enzymes that speed up the degradation of polysaccharides, lipids, DNA and RNA. Most, but not all, lysosomal enzymes are acid hydrolases and function at about pH 5.0.
Acidic conditions are maintained in the lysosome by proton pumps in the specialist membrane that surrounds it. The proton pumps transfer hydrogen ions from the cytosol, across the membrane and into the interior of the lysosome.

Material originating from 3 different sources requires dismantling and recycling. Substrates from two of these sources enter the cell from outside and the third originates from within.

1. From outside the cell the process of endocytosis, including pinocytosis (cellular drinking), admits liquids and small particles through the formation, in the plasma membrane, of small pits that are coated with protein. These seal up to form protein coated vesicles. Each vesicle develops to become an 'early endosome' and then a 'late endosome'.
2. Also from outside the cell phagocytosis (cellular eating) brings in relatively large particles (generally >250 nm in size), including bacteria and cell debris. Phagocytosis can be carried out by 'ordinary cells' but is mainly executed by macrophages that can contain up to 1,000 lysosomes per cell. The structure resulting from phagocytosis is called a phagosome.
3. From inside the cell autophagosomes are responsible for removing organelles, such as mitochondria and ribosomes, that are life expired. It is thought that a membranous structure surrounds and encloses the life expired organelle to form an autophagosome. This structure then fuses with a lysosome to form a 'hybrid organelle'.

Endolysosomal systems: 'kiss and run', full fusion activities and maturation models

Research has been carried out on tracing how materials taken into the cell by endocytosis are transported within the cell and eventually broken down. Much of the work has centred on early and late endosomes but with a measure of caution one can consider phagosomes, autophagosomes and late endosomes all as 'late endosomes' for the purpose of trying to understand the endolysosomal system.

There is now a considerable amount of evidence to show:

1. the main site for proteolysis is not the lysosome itself but an organelle which is more like the late endosome and contains about 20% of the available hydrolases.
2. lysosomes contain about 80% of the digestive enzymes.
3. lysosomes are probably storage organelles for hydrolases which they keep in an inactive form under acidic conditions at about pH 5.0.
4. lysosomes do not operate as independent organelles but meet with late endosomes to operate as an endolysosomal system.

These findings have led to the development of models based on the interplay of late endosomes and lysosomes displaying varying degrees of contact. One of these models is called 'kiss and run' and the other, 'fusion'

Kiss and Run
In this model, as the name suggests, the late endosome and lysosome make contact so that chemicals can be exchanged but after this encounter they separate fairly quickly. The lysosome is then available to 'kiss' another late endosome.

Fusion
More recent evidence has led to the 'fusion' hypothesis in which a late endosome and a lysosome completely fuse together to form a 'hybrid organelle'. During the fusion time molecular dismantling of the endocytic load takes place. The resulting amino acids and other molecules useful to the cell are taken by 'transporters' through the 'hybrid organelle' membrane into the cytoplasm. After dismantling and re-cycling the content of the organelle condenses, the lysosome is reformed and moves away to form a hybrid organelle with another late endosomes. Sometimes a small amount of residue is left. This is dealt with by the process of exocytosis in which the residue is ejected through the plasma membrane or it is sealed up in a pigment granule for the duration of the life of the organism.

Maturation system models
Models based on the principle of structures maturing to form lysosomes are not popular at present but two mentioned in some textbooks and are outlined here.
In both the maturation and vesicular transport models late endosomes develop to become a lysosome.
In the maturation model an early endosome is formed from vesicles originating in the plasma membrane combining together. Various other vesicles deliver and remove chemicals until the late endosome, and then the lysosome stage is reached.
In the vesicle transport model, early and late endosomes are considered stable separate organelles with vesicles carrying chemicals from early endosomes to late endosomes. Late endosomes then mature to become lysosomes

Lysosome function disorders
There are about 30 fairly rare disorders in humans that are due to defects in endolysosomal function. All are caused by errors in the genetic code and all are lysosomal storage disorders. In these disorders products accumulate in the lysosomes because the enzymes that would speed up their degradation are absent or defective.
Each disorder has a specific medical name, e.g. Inclusion-cell disease (I-cell disease), Tay-Sachs, Pompe and Gaucher's disease. Each disorder has a different outcome for the patient; some being more severe than others. In I-cell disease the lysosomes of fibroblast cells are deficient in nearly all the hydrolytic enzymes and large undigested 'inclusions' build up in the patients' cells. It is not yet clear whether lysosomes containing large amounts of undigested material re-cycle to take part in the formation of 'hybrid organelles'.

From a molecular biology point of view there are two groups of disorders; those associated with (1) destination label errors and (2) enzyme deficiency errors.
In I-cell disease the correct enzymes are produced but due to the molecular address label being 'wrong', they are routed away from the lysosome and probably to outside the cell. Tay-Sachs disease, a lysosomal storage disorder in nerve cells, nearly always causes early death but the incidence of death is now falling thanks to testing and genetic counselling. In Gaucher's disease large amounts of lipids accumulate in the lysosomes. Fortunately research in cell biology and biotechnology has produced an enzyme replacement therapy. It seems to be working but it is expensive and has to be administered by intravenous infusion. More interesting information about Gaucher's disorder can be obtained from (1) Gaucher's Association, www.gaucher.org.uk and (2) the National Gaucher Association, www.gaucherdisease.org.

SUMMARY

• There are two types of lysosomes; secretory lysosomes and conventional ones.
• Conventional lysosomes are involved in the dismantling and re-cycling of various substrates presented to them through endocytocis, phagocytosis and by autophagosomes. They are responsible for returning many amino acids to the system.
• The dismantling process is accelerated by the presence of enzymes. Many of these are acid hydrolases.
• The way lysosomes perform their function is the subject of a great deal of current research. Results to date suggest that the days of viewing lysosomes as 'stand alone' processing plants are numbered but perhaps we should take the view that there are probably different types of lysosomal systems and that no one model offers universal application.
• We need to view conventional lysosomes as part of an integrated endolysosomal system in which fusions of lysosomes with late endosomes appear to be taking centre stage. Lysosomes are experiencing a 'makeover'.