Lysosome: Difference between revisions

Latest revision as of 23:19, 13 May 2025

Template:Short description Template:Distinguish Template:Use dmy dates Template:Organelle diagram A lysosome (/ˈlaɪsəˌsoʊm/) is a membrane-bound organelle that is found in all mammalian cells, with the exception of red blood cells (erythrocytes).<ref>Template:Cite book Template:Pn</ref> There are normally hundreds of lysosomes in the cytosol, where they function as the cell’s degradation center. Their primary responsibility is catabolic degradation of proteins, polysaccharides and lipids into their respective building-block molecules: amino acids, monosaccharides, and free fatty acids. The breakdown is done by various enzymes, for example proteases, glycosidases and lipases.<ref name=hx>Template:Cite journal</ref>

With an acidic lumen limited by a single-bilayer lipid membrane, the lysosome holds an environment isolated from the rest of the cell. The lower pH creates optimal conditions for the over 60 different hydrolases inside.<ref name=":4">Template:Cite journal</ref>

Lysosomes receive extracellular particles through endocytosis, and intracellular components through autophagy.<ref name=hx/> They can also fuse with the plasma membrane and secrete their contents, a process called lysosomal exocytosis.<ref name=":4" /> After degradation lysosomal products are transported out of the lysosome through specific membrane proteins or via vesicular membrane trafficking to be recycled or to be utilized for energy.<ref name=hx/>

Aside from cellular clearance and secretion, lysosomes mediate biological processes like plasma membrane repair, cell homeostasis, energy metabolism, cell signaling, and the immune response.<ref name=":4" />

Discovery

File:Christian de Duve.tif

Christian de Duve, age 95, presenting his ideas on the origin of the eukaryotic cell (October 2012)

Christian de Duve, a Belgian scientist at the Laboratory of Physiological Chemistry at the Catholic University of Louvain, is credited with discovering lysosomes in the 1950s. De Duve and his team were studying the distribution of hydrolytic enzymes such as acid phosphatase within cells, using cell fractionation methods to isolate subcellular components. De Duve and his team identified an unknown organelle that was rich in acid phosphatase. This led them to propose the existence of lysosomes as membrane bound organelles containing digestive enzymes capable of breaking down a variety of biological molecules.

Using differential centrifugation and enzyme activity assays, the team confirmed the hypothesis and understood that these organelles play a crucial role in intracellular digestion processes, such as phagocytosis and autophagy. The presence of digestive enzymes was further validated using electron microscopy. De Duve’s discovery laid the foundation for new research into lysosomal functions and understanding disorders which could lead to undigested materials accumulating in the cell. De Duve was awarded the Nobel Prize in Physiology or Medicine in 1974.<ref name="Sabatini Adesnik Christian de Duve">Template:Cite journal</ref><ref>Template:Cite book</ref>

Function and structure

File:The Biological bulletin (19756543133).jpg

TEM views of various vesicular compartments. Lysosomes are denoted by "Ly". They are dyed dark due to their acidity; in the center of the top image, a Golgi Apparatus can be seen, distal from the cell membrane relative to the lysosome.

Lysosomes vary in shape and size depending on their state, what they are digesting, and the cell type they are in.<ref name=":0">Template:Cite book Template:Pn</ref> Their shape can differ from spherical and ovoid to occasionally tubular.<ref name="Bouhamdani A Compendium of Information on the Lysosome">Template:Cite journal</ref> The size of lysosomes ranges from 0.1-1.2 μm,<ref name=":0" /> with some tubular ones reaching up to 15 μm in phagocytes. Several hundred lysosomes can be found within a single cell. However, upon nutrient deprivation or induced autophagy, their numbers can drop below 50 in a cell.<ref name="Bouhamdani A Compendium of Information on the Lysosome"/>

Lysosomes contain a variety of enzymes that enable the cell to break down various biomolecules it engulfs, including peptides, nucleic acids, carbohydrates, and lipids. The enzymes responsible for this hydrolysis require an acidic environment for optimal activity, with a pH ranging from ~4.5–5.0. The interior of the lysosome is acidic compared to the slightly basic cytosol (pH 7.2).<ref>Template:Cite journal</ref>

The lysosomal membrane is a phospholipid bilayer with high carbohydrate content from heavily glycosylated membrane proteins. This forms a glycocalyx that protects the cell from the degradative enzymes held within the lysosome. Lysosomal hydrolases are pH-sensitive and do not function properly in the alkaline environment of the cytosol, ensuring that molecules and organelles in the cytosol are not degraded if there is leakage of hydrolytic enzymes from the lysosome.

In addition to breaking down polymers, lysosomes are capable of killing and digesting microbes, cells, or cellular debris. Through cooperation with phagosomes, lysosomes conduct autophagy, clearing out damaged structures and forming simple compounds, which are then used as new building materials. Similarly, lysosomes break down virus particles or bacteria during phagocytosis in macrophages.<ref>Template:Cite news</ref>

Lysosomes also help detect pathogens through toll-like receptors (TLRs), like TLR7 and TLR9. Microbes can be degraded into antigens, which are then loaded onto MHC molecules and presented to T-cells, a critical part of immune defense. Additionally, lysosomal enzymes can trigger lysosomal-mediated programmed cell death (LM-PCD) if released into the cytoplasm.

To maintain their acidic environment, lysosomes pump protons (H⁺ ions) from the cytosol into the lysosomal lumen via a proton pump in the lysosomal membrane. Vacuolar-ATPases are responsible for the transport of protons, while the counter transport of chloride ions is performed by ClC-7 Cl⁻/H⁺ antiporter.<ref>Template:Cite journal</ref> This mechanism helps maintain a steady acidic environment, as well as ionic homeostasis, within the lysosome.<ref>Template:Cite journal</ref><ref>Template:Cite journal</ref>

Lysosomes also help balance cellular metabolism by sensing nutrient availability. When nutrients are plentiful, they activate mTOR signaling to support anabolic (biosynthetic) processes. During starvation, lysosomes degrade autophagic material, recycling components to maintain cell survival.

Lysosomal degradation pathways

File:Lysosomes as catabolic centers of the cell.jpg

Cellular material is delivered to lysosomes in four different ways; (A) Macroautophagy, (B) Endosomal degradation, (C) Microautophagy and (D) Chaperone-mediated autophagy (CMA).

The lysosome is delivered material for degradation via transient interactions or complete fusion, forming endolysosomes and autolysosomes respectively. This way, the lysosomes act as reservoirs for acidic hydrolases, cycling through fusion and fission events with late endosomes and autophagosomes. The actual breakdown of endocytic and autophagic cargo primarily happens within these transient structures—endolysosomes and autolysosomes—under normal physiological conditions.<ref name="Platt Lysosomal storage disorders"/>

Endocytosed materials – such as complex lipids, membrane proteins, and polysaccharides – enter the endocytic pathway; moving first in early endosomes, then in late endosomes containing intraluminal vesicles (also referred to as multivesicular bodies, MVBs). Then they interact with lysosomes, either via full fusion, or via "kiss-and-run" events where brief membrane contact allows content exchange before the organelles separate. The resulting hybrid structure is called an endolysosome.<ref name="Samie Xu Lysosomal exocytosis">Template:Cite journal</ref><ref name="Platt Lysosomal storage disorders"/>

Intracellular materials – like damaged organelles or misfolded proteins – are processed through the autophagic pathway.<ref name="Samie Xu Lysosomal exocytosis"/> Autophagy, or “self-eating,” is a continuous cellular process that delivers cytosolic components to lysosomes for degradation. There are three main types of autophagy: macroautophagy, microautophagy, and chaperone-mediated autophagy (CMA)—each differing in how cargo is delivered to the lysosome.<ref name="Platt Lysosomal storage disorders"/> After merging with lysosomes they create hybrid organelles called autolysosomes.<ref name="Samie Xu Lysosomal exocytosis"/>

Macroautophagy involves the formation of isolated double- or multi-membranes that encapsulate portions of cytosolic material such as misfolded- or polyubiquitinated proteins, lipids, damaged or aged organelles, RNA, and fragments of the ER. These vesicles mature into autophagosomes and then fuse with lysosomes for degradation, creating an autolysosome. A key marker of autophagosomes is LC3-II, a lipidated form of microtubule-associated protein light chain 3 (MAP-LC3) that appears early in the process and is broken down during digestion
Microautophagy bypasses vesicle formation altogether, with lysosomes directly engulfing surrounding cytosolic material through membrane invagination; pinocytosis. This process is similar to the formation of intraluminal vesicles in MVBs.
Chaperone-mediated autophagy (CMA) selectively degrades proteins that contain a KFERQ motif. These substrates are recognized by Hsc70, which binds them and delivers them to lysosomes via the receptor LAMP-2A for degradation.<ref name="Platt Lysosomal storage disorders"/>

The resulting catabolites serve as building-block molecules for synthesizing complex macromolecules. These are exported from lysosomes via specific transporters or through vesicle trafficking. Once released into the cytosol or delivered to the Golgi apparatus, these catabolites are either further metabolized to generate energy or reused in biosynthetic pathways to form new complex molecules. Alternatively, some degradation products can be secreted out of the lysosomes through exocytosis.<ref name="Samie Xu Lysosomal exocytosis"/>

Because catabolic (degradative) and anabolic (biosynthetic) pathways are interconnected and tightly regulated, the flow of cargo through the endocytic and autophagic systems is modulated by cellular signaling and nutrient availability. Nutrient deprivation, for example, activates autophagy, which is then halted once lysosomal degradation is complete. Lysosomes themselves play a direct role in sensing nutrient levels through the lysosomal nutrient-sensing (LYNUS) system, which includes components such as V-ATPase, Rag GTPases, and the mTOR complex.<ref name="Samie Xu Lysosomal exocytosis"/>

Formation

File:Phagocytosis.svg

Process of phagocytosis: 1. A particle is ingested by a phagocyte after antigens are recognized which results in the formation of a phagosome. 2. The fusion of lysosomes with the phagosome creates a phagolysosome. The particle is broken down by the digestive enzymes found in the lysosomes.

The formation of lysosomes begins in the endoplasmic reticulum, where hydrolytic enzymes are synthesized. These enzymes are then transported to the Golgi apparatus (Golgi body), where they undergo modifications to ensure proper targeting and function. The enzymes are tagged with mannose-6-phosphate,<ref name="Coutinho Mannose-6-phosphate pathway">Template:Cite journal</ref> allowing them to be sorted into vesicles. These vesicles then bud off from the trans-Golgi network and fuse with early endosomes.<ref name="Coutinho Mannose-6-phosphate pathway"/><ref>Template:Cite journal</ref>

Early endosomes degrade cargo from the extracellular environment, and as they mature into late endosomes, proton pumps are activated, causing the internal environment to become acidic. This acidic environment activates the hydrolytic enzymes, which further mature the endosome into a lysosome.<ref>Template:Cite journal</ref> The lysosome then breaks down and recycles cellular waste.<ref name="Kaur Early Endosome Morphology">Template:Cite book</ref>

Disruptions in lysosomal formation can lead to dysfunctional lysosomes and the accumulation of undigested molecules, contributing to various lysosomal storage disorders.<ref name="Kaur Early Endosome Morphology"/>

Pathogen entry

Lysosomes play a crucial role in defending the cell against pathogens such as viruses or bacteria. When a pathogen enters the cell, it is often enclosed in a phagosome which then fuses with a lysosome to form a phagolysosome where the hydrolytic enzymes break down the pathogen.<ref name="Bird Edgington-Mitchell Newton Eat, prey, love">Template:Cite journal</ref><ref name="Sabatini Adesnik Christian de Duve"/> Lysosomes are a crucial part of innate immune system.<ref name="Bird Edgington-Mitchell Newton Eat, prey, love"/> Lysosomes also play a big role in adaptive immune system where fragments of pathogens that are broken down by phagolysosomes are sent to the major histocompatibility complex class II (MHC II) and presented on the surface of antigen presenting cells (APCs). Which then activates helper T cells and then causes an adaptive immune response.<ref>Template:Cite book</ref>

When viruses enter the cell via endocytosis, they get degraded in lysosomes but then some viruses have evolved strategies to escape lysosomes. During degradation of viruses in the lysosome, the virus can escape the lysosome before complete degradation and spreading viral material into the cytoplasm which then spreads viral infection in the cell. So, lysosomes need to effectively degrade all the biomolecules, in other words, poor lysosomal activity results in higher viral infections by viruses such as HIV.<ref name="Sabatini Adesnik Christian de Duve"/><ref>Template:Cite journal</ref>

Clinical significance

Lysosomal storage disorders are a group of metabolic disorders that stem from inherited genetic mutations that disrupt normal lysosomal function and homeostasis.<ref name="Samie Xu Lysosomal exocytosis"/><ref name="Platt Lysosomal storage disorders">Template:Cite journal</ref> Most frequently, the mutations are located in the acidic hydrolases but can also be found in non-enzymatic lysosomal proteins (soluble and membrane-bound) and non-lysosomal factors controlling lysosomal function.<ref>Template:Cite journal</ref> This leads to defective degradation, and therefore induces abnormal accumulations of un- or partially digested macromolecules within lysosomes. Lysosomal dysfunction also affect transport across the lysosomal membrane, vesicle trafficking, lysosome reformation and autophagy.<ref>Template:Cite journal</ref><ref name="Samie Xu Lysosomal exocytosis"/>

The stress of accumulated lysosomal substrates can lead to lysosomal membrane permeabilization, allowing hydrolytic enzymes to leak into the cytosol and initiate cell death. This cell loss particularly affects post-mitotic tissues such as the brain, liver, eyes, muscles, and spleen—resulting in the hallmark symptoms of lysosomal storage disorders, including neurodegeneration, cognitive impairment, and motor dysfunction.<ref name="Platt Lysosomal storage disorders"/><ref>Template:Cite journal</ref>

The age of onset and the specific symptoms in lysosomal storage disorders differ depending on the severity of the mutations, the cell types affected and what substrates accumulate. However, the clinical presentation is typically a neurodegenerative disease at childhood, with more variations presenting themselves in adulthood. In most cases, the central nervous system (CNS) is affected, causing the brain to experience global neurodegeneration, inflammation, activation of the innate immune system and astrogliosis.<ref name="Platt Lysosomal storage disorders"/>

Several therapeutic strategies have been developed to address lysosomal storage disorders. These include substrate reduction therapy, bone marrow transplantation, gene therapy, and enzyme replacement therapy. Currently, enzyme replacement therapy and substrate reduction are the most widely used. However, despite these advancements most lysosomal storage disorders still lack effective treatments as the existing ones often are limited by poor efficacy and are typically disease specific.<ref name="Platt Lysosomal storage disorders"/>

Lysosomotropism

Lysosomotropism refers to the tendency of lipophilic weak bases to accumulate in acidic organelles like lysosomes. While neutral forms of these compounds cross membranes easily, their protonated (charged) forms become trapped inside lysosomes, leading to concentrations up to 1000 times higher than outside the cell.<ref>Template:Cite journal</ref><ref>Template:Cite book</ref> This “acid trapping” or “proton pump” effect can be predicted using mathematical models.<ref>Template:Cite journal</ref>

Many approved drugs, including haloperidol,<ref>Template:Cite journal</ref> levomepromazine,<ref>Template:Cite journal</ref> and amantadine,<ref>Template:Cite journal</ref> exhibit lysosomotropic behavior. This helps explain their high tissue-to-blood concentration ratios and prolonged tissue retention, though fat solubility also contributes.

Some lysosomotropic drugs can interfere with lysosomal enzymes like acid sphingomyelinase.<ref>Template:Cite journal</ref><ref>Template:Cite journal</ref> Ambroxol, a mucolytic, promotes lysosomal exocytosis by neutralizing lysosomal pH and releasing stored calcium.<ref>Template:Cite journal</ref> This action may underlie its observed benefits in diseases linked to lysosomal dysfunction, including Parkinson's disease and lysosomal storage disorders.<ref>Template:Cite journal</ref><ref>Template:Cite journal</ref>

Systemic lupus erythematosus (Lupus)

Systemic lupus erythematosus (SLE) otherwise known as Lupus is an autoimmune disease where the immune system attacks healthy cells.<ref>Template:Cite web</ref> Lupus is prominent in systemic lupus erythematosus preventing macrophages and monocytes from degrading neutrophil extracellular traps<ref>Template:Cite journal</ref> and immune complexes.<ref name=":3">Template:Cite journal</ref><ref>Template:Cite journal</ref><ref>Template:Cite journal</ref> The failure to degrade internalized immune complexes rises from irregularly extended activity of mTORC2, which impairs lysosome acidification.<ref>Template:Cite journal</ref> As a result, immune complexes in the lysosome recycle to the surface of macrophages causing an accumulation of DNA fragments and nuclear complexes which triggers an immune response from the body which is leads to the multiple lupus-associated pathologies.<ref name=":3" /><ref>Template:Cite journal</ref><ref>Template:Cite journal</ref>

Different types of enzymes present in lysosomes

There are over 50 different types of hydrolytic enzymes in lysosomes, the table below shows a few of the main types and their substrates. It is important to keep in mind that each category below has multiple different types of enzymes.


Sr. No	Enzymes	Substrate
1	Proteases	Proteins and Peptides (breaks peptide bonds)<ref>Template:Cite book</ref>
2	Nucleases	DNA and RNA (cleaves phosphodiester bonds)<ref>Template:Cite journal</ref>
3	Glycosidases	Carbohydrates (breaks glycosidic bonds)<ref>Template:Cite book</ref>
4	Lipases	Lipids (breaks ester bonds)<ref>Template:Cite book</ref>
5	Phospholipases	Phospholipids (cleaves fatty acids from phospholipids)<ref>Template:Cite book</ref>
7	Phosphatases	Phosphorylated molecules (removes phosphate groups)<ref>Template:Cite journal</ref>
8	Sulfatases	Sulfated molecules (removes sulphate groups)<ref>Template:Cite book</ref>

References

External links

Template:Wiktionary

Template:Organelles

Template:Authority control