Podocyte
Encyclopedia
Podocytes are cells
in the Bowman's capsule
in the kidneys that wrap around the capillaries of the glomerulus
. The Bowman's capsule filters blood, holding back large molecules such as proteins, and passing through small molecules such as water, salts, and sugar, as the first step in forming urine.
The long processes, or "foot projections," of the podocytes wrap around the capillaries, and leave slits between them. Blood is filtered through these slits, each known as a slit diaphragm or filtration slit. Several proteins are required for the foot projections to wrap around the capillaries and function. When infants are born with certain defects in these proteins, such as nephrin
and CD2AP
, their kidneys cannot function. People have variations in these proteins, and some variations may predispose them to kidney failure later in life. Nephrin is a zipper-like protein that forms the slit diaphragm, with spaces between the teeth of the zipper, big enough to allow sugar and water through, but too small to allow proteins through. Nephron defects are responsible for congenital kidney failure. CD2AP regulates the podocyte cytoskeleton and stabilizes the slit diaphragm.
.
The slits are covered by slit diaphragms which are composed of a number of cell-surface proteins including nephrin
, podocalyxin
, and P-cadherin
, which ensure that large macromolecules such as serum albumin
and gamma globulin
remain in the bloodstream.
Small molecules such as water
, glucose
, and ion
ic salts are able to pass through the slit diaphragms and form an ultrafiltrate which is further processed by the nephron
to produce urine
.
Podocytes are also involved in regulation of glomerular filtration rate (GFR). When podocytes contract, they cause closure of filtration slits. This decreases the GFR by reducing the surface area available for filtration.
and coated pits can be seen along the basolateral domain of the podocytes.
In their cell bodies, podocytes possess a well-developed endoplasmic reticulum
and a large Golgi apparatus
, indicative of a high capacity for protein synthesis and post-translational modifications.
There is also growing evidence of a large number of multivesicular bodies and other lysosomal
components seen in these cells, indicating a high endocytic
activity.
"Pedicles" (or "foot processes") extend from the podocyte and increase the surface area which is crucial for the efficiency of ultrafiltration.
where large amounts of protein are lost from the blood.
An example of this occurs in the congenital disorder Finnish-type nephrosis, which is characterised by neonatal proteinuria leading to end-stage renal failure
. This disease has been found to be caused by a mutation in the nephrin
gene.
Cell (biology)
The cell is the basic structural and functional unit of all known living organisms. It is the smallest unit of life that is classified as a living thing, and is often called the building block of life. The Alberts text discusses how the "cellular building blocks" move to shape developing embryos....
in the Bowman's capsule
Bowman's capsule
The Bowman's capsule is a cup-like sac at the beginning of the tubular component of a nephron in the mammalian kidney that performs the first step in the filtration of blood to form urine. A glomerulus is enclosed in the sac...
in the kidneys that wrap around the capillaries of the glomerulus
Glomerulus
A glomerulus is a capillary tuft that is involved in the first step of filtering blood to form urine.A glomerulus is surrounded by Bowman's capsule, the beginning component of nephrons in the vertebrate kidney. A glomerulus receives its blood supply from an afferent arteriole of the renal...
. The Bowman's capsule filters blood, holding back large molecules such as proteins, and passing through small molecules such as water, salts, and sugar, as the first step in forming urine.
The long processes, or "foot projections," of the podocytes wrap around the capillaries, and leave slits between them. Blood is filtered through these slits, each known as a slit diaphragm or filtration slit. Several proteins are required for the foot projections to wrap around the capillaries and function. When infants are born with certain defects in these proteins, such as nephrin
Nephrin
Nephrin is a protein necessary for the proper functioning of the renal filtration barrier. The renal filtration barrier consists of fenestrated endothelial cells, the glomerular basement membrane, and the podocytes of epithelial cells...
and CD2AP
CD2AP
CD2-associated protein is a protein that in humans is encoded by the CD2AP gene.-Interactions:CD2AP has been shown to interact with NPHS2, Nephrin, Cbl gene and RAB4A.-Further reading:...
, their kidneys cannot function. People have variations in these proteins, and some variations may predispose them to kidney failure later in life. Nephrin is a zipper-like protein that forms the slit diaphragm, with spaces between the teeth of the zipper, big enough to allow sugar and water through, but too small to allow proteins through. Nephron defects are responsible for congenital kidney failure. CD2AP regulates the podocyte cytoskeleton and stabilizes the slit diaphragm.
Function
Adjacent podocytes interdigitate to cover the basal lamina which is intimately associated with the glomerular capillaries, but the podocytes leave gaps or thin filtration slitsFiltration slits
Foot processes of podocytes interdigitate with one another forming filtration slits that, in contrast to those in the glomerular endothelium, are spanned by slit diaphragms.-Structure:...
.
The slits are covered by slit diaphragms which are composed of a number of cell-surface proteins including nephrin
Nephrin
Nephrin is a protein necessary for the proper functioning of the renal filtration barrier. The renal filtration barrier consists of fenestrated endothelial cells, the glomerular basement membrane, and the podocytes of epithelial cells...
, podocalyxin
Podocalyxin
Podocalyxin, a sialoglycoprotein, is thought to be the major constituent of the glycocalyx of podocytes. It is a member of the CD34 family of transmembrane sialomucins. It coats the secondary foot processes of the podocytes. It is negatively charged and thus functions to keep adjacent foot...
, and P-cadherin
Cadherin
Cadherins are a class of type-1 transmembrane proteins. They play important roles in cell adhesion, ensuring that cells within tissues are bound together. They are dependent on calcium ions to function, hence their name.The cadherin superfamily includes cadherins, protocadherins, desmogleins, and...
, which ensure that large macromolecules such as serum albumin
Serum albumin
Serum albumin, often referred to simply as albumin is a protein that in humans is encoded by the ALB gene.Serum albumin is the most abundant plasma protein in mammals. Albumin is essential for maintaining the osmotic pressure needed for proper distribution of body fluids between intravascular...
and gamma globulin
Gamma globulin
Gamma globulins are a class of globulins, identified by their position after serum protein electrophoresis. The most significant gamma globulins are immunoglobulins , more commonly known as antibodies, although some Igs are not gamma globulins, and some gamma globulins are not Igs.-Use as medical...
remain in the bloodstream.
Small molecules such as water
Water
Water is a chemical substance with the chemical formula H2O. A water molecule contains one oxygen and two hydrogen atoms connected by covalent bonds. Water is a liquid at ambient conditions, but it often co-exists on Earth with its solid state, ice, and gaseous state . Water also exists in a...
, glucose
Glucose
Glucose is a simple sugar and an important carbohydrate in biology. Cells use it as the primary source of energy and a metabolic intermediate...
, and ion
Ion
An ion is an atom or molecule in which the total number of electrons is not equal to the total number of protons, giving it a net positive or negative electrical charge. The name was given by physicist Michael Faraday for the substances that allow a current to pass between electrodes in a...
ic salts are able to pass through the slit diaphragms and form an ultrafiltrate which is further processed by the nephron
Nephron
The renal tubule is the portion of the nephron containing the tubular fluid filtered through the glomerulus. After passing through the renal tubule, the filtrate continues to the collecting duct system, which is not part of the nephron....
to produce urine
Urine
Urine is a typically sterile liquid by-product of the body that is secreted by the kidneys through a process called urination and excreted through the urethra. Cellular metabolism generates numerous by-products, many rich in nitrogen, that require elimination from the bloodstream...
.
Podocytes are also involved in regulation of glomerular filtration rate (GFR). When podocytes contract, they cause closure of filtration slits. This decreases the GFR by reducing the surface area available for filtration.
Structure features
Structural features of podocytes indicate a high rate of vesicular traffic in these cells. Many coated vesiclesVesicle (biology)
A vesicle is a bubble of liquid within another liquid, a supramolecular assembly made up of many different molecules. More technically, a vesicle is a small membrane-enclosed sack that can store or transport substances. Vesicles can form naturally because of the properties of lipid membranes , or...
and coated pits can be seen along the basolateral domain of the podocytes.
In their cell bodies, podocytes possess a well-developed endoplasmic reticulum
Endoplasmic reticulum
The endoplasmic reticulum is an organelle of cells in eukaryotic organisms that forms an interconnected network of tubules, vesicles, and cisternae...
and a large Golgi apparatus
Golgi apparatus
The Golgi apparatus is an organelle found in most eukaryotic cells. It was identified in 1898 by the Italian physician Camillo Golgi, after whom the Golgi apparatus is named....
, indicative of a high capacity for protein synthesis and post-translational modifications.
There is also growing evidence of a large number of multivesicular bodies and other lysosomal
Lysosome
thumb|350px|Schematic of typical animal cell, showing subcellular components. [[Organelle]]s: [[nucleoli]] [[cell nucleus|nucleus]] [[ribosomes]] [[vesicle |vesicle]] rough [[endoplasmic reticulum]]...
components seen in these cells, indicating a high endocytic
Endocytosis
Endocytosis is a process by which cells absorb molecules by engulfing them. It is used by all cells of the body because most substances important to them are large polar molecules that cannot pass through the hydrophobic plasma or cell membrane...
activity.
"Pedicles" (or "foot processes") extend from the podocyte and increase the surface area which is crucial for the efficiency of ultrafiltration.
Pathology
Disruption of the slit diaphragms or destruction of the podocytes can lead to massive proteinuriaProteinuria
Proteinuria means the presence of anexcess of serum proteins in the urine. The protein in the urine often causes the urine to become foamy, although foamy urine may also be caused by bilirubin in the urine , retrograde ejaculation, pneumaturia due to a fistula, or drugs such as pyridium.- Causes...
where large amounts of protein are lost from the blood.
An example of this occurs in the congenital disorder Finnish-type nephrosis, which is characterised by neonatal proteinuria leading to end-stage renal failure
Renal failure
Renal failure or kidney failure describes a medical condition in which the kidneys fail to adequately filter toxins and waste products from the blood...
. This disease has been found to be caused by a mutation in the nephrin
Nephrin
Nephrin is a protein necessary for the proper functioning of the renal filtration barrier. The renal filtration barrier consists of fenestrated endothelial cells, the glomerular basement membrane, and the podocytes of epithelial cells...
gene.
External links
- "Mammal, renal vasculature (EM, High) - ". Ultrastructure of the Cell: podocytes and glomerular capillaries"- podocyte.ca at Samuel Lunenfeld Research InstituteSamuel Lunenfeld Research InstituteThe Samuel Lunenfeld Research Institute of Mount Sinai Hospital, Toronto, Ontario, Canada was established in 1985 by an endowment from the Lunenfeld and Kunin families. It comprises 36 principal investigators, has a budget of C$90 million , has over 200 trainees and approximately 600 staff...