Podocyte

Structure

A podocyte has a complex structure. Its cell body has extending major or primary processes that form secondary processes as podocyte foot processes or pedicels. The primary processes are held by microtubules and intermediate filaments. The foot processes have an actin-based cytoskeleton. Podocytes are found lining the Bowman's capsules in the nephrons of the kidney. The pedicels or foot processes wrap around the glomerular capillaries to form the filtration slits. The pedicels increase the surface area of the cells enabling efficient ultrafiltration.

Podocytes secrete and maintain the basement membrane.

There are numerous coated vesicles and coated pits along the basolateral domain of the podocytes which indicate a high rate of vesicular traffic.

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.

Energy needs

Podocytes require a significant amount of energy to preserve the structural integrity of their foot processes, given the substantial mechanical stress they endure during the glomerular filtration process.

Dynamic changes in glomerular capillary pressure exert both tensile and stretching forces on podocyte foot processes, and can lead to mechanical strain on their cytoskeleton. Concurrently, fluid flow shear stress is generated by the movement of glomerular ultrafiltrate, exerting a tangential force on the surface of these foot processes.

In order to preserve their intricate foot process architecture, podocytes require a substantial ATP expenditure to maintain their structure and cytoskeletal organization, counteract the elevated glomerular capillary pressure and stabilize the capillary wall.

Function

B. Glomerular basement membrane: 1. lamina rara interna 2. lamina densa 3. lamina rara externa

C. Podocytes: 1. enzymatic and structural protein 2. filtration slit 3. diaphragma]] Podocytes have primary processes called trabeculae, which wrap around the glomerular capillaries. The trabeculae in turn have secondary processes called pedicels or foot processes. Pedicels interdigitate, thereby giving rise to thin gaps called filtration slits. The slits are covered by slit diaphragms which are composed of a number of cell-surface proteins including nephrin, podocalyxin, and P-cadherin, which restrict the passage of large macromolecules such as serum albumin and gamma globulin and ensure that they remain in the bloodstream. Proteins that are required for the correct function of the slit diaphragm include nephrin, NEPH1, NEPH2, podocin, CD2AP. and FAT1.

Small molecules such as water, glucose, and ionic salts are able to pass through the filtration slits and form an ultrafiltrate in the tubular fluid, 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.

Clinical significance

A loss of the foot processes of the podocytes (i.e., podocyte effacement) is a hallmark of minimal change disease, which has therefore sometimes been called foot process disease.

Disruption of the filtration slits or destruction of the podocytes can lead to massive proteinuria, 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 kidney failure. This disease has been found to be caused by a mutation in the nephrin gene.

In 2002 Professor Moin Saleem at the University of Bristol made the first conditionally immortalised human podocyte cell line. This meant that podocytes could be grown and studied in the lab. Since then many discoveries have been made. Nephrotic syndrome occurs when there is a breakdown of the glomerular filtration barrier. The podocytes form one layer of the filtration barrier. Genetic mutations can cause podocyte dysfunction leading to an inability of the filtration barrier to restrict urinary protein loss. There are currently 53 genes known to play a role in genetic nephrotic syndrome. In idiopathic nephrotic syndrome, there is no known genetic mutation. It is thought to be caused by a hitherto unknown circulating permeability factor. Recent evidence suggests that the factor could be released by T-cells or B-cells, podocyte cell lines can be treated with plasma from patients with nephrotic syndrome to understand the specific responses of the podocyte to the circulating factor. There is growing evidence that the circulating factor could be signalling to the podocyte via the PAR-1 receptor.

Presence of podocytes in urine has been proposed as an early diagnostic marker for preeclampsia.

References

References

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