User:Kinkreet/Intermediate Filaments

Intermediate filaments (IFs) are a major component of the cytoskeleton, along with actin, microtubules and septins. IFs consists of over 65 different proteins, making up a filament 10-12nm in diameter. Unlike actin and microtubules, they have no polarity. They are dynamic structures, however, this dynamicity is not dependent on nucleotide hydrolysis.

Intermediate filaments acts as a scaffolding polymer, mechanical stress absorber, and structural support, both in the cytoplasm as well as the nucleus; IFs also play a role in integrating the entire cytoskeleton. Disease mutations indicate that intermediate filaments play an important role in cell-type-specific physiological functions. Intermediate filaments influence cell growth and death by interacting with non-structure-related proteins.

Vimentin keeps lymphocytes rigid while in circulation by forming a cytosolic spherical cage network; after receiving signals from chemokines and cytokines, plectin-mediated and phosphorylation-mediated vimentin retraction and condensation into a perinuclear aggregate occurs at the uropod (the trailing pole during migration), the cell becomes less rigid and able to vascularize.

All intermediate filament genes encode for cytoplasmic proteins, except lamins B1, B2 and A/C, which are found only in the nucleus, and interact with nuclear proteins.

Lamin IFs bind to chromatin directly as well as other nuclear proteins.

Certain intermediate filament proteins are preferentially expressed in certain cell types.

Crescentin is an intermediate filament-like protein found in prokaryotes, and is responsible for the shape of Caulobacter crescentus. Crescentin contains a heptad repeat, a α-helical rod domain. Crescentin can self-assemble into filaments in vivo. Crescentin is analogous to MreB and FtsZ, which are structurally-similar proteins for actin and tubulin, respectively, although similarity is lacking at the primary sequence-level.

The keratin K8/K18 pair is expressed in all epithelial cells from the four-cell stage and throughout early development, however, only expressed in epithelial linings during adulthood. Keratin K8/K18 is the sole component of liver hepatocytes, and so these cells have been used to study keratin K8/K18. In liver hepatocytes, keratin K8/K18 intermediate filaments act to protect the cell during phases of metabolic stress, by interacting with stress proteins such as hsp70, Mrj, Hsp27 and αB-crystallins, or kinases such as PKC-ε and c-Jun. Both K8 and K18 are rapidly and specifically (e.g. S33 [leads to interaction with 14-3-3ζ] and S52 on K18) phosphorylated when the cell is under stress.

The K8/K18 pair, as well as other keratins such as K17, are also involved in down-regulating apoptosis. Mice lacking K17 exhibit reversible alopecia, due to the apoptosis of matrix epithelial cells that are required for hair genesis. Intermediate filaments are thought to affect the display, densitiy or function of death receptors at the cell surface; for example, K8/K18 colocalize with tumour necrosis factor receptor 2 (TNFR2). K8/K18 mutants also exhibits a higher density of Fas receptors at the cell surface, increasing their sensitive to Fas-mediated apoptotic signals.

K8/K18 intermediate filaments can also down-regulate apoptosis by interacting with the downstream components after death-receptor engagement, such as the death-inducing signalling complex (DISC). K8, K14, K17 and K18 can interact with tumour-necrosis-factor-receptor-1-associated death domain protein (TRADD) through the highly conserved subdomain 1A of its rod domain. TRADD is an adaptor protein recruited to TNFR1 for downstream signalling; K8 and K18 can interact with TRADD and prevent its recruitment to TNFR1 to form the DISC, thus down-regulating TNFR1-mediated apoptosis.

Several intermediate filaments bind to, and are direct substrates for, caspases. K8/K18 are known to bind death-effector-domain-containing DNA-binding protein), active caspases 3 and 9, and ubiquitin during apoptosis; lamin A/C, vimentin and desmin intermediate filaments can be cleaved by caspases and this promotes apoptosis.

The intermediate filament network undergo extensive but reversible remodelling during mitosis. K10 and K1 are expressed in high levels post-mitosis. K10 sequesters Akt, a signal transduction molecule for cell growth/death, preventing its function and leads to reduced cell proliferation.

Intermediate filaments are involved in cell migration. After injury, keratinocytes next to the wounds becomes larger and polarized and reorganize their keratin network to the perinuclear area; this coincided with the up-regulation of K6 paralogues a and b, K16 and K17, and the down-regulation of K1/K10. Using K6 null mice, it was found that migration of these cells are enhanced, probably due to increased levels of total and tyrosine-phosphorylated p120 catenin. Thus, K6/K16 intermediate filaments provides mechanical resilience after injury, at the cost of reduced migration.

Vimentin is encoded by the Vim gene, located on chromosome 10 between position 17270258 and 1729592.

Withaferin A is a steroidal lactone from the plant Withania somnifera which binds to and inhibits vimentin. It causes vimentin to aggregate and retract from the cell periphery. The inhibition of vimentin led to a decreased interaction with NOD2 and subsequently the relocalization of NOD2 from the plasma membrane into the cytosol. Thus NOD2 localization at the plasma membrane depends on vimentin.

NOD2 forms a complex with receptor interacting protein 2, or caspase activation and recruitment domain of RIP-like interacting caspase-like apoptosis regulatory protein kinase (RICK)/RIP2, and activates nuclear factor (NF)-κB in epithelial cells and macrophages.

NOD1 and NOD2 binds to MDP and recruits the autophagy protein ATG16L1 to the plasma membrane.

Using withaferin A to inhibit vimentin, it was shown that NOD2-vimentin interaction, or NOD2 localization at the plasma membrane, limits the survival of E. coli, both AIEC and non-AIEC strains.

Cell-surface expressed vimentin is the binding target for several viruses, including porcine reproductive respiratory syndrome virus and Japanese encephalitis virus. The levels of vimentin is known to increase during ''Heliobacter pylori;; infection.

Disease
Disruption of the IF network are responsible for the fragile cell feature observed in several genetic diseases.

Cryptogenic cirrhosis

Hepatitis

Inflammatory bowel disease

Crohn's disease
Crohn's disease (CD) is a chronic gastrointestinal disease and a form of inflammatory bowel disease (IBD). Using genome-wide association scanning, several genes are identified as possible causes of CD, these include NOD2 (used in the innate immune response), IL23R (T cell response) and ATG16L ad IRGM (autophagy).

Mutations in the nucleotide-binding oligomerization domain-containing protein 2 (NOD2) gene creates the biggest genetic determinant of Crohn's disease (CD).

NOD2 is a receptor for muramyl dipeptide (MDP), a component of bacterial cell wall; thus NOD2 acts as a mammalian cytosolic pathogen receptor. NOD2 is localized and activated at the plasma membrane.

As deduced by yeast-2-hybrid and immunoprecipitation, it was shown that both vimentin and NOD2 colocalize at the plasma membrane. Vimentin interacts with the leucine-rich repeat (LRR) domain region of NOD2 using residues 52-283 on vimentin. Their interaction activates NF-κB in a NOD2-dependent manner. In Crohn's disease, polymorphism of the LRR domain (R702W and Leu1007fsinsCys but not G908R) disrupt this interaction, and carriage of two of these polymorphisms confers a 20-40 times increase in susceptibility to Crohn's disease. People with susceptibility to CD are prone to entatic bacteria, such as AIEC (adherent and invasive Escherichia coli), which have been consistently identified in patients with CD.

NOD2 activity also requires an intact actin cytoskeleton.

Vimentin is required for muramyl dipeptide-induced autophagy induction