Transthyretin

Transthyretin (TTR) is a serum and is_associated_with::cerebrospinal fluid carrier of the thyroid hormone is_associated_with::thyroxine (T4) and retinol-binding protein bound to is_associated_with::retinol. This is how transthyretin gained its name, transports thyroxine and retinol. The liver secretes transthyretin into the blood, and the choroid plexus secretes TTR into the is_associated_with::cerebrospinal fluid.

TTR was originally called prealbumin (or thyroxine-binding prealbumin) because it ran faster than is_associated_with::albumin on is_associated_with::electrophoresis gels.

Binding affinities
It functions in concert with two other thyroid hormone-binding proteins in the serum:

In cerebrospinal fluid TTR is the primary carrier of T4. TTR also acts as a carrier of is_associated_with::retinol (vitamin A) through its association with is_associated_with::retinol-binding protein (RBP) in the blood and the CSF. Less than 1% of TTR's T4 binding sites are occupied in blood, which is taken advantage of below to prevent TTRs dissociation, misfolding and aggregation which leads to the degeneration of post-mitotic tissue.

Numerous other small molecules are known to bind in the thyroxine binding sites, including many natural products (such as is_associated_with::resveratrol), drugs (is_associated_with::Tafamidis, or Vyndaqel, is_associated_with::diflunisal,  is_associated_with::flufenamic acid), and toxins (PCB ).

Structure
TTR is a 55kDa homotetramer with a dimer of dimers quaternary structure that is synthesized in the is_associated_with::liver, is_associated_with::choroid plexus and is_associated_with::retinal pigment epithelium for secretion into the bloodstream, cerebrospinal fluid and the eye, respectively. Each monomer is a 127-residue is_associated_with::polypeptide rich in is_associated_with::beta sheet structure. Association of two monomers via their edge beta-strands forms an extended beta sandwich. Further association of two of these dimers in a face-to-face fashion produces the homotetrameric structure and creates the two is_associated_with::thyroxine binding sites per tetramer. This dimer-dimer interface, comprising the two T4 binding sites, is the weaker dimer-dimer interface and is the one that comes apart first in the process of tetramer dissociation.

Role in disease
TTR misfolding and aggregation is known to be associated with the is_associated_with::amyloid diseases is_associated_with::senile systemic amyloidosis (SSA), is_associated_with::familial amyloid polyneuropathy (FAP), and is_associated_with::familial amyloid cardiomyopathy (FAC).

TTR tetramer dissociation is known to be rate-limiting for amyloid fibril formation. However, the monomer also must partially denature in order for TTR to be mis-assembly competent, leading to a variety of aggregate structures, including is_associated_with::amyloid fibrils.

While wild type TTR can dissociate, misfold, and aggregate, leading to SSA, is_associated_with::point mutations within TTR are known to destabilize the tetramer composed of mutant and wild-type TTR subunits, facilitating more facile dissociation and/or misfolding and amyloidogenesis. A replacement of valine by methionine at position 30 (TTR V30M) is the mutation most commonly associated with FAP. A position 122 replacement of valine by isoleucine (TTR V122I) is carried by 3.9% of the African-American population, and is the most common cause of FAC. SSA is estimated to affect over 25% of the population over age 80. Severity of disease varies greatly by mutation, with some mutations causing disease in the first or second decade of life, and others being more benign. Deposition of TTR amyloid is generally observed extracellularly, although TTR deposits are also clearly observed within the cardiomyocytes of the heart.

Treatment of familial TTR amyloid disease has historically relied on is_associated_with::liver transplantation as a crude form of gene therapy. Because TTR is primarily produced in the liver, replacement of a liver containing a mutant TTR gene with a normal gene is able to reduce the mutant TTR levels in the body to < 5% of pretransplant levels. Certain mutations, however, cause CNS amyloidosis, and due to the their production by the choroid plexus, the CNS TTR amyloid diseases do not respond to gene therapy mediated by liver transplantation.

In 2011, the is_associated_with::European Medicines Agency approved is_associated_with::Tafamidis or Vyndaqel for the amelioration of FAP. Vyndaqel kinetically stabilizes the TTR tetramer, preventing tetramer dissociation required for TTR amyloidogenesis and degradation of the autonomic nervous system and/or the peripheral nervous system and/or the heart. TTR is also thought to have beneficial side effects, by binding to the infamous is_associated_with::beta-amyloid protein, thereby preventing beta-amyloid's natural tendency to accumulate into the plaques associated with the early stages of is_associated_with::Alzheimer's Disease. Preventing plaque formation is thought to enable a cell to rid itself of this otherwise toxic protein form and, thus, help prevent and maybe even treat the disease.

There is now strong genetic and pharmacologic data (see is_associated_with::European Medicines Agency website for the Tafamidis clinical trial results) indicating that the process of amyloid fibril formation leads to the degeneration of post-mitotic tissue causing FAP and likely FAC and SSA. Evidence points to the oligomers generated in the process of amyloidogenicity leading to the observed is_associated_with::proteotoxicity.

Transthyretin level in cerebrospinal fluid has also been found to be lower in patients with some neurobiological disorders such as is_associated_with::schizophrenia. The reduced level of transthyretin in the CSF may indicate a lower thyroxine transport in brains of patients with schizophrenia.

Because transthyretin is made in part by the is_associated_with::choroid plexus, it can be used as an immunohistochemical marker for choroid plexus papillomas.

As of March 2015, there are two ongoing clinical trials undergoing recruitment in the United States and worldwide to evaluate potential treatments for TTR Amyloidosis.

Nutritional assessment
In medicine, nutritional status can be assessed by measuring the concentration of transthyretin in the is_associated_with::blood. In theory, other transport is_associated_with::proteins such as is_associated_with::albumin or is_associated_with::transferrin could be used, but transthyretin is preferred because of its shorter is_associated_with::half-life, although this means that its is_associated_with::concentration more closely reflects recent dietary intake rather than overall nutritional status. Transthyretin concentration has been shown to be a good indicator of whether or not a malnourished patient will develop is_associated_with::refeeding syndrome upon commencement of refeeding, via either the is_associated_with::enteral, is_associated_with::parenteral or oral routes.

Interactions
Transthyretin has been shown to interact with is_associated_with::Perlecan.