Structure Of Amyloidosis

Structure Of Amyloidosis

Discuss the chemical and physical structure of amyloidosis. What special stains are used to demonstrate amyloidosis?
Or
Write a brief on the staining characteristics of amyloid.
Or
Describe amyloidosis.
Answer:

Physical and Chemical Structure of Amyloidosis

It emerges that on the basis of morphology and physical characteristics, all forms of amyloid are similar in appearance but are chemically heterogeneous.

Amyloidosis Physical Structure

Amyloid consists of two types of proteins:

1. Fibrillar proteins: The majority of all forms of amyloid consist of a meshwork of fibrillar proteins. The girls are delicate random, dispersed, and non-branching, each measuring 7.5 to 10 nm in diameter and having an indefinite length. Each girl is further composed of the double helix of two pleated sheets in the form of twin filaments separated by a clear space. By X-ray crystallography and infra-red spectroscopy, fibrils are shown to have a cross-pleated sheet configuration which produces 1000 A° periodicity. These properties give amyloid its characteristic staining properties with Congo red dye and birefringence under polarising microscopy.

Amyloidosis structure

• AL protein: AL amyloid fibril protein is derived from the immunoglobulin light chain, which may be a complete light chain, or may include an amino-terminal segment and part of the C region of the immunoglobulin light chain. AL fibril protein is more frequently derived from the lambda light chain (twice more common) than kappa. AL-type official protein is produced by immunoglobulin-secreting cells and is therefore seen in association with plasma cell dyscrasias and is included in primary systemic amyloidosis.
• AA protein: AA fibril protein is composed of protein with a molecular weight of 8.5 kDa which is derived from a larger precursor protein in the serum called serum amyloid-associated protein with a molecular weight of 12.5 kDA. Deposits of AA amyloid do not have sequence homology. In the plasma, serum amyloid-associated protein circulates in association with HDL3 (high-density lipoprotein). Serum amyloid-associated protein is an acute phase reactant protein synthesized in the liver, in response to chronic inflammatory and traumatic conditions and thus the level of serum amyloid-associated protein is high in these conditions. AA fibril protein is found in secondary amyloidosis which is seen in association with several examples of chronic infectious and autoimmune inflammatory diseases and disseminated malignancies.

Beta pleated sheet in amyloidosis

2. Non­firillar components: They consist of 5% of amyloid material. They are:

• Amyloid P: It is synthesized from the liver and is structurally related to C reactive protein. On the basis of electron microscopy, it has a pentagonal profile or doughnut shape with an external diameter of 9 nm and an internal diameter of 4 nm.
• Apolipoprotein E: It is a regulator of lipoprotein metabolism and is found in all types of amyloid
• Sulfated glycosaminoglycans: They are constituents of matrix proteins, For Example. heparin sulfate
• Protein X: It is seen in cases of promises.

Amyloidosis Chemical Structure

Chemical analysis of fibril proteins of amyloid reveals the heterogeneous nature of amyloid. The three major forms of amyloid protein are:

1. AL protein, i.e. amyloid light protein: It is derived from lambda light chain and then kappa. AL type of viral protein is produced by immunoglobulin-secreting cells and is seen in plasma cell dyscrasias.

2. AA protein, i.e. amyloid associated protein: It is derived from a larger precursor protein in the serum called SAA. Deposits of AA amyloid don’t have sequence homology. It is found in secondary amyloidosis.

3. Other proteins:

• Transthyretin, i.e. TTR: It is a serum protein synthesized in the liver and normally transports thyroxine and retinol. Single amino acid substitution mutations in the structure of TTR results in a variant form of protein that is responsible for this form of amyloidosis termed ATTR. ATTR is the most common form of here do familial amyloidosis seen in familial amyloid
  polyneuropathies.
• A β2 microglobulin, i.e. A β2M: This form of amyloid is seen in cases of long-term hemodialysis (for 10-12 years). It is a small protein that is a normal component of major histocompatibility complex (MHC) class 1 and has a β-pleated sheet structure. β2M is an 11.8 kDa protein that is not filtered by the hemodialysis membrane and thus there is a high serum concentration of β2M protein in these patients. Although the deposit due to Aβ2M may be systemic in distribution, it has a predilection for bones and joints.
• Beta-amyloid protein (Aβ): Aβ is distinct from Aβ2M and is deposited in cerebral amyloid angiopathy and neurofibrillary tangles in Alzheimer’s disease. Aβ is derived from amyloid beta precursor protein (AβPP). The latter is a cell surface protein having a single transmembranous domain that functions as a receptor. The Aβ portion of this protein is seen extending into the extracellular region. Out of three intramembranous cleavage sites secretase—α, β, and γ, partial proteolysis of AβPP due to cleavage of β-secretase and γ-secretase sites generates Aβ, i.e. amyloidogenic protein in Alzheimer’s disease.
• IgG heavy chain amyloid (AH): It is derived from a truncated heavy chain of immunoglobulin.
• Amyloid from hormone precursor protein: These are procalcitonin, islet amyloid polypeptide, pro-insulin, and prolactin.
• Amyloid of prion protein(APrP): It is derived from precursor prion protein which is a plasma membrane glycoprotein. Prion proteins are proteinaceous infectious particles lacking in RNA or DNA. Amyloid in prognosis occurs due to an abnormally folded isoform of the PrP
• Miscellaneous here do familial forms of amyloid: It includes a variety of amyloid proteins reported recently.

Special Stains Used to Demonstrate Amyloidosis

• H and E stain: Amyloid by light microscopy with H and E stain appears as extracellular, structureless, and eosinophilic hyaline material. It appears pink in color.
• Metachromatic stains: The metachromatic stains are methyl violet/crystal violet which imparts rose pink color to amyloid deposit.
• Congo red stain: Under light microscopy while staining the amyloid with Congo red stain it gives pink red color. In polarized light, it provides re-green birefringence.
• Thioflavin-T and S: Under this amyloid fluoresces yellow color.
• Immunohistochemistry: Various antibody stains against specific antigenic protein types of amyloid are available. The most common antibodies are anti-AA, anti-lambda, and anti-kappa antibodies to differentiate between different types of amyloids.
• Non­specifi Stains:
  • Standard Toluidine Blue: It gives orthochromatic blue color to amyloid which under polarizing microscopy produces dark red birefringence.
  • Alcian Blue: Impart blue-green color and is used for mucopolysaccharide content in amyloid.
  • Periodic Schifftain (PAS): Used for demonstration of carbohydrate content. It appears pink in color.