Protein Misfolding Diseases and Therapeutic Approaches.

Protein folding is the process by which a polypeptide chain acquires its functional, native 3D structure. Protein misfolding, on the other hand, is a process in which proteins fails to fold into its native functional conformation. This misfolding of proteins may lead to precipitation of number of serious diseases such as Cystic fibrosis (CF), Alzheimer's disease (AD), Parkinson's disease (PD), and amyotrophic lateral sclerosis (ALS) etc. Protein quality-control (PQC) systems, consisting of molecular chaperones, proteases and regulatory factors, help in protein folding and prevent its aggregation. At the same time PQC syatems also do sorting and removal of improperly folded polypeptides. Among the major types of PQC systems involved in protein homeostasis are cytosolic, endoplasmic reticulum (ER) and mitochondrial ones. The cytosol PQC system includes a large number of component chaperones, such as nascent-polypeptide-associated complex (NAC), Hsp40, Hsp70, prefoldin and T Complex Protein-1 (TCP-1) ring complex (TRiC). Protein misfolding diseases caused due to defective cytosolic PQC system includes diseases involving keratin/collagen proteins, cardiomyopathies, phenylketonuria, PD and ALS. The components of PQC system of endoplasmic reticulum (ER) includes Binding immunoglobulin Protein (BiP), calnexin (CNX), calreticulin (CRT), glucose-regulated protein Grp94, the thiol-disulphide oxidoreductases, protein disulphide isomerase (PDI) and ERp57. ER-linked misfolding diseases include CF and familial neurohypophyseal diabetes insipidus (FNDI). The components of mitochondrial PQC system include mitochondrial chaperones such as the Hsp70, the Hsp60/Hsp10 and a set of proteases having AAA+ domains similar to the proteasome that are situated in the matrix or the inner membrane. Protein misfolding diseases caused due to defective mitochondrial PQC system include medium-chain acyl-CoA dehydrogenase (MCAD)/short-chain acyl-CoA dehydrogenase (SCAD) deficiency diseases, hereditary spastic paraplegia. Therapeutic approaches towards the treatment of various protein misfolding diseases using molecular, chemical and pharmacological classes of chaperones are also discussed.