Many diseases arise from mutations that primarily affect neither expression nor function, but result in a protein with decreased conformational stability.

This conformational instability can intermittently cause the affected protein to unfold and then undergo intermolecular linkage, which results in intracellular aggregation that causes cumulative cell damage.

The insidious nature of protein accumulation explains why some familial diseases characteristically affect individuals in middle- or old-age.

The aggregation of conformationally destabilized proteins is now known to be a feature of many of the neurodegenerative diseases, notably of Alzheimer disease and Parkinson disease and the spongiform encephalopathies. (see serpins, serpin family of protease inhibitors).

The common consequence of the mutations in all these diseases is the molecular instability of the encoded protein, which results in the formation of intermolecular beta-linkages, in which single peptide strands become aligned to form highly stable beta-sheets. These beta-linked structures then accumulate to cause cell death and hence disease.

Pathogenesis

Numerous diseases, including Alzheimer disease, Parkinson disease and other late-onset neurodegenerative diseases, arise from the conformationally driven aggregation of individual proteins.

Previous focus on just one end-product of such aggregation - extracellular deposits of amyloid - has diverted attention from what is now recognized as being primarily intracellular disease processes.

Recent structural findings show how cytotoxicity can result from even minor changes in conformation that do not lead to amyloid formation, as with the accumulation within the endoplasmic reticulum of intact mutant alpha-1-antitrypsin in hepatocytes and of neuroserpin in neurons.

Studies in Alzheimer disease and other dementias also indicate that the damage occurs at the stage of the initial intermolecular linkages that precede amyloid formation.

 The conformation of proteins is maintained by the tight packing of their amino-acid side-chains; even small changes that result from the replacement of a single amino acid can be sufficient to allow conformational unfolding. This is especially true of some key domains of functional proteins, and is well illustrated by the serpins.

 Propagation and prions

Structural studies of the serpins have also shown the feasibility of another perplexing feature of some of the familial and acquired encephalopathies: the ability of the underlying protein oligomers and filaments to self-propagate and even, with the prions, to propagate infectively.

Aggregation to form polymeric filaments is particularly likely to occur when there is a beta-strand receptor, as in the main beta-sheet of the conformationally unstable variants of alpha1-antitrypsin.

Both the serpins and the prions readily dimerize by domain swapping, and the initial oligomers, like those formed by serpins, act as a template for propagation of the conformational change.

The change is propagated from molecule to molecule, which then extend to give long-chain polymers. This is well illustrated by mutations of antithrombin.

Whereas mutations in alpha1-antitrypsin and neuroserpin result in the formation of long-chain polymers, those in antithrombin result in the formation of an inactive antithrombin monomer.

Moreover, in a process similar to that proposed for the prion encephalopathies, this aberrant form of antithrombin then binds to a normal antithrombin molecule, which leads to the propagation of conformational inactivation.

A more direct insight into the mechanisms of prion propagation that underlie the spongiform encephalopathies is provided by studies of the unrelated yeast prion Ure2. This normally soluble and highly ordered molecule undergoes a conformational change to form fibrils that bind the dye Congo red and show typical amyloid-like birefringence.

However, these fibrils do not have the cross-beta-structure of amyloid and, as is also observed with fibrils formed by orderly aggregation of serpins, the component molecules of the fibril retain their helical structure.

These amyloid-like fibrils are self-propagating but lose this ability over time, coincident with a transition of the fibril to give the typical amyloid X-ray fibre diffraction pattern.

These findings fit with a range of others and together they indicate that the neuronal pathology of the conformational dementias results from the early stages of intracellular aggregation, before or incidental to amyloid formation.

 Therapeutic pathways

The realization that the conformational diseases are due to aberrant beta-linkages opens the possibility of general as well as specific approaches to treatment. Current attempts are aimed at either blocking the formation of beta-linkages or increasing the turnover of the accumulated aggregates.

These approaches are typified by continuing studies that aim to block the linkage between the reactive-centre loop and beta-sheet A, which underlies the polymerization of the serpins.

The polymerization of Z alpha1-antitrypsin can be blocked by annealing reactive-loop peptides to beta-sheet A. These peptides were 11-13 residues long and able to anneal to other members of the serpin superfamily. This was most clearly shown by the finding that a reactive-loop peptide of antithrombin annealed more readily to beta-sheet A of alpha1-antitrypsin and vice versa.

These peptides, although useful in establishing the mechanism of polymerization, are too long and too promiscuous to be used for rational drug design.

 chemical chaperones can also be used to stabilize intermediates of the folding pathway.

The chaperone trimethyamine oxide has no effect on the secretion of Z alpha1-antitrypsin in cell culture as it allowed the conversion of unfolded Z alpha1-antitrypsin to polymers.

By contrast, the chaperone 4-phenylbutyrate (4-PBA) increases the secretion of Z alpha1-antitrypsin from cell lines and transgenic mice.

This agent has been used for several years to treat children with urea cycle disorders and more recently 4-PBA has been shown to increase the expression of mutant (deltaF508) cystic fibrosis transmembrane regulator protein in vivo.

These encouraging findings have led to a pilot study being carried out to evaluate the potential of 4-PBA to promote the secretion of 1-antitrypsin in patients with 1-antitrypsin deficiency.

The blockage of the beta-strand linkage that underlies the serpinopathies need not be complete, but merely sufficient, to allow the cell to degrade the aberrant protein. This is also likely to be the case for beta-strand blocking agents used to prevent the other beta-linked conformational diseases. Each disease will require a different beta-strand blocking agent targeted to the appropriate organelle of the cell or the extracellular matrix. is being illustrated by study of the serpins.

Types

 alpha1-antitrypsin deficiency
 conformational dementias

• late-onset dementia
• amyloidoses
• prion encephalopathies
• Huntington disease
• Alzheimer disease

See also

 conformational diversity

References

 Carrell RW. Cell toxicity and conformational disease. Trends Cell Biol. 2005 Nov;15(11):574-80. PMID: #16202603#

 Bernier V, Lagace M, Bichet DG, Bouvier M. Pharmacological chaperones: potential treatment for conformational diseases. Trends Endocrinol Metab. 2004 Jul;15(5):222-8. PMID: #15223052#

 Lomas DA, Carrell RW. Serpinopathies and the conformational dementias. Nat Rev Genet. 2002 Oct ;3(10):759-68. PMID : #12360234#

 Carrell RW, Lomas DA. Alpha1-antitrypsin deficiency: a model for conformational diseases. N Engl J Med. 2002 Jan 3;346(1):45-53. PMID: #11778003#

 Seidah NG, Prat A. Precursor convertases in the secretory pathway, cytosol and extracellular milieu. Essays Biochem. 2002;38:79-94. PMID: #12463163#