Department of Biochemistry, MSI/WTB Complex, University of Dundee Dow Street, Dundee DD1 5EH, Scotland
Intermediate filaments require small heat shock protein chaperones to maintain their structural integrity and prevent their aggregation; mutations in these proteins lead to myopathy and cataract.
Intermediate filaments have a central role in maintaining the structural integrity of cells and tissues. The different proteins that constitute intermediate filaments show cell type specific expression patterns, for example, desmin is expressed in cardiac and skeletal muscle and mutations in this intermediate filament protein cause cardiac and skeletal myopathy1. Keratins, on the other hand, are only expressed in epithelial tissues, and mutations in these proteins are associated with blistering disorders2. Aggregation of intermediate filaments is a marker of these diseases and is also observed in other disorders where mutations have yet to be identified. Curiously, the small heat shock protein (sHSP) chaperone, B-crystallin, is often associated with these intermediate filament aggregates3. The principal function of the sHSP chaperones is to prevent the unfolding of cellular proteins damaged by stress. Two recent papers in Nature Genetics1,
4 and one in Proc. Natl. Acad. Sci. USA (5), now tie these observations together. Examining families with desmin-related myopathy (DRM)—an inherited disease characterized by skeletal muscle weakness and heart failure—the investigators discovered mutations in either desmin1 or B-crystallin4. Intermediate filament aggregates containing B-crystallin as well as desmin were observed, confirming the notion that protein chaperones are important for maintaining the integrity of intermediate filament networks in cells.
Expression of B-crystallin is highest in the eye lens, but this chaperone is found in many other tissues, including skeletal and cardiac muscle, where it constitutes as much as two percent of the total muscle protein. In keeping with this expression pattern, patients with DRM not only show muscular weakness but also present with cataracts. No other phenotypes have been recorded, despite the abundant expression of B-crystallin in other cells, such as astrocytes, suggesting that the cell/tissue context in which the chaperone is expressed influences disease initiation.
The papers do not elucidate the precise mechanisms by which sHSPs and intermediate filaments interact. It is known that sHSPs can both modulate the equilibrium of intermediate filament assembly as well as bind to intermediate filaments themselves5. In the lens, a stable complex between intermediate filament proteins and A/B-crystallin is present as a unique structure called the beaded filament6. Alterations to the lenticular intermediate filament network cause cataracts in animal models. Cataracts in humans can be caused by mutations in either A-crystallin7 or B-crystallin4. In DRM, moreover, the structural integrity of the intermediate filament network in muscle cells is not maintained in the presence of mutant B-crystallin, perhaps as a result of the accumulation of exercise-induced damage. Furthermore, intermediate filament function is apparently unimpaired for the first few decades of life. Perhaps this late-onset characteristic is the result of functional redundancy among either heat shock proteins or intermediate filament proteins because the transfection of mutant B-crystallin into tissue culture cells causes intermediate filament aggregation with no apparent delay.
Mutations in B-crystallin may induce a slow accumulation of structurally distinct intermediate filaments in the affected tissues which, when a certain concentration is reached, form aggregates. Similarly, the desmin mutations may alter the intra-filament arrangement of protein subunits that facilitate filament-filament associations whereas they do not interfere with the binding of sHSPs. Intermediate filament-associated proteins2, such as plectin, crosslink the filament networks, and defective desmin or B-crystallin may alter such associations. In every example in which mutations in intermediate filament proteins or associated proteins cause disease, it is the appearance of intermediate filament aggregates rather than the loss of the filaments themselves that characterizes disease pathology.
Our knowledge of how the sHSPs act as chaperones is sparse. Research areas of intense investigation include: the sHSP substrate binding site, regulation of chaperone activity and target motifs, the multi-subunit functional complex and the requirement of nucleotides as a source of energy in chaperone activity. The solving of the crystal structure of Methanococcus jannaschii HSP16.5, a member of the sHSP protein family, should make it possible to model the structure of other sHSPs and to derive hypotheses about their mechanism of action8. The -crystallin domain is highly conserved within the sHSP family and is thought to be more important in the formation of the functional oligomeric complex than in chaperone activity. The crystal structure reveals that 24 monomers form a hollow, spherical complex, 120 Å in diameter, with eight-fold symmetry. Amino acid residues in the -crystallin domain that are found to be important in the interactions of sHSPs with other proteins can now be mapped using the crystal structure9.
Intermediate filaments require B-crystallin to remain functional and presumably to counteract the effects of stress because this increases the binding of B-crystallin to intermediate filaments. Besides DRM, there are several other diseases in which co-aggregates of intermediate filaments and sHSPs are characteristic of the disease pathology; these include Alexander's disease and alcoholic hepatitis in which Rosenthal fibers and Mallory bodies, respectively, are found. In both instances there exist animal models in which the intermediate filament complement has been altered. Neurofilaments, the neuron-specific intermediate filaments, also associate with B-crystallin in a range of neuropathologies such as those involving ballooned neurons, amyloid plaques associated with bulbous neurites and Lewy bodies. It is an open question whether sHSPs contribute to these various disorders, but this should be investigated further in the light of these recent results.
Mutations in either small heat shock proteins, such as B-crystallin, or intermediate filament proteins lead to collapse and aggregation of cellular intermediate filament networks resulting in skeletal muscle and cardiac myopathy. In the case of desmin-related myopathy, a link between protein chaperones and intermediate filaments has been established, although the mechanisms causing the pathology of the disease are still unknown. Possible factors contributing to this pathology include misfolding of filament-associated proteins and other proteins, and disruption of protein assembly pathways. Intermediate filament aggregates found in other human diseases (indicated by dashed arrows) should be re-examined given these recent results and the role of B-crystallin and its mutations determined.
Goldfarb, L.G. et al. Missense mutations in desmin associated with familial cardiac and skeletal myopathy. Nature Genet.19, 402-403 (1998). | Article | PubMed | ISI |
Fuchs, E. & Cleveland, D.W. A structural scaffolding of intermediate filaments in health and disease. Science279, 514-519 (1998). | Article | PubMed | ISI |
Iwaki, T., Kume-Iwaki, A., Liem, R.K.H. & Goldman, J.E. aB-crystallin is expressed in non-lenticular tissues and accumulates in Alexander's disease brain. Cell57, 71-78 (1989). | PubMed | ISI |
Vicart, P. et al. A missense mutation in the aB-crystallin chaperone gene causes a desmin-related myopathy. Nature Genet.20, 92-95 (1998). | Article | PubMed | ISI |
Muñoz-Mármol, A.M. et al. A dysfunctional desmin mutation in a patient with severe generalized myopathy. Proc. Natl. Acad. Sci. USA, 95, 11312-11317 (1998). | Article | PubMed | ISI |
Carter, J.M., Hutcheson, A.M. & Quinlan, R.A. In vitro studies on the assembly properties of the lens proteins CP49, CP115: Co-assembly with a-crystallin but not with vimentin. Exp. Eye Res.60, 181-192 (1995). | PubMed | ISI |
Litt, M. et al. Autosomal dominant mutation of congenital cataract associated with a missense mutation in the human a-crystallin gene CRYAA. Hum. Mol. Genet.7, 471-474 (1998). | Article | PubMed | ISI |
Kim, K.K., Kim, R. & Kim, S.H. Crystal structure of a small heat-shock protein. Nature394, 595-599 (1998). | Article | PubMed | ISI |
Boelens, W.C., Croes, Y., de Ruwe, M., de Reu, L. & de Jong, W.W. Negative charges in the C-terminal domain stabilize the B-crystallin complex. J. Biol. Chem.273, 28055-28090 (1998).
Muchowski, P.J., Valdez, M.M. & Clark, J.I. B-crystallin selectively targets intermediate filament proteins during thermal stress. Invest. Ophthalmol. Vis. Sci. 40, in the press. | PubMed |
B-crystallin, is often associated with these intermediate filament aggregates
