insight review articles Folding proteins in fatal ways Dennis J Selkoe Center for Neurologic Diseases Harvard Medical School Brigham and Women s Hospital Boston Massachusetts 02115 USA e mail dselkoe rics bwh harvard edu Human diseases characterized by insoluble extracellular deposits of proteins have been recognized for almost two centuries Such amyloidoses were once thought to represent arcane secondary phenomena of questionable pathogenic significance But it is has now become clear that many different proteins can misfold and form extracellular or intracellular aggregates that initiate profound cellular dysfunction Particularly challenging examples of such disorders occur in the post mitotic environment of the neuron and include Alzheimer s and Parkinson s diseases Understanding some of the principles of protein folding has helped to explain how such diseases arise with attendant therapeutic insights O ne of the most satisfying moments in scientific investigation occurs when previously disparate phenomena of unclear origin are shown to arise from a common principle During the past decade numerous aetiologically distinct diseases have been linked by the likelihood that they result from the progressive misfolding of specific proteins into aggregates that can injure and kill cells Together these disorders inflict enormous personal and societal burdens making it crucial to understand their genesis and to learn how to prevent them The amyloidoses have traditionally been defined as diseases in which normally soluble proteins accumulate in the extracellular space of various tissues as insoluble deposits of 10 nm fibrils that are rich in b sheet structure and have characteristic dye binding properties1 2 There are many examples of secreted circulating proteins that can under abnormal circumstances be converted in part to highly stable extracellular fibrils These include immunoglobulins in primary systemic amyloidosis and multiple myeloma amylin in the diabetic pancreas and small soluble proteins of uncertain function such as the amyloid b peptide Ab in Alzheimer s disease Although the specific polypeptides that comprise the deposits are different for each amyloidosis the disorders have several key features in common Perhaps foremost among them is the ability of proteins that are highly soluble in biological fluids to be converted gradually to insoluble filamentous polymers enriched in b pleated sheet conformation The common structural motif of virtually all amyloid fibrils consists of cross b sheets in which the peptide strands are arranged perpendicular to the long axis of the fibre Although it was once thought that relatively few proteins have this propensity recent data suggest that many soluble proteins can under certain circumstances undergo this conversion Of particular significance is the finding that globular proteins with diverse sequences that are not currently associated with a protein folding disease for example muscle myoglobin can undergo aggregation in vitro into fibrils indistinguishable from those found in the amyloidoses3 4 This finding supports the concept that aggregation into b sheet rich fibrils is a generic property of polypeptide chains regardless of sequence5 Another general feature of protein folding disorders is the prolonged period before clinical manifestations appear Although the age of onset of symptoms varies widely among the different diseases and even among cases of one disease most of these disorders become noticeable in middle or late life There is a prolonged preclinical phase during which 900 proteins misfold build up and progressively compromise cellular and tissue function A portion of this long prodrome derives from the energetic barriers to the formation of misfolded species including the fact that nucleation the initial development of very small metastable oligomers of a protein is a kinetically unfavourable requirement for fibrillogenesis6 7 It seems that time rather than great age is required in that some aggressive protein folding disorders can occur in young and early middle aged individuals In such cases time still has a role but the fibrillogenic process requires less time overall because particular biochemical circumstances promote accelerated nucleation One striking example is Down s syndrome in which patients with trisomy 21 develop abundant Ab aggregates in the brain as early as the age of ten owing to lifelong overexpression of the b amyloid precursor protein APP which is encoded on chromosome 21 Similarly inherited missense mutations in amyloid prone proteins can markedly accelerate their misfolding and fibrillogenesis producing earlier disease onset than occurs with the wild type isoform For example senile systemic amyloidosis arises late in life from the aggregation of wild type transthyretin whereas familial amyloidotic polyneuropathy I generally arises in mid life from the accelerated aggregation of mutant transthyretin In this review I describe briefly three types of amyloidosis systemic organ limited and intracellular I then examine how abberant protein folding may occur and how misfolded proteins may disrupt the cell Systemic amyloidoses The list of secreted circulating proteins that are capable of producing extracellular amyloid deposits in multiple organs is long and growing Table 1 Amyloidoses can arise when other pathological conditions cause a sharp increase in the concentration of an amyloid prone polypeptide as occurs for the serum amyloid A SAA protein during the acute phase response accompanying inflammatory disorders such as rheumatoid arthritis or chronic granulomatous infections such as tuberculosis A 76 residue proteolytic fragment of wild type SAA can then accumulate misfold aggregate and be deposited in the connective tissue of multiple organs including spleen kidney and liver Multiple myeloma is associated with the overproduction by plasma cells of monoclonal immunoglobulins that accumulate and form deposits as holoproteins and or proteolytic fragments in the extracellular space of various tissues Primary systemic amyloidosis also involves progressive multi tissue deposition of immunoglobulin light chains and their fragments In some systemic amyloidoses the basis for an NATURE VOL 426 18 25 DECEMBER 2003 www nature com nature 2003 Nature Publishing Group insight review articles Table 1 Some members of the family of systemic extracellular amyloidoses Clinical syndrome Fibril subunit Primary systemic