insights into the toxicity caused by the intracellular accumulation of mutant serpins largely comes from work on 1-antitrypsin deficiency , the best characterized of the serpinopathies .
1-antitrypsin is synthesized in the liver and is constitutively released into the circulation where it protects tissues against damage from the neutrophil enzyme elastase .
the z mutant of 1-antitrypsin is found in 4% of the north european population and causes some of the newly synthesized protein to misfold and form ordered polymers ( fig .
1 ) that are retained within the endoplasmic reticulum ( er ) of hepatocytes ( 3 , 4 ) . the resulting protein overload predisposes individuals homozygous for the z mutation to neonatal hepatitis , cirrhosis , and hepatocellular carcinoma .
other mutants of 1-antitrypsin ( 51phe , ser53phe ) also cause the protein to form ordered polymers and create a similar degree of protein overload .
these mutations probably also predispose to liver disease , but their rarity makes it difficult to undertake detailed epidemiological studies . the s ( glu264val ) and i ( arg39cys ) mutants of 1-antitrypsin form polymers much less readily than the z mutant and thus can be cleared by the normal disposal pathways within the cell .
these mutants are not associated with liver disease unless they are inherited along with the more severe z allele .
thus , there is a threshold effect whereby the mutation must cause sufficient accumulation of polymers to cause disease .
mutants of the brain - specific serpin neuroserpin also form ordered polymers that accumulate within the er of neurons .
these mutations cause an autosomal - dominant dementia known as familial encephalopathy with neuroserpin inclusion bodies ( fenib ) ( 5 ) .
similar to 1-antitrypsin mutants , the neuroserpin mutants display a striking genotype  phenotype correlation : more severe mutations lead to faster polymer formation , more inclusions , and disease onset at a younger age .
how intracellular accumulation of mutant 1-antitrypsin and neuroserpin polymers causes the cell death and inflammation that characterize cirrhosis and fenib is not completely clear .
a majority of the mutated protein fails to fold and is degraded by the proteasome ( 6 ) , and 1015% of the protein folds normally and is secreted into the plasma ( in the case of z 1-antitrypsin ) .
however , a small proportion of the mutant protein folds into ordered polymers , some of which are degraded by autophagy ( 7 ) and the remainder of which accumulates within the er . accumulation of the mutant protein in the er activates the transcription factor nuclear factor b ( nf-b ) and caspases ( 810 ) , thereby inducing inflammation and apoptosis . the mechanism that prevents the exit of polymers from the er and the consequent signaling that activates nf-b and caspases has yet to be elucidated .
not all pathogenic serpin mutations result in the build up of toxic polymers . in some cases
this is exemplified by naturally occurring mutations in the plasma proteins c1-inhibitor , antithrombin , and 1-antichymotrypsin , which control the complement , coagulation , and inflammatory cascades , respectively .
these mutations destabilize the protein 's architecture , allowing the formation of unstable intermediates and inactive polymers within the er of hepatocytes .
however , as these proteins are less abundant ( synthesized at 10% the rate of 1-antitrypsin ) and the mutations are usually heterozygous , the aberrant protein can be effectively cleared by degradative pathways and does not form toxic inclusions .
however , intrahepatic clearance of these mutant proteins leads to reduced secretion and a lack of functional protein in the circulation .
c1-inhibitor deficiency results in uncontrolled activity of the complement cascade and angio - edema ; antithrombin deficiency causes thrombosis ; and 1-antichymotrypsin deficiency renders tissues vulnerable to proteolytic attack , which can lead to inflammation and chronic obstructive pulmonary disease .
a mutation in the serpin heparin cofactor ii , which normally inhibits coagulation , is associated with plasma deficiency of the protein , but so far has not been shown to cause disease ( 11 ) .
this mutation is of particular interest as it is analogous to the z allele that causes polymerization of 1-antitrypsin .
the same mutation in the drosophila serpin necrotic causes temperature - dependent polymerization and inactivation of the protein ( 12 ) .
we therefore predict that episodes of fever will precipitate the inactivation of unstable serpins and thus exacerbate the tissue damage of the serpinopathies .
although mutation - induced polymerization often results in the retention of the protein within hepatocytes , some mutant protein can traffic through the secretory pathway and reach the circulation or local tissues .
this protein often retains function as a proteinase inhibitor but still carries the mutation and thus the propensity to form polymers once secreted .
indeed , polymers of 1-antitrypsin , antithrombin , and c1-inhibitor have been identified in the plasma of individuals with mutations in these proteins ( 1315 ) .
moreover , polymers of neuroserpin predominate in the culture media of cells transfected with mutants of neuroserpin ( 16 ) .
it is clearly important to consider whether extracellular polymers are inactive bystanders or whether they can themselves exacerbate the tissue damage of the serpinopathies .
recent work has shown that polymers of 1-antitrypsin are chemotactic for neutrophils in vitro and cause a neutrophil influx when instilled into the lungs of mice ( 1719 ) .
in contrast , the monomeric protein has little effect on the migration of neutrophils in vitro or in vivo .
the influx of inflammatory cells in mice treated with polymers is not mediated by chemokines , but appears to be a direct effect of 1-antitrypsin polymers on neutrophils .
neutrophil chemotaxis caused by systemic polymers may explain the well - recognized association between vascular disease associated with the presence of antibodies against neutrophil cytoplasmic components and the z allele of 1-antitrypsin ( 20 ) .
in this case , polymers may cause the aberrant leukocyte migration that underlies the arteritis and capillaritis that causes organ damage .
for example , the presence of neuroserpin polymers at the neuronal synapse may cause local inflammation , thus contributing to neuronal dysfunction and disruption of synaptic plasticity in fenib .
inflammation caused by the tissue deposition of c1-inhibitor polymers may exacerbate the vascular permeability that characterizes angio - edema .
it is clear that all three mechanisms of serpin - mediated tissue damage may contribute to the pathogenesis of disease , although their contribution is likely to vary between serpins and serpinopathies .
the interaction of the different pathways is perhaps best characterized for the type of emphysema that is associated with 1-antitrypsin deficiency .
individuals who are homozygous for the z mutation typically develop panlobular emphysema in their 40s and 50s ( or 30s in those who smoke ) . for many years
, this type of emphysema was largely attributed to the lack of effective antiproteinase activity and control of inflammation within the lung ( mechanism 2 ) .
however , the recognition that other pathways contribute to tissue damage in the serpinopathies requires a reconsideration of the pathogenesis of disease . in emphysema , noxious stimuli such as cigarette smoke are thought to trigger the initial inflammation within the lung .
a chemotactic gradient of interleukin ( il)-8 and leukotriene - b4 recruits large numbers of macrophages and neutrophils from capillaries into the small airways and alveoli ( fig .
2 ) . to reach the alveoli , the cells must migrate through an interstitial space that contains elastin , proteoglycans , and collagen , and then through the junctions between epithelial cells ( 21 , 22 ) . in patients with emphysema ,
neutrophils are initially concentrated in the centrilobular regions of the lung parenchyma where they release serine and cathepsin proteinases , which degrade elastin and other structural proteins and thus contribute to disease ( 23 ) . degraded elastin fragments themselves act as chemoattractants and
emphysema is further exacerbated by free radicals contained in cigarette smoke ( 10 per puff ) and superoxide released by activated neutrophils , which damage proteins , lipids , and dna ( 26 ) and cause cell death .
( a ) the alveolar wall is composed of structural type i ( green ) and type ii ( blue ) pneumocytes ( reference 22 ) .
these are separated from the capillary endothelium ( yellow ) by interstitial matrix that is maintained by fibroblasts ( red ) and also contains macrophages .
( b ) neutrophils ( light yellow ) migrate through the interstitium ( arrow ) into the lung ( black oval ) in response to inflammatory mediators , such as ltb4 ( blue ) , il-8 ( gold ) , and elastin fragments ( black ) .
interstitial polymers ( inset , repeating units of blue , red , and yellow ) bind to neutrophils and cause them to degranulate , thus amplifying and accelerating tissue destruction in emphysema . as the major inhibitor of neutrophil elastase , 1-antitrypsin is critical for defense against such proteolytic lung damage .
1-antitrypsin enters the lung by passive diffusion from the circulation and is also secreted locally by macrophages and bronchial and alveolar epithelial cells . in individuals homozygous for the z mutation , however , all of the 1-antitrypsin in the lungs has the propensity to form polymers , regardless of its source .
indeed , polymers of 1-antitrypsin are found in lavage ( 18 , 27 ) and tissue sections ( 19 ) from the lungs of these individuals .
deposits of polymers are particularly prominent around capillaries ( consistent with a circulating source of polymers ) and epithelial cells ( consistent with local synthesis ) . the greatest resistance to diffusion is the interstitium , where z 1-antitrypsin is concentrated and where it forms polymers . the chemotactic property of 1-antitrypsin polymers
can then trap neutrophils within the interstitium as they migrate from the vascular space to the alveolar compartment in response to chemokines ( fig .
2 ) . once neutrophils are retained within the interstitium , the polymers cause the cells to adhere , degranulate , and release proteolytic enzymes ( 17 ) , thereby maximizing damage to the extracellular matrix and spreading the focus of inflammation throughout the lobules of the lung .
the destructive effects of neutrophil - derived proteolytic enzymes are amplified in individuals with 1-antitrypsin deficiency , as the small amount of monomeric z 1-antitrypsin that is present is less efficient at inhibiting neutrophil elastase .
the activity of 1-antitrypsin is likely to be further reduced by oxidation of the key 358met residue by superoxide radicals .
in addition to exacerbating proteolysis , the lack of active 1-antitrypsin leads to uncontrolled activation of intracellular caspase-3 and hence alveolar cell apoptosis and emphysema ( 28 ) .
thus , there is evidence that both uncontrolled proteolytic activity due to loss of 1-antitrypsin function ( mechanism 2 ) and tissue damage due to extracellular deposition of z 1-antitrypsin polymers ( mechanism 3 ) contribute to the pathogenesis of emphysema .
although less well investigated , cell death and inflammation caused by intracellular accumulation of z 1-antitrypsin ( mechanism 1 ) is also likely to contribute to tissue damage in the alveolus and bronchial epithelium . as in the liver
, the intracellular production of z 1-antitrypsin may activate nf-b signaling cascades in lung cells ( 8 , 9 ) , which could increase the production of inflammatory mediators and further amplify neutrophil recruitment and tissue damage .
the chronic activation of nf-b is also likely to accelerate apoptosis within all alveolar cells , which would help explain the widespread destruction of the alveoli and the panlobular distribution of this type of emphysema .
although the relative importance of each of the three types of tissue damage remains to be clarified , this integrated model of emphysema associated with 1-antitrypsin deficiency suggests some novel therapeutic strategies .
small molecule inhibitors have been developed that block the polymerization of z 1-antitrypsin in vitro and clear protein aggregates in cell models of disease ( 29 ) .
however , the current generation of small molecules also inactivates 1-antitrypsin as a proteinase inhibitor .
thus , although these drugs may help reduce the risk of liver disease associated with 1-antitrypsin deficiency , they would be predicted to exacerbate emphysema .
it is therefore important to develop refined versions of these molecules that block polymerization of the protein without affecting its inhibitory activity .
such drugs could be administered directly into the lung to prevent polymerization and so ameliorate the inflammatory response .
small molecules that block the binding of polymers to neutrophils would be an alternative approach .
this strategy would block the ability of polymers to activate neutrophils and thus restore the normal migration pathway of neutrophils from the circulation to the alveoli .
strategies that target the downstream effects , rather than the polymer itself , provide a third possibility . given the central role of free radicals in emphysema , antioxidant agents are required to reduce oxidative stress within the lung .
although the current generation of antioxidants is not sufficiently potent , they may be effective if coadministered with agents that block polymer formation and/or the interaction of polymers with neutrophils . finally , inhibition of the inflammatory response by blocking intracellular ( nf-b  dependent ) and extracellular pathways that are activated by polymers might also be a successful approach to ameliorating the lung disease associated with 1-antitrypsin deficiency .
 members of the serpin ( serine proteinase inhibitor ) superfamily play a central role in the control of inflammatory , coagulation , and fibrinolytic cascades . 
 point mutations that cause abnormal conformational transitions in these proteins can trigger disease . 
 recent work has defined three pathways by which these conformers cause tissue damage . here 
 , we describe how these three mechanisms can be integrated into a new model of the pathogenesis of emphysema caused by mutations in the serpin 1-antitrypsin . 
Intracellular accumulation of mutant serpins: mechanism 1
Overactivity of proteolytic cascades: mechanism 2
Tissue damage caused by extracellular deposits: mechanism 3

Novel therapies for 