Thrombotic microangiopathies: multimers, metalloprotease, and beyond

Abstract

The pathophysiology of various types of thrombotic microangiopathies is coming progressively into focus. Therapeutic advances are likely to follow at a quickening pace. This discussion focuses on thrombotic thrombocytopenic purpura (TTP), the hemolytic-uremic syndrome (HUS), thrombotic microangiopathies associated with transplantation-immunosuppression or anti-angiogenesis therapy, and the preeclampsia/hemolysis-elevated liver enzymes and low platelets syndrome (HELLP).

Figure 1

Platelet adhesion and aggregation onto VWF multimeric strings. Stimulation of endothelial cells by a variety of agonists, including inflammatory cytokines and toxins, causes the cells to secrete long, hyperadhesive VWF strings (A, B). Circulating platelets instantly adhere to the cell‐anchored VWF strings (C, D), and then the platelets cohere to each other (aggregate) (E).

Figure 1

Platelet adhesion and aggregation onto VWF multimeric strings. Stimulation of endothelial cells by a variety of agonists, including inflammatory cytokines and toxins, causes the cells to secrete long, hyperadhesive VWF strings (A, B). Circulating platelets instantly adhere to the cell‐anchored VWF strings (C, D), and then the platelets cohere to each other (aggregate) (E).

Figure 1

Platelet adhesion and aggregation onto VWF multimeric strings. Stimulation of endothelial cells by a variety of agonists, including inflammatory cytokines and toxins, causes the cells to secrete long, hyperadhesive VWF strings (A, B). Circulating platelets instantly adhere to the cell‐anchored VWF strings (C, D), and then the platelets cohere to each other (aggregate) (E).

Figure 1

Platelet adhesion and aggregation onto VWF multimeric strings. Stimulation of endothelial cells by a variety of agonists, including inflammatory cytokines and toxins, causes the cells to secrete long, hyperadhesive VWF strings (A, B). Circulating platelets instantly adhere to the cell‐anchored VWF strings (C, D), and then the platelets cohere to each other (aggregate) (E).

Figure 1

Platelet adhesion and aggregation onto VWF multimeric strings. Stimulation of endothelial cells by a variety of agonists, including inflammatory cytokines and toxins, causes the cells to secrete long, hyperadhesive VWF strings (A, B). Circulating platelets instantly adhere to the cell‐anchored VWF strings (C, D), and then the platelets cohere to each other (aggregate) (E).

Figure 2

Domain structure of ADAMTS‐13, the VWF‐cleaving metalloprotease. MP = metalloprotease (proteolytic) domain; TSP = thrombospondin‐1‐like domain (a total of eight); CUB = two similar, non‐identical domains (CUB‐1 and CUB‐2) containing peptide sequences found in complement components C1r/C1s, a sea urchin protein, and a bone morphogenic protein. ADAMTS‐13 has a molecular mass of about 190 kD, and is produced in endothelial and hepatic stellate cells. The gene for ADAMTS‐13 is on chromosome #9.

Figure 3

(A) Normal protection from microvascular thrombosis: In normal individuals, ADAMTS‐13 enzyme molecules from the plasma attach to, and then rapidly cleave, long “sticky” VWF multimeric string‐like structures secreted from stimulated endothelial cells. Cleavage occurs at an exposed 1605–6 peptide bond in VWF monomeric subunits. (B) TTP = ADAMTS‐13‐deficient types (familial or autoantibody‐induced). Absent or severely reduced activity of ADAMTS‐13 prevents the timely cleavage of the long VWF multimeric strings secreted by stimulated endothelial cells. Platelets adhere and aggregate onto the uncleaved long VWF strings. Severe familial deficiency of ADAMTS‐13 activity caused by gene mutations, or profound inhibition of ADAMTS‐13 activity caused by acquired autoantibodies, result in a propensity for TTP. Episodes are especially likely to occur in the absence (or severe reduction) of ADAMTS‐13 if there is increased concurrent endothelial cell secretion of long VWF strings (e.g., in the presence of inflammatory cytokines, estrogen, or certain bacterial toxins). CUB represents the two nonidentical C‐terminal CUB domains of ADAMTS‐13; MP represents the N‐terminal metalloproteasse domain.

Figure 3

(A) Normal protection from microvascular thrombosis: In normal individuals, ADAMTS‐13 enzyme molecules from the plasma attach to, and then rapidly cleave, long “sticky” VWF multimeric string‐like structures secreted from stimulated endothelial cells. Cleavage occurs at an exposed 1605–6 peptide bond in VWF monomeric subunits. (B) TTP = ADAMTS‐13‐deficient types (familial or autoantibody‐induced). Absent or severely reduced activity of ADAMTS‐13 prevents the timely cleavage of the long VWF multimeric strings secreted by stimulated endothelial cells. Platelets adhere and aggregate onto the uncleaved long VWF strings. Severe familial deficiency of ADAMTS‐13 activity caused by gene mutations, or profound inhibition of ADAMTS‐13 activity caused by acquired autoantibodies, result in a propensity for TTP. Episodes are especially likely to occur in the absence (or severe reduction) of ADAMTS‐13 if there is increased concurrent endothelial cell secretion of long VWF strings (e.g., in the presence of inflammatory cytokines, estrogen, or certain bacterial toxins). CUB represents the two nonidentical C‐terminal CUB domains of ADAMTS‐13; MP represents the N‐terminal metalloproteasse domain.

Figure 4

(A) A “close‐up” of one of the many monomeric subunits that comprise a long VWF multimeric string. The VWF strings are secreted from endothelial cell Weibel–Palade bodies, and remain anchored in the endothelial cell membrane. The A1, A2 (containing the tyr1605–1606met ADAMTS‐13 proteolytic cleavage site), and A3 domains are shown. (B) Adequate quantities of ADAMTS‐13 enzymes are present in the plasma of normal individuals. The two carboxy‐terminal CUB domains are indicated, and the metalloprotease domain (MP) is drawn as a pincer‐like stricture on the amino‐terminal portion of the enzyme. Platelets from flowing blood adhere to the long VWF strings immediately after string secretion. Platelet adherence is via platelet GPIbα receptors to the A1 domain of VWF monomeric subunits. ADAMTS‐13 attaches via its spacer and CUB‐1 domain to the A3 domain of VWF monomeric subunits, and cleaves the adjacent tyr1605–1606met bond. ADAMTS‐13 cleavage occurs in various monomers along the length of VWF multimeric strings that have had their A2 domains unfolded. Unfolding occurs as a result of the propulsive force of VWF secretion from Weibel–Palade bodies, and is augmented by the shear stresses associated with fluid flow. (C) Both ATA and a pegylated oligonucleotide, ARC1779, block the interaction of the A1 domain of VWF monomers and GPIbα on platelets. ARC1779 is under development commercially for therapeutic use in TTP and arterial thrombosis.

Figure 4

(A) A “close‐up” of one of the many monomeric subunits that comprise a long VWF multimeric string. The VWF strings are secreted from endothelial cell Weibel–Palade bodies, and remain anchored in the endothelial cell membrane. The A1, A2 (containing the tyr1605–1606met ADAMTS‐13 proteolytic cleavage site), and A3 domains are shown. (B) Adequate quantities of ADAMTS‐13 enzymes are present in the plasma of normal individuals. The two carboxy‐terminal CUB domains are indicated, and the metalloprotease domain (MP) is drawn as a pincer‐like stricture on the amino‐terminal portion of the enzyme. Platelets from flowing blood adhere to the long VWF strings immediately after string secretion. Platelet adherence is via platelet GPIbα receptors to the A1 domain of VWF monomeric subunits. ADAMTS‐13 attaches via its spacer and CUB‐1 domain to the A3 domain of VWF monomeric subunits, and cleaves the adjacent tyr1605–1606met bond. ADAMTS‐13 cleavage occurs in various monomers along the length of VWF multimeric strings that have had their A2 domains unfolded. Unfolding occurs as a result of the propulsive force of VWF secretion from Weibel–Palade bodies, and is augmented by the shear stresses associated with fluid flow. (C) Both ATA and a pegylated oligonucleotide, ARC1779, block the interaction of the A1 domain of VWF monomers and GPIbα on platelets. ARC1779 is under development commercially for therapeutic use in TTP and arterial thrombosis.

Figure 4

(A) A “close‐up” of one of the many monomeric subunits that comprise a long VWF multimeric string. The VWF strings are secreted from endothelial cell Weibel–Palade bodies, and remain anchored in the endothelial cell membrane. The A1, A2 (containing the tyr1605–1606met ADAMTS‐13 proteolytic cleavage site), and A3 domains are shown. (B) Adequate quantities of ADAMTS‐13 enzymes are present in the plasma of normal individuals. The two carboxy‐terminal CUB domains are indicated, and the metalloprotease domain (MP) is drawn as a pincer‐like stricture on the amino‐terminal portion of the enzyme. Platelets from flowing blood adhere to the long VWF strings immediately after string secretion. Platelet adherence is via platelet GPIbα receptors to the A1 domain of VWF monomeric subunits. ADAMTS‐13 attaches via its spacer and CUB‐1 domain to the A3 domain of VWF monomeric subunits, and cleaves the adjacent tyr1605–1606met bond. ADAMTS‐13 cleavage occurs in various monomers along the length of VWF multimeric strings that have had their A2 domains unfolded. Unfolding occurs as a result of the propulsive force of VWF secretion from Weibel–Palade bodies, and is augmented by the shear stresses associated with fluid flow. (C) Both ATA and a pegylated oligonucleotide, ARC1779, block the interaction of the A1 domain of VWF monomers and GPIbα on platelets. ARC1779 is under development commercially for therapeutic use in TTP and arterial thrombosis.

Figure 5

Proposed pathophysiology of the diarrhea‐associated hemolytic‐uremic syndrome (HUS) that is most common in North America. (A) Enterohemorrhagic E. coli injure and efface colonic mucosa and induce bloody diarrhea, and produce Shiga toxins‐1 and ‐2. These toxins are transported by white cells and platelets through the bloodstream to globotriaosylceramide (Gb₃) receptors that are present in high concentrations on glomerular endothelial and other renal cells. (B) In response to the Shiga toxins, and elevated levels of circulating inflammatory cytokines (TNF‐α, IL‐8, IL‐6), glomerular endothelial cells profusely secrete long “sticky” VWF strings. The Shiga toxins, in conjunction with one of the inflammatory cytokines (IL‐6), also slow the rate of ADAMTS‐13‐mediated cleavage of cell‐bound VWF strings. The result is excessive platelet adhesion/aggregation on VWF strings atop glomerular microvascular endothelial cells over the course of about 1 week, progressive loss of nephron function, and increasing renal failure.

Figure 5

Proposed pathophysiology of the diarrhea‐associated hemolytic‐uremic syndrome (HUS) that is most common in North America. (A) Enterohemorrhagic E. coli injure and efface colonic mucosa and induce bloody diarrhea, and produce Shiga toxins‐1 and ‐2. These toxins are transported by white cells and platelets through the bloodstream to globotriaosylceramide (Gb₃) receptors that are present in high concentrations on glomerular endothelial and other renal cells. (B) In response to the Shiga toxins, and elevated levels of circulating inflammatory cytokines (TNF‐α, IL‐8, IL‐6), glomerular endothelial cells profusely secrete long “sticky” VWF strings. The Shiga toxins, in conjunction with one of the inflammatory cytokines (IL‐6), also slow the rate of ADAMTS‐13‐mediated cleavage of cell‐bound VWF strings. The result is excessive platelet adhesion/aggregation on VWF strings atop glomerular microvascular endothelial cells over the course of about 1 week, progressive loss of nephron function, and increasing renal failure.

Figure 6

The alternative complement pathway and atypical (recurrent) HUS. C3b = activated complement factor C3. B = complement factor B, which is activated to Bb by complement factor D. (C3b)₂Bb = the alternative complement pathway active C3 convertase that is stabilized by properdin (P). H = plasma factor H, which displaces Bb from C3b and allows its inactivation to iC3b by plasma complement factor I (C3b‐cleaving protease). MCP = membrane cofactor protein, which functions on cell membranes as factor H functions (predominantly) in plasma. Heterozygous mutations in factor H, less commonly MCP, or least commonly factor I cause increased susceptibility to a familial, recurrent type of HUS. Rare gain‐of‐function mutations in C3 or factor B may also cause the syndrome. The mutations known to be associated with atypical, recurrent HUS are shown in red. The rare production of autoantibodies to factor H also causes aytpical HUS.