Alternatively spliced isoforms of tissue factor pathway inhibitor

Abstract

Tissue factor pathway inhibitor (TFPI) is the major regulator of tissue factor (TF)-induced coagulation. It down regulates coagulation by binding to the TF/fVIIa complex in a fXa dependent manner. It is predominantly produced by microvascular endothelial cells, though it is also found in platelets, monocytes, smooth muscle cells, and plasma. Its physiological importance is demonstrated by the embryonic lethality observed in TFPI knockout mice and by the increase in thrombotic burden that occurs when heterozygous TFPI mice are bred with mice carrying genetic risk factors for thrombotic disease, such as factor V Leiden. Multiple TFPI isoforms, termed TFPIalpha, TFPIbeta, and TFPIdelta in humans and TFPIalpha, TFPIbeta, and TFPIgamma in mice, have been described, which differ in their domain structure and method for cell surface attachment. A significant functional difference between these isoforms has yet to be described in vivo. Both human and mouse tissues produce, on average, approximately 10 times more TFPIalpha message when compared to that of TFPIbeta. Consistent with this finding, several lines of evidence suggest that TFPIalpha is the predominant protein isoform in humans. In contrast, recent work from our laboratory demonstrates that TFPIbeta is the major protein isoform produced in adult mice, suggesting that TFPI isoform production is translationally regulated.

Figure 1

(A) Box diagram of the alternatively spliced forms of human TFPI. The amino acid sequence for the C-terminal region of each isoform is indicated. Only the first 12 amino acids of the C-terminal region of TFPIβ are predicted to be in the mature protein. The remainder of the C-terminal region encodes a GPI-anchor attachment sequence and is removed in the endoplasmic reticulum. (B) Diagram of the amino acid sequences of TFPIα, TFPIβ and TFPIδ showing the Kunitz domain structures and common 5′ splice donor site for TFPIα and TFPIβ, while the TFPIδ 5′ splice site is immediately following K2.

Figure 2

(A) Box diagram of the alternatively spliced forms of mouse TFPI. The amino acid sequence for the C-terminal region of each isoform is indicated. Only the first 8 amino acids of the C-terminal region of TFPIβ are predicted to be in the mature protein. The remainder of the C-terminal region encodes a GPI-anchor attachment sequence and is removed in the endoplasmic reticulum. The coding sequence for the C-terminal region of TFPIγ is located within the 3′UTR of TFPIβ. (B) Diagram of the amino acid sequences of TFPIα, TFPIβ and TFPIγ showing the Kunitz domain structures and common 5′ splice donor site for all three isoforms.