Expression of the plasma prekallikrein gene: utilization of multiple transcription start sites and alternative promoter regions

Human plasma kallikrein: roles in coagulation, fibrinolysis, inflammation pathways, and beyond

Human plasma kallikrein (PKa) is obtained by activating its precursor, prekallikrein (PK), historically named the Fletcher factor. Human PKa and tissue kallikreins are serine proteases from the same family, having high- and low-molecular weight kininogens (HKs and LKs) as substrates, releasing bradykinin (Bk) and Lys-bradykinin (Lys-Bk), respectively. This review presents a brief history of human PKa with details and recent observations of its evolution among the vertebrate coagulation proteins, including the relations with Factor XI. We explored the role of Factor XII in activating the plasma kallikrein-kinin system (KKS), the mechanism of activity and control in the KKS, and the function of HK on contact activation proteins on cell membranes. The role of human PKa in cell biology regarding the contact system and KSS, particularly the endothelial cells, and neutrophils, in inflammatory processes and infectious diseases, was also approached. We examined the natural plasma protein inhibitors, including a detailed survey of human PKa inhibitors' development and their potential market.

Plasma kallikrein: the bradykinin-producing enzyme

Plasma prekallikrein is the liver-derived precursor of the trypsin-like serine protease plasma kallikrein (PK) and circulates in plasma bound to high molecular weight kininogen. The zymogen is converted to PK by activated factor XII. PK drives multiple proteolytic reaction cascades in the cardiovascular system such as the intrinsic pathway of coagulation, the kallikrein-kinin system, the fibrinolytic system, the renin-angiotensin system and the alternative complement pathway. Here, we review the biochemistry and cell biology of PK and focus on recent in vivo studies that have established important functions of the protease in procoagulant and proinflammatory disease states. Targeting PK offers novel strategies not previously appreciated to interfere with thrombosis and vascular inflammation in a broad variety of diseases.

Identification of a cis-acting element which assists in the recruitment of alternative transcription start sites/promoters

The transcription start sites (TSSs) of the human plasma prekallikrein gene (KLKB1) determined in RNA from kidney are mostly localized within intron 1 and exon 2, suggesting that the region encompassing exon 1, intron 1, and exon 2 comprises an alternative promoter. Reporter gene analyses in HepG2 and immortalized human kidney epithelial cells confirmed a significant transcriptional activity of this putative promoter region. However, when the TSSs recruited for transcription of the reporter gene were determined, only a few transcripts starting within the insert were detected, whereas the majority of TSSs were located in the vector backbone up to about 2000 bp upstream of the insert. Further reporter gene studies with deletion mutants of the fragment exon 1-intron 1-exon 2 revealed that the 3'-terminal 13-bp segment of intron 1 is sufficient to promote transcriptional activity and induce upstream displacement of the TSS. We conclude that the 13-bp segment represents a cis-acting element which can displace TSSs by provoking the recruitment of alternative promoters and/or by masking intergenic transcription terminating signals.

Active plasma kallikrein localizes to mast cells and regulates epithelial cell apoptosis, adipocyte differentiation, and stromal remodeling during mammary gland involution

The plasminogen cascade of serine proteases directs both development and tumorigenesis in the mammary gland. Plasminogen can be activated to plasmin by urokinase-type plasminogen activator (uPA), tissue-type plasminogen activator (tPA), and plasma kallikrein (PKal). The dominant plasminogen activator for mammary involution is PKal, a serine protease that participates in the contact activation system of blood coagulation. We observed that the prekallikrein gene (Klkb1) is expressed highly in the mammary gland during stromal remodeling periods including puberty and postlactational involution. We used a variant of ecotin (ecotin-PKal), a macromolecular inhibitor of serine proteases engineered to be highly specific for active PKal, to demonstrate that inhibition of PKal with ecotin-PKal delays alveolar apoptosis, adipocyte replenishment, and stromal remodeling in the involuting mammary gland, producing a phenotype resembling that resulting from plasminogen deficiency. Using biotinylated ecotin-PKal, we localized active PKal to connective tissue-type mast cells in the mammary gland. Taken together, these results implicate PKal as an effector of the plasminogen cascade during mammary development.

T-SP1: a novel serine protease-like protein predominantly expressed in testis

Here, we describe a novel member in the group of membrane-anchored chymotrypsin (S1)-like serine proteases, namely testis serine protease 1 (T-SP1), as it is principally expressed in testis tissue. The human T-SP1 gene encompasses 28.7 kb on the short arm of chromosome 8 and consists of seven exons. Rapid amplification of cDNA ends (RACE) experiments revealed that due to alternative splicing three different variants (T-SP1/1, -2, -3) are detectable in testis tissue displaying pronounced heterogeneity at their 3'-end. T-SP1/1 consists of an 18 amino acid signal peptide and of a 49 amino acid propeptide. The following domain with the catalytic triad of His(108), Asp(156), and Ser(250) shares sequence identities of 42% and 40% with the blood coagulation factor XI and plasma kallikrein, respectively. Only T-SP1/1 contains a hydrophobic part at the C-terminus, which provides the basis for cell membrane anchoring. Using a newly generated polyclonal anti-T-SP1 antibody, expression of the T-SP1 protein was found in the Leydig and Sertoli cells of the testis and in the epithelial cells of the ductuli efferentes. Notably, T-SP1 protein was also detectable in prostate cancer and in some ovarian cancer tissues, indicating tumor-related synthesis of T-SP1 beyond testis tissue.

Full length cloning and expression analysis of splice variants of regulator of G-protein signaling RGS4 in human and murine brain

RGS4 (regulator of G protein signaling 4) protein is a GTPase-activating protein specific for Gi/o and Gq alpha subunits. It is highly expressed in brain but the mechanisms by which RGS4 expression is regulated remain unknown. RGS4 is associated with schizophrenia either through heritable genetic polymorphisms or as a co-regulated mediator of the pathology, and may play a role in other brain diseases. As a necessary step towards understanding the transcriptional regulation of RGS4, we isolated full-length splice variants of the human RGS4 and mouse Rgs4 gene using bioinformatic predictions, followed by RACE, RT-PCR, and sequencing. In human brain, we found five different isoforms RGS4-1, RGS4-2, RGS4-3, RGS4-4 and RGS4-5 of which RGS4-2, RGS4-3, RGS4-4 and RGS4-5 are novel. RGS4-1 and 2 encode a 205-amino acid protein, while RGS4-3 encodes a 302 aa protein with an N-terminal extension. RGS4-4 and RGS4-5 encode truncated proteins of 93 aa and 187 aa respectively. Our results indicate that RGS4-1, RGS4-2, RGS4-3 and RGS4-4 are translated into proteins. In contrast, the mouse brain has 3 different splice variants, Rgs4-1, Rgs4-2 and Rgs4-3 which encode the same 205 aa protein but vary in their 3'UTRs. Among the mouse isoforms, Rgs4-1 and Rgs4-3 are novel. Human RGS4 has four different transcription start sites and three different stop sites. We found differential expression of the human isoforms in dorsolateral prefrontal and visual cortex. All five RGS4 splice variants are expressed at high levels in human cortical areas although RGS4 isoforms 1, 2, and 3 are not expressed in the cerebellum. RGS4-2 is tissue-specific whereas RGS4-4 and RGS4-5 appear to be ubiquitously expressed. Our results suggest the intriguing possibility that RGS4 gene expression in the human brain is spatially and temporally regulated through differential transcription of isoforms from alternative promoters. This may have implications for the physiological role of RGS4 and in pathologies of the brain.