To explore the role of the key coagulation factor, fibrinogen, in development, hemostasis, wound repair, and disease pathogenesis, we disrupted the fibrinogen As chain gene in mice. Homozygous, As chain-deficient (As-/-) mice are born normal in appearance, and there is no evidence of fetal loss of these animals based on the Mendelian pattern of transmission of the mutant Ac~ chain allele. All of the component chains of fibrinogen (A¢~, B~, and y) are immunologically undetectable in the circulation of both neonatal and adult As -/-mice, and blood samples fail to either clot or support platelet aggregation in vitro. Overt bleeding events develop shortly after birth in --~30% of Ac~-/-mice, most frequently in the peritoneal cavity, skin, and soft tissues around joints. Remarkably, most newborns displaying signs of bleeding ultimately control the loss of blood, clear the affected tissues, and survive the neonatal period. Juveniles and young adult As-/-mice are predisposed to spontaneous fatal abdominal hemorrhage, but long-term survival is variable and highly dependent on genetic background. The periodic rupture of ovarian follicles in breeding-age As -/-females does not appear to significantly diminish life expectancy relative to males; however, pregnancy uniformly results in fatal uterine bleeding around the tenth day of gestation. Microscopic analysis of spontaneous lesions found in As -/-mice suggests that fibrin(ogen) plays a fundamental role in the organization of cells at sites of injury.[Key Words" Fibrinogen-deficient mice; coagulation; hemostasis; afibrinogenemia; platelet aggregation; wound healing; development] Received May 15, 1995; revised version accepted June 28, 1995.Fibrin(ogen) is the ultimate target of two sophisticated and opposing regulatory systems, the coagulation and fibrinolytic cascades, that together preserve vascular integrity and maintain hemostatic balance (Davie et al. 1991;Esmon 1993;Collen and Lijnen 1994). The coagulation system includes more than a dozen soluble and cell-associated factors that initiate, promote, and ultimately limit the formation of insoluble fibrin polymer (Davie et al. 1991}. A key step in coagulation is the generation of the serine protease, thrombin, which triggers platelet activation (Majerus 1994), converts fibrinogen into a spontaneously polymerizing fibrin monomer (Doolittle 1994), activates the transglutaminase (factor XIII) that covalently cross-links fibrin matrices (Chung and Ichinose 1995), and activates regulatory pathways that both promote and suppress coagulation (Davie et al.

The alpha2CDel322-325 and beta1Arg389 receptors act synergistically to increase the risk of heart failure in blacks. Genotyping at these two loci may be a useful approach for identification of persons at risk for heart failure or its progression, who may be candidates for early preventive measures.

Adrenergic receptors are expressed on virtually every cell type in the body and are the receptors for epinephrine and norepinephrine within the sympathetic nervous system. They serve critical roles in maintaining homeostasis in normal physiologic settings as well as pathologic states. These receptors are also targets for therapeutically administered agonists and antagonists. Recent studies have shown that at least seven adrenergic receptor subtypes display variation in amino acid sequence in the human population due to common genetic polymorphisms. Variations in potential regulatory domains in noncoding sequence are also present. Here, we review the consequences of these polymorphisms in terms of signaling, human physiology and disease, and response to therapy.

Hox All is one of the expanded set of vertebrate homeo box (Hox) genes with similarities to the Drosophila homeotic gene, Abdominal-B (Abd-B). These Abd-B-type Hox genes have been shown to be expressed in the most caudal regions of the developing vertebrate embryo and in overlapping domains within the developing limbs, suggesting that these genes play important roles in pattern formation in both appendicular and axial regions of the body. In this report whole-mount in situ hybridization in mouse embryos gave a precise description of Hox All gene expression in the developing limbs and in the axial domain of the developing body. In addition, we generated a targeted mutation in Hox All and characterized the resulting phenotype to begin to dissect developmental functions of the Abd-B subfamily of Hox genes. Hox All mutant mice exhibited double homeotic transformations, with the thirteenth thoracic segment posteriorized to form an additional first lumbar vertebra and with the sacral region anteriorized, generating yet another lumbar segment. Furthermore, skeletal malformations were observed in both forelimbs and hindlimbs. In mutant forelimbs, the ulna and radius were misshapen, the pisiform and triangular carpal bones were fused, and abnormal sesamoid bone development occurred. In mutant hindlimbs the tibia and fibula were joined incorrectly and malformed at their distal ends. Also, an enlarged sesamoid developed ventral to the tibiale bone. Both heterozygous and homozygous mice displayed mutant phenotypes adding an additional level of complexity to the Hox code hypothesis.

The ␣ 2 -adrenergic receptors (␣ 2 ARs) play a critical role in modulating neurotransmitter release in the central and peripheral sympathetic nervous systems. A polymorphism of the ␣ 2 AR subtype localized to human chromosome 4 (the pharmacologic ␣ 2C AR subtype) within an intracellular domain has been identified in normal individuals. The polymorphism (denoted Del322-325) is because of an in-frame 12-nucleic acid deletion encoding a receptor lacking Gly-Ala-Gly-Pro in the third intracellular loop. To delineate the functional consequences of this structural alteration, Chinese hamster ovary cells were permanently transfected with constructs encoding wild-type human ␣ 2C AR and the polymorphic receptor. The Del322-325 variant had decreased high affinity agonist binding (K H ‫؍‬ 7.3 ؎ 0.95 versus 3.7 ؎ 0.43 nM; %R H ‫؍‬ 31 ؎ 4 versus 49 ؎ 4) compared with wild-type indicating impaired formation of the agonist-receptor-G protein complex. The polymorphic receptor displayed markedly depressed epinephrine-promoted coupling to G i , inhibiting adenylyl cyclase by 10 ؎ 4.3% compared with 73 ؎ 2.4% for wild-type ␣ 2C AR. This also was so for the endogenous ligand norepinephrine and full and partial synthetic agonists. Depressed agonist-promoted coupling to the stimulation of MAP kinase (ϳ71% impaired) and inositol phosphate production (ϳ60% impaired) was also found with the polymorphic receptor. The Del322-325 receptor was ϳ10 times more frequent in African-Americans compared with Caucasians (allele frequencies 0.381 versus 0.040). Given this significant loss of function phenotype in several signal transduction cascades and the skewed ethnic prevalence, Del322-325 represents a pharmacoethnogenetic locus and may also be the basis for interindividual variation in cardiovascular or central nervous system pathophysiology.The ␣ 2 -adrenergic receptors (␣ 2 AR) 1 are cell surface receptors for catecholamines, which couple to the G i /G o family of G proteins. ␣ 2 AR are expressed at multiple sites within the central and peripheral sympathetic nervous systems and may also be expressed at noninnervated sites of peripheral tissues as well. In the central nervous system presynaptic ␣ 2 AR act to inhibit the release of neurotransmitters such as norepinephrine, serotonin, and dopamine. As such, a number of responses have been ascribed to activation of these receptors by endogenous catecholamines or exogenously administered agonists. These include modulation of blood pressure, sedation, analgesia, opiate withdrawal, and multiple complex cognitive and behavioral parameters (1-5).Three human ␣ 2 AR subtypes have been cloned and characterized and are denoted as the ␣ 2A , ␣ 2B , and ␣ 2C subtypes (6 -8). Based on chromosomal localization, these have previously been denoted as ␣ 2 C10, ␣ 2 C2, and ␣ 2 C4, respectively. Recent studies including those with genetically engineered mice have shown that the ␣ 2C subtype plays specific roles in modulation of the acoustic startle reflex, prepulse inhibition, isolation-induced aggression, spatial...

A polymorphic variant of the human ␣ 2B -adrenergic receptor (␣ 2B AR), which consists of a deletion of three glutamic acids (residues 301-303) in the third intracellular loop was found to be common in Caucasians (31%) and to a lesser extent in African-Americans (12%). The consequences of this deletion were assessed by expressing wild-type and the Del301-303 receptors in Chinese hamster ovary and COS cells. Ligand binding was not affected, although a small decrease in coupling efficiency to the inhibition of adenylyl cyclase was observed with the mutant. The deletion occurs within a stretch of acidic residues that is thought to establish the milieu for agonist-promoted phosphorylation and desensitization of the receptor by G protein-coupled receptor kinases (GRKs). Agonist-promoted phosphorylation studies carried out in cells coexpressing the ␣ 2B ARs and GRK2 revealed that the Del301-303 receptor displayed ϳ56% of wild-type phosphorylation. Furthermore, the depressed phosphorylation imposed by the deletion was found to result in a complete loss of short term agonistpromoted receptor desensitization. Thus the major phenotype of the Del301-303 ␣ 2B AR is one of impaired phosphorylation and desensitization by GRKs, and thus the polymorphisms renders the receptor incapable of modulation by this key mechanism of dynamic regulation.␣ 2 -Adrenergic receptors (␣ 2 AR) 1 are cell surface receptors for catecholamines that bind to the G i /G o family of G proteins, coupling to multiple effector systems including inhibition of adenylyl cyclase activity (1). ␣ 2 AR are widely expressed within the central and peripheral nervous system (2-4) and participate in a broad spectrum of physiologic functions such as regulation of blood pressure both centrally and within the vasculature, sedation, analgesia, regulation of insulin release, renal function, and cognitive and behavioral functions (5-12). Three human ␣ 2 AR subtypes have been cloned and characterized (␣ 2A , ␣ 2B , and ␣ 2C ). The ␣ 2B AR has a distinct pattern of expression within the brain, liver, lung, and kidney, and recent studies using gene knockouts in mice have shown that disruption of this receptor effects mouse viability (13), blood pressure responses to ␣ 2 AR agonists (13), and the hypertensive response to salt loading (14).Like the ␣ 2A AR subtype (15, 16), the ␣ 2B AR undergoes short term agonist promoted desensitization (17). This desensitization is due to phosphorylation of the receptor, which evokes a partial uncoupling of the receptor from functional interaction with G i /G o (18,19). Such phosphorylation appears to be due to G protein-coupled receptor kinases (GRKs), a family of serine/ threonine kinases that phosphorylate the agonist-occupied conformations of many G protein-coupled receptors (20). The process serves to finely regulate receptor function providing for rapid adaptation of the cell to its environment. Desensitization may also limit the therapeutic effectiveness of administered agonists. For the ␣ 2B AR, phosphorylation of serines/threonine...

Fragile X syndrome is caused by the transcriptional silencing of the FMR1 gene due to a trinucleotide repeat expansion. The encoded protein, Fmrp, has been found to be a nucleocytoplasmic RNA-binding protein containing both KH domains and RGG boxes that associates with polyribosomes as a ribonucleoprotein particle. RNA binding has previously been demonstrated with in vitro-translated Fmrp; however, it remained uncertain whether the selective RNA binding observed was an intrinsic property of Fmrp or required an associated protein(s). Here, baculovirus-expressed and affinity-purified FLAG-tagged murine Fmrp was shown to bind directly to both ribonucleotide homopolymers and human brain mRNA. FLAG-Fmrp exhibited selectivity for binding poly(G) > poly(U) > > poly(C) or poly(A). Moreover, purified FLAG-Fmrp bound to only a subset of brain mRNA, including the 3 untranslated regions of myelin basic protein message and its own message. Recombinant isoform 4, lacking the RGG boxes but maintaining both KH domains, was also purified and was found to only weakly interact with RNA. FLAG-purified I304N Fmrp, harboring the mutation of severe fragile X syndrome, demonstrated RNA binding, in contrast to previous suggestions. These data demonstrate the intrinsic property of Fmrp to selectively bind RNA and show FLAG-Fmrp as a suitable reagent for structural characterization and identification of cognate RNA ligands.Fragile X syndrome, a common mental retardation syndrome, results from an unstable CGG repeat expansion in the 5Ј-untranslated region of the FMR1 gene, leading to transcriptional silencing and the absence of Fmrp protein (1-4). Characterization of in vitro-translated Fmrp has shown it to be an RNA-associated protein with selectivity for homopolymer RNA and some human brain transcripts (5, 6). Fmrp contains nuclear localization and export sequences (7), as well as RGG box and KH domain motifs found in many RNA-binding proteins. The RGG box has been found in hnRNP 1 U, hnRNP A1, nucleolin, and fibrillarin and has been shown to autonomously bind homopolymeric RNA (8). However, the amino acid context surrounding the RGG box may influence the specificity and avidity of the RNA-nucleic acid interaction (8, 9). The KH domain is highly conserved in hnRNP K, yeast MER-1 splicing regulator, Sam68, and chicken vigillin, among other RNA-binding proteins (10).Fmrp expression is widespread but not ubiquitous. The majority of Fmrp is associated with polyribosomes and has been found to form an mRNP, which can be stripped from ribosomes by EDTA or RNase treatment (11)(12)(13)(14). A prevailing model suggests that Fmrp directly binds specific mRNAs in the nucleus as part of a hnRNP particle and then mediates its transport to the cytoplasm and delivery of the mRNP to the ribosome (4, 7).Homopolymer binding assays performed with in vitro-translated Fmrp and carboxyl-terminal truncated Fmrp proteins suggested a role for both the KH domain and RGG box in RNA binding (15). However, whether the observed RNA binding by Fmrp is mediated directly...

Emery-Dreifuss muscular dystrophy (EMD) is an X-linked disorder characterized by contractures, progressive muscle weakness and cardiomyopathy. The emerin gene, located in human Xq28, is approximately 2 kb in length, is composed of 6 exons and falls within a 219-kb region that has been completely sequenced. Immediately centromeric to emerin is the 26-kb filamin gene (FLN1), composed of 48 exons and encoding the actin-binding protein 280 (refs 7,8). Flanking this 48-kb FLN1/emerin region are two large inverted repeats, each 11.3 kb, that exhibit > 99% sequence identity. The high level of genomic detail in this region allowed us to characterize the first complete emerin gene deletion mutation that also involved a partial duplication of the nearby FLN1 gene. This rearrangement could be explained by mispairing of the large inverted repeats, followed by double recombination among one set of mispaired repeats and internal sequences. Furthermore, our characterization of this rare DNA rearrangement revealed a more common result of the mispairing of these large inverted repeats--recombination contained within the inverted repeats leading to the maintenance of repeat sequence homogeneity and inversion of the 48-kb FLN1/emerin region. The presence of this frequent inversion, found in the heterozygous state in 33% of females, helps to explain the discrepancies observed between the genetic and physical map distances in this region of the X chromosome. It also illustrates the biological insights which can be gleaned by sequencing the human genome.