Expression of beta-glucuronidase haplotypes in prototype and congenic mouse strains

Bone-Specific Metabolism of Dietary Polyphenols in Resorptive Bone Diseases

SCOPE

Curcumin prevents bone loss in resorptive bone diseases and inhibits osteoclast formation, a key process driving bone loss. Curcumin circulates as an inactive glucuronide that can be deconjugated in situ by bone's high β-glucuronidase (GUSB) content, forming the active aglycone. Because curcumin is a common remedy for musculoskeletal disease, effects of microenvironmental changes consequent to skeletal development or disease on bone curcumin metabolism are explored.

METHODS AND RESULTS

Across sexual/skeletal development or between sexes in C57BL/6 mice ingesting curcumin (500 mg kg-1 ), bone curcumin metabolism and GUSB enzyme activity are unchanged, except for >twofold higher (p < 0.05) bone curcumin-glucuronide substrate levels in immature (4-6-week-old) mice. In ovariectomized (OVX) or bone metastasis-bearing female mice, bone substrate levels are also >twofold higher. Aglycone curcumin levels tend to increase proportional to substrate such that the majority of glucuronide distributing to bone is deconjugated, including OVX mice where GUSB decreases by 24% (p < 0.01). GUSB also catalyzes deconjugation of resveratrol and quercetin glucuronides by bone, and a requirement for the aglycones for anti-osteoclastogenic bioactivity, analogous to curcumin, is confirmed.

CONCLUSION

Dietary polyphenols circulating as glucuronides may require in situ deconjugation for bone-protective effects, a process influenced by bone microenvironmental changes.

Beta-Glucuronidase Catalyzes Deconjugation and Activation of Curcumin-Glucuronide in Bone

The biological basis for documented in vivo bone-protective effects of turmeric-derived curcumin is unclear since curcumin is barely detectable in serum, being rapidly conjugated to form what is thought to be an inactive glucuronide. Studies were therefore undertaken to test the postulate that antiresorptive effects of curcumin require deconjugation within bone to form the bioactive aglycone and that β-glucuronidase (GUSB), a deconjugating enzyme expressed by hematopoietic marrow cells, facilitates this site-specific transformation. Consistent with this postulate, aglycone, but not glucuronidated, curcumin inhibited RANKL-stimulated osteoclastogenesis, a key curcumin target in bone. Aglycone curcumin, expressed relative to total curcumin, was higher in bone marrow than in serum of curcumin-treated C57BL/6J mice, while remaining a minor component. Ex vivo, under conditions preventing further metabolism of the unstable aglycone, the majority of curcumin-glucuronide delivered to marrow in vivo was hydrolyzed to the aglycone, a process that was inhibited by treatment with saccharolactone, a GUSB inhibitor, or in mice having reduced (C3H/HeJ) or absent (mps/mps) GUSB activity. These findings suggest that curcumin, despite low systemic bioavailability, may be enzymatically activated (deconjugated) within GUSB-enriched bone to exert protective effects, a metabolic process that could also contribute to bone-protective effects of other highly glucuronidated dietary polyphenols.

Stable Levels of Long-Term Transgene Expression Driven by the Latency-Associated Transcript Promoter in a Herpes Simplex Virus Type 1 Vector

Previous gene transfer studies of the herpes simplex virus type 1 (HSV-1) using the latency-associated transcript (LAT) promoter have reported a decrease in transgene expression in the brain over time, but the extent of this decrease has not been measured and it is unknown if expression eventually stabilizes. We examined LAT promoter-mediated transgene expression in the mouse brain for 1 year following intracranial injection with a HSV-1 vector expressing human beta-glucuronidase (GUSB). The vector genome copy number remained stable from 2 to 52 weeks. Quantitative reverse transcriptase PCR detected a peak of LAT intron expression at 2 weeks (corresponding to the end of the acute phase of viral infection), followed by stable expression during latency (13-52 weeks). The number of GUSB-positive cells also had a peak in the acute phase and then was stable during latency (13-52 weeks). GUSB enzymatic activity was maintained at 11% of normal at 6 and 12 months, indicating that the LAT promoter is capable of driving stable transgene expression in the brain.

Widespread gene delivery and structure-specific patterns of expression in the brain after intraventricular injections of neonatal mice with an adeno-associated virus vector

Developing a system for widespread somatic gene transfer in the central nervous system (CNS) would be beneficial for understanding the global influence of exogenous genes on animal models. We injected an adeno-associated virus serotype 2 (AAV2) vector into the cerebral lateral ventricles at birth and mapped its distribution and transduction pattern from a promoter capable of expression in multiple targets. The injections resulted in structure-specific patterns of expression that were maintained for at least 1 year in most regions, with efficient targeting of some of the major principal neuron layers. The patterns of transduction were explained by circulation of the viral vector in the subarachnoid space via CSF flow, followed by transduction of underlying structures, rather than by progenitor cell infection and subsequent migration. This study demonstrates that gene transfer throughout the CNS can be achieved without germ line transmission and establishes an experimental strategy for introducing genes to somatic cells in a highly predictable manner.

Recombinant adeno-associated virus-mediated correction of lysosomal storage within the central nervous system of the adult mucopolysaccharidosis type VII mouse

The central nervous system (CNS) is a predominant site of involvement in several lysosomal storage diseases (LSDs); and for many patients, these diseases are diagnosed only after the onset of symptoms related to the progressive accumulation of macromolecules within lysosomes. The mucopolysaccharidosis type VII (MPS VII) mice are deficient for the lysosomal enzyme beta-glucuronidase and, by early adulthood, develop a significant degree of glycosaminoglycan storage within neuronal, glial, and leptomeningeal cells. Using this animal model, we investigated whether gene transfer mediated by a recombinant adeno-associated virus (rAAV) vector is capable of reversing the progression of storage lesions within the CNS. Adult MPS VII mice received intracerebral injections of 4 X 10(7) infectious units of a rAAV vector carrying the murine beta-glucuronidase (gus-s(a)) cDNA under the transcriptional direction of the cytomegalovirus immediate-early promoter and enhancer. By 1 month after vector administration, transgene-derived beta-glucuronidase was present surrounding the injection site. Enzyme levels were between 50 and 240% of that found in wild-type mice. This level of beta-glucuronidase activity was sufficient to reduce the degree of lysosomal storage. Moreover, the reduction in storage was maintained for at least 3 months post-rAAV administration. These data demonstrate that rAAV vectors can transduce the diseased CNS of MPS VII mice and mediate levels of transgene expression necessary for a therapeutic response. Thus, rAAV vectors are potential tools in the treatment of the mucopolysaccharidoses and other lysosomal storage diseases.

Growth hormone and insulin-like growth factor-I enhance beta-glucuronidase gene activation by androgen in mouse kidney

Beta-glucuronidase (GUS) is a lysosomal enzyme that, in mouse kidney, is subject to control by multiple hormones: androgen, which increases GUS transcription; estrogen, which antagonizes androgen-mediated stimulation of GUS; and growth hormone (GH), which appears to be necessary for the full androgen effect. Neither estrogen nor GH affects GUS in the absence of androgen. In hypophysectomized or pituitary dwarf mice the reduced androgen stimulation of GUS can be partially restored with GH treatment. Androgen-induced GUS mRNA increased significantly with intermittent GH, compared to no GH or continuous GH. Intact mice subjected to continuous infusion of GH showed a depressed androgen effect on GUS similar to that seen in GH-deficient mice. Thus, pulsatile GH is required for the full androgen response. Insulin-like growth factor-I (IGF-I) also restored GUS induction by androgen in GH-deficient mice. We conclude that GH enhances the effect of androgen on the GUS gene via IGF-I. Using transgenic mice, we have also identified a genetic variant of the GUS gene that is insensitive to GH enhancement of the androgen effect.

Androgen responsiveness of mouse kidney beta-glucuronidase requires 5'-flanking and intragenic Gus-s sequences

Genetics studies of natural variants of the androgen response of mouse beta-glucuronidase (GUS) reveal a cis-active element closely linked to the GUS structural gene (Gus-s) that is necessary for this kidney-specific response. Results of our previous studies suggested sequences within or near an androgen-inducible deoxyribonuclease I-hypersensitive site (DH site) located in the ninth intron of Gus-s are associated with the androgen response of GUS. Using transgenic mice, we now demonstrate that at least two regions of sequence within Gus-s are involved in regulating the androgen response of GUS. The first, located within 3.8 kb of Gus-s 5'-flanking sequence, directs the response and its tissue specificity, while the second, located within a 6.4-kb fragment of Gus-s extending from the third through the ninth intron of Gus-s, protects the androgen responsiveness of the transgene from repressive influences of the insertion site.

The Gus-e locus regulates estrogen repression of androgen-induced beta-glucuronidase expression in mouse kidney

Both enzyme activity and mRNA concentration of beta-glucuronidase were measured in kidneys of mice treated with testosterone and the synthetic estrogen, diethylstilbestrol. Six congenic strains, all having a C57BL6/J genetic background but each having a different haplotype of the beta-glucuronidase gene complex, were compared. In each strain the induction caused by androgen was partially repressed by estrogen. The extent of this antagonism varied among the six haplotypes and was not coordinate with the extent of induction by androgen alone. Antagonism appears to be regulated by at least two alleles of a new locus, Gus-e, within the beta-glucuronidase gene complex. Repression by estrogen, like induction by androgen, appears to take place primarily at the transcriptional level. Kinetic studies revealed that estrogen causes the androgen response curve to plateau earlier and at a lower level. This suggests that estrogen increases the rate of gene deactivation rather than decreasing the rate of gene activation. Isoelectric focusing of beta-glucuronidase from Gus-ea and Gus-eb mice and their F1 progeny revealed that the genes are regulated in cis. Together, these findings support a model in which both sex hormones exert their effects on separate DNA response elements located in close proximity to the gene or within the gene itself.

The liver/erythrocyte pyruvate kinase gene complex [Pk-1] in the mouse: regulatory gene mutations

Nine enzyme activity variants and one charge variant of liver/erythrocyte pyruvate kinase have been found amongst laboratory and wild mice. Four of the enzyme activity variants were previously reported to be caused by allelic differences in the structural gene, Pk-1s. Analysis of two putative regulatory gene mutations is now reported, both of which map at, or close to, the structural gene on chromosome 3. One of these mutations, in the inbred strain SWR, is tissue specific, affecting enzyme concentration in the liver but not the erythrocyte the other, which arose in a mutation experiment, doubles the enzyme concentration in both tissues. The organization and the nomenclature in the [Pk-1] gene complex are discussed and are compared with the organization of other comprehensively analysed gene complexes in the mouse.

The N haplotype of the murine beta-glucuronidase gene is altered in both its systemic regulation and its response to androgen induction

A new haplotype of the beta-glucuronidase gene complex, [Gus]N, has been characterized following its transfer from the PAC/Cr strain to the standard strain C57BL/6J. The N haplotype contains a novel structural gene allele which encodes an allozyme differing from all previously characterized allozymes in both size and charge. Altered systemic regulation is exhibited by the [Gus]N haplotype. Multiple tissues contain levels of GUS protein that are 60 +/- 15% those found in the standard B haplotype. The regulatory mechanism for reduction is complex, involving tissue-specific changes in both enzyme synthesis and enzyme turnover. The changes in GUS protein synthesis do not result from changes in GUS mRNA levels. Instead, the amount of mature enzyme formed per mRNA molecule, or translational yield, is altered. These regulatory changes parallel those seen in other systemic regulatory variants of GUS which are also altered in translational yield. A commonality of mechanism among systemic regulatory variants of this gene is suggested. The N haplotype is also exceptional in the nature of its response to androgenic induction in kidney proximal tubule epithelial cells. The time course for GUS induction consists of a lag period followed by a progressive increase in mRNA, rate of enzyme synthesis, and enzyme activity. For the [Gus]N haplotype the lag is of an exceptionally short duration and the plateau is of a greater magnitude than for any haplotype previously described.

Genome organization and polymorphism of the murine beta-glucuronidase region

Thirty-eight kilobases of mouse genomic DNA which surround and include the coding sequences for beta-glucuronidase has been mapped. Intron-exon arrangements were determined by hybridization of genomic sequences with cDNA clones, and minimum estimates of gene length (11-17 kb) and intron number were obtained. Only a single gene was observed when genomic DNA was probed with subclones containing beta-glucuronidase coding sequence; there was no evidence of duplicated or pseudogenes. However, sequences distal to the 3' end of the gene are present elsewhere in the genome in a limited number of copies. Eight haplotypes of the beta-glucuronidase region with differing regulatory genotypes were compared for restriction fragment polymorphisms. Surprisingly little was found, considering the diverse origin of the haplotypes. Two of the polymorphisms that were found may be correlated with regulatory phenotypes. A BamHI site is missing from the CS and CL haplotypes that share regulatory properties, and a 0.2-kb insertion is consistently present in haplotypes showing increased response to induction by androgens in kidney.

DNA sequence variation within the beta-glucuronidase gene complex among inbred strains of mice

Tightly linked to the gene that encodes murine beta-glucuronidase (GUS) are three GUS-specific regulatory elements. Together, these elements define the GUS gene complex. Specific alleles of each regulatory element are associated with a specific GUS structural allele. These associations define the three common forms (haplotypes) of the GUS gene complex, designated A, B, and H. As an initial step in defining the DNA determinants of each regulatory element and to develop DNA markers for the common haplotypes, we have identified several DNA variants by blot hybridization analysis of restricted genomic DNA using GUS-specific cDNA probes. Of 30 tested restriction endonucleases, 24 reveal DNA polymorphisms that distinguish B- and H-haplotype DNA from that of the A haplotype. Of these 24, 18 uncover a restriction fragment length polymorphism in which the polymorphic fragment of A-haplotype DNA is 200-300 bp larger than the corresponding fragment of B- or H-haplotype DNA. DNA sequence analysis of this polymorphic region reveals the presence of a short, interspersed repetitive element of the B2 family within A-haplotype DNA which is absent in DNAs of B- or H-haplotype mice. None of the DNA variations revealed by these analyses can be associated at this time with variation in the regulatory or structural properties of GUS among the common haplotypes. Nevertheless, they do provide useful haplotype-specific markers within the GUS gene complex which are of critical importance for DNA transfer experiments in transgenic mice and in cultured cells.

Genetic variations in kinetic constants that describe beta-glucuronidase mRNA induction in androgen-treated mice

The kinetics of beta-glucuronidase mRNA induction by androgen in mouse kidney were determined for A, B, and CS haplotypes of the beta-glucuronidase gene. After a lag period, the kinetics of mRNA (R) induction are approximated by the turnover equation dR/dt = k1 - k2R. The A haplotype differs from the B primarily in the duration of the lag period and in k1, the rate constant determining the initial slope of the induction curve. The CS haplotype differs from B primarily in k2, the first-order rate constant that determines the half-time for induction. None of the haplotypes differs significantly in the half-life of beta-glucuronidase mRNA as measured by deinduction. Thus, there was no correlation between the half-time or extent of induction and the half-life of the RNA. Comparing half-times for induction with the half-life of the mRNA suggests that message stabilization can at most account for only part of the induction. We conclude that transcriptional activation of the beta-glucuronidase gene must be an important component of induction. Estimating absolute numbers of mRNA molecules and absolute rates of gene transcription, it appears that before induction there is approximately one molecule of beta-glucuronidase mRNA per cell and that each gene copy is transcribed once every 35 to 40 h. Depending on the haplotype examined, after induction, mRNA goes up to 80 to 400 molecules per induced cell. In the A haplotype, which has the highest induction, this corresponds to one transcript from each gene every 6 min if there is no induced stabilization of beta-glucuronidase mRNA, and one every 30 min if there is. Thus, it seems unlikely that more than one transcript is ever being synthesized at the same time from the beta-glucuronidase gene.

Genetic variations in kinetic constants that describe beta-glucuronidase mRNA induction in androgen-treated mice

The kinetics of beta-glucuronidase mRNA induction by androgen in mouse kidney were determined for A, B, and CS haplotypes of the beta-glucuronidase gene. After a lag period, the kinetics of mRNA (R) induction are approximated by the turnover equation dR/dt = k1 - k2R. The A haplotype differs from the B primarily in the duration of the lag period and in k1, the rate constant determining the initial slope of the induction curve. The CS haplotype differs from B primarily in k2, the first-order rate constant that determines the half-time for induction. None of the haplotypes differs significantly in the half-life of beta-glucuronidase mRNA as measured by deinduction. Thus, there was no correlation between the half-time or extent of induction and the half-life of the RNA. Comparing half-times for induction with the half-life of the mRNA suggests that message stabilization can at most account for only part of the induction. We conclude that transcriptional activation of the beta-glucuronidase gene must be an important component of induction. Estimating absolute numbers of mRNA molecules and absolute rates of gene transcription, it appears that before induction there is approximately one molecule of beta-glucuronidase mRNA per cell and that each gene copy is transcribed once every 35 to 40 h. Depending on the haplotype examined, after induction, mRNA goes up to 80 to 400 molecules per induced cell. In the A haplotype, which has the highest induction, this corresponds to one transcript from each gene every 6 min if there is no induced stabilization of beta-glucuronidase mRNA, and one every 30 min if there is. Thus, it seems unlikely that more than one transcript is ever being synthesized at the same time from the beta-glucuronidase gene.

A new regulatory gene in the histidine decarboxylase gene complex determines the responsiveness of the mouse kidney enzyme to testosterone

The level of histidine decarboxylase in mouse kidney normally differs between the sexes with females higher than males. In a strain derived from feral Danish mice (DAN), however, both males and females have the same, high, HDC activity due to the males being insensitive to repression by testosterone. Genetic analysis indicates that this insensitivity is caused by a variant allele of a new gene in the histidine decarboxylase gene complex,Hdc-a; theHdc-aballele in C57BL/10 confers high sensitivity to testosterone whereas theHdc-awallele in the DAN strain confers low sensitivity. In addition, the DAN strain has a novel haplotype for the other three known elements of [Hdc]: the alleleHdc-sdof the structural gene, theHdc-cdallele of the gene determining enzyme concentration, and the oestrogen-inducible alleleHdc-eb.

Genetic analysis of a new haplotype of the histidine decarboxylase gene complex in C57BL/6 mice

The histidine decarboxylase (HDC) gene complex, [Hdc], comprises the structural gene for mouse kidney HDC and closely linked regulatory elements which determine enzyme concentration and its response to hormones. One of these regulatory elements,Hdc-e, determines the response (induction or repression) of kidney HDC to oestrogen. HDC is oestrogen-inducible in C57BL/10 and oestrogen-repressible in DBA/2 and C57BL/6; alleles ofHdc-esegregate in crosses between C57BL/10 and DBA/2 and between the C57BL substrains. Two different haplotypes of[Hdc] have been defined previously, B.10 (Hdc-sb, Hdc-cb, Hdc-eb) in C57BL/10 and D (Hdc-sa, Hdc-cd, Hdc-ed) in DBA/2. C57BL/6 represents a third haplotype (B.6) (Hdc-sb, Hdc-cb, Hdc-ed) which differs from both B.10 and D.Hdc-emay therefore be a component of the complex independent ofHdc-sandHdc-c.

Weak androgen reduces the rate constant of beta-glucuronidase induction

Administration of androgen to mice induces kidney beta-glucuronidase. Measuring beta-glucuronidase activity, rate of beta-glucuronidase synthesis, beta-glucuronidase mRNA activity and beta-glucuronidase mRNA concentration, the time course of induction was compared using a strong androgen, dihydrotestosterone (DHT), and a weakly androgenic progestin, medroxyprogesterone acetate (MPA). Using MPA resulted in a longer lag, a 3-4-fold slower rate of induction as defined by the forward rate constant, ka, a lower final extent of induction, and a slightly lower turnover constant, kb. Differences in kinetics of induction were consistent for all 4 measured parameters, and mimicked previously described genetic differences in these rate constants. The coordinate induction of beta-glucuronidase protein and beta-glucuronidase mRNA indicates that the response to androgen is regulated at a pre-translational level. That substitution of MPA for DHT decreases ka, rather than increasing kb, suggests that induction of beta-glucuronidase follows an increased rate of mRNA synthesis rather than a decreased rate of mRNA turnover. Finally, the results are consistent with a model in which the kinetic constants for beta-glucuronidase induction are dependent on the concentration of receptor molecules in the active conformational state.

Properties of rat and mouse beta-glucuronidase mRNA and cDNA, including evidence for sequence polymorphism and genetic regulation of mRNA levels

cDNA clones containing partial sequences for beta-glucuronidase (beta G) were constructed from rat preputial gland RNA and identified by their ability to selectively hybridize beta G mRNA. One such rat clone was used to isolate several cross-hybridizing clones from a mouse-cDNA library prepared from kidney RNA from androgen-treated animals. Together, the set of mouse clones spans about 2.0 kb of the 2.6-kb beta G mRNA. Using these cDNA clones as probes, a genomic polymorphism for DNA restriction fragment size was found that proved to be genetically linked to the beta G gene complex. A fragment of beta G cDNA was subcloned into a vector carrying an SP6 polymerase promoter to provide a template for the in vitro synthesis of single-stranded RNA complementary to beta G mRNA. This provided an extremely sensitive probe for the assay of beta G mRNA sequences. Using either nick-translated cDNA or transcribed RNA as a hybridization probe, we found that mouse beta G RNA levels are strongly induced by testosterone, and that induction by testosterone is pituitary-dependent. During the lag period preceding induction, during the induction period itself, and during deinduction following removal of testosterone, beta G mRNA levels paralleled rates of beta G synthesis previously measured by in vivo pulse-labelling experiments. Genetic variation in the extent of induction affected either the level of beta G mRNA or its efficiency of translation depending on the strain of mice tested.

A structural gene (Hdc-s) for mouse kidney histidine decarboxylase

The concentration of mouse kidney histidine decarboxylase (HDC) is modulated by estrogen, testosterone, and thyroxine in a tissue-specific manner. Variation in HDC levels between strains of mice can be used to investigate the genetic regulation of enzyme structure, tissue specific expression, and induction and repression by hormones. Variation in the structure of HDC between different inbred strains of mice affecting its Km for the cofactor pyridoxal-5'-phosphate (PLP) and its heat stability has been discovered. The alternative phenotypes are additively inherited in crosses and the heat stability difference is due to alleles of a single structural gene, Hdc-s, which segregate among the BXD and BXH recombinant inbred strains. The allele Hdc-sb determines the heat-stable phenotype (C57BL substrains), and the allele Hdc-sd the heat-labile phenotype (DBA/2 and C3H/He strains). The alleles of the structural gene cosegregate with alleles of a regulatory gene previously named Hdc (determining kidney enzyme concentration); there were no recombinants among 38 RI strains. Therefore the two loci are less than 0.685 cM apart and comprise part of the HDC gene complex, [Hdc], on chromosome 2 of the mouse.

Trans-acting temporal locus within the beta-glucuronidase gene complex

Mice carrying the [Gus]H haplotype of the beta-glucuronidase gene complex have considerably decreased enzyme levels and a decreased rate of enzyme synthesis. This is now shown to result from the action of two regulatory loci within the gene complex. One is a systemic regulator, Gus-u, that acts cis to cause a uniform reduction in enzyme levels in all tissues. The other is a temporal locus, Gus-t, that acts trans to cause abrupt switches in the rate of enzyme synthesis in only certain tissues and at characteristic stages of development. The distinction between these two loci was made possible by the introduction of a method for quantitating the relative numbers of A and H allozyme subunits in beta-glucuronidase tetramers. The procedure involves purification of the enzyme, cleavage at methionyl residues with CNBr, isoelectric focusing to separate the peptides, and quantitation of the peptide containing the A/H amino acid substitution. The presence of a trans-acting regulatory locus within a gene complex raises evolutionary and functional questions about why it is located there and how it acts.

Progressive induction of beta-glucuronidase in individual kidney epithelial cells

The magnitude and kinetics of beta-glucuronidase induction in mouse kidney are determined by a cis-acting regulatory gene, Gus-r, that is closely linked to the enzyme structural gene. The accumulation of beta-glucuronidase mRNA during induction is much slower than the turnover time of the mRNA, suggesting progressive acquisition of mRNA synthesizing capacity during induction. Counts of the numbers of induced cells present at various times of induction in strains carrying three different alleles of Gus-r show that all potentially responsive cells respond immediately. The level of induction is progressive in individual cells and does not involve continued recruitment of new cells into the induced population. It appears that during induction each chromosome becomes progressively more active in directing the synthesis of beta-glucuronidase.