Genome analysis of collagen disease in MRL/lpr mice: polygenic inheritance resulting in the complex pathological manifestations

Genomic analysis of the appearance of ovarian mast cells in neonatal MRL/MpJ mice

In MRL/MpJ mice, ovarian mast cells (OMCs) are more abundant than in other mouse strains, and tend to distribute beneath the ovarian surface epithelium at birth. This study investigated the factors regulating the appearance of neonatal OMCs in progeny of the cross between MRL/MpJ and C57BL/6N strains. F1 neonates had less than half the number of OMCs than MRL/MpJ. Interestingly, MRLB6F1 had more neonatal OMCs than B6MRLF1, although they were distributed over comparable areas. Furthermore, in MRL/MpJ fetuses for which parturition was delayed until embryonic day 21.5, the number of OMCs was significantly higher than in age-matched controls at postnatal day 2. These results suggest that the number of OMCs was influenced by the environmental factors during pregnancy. Quantitative trait locus analysis using N2 backcross progeny revealed two significant loci on chromosome 8: D8Mit343–D8Mit312 for the number of OMCs and D8Mit86–D8Mit89 for their distribution, designated as mast cell in the ovary of MRL/MpJ 1 (mcom1) and mcom2, respectively. Among MC migration-associated genes, ovarian expression of chemokine (C-C motif) ligand 17 at mcom1 locus was significantly higher in MRL/MpJ than in C57BL/6N, and positively correlated with the expression of OMC marker genes. These results indicate that the appearance of neonatal OMCs in MRL/MpJ is controlled by environmental factors and filial genetic factors, and that the abundance and distribution of OMCs are regulated by independent filial genetic elements.

Relationship between numerous mast cells and early follicular development in neonatal MRL/MpJ mouse ovaries

In the neonatal mouse ovary, clusters of oocytes called nests break into smaller cysts and subsequently form individual follicles. During this period, we found numerous mast cells in the ovary of MRL/MpJ mice and investigated their appearance and morphology with follicular development. The ovarian mast cells, which were already present at postnatal day 0, tended to localize adjacent to the surface epithelium. Among 11 different mouse strains, MRL/MpJ mice possessed the greatest number of ovarian mast cells. Ovarian mast cells were also found in DBA/1, BALB/c, NZW, and DBA/2 mice but rarely in C57BL/6, NZB, AKR, C3H/He, CBA, and ICR mice. The ovarian mast cells expressed connective tissue mast cell markers, although mast cells around the surface epithelium also expressed a mucosal mast cell marker in MRL/MpJ mice. Some ovarian mast cells migrated into the oocyte nests and directly contacted the compressed and degenerated oocytes. In MRL/MpJ mice, the number of oocytes in the nest was significantly lower than in the other strains, and the number of oocytes showed a positive correlation with the number of ovarian mast cells. The gene expression of a mast cell marker also correlated with the expression of an oocyte nest marker, suggesting a link between the appearance of ovarian ? 4mast cells and early follicular development. Furthermore, the expression of follicle developmental markers was significantly higher in MRL/MpJ mice than in C57BL/6 mice. These results indicate that the appearance of ovarian mast cells is a unique phenotype of neonatal MRL/MpJ mice, and that ovarian mast cells participate in early follicular development, especially nest breakdown.

Quantitative trait locus analysis of ovarian cysts derived from rete ovarii in MRL/MpJ mice

MRL/MpJ (MRL) is a model mouse for autoimmune diseases such as dermatitis, vasculitis, arthritis, and glomerulonephritis. In addition to these immune-associated disorders, we found that older MRL mice develop ovarian cysts originating from the rete ovarii, which is lined by ciliated or nonciliated epithelium and considered remnants of mesonephric tubules. Ovarian cysts, which are reported to have several sources, are associated with female infertility, but information regarding the genetic etiology of ovarian cysts originating from the rete ovarii is rare. In this study, to elucidate the genetic background of development of ovarian cysts, we performed quantitative trait locus (QTL) analysis using 120 microsatellite markers, which cover the whole genome of murine chromosomes, and 213 backcross progenies between female MRL and male C57BL/6N mice. The quantitative trait measured was the circumferences of rete ovarii or ovarian cysts. As a result, suggestive linkages were detected on Chrs 3, 4, 6, and 11, but significant linkages were located on Chr 14 by interval mapping. We thereby designated the 27.5-cM region of Chr 14 "MRL Rete Ovarian Cysts (mroc)." The peak regions of Chrs 4 and 14 in particular showed a close additive interaction (p < 0.00001). From these results we concluded that multiple loci on Chrs 3, 4, 6, 11, and 14 interact to result in development of ovarian cysts in MRL mice.

Ovarian Cysts in MRL/MpJ Mice Originate from Rete Ovarii

In MRL mice aged more than 1 year, but not in C57BL/6 mice, ovaries had grossly visible cysts presenting unilaterally or bilaterally. Postnatally, all MRL mice developed ovarian cysts by 8 months of age. Observations by light microscopy, including lectin histochemistry, indicated that the cysts sometimes included papillomatous tissues located at the hilar region and were similar to the rete ovarii system, but not to follicles. Two types of epithelial cells, ciliated and non-ciliated, were arranged on the cysts, in which both cell types had many microvilli projecting in various directions and random ramifications in the cystic lumen. These characteristics suggest that ovarian cysts developing in MRL mice originate mostly from the rete ovarii. Cysts derived from the rete ovarii at 8 months of age were histologically detected in all C3H mice as well as MRL mice, with variable incidence in ICR, AKR, CBA/N and ddY, and none in C57L/6, DBA/2, BALB and A/J mice. However, measurement of the maximum diameters of the ovarian cysts indicated that MRL mice regularly possessed the largest cysts visible to the naked eye. This is the first report of ovarian cysts in this inbred strain, suggesting that ovarian cysts in MRL mice appear with stable incidence and development.

Sex-dependent effect of melatonin on systemic erythematosus lupus developed in Mrl/Mpj-Faslpr mice: it ameliorates the disease course in females, whereas it exacerbates it in males

In this study, the effect of chronic administration of melatonin on MRL/MpJ-Fas(lpr) mice has been studied. These mice spontaneously develop an autoimmune disease that has many features resembling human systemic lupus erythematosus. In fact, histological studies showed that all female mice and most male mice exhibited glomerular abnormalities, arteritic lesions, and cellular interstitial inflammatory infiltrate ranging from mild to severe patterns. Treatment with melatonin improved the histological pattern in females and worsened it in males. Moreover, female mice treated with melatonin showed a diminution of titers of total serum IgG, IgM, and anti-double-stranded DNA and anti-CII autoantibodies; a decrease in proinflammatory cytokines (IL-2, IL-6, interferon-gamma, TNF-alpha, and IL-1beta), an increase in antiinflammatory cytokines (IL-10), and a decrease in nitrite/nitrate. In male mice, treatment with melatonin exhibited the opposite effect, worsening all the immunological parameters with an elevation of titers of autoantibodies and a prevalence of proinflammatory vs. antiinflammatory cytokines. Similar results were obtained when lymphocytes from spleen and lymph nodes were cultured. Again, melatonin treatment in females decreased proinflammatory cytokines and increased antiinflammatory cytokines produced by lymphocytes; in males, the effect was the opposite. These findings suggest that melatonin action in MRL/MpJ-Fas(lpr) mice is gender dependent, probably through modulation and inhibition of sex hormones.

Morphogenetic investigation of metaphase-specific cell death in meiotic spermatocytes in mice

An MRL/MpJ strain of mice, including Ipr mutants, reveals the complex pathological manifestations of collagen disease, such as systemic vasculitis, glomerulonephritis, arthritis and sialoadenitis, in association with several autoimmune factors. Studies involving this mouse strain have shown that it exhibits a much-enhanced healing response compared with other mouse strains, together with reduced scarring in the periphery. Recently, unique characteristics were found in the testis of the MRL/MpJ mouse: metaphase-specific apoptosis (MSA) of meiotic spermatocytes, heat stress resistance in spermatocytes and the appearance of oocyte-like cells. The present review describes the morphological and genetic analysis of MSA, culminating in the conclusion that inherent mutation of exonuclease 1 induces checkpoint activity during meiotic division in MRL mice.

Quantitative trait loci analysis of heat stress resistance of spermatocytes in the MRL/MpJ mouse

The MRL/MpJ mouse has previously been reported to possess an interesting phenotype in which spermatocytes are resistant to the abdominal temperature heat shock. In this study genetic analysis for it was performed. The phenotypes of F2 progenies produced by mating MRL/MpJ and control strain C57BL/6 mice were not segregated into two types as parental phenotypes, suggesting that the phenotype is controlled by multiple genetic loci. Thus, quantitative trait loci (QTL) analysis was performed using 98 microsatellite markers. The weight ratio of the cryptorchid testis to the intact testis (testis weight ratio) and the Sertoli cell index were used for quantitative traits. QTL analysis revealed two significant QTLs located on Chrs 1 and 11 for testis weight ratio and one significant QTL located in the same region of Chr 1 for the Sertoli cell index. A microsatellite marker locus located in the peak of the QTL on Chr 1 did not recombine with the exonuclease 1 (Exo1) gene locus in 140 F2 progenies. Mutation of the Exo1 gene was previously reported to be responsible for metaphase-specific apoptosis (MSA) of spermatocytes in the MRL/MpJ mouse. These results raise the possibility that mutation of the Exo1 gene is responsible for both MSA and heat stress resistance of spermatocytes in the MRL/MpJ mouse.

Genetic mutation associated with meiotic metaphase-specific apoptosis in MRL/MpJ mice

It has been reported that the MRL/MpJ mouse strain shows several unique phenotypes, including rapid wound healing, inherent collagen disease, heat shock-resistant spermatocytes, and metaphase-specific apoptosis (Msa) in the testis. In the present study, we found the genetic mutation associated with Msa by chromosomal mapping with 555 backcross progeny. The Sertoli cell index of abnormal metaphasic spermatocytes was clearly divided into two groups in the first 200 male backcross progeny, which were created by mating female F1 (female C57BL/6 x male MRL/MpJ) with male MRL/MpJ mice, indicating that Msa was caused by only one gene. The result of chromosomal mapping throughout the 555 backcross progeny by using microsatellite markers and single nucleotide polymorphism (SNP) revealed that Msa was mapped on the telomeric region of chromosome 1 and was significantly linked with exonuclease 1 (Exo1) and choroideremia-like (rab escort protein 2) (Chml/Rep2) genes. It was found that the Chml/Rep2 gene was not a candidate for Msa by means of the nucleotide sequences of several inbred strains. On the Exo1 gene in strain MRL/MpJ, but not in other strains, it was surprisingly noted that the truncated forms (tr1-Exo1 and tr2-Exo1) were expressed in all tissues examined as well as normal Exo1 by reverse transcriptase-polymerase chain reaction (RT-PCR). Additionally, the truncated forms of the Exo1 gene were suggested to be transcribed by alternative splicing of the 9th exon, possibly resulting from nucleotide substitution of the branch site existing in the 8th intron. These results suggested that the testicular meiotic Msa in MRL/MpJ mice was a unique phenotype caused by incomplete alternative splicing of the Exo1 gene.

Dominant NZB contributions to lupus in the (SWR x NZB)F1 model

(SWR x NZB)F1 (or SNF1) mice succumb to lupus nephritis. Analysis of NZB x SNF1 backcross mice has recently revealed the existence of four dominant SWR loci (H2 on Chr 17, Swrl-1 on Chr 1, Swrl-2 on Chr 14 and Swrl-3 on Chr 18), and two NZB loci (Nba1 and Lbw2/Sbw2, both on Chr 4) conferring lupus susceptibility. The present study focusing on a panel of 88 SWR x SNF1 backcross mice reveals the existence of five suggestive loci for antinuclear antibody formation, consisting of three dominant NZB contributions (Nba4 on Chr 5, Lbw4 on Chr 6, and Nba5 on Chr 7), and two recessive SWR contributions (Swrl-1 on Chr 1, and Swrl-4 on Chr 10). In addition, this study reveals a dominant NZB locus for GN (Nba3 on Chr 7, peak at 31 cM), and a dominant NZB locus linked to early mortality, on Chr 10 (peak at 4 cM). Collectively, these studies suggest that lupus in the SNF1 strain is the epistatic end-product of four dominant SWR loci and four dominant NZB loci. The immunological functions and molecular identities of these loci await elucidation.

Genetic contributions of nonautoimmune SWR mice toward lupus nephritis

(SWR x New Zealand Black (NZB))F(1) (or SNF(1)) mice succumb to lupus nephritis. Although several NZB lupus susceptibility loci have been identified in other crosses, the potential genetic contributions of SWR to lupus remain unknown. To ascertain this, a panel of 86 NZB x F(1) backcross mice was immunophenotyped and genome scanned. Linkage analysis revealed four dominant SWR susceptibility loci (H2, Swrl-1, Swrl-2, and Swrl-3) and a recessive NZB locus, Nba1. Early mortality was most strongly linked to the H2 locus on chromosome (Chr) 17 (log likelihood of the odds (LOD) = 4.59 - 5.38). Susceptibility to glomerulonephritis was linked to H2 (Chr 17, LOD = 2.37 - 2.70), Swrl-2 (Chr 14, 36 cM, LOD = 2.48 - 2.71), and Nba1 (Chr 4, 75 cM, LOD = 2.15 - 2.23). IgG antinuclear autoantibody development was linked to H2 (Chr 17, LOD = 4.92 - 5.48), Swrl-1 (Chr 1, 86 cM, colocalizing with Sle1 and Nba2, LOD = 2.89 - 2.91), and Swrl-3 (Chr 18, 14 cM, LOD = 2.07 - 2.13). For each phenotype, epistatic interaction of two to three susceptibility loci was required to attain the high penetrance levels seen in the SNF(1) strain. Although the SWR contributions H2, Swrl-1, and Swrl-2 map to loci previously mapped in other strains, often linked to very similar phenotypes, Swrl-3 appears to be a novel locus. In conclusion, lupus in the SNF(1) strain is truly polygenic, with at least four dominant contributions from the SWR strain. The immunological functions and molecular identities of these loci await elucidation.

Murine lupus genetics: lessons learned

Recent reverse genetic studies in murine lupus have taught us the following lessons: (1) Lupus is extremely polygenic; (2) A single locus may be associated with many different phenotypes; (3) What appears to be a single locus may turn out to be a cluster of loci; (4) Different loci facilitate different immunologic steps leading to lupus; (5) Epistatic interactions between loci may engender novel autoimmune phenotypes; (6) Whereas some loci may be pathogenic, others may confer disease resistance; (7) Whereas the expression of some loci is sex-dependent, the expression of others clearly is not; (8) Two or more loci may have an impact on the same phenotype; (9) Lupus susceptibility loci appear to co-cluster with other autoimmunity susceptibility loci; (10) Lupus genes are likely to be polymorphic alleles with subtle impacts, rather than outright mutations with extreme functions. In contrast, forward genetic studies have revealed that molecules that impact apoptosis, the clearance of apoptotic cells, B-cell or T-cell function, and end-organ pathology can all potentially contribute to lupus. Collectively, the loci and genes identified by these two different approaches factorize into a few distinct pathways leading to lupus. Delineating the molecular mediators of these distinct checkpoints is the challenge that lies ahead.