Generation and Standardized, Systemic Phenotypic Analysis of Pou3f3L423P Mutant Mice

Increased levels of blood plasma urea were used as phenotypic parameter for establishing novel mouse models for kidney diseases on the genetic background of C3H inbred mice in the phenotype-driven Munich ENU mouse mutagenesis project. The phenotypically recessive mutant line HST011 was established and further analyzed. The causative mutation was detected in the POU domain, class 3 transcription factor 3 (Pou3f3) gene, which leads to the amino acid exchange Pou3f3L423P thereby affecting the conserved homeobox domain of the protein. Pou3f3 homozygous knockout mice are published and show perinatal death. Line Pou3f3L423P is a viable mouse model harboring a homozygous Pou3f3 mutation. Standardized, systemic phenotypic analysis of homozygous mutants was carried out in the German Mouse Clinic. Main phenotypic changes were low body weight and a state of low energy stores, kidney dysfunction and secondary effects thereof including low bone mineralization, multiple behavioral and neurological defects including locomotor, vestibular, auditory and nociceptive impairments, as well as multiple subtle changes in immunological parameters. Genome-wide transcriptome profiling analysis of kidney and brain of Pou3f3L423P homozygous mutants identified significantly regulated genes as compared to wild-type controls.


Introduction
Biomedical research with mice as animal models includes the search for and the analysis of alleles that predispose for or protect against specific diseases. A strategy for the search of novel disease-related alleles consists of the random chemical mutagenesis of a large number of animals followed by the systematic screening for clinically relevant disease phenotypes. The alkylating agent N-ethyl-N-nitrosourea (ENU) is mutagenic for premeiotic spermatogonial stem cells and allows the production of a large number of randomly mutagenized offspring from treated males. ENU predominantly induces point mutations [1]. In the phenotype-driven Munich ENU mouse mutagenesis project using C3HeB/FeJ (C3H) inbred mice as genetic background, a standardized screening profile of clinical chemical blood plasma parameters was established for the analysis of the offspring of the mutagenized mice in order to detect phenotypic variants [2,3]. Several mutant lines were established showing increased plasma urea levels as a parameter that is indicative of kidney diseases [4].
Pou3f3 (POU domain, class 3 transcription factor 3, also denominated as Brn1) is an intronless gene and is GC rich (74%) throughout the coding region [5,6]. It encodes a 497 amino acid protein that carries a common DNA binding motif called POU domain (aa 403-462). The amino acid sequence of POU3F3 is highly conserved in mouse and human thereby showing no differences in the POU domain but two amino acid exchanges as well as two sites harbouring glycine stretches with different length in the rest of the sequence (http://www.ensembl.org). POU transcription factors regulate a variety of developmental processes. POU3F3 plays a role in neuronal development and is expressed in the developing neocortex, both in the late precursor cells and in the migrating neurons (http://www.uniprot.org). The MGI database (http:// www.informatics.jax.org) harbouring knockout as well as mutant mouse alleles includes two complete POU3F3 knockouts. Pou3f3 homozygous knockout mice show neonatal mortality. One-day-old homozygous knockout mice have increased plasma urea and potassium levels with renal hypoplasia and show developmental defects in the forebrain and the loop of Henle [7,8].
The ENU mutagenesis-derived recessive mutant mouse line HST011 showing increased plasma urea levels was analyzed for the causative mutation. After the identification of the causative mutation in Pou3f3, a standardized, systemic phenotypic analysis of Pou3f3 L423P homozygous mutant mice was carried out in the German Mouse Clinic (http://www.mouseclinic.de) to examine organ systems and/or pathways that may be affected by the Pou3f3 mutation as primary or secondary effects.

Animals, Linkage Analysis, and Detection of the Causative Mutation
The recessive mutant line HST011 (= UREHR2) was established in the clinical chemical screen of the phenotype-based Munich ENU mouse mutagenesis project [9] on the C3HeB/FeJ (C3H) inbred genetic background by showing increased plasma urea values at the age of three months (cut-off level: 70 mg/dl = 11.7 mmol/l). Mouse husbandry, breeding, linkage analysis, and genome-wide mapping were performed as described previously [4]. All mice had free access to drinking water and a standard rodent diet (V1124; Ssniff, Soest, Germany) ad libitum.
For linkage analysis of the causative mutation, automated DNA extraction from the lysates was performed using the AGOWA Mag Maxi DNA Isolation Kit (AGOWA, Berlin, Germany). A genome-wide, evenly distributed mapping panel consisting of 158 single nucleotide polymorphism (SNP) markers was applied. The markers used are available upon request. Genotyping using this panel was performed by MassExtend, a MALDI-TOF high-throughput genotyping system supplied by Sequenom (San Diego, CA, USA). Additional fine mapping was performed using further SNP and microsatellite markers. Chromosomal positions of markers and genes are according to the GRCm38 mouse assembly (http://www.ensembl.org). All genes which are located in the identified defined chromosomal region were analyzed for published data about their wild-type and mutant function with respect to their potential impact on renal function and renal diseases.

Immunohistochemical Analysis
For immunohistochemical analysis, kidneys were fixed with 4% paraformaldehyde in PBS (pH 7.4) via orthograde vascular perfusion, postfixed by immersion in the same fixative for one day and embedded in paraffin. 3 μm thick paraffin sections were dewaxed and rehydrated, and heat induced antigen retrieval using Citrate buffer (pH 6.0) was performed. Immunohistochemistry was performed with a polyclonal guinea pig anti-POU3F3 antiserum (generous gift from Michael Wegner) [10] and HRP-labeled rabbit anti-guinea pig antibody (Dako, Hamburg, Germany). Immunoreactivity was visualized using 3,3-diaminobenzidine tetrahydrochloride dihydrate (DAB) (brown color) and nuclear counterstaining was done with haemalum (blue color).

Phenotypic Analysis in the German Mouse Clinic
Maintenance of the recessive mutant mouse line Pou3f3 L423P comprised the repeated backcross to C3H wild-type mice for more than ten generations leading to the subsequent loss of essentially all non-causative ENU mutations that were not linked to the Pou3f3 mutation. The comprehensive phenotypic analysis was carried out in the German Mouse Clinic at the Helmholtz Zentrum München by using standardized examination protocols (http://www.mouseclinic.de). The analysis covers several hundred parameters in the areas of allergy, behavior, cardiovascular analysis, clinical chemistry, dysmorphology including bone and cartilage, energy metabolism, eye analysis and vision, immunology, lung function, molecular phenotyping, neurology, nociception, steroid analysis, and pathology. The complete protocols of the examinations are described under http://www.mouseclinic.de [11][12][13].
Pou3f3 L423P homozygous mutant mice were analyzed with an age of 8-24 weeks. 27 homozygous mutants and 32 wild-type control littermates were used for phenotypic analyses. The number of animals analyzed was 13-16 animals per sex and genotype (except of otherwise stated in the text of the respective Results section).
Mouse husbandry was done under a continuously controlled specific pathogen free (SPF) hygiene standard according to the FELASA recommendations [14] (http://www.felasa.eu). All tests were carried out under the approval of the responsible animal welfare authority (Regierung von Oberbayern).
Data are shown as mean ± standard deviation. If not otherwise stated, data were analyzed using R, a language and environment for statistical computing. Tests for genotype effects were made by using t-test, Wilcoxon rank sum test, linear models, or ANOVA depending on the assumed distribution of the parameter and the questions addressed to the data. Significant differences are indicated for P < 0.05, 0.01, and 0.001.

Results
Generation of Line Pou3f3 L423P and Identification of the Causative Mutation urea values at the age of three months (cut-off level: 70 mg/dl = 11.7 mmol/l) in homozygous mutant mice. Homozygous mutants of both sexes were viable and fertile. They exhibited a smaller size but no other grossly apparent phenotype compared to wild-type controls.
Genome-wide linkage analysis of the causative mutation was carried out with phenotypically homozygous mutant G2 animals derived from the mating of phenotypically homozygous mutant animals on the inbred C3H genetic background with BALB/c inbred mice and the subsequent intercross of the phenotypically inconspicuous heterozygous mutant G1 mice. Using a panel of 116 genome-wide polymorphic markers, the mutant phenotype was mapped to a single locus on MMU1. Further fine mapping showed the highest linkage for the polymorphic marker D1Mit212 (40.0 Mb) (http://www.ensembl.org). The candidate gene Pou3f3 was selected for the sequence analysis, as it is located at 42.7 Mb on chromosome 1, and a published mutant phenotype has been described with increased plasma urea and potassium levels associated with renal hypoplasia (http://www.informatics.jax.org). Pou3f3 consists of a single exon coding for a 497 aa polypeptide. Sequence analysis of Pou3f3 revealed a T!C point mutation at nt 1268 (ENSMUST00000054883) which leads to the amino acid exchange from leucine to proline at aa position 423. Therefore, the name of line HST011 was designated as Pou3f3 L423P . The mutation affects the highly conserved DNA binding domain of the protein including aa 403-462. Allelic differentiation of the Pou3f3 L423P mutation was carried out by PCR-RFLP as the point mutation abolished the restriction site for the enzyme SmlI (Fig 1).
To evaluate the impact of the amino acid changing mutation POU3F3 L423P on cellular localization and abundance, immunohistochemical localization of POU3F3 in kidneys was performed (Fig 2). In wild-type controls, the transcription factor POU3F3 was immunostained in nuclei of numerous but not all tubule profiles in weak up to strong staining intensity. The kidneys of Pou3f3 L423P homozygous mutant mice exhibited similar staining pattern and staining intensity of POU3F3 compared to wild-type control kidneys.

Phenotypic Analysis in the German Mouse Clinic
Phenotypic analysis of mutant mice in the German Mouse Clinic includes the aim to collect and deliver by free access systemic phenome data of a high number of mutant mouse lines in a standardized manner. The complete phenotype reports will be deposited online (http://146. 107.6.187/phenomap/jsp/annotation/public/phenomap.jsf). Pou3f3 L423P homozygous mutant mice were analyzed under the preliminary line name HST011. Compared to wild-type controls, Pou3f3 L423P heterozygous mutant mice showed no differences in the guiding primary symptoms of plasma urea value and relative kidney weight. Thus, Pou3f3 L423P homozygous mutant mice were examined in the German Mouse Clinic with wild-type littermates as controls (S1 Table).

Behavioral and Neurological Analysis
Pou3f3 L423P homozygous mutant mice were tested at the age of eight weeks in the open field as a test of spontaneous unconditioned reactions to a novel environment allowing an evaluation of exploratory drive, reactivity to novelty and emotionality. Compared to controls, mutant mice generally showed an increased locomotor activity in this environment with increased distance moved forward and increased average speed of movement. They spent more time in the aversive zone of the center and less time in the periphery, however, this may be secondary due to their increased locomotor activity. In addition, female mutant mice exhibited a decreased rearing activity ( Table 1) that is likely due to the balance and vestibular deficits described below.
Body weights of mutant mice were lower than that of the controls. Already at eight weeks of age, the mean body weight reduction was 19% in male mutants and 24% in female mutants.
Significantly altered parameters of the SHIRPA analysis (at 8 weeks) were presence of lacrimation, presence of pelvic elevation during walking, and especially mutant mice showed trunk curl upon tail suspension and an impaired contact righting reflex when the mice were turned on their back. The other SHIRPA parameters (locomotor activity, body position, tremor, palpebral closure, defecation and urination during observation, transfer arousal, gait, tail elevation, startle response, touch escape, positional passivity, limb grasping, pinna reflex, corneal reflex, evidence of biting and vocalization in and above the arena) were without significant alterations ( Table 2). Subsequent measurement of the grip strength (g) for two paws and four paws to evaluate muscle performance revealed decreased values in the mutants (mean force for 2 paws: 101 ± 19 vs. 120 ± 20 in males, 84 ± 16 vs. 119 ± 16 in females (P = 0.09); mean force for 4 paws: 184 ± 28 vs. 207 ± 17 in males, 152 ± 8 vs. 198 ± 16 in females (P = 0.03); n = 12-16 per genotype and sex) which was attributed mainly to the decreased body weight.
Evaluation of motor coordination and balance in three consecutive trials on the accelerating rotarod at the age of 9 weeks revealed a tendency to increased latencies (sec) on the rod in the light-weight mutants (127 ± 53 vs. 103 ± 35 in males, 113 ± 55 vs. 97 ± 32 in females (P = 0.89); n = 12-16 per genotype and sex) and significantly increased passive rotations (45 in three trials of 27 mutants vs. 35 in three trials of 32 controls (P = 0.02)). The usual improvement in the performance of the task over the three trials was observed in both genotypes. Since the mutants showed movement deficits hinting towards coordination or balance deficits, additional tests especially sensitive to balance impairments were carried out with the same group of mice at the age of 19 weeks. Balance beam, beam ladder, swim ability as well as gait analysis were performed (n = 10-16 per sex and genotype) with alterations detected in all tests (Table 3).  Significance vs. wild-type controls: a, P < 0.05; c, P < 0.001.
Morphological analysis of the inner ears using the whole mount clearing method (n = 6 per genotype) showed abnormal superior semi-circular canal formation of the vestibular organ of the mutants (Fig 3). The canals appear intact on both sides but the thickness and the curvature of the superior semi-circular canal is impaired. The side of the superior semi-circular canal that meets the common crus presents an irregular curvature of the canal that might be responsible for the balance deficits in the mutants.
The prepulse inhibition (PPI) of the acoustic startle reflex test assessed the sensorimotor gating. Acoustic startle reactivity and prepulse inhibition were significantly decreased in mutant mice at all sound pressure levels, i.e. startle amplitude at 70, 80, 85, 90, 100, 110 and 120 dB, and prepulse inhibition at 4, 8 and 12 dB prepulse sound pressure level above background (P < 0.001 for all). The clear reduction of the acoustic startle amplitude suggested either impaired hearing ability or impaired neuromuscular recruitment. By measuring the auditory brain stem response (ABR), hearing sensitivity was assessed in a subset of mice (7 male and 3 female mutants as well as 9 male and 5 female controls; 16 weeks of age) by recording the minimum sound pressure required to elicit a physiological response to different auditory stimuli. There were significant (P 0.001) differences at all tone frequencies (6,12,18,24

Nociception
Analysis of nociception was carried out in 10-week-old mice by using the hot plate assay at 52°C for the detection of altered pain response. The reactions due to thermal pain by shaking and licking of the hind paws were measured in the mutants and controls (latency in seconds for shacking of hind paws: 15.4 ± 7.5 vs. 14.3 ± 4.1 in males, 18.8 ± 6.7 vs. 12.5 ± 4.3 in females (P = 0.02); latency in seconds for licking of hind paws: 24.6 ± 7.4 vs. 23.8 ± 5.7 in males, 28.0 ± 2.9 vs. 21.0 ± 5.7 in females (P = 0.01); n = 13-16 per genotype and sex). Thus, the female mutants showed a significant hypoalgesia.

Clinical Chemistry and Hematology Analysis
Line Pou3f3 L423P was established based on increased plasma urea levels in the homozygous mutant animals. Clinical chemical analysis of blood plasma at the age of 15 weeks revealed significantly increased values for creatinine, urea, chloride, potassium as well as alkaline phosphatase, aspartate aminotransferase and lactate dehydrogenase activity, and decreased values for glucose, albumin, total protein, cholesterol and triglycerides in the mutants (Table 4). In view of the lower body weight and increased locomotor activity, the plasma parameters with the decreased values might indicate a state of low energy stores in the mutants. In the hematology analysis, mutants showed decreased values for red blood cell count, hemoglobin, hematocrit, mean corpuscular volume and mean corpuscular hemoglobin. Thus, they showed erythropenic anemia with a trend towards microcytosis. This might be a secondary effect of the azotemia. In addition, platelets were also decreased, whereas white blood cells were increased in the mutants (Table 5). Clinical chemical analysis of the plasma parameters of an additional group of homozygous mutant and wild-type control mice (n = 5-10 per genotype and sex) not transferred to the German Mouse Clinic at the age of 12-14 weeks confirmed the results of the GMC analysis at the age of 15 weeks. Moreover, heterozygous mutants (17 males, 8 females) were also included in this analysis and showed no significant differences compared to the wild-type controls (data not shown). In addition, metabolic cage analysis was carried out in these mice at the age of 14-15 weeks (n = 4-10 per genotype and sex). The mutant mice harbored a lower body weight and showed an increased urine volume in the males as well as decreased urinary uric acid levels indicating kidney dysfunction, also together with the other parameters showing altered urine levels by tendency in both sexes when compared to the controls (Table 6).
The analysis was done under basal conditions and after deprivation of drinking water for 24 h in metabolic cages. 14-15-week-old mice were tested.

Dysmorphology, Bone and Cartilage
Pou3f3 L423P homozygous mutant mice were tested at the age of 10 (morphological analysis), 11 (click box test) and 19 (dual energy X-ray absorption (DXA) and X-ray analysis) weeks. The systematic morphological investigation via visual inspection (overall body shape, eye, coat, skin, vibrissae, extremities including limbs, digits and tail, teeth, ears, musculature, skeleton, neurology/behavior, respiratory system, reproductive system, other aberrant phenotype) found no genotype-specific differences. Analysis of the hearing ability in 11-week-old mice was examined by the click box test using a sound of 20 kHz, and the reaction of the animals (n = 13-16 per genotype and sex) was classified into six categories. We observed a decreased hearing reaction in the mutants by trend (P = 0.1 for males, P = 0.3 for females). DXA (dual energy X-ray absorption) analysis detected significantly decreased values for bone mineral density (BMD) in male mutants, bone mineral content (BMC) in all mutants, and bone content in male mutants compared to controls. This might be a secondary effect of the renal dysfunction leading to the impaired electrolyte balance in the mutant mice which was revealed in the clinical chemistry analysis. Concurrently body length, body weight, and fat mass were significantly decreased in mutants (Table 7). X-ray analysis of the skeleton (skull shape, mandibles, maxilla, teeth, orbit, number of vertebrae, vertebrae shape, number of ribs, rib shape, scapula, clavicle, pelvis, femur diameter, femur shape, tibia, fibula, humerus, ulna, radius, number of digits, completeness of digits, joints) found no genotype-specific differences between mutants and wild-type controls.

Energy Metabolism
Analysis of the energy metabolism was done on 11-week-old mice under ad libitum conditions. Indirect calorimetry revealed that mutants were considerably underweight and showed a marked reduction in rectal body temperature. The other effects on energy turnover were confounded by the difference in body weight. When adjusted for body weight, significant genotype effects on residual mean, minimum and maximum oxygen consumption were observed especially in the female mutants showing a slight hypometabolism. Despite lower metabolic rate, we observed a mild increase of locomotor activity but not rearing behavior. We could not detect a significant difference in the respiratory exchange ratio (RER) but the relation between the levels of oxygen consumption and metabolic fuel utilization indicated that metabolic flexibility may be impaired in mutant mice. At higher levels of VO 2 , the controls showed a wider range of respiratory exchange ratios than the mutant mice (not shown). In the determination of body composition by time domain nuclear magnetic resonance (TD-NMR), mutant mice also showed a significantly lower body weight than control mice. The statistical analysis did not reveal significant genotype related main effects on body fat content or lean mass when adjusted for body mass but it was evident that the relation between body weight and fat mass respectively lean mass was different between control and mutant mice (Table 8).

Immunology
The mice were tested at 15 weeks of age for leukocyte populations in peripheral blood and immunoglobulin levels (IgM, IgG1, IgG2a, IgG2b, IgG3, and IgA) as well as rheumatoid factor and anti-DNA antibodies in blood plasma. Flow cytometry observed a higher frequency of granulocytes as well as a lower frequency of CD8+ T cells resulting in the tendency of a higher CD4:CD8 ratio in female mutants. Furthermore, we found higher frequencies of CD44 expressing cells within both (CD4+ and CD8+) T cell clusters. Only in female mutants we found lower frequencies of B cells and NK cells and an increased frequency of CD25 expressing cells within the CD4+ T cell cluster ( Table 9). The frequency of granulocytes in blood showed changes which have been described for various pathological conditions (e.g. tumor development, infection, stress). Thus, the increased frequency of granulocytes might be a major factor for the changes in the main leukocyte populations in the mutants. The T cells showed differences in the frequencies of CD44 expressing cells. The high-surface expression of the CD44 is assigned to activated T cells; alternatively, these cells might represent cells that arise via physiological homeostatic proliferation [15]. For the immunoglobulin levels, slightly lower levels of immunoglobulins were found in male mutants compared to controls. Especially, the levels of IgG3 were significantly lower (data not shown). Analysis was done on 11-week-old mice under ad libitum conditions. No. per genotype and sex: n = 9-16. Substrate utilization rates during indirect calorimetry were calculated as described [27]. Data are presented as mean ± standard deviation. Significance vs. wild-type controls: a, P < 0.05; b, P < 0.01; c, P < 0.001. doi:10.1371/journal.pone.0150472.t008 In total, we found multiple changes in the frequencies of leukocyte subset proportions in peripheral blood. Most of the observed changes were subtle, however, the accumulation of the immunological findings may suggest a primary or secondary influence of the mutation on the immune system.

Molecular Phenotyping
Based on the results of the analysis in the German Mouse Clinic as well as on the published results of Pou3f3 knockout mice [7,8,16], the pathogenic effects of the Pou3f3 mutation were evaluated in brain, kidney and spleen by genome-wide transcriptome profiling analysis using Illumina MouseRef8 v2.0 Expression Bead Chips. Samples of the three selected organs of four mutant males were compared to four wild-type males as controls at the age of 24 weeks (in total 23 experiments as the RNA isolation of one control kidney sample failed).
Statistical analysis of the gene expression patterns [17,18] in the brain identified 104 significantly regulated genes in mutants vs. wild-type controls (false discovery rate (FDR) = 0.6%, fold change > 1.4). The range of the mean log 2 ratios was 1.6 to 2.0 for the 7 up-regulated genes as well as -2.0 to -1.4 for 96 down-regulated genes and -5.0 for Tshb (thyroid stimulating hormone, beta subunit) as additionally down-regulated gene. The genes are functionally associated with movement disorders, Huntington's disease, necrosis and RNA transcription. Statistical analysis of the gene expression patterns in the kidney identified 292 significantly regulated genes in mutants vs. wild-type controls (FDR = 1.5%, fold change > 1.4). The range of the mean log 2 ratios was 2.0 to 3.4 for the 22 up-regulated genes and -22.8 to -1.7 for the 270 down-regulated genes (with -22.8 to -5.0 for the genes Sycn, Wfdc15b, Cldn10, Shd, Higd1c, Gpx6, Popdc3, Slc6a12, and Mettl7a2). The genes are functionally associated with concentration of metal ions, lipids, proteins and steroids as well as renal impairment and failure.
Statistical analysis of gene expression patterns in the spleen identified 124 significantly regulated genes (FDR = 0%, fold change > 1.4), all of them were down-regulated. The range of the mean log 2 ratios was -3.9 to -1.5. The genes are functionally associated with inflammatory response including proliferation/migration/movement of leukocytes, phagocytes, granulocytes and lymphocytes.
The genes Asah1, Cnot2, Ctps2, Ddx5, Lgsn, Lypla1, Mtmr2 and Rcan1 were down-regulated in all three organs examined, the genes Armc 8, Atp6ap2, Dars, Hsd11b1, Sh3gl2, Snx14, Tmem49, Wdr37 and Yme1l1 were down-regulated in both brain and kidney, the gene Sec22a was down-regulated in both brain and spleen, and the genes Cav2, Cd9, Cyp2b10, Mal, Pira11, Stap1, Thbs1 and Vps41 were down-regulated in both kidney and spleen. The genes showed a range of their mean log 2 ratios between -2.8 to -1.4. Despite this overlap of differential gene expression in the organs, necrosis was found as the only gene-specific functional annotation over-represented in brain and kidney. The other enriched terms were associated with organsspecific functions.

Discussion
The recessive mutant line HST011 was established on the C3H inbred genetic background based on increased plasma urea levels, and the causative mutation was identified as Pou3f3 L423P thereby affecting the highly conserved homeobox domain of the protein. Pou3f3 is a transcription factor that is expressed in the central nervous system [7,19], the inner ear and some other organs like the kidney [8] during embryonic development. POU3F3 acts synergistically with SOX11 and SOX4 (http://www.uniprot.org).
Two complete POU3F3 knockout mutant mouse lines have been described to date. The first knockout mouse line was established by replacing the gene sequences which encode the first 476 amino acids with a PGK-neo cassette via homologous recombination on an unspecified genetic background [7]. The second knockout line was also established by using a PGK-neo cassette for replacing 1.2 kb of the coding region. Functional studies were performed on the mixed genetic background of 129S4/SvJae × C57BL/6J mice [8]. Heterozygous mutant mice were reported to be phenotypically normal in both knockout lines. Our heterozygous mutant mice also showed a grossly normal phenotype. Pou3f3 homozygous knockout mice showed neonatal mortality. In contrast, Pou3f3 L423P homozygous mutant mice are viable and fertile; however they exhibit a reduced body weight that was about 20-25% less than that of wild-type controls.
The neonatal homozygous knockout mice showed developmental defects in the forebrain [7]. In humans, a heterozygous de novo 360 kb deletion in chromosome band 2q12.1 resulting in the loss of the genes POU3F3 and MRPS9 was identified in a two-year-old boy with mild dysmorphic features of the head, mild motor delay and mild intellectual disability. The haploinsufficiency of POU3F3 was suggested to be the cause for the aberrant phenotype [20]. Eight-to-24 week-old Pou3f3 L423P homozygous mutant mice were phenotypically examined in the German Mouse Clinic. Our experiments observed deficits in locomotor and coordination behavior as well as in hearing ability. In the inner ear of rats, Pou3f3 was shown to be expressed specifically in nonsensory supporting cells and mesenchymal cells of the cochlea at early postnatal stages. Neonatal Pou3f3 homozygous knockout mice did not exhibit any abnormality in the cochlea [21]. The observed morphological alterations of the canals of the vestibular organ in Pou3f3 L423P homozygous mutant mice indicate that Pou3f3 is important for the proper development of the vestibular organ. The observed hearing impairment also might be attributed to alterations in the postnatal functional development or maintenance of the inner ear. Thus, the observed deficits in behavioral and neurological parameters of Pou3f3 L423P homozygous mutant mice might be the consequence of the morphological alterations and neuronal disorganization in the forebrain described previously as well as of the vestibular dysfunction.
One-day-old Pou3f3 homozygous knockout mice have been described to show a severe retardation of the normal development of Henle's loop, the distal convoluted tubule and the macula densa. They exhibited increased plasma urea and potassium levels with renal hypoplasia, and died within 24 hours of birth because of renal failure. In the kidneys of heterozygous knockout mice, the expression of a number of genes that are specific for the thick ascending limb of Henle's loop was reduced including Umod, Nkcc2/Slc12a1, Bsnd, Kcnj1 and Ptger3. These results demonstrate the crucial role for Pou3f3 in distal tubule formation and function in the mammalian kidney [8]. We already described the appearance of lower relative kidney weights in Pou3f3 L423P homozygous mutants [4]. Pou3f3 L423P homozygous mutant mice showed impaired renal function indicated by increased plasma levels of urea and creatinine, alterations in plasma potassium, chloride and protein levels, as well as an increased urine volume and altered renal excretion of uric acid. X-ray imaging did not reveal skeletal abnormalities, besides decreased body size and body weight, but bone mineral density was found to be decreased in homozygous mutants. The low bone mineralization may be a secondary effect due to kidney dysfunction. Altered bone mineralization was already found in previously described mouse lines showing impaired kidney function due to ENU induced mutations in the genes Slc12a1 [22] and Umod [23,24]. The observation of erythropenic anemia also may be a secondary effect due to kidney dysfunction.
The analysis of energy metabolism in Pou3f3 L423P homozygous mutant mice indicated reduced body temperature and reduced metabolic rate compared to wild-type controls. The results of the analysis of energy metabolism were supported by the changes of clinical chemical parameters associated with fat metabolism in homozygous mutants, namely blood lipid levels and possibly also protein concentrations. A lower body fat content was already found in previously described mouse lines showing impaired kidney function due to ENU induced mutations in the genes Slc12a1 [25] and Umod [23,24]. In addition, both Umod mutant mouse lines also showed a reduced body temperature [24,26].
The multiple subtle changes in immunological parameters may be caused by a primary or secondary effect of the mutation on the immune system. Genome-wide transcriptome profiling analysis of brain, kidney and spleen of Pou3f3 L423P homozygous mutants identified significantly regulated genes as compared to wild-type controls. They have to be examined in detail in further analyses. In addition, allergy, cardiovascular and steroid analyses of the German Mouse Clinic revealed no obvious differences of mutant and control mice (data not shown). Also the histological analysis (HE staining) of the organs of 24-week-old animals showed no genotype-specific differences (data not shown).
In total, as the first viable mouse line harbouring a Pou3f3 mutation, standardized, systemic phenotypic analysis of Pou3f3 L423P homozygous mutants at the age of 2-6 months in the German Mouse Clinic revealed major phenotypic changes including low body weight, kidney dysfunction and secondary effects thereof as well as neurological dysfunctions.
Supporting Information S1