Trapping genes expressed in the developing mouse inner ear

Fgf16(IRESCre) mice: a tool to inactivate genes expressed in inner ear cristae and spiral prominence epithelium

Fibroblast growth factors play important roles in inner ear development. Previous studies showed that mouse Fgf16 is expressed asymmetrically during the otic cup and vesicle stages of development, suggesting roles in regulating or responding to anteroposterior axial cues. Here, we studied otic Fgf16 expression throughout embryonic development and found transcripts in the developing cristae and in a few cells in the lateral wall of the cochlear duct. To determine the otic function of Fgf16 and to follow the fate of Fgf16-expressing cells, we generated an Fgf16(IRESCre) allele. We show that Fgf16 does not have a unique role in inner ear development and that the Fgf16 lineage is found throughout the three cristae, in portions of the semicircular canal ducts, and in the cochlear spiral prominence epithelial cells. This strain will be useful for gene ablations in these tissues.

Dusp6 (Mkp3) is a negative feedback regulator of FGF-stimulated ERK signaling during mouse development

Mitogen-activated protein kinase (MAPK) pathways are major mediators of extracellular signals that are transduced to the nucleus. MAPK signaling is attenuated at several levels, and one class of dual-specificity phosphatases, the MAPK phosphatases (MKPs), inhibit MAPK signaling by dephosphorylating activated MAPKs. Several of the MKPs are themselves induced by the signaling pathways they regulate, forming negative feedback loops that attenuate the signals. We show here that in mouse embryos, Fibroblast growth factor receptors (FGFRs) are required for transcription of Dusp6, which encodes MKP3, an extracellular signal-regulated kinase (ERK)-specific MKP. Targeted inactivation of Dusp6 increases levels of phosphorylated ERK, as well as the pERK target, Erm, and transcripts initiated from the Dusp6 promoter itself. Finally, the Dusp6 mutant allele causes variably penetrant, dominant postnatal lethality, skeletal dwarfism, coronal craniosynostosis and hearing loss; phenotypes that are also characteristic of mutations that activate FGFRs inappropriately. Taken together, these results show that DUSP6 serves in vivo as a negative feedback regulator of FGFR signaling and suggest that mutations in DUSP6 or related genes are candidates for causing or modifying unexplained cases of FGFR-like syndromes.

Gene trap mutagenesis

Our ability to genetically manipulate the mouse has had a great impact on medical research over the last few decades. Mouse genetics has developed into a powerful tool for dissecting the genetic causes of human disease and identifying potential targets for pharmaceutical intervention. With the recent sequencing of the human and mouse genomes, a large number of novel genes have been identified whose function in normal and disease physiology remains largely unknown. Government-sponsored multinational efforts are underway to analyze the function of all mouse genes through mutagenesis and phenotyping, making the mouse the interpreter of the human genome. A number of technologies are available for the generation of mutant mice, including gene targeting, gene trapping and transposon, chemical or radiation-induced mutagenesis. In this chapter, we review the current status of gene trapping technology, including its applicability to conditional mutagenesis.

Tools for targeted manipulation of the mouse genome

In the postgenomic era the mouse will be central to the challenge of ascribing a function to the 40,000 or so genes that constitute our genome. In this review, we summarize some of the classic and modern approaches that have fueled the recent dramatic explosion in mouse genetics. Together with the sequencing of the mouse genome, these tools will have a profound effect on our ability to generate new and more accurate mouse models and thus provide a powerful insight into the function of human genes during the processes of both normal development and disease.

Impaired motor coordination in mice that lack punc

The punc gene, encoding a member of the neural cell adhesion molecule family expressed in the developing central nervous system, limbs, and inner ear, was identified. To extend studies of the normal expression pattern of punc and to determine its function, a mouse strain bearing a lacZ/neo insertion in a 5' coding exon was created. The complex pattern of punc expression in embryos from embryonic day 9.5 (E9.5) to E11.5 was mimicked accurately by beta-galactosidase (beta-Gal) activity. As development proceeded, the distribution of beta-Gal activity was increasingly restricted, finally becoming confined to the brain and inner ear by E15.5. In the adult, beta-Gal activity was detected in several regions of the inner ear and brain and was particularly strong in the cerebellar Bergmann glia. Genetic analysis of this null allele demonstrated that punc is not required for normal embryogenesis. Interestingly, comparisons of beta-Gal activity and punc transcripts in heterozygous and homozygous mutant individuals demonstrated that punc is negatively autoregulated in some tissues. Adult punc-deficient mice were overtly normal and had normal hearing. Compared with control littermates, however, homozygous mutants had significantly reduced retention times on the Rotarod, suggesting a role for Bergmann glia-expressed Punc in the cerebellar control of motor coordination.

Gene trap: a way to identify novel genes and unravel their biological function

The gene trap methodology is a powerful tool to characterize novel genes and analyze their importance in biological phenomena. It is based on the use of mouse embryonic stem cells and reporter vectors designed to randomly integrate into the genome, tagging an insertion site and generating a mutation. Theoretically, all the 100,000 genes present in the mouse genome could be tagged and functionally inactivated at the same time. Here we describe the basic concepts and perspectives of this methodology and show some results obtained by the gene trap approach used to study molecular cascades in basic cell biology and in developmental processes.

Expression and genetic analysis of prtb, a gene that encodes a highly conserved proline-rich protein expressed in the brain

A mouse gene, designated prtb (proline codon-rich transcript, brain expressed) was identified and characterized from a gene trap embryonic stem cell line. It encodes a proline-rich protein of 168 amino acids that shares 99% amino acid sequence identity with its human homologue and is located on the distal region of mouse chromosome 15. To determine the expression pattern and function of prtb, mice that carry the prtb(gt) allele were generated. During embryogenesis,prtb gene expression as revealed by beta-galactosidase (beta-gal) marker gene activity was highly regulated. Between embryonic day (E) 11.5 and E12.5, beta-gal activity was restricted to the developing heart. From E13.5 on, expression in the heart was extinguished. However, very strong beta-gal activity could be detected in the brains of adult mice, suggesting a role for this gene in brain function. Mice homozygous for the mutation were viable, fertile, and did not display any obvious abnormalities. This could be due to functional redundancy as Northern blot hybridization analysis clearly demonstrated that prtb(gt) is likely to be a null allele.

Mouse gene trap approach: identification of novel genes and characterization of their biological functions

The mouse gene trap strategy is an insertional mutagenesis involving an exogenous DNA, termed the trap vector, as a mutagen that produces a mutation in the mouse genome and a sequence tag to facilitate the isolation of the mutated genes. The trap vector consists of a reporter gene whose expression mimics that of the endogenous genes mutated and a selection marker that sorts cells bearing the inserted vector. Gene trap is a powerful method for identifying genes important in biological phenomena. Moreover, the method produces mutant organisms whose phenotypes provide invaluable information about the biological functions of the genes responsible for these phenotypes. Indeed, a number of genes essential for mouse embryogenesis have been identified by the gene trap method. Here, we describe the principle, results, and perspectives for applications of gene trap approach to the study of cell differentiation and lineage commitment.Key words: gene trap, embryogenesis, jumonji.