The mammalian inner ear is a complex sensory organ comprised of auditory and vestibular structures that serve to coordinate the senses of hearing and balance, respectively. The inner ear develops over a protracted period originating from a thickening of surface ectoderm, the otic placode, which forms at the level of the prospective hindbrain upon inductive influences from neighboring tissues (Groves and Bronner-Fraser 2000;Ladher et al. 2000). Once induced, the otic placode invaginates to form the otic cup and shortly thereafter pinches off from the surface ectoderm to give rise to the otic vesicle. Over the next several days the otic vesicle undergoes an intense period of proliferation, differentiation, and morphogenesis culminating in the establishment of the ventrally derived auditory component of the inner ear, the cochlea, as well as the more dorsally derived vestibular apparatus, comprising the semicircular canals, utricle, and saccule (for review, see Torres and Giraldez 1998).Grafting and lineage tracing experiments performed in the chick, in addition to mutational analyses performed in the mouse, have confirmed that the fate of inner ear progenitors is specified early in development (Baker and Bronner-Fraser 2001). By the otic vesicle stage, numerous genes showing restricted patterns of expression compartmentalize the otic epithelium along its three major axes (Fekete and Wu 2002). With respect to the auditory component of the inner ear, the expression of several genes in the ventral and ventromedial regions of the otocyst, including the overlapping expression of the homeobox transcription factors Otx1 and Otx2 as well as the paired-box gene Pax2 mark the location of cochlear duct outgrowth (Fekete and Wu 2002). For vestibular development, the homeobox transcription factors Hmx2, Hmx3, and Dlx5 in the dorsolateral region of the otocyst mark the territory contributing to semicircular canal formation (Fekete and Wu 2002). Loss-of-function studies in the mouse confirm that each of these genes participates actively in establishing regional identity within the inner ear (Acampora et al. 1996(Acampora et al. , 1999Torres et al. 1996;Hadrys et al. 1998;Wang et al. 1998Wang et al. , 2001Depew et al. 1999;Morsli et al. 1999).In addition to the establishment of regional identity, a number of genes have also been identified that have an impact on the specification of distinct cell fates within the otocyst. The inner ear is a self-contained organ in that the majority of cell types contributing to its development including sensory, nonsensory, and neurogenic are derived from the otic epithelium (Torres and Giraldez 1998). For instance, within the anteroventral region of the otic vesicle, cells expressing the bHLH transcription factors Neurogenin-1 (Ngn1) and NeuroD form the neuronal lineage, giving rise to the cochleovestibular

The paired box transcription factor, Pax2, is important for cochlear development in the mouse inner ear. Two mutant alleles of Pax2, a knockout and a frameshift mutation (Pax21Neu), show either agenesis or severe malformation of the cochlea, respectively. In humans, mutations in the PAX2 gene cause renal coloboma syndrome that is characterized by kidney abnormalities, optic nerve colobomas and mild sensorineural deafness. To better understand the role of Pax2 in inner ear development, we examined the inner ear phenotype in the Pax2 knockout mice using paint-fill and gene expression analyses. We show that Pax2-/- ears often lack a distinct saccule, and the endolymphatic duct and common crus are invariably fused. However, a rudimentary cochlea is always present in all Pax2 knockout inner ears. Cochlear outgrowth in the mutants is arrested at an early stage due to apoptosis of cells that normally express Pax2 in the cochlear anlage. Lack of Pax2 affects tissue specification within the cochlear duct, particularly regions between the sensory tissue and the stria vascularis. Because the cochlear phenotypes observed in Pax2 mutants are more severe than those observed in mice lacking Otx1 and Otx2, we postulate that Pax2 plays a key role in regulating the differential growth within the cochlear duct and thus, its proper outgrowth and coiling.

(1996) J. Biol. Chem. 271, 6050 -6061). Cyclin G2 is highly expressed in the immune system where immunologic tolerance subjects self-reactive lymphocytes to negative selection and clonal deletion via apoptosis. Here we investigated the effect of growth inhibitory signals on cyclin G2 mRNA abundance in different maturation stage-specific murine B cell lines. Upon treatment of wild-type and p53 null B cell lines with the negative growth factor, transforming growth factor ␤1, or the growth inhibitory corticosteroid dexamethasone, cyclin G2 mRNA levels were increased in a time-dependent manner 5-14-fold over control cell levels. Proliferation signals promote the coordinated progression of a cell through the cell division cycle. In eukaryotes this process is controlled by the sequential formation, activation, and inhibition of cyclin-cyclin-dependent kinase (CDK) 1 complexes (1). Active cyclin-CDK complexes phosphorylate specific targets such as the tumor suppressor RB, various transcription factors, DNA polymerase ␣, and cytoskeletal proteins (2) and thus trigger progression through the cell cycle. The levels of many cyclins oscillate during the cell cycle and act as rate-limiting positive regulators of CDK activity. Mammalian cyclins are classified into different types based on their structural similarity, functional period in the cell division cycle, and regulated expression (1, 3, 4). 12 different cyclins in mammalian cells (cyclins A-I, some with multiple subtypes) have been identified (1, 5-7) either functionally or through an ϳ110-amino acid homologous region essential for cyclin-CDK complex formation (8 -10) referred to as the cyclin box (3, 11). Cyclin-CDK activity is also subject to regulation by CDK inhibitors (CDKIs) such as p15INK4 and p16 INK4, p21 WAF1/CIP1, and p27 KIP1 which, in response to negative stimuli, bind cyclin-CDK complexes and block cell cycle progression (5, 12). In addition to participation in cellular proliferation, CDKs and cyclin-CDK pairs may participate in processes not directly related to cell cycle regulation as evidenced by Pho80-Pho85 cyclin-CDK participation in yeast phosphate metabolism (13,14), the involvement of p35⅐CDK5 in promoting neurite outgrowth (15-17), the association of the cyclin H/CDK7 pair in the TFIIH transcription factor complex (18,19), and the cyclin C/CDK8 and SRB10/11 cyclin-CDK regulation of RNA polymerase II (20,21).We studied the effects of stimulatory and inhibitory signals on cell cycle components expressed in B lymphocytes representative of two different maturation stages of development. A robust immune system has to deliver specific and effective immune responses to foreign antigens and yet be immunologically tolerant of self-antigens. Such tolerance is achieved because T and B cells pass through stages in their development when ligation of their antigen receptors by self-antigens results in negative regulatory signals that induce either unresponsiveness and functional inactivation (clonal anergy) or their physical elimination (clonal deletion) (22-2...