Activins are dimeric (beta A beta A; beta B beta B; beta A beta B) members of the transforming growth factor-beta superfamily. They are widely expressed during murine development, are highly conserved during vertebrate evolution, and may be involved in mesoderm induction and neurulation in Xenopus laevis and Oryzias latipes. To investigate the function of mammalian activins in vivo, we generated mice with mutations either in activin-beta A or in both activin-beta A and activin-beta B. Activin-beta A-deficient mice develop to term but die within 24 h of birth. They lack whiskers and lower incisors and have defects in their secondary palates, including cleft palate, demonstrating that activin-beta A must have a role during craniofacial development. Mice lacking both activin subunits show the defects of both individual mutants but no additional defects, indicating that there is no functional redundancy between these proteins during embryogenesis. In contrast to observations in lower vertebrates, zygotic expression of activins is not essential for mesoderm formation in mice.

SUMMARY The repertoire of ~1200 odorant receptors (ORs) is mapped onto the array of ~1800 glomeruli in the mouse olfactory bulb (OB). The spatial organization of this array is influenced by the ORs. Here we show that glomerular mapping to broad domains in the dorsal OB is determined by two types of olfactory sensory neurons (OSNs), which reside in the dorsal olfactory epithelium. The OSN types express either Class I or Class II OR genes. Axons from the two OSN types segregate already within the olfactory nerve and form distinct domains of glomeruli in the OB. These class-specific anatomical domains correlate with known functional odorant response domains. However, axonal segregation and domain formation are not determined by the class of the expressed OR protein. Thus, the two OSN types are determinants of axonal wiring, operate at a higher level than ORs, and contribute to the functional organization of the glomerular array.

Odorant receptors (ORs) provide the core determinant of identity for axons of olfactory sensory neurons (OSNs) to coalesce into glomeruli in the olfactory bulb. Here, using gene targeting in mice, we examine how the OR protein determines axonal identity. An OR::GFP fusion protein is present in axons, consistent with a direct function of ORs in axon guidance. When the OR coding region is deleted, we observe OSNs that coexpress other ORs that function in odorant reception and axonal identity. It remains unclear if such coexpression is normally prevented by negative feedback on OR gene choice. A drastic reduction in OR protein level produces axonal coalescence into novel, remote glomeruli. By contrast, chimeric ORs and ORs with minor mutations perturb axon outgrowth. Strikingly, the beta2 adrenergic receptor can substitute for an OR in glomerular formation when expressed from an OR locus. Thus, ORs have not evolved a unique function in axon guidance.

An olfactory sensory neuron (OSN) expresses selectively one member from a repertoire of approximately 1000 odorant receptor (OR) genes and projects its axon to a specific glomerulus in the olfactory bulb. Both processes are here recapitulated by MOR23 and M71 OR minigenes, introduced into mice. Minigenes of 9 kb and as short as 2.2 kb are selectively expressed by neurons that do not coexpress the endogenous gene but coproject their axons to the same glomeruli. Deletion of a 395 bp upstream region in the MOR23 minigene abolishes expression. In this region we recognize sequence motifs conserved in many OR genes. Transgenic lines expressing the OR in ectopic epithelial zones form ectopic glomeruli, which also receive input from OSNs expressing the cognate endogenous receptor. This suggests a recruitment through homotypic interactions between OSNs expressing the same OR.