Ethanol hypersensitivity and olfactory discrimination defect in mice lacking a homolog of Drosophila neuralized

Abstract

Neurogenic genes in the Notch receptor-mediated signaling pathway play important roles in neuronal cell fate specification as well as neuronal differentiation. The Drosophila neuralized gene is one of the neurogenic genes. We have cloned a mouse homolog of Drosophila neuralized, m-neu1, and found that the m-neu1 transcript is expressed in differentiated neurons. Mice deficient for m-neu1 are viable and morphologically normal, but exhibit specific defects in olfactory discrimination and hypersensitivity to ethanol. These findings reveal an essential role of m-neu1 in ensuring proper processing of certain information in the adult brain.M any molecular mechanisms controlling neural development and neural function are evolutionarily conserved (1-3). One example is the Notch-mediated cell-cell interaction, a mechanism to mediate signaling between adjacent cells and thereby to specify cell fates in organisms ranging from worms and flies to mammals (4). In a given organism, the Notch signaling mechanism functions in many developmental processes as well as in adult life (5, 6). In the nervous system, the Notch signaling mechanism is involved in neurogenesis (1, 3), neuronal differentiation (2, 6), axon path finding (7,8), and neurite growth (9, 10). The importance of Notch in maintaining normal neuronal function in the adult has been revealed by a number of human neurological syndromes because of defects in Notch signaling (6).Like Notch, the Drosophila neuralized gene (neu) is a neurogenic gene whose function is to limit the number of neuronal precursor cells and to specify sensory organ as well as R8 photoreceptor cell fates (11)(12)(13)(14)(15)(16)(17). Genetic analyses suggest that neuralized interacts with other neurogenic genes, such as the Notch receptor and the Delta ligand for cell-cell interaction (18). Recent studies (15)(16)(17) revealed that neuralized functions cellautonomously to regulate a subset of Notch-dependent processes. How neuralized affects Notch signaling is unknown at present. An interesting possibility is that Neu protein with its C3HC4 RING finger domain functions as an E3 ubiquitin ligase to modulate Notch signaling (15)(16)(17)19).We have cloned a mouse homolog of Drosophila neuralized, m-neu1, and found that it is expressed in differentiated neurons. We examined its functional role in the nervous system by generating a loss-of-function allele of m-neu1. Whereas the m-neu1 null mutants exhibit normal performance in a number of behavioral tests, including Morris water maze assay for learning and memory, they display specific functional lesions in olfactory discrimination and ethanol effects on motor coordination. These studies identify specific functions of the adult brain that are critically dependent on m-neu1 activity. Materials and MethodsIsolation of a Mouse Neuralized Homolog. A mouse midgestation embryonic cDNA library (Stratagene) was screened with fragments of Drosophila neuralized cDNA as probes. In brief, the duplicated nylon filters containing mouse phage cDNAs were prehybridized...

Neuralized-2 Regulates a Notch Ligand in Cooperation with Mind Bomb-1

“…Targeted disruption of notch1 expression or disruption of its downstream targets in mice leads to embryonic lethality by developmental defects (43)(44)(45)(46). However, mice with disrupted neur1 exhibit no abnormal cell fate specifications during neurogenesis and somitogenesis, two processes in which Notch signaling has been shown to be involved (25,26). This discrepancy might be due to functional redundancy provided by Neur2.…”

“…1, A and B). Mouse Neur1 and Neur2 both have two Neuralized homology repeat (NHR) domains and a RING finger domain (25,26,30). Furthermore, comparison between mouse Neur1 and Neur2 subdomains showed a higher degree of homology, with 49, 42, and 70% identities in the NHR1, NHR2, and RING finger domains, respectively (data not shown).…”

“…neur2 was expressed at high levels in the brain and kidney and at low levels in the heart (Fig. 1F), whereas neur1 was expressed at high levels in the brain and skeletal muscle (25,26). The different expression patterns of these two genes suggest distinct roles of Neur1 and Neur2 in adults.…”

“…Recent studies have suggested that DNeur is a ubiquitin-protein isopeptide ligase (E3) that promotes the internalization and degradation of Delta to activate the Notch signaling pathway (10,11,24). However, mice with a disrupted neur1 (neur1 was heretofore known as neuralized in mice) gene are viable; they do not show any Notch-related phenotype during embryogenesis but do exhibit defects as adults, including impaired olfactory discrimination, hypersensitivity to ethanol, and abnormalities both in axoneme organization in the spermatozoa and in mammary gland maturation during pregnancy (25,26). Thus, we speculate that other vertebrate neur homolog(s) might compensate for loss of the neur gene in mammals based on the conservation between Drosophila and mammalian Notch signaling.…”

Endocytosis in Notch Signaling Activation

“…Quite surprisingly, inactivation of these genes does not result in major phenotypic defects [240][241][242]. Only subtle defects were scored in Neurl1 -/-mice: (i) male mutants are sterile due to a defect in the axonemal organization of spermatozoa that leads to immotile sperm [242]; (ii) female KOs are defective in the final stages of mammary gland maturation during pregnancy [242]; (iii) Neurl1 -/-mice are hypersensitive to ethanol effects on motor coordination and exhibit a defect in olfactory discrimination [241]. Only these latter defects can be putatively connected to an impairment of some subtle (yet to be defined) function of Notch in mammalian neurons, but no classical Notch signaling defects are identifiable in these mutants.…”