Ubiquitination of Bag-1

Intense apoptoses occur during the morphogenesis period of embryonic development. In the adult, and in the absence of genotoxic aggressions or degenerative pathologies, normal apoptoses strongly decrease in most organs and become restricted to self-renewing tissues containing progenitor stem cells, allowing continuous cell replacement. This is the case in the olfactory neuroepithelium that line the deep nasal cavity, which predominantly consists of odorant-receptive neurons. These sensory neurons project their axons via olfactory nerve towards a unique synaptic target, the ipsilateral olfactory bulb (Caggiano et al., 1994; Graziadei, 1973). The olfactory epithelium regenerates from local progenitors, termed globose basal cells, which reside in the depth of the epithelium and that exclusively give rise to neurons (Caggiano et al., 1994). It also regenerates from the remarkable ability of olfactory glial cells to stimulate axon growth (Franklin and Barnett, 2000). Adult olfactory neurons are shown to die via apoptosis either spontaneously (Mahalik, 1996) or after target removal (Michel et al., 1994; Holcomb et al., 1995). This population of sensory neurons is therefore more liable to go into apoptosis in vivo than other neurons of the adult nervous system. Several features make the sensory neurones amenable to studies on the mechanisms controlling the apoptotic process. First, apoptosis-liable neurons make up more than 80% of olfactory epithelium (Caggiano et al., 1994). Second, mature olfactory neurons can be made to go easily into apoptosis following bulbectomy (Graziadei, 1973). The wave of neuronal apoptosis is acute and terminates before stimulation of DNA replication in the progenitor cells (Schwartz-Levey et al., 1991; Michel et al., 1994). Third, the bulk of olfactory organ RNA can be easily and reproducibly sampled and analyzed (Michel et al., 1994; Michel et al., 1997; Farbman et al., 1999; Kastner et al., 2000). In addition, an immortalized rat olfactory neuronal cell line 13.S.1.24 exists that can be used as an in vitro model of apoptosis induction in olfactory neuron lineage (Coronas et al., 1997). In studies to determine what genes can regulate the olfactory neuronal apoptotic program, we investigated the action of the Bcl-2 associated factor Bag-1 because of its known effects in downregulating apoptosis. Bag-1 was originally identified as a Bcl-2-associated athanogene through its ability to interact with Bcl-2 (Takayama et al., 1995). It was found to enhance the anti-apoptotic action of Bcl-2 but was also shown to inhibit on its own apoptosis induced by several agents including staurosporine, hormones and growth factor withdrawal (Takayama et al., 1995; Clevenger et al., 1997). The mechanism by which Bag-1 exhibits its anti-apoptotic function is not known, but this protein has been shown to associate with several other factors involved in cell survival such as steroid receptors (Froesch et al., 1998), membrane-anchored HGF and PDGF receptors (Bardelli et al., 1996), the protein kinase Raf-1 (Wang et al., 1996), the protein chaperones hsc70/hsp70 (Zeiner et al., 1997) and the ring finger protein Siah-1A (Matsuzawa et al., 1998). In this communication we have asked whether Bag-1 is involved in regulating the onset of olfactory neuronal apoptosis. Our results show an inverse correlation between the onset of apoptosis and the level of Bag-1. Bag-1 1409