Unusual regulation of cyclin D1 and cyclin-dependent kinases cdk2 and cdk4during in vivo mitotic stimulation of olfactory neuron progenitors in adult mouse

The molecular mechanisms underlying cell cycle control in neuronal progenit ors have been investigated with adult mouse olfactory epithelium as a model system. Odor-receptive neurons of mammalian olfactory epithelium are short -lived and renewed in the adult by mitotic division of intrinsic neuronal p rogenitors. Ablation of the synaptic target, olfactory bulb, induces sequen tially extensive apoptosis of sensory neurons and then stimulation of proge nitor proliferation, peaking at 36 h and 4 days, respectively, postlesion. Known molecular effecters of G1 phase entry have been assessed on protein e xtracts of olfactory organs sampled at various postbulbectomy times in adul t mice, The decay of beta III-tubulin and olfactory marker protein levels a nd the rise of proliferating cell nuclear antigen (PCNA) levels, starting 1 and 3 days, respectively, postlesion, provided the kinetic frame of neuron al dynamics. Cyclin D1, cyclin E, and cyclin-dependent kinase cdk2 levels, low in olfactory organ of intact mice, increased 3 days after bulbectomy in parallel with PCNA levels; cdk4 content was initially high and unaffected by lesioning. Western blots of the known cdk inhibitors revealed proliferat ion-related decreases of p18, p21, and p27 from high expression in intact o rgans. Immunoprecipitation of cdk2 and cdk4 fractions of protein extracts a t 4 days postlesion (mitotic reaction peak) versus control, followed by cyc lin D1 immunoblotting, and vice versa, revealed that levels of both cyclin D1/cdk2 and cyclin D1/cdk4 complexes, as well as their kinase activities, w ere dramatically increased after lesion. In vivo proliferation of olfactory neuronal lineage cells thus involves functional binding of cyclin D1 with cdk2 and cdk4, with differential activation mechanisms for cdk2 and cdk4. I n addition, the RT-PCR-detected cyclin D1 mRNA level remained unaffected af ter bulbectomy, which indicated that the cyclin D1 rise should involve post transcriptional mechanisms in this in vivo neuronal system. These observati ons are discussed, along with their relevance to cell cycle control and to olfactory neuron dynamics.