CNP-induced cGMP signalling in neural differentiation and functional integration

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A cGMP signalling pathway triggered by binding of the secreted factor C-type natriuretic peptide (CNP) to its receptor guanylyl cyclase B (GC-B) has been linked to a remarkable variety of physiological functions. Our own investigations demonstrated that the bifurcation of axons from neurons of embryonic dorsaI root ganglia at the dorsal root entry zone of the spinal cord critically depends on a cGMP signalling pathway comprising CNP, GC-B and cGMP-dependent protein kinase I alpha (cGKI alpha). In the first funding period, we established a GC-B-LacZ-reporter mouse line for expression mapping and a GC-B-CreERT2 mouse line that in combination with an appropriate conditional reporter enables the visualization of individual GC-B-expressing neurons. We could demonstrate that, similar to DRG neurons, the CNP/GC-B/cGKI alpha signalling pathway also regulates axon bifurcation of neurons from cranial sensory ganglia at the embryonic hindbrain. The analysis of the expression patterns of CNP and GC-B in the central nervous system utilizing the respective LacZ-reporter mouse lines revealed an interesting distribution of the ligand and its receptor in the hippocampus, a brain structure involved in memory formation and prominent for adult neurogenesis. We determined the neuronal subpopulations that express CNP and GC-B in the hippocampus and the developmental time course of those expression patterns. Thereby, we observed an age-related decrease in the number of GC-B-expressing dentate granule cells (DGCs) in the hippocampal formation. Immunocytochemical studies following the application of BrdU to GC-B-LacZ-reporter mice demonstrated a replenishment of GC-B-positive DGCs in the adult brain. Moreover, we have established a genetic approach for fluorescent labelling of GC-B-positive DGCs with the aim to characterize their electrophysiological properties and connections. Preliminary results suggest a role of GC-B in the control of DGC excitability. These studies will be continued and complemented with behavioural tests. For this purpose, we have generated a conditional GC-B mouse model allowing tissue-specific inactivation of GC-B which will also be instrumental in collaborations with other groups within this network who will study GC-B function in the auditory system and in pain processing. Expression profiling of genetically-labelled neurons from GC-B heterozygous vs GC-B KO mice will be performed to identify further molecular components of cGMP signalling in GC-B-positive neurons and cGMP-dependent transcriptional programmes. These studies will further clarify the role of GC-B in the process of neuronal growth and synaptic integration and will improve our understanding of the neurological consequences of impaired cGMP signalling.