Gα_o1 and Gα_o1/Gα_o2 deletion differentially affect hippocampal mossy fiber tract anatomy and neuronal morphogenesis

The heterotrimeric G-protein αo subunit is ubiquitously expressed in the CNS as two splice variants Gα_o1 and Gα_o2, regulating various brain functions. Here, we investigated the effect of single Gα_o1, Gα_o2, and double Gα_o1/2 knockout on the postnatal development of the murine mossy fiber tract, a central pathway of the hippocampal connectivity circuit. The size of the hippocampal synaptic termination fields covered by mossy fiber boutons together with various fiber length parameters of the tract was analyzed by immunohistochemical staining of the vesicular Zinc transporter 3 (ZnT3) or Synaptoporin at postnatal days 2, 4, 8, 12, 16, and in the adult. Ultimately, Gα_o1 knockout resulted in a reduced developmental growth of synaptic mossy fiber terminal fields by 37% in the adult Stratum lucidum and by 30% in the total mossy fiber tract size. Other morphological parameters such as projection length of the infrapyramidal bundle of the tract were increased (+52% in Gα_o1 ^-/- mice). In contrast, Gα_o2 knockout had no effects on the mossy fiber tract. Moreover, by using primary heterozygous and homozygous Gα_o1 knockout hippocampal cultures, we detected a strongly pronounced reduction in axon and dendrite length (-50% and -38%, respectively) as well as axon and dendrite arborization complexity (-75% and -72% branch nodes, respectively) in the homozygous knockout. Deletion of both splice variants Gα_o1 and Gα_o2 partially rescued the in vivo and completely reconstituted the in vitro effects, indicating an opposing functional relevance of the two Gα_o splice variants for neuronal development and synaptic connectivity.