Stellate cells drive maturation of the entorhinal-hippocampal circuit

INTRODUCTION Maturation of cortical circuits depends on postnatal excitatory activity that spreads in a bottom-up manner from the sensory organs. Such activity is necessary for the formation and stabilization of computationally efficient synaptic networks. Whether similar activity is required to set up connectivity across the entire cortex, beyond the sensory regions, remains to be determined. In one high-end associative network involving the entorhinal cortex and hippocampus, the spatially tuned firing of place, grid, border, and head direction cells emerges during a protracted period of postnatal life. The appearance of a functional map of self-location in this system may reflect local computations enabled by the gradual maturation of local network topology. RATIONALE If spatial tuning arises as a consequence of specific network topology, then the emergence of functionally specific cell types during development should coincide with network maturation. We investigated microcircuit development during the second to fourth postnatal week in mice when the network of functionally specific cells in the medial entorhinal cortex takes on adult-like firing properties. Network-wide changes in the expression of maturation-related molecular markers (doublecortin, parvalbumin, and bassoon-positive synaptic puncta) were measured across multiple areas to reveal the temporal profile of maturation across subfields of the entorhinal-hippocampal circuit. To determine how circuit development is influenced by excitatory activity across subfields of the network, we used a targeted pharmacogenetic approach by which cells in different regions, with different projection patterns or different birth dates, could be silenced selectively during defined time windows. RESULTS Maturation of the hippocampal-entorhinal network followed a stereotyped sequence, where layer 2 of the medial entorhinal cortex (MEC-L2) was the first area to mature, followed by (in chronological order) CA3, CA1, dentate gyrus (DG), subiculum (SUB), layer 5 of the medial and lateral entorhinal cortex (MEC-L5 and LEC-L5, respectively), and, finally, layer 2 of the lateral entorhinal cortex (LEC-L2). At each stage of the circuit, excitatory activity was necessary for the maturation of downstream areas of the network, pointing to a linear and directional developmental sequence. This sequence originated in MEC-L2, where maturation of stellate cells preceded that of any other excitatory cell type in the circuit. Cell type–specific pharmacogenetic silencing revealed that activity in MEC-L2 stellate cells was necessary for maturation of the rest of the entorhinal-hippocampal network. Maturation of stellate cells themselves was independent of local excitatory activity but depended instead on when the stellate cells were born. Silencing layer 2 pyramidal cells had no effect on circuit maturation. The stellate cell–dependent arrest of maturation was more efficient if the silenced cells were born on the same day, suggesting that isochronic cohorts of stellate cells act synergistically to drive network maturation. CONCLUSION Our data show that the entorhinal-hippocampal circuit matures in a linear sequence that begins with stellate cells in MEC-L2 and propagates sequentially through the subfields of the transverse entorhinal-hippocampal circuit, recapitulating the flow of excitation through the circuit in adult animals. Maturation of stellate cells themselves may be determined intrinsically in a neurogenesis-dependent manner. The dependence on neurogenesis may parcel the stellate network into cohorts of isochronic neurons that act synergistically during development and give rise to independent subnetworks in the circuit. We propose that, during development, stellate cells are the source of an activity-dependent instructive signal that drives maturation of the entorhinal-hippocampal network underlying spatial representation in the brain. A small number of autonomously developing neuronal populations, similar to entorhinal stellate cells, may function as intrinsic drivers of maturation in different regions of the cortex.