Dentate Mossy Cells Control Memory Precision

Background: The hippocampus is part of the limbic system located between the thalamus and medial temporal lobe of the brain which plays a critical role in spatial navigation and episodic memory.In general,memory can be divided into highly accurate(Memory Precision)or more generalized(Memory Generalization)types.Studies on cell damage and tissue resection revealed that the memory precision-mediated neurons mainly existed in cerebral cortex and the dentate gyrus.The dentate gyrus,a key component of the hippocampus,contains about 2.5 to 3million neurons which are mainly divided into excitatory granule cells(GCs)and mossy cells(MCs),along with a subset of GABAergic inhibitory neurons.Due to the lack of a reliable method to identify and distinguish MCs from the other cell types,the specific function and circuit of MCs in the dentate gyrus remains unknown.Objectives: To establish the method for the identification of mossy cells,to confirm the specific functions and related circuits of mossy cells and to clarify the regulatory mechanism of mossy cells in memory precision.Methods: Mossy Cells(MCs)were specifically labeled and identified by constructing CR-cre mutant mice combined with Cre recombinase-dependent virus injection and immunofluorescence staining.In this study,3-month-old CR-Cre mice were used to inhibit the activity of MCs by infecting adeno-associated virus expressing Kir2.1,and a series of behavioral tests were conducted to explore the related functions of mossy cells in behavior.With the method of monosynaptic anterograde and retrograde tracing,the cell types and distribution of synaptic connections with MCs were investigated.Furthermore,by using patch-clamp combined with optogenetis,we verified the functional properties of the monosynapticconnections.Following the inhibition/activation of MCs circuits through virus injection along with optogenetic manipulation,we investigated the specific mechanism of the circuits in behavior.Results: With the methods of virus injection combined with immunofluorescence labeling,we found that neurons expressing CR protein in the dentate gyrus were excitatory mossy cells(MCs).Following the overexpression of Kir2.1 ion channels in MCs to inhibit MCs,we conducted a series of behavioral screening and found that inactivation of MCs caused the deficits of mice in the two-choice spatial discrimination tasks which indicated the impairment of accurate memory in mice.Through the anterograde and retrograde tracing,we identified the monosynaptic connections between MCs and SST cells and we verified that the connection was direct excitatory connections by whole-cell patch clamping along with optogenetics.By overexpressing Kir2.1 in SST cells,we identified the deficits of memory precision after the inhibition of SST cells.Mice with gene mutation in the double protein recombinase system(CR-Cre/SST-FLP)were treated with rAAV1/2-DIO-Kir2.1/tdT and rAAV1/2-fDIO-ChR2/GFP viruses by combining with optogenetics,which resulted in the expression of Kir2.1 in MCs and ChR2 in SST cells respectively.As a result,the behavioral phenotype of inactivated MCs was explicitly activated by SST cells,indicating that MCs regulated memory precision by establishing monosynaptic connection with SST cells.Conclusions:1.Mossy cells in dentate gyrus are involved in memory precision process in mice.2.Mossy cells establish functional monosynaptic connections with SST cells.3.Mossy cells control the process of memory precision through the monosynaptic connections with SST cells.