Since Ramon y Cajal, neuroscientists have speculated that even the most complex brain functions might eventuallybeunderstoodatthelevelofneuronalcelltypesandtheirconnections.Unfortunately,ourunder? standingofthebrain'sextensivediversityofcelltypesandelaboratecircuitryremainsfarfromcomplete.For example,despitethehippocampusbeingessentialforlearningandmemoryanditsdysfunctionunderlyinga widevarietyofdevastatingneuropsychiatricdisorders,westilllackfundamentalknowledgeaboutitsconstit? uentcelltypesandtheirmolecularprofilesandconnectivity,slowingprogresstowardsamechanisticcircuit? levelunderstandingofmemoryformation.WeproposetoidentifyalloftheinterneuroncelltypesinareaCA1 ofthehippocampusanddeciphertheircanonicalconnectivity.Wehaveassembledastronginterdisciplinary team capable of combining cutting?edge technologies including high?throughput multi?cell patch recordings, morphologicalreconstructions,single?cellRNAsequencing,andmachinelearningtoachievetwomaingoals: 1)dissectCA1microcircuitorganizationbygeneratingamorphologicalandelectrophysiologicaltaxonomyof allinterneuroncelltypesinmouseCA1andmaptheirconnections;and2)derivetranscriptomicsignaturesof morphologicallydefinedCA1interneuronsusingournewlydevelopedPatch?seqmethod.Ourteamrecently implemented this interdisciplinary experimental strategy to successfully identify cell types in the neocortex anddissecttheirmicrocircuitry.Usingmulti?cellpatchclamprecording,wewillcharacterizetheelectrophysi? ologicalproperties,morphology,laminarlocationandconnectivityofthousandsofneuronsfrommouseCA1 hippocampus.Wewillalsoutilizeanew,highlysensitive,costeffectivetechniqueforsingle?cellRNAsequenc? ing(Smart?seq2)tomaptheirtranscriptomesandmachinelearningtechniquestoclassifycellsintomolecular types. Importantly, our high?throughput method will allow us to obtain morphological, electrophysiological and complete transcriptome information for single neurons, which cannot be achieved using other methods suchasdissociatingtissueforsingle?cellsequencing.In?housecustomizationandautomationhasreducedour sequencingcosts,enablingustosequencethousandsofcellswithinareasonablebudget.Celltypeidentifica? tionwillbevalidatedusingmorphological,electrophysiological,andmoleculartests.Identifyingallofthein? terneuron cell types that comprise CA1 and determining how they connect to each other will have a broad, paradigm?shiftingimpact.Forinstance,itwillcontributetoacircuit?levelunderstandingofthecomputations thattakeplaceinCA1,suchasmemoryformation.Fromaclinicalperspective,single?celltranscriptomedata willyieldapowerfulatlastoinvestigaterelationshipsbetweencelltypesandgenesrelatedtoneuropsychiatric diseasesandfacilitatecell?typeengineering.
The hippocampus is a complex brain structure that mediates remarkable mental functions like learning and memory,whileitsmalfunctionisassociatedwithdevastatingneuropsychiatricdiseasesincludingAlzheimer's disease,schizophrenia,depression,andtemporallobeepilepsy.Formorethanacentury,neuroscientistshave marveled at the extensive diversity of neuronal cell types that comprise the hippocampus and other brain structures, yet we still know very little about their identity and how they are wired together to mediate informationprocessinginthebrain.Weproposeanovelresearchapproachthatwillenableustoidentifythe different interneuron cell types in the CA1 region of the hippocampus and determine their connectivity diagram, which will lead to a better circuit?level understanding of hippocampal function and more effective strategiesforthetreatmentofneuropsychiatricdisorders.
