Shared Transcriptional Features of Layer 3 Pyramidal Neurons Projecting to the Monkey Dorsolateral Prefrontal Cortex

OBJECTIVE: Layer 3 (L3) pyramidal neurons projecting to the primate dorsolateral prefrontal cortex (DLPFC) convey essential information for core cognitive processes. Alterations in these neurons could contribute to cognitive deficits in schizophrenia. This study was designed to determine whether DLPFC-projecting L3 pyramidal neurons have a distinctive transcriptional profile. METHODS: Laser microdissection was used to capture Nissl-stained and retrogradely labeled L3 pyramidal neurons that project to the DLPFC from the superior temporal cortex or posterior cingulate cortex of eight (four female) young adult (54-60 months of age) macaque monkeys. All samples were subjected to RNA sequencing. The results were compared to 1) data from L3 pyramidal neurons that project to the DLPFC from the posterior parietal cortex (lateral intraparietal cortex/area 7a [LIP/7a]) or contralateral DLPFC and 2) new single-nucleus RNA-sequencing data from multiple cortical regions of two female, young adult (77 and 96 months of age) monkeys and from similar publicly available human data. RESULTS: Numerous genes (N=2,876) were differentially expressed between Nissl-stained L3 pyramidal neurons from the superior temporal cortex and posterior cingulate cortex. In contrast, only 527 genes were differentially expressed between DLPFC-projecting L3 pyramidal neurons from these regions. Transcriptional profiles were concordant between genes enriched in DLPFC-projecting L3 pyramidal neurons from the superior temporal cortex and posterior cingulate cortex and those enriched in DLPFC-projecting L3 pyramidal neurons from LIP/7a and contralateral DLPFC. The 91 genes enriched in samples from all four regions represent a transcriptional profile of DLPFC-projecting L3 pyramidal neurons. This profile was detected in specific clusters in both new monkey data and published human single-nucleus RNA-sequencing data from cortical regions that project to the DLPFC, but not from primary visual cortex (V1), which lacks these projections. CONCLUSIONS: These findings indicate that DLPFC-projecting L3 pyramidal neurons can be identified by a shared transcriptional profile, regardless of cortical region. The transcriptional identification of these and other L3 pyramidal neurons with specific cortical projections will provide the means to determine which specific subtype or subtypes of pyramidal neurons contribute to cognitive impairments in schizophrenia.