In a study published in Cell on July 10, a team from the Center for Excellence in Brain Science and Intelligence Technology (CEBSIT) of the Chinese Academy of Sciences, along with a team from the HUST-Suzhou Institute for Brainsmatics reported the first comprehensive study of whole-brain projectomes of the macaque prefrontal cortex (PFC) at the single-neuron level and revealed the organization of macaque PFC connectivity. By comparing macaque and mouse PFC single-neuron projectomes, they revealed highly refined axon targeting and arborization in primates.
The PFC in primates including human has dramatically expanded over the course of evolution, which is believed to be the structural basis of high cognitive functions. Previous studies of PFC connectivity in non-human primates have mainly relied on population-level viral tracing and functional magnetic resonance imaging (fMRI), which in general lack single-cell resolution to examine projection diversity. Meanwhile, whole-brain imaging data for tracing axons in the primate brain are massive in size.
Dr. YAN Jun's team from CEBSIT has developed Fast Neurite Tracer (FNT) which enables the reconstruction of single-neurons projectomes in mouse PFC from TB-sized optical imaging datasets, and a high throughput single-neuron reconstruction system called Gapr which accelerates projectome reconstruction by integrating PB-sized data processing, AI-algorithm-based automatic reconstruction and large-scale collaborative proofreading, laying the groundwork for mapping single-neuron projectomes in the primate brains.
In this study, the researchers performed sparse labeling of single neurons in 19 subregions of the prefrontal cortex in macaques (Macaca fascicularis) by viral infection, and conducted fluorescence micro-optical sectioning tomography (fMOST) imaging. They realized large-scale three-dimensional (3D) reconstruction of whole-brain axon projectomes using Gapr system, yielding 2,231 single-neuron projectomes of the macaque PFC.
Based on axonal morphology in the whole brain, these neurons were classified into 32 projection subtypes, each exhibiting distinct projection pattern targeting different brain regions including the parietal and temporal cortices, contralateral hemisphere, striatum, thalamus, midbrain, and pons. Further AI-based functional predictions suggested that these projection subtypes were closely associated with sensory, motor, emotional, cognitive, and memory-related higher-order functions.
Through in-depth analysis of single-neuron projectomes in the macaque PFC, the researchers uncovered a set of organizational principles. Distinct PFC neuron subtypes are projected to either the parietal or temporal lobes, and neurons in different PFC subregions are projected to different subregions within these targets. The macaque PFC exhibited a modular network of intra-PFC connections, which may provide a structural basis for working memory.
Moreover, patchy terminal arbors were found in the PFC projections to the striatum and contralateral cortex in macaque but not in mouse, highlighting more spatially refined innervation pattern in the primate brain. Macaque PFC neurons with bilateral projections showed a stronger preference of contralateral targeting compared to those in mice, suggesting functional specialization of neurons projecting to the contralateral hemisphere in primates. The PFC neurons projecting to subcortical areas displayed a topographic relationship between their somata with their targets, suggesting differential downstream innervations across different prefrontal regions.
Through a systematic comparison of the single-neuron projectomes between macaque and mouse PFC, the researchers revealed that macaque PFC neurons shared similar target specificity but exhibited distinct morphological features including longer axon trunks, fewer collateral branches, and relatively smaller axon terminal arbors. These showed that compared to rodent neurons, primate neurons possessed simpler structures, more restricted projection targets, and more spatially refined innervation pattern. Such highly modular and selected connectivity of the primate PFC may provide the structural foundation for the emergence of advanced cognitive and executive functions in primate brains.
This study reveals the structural basis of the evolution of higher cognitive functions in the primate brains, provides important clues for exploring the neural origins of psychiatric disorders in human brain, and may inspire new design of artificial intelligence.
