Taken together, our findings reveal a hierarchical gradient of corticostriatal microstructural covariance that potentially underlies the functional specialization of corticostriatal neurocircuits. First, the corticostriatal microstructural covariance derived from the T1w/T2w map provides a hierarchical gradient along the anterior-posterior axis of the striatum. Second, the anterior parts of the corticostriatal hierarchy highly align with anatomic properties and the cell type-specific gene expression profiles across the human cortex, whereas these relationships diminish rapidly in posterior parts of the corticostriatal hierarchy. Third, the notable transcriptomic distinctions between the anterior and posterior parts of corticostriatal microstructural covariance indicate the critical roles of hierarchical gradients in the specific functions of corticostriatal networks. The spatiotemporal trajectories of transcriptomic signatures revealed that corticostriatal neurocircuits are affected by neurodevelopment. Additionally, the concordance between transcriptomic-level analyses and fMRI assessments suggests that corticostriatal neurocircuits are implicated in neuropsychiatric risks.
Recent MRI study have reported that the myelin content in the striatum shows a spatial gradient along the anterior-posterior axis, showing associations with cortical hierarchy demonstrated by corticostriatal microstructural covariation[15]. This finding is consistent with our observations that the myelin content in the anterior part of the striatum highly covaried with that in anterior parts of the cerebral cortex, such as the prefrontal, insular and limbic cortex. However, this covarying relationship attenuated in the posterior part of the striatum. Indeed, the myelin content in the human cortex follows a rostrocaudal gradient, indicating that the anterior cortex exhibits lower myelin content and more complex intracortical circuits[46]. This finding suggested that the anterior striatum is more likely to structurally connect to the anterior cortex, which supports functional specialization in the human brain. Notably, task fMRI studies have reported that the dorsomedial striatum, which is enhanced with rostrocaudal hierarchical organization within the lateral prefrontal cortex, supports increased levels of cognitive processing[47]. Furthermore, resting-state functional connectivity studies recapitulated the striatal functional hierarchy, indicating that the striatum contributes to processing motor, limbic, and heteromodal information through multiple large-scale corticostriatal circuits[13]. We also revealed that the patterns of corticostriatal microstructural covariation show gradient associations with cortical geodesic distance, demonstrating that hierarchical gradient corticostriatal microarchitecture may be constrained in anatomic properties. Thus, these findings suggest that the corticostriatal microarchitecture depends on human anatomic properties and could potentially play a significant role in shaping cognitive behaviors in humans.
The specific function of the corticostriatal circuitry may derive from neural projections between the cortex and striatum. For instance, excitatory neurons projecting to the striatum are predominantly located in the sensorimotor, associative and limbic cortical regions and play roles in motor functions, sensory processing and reward pathways[48–50]. Cerebral inhibitory interneurons can also shape corticostriatal function via synaptic connectivity[51], such that GABAergic interneurons in primates can establish long-range corticostriatal projections to affect physiological function[52]. On the other hand, the neural pathway from the prefrontal cortex to the dorsal striatum has been implicated in inhibitory control, suggesting a potential role in mediating cognitive behavioral control[53]. In the present study, we found that the anterior parts of corticostriatal covariance are strongly correlated with the cell type-specific gene expression profiles, such as excitatory and inhibitory neurons. Thus, the present findings provide evidence that excitatory and inhibitory neurons shape corticostriatal pathways at the genetic level. Notably, we also observed that the anterior parts of corticostriatal covariance are associated with the gene expression of astrocytes and microglia, indicating that these specific cell types also mediate cortical architecture via corticostriatal circuitry. Consistent with our findings, a previous study reported that astrocytes are a potential source of new neurons in the adult mammalian brain and may engage in a neurogenic program to provide excitatory input to the striatal circuitry[54]. In addition, microglial neurons have been suggested to be involved in the regulation of ongoing structural and functional processes though corticostriatal neural circuits[55]. Therefore, these findings suggested that cell type-specific neurons shape specific functions and contributes to the formation of hierarchical gradients in corticostriatal connectivity.
Distinct compartments of striatal tissue exhibit variations in dopaminergic and cholinergic expression along the main axes of the striatum[22–24], which are believed to underlie the organization of striatal connectivity[23, 56, 57]. Remarkably, our PLS analysis revealed significant differences in transcriptomic specialization, particularly in gene expression among excitatory, astrocytic, and inhibitory neurons, between the anterior and posterior striatum. These differences may explain why differences in neural projections along the anterior-posterior axis of the striatum are responsible for different aspects of human behaviors[58] and brain diseases[59]. In addition, PLS-1 in both the anterior and posterior striatum was mainly involved in ion transport and transmembrane transport, while PLS-1 in the posterior striatum was also related to the regulation of neurotransmitter levels and chemical synaptic transmission. Ion transport plays a critical role in maintaining ion homeostasis in the human body[60] and is implicated in neuropsychiatric disorders[61–63]. Dysfunctions in neurotransmitter regulation and chemical synaptic transmission in the brain are also considered to affect cognitive function in humans[64, 65]. For PLS-2, transcriptional specializations in the anterior striatum are primarily involved in behavior, synaptic signals, learning or memory, whereas those in the posterior striatum are mostly related to the regulation of transport, transsynaptic signaling and neurodevelopment. Our results further underscored the enrichment of both PLS-1 and PLS-2 in neuropsychiatric conditions, including schizophrenia, bipolar disorder, depressive disorders and mood disorders. Additionally, neural pathways from the medial frontal and limbic cortex to the striatum have been found to be associated with neuropsychiatric risks, such as depression, schizophrenia and bipolar disorders, as supported by both transcriptional correlation and fMRI analyses in the present study. Overall, these findings confirmed that circuits originating from distinct compartments of the striatum to the cerebral cortex may contribute to different functions and affect neuropsychiatric diseases.
The structural development of the brain extends into adulthood, followed by gradual aging of both cortical and subcortical structures[66]. Interindividual differences in brain development, particularly during periods from childhood to adulthood, have been linked to cognitive function[67, 68] and the risk of neuropsychiatric diseases[66, 69, 70]. Moreover, advancements in myelin-sensitive MRI study have revealed that the psychiatric traits of compulsivity and impulsivity are associated with attenuated developmental fronto-striatal myelination trajectories[71]. Task fMRI studies have also shown that the development of corticostriatal connectivity is associated with cognitive behaviors, such as goal-directed and reward-driven behaviors[72, 73]. Our findings illustrated both PLS-1 and PLS-2 weighted gene expression changes during neurodevelopment, revealing the spatiotemporal trajectories of transcriptomic signatures in corticostriatal microstructural covariance. Specifically, the analysis of PLS-1 weighted gene expression revealed a developmental cup-shaped pattern. This discovery aligns with a functional genomic analysis of human brain development which proposed a cup-shaped pattern of interregional transcriptomic variances[35]. Moreover, it is intriguing to note that PLS-2 weighted gene expression decreases in both the left and right posterior caudate throughout neurodevelopment, which diverges from the S-shaped pattern observed in other parts of the striatum. Previous studies have suggested that neurodevelopment in the anterior part of the caudate nucleus may begin earlier than in other brain regions, which undergo rapid growth and differentiation in the early stages of development. In contrast, neurodevelopment in the posterior part of the caudate nucleus may be slower and more prolonged[74, 75]. Thus, PLS-2 weighted gene expression may explain the difference in neurogenesis between the anterior and posterior caudate. Taken together, these findings collectively suggested that corticostriatal circuits may be influenced by interregional transcriptomic differences and may play a role in neuropsychiatric risk during the course of human brain development.
Several limitations of the present study should be noted. First, the current findings are based on small samples of donor brains. More comprehensive microarray gene expression datasets are necessary for future studies to ensure a more robust and reliable understanding of the genetic underpinnings of the observed phenomena. Second, spatiotemporal trajectories of corticostriatal microstructural covariance were analyzed based on BrainSpan data, which are limited by small cortical genetic samples. Last, in the split-half validation analysis, large deviations were observed between two random groups in the posterior caudate, possibly due to the extremely attenuated covarying relationships between the posterior striatum and the entire cerebral cortex compared to the anterior striatum. Thus, resting-state fMRI data should be used to further validate this phenomenon in future studies.
In summary, our study revealed a hierarchical gradient in corticostriatal microstructural covariance. By utilizing cell type-specific transcriptomic profile data, we illustrated that the hierarchical gradient of corticostriatal circuits is influenced by transcriptomic specialization, undergoes alterations during neurodevelopment, and ultimately plays a role in neuropsychiatric risks. These collective findings underscore the potential mechanism by which alterations in the hierarchical gradient of the corticostriatal network during neurodevelopment may be associated with neuropsychiatric risks.