The marmoset: An emerging model to unravel the evolution and development of the primate neocortex: The Marmoset to Model Brain Development

Evolutionary continuity and divergence of auditory dorsal and ventral pathways in primates revealed by ultra-high field diffusion MRI

Auditory dorsal and ventral pathways in the human brain play important roles in supporting speech and language processing. However, the evolutionary root of the dual auditory pathways in the primate brain is unclear. By parcellating the auditory cortex of marmosets (a New World monkey species), macaques (an Old World monkey species), and humans using the same individual-based analysis method and tracking the pathways from the auditory cortex based on multi-shell diffusion-weighted MRI (dMRI), homologous auditory dorsal and ventral fiber tracks were identified in these primate species. The ventral pathway was found to be well conserved in all three primate species analyzed but extend to more anterior temporal regions in humans. In contrast, the dorsal pathway showed a divergence between monkey and human brains. First, frontal regions in the human brain have stronger connections to the higher-level auditory regions than to the lower-level auditory regions along the dorsal pathway, while frontal regions in the monkey brain show opposite connection patterns along the dorsal pathway. Second, the left lateralization of the dorsal pathway is only found in humans. Moreover, the connectivity strength of the dorsal pathway in marmosets is more similar to that of humans than macaques. These results demonstrate the continuity and divergence of the dual auditory pathways in the primate brains along the evolutionary path, suggesting that the putative neural networks supporting human speech and language processing might have emerged early in primate evolution.

The marmoset as a model for investigating the neural basis of social cognition in health and disease

Social-cognitive processes facilitate the use of environmental cues to understand others, and to be understood by others. Animal models provide vital insights into the neural underpinning of social behaviours. To understand social cognition at even deeper behavioural, cognitive, neural, and molecular levels, we need to develop more representative study models, which allow testing of novel hypotheses using human-relevant cognitive tasks. Due to their cooperative breeding system and relatively small size, common marmosets (Callithrix jacchus) offer a promising translational model for such endeavours. In addition to having social behavioural patterns and group dynamics analogous to those of humans, marmosets have cortical brain areas relevant for the mechanistic analysis of human social cognition, albeit in simplified form. Thus, they are likely suitable animal models for deciphering the physiological processes, connectivity and molecular mechanisms supporting advanced cognitive functions. Here, we review findings emerging from marmoset social and behavioural studies, which have already provided significant insights into executive, motivational, social, and emotional dysfunction associated with neurological and psychiatric disorders.

Nonhuman Primates in Translational Research

Nonhuman primates are critically important animal models in which to study complex human diseases, understand biological functions, and address the safety of new diagnostics and therapies proposed for human use. They have genetic, physiologic, immunologic, and developmental similarities when compared to humans and therefore provide important preclinical models of human health and disease. This review highlights select research areas that demonstrate the importance of nonhuman primates in translational research. These include pregnancy and developmental disorders, infectious diseases, gene therapy, somatic cell genome editing, and applications of in vivo imaging. The power of the immune system and our increasing understanding of the role it plays in acute and chronic illnesses are being leveraged to produce new treatments for a range of medical conditions. Given the importance of the human immune system in health and disease, detailed study of the immune system of nonhuman primates is essential to advance preclinical translational research. The need for nonhuman primates continues to remain a high priority, which has been acutely evident during the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) global pandemic. Nonhuman primates will continue to address key questions and provide predictive models to identify the safety and efficiency of new diagnostics and therapies for human use across the lifespan.

Co-registration of Imaging Modalities (MRI, CT and PET) to Perform Frameless Stereotaxic Robotic Injections in the Common Marmoset

The common marmoset has emerged as a popular model in neuroscience research, in part due to its reproductive efficiency, genetic and neuroanatomical similarities to humans and the successful generation of transgenic lines. Stereotaxic procedures in marmosets are guided by 2D stereotaxic atlases, which are constructed with a limited number of animals and fail to account for inter-individual variability in skull and brain size. Here, we developed a frameless imaging-guided stereotaxic system that improves upon traditional approaches by using subject-specific registration of computed tomography (CT), magnetic resonance imaging (MRI) and positron emission tomography (PET) data to identify a surgical target, namely the putamen, in two marmosets. The skull surface was laser-scanned to create a point cloud that was registered to the 3D reconstruction of the skull from CT. Reconstruction of the skull, as well as of the brain from MR images, was crucial for surgical planning. Localisation and injection into the putamen was done using a 6-axis robotic arm controlled by a surgical navigation software (Brainsight™). Integration of subject-specific registration and frameless stereotaxic navigation allowed target localisation specific to each animal. Injection of alpha-synuclein fibrils into the putamen triggered progressive neurodegeneration of the nigro-striatal system, a key feature of Parkinson's disease. Four months post-surgery, a PET scan found evidence of nigro-striatal denervation, supporting accurate targeting of the putamen during co-registration and subsequent surgery. Our results suggest that this approach, coupled with frameless stereotaxic neuronavigation, is accurate in localising surgical targets and can be used to assess endpoints for longitudinal studies.

Light microscopic and heterogeneity analysis of astrocytes in the common marmoset brain

Astrocytes are abundant cells of the central nervous system (CNS) and are involved in processes including synapse formation/function, ion homeostasis, neurotransmitter uptake, and neurovascular coupling. Recent evidence indicates that astrocytes show diverse molecular, structural, and physiological properties within the CNS. This heterogeneity is reflected in differences in astrocyte structure, gene expression, functional properties, and responsiveness to injury/pathological conditions. Deeper investigation of astrocytic heterogeneity is needed to understand how astrocytes are configured to enable diverse roles in the CNS. While much has been learned about astrocytic heterogeneity in rodents, much less is known about astrocytic heterogeneity in the primate brain where astrocytes have greater size and complexity. The common marmoset (Callithrix jacchus) is a promising non‐human primate model because of similarities between marmosets and humans with respect to genetics, brain anatomy, and cognition/behavior. Here, we investigated the molecular and structural heterogeneity of marmoset astrocytes using an array of astrocytic markers, multi‐label confocal microscopy, and quantitative analysis. We used male and female marmosets and found that marmoset astrocytes show differences in expression of astrocytic markers in cortex, hippocampus, and cerebellum. These differences were accompanied by intra‐regional variation in expression of markers for glutamate/GABA transporters, and potassium and water channels. Differences in astrocyte structure were also found, along with complex interactions with blood vessels, microglia, and neurons. This study contributes to our knowledge of the cellular and molecular features of marmoset astrocytes and is useful for understanding the complex properties of astrocytes in the primate CNS.

The nonhuman primate neuroimaging and neuroanatomy project

Multi-modal neuroimaging projects such as the Human Connectome Project (HCP) and UK Biobank are advancing our understanding of human brain architecture, function, connectivity, and their variability across individuals using high-quality non-invasive data from many subjects. Such efforts depend upon the accuracy of non-invasive brain imaging measures. However, 'ground truth' validation of connectivity using invasive tracers is not feasible in humans. Studies using nonhuman primates (NHPs) enable comparisons between invasive and non-invasive measures, including exploration of how "functional connectivity" from fMRI and "tractographic connectivity" from diffusion MRI compare with long-distance connections measured using tract tracing. Our NonHuman Primate Neuroimaging & Neuroanatomy Project (NHP_NNP) is an international effort (6 laboratories in 5 countries) to: (i) acquire and analyze high-quality multi-modal brain imaging data of macaque and marmoset monkeys using protocols and methods adapted from the HCP; (ii) acquire quantitative invasive tract-tracing data for cortical and subcortical projections to cortical areas; and (iii) map the distributions of different brain cell types with immunocytochemical stains to better define brain areal boundaries. We are acquiring high-resolution structural, functional, and diffusion MRI data together with behavioral measures from over 100 individual macaques and marmosets in order to generate non-invasive measures of brain architecture such as myelin and cortical thickness maps, as well as functional and diffusion tractography-based connectomes. We are using classical and next-generation anatomical tracers to generate quantitative connectivity maps based on brain-wide counting of labeled cortical and subcortical neurons, providing ground truth measures of connectivity. Advanced statistical modeling techniques address the consistency of both kinds of data across individuals, allowing comparison of tracer-based and non-invasive MRI-based connectivity measures. We aim to develop improved cortical and subcortical areal atlases by combining histological and imaging methods. Finally, we are collecting genetic and sociality-associated behavioral data in all animals in an effort to understand how genetic variation shapes the connectome and behavior.

The Marmoset: The Next Frontier in Understanding the Development of the Human Brain

Rodent models, particularly mice, have dominated the field of developmental neuroscience for decades, like they have in most fields of biomedicine research. However, with 80 million years since rodents and primates last shared a common ancestor, the use of mice to model the development of the human brain is not without many shortcomings. The human brain diverges from the mouse brain in many aspects and is comprised of novel structures as well as diversified cellular subtypes. While these newly evolved features have no equivalent in rodents, they are observed in nonhuman primates. Therefore, elucidating the cellular mechanisms underlying the development and maturation of the healthy and diseased human brain can be achieved using less complex nonhuman primates. Historically, macaques were the preferred nonhuman primate model. However, over the past decade, the New World marmoset monkey (Callithrix jacchus) has gained more importance, particularly in the field of neurodevelopment. With its small size, twin or triplet birth, and prosocial behavior, the marmoset is an ideal model to study normal brain development as well as neurodevelopmental disorders, which are often associated with abnormal social behaviors. The growing interest in the marmoset has prompted many comparative studies, all demonstrating that the marmoset brain closely resembles that of the human and is perfectly suited to model human brain development. The marmoset is thus poised to extend its influence in the field of neurodevelopment and will hopefully fill the gaps that the mouse has left in our understanding of how our brain forms and how neurodevelopmental disorders originate.

Properties of visually guided saccadic behavior and bottom-up attention in marmoset, macaque, and human

Saccades are stereotypic behaviors whose investigation improves our understanding of how primate brains implement precise motor control. Furthermore, saccades offer an important window into the cognitive and attentional state of the brain. Historically, saccade studies have largely relied on macaques. However, the cortical network giving rise to the saccadic command is difficult to study in macaques because relevant cortical areas lie in deep sulci and are difficult to access. Recently, a New World monkey. the marmoset, has garnered attention as an alternative to macaques because of advantages including its smooth cortical surface. However, adoption of the marmoset for oculomotor research has been limited due to a lack of in-depth descriptions of marmoset saccade kinematics and their ability to perform psychophysical tasks. Here, we directly compare free-viewing and visually guided behavior of marmoset, macaque, and human engaged in identical tasks under similar conditions. In the video free-viewing task, all species exhibited qualitatively similar saccade kinematics up to 25° in amplitude although with different parameters. Furthermore, the conventional bottom-up saliency model predicted gaze targets at similar rates for all species. We further verified their visually guided behavior by training them with step and gap saccade tasks. In the step paradigm, marmosets did not show shorter saccade reaction time for upward saccades whereas macaques and humans did. In the gap paradigm, all species showed similar gap effect and express saccades. Our results suggest that the marmoset can serve as a model for oculomotor, attentional, and cognitive research while we need to be aware of their difference from macaque or human.NEW & NOTEWORTHY We directly compared the results of a video free-viewing task and visually guided saccade tasks (step and gap) among three different species: marmoset, macaque, and human. We found that all species exhibit qualitatively similar saccadic kinematics and saliency-driven saccadic behavior albeit with different parameters. Our results suggest that the marmoset possesses similar neural mechanisms to macaque and human for saccadic control, and it is an appropriate model to study neural mechanisms for active vision and attention.

Dynamic Changes in the Neurogenic Potential in the Ventricular-Subventricular Zone of Common Marmoset during Postnatal Brain Development

Even after birth, neuronal production continues in the ventricular-subventricular zone (V-SVZ) and hippocampus in many mammals. The immature new neurons ("neuroblasts") migrate and then mature at their final destination. In humans, neuroblast production and migration toward the neocortex and the olfactory bulb (OB) occur actively only for a few months after birth and then sharply decline with age. However, the precise spatiotemporal profiles and fates of postnatally born neurons remain unclear due to methodological limitations. We previously found that common marmosets, small nonhuman primates, share many features of V-SVZ organization with humans. Here, using marmosets injected with thymidine analogue(s) during various postnatal periods, we demonstrated spatiotemporal changes in neurogenesis during development. V-SVZ progenitor proliferation and neuroblast migration toward the OB and neocortex sharply decreased by 4 months, most strikingly in a V-SVZ subregion from which neuroblasts migrated toward the neocortex. Postnatally born neurons matured within a few months in the OB and hippocampus but remained immature until 6 months in the neocortex. While neurogenic activity was sustained for a month after birth, the distribution and/or differentiation diversity was more restricted in 1-month-born cells than in the neonatal-born population. These findings shed light on distinctive features of postnatal neurogenesis in primates.

Thalamocortical Afferents Innervate the Cortical Subplate much Earlier in Development in Primate than in Rodent

The current model, based on rodent data, proposes that thalamocortical afferents (TCA) innervate the subplate towards the end of cortical neurogenesis. This implies that the laminar identity of cortical neurons is specified by intrinsic instructions rather than information of thalamic origin. In order to determine whether this mechanism is conserved in the primates, we examined the growth of thalamocortical (TCA) and corticofugal afferents in early human and monkey fetal development. In the human, TCA, identified by secretagogin, calbindin, and ROBO1 immunoreactivity, were observed in the internal capsule of the ventral telencephalon as early as 7-7.5 PCW, crossing the pallial/subpallial boundary (PSB) by 8 PCW before the calretinin immunoreactive corticofugal fibers do. Furthermore, TCA were observed to be passing through the intermediate zone and innervating the presubplate of the dorsolateral cortex, and already by 10-12 PCW TCAs were occupying much of the cortex. Observations at equivalent stages in the marmoset confirmed that this pattern is conserved across primates. Therefore, our results demonstrate that in primates, TCAs innervate the cortical presubplate at earlier stages than previously demonstrated by acetylcholinesterase histochemistry, suggesting that pioneer thalamic afferents may contribute to early cortical circuitry that can participate in defining cortical neuron phenotypes.

Immortalized common marmoset (Callithrix jacchus) hepatic progenitor cells possess bipotentiality in vitro and in vivo

Common marmoset (Callithrix jacchus) is emerging as a clinically relevant nonhuman primate model for various diseases, but is hindered by the availability of marmoset cell lines, which are critical for understanding the disease pathogenesis and drug/toxicological screening prior to animal testing. Here we describe the generation of immortalized marmoset hepatic progenitor cells (MHPCs) by lentivirus-mediated transfer of the simian virus 40 large T antigen gene in fetal liver polygonal cells. MHPCs proliferate indefinitely in vitro without chromosomal alteration and telomere shortening. These cells possess hepatic progenitor cell-specific gene expression profiles with potential to differentiate into both hepatocytic and cholangiocytic lineages in vitro and in vivo and also can be genetically modified. Importantly, injected MHPCs repopulated the injured liver of fumarylacetoacetate hydrolase (Fah)-deficient mice with hepatocyte-like cells. MHPCs also engraft as cholangiocytes into bile ducts of 3,5-diethoxycarbonyl-1,4-dihydrocollidine (DDC)-induced bile ductular injured mice. MHPCs provide a tool to enable efficient derivation and genetic modification of both hepatocytes and cholangiocytes for use in disease modeling, tissue engineering, and drug screening.

A digital 3D atlas of the marmoset brain based on multi-modal MRI

The common marmoset (Callithrix jacchus) is a New-World monkey of growing interest in neuroscience. Magnetic resonance imaging (MRI) is an essential tool to unveil the anatomical and functional organization of the marmoset brain. To facilitate identification of regions of interest, it is desirable to register MR images to an atlas of the brain. However, currently available atlases of the marmoset brain are mainly based on 2D histological data, which are difficult to apply to 3D imaging techniques. Here, we constructed a 3D digital atlas based on high-resolution ex-vivo MRI images, including magnetization transfer ratio (a T1-like contrast), T2w images, and multi-shell diffusion MRI. Based on the multi-modal MRI images, we manually delineated 54 cortical areas and 16 subcortical regions on one hemisphere of the brain (the core version). The 54 cortical areas were merged into 13 larger cortical regions according to their locations to yield a coarse version of the atlas, and also parcellated into 106 sub-regions using a connectivity-based parcellation method to produce a refined atlas. Finally, we compared the new atlas set with existing histology atlases and demonstrated its applications in connectome studies, and in resting state and stimulus-based fMRI. The atlas set has been integrated into the widely-distributed neuroimaging data analysis software AFNI and SUMA, providing a readily usable multi-modal template space with multi-level anatomical labels (including labels from the Paxinos atlas) that can facilitate various neuroimaging studies of marmosets.

Diffusion-weighted tractography in the common marmoset monkey at 9.4T

The common marmoset (Callithrix jacchus) is a small New World primate that is becoming increasingly popular in the neurosciences as an animal model of preclinical human disease. With several major disorders characterized by alterations in neural white matter (e.g., multiple sclerosis, Alzheimer's disease, schizophrenia), proposed to be transgenically modeled using marmosets, the ability to isolate and characterize reliably major white matter fiber tracts with MRI will be of use for evaluating structural brain changes related to disease processes and symptomatology. Here, we propose protocols for isolating major white matter fiber tracts in the common marmoset using in vivo ultrahigh-field MRI (9.4T) diffusion-weighted imaging (DWI) data. With the use of a high angular-resolution DWI (256 diffusion-encoding directions) sequence, collected on four anesthetized marmosets, we provide guidelines for manually drawing fiber-tracking regions of interest, based on easily identified anatomical landmarks in DWI native space. These fiber-tract isolation protocols are expected to be experimentally useful for visualization and quantification of individual white matter fiber tracts in both control and experimental groups of marmosets (e.g., transgenic models). As disease models in the marmoset advance, the determination of how macroscopic white matter anatomy is altered as a function of disease state will be relevant in bridging the existing translational gap between preclinical rodent models and human patients.NEW & NOTEWORTHY Although significant progress has been made in mapping white matter connections in the marmoset brain using ex vivo tracing techniques, the application of in vivo virtual dissection of major white matter fiber tracts has been established by few studies in the marmoset literature. Here, we demonstrate the feasibility of whole-brain diffusion-weighted tractography in anesthetized marmosets at ultrahigh-field MRI (9.4T) and propose protocols for isolating nine major white matter fiber tracts in the marmoset brain.

Application of viral vectors to the study of neural connectivities and neural circuits in the marmoset brain

It is important to study the neural connectivities and functions in primates. For this purpose, it is critical to be able to transfer genes to certain neurons in the primate brain so that we can image the neuronal signals and analyze the function of the transferred gene. Toward this end, our team has been developing gene transfer systems using viral vectors. In this review, we summarize our current achievements as follows. 1) We compared the features of gene transfer using five different AAV serotypes in combination with three different promoters, namely, CMV, mouse CaMKII (CaMKII), and human synapsin 1 (hSyn1), in the marmoset cortex with those in the mouse and macaque cortices. 2) We used target‐specific double‐infection techniques in combination with TET‐ON and TET‐OFF using lentiviral retrograde vectors for enhanced visualization of neural connections. 3) We used an AAV‐mediated gene transfer method to study the transcriptional control for amplifying fluorescent signals using the TET/TRE system in the primate neocortex. We also established systems for shRNA mediated gene targeting in a neocortical region where a gene is significantly expressed and for expressing the gene using the CMV promoter for an unexpressed neocortical area in the primate cortex using AAV vectors to understand the regulation of downstream genes. Our findings have demonstrated the feasibility of using viral vector mediated gene transfer systems for the study of primate cortical circuits using the marmoset as an animal model. © 2016 Wiley Periodicals, Inc. Develop Neurobiol 77: 354–372, 2017