Corticospinal Tract Tracing in the Marmoset with a Clinical Whole-Body 3T Scanner Using Manganese-Enhanced MRI

Long-Term Intranasal Nerve Growth Factor Treatment Favors Neuron Formation in de novo Brain Tissue

Objective

To date, no safe and effective pharmacological treatment has been clinically validated for improving post-stroke neurogenesis. Growth factors are good candidates but low safety has limited their application in the clinic. An additional restraint is the delivery route. Intranasal delivery presents many advantages.

Materials and Methods

A brain lesion was induced in twenty-four rats. Nerve growth factor (NGF) 5 μg/kg/day or vehicle was given intranasally from day 10 post-lesion for two periods of five weeks, separated by a two-week wash out period with no treatment. Lesion volume and atrophy were identified by magnetic resonance imaging (MRI). Anxiety and sensorimotor recovery were measured by behavior tests. Neurogenesis, angiogenesis and inflammation were evaluated by histology at 12 weeks.

Results

Remarkable neurogenesis occurred and was visible at the second and third months after the insult. Tissue reconstruction was clearly detected by T2 weighted MRI at 8 and 12 weeks post-lesion and confirmed by histology. In the new tissue (8.1% of the lesion in the NGF group vs. 2.4%, in the control group at 12 weeks), NGF significantly increased the percentage of mature neurons (19% vs. 7%). Angiogenesis and inflammation were not different in the two groups. Sensorimotor recovery was neither improved nor hampered by NGF during the first period of treatment, but NGF treatment limited motor recovery in the second period.

Interpretation

The first five-week period of treatment was very well tolerated. This study is the first presenting the effects of a long treatment with NGF and has shown an important tissue regeneration rate at 8 and 12 weeks post-injury. NGF may have increased neuronal differentiation and survival and favored neurogenesis and neuron survival through subventricular zone (SVZ) neurogenesis or reprogramming of reactive astrocytes. For the first time, we evidenced a MRI biomarker of neurogenesis and tissue reconstruction with T2 and diffusion weighted imaging.

Magnetic Resonance Imaging of Marmoset Monkeys

The use of the common marmoset monkey (Callithrix jacchus) for neuroscientific research has grown markedly in the last decade. Magnetic resonance imaging (MRI) has played a significant role in establishing the extent of comparability of marmoset brain architecture with the human brain and brains of other preclinical species (eg, macaques and rodents). As a non-invasive technique, MRI allows for the flexible acquisition of the same sequences across different species in vivo, including imaging of whole-brain functional topologies not possible with more invasive techniques. Being one of the smallest New World primates, the marmoset may be an ideal nonhuman primate species to study with MRI. As primates, marmosets have an elaborated frontal cortex with features analogous to the human brain, while also having a small enough body size to fit into powerful small-bore MRI systems typically employed for rodent imaging; these systems offer superior signal strength and resolution. Further, marmosets have a rich behavioral repertoire uniquely paired with a lissencephalic cortex (like rodents). This smooth cortical surface lends itself well to MRI and also other invasive methodologies. With the advent of transgenic modification techniques, marmosets have gained significant traction as a powerful complement to canonical mammalian modelling species. Marmosets are poised to make major contributions to preclinical investigations of the pathophysiology of human brain disorders as well as more basic mechanistic explorations of the brain. The goal of this article is to provide an overview of the practical aspects of implementing MRI and fMRI in marmosets (both under anesthesia and fully awake) and discuss the development of resources recently made available for marmoset imaging.

A Reproducible New Model of Focal Ischemic Injury in the Marmoset Monkey: MRI and Behavioural Follow-Up

Ischemic stroke mostly affects the primary motor cortex and descending motor fibres, with consequent motor impairment. Pre-clinical models of stroke with reproducible and long-lasting sensorimotor deficits in higher-order animals are lacking. We describe a new method to induce focal brain damage targeting the motor cortex to study damage to the descending motor tracts in the non-human primate. Stereotaxic injection of malonate into the primary motor cortex produced a focal lesion in middle-aged marmosets (Callithrix jacchus). Assessment of sensorimotor function using a neurological scale and testing of forelimb dexterity and strength lasted a minimum of 12 weeks. Lesion evolution was followed by magnetic resonance imaging (MRI) at 24 h, 1 week, 4 and 12 weeks post-injury and before sacrifice for immunohistochemistry. Our model produced consistent lesions of the motor cortex, subcortical white matter and caudate nucleus. All animals displayed partial spontaneous recovery with long lasting motor deficits of force (54% loss) and dexterity (≈ 70% loss). Clearly visible T2 hypointensity in the white matter was observed with MRI and corresponded to areas of chronic gliosis in the internal capsule and lenticular fasciculus. We describe a straightforward procedure to reproducibly injure the motor cortex in the marmoset monkey, causing long-lasting motor deficits. The MRI signature reflects Wallerian degeneration and remote injury of corticospinal and corticopontine tracts, as well as subcortical motor loops. Our model may be suitable for the testing of therapies for post-stroke recovery, particularly in the chronic phase.

Manganese-Enhanced Magnetic Resonance Imaging: Application in Central Nervous System Diseases

Manganese-enhanced magnetic resonance imaging (MEMRI) relies on the strong paramagnetism of Mn2+. Mn2+ is a calcium ion analog and can enter excitable cells through voltage-gated calcium channels. Mn2+ can be transported along the axons of neurons via microtubule-based fast axonal transport. Based on these properties, MEMRI is used to describe neuroanatomical structures, monitor neural activity, and evaluate axonal transport rates. The application of MEMRI in preclinical animal models of central nervous system (CNS) diseases can provide more information for the study of disease mechanisms. In this article, we provide a brief review of MEMRI use in CNS diseases ranging from neurodegenerative diseases to brain injury and spinal cord injury.

Assessment of behavioural deficits following ischaemic stroke in the marmoset

Stroke is a common and devastating disease worldwide. Over the last two decades, many therapeutic approaches to ameliorate ischaemic stroke have been promising in animal studies but failed when transferred to the clinical situation. One of the possible explanations for these failures is the widespread use of animal models of cerebral ischemia that do not mimic the pathology encountered in the clinic. Accordingly, many expert committees recommended the integration of higher order species such as non-human primates in pre-clinical stroke studies. The common marmoset (Callithrix jacchus), a small New World monkey, start to stand out in the neuroscience field as a good compromise between larger primates and rodents. In this review, we discuss the relevance of the use of the marmoset in stroke studies. We will focus on behavioural tests developed in this species to assess sensorimotor deficits and their recovery during acute and chronic stages of brain ischaemia. The aim of this appraisal is to provide a comprehensive overview of the existing approaches to induce stroke in the marmoset as well as the paradigms for behavioural testing in this species. The data summarized in this review should contribute to the improvement of future stoke studies in the marmoset and accordingly improve the translation of the results from bench to bed.

Transcranial manganese delivery for neuronal tract tracing using MEMRI

There has been a growing interest in the use of manganese-enhanced MRI (MEMRI) for neuronal tract tracing in mammals, especially in rodents. For this MEMRI application, manganese solutions are usually directly injected into specific brain regions. Recently it was reported that manganese ions can diffuse through intact rat skull. Here the local manganese concentrations in the brain tissue after transcranial manganese application were quantified and the effectiveness of tracing from the area under the skull where delivery occurred was determined. It was established that transcranially applied manganese yields brain tissue enhancement dependent on the location of application on the skull and that manganese that enters the brain transcranially can trace to deeper brain areas.