Rapid mapping of spinal and supraspinal connectome via self-targeting glucose-based carbon dots

Abstract

The spinal cord is a highly dynamic network, playing significant roles in the vital functions of the brain. Disorders of the spinal cord, such as spinal cord injury and amyotrophic lateral sclerosis (ALS), are associated with neurodegeneration, often resulting in morbidity and mortality. The blood-brain barrier (BBB) poses a major challenge to imaging and therapeutic agents because less than 2% of small-molecule drugs and almost no large-molecule drugs can cross the BBB. Furthermore, spatial spectroscopy studies have shown highly heterogeneous BBB crossing with significant accumulation at the unintended brain regions. Thus, targeting systems that can cross the BBB at the spinal cord and precisely target specific cell types/populations are vitally needed. Carbon dots can be custom-designed to accumulate at the spinal cord; thus, they offer great potential as delivery platforms for imaging and therapeutic approaches. Since neurons are metabolically highly active and rely on glucose, we designed glucose-based carbon dots (GluCDs) with a diameter of ∼4 nm and glucose-like surface groups. We determined the CNS distribution of GluCDs on three scales: 1. brain regional distribution, 2. cellular tropism (e.g. neurons vs. glia), and 3. intracellular localization. We found that GluCDs (1) crossed the BBB at the spinal cord level, localized primarily to the spinal cord, and were quickly transported to higher centers in the brain, revealing supraspinal connectome within 4 hours after systemic delivery (minimally invasive and significantly faster than the available technologies); (2) almost exclusively localized to neurons without the need for a targeting ligand (neuronal self-targeting), and (3) were confined to late endosomal/lysosomal compartments in the neurons. Then, we verified our findings in a cervical spinal cord contusion injury model with GluCDs targeting the neurons at the injury epicenter. Therefore, GluCDs can be used as robust imaging agents to obtain rapid snapshots of the spinal/supraspinal network. GluCD nanoconjugates can open new avenues for targeted imaging of spinal cord injury. These findings can be extended to other spinal disorders such as ALS, spinal muscular atrophy, and spinal stroke.