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 vital functions of the brain. Disorders of the spinal cord, such as spinal cord injury (SCI) and amyotrophic lateral sclerosis (ALS), are associated with neurodegeneration, which often results in morbidity and mortality. The blood-brain barrier (BBB) represents a major challenge for imaging agents and therapeutics, as less than 2% of small-molecule drugs can cross the BBB. Furthermore, spatial spectroscopy studies show highly heterogeneous BBB crossing, with significant binding 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 target specific regions in the brain, offering 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 ~4 nm in diameter and glucose-like surface groups. Then, we determined the CNS distribution of GluCDs in three scales: 1. brain regional distribution, 2. cellular tropism (e.g. neurons vs glia), and 3. intracellular localization. We found that GluCDs: 1) cross the BBB at the spinal cord level, localize primarily to the spinal cord, and are quickly transported to higher centers in the brain, revealing supraspinal connectome within 4 hours after systemic delivery (minimally invasive and significantly faster than available technologies); 2) almost exclusively localize to neurons without the need for targeting ligand (neuronal self-targeting), 3) are confined to late endosomal/lysosomal compartment in neurons. We then verified our findings in a cervical spinal cord contusion injury, with GluCDs targeting neurons at the injury epicenter. Therefore, GluCDs can be used as robust imaging agents to take rapid snapshots of the spinal/supraspinal network. GluCD nanoconjugates can open new avenues for targeted imaging of SCI. These findings can be extended to other spinal disorders, such as ALS, spinal muscular atrophy, and spinal stroke.