The Role of Negative Charge in the Delivery of Quantum Dots to Neurons

Abstract

Despite our extensive knowledge of the structure of negatively charged cell surface proteoglycans and sialoglycoconjugates in the brain, we have little understanding of how their negative charge contributes to brain function. We have previously shown that intensely photoluminescent 9-nm diameter quantum dots (QDs) with a CdSe core, a ZnS shell, and a negatively charged compact molecular ligand coating (CL4) selectively target neurons rather than glia. We now provide an explanation for this selective neuronal delivery. In this study, we compared three zwitterionic QD coatings differing only in their regions of positive or negative charge, as well as a positively charged (NH2) polyethylene glycol (PEG) coat, for their ability to deliver the cell-membrane-penetrating chaperone lipopeptide JB577 (WG(Palmitoyl)VKIKKP9G2H6) to individual cells in neonatal rat hippocampal slices. We confirm both that preferential uptake in neurons, and the lack of uptake in glia, is strongly associated with having a region of greater negative charge on the QD coating. In addition, the role of negatively charged chondroitin sulfate of the extracellular matrix (ECM) in restricting uptake was further suggested by digesting neonatal rat hippocampal slices with chondroitinase ABC and showing increased uptake of QDs by oligodendrocytes. Treatment still did not affect uptake in astrocytes or microglia. Finally, the future potential of using QDs as vehicles for trafficking proteins into cells continues to show promise, as we show that by administering a histidine-tagged green fluorescent protein (eGFP-His6) to hippocampal slices, we can observe neuronal uptake of GFP.

Impairments of spatial learning and memory following intrahippocampal injection in rats of 3-mercaptopropionic acid-modified CdTe quantum dots and molecular mechanisms

“…Additionally, we found that the lysosomes might be long-term accumulation sites of CdTe QDs in neurons, similar to a previous study, 4 which could be partially explained by the findings of Walters et al that negative charge on the QD coating resulted in preferential uptake in neurons. 25 The pathological lesion findings could explain the impaired capabilities of learning and memory in MPAmodified CdTe QD-treated rats. However, the definite mechanisms causing this damage is not yet clear.…”

Quantum dot‐mediated delivery of siRNA to inhibit sphingomyelinase activities in brain‐derived cells

“…; Walters et al . , ), and it also might protect siRNA from digestion of intracellular RNase. Furthermore, QD‐siRNA constructs demonstrated robust knockdown of 72.0 ∓ 12.0% luciferase, which is similar or better than commercial transfection reagents.…”

“…Conversely, if the compact ligand or PEG was positively charged or the extracellular matrix was enzymatically digested with chondroitinases (Walters et al . ), the QDs were targeted more to oligodendrocytes. This makes QDs ideal multipurpose delivery vehicles because they not only facilitate specific cell‐type delivery (Walters ) and are non‐toxic, but also allow for real‐time tracking of their location in vitro , eliminating the need for the addition of fluorescent tags to track progress.…”

“…Further, in order to work with brain tissue (Walters et al . , ), we had to overcome two main obstacles to nanoparticle delivery, namely how to selectively target different neural cell types (Walters et al . , ) and how to get to subcellular and target organelles without toxicity.…”

“…, ), we had to overcome two main obstacles to nanoparticle delivery, namely how to selectively target different neural cell types (Walters et al . , ) and how to get to subcellular and target organelles without toxicity. We have previously shown JB577 functions as an endosomal release peptide/cytosolic delivery peptide and has neuronal selectivity in rat hippocampal slices (Walters et al .…”