To see or not to see: In vivo nanocarrier detection methods in the brain and their challenges.

Nanoparticles have a great potential to significantly improve the delivery of therapeutics to the brain and may also be equipped with properties to investigate brain function. The brain, being a highly complex organ shielded by selective barriers, requires its own specialized detection system. However, a significant hurdle to achieve these goals is still the identification of individual nanoparticles within the brain with sufficient cellular, subcellular, and temporal resolution. This review aims to provide a comprehensive summary of the current knowledge on detection systems for tracking nanoparticles across the blood-brain barrier and within the brain. We discuss commonly employed in vivo and ex vivo nanoparticle identification and quantification methods, as well as various imaging modalities able to detect nanoparticles in the brain. Advantages and weaknesses of these modalities as well as the biological factors that must be considered when interpreting results obtained through nanotechnologies are summarized. Finally, we critically evaluate the prevailing limitations of existing technologies and explore potential solutions.

Bradykinin 2 Receptors Mediate Long-Term Neurocognitive Deficits After Experimental Traumatic Brain Injury.

The kallikrein-kinin system is one of the first inflammatory pathways to be activated following traumatic brain injury (TBI) and has been shown to exacerbate brain edema formation in the acute phase through activation of bradykinin 2 receptors (B2R). However, the influence of B2R on chronic post-traumatic damage and outcome is unclear. In the current study, we assessed long-term effects of B2R-knockout (KO) after experimental TBI. B2R KO mice (heterozygous, homozygous) and wild-type (WT) littermates (n = 10/group) were subjected to controlled cortical impact (CCI) TBI. Lesion size was evaluated by magnetic resonance imaging up to 90 days after CCI. Motor and memory function were regularly assessed by Neurological Severity Score, Beam Walk, and Barnes maze test. Ninety days after TBI, brains were harvested for immunohistochemical analysis. There was no difference in cortical lesion size between B2R-deficient and WT animals 3 months after injury; however, hippocampal damage was reduced in B2R KO mice (p = 0.03). Protection of hippocampal tissue was accompanied by a significant improvement of learning and memory function 3 months after TBI (p = 0.02 WT vs. KO), whereas motor function was not influenced. Scar formation and astrogliosis were unaffected, but B2R deficiency led to a gene-dose-dependent attenuation of microglial activation and a reduction of CD45+ cells 3 months after TBI in cortex (p = 0.0003) and hippocampus (p < 0.0001). These results suggest that chronic hippocampal neurodegeneration and subsequent cognitive impairment are mediated by prolonged neuroinflammation and B2R. Inhibition of B2R may therefore represent a novel strategy to reduce long-term neurocognitive deficits after TBI.

Continued dysfunction of capillary pericytes promotes no-reflow after experimental stroke in vivo.

Incomplete reperfusion of the microvasculature ('no-reflow') after ischaemic stroke damages salvageable brain tissue. Previous ex vivo studies suggest pericytes are vulnerable to ischaemia and may exacerbate no-reflow, but the viability of pericytes and their association with no-reflow remains under-explored in vivo. Using longitudinal in vivo two-photon single-cell imaging over 7 days, we showed that 87% of pericytes constrict during cerebral ischaemia and remain constricted post reperfusion, and 50% of the pericyte population are acutely damaged. Moreover, we revealed ischaemic pericytes to be fundamentally implicated in capillary no-reflow by limiting and arresting blood flow within the first 24 h post stroke. Despite sustaining acute membrane damage, we observed that over half of all cortical pericytes survived ischaemia and responded to vasoactive stimuli, upregulated unique transcriptomic profiles and replicated. Finally, we demonstrated the delayed recovery of capillary diameter by ischaemic pericytes after reperfusion predicted vessel reconstriction in the subacute phase of stroke. Cumulatively, these findings demonstrate that surviving cortical pericytes remain both viable and promising therapeutic targets to counteract no-reflow after ischaemic stroke.

RIPK1 or RIPK3 deletion prevents progressive neuronal cell death and improves memory function after traumatic brain injury.

Traumatic brain injury (TBI) causes acute and subacute tissue damage, but is also associated with chronic inflammation and progressive loss of brain tissue months and years after the initial event. The trigger and the subsequent molecular mechanisms causing chronic brain injury after TBI are not well understood. The aim of the current study was therefore to investigate the hypothesis that necroptosis, a form a programmed cell death mediated by the interaction of Receptor Interacting Protein Kinases (RIPK) 1 and 3, is involved in this process. Neuron-specific RIPK1- or RIPK3-deficient mice and their wild-type littermates were subjected to experimental TBI by controlled cortical impact. Posttraumatic brain damage and functional outcome were assessed longitudinally by repetitive magnetic resonance imaging (MRI) and behavioral tests (beam walk, Barnes maze, and tail suspension), respectively, for up to three months after injury. Thereafter, brains were investigated by immunohistochemistry for the necroptotic marker phosphorylated mixed lineage kinase like protein(pMLKL) and activation of astrocytes and microglia. WT mice showed progressive chronic brain damage in cortex and hippocampus and increased levels of pMLKL after TBI. Chronic brain damage occurred almost exclusively in areas with iron deposits and was significantly reduced in RIPK1- or RIPK3-deficient mice by up to 80%. Neuroprotection was accompanied by a reduction of astrocyte and microglia activation and improved memory function. The data of the current study suggest that progressive chronic brain damage and cognitive decline after TBI depend on the expression of RIPK1/3 in neurons. Hence, inhibition of necroptosis signaling may represent a novel therapeutic target for the prevention of chronic post-traumatic brain damage.