The discovery of meningeal lymphatic vessels (LVs) has sparked interest in identifying their role in diseases of the central nervous system. Similar to peripheral LVs, meningeal LVs depend on vascular endothelial growth factor receptor-3 (VEGFR3) signaling for development. Here we characterize the effect of stroke on meningeal LVs, and the impact of meningeal lymphatic hypoplasia on post-stroke outcomes. We show that photothrombosis (PT), but not transient middle cerebral artery occlusion (tMCAo), induces meningeal lymphangiogenesis in young male C57Bl/J6 mice. We also show that Vegfr3 wt/mut mice develop significantly fewer meningeal LVs than Vegfr3 wt/wt mice. Again, meningeal lymphangiogenesis occurs in the alymphatic zone lateral to the sagittal sinus only after PT-induced stroke in Vegfr3 wt/wt mice. Interestingly, Vegfr3 wt/mut mice develop larger stroke volumes than Vegfr3 wt/wt mice after tMCAo, but not after PT. Our results reveal differences between PT and tMCAo models of stroke and underscore the need to consider method of stroke induction when investigating the role of meningeal lymphatics. Taken together, our data indicate that ischemic injury can induce the growth of meningeal LVs and that the absence of these LVs can impact post-stroke outcomes.
Hydrogels have been suggested as novel drug delivery systems for sustained release of therapeutic proteins in various neurological disorders. The main advantage these systems offer is the controlled, prolonged exposure to a therapeutically effective dose of the released drug after a single intracerebral injection. Characterization of controlled release of therapeutics from a hydrogel is generally performed in vitro, as current methods do not allow for in vivo measurements of spatiotemporal distribution and release kinetics of a loaded protein. Importantly, the in vivo environment introduces many additional variables and factors that cannot be effectively simulated under in vitro conditions. To address this, in the present contribution, we developed a noninvasive in vivo magnetic resonance imaging (MRI) method to monitor local protein release from two injected hydrogels of the same chemical composition but different initial water contents. We designed a biodegradable hydrogel formulation composed of low and high concentration thermosensitive polymer and thiolated hyaluronic acid, which is liquid at room temperature and forms a gel due to a combination of physical and chemical cross-linking upon injection at 37 °C. The in vivo protein release kinetics from these gels were assessed by MRI analysis utilizing a model protein labeled with an MR contrast agent, i.e. gadolinium-labeled albumin (74 kDa). As proof of principle, the release kinetics of the hydrogels were first measured with MRI in vitro. Subsequently, the protein loaded hydrogels were administered in male Wistar rat brains and the release in vivo was monitored for 21 days. In vitro, the thermosensitive hydrogels with an initial water content of 81 and 66% released 64 ± 3% and 43 ± 3% of the protein loading, respectively, during the first 6 days at 37 °C. These differences were even more profound in vivo, where the thermosensitive hydrogels released 83 ± 16% and 57 ± 15% of the protein load, respectively, 1 week postinjection. Measurement of volume changes of the gels over time showed that the thermosensitive gel with the higher polymer concentration increased more than 4-fold in size in vivo after 3 weeks, which was substantially different from the in vitro behavior where a volume change of 35% was observed. Our study demonstrates the potential of MRI to noninvasively monitor in vivo intracerebral protein release from a locally administered in situ forming hydrogel, which could aid in the development and optimization of such drug delivery systems for brain disorders.
Following stroke, B cells enter brain regions outside of the ischemic injury to mediate functional recovery. Although B cells produce neurotrophins that support remote plasticity, including brain-derived neurotrophic factor (BDNF), it remains unclear which signal(s) activate B cells in the absence of infarct-localized pro-inflammatory cues. Activation of N-methyl-d-aspartate (NMDA)-type receptor (NMDAR) subunits on neurons can upregulate mature BDNF (mBDNF) production from a pro-BDNF precursor, but whether this occurs in B cells is unknown. We identified GluN2A and GluN2B NMDAR subunits on B cells that respond to glutamate and mediate nearly half of the glutamate-induced Ca2+ responses in activated B cell subsets. Ischemic stroke recruits GluN2A+ B cells into the ipsilesional hemisphere and both stroke and neurophysiologic levels of glutamate regulate gene and surface expression. Regardless of injury, pro-BDNF+ B localize to spleen/circulation whereas mBDNF+ B cells localize to the brain, including in aged male and female mice. We confirmed B cell-derived BDNF was required for in vitro and in vivo B cell-mediated neuroprotection. Lastly, GluN2A, GluN2B, glutamate-induced Ca2+ responses, and BDNF expression were all clinically confirmed in B cells from healthy donors, with BDNF+ B cells present in post-stroke human parenchyma. These data suggest that B cells express functional NMDARs that respond to glutamate, enhance NMDAR signaling with activation, and upregulate mature BDNF expression within the brain. This study identifies potential glutamate-induced neurotrophic roles for B cells in the brain; an immune response to neurotransmitters unique from established pro-inflammatory stimuli and relevant to any CNS-localized injury or disease.
Abstract Background White Matter Hyperintensities (WMH) appear on T2‐weighted Fluid‐Attenuated Inversion Recovery (FLAIR) Magnetic Resonance Imaging (MRI) and are an important biomarker of cerebral small vessel disease (CSVD), cognitive decline, and stroke. However, manual delineation is laborious and bias‐prone, while automated segmentation has proven challenging. With the recent conclusion of the MICCAI‐Society WMH segmentation challenge, and our large clinical trials, “Risk Reduction for Alzheimer’s Disease” (rrAD) and “Hypertension, Intracranial Pulsatility and Brain Amyloid‐beta Clearance in older adults” (HIPAC), we investigated the differences in the total WMH volume segmented by various algorithms to ensure the extraction of accurate and meaningful image‐derived phenotypes. Methods The rrAD trial provides 512 2D axial T2 FLAIR scans, fat saturation off (ages 60‐85). The HIPAC study provides 84 3D T2 FLAIR scans, fat saturation on (ages 55‐79). The MICCAI WMH challenge provides a test set of 110 scans of 2D axial T2 FLAIR images, 30 without, and 80 with fat saturation. Our clinical trials and the MICCAI datasets comprise data from over 700 subjects, across 10 scanners and diverse sequences. Three of the best‐performing MICCAI algorithms were selected: PGS, Sysu‐Media‐2, FMRIB‐TrUE‐Net2, as well as the LPA algorithm from SPM. Segmentation results were randomized, and fully blinded expert raters scored a subset of 120 scans. Results Figure 1 depicts the average normalized images for each T2 FLAIR contrast. The MICCAI test set shows a median WMH burden of 10‐17 ml, while rrAD subjects presented with relative medians of ≈3−4 ml, and HIPAC subjects with ≈2−3 ml. Figure 2 shows the WMH volume for each segmentation algorithm and dataset. For HIPAC Sysu‐Media‐2 had high false pickup within grey matter regions, while LPA produced high false pickup at the posterior septum pellucidum. PGS scored best in manual ratings. Conclusions MRI sequences that use fat saturation change the T2 FLAIR image contrast substantially and impact segmentation performance, we observed better performance with fat saturation off. The MICCAI challenge algorithms outperformed classical methods (PGS leading), detecting small lesions, away from the ventricles. As WMH distribution, shape, and longitudinal tracking are likely more sensitive biomarkers than volume alone, further analysis is warranted.
