Continuous theta burst stimulation ameliorates cognitive deficits in microinfarcts mice via inhibiting glial activation and promoting paravascular CSF-ISF exchange.

Microinfarcts are widespread in the elderly, accompanied by varying degrees of cognitive decline. Continuous theta burst stimulation (cTBS) has been demonstrated to be neuroprotective on cognitive dysfunction, but the underlying cellular mechanism has been still not clear. In the present study, we evaluated the effects of cTBS on cognitive function and brain pathological changes in mice model of microinfarcts. The spatial learning and memory was assessed by Morris water maze (MWM), Glymphatic clearance efficiency was evaluated using in vivo two-photon imaging. The loss of neurons, activation of astrocytes and microglia, the expression and polarity distribution of the astrocytic aquaporin-4 (AQP4) were assessed by immunofluorescence staining. Our results showed that cTBS treatment significantly improved the spatial learning and memory, accelerated the efficiency of glymphatic clearance, up-regulated the AQP4 expression and improved the polarity distribution of AQP4 in microinfarcts mice. Besides, cTBS treatment increased the number of surviving neurons, whereas decreased the activated astrocytes and microglia. Our study suggested that cTBS accelerated glymphatic clearance and inhibited the excessive gliogenesis, which ultimately exerted neuroprotective effects on microinfarcts mice.

Intracisternal injection of beta-amyloid seeds promotes cerebral amyloid angiopathy.

Beta amyloid (Aß) is a key component of parenchymal Aß plaques and vascular Aß fibrils, which lead to cerebral amyloid angiopathy (CAA) in Alzheimer's disease (AD). Recent studies have revealed that Aß contained in the cerebrospinal fluid (CSF) can re-enter into brain through paravascular spaces. However, whether Aß in CSF may act as a constant source of pathogenic Aß in AD is still unclear. This study aimed to examine whether Aß pathology could be worsened when CSF Aß level was enhanced by intra-cisternal infusion of aged brain extract containing abundant Aß in TgCRND8 host mice. TgCRND8 mouse is an AD animal model which develops predominant parenchymal Aß plaques in the brain at as early as 3 months of age. Here, we showed that single intracisternal injection of Aß seeds into TgCRND8 mice before the presence of Aß pathology induced robust prion-like propagation of CAA within 90 days. The induced CAA is mainly distributed in the cerebral cortex, hippocampus and thalamus of TgCRND8 mice. Surprisingly, despite the robust increase in CAA levels, the TgCRND8 mice had a marked decrease in parenchymal Aß plaques and the plaques related neuroinflammation in the brains compared with the control mice. These results amply indicate that Aß in CSF may act as a source of Aß contributing to the growth of vascular Aß deposits in CAA. Our findings provide experimental evidence to unravel the mechanisms of CAA formation and the potential of targeting CSF Aß for CAA.

Uncertainty quantification of parenchymal tracer distribution using random diffusion and convective velocity fields.

BACKGROUND: Influx and clearance of substances in the brain parenchyma occur by a combination of diffusion and convection, but the relative importance of these mechanisms is unclear. Accurate modeling of tracer distributions in the brain relies on parameters that are partially unknown and with literature values varying by several orders of magnitude. In this work, we rigorously quantified the variability of tracer distribution in the brain resulting from uncertainty in diffusion and convection model parameters. METHODS: Using the convection-diffusion-reaction equation, we simulated tracer distribution in the brain parenchyma after intrathecal injection. Several models were tested to assess the uncertainty both in type of diffusion and velocity fields and also the importance of their magnitude. Our results were compared with experimental MRI results of tracer enhancement. RESULTS: In models of pure diffusion, the expected amount of tracer in the gray matter reached peak value after 15 h, while the white matter did not reach peak within 24 h with high likelihood. Models of the glymphatic system were similar qualitatively to the models of pure diffusion with respect to expected time to peak but displayed less variability. However, the expected time to peak was reduced to 11 h when an additional directionality was prescribed for the glymphatic circulation. In a model including drainage directly from the brain parenchyma, time to peak occured after 6-8 h for the gray matter. CONCLUSION: Even when uncertainties are taken into account, we find that diffusion alone is not sufficient to explain transport of tracer deep into the white matter as seen in experimental data. A glymphatic velocity field may increase transport if a large-scale directional structure is included in the glymphatic circulation.

Changing the Currently Held Concept of Cerebrospinal Fluid Dynamics Based on Shared Findings of Cerebrospinal Fluid Motion in the Cranial Cavity Using Various Types of Magnetic Resonance Imaging Techniques.

The "cerebrospinal fluid (CSF) circulation theory" of CSF flowing unidirectionally and circulating through the ventricles and subarachnoid space in a downward or upward fashion has been widely recognized. In this review, observations of CSF motion using different magnetic resonance imaging (MRI) techniques are described, findings that are shared among these techniques are extracted, and CSF motion, as we currently understand it based on the results from the quantitative analysis of CSF motion, is discussed, along with a discussion of slower water molecule motion in the perivascular, paravascular, and brain parenchyma. Today, a shared consensus regarding CSF motion is being formed, as follows: CSF motion is not a circulatory flow, but a combination of various directions of flow in the ventricles and subarachnoid space, and the acceleration of CSF motion differs depending on the CSF space. It is now necessary to revise the currently held concept that CSF flows unidirectionally. Currently, water molecule motion in the order of centimeters per second can be detected with various MRI techniques. Thus, we need new MRI techniques with high-velocity sensitivity, such as in the order of 10 µm/s, to determine water molecule movement in the vessel wall, paravascular space, and brain parenchyma. In this paper, the authors review the previous and current concepts of CSF motion in the central nervous system using various MRI techniques.

Rapid lymphatic efflux limits cerebrospinal fluid flow to the brain.

The relationships between cerebrospinal fluid (CSF) and brain interstitial fluid are still being elucidated. It has been proposed that CSF within the subarachnoid space will enter paravascular spaces along arteries to flush through the parenchyma of the brain. However, CSF also directly exits the subarachnoid space through the cribriform plate and other perineural routes to reach the lymphatic system. In this study, we aimed to elucidate the functional relationship between CSF efflux through lymphatics and the potential influx into the brain by assessment of the distribution of CSF-infused tracers in awake and anesthetized mice. Using near-infrared fluorescence imaging, we showed that tracers quickly exited the subarachnoid space by transport through the lymphatic system to the systemic circulation in awake mice, significantly limiting their spread to the paravascular spaces of the brain. Magnetic resonance imaging and fluorescence microscopy through the skull under anesthetized conditions indicated that tracers remained confined to paravascular spaces on the surface of the brain. Immediately after death, a substantial influx of tracers occurred along paravascular spaces extending into the brain parenchyma. We conclude that under normal conditions a rapid CSF turnover through lymphatics precludes significant bulk flow into the brain.

Characterizing the glymphatic influx by utilizing intracisternal infusion of fluorescently conjugated cadaverine.

AIMS: Accumulating evidence supports that cerebrospinal fluid (CSF) in the subarachnoid space (SAS) could reenter the brain parenchyma via the glymphatic influx. The present study was designed to characterize the detailed pathway of subarachnoid CSF influx by using a novel CSF tracer. MAIN METHODS: Fluorescently conjugated cadaverine (A488-ca), for the first time, was employed to investigate CSF movement in the brain. Following intracisternal infusion of CSF tracers, mice brain was sliced and prepared for fluorescence imaging. Some brain sections were immunostained in order to observe tracer distribution and cellular uptake. KEY FINDINGS: A488-ca moved into the brain parenchyma rapidly, and the influx was time and region dependent. A488-ca entered the mice brain more readily and spread more widely than another commonly used CSF tracer-fluorescently conjugated ovalbumin (OA-45). Furthermore, A488-ca could enter the brain parenchyma either along the paravascular space or across the pial surface. Suppression of glymphatic transport by administration with acetazolamide strikingly reduced the influx of A488-ca. More importantly, relative to OA-45 largely remained in the extracellular space, A488-ca exhibited obvious cellular uptake by astrocytes surrounding the blood vessels and neurons in the cerebral cortex. SIGNIFICANCE: Subarachnoid CSF could flow into the brain parenchyma via the glymphatic influx, in which the transcellular pathway was faithfully traced by intracisternal infusion with fluorescently conjugated cadaverine. These observations extend our comprehension on the glymphatic influx pathway.

Paravascular spaces at the brain surface: Low resistance pathways for cerebrospinal fluid flow.

Clearance of waste products from the brain is of vital importance. Recent publications suggest a potential clearance mechanism via paravascular channels around blood vessels. Arterial pulsations might provide the driving force for paravascular flow, but its flow pattern remains poorly characterized. In addition, the relationship between paravascular flow around leptomeningeal vessels and penetrating vessels is unclear. In this study, we determined blood flow and diameter pulsations through a thinned-skull cranial window. We observed that microspheres moved preferentially in the paravascular space of arteries rather than in the adjacent subarachnoid space or around veins. Paravascular flow was pulsatile, generated by the cardiac cycle, with net antegrade flow. Confocal imaging showed microspheres distributed along leptomeningeal arteries, while their presence along penetrating arteries was limited to few vessels. These data suggest that paravascular spaces around leptomeningeal arteries form low resistance pathways on the surface of the brain that facilitate cerebrospinal fluid flow.

Voluntary Exercise Promotes Glymphatic Clearance of Amyloid Beta and Reduces the Activation of Astrocytes and Microglia in Aged Mice.

Age is characterized by chronic inflammation, leading to synaptic dysfunction and dementia because the clearance of protein waste is reduced. The clearance of proteins depends partly on the permeation of the blood-brain barrier (BBB) or on the exchange of water and soluble contents between the cerebrospinal fluid (CSF) and the interstitial fluid (ISF). A wealth of evidence indicates that physical exercise improves memory and cognition in neurodegenerative diseases during aging, such as Alzheimer's disease (AD), but the influence of physical training on glymphatic clearance, BBB permeability and neuroinflammation remains unclear. In this study, glymphatic clearance and BBB permeability were evaluated in aged mice using in vivo two-photon imaging. The mice performed voluntary wheel running exercise and their water-maze cognition was assessed; the expression of the astrocytic water channel aquaporin 4 (AQP4), astrocyte and microglial activation, and the accumulation of amyloid beta (Aß) were evaluated with immunofluorescence or an enzyme-linked immunosorbent assay (ELISA); synaptic function was investigated with Thy1-green fluorescent protein (GFP) transgenic mice and immunofluorescent staining. Voluntary wheel running significantly improved water-maze cognition in the aged mice, accelerated the efficiency of glymphatic clearance, but which did not affect BBB permeability. The numbers of activated astrocytes and microglia decreased, AQP4 expression increased, and the distribution of astrocytic AQP4 was rearranged. Aß accumulation decreased, whereas dendrites, dendritic spines and postsynaptic density protein (PSD95) increased. Our study suggests that voluntary wheel running accelerated glymphatic clearance but not BBB permeation, improved astrocytic AQP4 expression and polarization, attenuated the accumulation of amyloid plaques and neuroinflammation, and ultimately protected mice against synaptic dysfunction and a decline in spatial cognition. These data suggest possible mechanisms for exercise-induced neuroprotection in the aging brain.

Paravascular channels, cisterns, and the subarachnoid space in the rat brain: A single compartment with preferential pathways.

Recent evidence suggests an extensive exchange of fluid and solutes between the subarachnoid space and the brain interstitium, involving preferential pathways along blood vessels. We studied the anatomical relations between brain vasculature, cerebrospinal fluid compartments, and paravascular spaces in male Wistar rats. A fluorescent tracer was infused into the cisterna magna, without affecting intracranial pressure. Tracer distribution was analyzed using a 3D imaging cryomicrotome, confocal microscopy, and correlative light and electron microscopy. We found a strong 3D colocalization of tracer with major arteries and veins in the subarachnoid space and large cisterns, attributed to relatively large subarachnoid space volumes around the vessels. Confocal imaging confirmed this colocalization and also revealed novel cisternal connections between the subarachnoid space and ventricles. Unlike the vessels in the subarachnoid space, penetrating arteries but not veins were surrounded by tracer. Correlative light and electron microscopy images indicated that this paravascular space was located outside of the endothelial layer in capillaries and just outside of the smooth muscle cells in arteries. In conclusion, the cerebrospinal fluid compartment, consisting of the subarachnoid space, cisterns, ventricles, and para-arteriolar spaces, forms a continuous and extensive network that surrounds and penetrates the rat brain, in which mixing may facilitate exchange between interstitial fluid and cerebrospinal fluid.