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Sleep is evolutionarily conserved across all species, and impaired sleep is a common trait of the diseased brain. Sleep quality decreases as we age, and disruption of the regular sleep architecture is a frequent antecedent to the onset of dementia in neurodegenerative diseases. The glymphatic system, which clears the brain of protein waste products, is mostly active during sleep. Yet the glymphatic system degrades with age, suggesting a causal relationship between sleep disturbance and symptomatic progression in the neurodegenerative dementias. The ties that bind sleep, aging, glymphatic clearance, and protein aggregation have shed new light on the pathogenesis of a broad range of neurodegenerative diseases, for which glymphatic failure may constitute a therapeutically targetable final common pathway [ABSTRACT FROM AUTHOR]

The glymphatic concept along with the discovery of meningeal lymphatic vessels have, in recent years, highlighted that fluid is directionally transported within the central nervous system (CNS). Imaging studies, as well as manipulations of fluid transport, point to a key role of the glymphatic–lymphatic system in clearance of amyloid-β and other proteins. As such, the glymphatic–lymphatic system represents a new target in combating neurodegenerative diseases. Not unexpectedly, introduction of a new plumbing system in the brain has stirred controversies. This opinion article will highlight what we know about the brain's fluid transport systems, where experimental data are lacking, and what is still debated. Several global models of brain fluid transport have been proposed. When assessing these models, it is imperative that they are based on observations in live animals. Cerebrospinal fluid (CSF) tracer distribution in histological sections mostly reflects nonphysiological events triggered after death. The literature suggests that diffusion and convective flow both contribute to clearance of CNS solutes. Experiments that are aimed at defining the relative contribution of diffusion versus convection are difficult to interpret because minor changes in physiological variables, such as body posture or respiratory rate, can significantly affect both pathways. Invasive procedures, such as tracer injection, will primarily suppress convective flow. The glymphatic system drives CSF into the brain along periarterial spaces and interstitial fluid (ISF) out along perivenous spaces. Aquaporin-4 (AQP4) water channels, densely expressed at the vascular endfeet of astrocytes, facilitate glymphatic transport, based on all studies on this topic (with one exception). Glymphatic–lymphatic efflux of amyloid-β contributes to diurnal variations in amyloid-β concentration in murine Alzheimer's disease models and represents a potential therapeutic target for Alzheimer's disease. [ABSTRACT FROM AUTHOR]

Pretargeting is a promising nuclear imaging technique that allows for the usage of antibodies (Abs) with enhanced imaging contrast and reduced patient radiation burden. It is based on bioorthogonal chemistry with the tetrazine ligation—a reaction between trans-cyclooctenes (TCOs) and tetrazines (Tzs)—currently being the most popular reaction due to its high selectivity and reactivity. As Abs can be designed to bind specifically to currently 'undruggable' targets such as protein isoforms or oligomers, which play a crucial role in neurodegenerative diseases, pretargeted imaging beyond the BBB is highly sought after, but has not been achieved yet. A challenge in this respect is that large molecules such as Abs show poor brain uptake. Uptake can be increased by receptor mediated transcytosis; however, it is largely unknown if the achieved brain concentrations are sufficient for pretargeted imaging. In this study, we investigated whether the required concentrations are feasible to reach. As a model Ab, we used the bispecific anti-amyloid beta (Aβ) anti-transferrin receptor (TfR) Ab 3D6scFv8D3 and conjugated it to a different amount of TCOs per Ab and tested different concentrations in vitro. With this model in hand, we estimated the minimum required TCO concentration to achieve a suitable contrast between the high and low binding regions. The estimation was carried out using pretargeted autoradiography on brain sections of an Alzheimer's disease mouse model. Biodistribution studies in wild-type (WT) mice were used to correlate how different TCO/Ab ratios alter the brain uptake. Pretargeted autoradiography showed that increasing the number of TCOs as well as increasing the TCO-Ab concentration increased the imaging contrast. A minimum brain concentration of TCOs for pretargeting purposes was determined to be 10.7 pmol/g in vitro. Biodistribution studies in WT mice showed a brain uptake of 1.1% ID/g using TCO-3D6scFv8D3 with 6.8 TCO/Ab. According to our estimations using the optimal parameters, pretargeted imaging beyond the BBB is not a utopia. Necessary brain TCO concentrations can be reached and are in the same order of magnitude as required to achieve sufficient contrast. This work gives a first estimate that pretargeted imaging is indeed possible with antibodies. This could allow the imaging of currently 'undruggable' targets and therefore be crucial to monitor (e.g., therapies for intractable neurodegenerative diseases). [ABSTRACT FROM AUTHOR]

• Cerebrospinal fluid in brain surface perivascular spaces flows and pulses. • Prior experiments show close links to wall motion, suggesting perivascular pumping. • Prior theory and simulations predict different pulsatility and phase than observed. • We add resistance and compliance as boundary conditions, using measured values. • Then, theory and simulations match observations much more closely. Cerebrospinal fluid (CSF) flows through the perivascular spaces (PVSs) surrounding cerebral arteries. Revealing the mechanisms driving that flow could bring improved understanding of brain waste transport and insights for disorders including Alzheimer's disease and stroke. In vivo velocity measurements of CSF in surface PVSs in mice have been used to argue that flow is driven primarily by the pulsatile motion of artery walls — perivascular pumping. However, fluid dynamics theory and simulation have predicted that perivascular pumping produces flows differing from in vivo observations starkly, particularly in the phase and relative amplitude of flow oscillation. We show that coupling theoretical and simulated flows to more realistic end boundary conditions, using resistance and compliance values measured in mice instead of using periodic boundaries, results in velocities that match observations more closely in phase and relative amplitude of oscillation, while preserving the existing agreement in mean flow speed. This quantitative agreement among theory, simulation, and in vivo measurement further supports the idea that perivascular pumping is an important CSF driver in physiological conditions. [ABSTRACT FROM AUTHOR]

Astrocytes have been presumed to be uninteresting for the better part of a century. Recent findings suggest unexpected new functions for these cells and highlight the importance of viewing most brain activities as a collaboration between astrocytes and neurons. Astrocytes have been implicated in dynamic regulation of neuron production, synaptic network formation, neuron electrical activity and specific neurological diseases. We are only at the threshold of understanding fully the nature and consequences of these new astrocyte functions, and there is still much to be learned about the older and better-known roles. But now, at last, astrocytes have our attention. [Copyright &y& Elsevier]

Drug delivery to the central nervous system remains a major problem due to biological barriers. The blood-brain-barrier can be bypassed by administering drugs intrathecally directly to the cerebrospinal fluid (CSF). The glymphatic system, a network of perivascular spaces promoting fluid exchange between CSF and interstitial space, could be utilized to enhance convective drug delivery from the CSF to the parenchyma. Glymphatic flow is highest during sleep and anesthesia regimens that induce a slow-wave sleep-like state. Here, using mass spectrometry and fluorescent imaging techniques, we show that the clinically used α 2 -adrenergic agonist dexmedetomidine that enhances EEG slow-wave activity, increases brain and spinal cord drug exposure of intrathecally administered drugs in mice and rats. Using oxycodone, naloxone, and an IgG-sized antibody as relevant model drugs we demonstrate that modulation of glymphatic flow has a distinct impact on the distribution of intrathecally administered therapeutics. These findings can be exploited in the clinic to improve the efficacy and safety of intrathecally administered therapeutics. Unlabelled Image • The glymphatic system drives cerebrospinal fluid influx into brain tissue. • Glymphatic flow is increased by drugs that decrease brain noradrenergic tone. • Dexmedetomidine is a clinically used α 2 -adrenergic agonist sedative. • Dexmedetomidine increases the brain delivery of intrathecally administered drugs. • Glymphatic flow can be manipulated to enhance the CNS delivery of intrathecal therapeutics. [ABSTRACT FROM AUTHOR]

Abstract Cerebral small vessel disease(s) (SVD) results from pathological changes of the small blood vessels in the brain and is common in older people. The diagnostic features by which SVD manifests in brain includes white matter hyperintensities, lacunes, dilated perivascular spaces, microbleeds, and atrophy. In the present study, we use in vivo magnetic resonance imaging (MRI) to characterize brain morphometry and longitudinal relaxation time (T1) of spontaneously hypertensive rats (SHRs) to study the contribution of chronic hypertension to SVD relevant pathology. Male SHR and Wistar-Kyoto (WKY) rats underwent 3D variable flip angle spoiled gradient echo brain MRI at 9.4 T at early (seven weeks old) and established (19 weeks old) stages of hypertension. The derived proton density weighted and T1 images were utilized for morphometry and to characterize T1 properties in gray matter (GM), white matter (WM) and cerebrospinal fluid (CSF). Custom tissue probability maps were constructed for accurate computerized whole brain tissue segmentations and voxel-wise analyses. Characteristic morphological differences between the two strains included enlarged ventricles, smaller corpus callosum (CC) volumes and general 'thinning' of CC in SHR compared to WKY rats at both age groups. While we did not observe parenchymal T1 differences, the T1 of CSF was elevated in SHR compared to controls. Collectively these findings indicate that SHRs develop WM atrophy which is a clinically robust MRI biomarker associated with WM degeneration. Highlights • VFA-SPGR MRI technique was applied in studying both morphology and longitudinal relaxation time. • SHRs exhibit enlarged ventricles and reduced corpus callosum volumes. • The longitudinal relaxation times in SHR and WKY rats were not significantly different in the brain. [ABSTRACT FROM AUTHOR]

The glymphatic pathway expedites clearance of waste, including soluble amyloid β (Aβ) from the brain. Transport through this pathway is controlled by the brain's arousal level because, during sleep or anesthesia, the brain's interstitial space volume expands (compared with wakefulness), resulting in faster waste removal. Humans, as well as animals, exhibit different body postures during sleep, which may also affect waste removal. Therefore, not only the level of consciousness, but also body posture, might affect CSF-interstitial fluid (ISF) exchange efficiency. We used dynamic-contrast-enhanced MRI and kinetic modeling to quantify CSF-ISF exchange rates in anesthetized rodents' brains in supine, prone, or lateral positions. To validate the MRI data and to assess specifically the influence of body posture on clearance of Aβ, we used fluorescence microscopy and radioactive tracers, respectively. The analysis showed that glymphatic transport was most efficient in the lateral position compared with the supine or prone positions. In the prone position, in which the rat's head was in the most upright position (mimicking posture during the awake state), transport was characterized by "retention" of the tracer, slower clearance, and more CSF efflux along larger caliber cervical vessels. The optical imaging and radiotracer studies confirmed that glymphatic transport and Aβ clearance were superior in the lateral and supine positions. We propose that the most popular sleep posture (lateral) has evolved to optimize waste removal during sleep and that posture must be considered in diagnostic imaging procedures developed in the future to assess CSF-ISF transport in humans. [ABSTRACT FROM AUTHOR]

An extensive literature shows that astrocytes exhibit metabotropic glutamate receptor 5 (mGluR5)-dependent increases in cytosolic calcium ions (Ca2+) in response to glutamatergic transmission and, in turn, modulate neuronal activity by their Ca2+-dependent release of gliotransmitters. These findings, based on studies of young rodents, have led to the concept of the tripartite synapse, in which astrocytes actively participate in neurotransmission. Using genomic analysis, immunoelectron microscopy, and two-photon microscopy of astrocytic Ca2+ signaling in vivo, we found that astrocytic expression of mGluR5 is developmentally regulated and is undetectable after postnatal week 3. In contrast, mGluR3, whose activation inhibits adenytate cyclase but not calcium signaling, was expressed in astrocytes at all developmental stages. Neuroglial signaling in the adult brain may therefore occur in a manner fundamentally distinct from that exhibited during development. [ABSTRACT FROM AUTHOR]

Microinfarcts are a common clinical feature of the aging brain, particularly in patients with cognitive decline or vascular or Alzheimer's dementia. However, the natural history of these lesions remains largely unexplored. Here we describe a mouse (C57BL/6J) model of multiple diffuse microinfarcts induced by unilateral internal carotid artery injection of cholesterol crystals (40 -70 /xm). Microinfarcts were spread throughout the deep cortex, subcortical tissue, and hippocampus and were comprised of a core positive for CD68 (a marker for reactive microglia and macrophages), surrounded by large regions of glial fibrillary acidic protein-positive reactive astrogliosis. Widespread reactive gliosis, including mislocalization of the astrocytic water channel aquaporin 4 persisted long after injury, recovering only after 1 month after stroke. Within the cortex, neuronal cell death progressed gradually over the first month, from ~35% at 3 d to 60% at 28 d after stroke. Delayed demyelination was also observed in lesions, beginning 28 d after stroke. These findings demonstrate that microinfarct development follows a distinct course compared to larger regional infarcts such as those induced by middle cerebral artery occlusion. The long-lasting gliosis, delayed neuronal loss, and demyelination suggest that the therapeutic window for microinfarcts may be much wider (perhaps days to weeks) than for larger strokes. [ABSTRACT FROM AUTHOR]

Protoplasmic astrocytes are critically important to energy metabolism in the CNS. Our current understanding of the metabolic interactions between neurons and glia is based on studies using cultured cells, from which mainly inferential conclusions have been drawn as to the relative roles of neurons and glia in brain metabolism. In this study, we used functional genomics to establish the relative compartmentalization of neuronal and astrocytic metabolic pathways in the adult brain. To this end, fluorescence-activated cell sorting was used to directly isolate neurons and protoplasmic astrocytes from the cortex of adult mice. Microarray analysis showed that astrocytes and neurons each express transcripts predicting individual self-sufficiency in both glycolysis and oxidative metabolism. Surprisingly, most enzymes in the tricarboxylic acid (TCA) cycle were expressed at higher relative levels in astrocytes than in neurons. Mass spectrometric analysis of the TCA cycle intermediates confirmed that freshly isolated adult astrocytes maintained an active TCA cycle, whereas immuno-electron microscopy revealed that fine astrocytic processes encompassing synapses contained a higher density of mitochondria than surrounding cells. These observations indicate that astrocytes exhibit robust oxidative metabolism in the intact adult brain and suggest a prominent contribution of astrocytic metabolism to functional brain imaging, including BOLD (blood-oxygen level-dependent) functional magnetic resonance imaging signals. [ABSTRACT FROM AUTHOR]

One of the most distinguishing features of the adult human brain is the complexity and diversity of its cortical astrocytes. Human protoplasmic astrocytes manifest a threefold larger diameter and have tenfold more primary processes than those of rodents. In all mammals, protoplasmic astrocytes are organized into spatially non-overlapping domains that encompass both neurons and vasculature. Yet unique to humans and primates are additional populations of layer 1 interlaminar astrocytes that extend long (millimeter) fibers, and layer 5–6 polarized astrocytes that also project distinctive long processes. We propose that human cortical evolution has been accompanied by increasing complexity in the form and function of astrocytes, which reflects an expansion of their functional roles in synaptic modulation and cortical circuitry. [Copyright &y& Elsevier]

Although astrocytes are the most abundant cell type in the brain, evidence for their activation during physiological sensory activity is lacking. Here we show that whisker stimulation evokes increases in astrocytic cytosolic calcium (Ca2+) within the barrel cortex of adult mice. Increases in astrocytic Ca2+ were a function of the frequency of stimulation, occurred within several seconds and were inhibited by metabotropic glutamate receptor antagonists. To distinguish between synaptic input and output, local synaptic activity in cortical layer 2 was silenced by iontophoresis of AMPA and NMDA receptor antagonists. The antagonists did not reduce astrocytic Ca2+ responses despite a marked reduction in excitatory postsynaptic currents in response to whisker stimulation. These findings indicate that astrocytes respond to synaptic input, by means of spillover or ectopic release of glutamate, and that increases in astrocytic Ca2+ occur independently of postsynaptic excitatory activity. [ABSTRACT FROM AUTHOR]

The subcortical white matter of the adult human brain harbors a pool of glial progenitor cells. These cells can be isolated by fluorescence-activated cell sorting (FACS) after either transfection with green fluorescent protein (GFP) under the control of theCNP2 promoter, or A2B5-targeted immunotagging. Although these cells give rise largely to oligodendrocytes, in low-density culture we observed that some also generated neurons. We thus asked whether these nominally glial progenitors might include multipotential progenitor cells capable of neurogenesis. We found that adult human white-matter progenitor cells (WMPCs) could be passaged as neurospheresin vitro and that these cells generated functionally competent neurons and glia both in vitro and after xenograft to the fetal rat brain. WMPCs were able to produce neurons after their initial isolation and did not requirein vitro expansion or reprogramming to do so. These experiments indicate that an abundant pool of mitotically competent neurogenic progenitor cells resides in the adult human white matter. [ABSTRACT FROM AUTHOR]

Background: Periarterial spaces (PASs) are annular channels that surround arteries in the brain and contain cerebrospinal fluid (CSF): a flow of CSF in these channels is thought to be an important part of the brain's system for clearing metabolic wastes. In vivo observations reveal that they are not concentric, circular annuli, however: the outer boundaries are often oblate, and the arteries that form the inner boundaries are often offset from the central axis. Methods: We model PAS cross-sections as circles surrounded by ellipses and vary the radii of the circles, major and minor axes of the ellipses, and two-dimensional eccentricities of the circles with respect to the ellipses. For each shape, we solve the governing Navier–Stokes equation to determine the velocity profile for steady laminar flow and then compute the corresponding hydraulic resistance. Results: We find that the observed shapes of PASs have lower hydraulic resistance than concentric, circular annuli of the same size, and therefore allow faster, more efficient flow of cerebrospinal fluid. We find that the minimum hydraulic resistance (and therefore maximum flow rate) for a given PAS cross-sectional area occurs when the ellipse is elongated and intersects the circle, dividing the PAS into two lobes, as is common around pial arteries. We also find that if both the inner and outer boundaries are nearly circular, the minimum hydraulic resistance occurs when the eccentricity is large, as is common around penetrating arteries. Conclusions: The concentric circular annulus assumed in recent studies is not a good model of the shape of actual PASs observed in vivo, and it greatly overestimates the hydraulic resistance of the PAS. Our parameterization can be used to incorporate more realistic resistances into hydraulic network models of flow of cerebrospinal fluid in the brain. Our results demonstrate that actual shapes observed in vivo are nearly optimal, in the sense of offering the least hydraulic resistance. This optimization may well represent an evolutionary adaptation that maximizes clearance of metabolic waste from the brain. [ABSTRACT FROM AUTHOR]

Background: Neurodegenerative diseases such as Alzheimer's are associated with the aggregation of endogenous peptides and proteins that contribute to neuronal dysfunction and loss. The glymphatic system, a brain-wide perivascular pathway along which cerebrospinal fluid (CSF) and interstitial fluid (ISF) rapidly exchange, has recently been identified as a key contributor to the clearance of interstitial solutes from the brain, including amyloid β. These findings suggest that measuring changes in glymphatic pathway function may be an important prognostic for evaluating neurodegenerative disease susceptibility or progression. However, no clinically acceptable approach to evaluate glymphatic pathway function in humans has yet been developed.Methods: Time-sequenced ex vivo fluorescence imaging of coronal rat and mouse brain slices was performed at 30-180 min following intrathecal infusion of CSF tracer (Texas Red- dextran-3, MW 3 kD; FITC- dextran-500, MW 500 kD) into the cisterna magna or lumbar spine. Tracer influx into different brain regions (cortex, white matter, subcortical structures, and hippocampus) in rat was quantified to map the movement of CSF tracer following infusion along both routes, and to determine whether glymphatic pathway function could be evaluated after lumbar intrathecal infusion.Results: Following lumbar intrathecal infusions, small molecular weight TR-d3 entered the brain along perivascular pathways and exchanged broadly with the brain ISF, consistent with the initial characterization of the glymphatic pathway in mice. Large molecular weight FITC-d500 remained confined to the perivascular spaces. Lumbar intrathecal infusions exhibited a reduced and delayed peak parenchymal fluorescence intensity compared to intracisternal infusions.Conclusion: Lumbar intrathecal contrast delivery is a clinically useful approach that could be used in conjunction with dynamic contrast enhanced MRI nuclear imaging to assess glymphatic pathway function in humans. [ABSTRACT FROM AUTHOR]