John Duncan

Multiplicity of cerebrospinal fluid functions: New challenges in health and disease

This review integrates eight aspects of cerebrospinal fluid (CSF) circulatory dynamics: formation rate, pressure, flow, volume, turnover rate, composition, recycling and reabsorption. Novel ways to modulate CSF formation emanate from recent analyses of choroid plexus transcription factors (E2F5), ion transporters (NaHCO3 cotransport), transport enzymes (isoforms of carbonic anhydrase), aquaporin 1 regulation, and plasticity of receptors for fluid-regulating neuropeptides. A greater appreciation of CSF pressure (CSFP) is being generated by fresh insights on peptidergic regulatory servomechanisms, the role of dysfunctional ependyma and circumventricular organs in causing congenital hydrocephalus, and the clinical use of algorithms to delineate CSFP waveforms for diagnostic and prognostic utility. Increasing attention focuses on CSF flow: how it impacts cerebral metabolism and hemodynamics, neural stem cell progression in the subventricular zone, and catabolite/peptide clearance from the CNS. The pathophysiological significance of changes in CSF volume is assessed from the respective viewpoints of hemodynamics (choroid plexus blood flow and pulsatility), hydrodynamics (choroidal hypo- and hypersecretion) and neuroendocrine factors (i.e., coordinated regulation by atrial natriuretic peptide, arginine vasopressin and basic fibroblast growth factor). In aging, normal pressure hydrocephalus and Alzheimers disease, the expanding CSF space reduces the CSF turnover rate, thus compromising the CSF sink action to clear harmful metabolites (e.g., amyloid) from the CNS. Dwindling CSF dynamics greatly harms the interstitial environment of neurons. Accordingly the altered CSF composition in neurodegenerative diseases and senescence, because of adverse effects on neural processes and cognition, needs more effective clinical management. CSF recycling between subarachnoid space, brain and ventricles promotes interstitial fluid (ISF) convection with both trophic and excretory benefits. Finally, CSF reabsorption via multiple pathways (olfactory and spinal arachnoidal bulk flow) is likely complemented by fluid clearance across capillary walls (aquaporin 4) and arachnoid villi when CSFP and fluid retention are markedly elevated. A model is presented that links CSF and ISF homeostasis to coordinated fluxes of water and solutes at both the blood-CSF and blood-brain transport interfaces.Outline1 Overview2 CSF formation2.1 Transcription factors2.2 Ion transporters2.3 Enzymes that modulate transport2.4 Aquaporins or water channels2.5 Receptors for neuropeptides3 CSF pressure3.1 Servomechanism regulatory hypothesis3.2 Ontogeny of CSF pressure generation3.3 Congenital hydrocephalus and periventricular regions3.4 Brain response to elevated CSF pressure3.5 Advances in measuring CSF waveforms4 CSF flow4.1 CSF flow and brain metabolism4.2 Flow effects on fetal germinal matrix4.3 Decreasing CSF flow in aging CNS4.4 Refinement of non-invasive flow measurements5 CSF volume5.1 Hemodynamic factors5.2 Hydrodynamic factors5.3 Neuroendocrine factors6 CSF turnover rate6.1 Adverse effect of ventriculomegaly6.2 Attenuated CSF sink action7 CSF composition7.1 Kidney-like action of CP-CSF system7.2 Altered CSF biochemistry in aging and disease7.3 Importance of clearance transport7.4 Therapeutic manipulation of composition8 CSF recycling in relation to ISF dynamics8.1 CSF exchange with brain interstitium8.2 Components of ISF movement in brain8.3 Compromised ISF/CSF dynamics and amyloid retention9 CSF reabsorption9.1 Arachnoidal outflow resistance9.2 Arachnoid villi vs. olfactory drainage routes9.3 Fluid reabsorption along spinal nerves9.4 Reabsorption across capillary aquaporin channels10 Developing translationally effective models for restoring CSF balance11 Conclusion

RAGE, LRP-1, and amyloid-beta protein in Alzheimer’s disease

The receptor for advanced glycation end products (RAGE) is thought to be a primary transporter of β-amyloid across the blood–brain barrier (BBB) into the brain from the systemic circulation, while the low-density lipoprotein receptor-related protein (LRP)-1 mediates transport of β-amyloid out of the brain. To determine whether there are Alzheimer’s disease (AD)-related changes in these BBB-associated β-amyloid receptors, we studied RAGE, LRP-1, and β-amyloid in human elderly control and AD hippocampi. In control hippocampi, there was robust RAGE immunoreactivity in neurons, whereas microvascular staining was barely detectable. LRP-1 staining, in contrast, was clearly evident within microvessels but only weakly stained neurons. In AD cases, neuronal RAGE immunoreactivity was significantly decreased. An unexpected finding was the strongly positive microvascular RAGE immunoreactivity. No evidence for colocalization of RAGE and β-amyloid was seen within either microvessels or senile plaques. A reversed pattern was evident for LRP-1 in AD. There was very strong staining for LRP-1 in neurons, with minimal microvascular staining. Unlike RAGE, colocalization of LRP-1 and β-amyloid was clearly present within senile plaques but not microvessels. Western blot analysis revealed a much higher concentration of RAGE protein in AD hippocampi as compared with controls. Concentration of LRP-1 was increased in AD hippocampi, likely secondary to its colocalization with senile plaques. These data confirm that AD is associated with changes in the relative distribution of RAGE and LRP-1 receptors in human hippocampus. They also suggest that the proportion of amyloid within the brains of AD patients that is derived from the systemic circulation may be significant.

The choroid plexus-cerebrospinal fluid system: from development to aging.

The function of the cerebrospinal fluid (CSF) and the tissue that secretes it, the choroid plexus (CP), has traditionally been thought of as both providing physical protection to the brain through buoyancy and facilitating the removal of brain metabolites through the bulk drainage of CSF. More recent studies suggest, however, that the CP-CSF system plays a much more active role in the development, homeostasis, and repair of the central nervous system (CNS). The highly specialized choroidal tissue synthesizes trophic and angiogenic factors, chemorepellents, and carrier proteins, and is strategically positioned within the ventricular cavities to supply the CNS with these biologically active substances. Through polarized transport systems and receptor-mediated transcytosis across the choroidal epithelium, the CP, a part of the blood-CSF barrier (BCSFB), controls the entry of nutrients, such as amino acids and nucleosides, and peptide hormones, such as leptin and prolactin, from the periphery into the brain. The CP also plays an important role in the clearance of toxins and drugs. During CNS development, CP-derived growth factors, such as members of the transforming growth factor-beta superfamily and retinoic acid, play an important role in controlling the patterning of neuronal differentiation in various brain regions. In the adult CNS, the CP appears to be critically involved in neuronal repair processes and the restoration of the brain microenvironment after traumatic and ischemic brain injury. Furthermore, recent studies suggest that the CP acts as a nursery for neuronal and astrocytic progenitor cells. The advancement of our knowledge of the neuroprotective capabilities of the CP may therefore facilitate the development of novel therapies for ischemic stroke and traumatic brain injury. In the later stages of life, the CP-CSF axis shows a decline in all aspects of its function, including CSF secretion and protein synthesis, which may in themselves increase the risk for development of late-life diseases, such as normal pressure hydrocephalus and Alzheimers disease. The understanding of the mechanisms that underlie the dysfunction of the CP-CSF system in the elderly may help discover the treatments needed to reverse the negative effects of aging that lead to global CNS failure.

Enhanced Prospects for Drug Delivery and Brain Targeting by the Choroid Plexus–CSF Route

The choroid plexus (CP), i.e., the blood–cerebrospinal fluid barrier (BCSFB) interface, is an epithelial boundary exploitable for drug delivery to brain. Agents transported from blood to lateral ventricles are convected by CSF volume transmission (bulk flow) to many periventricular targets. These include the caudate, hippocampus, specialized circumventricular organs, hypothalamus, and the downstream pia–glia and arachnoid membranes. The CSF circulatory system normally provides micronutrients, neurotrophins, hormones, neuropeptides, and growth factors extensively to neuronal networks. Therefore, drugs directed to CSF can modulate a variety of endocrine, immunologic, and behavioral phenomema; and can help to restore brain interstitial and cellular homeostasis disrupted by disease and trauma. This review integrates information from animal models that demonstrates marked physiologic effects of substances introduced into the ventricular system. It also recapitulates how pharmacologic agents administered into the CSF system prevent disease or enhance the brain’s ability to recover from chemical and physical insults. In regard to drug distribution in the CNS, the BCSFB interaction with the blood–brain barrier is discussed. With a view toward translational CSF pharmacotherapy, there are several promising innovations in progress: bone marrow cell infusions, CP encapsulation and transplants, neural stem cell augmentation, phage display of peptide ligands for CP epithelium, CSF gene transfer, regulation of leukocyte and cytokine trafficking at the BCSFB, and the purification of neurotoxic CSF in degenerative states. The progressively increasing pharmacological significance of the CP–CSF nexus is analyzed in light of treating AIDS, multiple sclerosis, stroke, hydrocephalus, and Alzheimer’s disease.

Early neutrophilic expression of vascular endothelial growth factor after traumatic brain injury

The formation of edema after traumatic brain injury (TBI) is in part associated with the disruption of the blood-brain barrier. However, the molecular and cellular mechanisms underlying these phenomena have not been fully understood. One possible factor involved in edema formation is vascular endothelial growth factor (VEGF). This growth factor has previously been demonstrated to increase the blood-brain barrier permeability to the low molecular weight markers and macromolecules. In this study, we analyzed the temporal changes in VEGF expression after TBI in rats. In the intact brain, VEGF was expressed at relatively low levels and was found in the cells located close to the cerebrospinal fluid space. These were the astrocytes located under the ependyma and the pia-glial lining, as well as the epithelial cells of the choroid plexus. In addition, several groups of neurons, including those located in the frontoparietal cortex and in all hippocampal regions, were VEGF-positive. The pattern of VEGF-immunopositive staining of neurons and choroidal epithelium suggested that in these cells, VEGF binds to the cell membrane-associated heparan sulfate proteoglycans. Following TBI, there was an early (within 4 h post-injury) increase in VEGF expression in the traumatized parenchyma associated with neutrophilic invasion. The ipsilateral choroid plexus appeared to play a role in facilitating the migration of neutrophils from blood into the cerebrospinal fluid space, from where many of these cells infiltrated the brain parenchyma. VEGF-immunopositive staining of neutrophils resembled haloes and was found ipsilaterally within the frontoparietal cortex and around the velum interpositum, a part of the subarachnoid space. These haloes likely represent the deposition of neutrophil-derived VEGF within the extracellular matrix, from where this growth factor may be gradually released during an early post-traumatic period. The maximum number of VEGF-secreting neutrophils was observed between 8 h and 1 day after TBI. In addition, from 4 h post-TBI, there was a progressive increase in the number of VEGF-immunoreactive astrocytes in the ipsilateral frontoparietal cortex. The maximum number of astrocytes expressing VEGF was observed 4 days after TBI, and then the levels of astroglial VEGF expression declined gradually. Early invasion of brain parenchyma by VEGF-secreting neutrophils together with a delayed increase in astrocytic synthesis of this growth factor correlate with the biphasic opening of the blood-brain barrier and formation of edema previously observed after TBI. Therefore, these findings suggest that VEGF plays an important role in promoting the formation of post-traumatic brain edema.

Increased expression of vasopressin V1a receptors after traumatic brain injury

Experimental evidence obtained in various animal models of brain injury indicates that vasopressin promotes the formation of cerebral edema. However, the molecular and cellular mechanisms underlying this vasopressin action are not fully understood. In the present study, we analyzed the temporal changes in expression of vasopressin V1a receptors after traumatic brain injury (TBI) in rats. In the intact brain, the V1a receptor was expressed in neurons located in all layers of the frontoparietal cortex. The V1a receptor-immunoreactive product was predominantly localized to neuronal nuclei and had both a diffused and punctate staining pattern. The V1a receptors were also expressed in astrocytes, especially in layer 1 of the frontoparietal cortex. In these cells, two distinctive patterns of immunopositive staining for V1a receptors were observed: a diffused cytosolic staining of cell bodies and processes and a clearly punctate staining pattern that was predominantly localized to the astrocytic cell bodies. The real-time reverse-transcriptase polymerase chain reaction analysis of changes in mRNA for the V1a receptor demonstrated that after TBI, there is an early (4 h post-TBI) increase in the number of transcripts in the ipsilateral frontoparietal cortex, when compared to the contralateral hemisphere or the sham-injured rats. This increase in the message was followed by the up-regulation of expression of the V1a receptors at the protein level. This was most evident in cortical astrocytes in the areas surrounding the lesion. The number of the V1a receptor-immunopositive astrocytes in the traumatized parenchyma gradually increased, starting at 8 h and peaking at 4-6 days after TBI. Furthermore, a redistribution of V1a receptors from the astrocytic cell bodies to the astrocytic processes was observed. In addition to astrocytes, an increased expression of V1a receptors was found in the endothelium of both blood microvessels and the large-diameter blood vessels in the frontoparietal cortex ipsilateral to injury. This increase in the V1a receptor expression was apparent between 2 and 4 days after TBI. As early as 1-2 h following the impact, there was also a striking increase in the number of the V1a receptor-immunopositive beaded axonal processes, with greatly enlarged varicosities, that were localized to various areas of the injured parenchyma. It is suggested that the increased expression of V1a receptors plays an important role in the vasopressin-mediated formation of edema in the injured brain.

Vertebrobasilar Thrombosis in Children: Report of Two Cases and Recommendations for Treatment

Two consecutive cases of children with vertebrobasilar thrombosis (VBT) were treated with high-dose intra-arterial urokinase within 4 h of presenting to the emergency room, after full evaluation by CT scan, MRI and MR angiography. Complete resolution of neurologic symptoms was achieved in both cases. Based on our limited pediatric experience, previous treatment of VBT at our institution and a review of the relevant literature, the authors suggest that VBT be specifically ruled out at initial diagnosis, and if present, full consideration be given to immediate treatment with intra-arterial thrombolytic therapy. This may lead to a significant reduction in the morbidity and mortality associated with VBT in children. A large prospective multi-institution study is needed to further evaluate the efficacy of this approach to childhood stroke.

Whole-body hyperthermia in the rat disrupts the blood-cerebrospinal fluid barrier and induces brain edema

The present investigation was undertaken to find out whether whole-body hyperthermia (WBH) alters blood-cerebrospinal fluid barrier (BCSFB) permeability to exogenously-administered tracers and whether choroid plexus and ependymal cells exhibit morphological alterations in hyperthermia. Rats subjected to 4 hours of heat stress at 38 degrees C in a biological oxygen demand (BOD) incubator exhibited a profound increase in the BCSFB to Evans blue and radioiodine. Blue staining of the dorsal surface of the hippocampus and caudate nucleus and a significant increase in Evans blue and [131]Iodine in cisternal cerebrospinal fluid were seen following 4-hour heat stress compared to control. Degeneration of choroidal epithelial cells and underlying ependyma, a dilated ventricular space, and degenerative changes in the underlying neuropil were frequent. Hippocampus, caudate nucleus, thalamus, and hypothalamus exhibited profound increases in water content after 4 hours of heat stress. These observations suggest that hyperthermia induced by WBH is capable of breaking down the BCSFB and contributing to cell and tissue injury in the central nervous system.

Altered expression of sialic acid-bearing glycoconjugates as revealed by lectin binding to choroid plexus in perinatal hydrocephalic rats

Glycoconjugates perform key roles in the development and maintenance of the CNS. A chief saccharide element in brain development is sialic acid, usually occurring as Nacetyl-neuraminic acid. Previous studies by H. Jones et al. demonstrated that glyco-conjugate secretions into CSF, by circumventricular organs like the SCO, may be associated with hydrocephalus development. Therefore, we used 4 lectins (LPA, SNA, MAL-II and WGA), specific for several forms of sialic acid, to probe for alterations in sialic acidbearing glycoconjugate profiles in the CSF-secreting choroid plexus (CP) of the HTx rat hydrocephalus model. Lectins are proteins that bind to carbohydrate residues in an antibody-like manner. We used 3 cohorts of rats: prenatal (20 days of gestation); neonatal (1 day after birth); and postnatal (4 days). Five choroid epithelial domains/ parameters were analyzed: apical cell surface; lateral surface; basal surface; cytoplasm; and all domains considered collectively. Staining intensity was judged visually on a 1+ to 4+ scale, at 400×. Means of sialic acid staining intensity were calculated using 5 animals from each cohort.

Hydrocephalus disorders: their biophysical and neuroendocrine impact on the choroid plexus epithelium

Publisher Summary This chapter discusses that choroid plexus (CP) epithelial cells carry out a wide variety of secretory and reabsorptive functions that, by way of cerebrospinal fluid (CSF) volume transmission, contribute to the stability of the neuronal extracellular environment. During fetal development, the brain critically depends upon the CP–CSF nexus for a steady supply of micronutrients and trophic factors to support normal growth. In adulthood, the CP reacts to biochemical and physical perturbations, associated with disease and trauma by secreting into CSF a wide array of growth factors, transport (carriage) proteins, and neuropeptides. Arginine vasopressin (AVP) and basic fibroblast growth factor (FGF–2) colocalize in the choroid epithelium. Release of these peptides at the blood–CSF barrier in response to ischemia and augmented intracranial pressure helps to repair injured tissue, and adjust CSF formation and pressure. The chapter discusses that as a part of homeostatic mechanisms to modulate extracellular fluid parameters, in the face of CSF distortions, such as ventriculomegaly, there is plasticity in regard to CP receptor targets for AVP and atrial natriuretic peptide (ANP). Functional and ultrastructural evidence is offered to support the model of a CSF neuroendocrine system, the histological substrate, of which is the “dark” epithelium in CP. The response of the AVP, ANP, and nitrergic (nNOS) systems to hydrocephalus provides insight on the fluid dynamics in the basolateral space between choroid epithelial cells.