Inflammation and Programmed Cell Death in Alzheimer’s Disease: Comparison of the Central Nervous System and Peripheral Blood

Although the central nervous system (CNS) has been defined as a privileged site in Alzheimer’s disease (AD), periphery can be more than simply witness of events leading to neurodegeneration. The CNS and peripheral blood can mutually communicate through cells and factors trafficking from the circulation into the brain and vice versa. A number of articles have reviewed inflammatory profiles and programmed cell death (PCD) in AD, separately in the CNS and at the peripheral level. This review does not provide an exhaustive account of what has been published on inflammation and PCD in AD. Rather, the aim of this review is to focus on possible linkages between the central and the peripheral compartments during AD progression, by critically analyzing, in a comparative manner, phenomena occurring in the CNS as well as the peripheral blood. In fact, growing evidence suggests that CNS and peripheral inflammation might present common features in the disease. Microarrays and metabolomics revealed that dysfunction of the glycolytic and oxidative pathways is similar in the brain and in the periphery. Moreover, dysregulated autophagosome/lysosomal molecular machinery, both at the CNS and the peripheral level, in AD-related cell damage, has been observed. Possible implications of these observations have been discussed.     

Experssion of α-Bungarotoxin Receptro Subtypes in Chick Central Nervous System During Development

Abstract

Chick central nervous system (CNS) expresses a-bungarotoxin (aBgtx) receptors. We have recently reported the purification and characterization of two aBgtx receptor subtypes, a7 and a7-a8 from chick optic lobe (COL). In order to study whether other aBgtx receptor subtypes are present in other areas of the chick CNS, as well as their developmental expression, we used anti-a7 and anti-a8 subunitspecific antibodies to study aBgtx receptors at different developmental stages in COL, brain and retina. We found that only the a7 and a7-a8 subtypes are present at all developmental stages in chick COL and brain, where they represent 90% of all the aBgtx receptors at embryonic day 19 and 1 day post hatching (Dl). In chick retina, an a8 subtype representing 50% of all aBgtx receptors at D1 is present in addition to the a7 and a7-a8 subtypes, and the expression of this a8 subtype increases during neurodevelopment.     

Steroid 5α-reductase Type 1 Immunolocalized in the Rat Peripheral Nervous System and Paraganglia

Steroid 5-reductase is an enzyme that converts a number of steroids with a C-4, 5 double bond and C-3 ketone to 5- reduced metabolites. This enzyme has been suggested to play a role in brain development and myelination in the rat nervous system. In the present study, we examined the cellular and subcellular localization of the enzyme immunocytochemically in the rat peripheral nervous system and paraganglia using a polyclonal antibody against rat 5-reductase type 1. Light and electron microscopical studies localized 5-reductase in the Schwann cells of myelinated and unmyelinated nerve fibres, the satellite cells of the ganglia, the enteric glial cells and the supporting/sustentacular cells of the paraganglia. In the myelinated nerve fibres, immunoreactivity was observed in the outer loops, the nodes of Ranvier and the Schmidt–Lanterman incisures. Subcellularly, the immunoreactivity was localized in the cytopl asm of various glial cells. No immunoreactivity was observed in the myelin membrane, the axon or the neuronal perikaryon. These findings suggest that 5-reductase is widely distributed in glial cells, and that, in addition to myelination, 5-reduced steroids play a role in some glial functions in the peripheral nervous system.     

Distribution Pattern of γ-amino Butyric Acid Immunoreactive Neural Structures in the Central and Peripheral Nervous System of the Tubificid Worm, Limnodrilus hoffmeisteri

By means of whole-mount immunohistochemistry, putative inhibitory (GABAergic) neural structures were identified in the central and peripheral nervous system of the tubificid worm, Limnodrilus hoffmeisteri. In the supraoesophageal ganglion (brain) only few strongly labelled cells were observed. However, in its commissural part a high number of stained nerve fibres, arising mainly from the ventral nerve cord and prostomium, occurred. Except for the suboesophageal ganglion the arrangement of γ-amino butyric acid-immunoreactive (GABA-IR) structures proved to be identical in each VNC ganglion. Behind the first segmental nerves three pairs of heavily stained neurones were located. Their processes (both ipsi- and contralateral) form four bundles of fine-fibred polysegmental interneuronal tracts that run close to the dorsal giant axons from the terminal ganglion to the suboesophageal one without interruption. A few small motoneurons and a pair of large ones with contralateral processes were also identified. A bipolar (presumably sensory) neuron was located at the root of each second segmental nerve. GABA-IR neurons were also found in the stomatogastric ganglia and pharyngeal wall; however, the latter structure had a well-developed fibre network, as well. Present results suggest that GABA acts as a common neurotransmitter in sensory, interneuronal and motor system of L. hoffmeisteri. The possible functional role of the identified GABA-IR neural structures in locomotion, escape and withdrawal reflexes in tubificid worms is discussed.     

2-Phenylethylamine: A Modulator of Catecholamine Transmission in the Mammalian Central Nervous System?

Since the identification of 2-phenylethylamine (β-phenylethylamine; PE) as a biogenic amine, there has been much discussion about what role, if any, it may have in the CNS. Indeed, the low endogenous concentration of PE in the brain and its relatively low potency in behavioral and pharmacological experiments have led some researchers to conclude that perhaps PE possessed no physiological role at all but that it was merely a metabolic by-product. Our findings have caused us to conclude otherwise, and in this article we review the neurochemical, neuropharmacological, and neurophysiological findings that lead us to propose that PE is a neuromodulator of catecholamine neurotransmission in the CNS.     

A Systematic Analysis of Buried Polar Side Chains and Their Interactions in α-Helical Proteins

Examination of 80 -helical proteins and domains demonstrates that they contain from 1 to more than 20 completely buried (water-inaccessible) polar side chains. As a rule the latter have partners for H-bonding but the resulting H-bond system is often not saturating. Basing on statistical analysis, we determined the optimal number of H-bonds for every type of polar side chain, and discuss the structural role of vacant donors and acceptors. About half of the H-bonds formed by buried side chains pertain to interhelix contacts of the (side chain)–(side chain) and (side chain)–(main chain) types. Such interactions appear to be a most important factor determining the mutual arrangement of -helices in proteins. Analysis of the frequency of occurrence of various interacting pairs reveals that these interactions are selective.     

Information and Efficiency in the Nervous System—A Synthesis

In systems biology, questions concerning the molecular and cellular makeup of an organism are of utmost importance, especially when trying to understand how unreliable components—like genetic circuits, biochemical cascades, and ion channels, among others—enable reliable and adaptive behaviour. The repertoire and speed of biological computations are limited by thermodynamic or metabolic constraints: an example can be found in neurons, where fluctuations in biophysical states limit the information they can encode—with almost 20–60% of the total energy allocated for the brain used for signalling purposes, either via action potentials or by synaptic transmission. Here, we consider the imperatives for neurons to optimise computational and metabolic efficiency, wherein benefits and costs trade-off against each other in the context of self-organised and adaptive behaviour. In particular, we try to link information theoretic (variational) and thermodynamic (Helmholtz) free-energy formulations of neuronal processing and show how they are related in a fundamental way through a complexity minimisation lemma.     

Neuron–Astroglial Interactions in Cell-Fate Commitment and Maturation in the Central Nervous System

Neuron–astroglia interactions play a key role in several events of brain development, such as neuronal generation, migration, survival, and differentiation; axonal growth; and synapse formation and function. While there is compelling evidence of the effects of astrocyte factors on neurons, their effects on astrocytes have not been fully determined. In this review, we will focus on the role of neurons in astrocyte generation and maturation. Further, we highlight the great heterogeneity and diversity of astroglial and neural progenitors such as radial glia cells, and discuss the importance of the variety of cellular interactions in controlling the structural and functional organization of the brain. Finally, we present recent data on a new role of astrocytes in neuronal maturation, as mediators of the action of biolipids in the cerebral cortex. We will argue that the functional architecture of the brain depends on an intimate neuron-glia partnership, by briefly discussing the emerging view of how neuron-astrocyte dysfunctions might be associated with neurodegenerative diseases and neurological disorders.     

Manifestations of Dissolution of the Central Nervous System in the Wakefulness–Sleep Cycle of Mammals

The data are presented on three stages of formation of the wakefulness–sleep cycle (WSC) in representatives of poikilothermal animals (fish, amphibians, and reptiles), which are formed in accordance with morphofunctional stages of CNS integration. Comparison of morphofunctional development of brain structures in ontogenesis of mammals with dynamics of formation of the WSC neurophysiological parameters allows revealing similarity and parallelism in phylo- and ontogenetic stages of development of this cycle. All these results confirm the statement that in the process of transition from wakefulness to sleep there occurs a gradual rearrangement of activity of the evolutionary formed levels of CNS integration from the cortico-striatal to the bulbar integration. It is emphasized that unlike the known classical concept of CNS dissolution in pathological catastrophes in its activity, processes of periodical functional dissolution of CNS perform an important protective-restorative function and are vitally important to an organism.     

The Microtubule-Associated Protein 1A (MAP1A) is an Early Molecular Target of Soluble Aβ-Peptide

A progressive accumulation of amyloid β-protein (Aβ) is widely recognized as a pathological hallmark of Alzheimer’s disease (AD). Substantial progress has been made toward understanding the neurodegenerative cascade initiated by small soluble species of Aβ and recent evidence supports the notion that microtubule rearrangements may be proximate to neuritic degeneration and deficits in episodic declarative memory. Here, we examined primary cortical neurons for changes in markers associated with synaptic function following exposure to sublethal concentrations of non-aggregated Aβ-peptide. This data show that soluble Aβ species at a sublethal concentration induce degradation of the microtubule-associated protein 1A (MAP1A) without concurrently affecting dendritic marker MAP2 and/or the pre-synaptic marker synaptophysin. In addition, MAP1A was found to highly co-localize with the postsynaptic density-95 (PSD-95) protein, proposing that microtubule perturbations might be central for the Aβ-induced neuronal dysfunctions as PSD-95 plays a key role in synaptic plasticity. In conclusion, this study suggests that disruption of MAP1A could be a very early manifestation of Aβ-mediated synaptic dysfunction—one that presages the clinical onset of AD by years. Moreover, our data support the notion of microtubule-stabilizing agents as effective AD drugs.     

The influence of α-tocopherol on arylsulfatases A and B in the liver of vitamin A-deficient rats

• 1.1. The effect of different levels of dietary α-tocopherol on arylsulfatases A and B (EC 3.1.6.1) was investigated in the liver of vitamin A-deficient rats and pair-fed controls.
• 2.2. Arylsulfatase A was increased in the liver of vitamin A-deficient rats receiving “normal” dietary α-tocopherol, but not when the rats were fed a high level of vitamin E.
• 3.3. Arylsulfatase B was increased in vitamin A deficiency regardless of the dietary α-tocopherol level.
• 4.4. Incubation at 37° of the lysosome-rich liver fraction caused more rapid release of both arylsulfatases from the lysosomes of the deficient rats receiving “normal” dietary α-tocopherol. High dietary α-tocopherol reversed this phenomenon.
• 5.5. The effect of retinol added in vitro appeared to be the opposite of its effect in vivo on sulfatase release from the lysosomes, whereas α-tocopherol had the same effect when added in vitro as when fed at high levels in vivo.
• 6.6. The effect of vitamin A deficiency on sulfate metabolism might be mediated through the role of retinol and perhaps of α-tocopherol on the stability of biological membranes.

     

Domain-Opening and Dynamic Coupling in the α-Subunit of Heterotrimeric G Proteins

Heterotrimeric G proteins are conformational switches that turn on intracellular signaling cascades in response to the activation of G-protein-coupled receptors. Receptor activation by extracellular stimuli promotes a cycle of GTP binding and hydrolysis on the G protein α-subunit (Gα). Important conformational transitions occurring during this cycle have been characterized from extensive crystallographic studies of Gα. However, the link between the observed conformations and the mechanisms involved in G-protein activation and effector interaction remain unclear. Here we describe a comprehensive principal component analysis of available Gα crystallographic structures supplemented with extensive unbiased conventional and accelerated molecular dynamics simulations that together characterize the response of Gα to GTP binding and hydrolysis. Our studies reveal details of activating conformational changes as well as the intrinsic flexibility of the α-helical domain that includes a large-scale 60° domain opening under nucleotide-free conditions. This result is consistent with the recently reported open crystal structure of Gs, the stimulatory G protein for adenylyl cyclase, in complex with the α2 adrenergic receptor. Sets of unique interactions potentially important for the conformational transition are also identified. Moreover simulations reveal nucleotide-dependent dynamical couplings of distal regions and residues potentially important for the allosteric link between functional sites.Heterotrimeric G proteins undergo cycles of GTP-dependent conformational rearrangements and alterations of their oligomeric αβγ form to convey receptor signals to downstream effectors that control diverse cellular processes ranging from movement to division and differentiation. Interaction with activated receptor promotes the exchange of GDP for GTP on the G protein α subunit (Gα) and its separation from its βγ subunit partners (Gβγ). Both isolated Gα and Gβγ then interact with downstream effectors. GTP hydrolysis deactivates Gα, which reassociates with Gβγ, becoming ready to restart the cycle. Each of these stages has been subjected to extensive crystallographic studies with high-resolution structures of Gα in complex with GDP, GTP analog, Gβγ, and, most recently, the G-protein-coupled receptors now available. These studies have provided extensive mechanistic insight. However, a number of important questions remain, including:

• How do the distinct conformations evident in the accumulated structures interconvert?
• How do disease-associated mutations affect the fidelity of these transitions?
• And, critically, how do distal functional sites responsible for nucleotide and protein partner binding allosterically coordinate their activities?

Here we describe a comprehensive analysis of the accumulated Gα crystallographic structures supplemented with extensive conventional (cMD) and accelerated molecular dynamics (aMD) simulations (1) that together map the structural and dynamical features of Gα in different nucleotide states. These enhanced sampling simulations reveal the spontaneous interconversion between GDP and GTP conformations and also characterize large-scale opening motions of the α-helical domain (HD) that were not accessible to previous simulation studies (2–5). Furthermore, the current simulations results reveal a distinctive pattern of collective motions that provide evidence for a nucleotide-dependent network of dynamic communication between the active site and the receptor and effector binding sites.Principal component analysis of 53 Gα experimental structures homologous to transducin (Gαt) reveals that the major variation in accumulated structures is the concerted association/disassociation of three nucleotide-binding site loops termed the switch regions (SI, SII, and SIII). An additional small-scale (<10°) rotation of the HD relative to the main catalytic Ras-like domain (RasD) is also apparent (see Fig. S1 in the Supporting Material). The distinct conformation of SI–SIII regions gives rise to nucleotide-associated segregation of GDP- and GTP-analog-bound experimental structures along the PC1-PC2 plane. Interestingly, both GDP- and GTP-bound structures display a skewed distribution along the PC1-PC2 plane that arises from HD rotation. In comparison, the distribution of the GTP-bound structures becomes more restricted and the skew decreases when the mapping is based on a principle component analysis that excludes the HD region (see Fig. S1).Recently, the HD region of Gαs (the α-subunit of the stimulatory G protein for adenyl cyclase) was shown to adopt a dramatically more open conformation in a crystal structure complex with the β2 adrenergic receptor (β2AR) (6). This clam-shell-like 127° opening in the absence of nucleotide and presence of receptor is consistent with electron microscopy (7) and double electron-electron resonance analysis (8). These results, together with recent hydrogen-deuterium exchange mass spectrometry data (9), indicate that there may be additional functional motions and inherent flexibility in the ensemble of native states beyond those apparent in the accumulated crystal structures of Gαt (9). To address this question, we performed multiple 100-ns aMD simulations of nucleotide-free Gαt. These simulations reveal a spontaneous large-scale opening and closing motion of larger magnitude (>60°) than those evident in the distribution of crystallographic structures (Fig. 1 A and see Fig. S2). In addition, the trajectory reveals two dominant modes of HD opening: an out-of-plane shifting (PC1 in Fig. S3) and an in-plane rotation (PC2 in Fig. S3). It is also notable that nucleotide-free aMD simulations sample both active (GTP-like) and inactive (GDP-like) structures (see Fig. S2) in an analogous manner to the spontaneous GDP to GTP interconversion sampled for Ras and Rho small G proteins with similar methods (10–12).Open in a separate windowFigure 1Nucleotide-associated differences in flexibility and dynamic coupling. (A) Mapping aMD simulation trajectories (blue points) onto the principal components obtained from analysis of Gα crystallographic GDP-bound (green) and GTP-analog bound (red) experimental structures. (Orange) Open β2AR-Gαs complex structure. (B) Results of dynamic coupling analysis mapped onto the average structure for each nucleotide state. (Spheres) Nodes for the nucleotide; the protein cartoon is colored by community structure. (C) Community network graph. (Circles) Communities, colored as in panel B. Radius of the circle indicates the number of residues in the community. Thickness of linking lines is determined by the maximum betweenness of the respective intercommunity edges (see the Supporting Material). (Red, blue, and green edges) Major topological difference between states.The low sequence identity between Gαt and Gαs (44.5%), as well as the absence of the receptor and Gβγ in the simulations, may explain the difference between the predicted ∼60° Gαt-HD rotation and that displayed in the β2AR-Gαs crystallographic structure (see Fig. S3). It is notable that, although the amplitude is much smaller, aMD simulations with bound nucleotide display similar dominant HD motions to those observed in the nucleotide-free simulations (see Fig. S4). This suggests that the interdomain flexibility of RasD and HD is likely an intrinsic feature of Gαt regardless of nucleotide state.The transition between distinct conformations (structural clusters; see Fig. S5) was observed to correspond to significant dynamical changes in side-chain contacts (see Fig. S6). Specifically, we found sequential contacts breaking during the HD in-plane rotation motion starting from the region between HD helix αD and RasD helix αG toward that between HD helix αE and RasD SIII and the P-loop. In comparison, for the out-of-plane shift, we found simultaneous breaking and formation of contacts in the region containing the loop between helices αB and αC, the N-terminus of αA, αE, and αF of HD; α1, SI, and the loop between strand β6 and helix α5 of RasD. Interactions highlighted in these regions as potentially important for the conformational transitions include D137::K276, S140::K273, S140::D227, Q143::R238, N145::E39, and D146::K266, the effect of which can be further evaluated by mutagenesis experiments and simulations.Dynamic network analysis methods developed by Sethi et al. (13) were used to examine whether the motions of one residue were correlated to the motions of another (distant) residue. In this approach, a weighted graph is constructed where each residue represents a node and the weight of the connection between nodes represents their respective correlation value. A clustering of edges is then used to define local communities of highly correlated residues that represent substructures that are highly intraconnected, but loosely interconnected. Applying this approach to multiple 40-ns cMD simulations initiated from GTP-, GDP-, and aMD-derived APO conformations revealed a consistent community composition as well as a distinct pattern of intercommunity connection between nucleotide states (Fig. 1, B and C).The dynamics of the RasD region can be decomposed into two main communities that stem from the nucleotide base and phosphate regions in GDP and GTP states: The first community is composed of residues from the P-loop, helix α1, strands β1–β3, and the phosphates of the nucleotide (orange in Fig. 1, B and C). The second community comprises residues from helix αG, strands β4–β6, and the nucleotide base region (tan in Fig. 1, B and C). This dynamic partitioning of the central β-sheet and central role of the nucleotide is consistent with the bilobal structure and dynamics previously reported for Ras (14). In the presence of GTP, the first community includes or is dynamically coupled to SI, SII, and SIII regions (see the orange node and the red edge in Fig. 1 C). Removal of the γ-phosphate of GTP disrupts this region, leading to decoupling of the switch regions from the nucleotide. Also evident for GDP states is an apparent tighter coupling of RasD and HD regions (blue edges in Fig. 1 C). We note that these findings are robust to the choice of initial simulation conditions and are observed in both cMD and aMD simulations (see Fig. S7 and Fig. S8). Nucleotide-free Gαt simulations display an altered dynamical network with respect to those of nucleotide bound states. In particular, RasD and HD regions lose connecting edges consistent with the large-scale opening of these domains (e.g., SIII-HD green edges in Fig. 1 C).A number of residues highlighted here as potentially important for mediating the coupling between prominent communities (see Table S1 in the Supporting Material) have been shown by previous mutagenesis studies to affect GDP release. For example, the double mutation A322S/R174M was found to significantly enhance the rate of GDP release (15). The current results indicate that these positions are involved in coupling the nucleotide and RasD. Also, mutations R144A and L232Q caused a faster basal GDP release rate in Gαi1 (16). The current analysis indicates that the equivalent positions in Gαt (S140 and M228) couple the RasD and HD, and suggests that their mutation could promote domain-domain motions. We also note the apparent coupling of α5 with the nucleotide base and Ploop-β1 with the phosphate regions of GDP. These direct connections of the receptor connecting N- and C-terminus to GDP are suggestive of potential routes for receptor-mediated GDP release. We expect further study of these sites and of receptor-bound dynamics to be informative in this regard.In conclusion, simulations suggest a flexible HD in Gαt similar to that found for Gαs. In particular, in the absence of nucleotide we observed the spontaneous large-scale opening and closing of HD relative to RasD, which was unseen in previous computational studies. Moreover, we found that the functional states of Gαt are associated with the distinct dynamical couplings of functional regions including SI–SIII, P-loop, α5, and the HD region. Finally, our results indicate that nucleotide may not directly induce large-scale conformational changes but, instead, act as a modulator of intrinsically accessible conformations and as a central participant in their associated dynamical couplings.     

Dual Function of Transforming Growth Factor-β in the Control of Proliferation and Apoptosis of Cells of the Nervous System

Transforming growth factor- (TGF-) is an agent that gave the name to an extensive superfamily of congeneric cytokines playing important roles in numerous physiological and pathological processes. TGF- is involved in a few signal pathways controlling growth, differentiation, and death (apoptosis) of the nerve cells. Yet, it was found that the role of TGF- in each of these processes is dual: it can act either as their stimulator or as an inhibitor. This review describes examples and principal mechanisms of the dual functions of TGF- in its regulatory influences realized in the mammalian nervous system.     

The α-keto-acids in the blood and urine of the fowl

• 1.1. The extraction, separation and quantitative estimation of α-ketoglutaric and pyruvic acids in the blood and urine of the domestic fowl are described.
• 2.2. The concentration of α-ketoglutaric acid in whole blood rose from 0·75 mg/ 100 ml at hatching to 1·53 mg/100 ml at 4 weeks of age. This was followed by a significant fall to 0·94 mg/100 ml at 5 weeks of age and thereafter it remained fairly constant.
• 3.3. The concentration of pyruvic acid in whole blood at hatching was 2·02 mg/100 ml. After an initial fall it rose to a maximum of 2·78 mg/100 ml at 4 weeks of age but thereafter there was a significant fall to the relatively constant level of approximately 1·3 mg/100 ml.
• 4.4. The concentration of pyruvic acid was found to be significantly higher in either the subclavian or jugular venous blood compared with that of heart blood. This was not so for α-ketoglutaric acid.
• 5.5. After 48 hr starvation there was a significant increase in the pyruvic acid content of whole blood. Several other keto-acids also appeared.
• 6.6. α-ketogkutaric acid was found to be the major keto-acid in the urine and accounter for approximately 44 per cent of the total.
• 7.7. These results are compared with values published for other species.