Cerebral hypoperfusion can occur in normal ageing and is proposed to underlie white matter disturbances observed in the ageing brain. Moreover, cerebral hypoperfusion and white matter attenuation are early events in the progression of Alzheimer’s disease (AD). White matter mostly consists of myelinated axons which have distinct protein architecture, segregated into defined regions; the axon initial segment (AIS), the node of Ranvier, paranode, juxtaparanode, and internode. These sites are essential for action potential initiation and/or propagation and subsequently effective brain function. At the outset of the studies in the thesis there was evidence that the different regions within the myelinated axons are vulnerable to injury and disease. Thus it is hypothesised that in response to normal ageing and/or cerebral hypoperfusion these structures are altered and associated with cognitive impairment and that these effects are exacerbated in a transgenic mouse model (APPSw,Ind, J9 line) which develops age-dependent amyloid-β (Αβ) pathology. The first study aims to investigate the effect of normal ageing and Aβ deposition on myelinated axons and on learning and memory. To address this, the effects of normal ageing on the integrity of the AIS, nodes of Ranvier, myelin, axons, synapses and spatial working memory are examined in young and aged wild-type and TgAPPSw,Ind mice. A significant reduction in the length of nodes of Ranvier is demonstrated in aged wild-type and TgAPPSw,Ind mice. In addition, the length of AIS, is significantly reduced in the aged wild-type animals while the young TgAPPSw,Ind have significantly shorter AIS than the young wild-type mice. These effects are not influenced by the presence of Aβ. Myelin integrity is affected by age but this is more prominent in the wild-type animals whilst axonal integrity is intact. Moreover, there is an age-related decrease of presynaptic boutons only in the TgAPPSw,Ind mice. Contrary to the original hypothesis, working memory performance is not altered with age or influenced by increasing Aβ levels. The second study aims to examine the effects of cerebral hypoperfusion in combination with Αβ pathology and/or ageing on cognitive performance and the structure of myelinated axons. To address this, the effects of surgically induced cerebral hypoperfusion on the integrity of the nodes of Ranvier, paranodes, myelin, axons and spatial working memory performance are investigated in young and aged wild-type and TgAPPSw,Ind mice. A decrease in nodal length is observed in response to hypoperfusion in young and aged animals. This effect is shown to be exacerbated in the young TgAPPSw,Ind animals. Moreover, the disruption of the nodal domain is shown to occur without any gross alterations in myelin and axonal integrity. It is also demonstrated that in response to hypoperfusion, spatial working memory performance is defected in young and aged animals of both genotypes. This deficit is exacerbated in the young TgAPPSw,Ind. The observed changes in the nodal structure are associated with poor working memory performance indicating functional implication for the nodal changes. These data highlight that structures within myelinated axons are vulnerable to ageing and cerebral hypoperfusion. Therefore, the development of strategies that minimize injury or drive repair to these regions is necessary together with therapeutic approaches against the vascular insults that induce hypoperfusion and lead to white matter attenuation and cognitive decline. In the future, it would be interesting to investigate how alterations at the AIS/nodes of Ranvier affect neuronal excitability.

Myelination of axons in the central and peripheral nervous system (CNS and PNS) is required for saltatory propagation of nerve impulses. Myelinated axons are organized in functionally distinct membrane domains and the correct formation and maintenance of these domains is fundamental for the correct propagation of the electrical impulse; however, the underlying mechanisms by which these domains are specified are just starting to be unravelled. The paranodal junctions (PNJs) have been shown to contribute to node formation in the CNS and to domain maintenance both in the CNS and PNS. In this thesis I have studied the function of the linkage of the PNJs to the axonal cytoskeleton in regulating these processes by using a combination of knock out and transgenic rescue strategies. Further, I have initiated studies on the live imaging of node assembly. I have shown that the link between the PNJ and the axonal cytoskeleton is required for both the correct timing of oligodendrocyte process migration and for clustering nodal proteins at heminodes, before nodes of Ranvier are formed. Failure to correctly regulate these events during development results in shorter internodes in adults. Further, I have shown the importance of the axonal paranodal cytoskeleton in the maintenance of the node of Ranvier, both in CNS and PNS. In the absence of a link between the PNJ and the axonal cytoskeleton, paranodes disassemble, which causes disruption of both nodal and juxtaparanodal domains. Electron microscopy shows that, despite paranodal disruption, transverse bands are preserved when the anchorage of the PNJ to the axonal cytoskeleton is removed. Surprisingly, the preservation of these structures is associated with the amelioration of the neurological defects seen in mice lacking PNJs. In order to study nodal assembly, I have initiated studies on the transport of the nodal proteins Neurofascin186 and β1Nav tagged with fluorescent tags in transgenic mice, in order to analyse axonal transport during development. I have exploited the triangularis muscle explant preparation and have analysed transport of nodal proteins in young and adult mice. I have shown that transport speeds decrease with age and that the two proteins are transported at different speeds in young animals, but these differences do not persist in adults. This suggests that during myelination these proteins are transported in different vesicles and that this may change during development.

Kürzlich wurden bei immunvermittelten Neuropathien Autoantikörper gegen Proteine des paranodalen axoglialen Komplexes beschrieben. Deren Charakteristika, Prävalenzen, pathophysiologische Relevanz sowie Bedeutung für Diagnostik und Therapie sind jedoch noch nicht abschließend erforscht. In dieser Studie wurden daher Seren und Plasmapheresematerial (PE-Material) von 150 Patienten mit inflammatorischen Neuropathien, nämlich 105 mit chronisch inflammatorischer demyelinisierender Polyneuropathie (CIDP), 21 mit Guillain- Barré-Syndrom (GBS) und 24 mit multifokaler motorischer Neuropathie (MMN), welche etablierte diagnostische Kriterien der jeweiligen Krankheit erfüllen, sowie 74 Kontrollen mittels immunhistochemischen Färbungen an murinen Zupfnervenpräparaten und/oder ELISA (Enzyme-linked Immunosorbent Assay) auf Autoantikörper gegen die paranodalen Proteine Caspr, Contactin-1 und Neurofascin- 155 untersucht. Bei positivem Ergebnis wurde deren Spezifität mittels immunhistochemischen Färbungen an transfizierten HEK (Human embryonic kidney)- 293-Zellen und Präinkubationsversuchen bestätigt. Es wurden die IgG-Subklassen und die Antikörpertiter bestimmt und das Komplementbindungsverhalten unter Zugabe von intravenösen Immunglobulinen (IVIG) mit zellbasierten und ELISA-basierten Methoden analysiert. Klinische Merkmale und das Therapieansprechen Antikörper-positiver Patienten wurden ermittelt und mit den experimentellen Ergebnissen in Zusammenhang gesetzt. IgG-Autoantikörper gegen Contactin-1 konnten bei vier Patienten mit CIDP nachgewiesen werden, IgG-Autoantikörper gegen Caspr bei einem Patienten mit CIDP und einer Patientin mit GBS. Es konnten keine weiteren Autoantikörper bei CIDP-Patienten, GBS-Patienten, MMN-Patienten oder bei den Kontrollen detektiert werden. Die Prävalenz von Autoantikörpern gegen axogliale paranodale Proteine liegt somit in dieser Studie bei jeweils 4,76% bei CIDP und GBS und 0% bei MMN. Die Antikörper gehörten bei Patienten in der akuten Erkrankungsphase (zwei der CIDP-Patienten mit Anti-Contactin-1-Autoantikörpern und eine GBS-Patientin mit Anti-Caspr-Autoantikörpern) hauptsächlich den Subklassen IgG1 und IgG3 an, bei Patienten in der chronischen Phase (zwei der CIDP-Patienten mit Anti-Contactin-1-Autoantikörpern, ein CIDP-Patient mit Anti-Caspr-Autoantikörpern) überwog die Subklasse IgG4. Experimentell kam es zur Komplementbindung und -aktivierung abhängig vom Gehalt der Subklassen IgG1-3, nicht aber IgG4; diese konnte durch die Zugabe von IVIG dosisabhängig gemindert werden. Alle Autoantikörper-positiven CIDP-Patienten zeigten einen GBS-artigen Beginn mit einer schweren motorischen Beteiligung. Anti-Contactin-1-positive Patienten kennzeichnete klinisch zusätzlich das Vorkommen einer Ataxie und eines Tremors, Anti-Caspr-positive Patienten das Vorkommen starker neuropathischer Schmerzen. Elektrophysiologisch standen neben Hinweisen auf eine Leitungsstörung Zeichen einer axonalen Schädigung im Vordergrund. Als histopathologisches Korrelat lagen eine nodale Architekturstörung und ein Axonverlust vor. Die Patienten zeigten nur in der Anfangsphase der Erkrankung ein Ansprechen auf IVIG. Bei drei CIDP-Patienten mit IgG4-Autoantikörpern (zwei Patienten mit Anti-Contactin-1-Antikörpern und ein Patient mit Anti-Caspr-Antikörpern) wurde eine Therapie mit Rituximab durchgeführt. Diese führte zu einer Titerreduktion und zur zeitgleichen klinischen und elektrophysiologischen Befundbesserung bei zwei Patienten. Die in dieser Arbeit angewandten Screeningmethoden führten zum erfolgreichen Nachweis von Autoantikörpern gegen paranodale axogliale Proteine. Die Patienten mit positivem Autoantikörpernachweis definieren eine kleine Untergruppe mit ähnlichen klinischen Merkmalen im Kollektiv der Patienten mit inflammatorischen Polyneuropathien. Histopathologische Merkmale sowie das Therapieansprechen auf antikörperdepletierende Therapie sprechen in Kombination mit den Ergebnissen weiterer Studien zu paranodalen Autoantikörpern für eine pathogenetische Relevanz der Autoantikörper. Mit einem charakteristischen, am Schnürring ansetzenden Pathomechanismus könnten Neuropathien mit Nachweis von paranodalen Autoantikörpern der kürzlich eingeführten Entität der Nodo-Paranodopathien angehören. Die Komplementaktivierung und das Therapieansprechen der Patienten auf IVIG stehen möglicherweise in Zusammenhang mit der prädominanten IgG-Subklasse. Diese könnte auch in Bezug auf die Chronifizierung eine Rolle spielen. Der Nachweis von Autoantikörpern gegen paranodale Proteine hat wohlmöglich in Zukunft direkte Konsequenzen auf das diagnostische und therapeutische Prozedere bei Patienten mit CIDP und GBS; weitere klinische und experimentelle Daten aus größeren, prospektiven Studien sind jedoch zum weiteren Verständnis und zur Charakterisierung dieser Entität notwendig. / Autoantibodies against proteins of the paranodal axoglial complex have been described in recent studies on immune-mediated neuropathies. Nevertheless, their characteristics, prevalences, pathophysiological relevance and impact on diagnostics and therapy have not been fully investigated. Therefore, sera and plasmapheresis material (PE-material) of 150 patients with inflammatory neuropathy, including 105 patients with chronic inflammatory demyelinating polyneuropathy (CIDP), 21 patients with Guillain-Barré-Syndrome (GBS) and 24 patients with multifocal motor neuropathy (MMN), fulfilling established diagnostic criteria for the respective disease, as well as 74 controls were screened for autoantibodies against the paranodal proteins caspr, contactin-1 and neurofascin-155 via immunohistochemic staining of murine teased fiber preparations and/or ELISA (Enzyme-linked Immunosorbent Assay). In the event of a positive result, their specificity was confirmed via immunohistochemic staining on transfected HEK (human embryonic kidney)-293-cells and preincubation experiments. IgG subclasses and antibody titers in human material were analysed and complement binding to the autoantibodies, also under the influence of therapeutic immunoglobulins (IVIG), was investigated in cell based assays and ELISA based assays. Clinical features and therapy response in antibody-positive patients were evaluated and compared to the experimental results. IgG-autoantibodies against contactin-1 were found in four patients with CIDP, IgG-autoantibodies against caspr were found in one patient with CIDP and one with GBS. No further autoantibodies were detected neither in patients with CIDP, GBS and MMN nor in the controls. The prevalences of autoantibodies against axoglial paranodal proteins in this study therefore are at 4,76% in CIDP and GBS and 0% in MMN. In the acute phase of the disease, autoantibodies of the IgG1 and IgG3 subclass could be detected (in two CIDP patients with anti-contactin-1 antibodies and one GBS patient with anti-caspr antibodies), whereas patients in the chronic phase of the disease showed IgG4-autoantibodies (two CIDP patients with anti-contactin-1 antibodies and one CIDP patient with anti-caspr antibodies). Complement binding and activation in vitro depended on the amount of the IgG subclasses IgG1-IgG3, but not IgG4. Complement binding could be reduced by IVIG dose-dependently. All CIDP-patients with autoantibodies showed a GBSlike onset with severe motor involvement. Additional features of anti-contactin-1 positive neuropathy were ataxia and tremor, of anti-caspr positive disease neuropathic pain. Electrophysiological studies revealed signs of conduction failure accompanied by striking signs of axonal damage. As a histopathologic correlate, a disruption of the nodal architecture and axonal loss were found. Patients only responded well to IVIG in the beginning of the disease. Three patients with autoantibodies of the IgG4 subclass (two patients with anti-contactin-1 and one patient with anti-caspr) were treated with rituximab, leading to a titer reduction accompanied by clinical and electrophysiological improvement in two patients. The screening methods used in this study are suitable for the detection of autoantibodies against paranodal proteins. Antibody-positive patients define a small subgroup of patients with inflammatory polyneuropathy that is characterized by distinct clinical features. Histopathological findings and therapy response to antibody- depleting treatment in this study as well as findings of further studies argue in favour of a pathogenetic relevance of the autoantibodies. Neuropathies associated with paranodal autoantibodies could belong to the new entity of nodo-paranodopathies, sharing a characteristic pathomechanism with the node of Ranvier being the site of attack. Complement binding and activation as well as response to IVIG could be related to the predominant IgG subclass of the autoantibodies. It could also influence the course and chronification of the disease. Therefore, detection of autoantibodies against paranodal proteins might have a direct impact on diagnostic and therapeutic strategies in patients with CIDP and GBS in the future. Nevertheless, further clinical and experimental data, including data from bigger and prospective studies are needed to understand and fully characterize this novel entity.

Titre de l'écran-titre (visionné le 22 mai 2023) / Les modèles mathématiques sont souvent employés en neurosciences pour mieux comprendre le comportement des neurones et des réseaux neuronaux. De nombreux outils mathématiques sont utilisés pour décrire les différents aspects de l'activité et des structures neuronales sur des échelles temporelles et temporelles s'étendant sur plusieurs ordres de grandeur. Par exemple, les systèmes d'équations différentielles ordinaires tels que le modèle de Hodgkin-Huxley sont utilisés depuis plusieurs décennies pour décrire les mécanismes de génération de potentiels d'action dans les neurones. À une échelle spatiale plus grande, les équations aux dérivées partielles (EDP) telles que les équations de Maxwell sont utilisées pour comprendre la distribution du champ électrique sur l'ensemble du cerveau. Un nombre moins important de recherches ont été consacrées à l'étude de la distribution des concentrations ioniques et du champ électrique dans les petites structures neuronales (∼ 1μm) telles que les nœuds de Ranvier, les épines dendritiques ou les vésicules présynaptiques. Une manière de modéliser ces structures est de résoudre le système EDP des équations de Poisson Nernst Planck. Ce système d'équations peut être utilisé pour calculer la distribution des concentrations ioniques en résolvant les équations de Nernst-Planck et résoudre la distribution des champs électriques par l'équation de Poisson. L'avantage d'une telle approche est qu'elle permet d'étudier des structures aux géométries arbitrairement complexes. L'objectif principal de cette thèse est d'utiliser le système d'équations de Poisson Nernst-Planck pour modéliser l'activité des épines dendritiques et des nœuds de Ranvier afin de mieux comprendre les les fluctuations des concentrations ioniques dans ces structures. Une contribution importante du projet projet est l'implémentation d'une méthode numériquement efficace pour résoudre ces équations. En effet, la résolution de l'EDP sur des géométries non triviales peut rapidement devenir coûteuse en termes de calcul ce qui rend important le choix d'une approche numérique efficace. Nous avons utilisé la méthode des éléments finis avec des éléments de second ordre. Notre code est implémenté sur le logiciel MEF++, un code développé par le groupe de recherche GIREF de l'Université Laval. Les deux structures d'intérêt, les épines dendritiques et les nœuds de Ranvier, ont été choisies parce qu'elles jouent des rôles importants dans la signalisation neuronale et parce que leurs fonctions sont susceptibles d'être modulées par des altérations de leurs géométries. Les épines dendritiques sont des structures en forme de champignon qui recouvrent les branches dendritiques. Une grande partie des synapses excitatrices sont situées sur les épines dendritiques et l'on pense donc que ces structures jouent un rôle dans la façon dont le signal électrique est transmis au corps cellulaire du neurone. Nous avons simulé des événements synaptiques se produisant sur des épines de géométries différentes afin de déchiffrer la relation entre leur forme et leur fonction. Les événements survenant au niveau des synapses excitatrices déclenchent deux types de réponses, une dépolarisation électrique et une augmentation de la concentration en calcium. Notre modèle décrit ces deux réponses. Nos simulations suggèrent que la forme des épines dendritiques est un déterminant important de la dynamique du calcium alors que son impact sur la signalisation électrique reste limité sur une large gamme de géométries. Les axones sont des structures filiformes qui transmettent des signaux électriques d'un neurone à d'autres. Les axones sont isolés électriquement par des gaines de myéline qui accélèrent la propagation des signaux. Les nœuds de Ranvier sont de petites sections non myélinisées de l'axone, espacées à des intervalles à peu près réguliers. Ces structures sont caractérisées par une forte densité de canaux commandés par le voltage qui maintiennent l'amplitude du potentiel d'action pendant sa propagation. Nous étudions numériquement l'effet de la longueur du nœud, de l'épaisseur de la myéline et de l'angle que fait la myéline avec le nœud de Ranvier sur la propagation du potentiel électrique dans la membrane de l'axone. Nous montrons que la perte de myéline dans le nœud de Ranvier pourrait avoir un impact important sur les potentiels extracellulaires. La méthodologie développée dans cette thèse pourrait être appliquée à de nombreuses autres structures telles que la fente synaptique ou les vésicules présynaptiques. / Mathematical models are often employed in neuroscience to better understand the behaviour of neurons and neural networks. Many mathematical tools are used to describe the different aspects of neural activity and structures over temporal and time scales spanning over several order of magnitudes. For example, systems of ordinary differential equations (ODE's) such as the Hodgkin-Huxley model have been used for several decades to describe the spike generating mechanisms in neurons. On a larger spatial scale, partial differential equations (PDE's) such as Maxwell equations are used to understand the distribution of the electrical field over the whole brain. A lesser amount of research has been devoted to the investigation of the distribution of ionic concentrations and electrical field in small neural structures (∼ 1 μm) such as nodes of Ranvier, dendritic spines or presynaptic vesicles. One way to perform such investigations is to solve the PDE system of Poisson Nernst Planck equations. This system of equations can be used to compute the distribution of ionic concentrations by solving the Nernst-Planck equations and resolve the distribution of electric fields through the Poisson equation. The advantage of such an approach is that it allows the investigation of structures with arbitrarily complex geometries. The main aim of this thesis is to use the Poisson Nernst-Planck system of equations to model the electrical activity of dendritic spines and nodes of Ranvier and to better understand the fluctuations of ionic concentrations in these structures. A significant contribution of the project is the implementation of a numerically efficient way to solve these equations. Indeed, the resolution of PDE on non trivial geometries can rapidly become computationally expensive making the choice of an efficient numerical approach important. We used the finite element method with second order elements. Our code is implemented on the MEF++ software, a code developed by the GIREF research group at Laval University. The two structures of interest, dendritic spines and nodes of Ranvier were chosen because they play important roles in signaling in neural signaling and because their functions is likely to be modulated by alterations in their geometries. Dendritic spines are mushroom like structures covering dendritic branches. A large proportion of excitatory synapses are located on dendritic spines and it is thus believed that these structures play a role in how the electric signal is transmitted to the neuron's cell body. We simulated synaptic events occurring on spines with many different geometries to decipher the elusive relationship between their shape and function. Events at excitatory synapses trigger two types of responses: an electrical depolarization and an increase in calcium concentration. Our model describes these two responses. Our simulations suggest that the shape of the spine is an important determinant of calcium dynamics while its impact on electric signaling remains limited over a wide range of geometries. Axons are wire like structures transmitting electric signals from one neuron to others. Axons are electrically insulated by myelin sheaths which accelerates signal propagation. Nodes of Ranvier are small unmyelinated sections of the axon spaced at roughly regular intervals. Theses structures are characterized by a high density of voltage gated channels which maintain the amplitude of the action potential during its propagation. Numerically, we investigate the effect of the node length, myelin thickness and the angle which the myelin makes with the node of Ranvier on the propagation of electric potential in the membrane of the axon. We show that loss of myelin in the node of Ranvier might have an important impact on extracellular potentials. The methodology developed in this thesis could be applied to many other structures such as the synaptic cleft or presynaptic vesicles.

Bilayer embedded proteins are affected by stress. This general affirmation is, in this thesis, embodied by two types of proteins: membrane spandex and voltage-gated sodium channels. In this work, we essentially explore, using methods from physics, the theoretical consequences of ideas drawn from experimental biology. Membrane spandex was postulated to exist and we study the theoretical implications and possible benefits for a cell to have such proteins embedded in its bilayer. There are no specific membrane spandex proteins, rather any protein with a transition involving a large enough area change between two non-conducting states could act as spandex. Bacterial cells have osmovalve channels which open at near-lytic tensions to protect themselves against rupture. Spandex expanding at tensions just below the osmovalves’ opening tension could relieve tension enough as to avoid costly accidental osmovalve opening due to transient bilayer tension excursions. Another possible role for spandex is a tension-damper: spandex could be used to maintain bilayer tension at a fixed level. This would be useful as many bilayer embedded channels are known to be modulated by tension. The Stress/shear experienced in traumatic brain injury cause an immediate (< 2 min) and irreversible TTX-sensitive rise in axonal calcium. In situ, this underlies an untreatable condition, diffuse axonal injury. TTX sensitivity indicates that leaky voltage-gated sodium (Nav) channels mediate the calcium increase. Wang et al. showed that the mammalian adult CNS Nav isoform, Nav1.6, expressed in Xenopus oocytes becomes “leaky” when subjected to bleb-inducing pipette aspiration. This “leaky” condition is caused by a hyperpolarized-shift (left-shift or towards lower potentials, typically 20 mV) of the kinetically coupled processes of activation and inactivation thus effectively degrading a well-confined window conductance into a TTX-sensitive Na leak. We propose experimental protocols to determine whether this left-shift is the result of an all-or-none or graded process and whether persistent Na currents are also left-shifted by trauma. We also use modeling to assess whether left-shifted Nav channel kinetics could lead to Na+ (and hence Ca2+ ) loading of axons and to study saltatory propagation after traumatizing a single node of Ranvier.

Bilayer embedded proteins are affected by stress. This general affirmation is, in this thesis, embodied by two types of proteins: membrane spandex and voltage-gated sodium channels. In this work, we essentially explore, using methods from physics, the theoretical consequences of ideas drawn from experimental biology. Membrane spandex was postulated to exist and we study the theoretical implications and possible benefits for a cell to have such proteins embedded in its bilayer. There are no specific membrane spandex proteins, rather any protein with a transition involving a large enough area change between two non-conducting states could act as spandex. Bacterial cells have osmovalve channels which open at near-lytic tensions to protect themselves against rupture. Spandex expanding at tensions just below the osmovalves’ opening tension could relieve tension enough as to avoid costly accidental osmovalve opening due to transient bilayer tension excursions. Another possible role for spandex is a tension-damper: spandex could be used to maintain bilayer tension at a fixed level. This would be useful as many bilayer embedded channels are known to be modulated by tension. The Stress/shear experienced in traumatic brain injury cause an immediate (< 2 min) and irreversible TTX-sensitive rise in axonal calcium. In situ, this underlies an untreatable condition, diffuse axonal injury. TTX sensitivity indicates that leaky voltage-gated sodium (Nav) channels mediate the calcium increase. Wang et al. showed that the mammalian adult CNS Nav isoform, Nav1.6, expressed in Xenopus oocytes becomes “leaky” when subjected to bleb-inducing pipette aspiration. This “leaky” condition is caused by a hyperpolarized-shift (left-shift or towards lower potentials, typically 20 mV) of the kinetically coupled processes of activation and inactivation thus effectively degrading a well-confined window conductance into a TTX-sensitive Na leak. We propose experimental protocols to determine whether this left-shift is the result of an all-or-none or graded process and whether persistent Na currents are also left-shifted by trauma. We also use modeling to assess whether left-shifted Nav channel kinetics could lead to Na+ (and hence Ca2+ ) loading of axons and to study saltatory propagation after traumatizing a single node of Ranvier.

Bilayer embedded proteins are affected by stress. This general affirmation is, in this thesis, embodied by two types of proteins: membrane spandex and voltage-gated sodium channels. In this work, we essentially explore, using methods from physics, the theoretical consequences of ideas drawn from experimental biology. Membrane spandex was postulated to exist and we study the theoretical implications and possible benefits for a cell to have such proteins embedded in its bilayer. There are no specific membrane spandex proteins, rather any protein with a transition involving a large enough area change between two non-conducting states could act as spandex. Bacterial cells have osmovalve channels which open at near-lytic tensions to protect themselves against rupture. Spandex expanding at tensions just below the osmovalves’ opening tension could relieve tension enough as to avoid costly accidental osmovalve opening due to transient bilayer tension excursions. Another possible role for spandex is a tension-damper: spandex could be used to maintain bilayer tension at a fixed level. This would be useful as many bilayer embedded channels are known to be modulated by tension. The Stress/shear experienced in traumatic brain injury cause an immediate (< 2 min) and irreversible TTX-sensitive rise in axonal calcium. In situ, this underlies an untreatable condition, diffuse axonal injury. TTX sensitivity indicates that leaky voltage-gated sodium (Nav) channels mediate the calcium increase. Wang et al. showed that the mammalian adult CNS Nav isoform, Nav1.6, expressed in Xenopus oocytes becomes “leaky” when subjected to bleb-inducing pipette aspiration. This “leaky” condition is caused by a hyperpolarized-shift (left-shift or towards lower potentials, typically 20 mV) of the kinetically coupled processes of activation and inactivation thus effectively degrading a well-confined window conductance into a TTX-sensitive Na leak. We propose experimental protocols to determine whether this left-shift is the result of an all-or-none or graded process and whether persistent Na currents are also left-shifted by trauma. We also use modeling to assess whether left-shifted Nav channel kinetics could lead to Na+ (and hence Ca2+ ) loading of axons and to study saltatory propagation after traumatizing a single node of Ranvier.

Bilayer embedded proteins are affected by stress. This general affirmation is, in this thesis, embodied by two types of proteins: membrane spandex and voltage-gated sodium channels. In this work, we essentially explore, using methods from physics, the theoretical consequences of ideas drawn from experimental biology. Membrane spandex was postulated to exist and we study the theoretical implications and possible benefits for a cell to have such proteins embedded in its bilayer. There are no specific membrane spandex proteins, rather any protein with a transition involving a large enough area change between two non-conducting states could act as spandex. Bacterial cells have osmovalve channels which open at near-lytic tensions to protect themselves against rupture. Spandex expanding at tensions just below the osmovalves’ opening tension could relieve tension enough as to avoid costly accidental osmovalve opening due to transient bilayer tension excursions. Another possible role for spandex is a tension-damper: spandex could be used to maintain bilayer tension at a fixed level. This would be useful as many bilayer embedded channels are known to be modulated by tension. The Stress/shear experienced in traumatic brain injury cause an immediate (< 2 min) and irreversible TTX-sensitive rise in axonal calcium. In situ, this underlies an untreatable condition, diffuse axonal injury. TTX sensitivity indicates that leaky voltage-gated sodium (Nav) channels mediate the calcium increase. Wang et al. showed that the mammalian adult CNS Nav isoform, Nav1.6, expressed in Xenopus oocytes becomes “leaky” when subjected to bleb-inducing pipette aspiration. This “leaky” condition is caused by a hyperpolarized-shift (left-shift or towards lower potentials, typically 20 mV) of the kinetically coupled processes of activation and inactivation thus effectively degrading a well-confined window conductance into a TTX-sensitive Na leak. We propose experimental protocols to determine whether this left-shift is the result of an all-or-none or graded process and whether persistent Na currents are also left-shifted by trauma. We also use modeling to assess whether left-shifted Nav channel kinetics could lead to Na+ (and hence Ca2+ ) loading of axons and to study saltatory propagation after traumatizing a single node of Ranvier.

No description available.

No description available.