Analyse der der BDNF-Aktivität zugrunde liegenden molekularen Mechanismen bei der Modulation der neuronalen Struktur

The adaptive function of the brain is ensured by modifying the neuronal circuits that underlie experience-dependent learning and memory processes. Neuronal plasticity is determined by the correlation of functional and structural changes of synapses or neurons. Modification of neuronal circuits is tightly controlled by extracellular signals maintaining the balance between excitation and inhibition. The neurotrophin BDNF has been implicated to regulate excitatory-inhibitory transmission of synaptic connections by modulating neuronal function and structure of glutamatergic and GABAergic neurons. The current study was aimed at gaining more insights into the role of BDNF in modulating the neuronal structure and function as well as its role during spatial learning by either deprivation or activity-dependent increase of BDNF. The molecular mechanism mediating BDNF function in glutamatergic and GABAergic neurons was addressed by examining changes in TrkB receptor activation upon a loss-of-function approach. TrkB receptor activation is impaired in inhibitory hippocampal neurons in absence of BDNF while excitatory neurons were not affected. Neither BDNF nor Neurotrophin-4 is required in regulating TrkB signalling in excitatory neurons. Analysis of the molecular mechanism in excitatory hippocampal neurons suggests that TrkB signalling is activated by a neurotrophin-independent, zinc-induced transactivation mechanism. Examination of cbdnf knockout mice revealed that in contrast to excitatory pyramidal neurons dentate gyrus granule cells require BDNF for their postnatal growth whereby this is attributed by a BDNF-dependent TrkB receptor activation. Not only the loss of BDNF signalling results in severe deficits in neuronal plasticity but also its elevation can elicit negative consequences. Synaptotagmin IV knockout mice were used as a gain-of-function approach for BDNF. Loss of this trafficking protein results in enhanced exocytosis of BDNF containing dense-core vesicles in response to neuronal activity. This elevated BDNF secretion elicits deficits in spatial learning and memory retention in the MWM. Analysis of the neuronal function revealed that activity-dependent increase of BDNF evoked by wheel running rescued a LTP deficit detected in sedentary SytIV knockout mice. The role of BDNF mediated by TrkB is extremely specific for distinct cell types and its disequilibrium could lead to an impaired neuronal plasticity provoking deficits in learning and memory.Die adaptive Funktion des Gehirns wird sichergestellt durch Modifikationen neuronaler Netzwerke, die erfahrungsbedingten Lern- und Gedächtnisprozessen zugrunde liegen. Dieser als neuronale Plastizität bezeichnete Prozess ist bestimmt durch die Korrelation funktioneller und struktureller Veränderungen von Synapsen bzw. Neuronen, die von extrazellulären Signalen gesteuert werden und ein Gleichgewicht zwischen Erregung und Hemmung aufrechterhalten. Das Neurotrophin BDNF spielt eine bedeutende Rolle bei der Regulierung der exzitatorischen-inhibitorischen Signalübertragung von Synapsen durch Modifizierung der neuronalen Funktion und Struktur von Glutamat- und GABAergen Neuronen. Ziel dieser Studie war es, weitere Erkenntnisse zur Rolle von BDNF als Regulator neuronaler Strukturen und Funktionen zu gewinnen sowie in Lernprozessen die Auswirkungen eines Verlustes bzw. einer aktivitäts-abhängigen Erhöhung zu betrachten. Es wurde der molekulare Mechanismus untersucht, der der Funktion von BDNF in Glutamat- und GABAergenen Neuronen zugrunde liegt, indem Veränderungen in der TrkB-Rezeptor-Aktivierung nach dem Verlust von BDNF betrachtet wurden. In inhibitorischen hippokampalen Neuronen führte dieser Verlust zu einer Beeinträchtigung der TrkB-Rezeptor-Aktivierung. Exzitatorische Neurone waren nicht betroffen. Weder BDNF noch Neurotrophin-4 werden zur Aktivierung des TrkB-Rezeptors benötigt. In diesen Neuronen kann TrkB von einem Neurotrophin-unabhängigen, Zink-vermittelten Transaktivierungsmechanismus aktiviert werden. Die Analyse von cbdnf ko Mäusen zeigte, dass im Gegensatz zu exzitatorischen Pyramidenzellen, Körnerzellen des Gyrus dentatus BDNF-vermittelte Aktivierung von TrkB für ihre postnatale Entwicklung benötigen. Nicht nur der Verlust von BDNF führt zu Defiziten in der neuronalen Plastizität, auch die Erhöhung kann negative Auswirkungen hervorrufen. Synaptotagmin IV ko Mäuse dienten als Versuchsmodell. Der Verlust dieses Proteins bewirkt eine erhöhte, aktivitäts-abhängige BDNF Sekretion, welches im MWM zu Defiziten im räumlichen Lernen und des Erinnerungsvermögens führte. Analysen der neuronalen Funktion belegten, dass eine aktivitäts-abhängige Erhöhung von BDNF durch Laufrad-Aktivität ein LTP-Defizit in sitzenden SytIV ko Mäusen revidierte. Die TrkB-vermittelte Rolle von BDNF, ist spezifisch für bestimmte Zelltypen und ein Ungleichgewicht kann zu Beeinträchtigungen in der neuronalen Plastizität und damit zu Defiziten in Lern- und Gedächtnisprozessen führen

Neuroligin-1 mediates presynaptic maturation through brain-derived neurotrophic factor signaling.

Background: Maturation is a process that allows synapses to acquire full functionality, optimizing their activity to diverse neural circuits, and defects in synaptic maturation may contribute to neurodevelopmental disorders. Neuroligin-1 (NL1) is a postsynaptic cell adhesion molecule essential for synapse maturation, a role typically attributed to binding to pre-synaptic ligands, the neurexins. However, the pathways underlying the action of NL1 in synaptic maturation are incompletely understood, and some of its previously observed effects seem reminiscent of those described for the neurotrophin brain-derived neurotrophic factor (BDNF). Here, we show that maturational increases in active zone stability and synaptic vesicle recycling rely on the joint action of NL1 and brain-derived neurotrophic factor (BDNF). Results: Applying BDNF to hippocampal neurons in primary cultures or organotypical slice cultures mimicked the effects of overexpressing NL1 on both structural and functional maturation. Overexpressing a NL1 mutant deficient in neurexin binding still induced presynaptic maturation. Like NL1, BDNF increased synaptic vesicle recycling and the augmentation of transmitter release by phorbol esters, both hallmarks of presynaptic maturation. Mimicking the effects of NL1, BDNF also increased the half-life of the active zone marker bassoon at synapses, reflecting increased active zone stability. Overexpressing NL1 increased the expression and synaptic accumulation of BDNF. Inhibiting BDNF signaling pharmacologically or genetically prevented the effects of NL1 on presynaptic maturation. Applying BDNF to NL1-knockout mouse cultures rescued defective presynaptic maturation, indicating that BDNF acts downstream of NL1 and can restore presynaptic maturation at late stages of network development. Conclusions: Our data introduce BDNF as a novel and essential component in a transsynaptic pathway linking NL1-mediated cell adhesion, neurotrophin action, and presynaptic maturation. Our findings connect synaptic cell adhesion and neurotrophin signaling and may provide a therapeutic approach to neurodevelopmental disorders by targeting synapse maturation

Long-Lasting Immunity Against SARS-CoV-2: Dream or Reality?

Since its declaration as a pandemic in March 2020, SARS-CoV-2 has infected more than 217 million people worldwide and despite mild disease in the majority of the cases, more than 4.5 million cases of COVID-19-associated death have been reported as of September 2021. The question whether recovery from COVID-19 results in prevention of reinfection can be answered with a "no" since cases of reinfections have been reported. The more important question is whether during SARS-CoV-2 infection, a protective immunity is built and maintained afterwards in a way which protects from possibly severe courses of disease in case of a reinfection. A similar question arises with respect to vaccination: as of September 2021, globally, more than 5.2 billion doses of vaccines have been administered. Therefore, it is of utmost importance to study the cellular and humoral immunity toward SARS-CoV-2 in a longitudinal manner. In this study, reconvalescent COVID-19 patients have been followed up for more than 1 year after SARS-CoV-2 infection to characterize in detail the long-term humoral as well as cellular immunity. Both SARS-CoV-2-specific T cells and antibodies could be detected for a period of more than 1 year after infection, indicating that the immune protection established during initial infection is maintained and might possibly protect from severe disease in case of reinfection or infection with novel emerging variants. Moreover, these data demonstrate the opportunity for immunotherapy of hospitalized COVID-19 patients via adoptive transfer of functional antiviral T cells isolated from reconvalescent individuals

COVID-19 immune signatures reveal stable antiviral T cell function despite declining humoral responses.

Cellular and humoral immunity to SARS-CoV-2 is critical to control primary infection and correlates with severity of disease. The role of SARS-CoV-2-specific T cell immunity, its relationship to antibodies, and pre-existing immunity against endemic coronaviruses (huCoV), which has been hypothesized to be protective, were investigated in 82 healthy donors (HDs), 204 recovered (RCs), and 92 active COVID-19 patients (ACs). ACs had high amounts of anti-SARS-CoV-2 nucleocapsid and spike IgG but lymphopenia and overall reduced antiviral T cell responses due to the inflammatory milieu, expression of inhibitory molecules (PD-1, Tim-3) as well as effector caspase-3, -7, and -8 activity in T cells. SARS-CoV-2-specific T cell immunity conferred by polyfunctional, mainly interferon-γ-secreting CD4+ T cells remained stable throughout convalescence, whereas humoral responses declined. Immune responses toward huCoV in RCs with mild disease and strong cellular SARS-CoV-2 T cell reactivity imply a protective role of pre-existing immunity against huCoV