Periodic Reporting for period 1 - Newron-TBI (Neurogenesis-related changes in hippocampal new neurons and circuits after traumatic brain injury)
Berichtszeitraum: 2018-04-01 bis 2020-03-31
Traumatic brain injury (TBI) is a major public health concern. Even when brain trauma is mild many individuals suffer from neuropsychological impairments such as memory loss, learning deficits, long-term disabling changes in cognition, sensorimotor function and personality. Over the past three decades, animal models have been developed to replicate the various aspects of human TBI and were designed to produce a relatively homogeneous type of injury, with age, gender, genetic background and the injury parameters all well controlled. Animal models may not be able to fully recapitulate all aspects of secondary injury development observed in human TBI, however, these models are essential for studying the biomechanical, cellular and molecular aspects of human TBI that cannot be addressed in the clinical setting, as well as for developing novel treatments. Animal studies have shown that TBI-induced wide range of effects may result from structural damage and functional deficits within the hippocampus. Functional plasticity of synaptic networks in the hippocampus has been implicated in the development of posttraumatic epilepsy after TBI. Within the hippocampus Dentate Gyrus (DG) acts as “gatekeeper” and “filter” of aberrant or excessive input information. DG function is directly determined by a delicate balance between neuronal excitation and inhibition and TBI can cause changes of this state of equilibrium. Moreover, TBI can affect adult hippocampal neurogenesis (AHN). AHN is the process of generating new neurons in the dentate gyrus (DG) from neural stem cells (NSCs) that integrate into the hippocampal circuitry by establishing synapses with existing neurons. AHN has been shown to participate in space navigation-related memory formation, learning, fear conditioning, anxiety, stress and pattern separation. NSCs are radial astrocytes like cells with radial glia properties. NSCs remain quiescent and are activated progressively to divide asymmetrically giving rise to neuronal precursors. Once they finish their round of divisions they differentiate into astrocytes losing their stem cell capabilities and because the activation of NSCs is coupled with their exhaustion, the population of NSCs declines over time. This decline of NSCs population and neurogenesis, that naturally occurs with aging, might also be accelerated due to neuronal hyperexcitation and excitotoxicity. We hypothesized, that TBI-induced hyperexcitation of existing granule cells (GCs) within DG can induce long-term changes in both NSCs and newborn neurons and those alterations can further contribute to hippocampal dysfunction.
The main goal of Newron-TBI was set to bring light to the long standing debate on how TBI affects DG circuitry and AHN. We aimed to investigate the short and long term effects of controlled cortical impact (CCI) as a model of TBI on NSCs and the neurogenic niche. We aimed to understand what particular changes TBI induces at the cellular, molecular and electrophysiological level in existing GCs, NSCs and newborn neurons.
The main goal of Newron-TBI was set to bring light to the long standing debate on how TBI affects DG circuitry and AHN. We aimed to investigate the short and long term effects of controlled cortical impact (CCI) as a model of TBI on NSCs and the neurogenic niche. We aimed to understand what particular changes TBI induces at the cellular, molecular and electrophysiological level in existing GCs, NSCs and newborn neurons.
AHN is a multi-step process that includes NSC activation, proliferation, apoptotic selection, migration and neuronal differentiation, and each individual stage is sensitive to dysregulation in a pathological environment such as after TBI. “Aberrant neurogenesis” can be provoked by TBI due to increased excitability of surviving granule cells (GCs) in DG, therefore it was important to assess TBI-induced changes in electrophysiological properties of pre-existing GCs. We focused on electrophysiological characteristics of immature as well as early mature neurons at short- and long-term timepoints post-TBI. In order to assess hallmarks of “aberrant neurogenesis” confocal microscopy and colocalization of bromodeoxyuridine (BrdU) with cell-type markers, such as doublecortin (DCX) was used. Morphological and anatomical properties of eGFP-expressing cells were analysed.
We have observed that, upon activation by TBI, NSCs do not only enter the cell cycle in much larger numbers, they also transform into reactive NSCs (React-NSCs). React-NSCs were characterized by significant increase in the thickness of the primary processes. Hallmarks of “aberrant neurogenesis” assessed in Newron-TBI project included increased numbers of newborn neurons, ectopic location of GCs and malformations of the cell soma. Obtained data showed increased density and the total number of newborn neurons. Moreover, other alterations in the newly generated GCs after TBI were present. We have observed ectopic displacement of such cells to the outer layers of DG and the increased diameter of the somas. Observed changes in spontaneous excitatory currents (sEPSCs) frequency and amplitude indicate remodeling of excitatory input likely expressed as an increase in the number of excitatory synapses and/or strengthening of active synapses. Interestingly we have observed that, in comparison with control animals, both TBI and SHAM surgeries are affecting electrophysiological and some morphological properties of GCs in DG, often in opposite manner.
Neuron-intrinsic alterations in baseline excitability and increased excitatory drive onto GCs together with “aberrant neurogenesis” are thought to change local neuronal rewiring and hence the structural and functional organization of the hippocampal network, which can potentially contribute to the formation of a pro-epileptogenic microenvironment.
In addition, we explored the possibility of nanoparticles (NPs) for therapeutic use. Used approach of 4 injections of NPs has been ineffective as a potential therapeutic treatment against TBI-induced alterations, however presented data are still preliminary and more experiments are planned for the future.
Newron-TBI project made a valuable contribution to extending our fundamental scientific knowledge about TBI induced alterations of AHN and DG excitatory circuitry. Project generated relevant outcomes and compiled a great amount of data that created high interest within the research community during scientific meetings.
We have observed that, upon activation by TBI, NSCs do not only enter the cell cycle in much larger numbers, they also transform into reactive NSCs (React-NSCs). React-NSCs were characterized by significant increase in the thickness of the primary processes. Hallmarks of “aberrant neurogenesis” assessed in Newron-TBI project included increased numbers of newborn neurons, ectopic location of GCs and malformations of the cell soma. Obtained data showed increased density and the total number of newborn neurons. Moreover, other alterations in the newly generated GCs after TBI were present. We have observed ectopic displacement of such cells to the outer layers of DG and the increased diameter of the somas. Observed changes in spontaneous excitatory currents (sEPSCs) frequency and amplitude indicate remodeling of excitatory input likely expressed as an increase in the number of excitatory synapses and/or strengthening of active synapses. Interestingly we have observed that, in comparison with control animals, both TBI and SHAM surgeries are affecting electrophysiological and some morphological properties of GCs in DG, often in opposite manner.
Neuron-intrinsic alterations in baseline excitability and increased excitatory drive onto GCs together with “aberrant neurogenesis” are thought to change local neuronal rewiring and hence the structural and functional organization of the hippocampal network, which can potentially contribute to the formation of a pro-epileptogenic microenvironment.
In addition, we explored the possibility of nanoparticles (NPs) for therapeutic use. Used approach of 4 injections of NPs has been ineffective as a potential therapeutic treatment against TBI-induced alterations, however presented data are still preliminary and more experiments are planned for the future.
Newron-TBI project made a valuable contribution to extending our fundamental scientific knowledge about TBI induced alterations of AHN and DG excitatory circuitry. Project generated relevant outcomes and compiled a great amount of data that created high interest within the research community during scientific meetings.
To date, knowledge of how TBI affects NSCs and AHN is limited and contradictory. Newron-TBI investigated for the first time the posttraumatic-induced changes in GCs, NSCs and newborn neurons, from molecular biology to electrophysiology in short and long term view. Although the project was aimed at describing basic mechanisms on how TBI affects AHN and DG excitatory circuitry, obtained results could have an impact in translational research with biomedical applications in developing novel strategic therapies against brain damage.