Department of Neurobiology, University of Osnabrück

Cellular and Molecular Neuroscience

(AG Roland Brandt)

Molecular mechanisms of neuronal development, aging and degeneration

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Figure 1: Microtubule network in a cultured neuron. Microtubules were visualized by expressing fluorescently labelled tubulin (meGFP-tubulin). Note the presence of abundant bundles of microtubules in the axon-like process on the right (image from N.I. Trushina, AG Brandt).

Neurons are one of the most extreme cell types because they contain processes that can reach a meter or longer and contain more than 99% of the cell’s volume. The processes and their synaptic contacts form the basis for a high degree of adaptive interconnectivity as a prerequisite for sophisticated behavioral repertoires. This requires the presence of a dynamic molecular machinery to establish and maintain such a morphology and to adapt to changes in the environment. The cytoskeleton is the most important intraneuronal structure that determines the shape of a neuron, and microtubules in particular are crucial in neuronal development and plasticity (Fig. 1). It is therefore not surprising that abnormalities in the organization of the cytoskeleton are a hallmark of many neurodegenerative diseases. On the other hand, microtubule-modulating drugs can slow down or even block the degeneration of neurons.

The group focuses on the function of microtubules and their associated proteins during neuronal development and neurodegeneration. In particular, a large part of the group concentrates on the investigation of the involvement of the neuronal microtubule-associated protein tau during neurodegenerative processes in Alzheimer's disease and other tauopathies as well as possible approaches to stop neurodegenerative processes. In addition, cellular mechanisms are investigated how neurons deal with and adapt to stress conditions.  

1. Cytoskeletal mechanisms of neuronal development and neurodegeneration

The assembly of microtubules is regulated largely by microtubule-associated proteins (MAPs). From the neuronal MAPs, the tau proteins have attracted particular interest due to their potential role in neurodegenerative disorders ("tauopathies") including Alzheimer's disease (AD) (for reviews see Bakota and Brandt, 2016; Brandt et al., 2020).

We were able to show that tau not only binds to microtubules, but also interacts with neural plasma membrane components via its amino-terminal, non-microtubule-binding projection domain (Brandt et al., 1995; Maas et al., 2000). In a cooperative approach with Cellzome GmbH, we showed that the non-microtubule binding N-terminus of tau interacts with the membrane component annexin A2 (AnxA2) and that this interaction is blocked by a tauopathy mutation (Gauthier-Kemper et al., 2011; Gauthier-Kemper et al., 2018). The data suggest that impaired membrane binding, which critically involves annexins as membrane-cytoskeleton linkers, contributes to the pathological effects in tauopathies such as AD. To understand the role of tau during the development of tauopathies, we created tau constructs in which we mutated disease-relevant phosphorylation sites to simulate permanent, high stoichiometric tau phosphorylation characteristic for AD ("pseudohyperphosphorylated (PHP)-tau"; Eidenmüller et al., 2000). We were able to show that PHP-tau exerts a neurotoxic effect in primary neurons and human model neurons ("hNT neurons") (Fath et al., 2002). In addition, we showed that phosphorylation of tau causes progressive neuronal degeneration in an authentic CNS environment, which is modulated by amyloid beta and mediated via glutamate receptor activation (Shahani et al., 2006; Tackenberg and Brandt, 2009). In collaboration with the laboratory of Dr. Harald Hutter (Simon Fraser University, Burnaby, Canada), we developed a Caenorhabditis elegans model of tau hyperphosphorylation (Brandt et al., 2009). In addition, we have developed a mouse model that expresses disease-like modified tau (PHP-tau) in forebrain neurons (Hundelt et al., 2011) to study tau-dependent mechanisms during neurodegeneration in a systemic environment.

Most neurodegenerative diseases are directly or indirectly linked to changes in the dynamics of cytoskeletal components  (Bakota and Brandt, 2010). Much of our recent work focuses on developing novel “live cell imaging” approaches in combination with molecular modeling to analyze protein dynamics in neurons (Weissmann et al., 2009). We have developed methods to quantitatively determine cytoskeletal dynamics from live cell imaging experiments (Gauthier and Brandt, 2010, Igaev et al., 2014). Using single-molecule tracking of tau in living neurons, we were able to show that tau interacts with microtubules via a "kiss and hop" mechanism (Janning et al., 2014) and that tau modifications affect tau's microtubule interactions in a potentially disease-relevant manner (Niewidok et al., 2016).

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Figure 2: Tau and the neurodegenerative triad in Alzheimer's disease (from: Bakota and Brandt, 2016).

2. Microtubule-directed approaches for the treatment of neurodegenerative diseases

Defects in the assembly and organization of microtubules occur as a result of axonal injury, in several neurodegenerative conditions and during ageing (Fig. 3). Systemic administration of microtubule-stabilizing drugs such as epothilones support the microtubule system and prevent axonopathies in animal models of tauopathies, indicating that microtubules are a potential target for preventing neurodegenerative processes (Brandt and Bakota, 2017).

We were able to show that amyloid beta and tau work together to induce dendritic simplification through dysregulation of microtubule dynamics (Golovyashkina et al., 2015) and provided evidence that stabilization of microtubules by subnanomolar concentrations of the drug epothilone D reverses amyloid beta-induced spine loss (Penazzi et al., 2016). The data indicate that microtubule stabilization could be a promising drug target to approach AD-related structural and functional changes.

We are currently focusing on the development and characterization of microtubule-modulating drugs with respect to modulating neurodegenerative processes as part of the EU-funded Innovative Training Network (ITN) "TubInTrain", a European Joint Doctorate on chemistry and biology that deals with the breakdown of microtubules related to neurodegenerative diseases and neurotoxicity.

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Figure 3: Disturbance of microtubule dynamics during de- and regeneration and potential therapeutic microtubule stabilization. AD, Alzheimer’s disease; PD, Parkinson’s disease (from: Brandt, 2017).

3. Molecular mechanisms of physiological and pathological adaption to stress in neurons

Stress granules are RNA-protein complexes that form in the cytosol of many cells in response to environmental stressors. The formation of stress granules is driven by liquid-liquid phase separation, creating non-membranous droplet-like structures that are held together by weak intermolecular interactions. The formation of stress granules is seen as a survival-promoting mechanism in order to adapt the translatome of a cell to adverse environmental conditions in a fast, adjustable, and reversible manner. Aberrant stress granule formation and dynamics has been linked to various neurodegenerative diseases such as amyotrophic lateral sclerosis, frontotemporal dementias and Alzheimer’s disease. In fact, some molecular compounds in stress granules, such as the multivalent RNA-binding proteins G3BP1 and IMP1, interact with the tau mRNA, suggesting a link between cell stress and tauopathies.  

We were able to show that the induction of stress granule formation by G3BP1 and IMP1 expression modulates the tau isoform expression and shifts tau expression to longer isoforms (Moschner et al., 2014). In order to follow the distribution and dynamics of G3BP1 and IMP1 in stressed neuronal cells, we carried out single-molecule imaging and detected hotspots of immobilized G3BP1 and IMP1, which we termed “nanocores” (Niewidok et al., 2018) (Fig. 4). Our live cell imaging approaches open up the possibility of analyzing the dynamics of protein exchange between stress granules and the local dynamics of protein interaction within stress granules (Niewidok et al., 2020).

We are currently focusing on the identification of conditions and factors involved in switching the material state of neuronal stress granules from a physiological, dynamic state to a potential disease-causing state as part of a coordinated research consortium (SFB 944) of the University of Osnabrück.

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Figure 4: Schematic representation of the presence of distributed nanocores in the mobile, liquid droplet-like phase of stress granules (from: Niewidok et al. 2018).

Selected References


Doctoral Theses