Understanding of the underlying mechanisms

Major Depression Disorder (MDD) is often accompanied by cognitive impairments, including concentration problems and attention deficits. These issues are related to the construct of working memory (WM). Additionally, a reduction in hippocampal volume is frequently observed in Major Depression. There is substantial evidence suggesting that physical exercise training can have positive effects on depressive and cognitive symptoms in MDD. This dissertation aims to integrate these areas of study to investigate the positive effects of physical fitness and exercise on WM in MDD patients, leading to a better understanding of the pathophysiology of the disease and its treatment through physical exercise training. The dissertation comprises three empirical studies that are part of the SPeED study (Sport/Exercise Therapy and Psychotherapy – evaluating treatment Effects in Depressive patients). In Study I (Heinzel et al. 2022), we examine whether a prior exercise intervention enhances the success of subsequent cognitive-behavioral therapy (CBT) and whether this effect is associated with specific physiological changes. Study II (Schwefel et al. 2023) analyzes neural activity and physical fitness in depressive patients during a WM task. Study III (Schwefel et al., sub) focuses on functional and structural neural changes following physical exercise training, with particular emphasis on the hippocampus. The n-back paradigm was used to measure WM function during functional magnetic resonance imaging (fMRI). The physical exercise intervention lasted 12 weeks and was supervised by sports therapists. The results indicate that physical fitness can be improved through training, and surprisingly, depressive symptoms improved in all groups. However, high exercise intensity did not lead to a general boosting effect for CBT. Nonetheless, regression analyses revealed that improvement of individual fitness predicted the success of CBT. MDD patients exhibited a specific activation pattern in frontoparietal brain regions, associated with longer reaction times and poorer performance during high demands in WM tasks compared to healthy controls. Additionally, a parietal fitness correlate was identified in the depressive sample. Improved performance and shorter reaction times were observed after the training intervention, particularly during high demands in the WM tasks. Furthermore, in patients who underwent intensive training, an increased activation of the hippocampus was observed as a result of the training. No structural changes in hippocampal volume were detected. The findings suggest that physical training holds promise as a treatment option for improving WM function in MDD patients. These insights may serve as a foundation for future research on the effects of physical fitness and exercise on mental health and cognition, offering valuable supplements to optimized physical exercise therapies for the treatment of MDD

Phospho-dependent Regulation of SAMHD1 Oligomerisation Couples Catalysis and Restriction.

SAMHD1 restricts HIV-1 infection of myeloid-lineage and resting CD4+ T-cells. Most likely this occurs through deoxynucleoside triphosphate triphosphohydrolase activity that reduces cellular dNTP to a level where reverse transcriptase cannot function, although alternative mechanisms have been proposed recently. Here, we present combined structural and virological data demonstrating that in addition to allosteric activation and triphosphohydrolase activity, restriction correlates with the capacity of SAMHD1 to form "long-lived" enzymatically competent tetramers. Tetramer disruption invariably abolishes restriction but has varied effects on in vitro triphosphohydrolase activity. SAMHD1 phosphorylation also ablates restriction and tetramer formation but without affecting triphosphohydrolase steady-state kinetics. However phospho-SAMHD1 is unable to catalyse dNTP turnover under conditions of nucleotide depletion. Based on our findings we propose a model for phosphorylation-dependent regulation of SAMHD1 activity where dephosphorylation switches housekeeping SAMHD1 found in cycling cells to a high-activity stable tetrameric form that depletes and maintains low levels of dNTPs in differentiated cells

Solution oligomeric state and steady state kinetics of SAMHD1.

<p>(<b>A</b>) SEC-MALLS analysis of SAMHD1 monomer-dimer-tetramer equilibrium for SAMHD1(115–583) (red) and SAMHD1(115–626) (black). The chromatograms are the output from the differential refractometer. The points are the weight-averaged molar masses determined at 1-second intervals throughout elution of chromatographic peaks. Dashed line chromatograms are apo-protein, solid lines are upon addition of 0.5 mM dGTP (<b>B</b>) Tetramer stability. SAMHD1(115–626) was incubated with 0.1 mM GTP and 0.5 mM dATP for the specified time intervals and then the oligomeric state analysed by SEC-MALLS. (<b>C</b>) Analysis of dATP hydrolysis during the time course shown in <b>B</b>. The chromatogram (black) is the recorded UV absorbance at 260 nm from the SEC-MALLS column for the five-minute incubation time point. Reference chromatograms (dashed lines) for substrate dATP (red) and product dA (blue) are overlaid. (<b>D</b>) Steady-state kinetic analysis of GTP stimulated hydrolysis of TTP by SAMHD1. The dependence of the rate of on substrate concentration for SAMHD1(115–626) (black) and SAMHD1(115–583) (red) are plotted. Solid lines are the best fit to the data using the Michaelis-Menten expression. Error bars represent the standard error of the mean (SEM) of three independent measurements. The derived constants K_M and k_cat for the reaction are displayed inset.</p

Model for phospho-regulation of SAMHD1 restriction.

<p>In the absence of dNTPs Apo-SAMHD1 is found in a monomer-dimer equilibrium regardless of the phosphorylation state (<b>1</b>). At high dNTP levels, typically in cycling cells, constitutively abundant GTP combines with dNTPs to fill allosteric sites. In both phosphorylated and un-phosphorylated SAMHD1 this results in the formation of an activated tetramer (<b>2</b>) that in the non-phosphorylated protein also includes additional intra-tetramer CtD interactions forming a stable activated tetramer (<b>3</b>). Under these conditions, both activated and stable activated tetramers hydrolyse the dNTP pool at comparable rates. At lower dNTP levels, the stabilisation afforded by the CtD interactions maintains enzyme activity in non-phosphorylated SAMHD1 by preventing the loss of dNTP-activator from the allosteric site. However, in phospho-SAMHD1, activating dNTPs dissociate from the allosteric site (<b>4</b>) resulting in disassembly of the tetramer and down-regulation of triphosphohydrolase activity. At very low levels, such as in differentiated myeloid cells, CtD-stabilised tetramers still retain activating dNTPs in the allosteric site (<b>5</b>) and SAMHD1 remains catalytically competent. It can therefore rapidly respond to any increase in intracellular dNTPs to maintain the dNTP levels below the threshold required for HIV-1 replication.</p

The effect of phosphorylation on SAMHD1 enzyme activity.

<p>(<b>A</b>) Steady-state kinetic analysis of GTP stimulated hydrolysis of TTP, by SAMHD1 (115–626) (black) and phospho-SAMHD1(115–626) (blue). The dependence of the rate of on substrate concentration is plotted. Error bars represent SEM of three independent measurements. Solid lines are the best fit to the data using the Michaelis-Menten expression. The derived constants K_M and k_cat are shown inset. (<b>B and C</b>) Effect of activator depletion on SAMHD1 and pSAMHD1 triphosphohydrolase activity. (<b>B</b>) IEX-HPLC analysis of nucleotide composition after incubation of SAMHD1(115–626) with GTP (upper), and pSAMHD1(115–626) (middle) or SAMHD1(115–626) (lower) with dATP and GTP for 0 and 300 seconds. (<b>C</b>) IEX-HPLC analysis of the hydrolysis of ddGTP added to the samples depleted of dATP in (<b>B</b>). The plot shows the quantification of the time dependent hydrolysis derived from integration of the ddGTP peak at each time point. Error bars represent the SEM from 3 independent experiments.</p

Different G based nucleotides can be accommodated in the SAMHD1 allosteric site.

<p>(<b>A</b>) The contents and conformation of the allosteric sites for structures of SAMHD1(115–583)-ddGTP (top), SAMHD1(115–583[R164A])-dGTP (middle) and SAMHD1(115–626)-GTP (bottom) are shown. Nucleotides are shown in stick representation, SAMHD1 residues making contacts with the nucleotides are labelled. (<b>B</b>) SEC-MALLS analysis of SAMHD1(115–626) incubated with ddGTP/dATP(blue), dGTP/dATP (green) or GTP/dATP (orange). The chromatograms are the output from the differential refractometer. The points are the weight-averaged molar masses determined at 1-second intervals throughout elution of chromatographic peaks.</p