Hippocampal neurons respond to brain activity with functional hypoxia

Abstract

Physical activity and cognitive challenge are established non-invasive methods to induce comprehensive brain activation and thereby improve global brain function including mood and emotional well-being in healthy subjects and in patients. However, the mechanisms underlying this experimental and clinical observation and broadly exploited therapeutic tool are still widely obscure. Here we show in the behaving brain that physiological (endogenous) hypoxia is likely a respective lead mechanism, regulating hippocampal plasticity via adaptive gene expression. A refined transgenic approach in mice, utilizing the oxygen-dependent degradation (ODD) domain of HIF-1α fused to CreERT2 recombinase, allows us to demonstrate hypoxic cells in the performing brain under normoxia and motor-cognitive challenge, and spatially map them by light-sheet microscopy, all in comparison to inspiratory hypoxia as strong positive control. We report that a complex motor-cognitive challenge causes hypoxia across essentially all brain areas, with hypoxic neurons particularly abundant in the hippocampus. These data suggest an intriguing model of neuroplasticity, in which a specific task-associated neuronal activity triggers mild hypoxia as a local neuron-specific as well as a brain-wide response, comprising indirectly activated neurons and non-neuronal cells.

Brain erythropoietin fine-tunes a counterbalance between neurodifferentiation and microglia in the adult hippocampus

“…Inspiratory and functional hypoxia induce endogenous EPO expression in brain and diminish microglia numbers in CA1 EPO is a hypoxia-inducible growth factor, swiftly upregulated upon reduced inspiratory oxygen availability or upon relative oxygen deficiency (more oxygen consumed than provided in steady state) in peripheral tissues as well as in brain (Brines and Cerami, 2005;Jelkmann, 1992). We compared by qPCR the EPO expression in CA1 after 6 h of strong exogenous hypoxia (reduced inspiratory oxygen; positive control; 6% O 2 ) with 6 h of functional hypoxia (i.e., relative oxygen deficiency), induced by using our CRW paradigm of motor-cognitive challenge (Butt et al, 2021a(Butt et al, , 2021bWakhloo et al, 2020). Under both conditions, EPO mRNA expression increased acutely compared to normoxia baseline (Figures 1A and 1B), supporting the next steps toward a more chronic approach.…”

“…Hypoxia scRNA-seq data support a role of the CSF1R system in the two-wire effect of EPO on microglia and neurogenesis We now wondered whether similar alterations in the CSF1R axis would be detectable in conditions of hypoxia-induced brain EPO, as demonstrated above (Figures 1A-1D). To address this question, we used our hypoxia scRNA-seq dataset, obtained from whole hippocampus after 5 days of daily exposure to 6% O 2 for 6 h each (Butt et al, 2021b) (Figures 4A and 4B). This dataset allowed us to explore the expression of Csf1r and its ligands Csf1 and Il34.…”

“…Regarding neuronal lineages, the picture is much more complex, with EPO apparently driving fast differentiation of diverse, non-dividing, local neuronal precursors (Hassouna et al, 2016;Wakhloo et al, 2020), similar to its established role in the hematopoietic system (Brown and Ceredig, 2019;Dessypris et al, 1988;Grover et al, 2014;Krantz, 1991). Exposure to motor-cognitive challenge, namely complex running wheels (CRW) (Liebetanz and Merkler, 2006), leads to endogenous hypoxia in pyramidal neurons, augmenting the expression of EPO and EPO receptor (EPOR) (Butt et al, 2021a(Butt et al, , 2021bWakhloo et al, 2020). These in turn prompt via autocrine/paracrine signaling the emergence of newly generated CA neurons, enhance dendritic spine densities, and improve performance (EPO brain doping).…”

Addressing the ‘hypoxia paradox’ in severe COVID-19: literature review and report of four cases treated with erythropoietin analogues

“…This is perhaps less surprising, considering that due to its very low expression, mRNA of EPO regularly escapes single cell mRNA-seq analysis, a known dropout effect of this methodology. We note that EPO is an extremely potent hypoxia-inducible factor, locally effective in auto-paracrine fashion at femtomol concentrations (Butt et al 2021 ; Wakhloo et al 2020 ). In the situation of COVID-19, however, the low expression is likely further inhibited as a result of the uncoupled hypoxia pathway.…”

Erythropoietin and Non-Erythropoietic Derivatives in Cognition

“…EPOR expression in the CA1 is among the highest in the brain, and expression of both EPO and EPOR are further elevated by the complex running wheel (CRW) cognitive challenge paradigm ( Wakhloo et al, 2020 ). This paradigm produces a mild functional hypoxia in CA1 pyramidal cells and is likely related to the higher sensitivity of CA1 neurons to oxygen levels ( Butt et al, 2021 ). As a gene that is highly induced by hypoxia, EPO levels rise rapidly in the CA1 as a protective physiological response.…”