In a recent study published in Nature Communications, researchers investigated whether platelet-released exerkines such as C-X-C motif ligand 4 (CXCXL4) or platelet factor 4 (PF4) can simulate the impacts of physical activity on the neuronal formation and prevent age-related cognitive decay.
Study: Platelet-derived exerkine CXCL4/platelet factor 4 rejuvenates hippocampal neurogenesis and restores cognitive function in aged mice. Image Credit: r.classen/Shutterstock.com
Exercise has been shown to counteract cognitive decay in healthy aging and neurodegenerative disorders such as Alzheimer's disease. Exerkines, secreted into the circulation, play a role in this response.
However, the molecular mechanisms underlying exercise's beneficial effects on brain health remain unclear, leading to the development of "exercise mimetics" or "exercise pills."
About the study
In the present study, researchers evaluated the impact of CXCL4 on cognition and aging using a neurogenesis ablation model of transgenic and aged mice.
PF4 (500.0 ng) was injected in the tail veins of young murine animals on alternate days for a week to investigate whether peripherally administered CXCL4 enhanced hippocampus neurogenesis.
A double thymidine analog [5-iodo-2'-deoxyuridine (IdU) and 5-chloro-2'-deoxyuridine (CldU)] labeling approach was used, and PF4 was injected systemically once to investigate the systemic impact of CXCL4. An in vitro adherent hippocampus monolayer culture model was employed to study further the effects of CXCL4 on neural precursor cell dynamics.
Confocal imaging was used to identify tagged PF4 within adult brain progenitor cells. A differentiation experiment was performed on adherent cultures to confirm CXCL4's effects on neuronal differentiation, and the counts of glial fibrillary acidic protein-positive (GFAP) astrocytes and beta-III-tubulin-expressing neuronal cells were assessed.
To determine whether cxcl4 is essential for preserving adult hippocampus neurogenesis, CXCL4 knockout animals with platelets devoid of CXCL4 messenger ribonucleic acid (mRNA) were used.
RNA sequencing was performed to acquire deeper insights into the mechanisms by which CXCL4 affected hippocampal neural cellular precursors in adult mice. PF4-treated neural precursor cells in the primary dentate gyrus of adult mice were separated using flow cytometry, followed by a gene ontology (GO) enrichment analysis.
The researchers did a western blot examination on dentate gyrus tissue extracted from PF4-treated C57BL/6J mice to see if it caused synaptic alterations in mature neurons.
In a mass spectrometry-based proteome study of platelets extracted from young and aged housed and running mice, the molecular alterations in platelets after four days of exercise were examined. Flow cytometry and pan-platelet markers, such as the cluster of differentiation 61 (CD61) and CD62P, were used to determine the active platelet proportion.
Platelet responses to short- and long-term exercise were assessed by proteomic screening on platelets from aged mice after a 4.0-week run. New object localization tasks, contextual fear conditioning, and active place avoidance tests were performed.
The team observed that platelets released anti-geronic exerkines. Acute exercise stimulated platelets, which released PF4, which increased hippocampus precursor cellular proliferation among old rats.
Administering PF4 to the brains of young murine animals through osmotic pumps provided sufficient PF4 to stimulate hippocampus regeneration. Furthermore, boosting PF4 systemic levels alleviated age-associated cognitive and regenerative deficits in a hippocampus neurogenesis-dependent way.
Neurogenesis was required for CXCL4-regulated cognitive restoration. In young murine animals, systemically administered CXCL4 enhanced the amount of doublecortin-positive (DCX+) immature neuronal cells within the dentate gyrus with no influence on the proliferation of Ki67-positive neural precursors.
Increasing serological CXCL4 levels in young mice did not lead to cell proliferation, or quiescent neural stem cell recruitment since there were no significant variations in the proportion of CXCL4-treated cells in various cell cycle stages such as gap 2 (G2), mitosis (M) and synthesis (S) stages.
Instead, through exerting pro-survival or differentiation effects on the precursors of neural cells, CXCL4 boosted neurogenic processes at advanced stages, including the survival and maturation of synthesized neurons.
The team observed a large increase in beta-III-tubulin-expressing cells in CXCL4-treated cultures while not influencing astrocyte differentiation, demonstrating PF4's pro-neurogenic function. The dentate gyrus of animals lacking CXCL4 had considerably reduced basal proliferation and neuronal formation levels.
This decrease was not related to volumetric alterations in the granular cells and hippocampus layer. CXCL4 did not impact cultures obtained from the subventricular region.
RNA sequencing and GO analysis results showed that CXCL4 increased the levels of genes related to synaptic transmission, memory, and learning in adult brain precursor cells. CXCL4 increased synaptic plasticity among dentate granule neuronal cells by increasing dendritic length and complexity and decreasing dendritic spine density.
However, CXCL4 did not impact adult neurons' synaptic composition or neurite complexity. After running, Properdin, progranulin, destrin, tropomyosin, and SH3 domain-containing and ubiquitin-related protein B were significantly elevated in young and old mice.
Several antioxidant proteins, including glutaredoxin-related protein 5 and selenoproteins involved in redox regulation, were also increased in the platelets of aged mice. Following 4.0 weeks of exercise, the number of bromodeoxyuridine (BrdU)- and the mature neuron marker (NeuN)-expressing neonatal neurons and CD62-activated platelets significantly increased.
Based on the study findings, exercise mediates rejuvenating effects on brain aging, with platelets restoring cognitive function and enhancing hippocampal neurogenesis in the aged brain.