Towards a comprehensive understanding of p75 neurotrophin receptor functions and interactions in the brain

Selective, Intrinsically Fluorescent Trk Modulating Probes

Neurotrophins (NTs) elicit the growth, survival, and differentiation of neurons and other neuroectoderm tissues via activation of Trk receptors. Hot spots for NT·Trk interactions involve three neurotrophin loops. Mimicry of these using "cyclo-organopeptides" comprising loop sequences cyclized onto endocyclic organic fragments accounts for a few of the low molecular mass Trk agonists or modulators reported so far; the majority are nonpeptidic small molecules accessed without molecular design and identified in random screens. It has proven difficult to verify activities induced by low molecular mass substances are due to Trk activation (rather than via other receptors), enhanced Trk expression, enhanced NT expression, or other pathways. Consequently, identification of selective probes for the various Trk receptors (e.g., A, B, and C) has been very challenging. Further, a key feature of probes for early stage assays is that they should be easily detectable, and none of the compounds reported to date are. In this work, we designed novel cyclo-organopeptide derivatives where the organic fragment is a BODIPY fluor and found ones that selectively, though not specifically, activate TrkA, B, or C. One of the assays used to reach this conclusion (binding to live Trk-expressing cells) relied on intrinsic fluorescence in the tested materials. Consequently, this work established low molecular mass Trk-selective probes exhibiting neuroprotective effects.

Brain plasticity and neuroinflammatory protein biomarkers with circulating MicroRNAs as predictors of acute brain injury outcome - A prospective cohort study

BACKGROUND

Brain recovery mechanisms after injuries like aneurysmal subarachnoid hemorrhage (aSAH), ischemic stroke (IS), and traumatic brain injury (TBI) involve brain plasticity, synaptic regeneration, and neuroinflammation. We hypothesized that serum levels of the p75 neurotrophic receptor (p75NTR) and associated signaling proteins, as well as differentially expressed (DE) microRNAs, could predict recovery outcomes irrespective of injury type.

METHODS

A prospective patient cohort with ischemic stroke (IS, n = 30), aneurysmal subarachnoid hemorrhage (aSAH, n = 31), and traumatic brain injury (TBI, n = 13) were evaluated (total n = 74). Serum samples were collected at two post-injury intervals (early: 1-3 days, late: 4-8 days), and outcomes were assessed after three months using the modified Rankin Scale (mRS), categorizing outcomes as favorable (mRS 0-3) or unfavorable (mRS 4-6). Six proteins were measured using ELISAs: p75NTR, NGF, sortilin, IL1β, TNFα, and cyclophilin. DE microRNAs were identified using DESeq2, and their target genes were predicted. Serum molecules between patients with differing outcomes were compared using a Kolmogorov-Smirnov test, 2-tailed t-test and multivariate linear discriminant analysis (LDA).

RESULTS

Favorable (n = 46) and unfavorable (n = 28) outcome cohorts were balanced with age and sex (p = 0.25 and 0.63). None of the studied proteins correlated with age. Combinatory LDA of the six protein biomarkers indicated strong prognostic value for favorable outcomes (OR 2.09; AUC = 70.3%, p = 0.0058). MicroRNA expression changes over time were identified in the aSAH, TBI, and IS groups (p < 0.05, FDR corrected). Twenty-three microRNAs were commonly DE across all brain injury groups when comparing favorable and unfavorable outcomes (p < 0.05). LDA of four microRNAs targeting the studied proteins showed high prognostic accuracy (OR 11.7; AUC = 94.1%, p = 0.016).

CONCLUSIONS

The combined prognostic microRNA and protein biomarker models demonstrated accurate outcome prognostication across diverse injury types, implying the presence of a common recovery mechanism. DE microRNAs were found to target the studied molecules, suggesting a potential mechanistic role in recovery. Further investigation is warranted to study these molecules in prognostication, as well as therapeutic targets for enhancing recovery.

Axonal injury mediated by neuronal p75NTR/TRAF6/JNK pathway contributes to cognitive impairment after repetitive mTBI

Repetitive mild traumatic brain injury (rmTBI) is one of the leading causes of cognitive disorders. The impairment of axonal integrity induced by rmTBI is speculated to underlie the progression of cognitive dysfunction. However, few studies have uncovered the cellular mechanism regulating axonal impairment. In this study, we showed that after rmTBI, the activation of neuronal p75NTR signaling contributes to abnormal axonal morphology and impaired axonal transport, which further leads to cognitive dysfunction in mice. By neuron-specific knockdown of p75NTR or treatment with p75NTR inhibitor LM11A-31, we observed better recovery of axonal integrity and cognitive function after brain trauma. Further analysis revealed that p75NTR relies on its adaptor protein TRAF6 to activate downstream signaling via TAK1 and JNK. Overall, our results provide novel insight into the role of neuronal p75NTR in axonal injury and suggest that p75NTR may be a promising target for cognitive function recovery after rmTBI.

Brain Plasticity Modulator p75 Neurotrophin Receptor in Human Urine after Different Acute Brain Injuries-A Prospective Cohort Study

Acute brain injuries (ABIs) pose a substantial global burden, demanding effective prognostic indicators for outcomes. This study explores the potential of urinary p75 neurotrophin receptor (p75NTR) concentration as a prognostic biomarker, particularly in relation to unfavorable outcomes. The study involved 46 ABI patients, comprising sub-cohorts of aneurysmal subarachnoid hemorrhage, ischemic stroke, and traumatic brain injury. Furthermore, we had four healthy controls. Samples were systematically collected from patients treated at the University Hospital of Turku between 2017 and 2019, at early (1.50 ± 0.70 days) and late (9.17 ± 3.40 days) post-admission time points. Urinary p75NTR levels, measured by ELISA and normalized to creatinine, were compared against patients' outcomes using the modified Rankin Scale (mRS). Early urine samples showed no significant p75NTR concentration difference between favorable and unfavorable mRS groups. In contrast, late samples exhibited a statistically significant increase in p75NTR concentrations in the unfavorable group (p = 0.033), demonstrating good prognostic accuracy (AUC = 70.9%, 95% CI = 53-89%, p = 0.03). Assessment of p75NTR concentration changes over time revealed no significant variation in the favorable group (p = 0.992) but a significant increase in the unfavorable group (p = 0.009). Moreover, p75NTR concentration was significantly higher in ABI patients (mean ± SD 40.49 ± 28.83-65.85 ± 35.04 ng/mg) compared to healthy controls (mean ± SD 0.54 ± 0.44 ng/mg), irrespective of sampling time or outcome (p < 0.0001). In conclusion, late urinary p75NTR concentrations emerged as a potential prognostic biomarker for ABIs, showing increased levels associated with unfavorable outcomes regardless of the specific type of brain injury. While early samples exhibited no significant differences, the observed late increases emphasize the time-dependent nature of this potential biomarker. Further validation in larger patient cohorts is crucial, highlighting the need for additional research to establish p75NTR as a reliable prognostic biomarker across various ABIs. Additionally, its potential role as a diagnostic biomarker warrants exploration.

Genetic and Pharmacological Modulation of P75 Neurotrophin Receptor Attenuate Brain Damage After Ischemic Stroke in Mice

The precursor nerve growth factor (ProNGF) and its receptor p75 neurotrophin receptor (p75NTR) are upregulated in several brain diseases, including ischemic stroke. The activation of p75NTR is associated with neuronal apoptosis and inflammation. Thus, we hypothesized that p75NTR modulation attenuates brain damage and improves functional outcomes after ischemic stroke. Two sets of experiments were performed. (1) Adult wild-type (WT) C57BL/6 J mice were subjected to intraluminal suture-middle cerebral artery occlusion (MCAO) to induce cerebral ischemia. Pharmacological inhibitor of p75NTR, LM11A-31 (50 mg/kg), or normal saline was administered intraperitoneally (IP) 1 h post-MCAO, and animals survived for 24 h. (2) Adult p75NTR heterozygous knockout (p75NTR+/-) and WT were subjected to photothrombotic (pMCAO) to induce ischemic stroke, and the animals survived for 72 h. The sensory-motor function of animals was measured using Catwalk XT. The brain samples were collected to assess infarction volume, edema, hemorrhagic transformation, neuroinflammation, and signaling pathway at 24 and 72 h after the stroke. The findings described that pharmacological inhibition and genetic knocking down of p75NTR reduce infarction size, edema, and hemorrhagic transformation following ischemic stroke. Additionally, p75NTR modulation significantly decreased several anti-apoptosis markers and improved sensory motor function compared to the WT mice following ischemic stroke. Our observations exhibit that the involvement of p75NTR in ischemic stroke and modulation of p75NTR could improve the outcome of ischemic stroke by increasing cell survival and enhancing motor performance. LM11A-31 has the potential to be a promising therapeutic agent for ischemic stroke. However, more evidence is needed to illuminate the efficacy of LM11A-31 in ischemic stroke.

A Possible Role for Nerve Growth Factor and Its Receptors in Human Sperm Pathology

Nerve growth factor (NGF) signalling affects spermatogenesis and mature sperm traits. In this paper, we aimed to evaluate the distribution and the role of NGF and its receptors (p75NTR and TrKA) on the reproductive apparatus (testis and epididymis) and sperm of fertile men (F) and men with different pathologies, namely varicocele (V) and urogenital infections (UGIs). We collected semen samples from 21 individuals (31-40 years old) subdivided as follows: V (n = 7), UGIs (n = 7), and F (n = 7). We submitted the semen samples to bacteriological analysis, leucocyte identification, and analysis of sperm parameters (concentration, motility, morphology, and viability). We determined the seminal plasma levels of NGF, interleukin 1β (IL-1β), and F2-isoprostanes (F2-IsoPs), and the gene and protein expression of NGF receptors on sperm. We also used immunofluorescence to examine NGF receptors on ejaculated sperm, testis, and epididymis. As expected, fertile men showed better sperm parameters as well as lower levels of NGF, F2-IsoPs, and IL-1β compared with men with infertility. Notably, in normal sperm, p75NTR and TrKA were localised throughout the entire tail. TrKA was also found in the post-acrosomal sheath. This localisation appeared different in patients with infertility: in particular, there was a strong p75NTR signal in the midpiece and the cytoplasmic residue or coiled tails of altered ejaculated sperm. In line with these findings, NGF receptors were intensely expressed in the epididymis and interstitial tissue of the testis. These data suggest the distinctive involvement of NGF and its receptors in the physiology of sperm from fertile men and men with infertility, indicating a possible role for new targeted treatment strategies.

The neuropeptide PACAP alleviates T. gondii infection-induced neuroinflammation and neuronal impairment

Background

Cerebral infection with the protozoan Toxoplasma gondii (T. gondii) is responsible for inflammation of the central nervous system (CNS) contributing to subtle neuronal alterations. Albeit essential for brain parasite control, continuous microglia activation and recruitment of peripheral immune cells entail distinct neuronal impairment upon infection-induced neuroinflammation. PACAP is an endogenous neuropeptide known to inhibit inflammation and promote neuronal survival. Since PACAP is actively transported into the CNS, we aimed to assess the impact of PACAP on the T. gondii-induced neuroinflammation and subsequent effects on neuronal homeostasis.

Methods

Exogenous PACAP was administered intraperitoneally in the chronic stage of T. gondii infection, and brains were isolated for histopathological analysis and determination of pathogen levels. Immune cells from the brain, blood, and spleen were analyzed by flow cytometry, and the further production of inflammatory mediators was investigated by intracellular protein staining as well as expression levels by RT-qPCR. Neuronal and synaptic alterations were assessed on the transcriptional and protein level, focusing on neurotrophins, neurotrophin-receptors and signature synaptic markers.

Results

Here, we reveal that PACAP administration reduced the inflammatory foci and the number of apoptotic cells in the brain parenchyma and restrained the activation of microglia and recruitment of monocytes. The neuropeptide reduced the expression of inflammatory mediators such as IFN-γ, IL-6, iNOS, and IL-1β. Moreover, PACAP diminished IFN-γ production by recruited CD4⁺ T cells in the CNS. Importantly, PACAP promoted neuronal health via increased expression of the neurotrophin BDNF and reduction of p75^NTR, a receptor related to neuronal cell death. In addition, PACAP administration was associated with increased expression of transporters involved in glutamatergic and GABAergic signaling that are particularly affected during cerebral toxoplasmosis.

Conclusions

Together, our findings unravel the beneficial effects of exogenous PACAP treatment upon infection-induced neuroinflammation, highlighting the potential implication of neuropeptides to promote neuronal survival and minimize synaptic prejudice.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12974-022-02639-z.

Matrix metalloproteinases, purinergic signaling, and epigenetics: hubs in the spinal neuroglial network following peripheral nerve injury

Activation of glial cells (reactive gliosis) and the purinergic pathway, together with metalloproteinase (MMP)-induced remodeling of the neural extracellular matrix (nECM), drive maladaptive changes in the spinal cord following peripheral nerve injury (PNI). We evaluated the effects on spinal maladaptive plasticity through administration of oxidized ATP (oxATP), an antagonist of P2X receptors (P2XR), and/or GM6001, an inhibitor of MMPs, in rats following spared nerve injury (SNI) of the sciatic nerve. With morpho-molecular techniques, we demonstrated a reduction in spinal reactive gliosis and changes in the neuro-glial-nECM crosstalk via expression remodeling of P2XR, nerve growth factor (NGF) receptors (TrkA and p75), and histone deacetylase 2 (HDAC2) after treatments with oxATP/GM6001. Altogether, our data suggest that MMPs and purinergic inhibition have a modulatory impact on key proteins in the neuro-glial-nECM network, acting at different levels from intracellular signaling to epigenetic modifications.

The effect of propofol on hypoxia- and TNF-α-mediated BDNF/TrkB pathway dysregulation in primary rat hippocampal neurons

AIMS

Hypoxia and inflammation may lead to BDNF/TrkB dysregulation and neurological disorders. Propofol is an anesthetic with neuroprotective properties. We wondered whether and how propofol affected BDNF/TrkB pathway in hippocampal neurons and astrocytes.

METHODS

Primary rat hippocampal neurons and astrocytes were cultured and exposed to propofol followed by hypoxia or TNF-α treatment. The expression of BDNF and the expression/truncation/phosphorylation of TrkB were measured. The underlying mechanisms were investigated.

RESULTS

Hypoxia and TNF-α reduced the expression of BDNF, which was reversed by pretreatment of 25 μM propofol in hippocampal neurons. Furthermore, hypoxia and TNF-α increased the phosphorylation of ERK and phosphorylation of CREB at Ser142, while reduced the phosphorylation of CREB at Ser133, which were all reversed by 25 μM propofol and 10 μM ERK inhibitor. In addition, hypoxia or TNF-α did not affect TrkB expression, truncation, or phosphorylation in hippocampal neurons and astrocytes. However, in hippocampal neurons, 50 μM propofol induced TrkB phosphorylation, which may be mediated by p35 expression and Cdk5 activation, as suggested by the data showing that blockade of p35 or Cdk5 expression mitigated propofol-induced TrkB phosphorylation.

CONCLUSIONS

Propofol modulated BDNF/TrkB pathway in hippocampal neurons via ERK/CREB and p35/Cdk5 under the condition of hypoxia or TNF-α exposure.