From Young to Old: Mimicking Neuronal Aging in Directly Converted Neurons from Young Donors

A substantial challenge in human brain aging is to find a suitable model to mimic neuronal aging in vitro as accurately as possible. Using directly converted neurons (iNs) from human fibroblasts is considered a promising tool in human aging since it retains the aging-associated mitochondrial donor signature. Still, using iNs from aged donors can pose certain restrictions due to their lower reprogramming and conversion efficacy than those from younger individuals. To overcome these limitations, our study aimed to establish an in vitro neuronal aging model mirroring features of in vivo aging by acute exposure on young iNs to either human stress hormone cortisol or the mitochondrial stressor rotenone, considering stress as a trigger of in vivo aging. The impact of rotenone was evident in mitochondrial bioenergetic properties by showing aging-associated deficits in mitochondrial respiration, cellular ATP, and MMP and a rise in glycolysis, mitochondrial superoxide, and mitochondrial ROS; meanwhile, cortisol only partially induced an aging-associated mitochondrial dysfunction. To replicate the in vivo aging-associated mitochondrial dysfunctions, using rotenone, a mitochondrial complex I inhibitor, proved to be superior to the cortisol model. This work is the first to use stress on young iNs to recreate aging-related mitochondrial impairments.

From Youthful Vigor to Aging Decline: Unravelling the Intrinsic and Extrinsic Determinants of Hippocampal Neural Stem Cell Aging

Since Joseph Altman published his pioneering work demonstrating neurogenesis in the hippocampus of adult rats, the number of publications in this field increased exponentially. Today, we know that the adult hippocampus harbors a pool of adult neural stem cells (NSCs) that are the source of life-long neurogenesis and plasticity. The functions of these NSCs are regulated by extrinsic cues arising from neighboring cells and the systemic environment. However, this tight regulation is subject to imbalance with age, resulting in a decline in adult NSCs and neurogenesis, which contributes to the progressive deterioration of hippocampus-related cognitive functions. Despite extensive investigation, the mechanisms underlying this age-related decline in neurogenesis are only incompletely understood, but appear to include an increase in NSC quiescence, changes in differentiation patterns, and NSC exhaustion. In this review, we summarize recent work that has improved our knowledge of hippocampal NSC aging, focusing on NSC-intrinsic mechanisms as well as cellular and molecular changes in the niche and systemic environment that might be involved in the age-related decline in NSC functions. Additionally, we identify future directions that may advance our understanding of NSC aging and the concomitant loss of hippocampal neurogenesis and plasticity.

In vitro modeling of the neurobiological effects of glucocorticoids: A review

Hypothalamic-pituitary adrenal (HPA)axis dysregulation has long been implicated in stress-related disorders such as major depression and post-traumatic stress disorder. Glucocorticoids (GCs) are released from the adrenal glands as a result of HPA-axis activation. The release of GCs is implicated with several neurobiological changes that are associated with negative consequences of chronic stress and the onset and course of psychiatric disorders. Investigating the underlying neurobiological effects of GCs may help to better understand the pathophysiology of stress-related psychiatric disorders. GCs impact a plethora of neuronal processes at the genetic, epigenetic, cellular, and molecular levels. Given the scarcity and difficulty in accessing human brain samples, 2D and 3D in vitro neuronal cultures are becoming increasingly useful in studying GC effects. In this review, we provide an overview of in vitro studies investigating the effects of GCs on key neuronal processes such as proliferation and survival of progenitor cells, neurogenesis, synaptic plasticity, neuronal activity, inflammation, genetic vulnerability, and epigenetic alterations. Finally, we discuss the challenges in the field and offer suggestions for improving the use of in vitro models to investigate GC effects.

Morphogenic Effect of Exogenous Glucocorticoid Hormones in the Girardia tigrina Planarian (Turbellaria, Tricladida)

We have studied the effect of two glucocorticoid hormones: hydrocortisone and its synthetic analogue methylprednisolone on the regeneration activity of head and tail blastema of the Girardia tigrina planarian. The regeneration activity was studied in head and tail blastema formed after resection by means of lifetime computer morphometry and immunohistochemical labeling of neoblasts. The search for orthologous proteins-glucocorticoid receptors (hydrocortisone) was performed using the SmedGD database of the Schmidtea mediterranea planarian. The results indicate that both hormones influence the recovery rate of the regenerating head and tail blastema. The worms with regenerating tail blastema have less sensitivity to the hormones' treatment compared to the ones with regenerating head blastema. Hydrocortisone at a high concentration (10-3 M) suppressed the regeneration rate, while stimulating it at lower concentrations (10-4-10-6 M). The same concentrations of methylprednisolone inhibited the regeneration of head blastema, but did not affect the tail blastema regeneration. The two hormones acted differently: while hydrocortisone stimulated the proliferation of neoblasts in the periwound region, methylprednisolone reduced the mitotic activity, mainly on the tail zone furthest from the wound surface. We suggest that exogenous glucocorticoids can influence endogenous mechanisms of hormone-dependent regeneration.

In vitro modeling of glucocorticoid mechanisms in stress-related mental disorders: Current challenges and future perspectives

In the last decade, in vitro models has been attracting a great deal of attention for the investigation of a number of mechanisms underlying neurological and mental disorders, including stress-related disorders, for which human brain material has rarely been available. Neuronal cultures have been extensively used to investigate the neurobiological effects of stress hormones, in particular glucocorticoids. Despite great advancements in this area, several challenges and limitations of studies attempting to model and investigate stress-related mechanisms in vitro exist. Such experiments often come along with non-standardized definitions stress paradigms in vitro, variations in cell models and cell types investigated, protocols with differing glucocorticoid concentrations and exposure times, and variability in the assessment of glucocorticoid-induced phenotypes, among others. Hence, drawing consensus conclusions from in-vitro stress studies is challenging. Addressing these limitations and aligning methodological aspects will be the first step towards an improved and standardized way of conducting in vitro studies into stress-related disorders, and is indispensable to reach the full potential of in vitro neuronal models. Here, we consider the most important challenges that need to be overcome and provide initial guidelines to achieve improved use of in vitro neuronal models for investigating mechanisms underlying the development of stress-related mental disorders.

Hydrocortisone Long-term Treatment Effect on Immunomodulatory Properties of Human Adipose-Derived Mesenchymal Stromal/Stem Cells

Cortisol is secreted in prolonged stress and has therapeutic effects in inflammatory diseases. Considering the immunomodulatory effects of mesenchymal stem cells, here we investigated the effect of hydrocortisone (HC) long-term treatment on immunomodulatory properties of human adipose-derived mesenchymal stromal/stem cells (ASCs). Isolated ASCs from healthy subjects were treated with different HC concentrations for 14 days. The effect of HC-treated ASCs on the proliferative response of peripheral blood mononuclear cells (PBMCs) was evaluated in ASCs/2-way mixed leukocyte reaction coculture using 2,3-bis-(2-methoxy-4-nitro-5-sulfophenyl)-2H-tetrazolium-5-carboxanilide (XTT)-assay. HC-treated ASCs were further divided into interferon gamma (IFN-γ) stimulated and unstimulated groups. Transforming growth factor beta 1 (TGF-β1) and interleukin (IL)-6 levels were measured in culture supernatants by enzyme-linked immunosorbent assay. Relative expression of cyclooxygenase-2 (COX-2), hepatocyte growth factor, indoleamine dioxygenase, and programmed death-ligand 1 genes was assessed by real-time PCR. Levels of TGF-β1 and COX-2 expression were elevated in unstimulated ASCs, while exposure to high concentration of HC significantly increased TGF-β1 levels and reduced COX-2 expression. Unstimulated HC-5-μM-treated ASCs increased PBMC proliferation ratio on day 2 of coculture compared to the control group (P = 0.05). In IFN-γ stimulated condition, pretreatment with HC-5 μM resulted in a significantly increased IL-6 and significantly decreased COX-2 expression compared to the HC untreated control group. In conclusion, our results showed various alterations of ASC immunomodulatory related features as a result of long-term exposure of different concentrations of HC. It seems that HC at low concentration pushed the balance toward extended immune response in ASCs, while this observation wasn't persistent in ASCs treated with higher concentrations of HC.

Glucocorticoid-mediated mechanisms of hippocampal damage: Contribution of subgranular neurogenesis

A comprehensive overview of the interplay between glucocorticoids (GCs) and adult hippocampal neurogenesis (AHN) is presented, particularly, in the context of a diseased brain. The effectors of GCs in the dentate gyrus neurogenic niche of the hippocampal are reviewed, and the consequences of the GC signaling on the generation and integration of new neurons are discussed. Recent findings demonstrating how GC signaling mediates impairments of the AHN in various brain pathologies are overviewed. GC-mediated effects on the generation and integration of adult-born neurons in the hippocampal dentate gyrus depend on the nature, severity, and duration of the acting stress factor. GCs realize their effects on the AHN primarily via specific glucocorticoid and mineralocorticoid receptors. Disruption of the reciprocal regulation between the hypothalamic-pituitary-adrenal (HPA) axis and the generation of the adult-born granular neurons is currently considered to be a key mechanism implicating the AHN into the pathogenesis of numerous brain diseases, including those without a direct hippocampal damage. These alterations vary from reduced proliferation of stem and progenitor cells to increased cell death and abnormalities in morphology, connectivity, and localization of young neurons. Although the involvement of the mutual regulation between the HPA axis and the AHN in the pathogenesis of cognitive deficits and mood impairments is evident, several unresolved critical issues are stated. Understanding the details of GC-mediated mechanisms involved in the alterations in AHN could enable the identification of molecular targets for ameliorating pathology-induced imbalance in the HPA axis/AHN mutual regulation to conquer cognitive and psychiatric disturbances.

The potential predictive value of salivary cortisol on the occurrence of secondary cognitive impairment after ischemic stroke

In this study, we aimed to investigate the relationship between salivary cortisol content and secondary mild cognitive impairment (MCI), thereby supporting the prediction of MCI in clinical practice. In this study, the salivary cortisol levels were examined in 120 patients with MCI after cerebral ischemic stroke (CIS) (CIS-MIC) and 80 CIS patients without MIC (CIS). The clinical data were compared among these patients with different cortisol levels. The salivary level of cortisol was significantly higher in patients with CIS-MIC (0.85-3.65 nmol/L) than that in those with CIS (0.52-1.21 nmol/L). The categorized analysis by CIS-MIC quartile showed that patient age, hyperlipidemia, total cholesterol (TC), low-density lipoprotein cholesterol (LDL-C), etc. were significantly increased with increasing salivary cortisol levels. Moreover, univariate and multivariate logistic regression analyses revealed that the MCI risk of patients in the first quartile was 0.35 and 0.41 times, respectively, of the fourth quartile. Multiple linear regression showed that patient age, the time of rescue, and the salivary cortisol level were independent factors in the Mini-Mental State Exam (MMSE) score of MCI patients. Meanwhile, the receiver operating characteristic (ROC) curve showed that the area under the curve of salivary cortisol as a diagnostic marker for MCI after CIS was 0.982, with sensitivity of 0.973 and specificity of 0.980. In this study, we found that salivary cortisol level was an independent risk factor of MCI after CIS. A higher salivary cortisol level indicated a higher probability of MCI occurrence, and salivary cortisol level can be used as a predictive marker for MCI occurrence.

Prolyl-hydroxyproline, a collagen-derived dipeptide, enhances hippocampal cell proliferation, which leads to antidepressant-like effects in mice

Depression has been a mental health issue worldwide. We previously reported that ginger-degraded collagen hydrolysate (GDCH) suppressed depression-like behavior in mice. Furthermore, prolyl-hydroxyproline (PO) and hydroxyprolyl-glycine (OG) were detected in the circulating blood after the oral administration of GDCH. In the present study, PO, but not OG, was detected in the cerebrospinal fluid of rats after the oral administration of GDCH, suggesting that PO is transported from blood to the brain. We then investigated the effects of PO and OG on the depression-like behavior of mice. The oral administration of PO significantly decreased depression-like behavior in the forced swim test. OG had no antidepressant-like effect. In addition, proline and hydroxyproline, components of PO, also had no antidepressant-like effect after their oral administration. PO significantly increased the gene expression of brain-derived neurotrophic factor and nerve growth factor in the hippocampus, and promoted the proliferation of neural progenitor cells in vivo and in vitro. PO also increased the dopamine concentration in the prefrontal cortex. Thus, PO-dependent regulation of neurotrophic function and neurotransmitter may be the mechanism for antidepressant-like behavior. Together, these results demonstrate that PO is an antidepressant bioactive peptide accompanying the proliferation of hippocampal neural progenitor cells.

Neuroprotective effects of selegiline on rat neural stem cells treated with hydrogen peroxide

Oxidative stress and reactive oxygen species generation have been implicated in the pathogenesis of several neurological disorders including Parkinson's disease, Alzheimer's disease, amyotrophic lateral sclerosis and multiple sclerosis. In the present study, the neuroprotective effects of selegiline against hydrogen peroxide-induced oxidative stress in hippocampus-derived neural stem cells (NSCs) were evaluated. NSCs isolated from neonatal Wistar rats were pretreated with different doses of selegiline for 48 h and then exposed to 125 µM H2O2 for 30 min. Using MTT and terminal deoxynucleotidyl transferase-mediated dUTP nick end labeling assays, acridine orange/ethidium bromide staining and reverse transcription-quantitative polymerase chain reaction, the effects of selegiline on cell survival, apoptosis and the expression of B-cell lymphoma 2 (Bcl-2) and heat shock protein 4 (Hspa4) in pretreated stem cells were assessed compared with a control group lacking pretreatment. The results indicated that the viability of cells pretreated with 20 µM selegiline was significantly increased compared with the control group (P<0.05). Additionally, 20 µM selegiline increased the mRNA expression of Bcl-2 and Hspa4 (P<0.05 vs. control) and suppressed oxidative stress-induced cell death (apoptosis and necrosis; P<0.05 vs. control and 10 µM groups). From these findings, it was concluded that selegiline may be a therapeutic candidate for the treatment of neurological diseases mediated by oxidative stress.

Neural Stem Cell-Based Regenerative Approaches for the Treatment of Multiple Sclerosis

Multiple sclerosis (MS) is a chronic, autoimmune, inflammatory, and demyelinating disorder of the central nervous system (CNS), which ultimately leads to axonal loss and permanent neurological disability. Current treatments for MS are largely comprised of medications that are either immunomodulatory or immunosuppressive and are aimed at reducing the frequency and intensity of relapses. Neural stem cells (NSCs) in the adult brain can differentiate into oligodendrocytes in a context-specific manner and are shown to be involved in the remyelination in these patients. NSCs may exert their beneficial effects not only through oligodendrocyte replacement but also by providing trophic support and immunomodulation, a phenomenon now known as "therapeutic plasticity." In this review, we first provided an update on the current knowledge regarding MS pathogenesis and the role of immune cells, microglia, and oligodendrocytes in MS disease progression. Next, we reviewed the current progress on research aimed toward stimulating endogenous NSC proliferation and differentiation to oligodendrocytes in vivo and in animal models of demyelination. In addition, we explored the neuroprotective and immunomodulatory effects of transplanted exogenous NSCs on T cell activation, microglial activation, and endogenous remyelination and their effects on the pathological process and prognosis in animal models of MS. Finally, we examined various protocols to generate genetically engineered NSCs as a potential therapy for MS. Overall, this review highlights the studies involving the immunomodulatory, neurotrophic, and regenerative effects of NSCs and novel methods aiming at stimulating the potential of NSCs for the treatment of MS.

Di-(2-ethylhexyl) Phthalate-Induced Hippocampus-Derived Neural Stem Cells Proliferation

The brain and spinal cord have a limited capacity for self-repair under damaged conditions. One of the best options to overcome these limitations involves the use of phytochemicals as potential therapeutic agents. In this study, we have aimed to investigate the effects of di-(2-ethylhexyl) phthalate (DEHP) on hippocampus-derived neural stem cells (NSCs) proliferation to search phytochemical candidates for possible treatment of neurological diseases using endogenous capacity. In this experimental study, neonatal rat hippocampus-derived NSCs were cultured and treated with various concentrations of DEHP (0, 100, 200, 400 and 600 µM) and Cirsium vulgare (C. vulgare) hydroethanolic extract (0, 200, 400, 600, 800 and 1000 µg/ml) for 48 hours under in vitro conditions. Cell proliferation rates and quantitative Sox2 gene expression were evaluated using MTT assay and real-time reverse transcription polymerase chain reaction (RT-PCR). We observed the highest average growth rate in the 400 µM DEHP and 800 µg/ml C. vulgare extract treated groups. Sox2 expression in the DEHP-treated NSCs significantly increased compared to the control group. Gas chromatography/mass spectrometry (GC/ MS) results demonstrated that the active ingredients that naturally occurred in the C. vulgare hydroethanolic extract were 2-ethyl-1-hexanamine, n-heptacosane, 1-cyclopentanecarboxylic acid, 1-heptadecanamine, 2,6-octadien-1-ol,2,6,10,14,18,22-tetracosahexaene, and DEHP. DEHP profoundly stimulated NSCs proliferation through Sox2 gene overexpression. These results provide and opportunity for further use of the C. vulgure phytochemicals for prevention and/or treatment of neurological diseases via phytochemical mediated-proliferation of endogenous adult NSCs.

Venlafaxine reverses decreased proliferation in the subventricular zone in a rat model of early life stress

It is believed that glucocorticoids control the proliferation of neural progenitor cells, and this process is highly involved in mood disorders and cognitive processes. Using the maternal separation model of chronic neonatal stress, it has been found that stress induced depressive-like behavior, cognitive deficits and a decrease in proliferation in the subventricular zone (SVZ). Venlafaxine reversed all deleterious effects of chronic stress by modulating HPA activity. These outcomes suggest modulation of stress-mediated glucocorticoid secretion as a target for the treatment of mood disorders and neurodegenerative processes.

Neural stem cells respond to stress hormones: distinguishing beneficial from detrimental stress

Neural stem cells (NSCs), the progenitors of the nervous system, control distinct, position-specific functions and are critically involved in the maintenance of homeostasis in the brain. The responses of these cells to various stressful stimuli are shaped by genetic, epigenetic, and environmental factors via mechanisms that are age and developmental stage-dependent and still remain, to a great extent, elusive. Increasing evidence advocates for the beneficial impact of the stress response in various settings, complementing the extensive number of studies on the detrimental effects of stress, particularly in the developing brain. In this review, we discuss suggested mechanisms mediating both the beneficial and detrimental effects of stressors on NSC activity across the lifespan. We focus on the specific effects of secreted factors and we propose NSCs as a “sensor,” capable of distinguishing among the different stressors and adapting its functions accordingly. All the above suggest the intriguing hypothesis that NSCs are an important part of the adaptive response to stressors via direct and indirect, specific mechanisms.