Neurogenesis, cellular plasticity and cognition: the impact of stem cells in the adult and aging brain--a mini-review

Repeated Intravenous Administration of Human Neural Stem Cells Producing Choline Acetyltransferase Exerts Anti-Aging Effects in Male F344 Rats

Major features of aging might be progressive decreases in cognitive function and physical activity, in addition to withered appearance. Previously, we reported that the intracerebroventricular injection of human neural stem cells (NSCs named F3) encoded the choline acetyltransferase gene (F3.ChAT). The cells secreted acetylcholine and growth factors (GFs) and neurotrophic factors (NFs), thereby improving learning and memory function as well as the physical activity of aged animals. In this study, F344 rats (10 months old) were intravenously transplanted with F3 or F3.ChAT NSCs (1 × 106 cells) once a month to the 21st month of age. Their physical activity and cognitive function were investigated, and brain acetylcholine (ACh) and cholinergic and dopaminergic system markers were analyzed. Neuroprotective and neuroregenerative activities of stem cells were also confirmed by analyzing oxidative damages, neuronal skeletal protein, angiogenesis, brain and muscle weights, and proliferating host stem cells. Stem cells markedly improved both cognitive and physical functions, in parallel with the elevation in ACh levels in cerebrospinal fluid and muscles, in which F3.ChAT cells were more effective than F3 parental cells. Stem cell transplantation downregulated CCL11 and recovered GFs and NFs in the brain, leading to restoration of microtubule-associated protein 2 as well as functional markers of cholinergic and dopaminergic systems, along with neovascularization. Stem cells also restored muscular GFs and NFs, resulting in increased angiogenesis and muscle mass. In addition, stem cells enhanced antioxidative capacity, attenuating oxidative damage to the brain and muscles. The results indicate that NSCs encoding ChAT improve cognitive function and physical activity of aging animals by protecting and recovering functions of multiple organs, including cholinergic and dopaminergic systems, as well as muscles from oxidative injuries through secretion of ACh and GFs/NFs, increased antioxidant elements, and enhanced blood flow.

Cognitive Effects of Montelukast: A Pharmaco-EEG Study

Montelukast is a well-established antiasthmatic drug with little side effects. It is a leukotriene receptor antagonist and recent research suggests cognitive benefits from its anti-inflammatory actions on the central nervous system. However, changes in brain activity were not directly shown so far in humans. This study aims to document changes in brain activity that are associated with cognitive improvement during treatment with Montelukast. We recorded EEG and conducted neuropsychological tests in 12 asthma-patients aged 38-73 years before and after 8 weeks of treatment with Montelukast. We found no significant changes on neuropsychological scales for memory, attention, and mood. In the EEG, we found decreased entropy at follow up during rest (p < 0.005). During episodic memory acquisition we found decreased entropy (p < 0.01) and acceleration of the background rhythm (p < 0.05). During visual attention performance, we detected an increase in gamma power (p < 0.005) and slowing of the background rhythm (p < 0.05). The study is limited by its small sample size, young age and absence of baseline cognitive impairment of the participants. Unspecific changes in brain activity were not accompanied by cognitive improvement. Future studies should examine elderly patients with cognitive impairment in a double-blind study with longer-term treatment by Montelukast.

An Analysis of Anesthesia Induction Dosing in Female Older Adults

BACKGROUND/OBJECTIVES

In the context of an aging surgical population, appropriate anesthetic induction dose adjustments for the older adult remain poorly defined. In the present study, we describe the prevalence of excess induction agent dose in reference to US Food and Drug Administration (FDA) guidance and seek to investigate the possible association of such excess dose with postinduction hypotension and postoperative acute kidney injury (AKI).

STUDY DESIGN

A retrospective observational study was conducted in a large tertiary teaching hospital in accordance with our a priori analytic protocol as registered on ClinicalTrials.gov (NCT03699696). For inclusion, patients had to be 65 years or older and to have received general anesthesia with propofol induction for gynecologic oncology surgery between December 1, 2014 and July 8, 2018. Descriptive variables of the patients, machine-captured perioperative vital signs, induction anesthetic, and vasopressor/inotrope administrations were recorded.

MAIN OUTCOME MEASURES

A total of 541 female patients met inclusion criteria. The mean (standard deviation) age of the cohort was 72.20 (5.93) years. Regarding the primary outcome, 65.43% (354 patients, 95% confidence interval 61.2-69.4) of the cohort received more than the FDA recommended 1-1.5 mg/kg induction dose for patients of advanced age undergoing general anesthesia.

RESULTS

The percentage of patients receiving doses in excess of the FDA guidance remained substantial across all age groups, but decreased progressively with increasing 5-year age intervals (from 74% among those aged 65-69 years to 44% among those aged > 80 years). Excess propofol dose in the present cohort was not associated with our a priori definition of postinduction hypotension. Regarding AKI, among the 30 patients suffering this outcome, it occurred less often in patients who received higher propofol doses (4.1% [9/217] vs. 15.3% [21/138], p < 0.001), a result that may have been confounded by differential rates of missingness.

CONCLUSIONS

Older adults commonly receive propofol induction doses in excess of the FDA guidance. The immediate hemodynamic effects of these doses on postinduction hypotension were not seen in the present cohort, suggesting that propofol dose adjustments may serve as a marker of physicians' judgments as to the frailty of patients. The relevance of the AKI association is difficult to interpret due to the non-differential missingness of AKI data between the two groups.

Hypoglycemia and Dementia Risk in Older Patients with Type 2 Diabetes Mellitus: A Propensity-Score Matched Analysis of a Population-Based Cohort Study

BACKGROUND

Type 2 diabetes mellitus (T2DM) is associated with an increased risk for dementia. The effects of hypoglycemia on dementia are controversial. Thus, we evaluated whether hypoglycemia increases the risk for dementia in senior patients with T2DM.

METHODS

We used the Korean National Health Insurance Service Senior cohort, which includes >10% of the entire senior population of South Korea. In total, 5,966 patients who had ever experienced at least one episode of hypoglycemia were matched with those who had not, using propensity score matching. The risk of dementia was assessed through a survival analysis of matched pairs.

RESULTS

Patients with underlying hypoglycemic events had an increased risk for all-cause dementia, Alzheimer's dementia (AD), and vascular dementia (VaD) compared with those who had not experienced a hypoglycemic event (hazard ratio [HR], 1.254; 95% confidence interval [CI], 1.166 to 1.349; P<0.001 for all-cause dementia; HR, 1.264; 95% CI, 1.162 to 1.375; P<0.001 for AD; HR, 1.286; 95% CI, 1.110 to 1.490; P<0.001 for VaD). According to number of hypoglycemic episodes, the HRs of dementia were 1.170, 1.201, and 1.358 in patients with one hypoglycemic episode, two or three episodes, and more than three episodes, respectively. In the subgroup analysis, hypoglycemia was associated with an increased risk for dementia in both sexes with or without T2DM microvascular or macrovascular complications.

CONCLUSION

Our findings suggest that patients with a history of hypoglycemia have a higher risk for dementia. This trend was similar for AD and VaD, the two most important subtypes of dementia.

Preoperative Management of the Geriatric Patient: Frailty and Cognitive Impairment Assessment

As the population ages, more geriatric patients will be presenting for surgical procedures. Preoperative evaluation seeks to assess patients for geriatric syndromes: frailty, sarcopenia, functional dependence, and malnutrition. Age-related changes in physiology increase risk for central nervous system, cardiovascular, pulmonary, renal, hepatic, and endocrine morbidity and mortality. Identification of various comorbidities allows for preoperative optimization and for opportunities for intervention including nutritional supplementation and prehabilitation, which may improve postoperative outcomes.

Magnitude Assessment of Adult Neurogenesis in the Octopus vulgaris Brain Using a Flow Cytometry-Based Technique

Adult neurogenesis is widespread among metazoans, it occurs in animals with a network nervous system, as cnidarians, and in animals with a complex and centralized brain, such as mammals, non-mammalian vertebrates, ecdysozoans, and a lophotrochozoan, Octopus vulgaris. Nevertheless, there are important differences among taxa, especially in the number of the regions involved and in cell proliferation rate during the life-cycle. The comparative evaluation of adult neurogenesis among different brain regions is an arduous task to achieve with only stereological techniques. However, in Octopus vulgaris we recently confirmed the presence of active proliferation in the learning-memory centers, multisensory integration centers, and the motor centers of the adult brain. Here, using a flow cytometry technique, we provide a method to quantify the active proliferation in octopus nervous system using a BrdU in vitro administration without exposing the animals to stress or painful injections usually used. This method is in line with the current animal welfare regulations regarding cephalopods, and the flow cytometry-based technique enabled us to measure adult neurogenesis more quickly and reliably than histological techniques, with the additional advantage of processing multiple samples in parallel. Flow cytometry is thus an appropriate technique for measuring and comparing adult neurogenesis in animals that are in a different physiological and/or environmental contexts. A BrdU immunoreactivity distribution, to define the neurogenic areas, and the effective penetration in vitro of the BrdU is also provided.

Limited daily feeding and intermittent feeding have different effects on regional brain energy homeostasis during aging

Albeit aging is an inevitable process, the rate of aging is susceptible to modifications. Dietary restriction (DR) is a vigorous nongenetic and nonpharmacological intervention that is known to delay aging and increase healthspan in diverse species. This study aimed to compare the impact of different restricting feeding regimes such as limited daily feeding (LDF, 60% AL) and intermittent feeding (IF) on brain energy homeostasis during aging. The analysis was focused on the key molecules in glucose and cholesterol metabolism in the cortex and hippocampus of middle-aged (12-month-old) and aged (24-month-old) male Wistar rats. We measured the impact of different DRs on the expression levels of AMPK, glucose transporters (GLUT1, GLUT3, GLUT4), and the rate-limiting enzyme in the cholesterol synthesis pathway (HMGCR). Additionally, we assessed the changes in the amounts of cholesterol, its metabolite, and precursors following LDF and IF. IF decreased the levels of AMPK and pAMPK in the cortex while the increased levels were detected in the hippocampus. Glucose metabolism was more affected in the cortex, while cholesterol metabolism was more influenced in the hippocampus. Overall, the hippocampus was more resilient to the DRs, with fewer changes compared to the cortex. We showed that LDF and IF differently affected the brain energy homeostasis during aging and that specific brain regions exhibited distinct vulnerabilities towards DRs. Consequently, special attention should be paid to the DR application among elderly as different phases of aging do not respond equally to altered nutritional regimes.

Vagal Nerve Stimulation for Treatment-Resistant Depression

Major depressive disorder (MDD) is prevalent. Although standards antidepressants are more effective than placebo, up to 35% of patients do not respond to 4 or more conventional treatments and are considered to have treatment-resistant depression (TRD). Considerable effort has been devoted to trying to find effective treatments for TRD. This review focuses on vagus nerve stimulation (VNS), approved for TRD in 2005 by the Food and Drugs Administration. Stimulation is carried by bipolar electrodes on the left cervical vagus nerve, which are attached to an implanted stimulator generator. The vagus bundle contains about 80% of afferent fibers terminating in the medulla, from which there are projections to many areas of brain, including the limbic forebrain. Various types of brain imaging studies reveal widespread functional effects in brain after either acute or chronic VNS. Although more randomized control trials of VNS need to be carried out before a definitive conclusion can be reached about its efficacy, the results of open studies, carried out over period of 1 to 2 years, show much more efficacy when compared with results from treatment as usual studies. There is an increase in clinical response to VNS between 3 and 12 months, which is quite different from that seen with standard antidepressant treatment of MDD. Preclinically, VNS affects many of the same brain areas, neurotransmitters (serotonin, norepinephrine) and signal transduction mechanisms (brain-derived neurotrophic factor-tropomyosin receptor kinase B) as those found with traditional antidepressants. Nevertheless, the mechanisms by which VNS benefits patients nonresponsive to conventional antidepressants is unclear, with further research needed to clarify this.

Cognitive performance in unipolar old-age depression: a longitudinal study

OBJECTIVE

Previous studies on cognitive deficits in acute and remitted states of old-age depression have shown mixed findings. The episodic nature of depression makes repeated assessment of cognitive performance important in order to address reversibility and stability of cognitive deficits.

METHODS

Dementia-free older participants (≥60 years) from the population-based Swedish National Study on Aging and Care in Kungsholmen who completed neuropsychological testing at baseline (T1) and follow-up (T2) formed the basis of the study sample. Participants were grouped according to depression status at T1 and T2: depressed-remitted (n = 32), remitted-depressed (n = 45), and nondepressed-depressed (n = 29). These groups were compared with a group of randomly selected and matched (age, gender, education, and follow-up time) healthy controls (n = 106) over a period of maximum 6 years.

RESULTS

Mixed ANCOVAs, controlling for age and gender, revealed depression-related deficits for processing speed, attention, executive function, and category fluency. In remitted states, only processing speed and attention were affected. However, these deficits were attenuated after exclusion of persons using benzodiazepine medications. A general pattern of cognitive decline was observed across all groups for processing speed, executive function, category fluency, and episodic and semantic memory; persons transitioning from a nondepressed to depressed state tended to show exacerbated cognitive decline.

CONCLUSIONS

The results support the notion that cognitive deficits in depression may be more transient than stable. Consequently, cognitive deficits in depression might be regarded as potential treatment targets rather than stable vulnerabilities. As such, repeated assessment of cognitive functioning may provide an additional marker of treatment response.

Opiate Analgesics as Negative Modulators of Adult Hippocampal Neurogenesis: Potential Implications in Clinical Practice

During the past decade, studies of the mechanisms and functional implications of adult hippocampal neurogenesis (ahNG) have significantly progressed. At present, it is proposed that adult born neurons may contribute to a variety of hippocampal-related functions, including specific cognitive aspects and mood regulation. Several groups focussed on the factors that regulate proliferation and fate determination of adult neural stem/progenitor cells (NSC/NPC), including clinically relevant drugs. Opiates were the first drugs shown to negatively impact neurogenesis in the adult mammalian hippocampus. Since that initial report, a vast array of information has been collected on the effect of opiate drugs, by either modulating proliferation of stem/progenitor cells or interfering with differentiation, maturation and survival of adult born neurons. The goal of this review is to critically revise the present state of knowledge on the effect of opiate drugs on the different developmental stages of ahNG, as well as the possible underlying mechanisms. We will also highlight the potential impact of deregulated hippocampal neurogenesis on patients undergoing chronic opiate treatment. Finally, we will discuss the differences in the negative impact on ahNG among clinically relevant opiate drugs, an aspect that may be potentially taken into account to avoid long-term deregulation of neural plasticity and its associated functions in the clinical practice.

Physical exercise induces hippocampal neurogenesis and prevents cognitive decline

Accumulating evidence from animal and human research indicate that adult hippocampal neurogenesis plays a key role in cognition. Meanwhile, cognitive decline is well known to associate with ageing-related neurodegenerative diseases such as Alzheimer's disease (AD) and Parkinson's disease (PD). Therefore, prevention of hippocampal neurogenesis reduction should be critical for these diseases. Physical exercise, a potent enhancer of adult hippocampal neurogenesis, has emerged as a potential therapy or an adjunctive therapeutic strategy for cognitive decline. In this review, we discuss the recent findings on hippocampal neurogenesis and the incorporation of new born neurons into the neuronal network in humans and in rodents. By focusing on hippocampal neurogenesis, we illustrate the role and possible mechanisms of physical exercise in cognition preservation.

Adult Neurogenesis in Mammals: Variations and Confusions

Mammalian adult neurogenesis has remained enigmatic. Two lines of research have emerged. One focuses on a potential repair mechanism in the human brain. The other aims at elucidating its functional role in the hippocampal formation, chiefly in cognitive processes; however, thus far it has been unsuccessful. Here, we try to recognize the sources of errors and conceptual confusion in comparative studies and neurobehavioral approaches with a focus on mice. Evolutionarily, mammalian adult neurogenesis appears as protracted juvenile neurogenesis originating from precursor cells in the secondary proliferation zones, from where newly formed cells migrate to target regions in the forebrain. This late developmental process is downregulated differentially in various brain structures depending on species and age. Adult neurogenesis declines substantially during early adulthood and persists at low levels into senescence. Short-lasting episodes in proliferation or reduction of adult neurogenesis may reflect a multitude of factors, and have been studied chiefly in mice and rats. Comparative studies face both species-specific variations in staining and technical abilities of laboratories, lacking quantification of important reference measures (e.g. granule cell number) and evaluation of maturational markers whose persistence might be functionally more relevant than proliferation rates. Likewise, the confusion about the functional role of variations in adult hippocampal neurogenesis has many causes. Prominent is an inferential statistical approach, usually with low statistical power. Interpretation is complicated by multiple theories about hippocampal function, often unrealistically extrapolating from humans to rodents. We believe that the field of mammalian adult neurogenesis needs more critical thinking, more sophisticated hypotheses, better statistical, technical and behavioral approaches, and a broader conceptual perspective incorporating comparative aspects rather than neglecting them.

Age-Dependent Neurogenesis and Neuron Numbers within the Olfactory Bulb and Hippocampus of Homing Pigeons

Many birds are supreme long-distance navigators that develop their navigational ability in the first months after fledgling but update the memorized environmental information needed for navigation also later in life. We studied the extent of juvenile and adult neurogenesis that could provide such age-related plasticity in brain regions known to mediate different mechanisms of pigeon homing: the olfactory bulb (OB), and the triangular area of the hippocampal formation (HP tr). Newly generated neurons (visualized by doublecortin, DCX) and mature neurons were counted stereologically in 35 pigeon brains ranging from 1 to 168 months of age. At the age of 1 month, both areas showed maximal proportions of DCX positive neurons, which rapidly declined during the first year of life. In the OB, the number of DCX-positive periglomerular neurons declined further over time, but the number of mature periglomerular cells appeared unchanged. In the hippocampus, the proportion of DCX-positive neurons showed a similar decline yet to a lesser extent. Remarkably, in the triangular area of the hippocampus, the oldest birds showed nearly twice the number of neurons as compared to young adult pigeons, suggesting that adult born neurons in these regions expanded the local circuitry even in aged birds. This increase might reflect navigational experience and, possibly, expanded spatial memory. On the other hand, the decrease of juvenile neurons in the aging OB without adding new circuitry might be related to the improved attachment to the loft characterizing adult and old pigeons.

The L-type calcium channel Cav1.3 is required for proper hippocampal neurogenesis and cognitive functions

L-type voltage gated Ca(2+) channels (LTCCs) are widely expressed within different brain regions including the hippocampus. The isoforms Cav1.2 and Cav1.3 have been shown to be involved in hippocampus-dependent learning and memory, cognitive functions that require proper hippocampal neurogenesis. In vitro, functional LTCCs are expressed on neuronal progenitor cells, where they promote neuronal differentiation. Expression of LTCCs on neural stem and progenitor cells within the neurogenic regions in the adult brain in vivo has not been examined so far, and a contribution of the individual isoforms Cav1.2 and Cav1.3 to adult neurogenesis remained to be clarified. To reveal the role of these channels we first evaluated the expression patterns of Cav1.2 and Cav1.3 in the hippocampal dentate gyrus and the subventricular zone (SVZ) in adult (2- and 3-month old) and middle-aged (15-month old) mice on mRNA and protein levels. We performed immunohistological analysis of hippocampal neurogenesis in adult and middle-aged Cav1.3(-/-) mice and finally addressed the importance of Cav1.3 for hippocampal function by evaluating spatial memory and depression-like behavior in adult Cav1.3(-/-) mice. Our results showed Cav1.2 and Cav1.3 expression at different stages of neuronal differentiation. While Cav1.2 was primarily restricted to mature NeuN(+) granular neurons, Cav1.3 was expressed in Nestin(+) neural stem cells and in mature NeuN(+) granular neurons. Adult and middle-aged Cav1.3(-/-) mice showed severe impairments in dentate gyrus neurogenesis, with significantly smaller dentate gyrus volume, reduced survival of newly generated cells, and reduced neuronal differentiation. Further, Cav1.3(-/-) mice showed impairment in the hippocampus dependent object location memory test, implicating Cav1.3 as an essential element for hippocampus-associated cognitive functions. Thus, modulation of LTCC activities may have a crucial impact on neurogenic responses and cognition, which should be considered for future therapeutic administration of LTCCs modulators.

Reduced plasma desmosterol-to-cholesterol ratio and longitudinal cognitive decline in Alzheimer's disease

Background

We here examined whether plasma desmosterol-to-cholesterol ratio (DES/CHO) is decreased in patients with Alzheimer's disease (AD) and investigated the association between plasma DES/CHO and longitudinal cognitive decline.

Methods

Plasma DES/CHO of AD patients and age-matched controls in a Japanese cross-sectional cohort was determined. Plasma DES/CHO at baseline and follow-up visits was assessed in relation to cognitive decline in Japanese and Swedish longitudinal cohorts.

Results

Plasma DES/CHO was significantly reduced in Japanese AD patients and significantly correlated with Mini-Mental State Examination (MMSE) score. The longitudinal analysis revealed that plasma DES/CHO in AD patients shows a significant decrease at follow-up intervals. The decline in plasma DES/CHO is larger in the AD group with rapid progression than in that with slow progression. The changes in plasma DES/CHO significantly correlated with changes in the MMSE score.

Conclusion

Plasma DES/CHO is decreased in AD patients and may serve as a longitudinal surrogate marker associated with cognitive decline.

Temporal profiles of synaptic plasticity-related signals in adult mouse hippocampus with methotrexate treatment

Methotrexate, which is used to treat many malignancies and autoimmune diseases, affects brain functions including hippocampal-dependent memory function. However, the precise mechanisms underlying methotrexate-induced hippocampal dysfunction are poorly understood. To evaluate temporal changes in synaptic plasticity-related signals, the expression and activity of N-methyl-D-aspartic acid receptor 1, calcium/calmodulin-dependent protein kinase II, extracellular signal-regulated kinase 1/2, cAMP responsive element-binding protein, glutamate receptor 1, brain-derived neurotrophic factor, and glial cell line-derived neurotrophic factor were examined in the hippocampi of adult C57BL/6 mice after methotrexate (40 mg/kg) intraperitoneal injection. Western blot analysis showed biphasic changes in synaptic plasticity-related signals in adult hippocampi following methotrexate treatment. N-methyl-D-aspartic acid receptor 1, calcium/calmodulin-dependent protein kinase II, and glutamate receptor 1 were acutely activated during the early phase (1 day post-injection), while extracellular signal-regulated kinase 1/2 and cAMP responsive element-binding protein activation showed biphasic increases during the early (1 day post-injection) and late phases (7–14 days post-injection). Brain-derived neurotrophic factor and glial cell line-derived neurotrophic factor expression increased significantly during the late phase (7–14 days post-injection). Therefore, methotrexate treatment affects synaptic plasticity-related signals in the adult mouse hippocampus, suggesting that changes in synaptic plasticity-related signals may be associated with neuronal survival and plasticity-related cellular remodeling.

The proliferation of amplifying neural progenitor cells is impaired in the aging brain and restored by the mTOR pathway activation

A decrease in neurogenesis in the aged brain has been correlated with cognitive decline. The molecular signaling that regulates age-related decline in neurogenesis is still not fully understood. We found that different subtypes of neural stem cells (NSCs) in the hippocampus were differentially impaired by aging. The quiescent NSCs decreased slowly, although the active NSCs exhibited a sharp and dramatic decline from the ages of 6-9 months and became more quiescent at an early stage during the aging process. The activity of the mammalian target of rapamycin (mTOR) signal pathway is compromised in the NSCs of the aged brain. Activating the mTOR signaling pathway increased NSC proliferation and promoted neurogenesis in aged mice. In contrast, inhibiting the mTOR signaling pathway decreased NSCs proliferation. These results indicate that an age-associated decline in neurogenesis is mainly because of the reduction in proliferation of active NSCs, at least partially because of the compromise in the mTOR signaling activity. Stimulating the mTOR signaling revitalizes the NSCs, restores their proliferation, and enhances neurogenesis in the hippocampus of the aged brain.

Modulation of neurogenesis by targeting epigenetic enzymes using small molecules: an overview

Neurogenesis consists of a plethora of complex cellular processes including neural stem cell (NSC) proliferation, migration, maturation or differentiation to neurons, and finally integration into the pre-existing neural circuits in the brain, which are temporally regulated and coordinated sequentially. Mammalian neurogenesis begins during embryonic development and continues in postnatal brain (adult neurogenesis). It is now evident that adult neurogenesis is driven by extracellular and intracellular signaling pathways, where epigenetic modifications like reversible histone acetylation, methylation, as well as DNA methylation play a vital role. Epigenetic regulation of gene expression during neural development is governed mainly by histone acetyltransferases (HATs), histone methyltransferase (HMTs), DNA methyltransferases (DNMTs), and also the enzymes for reversal, like histone deacetylases (HDACs), and many of these have also been shown to be involved in the regulation of adult neurogenesis. The contribution of these epigenetic marks to neurogenesis is increasingly being recognized, through knockout studies and small molecule modulator based studies. These small molecules are directly involved in regeneration and repair of neurons, and not only have applications from a therapeutic point of view, but also provide a tool to study the process of neurogenesis itself. In the present Review, we will focus on small molecules that act predominantly on epigenetic enzymes to enhance neurogenesis and neuroprotection and discuss the mechanism and recent advancements in their synthesis, targeting, and biology.

Indeterminate body growth and lack of gonadal decline in the brown ghost knifefish (Apteronotus leptorhynchus), an organism exhibiting negligible brain senescence

The brown ghost knifefish (Apteronotus leptorhynchus (Ellis in Eigenmann, 1912)) is the only vertebrate organism identified thus far that exhibits negligible brain senescence. The present study examines the basic growth patterns of this species, testing the hypothesis that indeterminate growth and lack of reproductive senescence correlate with negligible senescence. Analysis of length–mass relationships revealed negative allometric growth in males and isometric growth in females. Total length at first sexual maturity was 13.5 cm in males and 12.0 cm in females, whereas gonadal mass was 0.02 g in males and 0.2 g in females. Modelling of total length as a function of the number of otolith rings using attenuating growth equations revealed that lengths of up to 26.8 cm in males and 20.2 cm in females can be reached, indicating that the fish continue to grow throughout life. Gonadal mass increased significantly with age in sexually immature individuals of both sexes. In sexually mature fish, gonadal mass showed a marginal increase with age in males and no change in females. The demonstration of indeterminate growth of the fish and of the lack of gonadal regression with age has important implications for the characterization of brown ghost knifefish as a model of negligible senescence.

Exercise reduces diet-induced cognitive decline and increases hippocampal brain-derived neurotrophic factor in CA3 neurons

BACKGROUND

Previous studies have shown that a western diet impairs, whereas physical exercise enhances hippocampus-dependent learning and memory. Both diet and exercise influence expression of hippocampal brain-derived neurotrophic factor (BDNF), which is associated with improved cognition. We hypothesized that exercise reverses diet-induced cognitive decline while increasing hippocampal BDNF.

METHODS

To test the effects of exercise on hippocampal-dependent memory, we compared cognitive scores of Sprague-Dawley rats exercised by voluntary running wheel (RW) access or forced treadmill (TM) to sedentary (Sed) animals. Memory was tested by two-way active avoidance test (TWAA), in which animals are exposed to a brief shock in a specific chamber area. When an animal avoids, escapes or has reduced latency to do either, this is considered a measure of memory. In a second experiment, rats were fed either a high-fat diet or control diet for 16 weeks, then randomly assigned to running wheel access or sedentary condition, and TWAA memory was tested once a week for 7 weeks of exercise intervention.

RESULTS

Both groups of exercised animals had improved memory as indicated by reduced latency to avoid and escape shock, and increased avoid and escape episodes (p<0.05). Exposure to a high-fat diet resulted in poor performance during both the acquisition and retrieval phases of the memory test as compared to controls. Exercise reversed high-fat diet-induced memory impairment, and increased brain-derived neurotrophic factor (BDNF) in neurons of the hippocampal CA3 region.

CONCLUSIONS

These data suggest that exercise improves memory retrieval, particularly with respect to avoiding aversive stimuli, and may be beneficial in protecting against diet induced cognitive decline, likely via elevated BDNF in neurons of the CA3 region.

Cholesterol metabolism changes under long-term dietary restrictions while the cholesterol homeostasis remains unaffected in the cortex and hippocampus of aging rats

Maintaining cholesterol homeostasis in the brain is vital for its proper functioning. While it is well documented that dietary restriction modulates the metabolism of cholesterol peripherally, little is known as to how it can affect cholesterol metabolism in the brain. The present study was designed to elucidate the impact of long-term dietary restriction on brain cholesterol metabolism. Three-month-old male Wistar rats were exposed to long-term dietary restriction until 12 and 24 months of age. The concentrations of cholesterol, its precursors and metabolites, and food-derived phytosterols were measured in the serum, cortex, and hippocampus by gas chromatography/mass spectrometry. Relative changes in the levels of proteins involved in cholesterol synthesis, transport, and degradation were determined by Western blot analysis. Reduced food intake influenced the expression patterns of proteins implicated in cholesterol metabolism in the brain in a region-specific manner. Dietary restriction decreased the concentrations of cholesterol precursors, lanosterol in the cortex, and lanosterol and lathosterol in the hippocampus at 12 months, while the level of desmosterol was elevated in the hippocampus at 24 months. The concentrations of cholesterol and 24(S)-hydroxycholesterol remained unaffected. Food-derived phytosterols were significantly lower after dietary restriction in both the cortex and hippocampus at 12 and 24 months. These findings provide new insight into the effects of dietary restriction on cholesterol metabolism in the brain, lending further support to its neuroprotective effect.

Clinical and Cognitive Correlates of Structural Hippocampal Change in "At-Risk" Older Adults

With estimates of dementia expected to rise over the coming decades, there is interest in understanding the factors associated with promoting neuroprotection and limiting neurodegeneration. In this study, we examined the change in the volume of the hippocampus over a 2-month period in 34 older people "at risk" of cognitive decline (mean age = 66.8 years, 38% male). Factors that were examined included cognitive reserve, neuropsychological functioning, depression as well as a lifestyle (cognitive training) intervention. The results showed that over a 2-month period, increases in hippocampal size were associated with having higher premorbid intellect, greater occupational attainment, superior memory, and higher levels of functioning. Conversely, depression and disability were associated with decreases in hippocampal volume. Cognitive training was not associated with changes in hippocampal volume. These findings suggest that factors associated with cognitive reserve, cognition and depression may play an integral pathophysiological role in determining hippocampal volumes in "at-risk" older adults.

Should general anaesthesia be avoided in the elderly?

Surgery and anaesthesia exert comparatively greater adverse effects on the elderly than on the younger brain, manifest by the higher prevalence of postoperative delirium and cognitive dysfunction. Postoperative delirium and cognitive dysfunction delay rehabilitation, and are associated with increases in morbidity and mortality among elderly surgical patients. We review the aetiology of postoperative delirium and cognitive dysfunction in the elderly with a particular focus on anaesthesia and sedation, discuss methods of diagnosing and monitoring postoperative cognitive decline, and describe the treatment strategies by which such decline may be prevented.

Age-related changes in stem cell dynamics, neurogenesis, apoptosis, and gliosis in the adult brain: a novel teleost fish model of negligible senescence

Adult neurogenesis, the generation of new neurons in the adult central nervous system, is a reported feature of all examined vertebrate species. However, a dramatic decline in the rates of cell proliferation and neuronal differentiation occurs in mammals, typically starting near the onset of sexual maturation. In the present study, we examined possible age-related changes associated with adult neurogenesis in the brain of brown ghost knifefish (Apteronotus leptorhynchus), a teleost fish distinguished by its enormous neurogenic potential. Contrary to the well-established alterations in the mammalian brain during aging, in the brain of this teleostean species we could not find evidence for any significant age-related decline in the absolute levels of stem/progenitor cell proliferation, neuronal and glial differentiation, or long-term survival of newly generated cells. Moreover, there was no indication that the amount of glial fibrillary acidic protein or the number of apoptotic cells in the brain was altered significantly over the course of adult life. We hypothesize that this first demonstration of negligible cellular senescence in the vertebrate brain is related to the continued growth of this species and to the lack of reproductive senescence during adulthood. The establishment of the adult brain of this species as a novel model of negligible senescence provides new opportunities for the advancement of our understanding of the biology of aging and the fundamental mechanisms that underlie senescence in the brain.

Neurogenesis and neuronal regeneration in status epilepticus

Neurogenesis in the adult central nervous system has been well documented in several mammals including humans. By now, a plethora of data has been generated with the aim of understanding the molecular and cellular events governing neurogenesis. This growing comprehension will provide the basis for modulation of neurogenesis for therapeutic purposes, in particular in neurodegenerative diseases. Herein, we review the current knowledge on neurogenesis, in particular in the frame of epilepsy, since seizures have massive effects on neurogenesis. Conversely, some studies have suggested that aberrant neurogenesis might contribute to the development or manifestation of epilepsy and, moreover, chronic inhibition of neurogenesis in epilepsy might contribute to comorbidities of epilepsy such as cognitive deficits. Therefore, a better understanding of neurogenesis in the context of epilepsy is still required for future therapeutic purposes.

Modifications of hippocampal circuits and early disruption of adult neurogenesis in the tg2576 mouse model of Alzheimer's disease

At advanced stages of Alzheimer's disease, cognitive dysfunction is accompanied by severe alterations of hippocampal circuits that may largely underlie memory impairments. However, it is likely that anatomical remodeling in the hippocampus may start long before any cognitive alteration is detected. Using the well-described Tg2576 mouse model of Alzheimer's disease that develops progressive age-dependent amyloidosis and cognitive deficits, we examined whether specific stages of the disease were associated with the expression of anatomical markers of hippocampal dysfunction. We found that these mice develop a complex pattern of changes in their dentate gyrus with aging. Those include aberrant expression of neuropeptide Y and reduced levels of calbindin, reflecting a profound remodeling of inhibitory and excitatory circuits in the dentate gyrus. Preceding these changes, we identified severe alterations of adult hippocampal neurogenesis in Tg2576 mice. We gathered converging data in Tg2576 mice at young age, indicating impaired maturation of new neurons that may compromise their functional integration into hippocampal circuits. Thus, disruption of adult hippocampal neurogenesis occurred before network remodeling in this mouse model and therefore may account as an early event in the etiology of Alzheimer's pathology. Ultimately, both events may constitute key components of hippocampal dysfunction and associated cognitive deficits occurring in Alzheimer's disease.

Aging induces tissue-specific changes in cholesterol metabolism in rat brain and liver

Disturbance of cholesterol homeostasis in the brain is coupled to age-related brain dysfunction. In the present work, we studied the relationship between aging and cholesterol metabolism in two brain regions, the cortex and hippocampus, as well as in the sera and liver of 6-, 12-, 18- and 24-month-old male Wistar rats. Using gas chromatography-mass spectrometry, we undertook a comparative analysis of the concentrations of cholesterol, its precursors and metabolites, as well as dietary-derived phytosterols. During aging, the concentrations of the three cholesterol precursors examined (lanosterol, lathosterol and desmosterol) were unchanged in the cortex, except for desmosterol which decreased (44 %) in 18-month-old rats. In the hippocampus, aging was associated with a significant reduction in lanosterol and lathosterol concentrations at 24 months (28 and 25 %, respectively), as well as by a significant decrease of desmosterol concentration at 18 and 24 months (36 and 51 %, respectively). In contrast, in the liver we detected age-induced increases in lanosterol and lathosterol concentrations, and no change in desmosterol concentration. The amounts of these sterols were lower than in the brain regions. In the cortex and hippocampus, desmosterol was the predominant cholesterol precursor. In the liver, lathosterol was the most abundant precursor. This ratio remained stable during aging. The most striking effect of aging observed in our study was a significant decrease in desmosterol concentration in the hippocampus which could reflect age-related reduced synaptic plasticity, thus representing one of the detrimental effects of advanced age.

Of mice and men: neurogenesis, cognition and Alzheimer's disease

Neural stem cells are maintained in the subgranular layer of the dentate gyrus and in the subventricular zone in the adult mammalian brain throughout life. Neurogenesis is continuous, but its extent is tightly regulated by environmental factors, behavior, hormonal state, age, and brain health. Increasing evidence supports a role for new neurons in cognitive function in rodents. Recent evidence delineates significant similarities and differences between adult neurogenesis in rodents and humans. Being context-dependent, neurogenesis in the human brain might be manifested differently than in the rodent brain. Decline in neurogenesis may play a role in cognitive deterioration, leading to the development of progressive learning and memory disorders, such as Alzheimer’s disease. This review discusses the different observations concerning neurogenesis in the rodent and human brain, and their functional implications for the healthy and diseased brain.

Vascular endothelial growth factor inhibitors and cognitive impairment: evidence and controversies

INTRODUCTION

Chemotherapy-induced cognitive impairment, or 'chemobrain,' has been well established in the literature. However, neurocognitive toxic effect induced by targeted therapies such as anti-angiogenic agents is poorly investigated. Recently, emerging evidence suggests vascular endothelial growth factor (VEGF) to have a possible role in brain cognition giving rise to concerns whether VEGF inhibitors (VEGFIs) may induce neurotoxic effect on cancer patients' cognitive function.

AREAS COVERED

The aim of this review was to evaluate the plausible mechanisms underlying VEGF and cognition, and to highlight the evidence and controversies surrounding the cognitive issues associated with the use of VEGFIs.

EXPERT OPINION

This review paper has brought attention to the potential cognitive issues associated with the use of VEGFIs and has added a new, unexplored dimension to the problem of cancer treatment-related cognitive changes. However, the lack of evidence warrants the need for more well-designed studies to quantify the prevalence and severity of VEGFI-induced cognitive impairment in the cancer population, and to establish the role of VEGF in human cognitive function.

Novel functions of GABA signaling in adult neurogenesis

Neurotransmitter gamma-aminobutiric acid (GABA) through ionotropic GABA_A and metabotropic GABA_B receptors plays key roles in modulating the development, plasticity and function of neuronal networks. GABA is inhibitory in mature neurons but excitatory in immature neurons, neuroblasts and neural stem/progenitor cells (NSCs/NPCs). The switch from excitatory to inhibitory occurs following the development of glutamatergic synaptic input and results from the dynamic changes in the expression of Na⁺/K⁺/2Cl^- co-transporter NKCC1 driving Cl^- influx and neuron-specific K⁺/Cl^- co-transporter KCC2 driving Cl^- efflux. The developmental transition of KCC2 expression is regulated by Disrupted-in-Schizophrenia 1 (DISC1) and brain-derived neurotrophic factor (BDNF) signaling. The excitatory GABA signaling during early neurogenesis is important to the activity/experience-induced regulation of NSC quiescence, NPC proliferation, neuroblast migration and newborn neuronal maturation/functional integration. The inhibitory GABA signaling allows for the sparse and static functional networking essential for learning/memory development and maintenance.

The endocannabinoid system in normal and pathological brain ageing

The role of endocannabinoids as inhibitory retrograde transmitters is now widely known and intensively studied. However, endocannabinoids also influence neuronal activity by exerting neuroprotective effects and regulating glial responses. This review centres around this less-studied area, focusing on the cellular and molecular mechanisms underlying the protective effect of the cannabinoid system in brain ageing. The progression of ageing is largely determined by the balance between detrimental, pro-ageing, largely stochastic processes, and the activity of the homeostatic defence system. Experimental evidence suggests that the cannabinoid system is part of the latter system. Cannabinoids as regulators of mitochondrial activity, as anti-oxidants and as modulators of clearance processes protect neurons on the molecular level. On the cellular level, the cannabinoid system regulates the expression of brain-derived neurotrophic factor and neurogenesis. Neuroinflammatory processes contributing to the progression of normal brain ageing and to the pathogenesis of neurodegenerative diseases are suppressed by cannabinoids, suggesting that they may also influence the ageing process on the system level. In good agreement with the hypothesized beneficial role of cannabinoid system activity against brain ageing, it was shown that animals lacking CB1 receptors show early onset of learning deficits associated with age-related histological and molecular changes. In preclinical models of neurodegenerative disorders, cannabinoids show beneficial effects, but the clinical evidence regarding their efficacy as therapeutic tools is either inconclusive or still missing.

Peripheral immune system and neuroimmune communication impairment in a mouse model of Alzheimer's disease

Neurodegenerative diseases such as Alzheimer's disease (AD) can be understood in the context of the aging of neuroimmune communication. Although the contribution to AD of the immune cells present in the brain is accepted, the role of the peripheral immune system is less well known. The present review examines the behavior and the function and redox state of peripheral immune cells in a triple-transgenic mouse model (3×Tg-AD). These animals develop both beta-amyloid plaques and neurofibrillary tangles with a temporal- and regional-specific profile that closely mimics their development in the human AD brain. We have observed age and sex-related changes in several aspects of behavior and immune cell functions, which demonstrate premature aging. Lifestyle strategies such as physical exercise and environmental enrichment can improve these aspects. We propose that the analysis of the function and redox state of peripheral immune cells can be a useful tool for measuring the progression of AD.