Imaging and spectroscopic methods to investigate adult neurogenesis in vivo: New models and new avenues

Current understanding and prospects for targeting neurogenesis in the treatment of cognitive impairment

Adult hippocampal neurogenesis is linked to memory formation in the adult brain, with new neurons in the hippocampus exhibiting greater plasticity during their immature stages compared to mature neurons. Abnormal adult hippocampal neurogenesis is closely associated with cognitive impairment in central nervous system diseases. Targeting and regulating adult hippocampal neurogenesis have been shown to improve cognitive deficits. This review aims to expand the current understanding and prospects of targeting neurogenesis in the treatment of cognitive impairment. Recent research indicates the presence of abnormalities in AHN in several diseases associated with cognitive impairment, including cerebrovascular diseases, Alzheimer's disease, aging-related conditions, and issues related to anesthesia and surgery. The role of these abnormalities in the cognitive deficits caused by these diseases has been widely recognized, and targeting AHN is considered a promising approach for treating cognitive impairment. However, the underlying mechanisms of this role are not yet fully understood, and the effectiveness of targeting abnormal adult hippocampal neurogenesis for treatment remains limited, with a need for further development of treatment methods and detection techniques. By reviewing recent studies, we classify the potential mechanisms of adult hippocampal neurogenesis abnormalities into four categories: immunity, energy metabolism, aging, and pathological states. In immunity-related mechanisms, abnormalities in meningeal, brain, and peripheral immunity can disrupt normal adult hippocampal neurogenesis. Lipid metabolism and mitochondrial function disorders are significant energy metabolism factors that lead to abnormal adult hippocampal neurogenesis. During aging, the inflammatory state of the neurogenic niche and the expression of aging-related microRNAs contribute to reduced adult hippocampal neurogenesis and cognitive impairment in older adult patients. Pathological states of the body and emotional disorders may also result in abnormal adult hippocampal neurogenesis. Among the current strategies used to enhance this form of neurogenesis, physical therapies such as exercise, transcutaneous electrical nerve stimulation, and enriched environments have proven effective. Dietary interventions, including energy intake restriction and nutrient optimization, have shown efficacy in both basic research and clinical trials. However, drug treatments, such as antidepressants and stem cell therapy, are primarily reported in basic research, with limited clinical application. The relationship between abnormal adult hippocampal neurogenesis and cognitive impairment has garnered widespread attention, and targeting the former may be an important strategy for treating the latter. However, the mechanisms underlying abnormal adult hippocampal neurogenesis remain unclear, and treatments are lacking. This highlights the need for greater focus on translating research findings into clinical practice.

Circulating Neuronal Exosome Cargo as Biomarkers of Neuroplasticity in Cushing's Syndrome

The hippocampus is the main target of glucocorticoids (GCs) in the brain since it contains the greatest concentration of the specific receptors. GCs are among the factors modulating adult hippocampal neurogenesis (AHN), which occurs in mammalians, including humans. Prolonged exposure to high GC levels triggers AHN impairment and induces affective and cognitive deficits, consistently with hippocampal neurogenesis functions. Cushing's syndrome (CS) is a rare endocrine disorder characterized by persistently elevated GC levels, namely, cortisol, that also results in affective disorders and impairment of hippocampus-associated memory, suggesting a disruption of hippocampal neurogenesis. Players of adult neurogenesis process, such as Neural Stem/Progenitor Cells and differentiating neuronal cells, release exosomes able to cross brain blood barrier, reaching the peripheral blood. MicroRNAs are known to be selectively enriched in neuronal exosomes and to play a crucial role in adult neurogenesis regulation. The main question addressed in this exploratory study was whether neuroplasticity-related microRNAs (miRNAs), carried by neuronal-derived exosomes in peripheral blood, could reflect alterations in neurogenic processes associated with Cushing's syndrome. Hence, in the present work, we measured the content in selected miRNAs of neuronally derived exosomes in peripheral blood of patients affected by endogenous and active CS and age and sex-matched healthy subjects. The human miRNAs (miR-126, miR-9, miR-223, miR-34a, miR-124a, and miR-146a) were quantified by RT-qPCR. All the miRNAs analyzed were significantly differentially expressed in CS patients as compared to healthy subjects. Our findings support the following: (i) patients with Cushing's syndrome (CS) may exhibit a putative dysregulation of neurogenesis that could underlie the early-onset impairment of affective and cognitive functions; (ii) the exosomal cargo may represent a potential biomarker for monitoring functional and dysfunctional neuroplasticity processes in adult humans. Additional studies are needed to confirm and expand upon the findings across a wider cohort of patients.

Adult Hippocampal Neurogenesis in the Human Brain: Updates, Challenges, and Perspectives

The existence of neurogenesis in the adult human hippocampus has been under considerable debate within the past three decades due to the diverging conclusions originating mostly from immunohistochemistry studies. While some of these reports conclude that hippocampal neurogenesis in humans occurs throughout physiologic aging, others indicate that this phenomenon ends by early childhood. More recently, some groups have adopted next-generation sequencing technologies to characterize with more acuity the extent of this phenomenon in humans. Here, we review the current state of research on adult hippocampal neurogenesis in the human brain with an emphasis on the challenges and limitations of using immunohistochemistry and next-generation sequencing technologies for its study.

Neuroplasticity in Diffuse Low-grade Gliomas: Backward Modelling of Brain-tumor Interactions Prior to Diagnosis is Needed to Better Predict Recovery after Treatment

PURPOSE OF REVIEW

In low-grade glioma (LGG), besides the patient's neurological status and tumor characteristics on neuroimaging, current treatment guidelines mainly rely on the glioma's genetics at diagnosis to define therapeutic strategy, usually starting with surgical resection. However, this snapshot in time does not take into account the antecedent period of tumor progression and its interactions with the brain before presentation. This article reviews new concepts that pertain to reconstruct the history of previous interplay between the LGG's course and adaptive changes in the connectome within which the glioma is embedded over the years preceding the diagnosis.

RECENT FINDINGS

Microscale and macroscale parameters helpful for extrapolating backward in time are considered, both for the glioma (kinetics, migration vs. proliferation profile, metabolism with possible intratumoral heterogeneity, relationships with surrounding cerebral pathways) and for patterns of reconfiguration within and across neural networks in reaction to the LGG leading to considerable interindividual cerebral variability. Modelling these continuous variations at the time of LGG diagnosis is a prerequisite to predict recovery from treatment(s). It is important to go beyond the biology of the LGG at a given moment of its history, and instead construct a more comprehensive picture of the past and present dynamics of glioma-brain interactions, and their ongoing evolution, as a necessary stage to optimize a personalized management plan by thinking several steps ahead.

Emerging Pro-neurogenic Therapeutic Strategies for Neurodegenerative Diseases: A Review of Pre-clinical and Clinical Research

The neuroscience community has largely accepted the notion that functional neurons can be generated from neural stem cells in the adult brain, especially in two brain regions: the subventricular zone of the lateral ventricles and the subgranular zone in the dentate gyrus of the hippocampus. However, impaired neurogenesis has been observed in some neurodegenerative diseases, particularly in Alzheimer's, Parkinson's, and Huntington's diseases, and also in Lewy Body dementia. Therefore, restoration of neurogenic function in neurodegenerative diseases emerges as a potential therapeutic strategy to counteract, or at least delay, disease progression. Considering this, the present study summarizes the different neuronal niches, provides a collection of the therapeutic potential of different pro-neurogenic strategies in pre-clinical and clinical research, providing details about their possible modes of action, to guide future research and clinical practice.

Neural stem cells persist to generate new neurons in the hippocampus of adult and aged human brain - Fiction or accurate?

Adult neurogenesis, comprising the generation, differentiation and integration of new neurons in the mature brain, has emerged as a dynamic area of research over decades. The discovery of adult neurogenesis was a paradigm shift to comprehend mechanisms underlying brain plasticity, cognitive function, and neurological disorders. This review deliberates significant findings from articles published over four decades on adult neurogenesis, highlighting key milestones, methodological advances, and controversies that have shaped our comprehension of the phenomenon of adult neurogenesis. Early skepticism gave way to a rich body of evidence via various reliable approaches. Studies on neurogenic niches, microenvironmental factors, molecular regulators, and functional implications have uncovered the involvement of adult neurogenesis in learning, memory, mood, and even neurological and neurodegenerative conditions. Despite significant progress, several questions still need to be answered, including the exact contributions of new neurons to brain function, their integration into existing circuits, and the impact of enhancing adult neurogenesis in the human hippocampus. While the existence of robust neurogenesis in the adult and aged human hippocampus is yet to be confirmed, this review highlights evidence from a significant number of studies supporting the persistence of hippocampal neurogenesis during adulthood and aging in humans, including in some neurological conditions, such as epilepsy and Alzheimer's disease. Nonetheless, additional large-scale studies using single cell-RNA-seq, single nucleus-RNA-seq, and spatial transcriptomics are critical to validate the presence and contribution of hippocampal neurogenesis in the pathophysiology of various neurological and neurodegenerative conditions at different stages of the disease. There is also a need to develop standardized protocols for analyzing postmortem hippocampal tissues for cellular and molecular analyses.

Adult hippocampal neurogenesis in Alzheimer's disease: A roadmap to clinical relevance

Adult hippocampal neurogenesis (AHN) drops sharply during early stages of Alzheimer's disease (AD), via unknown mechanisms, and correlates with cognitive status in AD patients. Understanding AHN regulation in AD could provide a framework for innovative pharmacological interventions. We here combine molecular, behavioral, and clinical data and critically discuss the multicellular complexity of the AHN niche in relation to AD pathophysiology. We further present a roadmap toward a better understanding of the role of AHN in AD by probing the promises and caveats of the latest technological advancements in the field and addressing the conceptual and methodological challenges ahead.

Sirtuins and redox signaling interplay in neurogenesis, neurodegenerative diseases, and neural cell reprogramming

Since the discovery of Neural Stem Cells (NSCs) there are still mechanism to be clarified, such as the role of mitochondrial metabolism in the regulation of endogenous adult neurogenesis and its implication in neurodegeneration. Although stem cells require glycolysis to maintain their stemness, they can perform oxidative phosphorylation and it is becoming more and more evident that mitochondria are central players, not only for ATP production but also for neuronal differentiation's steps regulation, through their ability to handle cellular redox state, intracellular signaling, epigenetic state of the cell, as well as the gut microbiota-brain axis, upon dietary influences. In this scenario, the 8-oxoguanine DNA glycosylase (OGG1) repair system would link mitochondrial DNA integrity to the modulation of neural differentiation. On the other side, there is an increasing interest in NSCs generation, from induced pluripotent stem cells, as a clinical model for neurodegenerative diseases (NDs), although this methodology still presents several drawbacks, mainly related to the reprogramming process. Indeed, high levels of reactive oxygen species (ROS), associated with telomere shortening, genomic instability, and defective mitochondrial dynamics, lead to pluripotency limitation and reprogramming efficiency's reduction. Moreover, while a physiological or moderate ROS increase serves as a signaling mechanism, to activate differentiation and suppress self-renewal, excessive oxidative stress is a common feature of NDs and aging. This ROS-dependent regulatory effect might be modulated by newly identified ROS suppressors, including the NAD⁺-dependent deacetylase enzymes family called Sirtuins (SIRTs). Recently, the importance of subcellular localization of NAD synthesis has been coupled to different roles for NAD in chromatin stability, DNA repair, circadian rhythms, and longevity. SIRTs have been described as involved in the control of both telomere's chromatin state and expression of nuclear gene involved in the regulation of mitochondrial gene expression, as well as in several NDs and aging. SIRTs are ubiquitously expressed in the mammalian brain, where they play important roles. In this review we summarize the current knowledge on how SIRTs-dependent modulation of mitochondrial metabolism could impact on neurogenesis and neurodegeneration, focusing mainly on ROS function and their role in SIRTs-mediated cell reprogramming and telomere protection.

The critical periods of cerebral plasticity: A key aspect in a dialog between psychoanalysis and neuroscience centered on the psychopathology of schizophrenia

Through research into the molecular and cellular mechanisms that occur during critical periods, recent experimental neurobiological data have brought to light the importance of early childhood. These have demonstrated that childhood and early environmental stimuli play a part not only in our subjective construction, but also in brain development; thus, confirming Freud's intuition regarding the central role of childhood and early experiences of the environment in our psychological development and our subjective outcomes. "Critical periods" of cerebral development represent temporal windows that mark favorable, but also circumscribed, moments in developmental cerebral plasticity. They also vary between different cortical areas. There are, therefore, strictly defined temporal periods for learning language, music, etc., after which this learning becomes more difficult, or even impossible, to acquire. Now, research into these critical periods can be seen as having a significant part to play in the interdisciplinary dialog between psychoanalysis and neurosciences with regard to the role of early experiences in the etiology of some psychopathological conditions. Research into the cellular and molecular mechanisms controlling the onset and end of these critical periods, notably controlled by the maturation of parvalbumin-expressing basket cells, have brought to light the presence of anomalies in the maturation of these neurons in patients with schizophrenia. Starting from these findings we propose revisiting the psychoanalytic theories on the etiology of psychosis from an interdisciplinary perspective. Our study works from the observation, common to both psychoanalysis and neurosciences, that experience leaves a trace; be it a "psychic" or a "synaptic" trace. Thus, we develop a hypothesis for an "absence of trace" in psychosis; reexamining psychosis through the prism of the biological theory of critical periods in plasticity.

Depressive Pseudodementia with Reversible AD-like Brain Hypometabolism: A Case Report and a Review of the Literature

Recent European guidelines recommend using brain FDG-PET to differentiate between Alzheimer's disease (AD) and depressive pseudodementia (DP), with specific hypometabolism patterns across the former group, and typically normal or frontal hypometabolism in the latter. We report the case of a 74 years-old man with DP (MMSE 16/30), whose FDG-PET visual rating and semiquantitative analysis closely mimicked the typical AD pattern, showing severe hypometabolism in bilateral precuneus, parietal and temporal lobes, and sparing frontal areas, suggesting the diagnosis of moderate AD. Shortly after starting antidepressant polytherapy, he underwent formal NPS testing, which revealed moderate impairment of episodic memory and mild impairment on executive and visuospatial tests, judged consistent with neurodegenerative dementia and concomitant depression. Over the following two years, he improved dramatically: repeated NPS assessment did not show significant deficits, and FDG-PET showed restoration of cerebral metabolism. The confirmation of PET findings via semiquantitative analysis, and their reversion to normality with antidepressant treatment, proved the non-neurodegenerative origin of the initial AD-like FDG-PET abnormalities. We review similar cases and provide a comprehensive analysis of their implications, concluding that reversible FDG-PET widespread hypometabolism might represent a biomarker of pseudodementia. Therefore, we suggest caution when interpreting FDG-PET scans of depressed patients with cognitive impairment.