Reduced Hippocampal Dendrite Branching, Spine Density and Neurocognitive Function in Premature Rabbits, and Reversal with Estrogen or TrkB Agonist Treatment

Mechanisms Underlying Memory Impairment Induced by Fructose

Fructose consumption has increased over the years, especially in adolescents living in urban areas. Growing evidence indicates that daily fructose consumption leads to some pathological conditions, including memory impairment. This review summarizes relevant data describing cognitive deficits after fructose intake and analyzes the underlying neurobiological mechanisms. Preclinical experiments show sex-related deficits in spatial memory; that is, while males exhibit significant imbalances in spatial processing, females seem unaffected by dietary supplementation with fructose. Recognition memory has also been evaluated; however, only female rodents show a significant decline in the novel object recognition test performance. According to mechanistic evidence, fructose intake induces neuroinflammation, mitochondrial dysfunction, and oxidative stress in the short term. Subsequently, these mechanisms can trigger other long-term effects, such as inhibition of neurogenesis, downregulation of trophic factors and receptors, weakening of synaptic plasticity, and long-term potentiation decay. Integrating all these neurobiological mechanisms will help us understand the cellular and molecular processes that trigger the memory impairment induced by fructose.

Oestrogen treatment restores dentate gyrus development in premature newborns by IGF1 regulation

Prematurely-born infants cared for in the neonatal units suffer from memory and learning deficits. Prematurity diminishes neurogenesis and synaptogenesis in the hippocampal dentate gyrus (DG). This dysmaturation of neurons is attributed to elevated PSD95, NMDR2A, and IGF1 levels. Since oestrogen treatment plays key roles in the development and plasticity of DG, we hypothesized that 17β-estradiol (E2) treatment would ameliorate neurogenesis and synaptogenesis in the DG, reversing cognitive deficits in premature newborns. Additionally, E2-induced recovery would be mediated by IGF1 signalling. These hypotheses were tested in a rabbit model of prematurity and nonmaternal care, in which premature kits were gavage-fed and reared by laboratory personnel. We compared E2- and vehicle-treated preterm kits for morphological, molecular, and behavioural parameters. We also treated kits with oestrogen degrader, RAD1901, and assessed IGF1 signalling. We found that E2 treatment increased the number of Tbr2+ and DCX+ neuronal progenitors and increased the density of glutamatergic synapses in the DG. E2 treatment restored PSD95 and NMDAR2A levels and cognitive function in preterm kits. Transcriptomic analyses showed that E2 treatment contributed to recovery by influencing interactions between IGF1R and neurodegenerative, as well as glutamatergic genes. ERα expression was reduced on completion of E2 treatment at D7, followed by D30 elevation. E2-induced fluctuation in ERα levels was associated with a reciprocal elevation in IGF1/2 expression at D7 and reduction at D30. ERα degradation by RAD1901 treatment enhanced IGF1 levels, suggesting ERα inhibits IGF1 expression. E2 treatment alleviates the prematurity-induced maldevelopment of DG and cognitive dysfunctions by regulating ERα and IGF1 levels.

Elevated insulin growth factor-1 in dentate gyrus induces cognitive deficits in pre-term newborns

Prematurely born infants are deprived of maternal hormones and cared for in the stressful environment of Neonatal Intensive Care Units (NICUs). They suffer from long-lasting deficits in learning and memory. Here, we show that prematurity and associated neonatal stress disrupt dentate gyrus (DG) development and induce long-term cognitive deficits and that these effects are mediated by insulin growth factor-1 (IGF1). Nonmaternal care of premature rabbits increased the number of granule cells and interneurons and reduced neurogenesis, suggesting accelerated premature maturation of DG. However, the density of glutamatergic synapses, mature dendritic spines, and synaptic transmission were reduced in preterm kits compared with full-term controls, indicating that premature synaptic maturation was abnormal. These findings were consistent with cognitive deficits observed in premature rabbits and appeared to be driven by transcriptomic changes in the granule cells. Preterm kits displayed reduced weight, elevated serum cortisol and growth hormone, and higher IGF1 expression in the liver and DG relative to full-term controls. Importantly, blocking IGF-1 receptor in premature kits restored cognitive deficits, increased the density of glutamatergic puncta, and rescued NR2B and PSD95 levels in the DG. Hence, IGF1 inhibition alleviates prematurity-induced cognitive dysfunction and synaptic changes in the DG through modulation of NR2B and PSD95. The study identifies a novel strategy to potentially rescue DG maldevelopment and cognitive dysfunction in premature infants under stress in NICUs.

Severe intraventricular hemorrhage causes long-lasting structural damage in a preterm rabbit pup model

BACKGROUND

Intraventricular hemorrhage causes significant lifelong mortality and morbidity, especially in preterm born infants. Progress in finding an effective therapy is stymied by a lack of preterm animal models with long-term follow-up. This study addresses this unmet need, using an established model of preterm rabbit IVH and analyzing outcomes out to 1 month of age.

METHODS

Rabbit pups were delivered preterm and administered intraperitoneal injection of glycerol at 3 h of life and approximately 58% developed IVH. Neurobehavioral assessment was performed at 1 month of age followed by immunohistochemical labeling of epitopes for neurons, synapses, myelination, and interneurons, analyzed by means of digital quantitation and assessed via two-way ANOVA or Student's t test.

RESULTS

IVH pups had globally reduced myelin content, an aberrant cortical myelination microstructure, and thinner upper cortical layers (I-III). We also observed a lower number of parvalbumin (PV)-positive interneurons in deeper cortical layers (IV-VI) in IVH animals and reduced numbers of neurons, synapses, and microglia. However, there were no discernable changes in behaviors.

CONCLUSIONS

We have established in this preterm pup model that long-term changes after IVH include significant wide-ranging alterations to cortical organization and microstructure. Further work to improve the sensitivity of neurocognitive testing in this species at this age may be required.

IMPACT

This study uses an established animal model of preterm birth, in which the rabbit pups are truly born preterm, with reduced organ maturation and deprivation of maternally supplied trophic factors. This is the first study in preterm rabbits that explores the impacts of severe intraventricular hemorrhage beyond 14 days, out to 1 month of age. Our finding of persisting but subtle global changes including brain white and gray matter will have impact on our understanding of the best path for therapy design and interventions.

Rac-maninoff and Rho-vel: The symphony of Rho-GTPase signaling at excitatory synapses

Synaptic connections between neurons are essential for every facet of human cognition and are thus regulated with extreme precision. Rho-family GTPases, molecular switches that cycle between an active GTP-bound state and an inactive GDP-bound state, comprise a critical feature of synaptic regulation. Rho-GTPases are exquisitely controlled by an extensive suite of activators (GEFs) and inhibitors (GAPs and GDIs) and interact with many different signalling pathways to fulfill their roles in orchestrating the development, maintenance, and plasticity of excitatory synapses of the central nervous system. Among the mechanisms that control Rho-GTPase activity and signalling are cell surface receptors, GEF/GAP complexes that tightly regulate single Rho-GTPase dynamics, GEF/GAP and GEF/GEF functional complexes that coordinate multiple Rho-family GTPase activities, effector positive feedback loops, and mutual antagonism of opposing Rho-GTPase pathways. These complex regulatory mechanisms are employed by the cells of the nervous system in almost every step of development, and prominently figure into the processes of synaptic plasticity that underlie learning and memory. Finally, misregulation of Rho-GTPases plays critical roles in responses to neuronal injury, such as traumatic brain injury and neuropathic pain, and in neurodevelopmental and neurodegenerative disorders, including intellectual disability, autism spectrum disorder, schizophrenia, and Alzheimer's Disease. Thus, decoding the mechanisms of Rho-GTPase regulation and function at excitatory synapses has great potential for combatting many of the biggest current challenges in mental health.

Connecting the Neurobiology of Developmental Brain Injury: Neuronal Arborisation as a Regulator of Dysfunction and Potential Therapeutic Target

Neurodevelopmental disorders can derive from a complex combination of genetic variation and environmental pressures on key developmental processes. Despite this complex aetiology, and the equally complex array of syndromes and conditions diagnosed under the heading of neurodevelopmental disorder, there are parallels in the neuropathology of these conditions that suggest overlapping mechanisms of cellular injury and dysfunction. Neuronal arborisation is a process of dendrite and axon extension that is essential for the connectivity between neurons that underlies normal brain function. Disrupted arborisation and synapse formation are commonly reported in neurodevelopmental disorders. Here, we summarise the evidence for disrupted neuronal arborisation in these conditions, focusing primarily on the cortex and hippocampus. In addition, we explore the developmentally specific mechanisms by which neuronal arborisation is regulated. Finally, we discuss key regulators of neuronal arborisation that could link to neurodevelopmental disease and the potential for pharmacological modification of arborisation and the formation of synaptic connections that may provide therapeutic benefit in the future.

Therapeutic potential of stem cells for preterm infant brain damage: Can we move from the heterogeneity of preclinical and clinical studies to established therapeutics?

Acquired perinatal brain injuries are a set of conditions that remains a key challenge for neonatologists and that have significant social, emotional and financial implications for our communities. In our perspective article, we will introduce perinatal brain injury focusing specifically on the events leading to brain damage in preterm born infants and outcomes for these infants. Then we will summarize and discuss the preclinical and clinical studies testing the efficacy of stem cells as neuroprotectants in the last ten years in perinatal brain injury. There are no therapies to treat brain damage in preterm born infants and a primary finding from this review is that there is a scarcity of stem cell trials focused on overcoming brain injuries in these infants. Overall, across all forms of perinatal brain injury there is a remarkable heterogeneity in previous and on-going preclinical and clinical studies in terms of the stem cell type, animal models/patient selection, route and time of administration. Despite the quality of many of the studies this variation makes it difficult to reach a valid consensus for future developments. However, it is clear that stem cells (and stem cell derived exosomes) can reduce perinatal brain injury and our field needs to work collectively to refine an effective protocol for each type of injury. The use of standardized stem cell products and testing these products across multiple models of injury will provide a stronger framework for clinical trials development.

Cortical Gray Matter Injury in Encephalopathy of Prematurity: Link to Neurodevelopmental Disorders

Preterm-born infants frequently suffer from an array of neurological damage, collectively termed encephalopathy of prematurity (EoP). They also have an increased risk of presenting with a neurodevelopmental disorder (e.g., autism spectrum disorder; attention deficit hyperactivity disorder) later in life. It is hypothesized that it is the gray matter injury to the cortex, in addition to white matter injury, in EoP that is responsible for the altered behavior and cognition in these individuals. However, although it is established that gray matter injury occurs in infants following preterm birth, the exact nature of these changes is not fully elucidated. Here we will review the current state of knowledge in this field, amalgamating data from both clinical and preclinical studies. This will be placed in the context of normal processes of developmental biology and the known pathophysiology of neurodevelopmental disorders. Novel diagnostic and therapeutic tactics required integration of this information so that in the future we can combine mechanism-based approaches with patient stratification to ensure the most efficacious and cost-effective clinical practice.