Physiology seminars

Mariana Wolfner presents research on the molecular changes in eggs, sperm, and females that coordinate successful reproduction, drawing on genetic and physiological studies of fertilization. This NIH Wednesday Afternoon Lecture Series event is offered in person and through NIH VideoCast.

Neuroplasticity is essential for the establishment and strengthening of neural circuits. Repetitive transcranial magnetic stimulation (rTMS) is commonly used to modulate cortical excitability and shows promise in the treatment of some neurological disorders. Low intensity magnetic stimulation (LI-rTMS), which does not directly elicit action potentials in the stimulated neurons, have also shown some therapeutic effects, and it is important to determine the biological mechanisms underlying the effects of these low intensity magnetic fields, such as would occur in the regions surrounding the central high-intensity focus of rTMS. Our team has used a focal low-intensity (10mT) magnetic stimulation approach to address some of these questions and to identify cellular mechanisms. I will present several studies from our laboratory, addressing (1) effects of LIrTMS on neuronal activity and excitability ; and (2) neuronal morphology and post-lesion repair. The ensemble of our results indicate that the effects of LI-rTMS depend upon the stimulation pattern, the age of the animal, and the presence of cellular magnetoreceptors.

My laboratory has made an intriguing discovery that mitochondrial stress in one tissue can be communicated to distal tissues. We find that mitochondrial stress in the nervous system triggers the production of entities known as mitokines. These mitokines are discharged from the nervous system, orchestrating a response in peripheral tissues that extends the lifespan of C. elegans. The revelation came as a surprise, given the prevalent belief that cell autonomous mechanisms would underlie the relationship between mitochondrial function and aging. It was also surprising given the prevailing dogma that mitochondrial function must be increased, not decreased, to improve health and longevity. Our work also underscores the fact that mitochondria, which originated as a microbial entity and later evolved into an intracellular symbiont, have retained their capacity for intercommunication, now facilitated by signals from the nervous system. We hypothesize that this communication has evolved as a mechanism to reduce infection from pathogens.

The field of human biology faces three major technological challenges. Firstly, the causation problem is difficult to address in humans compared to model animals. Secondly, the complexity problem arises due to the lack of a comprehensive cell atlas for the human body, despite its cellular composition. Lastly, the heterogeneity problem arises from significant variations in both genetic and environmental factors among individuals. To tackle these challenges, we have developed innovative approaches. These include 1) mammalian next-generation genetics, such as Triple CRISPR for knockout (KO) mice and ES mice for knock-in (KI) mice, which enables causation studies without traditional breeding methods; 2) whole-body/brain cell profiling techniques, such as CUBIC, to unravel the complexity of cellular composition; and 3) accurate and user-friendly technologies for measuring sleep and awake states, exemplified by ACCEL, to facilitate the monitoring of fundamental brain states in real-world settings and thus address heterogeneity in human.

Humans and other animals evolved in habitats fraught with a range of environmental challenges to their bodies and brains. Accordingly, cells and organ systems possess adaptive stress-responsive signaling pathways that enable them to not only withstand environmental challenges, but also to prepare for future challenges and function more efficiently. These phylogenetically conserved processes are the foundation of the hormesis principle in which repeated exposures to low to moderate amounts of an environmental challenge improve cellular and organismal fitness. Here I describe cellular and molecular mechanisms by which cells in the brain and body respond to intermittent fasting and exercise in ways that enhance performance and counteract aging and disease processes. Switching back and forth between adaptive stress response (during fasting and exercise) and growth and plasticity (eating, resting, sleeping) modes enhances the performance and resilience of various organ systems. While pharmacological interventions that engage a particular hormetic mechanism are being developed, it seems unlikely that any will prove superior to fasting and exercise.

In the central nervous system, neuronal activity is a critical regulator of development and plasticity. Activity-dependent proliferation of healthy glial progenitors, oligodendrocyte precursor cells (OPCs), and the consequent generation of new oligodendrocytes contributes to adaptive myelination. This plasticity of myelin tunes neural circuit function and contributes to healthy cognition. The robust mitogenic effect of neuronal activity on normal oligodendroglial precursor cells, a putative cellular origin for many forms of glioma, suggests that dysregulated or “hijacked” mechanisms of myelin plasticity might similarly promote malignant cell proliferation in this devastating group of brain cancers. Indeed, neuronal activity promotes progression of both high-grade and low-grade glioma subtypes in preclinical models. Crucial mechanisms mediating activity-regulated glioma growth include paracrine secretion of BDNF and the synaptic protein neuroligin-3 (NLGN3). NLGN3 induces multiple oncogenic signaling pathways in the cancer cell, and also promotes glutamatergic synapse formation between neurons and glioma cells. Glioma cells integrate into neural circuits synaptically through neuron-to-glioma synapses, and electrically through potassium-evoked currents that are amplified through gap-junctional coupling between tumor cells This synaptic and electrical integration of glioma into neural circuits is central to tumor progression in preclinical models. Thus, neuron-glial interactions not only modulate neural circuit structure and function in the healthy brain, but paracrine and synaptic neuron-glioma interactions also play important roles in the pathogenesis of glial cancers. The mechanistic parallels between normal and malignant neuron-glial interactions underscores the extent to which mechanisms of neurodevelopment and plasticity are subverted by malignant gliomas, and the importance of understanding the neuroscience of cancer.

The transit of food through the gastrointestinal tract is critical for nutrient absorption and survival, and the gastrointestinal tract has the ability to initiate motility reflexes triggered by luminal distention. This complex function depends on the crosstalk between extrinsic and intrinsic neuronal innervation within the intestine, as well as local specialized enteroendocrine cells. However, the molecular mechanisms and the subset of sensory neurons underlying the initiation and regulation of intestinal motility remain largely unknown. Here, we show that humans lacking PIEZO2 exhibit impaired bowel sensation and motility. Piezo2 in mouse dorsal root but not nodose ganglia is required to sense gut content, and this activity slows down food transit rates in the stomach, small intestine, and colon. Indeed, Piezo2 is directly required to detect colon distension in vivo. Our study unveils the mechanosensory mechanisms that regulate the transit of luminal contents throughout the gut, which is a critical process to ensure proper digestion, nutrient absorption, and waste removal. These findings set the foundation of future work to identify the highly regulated interactions between sensory neurons, enteric neurons and non- neuronal cells that control gastrointestinal motility.

Gut-derived signals regulate metabolism, appetite, and behaviors important for mental health. We have performed a large-scale multidimensional screen to identify gut hormones and nutrient-sensing mechanisms in the intestine that regulate metabolism and behavior in the fruit fly Drosophila. We identified several gut hormones that affect fecundity, stress responses, metabolism, feeding, and sleep behaviors, many of which seem to act sex-specifically. We show that in response to nutrient intake, the enteroendocrine cells (EECs) of the adult Drosophila midgut release hormones that act via inter-organ relays to coordinate metabolism and feeding decisions. These findings suggest that crosstalk between the gut and other tissues regulates food choice according to metabolic needs, providing insight into how that intestine processes nutritional inputs and into the gut-derived signals that relay information regulating nutrient-specific hungers to maintain metabolic homeostasis.

Night shift work is associated with adverse health effects and leads to misalignment between timing cues from the environment and the endogenous circadian clock. In this presentation, I will discuss the effect of circadian misalignment induced by night shift work on peripheral 24-h rhythms on the transcriptome and metabolome in humans, presenting findings from both controlled laboratory studies and field studies. Furthermore, I will highlight the importance of taking into account interindividual differences in the response to circadian misalignment.

Laboratory studies have shown that meaningful changes in light exposure lead to phase shifts in melatonin rhythm. In natural settings, however, light is a very complex signal. How melatonin responds to weekly- and seasonal-dependent variations in light exposure is still poorly understood. In this talk I will present results from a series of observational and intervention studies on the relationship between melatonin and light exposure in the field.

Information processing is energetically expensive. In the mammalian brain, it is unclear how information coding and energy usage are regulated during food scarcity. I addressed this in the visual cortex of awake mice using whole-cell recordings and two-photon imaging to monitor layer 2/3 neuronal activity and ATP usage. I found that food restriction reduced synaptic ATP usage by 29% through a decrease in AMPA receptor conductance. Neuronal excitability was nonetheless preserved by a compensatory increase in input resistance and a depolarized resting membrane potential. Consequently, neurons spiked at similar rates as controls, but spent less ATP on underlying excitatory currents. This energy-saving strategy had a cost since it amplified the variability of visually-evoked subthreshold responses, leading to a 32% broadening in orientation tuning and impaired fine visual discrimination. This reduction in coding precision was associated with reduced levels of the fat mass-regulated hormone leptin and was restored by exogenous leptin supplementation. These findings reveal novel mechanisms that dynamically regulate energy usage and coding precision in neocortex.

Leonard Maler will focus on a specialized caudal portion of the cerebellum of teleost fish whose structure and physiology has been especially well studies to the point that we now have detailed computational analyses of its function. Idoia Quintana-Urzainqui will talk about what sharks can tell us about the evolution of the telencephalon, mainly focusing on the evolutionary expansion of the pallium and how shark embryos can hold key information to interpret the origin of the developmental processes that triggered this phenomenon.

Menstruation is a normal physiological process in women occurring as a result of changes in two ovarian produced hormones – estrogen and progesterone. As a result of these fluctuations, women experience different symptoms in their bodies – their immune system changes (Sekigawa et al, 2004), there are changes in their cardiovascular and digestive system (Millikan, 2006), as well as skin (Hall and Phillips, 2005). But these hormone fluctuations produce major changes in their behavioral pattern as well causing: anxiety, sadness, heightened irritability and anger (Severino and Moline, 1995) which is usually classified as premenstrual syndrome (PMS). In some cases these symptoms severely impair women’s lives and professional help is required. The official diagnosis according to DSM-5 (2013) is premenstrual dysphoric disorder (PMDD). Despite its ubiquitous presence the origins of PMS and PMDD are poorly understood. Some efforts to understand the underlying brain state during the menstruation cycle were performed by using TMS (Smith et al, 1999; 2002; 2003; Inghilleri et al, 2004; Hausmann et al, 2006). But all of these experiments suffer from major shortcomings - no control groups and small number of subjects. Our plan is to address all of these shortcomings and make this the biggest (to our knowledge) experiment of its kind which will, hopefully, provide us with some much needed answers.

The Hsieh lab focuses on the mechanisms that promote neural stem cell self-renewal and differentiation in embryonic and adult brain. Using mouse models, video-EEG monitoring, viral techniques, and imaging/electrophysiological approaches, we elucidated many of the key transcriptional/epigenetic regulators of adult neurogenesis and showed aberrant new neuron integration in adult rodent hippocampus contribute to circuit disruption and seizure development. Building on this work, I will present our recent studies describing how GABA-mediated Ca2+ activity regulates the production of aberrant adult-born granule cells. In a new direction of my laboratory, we are using human induced pluripotent stem cells and brain organoid models as approaches to understand brain development and disease. Mutations in one gene, Aristaless-related homeobox (ARX), are of considerable interest since they are known to cause a common spectrum of neurodevelopmental disorders including epilepsy, autism, and intellectual disability. We have generated cortical and subpallial organoids from patients with poly-alanine expansion mutations in ARX. To understand the nature of ARX mutations in the organoid system, we are currently performing cellular, molecular, and physiological analyses. I will present these data to gain a comprehensive picture of the effect of ARX mutations in brain development. Since we do not understand how human brain development is affected by ARX mutations that contribute to epilepsy, we believe these studies will allow us to understand the mechanism of pathogenesis of ARX mutations, which has the potential to impact the diagnosis and care of patients.

Establishment of precise neuronal connectivity in the neocortex relies on activity-dependent circuit reorganization during postnatal development. In the mouse somatosensory cortex layer 4, barrels are arranged in one-to-one correspondence to whiskers on the face. Thalamocortical axon termini are clustered in the center of each barrel. The layer 4 spiny stellate neurons are located around the barrel edge, extend their dendrites primarily toward the barrel center, and make synapses with thalamocortical axons corresponding to a single whisker. These organized circuits are established during the first postnatal week through activity-dependent refinement processes. However, activity pattern regulating the circuit formation is still elusive. Using two-photon calcium imaging in living neonatal mice, we found that layer 4 neurons within the same barrel fire synchronously in the absence of peripheral stimulation, creating a ''patchwork'' pattern of spontaneous activity corresponding to the barrel map. We also found that disruption of GluN1, an obligatory subunit of the N-methyl-D-aspartate (NMDA) receptor, in a sparse population of layer 4 neurons reduced activity correlation between GluN1 knockout neuron pairs within a barrel. Our results provide evidence for the involvement of layer 4 neuron NMDA receptors in spatial organization of the spontaneous firing activity of layer 4 neurons in the neonatal barrel cortex. In the talk I will introduce our strategy to analyze the role of NMDA receptor-dependent correlated activity in the layer 4 circuit formation.

All mammals transition from breastfeeding to independent feeding during the lactation period. In humans and other mammals, this critical transition is important for later in life metabolic control and, consequently, for the development of many chronic conditions. Here, Dr. Dietrich will discuss the work of his lab studying the function of hypothalamic neurons involved in homeostatic control during the transition from breastfeeding to independent feeding. His work illuminates novel properties of hypothalamic neurons in early life, suggesting mechanisms by which early life events shape homeostatic regulation throughout the individual’s lifespan.

Dendrites play a central role in neuronal computation, and many complex mechanisms shape their structure, function, and connectivity. Dendrites can undergo plastic changes during development and learning, as well as during neurodevelopmental and neurodegenerative disease. We will discuss how the molecular and electrophysiological properties of dendrites enable them to perform complex computations important for sensory-motor processing and higher cognitive function, and how these can go awry.