Periodic Reporting for period 4 - MacroStability (Stability and dynamics at different spatial scales: From physiology to Alzheimer's degeneration)
Reporting period: 2022-04-01 to 2023-09-30
A fundamental feature of neural circuits is the capacity for plasticity in response to experience. During the past 60 years, mounting evidence indicates that experience-dependent changes in synaptic transmission and neuronal wiring, phenomena collectively termed synaptic plasticity, underlie the cellular basis of neural computation, learning and memory. Hebbian-like plasticity, reflected by persistent changes in synaptic and intrinsic properties, is crucial for refinement of neural circuits and information storage, however, alone it is unlikely to account for the stable functioning of neural networks. Both, stability and plasticity are hallmarks of brain function that enable adaptations to unpredictable and dynamic environment, experience and learning. Coping with constantly changing environments, neural circuits need to spend a considerable amount of their available energy to maintain homeostasis and to minimize the effects of stochastic events. Destabilization of hippocampal and cortical circuits has been widely documented in neurodegenerative disorders, e.g. Alzheimer's disease (AD), the most frequent form of late-life dementia. However, the key mechanisms that underlie stability of activity patterns in central mammalian neural circuits are largely unknown. Furthermore, how disruption of these mechanisms affects the onset and the kinetics of AD-related neuronal dysfunctions remains an enigma.
In this project, we identified the key principles and molecular pathways regulating homeostasis of firing properties at the level of neuronal populations. We identified the key regulators of firing rate homeostasis: mitochondrial DHODH and IGF1R molecules. We found that these molecules regulate firing rate homeostasis via synaptic and intrinsic excitability mechanisms. Notably, DHODH inhibition suppressed aberrant pathological hippocampal activity in mouse models of intractable epilepsy and familial AD (fAD). Our results point to mitochondria as a key regulator of activity set-points, proposing a new strategy to treat brain disorders associated with hyperactivity of hippocampal circuits.
Furthermore, we studied firing rate homeostasis in the presymptomatic stage in AD mouse models. We demonstrated that before the onset of memory decline and sleep disturbances, AD model mice display no deficits in CA1 firing rate homeostasis during active wakefulness. However, homeostatic down-regulation of CA1 firing rates is disrupted during non-rapid-eye-movement (NREM) sleep and general anesthesia. fAD mutations impair downward firing rate homeostasis, resulting in pathological activity set points in response to anesthetic drug and inhibition blockade. Thus, firing rate dyshomeostasis of hippocampal circuits is masked during active wakefulness, but surfaces during low-arousal brain states, representing an early failure of the silent disease stage.
Finally, we searched for the functional significance of firing rate dyshomeostasis in the presymptomatic AD stage. We identified that epileptiform spikes emerge during low-arousal brain states in the CA1 and mPFC regions, leading to working memory disruptions. These epileptiform spikes are driven by inputs from the thalamic nucleus reuniens. Indeed, tonic deep brain stimulation (DBS) of the nucleus reuniens effectively suppresses epileptiform spikes and restores firing rate homeostasis under anesthesia, preventing further impairments in synaptic facilitation and working memory. Notably, applying DBS during the prodromal phase in young fAD mice mitigates age-dependent memory decline. The epileptiform spike rate during anesthesia in young model mice correlates with later working memory impairments. These findings highlight the midline thalamus as a central hub of functional resilience and underscore the clinical promise of DBS in conferring resilience to AD pathology by restoring circuit-level homeostasis.
These studies identified the fundamental mechanisms of stability in central neural circuits and provide a conceptual new way to boost cognitive resilience to AD by restoring activity homeostasis.
In this project, we identified the key principles and molecular pathways regulating homeostasis of firing properties at the level of neuronal populations. We identified the key regulators of firing rate homeostasis: mitochondrial DHODH and IGF1R molecules. We found that these molecules regulate firing rate homeostasis via synaptic and intrinsic excitability mechanisms. Notably, DHODH inhibition suppressed aberrant pathological hippocampal activity in mouse models of intractable epilepsy and familial AD (fAD). Our results point to mitochondria as a key regulator of activity set-points, proposing a new strategy to treat brain disorders associated with hyperactivity of hippocampal circuits.
Furthermore, we studied firing rate homeostasis in the presymptomatic stage in AD mouse models. We demonstrated that before the onset of memory decline and sleep disturbances, AD model mice display no deficits in CA1 firing rate homeostasis during active wakefulness. However, homeostatic down-regulation of CA1 firing rates is disrupted during non-rapid-eye-movement (NREM) sleep and general anesthesia. fAD mutations impair downward firing rate homeostasis, resulting in pathological activity set points in response to anesthetic drug and inhibition blockade. Thus, firing rate dyshomeostasis of hippocampal circuits is masked during active wakefulness, but surfaces during low-arousal brain states, representing an early failure of the silent disease stage.
Finally, we searched for the functional significance of firing rate dyshomeostasis in the presymptomatic AD stage. We identified that epileptiform spikes emerge during low-arousal brain states in the CA1 and mPFC regions, leading to working memory disruptions. These epileptiform spikes are driven by inputs from the thalamic nucleus reuniens. Indeed, tonic deep brain stimulation (DBS) of the nucleus reuniens effectively suppresses epileptiform spikes and restores firing rate homeostasis under anesthesia, preventing further impairments in synaptic facilitation and working memory. Notably, applying DBS during the prodromal phase in young fAD mice mitigates age-dependent memory decline. The epileptiform spike rate during anesthesia in young model mice correlates with later working memory impairments. These findings highlight the midline thalamus as a central hub of functional resilience and underscore the clinical promise of DBS in conferring resilience to AD pathology by restoring circuit-level homeostasis.
These studies identified the fundamental mechanisms of stability in central neural circuits and provide a conceptual new way to boost cognitive resilience to AD by restoring activity homeostasis.
Aim 1. Define the molecular architecture of a homeostatic control system underlying firing macro-stability at the level of neuronal populations.
1.1 Using genome-scale metabolic modelling, we identified the mitochondrial enzyme dihydroorotate dehydrogenase (DHODH) as one of the top predicted targets that transforms towards epilepsy-resistant metabolic state (Styr et al., Neuron 2019).
1.2 DHODH regulates MFR set point in hippocampal networks.
1.4 DHODH inhibition suppresses epileptic activity in model mice.
1.5 We identified IGF1Rs in the mitochondria and and found that they regulate MFR homeostatic response to inactivity (Katsenelson et al., PNAS 2022)
Aim 2. Exploring the mechanisms leading to destabilization of neural circuits' activity associated with Alzheimer's disease.
2.1 fAD mutations impair MFR homeostatic response of hippocampal network to hyperactivity, but not to inactivity (Zarhin et al., Cell Reports, 2022).
2.2 DHODH inhibition reduces the susceptibility to seizures in Dravet and AD models.
2.4. We demonstrated that deep brain stimulation of nucleus reuniens (DBS-nRE) boosts resilience to AD (Shoob et al., Nature Comm 2023).
2.5. We demonstrated that epileptiform spikes play a causal role in memory decline in AD.
Aim 3. Determining the basic principles stabilizing activity of hippocampal circuits in vivo.
3.1 Establishing an integrated system for imaging of neuronal activity in the CA1 hippocampus in behaving mice
Utilizing highly-sensitive GCaMP6f sensor for somatic Ca2+ imaging reflecting spiking activity, we established system for measuring neuronal Ca dynamics in the hippocampus of behaving mice based on miniaturized integrated head-mounted microscope in freely behaving mice.
3.2 We explored whether the hippocampus is capable of homeostatic regulation of MFR in response to a chronic perturbation. We applied long-term single-unit recordings in freely behaving mice. Our results show that activation of Gq-signaling in interneurons led to an acute suppression of MFR across a population of CA1 pyramidal cells that gradually recovers after 3 days returning to the original MFR set-point value. Interestingly, MFR returned to its brain state-specific set-point value. Additionally, mechanisms that restore MFR at the population level also restore contextual memory retrieval in fear conditioned mice. This study provides the first direct evidence on homeostatic regulation of MFR set points by brain states and points to the role of homeostatic mechanism in preserving hippocampus core function to retrieve stored memories (Atsmon et al., manuscript under preparation).
1.1 Using genome-scale metabolic modelling, we identified the mitochondrial enzyme dihydroorotate dehydrogenase (DHODH) as one of the top predicted targets that transforms towards epilepsy-resistant metabolic state (Styr et al., Neuron 2019).
1.2 DHODH regulates MFR set point in hippocampal networks.
1.4 DHODH inhibition suppresses epileptic activity in model mice.
1.5 We identified IGF1Rs in the mitochondria and and found that they regulate MFR homeostatic response to inactivity (Katsenelson et al., PNAS 2022)
Aim 2. Exploring the mechanisms leading to destabilization of neural circuits' activity associated with Alzheimer's disease.
2.1 fAD mutations impair MFR homeostatic response of hippocampal network to hyperactivity, but not to inactivity (Zarhin et al., Cell Reports, 2022).
2.2 DHODH inhibition reduces the susceptibility to seizures in Dravet and AD models.
2.4. We demonstrated that deep brain stimulation of nucleus reuniens (DBS-nRE) boosts resilience to AD (Shoob et al., Nature Comm 2023).
2.5. We demonstrated that epileptiform spikes play a causal role in memory decline in AD.
Aim 3. Determining the basic principles stabilizing activity of hippocampal circuits in vivo.
3.1 Establishing an integrated system for imaging of neuronal activity in the CA1 hippocampus in behaving mice
Utilizing highly-sensitive GCaMP6f sensor for somatic Ca2+ imaging reflecting spiking activity, we established system for measuring neuronal Ca dynamics in the hippocampus of behaving mice based on miniaturized integrated head-mounted microscope in freely behaving mice.
3.2 We explored whether the hippocampus is capable of homeostatic regulation of MFR in response to a chronic perturbation. We applied long-term single-unit recordings in freely behaving mice. Our results show that activation of Gq-signaling in interneurons led to an acute suppression of MFR across a population of CA1 pyramidal cells that gradually recovers after 3 days returning to the original MFR set-point value. Interestingly, MFR returned to its brain state-specific set-point value. Additionally, mechanisms that restore MFR at the population level also restore contextual memory retrieval in fear conditioned mice. This study provides the first direct evidence on homeostatic regulation of MFR set points by brain states and points to the role of homeostatic mechanism in preserving hippocampus core function to retrieve stored memories (Atsmon et al., manuscript under preparation).
Our project is at the crossroad of several disciplines, including optical imaging, electrophysiology, big data analysis and molecular biology. The new techniques and tools developed here will be (and already) useful for other biological disciplines, such as cell biology, metabolism and neurodegenerative disorders. We established large-scale Ca imaging with single-cell resolution, integrated with electrophysiology, in the hippocampus of behaving mice .