Fragmentation of mitochondrial network has been implicated in many neurodegenerative, renal, and metabolic diseases. However, a quantitative measure of the microscopic parameters resulting in the impaired balance between fission and fusion of mitochondria and consequently the fragmented networks in a wide range of pathological conditions does not exist. Here we present a comprehensive analysis of mitochondrial networks in cells with Alzheimer’s disease (AD), Huntington’s disease (HD), amyotrophic lateral sclerosis (ALS), Parkinson’s disease (PD), optic neuropathy (OPA), diabetes/cancer, acute kidney injury, Ca2+ overload, and Down Syndrome (DS) pathologies that indicates significant network fragmentation in all these conditions. Furthermore, we found key differences in the way the microscopic rates of fission and fusion are affected in different conditions. The observed fragmentation in cells with AD, HD, DS, kidney injury, Ca2+ overload, and diabetes/cancer pathologies results from the imbalance between the fission and fusion through lateral interactions, whereas that in OPA, PD, and ALS results from impaired balance between fission and fusion arising from longitudinal interactions of mitochondria. Such microscopic difference leads to major disparities in the fine structure and topology of the network that could have significant implications for the way fragmentation affects various cell functions in different diseases.

Alzheimer’s disease (AD) is characterized by the abnormal proteolytic processing of amyloid precursor protein, resulting in increased production of a self-aggregating form of beta amyloid (Aβ). Several lines of work on AD patients and transgenic mice with high Aβ levels exhibit altered rhythmicity, aberrant neuronal network activity and hyperexcitability reflected in clusters of hyperactive neurons, and spontaneous epileptic activity. Recent studies highlight that abnormal accumulation of Aβ changes intrinsic properties of inhibitory neurons, which is one of the main reasons underlying the impaired network activity. However, specific cellular mechanisms leading to interneuronal dysfunction are not completely understood. Using extended Hodgkin-Huxley (HH) formalism in conjunction with patch-clamp experiments, we investigate the mechanisms leading to the impaired activity of interneurons. Our detailed analysis indicates that increased Na+ leak explains several observations in inhibitory neurons, including their failure to reliably produce action potentials, smaller action potential amplitude, increased resting membrane potential, and higher membrane depolarization in response to a range of stimuli in a model of APPSWE/PSEN1DeltaE9 (APdE9) AD mice as compared to age-matched control mice. While increasing the conductance of hyperpolarization activated cyclic nucleotide-gated (HCN) ion channel could account for most of the observations, the extent of increase required to reproduce these observations render such changes unrealistic. Furthermore, increasing the conductance of HCN does not account for the observed changes in depolarizability of interneurons from APdE9 mice as compared to those from NTG mice. None of the other pathways tested could lead to all observations about interneuronal dysfunction. Thus we conclude that upregulated sodium leak is the most likely source of impaired interneuronal function.

Dravet syndrome (DS) is an epileptic encephalopathy that still lacks biomarkers for epileptogenesis and its treatment. Dysfunction of Na V 1.1 sodium channels, which are chiefly expressed in inhibitory interneurons, explains the epileptic phenotype. Understanding the network effects of these cellular deficits may help predict epileptogenesis. Here, we studied h-c coupling as a potential marker for altered inhibitory functioning and epileptogenesis in a DS mouse model. We found that cortical h-c coupling was reduced in both male and female juvenile DS mice and persisted only if spontaneous seizures occurred. h-c Coupling was partly restored by cannabidiol (CBD). Locally disrupting Na V 1.1 expression in the hippocampus or cortex yielded early attenuation of h-c coupling, which in the hippocampus associated with fast ripples, and which was replicated in a computational model when voltage-gated sodium currents were impaired in basket cells (BCs). Our results indicate attenuated h-c coupling as a promising early indicator of inhibitory dysfunction and seizure risk in DS.

Current approaches in treatment of Alzheimer’s disease (AD) is focused on early stages of cognitive decline. Identifying therapeutic targets that promote synaptic resilience during early stages may prevent progressive memory deficits by preserving memory mechanisms. We recently reported that the inducible isoform of phospholipase D (PLD1) was significantly increased in synaptosomes from post-mortem AD brains compared to age-matched controls. Using mouse models, we reported that the aberrantly elevated neuronal PLD1 is key for oligomeric amyloid driven synaptic dysfunction and underlying memory deficits. Here, we demonstrate that chronic inhibition using a well-tolerated PLD1 specific small molecule inhibitor is sufficient to prevent the progression of synaptic dysfunction during early stages in the 3xTg-AD mouse model. Firstly, we report prevention of cognitive decline in the inhibitor-treated group using novel object recognition (NOR) and fear conditioning (FC). Secondly, we provide electrophysiological assessment of better synaptic function in the inhibitor-treated group. Lastly, using Golgi staining, we report that preservation of dendritic spine integrity as one of the mechanisms underlying the action of the small molecule inhibitor. Collectively, these studies provide evidence for inhibition of PLD1 as a potential therapeutic strategy in preventing progression of cognitive decline associated with AD and related dementia.

Beta amyloid (Aβ) associated with Alzheimer’s disease (AD) leads to abnormal behavior in inhibitory neurons, resulting in hyperactive neuronal networks, epileptiform behavior, disrupted gamma rhythms, and aberrant synaptic plasticity. Previously, we used a dual modeling-experimental approach to explain several observations, including failure to reliably produce action potentials (APs), smaller AP amplitudes, higher resting membrane potential, and higher membrane depolarization in response to a range of stimuli in hippocampal inhibitory neurons from 12–16 month old female APPswe/PSEN1DeltaE9 (APdE9) AD mice as compared to age-matched non-transgenic (NTG) mice. Our experimental results also showed that AP initiation in interneurons from APdE9 mice are significantly different from that of NTG mice. APs in interneurons from NTG mice are characterized by abrupt onset and an upstroke that is much steeper and occurs with larger variability as compared to cells from APdE9 mice. The phase plot (the rate of change of membrane potential versus the instantaneous membrane potential) of APs produced by interneurons from APdE9 mice shows a biphasic behavior whereas that from NTG mice shows a monophasic behavior. Here we show that using the classic Hodgkin-Huxley (HH) formalism for the gating of voltage-gated sodium channels (VGSCs) in a single-compartment neuron, we cannot reproduce these features and a model that takes into account a cooperative activation of VGSCs is needed. We also argue that considering a realistic multi-compartment neuron where the kinetics of VGSC is modeled by HH formalism as done in the past, wouldn’t explain our observations when APs from both NTG and APdE9 mice are considered simultaneously. We further show that VGSCs in interneurons from APdE9 mice exhibit significantly lower cooperativity in their activation as compared to those form NTG mice.

21Spontaneous neuronal and astrocytic activity in the neonate forebrain is believed to drive 22 Author Summary 46Spontaneous neuronal and astrocytic activity during the early postnatal period is crucial to 47 the development and physiology of the neonate forebrain. Elucidating the origin of this activity is 48 key to our understanding of the cell maturation and formation of brain-region-specific networks. 49This study reports spontaneous, ultraslow, large-amplitude, long-lasting fluctuations in the 50 intracellular Na + concentration of neurons and astrocytes in the hippocampus of mice at postnatal 51 days 2-4 that mostly disappear after the first postnatal week. We combine ratiometric Na + imaging 52 and pharmacological manipulations with a detailed computational model of neuronal networks in 53 the neonatal and adult brain to provide key insights into the origin of these Na + fluctuations. 54 Furthermore, our model predicts that these periods of spontaneous Na + influx leave neonatal 55 neuronal networks more vulnerable to hyperactivity when compared to mature brain. 56 57 Spontaneous neuronal activity is a hallmark of the developing central nervous system [1], 58 and has been described in terms of intracellular Ca 2+ oscillations both in neurons and astrocytes 59 [2][3][4][5] and bursts of neuronal action potentials [6][7][8]. This activity is believed to promote the 60 maturation of individual cells and their integration into complex brain-region-specific networks 61 [1,[9][10][11]. In the rodent hippocampus, early network activity and Ca 2+ oscillations are mainly 62 attributed to the excitatory role of GABAergic transmission originating from inhibitory neurons 63 [7,[12][13][14]. 64The excitatory action of GABAergic neurotransmission is one of the most notable 65 characteristics that distinguish neonate brain from the mature brain, where GABA typically 66 inhibits neuronal networks [1, 7, 8,[10][11][12][15][16][17]. While recent work has also called the inhibitory 67 action of GABA on cortical networks into question [18], there are many other pathways that could 68 play a significant role in the observed spontaneous activity in neonate brain (discussed below). 69Additional key features of the early network oscillations in the hippocampus include their 70 synchronous behavior across most of the neuronal network, modulation by glutamate, recurrence 71 with regular frequency, and a limitation to early post-natal development [2, 7, 12]. 72More recently, Felix and co-workers [5] reported a new form of seemingly spontaneous 73 activity in acutely isolated tissue slices of hippocampus and cortex of neonatal mice. It consists of 74 spontaneous fluctuations in intracellular Na + both in astrocytes and neurons, which occur in ~25% 75 of pyramidal neurons and ~40% of astrocytes tested. Na + fluctuations are ultraslow in nature, 76 averaging ~2 fluctuations/hour, are not synchronized between cells, and are not significantly 77 affected by an array of pharmacological blockers for various channels, receptors, and transpo...

Background Phosphatidyl choline phospholipase D (PC‐PLD), a lipolytic enzyme that breaks down membrane phospholipids via two isoforms ‐ a constitutively expressed PLD2 and an inducible PLD1 isoform ‐ are also involved in developmentally important signaling mechanisms that regulate synaptic function. We were the first to propose and present a systematic study that established PLD1 as the aberrantly elevated isoform in AD and related dementia using human clinical samples and provided functional proof using mouse models to demonstrate the underlying synaptic dysfunction and memory deficits. Method Synaptosomal Western blot analysis on 3xTg‐AD mice hippocampi were used to investigate neuronal PLD1 expression and function. Long term potentiation of PLD1 dependent changes using pharmacological approaches in ex vivo slice preparations from wildtype and transgenic mouse models were used to assess synaptic perturbations that were first studied using the novel object recognition memory (NOR) and fear conditioning (FC) paradigms. Chronic PLD1 small molecule inhibitor treatment was assessed in different age groups to ascertain the efficacy of treatment at different stages of Aβ and tau‐driven AD‐like memory deficit progression in transgenic animal models. Lastly, brain tissues from these animals were subjected to Western Blot analyses, Golgi analysis, immunohistochemical analysis to ascertain the potential signaling mechanism(s) that is/are perturbed/altered by overexpression of PLD1 in such diseased states. Result Chronic PLD1 inhibition ameliorates the synaptic dysfunction and underlying memory deficits in the 3xTg‐AD mouse model by specific action on preserving mushroom spine dendritic spine integrity. Further analysis using Western Blots revealed an underlying mechanism that increases the levels of phosphocofilin in the crude synaptosomal fractions. Conclusion Using chronic administration of a well‐tolerated halopemide derivative of the specific PLD1 isoform inhibitor in the preclinical mouse models of synaptic dysfunction and memory deficits associated with amyloidogenic effects of Aβ and tau, we demonstrated neuroprotective aspects involving changes in the dendritic spine integrity that contributes to the preservation of memory. Furthermore, we also observed that the mechanism involves actin cytoskeletal dynamics that are modulated via phosphocofilin.

Spontaneous neuronal and astrocytic activity in the neonate forebrain is believed to drive the maturation of individual cells and their integration into complex brain-region-specific networks. The previously reported forms include bursts of electrical activity and oscillations in intracellular Ca2+ concentration. Here, we use ratiometric Na+ imaging to demonstrate spontaneous fluctuations in the intracellular Na+ concentration of CA1 pyramidal neurons and astrocytes in tissue slices obtained from the hippocampus of mice at postnatal days 2-4 (P2-4). These occur at very low frequency (~2/h), can last minutes with amplitudes up to several mM, and mostly disappear after the first postnatal week. To further investigate their mechanisms, we model a network consisting of pyramidal neurons and interneurons. Experimentally observed Na+ fluctuations are mimicked when GABAergic inhibition in the simulated network is made depolarizing. Our experiments and computational model show that blocking voltage-gated Na+ channels or GABAergic signaling significantly diminish the neuronal Na+ fluctuations. On the other hand, blocking a variety of other ion channels, receptors, or transporters including glutamatergic pathways, does not have significant effects. Our model also shows that the amplitude and duration of Na+ fluctuations decrease as we increase the strength of glial K+ uptake. Furthermore, neurons with smaller somatic volumes exhibit fluctuations with higher frequency and amplitude. As opposed to this, larger extracellular to intracellular volume ratio observed in neonatal brain exerts a dampening effect. Finally, our model predicts that these periods of spontaneous Na+ influx leave neonatal neuronal networks more vulnerable to seizure-like states when compared to mature brain.