Deficits in coordinated neuronal activity and network topology are striatal hallmarks in Huntington's disease

Criticality and universality in neuronal cultures during "up" and "down" states

The brain can be seen as a self-organized dynamical system that optimizes information processing and storage capabilities. This is supported by studies across scales, from small neuronal assemblies to the whole brain, where neuronal activity exhibits features typically associated with phase transitions in statistical physics. Such a critical state is characterized by the emergence of scale-free statistics as captured, for example, by the sizes and durations of activity avalanches corresponding to a cascading process of information flow. Another phenomenon observed during sleep, under anesthesia, and in in vitro cultures, is that cortical and hippocampal neuronal networks alternate between "up" and "down" states characterized by very distinct firing rates. Previous theoretical work has been able to relate these two concepts and proposed that only up states are critical whereas down states are subcritical, also indicating that the brain spontaneously transitions between the two. Using high-speed high-resolution calcium imaging recordings of neuronal cultures, we test this hypothesis here by analyzing the neuronal avalanche statistics in populations of thousands of neurons during "up" and "down" states separately. We find that both "up" and "down" states can exhibit scale-free behavior when taking into account their intrinsic time scales. In particular, the statistical signature of "down" states is indistinguishable from those observed previously in cultures without "up" states. We show that such behavior can not be explained by network models of non-conservative leaky integrate-and-fire neurons with short-term synaptic depression, even when realistic noise levels, spatial network embeddings, and heterogeneous populations are taken into account, which instead exhibits behavior consistent with previous theoretical models. Similar differences were also observed when taking into consideration finite-size scaling effects, suggesting that the intrinsic dynamics and self-organization mechanisms of these cultures might be more complex than previously thought. In particular, our findings point to the existence of different mechanisms of neuronal communication, with different time scales, acting during either high-activity or low-activity states, potentially requiring different plasticity mechanisms.

VPS13A knockdown impairs corticostriatal synaptic plasticity and locomotor behavior in a new mouse model of chorea-acanthocytosis

Chorea-acanthocytosis (ChAc) is an inherited neurodegenerative movement disorder caused by VPS13A gene mutations leading to the absence of protein expression. The striatum is the most affected brain region in ChAc patients. However, the study of the VPS13A function in the brain has been poorly addressed. Here we generated a VPS13A knockdown (KD) model and aimed to elucidate the contribution of VPS13A to synaptic plasticity and neuronal communication in the corticostriatal circuit. First, we infected primary cortical neurons with miR30-shRNA against VPS13A and analyzed its effects on neuronal plasticity. VPS13A-KD neurons showed a higher degree of branching than controls, accompanied by decreased BDNF and PSD-95 levels, indicative of synaptic alterations. We then injected AAV-KD bilaterally in the frontal cortex and two different regions of the striatum of mice and analyzed the effects of VPS13A-KD on animal behavior and synaptic plasticity. VPS13A-KD mice showed modification of the locomotor behavior pattern, with increased exploratory behavior and hyperlocomotion. Corticostriatal dysfunction in VPS13A-KD mice was evidenced by impaired striatal long-term depression (LTD) after stimulation of cortical afferents, which was partially recovered by BDNF administration. VPS13A-KD did not lead to neuronal loss in the cortex or the striatum but induced a decrease in the neuronal release of CX3CL1 and triggered a microglial reaction, especially in the striatum. Notably, CX3CL1 administration partially restored the impaired corticostriatal LTD in VPS13A-KD mice. Our results unveil the involvement of VPS13A in neuronal connectivity modifying BDNF and CX3CL1 release. Moreover, the involvement of VPS13A in synaptic plasticity and motor behavior provides key information to further understand not only ChAc pathophysiology but also other neurological disorders.

Neuron-derived extracellular vesicles contain synaptic proteins, promote spine formation, activate TrkB-mediated signalling and preserve neuronal complexity

Extracellular vesicles (EVs) play an important role in intercellular communication as carriers of signalling molecules such as bioactive miRNAs, proteins and lipids. EVs are key players in the functioning of the central nervous system (CNS) by influencing synaptic events and modulating recipient neurons. However, the specific role of neuron-to-neuron communication via EVs is still not well understood. Here, we provide evidence that primary neurons uptake neuron-derived EVs in the soma, dendrites, and even in the dendritic spines, and carry synaptic proteins. Neuron-derived EVs increased spine density and promoted the phosphorylation of Akt and ribosomal protein S6 (RPS6), via TrkB-signalling, without impairing the neuronal network activity. Strikingly, EVs exerted a trophic effect on challenged nutrient-deprived neurons. Altogether, our results place EVs in the spotlight for synaptic plasticity modulation as well as a possible therapeutic tool to fight neurodegeneration.

Neuronal Cultures: Exploring Biophysics, Complex Systems, and Medicine in a Dish

Neuronal cultures are one of the most important experimental models in modern interdisciplinary neuroscience, allowing to investigate in a control environment the emergence of complex behavior from an ensemble of interconnected neurons. Here, I review the research that we have conducted at the neurophysics laboratory at the University of Barcelona over the last 15 years, describing first the neuronal cultures that we prepare and the associated tools to acquire and analyze data, to next delve into the different research projects in which we actively participated to progress in the understanding of open questions, extend neuroscience research on new paradigms, and advance the treatment of neurological disorders. I finish the review by discussing the drawbacks and limitations of neuronal cultures, particularly in the context of brain-like models and biomedicine.

Long-term calcium imaging reveals functional development in hiPSC-derived cultures comparable to human but not rat primary cultures

Models for human brain-oriented research are often established on primary cultures from rodents, which fails to recapitulate cellular specificity and molecular cues of the human brain. Here we investigated whether neuronal cultures derived from human induced pluripotent stem cells (hiPSCs) feature key advantages compared with rodent primary cultures. Using calcium fluorescence imaging, we tracked spontaneous neuronal activity in hiPSC-derived, human, and rat primary cultures and compared their dynamic and functional behavior as they matured. We observed that hiPSC-derived cultures progressively changed upon development, exhibiting gradually richer activity patterns and functional traits. By contrast, rat primary cultures were locked in the same dynamic state since activity onset. Human primary cultures exhibited features in between hiPSC-derived and rat primary cultures, although traits from the former predominated. Our study demonstrates that hiPSC-derived cultures are excellent models to investigate development in neuronal assemblies, a hallmark for applications that monitor alterations caused by damage or neurodegeneration.

Rich dynamics and functional organization on topographically designed neuronal networks in vitro

Neuronal cultures are a prominent experimental tool to understand complex functional organization in neuronal assemblies. However, neurons grown on flat surfaces exhibit a strongly coherent bursting behavior with limited functionality. To approach the functional richness of naturally formed neuronal circuits, here we studied neuronal networks grown on polydimethylsiloxane (PDMS) topographical patterns shaped as either parallel tracks or square valleys. We followed the evolution of spontaneous activity in these cultures along 20 days in vitro using fluorescence calcium imaging. The networks were characterized by rich spatiotemporal activity patterns that comprised from small regions of the culture to its whole extent. Effective connectivity analysis revealed the emergence of spatially compact functional modules that were associated with both the underpinned topographical features and predominant spatiotemporal activity fronts. Our results show the capacity of spatial constraints to mold activity and functional organization, bringing new opportunities to comprehend the structure-function relationship in living neuronal circuits.

Calcium imaging: A versatile tool to examine Huntington's disease mechanisms and progression

Huntington's disease (HD) is a fatal, hereditary neurodegenerative disorder that causes chorea, cognitive deficits, and psychiatric symptoms. It is characterized by accumulation of mutant Htt protein, which primarily impacts striatal medium-sized spiny neurons (MSNs), as well as cortical pyramidal neurons (CPNs), causing synapse loss and eventually cell death. Perturbed Ca2+ homeostasis is believed to play a major role in HD, as altered Ca2+ homeostasis often precedes striatal dysfunction and manifestation of HD symptoms. In addition, dysregulation of Ca2+ can cause morphological and functional changes in MSNs and CPNs. Therefore, Ca2+ imaging techniques have the potential of visualizing changes in Ca2+ dynamics and neuronal activity in HD animal models. This minireview focuses on studies using diverse Ca2+ imaging techniques, including two-photon microscopy, fiber photometry, and miniscopes, in combination of Ca2+ indicators to monitor activity of neurons in HD models as the disease progresses. We then discuss the future applications of Ca2+ imaging to visualize disease mechanisms and alterations associated with HD, as well as studies showing how, as a proof-of-concept, Ca2+imaging using miniscopes in freely-behaving animals can help elucidate the differential role of direct and indirect pathway MSNs in HD symptoms.

Chemical and molecular tools to probe biological sex differences at multiple length scales

Biological sex differences are observed at multiple different length scales and across organ systems. Gaps in knowledge remain regarding our understanding of how molecular, cellular, and environmental factors contribute to physiological sex differences. Here, we provide our perspective on how chemical and molecular tools can be leveraged to explore sex differences in biology at the molecular, intracellular, extracellular, tissue, and organ length scales. We provide examples where chemical and molecular tools were used to explore sex differences in the cardiovascular, nervous, immune, and reproductive systems. We also provide a future outlook where chemical and molecular tools can be applied to continue investigating sex differences in biology, with the ultimate goal of addressing inequities in biomedical research and approaches to clinical treatments.

Restoration and targeting of aberrant neurotransmitters in Parkinson's disease therapeutics

Neurotransmitters are considered as a fundamental regulator in the process of neuronal growth, differentiation and survival. Parkinson's Disease (PD) occurs due to extensive damage of dopamine-producing neurons; this causes dopamine deficits in the midbrain, followed by the alternation of various other neurotransmitters (glutamate, GABA, serotonin, etc.). It has been observed that fluctuation of neurotransmission in the basal ganglia exhibits a great impact on the pathophysiology of PD. Dopamine replacement therapy, such as the use of L-DOPA, can increase the dopamine level, but it majorly ameliorates the motor symptoms and is also associated with long-term complications (for e.g., LID). While the non-dopaminergic system can efficiently target non-motor symptoms, for instance, the noradrenergic system regulates the synthesis of BDNF via the MAPK pathway, which is important in learning and memory. Herein, we briefly discuss the role of different neurotransmitters, implementation of neurotransmitter receptors in PD. We also illustrate the recent advances of neurotransmitter-based drugs, which are currently under in vivo and clinical studies. Reinstating normal neurotransmitter levels has been believed to be advantageous in the treatment of PD. Thus, there is an increasing demand for drugs that can specifically target the neurotransmission system and reinstate the normal levels of neurotransmitters, which might prevent or delay neurodegeneration in PD.

Unraveling the Spatiotemporal Distribution of VPS13A in the Mouse Brain

Loss-of-function mutations in the human vacuolar protein sorting the 13 homolog A (VPS13A) gene cause Chorea-acanthocytosis (ChAc), with selective degeneration of the striatum as the main neuropathologic feature. Very little is known about the VPS13A expression in the brain. The main objective of this work was to assess, for the first time, the spatiotemporal distribution of VPS13A in the mouse brain. We found VPS13A expression present in neurons already in the embryonic stage, with stable levels until adulthood. VPS13A mRNA and protein distributions were similar in the adult mouse brain. We found a widespread VPS13A distribution, with the strongest expression profiles in the pons, hippocampus, and cerebellum. Interestingly, expression was weak in the basal ganglia. VPS13A staining was positive in glutamatergic, GABAergic, and cholinergic neurons, but rarely in glial cells. At the cellular level, VPS13A was mainly located in the soma and neurites, co-localizing with both the endoplasmic reticulum and mitochondria. However, it was not enriched in dendritic spines or the synaptosomal fraction of cortical neurons. In vivo pharmacological modulation of the glutamatergic, dopaminergic or cholinergic systems did not modulate VPS13A concentration in the hippocampus, cerebral cortex, or striatum. These results indicate that VPS13A has remarkable stability in neuronal cells. Understanding the distinct expression pattern of VPS13A can provide relevant information to unravel pathophysiological hallmarks of ChAc.

Astrocytic and Neuronal Apolipoprotein E Isoforms Differentially Affect Neuronal Excitability

Synaptic changes and neuronal network dysfunction are among the earliest changes in Alzheimer’s disease (AD). Apolipoprotein E4 (ApoE4), the major genetic risk factor in AD, has been shown to be present at synapses and to induce hyperexcitability in mouse knock-in brain regions vulnerable to AD. ApoE in the brain is mainly generated by astrocytes, however, neurons can also produce ApoE under stress conditions such as aging. The potential synaptic function(s) of ApoE and whether the cellular source of ApoE might affect neuronal excitability remain poorly understood. Therefore, the aim of this study was to elucidate the synaptic localization and effects on neuronal activity of the two main human ApoE isoforms from different cellular sources in control and AD-like in vitro cultured neuron models. In this study ApoE is seen to localize at or near to synaptic terminals. Additionally, we detected a cellular source-specific effect of ApoE isoforms on neuronal activity measured by live cell Ca²⁺ imaging. Neuronal activity increases after acute but not long-term administration of ApoE4 astrocyte medium. In contrast, ApoE expressed by neurons appears to induce the highest neuronal firing rate in the presence of ApoE3, rather than ApoE4. Moreover, increased neuronal activity in APP/PS1 AD transgenic compared to wild-type neurons is seen in the absence of astrocytic ApoE and the presence of astrocytic ApoE4, but not ApoE3. In summary, ApoE can target synapses and differentially induce changes in neuronal activity depending on whether ApoE is produced by astrocytes or neurons. Astrocytic ApoE induces the strongest neuronal firing with ApoE4, while the most active and efficient neuronal activity induced by neuronal ApoE is caused by ApoE3. ApoE isoforms also differentially affect neuronal activity in AD transgenic compared to wild-type neurons.

In Vitro Development of Human iPSC-Derived Functional Neuronal Networks on Laser-Fabricated 3D Scaffolds

Neural progenitor cells generated from human induced pluripotent stem cells (hiPSCs) are the forefront of ″brain-on-chip″ investigations. Viable and functional hiPSC-derived neuronal networks are shaping powerful in vitro models for evaluating the normal and abnormal formation of cortical circuits, understanding the underlying disease mechanisms, and investigating the response to drugs. They therefore represent a desirable instrument for both the scientific community and the pharmacological industry. However, culture conditions required for the full functional maturation of individual neurons and networks are still unidentified. It has been recognized that three-dimensional (3D) culture conditions can better emulate in vivo neuronal tissue development compared to 2D cultures and thus provide a more desirable in vitro approach. In this paper, we present the design and implementation of a 3D scaffold platform that supports and promotes intricate neuronal network development. 3D scaffolds were produced through direct laser writing by two-photon polymerization (2PP), a high-resolution 3D laser microstructuring technology, using the biocompatible and nondegradable photoreactive resin Dental LT Clear (DClear). Neurons developed and interconnected on a 3D environment shaped by vertically stacked scaffold layers. The developed networks could support different cell types. Starting at the day 50 of 3D culture, neuronal progenitor cells could develop into cortical projection neurons (CNPs) of all six layers, different types of inhibitory neurons, and glia. Additionally and in contrast to 2D conditions, 3D scaffolds supported the long-term culturing of neuronal networks over the course of 120 days. Network health and functionality were probed through calcium imaging, which revealed a strong spontaneous neuronal activity that combined individual and collective events. Taken together, our results highlight advanced microstructured 3D scaffolds as a reliable platform for the 3D in vitro modeling of neuronal functions.