Overexpression of cypin alters dendrite morphology, single neuron activity, and network properties via distinct mechanisms

The role of snapin in regulation of brain homeostasis

Brain homeostasis refers to the normal working state of the brain in a certain period, which is important for overall health and normal life activities. Currently, there is a lack of effective treatment methods for the adverse consequences caused by brain homeostasis imbalance. Snapin is a protein that assists in the formation of neuronal synapses and plays a crucial role in the normal growth and development of synapses. Recently, many researchers have reported the association between snapin and neurologic and psychiatric disorders, demonstrating that snapin can improve brain homeostasis. Clinical manifestations of brain disease often involve imbalances in brain homeostasis and may lead to neurological and behavioral sequelae. This article aims to explore the role of snapin in restoring brain homeostasis after injury or diseases, highlighting its significance in maintaining brain homeostasis and treating brain diseases. Additionally, it comprehensively discusses the implications of snapin in other extracerebral diseases such as diabetes and viral infections, with the objective of determining the clinical potential of snapin in maintaining brain homeostasis.

Stretch-Induced Injury Affects Cortical Neuronal Networks in a Time- and Severity-Dependent Manner

Traumatic brain injury (TBI) is the leading cause of accident-related death and disability in the world and can lead to long-term neuropsychiatric symptoms, such as a decline in cognitive function and neurodegeneration. TBI includes primary and secondary injury, with head trauma and deformation of the brain caused by the physical force of the impact as primary injury, and cellular and molecular cascades that lead to cell death as secondary injury. Currently, there is no treatment for TBI-induced cell damage and neural circuit dysfunction in the brain, and thus, it is important to understand the underlying cellular mechanisms that lead to cell damage. In the current study, we use stretchable microelectrode arrays (sMEAs) to model the primary injury of TBI to study the electrophysiological effects of physically injuring cortical cells. We recorded electrophysiological activity before injury and then stretched the flexible membrane of the sMEAs to injure the cells to varying degrees. At 1, 24, and 72 h post-stretch, we recorded activity to analyze differences in spike rate, Fano factor, burstlet rate, burstlet width, synchrony of firing, local network efficiency, and Q statistic. Our results demonstrate that mechanical injury changes the firing properties of cortical neuron networks in culture in a time- and severity-dependent manner. Our results suggest that changes to electrophysiological properties after stretch are dependent on the strength of synchronization between neurons prior to injury.

Time-dependent homeostatic mechanisms underlie brain-derived neurotrophic factor action on neural circuitry

Plasticity and homeostatic mechanisms allow neural networks to maintain proper function while responding to physiological challenges. Despite previous work investigating morphological and synaptic effects of brain-derived neurotrophic factor (BDNF), the most prevalent growth factor in the central nervous system, how exposure to BDNF manifests at the network level remains unknown. Here we report that BDNF treatment affects rodent hippocampal network dynamics during development and recovery from glutamate-induced excitotoxicity in culture. Importantly, these effects are not obvious when traditional activity metrics are used, so we delve more deeply into network organization, functional analyses, and in silico simulations. We demonstrate that BDNF partially restores homeostasis by promoting recovery of weak and medium connections after injury. Imaging and computational analyses suggest these effects are caused by changes to inhibitory neurons and connections. From our in silico simulations, we find that BDNF remodels the network by indirectly strengthening weak excitatory synapses after injury. Ultimately, our findings may explain the difficulties encountered in preclinical and clinical trials with BDNF and also offer information for future trials to consider.

Decoding Natural Astrocyte Rhythms: Dynamic Actin Waves Result from Environmental Sensing by Primary Rodent Astrocytes

Astrocytes are key regulators of brain homeostasis, equilibrating ion, water, and neurotransmitter concentrations and maintaining essential conditions for proper cognitive function. Recently, it has been shown that the excitability of the actin cytoskeleton manifests in second-scale dynamic fluctuations and acts as a sensor of chemophysical environmental cues. However, it is not known whether the cytoskeleton is excitable in astrocytes and how the homeostatic function of astrocytes is linked to the dynamics of the cytoskeleton. Here it is shown that homeostatic regulation involves the excitable dynamics of actin in certain subcellular regions of astrocytes, especially near the cell boundary. The results further indicate that actin dynamics concentrate into "hotspot" regions that selectively respond to certain chemophysical stimuli, specifically the homeostatic challenges of ion or water concentration increases. Substrate topography makes the actin dynamics of astrocytes weaker. Super-resolution images demonstrate that surface topography is also associated with the predominant perpendicular alignment of actin filaments near the cell boundary, whereas flat substrates result in an actin cortex mainly parallel to the cell boundary. Additionally, coculture with neurons increases both the probability of actin dynamics and the strength of hotspots. The excitable systems character of actin thus makes astrocytes direct participants in neural cell network dynamics.

Cypin binds to tubulin heterodimers and microtubule protofilaments and regulates microtubule spacing in developing hippocampal neurons

Cytosolic PSD-95 interactor (cypin) is a multifunctional, guanine deaminase that plays a major role in shaping the morphology of the dendritic arbor of hippocampal and cortical neurons. Cypin catalyzes the Zn2+-dependent deamination of guanine to xanthine, which is then metabolized to uric acid by xanthine oxidase. Cypin binds to tubulin heterodimers via its carboxyl terminal region (amino acids (aa) 350-454), which contains a collapsin response mediator protein (CRMP) homology domain (aa 350-403). Moreover, this region alone is not sufficient to facilitate microtubule polymerization; therefore, additional cypin regions must be involved in this process. Here, we asked whether cypin binds to fully formed microtubules and how overexpression of cypin regulates the microtubule cytoskeleton in dendrites of cultured hippocampal neurons. Protein-protein docking strategies confirm that the cypin homodimer binds to tubulin heterodimers via amino acids within aa 350-454. Biochemical pull-down data suggest that aa 1-220 are necessary for cypin binding to soluble tubulin heterodimers and to taxol-stabilized microtubules. Molecular docking of the cypin homodimer to soluble tubulin heterodimers reveals a consistently observed docking pose using aa 47-71, 113-118, 174-178, and 411-418, which is consistent with our biochemical data. Additionally, overexpression of cypin in hippocampal neurons results in decreased spacing between microtubules. Our results suggest that several protein domains facilitate cypin-mediated polymerization of tubulin heterodimers into microtubules, possibly through a mechanism whereby cypin dimers bind to multiple tubulin heterodimers.

Real-Time Monitoring of Human Guanine Deaminase Activity by an Emissive Guanine Analog

Guanine deaminase (GDA) deaminates guanine to xanthine. Despite its significance, the study of human GDA remains limited compared to other metabolic deaminases. As a result, its substrate and inhibitor repertoire are limited, and effective real-time activity, inhibitory, and discovery assays are missing. Herein, we explore two emissive heterocyclic cores, based on thieno[3,4-d]pyrimidine (thN) and isothiazole[4,3-d]pyrimidine (tzN), as surrogate GDA substrates. We demonstrate that, unlike the thieno analog, thGN, the isothiazolo guanine surrogate, tzGN, does undergo effective enzymatic deamination by GDA and yields the spectroscopically distinct xanthine analog, tzXN. Further, we showcase the potential of this fluorescent nucleobase surrogate to provide a visible spectral window for a real-time study of GDA and its inhibition.

Cytosolic PSD-95 interactor alters functional organization of neural circuits and AMPA receptor signaling independent of PSD-95 binding

Cytosolic PSD-95 interactor (cypin) regulates many aspects of neuronal development and function, ranging from dendritogenesis to synaptic protein localization. While it is known that removal of postsynaptic density protein-95 (PSD-95) from the postsynaptic density decreases synaptic N-methyl-D-aspartate (NMDA) receptors and that cypin overexpression protects neurons from NMDA-induced toxicity, little is known about cypin's role in AMPA receptor clustering and function. Experimental work shows that cypin overexpression decreases PSD-95 levels in synaptosomes and the PSD, decreases PSD-95 clusters/μm², and increases mEPSC frequency. Analysis of microelectrode array (MEA) data demonstrates that cypin or cypinΔPDZ overexpression increases sensitivity to CNQX (cyanquixaline) and AMPA receptor-mediated decreases in spike waveform properties. Network-level analysis of MEA data reveals that cypinΔPDZ overexpression causes networks to be resilient to CNQX-induced changes in local efficiency. Incorporating these findings into a computational model of a neural circuit demonstrates a role for AMPA receptors in cypin-promoted changes to networks and shows that cypin increases firing rate while changing network functional organization, suggesting cypin overexpression facilitates information relay but modifies how information is encoded among brain regions. Our data show that cypin promotes changes to AMPA receptor signaling independent of PSD-95 binding, shaping neural circuits and output to regions beyond the hippocampus.

Cypin: A novel target for traumatic brain injury

Cytosolic PSD-95 interactor (cypin), the primary guanine deaminase in the brain, plays key roles in shaping neuronal circuits and regulating neuronal survival. Despite this pervasive role in neuronal function, the ability for cypin activity to affect recovery from acute brain injury is unknown. A key barrier in identifying the role of cypin in neurological recovery is the absence of pharmacological tools to manipulate cypin activity in vivo. Here, we use a small molecule screen to identify two activators and one inhibitor of cypin's guanine deaminase activity. The primary screen identified compounds that change the initial rate of guanine deamination using a colorimetric assay, and secondary screens included the ability of the compounds to protect neurons from NMDA-induced injury and NMDA-induced decreases in frequency and amplitude of miniature excitatory postsynaptic currents. Hippocampal neurons pretreated with activators preserved electrophysiological function and survival after NMDA-induced injury in vitro, while pretreatment with the inhibitor did not. The effects of the activators were abolished when cypin was knocked down. Administering either cypin activator directly into the brain one hour after traumatic brain injury significantly reduced fear conditioning deficits 5 days after injury, while delivering the cypin inhibitor did not improve outcome after TBI. Together, these data demonstrate that cypin activation is a novel approach for improving outcome after TBI and may provide a new pathway for reducing the deficits associated with TBI in patients.