Traumatic brain injury disrupts state-dependent functional cortical connectivity in a mouse model

Traumatic brain injury (TBI) is the leading cause of death in young people and can cause cognitive and motor dysfunction and disruptions in functional connectivity between brain regions. In human TBI patients and rodent models of TBI, functional connectivity is decreased after injury. Recovery of connectivity after TBI is associated with improved cognition and memory, suggesting an important link between connectivity and functional outcome. We examined widespread alterations in functional connectivity following TBI using simultaneous widefield mesoscale GCaMP7c calcium imaging and electrocorticography (ECoG) in mice injured using the controlled cortical impact (CCI) model of TBI. Combining CCI with widefield cortical imaging provides us with unprecedented access to characterize network connectivity changes throughout the entire injured cortex over time. Our data demonstrate that CCI profoundly disrupts functional connectivity immediately after injury, followed by partial recovery over 3 weeks. Examining discrete periods of locomotion and stillness reveals that CCI alters functional connectivity and reduces theta power only during periods of behavioral stillness. Together, these findings demonstrate that TBI causes dynamic, behavioral state-dependent changes in functional connectivity and ECoG activity across the cortex.

Evaluation of compact pulsed lasers for two-photon microscopy using a simple method for measuring two-photon excitation efficiency

Significance

Two-photon (2p) microscopy has historically relied on titanium sapphire pulsed lasers that are expensive and have a large footprint. Recently, several manufacturers have developed less expensive compact pulsed lasers optimized for 2p excitation of green fluorophores. However, quantitative evaluation of their quality is lacking.

Aim

We describe a simple approach to systematically evaluate 2p excitation efficiency, an empiric measure of the quality of a pulsed laser and its ability to elicit 2p induced fluorescence.

Approach

By measuring pulse width, repetition rate, and fluorescence output, we calculated a measure of 2p excitation efficiency η , which we compared for four commercially available compact pulsed lasers in the 920 to 930 nm wavelength range.

Results

2p excitation efficiency varied substantially among tested lasers. The Coherent Axon exhibited the best 2p excitation efficiency (1.09 ± 0.03 ), exceeding that of a titanium sapphire reference laser (defined to have efficiency = 1). However, its measured fluorescence was modest due to its long pulse width. Of the compact lasers, the Toptica Femtofiber Ultra exhibited the best combination of measured fluorescence (0.75 ± 0.01 ) and 2p excitation efficiency (0.86 ± 0.01 ).

Conclusions

We describe a simple method that both laser developers and end users can use to benchmark the 2p excitation efficiency of lasers used for 2p microscopy.

Proregenerative extracellular matrix hydrogel mitigates pathological alterations of pelvic skeletal muscles after birth injury

Pelvic floor disorders, including pelvic organ prolapse and urinary and fecal incontinence, affect millions of women globally and represent a major public health concern. Pelvic floor muscle (PFM) dysfunction has been identified as one of the leading risk factors for the development of these morbid conditions. Childbirth, specifically vaginal delivery, has been recognized as the most important potentially modifiable risk factor for PFM injury; however, the precise mechanisms of PFM dysfunction after parturition remain elusive. In this study, we demonstrated that PFMs exhibit atrophy and fibrosis in parous women with symptomatic pelvic organ prolapse. These pathological alterations were recapitulated in a preclinical rat model of simulated birth injury (SBI). The transcriptional signature of PFMs after injury demonstrated an impairment in muscle anabolism, persistent expression of genes that promote extracellular matrix (ECM) deposition, and a sustained inflammatory response. We also evaluated the administration of acellular injectable skeletal muscle ECM hydrogel for the prevention of these pathological alterations. Treatment of PFMs with the ECM hydrogel either at the time of birth injury or 4 weeks after injury mitigated PFM atrophy and fibrosis. By evaluating gene expression, we demonstrated that these changes are mainly driven by the hydrogel-induced enhancement of endogenous myogenesis, ECM remodeling, and modulation of the immune response. This work furthers our understanding of PFM birth injury and demonstrates proof of concept for future investigations of proregenerative biomaterial approaches for the treatment of injured pelvic soft tissues.

Treadmill-based task for assessing spatial memory in head-fixed mice

Several mouse in vivo neuronal recording techniques require head fixation. Head-fixed treadmill walking can be used to design tasks that enable the study of neural activity in the context of behavior. Here, we provide a detailed protocol for constructing a treadmill with tactile spatial cues, training mice on a rewarded behavioral task, and analyzing behavioral data. We discuss common problems and solutions we have developed to optimize training. Finally, we demonstrate how to test spatial memory performance using this task.

AAV ablates neurogenesis in the adult murine hippocampus

Recombinant adeno-associated virus (rAAV) has been widely used as a viral vector across mammalian biology and has been shown to be safe and effective in human gene therapy. We demonstrate that neural progenitor cells (NPCs) and immature dentate granule cells (DGCs) within the adult murine hippocampus are particularly sensitive to rAAV-induced cell death. Cell loss is dose dependent and nearly complete at experimentally relevant viral titers. rAAV-induced cell death is rapid and persistent, with loss of BrdU-labeled cells within 18 hr post-injection and no evidence of recovery of adult neurogenesis at 3 months post-injection. The remaining mature DGCs appear hyperactive 4 weeks post-injection based on immediate early gene expression, consistent with previous studies investigating the effects of attenuating adult neurogenesis. In vitro application of AAV or electroporation of AAV2 inverted terminal repeats (ITRs) is sufficient to induce cell death. Efficient transduction of the dentategyrus (DG)– without ablating adult neurogenesis– can be achieved by injection of rAAV2-retro serotyped virus into CA3. rAAV2-retro results in efficient retrograde labeling of mature DGCs and permits in vivo two-photon calcium imaging of dentate activity while leaving adult neurogenesis intact. These findings expand on recent reports implicating rAAV-linked toxicity in stem cells and other cell types and suggest that future work using rAAV as an experimental tool in the DG and as a gene therapy for diseases of the central nervous system should be carefully evaluated.

Acute ophthalmoplegia in a patient with anti-GQ1b antibody and chronic facial diplegia

A 56-year-old man with a remote history of bilateral recurrent facial palsies presented with a week of ophthalmoplegia with intact deep tendon reflexes and lack of ataxia, cerebrospinal fluid with albuminocytologic dissociation and elevated serum anti-ganglioside Q1b (GQ1b) IgG antibody. We diagnosed the patient with acute ophthalmoplegia without ataxia, a condition under the spectrum of anti-GQ1b antibody syndromes which also includes Miller Fisher syndrome. Given the rarity of recurrent facial palsies and anti-GQ1b antibody syndromes as well as reports associating facial palsies and this syndrome, we suggest that our case may be an unusual presentation of an anti-GQ1b antibody syndrome beginning with recurrent facial palsies several years prior to ophthalmoplegia. Prior studies of human nerves provide insight into the pathophysiology, including ganglioside distribution and cross-reactivities underlying the heterogeneity of anti-GQ1b antibody syndromes. This report may expand the differential diagnosis in patients with recurrent facial palsies and broaden the phenotype of anti-GQ1b syndromes.

Breakdown of spatial coding and interneuron synchronization in epileptic mice

Temporal lobe epilepsy causes severe cognitive deficits, but the circuit mechanisms remain unknown. Interneuron death and reorganization during epileptogenesis may disrupt the synchrony of hippocampal inhibition. To test this, we simultaneously recorded from the CA1 and dentate gyrus in pilocarpine-treated epileptic mice with silicon probes during head-fixed virtual navigation. We found desynchronized interneuron firing between the CA1 and dentate gyrus in epileptic mice. Since hippocampal interneurons control information processing, we tested whether CA1 spatial coding was altered in this desynchronized circuit, using a novel wire-free miniscope. We found that CA1 place cells in epileptic mice were unstable and completely remapped across a week. This spatial instability emerged around 6 weeks after status epilepticus, well after the onset of chronic seizures and interneuron death. Finally, CA1 network modeling showed that desynchronized inputs can impair the precision and stability of CA1 place cells. Together, these results demonstrate that temporally precise intrahippocampal communication is critical for spatial processing.

Breakdown of spatial coding and interneuron synchronization in epileptic mice

Temporal lobe epilepsy causes severe cognitive deficits, but the circuit mechanisms remain unknown. Interneuron death and reorganization during epileptogenesis may disrupt the synchrony of hippocampal inhibition. To test this, we simultaneously recorded from the CA1 and dentate gyrus in pilocarpine-treated epileptic mice with silicon probes during head-fixed virtual navigation. We found desynchronized interneuron firing between the CA1 and dentate gyrus in epileptic mice. Since hippocampal interneurons control information processing, we tested whether CA1 spatial coding was altered in this desynchronized circuit, using a novel wire-free miniscope. We found that CA1 place cells in epileptic mice were unstable and completely remapped across a week. This spatial instability emerged around 6 weeks after status epilepticus, well after the onset of chronic seizures and interneuron death. Finally, CA1 network modeling showed that desynchronized inputs can impair the precision and stability of CA1 place cells. Together, these results demonstrate that temporally precise intrahippocampal communication is critical for spatial processing.

Efficient Generation of CA3 Neurons from Human Pluripotent Stem Cells Enables Modeling of Hippocampal Connectivity In Vitro

Despite widespread interest in using human induced pluripotent stem cells (hiPSCs) in neurological disease modeling, a suitable model system to study human neuronal connectivity is lacking. Here, we report a comprehensive and efficient differentiation paradigm for hiPSCs that generate multiple CA3 pyramidal neuron subtypes as detected by single-cell RNA sequencing (RNA-seq). This differentiation paradigm exhibits characteristics of neuronal network maturation, and rabies virus tracing revealed synaptic connections between stem cell-derived dentate gyrus (DG) and CA3 neurons in vitro recapitulating the neuronal connectivity within the hippocampus. Because hippocampal dysfunction has been implicated in schizophrenia, we applied DG and CA3 differentiation paradigms to schizophrenia-patient-derived hiPSCs. We detected reduced activity in DG-CA3 co-culture and deficits in spontaneous and evoked activity in CA3 neurons from schizophrenia-patient-derived hiPSCs. Our approach offers critical insights into the network activity aspects of schizophrenia and may serve as a promising tool for modeling diseases with hippocampal vulnerability. VIDEO ABSTRACT.

What is memory? The present state of the engram

The mechanism of memory remains one of the great unsolved problems of biology. Grappling with the question more than a hundred years ago, the German zoologist Richard Semon formulated the concept of the engram, lasting connections in the brain that result from simultaneous “excitations”, whose precise physical nature and consequences were out of reach of the biology of his day. Neuroscientists now have the knowledge and tools to tackle this question, however, and this Forum brings together leading contemporary views on the mechanisms of memory and what the engram means today.

In vivo imaging of dendritic pruning in dentate granule cells

We longitudinally imaged the developing dendrites of adult-born mouse dentate granule cells (DGCs) in vivo and found that they underwent over-branching and pruning. Exposure to an enriched environment (EE) and constraining dendritic growth by disrupting Wnt signaling led to increased branch addition and accelerated growth, which were, however, counteracted by earlier and more extensive pruning. Our results indicate that pruning is regulated in a homeostatic fashion to oppose excessive branching and promote a similar dendrite structure in DGCs.

Paradox of pattern separation and adult neurogenesis: A dual role for new neurons balancing memory resolution and robustness

Hippocampal adult neurogenesis is thought to subserve pattern separation, the process by which similar patterns of neuronal inputs are transformed into distinct neuronal representations, permitting the discrimination of highly similar stimuli in hippocampus-dependent tasks. However, the mechanism by which immature adult-born dentate granule neurons cells (abDGCs) perform this function remains unknown. Two theories of abDGC function, one by which abDGCs modulate and sparsify activity in the dentate gyrus and one by which abDGCs act as autonomous coding units, are generally suggested to be mutually exclusive. This review suggests that these two mechanisms work in tandem to dynamically regulate memory resolution while avoiding memory interference and maintaining memory robustness.

Two-photon excitation microscopy and its applications in neuroscience

Two-photon excitation (2PE) overcomes many challenges in fluorescence microscopy. Compared to confocal microscopy, 2PE microscopy improves depth penetration, owing to the longer excitation wavelength required and to the ability to collect scattered emission photons as a useful signal. It also minimizes photodamage because lower energy photons are used and because fluorescence is confined to the geometrical focus of the laser spot. 2PE is therefore ideal for high-resolution, deep-tissue, time-lapse imaging of dynamic processes in cell biology. Here, we provide examples of important applications of 2PE for in vivo imaging of neuronal structure and signals; we also describe how it can be combined with optogenetics or photolysis of caged molecules to simultaneously probe and control neuronal activity.

Stick-and-diffuse and caged diffusion: a comparison of two models of synaptic vesicle dynamics

Two models were recently proposed to enable us to understand the dynamics of synaptic vesicles in hippocampal neurons. In the caged diffusion model, the vesicles diffuse in small circular cages located randomly in the bouton, while in the stick-and-diffuse model the vesicles bind and release from a cellular cytomatrix. In this article, we obtain analytic expressions for the fluorescence correlation spectroscopy (FCS) autocorrelation function for the two models and test their predictions against our earlier FCS measurements of the vesicle dynamics. We find that the stick-and-diffuse model agrees much better with the experiment. We find also that, due to the slow dynamics of the vesicles, the finite experimental integration time has an important effect on the FCS autocorrelation function and demonstrate its effect for the different models. The two models of the dynamics are also relevant to other cellular environments where mobile species undergo slow diffusionlike motion in restricted spaces or bind and release from a stationary substrate.

Probing vesicle dynamics in single hippocampal synapses

We use fluorescence correlation spectroscopy and fluorescence recovery after photobleaching to study vesicle dynamics inside the synapses of cultured hippocampal neurons labeled with the fluorescent vesicle marker FM 1-43. These studies show that when the cell is electrically at rest, only a small population of vesicles is mobile, taking seconds to traverse the synapse. Applying the phosphatase inhibitor okadaic acid causes vesicles to diffuse freely, moving 30 times faster than vesicles in control synapses. These results suggest that vesicles move sluggishly due to binding to elements of the synaptic cytomatrix and that this binding is altered by phosphorylation. Motivated by these results, a model is constructed consisting of diffusing vesicles that bind reversibly to the cytomatrix. This stick-and-diffuse model accounts for the fluorescence correlation spectroscopy and fluorescence recovery after photobleaching data, and also predicts the well-known exponential refilling of the readily releasable pool. Our measurements suggest that the movement of vesicles to the active zone is the rate-limiting step in this process.