Characterization of glutamatergic neurons in the rat atrial intrinsic cardiac ganglia that project to the cardiac ventricular wall

Molecular and functional characterization of the mouse intracardiac nervous system

BACKGROUND

The intracardiac nervous system (ICNS) refers to clusters of neurons, located within the heart, which participate to the neuronal regulation of cardiac functions and which are involved in the initiation of cardiac arrhythmias. Therefore, deciphering its role in cardiac physiology and physiopathology is mandatory.

OBJECTIVE

The aim of this study is to provide a phenotypic, electrophysiological and pharmacological characterization of the mouse ICNS, which is still poorly characterized.

METHODS

Global cardiac innervation and phenotypic diversity were investigated using immunohistochemistry on cleared murine heart and on tissue sections. Patch clamp technique was used for electrophysiological and pharmacological characterization of isolated mouse intracardiac neurons.

RESULTS

We have identified the expression of seven distinct neuronal markers within mouse ICNS, thus proving the neurochemical diversity of this network. Of note, it was the first time that the existence of neurons expressing the calcium binding protein calbindin, the neuropeptide Y (NPY) and the cocain and amphetamine regulated transcript (CART) peptide, was described in the mouse. Electrophysiological studies also revealed the existence of four different neuronal populations based on their electrical behavior. Finally, we showed that these neurons can be modulated by several neuromodulators.

CONCLUSION

This study showed that mouse ICNS presents a molecular and functional complexity similar to other species, and is therefore a suitable model to decipher the role of individual neuronal subtypes regarding the modulation of cardiac function and the initiation of cardiac arrhythmias.

Cell-surface tethered promiscuous biotinylators enable comparative small-scale surface proteomic analysis of human extracellular vesicles and cells

Characterization of cell surface proteome differences between cancer and healthy cells is a valuable approach for the identification of novel diagnostic and therapeutic targets. However, selective sampling of surface proteins for proteomics requires large samples (>10e6 cells) and long labeling times. These limitations preclude analysis of material-limited biological samples or the capture of rapid surface proteomic changes. Here, we present two labeling approaches to tether exogenous peroxidases (APEX2 and HRP) directly to cells, enabling rapid, small-scale cell surface biotinylation without the need to engineer cells. We used a novel lipidated DNA-tethered APEX2 (DNA-APEX2), which upon addition to cells promoted cell agnostic membrane-proximal labeling. Alternatively, we employed horseradish peroxidase (HRP) fused to the glycan-binding domain of wheat germ agglutinin (WGA-HRP). This approach yielded a rapid and commercially inexpensive means to directly label cells containing common N-Acetylglucosamine (GlcNAc) and sialic acid glycans on their surface. The facile WGA-HRP method permitted high surface coverage of cellular samples and enabled the first comparative surface proteome characterization of cells and cell-derived small extracellular vesicles (EVs), leading to the robust quantification of 953 cell and EV surface annotated proteins. We identified a newly recognized subset of EV-enriched markers, as well as proteins that are uniquely upregulated on Myc oncogene-transformed prostate cancer EVs. These two cell-tethered enzyme surface biotinylation approaches are highly advantageous for rapidly and directly labeling surface proteins across a range of material-limited sample types.

Subcellular localization of neuronal nuclei (NeuN) antigen in size and calcitonin gene-related peptide (CGRP) populations of dorsal root ganglion (DRG) neurons during acute peripheral inflammation

Pseudo-unipolar cell bodies of somatosensory primary neurons are located in the dorsal root ganglia (DRG). The somatic and peripheral domains of DRG neurons are often studied in sensory pain research to understand molecular mechanisms involved in the activation of pain and maintenance of inflammation. Adjuvant-induced arthritis (AIA) is an inflammatory model that elicits a robust and rapid onset immune response with a maximal swelling period of 24-48 h and persisting for several weeks. The AIA model in the hind paw of the rat elicits a potent inflammatory response of the dermis and epidermis, leading to protein expression changes for sensitization of many DRG neurons; however, it is unknown if the AIA model in the hind paw of the rat induces DRG neuronal injury, necrosis, or apoptosis at the somatic level. Neuronal nuclei (NeuN) antigen is a biomarker for post-mitotic neurons, neuronal identification, protein alterations, injury, and loss. Calcitonin gene-related peptide (CGRP) is expressed in C and Aδ DRG neurons, a subset of DRG neurons known to play a role in peripheral sensitization. The focus of this research was to evaluate the expression pattern of NeuN immunoreactivity, in size (soma) and CGRP subpopulations of DRG neurons in naïve and inflamed groups. Confirmed by both immunofluorescence and immunoprecipitation, DRG neuronal expression of NeuN was localized to nuclear and cytoplasmic subcellular compartments. NeuN increased within the nucleus of small CGRP positive DRG neurons during inflammation, indicating a potential role for NeuN in a subset of nociceptive neurons.

Evaluation of the bilateral cardiac afferent distribution at the spinal and vagal ganglia by retrograde labeling

The identity of sensory neurons innervating the heart tissue and the extent of information reported to the brain via these neurons are poorly understood. In order to evaluate the multidimensional distribution and abundance of the cardiac spinal and vagal afferents, we assessed the retrograde labeling efficiency of various tracers, and mapped the cardiac afferents qualitatively and quantitatively at the bilateral nodose ganglia (NGs) and dorsal root ganglia (DRGs). From the five different retrograde tracers evaluated, Di-8-ANEPPQ yielded reproducibly the highest labeling efficiency of cardiac afferents. We demonstrated specific cardiac afferents at NGs and C4 to T11 DRG segments. Next, the 2D reconstruction of the tissue sections and 3D imaging of the whole NGs and DRGs revealed homogeneous and bilateral distribution of cardiac afferents. The quantitative analyses of the labeled cardiac afferents demonstrated approximately 5-6% of the soma in NGs that were equally distributed bilaterally. The neuronal character of Di-8-ANEPPQ labeled cells were validated by coimmunostaning with pan-neuronal marker Tuj-1. In addition, the cell diameters of labeled cardiac sensory neurons were found smaller than 20 μm, implying the nociceptor phenotype confirmed by co-labeling with TRPV1 and Di-8-ANEPPQ. Importantly, co-labeling with two distinct tracers Di-8-ANEPPQ and WGA-647 demonstrated exclusively the same cardiac afferents in DRGs and NGs, validating our findings. Collectively, our findings revealed the cardiac afferents in NGs bilaterally and DRGs with the highest labeling efficiency reported, spatial distribution and quantitation at both 2D and 3D levels, furthering our understanding of this novel neuron population.

Chemogenetic activation of intracardiac cholinergic neurons improves cardiac function in pressure overload-induced heart failure

Heart failure (HF) is characterized by autonomic imbalance with sympathetic hyperactivity and loss of parasympathetic tone. Intracardiac ganglia (ICG) neurons represent the final common pathway for vagal innervation of the heart and strongly regulate cardiac functions. This study tests whether ICG cholinergic neuron activation mitigates the progression of cardiac dysfunction and reduces mortality that occurs in HF. HF was induced by transaortic constriction (TAC) in male transgenic Long-Evans rats expressing Cre recombinase within choline acetyltransferase (ChAT) neurons. ChAT neurons were selectively activated by expression and activation of excitatory designer receptors exclusively activated by designer receptors (DREADDs) by clozapine-N-oxide (TAC + treatment and sham-treated groups). Control animals expressed DREADDs but received saline (sham and TAC groups). A separate set of animals were telemetry instrumented to record blood pressure (BP) and heart rate (HR). Acute activation of ICG neurons resulted in robust reductions in BP (∼20 mmHg) and HR (∼100 beats/min). All groups of animals were subjected to weekly echocardiography and treadmill stress tests from 3 to 6 wk post-TAC/sham surgery. Activation of ICG cholinergic neurons reduced the left ventricular systolic dysfunction (reductions in ejection fraction, fractional shortening, stroke volume, and cardiac output) and cardiac autonomic dysfunction [reduced HR recovery (HRR) post peak effort] observed in TAC animals. Additionally, activation of ICG ChAT neurons reduced mortality by 30% compared with untreated TAC animals. These data suggest that ICG cholinergic neuron activation reduces cardiac dysfunction and improves survival in HF, indicating that ICG neuron activation could be a novel target for treating HF.NEW & NOTEWORTHY Intracardiac ganglia form the final common pathway for the parasympathetic innervation of the heart. This study has used a novel chemogenetic approach within transgenic ChAT-Cre rats [expressing only Cre-recombinase in choline acetyl transferase (ChAT) neurons] to selectively increase intracardiac cholinergic parasympathetic activity to the heart in a pressure overload-induced heart failure model. The findings from this study confirm that selective activation of intracardiac cholinergic neurons lessens cardiac dysfunction and mortality seen in heart failure, identifying a novel downstream cardiac-selective target for increasing cardioprotective parasympathetic activity in heart failure.

Glutaminase Increases in Rat Dorsal Root Ganglion Neurons after Unilateral Adjuvant-Induced Hind Paw Inflammation

Glutamate is a neurotransmitter used at both the peripheral and central terminals of nociceptive primary sensory neurons, yet little is known concerning regulation of glutamate metabolism during peripheral inflammation. Glutaminase (GLS) is an enzyme of the glutamate-glutamine cycle that converts glutamine into glutamate for neurotransmission and is implicated in producing elevated levels of glutamate in central and peripheral terminals. A potential mechanism for increased levels of glutamate is an elevation in GLS expression. We assessed GLS expression after unilateral hind paw inflammation by measuring GLS immunoreactivity (ir) with quantitative image analysis of L4 dorsal root ganglion (DRG) neurons after one, two, four, and eight days of adjuvant-induced arthritis (AIA) compared to saline injected controls. No significant elevation in GLS-ir occurred in the DRG ipsilateral to the inflamed hind paw after one or two days of AIA. After four days AIA, GLS-ir was elevated significantly in all sizes of DRG neurons. After eight days AIA, GLS-ir remained elevated in small (<400 µm²), presumably nociceptive neurons. Western blot analysis of the L4 DRG at day four AIA confirmed the elevated GLS-ir. The present study indicates that GLS expression is increased in the chronic stage of inflammation and may be a target for chronic pain therapy.