Nikola Jancovski

Angiotensin Type 1A Receptors in C1 Neurons of the Rostral Ventrolateral Medulla Modulate the Pressor Response to Aversive Stress

The rise in blood pressure during an acute aversive stress has been suggested to involve activation of angiotensin type 1A receptors (AT1ARs) at various sites within the brain, including the rostral ventrolateral medulla. In this study we examine the involvement of AT1ARs associated with a subclass of sympathetic premotor neurons of the rostral ventrolateral medulla, the C1 neurons. The distribution of putative AT1AR-expressing cells was mapped throughout the brains of three transgenic mice with a bacterial artificial chromosome-expressing green fluorescent protein under the control of the AT1AR promoter. The overall distribution correlated with that of the AT1ARs mapped by other methods and demonstrated that the majority of C1 neurons express the AT1AR. Cre-recombinase expression in C1 neurons of AT1AR-floxed mice enabled demonstration that the pressor response to microinjection of angiotensin II into the rostral ventrolateral medulla is dependent upon expression of the AT1AR in these neurons. Lentiviral-induced expression of wild-type AT1ARs in C1 neurons of global AT1AR knock-out mice, implanted with radiotelemeter devices for recording blood pressure, modulated the pressor response to aversive stress. During prolonged cage-switch stress, expression of AT1ARs in C1 neurons induced a greater sustained pressor response when compared to the control viral-injected group (22 ± 4 mmHg for AT1AR vs 10 ± 1 mmHg for GFP; p < 0.001), which was restored toward that of the wild-type group (28 ± 2 mmHg). This study demonstrates that AT1AR expression by C1 neurons is essential for the pressor response to angiotensin II and that this pathway plays an important role in the pressor response to aversive stress.

Central angiotensinergic mechanisms associated with hypertension.

Following its generation by both systemic and tissue-based renin-angiotensin systems, angiotensin II interacts with specific, G-protein coupled receptors to modulate multiple physiological systems, including the cardiovascular system. Genetic models in which the different components of the renin-angiotensin system have been deleted show large changes in resting blood pressure. Interruption of the generation of angiotensin II, or its interaction with these receptors, decreases blood pressure in hypertensive humans and experimental animal models of hypertension. Whilst the interaction of angiotensin II with the kidney is pivotal in this modulation of blood pressure, an involvement of the system in other tissues is important. Both systemic angiotensins, acting via the blood-brain barrier deficient circumventricular organs, and centrally-generated angiotensin modulate cardiovascular control by regulating fluid and electrolyte ingestion, autonomic activity and neuroendocrine function. This review discusses the pathways in the brain that are involved in this regulation of blood pressure as well as examining the sites in which altered angiotensin function might contribute to the development and maintenance of high blood pressure.

Excessive Respiratory Modulation of Blood Pressure Triggers Hypertension

The etiology of hypertension, the worlds biggest killer, remains poorly understood, with treatments targeting the established symptom, not the cause. The development of hypertension involves increased sympathetic nerve activity that, in experimental hypertension, may be driven by excessive respiratory modulation. Using selective viral and cell lesion techniques, we identify adrenergic C1 neurons in the medulla oblongata as critical for respiratory-sympathetic entrainment and the development of experimental hypertension. We also show that a cohort of young, normotensive humans, selected for an exaggerated blood pressure response to exercise and thus increased hypertension risk, has enhanced respiratory-related blood pressure fluctuations. These studies pinpoint a specific neuronal target for ameliorating excessive sympathetic activity during the developmental phase of hypertension and identify a group of pre-hypertensive subjects that would benefit from targeting these cells.

Stimulation of Angiotensin Type 1A Receptors on Catecholaminergic Cells Contributes to Angiotensin-Dependent Hypertension

Hypertension contributes to multiple forms of cardiovascular disease and thus morbidity and mortality. The mechanisms inducing hypertension remain unclear although the involvement of homeostatic systems, such as the renin–angiotensin and sympathetic nervous systems, is established. A pivotal role of the angiotensin type 1 receptor in the proximal tubule of the kidney for the development of experimental hypertension is established. Yet, other systems are involved. This study tests whether the expression of angiotensin type 1A receptors in catecholaminergic cells contributes to hypertension development. Using a Cre-lox approach, we deleted the angiotensin type 1A receptor from all catecholaminergic cells. This deletion did not alter basal metabolism or blood pressure but delayed the onset of angiotensin-dependent hypertension and reduced the maximal response. Cardiac hypertrophy was also reduced. The knockout mice showed attenuated activation of the sympathetic nervous system during angiotensin II infusion as measured by spectral analysis of the blood pressure. Increased reactive oxygen species production was observed in forebrain regions, including the subfornical organ, of the knockout mouse but was markedly reduced in the rostral ventrolateral medulla. These studies demonstrate that stimulation of the angiotensin type 1A receptor on catecholaminergic cells is required for the full development of angiotensin-dependent hypertension and support an important role for the sympathetic nervous system in this model.

Catecholaminergic C3 Neurons Are Sympathoexcitatory and Involved in Glucose Homeostasis

Brainstem catecholaminergic neurons play key roles in the autonomic, neuroendocrine, and behavioral responses to glucoprivation, yet the functions of the individual groups are not fully understood. Adrenergic C3 neurons project widely throughout the brain, including densely to sympathetic preganglionic neurons in the spinal cord, yet their function is completely unknown. Here we demonstrate in rats that optogenetic stimulation of C3 neurons induces sympathoexcitatory, cardiovasomotor functions. These neurons are activated by glucoprivation, but unlike the C1 cell group, not by hypotension. The cardiovascular activation induced by C3 neurons is less than that induced by optogenetic stimulation of C1 neurons; however, combined stimulation produces additive sympathoexcitatory and cardiovascular effects. The varicose axons of C3 neurons largely overlap with those of C1 neurons in the region of sympathetic preganglionic neurons in the spinal cord; however, regional differences point to effects on different sympathetic outflows. These studies definitively demonstrate the first known function of C3 neurons as unique cardiovasomotor stimulatory cells, embedded in the brainstem networks regulating cardiorespiratory activity and the response to glucoprivation.

Angiotensin type 1A receptor expression in C1 neurons of the rostral ventrolateral medulla contributes to the development of angiotensin‐dependent hypertension

What is the central question of this study? This study addresses the mechanism by which deletion of angiotensin II type 1A receptors from catecholaminergic neurons reduces angiotensin‐dependent hypertension, as well as the identity of the cells involved. What is the main finding and its importance? Deletion of angiotensin II type 1A receptors from catecholaminergic neurons results in reduced sympathetic nerve activation and fluid and electrolyte retention during angiotensin infusion. The C1 neurons of the rostral ventrolateral medulla are involved in the later phase of the hypertension. We demonstrate that at least two different populations of catecholaminergic neurons are involved in the sympathetic nerve activation required for the full development of angiotensin‐dependent hypertension.

Selective NaV1.1 activation rescues Dravet syndrome mice from seizures and premature death

Significance Spider venom is a rich source of peptides, many targeting ion channels. We assessed a venom peptide, Hm1a, as a potential targeted therapy for Dravet syndrome, the genetic epilepsy linked to a mutation in the gene encoding the sodium channel alpha subunit NaV1.1. Cell-based assays showed Hm1a was selective for hNaV1.1 over other sodium and potassium channels. Utilizing a mouse model of Dravet syndrome, Hm1a restored inhibitory neuron function and significantly reduced seizures and mortality in heterozygote mice. Evidence from the structure of Hm1a and modeling suggest Hm1a interacts with NaV1.1 inactivation domains, providing a structural correlate of the functional mechanisms. This proof-of-concept study provides a promising strategy for future drug development in genetic epilepsy and other neurogenetic disorders. Dravet syndrome is a catastrophic, pharmacoresistant epileptic encephalopathy. Disease onset occurs in the first year of life, followed by developmental delay with cognitive and behavioral dysfunction and substantially elevated risk of premature death. The majority of affected individuals harbor a loss-of-function mutation in one allele of SCN1A, which encodes the voltage-gated sodium channel NaV1.1. Brain NaV1.1 is primarily localized to fast-spiking inhibitory interneurons; thus the mechanism of epileptogenesis in Dravet syndrome is hypothesized to be reduced inhibitory neurotransmission leading to brain hyperexcitability. We show that selective activation of NaV1.1 by venom peptide Hm1a restores the function of inhibitory interneurons from Dravet syndrome mice without affecting the firing of excitatory neurons. Intracerebroventricular infusion of Hm1a rescues Dravet syndrome mice from seizures and premature death. This precision medicine approach, which specifically targets the molecular deficit in Dravet syndrome, presents an opportunity for treatment of this intractable epilepsy.

Orphan receptor GPR37L1 contributes to the sexual dimorphism of central cardiovascular control

BackgroundOver 100 mammalian G protein-coupled receptors are yet to be matched with endogenous ligands; these so-called orphans are prospective drug targets for the treatment of disease. GPR37L1 is one such orphan, abundant in the brain and detectable as mRNA in the heart and kidney. GPR37L1 ablation was reported to cause hypertension and left ventricular hypertrophy, and thus, we sought to further define the role of GPR37L1 in blood pressure homeostasis.MethodsWe investigated the cardiovascular effects of GPR37L1 using wild-type (GPR37L1wt/wt) and null (GPR37L1KO/KO) mice established on a C57BL/6J background, both under baseline conditions and during AngII infusion. We profiled GPR37L1 tissue expression, examining the endogenous receptor by immunoblotting and a β-galactosidase reporter mouse by immunohistochemistry.ResultsGPR37L1 protein was abundant in the brain but not detectable in the heart and kidney. We measured blood pressure in GPR37L1wt/wt and GPR37L1KO/KO mice and found that deletion of GPR37L1 causes a female-specific increase in systolic, diastolic, and mean arterial pressures. When challenged with short-term AngII infusion, only male GPR37L1KO/KO mice developed exacerbated left ventricular hypertrophy and evidence of heart failure, while the female GPR37L1KO/KO mice were protected from cardiac fibrosis.ConclusionsDespite its absence in the heart and kidney, GPR37L1 regulates baseline blood pressure in female mice and is crucial for cardiovascular compensatory responses in males. The expression of GPR37L1 in the brain, yet absence from peripheral cardiovascular tissues, suggests this orphan receptor is a hitherto unknown contributor to central cardiovascular control.

Role of angiotensin type 1A receptors in the sympathetic activation of angiotensin-dependent hypertension

head-up tilt test, referred for immune evaluation to the AllergyImmunology clinic at our institution (2007–2014). Their clinical history and immune laboratory studies (e.g. immunoglobulin levels, specific antibody responses, ANA, anti-SSA/SSO, dysautonomia autoimmune profile, serum cytokines) were reviewed. Results: Twenty patients (3 males, 17 females), mean age of 19.9 years (15–34), were evaluated. Symptoms suggestive of immune dysfunction included arthralgia (7), joint swelling (3), backache (2), dermatitis (5), alopecia (2), sicca (4), Raynaud’s (1), and recurrent infections (5). Five patients (25%) had autoantibodies (i.e. anti-SSA, ANA, anti-GAD65, folate receptor autoantibodies), and 9 of 14 patients (64%), had elevated serum cytokines. Severe dysautonomia was present in one patient with IDDM and 4 patients with B-cell immunodeficiency, all of which had other coexisting autoimmune problems. Conclusions: Young patients with dysautonomia may suffer from immune dysregulation and the severity of their disease is associated with severe dysautonomia. It remains to be determined if this immune dysregulation is a primary or secondary process.

Baroreceptor reflex control of heart rate in angiotensin type 1A receptor knockout mice

The baroreceptor reflex dampens the short‐term fluctuations in blood pressure by feedback modulation of heart rate (HR) and vascular resistance. Impairment of this reflex has been observed in hypertension and heart failure. Angiotensin II, a blood borne hormone, acts via its type 1A receptor to attenuate the baroreceptor reflex and this reflex is reported to be dramatically altered in angiotensin type 1A receptor knockout mice. This study sought to further investigate changes in the arterial and cardiopulmonary baroreceptor reflex control of HR in angiotensin II type 1A receptor knocked out mice. In artificially ventilated, isoflurane anesthetized mice, the arterial and cardiopulmonary baroreceptor reflexes were activated via injection or slow infusions, respectively, of phenylephrine and sodium nitroprusside through the jugular vein. We observed no impairment of either the arterial or cardiopulmonary baroreceptor reflex control of HR in angiotensin type 1A receptor knockout mice.