Interaction between Angiotensin Type 1, Type 2, and Mas Receptors to Regulate Adult Neurogenesis in the Brain Ventricular-Subventricular Zone

The cannabinoid CB1 receptor interacts with the angiotensin AT2 receptor. Overexpression of AT2-CB1 receptor heteromers in the striatum of 6-hydroxydopamine hemilesioned rats

It is of particular interest the potential of cannabinoid and angiotensin receptors as targets in the therapy of Parkinson's disease (PD). While endocannabinoids are neuromodulators that act through the CB₁ and CB₂ cannabinoid receptors, the renin angiotensin-system is relevant for regulation of the correct functioning of several brain circuits. Resonance energy transfer assays in a heterologous system showed that the CB₁ receptor (CB₁R) can directly interact with the angiotensin AT₂ receptor (AT₂R). Coactivation of the two receptors results in increased G_i-signaling. The AT₂-CB₁ receptor heteromer imprint consists of a blockade of AT₂R-mediated signaling by rimonabant, a CB₁R antagonist. Interestingly, the heteromer imprint, discovered in the heterologous system, was also found in primary striatal neurons thus demonstrating the expression of the heteromer in these cells. In situ proximity ligation assays confirmed the occurrence of AT₂-CB₁ receptor heteromers in striatal neurons. In addition, increased expression of the AT₂-CB₁ receptor heteromeric complexes was detected in the striatum of a rodent PD model consisting of rats hemilesioned using 6-hydroxydopamine. Expression of the heteromer was upregulated in the striatum of lesioned animals and, also, of lesioned animals that upon levodopa treatment became dyskinetic. In contrast, there was no upregulation in the striatum of lesioned rats that did not become dyskinetic upon chronic levodopa treatment. The results suggest that therapeutic developments focused on the CB₁R should consider that this receptor can interact with the AT₂R, which in the CNS is involved in mechanisms related to addictive behaviors and to neurodegenerative and neuroinflammatory diseases.

Angiotensin receptor blocker use is associated with upregulation of the memory-protective angiotensin type 4 receptor (AT4R) in the postmortem brains of individuals without cognitive impairment

The reported primary dementia-protective benefits of angiotensin II type 1 receptor (AT1R) blockers (ARB) are believed, at least in part, to arise from systemic effects on blood pressure. However, there is a specific and independently regulated brain renin-angiotensin system (RAS). Brain RAS acts mainly through three receptor subtypes; AT1R, AT2R, and AT4R. The AT1R promotes inflammation and mitochondrial reactive oxygen species generation. AT2R increases nitric oxide. AT4R is essential for dopamine and acetylcholine release. It is unknown whether ARB use is associated with changes in the brain RAS. Here, we compared the impact of treatment with ARB on not cognitively impaired individuals and individuals with Alzheimer's dementia using postmortem frontal-cortex samples of age- and sex-matched participants (70-90 years old, n = 30 in each group). We show that ARB use is associated with higher brain AT4R, lower oxidative stress, and amyloid-β burden in NCI participants. In AD, ARB use was associated with lower brain AT1R but had no impact on inflammation, oxidative stress, or amyloid-β burden. Our results may suggest a potential role for AT4R in the salutary effects for ARB on the brains of not cognitively impaired older adults.

Capsaicin combined with dietary fiber prevents high-fat diet associated aberrant lipid metabolism by improving the structure of intestinal flora

Capsaicin (CAP) and dietary fibers are natural active ingredients that given separately do positively affect obesity and metabolic diseases. However, it was unknown whether their combined administration might further improve blood lipids and gut flora composition. To test this hypothesis we administered capsaicin plus dietary fibers (CAP + DFs) to male rats on a high-fat diet and analyzed any changes in the intestinal microbiota make up, metabolites, and blood indexes. Our results showed that combining CAP with dietary fibers more intensely reduced total cholesterol (TC) and low-density lipoprotein cholesterol (LDL-C). CAP + DFs also increased gut bacteria variety, and the abundance of several beneficial bacterial strains, including Allobaculum and Akkermansia, while reducing harmful strains such as Desulfovibrio. Additionally, CAP + DFs significantly increased arginine levels and caused short-chain fatty acids accumulation in the contents of the cecal portion of rats' gut. In conclusion, notwithstanding the rats were kept on a high-fat diet, adding CAP + DFs to the chow further improved, as compared with CAP alone, the lipidemia and increased the gut beneficial bacterial strains, while reducing the harmful ones.

Inflammatory Role of Milk Fat Globule-Epidermal Growth Factor VIII in Age-Associated Arterial Remodeling

Background

Age‐associated aortic remodeling includes a marked increase in intimal medial thickness (IMT), associated with signs of inflammation. Although aortic wall milk fat globule–epidermal growth factor VIII (MFG‐E8) increases with age, and is associated with aortic inflammation, it is not known whether MFG‐E8 is required for the age‐associated increase in aortic IMT. Here, we tested whether MFG‐E8 is required for the age‐associated increase in aortic IMT.

Methods and Results

To determine the role of MFG‐E8 in the age‐associated increase of IMT, we compared aortic remodeling in adult (20‐week) and aged (96‐week) MFG‐E8 (−/−) knockout and age matched wild‐type (WT) littermate mice. The average aortic IMT increased with age in the WT from 50±10 to 70±20 μm (P<0.0001) but did not significantly increase with age in MFG‐E8 knockout mice. Because angiotensin II signaling is implicated as a driver of age‐associated increase in IMT, we infused 30‐week‐old MFG‐E8 knockout and age‐matched littermate WT mice with angiotensin II or saline via osmotic mini‐pumps to determine whether MFG‐E8 is required for angiotensin II–induced aortic remodeling. (1) In WT mice, angiotensin II infusion substantially increased IMT, elastic lamina degradation, collagen deposition, and the proliferation of vascular smooth muscle cells; in contrast, these effects were significantly reduced in MFG‐E8 KO mice; (2) On a molecular level, angiotensin II treatment significantly increased the activation and expression of matrix metalloproteinase type 2, transforming growth factor beta 1, and its downstream signaling molecule phosphorylated mother against decapentaplegic homolog 2, and collagen type I production in WT mice; however, in the MFG‐E8 knockout mice, these molecular effects were significantly reduced; and (3) in WT mice, angiotensin II increased levels of aortic inflammatory markers phosphorylated nuclear factor‐kappa beta p65, monocyte chemoattractant protein 1, tumor necrosis factor alpha, intercellular adhesion molecule 1, and vascular cell adhesion molecule 1 molecular expression, while in contrast, these inflammatory markers did not change in knockout mice.

Conclusions

Thus, MFG‐E8 is required for both age‐associated proinflammatory aortic remodeling and also for the angiotensin II–dependent induction in younger mice of an aortic inflammatory phenotype observed in advanced age. Targeting MFG‐E8 would be a novel molecular approach to curb adverse arterial remodeling.

Chicken Muscle-Derived ACE2 Upregulating Peptide VVHPKESF Inhibits Angiotensin II-Stimulated Inflammation in Vascular Smooth Muscle Cells via the ACE2/Ang (1-7)/MasR Axis

This study aimed to evaluate the modulatory effects of four chicken muscle-derived peptides [VRP, LKY, VRY, and VVHPKESF (V-F)] on angiotensin II (Ang II)-induced inflammation in rat vascular smooth muscle A7r5 cells. Only V-F could significantly attenuate Ang II-stimulated inflammation via the inhibition of NF-κB and p38 MAPK signaling, being dependent on the Mas receptor (MasR) not on the Ang II type 1 or type 2 receptor (AT₁R or AT₂R). V-F accelerated Ang II degradation by enhancing cellular ACE2 activity, which was due to ACE2 upregulation other than a direct ACE2 activation. These findings demonstrated that V-F ameliorated Ang II-induced inflammation in A7r5 cells via the ACE2/Ang (1-7)/MasR axis. Three peptide metabolites of V-F─VHPKESF, PKESF, and SF─were identified but were not considered major contributors to V-F's bioactivity. The regulation of peptide V-F on vascular inflammation supported its functional food or nutraceutical application in the prevention and treatment of hypertension and cardiovascular diseases.

Higher Angiotensin II Type 1 Receptor Levels and Activity in the Postmortem Brains of Older Persons with Alzheimer's Dementia

Aging is a key risk factor in Alzheimer's dementia (AD) development and progression. The primary dementia-protective benefits of angiotensin II subtype 1 receptor (AT1R) blockers are believed to arise from systemic effects on blood pressure. However, a brain-specific renin-angiotensin system (b-RAS) exists, which can be altered by AT1R blockers. Brain RAS acts mainly through 3 angiotensin receptors: AT1R, AT2R, and AT4R. Changes in these brain angiotensin receptors may accelerate the progression of AD. Using postmortem frontal cortex brain samples of age- and sex-matched cognitively normal individuals (n = 30) and AD patients (n = 30), we sought to dissect the b-RAS changes associated with AD and assess how these changes correlate with brain markers of oxidative stress, inflammation, and mitochondrial dysfunction as well as amyloid-β and paired helical filament tau pathologies. Our results show higher protein levels of the pro-inflammatory AT1R and phospho-ERK (pERK) in the brains of AD participants. Brain AT1R levels and pERK correlated with higher oxidative stress, lower cognitive performance, and higher tangle and amyloid-β scores. This study identifies molecular changes in b-RAS and offers insight into the role of b-RAS in AD-related brain pathology.

NRF2 Activation and Downstream Effects: Focus on Parkinson's Disease and Brain Angiotensin

Reactive oxygen species (ROS) are signalling molecules used to regulate cellular metabolism and homeostasis. However, excessive ROS production causes oxidative stress, one of the main mechanisms associated with the origin and progression of neurodegenerative disorders such as Parkinson's disease. NRF2 (Nuclear Factor-Erythroid 2 Like 2) is a transcription factor that orchestrates the cellular response to oxidative stress. The regulation of NRF2 signalling has been shown to be a promising strategy to modulate the progression of the neurodegeneration associated to Parkinson's disease. The NRF2 pathway has been shown to be affected in patients with this disease, and activation of NRF2 has neuroprotective effects in preclinical models, demonstrating the therapeutic potential of this pathway. In this review, we highlight recent advances regarding the regulation of NRF2, including the effect of Angiotensin II as an endogenous signalling molecule able to regulate ROS production and oxidative stress in dopaminergic neurons. The genes regulated and the downstream effects of activation, with special focus on Kruppel Like Factor 9 (KLF9) transcription factor, provide clues about the mechanisms involved in the neurodegenerative process as well as future therapeutic approaches.

Dopamine regulates adult neurogenesis in the ventricular-subventricular zone via dopamine D3 angiotensin type 2 receptor interactions

Adult neurogenesis is a dynamic and highly regulated process and different studies suggest that dopamine modulates ventricular-subventricular zone (V-SVZ) neurogenesis. However, the specific role of dopamine and the mechanisms/factors underlying its effects on physiological and pathological conditions such as Parkinson's disease (PD) are not fully understood. Recent studies have described counter-regulatory interactions between renin-angiotensin system (RAS) and dopamine in peripheral tissues and in the nigrostriatal system. We have previously demonstrated that angiotensin receptors regulate proliferation and generation of neuroblasts in the rodent V-SVZ. However, possible interactions between dopamine receptors and RAS in the V-SVZ and their role in alterations of neurogenesis in animal models of PD have not been investigated. In V-SVZ cultures, activation of dopamine receptors induced changes in the expression of angiotensin receptors. Moreover, dopamine, via D2-like receptors and particularly D3 receptors, increased generation of neurospheres derived from the V-SVZ and this effect was mediated by angiotensin type-2 (AT2) receptors. In rats, we observed a marked reduction in proliferation and generation of neuroblasts in the V-SVZ of dopamine-depleted animals, and inhibition of AT1 receptors or activation of AT2 receptors restored proliferation and generation of neuroblasts to control levels. Moreover, intrastriatal mesencephalic grafts partially restored proliferation and generation of neuroblasts observed in the V-SVZ of dopamine-depleted rats. Our data revealed that dopamine and angiotensin receptor interactions play a major role in the regulation of V-SVZ and suggest potential beneficial effects of RAS modulators on the regulation of adult V-SVZ neurogenesis.

(R,S)-Ketamine Promotes Striatal Neurogenesis and Sensorimotor Recovery Through Improving Poststroke Depression–Mediated Decrease in Atrial Natriuretic Peptide

Background

Poststroke social isolation could worsen poststroke depression and dampen neurogenesis. (R,S)-ketamine has antidepressant and neuroprotective effects; however, its roles and mechanisms in social isolation–mediated depressive-like behaviors and sensorimotor recovery remain unclear.

Methods

Mice were subjected to transient middle cerebral artery occlusion, and then were pair-housed with ovariectomized female mice or were housed isolated (ISO) starting at 3 days postischemia. ISO mice received 2 weeks of (R,S)-ketamine treatment starting at 14 days postischemia. Primary ependymal epithelial cells and choroid plexus epithelial cells were cultured and treated with recombinant human atrial natriuretic peptide (ANP) protein.

Results

The poststroke social isolation model was successfully established using middle cerebral artery occlusion combined with poststroke isolation, as demonstrated by a more prominent depression-like phenotype in ISO mice compared with pair-housed mice. (R,S)-ketamine reversed ISO-mediated depressive-like behaviors and increased ANP levels in the atrium. The depression-like phenotype was negatively correlated with ANP levels in both the atrium and plasma. Atrial GLP-1 and GLP-1 receptor signaling was essential to the promoting effects of (R,S)-ketamine on the synthesis and secretion of ANP from the atrium in ISO mice. (R,S)-ketamine also increased ANP and TGF-β1 levels in the choroid plexus of ISO mice. Recombinant human ANP increased TGF-β1 levels in both the primarily cultured ependymal epithelial cells and choroid plexus epithelial cells. Furthermore, (R,S)-ketamine increased TGF-β1 levels in the ischemic hemisphere and promoted striatal neurogenesis and sensorimotor recovery via ANP in ISO mice.

Conclusions

(R,S)-ketamine alleviated poststroke ISO-mediated depressive-like behaviors and thus promoted striatal neurogenesis and sensorimotor recovery via ANP.

The Renin-Angiotensin System Modulates Dopaminergic Neurotransmission: A New Player on the Scene

Parkinson’s disease (PD) is an extrapyramidal disorder characterized by neuronal degeneration in several regions of the peripheral and central nervous systems. It is the second most frequent neurodegenerative disease after Alzheimer’s. It has become a major health problem, affecting 1% of the world population over 60 years old and 3% of people beyond 80 years. The main histological findings are intracellular Lewy bodies composed of misfolded α-synuclein protein aggregates and loss of dopaminergic neurons in the central nervous system. Neuroinflammation, apoptosis, mitochondrial dysfunction, altered calcium homeostasis, abnormal protein degradation, and synaptic pathobiology have been put forward as mechanisms leading to cell death, α-synuclein deposition, or both. A progressive loss of dopaminergic neurons in the substantia nigra late in the neurodegeneration leads to developing motor symptoms like bradykinesia, tremor, and rigidity. The renin–angiotensin system (RAS), which is involved in regulating blood pressure and body fluid balance, also plays other important functions in the brain. The RAS is involved in the autocrine and paracrine regulation of the nigrostriatal dopaminergic synapses. Dopamine depletion, as in PD, increases angiotensin II expression, which stimulates or inhibits dopamine synthesis and is released via AT1 or AT2 receptors. Furthermore, angiotensin II AT1 receptors inhibit D1 receptor activation allosterically. Therefore, the RAS may have an important modulating role in the flow of information from the brain cortex to the basal ganglia. High angiotensin II levels might even aggravate neurodegeneration, activating the nicotinamide adenine dinucleotide phosphate (NADPH) oxidase complex, which leads to increased reactive oxygen species production.

Purinergic Signaling of ATP in COVID-19 Associated Guillain-Barré Syndrome

Declared as a global public health emergency, coronavirus disease 2019 (COVID-19) is presented as a disease of the respiratory tract, although severe cases can affect the entire organism. Several studies have shown neurological symptoms, ranging from dizziness and loss of consciousness to cerebrovascular and neurodegenerative diseases. In this context, Guillain-Barré syndrome, an immune-mediated inflammatory neuropathy, has been closely associated with critical cases of infection with "severe acute respiratory syndrome of coronavirus 2" (SARS-CoV-2), the etiological agent of COVID-19. Its pathophysiology is related to a generalized inflammation that affects the nervous system, but neurotropism was also revealed by the new coronavirus, which may increase the risk of neurological sequel, as well as the mortality of the disease. Thus, considering the comorbidities that SARS-CoV-2 infection can promote, the modulation of purinergic signaling can be applied as a potential therapy. In this perspective, given the role of adenosine triphosphate (ATP) in neural intercommunication, the P2X7 receptor (P2X7R) acts on microglia cells and its inhibition may be able to reduce the inflammatory condition of neurodegenerative diseases. Finally, alternative measures to circumvent the reality of the COVID-19 pandemic need to be considered, given the severity of critical cases and the viral involvement of multiple organs.

The role of peptidase neurolysin in neuroprotection and neural repair after stroke

Current experimental stroke research has evolved to focus on detailed understanding of the brain’s self-protective and restorative mechanisms, and harness this knowledge for development of new therapies. In this context, the role of peptidases and neuropeptides is of growing interest. In this focused review, peptidase neurolysin (Nln) and its extracellular peptide substrates are briefly discussed in relation to pathophysiology of ischemic stroke. Upregulation of Nln following stroke is viewed as a compensatory cerebroprotective mechanism in the acute phase of stroke, because the main neuropeptides inactivated by Nln are neuro/cerebrotoxic (bradykinin, substance P, neurotensin, angiotensin II, hemopressin), whereas the peptides generated by Nln are neuro/cerebroprotective (angiotensin-(1–7), Leu-/Met-enkephalins). This notion is confirmed by experimental studies documenting aggravation of stroke outcomes in mice after inhibition of Nln following stroke, and dramatic improvement of stroke outcomes in mice overexpressing Nln in the brain. The role of Nln in the (sub)chronic phase of stroke is less clear and it is likely, that this peptidase does not have a major role in neural repair mechanisms. This is because, the substrates of Nln are less uniform in modulating neurorestorative mechanisms in one direction, some appearing to have neural repair enhancing/stimulating potential, whereas others doing the opposite. Future studies focusing on the role of Nln in pathophysiology of stroke should determine its potential as a cerebroprotective target for stroke therapy, because its unique ability to modulate multiple neuropeptide systems critically involved in brain injury mechanisms is likely advantageous over modulation of one pathogenic pathway for stroke pharmacotherapy.

Application of stem cell biology in treating neurodegenerative diseases

BACKGROUND

The appropriate strategies are needed for stimulating the endogenous neurogenesis or introducing extrinsic neural progenitors, which could be harnessed as the regenerative resources for cueing the neurodegenerations. Adult neurogenesis is the endogenous continuing physiology in limited brain regions such as hippocampus, olfactory system, and hypothalamus. Besides adult neurogenesis, induced pluripotent stem cells (iPSCs) induced functional neurons could be another option for regenerative therapies.

OBJECTIVE

Current studies are trying to improve the adult neurogenesis and enable the iPSCs induced neurons into neural regeneration. Methods: Here in this review, we mainly introduced the recent progress in neural stem cell biology and its application in the treatment of the neurodegenerations. We main separated the strategy in summarizing the mediators and potential targets to promoting endogenous neural regeneration and transplantation of neural progenitors.

CONCLUSION

By collecting and comparing the advantages disadvantages between above-mentioned two strategies, we will offer the insight on future development of stem cell therapy in treating neurodegenerative patients.

Angiotensinergic receptors in the medial amygdaloid nucleus differently modulate behavioral responses in the elevated plus-maze and forced swimming test in rats

The brain renin-angiotensin system (RAS) has been implicated in anxiety and depression disorders, but the specific brain sites involved are poorly understood. The medial amygdaloid nucleus (MeA) is involved in expression of behavioral responses. However, despite evidence of the presence of all angiotensinergic receptors in this amygdaloid nucleus, regulation of anxiety- and depressive-like behaviors by angiotensinergic neurotransmissions within the MeA has never been reported. Thus, the present study aimed to investigate the role angiotensin II (AT₁ and AT₂ receptors) and angiotensin-(1-7) (Mas receptor) receptors present within the MeA in behavioral responses in the elevated plus-maze (EPM) and forced swimming test (FST). For this, male Wistar rats had cannula-guide bilaterally implanted into the MeA, and independent sets of animals received bilateral microinjections of either the selective AT₁ receptor antagonist losartan, the selective AT₂ receptor antagonist PD123319, the selective Mas receptor antagonist A-779 or vehicle into the MeA before the EPM and FST. Treatment of the MeA with either PD123319 or A-779 decreased the EPM open arms exploration, while losartan did not affect behavioral responses in this apparatus. However, intra-MeA microinjection of losartan decreased immobility in the FST. Administration of either PD123319 or A-779 into the MeA did not affect the immobility during the FST, but changed the pattern of the active behaviors swimming and climbing. Altogether, these results indicate the presence of different angiotensinergic mechanisms within the MeA controlling behavioral responses in the FST and EPM.

Brain Renin-Angiotensin System at the Intersect of Physical and Cognitive Frailty

The renin–angiotensin system (RAS) was initially considered to be part of the endocrine system regulating water and electrolyte balance, systemic vascular resistance, blood pressure, and cardiovascular homeostasis. It was later discovered that intracrine and local forms of RAS exist in the brain apart from the endocrine RAS. This brain-specific RAS plays essential roles in brain homeostasis by acting mainly through four angiotensin receptor subtypes; AT₁R, AT₂R, MasR, and AT₄R. These receptors have opposing effects; AT₁R promotes vasoconstriction, proliferation, inflammation, and oxidative stress while AT₂R and MasR counteract the effects of AT₁R. AT₄R is critical for dopamine and acetylcholine release and mediates learning and memory consolidation. Consequently, aging-associated dysregulation of the angiotensin receptor subtypes may lead to adverse clinical outcomes such as Alzheimer’s disease and frailty via excessive oxidative stress, neuroinflammation, endothelial dysfunction, microglial polarization, and alterations in neurotransmitter secretion. In this article, we review the brain RAS from this standpoint. After discussing the functions of individual brain RAS components and their intracellular and intracranial locations, we focus on the relationships among brain RAS, aging, frailty, and specific neurodegenerative diseases, such as Alzheimer’s disease, Parkinson’s disease, and vascular cognitive impairment, through oxidative stress, neuroinflammation, and vascular dysfunction. Finally, we discuss the effects of RAS-modulating drugs on the brain RAS and their use in novel treatment approaches.

The role of the brain renin-angiotensin system (RAS) in mild traumatic brain injury (TBI)

There is considerable interest in traumatic brain injury (TBI) induced by repeated concussions suffered by athletes in sports, military personnel from combat-and non-combat related activities, and civilian populations who suffer head injuries from accidents and domestic violence. Although the renin-angiotensin system (RAS) is primarily a systemic cardiovascular regulatory system that, when dysregulated, causes hypertension and cardiovascular pathology, the brain contains a local RAS that plays a critical role in the pathophysiology of several neurodegenerative diseases. This local RAS includes receptors for angiotensin (Ang) II within the brain parenchyma, as well as on circumventricular organs outside the blood-brain-barrier. The brain RAS acts primarily via the type 1 Ang II receptor (AT₁R), exacerbating insults and pathology. With TBI, the brain RAS may contribute to permanent brain damage, especially when a second TBI occurs before the brain recovers from an initial injury. Agents are needed that minimize the extent of injury from an acute TBI, reducing TBI-mediated permanent brain damage. This review discusses how activation of the brain RAS following TBI contributes to this damage, and how drugs that counteract activation of the AT₁R including AT₁R blockers (ARBs), renin inhibitors, angiotensin-converting enzyme (ACE) inhibitors, and agonists at type 2 Ang II receptors (AT₂) and at Ang (1-7) receptors (Mas) can potentially ameliorate TBI-induced brain damage.

Central Nervous System Targets and Routes for SARS-CoV-2: Current Views and New Hypotheses

As the coronavirus disease 2019 (COVID-19) pandemic unfolds, neurological signs and symptoms reflect the involvement of targets beyond the primary lung effects. The etiological agent of COVID-19, the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), exhibits neurotropism for central and peripheral nervous systems. Various infective mechanisms and paths can be exploited by the virus to reach the central nervous system, some of which bypass the blood-brain barrier; others alter its integrity. Numerous studies have established beyond doubt that the membrane-bound metalloprotease angiotensin-converting enzyme 2 (ACE2) performs the role of SARS-CoV-2 host-cell receptor. Histochemical studies and more recently transcriptomics of mRNA have dissected the cellular localization of the ACE2 enzyme in various tissues, including the central nervous system. Epithelial cells lining the nasal mucosae, the upper respiratory tract, and the oral cavity, bronchoalveolar cells type II in the pulmonary parenchyma, and intestinal enterocytes display ACE2 binding sites at their cell surfaces, making these epithelial mucosae the most likely viral entry points. Neuronal and glial cells and endothelial cells in the central nervous system also express ACE2. This short review analyzes the known entry points and routes followed by the SARS-CoV-2 to reach the central nervous system and postulates new hypothetical pathways stemming from the enterocytes lining the intestinal lumen.

COVID's Razor: RAS Imbalance, the Common Denominator Across Disparate, Unexpected Aspects of COVID-19

A modern iteration of Occam's Razor posits that "the simplest explanation is usually correct." Coronavirus Disease 2019 involves widespread organ damage and uneven mortality demographics, deemed unexpected from what was originally thought to be "a straightforward respiratory virus." The simplest explanation is that both the expected and unexpected aspects of COVID-19 share a common mechanism. Silent hypoxia, atypical acute respiratory distress syndrome (ARDS), stroke, olfactory loss, myocarditis, and increased mortality rates in the elderly, in men, in African-Americans, and in patients with obesity, diabetes, and cancer-all bear the fingerprints of the renin-angiotensin system (RAS) imbalance, suggesting that RAS is the common culprit. This article examines what RAS is and how it works, then from that baseline, the article presents the evidence suggesting RAS involvement in the disparate manifestations of COVID-19. Understanding the deeper workings of RAS helps one make sense of severe COVID-19. In addition, recognizing the role of RAS imbalance suggests potential routes to mitigate COVID-19 severity.

Effect of angiotensin II and angiotensin-(1-7) on proliferation of stem cells from human dental apical papilla

The effects of the renin-angiotensin system (RAS) on stem cells isolated from human dental apical papilla (SCAPs) are completely unknown. Therefore, the aim of this study was to identify RAS components expressed in SCAPs and the effects of angiotensin (Ang) II and Ang-(1-7) on cell proliferation. SCAPs were collected from third molar teeth of adolescents and maintained in cell culture. Messenger RNA expression and protein levels of angiotensin-converting enzyme (ACE), ACE2, and Mas, Ang II type I (AT1) and type II (AT2) receptors were detected in SCAPs. Treatment with either Ang II or Ang-(1-7) increased the proliferation of SCAPs. These effects were inhibited by PD123319, an AT2 antagonist. While Ang II augmented mTOR phosphorylation, Ang-(1-7) induced ERK1/2 phosphorylation. In conclusion, SCAPs produce the main RAS components and both Ang II and Ang-(1-7) treatments induced cell proliferation mediated by AT2 activation through different intracellular mechanisms.