Large saccular intracranial aneurysm in a child with RASA1-associated capillary malformation-arteriovenous malformation syndrome: illustrative case.

BACKGROUND: Large cerebral aneurysms are much less common in children than in adults. Thus, when present, these lesions require careful surgical evaluation and comprehensive genetic testing. RASA1-associated capillary malformation-arteriovenous malformation (RASA1-CM-AVM) syndrome is a rare disorder of angiogenic remodeling known to cause port-wine stains and arteriovenous fistulas but not previously associated with pediatric aneurysms. OBSERVATIONS: The authors report the case of a previously healthy 6-year-old boy who presented with seizure-like activity. Imaging demonstrated a lesion in the right ambient cistern with compression of the temporal lobe. Imaging characteristics were suggestive of a thrombosed aneurysm versus an epidermoid cyst. The patient underwent craniotomy, revealing a large saccular aneurysm, and clip ligation and excision were performed. Postoperative genetic analysis revealed a RASA1-CM-AVM syndrome. LESSONS: This is a rare case of a RASA1-associated pediatric cerebral aneurysm in the neurosurgical literature. This unique case highlights the need for maintaining a broad differential diagnosis as well as the utility of genetic testing for detecting underlying genetic syndromes in young children presenting with cerebral aneurysms.

Unraveling allostery within the angiotensin II type 1 receptor for Gαq and ß-arrestin coupling.

G protein-coupled receptors engage both G proteins and ß-arrestins, and their coupling can be biased by ligands and mutations. Here, to resolve structural elements and mechanisms underlying effector coupling to the angiotensin II (AngII) type 1 receptor (AT1R), we combined alanine scanning mutagenesis of the entire sequence of the receptor with pharmacological profiling of Gαq and ß-arrestin engagement to mutant receptors and molecular dynamics simulations. We showed that Gαq coupling to AT1R involved a large number of residues spread across the receptor, whereas fewer structural regions of the receptor contributed to ß-arrestin coupling regulation. Residue stretches in transmembrane domain 4 conferred ß-arrestin bias and represented an important structural element in AT1R for functional selectivity. Furthermore, we identified allosteric small-molecule binding sites that were enclosed by communities of residues that produced biased signaling when mutated. Last, we showed that allosteric communication within AT1R emanating from the Gαq coupling site spread beyond the orthosteric AngII-binding site and across different regions of the receptor, including currently unresolved structural regions. Our findings reveal structural elements and mechanisms within AT1R that bias Gαq and ß-arrestin coupling and that could be harnessed to design biased receptors for research purposes and to develop allosteric modulators.

Dynamic spatiotemporal determinants modulate GPCR:G protein coupling selectivity and promiscuity.

Recent studies have shown that G protein coupled receptors (GPCRs) show selective and promiscuous coupling to different Gα protein subfamilies and yet the mechanisms of the range of coupling preferences remain unclear. Here, we use Molecular Dynamics (MD) simulations on ten GPCR:G protein complexes and show that the location (spatial) and duration (temporal) of intermolecular contacts at the GPCR:Gα protein interface play a critical role in how GPCRs selectively interact with G proteins. We identify that some GPCR:G protein interface contacts are common across Gα subfamilies and others specific to Gα subfamilies. Using large scale data analysis techniques on the MD simulation snapshots we derive a spatio-temporal code for contacts that confer G protein selective coupling and validated these contacts using G protein activation BRET assays. Our results demonstrate that promiscuous GPCRs show persistent sampling of the common contacts more than G protein specific contacts. These findings suggest that GPCRs maintain contact with G proteins through a common central interface, while the selectivity comes from G protein specific contacts at the periphery of the interface.

Prostaglandin F2α and angiotensin II type 1 receptors exhibit differential cognate G protein coupling regulation.

Promiscuous G protein-coupled receptors (GPCRs) engage multiple Gα subtypes with different efficacies to propagate signals in cells. A mechanistic understanding of Gα selectivity by GPCRs is critical for therapeutic design, since signaling can be restrained by ligand-receptor complexes to preferentially engage specific G proteins. However, details of GPCR selectivity are unresolved. Here, we investigated cognate G protein selectivity using the prototypical promiscuous Gαq/11 and Gα12/13 coupling receptors, angiotensin II type I receptor (AT1R) and prostaglandin F2α receptor (FP), bioluminescence resonance energy transfer-based G protein and pathway-selective sensors, and G protein knockout cells. We determined that competition between G proteins for receptor binding occurred in a receptor- and G protein-specific manner for AT1R and FP but not for other receptors tested. In addition, we show that while Gα12/13 competes with Gαq/11 for AT1R coupling, the opposite occurs for FP, and Gαq-mediated signaling regulated G protein coupling only at AT1R. In cells, the functional modulation of biased ligands at FP and AT1R was contingent upon cognate Gα availability. The efficacy of AT1R-biased ligands, which poorly signal through Gαq/11, increased in the absence of Gα12/13. Finally, we show that a positive allosteric modulator of Gαq/11 signaling that also allosterically decreases FP-Gα12/13 coupling, lost its negative modulation in the absence of Gαq/11 coupling to FP. Together, our findings suggest that despite preferential binding of similar subsets of G proteins, GPCRs follow distinct selectivity rules, which may contribute to the regulation of ligand-mediated G protein bias of AT1R and FP.

Not Just Down and Out: Oculomotor Nerve Pathologic Spectrum.

The purpose of this article is to understand the complex pathologic spectrum of oculomotor nerve palsy. We review the detailed anatomy and function of the oculomotor nerve and demonstrate how the location of a lesion can drive the differential diagnosis. Lastly, we review atypical presentations of oculomotor nerve palsy to include oculomotor synkinesis and oculomotor nerve hyperactivity. Radiologists must be aware of the typical and atypical presentations of CN III palsy to accurately localize lesions as well as avoid premature exclusion of CN III pathology.

Utility of susceptibility weighted imaging (SWI) and pseudo-continuous arterial spin labeling (pCASL) in diagnosis of falcine venous thrombosis in a child with transient ischemic attack.

Pediatric stroke and transient ischemic attack (TIA) are uncommon but true emergencies with a wide differential diagnosis. Diagnostic imaging plays a critical role in differentiating the diverse range of etiologies. In this case, we report a 3-year-old female with no medical history who developed acute neurological deficits and demonstrate how adjunct advanced imaging including susceptibility weighted imaging (SWI) and pseudo-continuous arterial spin labeling (pCASL) can play a significant diagnostic role in the emergent setting. Imaging was performed with a Philips Ingenia 3.0T MRI. MRI brain, MR angiography (MRA), and phase contrast angiography MR Venography (PCA-MRV) were obtained. pCASL and SWI sequences were performed using SENSE (sensitivity encoding) parallel imaging techniques. MRI/MRA brain showed no restricted diffusion, abnormal T1/T2/FLAIR signal, arterial occlusion, or irregular angioarchitecture. SWI revealed increased susceptibility along the posterior falx cerebri and right posterior parietal and occipital lobes, and pCASL showed decreased blood flow within these same regions. No falcine sinus was visualized on PCA-MRV, but SWI and pCASL findings led to diagnosis of falcine sinus thrombosis and initiation of appropriate treatment. Repeat MRI one month later showed interval resolution of the abnormal SWI findings and a now patent persistent falcine sinus visualized on PCA-MRV imaging. Routine use of SWI imaging on all brain MRIs and addition of pCASL imaging when there is concern for ischemia or infarction in the emergent setting can limit the risk of missed occult diagnoses like a thrombosed falcine sinus.

Angiotensin II type 1 receptor variants alter endosomal receptor-ß-arrestin complex stability and MAPK activation.

The angiotensin II (AngII) type 1 receptor (AT1R), a member of the G protein-coupled receptor (GPCR) family, signals through G proteins and ß-arrestins, which act as adaptors to regulate AT1R internalization and mitogen-activated protein kinase (MAPK) ERK1/2 activation. ß-arrestin-dependent ERK1/2 regulation is the subject of important studies because its spatiotemporal control remains poorly understood for many GPCRs, including AT1R. To study the link between ß-arrestin-dependent trafficking and ERK1/2 signaling, we investigated three naturally occurring AT1R variants that show distinct receptor-ß-arrestin interactions: A163T, T282M, and C289W. Using bioluminescence resonance energy transfer (BRET)-based and conformational fluorescein arsenical hairpin-BRET sensors coupled with high-resolution fluorescence microscopy, we show that all AT1R variants form complexes with ß-arrestin2 at the plasma membrane and efficiently internalize into endosomes upon AngII stimulation. However, mutant receptors imposed distinct conformations in ß-arrestin2 and differentially impacted endosomal trafficking and MAPK signaling. Notably, T282M accumulated in endosomes, but its ability to form stable complexes following internalization was reduced, markedly impairing its ability to co-traffic with ß-arrestin2. We also found that despite ß-arrestin2 overexpression, T282M's and C289W's residency with ß-arrestin2 in endosomes was greatly reduced, leading to decreased ß-arrestin-dependent ERK1/2 activation, faster recycling of receptors to the plasma membrane, and impaired AngII-mediated proliferation. Our findings reveal that naturally occurring AT1R variants alter the patterns of receptor/ß-arrestin2 trafficking and suggest conformationally dependent ß-arrestin-mediated MAPK activation as well as endosomal receptor-ß-arrestin complex stabilization in the mitogenic response of AT1R.

Genetic code expansion and photocross-linking identify different ß-arrestin binding modes to the angiotensin II type 1 receptor.

The angiotensin II (AngII) type 1 receptor (AT1R) is a member of the G protein-coupled receptor (GPCR) family and binds ß-arrestins (ß-arrs), which regulate AT1R signaling and trafficking. These processes can be biased by different ligands or mutations in the AGTR1 gene. As for many GPCRs, the exact details for AT1R-ß-arr interactions driven by AngII or ß-arr-biased ligands remain largely unknown. Here, we used the amber-suppression technology to site-specifically introduce the unnatural amino acid (UAA) p-azido-l-phenylalanine (azF) into the intracellular loops (ICLs) and the C-tail of AT1R. Our goal was to generate competent photoreactive receptors that can be cross-linked to ß-arrs in cells. We performed UV-mediated photolysis of 25 different azF-labeled AT1Rs to cross-link ß-arr1 to AngII-bound receptors, enabling us to map important contact sites in the C-tail and in the ICL2 and ICL3 of the receptor. The extent of AT1R-ß-arr1 cross-linking among azF-labeled receptors differed, revealing variability in ß-arr's contact mode with the different AT1R domains. Moreover, the signature of ligated AT1R-ß-arr complexes from a subset of azF-labeled receptors also differed between AngII and ß-arr-biased ligand stimulation of receptors and between azF-labeled AT1R bearing and that lacking a bias signaling mutation. These observations further implied distinct interaction modalities of the AT1R-ß-arr1 complex in biased signaling conditions. Our findings demonstrate that this photocross-linking approach is useful for understanding GPCR-ß-arr complexes in different activation states and could be extended to study other protein-protein interactions in cells.

Cervical Spine Fracture With Vertebral Artery Dissection.

A 51-year-old man presented to a direct-access physical therapy clinic with persistent neck pain for 5 days after a fall in shallow water while surfing. Based on "dangerous mechanism of injury" from the Canadian cervical spine rule as being a high risk factor, the physical therapist ordered radiographs of the cervical spine, which were suggestive of a more serious injury. Computed tomography suggested and magnetic resonance imaging confirmed vertebral artery dissection. J Orthop Sports Phys Ther 2016;46(10):929. doi:10.2519/jospt.2016.0416.

Magnetic resonance imaging of uveitis.

INTRODUCTION: Uveitis is a term used to describe inflammation of the choroid, iris, or ciliary body, which make up the uveal tract. It can be idiopathic or associated with a systemic disease which may be infectious or noninfectious. With the exception of B-scan ultrasonography, current imaging methods for diagnosing and monitoring uveitis are predominately non-radiologic. Although MRI has been anecdotally shown to detect various inflammatory conditions of the globe, such as posterior scleritis, endophthalmitis, and posterior uveitis secondary to Vogt-Koyanagi-Harada disease, a more comprehensive review of the MRI findings in uveitis of various etiologies is presented here. METHODS: The MRI and CT studies of seven patients with uveitis and the clinical history of three of them (not available in four patients) were reviewed. Etiologies included ankylosing spondylitis, relapsing polychondritis, Vogt-Koyanagi-Harada disease, sarcoidosis, and tuberculosis. RESULTS: Increased gadolinium enhancement of the uveal tract, which is visualized as the enhancing layer immediately deep to the low-signal sclera, was seen on all six MRI studies. Diffusion-weighted imaging of a case with posterior uveitis and subretinal effusions revealed restriction within the uvea and effusions. Two patients had inflammatory nodules adherent to the uvea, two patients had vitreous humor abnormalities, and one patient exhibited proximal perineural and perimuscular spread of enhancement. Uveoscleral thickening and enhancement with a posterior calcification were observed in the patient with chronic uveitis imaged with CT. CONCLUSIONS: Increased uveal tract enhancement is a common finding in patients with uveitis, regardless of anatomic distribution and etiology. MRI can also further evaluate complications of uveitis and help differentiate it from masquerade syndromes.

An early venous abnormality: a potential cause of arteriovenous malformation recurrence.

In adults, complete surgical resection of arteriovenous malformations (AVMs) is generally curative. Recurrence of AVMs is extremely rare and most often delayed over many years. The authors report the case of a man in his 20s with rapid AVM recurrence and dual blood supply from the dura and intracerebral vessels. Early recurrence of the AVM allowed documentation of the early events associated with this recurrence. This was evidenced by the first appearance of an early vein without any signs of abnormal vasculature, suggesting that abnormality of the venous drainage system might be an inciting event in the recurrence and perhaps genesis of AVMs.