Identification of Small, Regularly Shaped Cerebral Aneurysms Prone to Rupture.

BACKGROUND AND PURPOSE: Many small, regularly shaped cerebral aneurysms rupture; however, they usually receive a low score based on current risk-assessment methods. Our goal was to identify patient and aneurysm characteristics associated with rupture of small, regularly shaped aneurysms and to develop and validate predictive models of rupture in this aneurysm subpopulation. MATERIALS AND METHODS: Cross-sectional data from 1079 aneurysms smaller than 7 mm with regular shapes (without blebs) were used to train predictive models for aneurysm rupture using machine learning methods. These models were based on the patient population, aneurysm location, and hemodynamic and geometric characteristics derived from image-based computational fluid dynamics models. An independent data set with 102 small, regularly shaped aneurysms was used for validation. RESULTS: Adverse hemodynamic environments characterized by strong, concentrated inflow jets, high speed, complex and unstable flow patterns, and concentrated, oscillatory, and heterogeneous wall shear stress patterns were associated with rupture in small, regularly shaped aneurysms. Additionally, ruptured aneurysms were larger and more elongated than unruptured aneurysms in this subset. A total of 5 hemodynamic and 6 geometric parameters along with aneurysm location, multiplicity, and morphology, were used as predictive variables. The best machine learning rupture prediction-model achieved a good performance with an area under the curve of 0.84 on the external validation data set. CONCLUSIONS: This study demonstrated the potential of using predictive machine learning models based on aneurysm-specific hemodynamic, geometric, and anatomic characteristics for identifying small, regularly shaped aneurysms prone to rupture.

Adaptive Remodeling in the Elastase-induced Rabbit Aneurysms.

BACKGROUND: Rupture of brain aneurysms is associated with high fatality and morbidity rates. Through remodeling of the collagen matrix, many aneurysms can remain unruptured for decades, despite an enlarging and evolving geometry. OBJECTIVE: Our objective was to explore this adaptive remodeling for the first time in an elastase induced aneurysm model in rabbits. METHODS: Saccular aneurysms were created in 22 New Zealand white rabbits and remodeling was assessed in tissue harvested 2, 4, 8 and 12 weeks after creation. RESULTS: The intramural principal stress ratio doubled after aneurysm creation due to increased longitudinal loads, triggering a remodeling response. A distinct wall layer with multi-directional collagen fibers developed between the media and adventitia as early as 2 weeks, and in all cases by 4 weeks with an average thickness of 50.6 ± 14.3 µm. Collagen fibers in this layer were multi-directional (AI = 0.56 ± 0.15) with low tortuosity (1.08 ± 0.02) compared with adjacent circumferentially aligned medial fibers (AI = 0.78 ± 0.12) and highly tortuous adventitial fibers (1.22 ± 0.03). A second phase of remodeling replaced circumferentially aligned fibers in the inner media with longitudinal fibers. A structurally motivated constitutive model with both remodeling modes was introduced along with methodology for determining material parameters from mechanical testing and multiphoton imaging. CONCLUSIONS: A new mechanism was identified by which aneurysm walls can rapidly adapt to changes in load, ensuring the structural integrity of the aneurysm until a slower process of medial reorganization occurs. The rabbit model can be used to evaluate therapies to increase aneurysm wall stability.

Effect of macro-calcification on the failure mechanics of intracranial aneurysmal wall tissue.

BACKGROUND: Calcification was recently found to be present in the majority of cerebral aneurysms, though how calcification and the presence or absence of co-localized lipid pools affect failure properties is still unknown. OBJECTIVE: The primary objective is to quantify the biomechanical effect of a macro-calcification with surrounding Near-Calcification Region (NCR) of varying mechanical properties on tissue failure behavior. METHODS: We utilized a structurally informed finite element model to simulate pre-failure and failure behavior of a human cerebral tissue specimen modeled as a composite containing a macro-calcification and surrounding NCR, embedded in a fiber matrix composite. Data from multiple imaging modalities was combined to quantify the collagen organization and calcification geometry. An idealized parametric model utilizing the calibrated model was used to explore the impact of NCR properties on tissue failure. RESULTS: Compared to tissue without calcification, peak stress was reduced by 82% and 49% for low modulus (representing lipid pool) and high modulus (simulating increase in calcification size) of the NCR, respectively. Failure process strongly depended on NCR properties with lipid pools blunting the onset of complete failure. When the NCR was calcified, the sample was able to sustain larger overall stress, however the failure process was abrupt with nearly simultaneous failure of the loaded fibers. CONCLUSIONS: Failure of calcified vascular tissue is strongly influenced by the ultrastructure in the vicinity of the calcification. Computational modeling of failure in fibrous soft tissues can be used to understand how pathological changes impact the tissue failure process, with potentially important clinical implications.

Regional Aneurysm Wall Enhancement is Affected by Local Hemodynamics: A 7T MRI Study.

BACKGROUND AND PURPOSE: Aneurysm wall enhancement has been proposed as a biomarker for inflammation and instability. However, the mechanisms of aneurysm wall enhancement remain unclear. We used 7T MR imaging to determine the effect of flow in different regions of the wall. MATERIALS AND METHODS: Twenty-three intracranial aneurysms imaged with 7T MR imaging and 3D angiography were studied with computational fluid dynamics. Local flow conditions were compared between aneurysm wall enhancement and nonenhanced regions. Aneurysm wall enhancement regions were subdivided according to their location on the aneurysm and relative to the inflow and were further compared. RESULTS: On average, wall shear stress was lower in enhanced than in nonenhanced regions (P = .05). Aneurysm wall enhancement regions at the neck had higher wall shear stress gradients (P = .05) with lower oscillations (P = .05) than nonenhanced regions. In contrast, aneurysm wall enhancement regions at the aneurysm body had lower wall shear stress (P = .01) and wall shear stress gradients (P = .008) than nonenhanced regions. Aneurysm wall enhancement regions far from the inflow had lower wall shear stress (P = .006) than nonenhanced regions, while aneurysm wall enhancement regions close to the inflow tended to have higher wall shear stress than the nonenhanced regions, but this association was not significant. CONCLUSIONS: Aneurysm wall enhancement regions tend to have lower wall shear stress than nonenhanced regions of the same aneurysm. Moreover, the association between flow conditions and aneurysm wall enhancement seems to depend on the location of the region on the aneurysm sac. Regions at the neck and close to the inflow tend to be exposed to higher wall shear stress and wall shear stress gradients. Regions at the body, dome, or far from the inflow tend to be exposed to uniformly low wall shear stress and have more aneurysm wall enhancement.

Wall Mechanical Properties and Hemodynamics of Unruptured Intracranial Aneurysms.

BACKGROUND AND PURPOSE: Aneurysm progression and rupture is thought to be governed by progressive degradation and weakening of the wall in response to abnormal hemodynamics. Our goal was to investigate the relationship between the intra-aneurysmal hemodynamic conditions and wall mechanical properties in human aneurysms. MATERIALS AND METHODS: A total of 8 unruptured aneurysms were analyzed. Computational fluid dynamics models were constructed from preoperative 3D rotational angiography images. The aneurysms were clipped, and the domes were resected and mechanically tested to failure with a uniaxial testing system under multiphoton microscopy. Linear regression analysis was performed to explore possible correlations between hemodynamic quantities and the failure characteristics and stiffness of the wall. RESULTS: The ultimate strain was correlated negatively to aneurysm inflow rate (P = .021), mean velocity (P = .025), and mean wall shear stress (P = .039). It was also correlated negatively to inflow concentration, oscillatory shear index, and measures of the complexity and instability of the flow; however, these trends did not reach statistical significance. The wall stiffness at high strains was correlated positively to inflow rate (P = .014), mean velocity (P = .008), inflow concentration (P = .04), flow instability (P = .006), flow complexity (P = .019), wall shear stress (P = .002), and oscillatory shear index (P = .004). CONCLUSIONS: In a study of 8 unruptured intracranial aneurysms, ultimate strain was correlated negatively with aneurysm inflow rate, mean velocity, and mean wall shear stress. Wall stiffness was correlated positively with aneurysm inflow rate, mean velocity, wall shear stress, flow complexity and stability, and oscillatory shear index. These trends and the impact of hemodynamics on wall structure and mechanical properties should be investigated further in larger studies.

Theory and application of arterial tissue in-host remodelling.

A central therapeutic goal in many applications of modern Biomedicine is the reconstruction of the diseased arterial sections via robust and viable tissue equivalents. In-host remodelling is an emerging technology that exploits the remodelling ability of the host to regenerate tissue. We develop a general theoretical framework of growth and remodeling of arterial tissue starting from a synthetic, degradable, acellularized graft and we demonstrate the potential of mechanistic models to guide the development and assisting in the design of arterial tissue engineered constructs.

Hemodynamics and anatomy of elastase-induced rabbit aneurysm models: similarity to human cerebral aneurysms?

BACKGROUND AND PURPOSE: Animal models provide a mechanism for fundamental studies of the coupling between hemodynamics and pathophysiology in diseases such as saccular aneurysms. In this work, we evaluated the capability of an elastase-induced saccular aneurysm model in rabbits to reproduce the anatomic and hemodynamic features typical for human intracranial aneurysms. MATERIALS AND METHODS: Saccular aneurysms were created in 51 rabbits at the origin of the RCCA. Twelve weeks' postcreation, the lumen geometry of the aneurysm and surrounding vasculature was acquired by using 3DRA. Geometric features of these models were measured. Pulsatile 3D CFD studies were performed with rabbit-specific inlet profiles. RESULTS: Geometric features, including aneurysm height, width, neck diameter, aspect ratio, and NSI of all 51 rabbit aneurysm models fell within the range reported for human IAs. The distribution and range in values of pressure, WSS, and OSI were also typical for human IAs. A single recirculation region was observed in 33 (65%) of 51 cases, whereas a second transient recirculation zone was observed in 18 (35%) cases. Both of these flow types are commonly observed in human IAs. CONCLUSIONS: Most hemodynamic and geometric features in a commonly used elastase-induced rabbit saccular aneurysm model are qualitatively and quantitatively similar to those seen in large numbers of human cerebral aneurysms.

An inelastic multi-mechanism constitutive equation for cerebral arterial tissue.

Intracranial aneurysms (ICA) are abnormal saccular dilations of cerebral arteries, commonly found at apices of arterial bifurcations and outer walls of curved arterial segments. Histological evidence suggests the stages in ICA development include the deformation of a segment of arterial wall into a "bleb" with no identifiable neck region followed by the development of an aneurysm with a clear neck. Afterwards, the aneurysm may undergo further enlargement, possibly with significant biological response including calcification and thrombosis. Past studies of the biomechanics of cerebral aneurysm tissue have been directed at modeling elastic deformations of pre-existing aneurysms. Taking this approach, the aneurysm wall is treated as a different entity than the arterial tissue from which it developed. In the current work, a nonlinear, inelastic, dual-mechanism constitutive equation for cerebral arterial tissue is developed. It is the first to model the recruitment of collagen fibers and degradation of the internal elastic lamina, two important characteristics of early stage aneurysm formation.

Comparison of a new pmp22 transgenic mouse line with other mouse models and human patients with CMT1A.

Charcot-Marie-Tooth disease type 1A is a dominantly inherited demyelinating disorder of the peripheral nervous system. It is most frequently caused by overexpression of peripheral myelin protein 22 (PMP22), but is also caused by point mutations in the PMP22 gene. We describe a new transgenic mouse model (My41) carrying the mouse, rather than the human, pmp22 gene. The My41 strain has a severe phenotype consisting of unstable gait and weakness of the hind limbs that becomes obvious during the first 3 weeks of life. My41 mice have a shortened life span and breed poorly. Pathologically, My41 mice have a demyelinating peripheral neuropathy in which 75% of axons do not have a measurable amount of myelin. We compare the peripheral nerve pathology seen in My41 mice, which carry the mouse pmp22 gene, with previously described transgenic mice over-expressing the human PMP22 protein and Trembler-J (TrJ) mice which have a P16L substitution. We also look at the differences between CMT1A duplication patients, patients with the P16L mutation and their appropriate mouse models.

Prevotella enzymes involved in mucin oligosaccharide degradation and evidence for a small operon of genes expressed during growth on mucin.

Mucin desulfation is believed to be a rate-limiting step in mucin degradation by colon bacteria. The activities of enzymes hydrolysing nine linkages found in mucin oligosaccharide chains were measured using model substrates, in extracts of two mucin-degrading bacteria, Prevotella strain RS2 and Bacteroides fragilis. Sulfatases desulfating N-acetylglucosamine-6-sulfate, galactose-6-sulfate and galactose-3-sulfate were found. The genomic DNA downstream from the gene encoding the mucin-desulfating sulfatase (N-acetylglucosamine-6-sulfatase) in Prevotella was sequenced, and two putative genes identified which are likely to be coexpressed with this sulfatase, though their activities are unknown. Northern and Western analyses showed that expression of this short operon of three genes is increased during growth on mucin.

Protein restriction for diabetic renal disease.

OBJECTIVES: To determine whether protein restriction slows or prevents progression of diabetic nephropathy towards renal failure. SEARCH STRATEGY: Computerised databases MEDLINE (1976-1996) and EMBASE (1974-1996) were searched using keywords diabetes mellitus, diabetic nephropathy, dietary proteins, diet, protein restricted and uremia. Recent issues of selected journals (Diabetic Medicine, Diabetologia, Diabetes Care, Kidney International, Nephrology Dialysis and Transplantation) were handsearched for papers not yet in the computerised databases. Reference lists of papers were also checked. SELECTION CRITERIA: This review was not limited to randomised controlled trials. All trials involving people with insulin-dependent diabetes following a lower protein diet for at least 4 months were considered since the straight line nature of progression as reflected by GFR means that patients can act as their own controls in a before and after comparison. DATA COLLECTION AND ANALYSIS: Data were extracted for length of follow up, level of protein restriction, renal function and dietary compliance. No studies of the impact of protein restriction on outcomes such as the need for dialysis or transplantation were found. The trials reported only the effect on short-term indicators such as creatinine clearance. MAIN RESULTS: Overall a protein restricted diet (0.3-0. 8g/kg) does appear to slow the progression of diabetic nephropathy towards renal failure. REVIEWER'S CONCLUSIONS: The results show that reducing protein intake appears to slow progression to renal failure, but some questions remain unanswered. The first is what level of protein restriction we should be used? The trials aimed for a daily intake of between 0.3 to 0.8g/kg of protein. The second concerns compliance in routine care - what level would be acceptable to patients? The third concerns long term outcomes -the present trials use proxy indicators such as creatinine clearance rather than outcomes such as time to dialysis or prevention of ESRF. All trials were carried out in subjects with insulin-dependent diabetes. It remains to be seen if a lower protein intake would slow the progression of nephropathy affecting the non-insulin dependent diabetic population.

Brefeldin A-dependent membrane tubule formation reconstituted in vitro is driven by a cell cycle-regulated microtubule motor.

Treatment of cultured cells with brefeldin A (BFA) induces the formation of extensive membrane tubules from the Golgi apparatus, trans-Golgi network, and early endosomes in a microtubule-dependent manner. We have reconstituted this transport process in vitro using Xenopus egg cytosol and a rat liver Golgi-enriched membrane fraction. The presence of BFA results in the formation of an intricate, interconnected tubular membrane network, a process that, as in vivo, is inhibited by nocodazole, the H1 anti-kinesin monoclonal antibody, and by membrane pretreatment with guanosine 5'-O-(3-thiotriphosphate). Surprisingly, membrane tubule formation is not due to the action of conventional kinesin or any of the other motors implicated in Golgi membrane dynamics. Two candidate motors of approximately 100 and approximately 130 kDa have been identified using the H1 antibody, both of which exhibit motor properties in a biochemical assay. Finally, BFA-induced membrane tubule formation does not occur in metaphase cytosol, and because membrane binding of both candidate motors is not altered after incubation in metaphase compared with interphase cytosol, these results suggest that either the ATPase or microtubule-binding activity of the relevant motor is cell cycle regulated.

Development of early postnatal peripheral nerve abnormalities in Trembler-J and PMP22 transgenic mice.

Mutations in the gene for peripheral myelin protein 22 (PMP22) are associated with peripheral neuropathy in mice and humans. Although PMP22 is strongly expressed in peripheral nerves and is localised largely to the myelin sheath, a dual role has been suggested as 2 differentially expressed promoters have been found. In this study we compared the initial stages of postnatal development in transgenic mouse models which have, in addition to the murine pmp22 gene, 7 (C22) and 4 (C61) copies of the human PMP22 gene and in homozygous and heterozygous Trembler-J (TrJ) mice, which have a point mutation in the pmp22 gene. The number of axons that were singly ensheathed by Schwann cells was the same in all groups indicating that PMP22 does not function in the initial ensheathment and separation of axons. At both P4 and P12 all mutants had an increased proportion of fibres that were incompletely surrounded by Schwann cell cytoplasm indicating that this step is disrupted in PMP22 mutants. C22 and homozygous TrJ animals could be distinguished by differences in the Schwann cell morphology at the initiation of myelination. In homozygous TrJ animals the Schwann cell cytoplasm had failed to make a full turn around the axon whereas in the C22 strain most fibres had formed a mesaxon. It is concluded that PMP22 functions in the initiation of myelination and probably involves the ensheathment of the axon by the Schwann cell, and the extension of this cell along the axon. Abnormalities may result from a failure of differentiation but more probably from defective interactions between the axon and the Schwann cell.

Correlation between varying levels of PMP22 expression and the degree of demyelination and reduction in nerve conduction velocity in transgenic mice.

Charcot-Marie-Tooth disease type 1A is most commonly caused by a duplication of a 1.5 Mb region of chromosome 17 which includes the peripheral myelin protein 22 gene (PMP22). Over-expression of this gene leads to a hypomyelinating/demyelinating neuropathy and to severely reduced nerve conduction velocity. Previous mouse and rat models have had relatively high levels of expression of the mouse or human PMP22 gene leading to severe demyelination. Here we describe five lines of transgenic mice carrying increasing copies of the human PMP22 gene (one to seven) and expressing increasing levels of the transgene. From histological and electrophysiological observations there appears to be a threshold below which expression of PMP22 has virtually no effect; below a ratio of human/mouse mRNA expression of approximately 0.8, little effect is observed. Between a ratio of 0.8 and 1.5, histological and nerve conduction velocity abnormalities are observed, but there are no behavioural signs of neuropathy. An expression ratio >1.5 leads to a severe neuropathy. A second observation concerns the histology of the different lines; the level of expression does not affect the type of demyelination, but influences the severity of involvement.

Bacterial glycosulphatases and sulphomucin degradation.

The presence of a high bacterial population in a region of the gastrointestinal tract is usually associated with the secretion of sulphomucins into the mucus gel covering that region. The term 'sulphomucin' is a histochemical description of the staining properties of mucin. At present this term can only be qualitatively related to the percentage of sulphate in the mucin molecule, which makes the term difficult to use in a biochemical and functional sense. Sulphomucins are thought to carry out the normal functions attributed to mucins; in addition, heavy sulphation rate-limits the degradation of mucins by bacterial mucin-degrading glycosidases. A number of mucin-specific glycosulphatases have been reported in bacteria, although only two such enzymes have been purified. These enzymes remove part of the sulphate content from sulphomucins and make them more susceptible to further enzymic degradation. The variety of chain locations and sugar attachment sites of sulphate esters on the mucin oligosaccharides, taken together with the data on the enzymes, suggest there will be a spectrum of bacterial glycosulphatases, with different properties, cellular locations and substrate specificities. Bacterial glycosulphatases have the potential to modify sulphated glycoconjugates at mucosal surfaces and should prove useful as biochemical tools for the study of sulphated glycoconjugates.

Abnormal Schwann cell/axon interactions in the Trembler-J mouse.

The Trembler-J (TrJ) mouse has a point mutation in the gene coding for peripheral myelin protein 22 (PMP22). Disturbances in PMP22 are associated with abnormal myelination in a range of inherited peripheral neuropathies both in mice and humans. PMP22 is produced mainly by Schwann cells in the peripheral nervous system where it is localised to compact myelin. The function of PMP22 is unclear but its low abundance (approximately 5% of total myelin protein) means that it is unlikely to play a structural role. Its inclusion in a recently discovered family of proteins suggests a function in cell proliferation/differentiation and possibly in adhesion. Nerves from TrJ and the allelic Trembler (Tr) mouse are characterised by abnormally thin myelin for the size of the axon and an increased number of Schwann cells. We report ultrastructural evidence of abnormal Schwann cell-axon interactions. Schwann cell nuclei have been found adjacent to the nodes of Ranvier whereas in normal animals they are located near the centre of the internodes. In some fibres the terminal myelin loops faced outwards into the extracellular space instead of turning inwards and terminating on the axon. In severely affected nerves many axons were only partially surrounded by Schwann cell cytoplasm. All these features suggest a failure of Schwann cell-axon recognition or interaction. In addition to abnormalities related to abnormal myelination there was significant axonal loss in the dorsal roots.

Cell cycle regulation of organelle transport.

Microtubule- and actin-based motors play a wide range of vital roles in the organisation and function of cells during both interphase and mitosis, all of which are likely to be under strict control. Here, we describe how one of these roles--the movement of membranes--is regulated through the cell cycle. Organelle movement in many species is greatly reduced in mitosis as compared to interphase, and this change occurs concomitantly with an inhibition of most membrane traffic functions. Data from in vitro studies is shedding light on how microtubule motor regulation may be achieved.

The pathology of charcot-marie-tooth disease and related disorders.

Approximately a quarter of a century ago, the disorders originally designated as Charcot-Marie-Tooth disease and Dejerine-Sottas disease were shown by combined clinical, electrophysiological and nerve biopsy studies to be genetically complex. In pathological terms they could be broadly classified into demyelinating neuropathies and axonopathies. Advances in the molecular genetics of these disorders, particularly for those with a demyelinating basis, have recently produced substantial new insights. The identification of mutations in genes for myelin proteins has provided the opportunity for investigating the precise mechanisms of these neuropathies, including the use of spontaneous and genetically engineered animal models.