Biomedical subjects

John G Flanagan

Publications and source records attributed to John G Flanagan.

At least 19 recordsLinked to original sources

PubMed2006

Follow your nose: axon pathfinding in olfactory map formation.

Two new studies report how discrete identities of olfactory sensory neurons are converted into a spatial map of axonal connections (Imai et al., 2006; Serizawa et al., 2006). They find that levels of cAMP signals derived from olfactory receptors (ORs) can direct targeting of axons along an axis, and that ORs and neural activity regulate expression of adhesion/guidance molecules in mosaic patterns that can sort axons into discrete locations.

Animals↗

PubMed2006

Novel methodology to comprehensively assess retinal arteriolar vascular reactivity to hypercapnia.

PURPOSE: (1) Describe a new methodology that permits the comprehensive assessment of retinal arteriolar vascular reactivity in response to a sustained and stable hypercapnic stimulus. (2) Determine the magnitude of the vascular reactivity response of the retinal arterioles to hypercapnic provocation in healthy, young subjects. METHODOLOGY: Eleven healthy subjects of mean age 27 years (SD 3.43) participated in the study and one eye was randomly selected. A mask attached to a sequential rebreathing circuit, and connected to a gas delivery system, was fitted to the face. To establish baseline values, subjects breathed bottled air for 15 min and at least 6 blood flow measurements of the supero-temporal arteriole were acquired using the Canon Laser Blood Flowmeter (CLBF). Air flow was then decreased until a stable increase in fractional end-tidal CO(2) concentration (F(ET)CO(2)) of 10-15% was achieved. CLBF measurements were acquired every minute (minimum of 6 measurements) during the 20-minute period of elevated F(ET)CO(2). F(et)CO(2) was then reduced to baseline levels, and 6 further CLBF measurements were acquired. Respiratory rate, blood pressure, pulse rate and oxygen saturation were monitored continuously. RESULTS: Retinal arteriolar diameter, blood velocity and blood flow increased during hypercapnia relative to baseline (p=0.0045, p<0.0001 and p<0.0001, respectively). Group mean F(ET)CO(2) showed an increase of 12.0% (SD 3.6) relative to baseline (p<0.0001). CONCLUSIONS: This study describes a new methodology that permits the comprehensive assessment of retinal arteriolar vascular reactivity in response to a sustained and stable hypercapnic stimulus. Retinal arteriolar diameter, blood velocity and blood flow increased significantly in response to a hypercapnic provocation in young, healthy subjects.

Adult↗

PubMed2006

The HSPGs Syndecan and Dallylike bind the receptor phosphatase LAR and exert distinct effects on synaptic development.

The formation and plasticity of synaptic connections rely on regulatory interactions between pre- and postsynaptic cells. We show that the Drosophila heparan sulfate proteoglycans (HSPGs) Syndecan (Sdc) and Dallylike (Dlp) are synaptic proteins necessary to control distinct aspects of synaptic biology. Sdc promotes the growth of presynaptic terminals, whereas Dlp regulates active zone form and function. Both Sdc and Dlp bind at high affinity to the protein tyrosine phosphatase LAR, a conserved receptor that controls both NMJ growth and active zone morphogenesis. These data and double mutant assays showing a requirement of LAR for actions of both HSPGs lead to a model in which presynaptic LAR is under complex control, with Sdc promoting and Dlp inhibiting LAR in order to control synapse morphogenesis and function.

Animals↗

PubMed2006

Neural map specification by gradients.

Topographic maps, in which the spatial order of neurons maps smoothly onto their axonal target, are a central feature of neural wiring. Ephrins and Eph receptors are well accepted as graded labels for map development, enabling current studies into molecular principles of mapping. Ephrins regulate axon growth either positively or negatively, leading to models in which axons terminate at a neutral or optimum point in the gradient. Axonal competition ensures the target is filled. Ephrins and Ephs are typically expressed in complex overlapping patterns, with implications for signaling mechanisms, scale of internal map features, and coordinated interconnection of multiple mapping modules. Recent studies of Wnt3 and En-2 show that topographic axon guidance cues may be as diverse as molecules previously regarded as morphogens and transcription factors.

Animals↗

PubMed2006

Agreement of the Heidelberg Retina Tomograph II macula edema module with fundus biomicroscopy in diabetic maculopathy.

OBJECTIVES: To estimate the agreement between the macular edema maps (MEMs) of the Retina Module of the Heidelberg Retina Tomograph II (Heidelberg Engineering, Heidelberg, Germany) and contact lens fundus biomicroscopy (FB) and to assess the influence of combining MEM data with the results of short-wavelength automated perimetry (SWAP) and fluorescein angiography (FA) on diagnostic test performance. DESIGN: Prospective, observational case series. METHODS: Twenty patients (20 eyes) with diabetic retinopathy with or without clinically manifest macular edema (11 and 9 eyes, respectively) were enrolled. All patients underwent full ophthalmologic examination and also MEM assessment, SWAP, and FA. RESULTS: Using FB as the "gold standard," the agreement between the MEMs and FB was very good (Kendall coefficient of concordance, 0.80). Macular edema maps showed good agreement with FA and SWAP (Kendall coefficient, 0.64 and 0.65). Virtually all of the edematous areas detected with MEM but not seen clinically had decreased sensitivity on SWAP and/or fluorescein leakage. CONCLUSIONS: Macular edema maps demonstrated very good agreement with FB. Combining the results of FA and SWAP with those of the MEMs provided supporting evidence of concomitant blood-retinal barrier leakage and visual dysfunction, respectively, in areas of early retinal thickening. Prospective studies are ongoing to fully assess the diagnostic test performance of MEMs in the detection of early and progressive diabetic macular edema.

Adult↗

PubMed2006

Retinal hemodynamics in early diabetic macular edema.

The objective of this study was to establish the baseline retinal hemodynamic characteristics of stratified groups of diabetic patients at increasing risk for the development of diabetic macular edema (DME). Group 1 had 50 control subjects, group 2 had 56 diabetic patients without clinically visible retinopathy, group 3 had 54 diabetic patients with microaneurysms and/or hard exudates within two disc diameters of the fovea in the absence of clinically manifest DME, and group 4 had 40 patients with clinically manifest DME. Retinal hemodynamics (diameter, velocity, maximum-to-minimum velocity ratio, and flow) were assessed. Intraocular pressure, blood pressure, and relevant systemic markers of diabetes control and complications were also undertaken. The maximum-to-minimum velocity ratio was elevated with increasing risk of clinically significant DME (P < 0.0001). No significant differences were found between the groups with respect to diameter, velocity, or flow. The maximum-to-minimum velocity ratio was correlated to age, duration of diabetes, blood pressure, pulse rate, intraocular pressure, and serum potassium levels. In conclusion, the maximum-to-minimum velocity ratio was significantly increased with increasing risk of development of DME. Retinal arteriolar hemodynamics were positively correlated to age, duration of diabetes, and blood pressure. These findings suggest a reduction in the compliance (i.e., an increase of vascular rigidity) of the arteriolar circulation with increasing risk of DME.

Adult↗

PubMed2005

Ephrin-A2 and -A5 influence patterning of normal and novel retinal projections to the thalamus: conserved mapping mechanisms in visual and auditory thalamic targets.

Sensory axons are targeted to modality-specific nuclei in the thalamus. Retinal ganglion cell axons project retinotopically to their principal thalamic target, the dorsal lateral geniculate nucleus (LGd), in a pattern likely dictated by the expression of molecular gradients in the LGd. Deafferenting the auditory thalamus induces retinal axons to innervate the medial geniculate nucleus (MGN). These retino-MGN projections also show retinotopic organization. Here we show that ephrin-A2 and -A5, which are expressed in similar gradients in the MGN and LGd, can be used to pattern novel retinal projections in the MGN. As in the LGd, retinal axons from each eye terminate in discrete eye-specific zones in the MGN of rewired wild-type and ephrin-A2/A5 knockout mice. However, ipsilateral eye axons, which arise from retinal regions of high EphA5 receptor expression and represent central visual field, terminate in markedly different ways in the two mice. In rewired wild-type mice, ipsilateral axons specifically avoid areas of high ephrin expression in the MGN. In rewired ephrin knockout mice, ipsilateral projections shift in location and spread more broadly, leading to an expanded representation of the ipsilateral eye in the MGN. Similarly, ipsilateral projections to the LGd in ephrin knockout mice are shifted and are more widespread than in the LGd of wild-type mice. In the MGN, as in the LGd, terminations from the two eyes show little overlap even in the knockout mice, suggesting that local interocular segregation occurs regardless of other patterning determinants. Our data demonstrate that graded topographic labels, such as the ephrins, can serve to shape multiple related aspects of afferent patterning, including topographic mapping and the extent and spread of eye-specific projections. Furthermore, when mapping labels and other cues are expressed in multiple target zones, novel projections are patterned according to rules that operate in their canonical targets.

Animals↗

PubMed2005

Ephrin-As and neural activity are required for eye-specific patterning during retinogeniculate mapping.

In mammals, retinal ganglion cell (RGC) projections initially intermingle and then segregate into a stereotyped pattern of eye-specific layers in the dorsal lateral geniculate nucleus (dLGN). Here we found that in mice deficient for ephrin-A2, ephrin-A3 and ephrin-A5, eye-specific inputs segregated but the shape and location of eye-specific layers were profoundly disrupted. In contrast, mice that lacked correlated retinal activity did not segregate eye-specific inputs. Inhibition of correlated neural activity in ephrin mutants led to overlapping retinal projections that were located in inappropriate regions of the dLGN. Thus, ephrin-As and neural activity act together to control patterning of eye-specific retinogeniculate layers.

Animals↗

PubMed2005

Ephrin-A2 reverse signaling negatively regulates neural progenitor proliferation and neurogenesis.

The number of cells in an organ is regulated by mitogens and trophic factors that impinge on intrinsic determinants of proliferation and apoptosis. We here report the identification of an additional mechanism to control cell number in the brain: EphA7 induces ephrin-A2 reverse signaling, which negatively regulates neural progenitor cell proliferation. Cells in the neural stem cell niche in the adult brain proliferate more and have a shorter cell cycle in mice lacking ephrin-A2. The increased progenitor proliferation is accompanied by a higher number of cells in the olfactory bulb. Disrupting the interaction between ephrin-A2 and EphA7 in the adult brain of wild-type mice disinhibits proliferation and results in increased neurogenesis. The identification of ephrin-A2 and EphA7 as negative regulators of progenitor cell proliferation reveals a novel mechanism to control cell numbers in the brain.

Animals↗

PubMed2005

The impact of hypercapnia on retinal capillary blood flow assessed by scanning laser Doppler flowmetry.

AIM: To determine the effect of hypercapnia on retinal capillary blood flow using scanning laser Doppler flowmetry (SLDF). METHODS: One randomly selected eye of each of 10 normal healthy subjects (mean age 25 years, SD 2.3) was studied. Subjects breathed unrestricted air for 15 min before (baseline) and after raising fractional (percent) end-tidal concentration of CO2 (FETCO2) for 15 min by adding low flows of CO2 to air entering a sequential gas delivery circuit attached to a nasal mask. Five good quality baseline SLDF images were acquired both of the optic nerve head (ONH) and of the macula. Subsequently, a minimum of 7 sequential images were acquired during hypercapnia. Five further images were acquired of the ONH, or of the macula, after returning to unlimited air breathing. The respiratory parameters of subjects were continually monitored. RESULTS: The group mean increase in end-tidal CO2 was 14.13% (SD 4.10) relative to baseline. The nasal macula (P = 0.028) and foveal (P = 0.042) areas showed a significant increase in retinal capillary blood flow in response to hypercapnia while no significant change was noted in the ONH or temporal macula areas. Change in blood flow significantly correlated with change of FETCO2 and/or end-tidal PO2 for 3 of the 4 locations. CONCLUSIONS: Hypercapnia provoked a significant increase in retinal capillary blood flow in 2 of 4 retinal locations. Hypercapnia also induced a change in respiratory parameters that significantly correlated with change in retinal capillary blood flow in 3 of the 4 locations.

Adult↗

PubMed2005

Relative change in diurnal mean ocular perfusion pressure: a risk factor for the diagnosis of primary open-angle glaucoma.

PURPOSE: To investigate diurnal change and pattern of variation in intraocular pressure (IOP) and systolic (SBP) and diastolic (DSP) blood pressures in a group with untreated primary open-angle glaucoma (uPOAG) and compare it with an age-matched, normal group. METHODS: IOP, SBP, and DBP were measured in 14 patients with uPOAG and in 14 normal subjects, every hour between 7 AM and 10 PM and the mean ocular perfusion pressure (MOPP) was calculated. Mixed-effect linear models were used to analyze the repeated-measures data in which both fixed and random effects were included. The relative diurnal change was calculated as the percentage decrease from maximum. RESULTS: The uPOAG group had the higher IOP (P < 0.001) and lower MOPP (P = 0.025). There was a significant diurnal change in IOP, SBP, DBP, and MOPP in both groups (P < 0.001). The pattern of diurnal variation in IOP (P = 0.137), SBP (P = 0.569), and DBP (P = 0.937) was not significantly different between groups but was significantly different for MOPP (P = 0.040). MOPP and IOP were most similar at 7 AM and 1 PM. Postprandial hypotension was significant for SBP, DBP, and MOPP (P < 0.001), but not IOP (P = 0.388) in both groups. The relative change in MOPP was larger in the uPOAG group (38% vs. 26%, P < 0.001), but the change in IOP was similar (42% vs. 41%, P = 0.786). There was a significant effect of DBP on IOP over the course of the day in the uPOAG group (P = 0.011) but not in the normal group (P = 0.733). CONCLUSIONS: The relative diurnal change in IOP was similar in both uPOAG and normal subjects but MOPP showed a significant difference. MOPP significantly decreased after lunch, and was at its lowest in uPOAG at 7 AM, when IOP was at its highest. A significant association was found between diurnal DBP and IOP in uPOAG.

Blood Pressure↗

PubMed2005

Anterior optic nerve capillary blood flow response to diurnal variation of mean ocular perfusion pressure in early untreated primary open-angle glaucoma.

PURPOSE: To examine the impact of diurnal variation in intraocular pressure (IOP) and mean ocular perfusion pressure (MOPP) on the variation in anterior optic nerve capillary blood flow (BF) in patients with untreated early primary open-angle glaucoma (uPOAG) and healthy volunteers. METHODS: Fourteen patients with uPOAG (age, 56.3 +/- 12 years [SD]; seven men) and 14 normal subjects (age, 57.6 +/- 9.9 years; five men) were examined. Diurnal IOP, systolic (SBP) and diastolic (DBP) blood pressures, and optic nerve head (ONH) topography were measured every hour; and diurnal BF was measured by flowmeter every 2 hours between 0700 and 2200 hours. A perfusion image analyzer was used to calculate the mean BF within the rim (mean rim flow, MRF). The local flow (LF) was calculated using the median and mean flow rates within a 10 x 10-pixel window placed on the rim in the area of maximum topography fluctuation (MTF). The MOPP was then calculated. Mixed-effect linear models were used to analyze the repeated measures data in which both fixed and random effects were included. RESULTS: IOP, BP, and MOPP had significant diurnal variation (P < 0.040). LF measured at the sector of MTF significantly changed in patients with uPOAG (P = 0.006) but not in normal subjects (P = 0.660). MRF did not show significant diurnal change in either group (P = 0.130, P = 0.770). LF increased significantly after lunch in the uPOAG group (P = 0.001). SBP had a significant effect on LF over the course of the day in the uPOAG group (P = 0.043). The diurnal change in IOP, BP, and MOPP did not have a significant effect on MTF in either group. In uPOAG, the local flow, in areas of greatest topographical change, correlated inversely with IOP at 0700 hours (P < or = 0.002). CONCLUSIONS: The mean rim flow did not change during the day, implying that the anterior optic nerve capillary blood flow was autoregulated in both normal subjects and in patients with uPOAG, despite significant changes in IOP and MOPP. However, the regions of greatest diurnal change in rim topography (MTF) had significant diurnal change in capillary blood flow in patients with uPOAG but not in normal subjects.

Blood Flow Velocity↗

PubMed2005

Factors influencing optic nerve head biomechanics.

PURPOSE: The biomechanical environment within the optic nerve head (ONH) may play a role in retinal ganglion cell loss in glaucomatous optic neuropathy. This was a systematic analysis in which finite element methods were used to determine which anatomic and biomechanical factors most influenced the biomechanical response of the ONH to acute changes in IOP. METHODS: Based on a previously described computational model of the eye, each of 21 input factors, representing the biomechanical properties of relevant ocular tissues, the IOP, and 14 geometric factors were independently varied. The biomechanical response of the ONH tissues was quantified through a set of 29 outcome measures, including peak and mean stress and strain within each tissue, and measures of geometric changes in ONH tissues. Input factors were ranked according to their aggregated influence on groups of outcome measures. RESULTS: The five input factors that had the largest influence across all outcome measures were, in ranked order: stiffness of the sclera, radius of the eye, stiffness of the lamina cribrosa, IOP, and thickness of the scleral shell. The five least influential factors were, in reverse ranked order: retinal thickness, peripapillary rim height, cup depth, cup-to-disc ratio, and pial thickness. Factor ranks were similar for various outcome measure groups and factor ranges. CONCLUSIONS: The model predicts that ONH biomechanics are strongly dependent on scleral biomechanical properties. Acute deformations of ONH tissues, and the consequent high levels of neural tissue strain, were less strongly dependent on the action of IOP directly on the internal surface of the ONH than on the indirect effects of IOP on the sclera. This suggests that interindividual variations in scleral properties could be a risk factor for the development of glaucoma. Eye size and lamina cribrosa biomechanical properties also have a strong influence on ONH biomechanics.

Biomechanical Phenomena↗

PubMed2005

Frequency doubling illusion: detection vs. form resolution.

PURPOSE: To investigate the difference between detection and form resolution thresholds for the frequency doubling (FD) illusion. METHOD: The right eyes of 20 normal healthy subjects (10 female; age range, 18 to 30 years; mean age, 24.05 years; SD, 1.39) were examined. Vertically oriented FD stimuli were presented at fixation and 10 degrees nasally using a dual temporal alternate forced choice method of constant stimuli (2AFC MOCS) to estimate the thresholds for detection and form resolution. Additional strategies for threshold estimation (yes/no, modified rapid estimation by binary search) were used to determine the detection threshold. The effect of subject instruction on the FD threshold was also examined. The test-retest characteristics were investigated by determining the coefficient of repeatability. RESULTS: Detection thresholds using 2AFC MOCS were 0.86 +/- 0.20 (+/-1 SE) Michelson contrast percentage (MC%) at fixation and 0.84 +/- 0.21 at 10 degrees. Form resolution thresholds using 2AFC MOCS were 1.08 +/- 0.23 MC% at fixation and 1.04 +/- 0.16 at 10 degrees . These thresholds were found to be significantly different at fixation (p = 0.004) and 10 degrees (p = 0.005). No difference was found between threshold estimation strategies, but subject instruction had a significant effect (fixation: 2.41 +/- 0.52 MC%, p < 0.001; 10 degrees : 2.19 +/- 0.48 MC%, p < 0.001). CONCLUSIONS: Detection thresholds were significantly lower than form resolution thresholds for the FD stimulus. This result is in agreement with classic FD studies and illustrates that the perception of flicker precedes the perception of the FD illusion at threshold.

Adult↗

PubMed2005

Reconstruction of human optic nerve heads for finite element modeling.

PURPOSE: Glaucoma is a common ocular disease whose pathogenesis is hypothesized to involve biomechanical damage to optic nerve tissues. Here we describe a method for the construction of patient-specific models that can be used to evaluate the biomechanical environment within the optic nerve head. We validate the method using a virtual eye, and demonstrate its use in computing optic nerve head biomechanics. METHODS: Human eyes were imaged and the optic nerve head region was processed to allow serial plastic histologic sections to be cut. These sections were photographed, unwarped and aligned so as to reconstruct three-dimensional patient-specific models incorporating sclera, pre- and post-laminar nerve, lamina cribrosa, and pia mater. Deformations, stresses and strains were computed in the resulting model using finite element techniques. RESULTS: The approach successfully reconstructed patient-specific optic nerve head models. Reconstruction of a virtual eye showed excellent agreement between the true and reconstructed geometries, and between the deformations and strains computed on the true and reconstructed geometries. A sample reconstruction showed reasonable agreement between computed and measured retinal surface deformations. CONCLUSION: The technique presented here is viable and can be used to accurately compute human optic nerve head biomechanics.

Biomechanical Phenomena↗

PubMed2004

Semaphorin 5A is a bifunctional axon guidance cue regulated by heparan and chondroitin sulfate proteoglycans.

The response of neuronal growth cones to axon guidance cues depends on the developmental context in which these cues are encountered. We show here that the transmembrane protein semaphorin 5A (Sema5A) is a bifunctional guidance cue exerting both attractive and inhibitory effects on developing axons of the fasciculus retroflexus, a diencephalon fiber tract associated with limbic function. The thrombospondin repeats of Sema5A physically interact with the glycosaminoglycan portion of both chondroitin sulfate proteoglycans (CSPGs) and heparan sulfate proteoglycans (HSPGs). CSPGs function as precisely localized extrinsic cues that convert Sema5A from an attractive to an inhibitory guidance cue. Therefore, glycosaminoglycan bound guidance cues provide a molecular mechanism for CSPG-mediated inhibition of axonal extension. Further, axonal HSPGs are required for Sema5A-mediated attraction, suggesting that HSPGs are components of functional Sema5A receptors. Thus, neuronal responses to Sema5A are proteoglycan dependent and interpreted according to the biological context in which this membrane bound guidance cue is presented.

Animals↗

PubMed2004

Generation of recognition diversity in the nervous system.

For decades, it has been suggested that complex neural wiring might be specified by extensive diversity in receptor isoforms. Dscam is a cell surface protein with 38,016 potential alternatively spliced isoforms in the fly nervous system. Remarkable binding studies now show that Dscam isoform diversity indeed results in an unprecedented level of recognition diversity, showing isoform-specific homophilic binding. In vivo studies have begun to suggest models for use of Dscam diversity in neuron-target recognition, axon fasciculation, and neuron self-recognition.

Animals↗

PubMed2004

Fiber order of the normal and regenerated optic tract of the frog (Rana pipiens).

In the normal frog, axons from the peripheral retina arising at the temporal pole course superficially in the middle stream of the diencephalic optic tract. Axons from the nasal pole course in two streams running in the opposite margins of the tract, dorsonasal axons ventrally, ventronasal axons dorsally. Axons from the dorsal and ventral poles of the retina occupy the intervals between the aforementioned middle and marginal streams. Axons from more central regions of the retina tend to occupy deeper levels of the optic tract. The regenerated optic tract does not regain its normal organization, e.g., axons of peripheral nasal origin are spread out widely over the entire width of the tract. However, axons from the temporal pole of the retina do return approximately to their original location in the middle stream. The concentration of temporal axons in the middle stream of the optic tract after regeneration may now be understood in terms of the expression pattern of the ephrin-A class of receptor tyrosine kinase ligands in the cellular matrix of the optic tract. The ephrin-As, which have a repellent effect on growing temporal retinal axons, are concentrated in and along the margins of the diencephalic optic tract and essentially absent from its middle stream. It is proposed here that peripheral temporal axons may be forced into this middle region by their avoidance of the higher levels of ephrin-A expression in the tract margins. In contrast, the growth pattern of regenerating peripheral nasal axons would not be affected by the ephrin-A gradient in the optic tract.

Animals↗