Hubertus Axer

A novel approach to the human connectome: Ultra-high resolution mapping of fiber tracts in the brain

Signal transmission between different brain regions requires connecting fiber tracts, the structural basis of the human connectome. In contrast to animal brains, where a multitude of tract tracing methods can be used, magnetic resonance (MR)-based diffusion imaging is presently the only promising approach to study fiber tracts between specific human brain regions. However, this procedure has various inherent restrictions caused by its relatively low spatial resolution. Here, we introduce 3D-polarized light imaging (3D-PLI) to map the three-dimensional course of fiber tracts in the human brain with a resolution at a submillimeter scale based on a voxel size of 100 μm isotropic or less. 3D-PLI demonstrates nerve fibers by utilizing their intrinsic birefringence of myelin sheaths surrounding axons. This optical method enables the demonstration of 3D fiber orientations in serial microtome sections of entire human brains. Examples for the feasibility of this novel approach are given here. 3D-PLI enables the study of brain regions of intense fiber crossing in unprecedented detail, and provides an independent evaluation of fiber tracts derived from diffusion imaging data.

Mapping of fiber orientation in human internal capsule by means of polarized light and confocal scanning laser microscopy.

The nervous fibers in the human internal capsule were mapped according to their three-dimensional orientation. Four human cadaver brains were cut into comparable and standardized sections parallel to the ACPC-plane, stained with DiI, and analyzed using a combination of confocal and polarized light microscopy at the same time. This combination provides information about the structure and orientation of the fibers in great detail with confocal microscopy, and information about the localization and orientation of long myelinated fiber tracts with polarization microscopy. The internal capsule was parcellated in the areas CI 1 to CI 4 containing fibers of distinct orientation and structure, which enriches the macroscopically definable parcellation in the anterior and posterior limb. Fibers of the anterior thalamic peduncle intermingle with frontopontine tract fibers. Single fibers connect the caudate and the lentiform nucleus. The pyramidal tract is located in the anterior half of the posterior limb intermingled with fibers of the superior thalamic peduncle. Parietooccipitopontine fibers are located in the posterior part of the posterior limb. The slopes of the different systems of fibers change continuously in the anterior posterior direction of the internal capsule. Using the 3D orientation of fibers as a criterion for parcellation, as well as the description of bundles as a collection of fibers belonging to particular tracts leads to a more function-related description of the anatomy of the internal capsule. The method can be used for interindividual, sex- or age-related comparisons of particular systems of fibers.

Characterization of human glucose transporter (GLUT) 11 (encoded by SLC2A11), a novel sugar-transport facilitator specifically expressed in heart and skeletal muscle.

Human GLUT11 (encoded by the solute carrier 2A11 gene, SLC2A11) is a novel sugar transporter which exhibits significant sequence similarity with the members of the GLUT family. The amino acid sequence deduced from its cDNAs predicts 12 putative membrane-spanning helices and all the motifs (sugar-transporter signatures) that have previously been shown to be essential for sugar-transport activity. The closest relative of GLUT11 is the fructose transporter GLUT5 (sharing 41.7% amino acid identity with GLUT11). The human GLUT11 gene (SLC2A11) consists of 12 exons and is located on chromosome 22q11.2. In human tissues, a 7.2 kb transcript of GLUT11 was detected exclusively in heart and skeletal muscle. Transfection of COS-7 cells with GLUT11 cDNA significantly increased the glucose-transport activity reconstituted from membrane extracts as well as the specific binding of the sugar-transporter ligand cytochalasin B. In contrast to that of GLUT4, the glucose-transport activity of GLUT11 was markedly inhibited by fructose. It is concluded that GLUT11 is a novel, muscle-specific transport facilitator that is a member of the extended GLUT family of sugar/polyol-transport facilitators.

Anatomy of the SMAS Revisited

Despite the relevance of the superficial musculoaponeurotic system (SMAS) in facial rejuvenation a clear anatomic definition of the SMAS is still lacking. Therefore, the morphology of the SMAS in 18 cadavers was investigated using different macroscopic and microscopic techniques. The region-specific anatomy of the SMAS is described in the forehead, parotid, zygomatic, and infraorbital regions, the nasolabial fold, and the lower lip. The SMAS is one continuous, organized fibrous network connecting the facial muscles with the dermis. It consists of a three-dimensional scaffold of collagen fibers, elastic fibers, and fat cells. Two different types of SMAS morphology were demonstrated: type 1 SMAS architecture is located lateral to the nasolabial fold with relatively small fibrous septa enclosing lobules of fat cells, whereas type 2 architecture is located medial to the nasolabial fold, where the SMAS consists of a dense collagen–muscle fiber meshwork. Overall, it was demonstrated that different facial regions show specific morphological characteristics, and thus region-specific surgical interventions may be necessary in facial rejuvenation.

NO-mediated cGMP synthesis in cholinergic neurons in the rat forebrain: effects of lesioning dopaminergic or serotonergic pathways on nNOS and cGMP synthesis

Nitric oxide synthase (NOS) activity and NO‐mediated cGMP synthesis were studied in the rat forebrain of control animals and animals which had received a unilateral lesioning of dopaminergic or serotonergic pathways. Lesioning of the dopaminergic innervation using 6‐hydroxydopamine resulted in a 50% decrease in NOS activity in the lesioned frontal cortex and caudate putamen. Lesioning of the serotonergic innervation using 5,7‐dihydroxytryptamine had no effect on NOS activity. NO‐mediated cGMP accumulation in rat forebrain slices was not affected by 6‐hydroxydopamine or 5,7,‐dihydroxytryptamine lesioning. Using cGMP immunocytochemistry, it was demonstrated that NO‐mediated cGMP synthesis was absent from dopaminergic, serotonergic, GABA‐ergic and neuronal NOS‐containing nerve fibres. A minor colocalization of cGMP immunoreactivity was found in parvalbumin‐containing fibres in the cortex. Extensive colocalization between cGMP immunoreactivity and the acetylcholine transporter was found in all cortical areas and in the caudate putamen. There was no effect of the lesions on this colocalization. These results demonstrate NO‐mediated cGMP accumulation in cholinergic fibres in the forebrain of the rat and suggest an anterograde signalling function of NO in cholinergic neuronal systems in the cortex and caudate putamen of the rat.

Localization of cGMP-dependent protein kinase type II in rat brain.

In brain, signaling pathways initiated by atrial natriuretic peptide, or transmitters which stimulate nitric oxide synthesis, increase cGMP as their second messenger. One important class of target molecules for cGMP is cGMP-dependent protein kinases, and in the present study, biochemical and immunocytochemical analyses demonstrate the widespread distribution of type II cGMP-dependent protein kinase in rat brain, from the cerebral cortex to the brainstem and cerebellum. Also, colocalization of cGMP-dependent protein kinase type II with its activator, cGMP, was found in several brain regions examined after in vitro stimulation of brain slices with sodium nitroprusside. In western blots, cGMP-dependent protein kinase type II was observed in all brain regions examined, although cerebellar cortex and pituitary contained comparatively less of the kinase. Immunocytochemistry revealed cGMP-dependent protein kinase type II in certain neurons, and occasionally in putative oligodendrocytes and astrocytes, however, its most striking and predominant localization was in neuropil. Electron microscopy examination of neuropil in the medial habenula showed localization of the kinase in both axon terminals and dendrites. As a membrane-associated protein, cGMP-dependent protein kinase type II often appeared to be transported to cell processes to a greater extent than being retained in the cell body. Thus, immunocytochemical labeling of cGMP-dependent protein kinase type II often did not coincide with the localization of kinase mRNA previously observed by others using in situ hybridization. We conclude that in contrast to cGMP-dependent protein kinase type I, which has a very restricted localization to cerebellar Purkinje cells and a few other sites, cGMP-dependent protein kinase type II is a very ubiquitous brain protein kinase and thus a more likely candidate for relaying myriad cGMP effects in brain requiring protein phosphorylation.

Falls and gait disorders in geriatric neurology.

Gait disturbances are frequent in older patients and lead to immobility, falls, and increased mortality. In gerontoneurologic patients a higher prevalence of risk factors for gait disturbances and falls has to be attributed due to neurodegenerative diseases, dementia, delirium, or psychotropic medication. The potential of neurological expertise to contribute to the evaluation and treatment of falls and gait disorders in geriatric patients is still not fully exploited. Sometimes a fall can be an index event to the diagnosis of the underlying disorder. This review, therefore, focuses on the relationship between falls, gait, and neurological diagnosis. It helps to find the correct diagnosis of the underlying disease as one major step in the management of gait disorders and fall prevention. From a pragmatic point of view falls can be classified according to loss or preservation of consciousness. Gait disturbances should be differentiated into gait disorders with and without cognitive impairment. Although gait impairments are influenced by multifactorial parameters, this differentiation may help to find a diagnosis and also to initiate an appropriate, disease-specific therapy. In addition, every fall patient has to be analyzed individually according to his individual risk factors, which all can potentially be influenced to improve mobility and to reduce falls.

The glucose transport facilitator GLUT8 is predominantly associated with the acrosomal region of mature spermatozoa.

Abstract. The glucose transporter 8 (GLUT8) is a recently identified member of the family of sugar transport facilitators. In human tissues GLUT8 is predominantly expressed in testis in a gonadotropin-dependent manner. It is shown here that the onset of mRNA synthesis of GLUT8 during the maturation of mouse testis coincides with the appearance of mature spermatozoa. Furthermore, immunohistochemistry with antiserum against the C-terminus of GLUT8 indicated that the protein was associated with spermatozoa within the seminiferous and the epididymal tubules. The GLUT8 immunoreactivity was detected within the head of mouse and human spermatozoa in the acrosomal region, and appeared to be located at the plasma membrane as well as within the cells. This specific expression and localization of GLUT8 suggests that the transport facilitator plays a major role in the fuel supply of mature spermatozoa, and that it is a potential target for inhibition of sperm cell function.

Collagen Fibers in Linea Alba and Rectus Sheaths

Background. After the description of a general scheme of the architecture of collagen fibers in linea alba and rectus sheaths, variability and differences of fiber architectures were analyzed to describe their functional role. Materials and methods. Using confocal laser scanning microscopy the diameter of each layer of fibril bundles was measured in linea alba and rectus sheaths of 12 human cadavers, and each fibril bundle was classified according to its orientation (oblique I and II, transverse). Results. The mean diameter of fibril bundles in the supraumbilical region of the linea alba was smaller than in the infraumbilical region, and in the supraumbilical region the thickness of the linea alba was smaller than in the infraumbilical region. Analyzing sex-dependent differences in the fiber architecture of the linea alba, a larger amount of transverse fibers relative to oblique fibers were found in females in infraumbilical regions. The thickness of the infraumbilical linea alba was smaller in females than in males, while its width was larger. Conclusions. There exist gender differences in the architecture of the linea alba. However, whether these morphological differences demonstrate the adaptability of this fiber architecture to biomechanical stress in raised intraabdominal pressure in pregnancy remains to be proven. The transverse fibers act as a counterpart to the intraabdominal pressure whereas the oblique fibers are involved mainly in movements of the trunk.

Fiber anatomy of dorsal and ventral language streams

Recent advances in neuroimaging have led to new insights into the organization of language related networks. Increasing evidence supports the model of dorsal and ventral streams of information flow between language-related areas. Therefore, a review of the descriptions of language-related fiber anatomy in the human and monkey brain was performed. In addition, case studies of macroscopical fiber dissection and polarized light imaging (PLI) with special focus on the ventral stream were done. Several fiber structures can be identified to play a role in language, i.e. the arcuate fasciculus as a part of the superior longitudinal fasciculus, the middle longitudinal fasciculus, the inferior fronto-occipital fasciculus, and extreme and external capsules. Substantial differences between human and monkey fiber architecture have been identified. Despite inconsistencies based on different terminologies used, there can be no doubt that dorsal and ventral language streams have a clear correlation in the structure of white matter tracts.