Studies of mdx mice

Deletion of aquaporin-4 improves capillary blood flow distribution in brain edema

Brain edema is a feared complication to disorders and insults affecting the brain. It can be fatal if the increase in intracranial pressure is sufficiently large to cause brain herniation. Moreover, accruing evidence suggests that even slight elevations of intracranial pressure have adverse effects, for instance on brain perfusion. The water channel aquaporin-4 (AQP4), densely expressed in perivascular astrocytic endfeet, plays a key role in brain edema formation. Using two-photon microscopy, we have studied AQP4-mediated swelling of astrocytes affects capillary blood flow and intracranial pressure (ICP) in unanesthetized mice using a mild brain edema model. We found improved regulation of capillary blood flow in mice devoid of AQP4, independently of the severity of ICP increase. Furthermore, we found brisk AQP4-dependent astrocytic Ca2+ signals in perivascular endfeet during edema that may play a role in the perturbed capillary blood flow dynamics. The study suggests that astrocytic endfoot swelling and pathological signaling disrupts microvascular flow regulation during brain edema formation.

Distribution of intraperitoneally administered deuterium-labeled water in aquaporin-4-knockout mouse brain after middle cerebral artery occlusion

Introduction

As the movement of water in the brain is known to be involved in neural activity and various brain pathologies, the ability to assess water dynamics in the brain will be important for the understanding of brain function and the diagnosis and treatment of brain diseases. Aquaporin-4 (AQP4) is a membrane channel protein that is highly expressed in brain astrocytes and is important for the movement of water molecules in the brain.

Methods

In this study, we investigated the contribution of AQP4 to brain water dynamics by administering deuterium-labeled water (D₂O) intraperitoneally to wild-type and AQP4 knockout (AQP4-ko) mice that had undergone surgical occlusion of the middle cerebral artery (MCA). Water dynamics in the infarct region and on either side of the anterior cerebral artery (ACA) was monitored with proton-density-weighted imaging (PDWI) performed on a 7T animal MRI.

Results

D₂O caused a negative signal change quickly after administration. The AQP4-ko mice showed a delay of the time-to-minimum in both the contralateral and ipsilateral ACA regions compared to wild-type mice. Also, only the AQP4- ko mice showed a delay of the time-to-minimum in the ipsilateral ACA region compared to the contralateral side. In only the wild-type mice, the signal minimum in the ipsilateral ACA region was higher than that in the contralateral ACA region. In the infarct region, the signal attenuation was slower for the AQP4-ko mice in comparison to the wild-type mice.

Discussion

These results suggest that AQP4 loss affects water dynamics in the ACA region not only in the infarct region. Dynamic PDWI after D₂O administration may be a useful tool for showing the effects of AQP4 in vivo.

Exploring cell membrane water exchange in aquaporin-4-deficient ischemic mouse brain using diffusion-weighted MRI

BACKGROUND

Aquaporin-4 is a membrane channel protein that is highly expressed in brain astrocytes and facilitates the transport of water molecules. It has been suggested that suppression of aquaporin-4 function may be an effective treatment for reducing cellular edema after cerebral infarction. It is therefore important to develop clinically applicable measurement systems to evaluate and better understand the effects of aquaporin-4 suppression on the living body.

METHODS

Animal models of focal cerebral ischemia were created by surgically occluding the middle cerebral artery of wild-type and aquaporin-4 knockout mice, after which multi-b-value multi-diffusion-time diffusion-weighted imaging measurements were performed. Data were analyzed with both the apparent diffusion coefficient (ADC) model and a compartmental water-exchange model.

RESULTS

ADCs were estimated for five different b value ranges. The ADC of aquaporin-4 knockout mice in the contralateral region was significantly higher than that of wild-type mice for each range. In contrast, aquaporin-4 knockout mice had significantly lower ADC than wild-type mice in ischemic tissue for each b-value range. Genotype-dependent differences in the ADC were particularly significant for the lowest ranges in normal tissue and for the highest ranges in ischemic tissue. The ADCs measured at different diffusion times were significantly different for both genotypes. Fitting of the water-exchange model to the ischemic region data found that the water-exchange time in aquaporin-4 knockout mice was approximately 2.5 times longer than that in wild-type mice.

CONCLUSIONS

Multi-b-value multi-diffusion-time diffusion-weighted imaging may be useful for in vivo research and clinical diagnosis of aquaporin-4-related diseases.

Insights into Cell Surface Expression, Supramolecular Organization, and Functions of Aquaporin 4 Isoforms in Astrocytes

Aquaporin 4 (AQP4) is the most abundant water channel in the central nervous system (CNS). Its expression is confined to non-neuronal glial cells, predominantly to astrocytes that represent a heterogeneous glial cell type in the CNS. The membrane of astrocyte processes, which align brain capillaries and pia, is particularly rich in AQP4. Several isoforms of AQP4 have been described; however, only some (AQP4a (M1), AQP4 c (M23), AQP4e, and AQP4ex) have been identified in the plasma membrane assemblies of astrocytes termed orthogonal arrays of particles (OAPs). Intracellular splicing isoforms (AQP4b, AQP4d, AQP4f, AQP4-Δ4) have been documented, and most of them are postulated to have a role in the cell surface distribution of the plasma membrane isoforms and in the formation of OAPs in murine and human astrocytes. Although OAPs have been proposed to play various roles in the functioning of astrocytes and CNS tissue as a whole, many of these still need to be described. OAPs are studied primarily from the perspective of understanding water permeability regulation through the plasma membrane and of their involvement in cell adhesion and in the dynamics of astrocytic processes. This review describes the cellular distribution of various AQP4 isoforms and their implications in OAP assembly, which is regulated by several intracellular and extracellular proteins.

Diverse application of MRI for mouse phenotyping

Small animal models, particularly mouse models, of human diseases are becoming an indispensable tool for biomedical research. Studies in animal models have provided important insights into the etiology of diseases and accelerated the development of therapeutic strategies. Detailed phenotypic characterization is essential, both for the development of such animal models and mechanistic studies into disease pathogenesis and testing the efficacy of experimental therapeutics. MRI is a versatile and noninvasive imaging modality with excellent penetration depth, tissue coverage, and soft tissue contrast. MRI, being a multi-modal imaging modality, together with proven imaging protocols and availability of good contrast agents, is ideally suited for phenotyping mutant mouse models. Here we describe the applications of MRI for phenotyping structural birth defects involving the brain, heart, and kidney in mice. The versatility of MRI and its ease of use are well suited to meet the rapidly increasing demands for mouse phenotyping in the coming age of functional genomics. Birth Defects Research 109:758-770, 2017. © 2017 Wiley Periodicals, Inc.

Factors determining the density of AQP4 water channel molecules at the brain-blood interface

Perivascular endfeet of astrocytes are enriched with aquaporin-4 (AQP4)-a water channel that is critically involved in water transport at the brain-blood interface and that recently was identified as a key molecule in a system for waste clearance. The factors that determine the size of the perivascular AQP4 pool remain to be identified. Here we show that the size of this pool differs considerably between brain regions, roughly mirroring regional differences in Aqp4 mRNA copy numbers. We demonstrate that a targeted deletion of α-syntrophin-a member of the dystrophin complex responsible for AQP4 anchoring-removes a substantial and fairly constant proportion (79-94 %) of the perivascular AQP4 pool across the central nervous system (CNS). Quantitative immunogold analyses of AQP4 and α-syntrophin in perivascular membranes indicate that there is a fixed stoichiometry between these two molecules. Both molecules occur at higher densities in endfoot membrane domains facing pericytes than in endfoot membrane domains facing endothelial cells. Our data suggest that irrespective of region, endfoot targeting of α-syntrophin is the single most important factor determining the size of the perivascular AQP4 pool and hence the capacity for water transport at the brain-blood interface.

Absence of aquaporin-4 in skeletal muscle alters proteins involved in bioenergetic pathways and calcium handling

Aquaporin-4 (AQP4) is a water channel expressed at the sarcolemma of fast-twitch skeletal muscle fibers, whose expression is altered in several forms of muscular dystrophies. However, little is known concerning the physiological role of AQP4 in skeletal muscle and its functional and structural interaction with skeletal muscle proteome. Using AQP4-null mice, we analyzed the effect of the absence of AQP4 on the morphology and protein composition of sarcolemma as well as on the whole skeletal muscle proteome. Immunofluorescence analysis showed that the absence of AQP4 did not perturb the expression and cellular localization of the dystrophin-glycoprotein complex proteins, aside from those belonging to the extracellular matrix, and no alteration was found in sarcolemma integrity by dye extravasation assay. With the use of a 2DE-approach (BN/SDS-PAGE), protein maps revealed that in quadriceps, out of 300 Coomassie-blue detected and matched spots, 19 proteins exhibited changed expression in AQP4(-/-) compared to WT mice. In particular, comparison of the protein profiles revealed 12 up- and 7 down-regulated protein spots in AQP4-/- muscle. Protein identification by MS revealed that the perturbed expression pattern belongs to proteins involved in energy metabolism (i.e. GAPDH, creatine kinase), as well as in Ca(2+) handling (i.e. parvalbumin, SERCA1). Western blot analysis, performed on some significantly changed proteins, validated the 2D results. Together these findings suggest AQP4 as a novel determinant in the regulation of skeletal muscle metabolism and better define the role of this water channel in skeletal muscle physiology.

Higher order structure of aquaporin-4

Unlike other mammalian AQPs, multiple tetramers of AQP4 associate in the plasma membrane to form peculiar structures called Orthogonal Arrays of Particles (OAPs), that are observable by freeze-fracture electron microscopy (FFEM). However, FFEM cannot give information about the composition of OAPs of different sizes, and due to its technical complexity is not easily applicable as a routine technique. Recently, we employed the 2D gel electrophoresis BN-SDS/PAGE that for the first time enabled the biochemical isolation of AQP4-OAPs from several tissues. We found that AQP4 protein is present in several higher-order complexes (membrane pools of supra-structures) which contain different ratios of M1/M23 isoforms corresponding to AQP4-OAPs of different size. In this paper, we illustrate in detail the potentiality of 2D BN/SDS-PAGE for analyzing AQP4 supra-structures, their relationship with the dystrophin glycoprotein complex and other membrane proteins, and their role as a specific target of Neuromyelitis Optica autoantibodies.

Dystrophin-dependent and -independent AQP4 pools are expressed in the mouse brain

In a recent study, we demonstrated that in the plasma membrane AQP4 is organized into several distinct large multisubunit complexes. In this study, we analysed whether these pools are similarly affected in dystrophin-deficient mice and immunolocalized the sites of dystrophin-dependent and -independent AQP4 pools. Western blot performed on two-dimensional Blue Native/SDS-PAGE membranes indicated that, among the AQP4 pools, it was mainly a large multisubunit complex that was specifically affected in dystrophin-deficient mice (DP71 and mdx3cv mice). This dystrophin-dependent AQP4 pool was immunolocalized in perivascular astrocytes, since it was found to be significantly altered in both types of dystrophin-deficient mice. Dystrophin-independent pools were immunolocalized in the granular cell layer of the cerebellum and in the subpial endfoot layer and ependymal cells in the brain. These data provide a better understanding on the association between AQP4 and the dystrophin-glycoprotein complex in the central nervous system.

Morphological changes in the trigemino-rubral pathway in dystrophic (mdx) mice

The lack of dystrophin that causes Duchenne muscle disease affects not only the muscles but also the central nervous system. Dystrophin-deficient mdx mice present changes in several brain fiber systems. We compared the projections from the trigeminal sensory nuclear complex to the red nucleus in control and mdx mice using retrograde tracers. Injection of 200 nL 2% fluorogold into the red nucleus caused labeling in the mesencephalic trigeminal nucleus, the principal sensory nucleus and the oral, interpolar, and caudal subnuclei of the spinal trigeminal nucleus in both control and mdx mice. Injection of latex microbeads labeled with rhodamine and fluorescein gave results similar to those seen with fluorogold. The number of labeled neurons in the trigeminal sensory nuclear complex was significantly reduced in mdx mice. In the oral subnucleus of the spinal trigeminal nucleus this reduction was 50%. These results indicate that the trigemino-rubral pathway is reduced in dystrophin-deficient mice.

Different pattern of aquaporin-4 expression in extensor digitorum longus and soleus during early development

Aquaporin-4 (AQP4) is the neuromuscular water channel expressed at the sarcolemma of mammalian fast-twitch fibers that mediates a high water transport rate, which is important during muscle activity. Clinical interest in the neuromuscular expression of AQP4 has increased as it is associated with the protein complex formed by dystrophin, the product of the gene affected in Duchenne muscular dystrophy. The expression of AQP4 during development has not been characterized. In this study, we analyzed the expression of AQP4 in extensor digitorum longus (EDL) and soleus, a fast- and slow-twitch muscle, respectively, during the first weeks after birth. The results show that AQP4 expression in both types of skeletal muscle occurs postnatally. The time course of expression of AQP4 in the two types of muscles was also different. Whereas the expression of AQP4 protein levels in the EDL showed a progressive increase during the first month after birth, reaching levels found in adults by day 24, the levels of the protein in the soleus showed a transient peak between day 12 and day 24 and declined thereafter, an effect that may be related to the transient high number of fast motor units innervating the soleus muscle during this time. The results suggest that AQP4 expression in skeletal muscle is under neuronal influence and contribute to the understanding of the molecular events of fiber differentiation during development.