Microglial mTOR Activation Upregulates Trem2 and Enhances ß-Amyloid Plaque Clearance in the 5XFAD Alzheimer's Disease Model.

The mechanistic target of rapamycin (mTOR) signaling pathway plays a major role in key cellular processes including metabolism and differentiation; however, the role of mTOR in microglia and its importance in Alzheimer's disease (AD) have remained largely uncharacterized. We report that selective loss of Tsc1, a negative regulator of mTOR, in microglia in mice of both sexes, caused mTOR activation and upregulation of Trem2 with enhanced ß-Amyloid (Aß) clearance, reduced spine loss, and improved cognitive function in the 5XFAD AD mouse model. Combined loss of Tsc1 and Trem2 in microglia led to reduced Aß clearance and increased Aß plaque burden revealing that Trem2 functions downstream of mTOR. Tsc1 mutant microglia showed increased phagocytosis with upregulation of CD68 and Lamp1 lysosomal proteins. In vitro studies using Tsc1-deficient microglia revealed enhanced endocytosis of the lysosomal tracker indicator Green DND-26 suggesting increased lysosomal activity. Incubation of Tsc1-deficient microglia with fluorescent-labeled Aß revealed enhanced Aß uptake and clearance, which was blunted by rapamycin, an mTOR inhibitor. In vivo treatment of mice of relevant genotypes in the 5XFAD background with rapamycin, affected microglial activity, decreased Trem2 expression and reduced Aß clearance causing an increase in Aß plaque burden. Prolonged treatment with rapamycin caused even further reduction of mTOR activity, reduction in Trem2 expression, and increase in Aß levels. Together, our findings reveal that mTOR signaling in microglia is critically linked to Trem2 regulation and lysosomal biogenesis, and that the upregulation of Trem2 in microglia through mTOR activation could be exploited toward better therapeutic avenues to Aß-related AD pathologies.SIGNIFICANCE STATEMENT Mechanistic target of rapamycin (mTOR) signaling pathway is a key regulator for major cellular metabolic processes. However, the link between mTOR signaling and Alzheimer's disease (AD) is not well understood. In this study, we provide compelling in vivo evidence that mTOR activation in microglia would benefit ß-Amyloid (Aß)-related AD pathologies, as it upregulates Trem2, a key receptor for Aß plaque uptake. Inhibition of mTOR pathway with rapamycin, a well-established immunosuppressant, downregulated Trem2 in microglia and reduced Aß plaque clearance indicating that mTOR inactivation may be detrimental in Aß-associated AD patients. This finding will have a significant public health impact and benefit, regarding the usage of rapamycin in AD patients, which we believe will aggravate the Aß-related AD pathologies.

Drosophila Homolog of the Human Carpenter Syndrome Linked Gene, MEGF8, Is Required for Synapse Development and Function.

Drosophila multiple epidermal growth factor-like domains 8 (dMegf8) is a homolog of human MEGF8 MEGF8 encodes a multidomain transmembrane protein which is highly conserved across species. In humans, MEGF8 mutations cause a rare genetic disorder called Carpenter syndrome, which is frequently associated with abnormal left-right patterning, cardiac defects, and learning disabilities. MEGF8 is also associated with psychiatric disorders. Despite its clinical relevance, MEGF8 remains poorly characterized; and although it is highly conserved, studies on animal models of Megf8 are also very limited. The presence of intellectual disabilities in Carpenter syndrome patients and association of MEGF8 with psychiatric disorders indicate that mutations in MEGF8 cause underlying defects in synaptic structure and functions. In this study, we investigated the role of Drosophila dMegf8 in glutamatergic synapses of the larval neuromuscular junctions (NMJ) in both males and females. We show that dMegf8 localizes to NMJ synapses and is required for proper synaptic growth. dMegf8 mutant larvae and adults show severe motor coordination deficits. At the NMJ, dMegf8 mutants show altered localization of presynaptic and postsynaptic proteins, defects in synaptic ultrastructure, and neurotransmission. Interestingly, dMegf8 mutants have reduced levels of the Type II BMP receptor Wishful thinking (Wit). dMegf8 displays genetic interactions with neurexin-1 (dnrx) and wit, and in association with Dnrx and Wit plays an essential role in synapse organization. Our studies provide insights into human MEGF8 functions and potentially into mechanisms that may underlie intellectual disabilities observed in Carpenter syndrome as well as MEGF8-related synaptic structural and/or functional deficits in psychiatric disorders.SIGNIFICANCE STATEMENT Carpenter syndrome, known for over a century now, is a genetic disorder linked to mutations in Multiple Epidermal Growth Factor-like Domains 8 (MEGF8) gene and associated with intellectual disabilities among other symptoms. MEGF8 is also associated with psychiatric disorders. Despite the high genetic conservation and clinical relevance, the functions of MEGF8 remain largely uncharacterized. Patients with intellectual disabilities and psychiatric diseases often have an underlying defect in synaptic structure and function. This work defines the role of the fly homolog of human MEGF8, dMegf8, in glutamatergic synapse growth, organization, and function and provide insights into potential functions of MEGF8 in human central synapses and synaptic mechanisms that may underlie psychiatric disorders and intellectual disabilities seen in Carpenter syndrome.

Tbx1, a gene encoded in 22q11.2 copy number variant, is a link between alterations in fimbria myelination and cognitive speed in mice.

Copy number variants (CNVs) have provided a reliable entry point to identify the structural correlates of atypical cognitive development. Hemizygous deletion of human chromosome 22q11.2 is associated with impaired cognitive function; however, the mechanisms by which the CNVs contribute to cognitive deficits via diverse structural alterations in the brain remain unclear. This study aimed to determine the cellular basis of the link between alterations in brain structure and cognitive functions in mice with a heterozygous deletion of Tbx1, one of the 22q11.2-encoded genes. Ex vivo whole-brain diffusion-tensor imaging (DTI)-magnetic resonance imaging (MRI) in Tbx1 heterozygous mice indicated that the fimbria was the only region with significant myelin alteration. Electron microscopic and histological analyses showed that Tbx1 heterozygous mice exhibited an apparent absence of large myelinated axons and thicker myelin in medium axons in the fimbria, resulting in an overall decrease in myelin. The fimbria of Tbx1 heterozygous mice showed reduced mRNA levels of Ng2, a gene required to produce oligodendrocyte precursor cells. Moreover, postnatal progenitor cells derived from the subventricular zone, a source of oligodendrocytes in the fimbria, produced fewer oligodendrocytes in vitro. Behavioral analyses of these mice showed selectively slower acquisition of spatial memory and cognitive flexibility with no effects on their accuracy or sensory or motor capacities. Our findings provide a genetic and cellular basis for the compromised cognitive speed in patients with 22q11.2 hemizygous deletion.

Reorganization of Destabilized Nodes of Ranvier in ßIV Spectrin Mutants Uncovers Critical Timelines for Nodal Restoration and Prevention of Motor Paresis.

Disorganization of nodes of Ranvier is associated with motor and sensory dysfunctions. Mechanisms that allow nodal recovery during pathological processes remain poorly understood. A highly enriched nodal cytoskeletal protein ßIV spectrin anchors and stabilizes the nodal complex to actin cytoskeleton. Loss of murine ßIV spectrin allows the initial nodal organization, but causes gradual nodal destabilization. Mutations in human ßIV spectrin cause auditory neuropathy and impairment in motor coordination. Similar phenotypes are caused by nodal disruption due to demyelination. Here we report on the precise timelines of nodal disorganization and reorganization by following disassembly and reassembly of key nodal proteins in ßIV spectrin mice of both sexes before and after ßIV spectrin re-expression at specifically chosen developmental time points. We show that the timeline of nodal restoration has different outcomes in the PNS and CNS with respect to nodal reassembly and functional restoration. In the PNS, restoration of nodes occurs within 1 month regardless of the time of ßIV spectrin re-expression. In contrast, the CNS nodal reorganization and functional restoration occurs within a critical time window; after that, nodal reorganization diminishes, leading to less efficient motor recovery. We demonstrate that timely restoration of nodes can improve both the functional properties and the ultrastructure of myelinated fibers affected by long-term nodal disorganization. Our studies, which indicate a critical timeline for nodal restoration together with overall motor performance and prolonged life span, further support the idea that nodal restoration is more beneficial if initiated before any axonal damage, which is critically relevant to demyelinating disorders.SIGNIFICANCE STATEMENT Nodes of Ranvier are integral to efficient and rapid signal transmission along myelinated fibers. Various demyelinating disorders are characterized by destabilization of the nodal molecular complex, accompanied by severe reduction in nerve conduction and the onset of motor and sensory dysfunctions. This study is the first to report in vivo reassembly of destabilized nodes with sequential improvement in overall motor performance. Our study reveals that nodal restoration is achievable before any axonal damage, and that long-term nodal destabilization causes irreversible axonal structural changes that prevent functional restoration. Our studies provide significant insights into timely restoration of nodal domains as a potential therapeutic approach in treatment of demyelinating disorders.

Ablation of cytoskeletal scaffolding proteins, Band 4.1B and Whirlin, leads to cerebellar purkinje axon pathology and motor dysfunction.

The cerebellar cortex receives neural information from other brain regions to allow fine motor coordination and motor learning. The primary output neurons from the cerebellum are the Purkinje neurons that transmit inhibitory responses to deep cerebellar nuclei through their myelinated axons. Altered morphological organization and electrical properties of the Purkinje axons lead to detrimental changes in locomotor activity often leading to cerebellar ataxias. Two cytoskeletal scaffolding proteins Band 4.1B (4.1B) and Whirlin (Whrn) have been previously shown to play independent roles in axonal domain organization and maintenance in myelinated axons in the spinal cord and sciatic nerves. Immunoblot analysis had indicated cerebellar expression for both 4.1B and Whrn; however, their subcellular localization and cerebellum-specific functions have not been characterized. Using 4.1B and Whrn single and double mutant animals, we show that both proteins are expressed in common cellular compartments of the cerebellum and play cooperative roles in preservation of the integrity of Purkinje neuron myelinated axons. We demonstrate that both 4.1B and Whrn are required for the maintenance of axonal ultrastructure and health. Loss of 4.1B and Whrn leads to axonal transport defects manifested by formation of swellings containing cytoskeletal components, membranous organelles, and vesicles. Moreover, ablation of both proteins progressively affects cerebellar function with impairment in locomotor performance detected by altered gait parameters. Together, our data indicate that 4.1B and Whrn are required for maintaining proper axonal cytoskeletal organization and axonal domains, which is necessary for cerebellum-controlled fine motor coordination.

Early and Late Loss of the Cytoskeletal Scaffolding Protein, Ankyrin G Reveals Its Role in Maturation and Maintenance of Nodes of Ranvier in Myelinated Axons.

The mechanisms that govern node of Ranvier organization, stability, and long-term maintenance remain to be fully elucidated. One of the molecular components of the node is the cytoskeletal scaffolding protein, ankyrin G (AnkG), which interacts with multiple members of the nodal complex. The role of AnkG in nodal organization and maintenance is still not clearly defined as to whether AnkG functions as an initial nodal organizer or whether it functions as a nodal stabilizer after the nodal complex has been assembled. Using a mouse model system, we report here that perinatal and juvenile neuronal ablation of AnkG has differential consequences on nodal stability. Early loss of AnkG creates immature nodes with abnormal morphology, which undergo accelerated destabilization within a month, resulting in rapid voltage-gated sodium (NaV) channel and ßIV spectrin loss with reduced effects on neurofascin 186. On the other hand, late ablation of AnkG from established nodal complexes leads to slow but progressive nodal destabilization over 10 months, primarily affecting ßIV spectrin, followed by NaV channels, with modest impact on neurofascin 186. We also show that ankyrin R and ßI spectrin are not sufficient to prevent nodal disorganization after AnkG ablation. Additionally, nodal disorganization in both early and late AnkG mutants is accompanied by axonal pathology and neurological dysfunction. Together, our results suggest that AnkG plays an indispensable role in the maturation and long-term stabilization of the newly assembled nodal complex, and that loss of AnkG after nodal stabilization does not lead to rapid nodal disassembly but to loss of specific nodal components in a time-dependent manner.SIGNIFICANCE STATEMENT Nodes of Ranvier are the myelin-free gaps along myelinated axons that allow fast communication between neurons and their target cells by propagating action potentials in a saltatory manner. The cytoskeletal scaffolding protein ankyrin G (AnkG) has been thought to play an important role in node formation; however, its precise role in nodal assembly, stability, and maintenance is still not clear. By using spatiotemporal ablation of AnkG, we report its differential role in nodal maturation and stabilization. We show that early AnkG-deficient nodes fail to mature and undergo rapid destabilization. In contrast, nodes that assemble with AnkG are much more stable and undergo gradual disintegration with sequential loss of nodal components in the absence of AnkG.

A versatile genetic tool to study midline glia function in the Drosophila CNS.

Neuron-glial interactions are crucial for growth, guidance and ensheathment of axons across species. In the Drosophila CNS midline, neuron-glial interactions underlie ensheathment of commissural axons by midline glial (MG) cells in a manner similar to mammalian oligodendrocytes. Although there has been some advance in the study of neuron-glial interactions and ensheathment of axons in the CNS midline, key aspects of axonal ensheathment are still not fully understood. One of the limitations has been the unavailability of MG membrane markers that could highlight the glial processes wrapping the axons. Previous studies have identified two key molecular players from the neuronal and glial cell types in the CNS midline. These are the neuronal transmembrane protein Neurexin IV (Nrx IV) and the membrane-anchored MG protein Wrapper, both of which interact in trans to mediate neuron-glial interactions and ensheathment of commissural axons. In the current study, we attempt to further our understanding of MG biology and try to overcome some of the technical difficulties posed by the lack of a robust MG driver that will specifically allow expression or knockdown of genes in MG. We report the generation of BAC transgenic flies of wrapper-GAL4 and demonstrate how these flies could be used as a genetic tool to understand MG biology. We have utilized the GAL4/UAS system to drive GFP-reporter lines (membrane-bound mCD8-GFP; microtubule-associated tau-GFP) and nuclear lacZ using wrapper-GAL4 to highlight the MG cells and/or their processes that surround and perform axonal ensheathment functions in the embryonic midline. We also describe the utility of the wrapper-GAL4 driver line to down-regulate known MG genes specifically in Wrapper-positive cells. Finally, we validate the functionality of the wrapper-GAL4 driver by rescue of wrapper mutant phenotypes and lethality. Together, these studies provide us with a versatile genetic tool to investigate MG functions and will aid in future investigations where genetic screens using wrapper-GAL4 could be designed to identify novel molecular players at the Drosophila midline and unravel key aspects of MG biology.

Dysregulation of schizophrenia-related aquaporin 3 through disruption of paranode influences neuronal viability.

Myelinated axons segregate the axonal membrane into four defined regions: the node of Ranvier, paranode, juxtaparanode, and internode. The paranodal junction consists of specific component proteins, such as neurofascin155 (NF155) on the glial side, and Caspr and Contactin on the axonal side. Although paranodal junctions are thought to play crucial roles in rapid saltatory conduction and nodal assembly, the role of their interaction with neurons is not fully understood. In a previous study, conditional NF155 knockout in oligodendrocytes led to disorganization of the paranodal junctions. To examine if disruption of paranodal junctions affects neuronal gene expression, we prepared total RNA from the retina of NF155 conditional knockout, and performed expression analysis. We found that the expression level of 433 genes changed in response to paranodal junction ablation. Interestingly, expression of aquaporin 3 (AQP3) was significantly reduced in NF155 conditional knockout mice, but not in cerebroside sulfotransferase knockout (CST-KO) mice, whose paranodes are not originally formed during development. Copy number variations have an important role in the etiology of schizophrenia (SCZ). We observed rare duplications of AQP3 in SCZ patients, suggesting a correlation between abnormal AQP3 expression and SCZ. To determine if AQP3 over-expression in NF155 conditional knockout mice influences neuronal function, we performed adeno-associated virus (AAV)-mediated over-expression of AQP3 in the motor cortex of mice and found a significant increase in caspase 3-dependent neuronal apoptosis in AQP3-transduced cells. This study may provide new insights into therapeutic approaches for SCZ by regulating AQP3 expression, which is associated with paranodal disruption.

Drosophila Ringmaker regulates microtubule stabilization and axonal extension during embryonic development.

Axonal growth and targeting are fundamental to the organization of the nervous system, and require active engagement of the cytoskeleton. Polymerization and stabilization of axonal microtubules is central to axonal growth and maturation of neuronal connectivity. Studies have suggested that members of the tubulin polymerization promoting protein (TPPP, also known as P25α) family are involved in cellular process extension. However, no in vivo knockout data exists regarding its role in axonal growth during development. Here, we report the characterization of Ringmaker (Ringer; CG45057), the only Drosophila homolog of long p25α proteins. Immunohistochemical analyses indicate that Ringer expression is dynamically regulated in the embryonic central nervous system (CNS). ringer-null mutants show cell misplacement, and errors in axonal extension and targeting. Ultrastructural examination of ringer mutants revealed defective microtubule morphology and organization. Primary neuronal cultures of ringer mutants exhibit defective axonal extension, and Ringer expression in cells induced microtubule stabilization and bundling into rings. In vitro assays showed that Ringer directly affects tubulin, and promotes microtubule bundling and polymerization. Together, our studies uncover an essential function of Ringer in axonal extension and targeting through proper microtubule organization.

Neurexin, Neuroligin and Wishful Thinking coordinate synaptic cytoarchitecture and growth at neuromuscular junctions.

Trans-synaptic interactions involving Neurexins and Neuroligins are thought to promote adhesive interactions for precise alignment of the pre- and postsynaptic compartments and organize synaptic macromolecular complexes across species. In Drosophila, while Neurexin (Dnrx) and Neuroligins (Dnlg) are emerging as central organizing molecules at synapses, very little is known of the spectrum of proteins that might be recruited to the Dnrx/Dnlg trans-synaptic interface for organization and growth of the synapses. Using full length and truncated forms of Dnrx and Dnlg1 together with cell biological analyses and genetic interactions, we report novel functions of Dnrx and Dnlg1 in clustering of pre- and postsynaptic proteins, coordination of synaptic growth and ultrastructural organization. We show that Dnrx and Dnlg1 extracellular and intracellular regions are required for proper synaptic growth and localization of Dnlg1 and Dnrx, respectively. dnrx and dnlg1 single and double mutants display altered subcellular distribution of Discs large (Dlg), which is the homolog of mammalian post-synaptic density protein, PSD95. dnrx and dnlg1 mutants also display ultrastructural defects ranging from abnormal active zones, misformed pre- and post-synaptic areas with underdeveloped subsynaptic reticulum. Interestingly, dnrx and dnlg1 mutants have reduced levels of the Bone Morphogenetic Protein (BMP) receptor Wishful thinking (Wit), and Dnrx and Dnlg1 are required for proper localization and stability of Wit. In addition, the synaptic overgrowth phenotype resulting from the overexpression of Dnrx fails to manifest in wit mutants. Phenotypic analyses of dnrx/wit and dnlg1/wit mutants indicate that Dnrx/Dnlg1/Wit coordinate synaptic growth and architecture at the NMJ. Our findings also demonstrate that loss of Dnrx and Dnlg1 leads to decreased levels of the BMP co-receptor, Thickveins and the downstream effector phosphorylated Mad at the Neuromuscular Junction (NMJ) synapses indicating that Dnrx/Dnlg1 regulate components of the BMP signaling pathway. Together our findings reveal that Dnrx/Dnlg are at the core of a highly orchestrated process that combines adhesive and signaling mechanisms to ensure proper synaptic organization and growth during NMJ development.

Axonal domain disorganization in Caspr1 and Caspr2 mutant myelinated axons affects neuromuscular junction integrity, leading to muscle atrophy.

Bidirectional interactions between neurons and myelinating glial cells result in formation of axonal domains along myelinated fibers. Loss of axonal domains leads to detrimental consequences on nerve structure and function, resulting in reduced conductive properties and the diminished ability to reliably transmit signals to the targets they innervate. Thus, impairment of peripheral myelinated axons that project to the surface of muscle fibers and form neuromuscular junction (NMJ) synapses leads to muscle dysfunction. The goal of our studies was to determine how altered electrophysiological properties due to axonal domain disorganization lead to muscle pathology, which is relevant to a variety of peripheral neuropathies, demyelinating diseases, and neurodegenerative disorders. Using conventional Contactin-Associated Protein 1 (Caspr1) and Caspr2 single or double mutants with disrupted paranodal, juxtaparanodal, or both regions, respectively, in peripheral myelinated axons, we correlated defects in NMJ integrity and muscle pathology. Our data show that loss of axonal domains in Caspr1 and Caspr2 single and double mutants primarily alters distal myelinated fibers together with presynaptic terminals, eventually leading to NMJ denervation and reduction in postsynaptic endplate areas. Moreover, reduction in conductive properties of peripheral myelinated fibers together with NMJ disintegration leads to muscle atrophy in Caspr1 mutants or muscle fiber degeneration accompanied by mitochondrial dysfunction in Caspr1/Caspr2 double mutants. Together, our data indicate that proper organization of axonal domains in myelinated fibers is critical for optimal propagation of electrical signals, NMJ integrity, and muscle health, and provide insights into a wide range of pathologies that result in reduced nerve conduction leading to muscle atrophy. © 2017 Wiley Periodicals, Inc.

The relationship of nerve fibre pathology to sensory function in entrapment neuropathy.

Surprisingly little is known about the impact of entrapment neuropathy on target innervation and the relationship of nerve fibre pathology to sensory symptoms and signs. Carpal tunnel syndrome is the most common entrapment neuropathy; the aim of this study was to investigate its effect on the morphology of small unmyelinated as well as myelinated sensory axons and relate such changes to somatosensory function and clinical symptoms. Thirty patients with a clinical and electrophysiological diagnosis of carpal tunnel syndrome [17 females, mean age (standard deviation) 56.4 (15.3)] and 26 age and gender matched healthy volunteers [18 females, mean age (standard deviation) 51.0 (17.3)] participated in the study. Small and large fibre function was examined with quantitative sensory testing in the median nerve territory of the hand. Vibration and mechanical detection thresholds were significantly elevated in patients with carpal tunnel syndrome (P<0.007) confirming large fibre dysfunction and patients also presented with increased thermal detection thresholds (P<0.0001) indicative of C and Aδ-fibre dysfunction. Mechanical and thermal pain thresholds were comparable between groups (P>0.13). A skin biopsy was taken from a median nerve innervated area of the proximal phalanx of the index finger. Immunohistochemical staining for protein gene product 9.5 and myelin basic protein was used to evaluate morphological features of unmyelinated and myelinated axons. Evaluation of intraepidermal nerve fibre density showed a striking loss in patients (P<0.0001) confirming a significant compromise of small fibres. The extent of Meissner corpuscles and dermal nerve bundles were comparable between groups (P>0.07). However, patients displayed a significant increase in the percentage of elongated nodes (P<0.0001), with altered architecture of voltage-gated sodium channel distribution. Whereas neither neurophysiology nor quantitative sensory testing correlated with patients' symptoms or function deficits, the presence of elongated nodes was inversely correlated with a number of functional and symptom related scores (P<0.023). Our findings suggest that carpal tunnel syndrome does not exclusively affect large fibres but is associated with loss of function in modalities mediated by both unmyelinated and myelinated sensory axons. We also document for the first time that entrapment neuropathies lead to a clear reduction in intraepidermal nerve fibre density, which was independent of electrodiagnostic test severity. The presence of elongated nodes in the target tissue further suggests that entrapment neuropathies affect nodal structure/myelin well beyond the focal compression site. Interestingly, nodal lengthening may be an adaptive phenomenon as it inversely correlates with symptom severity.

Protocol for isolating and processing mouse sciatic nerve fibers for confocal immunohistochemistry.

Many motor and neurodegenerative diseases affect the peripheral nervous system (PNS). The myelinated axons in the sciatic nerves offer valuable insights into the pathology of these diseases. Here, we present a protocol for isolating and processing mouse sciatic nerves for confocal immunohistochemistry. We describe steps for mouse perfusion, removing and fixing the sciatic nerve, transferring nerves onto slides, staining, and imaging. This protocol can assist in characterizing pathologies of myelinated fibers resulting from diseases affecting the PNS. For complete details on the use and execution of this protocol, please refer to Chang et al. (2023).1.

Microglia, Trem2, and Neurodegeneration.

Microglia are a specialized type of neuroimmune cells that undergo morphological and molecular changes through multiple signaling pathways in response to pathological protein aggregates, neuronal death, tissue injury, or infections. Microglia express Trem2, which serves as a receptor for a multitude of ligands enhancing their phagocytic activity. Trem2 has emerged as a critical modulator of microglial activity, especially in many neurodegenerative disorders. Human TREM2 mutations are associated with an increased risk of developing Alzheimer disease (AD) and other neurodegenerative diseases. Trem2 plays dual roles in neuroinflammation and more specifically in disease-associated microglia. Most recent developments on the molecular mechanisms of Trem2, emphasizing its role in uptake and clearance of amyloid ß (Aß) aggregates and other tissue debris to help protect and preserve the brain, are encouraging. Although Trem2 normally stimulates defense mechanisms, its dysregulation can intensify inflammation, which poses major therapeutic challenges. Recent therapeutic approaches targeting Trem2 via agonistic antibodies and gene therapy methodologies present possible avenues for reducing the burden of neurodegenerative diseases. This review highlights the promise of Trem2 as a therapeutic target, especially for Aß-associated AD, and calls for more mechanistic investigations to understand the context-specific role of microglial Trem2 in developing effective therapies against neurodegenerative diseases.

Pinceau organization in the cerebellum requires distinct functions of neurofascin in Purkinje and basket neurons during postnatal development.

Basket axon collaterals synapse onto the Purkinje soma/axon initial segment (AIS) area to form specialized structures, the pinceau, which are critical for normal cerebellar function. Mechanistic details of how the pinceau become organized during cerebellar development are poorly understood. Loss of cytoskeletal adaptor protein Ankyrin G (AnkG) results in mislocalization of the cell adhesion molecule Neurofascin (Nfasc) at the Purkinje AIS and abnormal organization of the pinceau. Loss of Nfasc in adult Purkinje neurons leads to slow disorganization of the Purkinje AIS and pinceau morphology. Here, we used mouse conditional knock-out techniques to show that selective loss of Nfasc, specifically in Purkinje neurons during early development, prevented maturation of the AIS and resulted in loss of Purkinje neuron spontaneous activity and pinceau disorganization. Loss of Nfasc in both Purkinje and basket neurons caused abnormal basket axon collateral branching and targeting to Purkinje soma/AIS, leading to extensive pinceau disorganization, Purkinje neuron degeneration, and severe ataxia. Our studies reveal that the Purkinje Nfasc is required for AIS maturation and for maintaining stable contacts between basket axon terminals and the Purkinje AIS during pinceau organization, while the basket neuron Nfasc in combination with Purkinje Nfasc is required for proper basket axon collateral outgrowth and targeting to Purkinje soma/AIS. Thus, cerebellar pinceau organization requires coordinated mechanisms involving specific Nfasc functions in both Purkinje and basket neurons.

Localization of PDZD7 to the stereocilia ankle-link associates this scaffolding protein with the Usher syndrome protein network.

Usher syndrome is the leading cause of genetic deaf-blindness. Monoallelic mutations in PDZD7 increase the severity of Usher type II syndrome caused by mutations in USH2A and GPR98, which respectively encode usherin and GPR98. PDZ domain-containing 7 protein (PDZD7) is a paralog of the scaffolding proteins harmonin and whirlin, which are implicated in Usher type 1 and type 2 syndromes. While usherin and GPR98 have been reported to form hair cell stereocilia ankle-links, harmonin localizes to the stereocilia upper tip-link density and whirlin localizes to both tip and ankle-link regions. Here, we used mass spectrometry to show that PDZD7 is expressed in chick stereocilia at a comparable molecular abundance to GPR98. We also show by immunofluorescence and by overexpression of tagged proteins in rat and mouse hair cells that PDZD7 localizes to the ankle-link region, overlapping with usherin, whirlin, and GPR98. Finally, we show in LLC-PK1 cells that cytosolic domains of usherin and GPR98 can bind to both whirlin and PDZD7. These observations are consistent with PDZD7 being a modifier and candidate gene for USH2, and suggest that PDZD7 is a second scaffolding component of the ankle-link complex.

Drosophila neuroligin 2 is required presynaptically and postsynaptically for proper synaptic differentiation and synaptic transmission.

Trans-synaptic adhesion between Neurexins (Nrxs) and Neuroligins (Nlgs) is thought to be required for proper synapse organization and modulation, and mutations in several human Nlgs have shown association with autism spectrum disorders. Here we report the generation and phenotypic characterization of Drosophila neuroligin 2 (dnlg2) mutants. Loss of dnlg2 results in reduced bouton numbers, aberrant presynaptic and postsynaptic development at neuromuscular junctions (NMJs), and impaired synaptic transmission. In dnlg2 mutants, the evoked responses are decreased in amplitude, whereas the total active zone (AZ) numbers at the NMJ are comparable to wild type, suggesting a decrease in the release probability. Ultrastructurally, the presynaptic AZ number per bouton area and the postsynaptic density area are both increased in dnlg2 mutants, whereas the subsynaptic reticulum is reduced in volume. We show that both presynaptic and postsynaptic expression of Dnlg2 is required to restore synaptic growth and function in dnlg2 mutants. Postsynaptic expression of Dnlg2 in dnlg2 mutants and wild type leads to reduced bouton growth whereas presynaptic and postsynaptic overexpression in wild-type animals results in synaptic overgrowth. Since Nlgs have been shown to bind to Nrxs, we created double mutants. These mutants are viable and display phenotypes that closely resemble those of dnlg2 and dnrx single mutants. Our results provide compelling evidence that Dnlg2 functions both presynaptically and postsynaptically together with Neurexin to determine the proper number of boutons as well as the number of AZs and size of synaptic densities during the development of NMJs.

Organization and maintenance of molecular domains in myelinated axons.

Over a century ago, Ramon y Cajal first proposed the idea of a directionality involved in nerve conduction and neuronal communication. Decades later, it was discovered that myelin, produced by glial cells, insulated axons with periodic breaks where nodes of Ranvier (nodes) form to allow for saltatory conduction. In the peripheral nervous system (PNS), Schwann cells are the glia that can either individually myelinate the axon from one neuron or ensheath axons of many neurons. In the central nervous system (CNS), oligodendrocytes are the glia that myelinate axons from different neurons. Review of more recent studies revealed that this myelination created polarized domains adjacent to the nodes. However, the molecular mechanisms responsible for the organization of axonal domains are only now beginning to be elucidated. The molecular domains in myelinated axons include the axon initial segment (AIS), where various ion channels are clustered and action potentials are initiated; the node, where sodium channels are clustered and action potentials are propagated; the paranode, where myelin loops contact with the axolemma; the juxtaparanode (JXP), where delayed-rectifier potassium channels are clustered; and the internode, where myelin is compactly wrapped. Each domain contains a unique subset of proteins critical for the domain's function. However, the roles of these proteins in axonal domain organization are not fully understood. In this review, we highlight recent advances on the molecular nature and functions of some of the components of each axonal domain and their roles in axonal domain organization and maintenance for proper neuronal communication.

Whirlin, a cytoskeletal scaffolding protein, stabilizes the paranodal region and axonal cytoskeleton in myelinated axons.

BACKGROUND: Myelinated axons are organized into distinct subcellular and molecular regions. Without proper organization, electrical nerve conduction is delayed, resulting in detrimental physiological outcomes. One such region is the paranode where axo-glial septate junctions act as a molecular fence to separate the sodium (Na+) channel-enriched node from the potassium (K+) channel-enriched juxtaparanode. A significant lack of knowledge remains as to cytoskeletal proteins which stabilize paranodal domains and underlying cytoskeleton. Whirlin (Whrn) is a PDZ domain-containing cytoskeletal scaffold whose absence in humans results in Usher Syndromes or variable deafness-blindness syndromes. Mutant Whirlin (Whrn) mouse model studies have linked such behavioral deficits to improper localization of critical transmembrane protein complexes in the ear and eye. Until now, no reports exist about the function of Whrn in myelinated axons. RESULTS: RT-PCR and immunoblot analyses revealed expression of Whrn mRNA and Whrn full-length protein, respectively, in several stages of central and peripheral nervous system development. Comparing wild-type mice to Whrn knockout (Whrn-/-) mice, we observed no significant differences in the expression of standard axonal domain markers by immunoblot analysis but observed and quantified a novel paranodal compaction phenotype in 4 to 8 week-old Whrn-/- nerves. The paranodal compaction phenotype and associated cytoskeletal disruption was observed in Whrn-/- mutant sciatic nerves and spinal cord fibers from early (2 week-old) to late (1 year-old) stages of development. Light and electron microscopic analyses of Whrn knockout mice reveal bead-like swellings in cerebellar Purkinje axons containing mitochondria and vesicles by both. These data suggest that Whrn plays a role in proper cytoskeletal organization in myelinated axons. CONCLUSIONS: Domain organization in myelinated axons remains a complex developmental process. Here we demonstrate that loss of Whrn disrupts proper axonal domain organization. Whrn likely contributes to the stabilization of paranodal myelin loops and axonal cytoskeleton through yet unconfirmed cytoskeletal proteins. Paranodal abnormalities are consistently observed throughout development (2 wk-1 yr) and similar between central and peripheral nervous systems. In conclusion, our observations suggest that Whrn is not required for the organization of axonal domains, but once organized, Whrn acts as a cytoskeletal linker to ensure proper paranodal compaction and stabilization of the axonal cytoskeleton in myelinated axons.

Pattern of adrenal morphology and function in pulmonary tuberculosis: response to treatment with antitubercular therapy.

OBJECTIVE: Although previous studies have shown enlargement of adrenal glands in patients with tuberculosis, only few have studied its natural history. Similarly, there have been variable reports of adrenal function in these patients. Some but not all the studies revealed a high prevalence of adrenal insufficiency. The aim of this study was to prospectively study the adrenal cortical function and morphology before and after treatment of pulmonary tuberculosis. DESIGN AND PATIENTS: In a prospective case-control study, 45 patients with acute pulmonary tuberculosis and 45 healthy age- and gender-matched controls were studied. Adrenal function was tested using a short Synacthen test at the baseline and after 6 months of antitubercular treatment. Size of both adrenal glands was studied using 64 slice MDCT (Somatom Sensation, Siemens) before and after 6 months of antitubercular treatment. The findings were compared with a group of healthy controls. RESULTS: The mean age of cases was 42·4 ± 20·4 years and that of controls was 41·1 ± 17·7 years. The various clinical and biochemical parameters were comparable between cases and controls. The mean cortisol both basal and one-hour postintravenous Synacthen was significantly lower in cases as compared to controls (mean ± SD basal cortisol of 413·57 ± 108·42 nmol/l in cases vs 615·26 ± 108·15 nmol/l in controls and post Synacthen cortisol of 685·61 ± 83·87 nmol/l in cases vs 1497·30 ± 734·44 nmol/l in controls). Both the adrenal length and thickness of the right and left adrenal gland were greater in patients with active tuberculosis compared with controls and reversed to baseline after completion of antitubercular treatment. CONCLUSION: Adrenal enlargement demonstrated by CT is common in active tuberculosis and is reversible. Basal and stimulated cortisol is less in patients with pulmonary tuberculosis as compared with healthy controls and increases after antitubercular treatment, although unlike previous studies, none of the patients had adrenal insufficiency.