Bidirectional Signaling of Neuregulin-2 Mediates Formation of GABAergic Synapses and Maturation of Glutamatergic Synapses in Newborn Granule Cells of Postnatal Hippocampus

Skeletal muscle reprogramming enhances reinnervation after peripheral nerve injury

Peripheral Nerve Injuries (PNI) affect more than 20 million Americans and severely impact quality of life by causing long-term disability. The onset of PNI is characterized by nerve degeneration distal to the nerve injury resulting in long periods of skeletal muscle denervation. During this period, muscle fibers atrophy and frequently become incapable of "accepting" innervation because of the slow speed of axon regeneration post injury. We hypothesize that reprogramming the skeletal muscle to an embryonic-like state may preserve its reinnervation capability following PNI. To this end, we generated a mouse model in which NANOG, a pluripotency-associated transcription factor can be expressed locally upon delivery of doxycycline (Dox) in a polymeric vehicle. NANOG expression in the muscle upregulated the percentage of Pax7+ nuclei and expression of eMYHC along with other genes that are involved in muscle development. In a sciatic nerve transection model, NANOG expression led to upregulation of key genes associated with myogenesis, neurogenesis and neuromuscular junction (NMJ) formation, and downregulation of key muscle atrophy genes. Further, NANOG mice demonstrated extensive overlap between synaptic vesicles and NMJ acetylcholine receptors (AChRs) indicating restored innervation. Indeed, NANOG mice showed greater improvement in motor function as compared to wild-type (WT) animals, as evidenced by improved toe-spread reflex, EMG responses and isometric force production. In conclusion, we demonstrate that reprogramming the muscle can be an effective strategy to improve reinnervation and functional outcomes after PNI.

APOE expression and secretion are modulated by mitochondrial dysfunction

Mitochondria influence cellular function through both cell-autonomous and non-cell autonomous mechanisms, such as production of paracrine and endocrine factors. Here, we demonstrate that mitochondrial regulation of the secretome is more extensive than previously appreciated, as both genetic and pharmacological disruption of the electron transport chain caused upregulation of the Alzheimer's disease risk factor apolipoprotein E (APOE) and other secretome components. Indirect disruption of the electron transport chain by gene editing of SLC25A mitochondrial membrane transporters as well as direct genetic and pharmacological disruption of either complexes I, III, or the copper-containing complex IV of the electron transport chain elicited upregulation of APOE transcript, protein, and secretion, up to 49-fold. These APOE phenotypes were robustly expressed in diverse cell types and iPSC-derived human astrocytes as part of an inflammatory gene expression program. Moreover, age- and genotype-dependent decline in brain levels of respiratory complex I preceded an increase in APOE in the 5xFAD mouse model. We propose that mitochondria act as novel upstream regulators of APOE-dependent cellular processes in health and disease.

Alternative Brain Slice-on-a-Chip for Organotypic Culture and Effective Fluorescence Injection Testing

Mouse brain slices are one of the most common models to study brain development and functioning, increasing the number of study models that integrate microfluidic systems for hippocampal slice cultures. This report presents an alternative brain slice-on-a-chip, integrating an injection system inside the chip to dispense a fluorescent dye for long-term monitoring. Hippocampal slices have been cultured inside these chips, observing fluorescence signals from living cells, maintaining the cytoarchitecture of the slices. Having fluorescence images of biological samples inside the chip demonstrates the effectiveness of the staining process using the injection method avoiding leaks or biological contamination. The technology developed in this study presents a significant improvement in the local administration of reagents within a brain slice-on-a-chip system, which could be a suitable option for organotypic cultures in a microfluidic chip acting as a highly effective bioreactor.

Neuregulins in Neurodegenerative Diseases

Neurodegenerative diseases, including Alzheimer's disease (AD), Parkinson's disease (PD) and amyotrophic lateral sclerosis (ALS), are typically characterized by progressive neuronal loss and neurological dysfunctions in the nervous system, affecting both memory and motor functions. Neuregulins (NRGs) belong to the epidermal growth factor (EGF)-like family of extracellular ligands and they play an important role in the development, maintenance, and repair of both the central nervous system (CNS) and peripheral nervous system (PNS) through the ErbB signaling pathway. They also regulate multiple intercellular signal transduction and participate in a wide range of biological processes, such as differentiation, migration, and myelination. In this review article, we summarized research on the changes and roles of NRGs in neurodegenerative diseases, especially in AD. We elaborated on the structural features of each NRG subtype and roles of NRG/ErbB signaling networks in neurodegenerative diseases. We also discussed the therapeutic potential of NRGs in the symptom remission of neurodegenerative diseases, which may offer hope for advancing related treatment.

Neuregulins 1, 2, and 3 Promote Early Neurite Outgrowth in ErbB4-Expressing Cortical GABAergic Interneurons

The neuregulins (Nrgs 1-4) are a family of signaling molecules that play diverse roles in the nervous system. Nrg1 has been implicated in the formation of synapses and in synaptic plasticity. Previous studies have shown Nrg1 can affect neurite outgrowth in several neuronal populations, while the role of Nrg2 and Nrg3 in this process has remained understudied. The Nrgs can bind and activate the ErbB4 receptor tyrosine kinase which is preferentially expressed in GABAergic interneurons in the rodent hippocampus and cerebral cortex. In the present study, we evaluated the effects of Nrgs 1, 2, and 3 on neurite outgrowth of dissociated rat cortical ErbB4-positive (⁺)/GABA⁺ interneurons in vitro. All three Nrgs were able to promote neurite outgrowth during the first 2 days in vitro, with increases detected for both the axon (116-120%) and other neurites (100-120%). Increases in the average number of primary and secondary neurites were also observed. Treatment with the Nrgs for an additional 3 days promoted an increase in axonal length (86-96%), with only minimal effects on the remaining neurites (8-13%). ErbB4 expression persisted throughout the dendritic arbor and cell soma at all stages examined, while its expression in the axon was transient and declined with cell maturation. ErbB4 overexpression in GABAergic neurons promoted neurite outgrowth, an effect that was potentiated by Nrg treatment. These results show that Nrgs 1, 2, and 3 are each capable of influencing dendritic and axonal growth at early developmental stages in GABAergic neurons grown in vitro.

p140Cap Regulates GABAergic Synaptogenesis and Development of Hippocampal Inhibitory Circuits

The neuronal scaffold protein p140Cap was investigated during hippocampal network formation. p140Cap is present in presynaptic GABAergic terminals and its genetic depletion results in a marked alteration of inhibitory synaptic activity. p140Cap-/- cultured neurons display higher frequency of miniature inhibitory postsynaptic currents (mIPSCs) with no changes of their mean amplitude. Consistent with a potential presynaptic alteration of basal GABA release, p140Cap-/- neurons exhibit a larger synaptic vesicle readily releasable pool, without any variation of single GABAA receptor unitary currents and number of postsynaptic channels. Furthermore, p140Cap-/- neurons show a premature and enhanced network synchronization and appear more susceptible to 4-aminopyridine-induced seizures in vitro and to kainate-induced seizures in vivo. The hippocampus of p140Cap-/- mice showed a significant increase in the number of both inhibitory synapses and of parvalbumin- and somatostatin-expressing interneurons. Specific deletion of p140Cap in forebrain interneurons resulted in increased susceptibility to in vitro epileptic events and increased inhibitory synaptogenesis, comparable to those observed in p140Cap-/- mice. Altogether, our data demonstrate that p140Cap finely tunes inhibitory synaptogenesis and GABAergic neurotransmission, thus regulating the establishment and maintenance of the proper hippocampal excitatory/inhibitory balance.

Proteolytic Processing of Neuregulin 2

Neuregulin 2 (NRG2) belongs to the EGF family of growth factors. Most of this family members require proteolytic cleavage to liberate their ectodomains capable of binding and activating their cognate ErbB receptors. To date, most of the studies investigating proteolytic processing of neuregulins focused on NRG1, which was shown to undergo ectodomain shedding by several ADAM proteases and BACE1 and the remaining fragment was further cleaved by γ-secretase. Recently, NRG2 attracted more attention due to its role in the neurogenesis and modulation of behaviors associated with psychiatric disorders. In this study, we used genetic engineering methods to identify proteases involved in proteolytic processing of murine NRG2. Using non-neuronal cell lines as well as cultures of primary hippocampal neurons, we demonstrated that the major proteases responsible for releasing NRG2 ectodomain are ADAM10 and BACE2. Co-expression of NRG2 and BACE2 in neurons of certain brain structures including medulla oblongata and cerebellar deep nuclei was confirmed via immunohistochemical staining. The cleavage of NRG2 by ADAM10 or BACE2 generates a C-terminal fragment that serves as a substrate for γ-secretase. We also showed that murine NRG2 is subject to post-translational modifications, substantial glycosylation of its extracellular part, and phosphorylation of the cytoplasmic tail.

Neuregulin and ErbB expression is regulated by development and sensory experience in mouse visual cortex

Neuregulins (NRGs) are protein ligands that impact neural development and circuit function. NRGs signal through the ErbB receptor tyrosine kinase family. NRG1/ErbB4 signaling in parvalbumin-expressing (PV) inhibitory interneurons is critical for visual cortical plasticity. There are multiple types of NRGs and ErbBs that can potentially contribute to visual cortical plasticity at different developmental stages. Thus, it is important to understand the normal developmental expression profiles of NRGs and ErbBs in specific neuron types in the visual cortex, and to study whether and how their expression changes in PV inhibitory neurons and excitatory neurons track with sensory perturbation. Cell type-specific translating ribosome affinity purification and qPCR was used to compare mRNA expression of nrg1,2,3,4 and erbB1,2,3,4 in PV and excitatory neurons in mouse visual cortex. We show that the expression of nrg1 and nrg3 decreases in PV neurons at the critical period peak, postnatal day 28 (P28) after monocular deprivation and dark rearing, and in the adult cortex (at P104) after 2-week long dark exposure. In contrast, nrg1 expression by excitatory neurons is unchanged at P28 and P104 following sensory deprivation, whereas nrg3 expression by excitatory neurons shows changes depending on the age and the mode of sensory deprivation. ErbB4 expression in PV neurons remains consistently high and does not appear to change in response to sensory deprivation. These data provide new important details of cell type-specific NRG/ErbB expression in the visual cortex and support that NRG1/ErbB4 signaling is implicated in both critical period and adult visual cortical plasticity.

Gene expression analysis reveals early dysregulation of disease pathways and links Chmp7 to pathogenesis of spinal and bulbar muscular atrophy

Spinal and bulbar muscular atrophy (SBMA) results from a CAG repeat expansion within the androgen receptor gene (AR). It is unclear why motor neurons selectively degenerate and there are currently no treatments for this debilitating disease. To uncover the causative genes and pathways involved in motor neuron dysfunction, we undertook transcriptomic profiling of primary embryonic motor neurons from SBMA mice. We show that transcriptional dysregulation occurs early during development in SBMA motor neurons. One gene found to be dysregulated, Chmp7, was also altered in vivo in spinal cord before symptom onset in SBMA mice, and crucially in motor neuron precursor cells derived from SBMA patient stem cells, suggesting that Chmp7 may play a causal role in disease pathogenesis by disrupting the endosome-lysosome system. Furthermore, genes were enriched in SBMA motor neurons in several key pathways including p53, DNA repair, WNT and mitochondrial function. SBMA embryonic motor neurons also displayed dysfunctional mitochondria along with DNA damage, possibly resulting from DNA repair gene dysregulation and/or mitochondrial dysfunction. This indicates that a coordinated dysregulation of multiple pathways leads to development of SBMA. Importantly, our findings suggest that the identified pathways and genes, in particular Chmp7, may serve as potential therapeutic targets in SBMA.

H, Nadella RK, More RP, Seshadri M, Viswanath B, Rao M, Jain S, Mukherjee O. Exome sequencing in families with severe mental illness identifies novel and rare variants in genes implicated in Mendelian neuropsychiatric syndromes

AIM

Severe mental illnesses (SMI), such as bipolar disorder and schizophrenia, are highly heritable, and have a complex pattern of inheritance. Genome-wide association studies detect a part of the heritability, which can be attributed to common genetic variation. Examination of rare variants with next-generation sequencing may add to the understanding of the genetic architecture of SMI.

METHODS

We analyzed 32 ill subjects from eight multiplex families and 33 healthy individuals using whole-exome sequencing. Prioritized variants were selected by a three-step filtering process, which included: deleteriousness by five in silico algorithms; sharing within families by affected individuals; rarity in South Asian sample estimated using the Exome Aggregation Consortium data; and complete absence of these variants in control individuals from the same gene pool.

RESULTS

We identified 42 rare, non-synonymous deleterious variants (~5 per pedigree) in this study. None of the variants were shared across families, indicating a 'private' mutational profile. Twenty (47.6%) of the variant harboring genes were previously reported to contribute to the risk of diverse neuropsychiatric syndromes, nine (21.4%) of which were of Mendelian inheritance. These included genes carrying novel deleterious variants, such as the GRM1 gene implicated in spinocerebellar ataxia 44 and the NIPBL gene implicated in Cornelia de Lange syndrome.

CONCLUSION

Next-generation sequencing approaches in family-based studies are useful to identify novel and rare variants in genes for complex disorders like SMI. The findings of the study suggest a potential phenotypic burden of rare variants in Mendelian disease genes, indicating pleiotropic effects in the etiology of SMI.

An essential role for neuregulin-4 in the growth and elaboration of developing neocortical pyramidal dendrites

Neuregulins, with the exception of neuregulin-4 (NRG4), have been shown to be extensively involved in many aspects of neural development and function and are implicated in several neurological disorders, including schizophrenia, depression and bipolar disorder. Here we provide the first evidence that NRG4 has a crucial function in the developing brain. We show that both the apical and basal dendrites of neocortical pyramidal neurons are markedly stunted in Nrg4^-/- neonates in vivo compared with Nrg4^+/+ littermates. Neocortical pyramidal neurons cultured from Nrg4^-/- embryos had significantly shorter and less branched neurites than those cultured from Nrg4^+/+ littermates. Recombinant NRG4 rescued the stunted phenotype of embryonic neocortical pyramidal neurons cultured from Nrg4^-/- mice. The majority of cultured wild type embryonic cortical pyramidal neurons co-expressed NRG4 and its receptor ErbB4. The difference between neocortical pyramidal dendrites of Nrg4^-/- and Nrg4^+/+ mice was less pronounced, though still significant, in juvenile mice. However, by adult stages, the pyramidal dendrite arbors of Nrg4^-/- and Nrg4^+/+ mice were similar, suggesting that compensatory changes in Nrg4^-/- mice occur with age. Our findings show that NRG4 is a major novel regulator of dendritic arborisation in the developing cerebral cortex and suggest that it exerts its effects by an autocrine/paracrine mechanism.

Functional dissection of astrocyte-secreted proteins: Implications in brain health and diseases

Astrocytes, which are homeostatic cells of the central nervous system (CNS), display remarkable heterogeneity in their morphology and function. Besides their physical and metabolic support to neurons, astrocytes modulate the blood-brain barrier, regulate CNS synaptogenesis, guide axon pathfinding, maintain brain homeostasis, affect neuronal development and plasticity, and contribute to diverse neuropathologies via secreted proteins. The identification of astrocytic proteome and secretome profiles has provided new insights into the maintenance of neuronal health and survival, the pathogenesis of brain injury, and neurodegeneration. Recent advances in proteomics research have provided an excellent catalog of astrocyte-secreted proteins. This review categorizes astrocyte-secreted proteins and discusses evidence that astrocytes play a crucial role in neuronal activity and brain function. An in-depth understanding of astrocyte-secreted proteins and their pathways is pivotal for the development of novel strategies for restoring brain homeostasis, limiting brain injury/inflammation, counteracting neurodegeneration, and obtaining functional recovery.

Neural mechanisms underlying GABAergic regulation of adult hippocampal neurogenesis

Within the dentate gyrus of the adult hippocampus is the subgranular zone, which contains a neurogenic niche for radial-glia like cells, the most primitive neural stem cells in the adult brain. The quiescence of neural stem cells is maintained by tonic gamma-aminobutyric acid (GABA) released from local interneurons. Once these cells differentiate into neural progenitor cells, GABA continues to regulate their development into mature granule cells, the principal cell type of the dentate gyrus. Here, we review the role of GABA circuits, signaling, and receptors in regulating development of adult-born cells, as well as the molecular players that modulate GABA signaling. Furthermore, we review recent findings linking dysregulation of adult hippocampal neurogenesis to the altered GABAergic circuitry and signaling under various pathological conditions.

RNA localization is a key determinant of neurite-enriched proteome

Protein subcellular localization is fundamental to the establishment of the body axis, cell migration, synaptic plasticity, and a vast range of other biological processes. Protein localization occurs through three mechanisms: protein transport, mRNA localization, and local translation. However, the relative contribution of each process to neuronal polarity remains unknown. Using neurons differentiated from mouse embryonic stem cells, we analyze protein and RNA expression and translation rates in isolated cell bodies and neurites genome-wide. We quantify 7323 proteins and the entire transcriptome, and identify hundreds of neurite-localized proteins and locally translated mRNAs. Our results demonstrate that mRNA localization is the primary mechanism for protein localization in neurites that may account for half of the neurite-localized proteome. Moreover, we identify multiple neurite-targeted non-coding RNAs and RNA-binding proteins with potential regulatory roles. These results provide further insight into the mechanisms underlying the establishment of neuronal polarity.

Subcellular localization of RNAs and proteins is important for polarized cells such as neurons. Here the authors differentiate mouse embryonic stem cells into neurons, and analyze the local transcriptome, proteome, and translated transcriptome in their cell bodies and neurites, providing a unique resource for future studies on neuronal polarity.