Gene Therapy Overexpressing Neuregulin 1 Type I in Combination With Neuregulin 1 Type III Promotes Functional Improvement in the SOD1G93A ALS Mice

Multiple Roles for Neuregulins and Their ERBB Receptors in Neurodegenerative Disease Pathogenesis and Therapy

The role that neurotrophins, such as nerve growth factor, play in the pathogenesis of neurodegenerative diseases has long been appreciated. However, the neuregulin (NRG) family of growth factors and/or their v-erb-B2 avian erythroblastic leukemia viral oncogene homolog (ERBB) receptors have also been implicated in the pathogenesis of conditions, such as Alzheimer disease (AD), frontotemporal lobar degeneration (FTLD), and amyotrophic lateral sclerosis (ALS). In this review, we consider i) the structural variability of NRG isoforms generated by alternative RNA splicing, the use of multiple promoters and proteolysis, and the impact that this structural variability has on neuronal and glial physiology during development and adulthood. We discuss ii) the NRG receptors ERBB2, ERBB3, and ERBB4, how activation of each of these receptors further diversifies NRG actions in the central nervous system, and how dementia-related proteins, such as γ-secretase modulate the action of NRGs and their ERBB receptors. We then iii) turn to the abnormalities in NRG and ERBB expression and function evident in human AD and mouse AD models, how these abnormalities affect brain function, and attempts to use NRGs to treat AD. Finally, iv) we discuss NRG effects on the survival and function of neurons relevant to FTLD and ALS, alterations in NRG/ERBB signaling identified in these conditions, and the recent discovery of multiple human pedigrees in which autosomal dominant FTLD/ALS potentially results from point mutations in ERBB4.

Pirfenidone mitigates demyelination and electrophysiological alterations in multiple sclerosis: Targeting NF-κB, sirt1, and neurotrophic genes

Multiple sclerosis (MS) is a demyelinating disease affecting the central nervous system associated with progressive neurodegeneration. Pirfenidone (Pir) is a well-known antifibrotic agent; however, Pir's function in MS is little explored. We evaluated the neuroprotective effects of Pir in MS and its possible underlying mechanisms. Forty male Swiss mice were divided equally into control, cuprizone (CPZ), Pir, and CPZ + Pir groups. Assessment of motor function was conducted using neurobehavioral tests, EMG, and nerve conduction velocity (NCV). Mice's brains were extracted to measure oxidative stress, neuroinflammatory markers, and the expression of neurotrophic genes. The corpus callosum and the sciatic nerve were subjected to histopathological and immunohistochemical studies. The CPZ group was associated with significant reductions in muscle power, frequency of contraction, sciatic NCV, SOD, IL-10, SIRT1, NGF, and neuregulin-1. Significant increases in MDA, TNF-α, INF-γ, IL-17, TGF-β, and NF-κB were also detected. Multiple areas of partially demyelinated nerve fibers in the corpus callosum, the loss of oligodendrocyte nuclei, and increased microglia and astrocytes were also observed. The sciatic nerve revealed partial demyelination with significantly reduced myelin basic protein (MBP) expression. Pir significantly restored motor function, demyelination, and neurodegenerative changes induced by CPZ. Besides the antifibrotic action of Pir, we concluded that it improves motor function in MS by alleviating the demyelinating process and neurodegeneration. Its potential anti-inflammatory, antioxidant, and antifibrotic properties could be contributing factors. These effects could be mediated by modulating the NF-κB, SIRT1, NGF, and neuregulin-1 pathways. Pir is a promising agent for treating MS.

Skeletal muscle dysfunction in amyotrophic lateral sclerosis: a mitochondrial perspective and therapeutic approaches

Amyotrophic lateral sclerosis (ALS) is a progressive and fatal neuromuscular disease that results in the loss of motor neurons and severe skeletal muscle atrophy. The etiology of ALS is linked to skeletal muscle, which can activate a retrograde signaling cascade that destroys motor neurons. This is why satellite cells and mitochondria play a crucial role in the health and performance of skeletal muscles. This review presents current knowledge on the involvement of mitochondrial dysfunction, skeletal muscle atrophy, muscle satellite cells, and neuromuscular junction (NMJ) in ALS. It also discusses current therapeutic strategies, including exercise, drugs, stem cells, gene therapy, and the prospective use of mitochondrial transplantation as a viable therapeutic strategy.

A Novel Form of Neuregulin 1 Type III Caused by N-Terminal Processing

Nrg1 (Neuregulin 1) type III, a susceptible gene of schizophrenia, exhibits a critical role in the central nervous system and is essential at each stage of Schwann's cell development. Nrg1 type III comprises double-pass transmembrane domains, with the N-terminal and C-terminal localizing inside the cells. The N-terminal transmembrane helix partially overlaps with the cysteine-rich domain (CRD). In this study, Nrg1 type III constructs with different tags were transformed into cultured cells to verify whether CRD destroyed the transmembrane helix formation. We took advantage of immunofluorescent and immunoprecipitation assays on whole cells and analyzed the N-terminal distribution. Astonishingly, we found that a novel form of Nrg1 type III, about 10% of Nrg1 type III, omitted the N-terminal transmembrane helix, with the N-terminal positioning outside the membrane. The results indicated that the novel single-pass transmembrane status was a minor form of Nrg1 type III caused by N-terminal processing, while the major form was a double-pass transmembrane status.

Targeting neuromuscular junction to treat neuromuscular disorders

The integrity and preservation of the neuromuscular junction (NMJ), the interface between the motor neuron and skeletal muscle, is critical for maintaining a healthy skeletal muscle. The structural and/or functional defects in the three cellular components of NMJ, namely the pre-synaptic terminal, synaptic cleft, and post-synaptic region, negatively affect skeletal muscle mass and/or strength. Therefore, NMJ repair appears to be an appropriate therapy for muscle disorders. Mouse models provide a detailed molecular characterization of various cellular components of NMJ with relevance to human diseases. This review discusses different molecular targets on the three cellular components of NMJ for treating muscle diseases. The potential effects of these therapies on NMJ morphology and motor performance, their therapeutic efficacy, and clinical relevance are discussed. Collectively, the available data supports targeting NMJ alone or as an adjunct therapy in treating muscle disorders. However, the potential impact of such interventions on human patients with muscle disorders requires further investigation.

RNA Transcript Diversity in Neuromuscular Research

Three decades since the Human Genome Project began, scientists have now identified more then 25,000 protein coding genes in the human genome. The vast majority of the protein coding genes (>  90%) are multi-exonic, with the coding DNA being interrupted by intronic sequences, which are removed from the pre-mRNA transcripts before being translated into proteins, a process called splicing maturation. Variations in this process, i.e. by exon skipping, intron retention, alternative 5' splice site (5'ss), 3' splice site (3'ss), or polyadenylation usage, lead to remarkable transcriptome and proteome diversity in human tissues. Given its critical biological importance, alternative splicing is tightly regulated in a tissue- and developmental stage-specific manner. The central nervous system and skeletal muscle are amongst the tissues with the highest number of differentially expressed alternative exons, revealing a remarkable degree of transcriptome complexity. It is therefore not surprising that splicing mis-regulation is causally associated with a myriad of neuromuscular diseases, including but not limited to amyotrophic lateral sclerosis (ALS), spinal muscular atrophy (SMA), Duchenne muscular dystrophy (DMD), and myotonic dystrophy type 1 and 2 (DM1, DM2). A gene's transcript diversity has since become an integral and an important consideration for drug design, development and therapy. In this review, we will discuss transcript diversity in the context of neuromuscular diseases and current approaches to address splicing mis-regulation.

Boosting neuregulin 1 type-III expression hastens SMA motor axon maturation

Intercellular communication between axons and Schwann cells is critical for attaining the complex morphological steps necessary for axon maturation. In the early onset motor neuron disease spinal muscular atrophy (SMA), many motor axons are not ensheathed by Schwann cells nor grow sufficiently in radial diameter to become myelinated. These developmentally arrested motor axons are dysfunctional and vulnerable to rapid degeneration, limiting efficacy of current SMA therapeutics. We hypothesized that accelerating SMA motor axon maturation would improve their function and reduce disease features. A principle regulator of peripheral axon development is neuregulin 1 type III (NRG1-III). Expressed on axon surfaces, it interacts with Schwann cell receptors to mediate axon ensheathment and myelination. We examined NRG1 mRNA and protein expression levels in human and mouse SMA tissues and observed reduced expression in SMA spinal cord and in ventral, but not dorsal root axons. To determine the impact of neuronal NRG1-III overexpression on SMA motor axon development, we bred NRG1-III overexpressing mice to SMA∆7 mice. Neonatally, elevated NRG1-III expression increased SMA ventral root size as well as axon segregation, diameter, and myelination resulting in improved motor axon conduction velocities. NRG1-III was not able to prevent distal axonal degeneration nor improve axon electrophysiology, motor behavior, or survival of older mice. Together these findings demonstrate that early SMA motor axon developmental impairments can be ameliorated by a molecular strategy independent of SMN replacement providing hope for future SMA combinatorial therapeutic approaches.

A Glance at the Molecules That Regulate Oligodendrocyte Myelination

Oligodendrocyte (OL) myelination is a critical process for the neuronal axon function in the central nervous system. After demyelination occurs because of pathophysiology, remyelination makes repairs similar to myelination. Proliferation and differentiation are the two main stages in OL myelination, and most factors commonly play converse roles in these two stages, except for a few factors and signaling pathways, such as OLIG2 (Oligodendrocyte transcription factor 2). Moreover, some OL maturation gene mutations induce hypomyelination or hypermyelination without an obvious function in proliferation and differentiation. Herein, three types of factors regulating myelination are reviewed in sequence.