What could be the function of the spinal muscular atrophy-causing protein SMN in macrophages?

Spinal Muscular Atrophy (SMA), a neurodegenerative disorder, extends its impact beyond the nervous system. The central protein implicated in SMA, Survival Motor Neuron (SMN) protein, is ubiquitously expressed and functions in fundamental processes such as alternative splicing, translation, cytoskeletal dynamics and signaling. These processes are relevant for all cellular systems, including cells of the immune system such as macrophages. Macrophages are capable of modulating their splicing, cytoskeleton and expression profile in order to fulfil their role in tissue homeostasis and defense. However, less is known about impairment or dysfunction of macrophages lacking SMN and the subsequent impact on the immune system of SMA patients. We aimed to review the potential overlaps between SMN functions and macrophage mechanisms highlighting the need for future research, as well as the current state of research addressing the role of macrophages in SMA.

Dysregulation of innate immune signaling in animal models of spinal muscular atrophy.

BACKGROUND: Spinal muscular atrophy (SMA) is a devastating neuromuscular disease caused by hypomorphic loss of function in the survival motor neuron (SMN) protein. SMA presents across a broad spectrum of disease severity. Unfortunately, genetic models of intermediate SMA have been difficult to generate in vertebrates and are thus unable to address key aspects of disease etiology. To address these issues, we developed a Drosophila model system that recapitulates the full range of SMA severity, allowing studies of pre-onset biology as well as late-stage disease processes. RESULTS: Here, we carried out transcriptomic and proteomic profiling of mild and intermediate Drosophila models of SMA to elucidate molecules and pathways that contribute to the disease. Using this approach, we elaborated a role for the SMN complex in the regulation of innate immune signaling. We find that mutation or tissue-specific depletion of SMN induces hyperactivation of the immune deficiency (IMD) and Toll pathways, leading to overexpression of antimicrobial peptides (AMPs) and ectopic formation of melanotic masses in the absence of an external challenge. Furthermore, the knockdown of downstream targets of these signaling pathways reduced melanotic mass formation caused by SMN loss. Importantly, we identify SMN as a negative regulator of a ubiquitylation complex that includes Traf6, Bendless, and Diap2 and plays a pivotal role in several signaling networks. CONCLUSIONS: In alignment with recent research on other neurodegenerative diseases, these findings suggest that hyperactivation of innate immunity contributes to SMA pathology. This work not only provides compelling evidence that hyperactive innate immune signaling is a primary effect of SMN depletion, but it also suggests that the SMN complex plays a regulatory role in this process in vivo. In summary, immune dysfunction in SMA is a consequence of reduced SMN levels and is driven by cellular and molecular mechanisms that are conserved between insects and mammals.

SMN deficiency negatively impacts red pulp macrophages and spleen development in mouse models of spinal muscular atrophy.

Spinal muscular atrophy (SMA) is a progressive neurodegenerative disease that is the leading genetic cause of infantile death. It is caused by a severe deficiency of the ubiquitously expressed Survival Motor Neuron (SMN) protein. SMA is characterized by α-lower motor neuron loss and muscle atrophy, however, there is a growing list of tissues impacted by a SMN deficiency beyond motor neurons. The non-neuronal defects are observed in the most severe Type I SMA patients and most of the widely used SMA mouse models, however, as effective therapeutics are developed, it is unclear whether additional symptoms will be uncovered in longer lived patients. Recently, the immune system and inflammation has been identified as a contributor to neurodegenerative diseases such as ALS. To determine whether the immune system is comprised in SMA, we analyzed the spleen and immunological components in SMA mice. In this report, we identify: a significant reduction in spleen size in multiple SMA mouse models and a pathological reduction in red pulp and extramedullary hematopoiesis. Additionally, red pulp macrophages, a discrete subset of yolk sac-derived macrophages, were found to be altered in SMA spleens even in pre-symptomatic post-natal day 2 animals. These cells, which are involved in iron metabolism and the phagocytosis of erythrocytes and blood-borne pathogens are significantly reduced prior to the development of the neurodegenerative hallmarks of SMA, implying a differential role of SMN in myeloid cell ontogeny. Collectively, these results demonstrate that SMN deficiency impacts spleen development and suggests a potential role for immunological development in SMA.

Endoneurial edema in sural nerve may indicate recent onset inflammatory neuropathy.

INTRODUCTION: Sural nerve biopsy is an important means of establishing the diagnosis of inflammatory neuropathies. We investigated the diagnostic value of endoneurial edema. METHODS: Diagnostic sural nerve biopsies from 42 patients with inflammatory and 28 patients with noninflammatory neuropathies were re-evaluated for the presence of endoneurial edema. Edema was assessed on hematoxylin-eosin stained paraffin and frozen sections and on azure II-methylene blue stained semithin sections. We determined the area of endoneurial edema on digitized images in relation to the entire endoneurial area of each fascicle. RESULTS: Edema was more extensive in neuropathies with short disease duration (≤12 months) as compared to long duration (>12 months; P < 0.01). Edema in inflammatory neuropathies of ≤12 months duration covered a larger area than in noninflammatory neuropathies (P < 0.01), and the extent of edema correlated negatively with disease duration (P < 0.05). CONCLUSIONS: Endoneurial edema may be a useful additional disease marker in inflammatory neuropathies of recent onset.

Seemingly neutral polymorphic variants may confer immunity to splicing-inactivating mutations: a synonymous SNP in exon 5 of MCAD protects from deleterious mutations in a flanking exonic splicing enhancer.

The idea that point mutations in exons may affect splicing is intriguing and adds an additional layer of complexity when evaluating their possible effects. Even in the best-studied examples, the molecular mechanisms are not fully understood. Here, we use patient cells, model minigenes, and in vitro assays to show that a missense mutation in exon 5 of the medium-chain acyl-CoA dehydrogenase (MCAD) gene primarily causes exon skipping by inactivating a crucial exonic splicing enhancer (ESE), thus leading to loss of a functional protein and to MCAD deficiency. This ESE functions by antagonizing a juxtaposed exonic splicing silencer (ESS) and is necessary to define a suboptimal 3' splice site. Remarkably, a synonymous polymorphic variation in MCAD exon 5 inactivates the ESS, and, although this has no effect on splicing by itself, it makes splicing immune to deleterious mutations in the ESE. Furthermore, the region of MCAD exon 5 that harbors these elements is nearly identical to the exon 7 region of the survival of motor neuron (SMN) genes that contains the deleterious silent mutation in SMN2, indicating a very similar and finely tuned interplay between regulatory elements in these two genes. Our findings illustrate a mechanism for dramatic context-dependent effects of single-nucleotide polymorphisms on gene-expression regulation and show that it is essential that potential deleterious effects of mutations on splicing be evaluated in the context of the relevant haplotype.

[An autopsy case with lower motor neuron disease showing a transient-appearance of anti-GM1 antibody and an improvement of conduction block after gamma-globulin administration].

We report a 63-year-old man who died of respiratory failure. He was well until 1992 (57 years of his age), when he had an onset of progressive weakness of the bilateral upper limbs. He showed no improvement with TRH administration in other hospital. On January 12, 1994, he admitted to our department because of the progressive muscle weakness. Neurologic examination revealed a muscular atrophy associated with severe weakness and hyporeflexia in both upper limbs, and fasciculation were seen in his tongue. Electrophysiological studies revealed mild conduction block in the left medial nerve, and F-waves were not evoked in the left ulnar nerve and bilateral median nerves. After an administration of 25 g/day of human gamma-immunoglobulin for 5 days, conduction block as well as F-wave abnormalities in the left median and left ulnar nerve were improved, yet no improvement of muscle weakness was seen. The anti-GM1 IgG titer was transiently elevated in the patient's serum after gamma-immunoglobulin therapy. On September 8, 1994, subtotal gastrectomy was performed because of the early stage gastric cancer. Histological examination showed poorly differentiated adenocarcinoma (signet-ring cell carcinoma). His muscle weakness had been gradually extended to the lower limbs and he couldn't walk himself on January, 1998. On March, 1998, he developed tetraplegia, mild dysphagia, dysuria and the respiratory disturbance. On April 12, 1998, he admitted to our department for the second time. Neurologic examination revealed a muscular atrophy and fasciculation associated with severe weakness in all of his limbs, tongue and musclus masseter. Neither deep tendon reflex nor pathologic reflex was evoked in his upper and lower extremities. His ocular movements and sensations were well preserved. He died of respiratory failure on May 1, 1998. The patient was presented in a neurological CPC. Neurological and laboratory findings suggested a spinal progressive muscular atrophy (SPMA). However, there were several unusual points as a typical SPMA in this case, that is, an improvement of the electrophysiological abnormalities by gamma-globulin treatment, as well as transient elevation of anti-GM1 antibody. The clinical neurologists have arrived at the conclusion that the patient had lower motor neuron syndrome associated with anti-ganglioside antibody and cause of death was ascribed to the respiratory failure. We discussed whether this case was SPMA or multifocal motor neuropathy. Postmortem examination revealed numerous diverticulums in the ascending colon and lymphothyroiditis. No recurrent carcinoma was detected. Neuropathologically, both severe atrophy of the anterior spinal roots, and severe gliosis and neuronal loss in the anterior horn of the spinal cord were observed. Onuf nuclei were not affected. Neurogenic muscular atrophy was detected in the tongue, diaphragm, and limb muscles. Motor neurons of the brainstem were relatively preserved, but skein-like inclusions as detected by anti-ubiquitin antibody, were present in the facial and hypoglossal nuclei. Neither motor cortex nor cortico-spinal tracts were affected. Demyelination, remyelination or cellular infiltrations were not apparent in the right median nerve and sciatic nerves. The neuropathologic features were compatible with SPMA.

Serum antibodies to HTLV-I in Thai patients with chronic progressive myelopathy, multiple sclerosis, myopathy and in HIV-seropositive intravenous drug abusers.

Antibodies to HTLV-I were assayed in sera of 9 patients with progressive myelopathy, 11 with multiple sclerosis, 5 with myopathy and in 10 HIV-seropositive intravenous heroin abusers. Clinical features in 9 cases with progressive myelopathy were not different from those previously described in tropical spastic paraparesis associated with HTLV-I infection. No detectable HTLV-1 antibody was found in the sera of any of the 35 patients studied.

Activated T cells in type I Charcot-Marie-Tooth disease: evidence for immunologic heterogeneity.

Common recognized variability in the familial peripheral neuropathy, type I Charcot-Marie-Tooth disease (CMT I), led to an examination of cell-mediated immune responses in 23 CMT I patients. Increased numbers of activated T cells were found in the peripheral blood of 14 (61%) patients using fluorescent monoclonal Ta1 antibody as quantitated by flow cytometry. Altered immunoregulation was also suggested by increased levels of prostaglandin-mediated lymphocyte suppression. In the other nine CMT I patients, immune responses were normal. Lack of a relationship between Ta1 expression and CMT clinical symptoms, but with consistency within six CMT families, support the concept of immunologic heterogeneity in type I CMT with a possible genetic component.