Cells dedicate significant energy to build proteins often organized in multiprotein assemblies with tightly regulated stoichiometries. As genes encoding subunits assembling in a multisubunit complex are dispersed in the genome of eukaryotes, it is unclear how these protein complexes assemble. Here, we show that mammalian nuclear transcription complexes (TFIID, TREX-2 and SAGA) composed of a large number of subunits, but lacking precise architectural details are built co-translationally. We demonstrate that dimerization domains and their positions in the interacting subunits determine the co-translational assembly pathway (simultaneous or sequential). The lack of co-translational interaction can lead to degradation of the partner protein. Thus, protein synthesis and complex assembly are linked in building mammalian multisubunit complexes, suggesting that co-translational assembly is a general principle in mammalian cells to avoid non-specific interactions and protein aggregation. These findings will also advance structural biology by defining endogenous co-translational building blocks in the architecture of multisubunit complexes.

In this study, we describe the phenotypic spectrum of distal hereditary motor neuropathy caused by mutations in the small heat shock proteins HSPB1 and HSPB8 and investigate the functional consequences of newly discovered variants. Among 510 unrelated patients with distal motor neuropathy, we identified mutations in HSPB1 (28 index patients/510; 5.5%) and HSPB8 (four index patients/510; 0.8%) genes. Patients have slowly progressive distal (100%) and proximal (13%) weakness in lower limbs (100%), mild lower limbs sensory involvement (31%), foot deformities (73%), progressive distal upper limb weakness (29%), mildly raised serum creatine kinase levels (100%), and central nervous system involvement (9%). We identified 12 HSPB1 and four HSPB8 mutations, including five and three not previously reported. Transmission was either dominant (78%), recessive (3%), or de novo (19%). Three missense mutations in HSPB1 (Pro7Ser, Gly53Asp, and Gln128Arg) cause hyperphosphorylation of neurofilaments, whereas the C-terminal mutant Ser187Leu triggers protein aggregation. Two frameshift mutations (Leu58fs and Ala61fs) create a premature stop codon leading to proteasomal degradation. Two mutations in HSPB8 (Lys141Met/Asn) exhibited increased binding to Bag3. We demonstrate that HSPB1 and HSPB8 mutations are a major cause of inherited motor axonal neuropathy. Mutations lead to diverse functional outcomes further demonstrating the pleotropic character of small heat shock proteins.

The objective of this study is to assess the genetic distribution of Charcot-Marie-Tooth (CMT) disease in Campania, a region of Southern Italy. We analyzed a cohort of 197 index cases and reported the type and frequency of mutations for the whole CMT population and for each electrophysiological group (CMT1, CMT2, and hereditary neuropathy with susceptibility to pressure palsies [HNPP]) and for familial and isolated CMT cases. Genetic diagnosis was achieved in 148 patients (75.1%) with a higher success rate in HNPP and CMT1 than CMT2. Only four genes (PMP22, GJB1, MPZ, and GDAP1) accounted for 92% of all genetically confirmed CMT cases. In CMT1, PMP22 duplication was the most common mutation while the second gene in order of frequency was MPZ in familial and SH3TC2 in isolated cases. In CMT2, GJB1 was the most frequent mutated gene and GJB1 with GDAP1 accounted for almost 3/4 of genetically defined CMT2 patients. The first gene in order of frequency was GJB1 in familial and GDAP1 in isolated cases. In HNPP, the majority of patients harbored the PMP22 gene deletion. The novelty of our data is the relatively high frequency of SH3TC2 and GDAP1 mutations in demyelinating and axonal forms, respectively. These epidemiological data can help in panel design for our patients' population.

Mutations in the small heat-shock protein 27 kDa protein 1 (HSPB1) and 22 kDa protein 8 (HSPB8) genes were associated with distal hereditary motor neuropathy (dHMN) and with the axonal form of Charcot-Marie-Tooth disease type 2 (CMT2). Here we report the clinical and molecular evaluation of an Italian dHMN and CMT2 cohort to establish HSPB1 and HSPB8 mutation occurrence and associated clinical features. One hundred and sixty-seven patients with dHMN or CMT2 were studied. HSPB1 and HSPB8 exons 1 and 3 molecular analysis was carried out through DHPLC and direct sequencing of each variant chromatogram. HSPB8 exon 2 was analyzed by direct sequencing. Four mutations in five unrelated dHMN patients and four mutations in four unrelated CMT2 cases were found in HSPB1. The p.Arg136Leu mutation was found in two patients with different phenotypes. Electroneurographical follow-up study in a dHMN patient revealed that sensory impairment occurred with disease progression. The HSPB1 mutation frequency was 8% in dHMN and 4% in CMT2 patients. The significant HSPB1 mutation frequency in both phenotypes indicates its relevance in the pathogenesis of these neuropathies. Recent literature data suggest a continuum between dHMN and CMT2. We confirm this finding in our cohort, proposing a definite relationship between these disorders.

Genetic discoveries in amyotrophic lateral sclerosis (ALS) have a significant impact on deciphering molecular mechanisms of motor neuron degeneration but, despite recent advances, the etiology of most sporadic cases remains elusive. Several cellular mechanisms contribute to the motor neuron degeneration in ALS, including RNA metabolism, cellular interactions between neurons and nonneuronal cells, and seeding of misfolded protein with prion‐like propagation. In this scenario, the importance of protein turnover and degradation in motor neuron homeostasis gained increased recognition. In this study, we evaluated the role of the candidate gene HSPB1, a molecular chaperone involved in several proteome‐maintenance functions. In a cohort of 247 unrelated Italian ALS patients, we identified two variants (c.570G>C, p.Gln190His and c.610dupG, p.Ala204Glyfs*6). Functional characterization of the p.Ala204Glyfs*6 demonstrated that the mutant protein alters HSPB1 dynamic equilibrium, sequestering the wild‐type protein in a stable dimer and resulting in a loss of chaperone‐like activity. Our results underline the relevance of identifying rare but pathogenic variations in sporadic neurodegenerative diseases, suggesting a possible correlation between specific pathomechanisms linked to HSPB1 mutations and the associated neurological phenotype. Our study provides additional lines of evidence to support the involvement of HSPB1 in the pathogenesis of sporadic ALS.

We report the first case of a missense mutation in MPZ causing a gain of glycosylation in Myelin Protein Zero (P0), the main protein of peripheral nervous system myelin. The patient was affected by a severe demyelinating neuropathy caused by a missense mutation, D32N, that created a new glycosylation sequence. We confirmed that the mutant protein is hyperglycosylated, is partially retained into the Golgi apparatus in vitro and disrupts intercellular adhesion. By sequential experiments, we demonstrated that hyperglycosylation is the main mechanism of this mutation. Gain of glycosylation is a new mechanism in CMT1B.

No abstract

Neuronal microexons represent the most highly conserved class of alternative splicing events and their timed expression shapes neuronal biology, including neuronal commitment and differentiation. The six-nt microexon 34ʹ is included in the neuronal form of TAF1 mRNA, which encodes the largest subunit of the basal transcription factor TFIID. In this study, we investigate the tissue distribution of TAF1-34ʹ mRNA and protein and the mechanism responsible for its neuronal-specific splicing. Using isoform-specific RNA probes and antibodies, we observe that canonical TAF1 and TAF1-34ʹ have different distributions in the brain, which distinguish proliferating from post-mitotic neurons. Knockdown and ectopic expression experiments demonstrate that the neuronal-specific splicing factor SRRM4/nSR100 promotes the inclusion of microexon 34ʹ into TAF1 mRNA, through the recognition of UGC sequences in the poly-pyrimidine tract upstream of the regulated microexon. These results show that SRRM4 regulates temporal and spatial expression of alternative TAF1 mRNAs to generate a neuronal-specific TFIID complex.