Intermediate filaments are the most heterogeneous class among cytoskeletal elements. While some of them have been well-characterized, little is known about peripherin. Peripherin is a class III intermediate filament protein with a specific expression in the peripheral nervous system. Epigenetic modifications are involved in this cell-type-specific expression. Peripherin has important roles in neurite outgrowth and stability, axonal transport, and axonal myelination. Moreover, peripherin interacts with proteins involved in vesicular trafficking, signal transduction, DNA/RNA processing, protein folding, and mitochondrial metabolism, suggesting a role in all these processes. This review collects information regarding peripherin gene regulation, post-translational modifications, and functions and its involvement in the onset of a number of diseases.

Charcot-Marie-Tooth type 2B (CMT2B) disease is a dominant axonal peripheral neuropathy caused by five mutations in the RAB7A gene. Autophagy and late endocytic trafficking were already characterized in CMT2B. Indeed, impairment of autophagy and an increase in lysosomal degradative activity were found in cells expressing the mutant proteins. Recently, we described a novel RAB7 mutation associated with predominantly motor CMT2 and impaired EGFR trafficking. With the aim to analyze the autophagy process and lysosomal activity in CMT2B fibroblasts carrying the p.K126R RAB7 novel mutation and to investigate further the causes of the different phenotype, we have performed Western blot, immunofluorescence and cytometric analyses monitoring autophagic markers and endocytic proteins. Moreover, we investigated lipophagy by analyzing accumulation of lipid droplets and their co-localization with endolysosomal degradative compartments. We found that cells expressing the RAB7^K126R mutant protein were characterized by impairment of autophagy and lipophagy processes and by a moderate increase in lysosomal activity compared to the previously studied cells carrying the RAB7^V162M mutation. Thus, we concluded that EGFR trafficking alterations and a moderate increase in lysosomal activity with concomitant impairment of autophagy could induce the specific predominantly motor phenotype observed in K126R patients.

Amyotrophic lateral sclerosis (ALS) is a neurodegenerative disease caused by the degeneration of upper and lower motor neurons in the brain, brainstem and spinal cord. About 10% of familial ALS cases are linked to pathogenetic substitution in TARDBP, the gene encoding the TDP-43 protein. A novel rare causative variant in TARDBP (p.G376D) was recently reported in ALS patients. It leads to TDP-43 cytoplasmic mislocalization, increased oxidative stress and reduced cell viability. However, functional studies on the effects of this molecular defect have not yet been carried out. Mitochondria are highly dynamic organelles, and their deregulation has emerged as a key factor in many diseases, among which is ALS. Therefore, this study aimed at determining the impact of this causative variant on mitochondria. In cellular models expressing TDP-43^G376D and in fibroblasts derived from patients carrying this molecular defect, we observed alterations of mitochondrial functionality. We demonstrated increased localization of the mutated protein to mitochondria and a reduced abundance of subunits of complex I and complex II of the mitochondrial respiratory chain, associated with a decrease in mitochondrial membrane potential, in cellular respiration and in cytochrome C oxidase (COX) activity. Moreover, ALS cells showed increased mitochondrial fragmentation and reduced abundance of antioxidant enzymes causing increased oxidative stress. These results expand our knowledge about the molecular mechanisms underlying ALS pathogenesis associated with TDP-43 p.G376D and could help to identify new therapeutic strategies to counteract this disease.

Amyotrophic lateral sclerosis (ALS) is a fatal neurodegenerative disease affecting motor neurons. Consequent to the loss of these cells, neuromuscular functions decline, causing progressive weakness, muscle wasting, and paralysis, leading to death in 2 to 5 years. More than 90% of ALS cases are sporadic, while the remaining 10% of cases are familial, due to mutations in 40 different genes. One of the most common genes to be mutated in ALS is TARDBP (transactive response DNA binding protein 43), which encodes TDP-43 (TAR DNA-binding protein 43). A mutation in exon 6 of TARDBP causes the aminoacidic substitution G376D in the C-terminal region of TDP-43, leading to its cytoplasmic mislocalization and aggregation. In fibroblasts derived from patients carrying this mutation, we found a strong increase in lysosome number, with overexpression and higher nuclear translocation of the transcription factor TFEB. In contrast, lysosomal functionality was deeply compromised. Interestingly, lysosomal activity was unaffected at an early stage of the disease, worsening in more advanced stages. Moreover, we observed the same pathological phenotype in iPSC (induced pluripotent stem cells)-derived patient motor neurons carrying the G376D mutation. Therefore, this mutation compromises the functionality of lysosomes, possibly contributing to neurodegeneration.

Amyotrophic Lateral Sclerosis (ALS) is a progressive neurodegenerative disorder strongly associated with mitochondrial dysfunction and impaired proteostasis. Mutations in TARDBP, encoding TAR DNA-binding protein 43 (TDP-43), contribute to disease pathogenesis through cytoplasmic mislocalization and aggregation. Among these, the ALS-linked TDP-43^G376D mutation has been previously associated with oxidative stress, mitochondrial fragmentation, and impaired oxidative phosphorylation. Here, we investigated the impact of TDP-43^G376D on Mitochondrial Quality Control (MQC) pathways using patient-derived dermal fibroblasts carrying the mutation at early and advanced disease stages, complemented by HEK293T and Neuro2a cellular models expressing mutant TDP-43. We show that TDP-43^G376D impairs mitophagic flux, as evidenced by reduced delivery of damaged mitochondria to lysosomes. This was accompanied by pronounced disruption of mitochondrial cristae architecture and accumulation of mitochondrial DNA damage, indicating compromised mitochondrial genome integrity. Furthermore, TDP-43^G376D induces sustained activation of the mitochondrial Unfolded Protein Response (UPR^mt), consistent with persistent mitochondrial stress, while selectively impairing the sirtuin-dependent antioxidant branch. In parallel, activation of the Endoplasmic Reticulum UPR (UPR^ER) was observed, indicating a coordinated engagement of cellular stress pathways. Collectively, our findings identify coordinated alterations in multiple MQC pathways associated with TDP-43^G376D rather than isolated mitochondrial defects, supporting further investigation of these pathways in larger and disease-relevant ALS models.