REEP1 Preserves Motor Function in SOD1G93A Mice by Improving Mitochondrial Function via Interaction with NDUFA4

A decline in the activities of oxidative phosphorylation (OXPHOS) complexes has been consistently reported in amyotrophic lateral sclerosis (ALS) patients and animal models of ALS, although the underlying molecular mechanisms are still elusive. Here, we report that receptor expression enhancing protein 1 (REEP1) acts as an important regulator of complex IV assembly, which is pivotal to preserving motor neurons in SOD1G93A mice. We found the expression of REEP1 was greatly reduced in transgenic SOD1G93A mice with ALS. Moreover, forced expression of REEP1 in the spinal cord extended the lifespan, decelerated symptom progression, and improved the motor performance of SOD1G93A mice. The neuromuscular synaptic loss, gliosis, and even motor neuron loss in SOD1G93A mice were alleviated by increased REEP1 through augmentation of mitochondrial function. Mechanistically, REEP1 associates with NDUFA4, and plays an important role in preserving the integrity of mitochondrial complex IV. Our findings offer insights into the pathogenic mechanism of REEP1 deficiency in neurodegenerative diseases and suggest a new therapeutic target for ALS.

The interconnection of endoplasmic reticulum and microtubule and its implication in Hereditary Spastic Paraplegia

The endoplasmic reticulum (ER) and microtubule (MT) network form extensive contact with each other and their interconnection plays a pivotal role in ER maintenance and distribution as well as MT stability. The ER participates in a variety of biological processes including protein folding and processing, lipid biosynthesis, and Ca²⁺ storage. MTs specifically regulate cellular architecture, provide routes for transport of molecules or organelles, and mediate signaling events. The ER morphology and dynamics are regulated by a class of ER shaping proteins, which also provide the physical contact structure for linking of ER and MT. In addition to these ER-localized and MT-binding proteins, specific motor proteins and adaptor-linking proteins also mediate bidirectional communication between the two structures. In this review, we summarize the current understanding of the structure and function of ER-MT interconnection. We further highlight the morphologic factors which coordinate the ER-MT network and maintain the normal physiological function of neurons, with their defect causing neurodegenerative diseases such as Hereditary Spastic Paraplegia (HSP). These findings promote our understanding of the pathogenesis of HSP and provide important therapeutic targets for treatment of these diseases.

Exploring the eukaryotic Yip and REEP/Yop superfamily of membrane-shaping adapter proteins (MSAPs): A cacophony or harmony of structure and function?

Polytopic cargo proteins are synthesized and exported along the secretory pathway from the endoplasmic reticulum (ER), through the Golgi apparatus, with eventual insertion into the plasma membrane (PM). While searching for proteins that could enhance cell surface expression of olfactory receptors, a new family of proteins termed “receptor expression-enhancing proteins” or REEPs were identified. These membrane-shaping hairpin proteins serve as adapters, interacting with intracellular transport machinery, to regulate cargo protein trafficking. However, REEPs belong to a larger family of proteins, the Yip (Ypt-interacting protein) family, conserved in yeast and higher eukaryotes. To date, eighteen mammalian Yip family members, divided into four subfamilies (Yipf, REEP, Yif, and PRAF), have been identified. Yeast research has revealed many intriguing aspects of yeast Yip function, functions that have not completely been explored with mammalian Yip family members. This review and analysis will clarify the different Yip family nomenclature that have encumbered prior comparisons between yeast, plants, and eukaryotic family members, to provide a more complete understanding of their interacting proteins, membrane topology, organelle localization, and role as regulators of cargo trafficking and localization. In addition, the biological role of membrane shaping and sensing hairpin and amphipathic helical domains of various Yip proteins and their potential cellular functions will be described. Lastly, this review will discuss the concept of Yip proteins as members of a larger superfamily of membrane-shaping adapter proteins (MSAPs), proteins that both shape membranes via membrane-sensing and hairpin insertion, and well as act as adapters for protein-protein interactions. MSAPs are defined by their localization to specific membranes, ability to alter membrane structure, interactions with other proteins via specific domains, and specific interactions/effects on cargo proteins.

ER Morphology in the Pathogenesis of Hereditary Spastic Paraplegia

The endoplasmic reticulum (ER) is the most abundant and widespread organelle in cells. Its peculiar membrane architecture, formed by an intricate network of tubules and cisternae, is critical to its multifaceted function. Regulation of ER morphology is coordinated by a few ER-specific membrane proteins and is thought to be particularly important in neurons, where organized ER membranes are found even in the most distant neurite terminals. Mutation of ER-shaping proteins has been implicated in the neurodegenerative disease hereditary spastic paraplegia (HSP). In this review we discuss the involvement of these proteins in the pathogenesis of HSP, focusing on the experimental evidence linking their molecular function to disease onset. Although the precise biochemical activity of some ER-related HSP proteins has been elucidated, the pathological mechanism underlying ER-linked HSP is still undetermined and needs to be further investigated.

Non-Rodent Genetic Animal Models for Studying Tauopathy: Review of Drosophila, Zebrafish, and C. elegans Models

Tauopathy refers to a group of progressive neurodegenerative diseases, including frontotemporal lobar degeneration and Alzheimer's disease, which correlate with the malfunction of microtubule-associated protein Tau (MAPT) due to abnormal hyperphosphorylation, leading to the formation of intracellular aggregates in the brain. Despite extensive efforts to understand tauopathy and develop an efficient therapy, our knowledge is still far from complete. To find a solution for this group of devastating diseases, several animal models that mimic diverse disease phenotypes of tauopathy have been developed. Rodents are the dominating tauopathy models because of their similarity to humans and established disease lines, as well as experimental approaches. However, powerful genetic animal models using Drosophila, zebrafish, and C. elegans have also been developed for modeling tauopathy and have contributed to understanding the pathophysiology of tauopathy. The success of these models stems from the short lifespans, versatile genetic tools, real-time in-vivo imaging, low maintenance costs, and the capability for high-throughput screening. In this review, we summarize the main findings on mechanisms of tauopathy and discuss the current tauopathy models of these non-rodent genetic animals, highlighting their key advantages and limitations in tauopathy research.

Inhibition of ER stress improves progressive motor deficits in a REEP1-null mouse model of hereditary spastic paraplegia

Hereditary spastic paraplegias (HSPs) are genetic neurodegenerative diseases. HSPs are characterized by lower-extremity weakness and spasticity. However, there is no specific clinical treatment strategy to prevent or reverse nerve degeneration in HSPs. Mutations in receptor expression-enhancing protein 1 (REEP1) are well-recognized and relatively common causes of autosomal dominant HSPs. REEP1 modifies the endoplasmic reticulum (ER) shape, and is implicated in the ER stress response. Defects in the ER stress response seem to be crucial mechanisms underlying HSP neurodegeneration. Here, we report that REEP1-/- mice exhibit progressive motor deficits, along with denervation of neuromuscular junctions and increased ER stress. Moreover, marked axonal degeneration and morphological abnormalities are observed. In this study, we treated both REEP1-/- and wild-type (WT) mice with salubrinal, which is a specific inhibitor of ER stress, and we observed increased nerve-muscle connections and enhanced motor functions. Our data highlight the importance of ER homeostasis in HSPs, providing new opportunities for HSP treatment.

A complete overview of REEP1: old and new insights on its role in hereditary spastic paraplegia and neurodegeneration

At the end of 19th century, Adolf von Strümpell and Sigmund Freud independently described the symptoms of a new pathology now known as hereditary spastic paraplegia (HSP). HSP is part of the group of genetic neurodegenerative diseases usually associated with slow progressive pyramidal syndrome, spasticity, weakness of the lower limbs, and distal-end degeneration of motor neuron long axons. Patients are typically characterized by gait symptoms (with or without other neurological disorders), which can appear both in young and adult ages depending on the different HSP forms. The disease prevalence is at 1.3-9.6 in 100 000 individuals in different areas of the world, making HSP part of the group of rare neurodegenerative diseases. Thus far, there are no specific clinical and paraclinical tests, and DNA analysis is still the only strategy to obtain a certain diagnosis. For these reasons, it is mandatory to extend the knowledge on genetic causes, pathology mechanism, and disease progression to give clinicians more tools to obtain early diagnosis, better therapeutic strategies, and examination tests. This review gives an overview of HSP pathologies and general insights to a specific HSP subtype called spastic paraplegia 31 (SPG31), which rises after mutation of REEP1 gene. In fact, recent findings discovered an interesting endoplasmic reticulum antistress function of REEP1 and a role of this protein in preventing τ accumulation in animal models. For this reason, this work tries to elucidate the main aspects of REEP1, which are described in the literature, to better understand its role in SPG31 HSP and other pathologies.

Naringenin Ameliorates Drosophila ReepA Hereditary Spastic Paraplegia-Linked Phenotypes

Defects in the endoplasmic reticulum (ER) membrane shaping and interaction with other organelles seem to be a crucial mechanism underlying Hereditary Spastic Paraplegia (HSP) neurodegeneration. REEP1, a transmembrane protein belonging to TB2/HVA22 family, is implicated in SPG31, an autosomal dominant form of HSP, and its interaction with Atlastin/SPG3A and Spastin/SPG4, the other two major HSP linked proteins, has been demonstrated to play a crucial role in modifying ER architecture. In addition, the Drosophila ortholog of REEP1, named ReepA, has been found to regulate the response to ER neuronal stress. Herein we investigated the role of ReepA in ER morphology and stress response. ReepA is upregulated under stress conditions and aging. Our data show that ReepA triggers a selective activation of Ire1 and Atf6 branches of Unfolded Protein Response (UPR) and modifies ER morphology. Drosophila lacking ReepA showed Atf6 and Ire1 activation, expansion of ER sheet-like structures, locomotor dysfunction and shortened lifespan. Furthermore, we found that naringenin, a flavonoid that possesses strong antioxidant and neuroprotective activity, can rescue the cellular phenotypes, the lifespan and locomotor disability associated with ReepA loss of function. Our data highlight the importance of ER homeostasis in nervous system functionality and HSP neurodegenerative mechanisms, opening new opportunities for HSP treatment.

NeurodegenERation: The Central Role for ER Contacts in Neuronal Function and Axonopathy, Lessons From Hereditary Spastic Paraplegias and Related Diseases

The hereditary spastic paraplegias (HSPs) are a group of inherited neurodegenerative conditions whose characteristic feature is degeneration of the longest axons within the corticospinal tract which leads to progressive spasticity and weakness of the lower limbs. Though highly genetically heterogeneous, the majority of HSP cases are caused by mutations in genes encoding proteins that are responsible for generating and organizing the tubular endoplasmic reticulum (ER). Despite this, the role of the ER within neurons, particularly the long axons affected in HSP, is not well understood. Throughout axons, ER tubules make extensive contacts with other organelles, the cytoskeleton and the plasma membrane. At these ER contacts, protein complexes work in concert to perform specialized functions including organelle shaping, calcium homeostasis and lipid biogenesis, all of which are vital for neuronal survival and may be disrupted by HSP-causing mutations. In this article we summarize the proteins which mediate ER contacts, review the functions these contacts are known to carry out within neurons, and discuss the potential contribution of disruption of ER contacts to axonopathy in HSP.

DNA damage triggers tubular endoplasmic reticulum extension to promote apoptosis by facilitating ER-mitochondria signaling

The endoplasmic reticulum (ER) is composed of the nuclear envelope, perinuclear sheets and a peripheral tubular network. The peripheral ER and mitochondria form tight contacts at specific subdomains, which coordinate the functions of the two organelles and are required for multiple cellular processes such as Ca²⁺ transfer and apoptosis. However, it is largely unknown how ER morphology and ER-mitochondria signaling are dynamically regulated under different physiological or pathological conditions such as DNA damage. Here we show that the peripheral, tubular ER undergoes significant extension in response to DNA damage, and that this process is dependent on p53-mediated transcriptional activation of the ER-shaping proteins REEP1, REEP2 and EI24 (alias PIG8). This promotes the formation of ER-mitochondria contacts through EI24 and the mitochondrial outer membrane protein VDAC2, facilitates Ca²⁺ transfer from ER to mitochondria and promotes DNA damage-induced apoptosis. Thus, we identify a unique DNA damage response pathway involving alterations in ER morphology, ER-mitochondria signaling, and apoptosis.

Reep1 null mice reveal a converging role for hereditary spastic paraplegia proteins in lipid droplet regulation

Hereditary spastic paraplegias (HSPs; SPG1-76 plus others) are length-dependent disorders affecting long corticospinal axons, and the most common autosomal dominant forms are caused by mutations in genes that encode the spastin (SPG4), atlastin-1 (SPG3A) and REEP1 (SPG31) proteins. These proteins bind one another and shape the tubular endoplasmic reticulum (ER) network throughout cells. They also are involved in lipid droplet formation, enlargement, or both in cells, though mechanisms remain unclear. Here we have identified evidence of partial lipoatrophy in Reep1 null mice in addition to prominent spastic paraparesis. Furthermore, Reep1-/- embryonic fibroblasts and neurons in the cerebral cortex both show lipid droplet abnormalities. The apparent partial lipodystrophy in Reep1 null mice, although less severe, is reminiscent of the lipoatrophy phenotype observed in the most common form of autosomal recessive lipodystrophy, Berardinelli-Seip congenital lipodystrophy. Berardinelli-Seip lipodystrophy is caused by autosomal recessive mutations in the BSCL2 gene that encodes an ER protein, seipin, that is also mutated in the autosomal dominant HSP SPG17 (Silver syndrome). Furthermore, REEP1 co-immunoprecipitates with seipin in cells. This strengthens the link between alterations in ER morphogenesis and lipid abnormalities, with important pathogenic implications for the most common forms of HSP.

Recessive REEP1 mutation is associated with congenital axonal neuropathy and diaphragmatic palsy

OBJECTIVE

To identify the underlying genetic cause of a congenital neuropathy in a 5-year-old boy as part of a cohort of 32 patients from 23 families with genetically unresolved neuropathies.

METHODS

We used autozygosity mapping coupled with next-generation sequencing to investigate a consanguineous family from Lebanon with 1 affected and 2 healthy children. Variants were investigated for segregation in the family by Sanger sequencing. A splice site mutation was further evaluated on the messenger RNA level by quantitative reverse transcription PCR. Subsequently, a larger cohort was specifically screened for receptor expression-enhancing protein 1 (REEP1) gene mutations.

RESULTS

We detected a homozygous splice donor mutation in REEP1 (c.303+1-7GTAATAT>AC, p.F62Kfs23*; NM_022912) that cosegregated with the phenotype in the family, leading to complete skipping of exon 4 and a premature stop codon. The phenotype of the patient is similar to spinal muscular atrophy with respiratory distress type 1 (SMARD1) with additional distal arthrogryposis and involvement of the upper motor neuron manifested by pronounced hyperreflexia.

CONCLUSION

To date, only dominant REEP1 mutations have been reported to be associated with a slowly progressive hereditary spastic paraplegia. The findings from our patient expand the phenotypical spectrum and the mode of inheritance of REEP1-associated disorders. Recessive mutations in REEP1 should be considered in the molecular genetic workup of patients with a neuromuscular disorder resembling SMARD1, especially if additional signs of upper motor neuron involvement and distal arthrogryposis are present.

Induction of excessive endoplasmic reticulum stress in the Drosophila male accessory gland results in infertility

Endoplasmic reticulum (ER) stress occurs when misfolded proteins accumulate in the lumen of the ER. A cell responds to ER stress with the unfolded protein response (UPR), a complex program of transcriptional and translational changes aimed at clearing misfolded proteins. Secretory tissues and cells are particularly well adapted to respond to ER stress because their function requires high protein production and secretory load. The insect male accessory gland (AG) is a secretory tissue involved in male fertility. The AG secretes many seminal fluid proteins (SFPs) essential for male reproduction. Among adult Drosophila tissues, we find that genes upregulated by ER stress are most highly expressed in the AG, suggesting that the AG is already undergoing high levels of ER stress due to its normal secretory functions. We hypothesized that induction of excessive ER stress in the AG above basal levels, would perturb normal function and provide a genetic tool for studying AG and SFP biology. To test this, we genetically induced excessive ER stress in the AG by conditional 1) expression of a misfolded protein or 2) knockdown of the UPR regulatory protein, BiP. Both genetic manipulations induced excessive ER stress in the AG, as indicated by the increase in Xbp1 splicing, a marker of ER stress. Both models resulted in a large decrease in or loss of SFP production and male infertility. Sperm production, motility, and transfer appeared unaffected. The induction of strong ER stress in the insect male AG may provide a simple way for studying or manipulating male fertility, as it eliminates AG function while preserving sperm production.