The Reelin receptor ApoER2 is a cargo for the adaptor protein complex AP-4: Implications for Hereditary Spastic Paraplegia

Adaptor protein complex 4 (AP-4) is a heterotetrameric complex that promotes export of selected cargo proteins from the trans-Golgi network. Mutations in each of the AP-4 subunits cause a complicated form of Hereditary Spastic Paraplegia (HSP). Herein, we report that ApoER2, a receptor in the Reelin signaling pathway, is a cargo of the AP-4 complex. We identify the motif ISSF/Y within the ApoER2 cytosolic domain as necessary for interaction with the canonical signal-binding pocket of the µ4 (AP4M1) subunit of AP-4. AP4E1- knock-out (KO) HeLa cells and hippocampal neurons from Ap4e1-KO mice display increased co-localization of ApoER2 with Golgi markers. Furthermore, hippocampal neurons from Ap4e1-KO mice and AP4M1-KO human iPSC-derived cortical i3Neurons exhibit reduced ApoER2 protein expression. Analyses of biosynthetic transport of ApoER2 reveal differential post-Golgi trafficking of the receptor, with lower axonal distribution in KO compared to wild-type neurons, indicating a role of AP-4 and the ISSF/Y motif in the axonal localization of ApoER2. Finally, analyses of Reelin signaling in mouse hippocampal and human cortical KO neurons show that AP4 deficiency causes no changes in Reelin-dependent activation of the AKT pathway and only mild changes in Reelin-induced dendritic arborization, but reduces Reelin-induced ERK phosphorylation, CREB activation, and Golgi deployment. This work thus establishes ApoER2 as a novel cargo of the AP-4 complex, suggesting that defects in the trafficking of this receptor and in the Reelin signaling pathway could contribute to the pathogenesis of HSP caused by mutations in AP-4 subunits.

Atl (atlastin) regulates mTor signaling and autophagy in Drosophila muscle through alteration of the lysosomal network

ABBREVIATIONS

atl atlastin; ALR autophagic lysosome reformation; ER endoplasmic reticulum; GFP green fluorescent protein; HSP hereditary spastic paraplegia; Lamp1 lysosomal associated membrane protein 1 PolyUB polyubiquitin; RFP red fluorescent protein; spin spinster; mTor mechanistic Target of rapamycin; VCP valosin containing protein.

Syringomyelia: A New Phenotype of SPG11-Related Hereditary Spastic Paraplegia?

Hereditary spastic paraplegia (HSP) refers to a group of neurodegenerative disorders affecting motor neurons in the central nervous system. HSP type 11 is the most frequent subtype of autosomal recessive HSPs. Caused by pathogenic variants in SPG11, HSP type 11 has a heterogeneous clinical presentation, including various degrees of cognitive dysfunction, spasticity and weakness predominantly in the lower extremities among other features. An 8-year-old boy visited our rehabilitation clinic with a chief complaint of intellectual impairment. Motor weakness was not apparent, but he exhibited a mild limping gait with physical signs of upper motor neuron involvement. Next generation sequencing revealed biallelic pathogenic variants, c.2163dupT and c.5866+1G>A in SPG11, inherited biparentally which was confirmed by Sanger sequencing. Brain imaging study showed thinning of corpus callosum, consistent with previous reports, however whole spine imaging study revealed extensive syringomyelia in his spinal cord, a rare finding in HSP type 11. Further studies are needed to determine whether this finding is a true phenotype associated with HSP type 11.

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.

Neuroimaging in hereditary spastic paraplegias: from qualitative cues to precision biomarkers

INTRODUCTION

: Hereditary spastic paraplegias (HSP) include a clinically and genetically heterogeneous group of conditions. Novel imaging modalities have been increasingly applied to HSP cohorts which helps to quantitatively evaluate the integrity of specific anatomical structures and develop monitoring markers for both clinical care and future clinical trials.

AREAS COVERED

: Advances in HSP imaging are systematically reviewed with a focus on cohort sizes, imaging modalities, study design, clinical correlates, methodological approaches, and key findings.

EXPERT OPINION

: A wide range of imaging techniques have been recently applied to HSP cohorts. Common shortcomings of existing studies include the evaluation of genetically unconfirmed or admixed cohorts, limited sample sizes, unimodal imaging approaches, lack of postmortem validation, and a limited clinical battery, often exclusively focusing on motor aspects of the condition. A number of innovative methodological approaches have also be identified, such as robust longitudinal study designs, the implementation of multimodal imaging protocols, complementary cognitive assessments, and the comparison of HSP cohorts to MND cohorts. Collaborative multicentre initiatives may overcome sample limitations, and comprehensive clinical profiling with motor, extrapyramidal, cerebellar, and neuropsychological assessments would permit systematic clinico-radiological correlations. Academic achievements in HSP imaging have the potential to be developed into viable clinical applications to expedite the diagnosis and monitor disease progression.

Therapeutic Strategies for Mutant SPAST-Based Hereditary Spastic Paraplegia

Mutations of the SPAST gene that encodes the microtubule-severing enzyme called spastin are the chief cause of Hereditary Spastic Paraplegia. Growing evidence indicates that pathogenic mutations functionally compromise the spastin protein and endow it with toxic gain-of-function properties. With each of these two factors potentially relevant to disease etiology, the present article discusses possible therapeutic strategies that may ameliorate symptoms in patients suffering from SPAST-based Hereditary Spastic Paraplegia, which is usually termed SPG4-HSP.

Exome sequencing of a Pakistani family with spastic paraplegia identified an 18 bp deletion in the cytochrome B5 domain of FA2H

Hereditary spastic paraplegias (HSPs) are a diverse class of neurodegenerative disorders that mainly affect the corticospinal tract of the body and result in various clinical conditions such as lower limb spasticity and muscle weakness in the lower extremities. Worldwide, more than 70 chromosomal loci/genes have been reported to be associated with HSPs, out of which, six genes viz., ATL1, FA2H, GJC2, AP4E1, ALDH18A1 and ATP13A2 have been mapped in Pakistani families. In the present genetic study, we report on a large consanguineous Pakistani family with a complex form of HSP segregating with a 18 bp deletion in the first exon of the Fatty Acid 2-Hydroxylase (FA2H) gene (NM_024306.5:c.159_176del). The identified in-frame deletion results in loss of six amino acids (p.Arg53_Ile58del) within the cytochrome B5 domain of the protein. FA2H is required for alpha-hydroxylation of free fatty acids to form alpha-hydroxylated sphingolipids. Its cytochrome b5-like heme-binding domain, which spans from residues 15 to 85, imparts the redox activity to FA2H. This mutation has previously been reported in a Pakistani family presenting with a similar form of complex HSP. Together with our findings the pathogenic role of the observed variant is further supported. Mutation studies on additional Pakistani families for FA2H will further elucidate its mutational spectrum, which may help in developing a prenatal diagnostic test for Khyber Pakhtunkhwa resident Pakistani families.

Hereditary Spastic Paraplegia Is a Common Phenotypic Finding in ARG1 Deficiency, P5CS Deficiency and HHH Syndrome: Three Inborn Errors of Metabolism Caused by Alteration of an Interconnected Pathway of Glutamate and Urea Cycle Metabolism

Hereditary Spastic Paraplegias (HSPs) are a clinically and genetically heterogeneous group of neurodegenerative disorders characterized by a progressive rigidity and weakness of the lower limbs, caused by pyramidal tract lesions. As of today, 80 different forms of HSP have been mapped, 64 genes have been cloned, and new forms are constantly being described. HSPs represent an intensively studied field, and the functional understanding of the biochemical and molecular pathogenetic pathways are starting to be elucidated. Recently, dominant and recessive mutations in the ALDH18A1 gene resulting in the deficiency of the encoded enzyme (delta-1-pyrroline-5-carboxylate synthase, P5CS) have been pathogenetically linked to HSP. P5CS is a critical enzyme in the conversion of glutamate to pyrroline-5-carboxylate, an intermediate that enters in the proline biosynthesis and that is connected with the urea cycle. Interestingly, two urea cycle disorders, Argininemia and Hyperornithinemia-Hyperammonemia-Homocitrullinuria syndrome, are clinically characterized by highly penetrant spastic paraplegia. These three diseases represent a peculiar group of HSPs caused by Inborn Errors of Metabolism. Here we comment on these forms, on the common features among them and on the hypotheses for possible shared pathogenetic mechanisms causing the HSP phenotype.

Rare disease models provide insight into inherited forms of neurodegeneration

Hereditary spastic paraplegias (HSPs) are a group of inherited neurodegenerative conditions characterised by retrograde degeneration of the longest motor neurons in the corticospinal tract, resulting in muscle weakness and spasticity of the lower limbs. To date more than 70 genetic loci have been associated with HSP, however the majority of cases are caused by mutations that encode proteins responsible for generating and maintaining tubular endoplasmic reticulum (ER) structure. These ER-shaping proteins are vital for the long-term survival of axons, however the mechanisms by which mutations in these proteins give rise to HSP remain poorly understood. To begin to address this we have characterized in vivo loss of function models of two very rare forms of HSP caused by loss of the ER-shaping proteins ARL6IP1 (SPG61) and RTN2 (SPG12). These models display progressive locomotor defects, disrupted organisation of the tubular ER and length-dependant defects in the axonal mitochondrial network. Here we compare our findings with those associated with more common forms HSP including: Spastin, Atlastin-1 and REEP 1 which together account for over half of all cases of autosomal dominant HSP. Furthermore, we discuss recent observations in other HSP models which are directly implicated in mitochondrial function and localization. Overall, we highlight the common features of our rare models of HSP and other models of disease which could indicate shared mechanisms underpinning neurodegeneration in these disorders.

Low dose tubulin-binding drugs rescue peroxisome trafficking deficit in patient-derived stem cells in Hereditary Spastic Paraplegia

Hereditary Spastic Paraplegia (HSP) is a genetically heterogeneous group of disorders, diagnosed by progressive gait disturbances with muscle weakness and spasticity, for which there are no treatments targeted at the underlying pathophysiology. Mutations in spastin are a common cause of HSP. Spastin is a microtubule-severing protein whose mutation in mouse causes defective axonal transport. In human patient-derived olfactory neurosphere-derived (ONS) cells, spastin mutations lead to lower levels of acetylated α-tubulin, a marker of stabilised microtubules, and to slower speed of peroxisome trafficking. Here we screened multiple concentrations of four tubulin-binding drugs for their ability to rescue levels of acetylated α-tubulin in patient-derived ONS cells. Drug doses that restored acetylated α-tubulin to levels in control-derived ONS cells were then selected for their ability to rescue peroxisome trafficking deficits. Automated microscopic screening identified very low doses of the four drugs (0.5 nM taxol, 0.5 nM vinblastine, 2 nM epothilone D, 10 µM noscapine) that rescued acetylated α-tubulin in patient-derived ONS cells. These same doses rescued peroxisome trafficking deficits, restoring peroxisome speeds to untreated control cell levels. These results demonstrate a novel approach for drug screening based on high throughput automated microscopy for acetylated α-tubulin followed by functional validation of microtubule-based peroxisome transport. From a clinical perspective, all the drugs tested are used clinically, but at much higher doses. Importantly, epothilone D and noscapine can enter the central nervous system, making them potential candidates for future clinical trials.