Widespread thalamic and cerebellar degeneration in a patient with a complicated hereditary spastic paraplegia (HSP)

Betz cells of the primary motor cortex

Betz cells, named in honor of Volodymyr Betz (1834-1894), who described them as "giant pyramids" in the primary motor cortex of primates and other mammalian species, are layer V extratelencephalic projection (ETP) neurons that directly innervate α-motoneurons of the brainstem and spinal cord. Despite their large volume and circumferential dendritic architecture, to date, no single molecular criterion has been established that unequivocally distinguishes adult Betz cells from other layer V ETP neurons. In primates, transcriptional signatures suggest the presence of at least two ETP neuron clusters that contain mature Betz cells; these are characterized by an abundance of axon guidance and oxidative phosphorylation transcripts. How neurodevelopmental programs drive the distinct positional and morphological features of Betz cells in humans remains unknown. Betz cells display a distinct biphasic firing pattern involving early cessation of firing followed by delayed sustained acceleration in spike frequency and magnitude. Few cell type-specific transcripts and electrophysiological characteristics are conserved between rodent layer V ETP neurons of the motor cortex and primate Betz cells. This has implications for the modeling of disorders that affect the motor cortex in humans, such as amyotrophic lateral sclerosis (ALS). Perhaps vulnerability to ALS is linked to the evolution of neural networks for fine motor control reflected in the distinct morphomolecular architecture of the human motor cortex, including Betz cells. Here, we discuss histological, molecular, and functional data concerning the position of Betz cells in the emerging taxonomy of neurons across diverse species and their role in neurological disorders.

A Rare Patient with Hereditary Spastic Paraparesis with Parkinsonism

Herein, we present a rare patient with hereditary spastic paraparesis (HSP) in whom significant parkinsonism was involved in the clinic. Besides, the dopamine transport single-photon emission computed tomography scan also showed decreased tracer uptake in the bilateral striatum. Via the presentation of this patient, we discuss the parkinsonian findings in patients with HSP. We think that the observations of dopaminergic neuron vulnerability in HSP patients raise the possibility that degeneration of central dopaminergic neurons may contribute to the phenotype of HSP. The documentation of these rare variants will aid to understand the unknown pathophysiology of the disease course.

Cerebellar pathology in motor neuron disease: neuroplasticity and neurodegeneration

Amyotrophic lateral sclerosis is a relentlessly progressive multi-system condition. The clinical picture is dominated by upper and lower motor neuron degeneration, but extra-motor pathology is increasingly recognized, including cerebellar pathology. Post-mortem and neuroimaging studies primarily focus on the characterization of supratentorial disease, despite emerging evidence of cerebellar degeneration in amyotrophic lateral sclerosis. Cardinal clinical features of amyotrophic lateral sclerosis, such as dysarthria, dysphagia, cognitive and behavioral deficits, saccade abnormalities, gait impairment, respiratory weakness and pseudobulbar affect are likely to be exacerbated by co-existing cerebellar pathology. This review summarizes in vivo and post mortem evidence for cerebellar degeneration in amyotrophic lateral sclerosis. Structural imaging studies consistently capture cerebellar grey matter volume reductions, diffusivity studies readily detect both intra-cerebellar and cerebellar peduncle white matter alterations and functional imaging studies commonly report increased functional connectivity with supratentorial regions. Increased functional connectivity is commonly interpreted as evidence of neuroplasticity representing compensatory processes despite the lack of post-mortem validation. There is a scarcity of post-mortem studies focusing on cerebellar alterations, but these detect pTDP-43 in cerebellar nuclei. Cerebellar pathology is an overlooked facet of neurodegeneration in amyotrophic lateral sclerosis despite its contribution to a multitude of clinical symptoms, widespread connectivity to spinal and supratentorial regions and putative role in compensating for the degeneration of primary motor regions.

Thalamic atrophy in patients with pure hereditary spastic paraplegia type 4

SPG4 is an autosomal dominant pure form of hereditary spastic paraplegia (HSP) caused by mutations in the SPAST gene. HSP is considered an upper motor neuron disorder characterized by progressive spasticity and weakness of the lower limbs caused by degeneration of the corticospinal tract. In other neurodegenerative motor disorders, the thalamus and basal ganglia are affected, with a considerable impact on disease progression. However, only a few works have studied these brain structures in HSP, mainly in complex forms of this disease. Our research aims to detect potential alterations in the volume and shape of the thalamus and various basal ganglia structures by comparing 12 patients with pure HSP and 18 healthy controls. We used two neuroimaging procedures: automated segmentation of the subcortical structures (thalamus, hippocampus, caudate nucleus, globus pallidus, and putamen) in native space and shape analysis of the structures. We found a significant reduction in thalamic volume bilaterally, as well as an inward deformation, mainly in the sensory-motor thalamic regions in patients with pure HSP and a mutation in SPG4. We also observed a significant negative correlation between the shape of the thalamus and clinical scores (the Spastic Paraplegia Rating Scale score and disease duration). Moreover, we found a 'Group × Age' interaction that was closely related to the severity of the disease. No differences in volume or in shape were found in the remaining subcortical structures studied. Our results suggest that changes in structure of the thalamus could be an imaging biomarker of disease progression in pHSP.

A Newly Identified Missense Mutation in FARS2 Causes Autosomal-Recessive Spastic Paraplegia

Hereditary spastic paraplegia (HSP) is a clinically and genetically heterogeneous group of neurodegenerative disorders characterized by spasticity of the lower limbs due to pyramidal tract dysfunction. Here, we report that a missense homozygous mutation c.424G>T (p.D142Y) in the FARS2 gene, which encodes a mitochondrial phenylalanyl tRNA synthetase (mtPheRS), causes HSP in a Chinese consanguineous family by using combination of homozygous mapping and whole-exome sequencing. Immunohistochemical experiments were performed showing that the FARS2 protein was highly expressed in the Purkinje cells of rat cerebellum. The aminoacylation activity of mtPheRS was severely disrupted by the p.D142Y substitution in vitro not only in the first aminoacylation step but also in the last transfer step. Taken together, our results indicate that a missense mutation in FARS2 contributes to HSP, which has the clinical significance of the regulation of tRNA synthetases in human neurodegenerative diseases.

Gray and white matter alterations in hereditary spastic paraplegia type SPG4 and clinical correlations

Hereditary spastic paraplegias (HSP) are a group of clinically and genetically heterogeneous disorders with the hallmark of progressive spastic gait disturbance. We used advanced neuroimaging to identify brain regions involved in SPG4, the most common HSP genotype. Additionally, we analyzed correlations between imaging and clinical findings. We performed 3T MRI scans including isotropic high-resolution 3D T1, T2-FLAIR, and DTI sequences in 15 adult patients with genetically confirmed SPG4 and 15 age- and sex-matched healthy controls. Brain volume loss of gray and white matter was evaluated through voxel-based morphometry (VBM) for supra- and infratentorial regions separately. DTI maps of axial diffusivity (AD), radial diffusivity (RD), mean diffusivity (MD), fractional anisotropy (FA), and measured anisotropy (MA1) were analyzed through tract-based special statistics (TBSS). VBM and TBSS revealed a widespread affection of gray and white matter in SPG4 including the corpus callosum, medio-dorsal thalamus, parieto-occipital regions, upper brainstem, cerebellum, and corticospinal tract. Significant correlations with correlation coefficients r > 0.6 between clinical data and DTI findings could be demonstrated for disease duration and disease severity as assessed by the spastic paraplegia rating scale for the pontine crossing tract (AD) and the corpus callosum (RD and FA). Imaging also provided evidence that SPG4 underlies a primarily axonal rather than demyelinating damage in accordance with post-mortem data. DTI is an attractive tool to assess subclinical affection in SPG4. The correlation of imaging findings with disease duration and severity suggests AD, RD, and FA as potential progression markers in interventional studies.

Striatal dopaminergic functioning in patients with sporadic and hereditary spastic paraplegias with parkinsonism

Sporadic spastic paraplegia (SSP) and hereditary spastic paraplegia (HSP) belong to a clinical and genetically heterogeneous group of disorders characterized by progressive spasticity and weakness in the lower extremities. The symptoms are associated with pyramidal tract dysfunction and degeneration of the corticospinal tracts. Parkinsonism is uncommon in SSP/HSP patients. However, both disorders are associated with damage to the nigrostriatal dopaminergic system. In the present study, the clinical features of patients with SSP/HSP were investigated, and nigrostriatal dopaminergic binding potential was assessed using dopamine transporter (DAT) single-photon emission computer tomography (SPECT). Nine patients with spastic paraplegia participated in the present study. The subjects underwent DAT SPECT using the agent [2-[[2-[[[3-(4-chlorophenyl)-8-methyl-8-azabicyclo[3,2,1]oct-2-yl]methyl](2-mercaptoethyl)amino]ethyl]amino]ethanethiolato (3-)-N2,N20,S2,S20]oxo-[IR-(exo-exo)])-[(99)mTc]technetium ([(99)mTc]TRODAT-1). The [(99)mTc]TRODAT-1 SPECT images of five patients appeared normal, whereas the images of four patients revealed reduced striatal ligand uptake. Among the four patients with reduced uptake, two had parkinsonism, and one exhibited periodic limb movements and restless leg syndrome. Our DAT SPECT imaging study shows that reduced DAT density may be observed in patients with parkinsonism. The results of the present study offer an explanation for the spectrum of spastic paraplegia symptoms and the progression of the disorder.

Electrophysiological characterisation of motor and sensory tracts in patients with hereditary spastic paraplegia (HSP)

Background

Hereditary spastic paraplegias (HSPs) are characterised by lower limb spasticity due to degeneration of the corticospinal tract. We set out for an electrophysiological characterisation of motor and sensory tracts in patients with HSP.

Methods

We clinically and electrophysiologically examined a cohort of 128 patients with genetically confirmed or clinically probable HSP. Motor evoked potentials (MEPs) to arms and legs, somato-sensory evoked potentials of median and tibial nerves, and nerve conduction studies of tibial, ulnar, sural, and radial nerves were assessed.

Results

Whereas all patients showed clinical signs of spastic paraparesis, MEPs were normal in 27% of patients and revealed a broad spectrum with axonal or demyelinating features in the others. This heterogeneity can at least in part be explained by different underlying genotypes, hinting for distinct pathomechanisms in HSP subtypes. In the largest subgroup, SPG4, an axonal type of damage was evident. Comprehensive electrophysiological testing disclosed a more widespread affection of long fibre tracts involving peripheral nerves and the sensory system in 40%, respectively. Electrophysiological abnormalities correlated with the severity of clinical symptoms.

Conclusions

Whereas HSP is primarily considered as an upper motoneuron disorder, our data suggest a more widespread affection of motor and sensory tracts in the central and peripheral nervous system as a common finding in HSP. The distribution patterns of electrophysiological abnormalities were associated with distinct HSP genotypes and could reflect different underlying pathomechanisms. Electrophysiological measures are independent of symptomatic treatment and may therefore serve as a reliable biomarker in upcoming HSP trials.