Biology subjects

Orr, H. T.

Publications and source records attributed to Orr, H. T..

7 recordsLinked to original sources

bioRxiv2026

Impaired motor activity in a CRISPR SCA5 L253P knock-in mouse is associated with selective beta-III-spectrin subcellular redistribution in the cerebellum

The spinocerebellar ataxia type 5 (SCA5) L253P mutation in {beta}-III-spectrin causes high-affinity actin binding. Here we developed a CRISPR knock-in mouse to determine the in vivo impact of L253P on Purkinje neurons and motor activity, and to establish a model for future testing of SCA5 therapeutics. Significantly, the knock-in mouse shows impaired motor activity on elevated beam assays at 20 weeks. In the cerebellum, L253P causes a subcellular redistribution of {beta}-III-spectrin in Purkinje neurons. This is marked by loss of {beta}-III-spectrin in distal dendrites, accumulation of {beta}-III-spectrin at the plasma membrane of the soma and proximal dendrites, and formation of inclusions in the soma. The inclusions additionally contain F-actin and -II-spectrin, accumulate around the nucleus, form at an early age, and are larger in homozygous {beta}-III-spectrinL253P/L253P compared to heterozygous {beta}-III-spectrinL253P/+ mice. In contrast, neurons of the hippocampus and cerebral cortex, where {beta}-III-spectrin is also known to be expressed, abnormally accumulate {beta}-III-spectrin at the plasma membrane but do not form inclusions. To gain greater insight into disease mechanisms, unbiased proteomics identified over 150 cerebellar proteins that physically associate with {beta}-III-spectrin. Of these, cluster analysis revealed a group of 41 proteins, including glutamate receptors, SERCA2, and CaMKII, linked to synaptic transmission. Thus, the effect of the L253P to alter {beta}-III-spectrin localization, including decreased levels in distal dendrites, is likely associated with a disruption of {beta}-III-spectrin function in postsynaptic signaling. Consistent with this, and in agreement with prior findings in knockout mice, the L253P {beta}-III-spectrin knock-in mouse here shows that CaMKII, a calcium sensor and key mediator of glutamate signaling, is ~2-fold activated. Further, the abundance of EAAT4, a glutamate transporter, is significantly reduced. The L253P knock-in mouse primes future preclinical testing of SCA5 therapeutics, such as small molecule modulators of spectrin-actin binding, and glutamate and calcium signaling pathways.

neuroscience↗

bioRxiv2025

Striatal pathology in Spinocerebellar Ataxia Type 1 mice: A comparative study with Huntingtons disease

Spinocerebellar ataxia type 1 (SCA1) and Huntingtons disease (HD), are motor diseases caused by CAG expansions in ATXN1 and HTT, where SCA1 shows prominent cerebellar neurodegeneration and HD shows prominent striatal neurodegeneration, particularly in the Medium Spiny Neurons (MSNs). Since human and mouse studies demonstrate progressive striatal vulnerability in SCA1, we examined age-dependent molecular, cellular and functional striatal attributes in SCA1 (f-ATXN1146Q/2Q) knockin mice, by assessing RNA-sequencing, immunohistochemistry and electrophysiology. Striatal mRNAs are downregulated in SCA1 mice, many in common with HD mice, and specificity in MSNs is supported by the rescue of transcriptomic dysregulation with deletion of mutant Ataxin1 from MSNs. Immunohistochemistry assessed dopamine receptor 1 (D1R) and 2 (D2R) expression in indirect and direct MSNs. In HD mice (HttQ175/Q7), expression of both D1R and D2R proteins in MSNs decreased with age in parallel with their RNA levels. In the SCA1 mouse striatum, D1R protein expression decreased with age as seen in murine HD striatum. In contrast, while D2R protein level was decreased similar to D1R protein at 5-weeks of age, by 40-weeks expression of D2R protein recovered to levels recorded in WT mice. Electrophysiological assessment showed a reduction of excitatory synaptic transmission in SCA1 mouse MSNs, indicating functional deficits early in disease. In contrast to cerebellar and many other aspects of SCA1 pathology known to depend on proper nuclear localization of ATXN1 with an expanded polyglutamine, mutating ATXN1s nuclear localization failed to correct striatal MSN RNA and protein downregulations, indicating a difference in how ATXN1 exerts its pathological effects between the cerebellum and the striatum. Together, these data provide a molecular and cellular basis of striatal pathology in SCA1.

neuroscience↗

bioRxiv2023

Increased intrinsic membrane excitability is associated with hypertrophic olivary degeneration in spinocerebellar ataxia type 1

One of the characteristic areas of brainstem degeneration across multiple spinocerebellar ataxias (SCAs) is the inferior olive (IO), a medullary nucleus that plays a key role in motor learning. In addition to its vulnerability in SCAs, the IO is also susceptible to a distinct pathology known as hypertrophic olivary degeneration (HOD). Clinically, HOD has been exclusively observed after lesions in the brainstem disrupt inhibitory afferents to the IO. Here, for the first time, we describe HOD in another context: spinocerebellar ataxia type 1 (SCA1). Using the genetically-precise SCA1 knock-in mouse model (SCA1-KI; both sexes used), we assessed SCA1-associated changes in IO neuron structure and function. Concurrent with degeneration, we found that SCA1-KI IO neurons are hypertrophic, exhibiting early dendrite lengthening and later somatic expansion. Unlike in previous descriptions of HOD, we observed no clear loss of IO inhibitory innervation; nevertheless, patch-clamp recordings from brainstem slices reveal that SCA1-KI IO neurons are hyperexcitable. Rather than synaptic disinhibition, we identify increases in intrinsic membrane excitability as the more likely mechanism underlying this novel SCA1 phenotype. Specifically, transcriptome analysis indicates that SCA1-KI IO hyperexcitability is associated with a reduced medullary expression of ion channels responsible for spike afterhyperpolarization (AHP) in IO neurons - a result that has a functional consequence, as SCA1-KI IO neuron spikes exhibit a diminished AHP. These results reveal membrane excitability as a potential link between disparate causes of IO degeneration, suggesting that HOD can result from any cause, intrinsic or extrinsic, that increases excitability of the IO neuron membrane. Significance statementLittle is known about the factors that make inferior olive (IO) neurons susceptible to degeneration in the spinocerebellar ataxias (SCAs), a group of inherited neurodegenerative movement disorders. Another well-described form of IO degeneration, known as hypertrophic olivary degeneration (HOD), results from inhibitory denervation of the IO after brainstem injury. Here, we describe a novel finding of HOD in SCA1 without inhibitory denervation, in association with increased intrinsic membrane excitability and reduced potassium channel transcripts. This suggests that increased membrane excitability may be the underlying primary mechanism of HOD. Identifying hyperexcitability as the mechanistic driver of HOD would imply that reducing intrinsic IO excitability could be an effective strategy for treating diverse causes of both inherited and sporadic olivary degeneration.

neuroscience↗

bioRxiv2023

Purkinje-Enriched snRNA-seq in SCA7 Cerebellum Reveals Zebrin Identity Loss as a Central Feature of Polyglutamine Ataxias

Spinocerebellar ataxia type 7 (SCA7) is an inherited neurodegenerative disorder caused by a CAG-polyglutamine repeat expansion. SCA7 patients display a striking loss of Purkinje cell (PC) neurons with disease progression; however, PCs are rare, making them difficult to characterize. We developed a PC nuclei enrichment protocol and applied it to single-nucleus RNA-seq of a SCA7 knock-in mouse model. Our results unify prior observations into a central mechanism of cell identity loss, impacting both glia and PCs, driving accumulation of inhibitory synapses and altered PC spiking. Zebrin-II subtype dysregulation is the predominant signal in PCs, leading to complete loss of zebrin-II striping at motor symptom onset in SCA7 mice. We show this zebrin-II subtype degradation is shared across Polyglutamine Ataxia mouse models and SCA7 patients. It has been speculated that PC subtype organization is critical for cerebellar function, and our results suggest that a breakdown of zebrin-II parasagittal striping is pathological.

genomics↗

bioRxiv2023

Regional vulnerability in a neurodegenerative disease: Delineating SCA1 CNS and muscle therapeutic targets using a conditional mutant ATXN1 mouse

Spinocerebellar ataxia type 1 (SCA1) is a fatal neurodegenerative disease caused by an expanded polyglutamine tract in the widely expressed ATXN1 protein. To elucidate anatomical regions and cell types that underlie mutant ATXN1-induced disease phenotypes, we developed a floxed conditional knockout mouse model (f-ATXN1146Q/2Q) having mouse Atxn1 coding exons replaced by human exons encoding 146 glutamines. F-ATXN1146Q/2Q mice manifest SCA1-like phenotypes including motor and cognitive deficits, wasting, and decreased survival. CNS contributions to disease were revealed using ATXN1146Q/2Q;Nestin-Cre mice, that showed improved rotarod, open field and Barnes maze performances. Striatal contributions to motor deficits were examined using f-ATXN1146Q/2Q;Rgs9-Cre mice. Mice lacking striatal ATXN1146Q/2Q had improved rotarod performance late in disease. Muscle contributions to disease were revealed in f-ATXN1146Q/2Q;ACTA1-Cre mice which lacked muscle pathology and kyphosis seen in f-ATXN1146Q/2Q mice. Kyphosis was not improved in f-ATXN1146Q/2Q;Nestin-Cre mice. Thus, optimal SCA1 therapeutics will require targeting mutant ATXN1 toxic actions in multiple brain regions and muscle.

neuroscience↗

bioRxiv2021

Disrupting ATXN1 Nuclear Localization in a Knock-in SCA1 Mouse Model Improves a Spectrum of SCA1-Like Phenotypes and their Brain Region Associated Transcriptomic Profiles

Spinocerebellar ataxia type 1 (SCA1) is a dominant trinucleotide repeat neurodegenerative disease characterized by motor dysfunction, cognitive impairment, and premature death. Degeneration of cerebellar Purkinje cells is a frequent and prominent pathological feature of SCA1. We previously showed that transport of ATXN1 to Purkinje cell nuclei is required for pathology, where mutant ATXN1 alters transcription. To examine the role of ATXN1 nuclear localization broadly in SCA1-like disease pathogenesis, CRISPR-Cas9 was used to develop a mouse with the amino acid alteration (K772T) in the nuclear localization sequence of the expanded ATXN1 protein. Characterization of these mice indicates proper nuclear localization of mutant ATXN1 contributes to many disease-like phenotypes including motor dysfunction, cognitive deficits, and premature lethality. RNA sequencing analysis of genes whose expression was corrected to WT levels in Atxn1175QK772T/2Q mice indicates that transcriptomic aspects of SCA1 pathogenesis differ between the cerebellum, brainstem, cerebral cortex, hippocampus, and striatum.

neuroscience↗

bioRxiv2021

Cholecystokinin 1 Receptor (Cck1R) Activates mTORC1 signaling and is Protective to Purkinje cells in SCA Mice

Spinocerebellar Ataxias (SCAs) are a group of genetic diseases characterized by progressive ataxia and neurodegeneration, often in cerebellar Purkinje neurons. A SCA1 mouse model, Pcp2-ATXN1[30Q]D776, has severe ataxia in absence of progressive Purkinje neuron degeneration and death. Previous RNA-seq analyses identified cerebellar up-regulation of the peptide hormone Cholecystokinin (Cck) in Pcp2-ATXN1[30Q]D776 mice. Importantly, absence of Cck1 receptor (Cck1R) in Pcp2-ATXN1[30Q]D776 mice confers a progressive disease with Purkinje neuron death. A Cck1R agonist, A71623 administered to Pcp2-ATXN1[30Q]D776;Cck-/- and Pcp2-AXTN1[82Q] mice dampened Purkinje neuron pathology and associated deficits in motor performance. In addition, A71623 administration improved motor performance of Pcp2-ATXN2[127Q] SCA2 mice. Moreover, the Cck1R agonist A71623 corrected mTORC1 signaling and improved expression of calbindin in cerebella of AXTN1[82Q] and ATXN2[127Q] mice. These results indicate that manipulation of the Cck-Cck1R pathway is a potential therapeutic target for treatment of diseases involving Purkinje neuron degeneration.

neuroscience↗