Perivascular nerve fiber α-synuclein regulates contractility of mouse aorta: a link to autonomic dysfunction in Parkinson's disease

Distribution of alpha-synuclein in rat salivary glands

Expression of alpha-synuclein (Syn), a presynaptic neuronal protein, was immunohistochemically examined in intact rat submandibular, sublingual, and lingual glands. The submandibular gland contained abundant periductal Syn-immunoreactive (-ir) nerve fibers. Abundant Syn-ir varicosities were present in acini of the sublingual and serous lingual glands. By confocal laser scanning microscopy, Syn-ir nerve fibers around smooth muscle actin (SMA)-ir cells alone were infrequent; however, those around aquaporin-5 (AQP5)-ir cells alone and both SMA- and AQP5-ir cells were abundant in the sublingual and serous lingual glands. SMA-ir cells were occasionally immunoreactive for toll-like receptor 4, a Syn receptor. Syn-ir nerve fibers contained tyrosine hydroxylase (TH) in the submandibular gland and choline acetyltransferase (ChAT) in all examined salivary glands. In the superior cervical (SCG), submandibular, and intralingual ganglia, sympathetic and parasympathetic neurons co-expressed Syn with TH and ChAT, respectively. SCG neurons innervating the submandibular gland contained mostly Syn. In the thoracic spinal cord, 14.7% of ChAT-ir preganglionic sympathetic neurons co-expressed Syn. In the superior salivatory nucleus, preganglionic parasympathetic neurons projecting to the lingual nerve co-expressed Syn and ChAT. The present findings indicate that released Syn acts on myoepithelial cells. Syn in pre- and post-ganglionic neurons may regulate neurotransmitter release and salivary volume and composition.

α-Synuclein pathology from the body to the brain: so many seeds so close to the central soil

ABSTRACT

α-Synuclein is a protein that mainly exists in the presynaptic terminals. Abnormal folding and accumulation of α-synuclein are found in several neurodegenerative diseases, including Parkinson's disease. Aggregated and highly phosphorylated α-synuclein constitutes the main component of Lewy bodies in the brain, the pathological hallmark of Parkinson's disease. For decades, much attention has been focused on the accumulation of α-synuclein in the brain parenchyma rather than considering Parkinson's disease as a systemic disease. Recent evidence demonstrates that, at least in some patients, the initial α-synuclein pathology originates in the peripheral organs and spreads to the brain. Injection of α-synuclein preformed fibrils into the gastrointestinal tract triggers the gut-to-brain propagation of α-synuclein pathology. However, whether α-synuclein pathology can occur spontaneously in peripheral organs independent of exogenous α-synuclein preformed fibrils or pathological α-synuclein leakage from the central nervous system remains under investigation. In this review, we aimed to summarize the role of peripheral α-synuclein pathology in the pathogenesis of Parkinson's disease. We also discuss the pathways by which α-synuclein pathology spreads from the body to the brain.

α-Synuclein in traumatic and vascular diseases of the central nervous system

α-Synuclein (α-Syn) is a small, soluble, disordered protein that is widely expressed in the nervous system. Although its physiological functions are not yet fully understood, it is mainly involved in synaptic vesicle transport, neurotransmitter synthesis and release, cell membrane homeostasis, lipid synthesis, mitochondrial and lysosomal activities, and heavy metal removal. The complex and inconsistent pathological manifestations of α-Syn are attributed to its structural instability, mutational complexity, misfolding, and diverse posttranslational modifications. These effects trigger mitochondrial dysfunction, oxidative stress, and neuroinflammatory responses, resulting in neuronal death and neurodegeneration. Several recent studies have discovered the pathogenic roles of α-Syn in traumatic and vascular central nervous system diseases, such as traumatic spinal cord injury, brain injury, and stroke, and in aggravating the processes of neurodegeneration. This review aims to highlight the structural and pathophysiological changes in α-Syn and its mechanism of action in traumatic and vascular diseases of the central nervous system.

Effects of Alpha-Synuclein on Primary Spinal Cord Neurons Associated with Apoptosis and CNTF Expression

Spinal cord injury (SCI) often causes neurological deficits with poor recovery; the treatment, however, is far from satisfaction, and the mechanisms remain unclear. Using immunohistochemistry and western blotting analysis, we found α-synuclein (SNCA) was significantly up-regulated in the spinal caudal segment of rats subjected to spinal cord transection at 3 days post-operation. Moreover, the role of SNCA on neuronal growth and apoptosis in vitro was determined by using overexpressing and interfering SNCA recombined plasmid vectors, and the underlying mechanism was detected by QRT-PCR and western blotting. Spinal neurons transfected with SNCA-shRNA lentivirus gave rise to an optimal neuronal survival, while it results in cell apoptosis in SNCA-ORF group. In molecular level, SNCA silence induced the up-regulation of CNTF and down-regulation of Caspase7/9. Together, endogenous SNCA plays a crucial role in spinal neuronal survival, in which the underlying mechanism may be linked to the regulation both apoptotic genes (Caspase7/9) and CNTF. The present findings therefore provide novel insights into the role of SNCA in spinal cord and associated mechanism, which may provide novel cue for the treatment of SCI in future clinic trials.

Sepiapterin reductase gene-disrupted mice suffer from hypertension with fluctuation and bradycardia

(6R)‐l‐erythro‐5,6,7,8‐Tetrahydrobiopterin (BH4) is an essential cofactor for monoamine and nitric oxide (NO) production. Sepiapterin reductase (SPR) catalyzes the final step in BH4 biosynthesis. We analyzed the cardiovascular function of adult Spr gene‐disrupted (Spr^−/−) mice for the first time. After weaning, Spr^−/− mice suffered from hypertension with fluctuation and bradycardia, while the monoamine contents in these mice were less than 10% of those in the wild‐type mice as a result of BH4 depletion. Heart rate variability analysis indicated the sympathetic dominant state in Spr^−/− mice. The endothelium‐dependent vascular relaxation in response to acetylcholine was significantly impaired in Spr^−/− mice after sexual maturation (above 4 months old). Protein amounts of α₁ adrenergic receptor and eNOS in the aorta were not altered. Spr^−/− mice exhibited hypoglycemia and elevation of plasma renin activity. Our results suggest that the hypertension with fluctuation and bradycardia of Spr^−/− mice would be caused by an imbalance of sympathetic and parasympathetic input and impaired nitric oxide production in endothelial cells. We suggest an important role of BH4 and SPR in age‐related hypertension and a possible relationship with the cardiovascular instabilities in autonomic diseases, including Parkinson's disease and spinal cord injury.

Alpha-synuclein pathology and axonal degeneration of the peripheral motor nerves innervating pharyngeal muscles in Parkinson disease

Parkinson disease (PD) is a neurodegenerative disease primarily characterized by cardinal motor manifestations and CNS pathology. Current drug therapies can often stabilize these cardinal motor symptoms, and attention has shifted to the other motor and nonmotor symptoms of PD that are resistant to drug therapy. Dysphagia in PD is perhaps the most important drug-resistant symptom because it leads to aspiration and pneumonia, the leading cause of death. Here, we present direct evidence for degeneration of the pharyngeal motor nerves in PD. We examined the cervical vagal nerve (cranial nerve X), pharyngeal branch of nerve X, and pharyngeal plexus innervating the pharyngeal muscles in 14 postmortem specimens, that is, from 10 patients with PD and 4 age-matched control subjects. Synucleinopathy in the pharyngeal nerves was detected using an immunohistochemical method for phosphorylated α-synuclein. Alpha-synuclein aggregates were revealed in nerve X and the pharyngeal branch of nerve X, and immunoreactive intramuscular nerve twigs and axon terminals within the neuromuscular junctions were identified in all of the PD patients but in none of the controls. These findings indicate that the motor nervous system of the pharynx is involved in the pathologic process of PD. Notably, PD patients who have had dysphagia had a higher density of α-synuclein aggregates in the pharyngeal nerves than those without dysphagia. These findings indicate that motor involvement of the pharynx in PD is one of the factors leading to oropharyngeal dysphagia commonly seen in PD patients.

Mice overexpressing wild-type human alpha-synuclein display alterations in colonic myenteric ganglia and defecation

BACKGROUND

Prevalent non-motor symptoms of Parkinson's disease (PD) include gastrointestinal motor impairments and advanced stage PD displays pathological aggregates of α-synuclein in colonic enteric neurons. We previously showed that 12 months old mice overexpressing human wild type (WT) α-synuclein under the Thy1 promoter (Thy1-aSyn) displayed colonic motor dysfunction. We investigated functional gut alterations at earlier ages and histological correlates.

METHODS

Defecation, gastric emptying (GE), and immunostaining for α-synuclein, peripheral choline acetyltransferase (pChAT), tyrosine hydroxylase (TH), neuronal nitric oxide synthase (nNOS), and vasoactive intestinal peptide (VIP) in distal colon myenteric plexuses were assessed in male Thy1-aSyn compared to littermate WT mice.

KEY RESULTS

Thy1-aSyn mice aged 2.5-3 or 7-8 months old had 81% and 55% reduction in fecal pellet output, respectively, in the first 15 min of exposure to a novel environment. The reduction remained significant in the older group for 2-h, and subsequent refeeding resulted also in a 60% and 69% reduction of defecation in the first hour, respectively. Thy1-aSyn mice (8-10 months) displayed increased α-synuclein in the myenteric plexuses with abundant varicose terminals surrounding pChAT-immunoreactive (ir) neurons, and only a few, nNOS-ir neurons. There were no conspicuous changes in pChAT- and nNOS-ir neurons, or TH- and VIP-ir nerve fibers. Thy1-aSyn mice aged 4-18 months had normal GE.

CONCLUSIONS & INFERENCES

The occurrence of over-production of pre-synaptic α-synuclein in colonic myenteric ganglia several months before the loss of striatal dopamine may provide an anatomical basis for interference with cholinergic neuronal activation, causing an early impairment in defecation to stimuli.

Neuromuscular pathology in mice lacking alpha-synuclein

This work was undertaken in order to study the possible role of alpha-synuclein in the function of the neuro-muscular junction in skeletal muscles. Repeated stimulation of skeletal muscle motor neurons revealed signs of neuromuscular pathology in alpha-synuclein null mutated (C57Bl/6JOlaHsd) and knockout (B6;129X1-Snca(tm1Rosl)/J) mice. This stimulation produced repetitive compound muscle action potentials in both lines of alpha-synuclein deficient mice. Muscle strength and muscle coordination during ambulation were unaffected, though motor learning was slower in alpha-synuclein deficient mice in the Rotarod test. We conclude that alpha-synuclein may play a role in acetylcholine compartmentalization at the neuromuscular junction, and in the fine control of activity of skeletal muscles.