Search

Summary: Myocardial infarction (MI) triggers an inflammatory response that transitions from pro-inflammatory to reparative over time. Restoring sympathetic nerves in the heart after MI prevents arrhythmias. This study investigated if reinnervation altered the immune response after MI. This study used quantitative multiplex immunohistochemistry to identify the immune cells present in the heart 2 weeks after ischemia-reperfusion. Two therapeutics stimulated reinnervation, preventing arrhythmias and shifting the immune response from inflammatory to reparative, with fewer pro-inflammatory macrophages and more regulatory T cells and reparative macrophages. Treatments did not alter macrophage phenotype in vitro, which suggested reinnervation contributed to the altered immune response.

Abstract Sympathetic nerve loss in the heart predicts the risk of ventricular arrhythmias after myocardial infarction (MI) in patients. Sympathetic denervation after cardiac ischemia–reperfusion is sustained by matrix components chondroitin sulfate proteoglycans (CSPGs) in the cardiac scar. We showed that 4,6‐sulfation of CSPGs was critical for preventing nerve growth into the scar. Promoting early reinnervation with therapeutics reduces arrhythmias during the first 2 weeks after MI, but the longer‐term consequences of restoring innervation are unknown. Therefore, we asked if the beneficial effects of early reinnervation were sustained. We compared cardiac function and arrhythmia susceptibility 40 days after MI in mice treated on Days 3–10 with vehicle or with intracellular sigma peptide to restore innervation. Surprisingly, both groups had normal innervation density in the cardiac scar 40 days after MI, indicating delayed reinnervation of the infarct in vehicle‐treated mice. That coincided with similar cardiac function and arrhythmia susceptibility in the two groups. We investigated the mechanism allowing delayed reinnervation of the cardiac scar. We found that CSPG 4,6‐sulfation, which is elevated early after ischemia–reperfusion, was reduced to control levels allowing reinnervation of the infarct. Thus, remodeling of extracellular matrix weeks after injury leads to remodeling of sympathetic neurons in the heart.

Alzheimer's disease (AD) is a neurodegenerative disease characterized by severe cognitive deterioration. While causes of AD pathology are debated, a large body of evidence suggests that increased cleavage of Amyloid Precursor Protein (APP) producing the neurotoxic Amyloid-β (Aβ) peptide plays a fundamental role in AD pathogenesis. One of the detrimental behavioral symptoms commonly associated with AD is the fragmentation of sleep-activity cycles with increased nighttime activity and daytime naps in humans. Sleep-activity cycles, as well as physiological and cellular rhythms, which may be important for neuronal homeostasis, are generated by a molecular system known as the circadian clock. Links between AD and the circadian system are increasingly evident but not well understood. Here we examined whether genetic manipulations of APP-like (APPL) protein cleavage in Drosophila melanogaster affect rest-activity rhythms and core circadian clock function in this model organism. We show that the increased β-cleavage of endogenous APPL by the β-secretase (dBACE) severely disrupts circadian behavior and leads to reduced expression of clock protein PER in central clock neurons of aging flies. Our data suggest that behavioral rhythm disruption is not a product of APPL-derived Aβ production but rather may be caused by a mechanism common to both α and β-cleavage pathways. Specifically, we show that increased production of the endogenous Drosophila Amyloid Intracellular Domain (dAICD) caused disruption of circadian rest-activity rhythms, while flies overexpressing endogenous APPL maintained stronger circadian rhythms during aging. In summary, our study offers a novel entry point toward understanding the mechanism of circadian rhythm disruption in Alzheimer's disease.

Sympathetic denervation of the heart following ischemia/reperfusion induced myocardial infarction (MI) is sustained by chondroitin sulfate proteoglycans (CSPGs) in the cardiac scar. Denervation predicts risk of sudden cardiac death in humans. Blocking CSPG signaling restores sympathetic axon outgrowth into the cardiac scar, decreasing arrhythmia susceptibility. Axon growth inhibition by CSPGs can depend on the sulfation status of the glycosaminoglycan (CS-GAG) side chains. Tandem sulfation of CS-GAGs at the 4th (4S) and 6th (6S) positions of n-acetyl-galactosamine inhibits outgrowth in several types of central neurons, but we don’t know if sulfation is similarly critical during peripheral nerve regeneration. We asked if CSPG sulfation prevented sympathetic axon outgrowth after MI. Reducing 4S with the 4-sulfatase enzyme Arylsulfatase-B (ARSB) enhanced outgrowth of dissociated rat sympathetic neurons over CSPGs. Likewise, reducing 4S with ARSB restored axon outgrowth from mouse sympathetic ganglia co-cultured with cardiac scar tissue. We quantified enzymes responsible for adding and removing sulfation, and found that CHST15 (4S dependent 6-sulfotransferase) was upregulated, and ARSB was downregulated after MI. This suggests a mechanism for production and maintenance of sulfated CSPGs in the cardiac scar. We decreased 4S,6S CS-GAGs in vivo by transient siRNA knockdown of Chst15 after MI, and found that reducing 4S,6S restored tyrosine hydroxylase (TH) positive sympathetic nerve fibers in the cardiac scar. Reinnervation reduced isoproterenol induced arrhythmias. Our results suggest that modulating CSPG-sulfation after MI may be a therapeutic target to promote sympathetic nerve regeneration in the cardiac scar and reduce post-MI cardiac arrhythmias.

Circadian clocks coordinate physiological, neurological, and behavioral functions into circa 24 hour rhythms, and the molecular mechanisms underlying circadian clock oscillations are conserved from Drosophila to humans. Clock oscillations and clock-controlled rhythms are known to dampen during aging; additionally, genetic or environmental clock disruption leads to accelerated aging and increased susceptibility to age-related pathologies. Neurodegenerative diseases, such as Alzheimer's disease (AD), are associated with a decay of circadian rhythms, but it is not clear whether circadian disruption accelerates neuronal and motor decline associated with these diseases. To address this question, we utilized transgenic Drosophila expressing various Amyloid-β (Aβ) peptides, which are prone to form aggregates characteristic of AD pathology in humans. We compared development of AD-like symptoms in adult flies expressing Aβ peptides in the wild type background and in flies with clocks disrupted via a null mutation in the clock gene period (per01). No significant differences were observed in longevity, climbing ability and brain neurodegeneration levels between control and clock-deficient flies, suggesting that loss of clock function does not exacerbate pathogenicity caused by human-derived Aβ peptides in flies. However, AD-like pathologies affected the circadian system in aging flies. We report that rest/activity rhythms were impaired in an age-dependent manner. Flies expressing the highly pathogenic arctic Aβ peptide showed a dramatic degradation of these rhythms in tune with their reduced longevity and impaired climbing ability. At the same time, the central pacemaker remained intact in these flies providing evidence that expression of Aβ peptides causes rhythm degradation downstream from the central clock mechanism.