Biology subjects

Loreto, A.

Publications and source records attributed to Loreto, A..

9 recordsLinked to original sources

bioRxiv2025

RNA-ligand complexes and the attenuation of neutral confinement in the evolution of RNA secondary structures

RNA molecules with identical nucleotide sequence can adopt different structures. Mutations can alter their properties; for example, some mutations increase the stability of a functionally relevant structure at the expense of other structures stability. Interestingly, the structural diversity that a sequence produces is correlated to the number of structures that it can access upon mutation. Thus, enhancing a structures stability can lead to neutral confinement, an evolutionary dead-end in which mutational access to novel structures is increasingly difficult. If structure is critical to biological function, how do RNA molecules escape neutral confinement? We developed a model in which an RNA molecules function depends on binding to a ligand and we applied it to study sequences that fold according to RNA biophysics, also simulating their evolution. Our analyses and simulations identified effects that decrease the selective advantage of augmenting a structures stability. By disfavouring evolution of highly stable structures and favouring the accumulation of genetic variation, these effects hinder neutral confinement. The most important effect stems from the sequestration of high affinity structures in RNA-ligand complexes and their replenishment through thermal fluctuations. In this perspective, a common scenario may help to explain how RNA evolution avoids coming to a halt.

evolutionary biology↗

bioRxiv2024

SARM1 activation induces reversible mitochondrial dysfunction and can be prevented in human neurons by antisense oligonucleotides

SARM1 is a key regulator of a conserved program of axon degeneration increasingly linked to human neurodegenerative diseases. Pathological SARM1 activation causes rapid NAD consumption, disrupting cellular homeostasis and leading to axon degeneration. In this study, we develop antisense oligonucleotides targeting human SARM1, demonstrating robust neuroprotection against morphological, metabolic, and mitochondrial impairment in human iPSC-derived dopamine neurons induced by the lethal neurotoxin vacor, a potent SARM1 activator. Furthermore, our findings reveal that axon fragmentation can be prevented, and mitochondrial dysfunction reversed using the NAD precursor nicotinamide, a form of vitamin B3, even after SARM1 activation has occurred, when neurons are already unhealthy. This research identifies ASOs as a promising therapeutic strategy to block SARM1, and provides an extensive characterisation and further mechanistic insights that demonstrate the reversibility of SARM1 toxicity in human neurons. It also identifies the SARM1 activator vacor as a specific and reversible neuroablative agent in human neurons.

neuroscience↗

bioRxiv2024

Chronically low NMNAT2 expression causes sub-lethal SARM1 activation and altered response to nicotinamide riboside in axons

Nicotinamide mononucleotide adenylyltransferase 2 (NMNAT2) is an endogenous axon survival factor that maintains axon health by blocking activation of the downstream pro-degenerative protein SARM1 (sterile alpha and TIR motif containing protein 1). While complete absence of NMNAT2 in mice results in extensive axon truncation and perinatal lethality, the removal of SARM1 completely rescues these phenotypes. Reduced levels of NMNAT2 can be compatible with life, however they compromise axon development and survival. Mice born expressing sub-heterozygous levels of NMNAT2 remain overtly normal into old age but develop axonal defects in vivo and in vitro as well as behavioural phenotypes. Therefore, it is important to examine the effects of constitutively low NMNAT2 expression on SARM1 activation and disease susceptibility. Here we demonstrate that chronically low NMNAT2 levels reduce prenatal viability in mice in a SARM1-dependent manner and lead to sub-lethal SARM1 activation in morphologically intact axons of superior cervical ganglion (SCG) primary cultures. This is characterised by a depletion in NAD(P) and compromised neurite outgrowth. We also show that chronically low NMNAT2 expression reverses the NAD-enhancing effect of nicotinamide riboside (NR) in axons in a SARM1-dependent manner. These data indicate that low NMNAT2 levels can trigger sub-lethal SARM1 activation which is detectable at the molecular level and could predispose to human axonal disorders.

neuroscience↗

bioRxiv2023

Distinct axo-protective and axo-destructive roles for Schwann cells after injury in a novel compartmentalised mouse myelinating coculture system.

Myelinating Schwann cell (SC)- dorsal root ganglion (DRG) neuron cocultures have been an important technique over the last four decades in understanding cell-cell signalling and interactions during peripheral nervous system (PNS) myelination, injury, and regeneration. While methods using rat SCs and rat DRG neurons are commonplace, there are no established protocols in the field describing the use of mouse SCs with mouse DRG neurons in dissociated myelinating cocultures. There is a great need for such a protocol as this would allow the use of cells from many different transgenic mouse lines. Here we describe a protocol to coculture dissociated mouse SCs and DRG neurons and induce robust myelination. Use of microfluidic chambers permits fluidic isolation for drug treatments, allows cultures to be axotomised to study injury responses, and cells can readily be transfected with lentiviruses to permit live imaging. We used this model to quantify the rate of degeneration after traumatic axotomy in the presence and absence of myelinating SCs and axon aligned SCs that were not induced to myelinate. We find that SCs, irrespective of myelination status, are axo-protective and delay axon degeneration early on. At later time points after injury, we use live imaging of cocultures to show that once axonal degeneration has commenced SCs break up, ingest, and clear axonal debris. Summary statementA novel compartmentalised dissociated mouse myelinating SC-DRG coculture system reveals distinct axo-protective and axo-destructive phases of Schwann cells on axon integrity after trauma.

neuroscience↗

bioRxiv2022

SARM1 detection in oligodendrocytes but not Schwann cells though sarm1/Sarm1 deletion does not perturb CNS nor PNS myelination in zebrafish and mice.

SARM1 is a central regulator of programmed axon death and is required to initiate axon self-destruction after traumatic and toxic insults to the nervous system. Abnormal activation of this axon degeneration pathway is increasingly recognized as a contributor to human neurological disease and SARM1 knockdown or inhibition has become an attractive therapeutic strategy to preserve axon loss in a variety of disorders of the peripheral and central nervous system. Despite this, it remains unknown whether Sarm1/SARM1 is present in myelinating glia and whether it plays a role in myelination in the PNS or CNS. It is important to answer these questions to understand whether future therapies inhibiting SARM1 function may have unintended deleterious impacts on myelination. Here we show that Sarm1 mRNA is present in oligodendrocytes in zebrafish but only detectable at low levels in Schwann cells in both zebrafish and mice. We find SARM1 protein is readily detectable in murine oligodendrocytes in vitro and in vivo and activation of endogenous SARM1 in oligodendrocytes induces cell death. In contrast, SARM1 protein is not detectable in Schwann cells and satellite glia in the adult murine nervous system. Cultured Schwann cells contain negligible functional SARM1 and are insensitive to specific SARM1 activators. Using zebrafish and mouse Sarm1 mutants, we show that SARM1 is not required for initiation of myelination nor myelin sheath maintenance by oligodendrocytes and Schwann cells. Thus, strategies to inhibit SARM1 function in the nervous system to treat neurological disease are unlikely to perturb myelination in humans. Main PointsO_LISARM1 protein is detectable in oligodendrocytes but not in Schwann cells C_LIO_LIOligodendrocytes but not Schwann cells die in response to endogenous SARM1 activation C_LIO_LICNS nor PNS myelination, in zebrafish and mice, is hindered by loss of sarm1/Sarm1 C_LI

neuroscience↗

bioRxiv2022

The NAD+ precursor NMN activates dSarm to trigger axon degeneration in Drosophila

Axon degeneration contributes to the disruption of neuronal circuit function in diseased and injured nervous systems. Severed axons degenerate following the activation of an evolutionarily conserved signaling pathway, which culminates in the activation of SARM1 in mammals to execute the pathological depletion of the metabolite NAD+. SARM1 NADase activity is activated by the NAD+ precursor nicotinamide mononucleotide (NMN). In mammals, keeping NMN levels low potently preserves axons after injury, however, it remains unclear whether NMN is also a key mediator of axon degeneration, and dSarm activation, in flies. Here, we demonstrate that lowering NMN levels in Drosophila through the expression of a newly generated prokaryotic NMN-Deamidase (NMN-D) preserves severed axons for months and keeps them circuit-integrated for weeks. NMN-D alters the NAD+ metabolic flux by lowering NMN, while NAD+ remains unchanged in vivo. Increased NMN synthesis, by the expression of mouse nicotinamide phosphoribosyltransferase (mNAMPT), leads to faster axon degeneration after injury. We also show that NMN-induced activation of dSarm mediates axon degeneration in vivo. Finally, NMN-D delays neurodegeneration caused by loss of the sole NMN-consuming and NAD+-synthesizing enzyme dNmnat. Our results reveal a critical role for NMN in neurodegeneration in the fly, which extends beyond axonal injury. The potent neuroprotection by reducing NMN levels is similar or even stronger than the interference with other essential mediators of axon degeneration in Drosophila.

neuroscience↗

bioRxiv2021

PROGRAMMED AXON DEATH EXECUTOR SARM1 IS A MULTI-FUNCTIONAL NAD(P)ASE WITH PROMINENT BASE EXCHANGE ACTIVITY, ALL REGULATED BY PHYSIOLOGICAL LEVELS OF NMN, NAD, NADP AND OTHER METABOLITES

SARM1 is an NAD glycohydrolase and TLR adapter with an essential, prodegenerative role in programmed axon death (Wallerian degeneration). It has low basal NADase activity that becomes strongly activated by NAD precursor NMN. Very high levels of NAD oppose this activation, competing for the same allosteric site on SARM1s regulatory ARM domain. Injury or diseases that deplete axons of NMNAT2, an essential enzyme converting NMN to NAD, cause SARM1 activation. The resulting NAD degradation by SARM1, combined with loss of NAD synthesis by NMNAT2, causes rapid depletion of axonal NAD. This NAD loss is widely assumed to mediate axon death and is consequently a key focus for therapeutic strategies for axonopathies. However, like other NAD(P) glycohydrolases, SARM1 has additional enzyme activities whose contributions, consequences and regulation need to be fully understood. Here, we compare the multiple actions and regulation of recombinant human SARM1 with those of two other NAD(P) glycohydrolases, human CD38 and Aplysia californica ADP ribosyl cyclase. We find that SARM1 has the highest transglycosidation (base exchange) activity of these enzymes at neutral pH and with some bases this dominates NAD(P) hydrolysis and cyclisation. Moreover, like its NADase and NADPase reactions, SARM1-mediated base exchange at neutral pH is activated by increases in the NMN:NAD ratio, which we show for the first time can act in the presence of physiological levels of both metabolites. We establish that SARM1 base exchange is the most likely physiological source of calcium mobilizing agent NaADP, and potentially of other NAD(P) analogues, which could contribute to axon and cell death. We also identify regulatory effects of free pyridine bases, of NADP and of nicotinic acid riboside (NaR) on SARM1 that represent further therapeutic opportunities. These data will help to pinpoint which of the multiple functions of SARM1 is responsible for axon degeneration and how it can be optimally targeted to block axon degeneration in disease.

biochemistry↗

bioRxiv2020

Potent activation of SARM1 by NMN analogue VMN underlies vacor neurotoxicity

Axon loss underlies symptom onset and progression in many neurodegenerative disorders. Axon degeneration in injury and disease is promoted by activation of the nicotinamide adenine dinucleotide (NAD)-consuming enzyme SARM1 (sterile alpha and TIR motif-containing protein 1). Here, we report vacor mononucleotide (VMN), a metabolite of the pesticide and neurotoxin vacor, as the most potent yet SARM1 activator. Removal of SARM1 shows complete rescue from vacor-induced neuron and axon death in vitro and in vivo. We present the crystal structure of VMN bound to the Drosophila SARM1 regulatory armadillo-repeat domain, thus facilitating drug development to prevent SARM1 activation in human disease. This study indicates the likely mechanism of action of vacor as a pesticide and lethal neurotoxin in humans, provides important new tools for drug discovery, and further demonstrates that SARM1 removal can permanently block programmed axon death specifically induced by toxicity as well as genetic mutation.

neuroscience↗

bioRxiv2019

Mitochondrial impairment activates the Wallerian pathway through depletion of NMNAT2 leading to SARM1-dependent axon degeneration

Wallerian degeneration of physically injured axons involves a well-defined molecular pathway linking loss of axonal survival factor NMNAT2 to activation of pro-degenerative protein SARM1. Manipulating the pathway through these proteins led to the identification of non-axotomy insults causing axon degeneration by a Wallerian-like mechanism, including several involving mitochondrial impairment. Mitochondrial dysfunction is heavily implicated in Parkinsons disease, Charcot-Marie-Tooth disease, hereditary spastic paraplegia and other axonal disorders. However, whether and how mitochondrial impairment activates Wallerian degeneration has remained unclear. Here, we show that disruption of mitochondrial membrane potential leads to axonal NMNAT2 depletion in mouse sympathetic neurons, increasing the substrate-to-product ratio (NMN/NAD) of this NAD-synthesising enzyme, a metabolic fingerprint of Wallerian degeneration. The mechanism appears to involve both impaired NMNAT2 synthesis and reduced axonal transport. Expression of WLDS and Sarm1 deletion both protect axons after mitochondrial uncoupling. Blocking the pathway also confers neuroprotection and increases the lifespan of flies with Pink1 loss-of-function mutation, which causes severe mitochondrial defects. These data indicate that mitochondrial impairment replicates all the major steps of Wallerian degeneration, placing it upstream of NMNAT2 loss, with the potential to contribute to axon pathology in mitochondrial disorders.

neuroscience↗