Finding the right match fast.

DNA recombinases face the daunting task of locating and pairing up specific sequences among millions of base pairs in a genome, all within about an hour. Qi et al. show that recombinases solve this problem by searching in 8-nt microhomology units, reducing the search space and accelerating the homology search.

Structures illustrate step-by-step mitochondrial transcription initiation.

Transcription initiation is a key regulatory step in gene expression during which RNA polymerase (RNAP) initiates RNA synthesis de novo, and the synthesized RNA at a specific length triggers the transition to the elongation phase. Mitochondria recruit a single-subunit RNAP and one or two auxiliary factors to initiate transcription. Previous studies have revealed the molecular architectures of yeast1 and human2 mitochondrial RNAP initiation complexes (ICs). Here we provide a comprehensive, stepwise mechanism of transcription initiation by solving high-resolution cryogenic electron microscopy (cryo-EM) structures of yeast mitochondrial RNAP and the transcription factor Mtf1 catalysing two- to eight-nucleotide RNA synthesis at single-nucleotide addition steps. The growing RNA-DNA is accommodated in the polymerase cleft by template scrunching and non-template reorganization, creating stressed intermediates. During early initiation, non-template strand scrunching and unscrunching destabilize the short two- and three-nucleotide RNAs, triggering abortive synthesis. Subsequently, the non-template reorganizes into a base-stacked staircase-like structure supporting processive five- to eight-nucleotide RNA synthesis. The expanded non-template staircase and highly scrunched template in IC8 destabilize the promoter interactions with Mtf1 to facilitate initiation bubble collapse and promoter escape for the transition from initiation to the elongation complex (EC). The series of transcription initiation steps, each guided by the interplay of multiple structural components, reveal a finely tuned mechanism for potential regulatory control.

Cryo-EM Structures Reveal Transcription Initiation Steps by Yeast Mitochondrial RNA Polymerase.

Mitochondrial RNA polymerase (mtRNAP) is crucial in cellular energy production, yet understanding of mitochondrial DNA transcription initiation lags that of bacterial and nuclear DNA transcription. We report structures of two transcription initiation intermediate states of yeast mtRNAP that explain promoter melting, template alignment, DNA scrunching, abortive synthesis, and transition into elongation. In the partially melted initiation complex (PmIC), transcription factor MTF1 makes base-specific interactions with flipped non-template (NT) nucleotides "AAGT" at -4 to -1 positions of the DNA promoter. In the initiation complex (IC), the template in the expanded 7-mer bubble positions the RNA and NTP analog UTPαS, while NT scrunches into an NT loop. The scrunched NT loop is stabilized by the centrally positioned MTF1 C-tail. The IC and PmIC states coexist in solution, revealing a dynamic equilibrium between two functional states. Frequent scrunching/unscruching transitions and the imminent steric clashes of the inflating NT loop and growing RNA:DNA with the C-tail explain abortive synthesis and transition into elongation.

The intrinsically disordered CARDs-Helicase linker in RIG-I is a molecular gate for RNA proofreading.

The innate immune receptor RIG-I provides a first line of defense against viral infections. Viral RNAs are recognized by RIG-I's C-terminal domain (CTD), but the RNA must engage the helicase domain to release the signaling CARD (Caspase Activation and Recruitment Domain) domains from their autoinhibitory CARD2:Hel2i interactions. Because the helicase itself lacks RNA specificity, mechanisms to proofread RNAs entering the helicase domain must exist. Although such mechanisms would be crucial in preventing aberrant immune responses by non-specific RNAs, they remain largely uncharacterized to date. This study reveals a previously unknown proofreading mechanism through which RIG-I ensures that the helicase engages RNAs explicitly recognized by the CTD. A crucial part of this mechanism involves the intrinsically disordered CARDs-Helicase Linker (CHL), which connects the CARDs to the helicase subdomain Hel1. CHL uses its negatively charged regions to antagonize incoming RNAs electrostatically. In addition to this RNA gating function, CHL is essential for stabilization of the CARD2:Hel2i interface. Overall, we uncover that the CHL and CARD2:Hel2i interface work together to establish a tunable gating mechanism that allows CTD-chosen RNAs to bind the helicase domain, while at the same time blocking non-specific RNAs. These findings also indicate that CHL could represent a novel target for RIG-I-based therapeutics.

RIG-I Uses an ATPase-Powered Translocation-Throttling Mechanism for Kinetic Proofreading of RNAs and Oligomerization.

RIG-I has a remarkable ability to specifically select viral 5'ppp dsRNAs for activation from a pool of cytosolic self-RNAs. The ATPase activity of RIG-I plays a role in RNA discrimination and activation, but the underlying mechanism was unclear. Using transient-state kinetics, we elucidated the ATPase-driven "kinetic proofreading" mechanism of RIG-I activation and RNA discrimination, akin to DNA polymerases, ribosomes, and T cell receptors. Even in the autoinhibited state of RIG-I, the C-terminal domain kinetically discriminates against self-RNAs by fast off rates. ATP binding facilitates dsRNA engagement but, interestingly, makes RIG-I promiscuous, explaining the constitutive signaling by Singleton-Merten syndrome-linked mutants that bind ATP without hydrolysis. ATP hydrolysis dissociates self-RNAs faster than 5'ppp dsRNA but, more importantly, drives RIG-I oligomerization through translocation, which we show to be regulated by helicase motif IVa. RIG-I translocates directionally from the dsRNA end into the stem region, and the 5'ppp end "throttles" translocation to provide a mechanism for threading and building a signaling-active oligomeric complex.

Correlating Transcription Initiation and Conformational Changes by a Single-Subunit RNA Polymerase with Near Base-Pair Resolution.

We provide a comprehensive analysis of transcription in real time by T7 RNA Polymerase (RNAP) using single-molecule fluorescence resonance energy transfer by monitoring the entire life history of transcription initiation, including stepwise RNA synthesis with near base-pair resolution, abortive cycling, and transition into elongation. Kinetically branching pathways were observed for abortive initiation with an RNAP either recycling on the same promoter or exchanging with another RNAP from solution. We detected fast and slow populations of RNAP in their transition into elongation, consistent with the efficient and delayed promoter release, respectively, observed in ensemble studies. Real-time monitoring of abortive cycling using three-probe analysis showed that the initiation events are stochastically branched into productive and failed transcription. The abortive products are generated primarily from initiation events that fail to progress to elongation, and a majority of the productive events transit to elongation without making abortive products.

Unraveling blunt-end RNA binding and ATPase-driven translocation activities of the RIG-I family helicase LGP2.

The RIG-I family helicases, comprising RIG-I, MDA5 and LGP2, are cytoplasmic RNA sensors that trigger an antiviral immune response by specifically recognizing foreign RNAs. While LGP2 lacks the signaling domain necessary for immune activation, it plays a vital role in regulating the RIG-I/MDA5 signaling pathway. In this study, we investigate the mechanisms underlying this regulation by examining the oligomeric state, RNA binding specificity, and translocation activity of human LGP2 and the impact of ATPase activity. We show that LGP2, like RIG-I, prefers binding blunt-ended double-stranded (ds) RNAs over internal dsRNA regions or RNA overhangs and associates with blunt-ends faster than with overhangs. Unlike RIG-I, a 5'-triphosphate (5'ppp), Cap0, or Cap1 RNA-end does not influence LGP2's RNA binding affinity. LGP2 hydrolyzes ATP in the presence of RNA but at a 5-10 fold slower rate than RIG-I. Nevertheless, LGP2 uses its ATPase activity to translocate and displace biotin-streptavidin interactions. This activity is significantly hindered by a methylated RNA patch, particularly on the 3'-strand, suggesting a 3'-strand tracking mechanism like RIG-I. The preference of LGP2 for blunt-end RNA binding, its insensitivity to Cap0/Cap1 modification, and its translocation/protein displacement ability have substantial implications for how LGP2 regulates the RNA sensing process by MDA5/RIG-I.

RIG-I recognizes metabolite-capped RNAs as signaling ligands.

The innate immune receptor RIG-I recognizes 5'-triphosphate double-stranded RNAs (5' PPP dsRNA) as pathogenic RNAs. Such RNA-ends are present in viral genomes and replication intermediates, and they activate the RIG-I signaling pathway to produce a potent interferon response essential for viral clearance. Endogenous mRNAs cap the 5' PPP-end with m7G and methylate the 2'-O-ribose to evade RIG-I, preventing aberrant immune responses deleterious to the cell. Recent studies have identified RNAs in cells capped with metabolites such as NAD+, FAD and dephosphoCoA. Whether RIG-I recognizes these metabolite-capped RNAs has not been investigated. Here, we describe a strategy to make metabolite-capped RNAs free from 5' PPP dsRNA contamination, using in vitro transcription initiated with metabolites. Mechanistic studies show that metabolite-capped RNAs have a high affinity for RIG-I, stimulating the ATPase activity at comparable levels to 5' PPP dsRNA. Cellular signaling assays show that the metabolite-capped RNAs potently stimulate the innate antiviral immune response. This demonstrates that RIG-I can tolerate diphosphate-linked, capped RNAs with bulky groups at the 5' RNA end. This novel class of RNAs that stimulate RIG-I signaling may have cellular roles in activating the interferon response and may be exploited with proper functionalities for RIG-I-related RNA therapeutics.

The N-terminal domain of human mitochondrial helicase Twinkle has DNA-binding activity crucial for supporting processive DNA synthesis by polymerase γ.

Twinkle is the ring-shaped replicative helicase within the human mitochondria with high homology to bacteriophage T7 gp4 helicase-primase. Unlike many orthologs of Twinkle, the N-terminal domain (NTD) of human Twinkle has lost its primase activity through evolutionarily acquired mutations. The NTD has no demonstrated activity thus far; its role has remained unclear. Here, we biochemically characterize the isolated NTD and C-terminal domain (CTD) with linker to decipher their contributions to full-length Twinkle activities. This novel CTD construct hydrolyzes ATP, has weak DNA unwinding activity, and assists DNA polymerase γ (Polγ)-catalyzed strand-displacement synthesis on short replication forks. However, CTD fails to promote multikilobase length product formation by Polγ in rolling-circle DNA synthesis. Thus, CTD retains all the motor functions but struggles to implement them for processive translocation. We show that NTD has DNA-binding activity, and its presence stabilizes Twinkle oligomerization. CTD oligomerizes on its own, but the loss of NTD results in heterogeneously sized oligomeric species. The CTD also exhibits weaker and salt-sensitive DNA binding compared with full-length Twinkle. Based on these results, we propose that NTD directly contributes to DNA binding and holds the DNA in place behind the central channel of the CTD like a "doorstop," preventing helicase slippages and sustaining processive unwinding. Consistent with this model, mitochondrial single-stranded DNA-binding protein (mtSSB) compensate for the NTD loss and partially restore kilobase length DNA synthesis by CTD and Polγ. The implications of our studies are foundational for understanding the mechanisms of disease-causing Twinkle mutants that lie in the NTD.

Excessive excision of correct nucleotides during DNA synthesis explained by replication hurdles.

The proofreading exonuclease activity of replicative DNA polymerase excises misincorporated nucleotides during DNA synthesis, but these events are rare. Therefore, we were surprised to find that T7 replisome excised nearly 7% of correctly incorporated nucleotides during leading and lagging strand syntheses. Similar observations with two other DNA polymerases establish its generality. We show that excessive excision of correctly incorporated nucleotides is not due to events such as processive degradation of nascent DNA or spontaneous partitioning of primer-end to the exonuclease site as a "cost of proofreading". Instead, we show that replication hurdles, including secondary structures in template, slowed helicase, or uncoupled helicase-polymerase, increase DNA reannealing and polymerase backtracking, and generate frayed primer-ends that are shuttled to the exonuclease site and excised efficiently. Our studies indicate that active-site shuttling occurs at a high frequency, and we propose that it serves as a proofreading mechanism to protect primer-ends from mutagenic extensions.

Proofreading mechanisms of the innate immune receptor RIG-I: distinguishing self and viral RNA.

The RIG-I-like receptors (RLRs), comprising retinoic acid-inducible gene I (RIG-I), melanoma differentiation-associated gene 5 (MDA5), and laboratory of genetics and physiology 2 (LGP2), are pattern recognition receptors belonging to the DExD/H-box RNA helicase family of proteins. RLRs detect viral RNAs in the cytoplasm and respond by initiating a robust antiviral response that up-regulates interferon and cytokine production. RIG-I and MDA5 complement each other by recognizing different RNA features, and LGP2 regulates their activation. RIG-I's multilayered RNA recognition and proofreading mechanisms ensure accurate viral RNA detection while averting harmful responses to host RNAs. RIG-I's C-terminal domain targets 5'-triphosphate double-stranded RNA (dsRNA) blunt ends, while an intrinsic gating mechanism prevents the helicase domains from non-specifically engaging with host RNAs. The ATPase and RNA translocation activity of RIG-I adds another layer of selectivity by minimizing the lifetime of RIG-I on non-specific RNAs, preventing off-target activation. The versatility of RIG-I's ATPase function also amplifies downstream signaling by enhancing the signaling domain (CARDs) exposure on 5'-triphosphate dsRNA and promoting oligomerization. In this review, we offer an in-depth understanding of the mechanisms RIG-I uses to facilitate viral RNA sensing and regulate downstream activation of the immune system.

The efficacy and safety of systemic corticosteroids as first line treatment for granulomatous lymphocytic interstitial lung disease.

BACKGROUND: Granulomatous and lymphocytic interstitial lung disease (gl-ILD) is a major cause of morbidity and mortality among patients with common variable immunodeficiency. Corticosteroids are recommended as first-line treatment for gl-ILD, but evidence for their efficacy is lacking. OBJECTIVES: This study analyzed the effect of high-dose corticosteroids (≥0.3 mg/kg prednisone equivalent) on gl-ILD, measured by high-resolution computed tomography (HRCT) scans, and pulmonary function test (PFT) results. METHODS: Patients who had received high-dose corticosteroids but no other immunosuppressive therapy at the time (n = 56) and who underwent repeated HRCT scanning or PFT (n = 39) during the retrospective and/or prospective phase of the Study of Interstitial Lung Disease in Primary Antibody Deficiency (STILPAD) were included in the analysis. Patients without any immunosuppressive treatment were selected as controls (n = 23). HRCT scans were blinded, randomized, and scored using the Hartman score. Differences between the baseline and follow-up HRCT scans and PFT were analyzed. RESULTS: Treatment with high-dose corticosteroids significantly improved HRCT scores and forced vital capacity. Carbon monoxide diffusion capacity significantly improved in both groups. Of 18 patients, for whom extended follow-up data was available, 13 achieved a long-term, maintenance therapy independent remission. All patients with relapse were retreated with corticosteroids, but only one-fifth of them responded. Two opportunistic infections were found in the corticosteroid treatment group, while overall infection rate was similar between cohorts. CONCLUSIONS: Induction therapy with high-dose corticosteroids improved HRCT scans and PFT results of patients with gl-ILD and achieved long-term remission in 42% of patients. It was not associated with major side effects. Low-dose maintenance therapy provided no benefit and efficacy was poor in relapsing disease.

Twinkle-Catalyzed Toehold-Mediated DNA Strand Displacement Reaction.

Strand exchange between homologous nucleic acid sequences is the basis for cellular DNA repair, recombination, and genome editing technologies. Specialized enzymes catalyze cellular strand exchange; however, the reaction occurs spontaneously when a single-stranded DNA toehold can dock the invader strand on the target DNA to initiate strand exchange through branch migration. Due to its precise response, the spontaneous toehold-mediated strand displacement (TMSD) reaction is widely employed in DNA nanotechnology. However, enzyme-free TMSD suffers from slow rates, resulting in slow response times. Here, we show that human mitochondrial DNA helicase Twinkle can accelerate TMSD up to 6000-fold. Mechanistic studies indicate that Twinkle accelerates TMSD by catalyzing the docking step, which typically limits spontaneous reactions. The catalysis occurs without ATP, and Twinkle-catalyzed TMSD rates remain sensitive to base-pair mismatches. The simple catalysis, tunability, and speed improvement of the catalyzed TMSD can be leveraged in nanotechnology, requiring sensitive detection and faster response times.

Kangaroo mother care utilization at stepdown ward of a tertiary care teaching hospital: a quality improvement study.

PURPOSE: India has the highest burden of preterm/low birth weight newborns. To tackle this, Kangaroo Mother Care (KMC) needs to be scaled up. We did a quality improvement (QI) study to increase KMC coverage to 80% and its utilization to at least 4 h/infant/day. METHODS: This study was conducted at a stepdown ward (KMC ward) of a tertiary care teaching institute over a period of four months. All babies with birth weight <2.5 kg were eligible. The QI team included faculty in-charge, one senior resident and three senior staff nurses. Potential barriers were listed using fish-bone analysis. Four possible interventions were identified (daily documentation of total KMC hours by doctor, providing KMC during all the nursing duty shifts, counseling and education to mothers and family members), introduced, and then subsequently tested by four Plan-Do-Study-Act (PDSA) cycles and sustenance was assessed over three months. RESULTS: A total of 93 infants were included in this QI study. During baseline phase, the KMC coverage was 50% which increased to 100% by the end of fourth PDSA cycle and remained 100% during the sustenance phase. During baseline period, KMC was given for ≥ 4 h in 18.8% (28 of 149) patient days which increased to 88.96% (137 of 154) during the sustenance phase. The mean KMC utilization increased from 1.97 (1.57) h/infant/day to 5.65 (1.20) h/infant/day in the sustenance phase. CONCLUSION: QI study incorporating PDSA cycles helped improve coverage and utilization of KMC.

Resolving the polygenic aetiology of a late onset combined immune deficiency caused by NFKB1 haploinsufficiency and modified by PIK3R1 and TNFRSF13B variants.

Genetic variants in PIK3CD, PIK3R1 and NFKB1 cause the primary immune deficiencies, activated PI3Kδ syndrome (APDS) 1, APDS2 and NFκB1 haploinsufficiency, respectively. We have identified a family with known or potentially pathogenic variants NFKB1, TNFRSF13B and PIK3R1. The study's aim was to describe their associated immune and cellular phenotypes and compare with individuals with monogenic disease. NFκB1 pathway function was measured by immunoblotting and PI3Kδ pathway activity by phospho-flow cytometry. p105/p50 expression was absent in two individuals but elevated pS6 only in the index case. Transfection of primary T cells demonstrated increased basal pS6 signalling due to mutant PIK3R1, but not mutant NFKB1 or their wildtype forms. We report on the presence of pathogenic variant NFKB1, with likely modifying variants in TNFRSF13B and PIK3R1 in a family. We describe immune features of both NFκB1 haploinsufficiency and APDS2, and the inhibition of excessive PI3K signalling by rapamycin in vitro.

SARS-CoV-2 Vaccine Responses in Individuals with Antibody Deficiency: Findings from the COV-AD Study.

BACKGROUND: Vaccination prevents severe morbidity and mortality from COVID-19 in the general population. The immunogenicity and efficacy of SARS-CoV-2 vaccines in patients with antibody deficiency is poorly understood. OBJECTIVES: COVID-19 in patients with antibody deficiency (COV-AD) is a multi-site UK study that aims to determine the immune response to SARS-CoV-2 infection and vaccination in patients with primary or secondary antibody deficiency, a population that suffers from severe and recurrent infection and does not respond well to vaccination. METHODS: Individuals on immunoglobulin replacement therapy or with an IgG less than 4 g/L receiving antibiotic prophylaxis were recruited from April 2021. Serological and cellular responses were determined using ELISA, live-virus neutralisation and interferon gamma release assays. SARS-CoV-2 infection and clearance were determined by PCR from serial nasopharyngeal swabs. RESULTS: A total of 5.6% (n = 320) of the cohort reported prior SARS-CoV-2 infection, but only 0.3% remained PCR positive on study entry. Seropositivity, following two doses of SARS-CoV-2 vaccination, was 54.8% (n = 168) compared with 100% of healthy controls (n = 205). The magnitude of the antibody response and its neutralising capacity were both significantly reduced compared to controls. Participants vaccinated with the Pfizer/BioNTech vaccine were more likely to be seropositive (65.7% vs. 48.0%, p = 0.03) and have higher antibody levels compared with the AstraZeneca vaccine (IgGAM ratio 3.73 vs. 2.39, p = 0.0003). T cell responses post vaccination was demonstrable in 46.2% of participants and were associated with better antibody responses but there was no difference between the two vaccines. Eleven vaccine-breakthrough infections have occurred to date, 10 of them in recipients of the AstraZeneca vaccine. CONCLUSION: SARS-CoV-2 vaccines demonstrate reduced immunogenicity in patients with antibody deficiency with evidence of vaccine breakthrough infection.

Outcomes following SARS-CoV-2 infection in patients with primary and secondary immunodeficiency in the UK.

In March 2020, the United Kingdom Primary Immunodeficiency Network (UKPIN) established a registry of cases to collate the outcomes of individuals with PID and SID following SARS-CoV-2 infection and treatment. A total of 310 cases of SARS-CoV-2 infection in individuals with PID or SID have now been reported in the UK. The overall mortality within the cohort was 17.7% (n = 55/310). Individuals with CVID demonstrated an infection fatality rate (IFR) of 18.3% (n = 17/93), individuals with PID receiving IgRT had an IFR of 16.3% (n = 26/159) and individuals with SID, an IFR of 27.2% (n = 25/92). Individuals with PID and SID had higher inpatient mortality and died at a younger age than the general population. Increasing age, low pre-SARS-CoV-2 infection lymphocyte count and the presence of common co-morbidities increased the risk of mortality in PID. Access to specific COVID-19 treatments in this cohort was limited: only 22.9% (n = 33/144) of patients admitted to the hospital received dexamethasone, remdesivir, an anti-SARS-CoV-2 antibody-based therapeutic (e.g. REGN-COV2 or convalescent plasma) or tocilizumab as a monotherapy or in combination. Dexamethasone, remdesivir, and anti-SARS-CoV-2 antibody-based therapeutics appeared efficacious in PID and SID. Compared to the general population, individuals with PID or SID are at high risk of mortality following SARS-CoV-2 infection. Increasing age, low baseline lymphocyte count, and the presence of co-morbidities are additional risk factors for poor outcome in this cohort.

Kinetics of α-synuclein prions preceding neuropathological inclusions in multiple system atrophy.

Multiple system atrophy (MSA), a progressive neurodegenerative disease characterized by autonomic dysfunction and motor impairment, is caused by the self-templated misfolding of the protein α-synuclein. With no treatment currently available, we sought to characterize the spread of α-synuclein in a transgenic mouse model of MSA prion propagation to support drug discovery programs for synucleinopathies. Brain homogenates from MSA patient samples or mouse-passaged MSA were inoculated either by standard freehand injection or stereotactically into TgM83+/- mice, which express human α-synuclein with the A53T mutation. Following disease onset, brains from the mice were tested for biologically active α-synuclein prions using a cell-based assay and examined for α-synuclein neuropathology. Inoculation studies using homogenates prepared from brain regions lacking detectable α-synuclein neuropathology transmitted neurological disease to mice. Terminal animals contained similar concentrations of α-synuclein prions; however, a time-course study where mice were terminated every five days through disease progression revealed that the kinetics of α-synuclein prion replication in the mice were variable. Stereotactic inoculation into the thalamus reduced variability in disease onset in the mice, although incubation times were consistent with standard inoculations. Using human samples with and without neuropathological lesions, we observed that α-synuclein prion formation precedes neuropathology in the brain, suggesting that disease in patients is not limited to brain regions containing neuropathological lesions.

Anatomy of the proximal septal vein in patients with focal intramural ventricular arrhythmias.

BACKGROUND: Focal ventricular arrhythmias (VAs) originating from the intramural myocardium of the basal septum are difficult to localize and ablate. Proximal septal veins emptying into the great cardiac vein can reach close to the origin of intramural arrhythmias. OBJECTIVE: To assess characteristics of proximal septal coronary veins in patients with intramural VAs. METHODS AND RESULTS: Among 84 consecutive patients with intramural VAs, 29 patients (age 60 ± 11years, 16 males, ejection fraction 47 ± 13%) underwent preprocedural cardiac computed tomographic angiography (CTA). In 14 of these patients, the intramural site of origin (SOO) was identified with multipolar catheters. The intramural SOO could not be accessed with mapping catheters in the other 15 patients while mapping the coronary venous system. The CTA identified sizable proximal septal veins in all patients in whom the SOO could be accessed with mapping catheters. In the patients in whom the intramural SOO was not identified, the proximal septal veins were often either small (<2 mm at the branching site) or non-existent (n = 9, p = .001). The proximal septal veins in patients in whom the SOO was identified were larger than in the patients in whom the SOO could not be identified (3.0 ± 0.6 mm vs. 2.1 ± 0.9 mm, p = .01). CONCLUSIONS: Preprocedural imaging with CTAs can be beneficial in identifying the anatomy of proximal septal coronary veins that allow adequate mapping of patients with suspected intramural VAs.

Progesterone rapidly alters the use of place and response memory during spatial navigation in female rats.

17ß-Estradiol (E2) and progesterone (P) influence place and response memory in female rats in spatial navigation tasks. Use of these memory systems is associated with the hippocampus and the dorsal striatum, respectively. Injections of E2 result in a well-established bias to use place memory, while much less is understood about the role of P. A total of 120 ovariectomized female rats were tested within a dual-solution T-maze task and treated with either low E2 (n = 24), high E2 (10 µg/kg; n = 24), or high E2 in combination with P (500 µg/kg) at three time points before testing: 15 min (n = 24), 1 h (n = 24), and 4 h (n = 24). Given alone, high E2 biases rats to the use of place memory, but this effect is reversed when P is given 1 h or 4 h before testing. This indicates that P may be playing an inhibitory role in the hippocampus during spatial tasks, which is consistent with past findings. Our findings show that P acts rapidly (within an hour) to affect performance during spatial tasks.