Spatial distribution of Mycobacterium tuberculosis mRNA and secreted antigens in acid-fast negative human antemortem and resected tissue.

BACKGROUND: The ability to detect evidence of Mycobacterium tuberculosis (Mtb) infection within human tissues is critical to the study of Mtb physiology, tropism, and spatial distribution within TB lesions. The capacity of the widely-used Ziehl-Neelsen (ZN) staining method for identifying Mtb acid-fast bacilli (AFB) in tissue is highly variable, which can limit detection of Mtb bacilli for research and diagnostic purposes. Here, we sought to circumvent these limitations via detection of Mtb mRNA and secreted antigens in human tuberculous tissue. METHODS: We adapted RNAscope, an RNA in situ hybridisation (RISH) technique, to detect Mtb mRNA in ante- and postmortem human TB tissues and developed a dual ZN/immunohistochemistry staining approach to identify AFB and bacilli producing antigen 85B (Ag85B). FINDINGS: We identified Mtb mRNA within intact and disintegrating bacilli as well as extrabacillary mRNA. Mtb mRNA was distributed zonally within necrotic and non-necrotic granulomas. We also found Mtb mRNA within, and adjacent to, necrotic granulomas in ZN-negative lung tissue and in Ag85B-positive bronchiolar epithelium. Intriguingly, we observed accumulation of Mtb mRNA and Ag85B in the cytoplasm of host cells. Notably, many AFB were negative for Ag85B staining. Mtb mRNA was observed in ZN-negative antemortem lymph node biopsies. INTERPRETATION: RNAscope and dual ZN/immunohistochemistry staining are well-suited for identifying subsets of intact Mtb and/or bacillary remnants in human tissue. RNAscope can identify Mtb mRNA in ZN-negative tissues from patients with TB and may have diagnostic potential in complex TB cases. FUNDING: Wellcome Leap Delta Tissue Program, Wellcome Strategic Core Award, the National Institutes of Health (NIH, USA), the Mary Heersink Institute for Global Health at UAB, the UAB Heersink School of Medicine.

LiLA: lipid lung-based ATLAS built through a comprehensive workflow designed for an accurate lipid annotation.

Accurate lipid annotation is crucial for understanding the role of lipids in health and disease and identifying therapeutic targets. However, annotating the wide variety of lipid species in biological samples remains challenging in untargeted lipidomic studies. In this work, we present a lipid annotation workflow based on LC-MS and MS/MS strategies, the combination of four bioinformatic tools, and a decision tree to support the accurate annotation and semi-quantification of the lipid species present in lung tissue from control mice. The proposed workflow allowed us to generate a lipid lung-based ATLAS (LiLA), which was then employed to unveil the lipidomic signatures of the Mycobacterium tuberculosis infection at two different time points for a deeper understanding of the disease progression. This workflow, combined with manual inspection strategies of MS/MS data, can enhance the annotation process for lipidomic studies and guide the generation of sample-specific lipidome maps. LiLA serves as a freely available data resource that can be employed in future studies to address lipidomic alterations in mice lung tissue.

Detection of Mycobacterium tuberculosis in human tissue via RNA in situ hybridization.

Rationale: Accurate TB diagnosis is hampered by the variable efficacy of the widely-used Ziehl-Neelsen (ZN) staining method to identify Mycobacterium tuberculosis ( Mtb ) acid-fast bacilli (AFB). Here, we sought to circumvent this current limitation through direct detection of Mtb mRNA. Objectives: To employ RNAscope to determine the spatial distribution of Mtb mRNA within tuberculous human tissue, to appraise ZN-negative tissue from confirmed TB patients, and to provide proof-of-concept of RNAscope as a platform to inform TB diagnosis and Mtb biology. Methods: We examined ante- and postmortem human TB tissue using RNAscope to detect Mtb mRNA and a dual ZN/immunohistochemistry staining approach to identify AFB and bacilli producing antigen 85B (Ag85B). Measurements and main results: We adapted RNAscope for Mtb and identified intact and disintegrated Mtb bacilli and intra- and extracellular Mtb mRNA. Mtb mRNA was distributed zonally within necrotic and non-necrotic granulomas. We also found Mtb mRNA within, and adjacent to, necrotic granulomas in ZN-negative lung tissue and in Ag85B-positive bronchial epithelium. Intriguingly, we observed accumulation of Mtb mRNA and Ag85B in the cytoplasm of host cells. Notably, many AFB were negative for Ag85B staining. Mtb mRNA was observed in ZN-negative antemortem lymph node biopsies. Conclusions: RNAscope has diagnostic potential and can guide therapeutic intervention as it detects Mtb mRNA and morphology in ZN-negative tissues from TB patients, and Mtb mRNA in ZN-negative antemortem biopsies, respectively. Lastly, our data provide evidence that at least two phenotypically distinct populations of Mtb bacilli exist in vivo .

NAD(H) homeostasis underlies host protection mediated by glycolytic myeloid cells in tuberculosis.

Mycobacterium tuberculosis (Mtb) disrupts glycolytic flux in infected myeloid cells through an unclear mechanism. Flux through the glycolytic pathway in myeloid cells is inextricably linked to the availability of NAD+, which is maintained by NAD+ salvage and lactate metabolism. Using lung tissue from tuberculosis (TB) patients and myeloid deficient LDHA (LdhaLysM-/-) mice, we demonstrate that glycolysis in myeloid cells is essential for protective immunity in TB. Glycolytic myeloid cells are essential for the early recruitment of multiple classes of immune cells and IFNγ-mediated protection. We identify NAD+ depletion as central to the glycolytic inhibition caused by Mtb. Lastly, we show that the NAD+ precursor nicotinamide exerts a host-dependent, antimycobacterial effect, and that nicotinamide prophylaxis and treatment reduce Mtb lung burden in mice. These findings provide insight into how Mtb alters host metabolism through perturbation of NAD(H) homeostasis and reprogramming of glycolysis, highlighting this pathway as a potential therapeutic target.

A high-resolution 3D atlas of the spectrum of tuberculous and COVID-19 lung lesions.

Our current understanding of the spectrum of TB and COVID-19 lesions in the human lung is limited by a reliance on low-resolution imaging platforms that cannot provide accurate 3D representations of lesion types within the context of the whole lung. To characterize TB and COVID-19 lesions in 3D, we applied micro/nanocomputed tomography to surgically resected, postmortem, and paraffin-embedded human lung tissue. We define a spectrum of TB pathologies, including cavitary lesions, calcium deposits outside and inside necrotic granulomas and mycetomas, and vascular rearrangement. We identified an unusual spatial arrangement of vasculature within an entire COVID-19 lobe, and 3D segmentation of blood vessels revealed microangiopathy associated with hemorrhage. Notably, segmentation of pathological anomalies reveals hidden pathological structures that might otherwise be disregarded, demonstrating a powerful method to visualize pathologies in 3D in TB lung tissue and whole COVID-19 lobes. These findings provide unexpected new insight into the spatial organization of the spectrum of TB and COVID-19 lesions within the framework of the entire lung.

Mycobacterium tuberculosis DosS binds H2S through its Fe3+ heme iron to regulate the DosR dormancy regulon.

Mycobacterium tuberculosis (Mtb) senses and responds to host-derived gasotransmitters NO and CO via heme-containing sensor kinases DosS and DosT and the response regulator DosR. Hydrogen sulfide (H2S) is an important signaling molecule in mammals, but its role in Mtb physiology is unclear. We have previously shown that exogenous H2S can modulate expression of genes in the Dos dormancy regulon via an unknown mechanism(s). Here, we test the hypothesis that Mtb senses and responds to H2S via the DosS/T/R system. Using UV-Vis and EPR spectroscopy, we show that H2S binds directly to the ferric (Fe3+) heme of DosS (KDapp = 5.30 µM) but not the ferrous (Fe2+) form. No interaction with DosT(Fe2+-O2) was detected. We found that the binding of sulfide can slowly reduce the DosS heme iron to the ferrous form. Steered Molecular Dynamics simulations show that H2S, and not the charged HS- species, can enter the DosS heme pocket. We also show that H2S increases DosS autokinase activity and subsequent phosphorylation of DosR, and H2S-mediated increases in Dos regulon gene expression is lost in Mtb lacking DosS. Finally, we demonstrate that physiological levels of H2S in macrophages can induce DosR regulon genes via DosS. Overall, these data reveal a novel mechanism whereby Mtb senses and responds to a third host gasotransmitter, H2S, via DosS(Fe3+). These findings highlight the remarkable plasticity of DosS and establish a new paradigm for how bacteria can sense multiple gasotransmitters through a single heme sensor kinase.

Mycobacterium tuberculosis H2S Functions as a Sink to Modulate Central Metabolism, Bioenergetics, and Drug Susceptibility.

H2S is a potent gasotransmitter in eukaryotes and bacteria. Host-derived H2S has been shown to profoundly alter M. tuberculosis (Mtb) energy metabolism and growth. However, compelling evidence for endogenous production of H2S and its role in Mtb physiology is lacking. We show that multidrug-resistant and drug-susceptible clinical Mtb strains produce H2S, whereas H2S production in non-pathogenic M. smegmatis is barely detectable. We identified Rv3684 (Cds1) as an H2S-producing enzyme in Mtb and show that cds1 disruption reduces, but does not eliminate, H2S production, suggesting the involvement of multiple genes in H2S production. We identified endogenous H2S to be an effector molecule that maintains bioenergetic homeostasis by stimulating respiration primarily via cytochrome bd. Importantly, H2S plays a key role in central metabolism by modulating the balance between oxidative phosphorylation and glycolysis, and it functions as a sink to recycle sulfur atoms back to cysteine to maintain sulfur homeostasis. Lastly, Mtb-generated H2S regulates redox homeostasis and susceptibility to anti-TB drugs clofazimine and rifampicin. These findings reveal previously unknown facets of Mtb physiology and have implications for routine laboratory culturing, understanding drug susceptibility, and improved diagnostics.

Micro-Computed Tomography Analysis of the Human Tuberculous Lung Reveals Remarkable Heterogeneity in Three-dimensional Granuloma Morphology.

Rationale: Our current understanding of tuberculosis (TB) pathophysiology is limited by a reliance on animal models, the paucity of human TB lung tissue, and traditional histopathological analysis, a destructive two-dimensional approach that provides limited spatial insight. Determining the three-dimensional (3D) structure of the necrotic granuloma, a characteristic feature of TB, will more accurately inform preventive TB strategies.Objectives: To ascertain the 3D shape of the human tuberculous granuloma and its spatial relationship with airways and vasculature within large lung tissues.Methods: We characterized the 3D microanatomical environment of human tuberculous lungs by using micro computed tomography, histopathology, and immunohistochemistry. By using 3D segmentation software, we accurately reconstructed TB granulomas, vasculature, and airways in three dimensions and confirmed our findings by using histopathology and immunohistochemistry.Measurements and Main Results: We observed marked heterogeneity in the morphology, volume, and number of TB granulomas in human lung sections. Unlike depictions of granulomas as simple spherical structures, human necrotic granulomas exhibit complex, cylindrical, branched morphologies that are connected to the airways and shaped by the bronchi. The use of 3D imaging of human TB lung sections provides unanticipated insight into the spatial organization of TB granulomas in relation to the airways and vasculature.Conclusions: Our findings highlight the likelihood that a single, structurally complex lesion could be mistakenly viewed as multiple independent lesions when evaluated in two dimensions. In addition, the lack of vascularization within obstructed bronchi establishes a paradigm for antimycobacterial drug tolerance. Lastly, our results suggest that bronchogenic spread of Mycobacterium tuberculosis reseeds the lung.

The Role of Host-Generated H2S in Microbial Pathogenesis: New Perspectives on Tuberculosis.

For centuries, hydrogen sulfide (H2S) was considered primarily as a poisonous gas and environmental hazard. However, with the discovery of prokaryotic and eukaryotic enzymes for H2S production, breakdown, and utilization, H2S has emerged as an important signaling molecule in a wide range of physiological and pathological processes. Hence, H2S is considered a gasotransmitter along with nitric oxide (•NO) and carbon monoxide (CO). Surprisingly, despite having overlapping functions with •NO and CO, the role of host H2S in microbial pathogenesis is understudied and represents a gap in our knowledge. Given the numerous reports that followed the discovery of •NO and CO and their respective roles in microbial pathogenesis, we anticipate a rapid increase in studies that further define the importance of H2S in microbial pathogenesis, which may lead to new virulence paradigms. Therefore, this review provides an overview of sulfide chemistry, enzymatic production of H2S, and the importance of H2S in metabolism and immunity in response to microbial pathogens. We then describe our current understanding of the role of host-derived H2S in tuberculosis (TB) disease, including its influences on host immunity and bioenergetics, and on Mycobacterium tuberculosis (Mtb) growth and survival. Finally, this review discusses the utility of H2S-donor compounds, inhibitors of H2S-producing enzymes, and their potential clinical significance.

Bedaquiline reprograms central metabolism to reveal glycolytic vulnerability in Mycobacterium tuberculosis.

The approval of bedaquiline (BDQ) for the treatment of tuberculosis has generated substantial interest in inhibiting energy metabolism as a therapeutic paradigm. However, it is not known precisely how BDQ triggers cell death in Mycobacterium tuberculosis (Mtb). Using 13C isotopomer analysis, we show that BDQ-treated Mtb redirects central carbon metabolism to induce a metabolically vulnerable state susceptible to genetic disruption of glycolysis and gluconeogenesis. Metabolic flux profiles indicate that BDQ-treated Mtb is dependent on glycolysis for ATP production, operates a bifurcated TCA cycle by increasing flux through the glyoxylate shunt, and requires enzymes of the anaplerotic node and methylcitrate cycle. Targeting oxidative phosphorylation (OXPHOS) with BDQ and simultaneously inhibiting substrate level phosphorylation via genetic disruption of glycolysis leads to rapid sterilization. Our findings provide insight into the metabolic mechanism of BDQ-induced cell death and establish a paradigm for the development of combination therapies that target OXPHOS and glycolysis.

Hydrogen sulfide stimulates Mycobacterium tuberculosis respiration, growth and pathogenesis.

Hydrogen sulfide (H2S) is involved in numerous pathophysiological processes and shares overlapping functions with CO and •NO. However, the importance of host-derived H2S in microbial pathogenesis is unknown. Here we show that Mtb-infected mice deficient in the H2S-producing enzyme cystathionine ß-synthase (CBS) survive longer with reduced organ burden, and that pharmacological inhibition of CBS reduces Mtb bacillary load in mice. High-resolution respirometry, transcriptomics and mass spectrometry establish that H2S stimulates Mtb respiration and bioenergetics predominantly via cytochrome bd oxidase, and that H2S reverses •NO-mediated inhibition of Mtb respiration. Further, exposure of Mtb to H2S regulates genes involved in sulfur and copper metabolism and the Dos regulon. Our results indicate that Mtb exploits host-derived H2S to promote growth and disease, and suggest that host-directed therapies targeting H2S production may be potentially useful for the management of tuberculosis and other microbial infections.

Microanatomic Distribution of Myeloid Heme Oxygenase-1 Protects against Free Radical-Mediated Immunopathology in Human Tuberculosis.

Heme oxygenase-1 (HO-1) is a cytoprotective enzyme that controls inflammatory responses and redox homeostasis; however, its role during pulmonary tuberculosis (TB) remains unclear. Using freshly resected human TB lung tissue, we examined the role of HO-1 within the cellular and pathological spectrum of TB. Flow cytometry and histopathological analysis of human TB lung tissues showed that HO-1 is expressed primarily in myeloid cells and that HO-1 levels in these cells were directly proportional to cytoprotection. HO-1 mitigates TB pathophysiology by diminishing myeloid cell-mediated oxidative damage caused by reactive oxygen and/or nitrogen intermediates, which control granulocytic karyorrhexis to generate a zonal HO-1 response. Using whole-body or myeloid-specific HO-1-deficient mice, we demonstrate that HO-1 is required to control myeloid cell infiltration and inflammation to protect against TB progression. Overall, this study reveals that zonation of HO-1 in myeloid cells modulates free-radical-mediated stress, which regulates human TB immunopathology.

Ferritin H Deficiency in Myeloid Compartments Dysregulates Host Energy Metabolism and Increases Susceptibility to Mycobacterium tuberculosis Infection.

Iron is an essential factor for the growth and virulence of Mycobacterium tuberculosis (Mtb). However, little is known about the mechanisms by which the host controls iron availability during infection. Since ferritin heavy chain (FtH) is a major intracellular source of reserve iron in the host, we hypothesized that the lack of FtH would cause dysregulated iron homeostasis to exacerbate TB disease. Therefore, we used knockout mice lacking FtH in myeloid-derived cell populations to study Mtb disease progression. We found that FtH plays a critical role in protecting mice against Mtb, as evidenced by increased organ burden, extrapulmonary dissemination, and decreased survival in Fth-/- mice. Flow cytometry analysis showed that reduced levels of FtH contribute to an excessive inflammatory response to exacerbate disease. Extracellular flux analysis showed that FtH is essential for maintaining bioenergetic homeostasis through oxidative phosphorylation. In support of these findings, RNAseq and mass spectrometry analyses demonstrated an essential role for FtH in mitochondrial function and maintenance of central intermediary metabolism in vivo. Further, we show that FtH deficiency leads to iron dysregulation through the hepcidin-ferroportin axis during infection. To assess the clinical significance of our animal studies, we performed a clinicopathological analysis of iron distribution within human TB lung tissue and showed that Mtb severely disrupts iron homeostasis in distinct microanatomic locations of the human lung. We identified hemorrhage as a major source of metabolically inert iron deposition. Importantly, we observed increased iron levels in human TB lung tissue compared to healthy tissue. Overall, these findings advance our understanding of the link between iron-dependent energy metabolism and immunity and provide new insight into iron distribution within the spectrum of human pulmonary TB. These metabolic mechanisms could serve as the foundation for novel host-directed strategies.

The emerging role of gasotransmitters in the pathogenesis of tuberculosis.

Mycobacterium tuberculosis (Mtb) is a facultative intracellular pathogen and the second largest contributor to global mortality caused by an infectious agent after HIV. In infected host cells, Mtb is faced with a harsh intracellular environment including hypoxia and the release of nitric oxide (NO) and carbon monoxide (CO) by immune cells. Hypoxia, NO and CO induce a state of in vitro dormancy where Mtb senses these gases via the DosS and DosT heme sensor kinase proteins, which in turn induce a set of ∼47 genes, known as the Mtb Dos dormancy regulon. On the contrary, both iNOS and HO-1, which produce NO and CO, respectively, have been shown to be important against mycobacterial disease progression. In this review, we discuss the impact of O2, NO and CO on Mtb physiology and in host responses to Mtb infection as well as the potential role of another major endogenous gas, hydrogen sulfide (H2S), in Mtb pathogenesis.

Ergothioneine Maintains Redox and Bioenergetic Homeostasis Essential for Drug Susceptibility and Virulence of Mycobacterium tuberculosis.

The mechanisms by which Mycobacterium tuberculosis (Mtb) maintains metabolic equilibrium to survive during infection and upon exposure to antimycobacterial drugs are poorly characterized. Ergothioneine (EGT) and mycothiol (MSH) are the major redox buffers present in Mtb, but the contribution of EGT to Mtb redox homeostasis and virulence remains unknown. We report that Mtb WhiB3, a 4Fe-4S redox sensor protein, regulates EGT production and maintains bioenergetic homeostasis. We show that central carbon metabolism and lipid precursors regulate EGT production and that EGT modulates drug sensitivity. Notably, EGT and MSH are both essential for redox and bioenergetic homeostasis. Transcriptomic analyses of EGT and MSH mutants indicate overlapping but distinct functions of EGT and MSH. Last, we show that EGT is critical for Mtb survival in both macrophages and mice. This study has uncovered a dynamic balance between Mtb redox and bioenergetic homeostasis, which critically influences Mtb drug susceptibility and pathogenicity.

Fiber-modified adenovirus for central nervous system Parkinson's disease gene therapy.

Gene-based therapies for neurological diseases continue to develop briskly. As disease mechanisms are elucidated, flexible gene delivery platforms incorporating transcriptional regulatory elements, therapeutic genes and targeted delivery are required for the safety and efficacy of these approaches. Adenovirus serotype 5 (Ad5)-based vectors can carry large genetic payloads to provide this flexibility, but do not transduce neuronal cells efficiently. To address this, we have developed a tropism-modified Ad5 vector with neuron-selective targeting properties for evaluation in models of Parkinson disease therapy. A panel of tropism-modified Ad5 vectors was screened for enhanced gene delivery in a neuroblastoma cell line model system. We used these observations to design and construct an unbiased Ad vector platform, consisting of an unmodified Ad5 and a tropism-modified Ad5 vector containing the fiber knob domain from canine Ad serotype 2 (Ad5-CGW-CK2). Delivery to the substantia nigra or striatum showed that this vector produced a neuronally-restricted pattern of gene expression. Many of the transduced neurons were from regions with afferent projections to the injection site, implicating that the vector binds the presynaptic terminal resulting in presynaptic transduction. We show that Ad5-CGW-CK2 can selectively transduce neurons in the brain and hypothesize that this modular platform is potentially adaptable to clinical use.

An adenovirus vector incorporating carbohydrate binding domains utilizes glycans for gene transfer.

BACKGROUND: Vectors based on human adenovirus serotype 5 (HAdV-5) continue to show promise as delivery vehicles for cancer gene therapy. Nevertheless, it has become clear that therapeutic benefit is directly linked to tumor-specific vector localization, highlighting the need for tumor-targeted gene delivery. Aberrant glycosylation of cell surface glycoproteins and glycolipids is a central feature of malignant transformation, and tumor-associated glycoforms are recognized as cancer biomarkers. On this basis, we hypothesized that cancer-specific cell-surface glycans could be the basis of a novel paradigm in HAdV-5-based vector targeting. METHODOLOGY/PRINCIPAL FINDINGS: As a first step toward this goal, we constructed a novel HAdV-5 vector encoding a unique chimeric fiber protein that contains the tandem carbohydrate binding domains of the fiber protein of the NADC-1 strain of porcine adenovirus type 4 (PAdV-4). This glycan-targeted vector displays augmented CAR-independent gene transfer in cells with low CAR expression. Further, we show that gene transfer is markedly decreased in cells with genetic glycosylation defects and by inhibitors of glycosylation in normal cells. CONCLUSIONS/SIGNIFICANCE: These data provide the initial proof-of-concept for HAdV-5 vector-mediated gene delivery based on the presence of cell-surface carbohydrates. Further development of this new targeting paradigm could provide targeted gene delivery based on vector recognition of disease-specific glycan biomarkers.

Iron sulfur cluster proteins and microbial regulation: implications for understanding tuberculosis.

All pathogenic and nonpathogenic microbes are continuously exposed to environmental or endogenous reactive oxygen and nitrogen species, which can critically effect survival and disease. Iron-sulfur [Fe-S] cluster containing prosthetic groups provide the microbial cell with a unique capacity to sense and transcriptionally respond to diatomic gases (e.g. NO and O2) and redox-cycling agents. Recent advances in our understanding of the mechanisms for how the FNR and SoxR [Fe-S] cluster proteins respond to NO and O2 have provided new insights into the biochemical mechanism of action of the Mycobacterium tuberculosis (Mtb) family of WhiB [Fe-S] cluster proteins. These insights have provided the basis for establishing a unifying paradigm for the Mtb WhiB family of proteins. Mtb is the etiological agent for tuberculosis (TB), a disease that affects nearly one-third of the world's population.

Transduction of brain dopamine neurons by adenoviral vectors is modulated by CAR expression: rationale for tropism modified vectors in PD gene therapy.

BACKGROUND: Gene-based therapy is a new paradigm for the treatment of Parkinson disease (PD) and offers considerable promise for precise targeting and flexibility to impact multiple pathobiological processes for which small molecule agents are not available. Some success has been achieved utilizing adeno-associated virus for this approach, but it is likely that the characteristics of this vector system will ultimately create barriers to progress in clinical therapy. Adenovirus (Ad) vector overcomes limitations in payload size and targeting. The cellular tropism of Ad serotype 5 (Ad5)-based vectors is regulated by the Ad attachment protein binding to its primary cellular receptor, the coxsackie and adenovirus receptor (CAR). Many clinically relevant tissues are refractory to Ad5 infection due to negligible CAR levels but can be targeted by tropism-modified, CAR-independent forms of Ad. Our objective was to evaluate the role of CAR protein in transduction of dopamine (DA) neurons in vivo. METHODOLOGY/PRINCIPAL FINDINGS: Ad5 was delivered to the substantia nigra (SN) in wild type (wt) and CAR transgenic animals. Cellular tropism was assessed by immunohistochemistry (IHC) in the SN and striatal terminals. CAR expression was assessed by western blot and IHC. We found in wt animals, Ad5 results in robust transgene expression in astrocytes and other non-neuronal cells but poor infection of DA neurons. In contrast, in transgenic animals, Ad5 infects SNc neurons resulting in expression of transduced protein in their striatal terminals. Western blot showed low CAR expression in the ventral midbrain of wt animals compared to transgenic animals. Interestingly, hCAR protein localizes with markers of post-synaptic structures, suggesting synapses are the point of entry into dopaminergic neurons in transgenic animals. CONCLUSIONS/SIGNIFICANCE: These findings demonstrate that CAR deficiency limits infection of wild type DA neurons by Ad5 and provide a rationale for the development of tropism-modified, CAR-independent Ad-vectors for use in gene therapy of human PD.