Inducible control of gene expression with destabilized Cre

Delivering large genes using adeno-associated virus and the CRE-lox DNA recombination system

Adeno-associated virus (AAV) is a safe and efficient gene delivery vehicle for gene therapies. However, its relatively small packaging capacity limits its use as a gene transfer vector. Here, we describe a strategy to deliver large genes that exceed the AAV's packaging capacity using up to four AAV vectors and the CRE-lox DNA recombination system. We devised novel lox sites by combining non-compatible and reaction equilibrium-modifying lox site variants. These lox sites facilitate sequence-specific and near-unidirectional recombination of AAV vector genomes, enabling efficient reconstitution of up to 16 kb of therapeutic genes in a pre-determined configuration. Using this strategy, we have developed AAV gene therapy vectors to deliver IFT140, PCDH15, CEP290, and CDH23 and demonstrate efficient production of full-length proteins in cultured mammalian cells and mouse retinas. Notably, AAV-IFT140 gene therapy vectors ameliorated retinal degeneration and preserved visual functions in an IFT140-associated retinitis pigmentosa mouse model. The CRE-lox approach described here provides a simple, flexible, and effective platform for generating AAV gene therapy vectors beyond AAV's packaging capacity.

Generation of an inducible destabilized-domain Cre mouse line to target disease associated microglia

The function of microglia during progression of Alzheimer's disease (AD) can be investigated using mouse models that enable genetic manipulation of microglial subpopulations in a temporal manner. We developed a mouse strain that expresses destabilized-domain Cre recombinase (DD-Cre) from the Cst7 locus ( Cst7 DD-Cre ) and tested this in 5xFAD amyloidogenic, Ai14 tdTomato cre-reporter line mice. Dietary administration of trimethoprim to induce DD-Cre activity produces long-term labeling in disease associated microglia (DAM) without evidence of leakiness, with tdTomato-expression restricted to cells surrounding plaques. Using this model, we found that DAMs are a subset of plaque-associated microglia (PAMs) and their transition to DAM increases with age and disease stage. Spatial transcriptomic analysis revealed that tdTomato+ cells show higher expression of disease and inflammatory genes compared to other microglial populations, including non-labeled PAMs. This model should allow inducible cre-loxP targeting of DAMs, without leakiness.

Highlights

We developed a new mouse strain which specifically enables recombination of loxP sites in disease associated microglia (DAMs) and can be used to manipulate DAM-gene expression.DAMs represent a subset of plaque associated microglia (PAMs), and DAM expression increases with disease progression.Spatial transcriptomic analyses reveal that DAMs have higher expression of disease and inflammatory genes compared to other PAMs.

Synaptic architecture of a memory engram in the mouse hippocampus

Memory engrams are formed through experience-dependent remodeling of neural circuits, but their detailed architectures have remained unresolved. Using 3D electron microscopy, we performed nanoscale reconstructions of the hippocampal CA3-CA1 pathway following chemogenetic labeling of cellular ensembles with a remote history of correlated excitation during associative learning. Projection neurons involved in memory acquisition expanded their connectomes via multi-synaptic boutons without altering the numbers and spatial arrangements of individual axonal terminals and dendritic spines. This expansion was driven by presynaptic activity elicited by specific negative valence stimuli, regardless of the co-activation state of postsynaptic partners. The rewiring of initial ensembles representing an engram coincided with local, input-specific changes in the shapes and organelle composition of glutamatergic synapses, reflecting their weights and potential for further modifications. Our findings challenge the view that the connectivity among neuronal substrates of memory traces is governed by Hebbian mechanisms, and offer a structural basis for representational drifts.

Delivering large genes using adeno-associated virus and the CRE-lox DNA recombination system

Adeno-associated virus (AAV) is a safe and efficient gene delivery vehicle for gene therapies. However, its relatively small packaging capacity limits its use as a gene transfer vector. Here, we describe a strategy to deliver large genes that exceed the AAV's packaging capacity using up to four AAV vectors and the CRE-lox DNA recombination system. We devised novel lox sites by combining non-compatible and reaction equilibrium-modifying lox site variants. These lox sites facilitate sequence-specific and near-unidirectional recombination of AAV vector genomes, enabling efficient reconstitution of up to 16 kb of therapeutic genes in a pre-determined configuration. Using this strategy, we have developed AAV gene therapy vectors to deliver IFT140 , PCDH15 , CEP290 , and CDH23 and demonstrate efficient production of full-length proteins in cultured mammalian cells and mouse retinas. Notably, this approach significantly surpasses the trans-splicing and split-intein-based reconstitution methods in efficiency, requiring lower doses, minimizing or eliminating the production of truncated protein products, and offering flexibility in selecting splitting positions. The CRE-lox approach described here provides a simple and effective platform for producing AAV gene therapy vectors beyond AAV's packaging capacity.

An Approach to Intersectionally Target Mature Enteroendocrine Cells in the Small Intestine of Mice

Enteroendocrine cells (EECs) constitute only a small proportion of Villin-1 (Vil1)-expressing intestinal epithelial cells (IECs) of the gastrointestinal tract; yet, in sum, they build the largest endocrine organ of the body, with each of them storing and releasing a distinct set of peptides for the control of feeding behavior, glucose metabolism, and gastrointestinal motility. Like all IEC types, EECs are continuously renewed from intestinal stem cells in the crypt base and terminally differentiate into mature subtypes while moving up the crypt-villus axis. Interestingly, EECs adjust their hormonal secretion according to their migration state as EECs receive altering differentiation signals along the crypt-villus axis and thus undergo functional readaptation. Cell-specific targeting of mature EEC subtypes by specific promoters is challenging because the expression of EEC-derived peptides and their precursors is not limited to EECs but are also found in other organs, such as the brain (e.g., Cck and Sst) as well as in the pancreas (e.g., Sst and Gcg). Here, we describe an intersectional genetic approach that enables cell type-specific targeting of functionally distinct EEC subtypes by combining a newly generated Dre-recombinase expressing mouse line (Vil1-2A-DD-Dre) with multiple existing Cre-recombinase mice and mouse strains with rox and loxP sites flanked stop cassettes for transgene expression. We found that transgene expression in triple-transgenic mice is highly specific in I but not D and L cells in the terminal villi of the small intestine. The targeting of EECs only in terminal villi is due to the integration of a defective 2A separating peptide that, combined with low EEC intrinsic Vil1 expression, restricts our Vil1-2A-DD-Dre mouse line and the intersectional genetic approach described here only applicable for the investigation of mature EEC subpopulations.

Mouse models of diffuse large B cell lymphoma

Diffuse large B cell lymphoma (DLBCL) is a genetically highly heterogeneous disease. Yet, to date, the vast majority of patients receive standardized frontline chemo-immune-therapy consisting of an anthracycline backbone. Using these regimens, approximately 65% of patients can be cured, whereas the remaining 35% of patients will face relapsed or refractory disease, which, even in the era of CAR-T cells, is difficult to treat. To systematically tackle this high medical need, it is important to design, generate and deploy suitable in vivo model systems that capture disease biology, heterogeneity and drug response. Recently published, large comprehensive genomic characterization studies, which defined molecular sub-groups of DLBCL, provide an ideal framework for the generation of autochthonous mouse models, as well as an ideal benchmark for cell line-derived or patient-derived mouse models of DLBCL. Here we discuss the current state of the art in the field of mouse modelling of human DLBCL, with a particular focus on disease biology and genetically defined molecular vulnerabilities, as well as potential targeting strategies.

MicroRNAs are necessary for the emergence of Purkinje cell identity

Brain computations are dictated by the unique morphology and connectivity of neuronal subtypes, features established by closely timed developmental events. MicroRNAs (miRNAs) are critical for brain development, but current technologies lack the spatiotemporal resolution to determine how miRNAs instruct the steps leading to subtype identity. Here, we developed new tools to tackle this major gap. Fast and reversible miRNA loss-of-function revealed that miRNAs are necessary for cerebellar Purkinje cell (PC) differentiation, which previously appeared miRNA-independent, and resolved distinct miRNA critical windows in PC dendritogenesis and climbing fiber synaptogenesis, key determinants of PC identity. To identify underlying mechanisms, we generated a mouse model, which enables precise mapping of miRNAs and their targets in rare cell types. With PC-specific maps, we found that the PC-enriched miR-206 drives exuberant dendritogenesis and modulates synaptogenesis. Our results showcase vastly improved approaches for dissecting miRNA function and reveal that many critical miRNA mechanisms remain largely unexplored.

Highlights

Fast miRNA loss-of-function with T6B impairs postnatal Purkinje cell developmentReversible T6B reveals critical miRNA windows for dendritogenesis and synaptogenesisConditional Spy3-Ago2 mouse line enables miRNA-target network mapping in rare cellsPurkinje cell-enriched miR-206 regulates its unique dendritic and synaptic morphology.

Defining Gene Function in Spermatogonial Stem Cells Through Conditional Knockout Approaches

Mammalian male fertility is maintained throughout life by a population of self-renewing mitotic germ cells known as spermatogonial stem cells (SSCs). Much of our current understanding regarding the molecular mechanisms underlying SSC activity is derived from studies using conditional knockout mouse models. Here, we provide a guide for the selection and use of mouse strains to develop conditional knockout models for the study of SSCs, as well as their precursors and differentiation-committed progeny. We describe Cre recombinase-expressing strains, breeding strategies to generate experimental groups, and treatment regimens for inducible knockout models and provide advice for verifying and improving conditional knockout efficiency. This resource can be beneficial to those aiming to develop conditional knockout models for the study of SSC development and postnatal function.

Selective Menin Deletion in the Hippocampal CA1 Region Leads to Disruption of Contextual Memory in the MEN1 Conditional Knockout Mouse: Behavioral Restoration and Gain of Function following the Reintroduction of MEN1 Gene

Cholinergic neuronal networks in the hippocampus play a key role in the regulation of learning and memory in mammals. Perturbations of these networks, in turn, underlie neurodegenerative diseases. However, the mechanisms remain largely undefined. We have recently demonstrated that an in vitro MEN1 gene deletion perturbs nicotinic cholinergic plasticity at the hippocampal glutamatergic synapses. Furthermore, MEN1 neuronal conditional knockout in freely behaving animals has also been shown to result in learning and memory deficits, though the evidence remains equivocal. In this study, using an AVV viral vector transcription approach, we provide direct evidence that MEN1 gene deletion in the CA1 region of the hippocampus indeed leads to contextual fear conditioning deficits in conditional knockout animals. This loss of function was, however, recovered when the same animals were re-injected to overexpress MEN1. This study provides the first direct evidence for the sufficiency and necessity of MEN1 in fear conditioning, and further endorses the role of menin in the regulation of cholinergic synaptic machinery in the hippocampus. These data underscore the importance of further exploring and revisiting the cholinergic hypothesis that underlies neurodegenerative diseases that affect learning and memory.

Stable Transgenic Mouse Strain with Enhanced Photoactivatable Cre Recombinase for Spatiotemporal Genome Manipulation

Optogenetic genome engineering is a powerful technology for high-resolution spatiotemporal genetic manipulation, especially for in vivo studies. It is difficult to generate stable transgenic animals carrying a tightly regulated optogenetic system, as its long-term expression induces high background activity. Here, the generation of an enhanced photoactivatable Cre recombinase (ePA-Cre) transgenic mouse strain with stringent light responsiveness and high recombination efficiency is reported. Through serial optimization, ePA-Cre is developed to generate a transgenic mouse line that exhibits 175-fold induction upon illumination. Efficient light-dependent recombination is detected in embryos and various adult tissues of ePA-Cre mice crossed with the Ai14 tdTomato reporter. Importantly, no significant background Cre activity is detected in the tested tissues except the skin. Moreover, efficient light-inducible cell ablation is achieved in ePA-Cre mice crossed with Rosa26-LSL-DTA mice. In conclusion, ePA-Cre mice offer a tightly inducible, highly efficient, and spatiotemporal-specific genome engineering tool for multiple applications.

Chronic Ca2+ imaging of cortical neurons with long-term expression of GCaMP-X

Dynamic Ca2+ signals reflect acute changes in membrane excitability, and also mediate signaling cascades in chronic processes. In both cases, chronic Ca2+ imaging is often desired, but challenged by the cytotoxicity intrinsic to calmodulin (CaM)-based GCaMP, a series of genetically-encoded Ca2+ indicators that have been widely applied. Here, we demonstrate the performance of GCaMP-X in chronic Ca2+ imaging of cortical neurons, where GCaMP-X by design is to eliminate the unwanted interactions between the conventional GCaMP and endogenous (apo)CaM-binding proteins. By expressing in adult mice at high levels over an extended time frame, GCaMP-X showed less damage and improved performance in two-photon imaging of sensory (whisker-deflection) responses or spontaneous Ca2+ fluctuations, in comparison with GCaMP. Chronic Ca2+ imaging of one month or longer was conducted for cultured cortical neurons expressing GCaMP-X, unveiling that spontaneous/local Ca2+ transients progressively developed into autonomous/global Ca2+ oscillations. Along with the morphological indices of neurite length and soma size, the major metrics of oscillatory Ca2+, including rate, amplitude and synchrony were also examined. Dysregulations of both neuritogenesis and Ca2+ oscillations became discernible around 2-3 weeks after virus injection or drug induction to express GCaMP in newborn or mature neurons, which were exacerbated by stronger or prolonged expression of GCaMP. In contrast, neurons expressing GCaMP-X were significantly less damaged or perturbed, altogether highlighting the unique importance of oscillatory Ca2+ to neural development and neuronal health. In summary, GCaMP-X provides a viable solution for Ca2+ imaging applications involving long-time and/or high-level expression of Ca2+ probes.

Rhesus macaque versus rat divergence in the corticospinal projectome

We used viral intersectional tools to map the entire projectome of corticospinal neurons associated with fine distal forelimb control in Fischer 344 rats and rhesus macaques. In rats, we found an extraordinarily diverse set of collateral projections from corticospinal neurons to 23 different brain and spinal regions. Remarkably, the vast weighting of this "motor" projection was to sensory systems in both the brain and spinal cord, confirmed by optogenetic and transsynaptic viral intersectional tools. In contrast, rhesus macaques exhibited far heavier and narrower weighting of corticospinal outputs toward spinal and brainstem motor systems. Thus, corticospinal systems in macaques primarily constitute a final output system for fine motor control, whereas this projection in rats exerts a multi-modal integrative role that accesses far broader CNS regions. Unique structural-functional correlations can be achieved by mapping and quantifying a single neuronal system's total axonal output and its relative weighting across CNS targets.

Development and transdifferentiation into inner hair cells require Tbx2

Atoh1 is essential for the development of both outer hair cells (OHCs) and inner hair cells (IHCs) in the mammalian cochlea. Whereas Ikzf2 is necessary for OHC development, the key gene required for IHC development remains unknown. We found that deletion of Tbx2 in neonatal IHCs led to their transdifferentiation into OHCs by repressing 26.7% of IHC genes and inducing 56.3% of OHC genes, including Ikzf2. More importantly, persistent expression of Tbx2 coupled with transient Atoh1 expression effectively reprogrammed non-sensory supporting cells into new IHCs expressing the functional IHC marker vGlut3. The differentiation status of these new IHCs was considerably more advanced than that previously reported. Thus, Tbx2 is essential for IHC development and co-upregulation of Tbx2 with Atoh1 in supporting cells represents a new approach for treating deafness related to IHC degeneration.

Transgenic mice encoding modern imaging probes: Properties and applications

Modern biology is increasingly reliant on optical technologies, including visualization and longitudinal monitoring of cellular processes. The major limitation here is the availability of animal models to track the molecules and cells in their natural environment in vivo. Owing to the integrity of the studied tissue and the high stability of transgene expression throughout life, transgenic mice encoding fluorescent proteins and biosensors represent unique tools for in vivo studies in norm and pathology. We review the strategies for targeting probe expression in specific tissues, cell subtypes, or cellular compartments. We describe the application of transgenic mice expressing fluorescent proteins for tracking protein expression patterns, apoptotic events, tissue differentiation and regeneration, neurogenesis, tumorigenesis, and cell fate mapping. We overview the possibilities of functional imaging of secondary messengers, neurotransmitters, and ion fluxes. Finally, we provide the rationale and perspectives for the use of transgenic imaging probes in translational research and drug discovery.

A Genetic Toolkit for Simultaneous Generation of LexA- and QF-Expressing Clones in Selected Cell Types in Drosophila

Visualization and manipulation of defined motoneurons have provided significant insights into how motor circuits are assembled in Drosophila. A conventional approach for molecular and cellular analyses of subsets of motoneurons involves the expression of a wide range of UAS transgenes using available GAL4 drivers (eg, eve promoter-fused GAL4). However, a more powerful toolkit could be one that enables a single-cell characterization of interactions between neurites from neurons of interest. Here we show the development of a UAS > LexA > QF expression system to generate randomly selected neurons expressing one of the 2 binary expression systems. As a demonstration, we apply it to visualize dendrite-dendrite interactions by genetically labeling eve⁺ neurons with distinct fluorescent reporters.

RB depletion is required for the continuous growth of tumors initiated by loss of RB

The retinoblastoma (RB) tumor suppressor is functionally inactivated in a wide range of human tumors where this inactivation promotes tumorigenesis in part by allowing uncontrolled proliferation. RB has been extensively studied, but its mechanisms of action in normal and cancer cells remain only partly understood. Here, we describe a new mouse model to investigate the consequences of RB depletion and its re-activation in vivo. In these mice, induction of shRNA molecules targeting RB for knock-down results in the development of phenotypes similar to Rb knock-out mice, including the development of pituitary and thyroid tumors. Re-expression of RB leads to cell cycle arrest in cancer cells and repression of transcriptional programs driven by E2F activity. Thus, continuous RB loss is required for the maintenance of tumor phenotypes initiated by loss of RB, and this new mouse model will provide a new platform to investigate RB function in vivo.

A genetically targeted reporter for PET imaging of deep neuronal circuits in mammalian brains

Positron emission tomography (PET) allows biomolecular tracking but PET monitoring of brain networks has been hampered by a lack of suitable reporters. Here, we take advantage of bacterial dihydrofolate reductase, ecDHFR, and its unique antagonist, TMP, to facilitate in vivo imaging in the brain. Peripheral administration of radiofluorinated and fluorescent TMP analogs enabled PET and intravital microscopy, respectively, of neuronal ecDHFR expression in mice. This technique can be used to the visualize neuronal circuit activity elicited by chemogenetic manipulation in the mouse hippocampus. Notably, ecDHFR-PET allows mapping of neuronal projections in non-human primate brains, demonstrating the applicability of ecDHFR-based tracking technologies for network monitoring. Finally, we demonstrate the utility of TMP analogs for PET studies of turnover and self-assembly of proteins tagged with ecDHFR mutants. These results establish opportunities for a broad spectrum of previously unattainable PET analyses of mammalian brain circuits at the molecular level.

Engineering living therapeutics with synthetic biology

The steadfast advance of the synthetic biology field has enabled scientists to use genetically engineered cells, instead of small molecules or biologics, as the basis for the development of novel therapeutics. Cells endowed with synthetic gene circuits can control the localization, timing and dosage of therapeutic activities in response to specific disease biomarkers and thus represent a powerful new weapon in the fight against disease. Here, we conceptualize how synthetic biology approaches can be applied to programme living cells with therapeutic functions and discuss the advantages that they offer over conventional therapies in terms of flexibility, specificity and predictability, as well as challenges for their development. We present notable advances in the creation of engineered cells that harbour synthetic gene circuits capable of biological sensing and computation of signals derived from intracellular or extracellular biomarkers. We categorize and describe these developments based on the cell scaffold (human or microbial) and the site at which the engineered cell exerts its therapeutic function within its human host. The design of cell-based therapeutics with synthetic biology is a rapidly growing strategy in medicine that holds great promise for the development of effective treatments for a wide variety of human diseases.

Genetic targeting of adult Renshaw cells using a Calbindin 1 destabilized Cre allele for intersection with Parvalbumin or Engrailed1

Renshaw cells (RCs) are one of the most studied spinal interneurons; however, their roles in motor control remain enigmatic in part due to the lack of experimental models to interfere with RC function, specifically in adults. To overcome this limitation, we leveraged the distinct temporal regulation of Calbindin (Calb1) expression in RCs to create genetic models for timed RC manipulation. We used a Calb1 allele expressing a destabilized Cre (dgCre) theoretically active only upon trimethoprim (TMP) administration. TMP timing and dose influenced RC targeting efficiency, which was highest within the first three postnatal weeks, but specificity was low with many other spinal neurons also targeted. In addition, dgCre showed TMP-independent activity resulting in spontaneous recombination events that accumulated with age. Combining Calb1-dgCre with Parvalbumin (Pvalb) or Engrailed1 (En1) Flpo alleles in dual conditional systems increased cellular and timing specificity. Under optimal conditions, Calb1-dgCre/Pvalb-Flpo mice targeted 90% of RCs and few dorsal horn neurons; Calb1-dgCre/En1-Flpo mice showed higher specificity, but only a maximum of 70% of RCs targeted. Both models targeted neurons throughout the brain. Restricted spinal expression was obtained by injecting intraspinally AAVs carrying dual conditional genes. These results describe the first models to genetically target RCs bypassing development.

Current tools to interrogate microglial biology

Microglial cells perform a plethora of functions in the central nervous system (CNS), involving them in brain development, maintenance of homeostasis in adulthood, and CNS diseases. Significant technical advancements have prompted the development of novel systems adapted to analyze microglia with increasing specificity and intricacy. The advent of single-cell technologies combined with targeted mouse models has been decisive in deciphering microglia heterogeneity and dissecting microglial functions. However sophisticated these tools have become, clear limitations remain. Understanding their pitfalls and advantages ensures their correct application. Therefore, we provide a guide to the cutting-edge methods currently available to dissect microglial biology.

Intersectional targeting of defined neural circuits by adeno-associated virus vectors

The mammalian nervous system is a complex network of interconnected cells. We review emerging techniques that use the axonal transport of adeno-associated virus (AAV) vectors to dissect neural circuits. These intersectional approaches specifically target AAV-mediated gene expression to discrete neuron populations based on their axonal connectivity, including: (a) neurons with one defined output, (b) neurons with one defined input, (c) neurons with one defined input and one defined output, and (d) neurons with two defined inputs or outputs. The number of labeled neurons can be directly controlled to trace axonal projections and examine cellular morphology. These approaches can precisely target the expression of fluorescent reporters, optogenetic ion channels, chemogenetic receptors, disease-associated proteins, and other factors to defined neural circuits in mammals ranging from mice to macaques, and thereby provide a powerful new means to understand the structure and function of the nervous system.

Distinct subtypes of proprioceptive dorsal root ganglion neurons regulate adaptive proprioception in mice

Proprioceptive neurons (PNs) are essential for the proper execution of all our movements by providing muscle sensory feedback to the central motor network. Here, using deep single cell RNAseq of adult PNs coupled with virus and genetic tracings, we molecularly identify three main types of PNs (Ia, Ib and II) and find that they segregate into eight distinct subgroups. Our data unveil a highly sophisticated organization of PNs into discrete sensory input channels with distinct spatial distribution, innervation patterns and molecular profiles. Altogether, these features contribute to finely regulate proprioception during complex motor behavior. Moreover, while Ib- and II-PN subtypes are specified around birth, Ia-PN subtypes diversify later in life along with increased motor activity. We also show Ia-PNs plasticity following exercise training, suggesting Ia-PNs are important players in adaptive proprioceptive function in adult mice.

Molecular diversity of proprioceptive neuron types (Ia, Ib and II PNs) is unclear. Here, the authors characterized the functional organization and development of eight subtypes of PNs in mice. Importantly, Ia subtypes are plastic, suggesting a role in adaptive proprioception during motor behavior.

Efficient photoactivatable Dre recombinase for cell type-specific spatiotemporal control of genome engineering in the mouse

Precise genetic engineering in specific cell types within an intact organism is intriguing yet challenging, especially in a spatiotemporal manner without the interference caused by chemical inducers. Here we engineered a photoactivatable Dre recombinase based on the identification of an optimal split site and demonstrated that it efficiently regulated transgene expression in mouse tissues spatiotemporally upon blue light illumination. Moreover, through a double-floxed inverted open reading frame strategy, we developed a Cre-activated light-inducible Dre (CALID) system. Taking advantage of well-defined cell-type-specific promoters or a well-established Cre transgenic mouse strain, we demonstrated that the CALID system was able to activate endogenous reporter expression for either bulk or sparse labeling of CaMKIIα-positive excitatory neurons and parvalbumin interneurons in the brain. This flexible and tunable system could be a powerful tool for the dissection and modulation of developmental and genetic complexity in a wide range of biological systems.

Regional Differences in Penetration of the Protein Stabilizer Trimethoprim (TMP) in the Rat Central Nervous System

Regulating gene expression at the protein level is becoming increasingly important for answering basic questions in neurobiology. Several techniques using destabilizing domains (DD) on transgenes, which can be activated or deactivated by specific drugs, have been developed to achieve this goal. A DD from bacterial dihydrofolate reductase bound and stabilized by trimethoprim (TMP) represents such a tool. To control transgenic protein levels in the brain, the DD-regulating drugs need to have sufficient penetration into the central nervous system (CNS). Yet, very limited information is available on TMP pharmacokinetics in the CNS following systemic injection. Here, we performed a pharmacokinetic study on the penetration of TMP into different CNS compartments in the rat. We used mass spectrometry to measure TMP concentrations in serum, cerebrospinal fluid (CSF) and tissue samples of different CNS regions upon intraperitoneal TMP injection. We show that TMP quickly (within 10 min) penetrates from serum to CSF through the blood-CSF barrier. TMP also shows quick penetration into brain tissue but concentrations were an order of magnitude lower compared to serum or CSF. TMP concentration in spinal cord was lower than in any other analyzed CNS area. Nevertheless, effective levels of TMP to stabilize DDs can be reached in the CNS with half-lives around 2 h. These data show that TMP has good and fast penetration properties into the CNS and is therefore a valuable ligand for precisely controlling protein expression in the CNS in rodents.

Protocol for In Vivo Evaluation and Use of Destabilizing Domains in the Eye, Liver, and Beyond

Destabilizing domains (DDs) have been used successfully to conditionally control the abundance of proteins of interest (POIs) in a small-molecule-dependent manner in mice, worms (Caenorhabditis elegans), and Drosophila. However, development of such systems must account for delivery of the DD-POIs to the target tissue, accessibility of the target tissue to the small molecule, and quantification of stabilization. Here, we describe the considerations and steps to take in order to effectively implement a DD-POI in mouse ocular and hepatic tissue. For complete details on the use and execution of this protocol, please refer to Datta et al. (2018), Ramadurgum and Hulleman (2020), and Ramadurgum et al. (2020).

To Kill a Microglia: A Case for CSF1R Inhibitors

Microglia, the brain's immune sentinels, have garnered much attention in recent years. Researchers have begun to identify the manifold roles that these cells play in the central nervous system (CNS), and this work has been greatly facilitated by microglial depletion paradigms. The varying degrees of spatiotemporal manipulation afforded by such techniques allow microglial ablation before, during, and/or following insult, injury, or disease. We review the major methods of microglial depletion, including toxin-based, genetic, and pharmacological approaches, which differ in key factors including depletion onset, duration, and off-target effects. We conclude that pharmacological CSF1R inhibitors afford the most extensive versatility in manipulating microglia, making them ideal candidates for future studies investigating microglial function in health and disease.

Representation of Olfactory Information in Organized Active Neural Ensembles in the Hypothalamus

The internal representation of sensory information via coherent activation of specific pathways in the nervous system is key to appropriate behavioral responses. Little is known about how chemical stimuli that elicit instinctive behaviors lead to organized patterns of activity in the hypothalamus. Here, we study how a wide range of chemosignals form a discernible map of olfactory information in the ventromedial nucleus of the hypothalamus (VMH) and show that different stimuli entail distinct active neural ensembles. Importantly, we demonstrate that this map depends on functional inputs from the vomeronasal organ. We present evidence that the spatial locations of active VMH ensembles are correlated with activation of distinct vomeronasal receptors and that disjunct VMH ensembles exhibit differential projection patterns. Moreover, active ensembles with distinct spatial locations are not necessarily associated with different behavior categories, such as defensive or social, calling for a revision of the currently accepted model of VMH organization.

A non-invasive far-red light-induced split-Cre recombinase system for controllable genome engineering in mice

The Cre-loxP recombination system is a powerful tool for genetic manipulation. However, there are widely recognized limitations with chemically inducible Cre-loxP systems, and the UV and blue-light induced systems have phototoxicity and minimal capacity for deep tissue penetration. Here, we develop a far-red light-induced split Cre-loxP system (FISC system) based on a bacteriophytochrome optogenetic system and split-Cre recombinase, enabling optogenetical regulation of genome engineering in vivo solely by utilizing a far-red light (FRL). The FISC system exhibits low background and no detectable photocytotoxicity, while offering efficient FRL-induced DNA recombination. Our in vivo studies showcase the strong organ-penetration capacity of FISC system, markedly outperforming two blue-light-based Cre systems for recombination induction in the liver. Demonstrating its strong clinical relevance, we successfully deploy a FISC system using adeno-associated virus (AAV) delivery. Thus, the FISC system expands the optogenetic toolbox for DNA recombination to achieve spatiotemporally controlled, non-invasive genome engineering in living systems.

Current light-inducible Cre-loxP systems have minimal capacity for deep tissue penetration. Here, the authors present a far-red light-induced split Cre-loxP system for in vivo genome engineering.

Simultaneous Control of Endogenous and User-Defined Genetic Pathways Using Unique ecDHFR Pharmacological Chaperones

Destabilizing domains (DDs), such as a mutated form of Escherichia coli dihydrofolate reductase (ecDHFR), confer instability and promote protein degradation. However, when combined with small-molecule stabilizers (e.g., the antibiotic trimethoprim), DDs allow positive regulation of fusion protein abundance. Using a combinatorial screening approach, we identified and validated 17 unique 2,4-diaminopyrimidine/triazine-based ecDHFR DD stabilizers, at least 15 of which were ineffective antibiotics against E. coli and S. aureus. Identified stabilizers functioned in vivo to control an ecDHFR DD-firefly luciferase in the mouse eye and/or the liver. Next, stabilizers were leveraged to perform synergistic dual functions in vitro (HeLa cell death sensitization) and in vivo (repression of ocular inflammation) by stabilizing a user-defined ecDHFR DD while also controlling endogenous signaling pathways. Thus, these newly identified pharmacological chaperones allow for simultaneous control of compound-specific endogenous and user-defined genetic pathways, the combination of which may provide synergistic effects in complex biological scenarios.

Achieving tight control of a photoactivatable Cre recombinase gene switch: new design strategies and functional characterization in mammalian cells and rodent

A common mechanism for inducibly controlling protein function relies on reconstitution of split protein fragments using chemical or light-induced dimerization domains. A protein is split into fragments that are inactive on their own, but can be reconstituted after dimerization. As many split proteins retain affinity for their complementary half, maintaining low activity in the absence of an inducer remains a challenge. Here, we systematically explore methods to achieve tight regulation of inducible proteins that are effective despite variation in protein expression level. We characterize a previously developed split Cre recombinase (PA-Cre2.0) that is reconstituted upon light-induced CRY2-CIB1 dimerization, in cultured cells and in vivo in rodent brain. In culture, PA-Cre2.0 shows low background and high induced activity over a wide range of expression levels, while in vivo the system also shows low background and sensitive response to brief light inputs. The consistent activity stems from fragment compartmentalization that shifts localization toward the cytosol. Extending this work, we exploit nuclear compartmentalization to generate light-and-chemical regulated versions of Cre recombinase. This work demonstrates in vivo functionality of PA-Cre2.0, describes new approaches to achieve tight inducible control of Cre DNA recombinase, and provides general guidelines for further engineering and application of split protein fragments.

Engineering Photoactivatability in Genetically Encoded Voltage and pH Indicators

Genetically-encoded indicators of neuronal activity enable the labeling of a genetically defined population of neurons to optically monitor their activities. However, researchers often find difficulties in identifying relevant signals from excessive background fluorescence. A photoactivatable version of a genetically encoded calcium indicator, sPA-GCaMP6f is a good example of circumventing such an obstacle by limiting the fluorescence to a region of interest defined by the user. Here, we apply this strategy to genetically encoded voltage (GEVI) and pH (GEPI) indicators. Three photoactivatable GEVI candidates were considered. The first one used a circularly-permuted fluorescent protein, the second design involved a Förster resonance energy transfer (FRET) pair, and the third approach employed a pH-sensitive variant of GFP, ecliptic pHluorin. The candidate with a variant of ecliptic pHluorin exhibited photoactivation and a voltage-dependent fluorescence change. This effort also yielded a pH-sensitive photoactivatable GFP that varies its brightness in response to intracellular pH changes.

High-performance chemical- and light-inducible recombinases in mammalian cells and mice

Site-specific DNA recombinases are important genome engineering tools. Chemical- and light-inducible recombinases, in particular, enable spatiotemporal control of gene expression. However, inducible recombinases are scarce due to the challenge of engineering high performance systems, thus constraining the sophistication of genetic circuits and animal models that can be created. Here we present a library of >20 orthogonal inducible split recombinases that can be activated by small molecules, light and temperature in mammalian cells and mice. Furthermore, we engineer inducible split Cre systems with better performance than existing systems. Using our orthogonal inducible recombinases, we create a genetic switchboard that can independently regulate the expression of 3 different cytokines in the same cell, a tripartite inducible Flp, and a 4-input AND gate. We quantitatively characterize the inducible recombinases for benchmarking their performances, including computation of distinguishability of outputs. This library expands capabilities for multiplexed mammalian gene expression control.

Optogenetic Medicine: Synthetic Therapeutic Solutions Precision-Guided by Light

Gene- and cell-based therapies are well recognized as central pillars of next-generation medicine, but controllability remains a critical issue for clinical applications. In this context, optogenetics is opening up exciting new opportunities for precision-guided medicine by using illumination with light of appropriate intensity and wavelength as a trigger signal to achieve pinpoint spatiotemporal control of cellular activities, such as transgene expression. In this review, we highlight recent advances in optogenetics, focusing on devices for biomedical applications. We introduce the construction and applications of optogenetic-based biomedical tools to treat neurological diseases, diabetes, heart diseases, and cancer, as well as bioelectronic implants that combine light-interfaced electronic devices and optogenetic systems into portable personalized precision bioelectronic medical tools. Optogenetics-based technology promises the capability to achieve traceless, remotely controlled precision dosing of an enormous range of therapeutic outputs. Finally, we discuss the prospects for optogenetic medicine, as well as some emerging challenges.

Class IIa HDACs regulate learning and memory through dynamic experience-dependent repression of transcription

The formation of new memories requires transcription. However, the mechanisms that limit signaling of relevant gene programs in space and time for precision of information coding remain poorly understood. We found that, during learning, the cellular patterns of expression of early response genes (ERGs) are regulated by class IIa HDACs 4 and 5, transcriptional repressors that transiently enter neuronal nuclei from cytoplasm after sensory input. Mice lacking these repressors in the forebrain have abnormally broad experience-dependent expression of ERGs, altered synaptic architecture and function, elevated anxiety, and severely impaired memory. By acutely manipulating the nuclear activity of class IIa HDACs in behaving animals using a chemical-genetic technique, we further demonstrate that rapid induction of transcriptional programs is critical for memory acquisition but these programs may become dispensable when a stable memory is formed. These results provide new insights into the molecular basis of memory storage.

The molecular mechanisms of memory storage remain poorly understood. In this study, authors describe a new mechanism that regulates the cellular patterns of early response gene signaling during learning via the recruitment of two functionally redundant nuclear repressors, class IIa histone deacetylases (HDACs) 4 and 5

Drug Inducible CRISPR/Cas Systems

Clustered, regularly interspaced, short palindromic repeats (CRISPR)/CRISPR-associated protein (Cas) systems have been employed as a powerful versatile technology for programmable gene editing, transcriptional modulation, epigenetic modulation, and genome labeling, etc. Yet better control of their activity is important to accomplish greater precision and to reduce undesired outcomes such as off-target events. The use of small molecules to control CRISPR/Cas activity represents a promising direction. Here, we provide an updated review on multiple drug inducible CRISPR/Cas systems and discuss their distinct properties. We arbitrarily divided the emerging drug inducible CRISPR/Cas systems into two categories based on whether at transcription or protein level does chemical control occurs. The first category includes Tet-On/Off system and Cre-dependent system. The second category includes chemically induced proximity systems, intein splicing system, 4-Hydroxytamoxifen-Estrogen Receptor based nuclear localization systems, allosterically regulated Cas9 system, and destabilizing domain mediated protein degradation systems. Finally, the advantages and limitations of each system were summarized.

Preactivation of β-catenin in osteoblasts improves the osteoanabolic effect of PTH in type 1 diabetic mice

Type 1 diabetes (T1D) is correlated with osteopenia primarily due to low bone formation. Parathyroid hormone (PTH) is a known anabolic agent for bone, the anabolic effects of which are partially mediated through the Wnt/β-catenin signaling pathway. In the present study, we first determined the utility of intermittent PTH treatment in a streptozotocin-induced T1D mouse model. It was shown that the PTH-induced anabolic effects on bone mass and bone formation were attenuated in T1D mice compared with nondiabetic mice. Further, PTH treatment failed to activate β-catenin signaling in osteoblasts of T1D mice and was unable to improve osteoblast proliferation and differentiation. Next, the Col1-3.2 kb-CreERTM; β-cateninfx(ex3) mice were used to conditionally activate β-catenin in osteoblasts by injecting tamoxifen, and we addressed whether or not preactivation of β-catenin boosted the anabolic action of PTH on T1D-related bone loss. The results demonstrated that pretreatment with activation of osteoblastic β-catenin followed by PTH treatment outperformed PTH or β-catenin activation monotherapy and led to greatly improved bone structure, bone mass, and bone strength in this preclinical model of T1DM. Further analysis demonstrated that osteoblast proliferation and differentiation, as well as osteoprogenitors in the marrow, were all improved in the combination treatment group. These findings indicated a clear advantage of developing β-catenin as a target to improve the efficacy of PTH in the treatment of T1D-related osteopenia.

Deep Sequencing of Somatosensory Neurons Reveals Molecular Determinants of Intrinsic Physiological Properties

Dorsal root ganglion (DRG) sensory neuron subtypes defined by their in vivo properties display distinct intrinsic electrical properties. We used bulk RNA sequencing of genetically labeled neurons and electrophysiological analyses to define ion channel contributions to the intrinsic electrical properties of DRG neuron subtypes. The transcriptome profiles of eight DRG neuron subtypes revealed differentially expressed and functionally relevant genes, including voltage-gated ion channels. Guided by these data, electrophysiological analyses using pharmacological and genetic manipulations as well as computational modeling of DRG neuron subtypes were undertaken to assess the functions of select voltage-gated potassium channels (Kv1, Kv2, Kv3, and Kv4) in shaping action potential (AP) waveforms and firing patterns. Our findings show that the transcriptome profiles have predictive value for defining ion channel contributions to sensory neuron subtype-specific intrinsic physiological properties. The distinct ensembles of voltage-gated ion channels predicted to underlie the unique intrinsic physiological properties of eight DRG neuron subtypes are presented.

A Suite of Transgenic Driver and Reporter Mouse Lines with Enhanced Brain-Cell-Type Targeting and Functionality

Modern genetic approaches are powerful in providing access to diverse cell types in the brain and facilitating the study of their function. Here, we report a large set of driver and reporter transgenic mouse lines, including 23 new driver lines targeting a variety of cortical and subcortical cell populations and 26 new reporter lines expressing an array of molecular tools. In particular, we describe the TIGRE2.0 transgenic platform and introduce Cre-dependent reporter lines that enable optical physiology, optogenetics, and sparse labeling of genetically defined cell populations. TIGRE2.0 reporters broke the barrier in transgene expression level of single-copy targeted-insertion transgenesis in a wide range of neuronal types, along with additional advantage of a simplified breeding strategy compared to our first-generation TIGRE lines. These novel transgenic lines greatly expand the repertoire of high-precision genetic tools available to effectively identify, monitor, and manipulate distinct cell types in the mouse brain.

Single-Neuron Level One-Photon Voltage Imaging With Sparsely Targeted Genetically Encoded Voltage Indicators

Voltage imaging of many neurons simultaneously at single-cell resolution is hampered by the difficulty of detecting small voltage signals from overlapping neuronal processes in neural tissue. Recent advances in genetically encoded voltage indicator (GEVI) imaging have shown single-cell resolution optical voltage recordings in intact tissue through imaging naturally sparse cell classes, sparse viral expression, soma restricted expression, advanced optical systems, or a combination of these. Widespread sparse and strong transgenic GEVI expression would enable straightforward optical access to a densely occurring cell type, such as cortical pyramidal cells. Here we demonstrate that a recently described sparse transgenic expression strategy can enable single-cell resolution voltage imaging of cortical pyramidal cells in intact brain tissue without restricting expression to the soma. We also quantify the functional crosstalk in brain tissue and discuss optimal imaging rates to inform future GEVI experimental design.

Noninvasive optical activation of Flp recombinase for genetic manipulation in deep mouse brain regions

Spatiotemporal control of gene expression or labeling is a valuable strategy for identifying functions of genes within complex neural circuits. Here, we develop a highly light-sensitive and efficient photoactivatable Flp recombinase (PA-Flp) that is suitable for genetic manipulation in vivo. The highly light-sensitive property of PA-Flp is ideal for activation in deep mouse brain regions by illumination with a noninvasive light-emitting diode. In addition, PA-Flp can be extended to the Cre-lox system through a viral vector as Flp-dependent Cre expression platform, thereby activating both Flp and Cre. Finally, we demonstrate that PA-Flp–dependent, Cre-mediated Ca_v3.1 silencing in the medial septum increases object-exploration behavior in mice. Thus, PA-Flp is a noninvasive, highly efficient, and easy-to-use optogenetic module that offers a side-effect-free and expandable genetic manipulation tool for neuroscience research.

Most approaches to control gene expression in vivo require generation of knock-in mouse lines and often lack spatiotemporal control. Here the authors develop a photo-activatable Flp recombinase system and demonstrate its use by controlling object-exploration behavior in mice through Cav3.1 silencing.

Towards an Understanding of Synapse Formation

Synapses are intercellular junctions specialized for fast, point-to-point information transfer from a presynaptic neuron to a postsynaptic cell. At a synapse, a presynaptic terminal secretes neurotransmitters via a canonical release machinery, while a postsynaptic specialization senses neurotransmitters via diverse receptors. Synaptic junctions are likely organized by trans-synaptic cell-adhesion molecules (CAMs) that bidirectionally orchestrate synapse formation, restructuring, and elimination. Many candidate synaptic CAMs were described, but which CAMs are central actors and which are bystanders remains unclear. Moreover, multiple genes encoding synaptic CAMs were linked to neuropsychiatric disorders, but the mechanisms involved are unresolved. Here, I propose that engagement of multifarious synaptic CAMs produces parallel trans-synaptic signals that mediate the establishment, organization, and plasticity of synapses, thereby controlling information processing by neural circuits. Among others, this hypothesis implies that synapse formation can be understood in terms of inter- and intracellular signaling, and that neuropsychiatric disorders involve an impairment in such signaling.

Chemogenetic Tools for Causal Cellular and Neuronal Biology

Chemogenetic technologies enable selective pharmacological control of specific cell populations. An increasing number of approaches have been developed that modulate different signaling pathways. Selective pharmacological control over G protein-coupled receptor signaling, ion channel conductances, protein association, protein stability, and small molecule targeting allows modulation of cellular processes in distinct cell types. Here, we review these chemogenetic technologies and instances of their applications in complex tissues in vivo and ex vivo.

Mapping projections of molecularly defined dopamine neuron subtypes using intersectional genetic approaches

Midbrain dopamine (DA) neurons have diverse functions that can in part be explained by their heterogeneity. Although molecularly distinct subtypes of DA neurons have been identified by single-cell gene expression profiling, fundamental features such as their projection patterns have not been elucidated. Progress in this regard has been hindered by the lack of genetic tools for studying DA neuron subtypes. Here we develop intersectional genetic labeling strategies, based on combinatorial gene expression, to map the projections of molecularly defined DA neuron subtypes. We reveal distinct genetically defined dopaminergic pathways arising from the substantia nigra pars compacta and from the ventral tegmental area that innervate specific regions of the caudate putamen, nucleus accumbens and amygdala. Together, the genetic toolbox and DA neuron subtype projections presented here constitute a resource that will accelerate the investigation of this clinically significant neurotransmitter system.

Spatiotemporal compartmentalization of hepatic NADH and NADPH metabolism

Compartmentalization is a fundamental design principle of eukaryotic metabolism. Here, we review the compartmentalization of NAD+/NADH and NADP+/NADPH with a focus on the liver, an organ that experiences the extremes of biochemical physiology each day. Historical studies of the liver, using classical biochemical fractionation and measurements of redox-coupled metabolites, have given rise to the prevailing view that mitochondrial NAD(H) pools tend to be oxidized and important for energy homeostasis, whereas cytosolic NADP(H) pools tend to be highly reduced for reductive biosynthesis. Despite this textbook view, many questions still remain as to the relative size of these subcellular pools and their redox ratios in different physiological states, and to what extent such redox ratios are simply indicators versus drivers of metabolism. By performing a bioinformatic survey, we find that the liver expresses 352 known or predicted enzymes composing the hepatic NAD(P)ome, i.e. the union of all predicted enzymes producing or consuming NADP(H) or NAD(H) or using them as a redox co-factor. Notably, less than half are predicted to be localized within the cytosol or mitochondria, and a very large fraction of these genes exhibit gene expression patterns that vary during the time of day or in response to fasting or feeding. A future challenge lies in applying emerging new genetic tools to measure and manipulate in vivo hepatic NADP(H) and NAD(H) with subcellular and temporal resolution. Insights from such fundamental studies will be crucial in deciphering the pathogenesis of very common diseases known to involve alterations in hepatic NAD(P)H, such as diabetes and fatty liver disease.

A versatile genetic tool for post-translational control of gene expression in Drosophila melanogaster

Several techniques have been developed to manipulate gene expression temporally in intact neural circuits. However, the applicability of current tools developed for in vivo studies in Drosophila is limited by their incompatibility with existing GAL4 lines and side effects on physiology and behavior. To circumvent these limitations, we adopted a strategy to reversibly regulate protein degradation with a small molecule by using a destabilizing domain (DD). We show that this system is effective across different tissues and developmental stages. We further show that this system can be used to control in vivo gene expression levels with low background, large dynamic range, and in a reversible manner without detectable side effects on the lifespan or behavior of the animal. Additionally, we engineered tools for chemically controlling gene expression (GAL80-DD) and recombination (FLP-DD). We demonstrate the applicability of this technology in manipulating neuronal activity and for high-efficiency sparse labeling of neuronal populations.

Transgenic Strategies for Sparse but Strong Expression of Genetically Encoded Voltage and Calcium Indicators

Rapidly progressing development of optogenetic tools, particularly genetically encoded optical indicators, enables monitoring activities of neuronal circuits of identified cell populations in longitudinal in vivo studies. Recently developed advanced transgenic approaches achieve high levels of indicator expression. However, targeting non-sparse cell populations leads to dense expression patterns such that optical signals from neuronal processes cannot be allocated to individual neurons. This issue is particularly pertinent for the use of genetically encoded voltage indicators whose membrane-delimited signals arise largely from the neuropil where dendritic and axonal membranes of many cells intermingle. Here we address this need for sparse but strong expression of genetically encoded optical indicators using a titratable recombination-activated transgene transcription to achieve a Golgi staining-type indicator expression pattern in vivo. Using different transgenic strategies, we also illustrate that co-expression of genetically encoded voltage and calcium indicators can be achieved in vivo for studying neuronal circuit input–output relationships.

Dopamine Inhibition Differentially Controls Excitability of Substantia Nigra Dopamine Neuron Subpopulations through T-Type Calcium Channels

While there is growing appreciation for diversity among ventral tegmental area dopamine neurons, much less is known regarding functional heterogeneity among the substantia nigra pars compacta (SNc) neurons. Here, we show that calbindin-positive dorsal tier and calbindin-negative ventral tier SNc dopaminergic neurons in mice comprise functionally distinct subpopulations distinguished by their dendritic calcium signaling, rebound excitation, and physiological responses to dopamine D2-receptor (D2) autoinhibition. While dopamine is known to inhibit action potential backpropagation, our experiments revealed an unexpected enhancement of excitatory responses and dendritic calcium signals in the presence of D2-receptor inhibition. Specifically, dopamine inhibition and direct hyperpolarization enabled the generation of low-threshold depolarizations that occurred in an all-or-none or graded manner, due to recruitment of T-type calcium channels. Interestingly, these effects occurred selectively in calbindin-negative dopaminergic neurons within the SNc. Thus, calbindin-positive and calbindin-negative SNc neurons differ substantially in their calcium channel composition and efficacy of excitatory inputs in the presence of dopamine inhibition.SIGNIFICANCE STATEMENT Substantia nigra dopaminergic neurons can be divided into two populations: the calbindin-negative ventral tier, which is vulnerable to neurodegeneration in Parkinson's disease, and the calbindin-positive dorsal tier, which is relatively resilient. Although tonic firing is similar in these subpopulations, we find that their responses to dopamine-mediated inhibition are strikingly different. During inhibition, calbindin-negative neurons exhibit increased sensitivity to excitatory inputs, which can then trigger large dendritic calcium transients due to strong expression of T-type calcium channels. Therefore, SNc neurons differ substantially in their calcium channel composition, which may contribute to their differential vulnerability. Furthermore, T-currents increase excitation efficacy onto calbindin-negative cells during dopamine inhibition, suggesting that shared inputs are differentially processed in subpopulations resulting in distinct downstream dopamine signals.

A distinct entorhinal cortex to hippocampal CA1 direct circuit for olfactory associative learning

Lateral and medial parts of entorhinal cortex (EC) convey nonspatial 'what' and spatial 'where' information, respectively, into hippocampal CA1, via both the indirect EC layer 2→ hippocampal dentate gyrus→CA3→CA1 and the direct EC layer 3→CA1 paths. However, it remains elusive how the direct path transfers distinct information and contributes to hippocampal learning functions. Here we report that lateral EC projection neurons selectively form direct excitatory synapses onto a subpopulation of morphologically complex, calbindin-expressing pyramidal cells (PCs) in the dorsal CA1 (dCA1), while medial EC neurons uniformly innervate all dCA1 PCs. Optogenetically inactivating the distinct lateral EC-dCA1 connections or the postsynaptic dCA1 calbindin-expressing PC activity slows olfactory associative learning. Moreover, optetrode recordings reveal that dCA1 calbindin-expressing PCs develop more selective spiking responses to odor cues during learning. Thus, our results identify a direct lateral EC→dCA1 circuit that is required for olfactory associative learning.