Genetic Approaches to Reveal the Connectivity of Adult-Born Neurons

Much has been learned about the environmental and molecular factors that influence the division, migration, and programmed cell death of adult-born neurons in the mammalian brain. However, detailed knowledge of the mechanisms that govern the formation and maintenance of functional circuit connectivity via adult neurogenesis remains elusive. Recent advances in genetic technologies now afford the ability to precisely target discrete brain tissues, neuronal subtypes, and even single neurons for vital reporter expression and controlled activity manipulations. Here, I review current viral tracing methods, heterologous receptor expression systems, and optogenetic technologies that hold promise toward elucidating the wiring diagrams and circuit properties of adult-born neurons

Chemical genetics : receptor-ligand pairs for rapid manipulation of neuronal activity

PMID: 22119143 [PubMed - indexed for MEDLINE] PMCID: PMC3294416 Free PMC ArticlePeer reviewedPublisher PD

Activity-Induced Remodeling of Olfactory Bulb Microcircuits Revealed by Monosynaptic Tracing

The continued addition of new neurons to mature olfactory circuits represents a remarkable mode of cellular and structural brain plasticity. However, the anatomical configuration of newly established circuits, the types and numbers of neurons that form new synaptic connections, and the effect of sensory experience on synaptic connectivity in the olfactory bulb remain poorly understood. Using in vivo electroporation and monosynaptic tracing, we show that postnatal-born granule cells form synaptic connections with centrifugal inputs and mitral/tufted cells in the mouse olfactory bulb. In addition, newly born granule cells receive extensive input from local inhibitory short axon cells, a poorly understood cell population. The connectivity of short axon cells shows clustered organization, and their synaptic input onto newborn granule cells dramatically and selectively expands with odor stimulation. Our findings suggest that sensory experience promotes the synaptic integration of new neurons into cell type-specific olfactory circuits

PhD

dissertationHox proteins are homeodomain containing, sequence specific, DNA-binding transcription factors that play a crucial role in the specification of antero-posterior identity in the animal. Loss of function analysis, through targeted gene inactivation, has provided valuable insight as to where individual Hox genes are acting to specify cell fates along the axis of the developing mouse embryo. It has been demonstrated that mutations in 3' genes directly affect the development of anterior embryonic structures, whereas inactivation of 5' genes results in abnormal development of posterior structures. Mouse models generated with mutations in the 3' paralogs result in cranio-facial defects and loss of various neuronal populations along the head and neck region, phenotypes typical of deficiencies in neural crest and/or hindbrain derivatives. Hoxb1 is one such 3' paralog. Targeted inactivation of the Hoxb1 gene has provided an ideal model for studying the molecular mechanisms associated with neuronal specification, maturation, and/or survival. Homozygous mutant mice harboring null alleles of Hoxb1 fail to form the facial branchio-motor components of the VII cranial nerve, a specific population of neurons born in the fourth rhombomeric segment (r4) of the hindbrain that are destined to innervate target tissues of the second branchial arch. In wildtype animals, Hoxb1 is regionally restricted in expression and function to the neural tube and migrating neural crest cells within r4 of the mouse hindbrain, implicating two populations of cells that may contribute to the normal development of the VIIth cranial nerve circuitry. To date, analysis of this phenotype has focused predominantly on the progenitor pools within the neural tube that become the VIIth nerve motoneurons and has included the characterization of anatomical and molecular differences between mutant and wildtype cells. The focus of this thesis has been to address the novel, pleiotropic roles for Hoxb1 in the formation and maintenance of the VIIth cranial nerve circuitry by uncoupling the different functions for Hoxb1 in motoneuron specification within the CNS and the neural crest cell programming in the periphery

Hoxb1 functions in both motoneurons and in tissues of the periphery to establish and maintain the proper neuronal circuitry.

Journal ArticleFormation of neuronal circuits in the head requires the coordinated development of neurons within the central nervous system (CNS) and neural crest-derived peripheral target tissues. Hoxb1, which is expressed throughout rhombomere 4 (r4), has been shown to be required for the specification of facial branchiomotor neuron progenitors that are programmed to innervate the muscles of facial expression. In this study, we have uncovered additional roles for Hoxb1-expressing cells in the formation and maintenance of the VIIth cranial nerve circuitry. By conditionally deleting the Hoxb1 locus in neural crest, we demonstrate that Hoxb1 is also required in r4-derived neural crest to facilitate and maintain formation of the VIIth nerve circuitry. Genetic lineage analysis revealed that a significant population of r4-derived neural crest is fated to generate glia that myelinate the VIIth cranial nerve. Neural crest cultures show that the absence of Hoxb1 function does not appear to affect overall glial progenitor specification, suggesting that a later glial function is critical for maintenance of the VIIth nerve. Taken together, these results suggest that the molecular program governing the development and maintenance of the VIIth cranial nerve is dependent upon Hoxb1, both in the neural crest-derived glia and in the facial branchiomotor neurons

Neuropeptide Signaling Networks and Brain Circuit Plasticity

The brain is a remarkable network of circuits dedicated to sensory integration, perception, and response. The computational power of the brain is estimated to dwarf that of most modern supercomputers, but perhaps its most fascinating capability is to structurally refine itself in response to experience. In the language of computers, the brain is loaded with programs that encode when and how to alter its own hardware. This programmed “plasticity” is a critical mechanism by which the brain shapes behavior to adapt to changing environments. The expansive array of molecular commands that help execute this programming is beginning to emerge. Notably, several neuropeptide transmitters, previously best characterized for their roles in hypothalamic endocrine regulation, have increasingly been recognized for mediating activity-dependent refinement of local brain circuits. Here, we discuss recent discoveries that reveal how local signaling by corticotropin-releasing hormone reshapes mouse olfactory bulb circuits in response to activity and further explore how other local neuropeptide networks may function toward similar ends

Genetic strategies to investigate neuronal circuit properties using stem cell-derived neurons

The mammalian brain is anatomically and functionally complex, and prone to diverse forms of injury and neuropathology. Scientists have long strived to develop cell replacement therapies to repair damaged and diseased nervous tissue. However, this goal has remained unrealized for various reasons, including nascent knowledge of neuronal development, the inability to track and manipulate transplanted cells within complex neuronal networks, and host graft rejection. Recent advances in embryonic stem cell (ESC) and induced pluripotent stem cell (iPSC) technology, alongside novel genetic strategies to mark and manipulate stem cell-derived neurons now provide unprecedented opportunities to investigate complex neuronal circuits in both healthy and diseased brains. Here, we review current technologies aimed at generating and manipulating neurons derived from ESCs and iPSCs towards investigation and manipulation of complex neuronal circuits, ultimately leading to the design and development of novel cell-based therapeutic approaches