title: Genetically-directed, cell type-specific sparse labeling for the analysis of neuronal morphology.
abstract: BACKGROUND
  In mammals, genetically-directed cell labeling technologies have not yet been applied to the morphologic analysis of neurons with very large and complex arbors, an application that requires extremely sparse labeling and that is only rendered practical by limiting the labeled population to one or a few predetermined neuronal subtypes.
  
  
  RESULTS
  In the present study we have addressed this application by using CreER technology to non-invasively label very small numbers of neurons so that their morphologies can be fully visualized. Four lines of IRES-CreER knock-in mice were constructed to permit labeling selectively in cholinergic or catecholaminergic neurons [choline acetyltransferase (ChAT)-IRES-CreER or tyrosine hydroxylase (TH)-IRES-CreER], predominantly in projection neurons [neurofilament light chain (NFL)-IRES-CreER], or broadly in neurons and some glia [vesicle-associated membrane protein2 (VAMP2)-IRES-CreER]. When crossed to the Z/AP reporter and exposed to 4-hydroxytamoxifen in the early postnatal period, the number of neurons expressing the human placental alkaline phosphatase reporter can be reproducibly lowered to fewer than 50 per brain. Sparse Cre-mediated recombination in ChAT-IRES-CreER;Z/AP mice shows the full axonal and dendritic arbors of individual forebrain cholinergic neurons, the first time that the complete morphologies of these very large neurons have been revealed in any species.
  
  
  CONCLUSIONS
  Sparse genetically-directed, cell type-specific neuronal labeling with IRES-creER lines should prove useful for studying a wide variety of questions in neuronal development and disease.
authors.affiliation: Department of Molecular Biology and Genetics, Johns Hopkins University School of Medicine, Baltimore, MD, USA.
authors.forename: Thomas
authors.initials: T
authors.lastname: Rotolo
authors.forename: Philip M
authors.initials: PM
authors.lastname: Smallwood
authors.forename: John
authors.initials: J
authors.lastname: Williams
authors.forename: Jeremy
authors.initials: J
authors.lastname: Nathans
mesh_terms: D000818:Animals; D002395:Catecholamines / Q000378:metabolism; D002795:Choline O-Acetyltransferase / Q000235:genetics / Q000378:metabolism; D002999:Clone Cells; D051379:Mice; D008957:Models, Genetic; D016900:Neurofilament Proteins / Q000235:genetics / Q000378:metabolism; D009474:Neurons / Q000737:chemistry / Q000201:enzymology / Q000648:ultrastructure*; D014446:Tyrosine 3-Monooxygenase / Q000235:genetics / Q000378:metabolism
chemical_list: D002395:Catecholamines; D016900:Neurofilament Proteins; C044882:neurofilament protein L; D014446:Tyrosine 3-Monooxygenase; D002795:Choline O-Acetyltransferase