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Referee comments: Referee 1

Posted by PLOS_ONE_Group on 11 Jan 2008 at 17:16 GMT

Referee 1's review:

The manuscript by Freed and colleagues examines gene expression in FACS-sorted PSA-NCAM+ cells generated from human embryonic stem cells (hESC) with the goal of relating gene expression in these precursor cells to gene expression in embryonic and adult dopamine neurons. In the introduction, the authors describe shortcomings of previous genetic analysis of developing dopaminergic neurons. The rationale indicated for selecting PSA NCAM cells for study is the presence of a cell surface protein and the fact that a fraction of the cells will become dopaminergic neurons. While the authors acknowledge that PSA-NCAM+ cells are precursors for a number of cell lineages, they have done a technically appropriate analysis of gene expression in these cells compared to undifferentiated hESC.

Results cover a range of topics from histochemical and electrophysiologic analysis of cells subsequently differentiated from sorted PSA-NCAM cells to demonstrate neuronal characteristics. The heart of the paper is the identification of genes using differential display methodology. The most abundant genes were H19 and IGF2. H19 was not expressed in adult dopamine neurons and IGF2 was expressed in one of two postmortem human dopamine neuron samples. Several genes known to be present in human dopamine neurons at early or late stages of development were identified in the NCAM samples, although tyrosine hydroxylase was not.

Interestingly, many of the PSA NCAM genes were associated with the 11p15.5 chromosome imprinting region, which is also known to contain many genes related to dopamine neurons.

This is a worthwhile manuscript with an abundance of genetic data on PSA NCAM cells. While much emphasis is placed on the link to dopamine neurons, those data are more confirmatory of previously identified dopamine genes.

This description of neural progenitor genes will be of particular interest to researchers interested in PSA NCAM neuroprogenitors as well as development of dopamine neurons.

The paper is well written with appropriate methods and data clearly presented. Full data sets are presented in supplementary and web-based forms.

N.B. These are the general comments made by the reviewer when reviewing this paper in light of which the manuscript was revised. Specific points addressed during revision of the paper are not shown.