DIO-SPOTlight Transgenic Mouse to Functionally Monitor Protein Synthesis Regulated by the Integrated Stress Response

Reviewer #1 (Public review):

Summary:

The authors created a transgenic mouse line to read out integrated stress responses with single-cell resolution.

Strengths:

ISR plays an important role in the development, maintenance, and degeneration of the nervous system. This mouse line represents a potentially important tool to understand ISR in situ.

Weaknesses:

The current manuscript is clearly written. However, more validation experiments should be performed to understand the exact meaning of the fluorescence intensity of GFP and RFP channels. This is important because these results will define how this tool will be used in the future and in the field.

Reviewer #2 (Public review):

Summary:

In this paper, the authors create transgenic animals with a CMV promoter driving expression of their DIO-SPOTlight construct in which uORF2 and the authentic ORF of Atf4 are replaced by GFP and tdTomato respectively, such that ISR activation is predicted to diminish GFP expression and enhance RFP expression. The major experimental finding of the paper is that cholinergic neurons have the most robust activation of the reporter, consistent with and extending upon their previous work.

Strengths:

It is very likely that the reporter does indeed read out on ISR activation at some level. It is mostly likely to be useful for screening and hypothesis testing than for gaining mechanistic insight, because, as the authors note in the present version, ATF4 itself is but one component of ISR activation. Cells might have robust eIF2a phosphorylation but have suppressed translational regulation (for instance by regulating the expression of eIF2B). The mRNA and protein half-lives of the GFP and Tomato are likely quite different from that of the equivalent components in ATF4, which means that the reporter is likely to behave differently from ATF4 itself over time.

Weaknesses:

The major element that the current manuscript lacks is a detailed comparison between how the reporter behaves and how it tracks with eIF2a phosphorylation, ATF4, and the initiation of the gene expression program downstream of ATF4. While this would be difficult to do in vivo, it would seem much more feasible to isolate primary cells (neurons, fibroblasts, hepatocytes, etc.) from the animals and thoroughly characterize the kinetics of reporter-versus-ISR activation. In that way, the reader can have a better idea of how to interpret the behavior of the reporter. As it is, the authors' attempt to account for the reporter's behavior in Figure 3F is purely speculative and not backed by experiment or modeling.

Reviewer #3 (Public review):

Summary:

The previously described reporter SPOTlight is a fluorescence-based reporter of the integrated stress response, specifically, protein synthesis initiation dynamics. In the current study from the same lab, the authors describe the creation and characterization of a transgenic mouse that expresses SPOTlight.

Strengths:

The previously described reporter has now been made into a Cre-dependent transgene in mice. The authors replicate previous findings from their lab that were acquired using viral vector-mediated delivery of their reporter.

Weaknesses:

There is not a clear advantage to having the Cre-dependent SPOTlight reporter in a transgenic mouse over using a viral vector to deliver the same Cre-dependent SPOTlight based on the experiments presented. There are potential general advantages and disadvantages to virus vs transgenic mouse but no side-by-side comparisons are performed here.

It is not clear whether overexpressing the reporter alters basal ISR/UPR function and gene expression. The CAG is a strong promoter and overexpression of fluorescent proteins (or any protein) can potentially stress protein synthesis and processing mechanisms. The use of the animal as a reporter may be misleading if the presence of the reporter is already altering ISR/UPR.