Origin and Emergence of Microglia in the CNS—An Interesting (Hi)story of an Eccentric Cell

Round 1

Reviewer 1 Report

In the manuscript (ID cimb-2274673), Dermitzakis and colleagues systematically introduced the ontogeny of microglia, molecular cues orchestrating microgliogenesis, and dynamic changes in microglial population under conditions of CNS diseases. Finally, the authors offered some insights into future research directions in the field. The topic is timely and interesting. The manuscript is overall balanced and well-organized. I have the following concerns:

1.         Please include newly-identified genes (such as EED, ARID1A, TGM2, MECP2) that also play pivotal roles in microglia during development.

2.         To claim ‘this collection of data can potentially shed light on the therapeutic potential of microglia for CNS perturbations across various levels of severity’, the authors should provide further discussion with the relationship between microglia and neurological disorders.

3.         I, or probably most readers, would like to know how to distinguish microglia from macrophages under both physiological and disease conditions.

4.         It will be great to highlight remaining challenges and opportunities of microglia in the Conclusions section.

5.         Please have the manuscript proofread for typos as well as grammatical revisions.  

Author Response

Reviewer 1

In the manuscript (ID cimb-2274673), Dermitzakis and colleagues systematically introduced the ontogeny of microglia, molecular cues orchestrating microgliogenesis, and dynamic changes in microglial population under conditions of CNS diseases. Finally, the authors offered some insights into future research directions in the field. The topic is timely and interesting. The manuscript is overall balanced and well-organized. I have the following concerns:

1. Please include newly-identified genes (such as EED, ARID1A, TGM2, MECP2) that also play pivotal roles in microglia during development.

Response:

We would like to thank Reviewer 1 as the statement brought forth have been very useful in improving the manuscript. We acknowledge the newly-identified genes thus we were encouraged to include them in section 3 of the revised manuscript (additions are highlighted).

New bibliography references are:

Loss of microglial EED impairs synapse density, learning, and memory (https://pubmed.ncbi.nlm.nih.gov/35484239/)

Microglia homeostasis mediated by epigenetic ARID1A regulates neural progenitor cells response and leads to autism-like behaviors (https://pubmed.ncbi.nlm.nih.gov/35858990/)

Abnormal microglial polarization induced by Arid1a deletion leads to neuronal differentiation deficits (https://pubmed.ncbi.nlm.nih.gov/35854653/)

Microglial transglutaminase 2 deficiency causes impaired synaptic remodelling and cognitive deficits in mice (https://pubmed.ncbi.nlm.nih.gov/36878712/)

MECP2 Increases the Pro-Inflammatory Response of Microglial Cells and Phosphorylation at Serine 423 Regulates Neuronal Gene Expression upon Neuroinflammation (https://pubmed.ncbi.nlm.nih.gov/33918872/)

Methyl-CpG Binding Protein 2 Regulates Microglia and Macrophage Gene Expression in Response to Inflammatory Stimuli (https://pubmed.ncbi.nlm.nih.gov/25902482/)

2. To claim ‘this collection of data can potentially shed light on the therapeutic potential of microglia for CNS perturbations across various levels of severity’, the authors should provide further discussion with the relationship between microglia and neurological disorders.

Response:

More information has been included and discussed in the relevant sections (5. Proliferation in the adult compromised CNS and 6. Discussion), as per suggestion of the Reviewer. Briefly, we pointed out to the accumulation of reactive oxygen species which in turn propagate the inflammatory neurodegeneration and how identification of developmental microglia states can be selectively targeted to induce neurorepair.

New bibliographic references are:

Fragmented mitochondria released from microglia trigger A1 astrocytic response and propagate inflammatory neurodegeneration (https://pubmed.ncbi.nlm.nih.gov/31551592)

Mitochondrial control of inflammation (https://www.nature.com/articles/s41577-022-00760-x)

3. I, or probably most readers, would like to know how to distinguish microglia from macrophages under both physiological and disease conditions.

Response:

We appreciate the suggestion thus we modified the relevant text accordingly. Changes are highlighted in section 3., providing additional information about the marker selectiveness for each type.

New bibliographic references are:

Microglia states and nomenclature: A field at its crossroads (https://pubmed.ncbi.nlm.nih.gov/36327895/)

Novel Hexb-based tools for studying microglia in the CNS (https://pubmed.ncbi.nlm.nih.gov/32541832/)

Overview of General and Discriminating Markers of Differential Microglia Phenotypes (https://pubmed.ncbi.nlm.nih.gov/32848611/)

Siglec-H Is a Microglia-Specific Marker That Discriminates Microglia from CNS-Associated Macrophages and CNS-Infiltrating Monocytes (https://pubmed.ncbi.nlm.nih.gov/28836308/)

New tools for studying microglia in the mouse and human CNS (https://pubmed.ncbi.nlm.nih.gov/26884166/)

Modulating inflammatory monocytes with a unique microRNA gene signature ameliorates murine ALS (https://pubmed.ncbi.nlm.nih.gov/22863620/)

4. It will be great to highlight remaining challenges and opportunities of microglia in the Conclusions section.

Response:

The remaining challenges and opportunities of microglia have been briefly discussed and are highlighted in the relevant section 6.

5. Please have the manuscript proofread for typos as well as grammatical revisions.

Response:

The manuscript has been thoroughly proofread for typos and grammatical errors.

All comments have been addressed and changes are now reflected in the revised manuscript.

Reviewer 2 Report

The authors reviewed the origin and emergence of microglia in the CNS, an issue that has been debated for many years with the birth and death of several hypotheses

 This review confirms the validity of the Yolk Sac origin of microglia, in agreement with  most researchers. The review is narrative with a thematic analysis (see: Grant, M.J. and Booth, A. (2009), A typology of reviews: an analysis of 14 review types and associated methodologies. Health Information & Libraries Journal, 26: 91-108. https://doi.org/10.1111/j.1471-1842.2009.00848.x) dividing the article into  6  chapters. After introduction, that examines the origin of the term microglia and the importance of tissue resident macrophages, the article goes on with ontogeny of microglia and their molecular cues, then  spatiotemporal distribution in various species. Last chapter is about pathology and proliferation, then the conclusions.

It is true that in literature we can find other review on this topic, but also the last I know (  Zhang, L., Cao, Y., Zhang, X., Gu, X., Mao, Y., & Peng, B. (2022). The origin and repopulation of microglia. Developmental Neurobiology, 82, 112– 124. https://doi.org/10.1002/dneu.22862), although interesting, did not show the did not show the same thoroughness as this.

The authors made a good job, the goal of defining “existing data on the origin of microglia since there  has been controversy over their ontogeny” was fully achieved.  Of course, not everything is conclusively clear, especially with regards to  the turn over of the  microglia in the brain and the origin of the newborn cells( Zhang, L., Cao, Y., Zhang, X., Gu, X., Mao, Y., & Peng, B. (2022). The origin and repopulation of microglia. Developmental Neurobiology, 82, 112– 124. https://doi.org/10.1002/dneu.22862) as you wrote, line 453 : ..The final confirmation of the exact repopulation pattern necessitates further investigation...

Your optimistic conclusions that understanding the role of molecules acting in embryogenesis can help develop new therapies for neurodegenerative diseases, but also be part of research on aging, and allow laboratory manipulations aimed at a beneficial microglial renewal in the central nervous system, are acceptable and justified.

I have no other remarks to make. The authors reviewed the origin and emergence of microglia in the CNS, an issue that has been debated for many years with the birth and death of several hypotheses

Author Response

Reviewer 2

This review confirms the validity of the Yolk Sac origin of microglia, in agreement with most researchers. The review is narrative with a thematic analysis (see: Grant, M.J. and Booth, A. (2009), A typology of reviews: an analysis of 14 review types and associated methodologies. Health Information & Libraries Journal, 26: 91-108. https://doi.org/10.1111/j.1471-1842.2009.00848.x) dividing the article into 6 chapters. After introduction, that examines the origin of the term microglia and the importance of tissue resident macrophages, the article goes on with ontogeny of microglia and their molecular cues, then spatiotemporal distribution in various species. Last chapter is about pathology and proliferation, then the conclusions.

It is true that in literature we can find other review on this topic, but also the last I know (Zhang, L., Cao, Y., Zhang, X., Gu, X., Mao, Y., & Peng, B. (2022). The origin and repopulation of microglia. Developmental Neurobiology, 82, 112– 124. https://doi.org/10.1002/dneu.22862), although interesting, did not show the did not show the same thoroughness as this.

The authors made a good job, the goal of defining “existing data on the origin of microglia since there has been controversy over their ontogeny” was fully achieved.  Of course, not everything is conclusively clear, especially with regards to the turnover of the microglia in the brain and the origin of the newborn cells (Zhang, L., Cao, Y., Zhang, X., Gu, X., Mao, Y., & Peng, B. (2022). The origin and repopulation of microglia. Developmental Neurobiology, 82, 112– 124. https://doi.org/10.1002/dneu.22862) as you wrote, line 453: The final confirmation of the exact repopulation pattern necessitates further investigation...

Your optimistic conclusions that understanding the role of molecules acting in embryogenesis can help develop new therapies for neurodegenerative diseases, but also be part of research on aging, and allow laboratory manipulations aimed at a beneficial microglial renewal in the central nervous system, are acceptable and justified.

I have no other remarks to make. The authors reviewed the origin and emergence of microglia in the CNS, an issue that has been debated for many years with the birth and death of several hypotheses.

Response:

We thank Reviewer 2 for the diligent scrutiny of the submitted manuscript and appreciate the positive feedback. All in all, we very much appreciate the encouraging, critical and constructive comments on this manuscript.