Knockdown of PHOX2B in the Retrotrapezoid Nucleus Impairs Central CO2 Chemoreflex in Rats

Core Concepts

Knockdown of PHOX2B in the retrotrapezoid nucleus (RTN) of adult rats reduces the central CO2 chemoreflex response, likely by decreasing the expression of key CO2/pH sensors GPR4 and TASK-2 in RTN neurons.

Abstract

This study investigated the role of the transcription factor PHOX2B in the adult brain, particularly in the retrotrapezoid nucleus (RTN), a key structure for central CO2 chemoreception. The authors used a lentiviral shRNA approach to progressively knockdown PHOX2B expression in the RTN of adult rats over 2 and 4 weeks. At 2 weeks post-infection, there was a modest reduction in the number of RTN neurons expressing both PHOX2B and the RTN marker Nmb, but no significant impairment in ventilation or the CO2 chemoreflex. However, at 4 weeks post-infection, the knockdown of PHOX2B was more pronounced, with a 67% reduction in Nmb+/PHOX2B+ neurons compared to naive rats. This was accompanied by a significant impairment in the hypercapnic ventilatory response, without affecting ventilation in room air or hypoxia. The authors found that the reduction in PHOX2B expression was specifically associated with decreased mRNA levels of the CO2/pH sensors GPR4 and TASK-2 in the Nmb+/PHOX2B- neurons of the RTN. In contrast, Nmb expression itself was not affected by PHOX2B knockdown. These results suggest that PHOX2B plays an important role in maintaining the chemosensitive properties of RTN neurons by regulating the expression of key CO2 sensors. The progressive knockdown of PHOX2B leads to impairment of the central CO2 chemoreflex, which may contribute to the respiratory disturbances observed in Congenital Central Hypoventilation Syndrome (CCHS) caused by PHOX2B mutations.

Stats

Nmb+/PHOX2B+ neurons in the RTN were reduced by 67% in PHOX2B-shRNA rats compared to naive rats. The hypercapnic ventilatory response (HCVR) was reduced by 31% in PHOX2B-shRNA rats compared to baseline, naive and NT-shRNA rats. Gpr4 and Task2 mRNA expression was reduced by 34.4% and 39% respectively in PHOX2B-shRNA rats compared to naive and NT-shRNA rats.

Quotes

"Knockdown of PHOX2B in RTN causes a reduction in the chemoreflex response that was proportional to the decrease in the fraction of Nmb/PHOX2B expressing RTN neurons." "The level of expression of Gpr4 and Task2 mRNAs, the two proton sensors responsible for the RTN neurons chemosensory properties were significantly reduced in RTN neurons that lacked PHOX2B protein expression."

Key Insights Distilled From

Knockdown of PHOX2B in the retrotrapezoid nucleus reduces the central CO2 chemoreflex in rats

by Cardani,S., ... at www.biorxiv.org 12-14-2023

https://www.biorxiv.org/content/10.1101/2023.12.13.571591v2

Deeper Inquiries

How might the findings of this study inform potential therapeutic strategies for respiratory disorders like Congenital Central Hypoventilation Syndrome (CCHS) caused by PHOX2B mutations

The findings of this study provide valuable insights into potential therapeutic strategies for respiratory disorders like Congenital Central Hypoventilation Syndrome (CCHS) caused by PHOX2B mutations. The study demonstrated that a reduction in PHOX2B expression in chemosensitive neuromedin-B (NMB) expressing neurons in the retrotrapezoid nucleus (RTN) led to impaired central CO2 chemoreception. This suggests that targeting PHOX2B expression in RTN neurons could be a potential therapeutic approach for CCHS. By understanding the specific role of PHOX2B in the chemosensitive properties of RTN neurons, researchers and clinicians could potentially develop targeted therapies that aim to modulate PHOX2B expression or activity in these neurons to improve respiratory function in individuals with CCHS.

What other transcriptional targets of PHOX2B in RTN neurons could be investigated to further elucidate its role in central CO2 chemoreception

To further elucidate the role of PHOX2B in central CO2 chemoreception, investigating other transcriptional targets of PHOX2B in RTN neurons could provide valuable insights. Some potential transcriptional targets to explore could include genes involved in pH sensing, ion channel regulation, neurotransmitter release, and neuronal excitability. By identifying and studying these transcriptional targets, researchers can gain a deeper understanding of the molecular mechanisms through which PHOX2B influences the chemosensitive properties of RTN neurons. This comprehensive approach could help uncover additional pathways and processes that contribute to the central CO2 chemoreflex and respiratory control.

Given the widespread expression of PHOX2B in the adult brain, what are the potential implications of PHOX2B knockdown in other neuronal populations beyond the RTN

Given the widespread expression of PHOX2B in the adult brain, the implications of PHOX2B knockdown in other neuronal populations beyond the RTN could be significant. PHOX2B is known to play a crucial role in the development and maintenance of various neuronal phenotypes in both the central and peripheral nervous systems. Therefore, knockdown of PHOX2B in other neuronal populations could potentially impact functions such as autonomic control, respiratory rhythm generation, and chemosensitivity in different brain regions. Investigating the effects of PHOX2B knockdown in these neuronal populations could provide valuable insights into the broader role of PHOX2B in adult brain function and its potential implications for various physiological processes beyond respiratory control.