TrkB expression and dependence divides gustatory neurons into three subpopulations

Area of Science:

• Neuroscience
• Developmental Biology
• Sensory Systems

Background:

• Gustatory neuron development involves complex changes, with limited understanding of regulatory factors.
• Brain-derived neurotrophic factor (BDNF) is crucial for embryonic geniculate neuron development.
• Postnatal BDNF expression is restricted, suggesting developmental regulation of its receptor, TrkB.

Purpose of the Study:

• To investigate the developmental restriction of tropomyosin kinase B receptor (TrkB) in gustatory neurons.
• To determine if TrkB expression levels differentiate taste neuron subsets.
• To explore the role of TrkB in gustatory neuron survival and innervation.

Main Methods:

• Utilized transgenic mouse models to label geniculate neurons (Phox2b+) and TrkB-expressing neurons (TrkB^GFP).
• Compared neuron and taste bud numbers in wild-type and conditional TrkB knockout mice.
• Analyzed TrkB expression dynamics in oral and auricular geniculate neurons during development.

Main Results:

• TrkB receptor expression becomes restricted to 50% of oral gustatory neurons between embryonic days 15.5-17.5.
• TrkB downregulation was specific to oral neurons; auricular neurons maintained constant TrkB expression.
• Conditional TrkB removal revealed TrkB-dependent survival for 92% of oral sensory neurons, with remaining neurons failing to innervate taste buds.

Conclusions:

• TrkB expression and dependence delineate three gustatory neuron subpopulations: always TrkB-expressing (50%), TrkB-downregulating (41%), and TrkB-independent (9%).
• TrkB-independent neurons are likely non-gustatory as they do not innervate taste buds.
• Differential TrkB dependence highlights distinct developmental trajectories and functional specializations within gustatory neurons.