A new study from the Dulac Lab explores how one small brain region called the preoptic area (POA) of the hypothalamus changes through early life in mice. Though tiny, the POA is a known hub for neurons that govern basic needs like thirst and hunger, as well as neurons that regulate social behavior. Scientists have already studied POA neurons in adult mice, but not juveniles. The team, led by Dulac Lab postdoc Harris Kaplan, was particularly interested in how cell types in the POA manifest during infancy.
“I was initially trained as a developmental biologist,” says MCB faculty and study senior author Catherine Dulac. “And, from the time I started to work on the neural control of instinctive behavior in my lab, it has always been a dream for me to explore how the corresponding neural circuits emerge during development and to assess the respective contributions of genetic versus environmental information. These questions have become accessible in the last few years thanks to the amazing power of the single cell sequencing technology and the ingenious experimental paradigms designed by Harris [Kaplan] in the lab.”
Baby mice have very different lifestyles from adult mice. “Mammals, as infants, obtain food or water from their mother,” Kaplan explains. “They obtain warmth through their parents or their siblings. Their social behavior is really different as young animals. They rely on social behavior for these basic survival needs.”
Kaplan and his colleagues used single-nucleus RNA sequencing at 9 different ages to catalogue over 150 different neuron types that reside in the POA. In most brain regions, neuronal diversity emerges gradually around the time of birth. However, Kaplan and colleagues found that, in the POA, the diversity of cell types emerged much earlier than expected, on embryonic day 14.
However, despite this early diversification, gene expression in the POA neurons did change over the course of infancy. The biggest waves of changes align with the timing of birth, weaning, and puberty. All three of these periods are marked by major behavioral changes. Moreover, neuron types mature, or express genes the same way their adult counterparts do, when they become behaviorally relevant. Kaplan gives the example of thirst neurons that mature around birth when the mouse starts drinking milk. However, many neuron types that are important for social interaction don’t mature until puberty.
“The timing of gene expression changes correlates with behavior changes, which is really interesting,” Kaplan says. “So, the cell types mature when the behaviors mature. It leaves a little bit of an open question: How do gene expression changes affect how these cell types drive behavior? From these data alone, it’s a hard question to answer, and it’s what I want to look at in the future.”
Kaplan says it’s not clear whether the gene expression changes in the neurons are causing the behavioral shifts, or vice versa. He adds that he dislikes the “nature versus nurture” argument, because in the vast majority of cases, both genetics and experience are important for shaping organisms.
The team’s analysis also revealed that striking sex differences in the POA neurons are present from the time of birth, even though many social differences between sexes don’t appear until puberty.
The study’s second author, postdoc Brandon Logeman of the Dulac Lab, tackled a second data set. This data focused on chromatin accessibility. Logeman was able to show that changes in gene expression depend on networks of molecules that bind to DNA called transcription factors.
The team also wanted to know how differences in experience or sensation affect the development of POA neurons, so they repeated the experiment with five types of mutant mice and one group of mice that were reared in total darkness. They only found significant differences in one of the mutants they tested–Trpc2 mutants. Trcp2 knockout mice do not have a functioning vomeronasal organ (VNO), an organ that detects pheromones.
“In these infants that lack VNO sensation, their adult behavior is completely weird,” Kaplan says. “Sex-specific behaviors like parenting or aggression or mating are really different in these animals. And Catherine’s lab was the first to show this. It’s really serendipitous that out of all the mutants we tested, this was the one that showed an effect. It’s full circle for Catherine’s lab.”
“The data we have obtained are extraordinarily rich,” Dulac says. “And are opening the field to a whole range of new fascinating questions on how physiological needs and instincts develop.”
Further experiments to understand how a lack of VNO input affects gene expression are in the works. Kaplan, who is preparing to launch his own lab, also wants to follow up on the POA neurons that regulate sleep and how they change over the course of growing up.
by Harris Kaplan, Catherine Dulac, and Diana Crow
