L6 - Hypothalamus - Gene environment interactions and stress

explain the hypothalamic stress axis - hypothalamus-pituitary-adrenal (HPA) axis

• within the paraventricular nucleus of the hypothalamus there are specialised neurons that produce cotricotropin-releasing-hormone (CRH)

• these neurons release CRH from their axon terminal into the portal capillary network, a specialised brain-body interface

• CRH is transported to the anterior pituitary where it stimulates the release of adrenocorticotropic hormone (ACTH)

• ACTH enters the circulation and acts on peripheral organs to mediate the body’s stress response

where are cotricotropin releasing hormone neurons located and what do they arise from?

• located in the paraventricular nucleus (PVN)

• arise from paraventricular nucleus progenitors

where are paraventricular neurons born and what do they do?

• born in the paraventricular nucleus

• they extend axons to the median eminence (ME) and posterior pituitary

• the axons projecting to the median eminence release hormones including cotricotropin releasing hormone into the portal capillary network

where is the corticotropin releasing hormone transported and what do they stimulate?

• transported to the anterior pituitary

• stimulates corticotrophs (specialised class of endocrine cells) which regulates release of pituitary hormones like adrenocorticotropic hormone (ACTH)

what are mouse models used for?

• research into feeding and energy homeostasis because changes in body weight are easy to measure and provide a clear behavioural readout

• relative small samples can be used

why can’t mouse models be used when studying behaviours such as stress response?

• such studies require larger animal numbers to achieve statistical effects

• this would make mouse studies expensive

what animal model is used to study stress pathways and why?

• zebrafish

• as they are easy to maintain in large numbers

• allow efficient analysis of hypothalamic neurons involved in stress regulation

explain the stress axis (HPA) regulation in response to stress

• cortisol is the main stress hormone in the body released from the adrenal gland

• in response to stress, CRH neurons in the hypothalamus release CRH into the portal circulation

• this stimulates corticotrophs in the anterior pituitary to release ACTH

• ACTH travels through the bloodstream to the adrenal glands triggering the release of cortisol

• cortisol exerts a negative feedback on the hypothalamus and pituitary, inhibiting further CRH and ACTH release, thereby regulating the stress response

how was the gene DISC1 (Distributed In Schizophrenia 1) first identified?

• through genetic analysis of an inbred family with high incidence of mental health disorders, like schizophrenia, depression, bipolar, and anxiety

• by tracing the family lineage and sequencing DNA across generations, researchers found that all affected individuals had a shared mutation in the DISC1 gene

• this linked DISC1 to stress-related psychiatric illness

how was the expression of DISC1 in development studied?

• in situ hybridisation was used to determine where DISC1 mRNA is expressed during development

• in zebrafish embryos, DISC1 is expressed in the developing hypothalamus, with mRNA present early in embryogenesis

when is DISC1 expressed?

• DISC1 expression was detected in progenitor cells at the stage when they are deciding between tuberal or PVN/mammillary progenitor fates

• suggesting a role for DISC1 in early hypothalamic cell fate decisions

what did the generation of DISC1 mutant zebrafish show?

• two DISC1 mutant zebrafish lines were generated

• both survived through embryonic, larval and adult stages

• however neither mutant produced full length DISC1 mRNA or full length DISC1 protein

• the viability of DISC1 mutants indicates that DISC1 is not required for survival but instead regulates specific aspects of brain development and stress-related neural function

how was consequences of DISC1 loss studied and what are the consequences of DISC1 loss on hypothalamic progenitors?

• wild type and DISC1 mutant zebrafish were compared to study hypothalamic development focusing on PVN/mammillary and tuberal progenitors

• Rax1 was used as a marker for tuberal progenitors

• DISC1 mutants had fewer or weaker Rax1+ cells compared to wild-type

• this indicates that DISC1 is important for normal specification or maintanence of tuberal hypothalamic progenitors

what are the effects of DISC1 mutations on hypothalamic neurons?

• Pomc neurons originate from Islet1 progenitors which arise from earlier Rax expressing progenitors

• in DISC1 mutants Pomc expression is weaker and there are less Pomc neurons compared to wild-type

• in DISC1 mutants CRH + PVN neurons are increased

• this indicates a shift in hypothalamic cell fate from tuberal (Pomc) to paraventricular (CRH) lineages

• this validates that DISC1 regulates hypothalamic development by controlling progenitor cell fate between tuberal/Pomc and PVN/CRH neurons

in DISC1 mutants what causes an exaggerated stress response ?

• excess CRH PVN neurons in DISC1 mutants leads to elevated CRH, ACTH and cortisol which causes an exaggerated stress response

explain the behaviour assay in DISC1 mutants vs wildtype

• in zebrafish, exposure to stress related pheromone Schreckstoff normally triggers a sequence of behaviours - dating around followed by freezing

• in DISC1 mutants, fish fail to respond to Schreckstoff and continue to swim indicating a disrupted stress response

• this assay showed that DISC1 is critical for a normal stress response

explain the neuroendocrine stress assay in DISC1 mutants

• two DISC1 mutant zebrafish lines were exposed to a stressor and then immediately sacrificed to measure cortisol levels

• in wild type, stress caused a clear increase in cortisol

• in DISC1 mutants, failed to upregulate cortisol in response to the same stressor

• indicating that DISC1 is necessary for a normal neuroendocrine stress response