Roles for PKC signaling in chromaffin cell exocytosis.

Area of Science:

• Neuroendocrinology
• Cellular Signaling
• Molecular Biology

Background:

• Chromaffin cells in the adrenal medulla mediate the sympathetic stress response by secreting catecholamines.
• Pituitary adenylate cyclase-activating polypeptide (PACAP) is a neurotransmitter that elicits a potent secretory response from chromaffin cells.
• The precise intracellular mechanisms linking PACAP stimulation to secretion, particularly downstream of phospholipase C epsilon (PLCε), are not fully understood.

Purpose of the Study:

• To elucidate the role of signaling events downstream of PLCε in PACAP-mediated chromaffin cell secretion.
• To investigate the involvement of diacylglycerol (DAG) and protein kinase C (PKC) in the PACAP secretory pathway.

Main Methods:

• Measurement of DAG production following PACAP stimulation in chromaffin cells.
• Pharmacological inhibition of PKC using NPC 15437, a DAG-binding inhibitor.
• Assessment of exocytosis, Ca2+ dynamics, and readily releasable pool size.
• Quantitative PCR to determine PKC isoform expression in mouse chromaffin cells.
• Genetic knockdown of specific PKC isoforms (PKCβ, PKCε, PKCμ).

Main Results:

• PACAP stimulation led to PLCε-dependent DAG production, activating PKC and its translocation to the plasma membrane.
• PKC activation was essential for PACAP-induced increases in cytosolic Ca2+ and exocytosis.
• Inhibition of PKC significantly impaired PACAP-evoked secretion, affecting granule pool size, Ca2+ sensitivity, and channel activation.
• PKCβ and PKCε isoforms were highly expressed and critical for PACAP-stimulated secretion, unlike PKCμ.

Conclusions:

• Protein kinase C signaling, initiated by DAG production downstream of PLCε, plays a pivotal role in PACAP-stimulated exocytosis from chromaffin cells.
• Specific PKC isoforms, particularly PKCβ and PKCε, are key regulators of the PACAP-driven secretory pathway.
• This study clarifies a critical signaling cascade involved in the adrenal medulla's stress response.