Differential expression of CaMKII isoforms and overall kinase activity in rat dorsal root ganglia after injury

- DRG neurons express mRNA for all CaMKII isoforms in the order of γ > α > β = δ.
- Painful nerve injury reduces γ and α CaMKII in axotomized neurons, but elevates δ.
- Neurons adjacent to axotomized neurons show depressed αCaMKII mRNA.
- Total CaMKII protein and activity fall in axotomized more than in adjacent neurons.
- Ca2+-independent catalytic activity falls only in axotomized sensory neurons.

Ca2+/calmodulin-dependent protein kinase II (CaMKII) decodes neuronal activity by translating cytoplasmic Ca2+ signals into kinase activity that regulates neuronal functions including excitability, gene expression, and synaptic transmission. Four genes lead to developmental and differential expression of CaMKII isoforms (α, β, γ, δ). We determined mRNA levels of these isoforms in the dorsal root ganglia (DRG) of adult rats with and without nerve injury in order to determine if differential expression of CaMKII isoforms may contribute to functional differences that follow injury. DRG neurons express mRNA for all four isoforms, and the relative abundance of CaMKII isoforms was γ > α > β = δ, based on the CT values. Following ligation of the 5th lumbar (L5) spinal nerve (SNL), the β isoform did not change, but mRNA levels of both the γ and α isoforms were reduced in the directly injured L5 neurons, and the α isoform was reduced in L4 neurons, compared to their contemporary controls. In contrast, expression of the δ isoform mRNA increased in L5 neurons. CaMKII protein decreased following nerve injury in both L4 and L5 populations. Total CaMKII activity measured under saturating Ca2+/CaM conditions was decreased in both L4 and L5 populations, while autonomous CaMKII activity determined in the absence of Ca2+ was selectively reduced in axotomized L5 neurons 21 days after injury. Thus, loss of CaMKII signaling in sensory neurons after peripheral nerve injury may contribute to neuronal dysfunction and pain.