Intracellular Distribution of Fluorescent Copper and Zinc Bis(thiosemicarbazonato) Complexes Measured with Fluorescence Lifetime Spectroscopy

The
intracellular distribution of fluorescently labeled copper and zinc
bis­(thiosemicarbazonato) complexes was investigated in M17 neuroblastoma
cells and primary cortical neurons with a view to providing insights
into the neuroprotective activity of a copper bis­(thiosemicarbazonato)
complex known as CuII(atsm). Time-resolved fluorescence
measurements allowed the identification of the CuII and
ZnII complexes as well as the free ligand inside the cells
by virtue of the distinct fluorescence lifetime of each species. Confocal
fluorescent microscopy of cells treated with the fluorescent copper­(II)­bis­(thiosemicarbazonato)
complex revealed significant fluorescence associated with cytoplasmic
puncta that were identified to be lysosomes in primary cortical neurons
and both lipid droplets and lysosomes in M17 neuroblastoma cells.
Fluorescence lifetime imaging microscopy confirmed that the fluorescence
signal emanating from the lipid droplets could be attributed to the
copper­(II) complex but also that some degree of loss of the metal
ion led to diffuse cytosolic fluorescence that could be attributed
to the metal-free ligand. The accumulation of the copper­(II) complex
in lipid droplets could be relevant to the neuroprotective activity
of CuII(atsm) in models of amyotrophic lateral sclerosis
and Parkinson’s disease.