Jaarboek no. 89. 2010/2011 - Koninklijke Maatschappij voor ...

Natuurkundige voordrachten I Nieuwe reeks 89
Migraine: de ontrafeling van een complexe ziekte
116
Figure 1
Schematic topographic drawing of the CaV2.1 (α1A ) subunit of the P/Q-type voltagegated calcium channel encoded by
the FHM1 gene CACNA1A. The protein consists of four repeating domains (I–IV), each containing six transmembrane
helices (S1–S6). The approximate locations of mutations linked to FHM are indicated. Amino acid positions refer to
GenBank accession number X99897.
the results and their interpretation. In addition,
single-channel measurements (which were so effective
in revealing gain-of-function effects for FHM
mutations, as discussed above) are technically very
challenging to perform and analyze. To help overcome
these factors, the mutation can be introduced
into the orthologous gene in the mouse genome by
homologous recombination (the so-called gene-targeting
strategy, and more specifically the knockin
strategy). The advantages of this approach are that
the mutation is present in all relevant splice forms
of the gene, and the mutant protein is expressed in
the appropriate tissues, at the appropriate levels,
at the appropriate developmental stages, and with
its appropriate binding partners. This strategy was
employed for the R192Q mutation, which neutralizes
a positive charge in the first voltage sensor (see
Fig. 1). In principle, one would predict this mutation
to decrease voltage sensitivity. However, a detailed
examination of the channel properties revealed a
clear gain of function: despite being expressed at
levels equivalent to wild-type channels, much larger
currents were observed at negative voltages (at −30
mV, the current through mutant channels was bet-
ter than three times that of wild-type channels); at
positive voltages, mutant channels carried the same
current as wild-type channels. Thus, the increased
currents at negative voltages could be explained in
full by a negative shift in activation for the mutant
channels. This shift is consistent with similar effects
observed for other FHM1 mutations.
Consistent with the role of CaV2.1 channels in
mediating transmitter release at the neuromuscular
junction, the R192Q mutant mice exhibited
increased transmission from motor neurons. This
increase was reflected in both a higher frequency
of miniature (spontaneous) endplate potentials and
larger evoked endplate potentials, both of which
showed a gene-dosage effect. Interestingly, the
effect of the mutation on evoked endplate potentials
was observed only when extracellular Ca2+ was
reduced to subphysiological levels, suggesting a
ceiling effect in physiological Ca2+ concentrations,
and perhaps explaining why R192Q patients do not
present with chronic neuromuscular deficits. As
discussed in Section III, CSD is believed to be the
physiological correlate of the migraine aura. The
naturally occurring mouse Cacna1a mutants tot-