Question

If NAD+ levels are low, what pathways are being affected?

Answer 1

The fermentation pathway kicks in and in that way restore NAD+.

Answer 2

There are a number of enzymes which act as deacetylases, including SIRT1 which depend on NAD+. These are of particular interest because they are involved in aging, transcriptional control and apoptosis.
Apoptosis is programmed cell death and normally takes place in cells with overexpressed oncogenes. However, overexpression of the SIRT1 deacetylase can inactivate the mechanism which normally induces apoptosis, allowing the cells to carry on dividing. Hence SIRT1 is a molecule which is being studied extensively at the moment which depends on NAD+.
It is thought that when a cell is low on energy the NAD+ increases the SIRT1 activity. This perhaps may be part of the mechanism by which SIRT1 can expand lifespan and so agents which can enhance SIRT1 activity are of interest (eg resveratrol). Here's a link if you're interested:

http://en.wikipedia.org/wiki/Sirtuins